EPA/600/2-88/005
January 1988
COPPER DUMP LEACHING AND
MANAGEMENT PRACTICES THAT MINIMIZE
THE POTENTIAL FOR ENVIRONMENTAL RELEASES
by
Robert Hearn and Robert Hoye
PEI Associates, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
EPA Contract No. 68-02-3995
Work Assignment Nos. 1/025 and 2/047
Project Officer
S. Jackson Hubbard
Manufacturing and Service Industries Branch
Water Engineering Research Laboratory
Cincinnati, Ohio 45268
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45246
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completingj
1. REPORT NO. 2. 1
EPA/600/2-88/005
3. RECIPIENT'S ACCESSION NO. .
PBS 8 1 Eli 4/AS
A. TITLE AND SUBTITLE
COPPER DUMP LEACHING AND MANAGEMENT PRACTICES THAT
MINIMIZE THE POTENTIAL FOR ENVIRONMENTAL RELEASES
5. REPORT DATE
January 1988
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert Heam
Robert Hoye
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEI Associates, Inc.
11499 Chester Road
Cincinnati, OH 45246
10. PROGRAM ELEMENT NO.
1 1. CONTRACT/GRANT NO.
68-02-3995
12. SPONSORING AGENCY NAME AND ADDRESS
Hazardous Waste Engineering Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
Project Officer: Jackson Hubbard
16. ABSTRACT
This report presents a description of the magnitude and distribution of
copper dump leaching, the design and operation of leaching facilities, the
potential for environmental impact, and management practices that can be used
to minimize environmental releases. Ttie information contained in the report
was obtained through searches of published and unpublished literature and
through contact with knowledgeable individuals involved in the dump leaching
industry, Tten leaching operations were visited to acquire firsthand knowl-
edge and site-specific information.
Seepage frcm leach dumps and process solution collection systems is the
most significant potential mechanism for the release on contaminants. These
solutions have low pH and high concentrations of metals and total dissolved
solids (TDS). Ground-water impacts have been documented. -The application
and efficiency of standard waste management practices at dump leach opera-
tions are site-specific and are limited by the magnitude of these facilities.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b. IDENTIF IE RS/OPEN ENDEDTERMS
c. cosati Field/Group
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Reportj
UNCLASSIFIED
21. NO. OF PAGES
168
20. aeCURiT Y CLASS (This page)
UNCLASSIFIED
22. PRICE
CPA F*rm 2220-1 (R«v. 4-77) previous edition is O0solete
1
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FOREWARD
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation of
solid and hazardous wastes. These materials, if improperly dealt with, can
threaten both public health and tne environment. Abandoned waste sites and
accidental releases of toxic and hazardous substances to the environment also
have important environmental and public health implications. The Hazardous
Waste Engineering Research Laboratory assists in providing an authoritative
and defensible engineering basis for assessing and solving these problems.
Its products support the policies, programs and regulations of the
Environmental Protection Agency, the permitting and other responsibilities of
the State and local governments, and the needs of both large and small
businesses in handling their wastes responsibly and economically.
This report presents a description of the magnitude and distribution of
copper dump leaching, the design and operation of Teaching facilities, the
potential for environmental impact, and management practices that can be used
to minimize environmental releases. The information contained in the report
was obtained through searches of published and unpublished literature and
through contact with knowledgeable individuals involved in the dump leaching
industry. Ten leaching operations were visited to acquire firsthand knowledge
and site-specific information. Seepage from leach dumps and process solution
collection systems is the most significant potential mechanism for the release
of contaminants. These solutions have low pH and high concentrations of
metals and total dissolved solids (TDS). Ground-water impacts have been
documented. The application and efficiency of standard waste management
practices at dump leach operations are site specific and are limited by the
magnitude of these facilities.
Thomas Hauser, Director
Hazardous Waste Engineering Research Laboratory
iii
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ABSTRACT
This report presents a description of the magnitude and distribution of
copper dump leaching, the design and operation of leaching facilities, the
potential for environmental impact, and management practices that can be used
to minimize environmental releases. The information contained in the report
was obtained through searches of published and unpublished literature and
through contact with knowledgeable individuals involved in the dump leaching
industry. Ten leaching operations were visited to acquire firsthand knowl-
edge and site-specific information.
Seepage from leach dumps and process solution collection systems is the
most significant potential mechanism for the release of contaminants. These
solutions have low pH and high concentrations of metals and total dissolved
solids (TDS). Ground-water impacts have been documented. The application
and efficiency of standard waste management practices at dump leach opera-
tions are site-specific and are limited by the magnitude of these facilities.
iv
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CONTENTS
Page
Foreword i 1 i
Abstract i v
Figures vii
Tables viii
1. Introduction 1
Background 1
Purpose and scope 3
Content 4
2. Overview of Leaching Practices in the Copper Industry 6
Industry characterization 6
Fundamentals of copper leaching 9
Characteristics and geographic distribution of copper
leaching sites 17
3. Design and Operation of Copper Leaching Systems 24
Dump leaching 24
Heap leaching 28
Other leaching processes 31
Copper recovery processes 34
4. Potential for Environmental Impact 39
Potential mechanisms for the release of contamination 40
Sources and characteristics of potential ground-water
contamination 41
Ground-water contamination data 51
5. Alternative Management Practices 61
Sit? characterization and monitoring 64
Leachate control systems 71
Ground-water management systems 76
Surface-water management systems 84
Reclamation and closure activities 91
Other alternative management practices 96
(continued)
v
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CONTENTS (continued)
Page
6. Conclusions and Recommendations 100
Conclusions 100
Recommendations 104
References 107
Bibliography 110
Appendix A Trip Reports 119
Cyrpus Bagdad Mining Company 120
Noranda Lakeshores Mines, Inc. 124
Silver Bell Mine 127
Inspiration Consolidated Copper Company 131
Pinto Valley Copper Corporation 135
Ray Mines Division 139
San Manuel Mine 144
Tyrone Mine 148
Cyprus Johnson Copper Company 152
Bingham Canyon Mine 155
vi
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
FIGURES
Page
Hydrometallurgical Processing of Copper 7
Geographic Distribution of Active Copper Leaching
Operations in the United States 20
Geographic Distribution of Inactive Copper Leaching
Operations in the United States 23
Typical Leach Dump 26
Typi cal Preci pi tation PI ant 36
Typical Solvent Extraction/Electrowinning Plant 38
Solubilities of Oxides and Hydroxides of Various Metals 47
Map of the Chino Operations 53
Location of the Ground-Water Monitoring Wells in Relation
to the Leach Dumps at the Tyrone Mine 55
Concentration of Total Dissolved Solids in Wells Around
Tyrone's No. 2 Leach Dump 56
Concentration of Dissolved Iron in Wells Around Tyrone's
No. 2 Leach Dump 56
pH level in Hells Around Tyrone's No. 2 Leach Dump 57
Globe/Miami Mining District 58
Past and Present Mining Activities in the Globe/Miami Area 60
vii
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TABLES
Number Page
1 Annual Production of Primary Copper, 1975-1984 9
2 Salient Characteristics of Copper Leaching Methods 11
3 Principal Copper Minerals 12
4 Inventory of Active Copper Leaching Operations in the
United States 18
5 Inventory of Inactive Copper Leaching Operations in the
United States 21
6 Potential Acidity and EP Toxic Characteristics of Spent
Leach Material 43
7 Elemental Composition of Barren Aqueous Solution From
Cementation Process 44
8 Concentration of EP Toxic Metals in Samples of Pregnant
Leach Liquors 48
9 Relative Mobilities of the Elements 49
10 Elemental Composition of Pregnant Leach Liquor 52
11 Comparison of Surface Runoff, Ground Water, and Arizona
Stream Standards, Bellview-Gibson Area Sampling 59
12 Management Practices by Operational Phase 63
13 Methods of Well Installation 69
14 1986 Costs for Drilling and Installing 2- to 4-Inch-
Diameter Wei Is 70
15 1986 Liner Installation Costs 75
16 Criteria for Well Selection 78
(continued)
viii
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TABLES (continued)
Number Page
17 1986 Costs for Selected Pumps and Accessories 82
18 Summary of Seven Recovery System Cost Scenarios 83
19 1986 Costs of Materials and Installation for Subsurface
Drains 85
20 1986 Costs of Installing a Slurry Wall 87
21 1986 Costs of Common Grouts 88
22 1986 Costs for Ground Barrier in Rock 88
23 1986 Costs for Establishing Surface Water Controls 92
24 Typical Costs for Capping and Revegetation 95
ix
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SECTION 1
INTRODUCTION
BACKGROUND
The copper industry in the United States has been piling mine wastes
(i.e., overburden and mine waste rock) and low-grade ores in and around
mining sites for much of this century. Most of these wastes contain signif-
icant amounts of pyrites and other naturally occurring metal sulfides, but
they contain too little copper for recovery by conventional milling. With
the addition of sufficient water through precipitation, air, and the activity
of autotrophic bacteria, this material can generate a leachate that has a low
pH and contains high concentrations of copper and other metals.
In the 1920's, large-scale commercial leaching of these waste piles was
initiated to recover the copper. These operations entailed the addition of
sulfuric acid to the piles to accelerate the leaching process, collection of
the leachate, and extraction of the copper by iron precipitation. The sites
for these leach dumps were selected primarily to minimize haulage distances
and thereby reduce costs. Very little consideration was given to how such
sites would affect the environment. Consequently, most of the copper leach-
ing operations were uncontrolled from an environmental standpoint, and at
least some leachate entered the surface and ground waters surrounding the
s i te.
Concerns about the environmental impact of mining operations began to
gain public attention around 1970. Specific problems, such as discharges
into surface waters and emissions into the air from copper recovery processes,
were addressed by the Federal Water Pollution Control Act, the Clean Air Act,
and various other Federal and State laws. The environmental effects of the
solid waste management practices used by the mining industry at such sites,
however, were not addressed until 1976, when the Resource Conservation and
1
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Recovery Act (RCRA) was enacted. Section 8002(f) of RCRA required the U.S.
Environmental Protection Agency (EPA) to conduct an investigation of all
solid waste management practices in the mining industry. That mandate
specifically directed EPA to conduct "a detailed and comprehensive study on
the adverse effects of solid wastes from active and abandoned surface and
underground mines on the environment, including, but not limited to, the
effect of such wastes on humans, water, air, health, welfare, and natural
resources.
In 1980, Congress amended RCRA to exclude waste materials generated by
the "extraction, beneficiation, and processing of ores and minerals" from
many of the requirements of Subtitle C. The 1980 amendments also added
Section 8002(p), which directed EPA to conduct a "detailed and comprehensive
study on the adverse effects on human health and the environment, if any, of
the disposal and utilization of solid wastes from the extraction, benefici-
ation, and processing cf ores and minerals." In addition, Section 7 of these
amendments (the "Bevill Amendment") amended Section 3001 to exclude these
wastes from regulation under Subtitle C pending completion of the studies
required by Sections 8002(f) and (p). The EPA was required to make a regu-
latory determination within 6 months after submitting the study to Congress
as to whether regulations would be promulgated or if regulations were unwar-
2
ranted for such mining and beneficiation wastes.
A report mandated by Sections 8002(f) and (p) was submitted to Congress
3
on December 31, 1985. Copper dump leaching practices were among the extrac-
tion and beneficiation processes discussed in the report. The report treated
copper leaching as a waste management system, which would make it subject to
regulation under RCRA. The report concluded that the low pH and the poten-
tially high concentrations of metals found in the leachates and leach mate-
rial used in copper leaching operations made these wastes potentially hazard-
ous to human health and the environment and possibly justify the listing of
dump leaching wastes as hazardous.
On July 3, 1986, the EPA issued the regulatory determination required
4
by Congress. After reviewing the comments received in connection with its
report to Congress, the EPA concluded that regulation of mining wastes under
Subtitle C of RCRA was not warranted at this time. The notice indicated that
RCRA's hazardous waste management standards "are likely to be environmentally
2
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unnecessary, technically infeasible, or economically impractical when applied
to mining wastes." The EPA stated that "dump and heap leach piles are not
wastes; rather, they are raw materials used in the production process.
Similarly, the leach liquor that is captured and processed to recover metal
values is a product, not a waste. Only the leach liquor which escapes from
the production process and abandoned heap and dump leach piles are wastes."
The EPA expressed continued concern, however, about problems associated with
mining wastes, such as high acid-generation potential, radioactivity, asbes-
tos content, and cyanide content. Because of these concerns, the EPA indi-
cated that it would develop a program for mining wastes under Subtitle D of
RCRA. This program will be designed to investigate and address the problems
associated with mining wastes and will include criteria specifically tailored
to the "unique characteristics" of mining operations.
To develop a program under Subtitle D that appropriately addresses the
problems associated with mining wastes, the EPA must collect additional
information on the nature of mining wastes, current waste management prac-
tices, and the potential for exposure to these wastes. This report addresses
these issues with regard to the development, operation, and closure activities
associated with copper dump leaching operations.
PURPOSE AND SCOPE
The purpose of this report is to describe the characteristics of current
copper dump leaching operations in the Western United States. This analysis
includes an inventory of active, inactive, and abandoned copper leaching
sites and a sumnary of available environmental impact data from such sites.
It also examines current management practices and possible alternative prac-
tices that could be used to reduce the environmental impact of copper leach-
ing operations. In addition to dump leaching, other leaching methods that
are being used are also described.
The data included in this report were obtained primarily from a litera-
ture search and from visits to the major copper leaching operations in the
Southwest. The intent of the literature search was to collect information on
dump leaching practices, recovery technologies, and the environmental impact
of leaching operations in the copper mining industry. State environmental
3
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personnel and experts representing such organizations as the Bureau of Mines
were also consulted to identify any ongoing research activities dealing with
copper leach practices and control technologies. Ten active and inactive
copper leach sites were visited to obtain information on the operation and
characteristics of the leaching operations. A bibliography of selected
references identified during the literature search is included at the end of
this report. The findings from the site visits are presented in the trip
reports contained in Appendix A.
CONTENT
Section 2 provides an overview of the copper industry and the signifi-
cance of leaching practices within that industry. It also provides a de-
scription of the basic characteristics of copper leaching and presents an
inventory of active, inactive, and abandoned sites, including information on
the location and production capacity of each of the sites.
Section 3 provides a more detailed description of the copper dump leach-
ing practices currently used in the United States. In addition to a detailed
discussion of dump leaching operations (the most prevalent method in the
United States), it includes a description of heap, in situ, vat, and agita-
tion leaching operations. Descriptions of methods used to recover copper
from solution are also presented.
Section 4 presents a summary of existing monitoring data regarding the
environmental impact of copper leaching operations. The summary includes a
description of the characteristics of the leaching material, the leaching
solutions, and the copper ladden liquids (i.e..pregnant liquor solutions),
which are the primary sources of potential ground-water contamination.
Section 5 presents a discussion on the following management practices
that may be used to reduce the potential for ground-water contamination by
leachates released by copper dump leaching operations: site characterization
and ground-water monitoring techniques, ground-water management systems,
surface-water management practices, leachate control systems, and reclamation
and closure activities. Illustrative costs associated with the implementa-
tion of each of these practices are also presented and discussed.
4
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Section 6 summarizes the information provided in the report and presents
conclusions concerning the potential impact of copper leaching operations on
human health and the environment and the effect of existing and proposed
management practices on surface-water and ground-water contamination. Areas
in which additional information is required are also identified.
5
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SECTION 2
OVERVIEW OF LEACHING PRACTICES IN THE COPPER INDUSTRY
Leaching is a hydrometallurgical process that separates a valuable
product from the gangue materials or host rock by dissolving the product in a
solvent solution. The product is then recovered from solution in a relative-
ly pure form by a chemical or electrolytic process. In the copper industry,
dump leaching methods are used to extract copper from ores too low in grade
to concentrate by conventional beneficiation and froth flotation. The
dissolved copper is subsequently recovered from solution by precipitation
onto scrap iron or by solvent extraction and electrowinning. The flow dia-
gram in Figure 1 depicts dump leaching and hydrometallurgical processing of
copper.
This section presents an overview of the copper mining industry in the
United States and the significance of dump leaching operations to the indus-
try. The chemistry and basic operating characteristics of the leaching
methods most commonly practiced in the United States are also described.
Finally, a list of currently active, inactive, and abandoned copper leaching
sites is presented.
INDUSTRY CHARACTERIZATION
The first use of leaching to recover copper from ores is believed to
5
have occurred in the Rio Tinto area of Spain. Records in that area indicate
that concessions were granted for the recovery of copper from leach liquors
in 1752.6 The controlled leaching of uncrushed, low-grade copper ore was not
introduced into the United States, however, until 1914•^ Today, dump leach-
ing is an integral part of most active mining operations.
Copper mining is centered in three States: Arizona, New Mexico, and
3
Utah. Other States where copper is mined include Nevada, Montana, Michigan,
6
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CONVENTIONAL
PROCESSING
HIGH GRADE ORE
LOW
GRADE
ORE
POTENTIAL
SEEPAGE AND
LEACHATE
RELEASE
SURFACE
MINE
LEACH ORE
DUMP
PREGNANT
LIQUOR
BARREN SOLUTION
BARREN SOLUTION
a CEMENT
COPPER
SLURRY
BARREN
SOLVENT
LOAOED
SOLVENT
WATER
ELECTROLYTE
STRIPPING
PREGNANT
ELECTROLYTE
SPENT
ELECTROLYTE
COPPER
CATHODES
CEMENT
COPPER
ACID OR
MAKEUP WATER
ADDITION
DECANTING
DRYING
ELECTROWINNING
SOLVENT
EXTRACTION
PRECIPITATION
RECYCLE
TO LEACH
OPERATION
Figure 1. Hydrometallurgical processing of copper.
7
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and Tennessee. Copper is also frequently recovered as a byproduct from the
processing of silver, lead, and zinc ores. These operations are centered in
Idaho, Missouri, and Tennessee, respectively. In 1985, only 27 primary and
41 byproduct copper mines were active. By comparison, 48 primary and 65
byproduct copper mines were active in 1975 (personal cormiunication from J. L.
W. Jolly, U.S. Bureau of Mines, Washington, D.C., August 1986). The number
of active copper mines in the United States has and continues to decline
o
because of world overproduction and accompanying low prices.
Of the active operations, surface mining accounts for approximately 90
Q
percent of the copper ore produced annually. This method of mining dis-
places enormous amounts of soil and rock in the process of gaining access to
and exploiting the principal ore body. In 1983, more than 234 million metric
tons of such waste rock was generated by copper mining operations.^ In
Arizona alone, about 2.5 billion metric tons of waste rock were generated
during the 10-year period from 1975 to 1984 (personal communication from J.
B. Hiskey, Arizona Bureau of Geology and Mineral Technology, Tucson, Arizona,
August 20, 1986). Because much of this material contains small amounts of
copper (less than 0.3 percent), it is often segregated from the barren waste
rock, deposited in huge dumps adjacent to the mine pit, and leached with a
dilute acid solution to recover additional copper values. Cement copper
(copper that has been recovered from the leach solution by precipitation onto
iron) generally requires smelting and refining by conventional means to
produce high-grade copper. The cathode copper obtained by electrowinning,
however, is generally of sufficient grade for direct casting without addi-
tional processing. Many of the larger copper-producing companies are fully
D
integrated from mine to refinery.
Table 1 presents annual primary copper production figures for 1975
through 1984. Although total primary production has declined over the last
10 years, the percentage of copper produced by leaching operations has in-
creased during the same period. In the future, as lower grade ores are mined
and the costs of conventional milling and smelting continue to rise, 7eaching
is expected to account for an increasing percentage of the total primary
copper production in the United States.*® Some researchers estimate that by
8
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1990 leaching will account for as much as 25 to 30 percent of the annual
copper production (personal communication from J. B. Hiskey, Arizona Bureau
of Geology and Mineral Technology, Tucson, Arizona, August 20, 1986).
TABLE 1.
ANNUAL PRODUCTION OF PRIMARY
COPPER, 1975*
-1984a
Combined
precipitate
and electrowon
No. of
production,
principal
Total primary
Precipitate
Electrowon
% of
copper
production,
production,
production,
total primary
Year
mines
metric tons
metric tons
metric tons
production
1975
48
1,280,000
131,000
42,900
13.6
1976
46
1,460,000
114,000
71,900
12--8
1977
44
1,360,000
122,000
81,100
14.9
1978
42
1,360,000
111,000
98,400
15.4
1979
40
1,450,000
127,000
100,000
15.7
1980
41
1,180,000
102,000
118,000
18.6
1981
41
1,540,000
114,000
161,000
17.9
1982
36
1,150,000
105,000
132,000
20.6
1983
31
1,040,000
89,300
102,000
18.4
1984
31
1,090,000
80,800
110,000
17.5
a Source: Reference 9.
FUNDAMENTALS OF COPPER LEACHING
In recent years, world overproduction of copper has driven its price to
post-World War II lows. Further, stricter environmental laws in this country
have increased procuction costs. Hydrometallurgical processing of copper,
which has been commercially developed over the last 20 years, provides and
economical alternative to conventional smelting and refining operations.
These developments have had an important impact on U.S. copper production and
have resulted in a significant increase in the application of leaching technol-
ogy to low-grade ores.
Leaching Methods
Dump leaching is the principal method by which copper values are leached
from low-grade (i.e., less than 0.5% copper) ore. Other methods that are
9
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used to a lesser extent or are currently under development include heap vat,
agitation, and in situ leaching. These methods differ with respect to the
degree of site preparation, the type and grade of ore leached, ore prepara-
tion, the characteristics of the leach solution, the duration of the leach
cycle, and the size of the operation.** The salient characteristics of the
various leaching methods are summarized in Table 2.
Ores and Reagents
The U.S. copper industry is based predominantly on production from
porphyry deposits (large, relatively low-grade occurrences of disseminated
12
mineralization). Copper occurs in these deposits primarily as sulfide or
oxide minerals. The principal copper minerals are listed in Table 3. Ap-
proximately 90 percent of the copper ore mined in the United States occurs in
sulfide ore.** Oxide ores originated mainly from supergene chalcocite depos-
its (those formed by descending solutions). Circulating surface water caused
the copper sulfide deposits to oxidize or weather and, under various condi-
tions, to precipitate as copper oxide minerals.
Historically, sulfide and oxide ores containing less than about 0.3
percent copper were deemed too lean in copper to be smelted directly or to be
processed into a concentrate. Heating and melting of huge quantities of
worthless material would have required too much energy and too great a fur-
nace capacity. Isolation of the copper minerals in a concentrate also would
have required considerable amounts of energy for adequate crushing and grind-
ing of the ore and effective separation of the copper from other minerals in
the ore. Hence, these materials were generally discarded in large dumps in
and around mining sites and were leached as an afterthought. Dump leaching
of low-grade ores is an integral part of most current copper mining opera-
tions.
The mineralization of these leach materials is important to the leaching
process. The type of copper mineral controls the reaction chemistry and the
rate at which copper is dissolved. In the case of sulfide ores, pyrite
(FeS^), a common constituent, is important because it leads to the formation
of ferric sulfate, sulfuric acid, and heat, which aid in the leaching of the
copper minerals.** Host rock gangue minerals also have an important role in
10
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TABLE 2. SALIENT CHARACTERISTICS OF COPPER LEACHING METHODS3
Leaching
method
Mineralization
% Cu
in ore
H2S0i4 con-
centration
in leachant,
kg/m3
Cu concen-
tration in
pregnant
solution,
kg/m3
Leach
cycle
Representative
size of
operation
Copper
leached,
metric
tons/day
Dump
Sulfide or mixed
oxide/sulfide
wastes
0.2-1
1-5
1-2
3-30
years
5 x 10^ metric
tons of ore
100
Heap
Oxide
0.5-1
2-10
2-5
4-6
mos.
3 x 10^ metric
tons of ore
20
In situ
Oxide (with some
sulfide)
0.5-1
1-5
1-2
5-25
years
4 x 10^ metric
tons of ore
20
Vat
Oxide
1-2
50-100
30-40
5-10
days
6-12 vats
10-120
Agita-
tion
Oxide (concentrate) 20-30
50-100
30-50
2-5
h
45 leach tanks
47 thickeners
350
Roaster calcines
30-40
50-100
30-50
2-5
h
3 Source: Reference 11.
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the leaching of low-grade ores. Gangue minerals can neutralize large quanti-
ties of acid and can, by their decomposition, add various species to the
leaching solution.
TABLE 3. PRINCIPAL COPPER MINERALS3
Composition
CuFeS^
Cu2S
CuS
Cu5FeS4
2CuC03-Cu(0H),
CuC03-Cu(0H)2
CuSi03-2H20
CU2O
Mineral
Sulfides
Chalcopyrite
Chalcocite
Covellite
Bornite
Oxides
Azurite
Malachite
Chrysocolla
Cuprite
a Source: Reference 8.
In dump leaching of sulfide ores, the principal leaching agent is ferric
ion generated by autotrophic bacteria (personal communication from M. E. Wads-
worth, Dean at the College of Mines and Mineral Industries, University of
Utah to Dr. Abron-Robinson of Peter Consultants, Inc., letter dated October
9, 1986). These bacteria generate acid needed for acid-consuming reactions
including oxygen reduction. The bacteria also oxidize sulfate and thus forir
more sulfuric acid. Sulfuric acid also may be added to the barren leach solu-
tion applied to the dump. Most leaching operations can obtain sulfuric acid
rather inexpensively from copper smelters that produce large amounts of the
acid from the sulfur dioxide (SO^) gases generated in the smelting, roasting,
and converting operations. Another advantage of sulfuric acid is that it
rapidly solubilizes copper oxides and is regenerated when sulfide minerals
12
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are leached. Other leaching agents include solutions of ammonia hydroxide
and ammonia carbonate, which enhance the solubility of copper oxides through
complex formation, and solutions of ferric sulfate and ferric chloride, which
oxidize insoluble copper sulfides to soluble species.
Chemistry of the Leaching Process
13
The chemistry of leaching copper minerals is well documented. Most
oxidized copper ores dissolve rapidly in dilute sulfuric acid. Sulfide
minerals, on the other hand, are not soluble in sulfuric acid unless oxidiz-
ing conditions exist. Even then, the reaction is much slower unless the
oxidizing conditions are very strong.** The rate at which copper is leached
from dumps is diffusion-controlled. The rate of dissolution depends upon a
variety of factors, including the amount of contact area between the leaching
solution and the mineral solids, the effect of bacterial activity, the types
of copper minerals present, the acid strength, and the reaction temperature.
Some of the reactions by which copper oxide ores are leached with sul-
furic acid are presented in Equations 1 through 3. Cuprite and native copper
are common nonsulfide minerals that require some oxidation to dissolve the
copper. The reactions by which these minerals are dissolved are presented in
13
Equations 4 through 6.
2CuC03-Cu(0H)2(s) + 3H2S04(aq) t 3CuS04(aq) + 2C02(g) + 4H20(1) (Eq. 1)
CuC03-Cu(0H)2(s) + 2H2S04(aq) J 2CuS04(aq) + C0?(g) + 3H20(1) (Eq. 2)
Azurite
Malachite
Chrysocolla
CuSi03-2H20(s) + H2S04(aq) t CuS04(aq) + SiOgCs) + 3H20(1)
Cuprite
(Eq. 3)
Cu20(s) + H2S04(aq) t CuS04(aq) + Cu(s.) + H20(1)
Cu^O(s) + H2S04(aq) + Fe2(S04)3(aq) * 2CuS04(aq) + H20(1)
+ 2FeS04(aq)
(Eq. 4)
(Eq. 5)
13
-------
Native Copper
Cu(s) + Fe2(S04)3(aq) t CuS04(aq) + 2Fe$04(aq) (Eq. 6)
The oxidizing conditions required for copper sulfide minerals are
provided by the presence of ferric sulfate, which is generated when pyrite in
the ore is exposed to moisture and oxygen in the atmosphere and to bacterial
activity. Some of the primary reactions by which the major copper sulfide
13
minerals are leached are presented in Equations 7 through 12.
Chalcopyrite
CuFeS2(s) + 2Fe2(S04)(aq) = CuS04(aq) + 5FeS04(aq) + 25° (Eq. 7)
S° + 1.5 02(g) + H^O + bacterial activity = f^SO^aq) (Eq. 8)
Chalcocite
Cu2S(s) + Fe2(S04)3(aq) £ CuS(s) + CuS04(aq) + 2FeS04(s) (Eq. 9)
Cu2S(s) + 2Fe2(S04)3(aq) £ 2CuS04(aq) + 4FeS04(aq) + S(s) (Eq. 10)
Covel1ite
CuS(s) + Fe2(S04)3(aq) £ CuS04(aq) + 2FeS04(aq) + S(s) (Eq. 11)
CuS(s) + 202(g) £ CuS04(aq) (Eq. 12)
The amount of copper released during leaching of low-grade sulfide ores
has been found to be directly proportional to the quantity of oxygen reacting
with the ore. The rate of oxidation depends on a variety of factors. The
rate can be maximized, however, by maintaining the pH of the solution rela-
tively low; the lower the pH, the faster the rate of oxidation. At pH levels
14
above 2.5 to 2.6, the leaching of copper appears to slow considerably.
The process of leaching copper from sulfide minerals requires the pres-
ence of autotrophic bacteria. Although the mechanism of this process is not
completely understood, it is believed that most sulfide mines contain this
bacteria and that use of mine waters for the leach solution provides the
initial cultures of the bacteria for the leach systems.11 Autotrophic bac-
teria, such as Thiobaci11 us ferrooxidans, promote the oxidation of pyrite to
ferrous sulfate and sulfuric acid, which, in turn, react with the copper
14
-------
13
sulfide minerals to produce copper sulfate in solution. The bacteria
require oxygen to meet metabolic needs. The coupled reactions are:
bac
1/2 02(aq) + H2S04(aq) + 2FeS04(aq) = Fe2(S04)3(aq) + H20 (Eq. 13)
This reaction is acid-consuming. The bacterial reactions, coupled with other
nonbioextractive reactions within the dump, result in a dynamic buffering
condition that causes a constant pH.
The production of sulfuric acid is a requirement for effective dump
leaching. Acid generation makes it possible for oxygen discharge and ferric
ion production to occur under microbial activity. The direct oxidation of
pyrite results in the formation of two equivalents of acid per molecule of
original pyrite:
FeS2 + 3.502 + H20 * Fe2+ + 2S042" + 2H+ (Eq. 14)
In addition, the ferrous iron produced by this reaction undergoes oxidation
to ferric iron and precipitates as a hydroxide, which results in two addi-
tional acid equivalents:
14Fe2+ + 3.502 + 14H+ t 14Fe3+ + 7H20 (Eq. 15)
Fe+3 + 3H20 + Fe(0H)3 + 3H+ (Eq:. 16)
Thus, a total of four equivalents of acid are produced from a single molecule
of pyrite.^ The hydrolysis reaction (Eq. 16) also forms hydrated hematite
(boemite) or iron jarosites.
During the active life of a leaching operation, the addition of sulfuric
acid may be useful because it offsets acid losses in gangue materials. If
the leaching operation is suspended or abandoned, acid may continue to be
produced if sufficient water from precipitation is present. As the solution
percolates through the ore, it will continue to leach out metals and other
potentially toxic ground-water contaminants if the acid generated exceeds the
neutralization capacity of the gangue. This process may continue for decades
under suitable environmental conditions (e.g., moist climates).
The precipitation of basic iron salts in pipelines, on the surface of
dumps, and within the dumps is a problem primarily during the active life of
15
-------
the leaching operation. These salts tend to constrict pipes and form imper-
vious layers within the dumps that restrict the movement of leach solutions.
This problem is typically addressed by controlling the pH and iron content of
13
the leach solution. This problem may also be addressed by passing a reduc-
ing solution through the pile. Because of the huge volume of solution,
however, controlling its chemical balance is very costly. After closure of
the operation, the precipitation of iron salts may serve a useful purpose by
preventing moisture and leachate generated within the pile from contacting
some of the metal-bearing minerals and thereby reducing the acid generation
capability of the abandoned leach site and its resulting impact on the envi-
ronment. The precipitated iron salts and the increased weathering of the
rock due to acid reaction with gangue minerals may decrease the permeability
of the dump. Natural settling and compression of the dump over time may also
decrease its permeability.
Dump leaching refers to the leaching of run-of-mine, low-grade, copper
ore that has been deposited on the ground surface without any site prepara-
tion. Copper leach dumps are massive. They are typically over 100 feet high
and cover hundreds of acres. Dumps are placed in an area where the slope of
the native terrain provides the means for collection of pregnant liquor. The
leach solution flows by gravity through the dump and then over the slope of
the native ground beneath the dump to a collection point, usually a pond, at
the downgrade toe of the dump. Historically, dump leaching evolved from
secondary efforts to recover copper from waste rock and mine overburden
generated by conventional mining methods (principally open-pit operations).
Most newer copper mining operations have included recoveries from leaching in
premining planning and economic evaluations. In some cases, the ability to
recover additional copper by dump leaching may have been a key element in the
decision to mine a particular ore body.
In contrast to dump leaching, heap leaching refers to the leaching of
low-grade ore that has been deposited on a specially prepared pad (e.g.,
synthetic, asphalt, or compacted clay pad). In heap leaching, the ore is
frequently crushed prior to emplacement. Site-specific characterisitcs
determine the nature and extent of the leaching operations used. Other
leaching methods are used to lesser extents. In situ leaching involves the
16
-------
leaching of low-grade copper ore without its removal from the ground, i.e.,
in abandoned underground mine workings, pit walls, and subsidence zones. In
some cases, the ore may be prepared for leaching by blasting or hydraulic
fracturing. Vat leaching is used to extract copper from crushed, nonporous,
oxide ore in large tanks or vats. Agitation leaching involves the rapid
leaching of ore concentrate or roaster calcine in agitated tanks. Vat and
agitation leaching are generally more rapid, more efficient, and much more
costly than dump leaching. Each of these methods is described in greater
detail in Section 3.
CHARACTERISTICS AND GEOGRAPHIC DISTRIBUTION OF COPPER LEACHING SITES
As of this writing, there are 18 commercially active copper leaching
operations in the United States with a total production capacity of 277,300
metric tons of copper per year (personal communication from J. L. W. Jolly,
U.S. Bureau of Mines, Washington, D.C., August 1986). Arizona has 14 active
sites, New Mexico has two, and Utah and Nevada each have one. Historically,
the southwestern United States has been the principal copper-producing region
of the country. The topography of most of the mines in this area is gently
rolling to mountainous, vegetation is relatively sparse, and the climate is
generally arid to semiarid. The average annual precipitation at most sites
is less than 20 inches. As a result, there is very little surface water in
these areas, and most leaching operations rely on ground water as a source of
water. On the other hand, surface-water runoff from winter snow accumulation
around sites in mountainous areas (such as Bingham Canyon, Utah) is much
15
greater than that found in southern Arizona and western New Mexico.
Copper dump and heap leaching sites are located in proximity to the
mining operation, as haulage costs quickly offset the value of the copper
recovered. The land around most of these mines is primarily undisturbed and
vacant, with a few scattered farms and ranches. Livestock grazing and agri-
cultural industries are the primary uses of the land. In some cases,
however, the facilities are located near urban or residential areas. For
example, Kennecott's Bingham Canyon operation is about 15 miles west of Salt
Lake City, and the Inspiration mine is less than 1 mile from the towns of
Globe and Miami in eastern Arizona. Table 4 provides an inventory of these
active
17
-------
TABLE 4. INVENTORY OF ACTIVE COPPER LEACHING OPERATIONS IN THE UNITED STATES
Leaching method
Recovery
method
n
4-»
c
o
~j
•a
c
o
4->
TJ
4-»
a.
:*
Operation
Location
i
o
o.
aj
3C
c
4-»
m
>
CT>
<
0
01
u
a_
UJ
1
X
IS]
Capacity, metric tons
ASARCO. Inc.
Stiver Bell
Marana, Arizona
X
X
6,000
Battle Mountain Gold Co.
Battle Mountain
Battle Mountain, Nevada
X
X
5,000
Cyprus Minerals Co.
Bagdad
Mineral Park
Sierrita/Esperanza
Bagdad, Arizona
Kingman, Arizona
Sahuarita, Arizona
X
X
X
X
X
X
6,800
3,500
6,000
Inspiration Consolidated Copper Co.
Inspiration
Oxhide
Claypool, Arizona
Claypool, Arizona
X
X
X
X
X
42,000
500
Kennecott
Bingham Canyon
Chino
Ray
Bingham Canyon, Utah
Santa Rita, New Mexico
Ray, Arizona
X
X
X
X
xa
X
X
X
X
36,000
20,000
15.000/29,000
Kocide Chemical
Van Dyke
Casa Grande, Arizona
xa
NAb
Leaching Technology, Inc.
Naclmlento
Cuba, New Mexico
xa
NA
Newnont Mining Co.
Miami Leach
Pinto Valley
San Manuel
Miami, Arizona
Miami, Arizona
San Manuel, Arizona
X
X
X
X
X
X
5,000
16,000
25,000
Noranda Lakeshore Mines, Inc.
Lakeshore
Casa Grande, Arizona
x
X
10,000
Phelps Dodge Corp.
Copper Queen
Horenci/Metcalf
Tyrone
Bisbee, Arizona
Morenci, Arizona
Tyrone, New Mexico
X
X
X
x
X
X
X
X
2,500
10,000
5,000/30,000
Experimental only.
bNA - not available.
-------
operations. Figure 2 shows the geographic distribution of the active leach-
ing operations in the Western States.
Table 5 provides an inventory of the inactive and abandoned copper
leaching sites in the United States. These sites currently number about 23,
including one new site that is in the permitting stage. It is difficult to
assess the precise number of abandoned leaching operations. Many of the
sites are very small and ceased operation many years ago. Others that are
now closed may not be permanently abandoned. Just as dumps containing what
was once considered waste rock are now being leached, improved leaching
techniques and copper recovery methods may result in the reactivation of
currently inactive sites. Also, rising copper prices may make the operation
of an inactive site economically feasible once again. In this case, produc-
tion would likely resume. Figure 3 shows the geographic distribution of the
inactive and abandoned leaching operations identified in the Western States.
19
-------
lake en*
,Lk5
• 4
5-6
* ?-$
L 9*
J • U
10-" * •
M *.
TUCSON u
^SAhtt ^
#18
»
U
KEV
2] Bliww* oKat
3l HlH£l%At P***
v»
^ pikto v*u-^
\|) ¥WW1P* fU
*t> siuvt* Btr^
sSSgr»
16 COWER »sw
\l\ T1R0HE
is)
^ nfcCW^NTO
teog«pMc '"'JJmf®1'5-
copper «aO""5
ions
20
-------
TABLE 5. INVENTORY OF INACTIVE COPPER LEACHING OPERATIONS IN THE UNITED STATES
Leaching method
Recovery
method
Operation
Location
a
§
o
Heap
In situ
>
Agi tation
Precipi tation
SX-EW
Capacity, metric tons
The Anaconda Minerals Co.
Yerrington
Weed Heights, Nevada
X
X
NAa
Anamax Mining Co.
Twin Buttes
Sahuartta, Arizona
X
X
X
33,000
ASARCO, Inc.
San Xavler
Sahuarita, Arizona
X
X
NA
Cochise Mining Corp.
Peacock
Safford, Arizona
X
NA
Cyprus Minerals Co.
Johnson
Benson, Arizona
X
X
4,000
Essex International, Inc.
Mil ford
Mil ford, Utah
X
NA
Inspiration Consolidated Copper Co.
Bluebird
Inspiration
Claypool, Arizona
Claypool, Arizona
X
X
X
X
7,300
NA
Kelnine Corp. .
Lisbon Valley"
Moab, Utah
X
NA
Kennecott
Klmberly-Sunshine
Ray
Ely, Nevada
Ray, Arizona
X
X
X
NA
NA
McAlester Fuel Co.
Zonia
Kirkland, Arizona
X
X
NA
Hewmont Mining Co.
Copper Cities
Miami, Arizona
X
X
2,000
Noranda Lakeshore Mines, Inc.
Lakeshore
Casa Grande, Arizona
X
X
NA
Ohio Copper Co.
Ohio Copper
Bingham Canyon, Utah
X
NA
(continued)
-------
TABLE 5 (continued)
Operation
Location
Leaching method
Recovery
method
Capacity, metric tons
Dump
Heap
In situ
Vat
Agitation
Precipi tation
SX-EW
Osceola Gold and Silver, Inc.
Rio Tinto
Mountain City, Nevada
X
NA
Phelps Dodge Corp.
hew Cornelia (Ajo)
Ajo, Arizona
X
X
500
United Vurde
Jerome, Arizona
X
NA
Ranchers Exploration and Development
Corp.
Big Mike
Winnemuccd, Nevada
X
X
NA
Old Keliable
Mammoth, Arizona
X
NA
Washington Corp.
Arbiter Leach
Butte, Montana
X
NA
Berkeley
Butte, Montana
X
X
10,000
Butte Hill Leach
Butte, Montana
X
X
10,000
®NA - not available.
bPerii1tt1ng stage.
-------
A HEL ENA
1-3
A RENO
6
•4
7 •
A SALT lake CITY
8 •
9 • 10
A LAS VEGAS'
12
15
13-14
ASANTE Ft
PHOENIX A * ALBI'QUFRQUEj
1? •
19 20
UCSONA
KEY
1) ARBITER LEACH
2) BERKELEY
3) BUTTE HILL LEACH
4) RIO TINTO
5) BIG KIKE
6) rCRRlNGTON
7) KIHBERLY/SUSSHINE
8) OHIO COPPER
9) MlLLfORO
10) LISBON VALLEY
11) UNITED VERDE
12) ZQNlA
13) BLUEBIRD
14) INSPIRATION
15) COPPER CITIES
16) RAY
17) LAKESHORE
18) NEW CORNELIA (AJO)
19) OLD RELIABLE
20) PEACOCK
21) JOHNSON
22) SAN XAVIER
23) TWIN BUTTES
Figure 3. Geographic distribution of inactive copper leaching operations
in the United States.
23
-------
SECTION 3
DESIGN AND OPERATION OF COPPER LEACHING SYSTEMS
This section discusses the design and operating characteristics of
typical dump leaching systems. It includes a detailed description of the
site characteristics, construction practices, leaching solution, and process
steps. A brief discussion of heap, in situ, vat, and agititation leaching is
also presented. The section concludes with a description of recovery of
copper from solution by the cementation and solvent extraction recovery
processes.
DUMP LEACHING
Dump leaching refers to percolation leaching of copper from run-of-mine
low-grade ores that has been piled on unprepared ground. Enormous quantities
of mine overburden and of nonmil1-grade ore (i.e., <0.5% copper) have been
accumulated and leached in dumps around copper mining sites. The leach cycle
for this type of operation is extremely long, usually measured in decades.
Current operations place leach-grade ore on their dumps in contrast to older
operations that leached former waste dumps. When an ore body is such that
mill-grade ore, leach-grade ore, and waste rock containing insufficient
copper for economical recovery can each be mined separately, a waste dump
is constructed in addition to a leach operation. Because ore characteristics
change gradationally rather than abruptly, however, such segregation is not
always possible. In these cases, the cost in time and analyses required to
define what is leach ore and what is waste may be greater than the opera-
tional cost savings. Therefore, operations may opt to put all nonmill-grade
ore in a leach dump (personal communication from N. Greenwald, Newmont Serv-
ices, Ltd., October 30, 1986).
24
-------
Site Selection and Preparation
Leach dumps are located adjacent to the mine site to minimize haulage
costs and to increase the economic efficiency of the operation. Naturally
sloping terrain is selected to facilitate the collection and recovery of the
pregnant leach liquors.
When many of the older dumps were started, the dump leaching of copper
from low-grade sulfide ores was not practiced; therefore, the overburden
containing these minerals was treated as mine waste. As a result, the char-
acteristics of the sites chosen to dump this material was not a major factor
in the selection process. In recent years, the leaching of low-grade ores
has become an integral part of the copper production process at most copper
mining operations. Greater importance has been placed on the selection of
sites (e.g., suitable slopes, low permeability, proximity to ground water and
surface water are considered). Because of haulage costs, however, potential
sites are limited to those in the immediate vicinity of the mine.
Pile Size and Construction
Leach dumps typically cover hundreds of hectares, rise to heights of 60
meters or more, and contain several million metric tons of uncrushed, low-
grade ore (Figure 4). The copper content of material deposited in leach
13
dumps averages about 0.3 percent. The materials generally vary consider-
ably in particle size, from large angular blocks of hard rock to highly
weathered fine-grained soils. Most of the material is less than 0.6 meter in
diameter.^
In most dump leach operations, the material is hauled to the top of the
dump by trucks. Bulldozers are used to level the surfaces and edges of the
dump. The material is typically deposited by end-dumping in lifts on top of
an existing dump that has already been leached. Large dumps are usually
raised in lifts of 15 to 30 meters. Some sorting of materials occurs when
this method of deposition is used. Coarser fragments tend to roll down to
the bottom of the slope, whereas finer materials accumilate near the surface
of the dump. A degree of compacting in the top meter of each lift results
from the heavy equipment and truck traffic. After the lift is completed, the
top layer is scarified (by a bulldozer and ripper) to facilitate infiltration
of the leach solution.^
25
-------
I AO
1S-TO 30-
HETCR •
HfTS
ISO
150
OUMP MATERIAL varies in TCXTURE
FROM LARGE ANGULAR BLOCKS Of
HARO ROCK TO FlNEGRAIN£0 SOIL
AHO SANOt GRMfl. MOST MATERIAL
IS LESS THAN 0.6 KTER IN
OIAMETCR.
1?0
l?0
90
PREGNANT
LIQUOR PQKO
60
APPBOUNATE CONTACT OF LEACH
CX*P AND BEDROCK.
RELATIVELY IMPERVIOUS MATERIAL
SUCH AS BEDROCK
JO
0
Figure 4.
Typical leach dump.
-------
Most leach dumps begin to settle as they are built and continue to
settle after the leach solutions have been applied. This continued settling
results in part from the percolating liquid moving finer particles into the
void spaces between larger particles. The dump is also compressed by the
added weight of the solutions and the destruction of the competency of the
bridging rocks by chemical reactions that decrepitate the rock.17
Characteristics of the Leach Solution
The leach solution for dump leaching of low-grade copper ores typically
consists of the barren solution from the precipitation or solvent extraction
process used to recover the copper from the pregnant leach liquor plus makeup
water to replenish water lost by evaporation and seepage. Sulfuric acid also
may be added; however, the need for extra acid for sulfate ores characteristic
of the Western United States is controversial (personal communication from
M. W. Wadsworth, University of Utah, to Peer Consultants, October 6, 1986).
Frequently, only makeup water is added because the bacterial activity on the
sulfide minerals that predominate within the dump generates the necessary
acid in the leach solution. The typical rate of application of leaching
3
solution on copper dumps is on the order of 0.01 to 0.05 m of leachant per
11 13
day per square meter of horizontal surface. '
Process Description
Leach solutions are introduced onto or into dumps by a variety of
methods. These include:
1) Flooding the surface by use of a series of small diked ponds.
2) Spraying the solution from hoses or through metal or plastic
sprinkler heads.
3) Injecting the solution through holes drilled in the dump and cased
with perforated plastic pipe.
4) A combination of these methods.
The distribution method depends on the climatic conditions, dump height,
surface area, scale of operation, mineralogy, and size of the leach
material.^
Because most distribution methods do not provide completely uniform
coverage, the application rate of the solution to the dump will vary. The
application rate is generally defined as the volumetric flow rate of the
27
-------
leach solution divided by the surface area to which the solution is actually
2
being applied. The average applicaton rate varies between 20 liters/m per
p i o
hour for sprinklers to as much as 200 liters/m per hour for pond leaching.
In practice, most dumps are leached in sections. The leaching period
for each section generally takes 4 to 6 weeks, depending on the efficiency of
the surface infiltration. Leaching is generally stopped when either the
copper content of the pregnant liquor from the section falls below a predeter-
mined concentration or when permeability diminishes because of the accumula-
tion of decomposed clay materials and iron salt precipitates. After leaching
of a section has been discontinued, the surface is scarified by ripping, and
either the leaching process is resumed or another lift is begun on the surface.
The alternate wetting and resting during the leach cycle promotes efficient
13
leaching of sulfide minerals within the dump.
Under the influence of gravity, the leaching solution percolates down
through the ore and carries the dissolved copper along with it. Total dis-
tribution of the leach solution throughout a dump, however, is difficult to
achieve. In sloped areas, channeling of the solution down the slope acceler-
ates runoff. Within the dump, alternate layering of coarse and fine mate-
rials as a result of poor dump construction promotes horizontal solution
flow, which may result in the discharge of the copper-bearing liquor from the
sides of the dump rather than from the base. The total volume of leach
solutions added to dumps must be limited to prevent certain areas, particu-
larly the sloped areas, from becoming saturated. Excess moisture in the pile
can lead to slumping of large tonnages of material.^
When the pregnant leach solution reaches the bottom of the dump, it
flows over the native ground into a collection channel and/or holding pond at
the downgrade toe of the dump. Holding ponds are generally located in natural
drainage basins enclosed by a dam. The pregnant solution is pumped from the
dam to the precipitation or solvent extraction plant, where the copper is
recovered from solution.
HEAP LEACHING
Design and Construction
Heap leaching involves placement of leach-grade ore, which has typically
been crushed, on a specially prepared surface (i.e., a pad). Because heaps
28
-------
are smaller than dumps, the use of low-permeability pads is permitted. (The
use of such pads under dumps is prohibited because the weight of massive
dumps would likely result in movement exceeding the shear strength of liner
materials.) The application of heap leaching is determined by site-specific
economics. Heap leaching is generally practiced with oxide ores because
these types of deposits are smaller; because oxides leach more rapidly than
sulfides, which allows quicker cost recoveries; and because the oxide leachate
has a higher copper constant than sulfide leachate, which provides greater
incentive to recover as much solution as possible (personal communication
from N. Greenwald, Newmont Services, Ltd., October 30, 1986).
Site preparation involves clearing the site of vegetation and grading
the surface toward a collection sump. The native soil may be treated with
some type of binder, or a natural (clay), asphalt, or synthetic (e.g., poly-
13
ethylene) liner may be installed.
Heaps are much smaller than copper leach dumps. On the average, they
contain between 100,000 and 500,000 metric tons of ore. Because the ore is
crushed, the size of the particles in the leach material is also considerably
13
smaller--generally less than 10 cm in diameter. Copper values are generally
1 percent or better.
In most heap leaching operations, the ore is blasted or ripped from
surface deposits and then hauled to the heap in trucks. Heap leach piles are
generally built in lifts of 4 to 6 meters. The size of the lifts will vary,
however, depending on the size of the particles in the leach material. As
the average particle size decreases, the size of each lift is generally
reduced to improve extraction rates. An optimum lift height is believed to
exist for ore within a given particle size ranged
Because of the relatively high copper content of the oxide ore and the
size of the particles in the leach material, copper heaps are designed and
operated to minimize truck traffic and dozer work on the surface. The pur-
pose of such procedures is to reduce the compaction resulting from these
activities and thereby improve the permeability of the heap.
One method of constructing a new heap involves placement of the leach
material in a strip along the centerline of the new heap. Subsequent loads
29
-------
are then dumped along the outer edge of the strip and pushed over the side
with a bulldozer to build the heap to its full width. With this method of
material emplacement, only the top meter of the heap becomes compacted. This
layer is subsequently scarified to promote infiltration of the leach solu-
tion.^
Characteristics of the Leach Solution
Sulfuric acid is the lixiviant used in heap leaching. The rate of
leaching is proportional to the acid concentration (up to 5 percent H^SO^ is
used). The acid strength of the leach solution varies proportionately with
the rate of leaching,-the grade of the copper ore leached, and the amount of
acid-consuming gangue present in the ore.10 The pregnant liquors produced by
heap leaching are generally much higher in copper content than those produced
by dump leaching because ore grades are higher, the acid solutions are stronger,
and the leach materials contain higher proportions of readily soluble oxide
minerals.*^
Process Description
Leach solutions are introduced onto or into the heaps in much the same
manner as they are in dump leaching operations; i.e., by flooding, spraying,
or injecting the solution or by a combination of these methods.
Heap leaching is a relatively continuous process. Alternate wetting and
aerating of the leach material is generally unnecessary because the dissolu-
tion of copper in oxide minerals does not require oxidation and because
surface infiltration and subsurface flow are relatively unaffected by the
precipitation of iron oxfde. The mechanical action of the droplets of leach
solution contacting the heap surface, however, will cause some decrepitation
3
of the ore and compaction of the surface and will promote crust formation.
Leaching is generally stopped after a given period dictated by the leaching
cycle or when the copper content of the pregnant liquor falls below a prede-
termined concentration. When no further leaching occurs, the surface of the
area is scarified by ripping and another lift is begun on the surface.
The heap leaching cycle typically lasts between 60 and 180 days. The
solution percolates through the ore and dissolves the copper minerals along
its path. The total distribution of leach solution throughout a heap, however,
30
-------
is more easily achieved than in leach dumps because of the greater consistency
of the size and distribution of particles within the heap and the absence of
channeling caused by iron salt precipitates. The total volume of leach
solutions added to dumps still must be limited to prevent areas from becoming
saturated.
When the pregnant leach solution reaches the bottom of the heap, it
flows over the pad into a collection trough and/or holding pond, from which
it is pumped to a precipitation or solvent extraction plant for recovery of
the copper from solution.
OTHER LEACHING PROCESSES
In Situ Leaching
In situ leaching, also called solution mining, refers to the leaching of
low-grade copper ore without removing it from the ground. The economics of
current mining and recovery methods often prevents the mining of ore that
either contains insufficient metal values or entails extensive site prepara-
tion or operating expense. For this reason, the use of the in situ method is
increasing as a means of recovering additional copper from old mine workings
(open pits, block caved areas, backfilled stopes) from which the primary
sulfide deposit has been removed. These types of operations tend to leave
behind considerable fractured copper-bearing rock that is uneconomical to
mine and recover by conventional means. In situ leaching is also being
considered as a method for extracting copper from deposits that have been
19
fragmented by blasting, previous block-caving mining, or hydrofacting. It
should also be applicable to highly porous deposits without fragmentation.
Most abandoned underground mining operations leave halos of low-grade
ore surrounding tunnels, stopes, rises, and pillars, and the engineering
required in such mines normally provides the basic needs for a leaching
operation. The main shaft is almost always used as a main drainage reser-
voir. Because tunnels always run upgrade, water or leach solutions flow
naturally by gravity to the main shaft for recovery. Fluids flowing from the
extraction drifts and haulage drifts are usually collected behind a dam
14
placed across the main shaft and pumped to the surface. The block caving
31
-------
process causes the area above the cave to fracture and expand in volume,
which results in a greater porosity of the mineralized ore body than in the
surrounding undisturbed rock. Good examples of this technique may be found
ot the Lakeshore mine near Casa Grande, Arizona, and the Miami mine in Miami,
Arizona.
The chemistry of in situ leaching is similar to that of dump leaching of
sulfidic ores and heap leaching of oxidized ores. So that the solution will
be exposed to as much mineralization as possible, the ore body must have
adequate permeability. Although natural fractures and interconnected pore
spaces may increase the porosity, blasting, caving, and hydrofacing also may
be required. As noted, the subsidence of material around block caving opera-
tions may create sufficient fracturing to make in situ leaching feasible.
The leaching solution can be applied to the area either by percolation (using
spraying or flooding) or by injection techniques. As the leach solution
percolates through the ore, it dissolves the copper minerals, and the resul-
tant pregnant liquor is collected from recovery wells or underground work-
18
ings. Because copper may be leached from both oxide and sulfide minerals,
13
the leaching process may continue for several years.
In situ leaching can be very economical when applied to previously mined
ore zones, but it is very expensive when applied to new ores. The impact on
the surrounding environment is lowered when the ore body is surrounded by a
low-permeability layer that minimizes the loss of leach solutions. To be
successful, an in situ leaching operation generally must have the following
characteristics:
° Host rock that does not consume acid
0 Host rock that will not decrepitate to seal intrarock fractures
° Sufficient rock fracturing to permit solution to reach copper
minerals
0 Copper minerals concentrated along fracture rock
° Copper minerals that dissolve within required time
0 Ability to recirculate the solution through the ore
19
° Availability of adequate water
32
-------
For effective leaching of sulfide ores, good aeration and active bac-
teria are required, as in dump leaching.
Vat Leaching
Vat leaching is used to extract copper from predominantly oxide ores by
subjecting the ore to concentrated sulfuric acid in a series of large tanks
or vats. Vat leaching has been preferred to heap leaching in cases where
high-grade ore requires crushing to permit adequate contact between the leach
solution and the copper minerals. The advantages of this method are high
13
copper extraction rates, short leach cycles, and negligible solution losses.
To expose the copper minerals, the ore is crushed to an approximate size
of less than 1 cm and screened to separate the fines (which prevent good
13
distribution of the leach solution) before it is placed into the vats.
Most vat leaching operations use several large rectangular tanks having
floors that act as filters to facilitate the upflow and downflow of solu-
20
tions. A typical vat measures 25 meters long, 15 meters wide, and 6 meters
deep and contains between 3000 and 5000 metric tons of material. Vat leach-
ing is a batch countercurrent operation with a complete cycle that involves
vat loading; ore leaching, washing, draining; and vat excavating. The overall
cycle may take 10 to 14 days. The pregnant solutions collected from the most
recently filled vats are sufficiently high in copper to permit the direct
recovery of copper by electrowinning. The pregnant liquor solution may be
sent to a solvent extraction step prior to electrowinning, however, if the
iron content of the solution is high. Iron reduces the efficiency in the
electrowinning process, and solvent extraction can eliminate this problem.
The solutions from the remaining soaks are recycled as leachant for subse-
quent batches of fresh ore. Continuous leaching, a method in which the
leachant continuously flows through the ore in a sequence of vats, has also
been practiced.** Factors that affect the leach rate (in both batch and
continuous leaching) include particle size and porosity, temperature, and
acid strength.
Agitation Leaching
Agitation leaching refers to the rapid leaching of fine particles of
oxide ore or roaster calcines with a strong sulfuric acid solution in agi-
tated tanks. This leaching method has been used primarily in conjunction
33
-------
with vat leaching operations to recover copper from the fines filtered out of
the vat material. Additional lean material is crushed and ground to a fine
particle size (90 percent less than 75 urn) and combined with the fines from
the vat operation. This material is then mixed with the leach solution to
form a pulp or a slurry having a density of between 30 and 40 percent. The
mixture is agitated by air or mechanical means in a series of three or six
tanks (volume 50-200 m ) for a period of 2 to 5 hours. Upon completion of
the leach cycle, the pregnant liquor is separated from the acid-insoluble
residue by cocurrent or countercurrent washing.**
Because of the fine particle size of the solids, the strength of the
acid solution, and the agitation of the leach slurry, which promotes better
liquid-solid contact, agitation leaching has the highest level of copper
extraction (in some instances greater than 95 percent extraction).**
COPPER RECOVERY PROCESSES
The traditional method for processing and recovering copper from heap
and dump leaching operations has been cementation. The main advantage of
cementation is its simplicity. The process uses scrap iron to precipitate
copper from the pregnant leach solution according to the following substitu-
tion reaction:^
Fe + CuS04 ^ Cu + FeS04 (Eq. 17)
The copper precipitates on the iron surfaces and is detached in flake or
powder form under the influence of the solution flow. The overall recovery
of copper is approximately 90 percent, and the precipitate generally contains
91
between 85 and 90 percent copper. The recovered copper is relatively
impure, however, and subsequent refining is required, usually by smelting and
electro-refinning.
One of the problems with the cementation process is that some of the
iron scrap dissolves into the pregnant leach liquor as the copper is being
removed. Iron is consumed according to the following reactions:
Fe + Fe2(S04)3 J 3FeS04 (Eq. 18)
Fe + H2S04 t FeS04 + H2 (Eq. 19)
34
-------
Inasmuch as the barren solution produced by this process is generally
returned to the top of the leach pile as part of the leaching solution, the
precipitation of iron salts on the surface of the dump is exacerbated, which
significantly restricts the passage of leaching solution into the dump.
A typical precipitation plant, illustrated in Figure 5, uses gravity
launders constructed of concrete. Wood and/or stainless steel gratings are
used to divert solution flow and support the iron precipitant. The launders
are charged with iron scrap by a variety of mechanical means, including belt
conveyors, crane-mounted magnets, and clamshell buckets. Solution is intro-
duced at the upper end of the plant and allowed to trickle downward through
the scrap by gravity. Most plants precipitate more than 60 percent of the
recoverable copper in the first few launders. The iron scrap in these laun-
ders is washed with high-pressure streams of water several times a week to
remove the copper. Copper precipitated in the remaining launders is usually
removed from once a week to once a month. The barren solution is discharged
13
by gravity from the lower end of the plant.
The resultant slurry of water from the washing process is emptied into
decant basins through drain valves, and the cement copper is allowed to
settle. The clear water is decanted and returned to the launders. The
cement copper is removed from the basins by various mechanical means and
dried (e.g., on concrete drying pads) before being shipped to a smelter or
other market.
In recent years, solvent extraction increasingly has been used for
selective recovery of copper from the dilute leach liquors, particularly
those recovered from low-grade dump leaching operations. Although the capi-
tal cost of building a solvent extraction (SX) plant is significantly higher
than that required to build a cementation plant, the problems associated with
the buildup of iron precipitates in the leaching solution are eliminated.
Also, the continually rising cost of iron scrap may make the operation of an
SX plant more economical.**
In solvent extraction, an organic solvent containing a copper-specific
chelating agent that complexes wtih copper ion is used to extract the copper
from the leach solution, after which the copper is stripped from the organic
phase by a strong acid solution. The resultant solution contains a high
concentration of copper that serves as an electrolyte for electrowinning.
35
-------
DRAINS
SIDE VIEW
DRYING
PAD
UMm
IK.
c:;
DRAINS ["
c::
1:2
c::
c::?
[;¦.
CELL
CELL
SOLUTION
FLOW "
o
m
TO
TOP VIEW
CEMENT/
COPPER
ov.v\
o»«C
DECANT
BASIN
END VIEW
SOLUTION
FLOW
PERFORATED
SCREEN
Figure 5. Typical precipitation plant
Source: Reference 13.
36
-------
Various chelating agents are currently in use. Solvent extraction
requires chelates that have a greater affinity for copper than for other
metals. So that a low-viscosity liquid can be obtained, the chelating agents
are always dissolved (5 to 20 percent by volume) in an organic carrier such
as kerosene. A modifier is often added to improve reaction rates and phase
separation.**
The extraction of copper from the leach solution into an immiscible
organic phase is usually accomplished in a series of mixer/settler stages.
The mixing stage causes the leach solution to contact the unloaded solvent.
This produces an emulsion, which is pumped into a settling tank, where the
loaded organic phase and the barren solution (raffinate) are separated by
gravity. The loaded organic phase is then stripped of its copper by a high-
acid aqueous phase. The concentration of acid in this aqueous phase is much
greater than that of the original leach solution (150 to 185 kg/m H^SO^) and
is suitable for electrowinning high-purity copper cathodes. The raffinate
(which contains almost all of the impurity metals in the original pregnant
liquor solution) is returned to the leach operation for further use.**
Figure 6 illustrates a typical solvent extraction/electrowinning plant.
Typically, the area selected for a solvent extraction plant is located
above the proposed raffinate pond so that any spills or overflows from the
process will drain into the pond and be recovered. Newer facilities locate
the mixers and settlers on concrete pads at ground level. The associated
tank farm and related equipment are located in a hole lined with gunite or
some other impervious material. A typical plant includes extract settlers
and strip settlers with primary and auxiliary mixers, a diluent (generally
kerosene) storage tank, a barren or stripped organic storage tank, an elec-
trolyte coalescer, and a series of tanks used in connection with back wash-
ing, crud (a semisolid residue formed in the mixers/settlers by the reaction
of the solvent with organic constituents in the leaching solution) process-
ing, pH neutralization, and emergency dumping.
37
-------
SOLVENT EXTRACTION RAFF I NATE
oo
oo
LEACH DUMP
PREGNANT LIQUOR
ORGANIC/
AQUEOUS
MIXTURE
ORGANIC/
AQUEOUS
MIXTURE
ORGANIC
ORGANIC
AQUEOUS
AQUEOUS
SECOND STAGE
EXTRACTION
FIRST STAGE
EXTRACTION
f\
ORGANIC/
AQUEOUS
MIXTURE
MAKEUP ORGANIC
EXTRACTANT
ORGANIC
AQUEOUS
STRIPPING STAGE
COPPER CATHODES
ELECTROLYTE BLEED STREAM
POWER AND MAKEUP
WATER AND ACID
ELECTROWINNING
ELECTROWINNING
Figure 6. Typical solvent extraction/electrowinning plant.
Source: Courtesy of Phelps Dodge Corporation.
-------
SECTION 4
POTENTIAL FOR ENVIRONMENTAL IMPACT
This section discusses the potential impact of copper leaching opera-
tions on the environment. An initial environment assessment requires identi-
fication of the mechanisms that could result in the release of contaminants.
These mechanisms may be a natural part of the leaching operation or the
unintended effect of outside influences or unexpected events. Generally, the
most significant mechanisms are those related to a natural part of the leach-
ing operation, as they are the most difficult to change and will have the
greatest long-term impact on the environment. Mitigative systems designed to
control these types of releases may be required to function for years or even
decades. Consequently, the cost of constructing and maintaining such systems
may be relatively unlimited. On the other hand, changes in the uses or
characteristics of the land surrounding a leaching site or unexpected failures
within the leaching circuit itself tend to have a relatively short-term
impact on the environment. Although an immediate response may be required,
the cost of implementing and maintaining the mitigative activities is gen-
erally limited.
After the release mechanisms from the leaching operation have been
identified, the potential constituents of the released substances are dis-
cussed. These generally depend on the characteristics of the leach material,
the nature of the leaching solution, and the type of copper recovery process
used in the operation. The mobility of many metals and other minerals is
enhanced by the low-pH solutions typically generated in leaching operations.
The mobility of minerals in the leach material may also be enhanced by contam-
inants introduced by the copper recovery process and leaching solution. The
actual characteristics of the potential releases from a copper leaching
operation also are affected, however, by other site-specific factors that may
offset the impact of these processes and minimize the potential harmful
effects of the release. It is not feasible to quantify national source terms
39
-------
for acid released from these wastes.*® Such data are highly site-specific
and generally unavailable.*®
The final portion of this section reviews actual environmental monitor-
ing data gathered from sites in the Southwest. A brief discussion of each
site and a summary of the monitoring results are provided.
POTENTIAL MECHANISMS FOR THE RELEASE OF CONTAMINATION
Most copper dump leaching operations are designed to be closed systems;
the pregnant liquor from the leach pile is pumped to the copper recovery
process, and the barren aqueous solution discharged from the copper recovery
process is recycled back onto the leach pile. Nevertheless, these systems
include the following potential mechanisms for the release of contamination
into the ground water:
° Seepage of acid solutions through soils or liners beneath the leach
piles
° Leakage from solution holding ponds and transfer channels
0 Spills from ruptured pipes and copper-recovery equipment
° Pond overflow or solution discharge caused by excessive liquid in
the solution cycle
0 Failure of the dams or liners of solution holding ponds.*®
Seepage can occur from leach dumps, holding ponds, and transfer channels
because most are built directly on the existing topography. Natural drainage
basins are generally used to convey and hold the leach solutions. The perme-
ability of the surfaces on which these were built and the resulting environ-
mental impact generally were not investigated extensively at sites selected
prior to around 1970. Ravines and gullies were usually selected because they
were the shortest and most easily traversed distance from the mine operation.
Since 1970, however, the shrinking price of copper, coupled with increased
concerns about the environmental impact of copper leaching operations, has
spurred efforts to improve the efficiency of these operations. As a result,
most new dumps are located on relatively impermeable ground and the drainage
40
-------
pattern is designed to allow the easiest and most effective collection of
leach solution.
Excessive liquid in the solution cycle, a potential release mechanism,
can occur as a result of changes in climatic conditions, unusual storm events,
or mechanical equipment failure. Because most leach piles are located in
natural drainage basins, runoff from rainfall and snow melt within these
basins will increase the flow rates of liquids into the holding ponds. If
this additional flow cannot be handled by the copper recovery process or be
diverted into secondary containment areas, the liquid may overflow the banks
of the pond onto the surrounding property. A similar problem may arise if
the pumping equipment used to drain the holding ponds malfunctions as a
result of mechanical malfunction or power failure.
The failure of transfer pipes or holding pond dams and liners can be.
attributed to a variety of factors. These may include weathering and decompo-
sition caused by contact with the acid leaching solutions and exposure to
air. Seismic activity, rock slides, and operator error are also potential
failure mechanisms.
SOURCES AND CHARACTERISTICS OF POTENTIAL GROUND-WATER CONTAMINATION
Spent Leach Material
When the copper content of the pregnant liquor from dump leaching opera-
tions drops below a value that can be economically recovered, leaching of the
ore/waste rock is discontinued. Because of the enormous size of the leach
dumps, the spent leach material usually is not removed after operations have
ceased. In areas where evaporation exceeds precipitation (as in the desert
Southwest where most dumps are located), the leach piles tend to dry out over
a period of several months. Because of the huge absorptive capacity of these
piles, many inactive leach dumps and heaps do not generate leachate, even
after major storm events or prolonged periods of above-average precipitation.
Conversely, in areas of high precipitation, infiltrating rainwater or snowmelt
may cause acid generation and leaching of copper and other minerals (particu-
larly sulfide minerals that can be oxidized to water-soluble sulfates or
41
-------
multivalent minerals that can be oxidized from a lower-valent water-insoluble
state to a higher-valent water-soluble state) to continue for several years.
Data from previous EPA characterization studies (Table 6) indicate the
potential acidity (total sulfur content as determined by peroxide oxidation)
of spent leach material varies widely (from less than 10 to greater than
10,000 ug C0o/g material). Although none of the samples analyzed exhibited
the characteristic of EP toxicity, several extracts contained slightly ele-
22
vated concentrations of cadmium, chromium, lead, selenium, and/or silver.
Leaching Solution
The primary constituent of the leaching solution distributed on dumps is
the barren aqueous solution discharged from the copper recovery process.
Makeup water may be added to replenish the water lost through evaporation and
seepage during leaching, and concentrated sulfuric acid may be added to
reduce the pH of the leach solution. The copper recovery process leaves many
dissolved substances in the barren solution. Although both cementation and
solvent extraction are reasonably effective in recovering the copper, the
other substances continue to accumulate until their concentration reaches
saturation values. The nature of these other substances is directly related
to the composition of both the ore body and the type of copper recovery
process in use. As a result, generalizations concerning the trace metal
composition of the leaching solution, which is considered a process material
until such time as it is lost to the environment, are difficult.
In addition to leaching pyrite and other iron-bearing minerals, the
cementation process (Equations 18 and 19) results in a buildup of dissolved
iron in the leach solution. The typical brown color of the water in barren
solution ponds is indicative of low copper content and high iron content.
The iron concentration is controlled by precipitation of iron oxides and
jarosites within the dump and by the steady-state pH of the system.
The barren solution from the cementation process will also contain most
of the original sulfate salts present in the leach liquor. Table 7 presents
the results of an analysis of the elemental composition of the barren aqueous
23
solution from a representative leaching operation.
42
-------
TABLE 6. POTENTIAL ACIDITY AND EP TOXIC CHARACTERISTICS OF SPENT LEACH MATERIAL3
(mg/liter except as noted)
Constituent
RCRA
hazard
criterion
Sample
1
Sample
2
Sample
3
Sample
4
Sample
5
Sample
6
Sample
7
Sample
8
Potential
acidity,
ug C03/g
NAb
10,220
<10
1370
3458
2910
2300
<10
<10
Arsenic
>5.0
0.0341
<0.0042
<0.0030
0.0205
<0.012
<0.012
<0.0015
<0.0015
Barium
>100.0
0.019
0.04
0.11
0.12
0.21
0.19
<0.001
0.038
Cadmium
>1.0
0.049
<0.008
0.012
0.016
<0.008
0.015
0.025
<0.008
Chromium
>5.0
0.150
<0.001
0.004
0.004
0.009
<0.001
0.073
0.004
Lead
>5.0
<0.084
<0.084
<0.084
<0.084
<0.084
<0.084
0.260
<0.084
Mercury
>0.2
<0.0008
<0.0008
<0.0002
<0.0002
0.0022
<0.0002
<0.0010
<0.0010
Selenium
>1.0
0.0594
0.0060
0.0092
0.0356
<0.005
<0.005
<0.0036
<0.0057
Si 1ver
>5.0
0.041
<0.002
0.006
0.005
<0.002
0.012
0.120
<0.002
a Source: Reference 22.
k NA = not applicable.
-------
TABLE 7. ELEMENTAL COMPOSITION OF BARREN AQUEOUS
SOLUTION FROM CEMENTATION PROCESS3
Element
Concentration
Q or
Aluminum
(Al)
5.90
Q
Arsenic
(As)
<0.01
S
Beryl 1ium
(Be)
0.00008
s
Bismuth
(Bi)
-
s
Calcium
(Ca)
0.4
s
Chromium
(Cr)
-
s
Cobalt
(Co)
0.005-.01
s
Columbium
(Cb)
0.002
s
Copper
(Cu)
0.09
s
Gallium
(Ga)
-
s
Germanium
(Ge)
<0.001
s
Iron
(Fe)
1.-5.
s
Lanthanum
(La)
0.01
s
Lead
(Pb)
0.003
Q
Magnesium
(Mg)
2.52
Q
Manganese
(Mn
0.35
Q
Molybdenum
(Mo)
<0.001
s
Nickel
(Ni)
0.011
Q
Si 1 icon
Silver0
(Si)
0.56
Q
(Ag)
0.05-0.1
s
Sodium
(Na)
0.1
s
Tin
(Sn)
0.0002
s
Titanium
(Ti)
<0.01
Q
Uranium
(U)
0.0087
Q
Vanadium
(V)
<0.002
S
Yttrium
(Y)
0.003
S
Zinc
(Zn)
0.17
Q
Total residue, g/liter 185
a Source: Reference 23.
b Q = quantitative chemical analysis; S = semiquantitative
analysis.
c Silver reported in ounces per ton, all others in weight-
percent.
44
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The constituents of the raffinate from the solvent extraction process
depends on the following:
° Type of solvent used
° Type of carrier used (e.g., kerosene)
0 Rate at which equilibrium conditions are approached
0 Rate and extent to which organic and aqueous phases disengage from
the emulsion created during the mixing stage
° Mechanisms designed to remove entrained organics from the raffinate
The organic solvents and carriers currently used for solvent extraction are
specifically formulated to have very low solubility for impurity metals.**
Consequently, these metals remain almost entirely in the raffinate. Contam-
ination of the raffinate by the organic solvent is also minimized, as exces-
sive use of solvents can have a significant impact on the cost of producing
copper. Typically, the amount of solvent in the raffinate is estimated to be
around 100 ppm. Newer organic solvents (e.g., LIX 84) have been used success-
fully (for example, at the Tyrone Mine near Silver City, New Mexico) to
reduce solvent loss to the raffinate to less than 30 ppm. The raffinate may
also contain degradation products of the solvent. No simple analytical
method, however, appears to have been found to determine the amount of the
organic solvent and carrier materials lost by entrainment and dissolution in
the raffinate.
Pregnant Leach Liquor
Dissolution of copper minerals produces copper, ferric and ferrous ions,
and sulfuric acid. The amounts of these and other constituents dissolved in
the pregnant liquor vary widely with the type of mineralization being leached,
the characteristics of the leach solution, and the leaching method used.
The type of copper mineralization affects the rate of leaching and the
subsequent copper content of the pregnant leach liquor. Ores and waste rock
containing predominantly oxidized copper minerals are more readily leached
and give rise to more highly concentrated pregnant liquors than do copper
45
a
a
-------
sulfide ores. On the other hand, reduced copper ores containing pyrite
(FeS^) and other base metal sulfides give rise to pregnant solutions with a
higher iron content and a lower pH.
The pH of the leaching solution is indicative of bacterial activity and,
subsequently, the generation of ferric sulfate, which is the principal factor
controlling the rate of dissolution of copper and other metals contained in
the host rock and thus their occurrence in the pregnant liquor. As noted
above, the oxidation of sulfide minerals, principally pyrite, produces sul-
furic acid, which lowers the pH of the leach solution. The pH of pregnant
liquors averages around 2.4, but it may range between 1.9 and 3.0.^ At
these low pH's, the solubilities of many toxic metals are increased (arsenic
and selenium are exceptions, although they still may be found in significant
concentrations in acidic solutions). Figure 7 illustrates the effect of pH
reduction on the aqueous solubility of various metals.^ Data on the concen-
tration of EP toxic metals in several samples of dump and heap leach liquors,
which (like the leaching solutions) are considered process materials until
22
such time as they are lost to the environment, are presented in Table 8.
The relative mobilities of the elements in different subsurface envi-
ronments (oxidizing, acid, neutral-alkaline, reducing) are presented in
25
Table 9. This table indicates the effect of pH and oxidizing/reducing
conditions within leach dumps on the leaching potential of copper and other
minerals. Other phenomena that may increase the mobility of constituents in
the host rock and, consequently, their occurrence in the pregnant liquor
include metal complexation and ion pairing.
The leaching method used to extract copper from the ore/waste rock has a
significant impact on the characteristics of the pregnant liquor. As noted
in Table 3, the copper content of the pregnant solution from vat and agita-
3
tion operations ranges from 30 to 50 kg/m , whereas the copper content of the
pregnant solution from dump, heap, and in situ operations is generally less
than 5 kg/m . This difference is attributable to the relative particle size
of the leach material and the efficacy of contact between the leach solution
and the host rock. Similar differences in the mineral content of the pregnant
liquor may be observed for other minerals for the same reasons. Other aspects
of the various leaching methods that determine the characteristics of the
46
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pH
Figure 7. Solubilities of oxides and hydroxides of various metals
Source: Reference 10.
47
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TABLE 8. CONCENTRATION OF EP TOXIC METALS
IN SAMPLES OF PREGNANT LEACH LIQUORS
(rr.g/1 iter)
RCRA
hazard
Constituent criterion Sample 1 Sample 2 Sample 3 Sample 4 Sample 5
pH
<2 or >12.5
1.5C
2.60
1.82
1.95
2.49
Arsenic
>5.0
4.23 ¦
2.15
CO
(N.
3.5
2.5
Barium
>100.C
<0.001
<0.02
<1.0
<1.0
<1.0
Cadmium
1 v
t—•
O
1.72
1.40
1.8
0.82
0.55
Chromium
>5.0
<0.001
0.45
3.4
1.2
0.81
Lead
>5.0
3.30
<4.20
<0.05
<0.05
<0.05
Mercury
>0.2
0.001
<0.0002
<0.002
<0.002
<0.002
Selenium
>1.0
2.74
2.13
0.57
0.35
<0.01
Silver
>5.0
0.22
0.64
<0.05
<0.05
0.13
a Source: Reference 22.
48
-------
TABLE 9. RELATIVE MOBILITIES OF THE ELEMENTS
Chemical
environment
Relative
mobi1i ties
Oxidizing
Acid
Neutral to alkaline
Reduci ng
Very High
CI, I, Br
CI , I, Br
CI, I, Br
CI , I, Br
S, B
S, B
S, B
Mo, V, U, Sc, Re
S, B
High
Mo, V, U, Sc, Re
Mo, V, U, Sc, Re
Ca, Na, Mg, F, Sr, Ra
Ca, Na, Mg, F, Sr, Ra
Ca, Na, Mg, F, Sr, Ra
Ca, Na, Mg, F, Sr, Ra
Zn
Zn
Cu, Co, Ni, Hg, Ag, Au
Medium
Cu, Co, Ni, Hg, Ag, Au
As, Cd
As, Cd
As, Cd
Low
Si, P, K
Si, P, K
Si, P, K
Si, P, K
Pb, Li, Rb, Ba, Be
Pb, Li, Rb, Ba, Be
Pb, Li, Rb, Ba, Be
Bi, Sb, Ge, Cs, Ti
Bi, Sb, Ge, Cs, Ti
Bi, Sb, Ge, Cs, Ti
Fe, Mn
Fe, Mn
Fe, Mn
(continued)
-------
TABLE 9 (continued)
Chemical
envi ronment
Relative
mobi1ities
Oxidizing
Acid
Neutral to alkaline
Reducing
Fe, Mn
Very low to
A1, Ti, Sn, Te, W
A1, Ti, Sn, Te, W
A1, Ti, Sn, Te, W
A1, Ti, Sn, Te, W
Immobile
Nb, Ta, Pt, Cr, Zr
Nb, Ta, Pt, Cr, Zr
Nb, Ta, Pt, Cr, Zr
Nb, Ta, Pt, Cr, Zr
Th, Rare Earths
Th, Rare Earths
Th, Rare Earths
Th, Rare Earths
S, B
Mo, V, Li,'Sc , Re
Zn
Zn
Cu , Co, Ni, Hg, Ag, Au
Co, Cu, Ni , Hg, Ag, Au
As, Cd
Pb, Li, Rb, Ba , Be
Bi, Sb, Ge, Cs, Ti
a Source: Reference 25.
-------
pregnant liquor include temperature and rate of application of the leach
solution.
The elemental composition of the pregnant liquor from a typical durrp
23
leach operation is presented in Table 10.
GROUND-WATER CONTAMINATION DATA
Chino Lampbright Leaching Area
A study of the ground-water quality at the Lampbright leaching area of
Kennecott Corporation's Chino mine was conducted between December 15, 1981,
pc
and August 25, 1982. A map of the Chino operations and the Lampbright
leaching area is presented in Figure 8. The Lampbright leach area is con-
structed on an unlined natural drainage basin. Pregnant liquors are collect-
ed at the toe of the leach pile in an unlined leachate collection pond. The
pregnant liquors are then pumped to a cementation plant for copper recovery.
At the time of the study, the Lampbright leach area occupied approximately
500 acres.
Nine parameters identified as potential indicators of seepage from the
leach pile [calcium, magnesium, manganese, nickel, zinc, pH, total dissolved
solids (TDS), sulfate, and fluoride] were found in one well at levels that
indicated an impact from the dump leaching operation. Levels of calcium,
TDS, and sulfate indicative of impact were also found to be present in another
well. Thus, the study concluded that the leaching operation had an impact on
the quality of water in these two wells.
Tyrone No. 2 Leach Dump
Copper leaching operations in New Mexico are subject to the State's
Ground-Water Quality Protection Regulations, which require any person who
discharges effluent or leachate that may move directly or indirectly into
ground water to obtain and operate within the conditions of a permit or
"discharge plan." All discharge plans contain monitoring provisions that
require the sampling, analysis, and reporting of ground-water end leachate
quality.
In accordance with these provisions, the Phelps Dodge Corporation has
collected 5 years of ground-water quality data from monitoring wells in and
around the dump leach areas at the Tyrone mine. The locations of some of the
-------
TABLE 10. ELEMENTAL COMPOSITION OF PREGNANT LEACH LIQUOR3
Element
Concentration
Q or Sb
Aluminum
(Al)
0.05-0.1
S
Arsenic
(As)
5.90
Q
Beryl 1ium
(Be)
<0.01
S
Bismuth
(Bi)
0.00008
S
Calcium
(Ca)
-
s
Chromium
(Cr)
0.4
s
Cobalt
(Co)
0.002
s
Columbium
(Cb)
0.005-0.01
s
Copper
(Cu)
-
s
Gal 1ium
(6a)
0.09
s
Germanium
(Ge)
1.-5.
s
Iron
(Fe)
-
s
Lanthanum
Ua)
<0.001
s
Lead
(Pb)
0.01
s
Magnesium
(Kg)
2.52
Q
Manganese
(Mn)
0.35
Q
¦ Molybdenum
(Mo)
<0.001
s
Nickel
(Ni)
0.1
s
Si 1 icon
Silver
(Si)
0.011
Q
(Ag)
0.003
Q
Sodium
(Na)
0.56
Q
Tin
(Sn)
0.0002
s
Titanium
(Ti)
<0.01
Q
Uranium
(U)
0.0087
Q
Vanadium
(V)
<0.002
s
Yttrium
(Y)
0.003
s
Zinc
(Zn)
0.17
Q
Total residue, g/liter 185
a Source: Reference 23-
b Q = quantitative chemical analysis; S = semiquantitative
analysis.
c Silver reported in ounces per ton, all others in weight-
percent.
52
-------
MINE PIT
WHITEWATER
LEACH AREA
LAMPBRIGHT
LEACH AREA
O LEACHATE
COLLECTION
POND
BAYARD
HURLEY
;iCHIN0 MILL AND
- SINTER PLANT
TAILINGS
PONDS
Q
~n
SCALE: 1 in
= 1.5 mile
Figure 8. Map of the Chino operations.
Source: Reference 26.
53
-------
monitoring wells are shown in Figure 9. The data indicate that some contam-
ination of the ground water has occurred in the vicinity of the No. 2 dump,
as evidenced by an increase in the concentration of total dissolved solids
and dissolved iron and a decrease in the pH in Wells 6-3, 6-4, and 6-5 over
background conditions (Well 4-1). These data are presented graphically in
Figures 10 through 12.
Globe/Miami Area
The Globe/Miami mining district east of Phoenix, Arizona (Figure 13) was
the focus of a recent water quality study by the Central Arizona Association
28
of Governments. The objective of the study was to assess the impact of
past and present mining activities on water quality in the Pinto and Pinal
Creek drainage basins. The study concluded that surface runoff from past
dump leach activities at the Bellview-Gibson mine site has degraded surface-
water quality in Pinto Creek; however, ground water in the Pinto Creek basin
is generally of high quality. Table 11 presents analytical results from
surface runoff and ground-water samples in the Bell view area as well as the
appropriate stream standards. The Globe/Miami water quality study also
reported that surface and ground water in the Pinal Creek basin have exhib-
ited significant deterioration over the past 40 years but that the exact
source(s) of contamination are difficult to identify because of the large
number (greater than 200) of active and inactive mining sites in the area
(see Figure 14).
54
-------
w*lt LOM'ICI
¥»#'! N9TP
Qr0u"id-w#;«i £
Fill!
M Da*« Aval »B • 9'f.b •© *?'3t
'•~Svv^ *f: r
Qrou^«-W«l»r ?•«*•?.C< C0*fOu'
In F««' «t?o*e W«an 5*t L*
o««h»a
/(«¥%'
j&/§5
641
5f
iS#*
p> SCALE
P'BS
Reproduced from
best available copy
Figure 9. Location of the ground-water monitoring wells in relation
to the leach dumps at the Tyrone mine.
55
-------
o>
£
to
Q
-a- WELL 4-1
-J- WELL 6-3
o WELL 6-4
«*- WELL 6-5
DATE
Figure 10. Concentration of total dissolved solids in
wells around Tyrone's No. 2 leach dunp.
*- CM
CO CO
cO n
DATE
Figure 11. Concentration of dissolved iron in
wells around Tyrone's No. 2 leach dump.
-B- WELL 4-"
•J- WELL 6-3
-0- WELL 6-4
WELL 6*5
56
-------
CM
C\J
CM
C*5
n
D
GO
CO
GO
CO
CO
CO
CM
T
GO
CM
C\J
-O- WELL 4-1
-O" WELL 6-3
-0- WELL 6-4
WELL 6-5
DATE
Figure 12. pH level in wells around
Tyrone's No. 2 leach dump.
57
-------
Copper Cities
Pinto
•-Volley*
vv, N
¦,
-------
TABLE 11. COMPARISON OF SURFACE RUNOFF, GROUND WATER, AND ARIZONA STREAM STANDARDS
BELLVIEW-GIBSON AREA SAMPLING3
(mg/liter except as noted)
Pinto Cr.
at old
Hwy. 60
Stream
Well
Wei 1
Well
Well
No. 12
Parameter
Mineral
Creek at
Belleview
Bridge
standards
No. 1
No. 2
No. 12
duplicate
Date
7/21/81
8/10/81
8/10/81
8/10/81
__
3/6/82
3/6/82
3/7/82
3/7/82
Time
2345
1650
1730
Comp.
__
1200
1607
1102
1102
Temp, °C
--
9
9
--
16
16.5
13
13
Conductivity,
390
420
—
2400
1050
1900
1900
umho/cm2
pH, S.U.
4-§
4.2
6.3
--
6.5-9.0
6.3
5.8
6.0
6.0
Arsenic
<0.02°
<0.02
<0.02
<0.02
0.05
<0.02
<0.02
<0.02
<0.02
Nitrate
Jb
0.6
0.7
0.6
<0.1
0.1
0.2
0.2
Silica
89b
0.9
10
24
19
51
54
51
Alkalinity
<2b
6.7
1.1
26.7
—
276
25
<1
<1
Calcium
53b
63
43
15
509
171
459
466
Chloride
<2b
3
1
5.9
23
8
31
31
Copper
36<1b
17.2
14.4
0.16
0.05
0.17
8
0.06
0.02
Iron
21b
0.8
0.7
0.1
12
8
115
117
Magnesium
Hb
10
7
4
131
48
140
137
Manganese
K7b
1.1
0.98
0.16
10
19
9
13
13
Potassium
3*7b
2.7
2.1
2.2
--
3
2
2
2
Sodium
1 2
h
3
1
6
52
31
79
82
Sulfate
230°
228
330
313
--
1693
725
2172
1760
Solids
1662h
152
256
186
2824
976
2848
2702
Zinc
0.4
0.2
0.5
0.1
0.5
0.09
0.24
0.12
0.12
a Source: Reference 28.
^ Total sample
c Suspended solids.
-------
TOKYO
1VATIONAL
i
SCALE
i > 1 » 1 -?—?
•T*TUTi MILlt
O PROSPECTS
• PRODUCERS
Figure 14. Past and present mining activities in the Globe/Miami area
Source: Reference 27.
60
-------
SECTION 5
ALTERNATIVE MANAGEMENT PRACTICES
An array of alternative management practices are available for possible
implementation at copper dump leaching operations to mitigate actual or
potential environmental releases. These practices are capable of addressing
a wide range of surface and subsurface conditions associated with active
leaching, closure procedures, and the postclosure period. Most leaching
operations have adopted a subset of these practices to control leachate
production and minimize solution loss. The techniques are controlled by the
29
characteristics of the site and are generally unique. A particular type or
combination of management practices is seldom adequate for all leaching
operations because of differences in the topography, geology, hydrogeology,
meteorology, and detailed operating characteristics of the site. External
factors such as the price of copper and the competitiveness of the operation
in the world market are also important considerations in determining the
economic feasibility of a particular technique. As a result, each leaching
operation has implemented a set of management practices that provides the
most efficient, cost- effective recovery of copper at that facility. These
practices are designed to manage solution losses ana monitor fluid balances
as an integral part of the leaching operation.
The relevancy of a particular management practice to a specific copper
leaching site depends, at least in part, on the operational phase of the
site. Three operational phases are distinguished here:
° The active phase includes the development of the site and the
period during which copper is being recovered from solution.
During this period, new ore may or may not be added to the leach
piles. This phase also includes periods during which copper is
recovered from liquids that have percolated through the piles, even
though the distribution of leaching solutions to the surfaces of
the copper heaps or dumps has ceased.
61
-------
0 The closure phase immediately follows the active phase and is the
period during which various activities are undertaken to ensure
adequate protection of human health and the environment during the
post-closure period. During this period, no further additions are
made to the leach piles and copper is not recovered from the leach-
ates produced by the piles.
0 The post-closure phase, which follows the closure phase, is the
period during which the primary activities are the monitoring of
the site and the maintenance of the closure technologies implemented
during the previous phase.
In addition to these operational management practices, it may be neces-
sary to implement corrective actions to control prior releases or newly
discovered releases of contaminants into surface water or ground water.
Corrective actions may be required at any point in the operational or closure
phases of a copper leaching site. The need for corrective action, however,
may not be determined to be necessary until the contamination threatens human
health or the environment.
Most of the management practices described in this section can be imple-
mented, to a greater or lesser extent, during any of the operational phases
of a copper heap or dump leaching operation. Implementation of some of the
practices (e.g., the installation of a liner) is feasible only during the
design and initial operating phases. The application of several other tech-
nologies is inappropriate during active operations because of the ongoing
nature of the material deposition and leaching process. For example, revege-
tation and capping practices designed to reduce the percolation of liquids
and to prevent the flow of air within leach piles are appropriate only at
closure. Table 12 lists the various management practices discussed in this
section and the operational phases during which they are applied.
During the evaluation of the management practices at copper leaching
operations, little information was available as to the effectiveness of these
techniques relative to surface-water and ground-water quality directly associ-
ated with actual leaching operations. Most mining operations have a variety
of potential sources of ground-water contamination, and available information
is insufficient for specific determination of the extent, nature, and source
of water quality degradation due to the leaching operation. Consequently,
implementation of a particular type or combination of management practices
62
-------
TABLE 12. MANAGEMENT PRACTICES BY OPERATIONAL PHASE
Management
Active
Post-
Corrective
practices
Niitigative measures
1 ife
Closure
closure
action
Leachate con-
Subgrade liners
X
trol systems
Pond and trench liners
X
X
Ground-water
Ground-water monitoring
X
X
X
X
management
Ground-water control
X
systems
Subsurface drains
X
X
X
X
Subsurface barriers
X
X
X
X
Surface-water
Diversions
X
X
management
Containment systems
X
X
Reclamation
Revegetation
X
X
X
and closure
Capping
X
X
X
activities
Securi ty
X
X
X
X
-------
that focuses exclusively on the leaching operation may not effectively address
the ground-water contamination problems at the site.
An additional factor in the evaluation of alternative management prac-
tices for copper dump leaching operations is the cost of implementation and
maintenance. As much real cost data as possible have been obtained from the
mining industry sources. In many cases, site-specific characteristics of
leaching operations that affect the cost of basic operations also have been
identified. Where possible, a range of costs is given to reflect the effect
of these variables on the implementation of the management practice.
SITE CHARACTERIZATION AND MONITORING
The implementation of an effective system of mitigative management
practices is typically preceded by a preliminary evaluation of the geologic
and hydrogeologic characteristics of the copper dump leaching operation. The
purpose of this evaluation is to identify conditions in and around the leach-
ing operation that may affect the production of leachates and characterize
the pathways for contaminant transport within the surface-water and ground-
water systems. The size and complexity of the system of mitigative manage-
ment practices implemented by a particular site will be based primarily on
the information and conclusions drawn from this evaluation. The following
subsections discuss the nature and type of information required during this
preliminary evaluation and the methods for gathering these data.
Geologic and Hydrogeologic Evaluation
The geologic and hydrogeologic setting of the copper leaching operation
is probably the most important factor in determining the need for and design
configuration of a mitigative management system. This determination requires
the careful collection and evaluation of both regional and site-specific
information.
The initial step in the geologic and hydrogeologic evaluation of a
copper leaching operation often is a survey of the site's physical and opera-
tional characteristics. The information gathered during this survey is used
to estimate the potential volume and flow pattern of leachate in and around
the copper leaching operation. Components of such a survey include:
64
-------
0 Nature, history, and location of the leaching operation
0 Characteristics of the leach material and deposition practices
0 Size and location of the leach piles in relation to the existing
topography and other mining operations
° Existence of liners or other low-permeability barriers around the
leach operation
° Current uses of surface and ground waters in the vicinity of the
operations and the proximity of these waters to populated areas.
Any historical precipitation records and existing geologic and topographic
maps also should be consulted. Information concerning the characteristics of
the material used in copper leaching operations aids in the identification of
the nature and amount of leachate produced. The leach material is the princi-
pal source of the constituents in the leachate; however, as discussed in
Section 4, some contaminants may be introduced by the leaching solution and
the recycled liquids from the copper recovery process. The solubility of
these contaminants depends, in part, on the mineralization of the ore and the
leaching conditions (i.e., pH, Eh, etc.). For example, when exposed to air
and moisture, sulfide ores containing high concentrations of pyritic minerals
tend to lower the pH of the leachate and increase the solubility of certain
heavy metals. The presence of alkaline substances in the leach material or
in the surface material surrounding the leach pile, however, may neutralize
the acid arid reduce metal solubility.^ The mineralization of the leach
material, as well as its physical properties, will also affect the permeabil-
ity of the leach piles and the extent of contact between the acid solution
and the leach material. The weathering and decomposition of certain ores
(caused by saturation with acid solutions and exposure to air) result in the
formation of fine, clayey materials that tend to plug voids in the rock and
restrict both the flow of solutions and the flow of air into the dump.^
The size and configuration of a leach pile, including the characteristics
of the embankments, are important factors in estimating the volume and flow
direction of leachate generated by the pile, as well as the concentration of
contaminants. The thickness and cross-sectional area of the leach piles
affect the amount of leachate produced, whereas the general configuration of
65
-------
the pile's foundation area and the drainage pattern of the old topography
affect the flow patterns of the leachate. This process is complicated by the
fact that many copper leach piles cover hundreds of hectares and contain tens
of millions of metric tons of low-grade ore. As a result, horizontal and
vertical distances between hydraulically upgradient areas and downgradient
areas can be great. The variations in the natural conaitions over such
large distances (thousands of meters) can greatly complicate the hydrogeo-
logic evaluation.
Estimations of the travel rate, direction, and distance of potential
contamination from copper heap and dump leaching operations must consider the
effect of other mining operations located in the vicinity. Active, inactive,
or abandoned mines and/or waste disposal sites may complicate flow patterns
when the bottom elevation of underground or open pit mining is below the
water table. These mines may act as ground-water sinks, which can control
the movement of a plume. Abandoned mine sites and waste rock dumps, particu-
larly those containing a high percentage of sulfide minerals, also may compli-
cate the hydrogeologic evaluation because they represent another potential
source of contamination that must be considered.
The volume and characteristics of the leachate will also be affected by
the procedures used to place the leach material on the heap or dump site.
These procedures affect the relative compaction of various areas of the leach
pile. The compression of the leach materials by vehicular traffic and by
increasingly greater overburden loads of new leach material may be quite
high. In addition, if the ore and gangue material contain carbonates that
have the capacity to neutralize sulfuric acid, the deposition of these mate-
rials above pyritic materials in the water infiltration pathway can raise the
ph of the solution and reduce the potential dissolution of contaminants.
Although a physical survey of the site will provide information on the
potential volume and flow patterns of the leachate, monitoring wells are
generally used to obtain site-specific data on the geologic and hydrologic
characteristics of the site, surface- and ground-water transport mechanisms,
and potential human health and environmental receptors. These data will also
provide evidence of current environmental damage and water contamination and
aid in estimating travel times for contaminants. Often, however, these data
66
-------
will yield only qualitative findings that provide an initial estimate of the
impact of copper leaching operations on the surrounding environment. Esti-
mates obtained through simple water balance methods or qualitative transport
modeling provide the framework for determining the need and configuration of
a more quantitative monitoring program. Preliminary design parameters for
monitoring wells can be established through these evaluations, including
which water bearing zones to monitor, the approximate number and location of
monitoring wells, the parameters to be sampled, the duration and number of
sampling events, the location of surface-water sampling points, and the
rationale for establishing the monitoring program.
Ground-water Monitoring Program
The obvious objective of a ground-water monitoring program is to detect
subsurface releases of leaching solutions and, if necessary, to generate the
data required to select and implement a corrective many action strategy.
Because ground-water monitoring usually is not practiced at many active
copper heap and dump leaching operations, only a limited amount of background
information is available from which to determine the leaching operation's
contribution to the existing constituents in the ground water. Therefore,
ground-water monitoring programs at existing operations will only determine
if additional degradation of the ground-water quality is occurring. At new
facilities, ground-water monitoring can provide a more accurate measurement
of conditions prior to the impact of seepage from the leach piles and collec-
tion ponds.
A typical monitoring network consists of a series of nonpumping wells
located downgradient from the leach piles and at least one well upgradient in
32
an area that has not been affected by potential contaminant migration. The
number of monitoring wells and the complexity of the well network will depend
on the size of the copper leaching operation and the presence of other poten-
tial sources of contamination in the vicinity. For example, a comprehensive
water quality study around several active and abandoned mines 1n Arizona's
Miami/Globe area required a monitoring network composed of 113 new wells.
This network included existing water supply wells and wells converted to
hydrologic monitoring nests, shallow and small-diameter wells, wells drilled
67
-------
adjacent to deeper existing wells, and deep wells. Kennecott Copper Corpora-
tion currently has a monitoring program underway that includes more than 200
wells located in and around the leaching areas.
A comprehensive ground-water monitoring program will initially comprise
an array of wells sited according to the information and conclusions drawn
from the geologic and hydrogeologic evaluation previously described. If
ground-water contamination is detected, however, additional wells may be
constructed to gauge the dispersion and attenuation of the leachate. This
30
approach can be a time-consuming and expensive process.
The depth of each well in the monitoring network will depend primarily
on the depth and characteristics of the underlying aquifer and the vertical
30
spread of potential contamination. The depth of aquifers under copper
leach sites varies considerably, and they are often very deep, particularly
in the arid regions of the Southwest. Wells ranging in depth from 30 meters
to more than 3000 meters have been required at Kennecott Corporation's Bing-
ham Canyon operation to provide adequate monitoring of ground-water quality.
The size of a monitoring well will depend on the sampling method used,
flow rates in the aquifer, and the characteristics of the material surround-
ing the site of the proposed well. Vacuum and pressure sampling methods
that require relatively small holes (51 mm) cannot be used in wells deeper
than about 9 meters. Consequently, bailers or submersible pumps are required
to withdraw samples from the wells. These generally require a relatively
large well diameter (102 mm). Larger-diameter holes also may be required if
the well is located in tight materials where recovery is slow or if ground-
30
water flow rates are extremely slow.
Monitoring wells may be installed by a variety of methods. Shallow
wells (less than 30 meters) are generally augered, driven, or jetted. For
deep wells (greater than 30 meters), rotary drilling, jetting, and cable
30
tooling are generally used. Table 13 briefly describes the application of
31
these methods under various geologic and hydrogeologic conditions.
Cost of Site Characterization and Monitoring
The cost of installing well systems at copper leaching operations will
vary greatly from site to site. The primary factors that determine these
costs are the size of the leaching operation and the complexity of the local
68
-------
TABLE 13. METHODS OF WELL INSTALLATION3
Applications
Method
Basic
Variations
Geologic material
Max. well
dia., in.
Max. well
depth, ft
Hand
Augers
Soft soils without
excess sand and
water; no boulders
6
20
Driving points
Soft soils free of
boulders
3
30
Boring
Rotary auger
bucket
Soft soils without
excess boulders
48
90
Spiral auger
Soft soils without
excess sand and
water; no boulders
6
90
Jetting
Self-jetting
Soft soils free of
boulders
8
50
WeiIpoint/riser
unit
Soft soils free of
boulders
8
50
Separate tem-
porary jetting
pipe
Soft soils free of
boulders
8
100
Separate permanent
jetting pipe
Soft soils free of
boulders
8
100
aSource: Reference 31.
69
-------
hydrology. Costs will also be affected by the characteristics of the ground
water, the extent of contamination (if any), the availability of supplies and
equipment, and local wage rates.
Installation costs include the costs of drilling, well materials, crews,
and equipment. The principal parameters that will affect installation costs
0 Well diameter
Well depth
0 Well components
0 Drilling specifications
0 Geologic material being drilled
0 Sampling requirements
0 Site access-0
Table 14 presents some typical costs for drilling and installing well
are:
systems.
31
TABLE 14. 1986 COSTS FOR DRILLING . .
AND INSTALLING 2- TO 4-INCH-DIAMETER WELLS9'D
Drilling method
Cost, $/m
Conventional hydraulic rotary
Reverse circulation hydraulic roters
Air rotary
Auger (hollow stem)
Cable tool
Hole puncher (jetting)0
Self jetting0
Mobilization
1600-1900
80-130
110-145
55-80
35-70
50-55
130
70
a Source: Reference 31 (modified).
k Includes drilling, well material, and installation costs.
c Includes rental of all necessary equipment, e.g., well points, pumps, and
header.
70
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LEACHATE CONTROL SYSTEMS
Leach Pile Surface Preparation and Liners
Leach dumps are built on unprepared existing topography. Generally, the
sites continue to add fresh ore to these established dumps and leach them
indefinitely. As a result, dump sites frequently cover hundreds of hectares
and contain tens of millions of metric tons of low-grade ore. Whereas heap
leaching (use of specially constructed pads) is practiced to some extent, it
has not been demonstrated that pads (i.e., liners) are applicable to practices
covering hundres of hectares and containing millions of tons of ore. The
massive size of such practices may result in shear forces that would destroy
the integrity of a liner.
Heap leaching has been used in place of dump leaching generally because
of high permeability or neutralizing characteristics of the area selected for
the operation. Most leach sites are selected to take advantage of existing
impermeable surfaces and to utilize the natural slope-of ridges and valleys
for the collection of pregnant leach solutions. Land having this type of
geology and terrain, however, is not always within a reasonable hauling
distance from the mining operation. For example, Newmont Mining Corporation
investigated several sites on which to locate a new copper oxide ore leaching
operation for its San Manuel mine. The nearest site having a reasonably
impermeable surface and sloping terrain was about 2 miles from the mine pit,
and haulage costs would have made a leaching dump operation at that site
uneconomical. Therefore, Newmont evaluated a site within 1/2 mile of the
pit. This site was covered with a Gila Conglomerate that was very alkaline.
It was estimated that each ton of the Gila Conglomerate would have consumed
up to 200 lb of sulfuric acid, which would effectively neutralize the leach-
ing solution and cause copper to precipitate at the base of the pile. The
surface was also poorly stratified, and permeability ranged between 10"^ to
10 cm/s. These characteristics would have resulted in excessive solution
loss and significantly reduced the efficiency of a dump leach operation. The
cost of installing a liner and conducting heap leaching as opposed to dump
leaching, however, was determined to be a more cost-effective alternative
than the extra haulage costs that would have been incurred if the original
site had been selected.
71
-------
'Potential solution loss was also a principal reason for the selection of
heap leaching at the Anaconda Company mine in Butte, Montana. At that site,
the surface of the area selected for the operation was covered with an allu-
vial material between 5 and 80 feet deep that had been deposited on a quartz
monzonite.
In heap leaching, the installation of a liner is generally accomplished
in three phases:
1) Excavation and grading of the proposed leach area
2) Preparation of the supporting subgrade surface
3) Installation of the liner^2
The techniques used during the excavation and grading phase are fairly
typical. The site is first scraped and graded by bulldozers to remove all
vegetation and to contour the surface to channel the leaching solution toward
one or more collection points. Equipment such as sheepsfoot and vibratory
rollers also can be used for further grading and compacting of the surface.
The purpose of the subgrade is to proviae a relatively firm and unyield-
32
ing support for the liner material. The Butte mine selected slag with a
particle size of minus 4 cm. The material was spread to a depth of approxi-
mately 10 cm and compacted with a vibratory roller. The San Manuel mine used
Gila Conglomerate that had been well worked and compacted. Generally, the
surface of the subgrade must be finished to create a regular, flat surface,
regardless of the type of subgrade material used. Rocks and irregularities
with sharp edges must be eliminated, although the required regularity and
texture of the surface will depend on the type of liner used.
Liners for new copper heap leach sites can be formed from natural earthen
(clay) materials, admixed materials, synthetic materials, or a combination of
these. The type of liner used will depend on a variety of site-specific
factors, including the existing topography, the surrounding climatic condi-
tions, the geologic structure, and the geohydrologic characteristics.
The Tyrone mine near Silver City, New Mexico, recently installed a clay
pad over portions of the surface under its proposed No. 3 leach area. Because
of the difficulties associated with the excavation, grading, and compaction
of the steep slopes, the installation of pads was limited to surfaces having
72
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a slope of less than 5:1. The pad installed covered 59 hectares and consisted
of 45 cm of compacted soil built in 15-cm lifts. The soil, which was obtained
on site, had a minimum fines content of 12 percent and a moisture content of
±2 percent of the optimum level. Each lift was compacted to 98 percent of
its maximum dry density.
The Butte mine installed an asphalt liner. Initially, a coating of
asphalt primer was sprayed over the slag subgrade to prepare the surface for
the deposition of the asphalt pad. The pad itself consisted of a compacted
layer of asphalt 8 cm thick. The surface of the pad was cured with a 0.3-cm
layer of sealant.
At the San Manuel mine, a synthetic liner was used to cover an area
totaling approximately 34 hectares. The material (60-mil, high-density
polyethylene) was chosen partly because of its resistance to the corrosive
effects of the pregnant liquor solution and partly because of its tensile
strength. In the more critical areas where solutions gather, such as collec-
tion ditches, 100-mil material was used. The liner material was received in
rectangular sheets measuring approximately 8 m by 60 m and heat-welded to-
gether.
Prior to deposition of the heap leach material, the liner is covered
with sand or gravel to provide a drainage blanket and to protect the surface
32
of the pad from heavy truck travel and damage from boulders. The Tyrone
mine spread 46 cm of soil over the clay liner. The Butte mine spread two
layers of cover material prior to the deposition of the leaching ore. The
first layer consisted of at least 30 cm of fine mine-run material. This was
followed by.a 1.5- to 1.8-meter-thick layer of coarse mine-run material. The
San Manuel mine installed 0.5 meter of a sand and graded gravel mixture. The
gravel was approximately 2.5 to 7.5 cm in size. Coarser gravel was used
around the edges of the liner to promote drainage of the pregnant liquor
solution.
Pond and Trench Liners
Copper leaching operations typically use ponds to collect the pregnant
leach liquors from the heaps or dumps and to hold the barren solution from
the copper recovery process prior to recirculating it onto the leach heaps.
Several operations also use evaporation ponds to collect and evaporate excess
73
-------
solution in the leaching circuit to prevent surface discharges. These ponds
generally measure several hectares in size and, where the topography permits,
are built into natural drainage basins. A dam is constructed between the
valley walls and across the valley floor to form the pond.
At most of the older copper leaching operations, the collection ponds
and trenches through which the solutions flow are unlined. In addition,
these areas received little or no surface preparation before leaching opera-
tions were initiated. The dams are generally constructed of concrete or rock
with clay cores. When feasible, the dams are also grouted into bedrock to
minimize subsurface seepage. As a result, the amount of leachate discharged
into the ground water around these structures is primarily dependent on the
permeability of the underlying surface and the integrity of the dams.
At several leaching operations, liners have been installed in the collec-
tion ponds and diversion channels to reduce seepage from the site and to
increase the amount of copper recovery. This is particularly true of the
raffinate ponds that have been constructed within the last 10 years in conjunc-
tion with a solvent-extraction plant. Several facilities have also lined the
pregnant liquid collection trenches and ponds. Generally, the trenches have
been lined with concrete or a synthetic liner such as polyethylene. The
collection ponds are typically lined with gunite, clay, or synthetics.
Cyprus Bagdad, for example, recently replaced its principal, unlined,
pregnant-liquor collection pond with a new, lined, collection system.
Both the trench and pregnant-liquor collection pond were lined with 100-mil
polyethylene. The techniques used to install the liner in the collection
trench and pond were similar to those discussed in connection with the lining
of leach piles. The area chosen for the trench and pond was first excavated
and rough graded. After the excavation and grading were completed, a fill
subgrade material was hauled to the site and compacted in layers. After the
subgrade material was in place and had been adequately compacted and finished,
the liner was installed. The liner was cut and spread by hand in the trench
and collection pond areas. The seams were then sealed and tested to ensure
their integrity. After the liner had been installed, the pipes were laid on
the bottom of the pond and a pumping system was installed to carry the pregnant
liquor to the copper-recovery plant.
74
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Cost Analysis of Leachate Control Systems
A variety of factors affect the cost of lining the surface of a proposed
leach pile or the trenches and ponds of the solution collection system,
including the following:
Type of material used
Location of the leaching operation and the associated cost of
transporting the lining material to the site
Size of the leaching operation; the economics of scale will gener-
ally lower the unit costs for large projects
Type and consistency of the surface material at the site
Installation costs associated with the type of material and the
quality control procedures required32
The estimated cost of installing several different types of liners is
presented in Table 15 These costs do not reflect other system components
that should be included with a liner system, such as ground-water monitoring
wells and diversion systems for surface runoff. Neither do they reflect the
cost of installing a system to divert and hold the leaching solution during
construction activities in the ponds and trenches.
TABLE 15. 1986 LINER INSTALLATION COSTS3
Liner type
Installed cost, $/m3
Soil-bentonite
Soil-cement
(15 cm thick and sealer)
Asphalt-concrete
(10 cm thick, hot mix)
Chlorinated polyethylene (CPE)
Chlorosulfinat^d polyethylene (CSPE)
High-density polyethylene (HDPE)
7.40-12.7
7.40-12.7
7.30-10.2
1.90
3.30
60 mil
8.60
9.20-9.70
80 mil
a Source: Reference 32.
75
-------
GROUND-WATER MANAGEMENT SYSTEMS
Adequate evaluation and management of ground-water contamination at
copper leach facilities normally involves either 1) monitoring systems de-
signed to detect and evaluate changes in concentrations of chemical constit-
uents in the ground water (discussed on page 64), or 2) control systems
designed to manipulate the ground water to contain or remove contaminants or
to adjust ground-water levels to prevent formation of a plume. Ground-water
monitoring systems can be used during any phase of a copper heap or dump
leaching operation to identify the nature or extent of contamination. Ground-
water control systems, on the other hand, are used primarily as corrective
action measures and may include pumping systems, subsurface drains, or subsur-
face barriers, used alone or in combination with each other.
Ground-Water Pumping Systems
Ground-water pumping techniques generally involve one or more of the
following options: 1) containment of a plume, 2) removal of a plume after
measures have been taken to halt the source of the contamination, and 3)
diversion of ground water to prevent clean ground water from flowing through
a source of contamination or to prevent contaminated ground water from con-
31
tacting a drinking water supply.
In a typical ground-water pumping system, extraction wells or a combina-
tion of extraction and injection wells are used to reduce or control seepage
losses through the foundation of the leach dump and collection ponds. The
wells must be located at points that intersect the plumes of contaminated
seepage. These types of systems are most effective, however, at sites where
the underlying aquifers have high intergranular conductivity. They have also
been used with some effectiveness at sites with moderate hydraulic conductiv-
ity and sites where movement of the leachate is occurring along fractured or
jointed bedrock. Ground-water control systems perform poorly in low-transmis-
sivity aquifers.
The use of extraction wells alone is best suited for situations where
the hydraulic gradient is steep and the hydraulic conductivity is high. A
combination of extraction and injection wells is frequently used when the
hydraulic gradient is relatively flat and hydraulic conductivities are only
76
-------
moderate. The function of the injection wells is to direct contaminants to
31
the extraction wells.
Extraction wells have been effective in altering the direction of ground-
water movement around tailings ponds in both the Tucson and Globe/Miami
copper mining districts. Experience has indicated, however, that adequate
monitoring must be provided in areas such as this to assure that sufficient
water is pumped from the wells to offset the recharge from the leaching
33
operation and other potential sources of ground-water contamination.
Leaching solutions appear to be readily transportable, as evidenced by
the low-pH ground water found around some dump leaching operations. The
movement of this ground water can be extremely slow, however, and flow pat-
terns may be difficult to predict, particularly at sites located on fractured
bedrock. Also, the flow of ground water may be diverted or distorted by
underground or open pit mining operations in the vicinity of the leaching
operation that have been excavated below the water table. A major problem in
that type of environment is that it tends to interfere with the normal flow
pattern of the potentially contaminated water (i.e., through the fractures)
and it does not always reach a monitoring well. Extensive hydrogeologic
analysis may be reouired to predict ground-water flow directions.
Four types of wells possibly can be used for ground-water pumping: 1)
deep wells, 2) ejector wells, 3) wellpoints, and 4) suction wells. The
latter two have much less application. Table 16 summarizes the conditions
31
under which each of these well types is most applicable. Deep wells and
ejector wells are used when extraction depths are greater than about 6 meters.
Ejector wells generally require less piping and a smaller-diameter casing
than deep wells, but they are very inefficient (typically less than 15 percent
efficiency) and susceptible to clogging in some environments. Wellpoint and
suction well systems are best suited for shallow aquifers where extraction is
not required below 6 meters. These systems differ primarily in the size and
31
consequent pumping capacity of the well.
In the selection of the components for any of these systems, the nature
of the environment in which they will be operating must be considered. The
low-pH leachate recovered from the ground water surrounding copper leaching
operations may be particularly corrosive to the casings, screens, pumps, and
77
-------
TABLE 16. CRITERIA FOR WELL SELECTION3
Parameters
Weilpoints
Suction wells
Ejector wells
Deep welIs
Hydrology
Low hydraulic conductivities
(e.g., silty or clayey sands)
Good
Poor
Good
Fair to poor
High hydraulic conductivities
(e.g., clean sands and gravel)
Good
Good
Poor
Good
Heterogeneous materials
(e.g., stratified soils)
Good
Poor
Good
Fair to poor
Proximate recharge
Good
Poor
Good to fair
Poor
Remote recharge
Good
Good
Good
Good
Depth of well
Shallow <20 ft
Shallow <20 ft
Deep >20 ft
Deep >20 ft
Normal spacing
5 - 10 ft
20 - 40 ft
10 - 20 ft
>50 ft
Normal range of capacity
(per unit)
0.1 - 25 gpm
50 - 400 gpm
0.1 - 40 gpm
25 - 3000 gpm
Efficiency
Good
Good
Poor
Fair
aSource: Reference 31.
-------
other equipment used in their construction. High concentrations of iron or
other potential precipitates in the water may tend to clog lines and reduce
the efficiency of the system.
Subsurface Drainage Systems
Subsurface drainage systems use some type of buried conduit to collect
and convey discharges from the leaching operation. The drains essentially
function as a continuous line of extraction wells that channel the collected
liquid to a treatment or disposal system. Consequently, subsurface drains
31
can be used to contain or remove a plume.
Drains are generally more cost-effective than pumping systems when
depths are shallow, particularly in strata with low or variable hydraulic
conductivity. Frequently used where the depth to a low permeable barrier is
relatively shallow, the drains are laid above the barrier. This approach can
be particularly applicable at copper leaching sites where many of the piles
are located over bedrock covered with a thin intervening layer of porous
alluvial material through which leaching solution may be seeping. As the
depth to the impermeable barrier increases, however, the costs of shoring,
dewatering, and excavating the hard rock can make such a drain cost-prohibi-
tive. The practical depth limit is about 25 meters. Subsurface drains are
also easier to operate and more reliable than pumping systems. Because water
is collected by gravity flow and hydraulic pressure, there are no pumps or
other electrical components to fail. Operation and maintenance procedures
are relatively simple; however, clogging or breaks in pipes can be very
costly and time-consuming to repair.
Another potential disadvantage of subsurface drains at copper leaching
sites is the clogging of pipes and drains due to the precipitation of iron,
manganese, and other minerals dissolved in the solution. This may be caused
by excursions of the leachate pH, the presence of iron-reducing bacteria, or
the presence of other minerals that form soluble or insoluble iron complexes.
Frequent and potentially expensive cleaning of pipes constricted by these
materials would be necessary to maintain the effectiveness of the system.
A subsurface drainage system includes the following major components: a
drain channel consisting of a pipe or a gravel bed; an envelope to convey
79
-------
flow from the aquifer to the drain pipe or bed; a filter to prevent fine
particles from clogging the system; and wells to collect the flow and to pump
31
the discharge to a treatment or disposal process. The pipe or gravel is
laid in a trench that has been excavated and graded to prevent ponding and to
minimize potential clogging. Maintaining a dry environment during excavation
and placement of the pipe generally requires some type of dewatering system.
Some type of wall stabilization also may be required in deep trenches or in
relatively unstable soils.
Subsurface Barriers
Vertical subsurface flow barriers can be effective in stopping ground-
water drainage or diverting it around a leaching site at depths less than 75
feet. These barriers are particularly effective where inflow occurs only at
27
a few isolated locations. These barriers are installed below ground to
contain, capture, or redirect ground-water flow in the vicinity of the site.
The most common subsurface barriers are slurry walls and grouting.
Slurry walls provide a relatively inexpensive means of reducing seepage
31
in embankments or foundations. Slurry walls are constructed in a vertical
trench that is excavated under a slurry and backfilled with a material having
a low permeability. The slurry acts essentially as a drilling fluid for
shoring the trench hydraulically to prevent collapse; however, it also forms
a filter cake to prevent fluid losses into the surrounding ground. The
backfill material commonly consists of concrete, a concrete-bentonite mix-
31
ture, a bentonite-soil blend, or a hybrid of these.
The most important consideration in designing a slurry wall is the
permeability of the completed wall. For control of seepage, the wall is
keyed into a low-permeabi1ity confining layer beneath the site. The depth
and nature of this layer, however, will significantly affect both the cost
and effectiveness of the wall.
Where the subsurface barrier is to be installed in rock, grouting is
usually selected because excavating or driving through this type of material
27
is difficult. Grouting is a process whereby one of a variety of fluids is
injected into crevices and joints in a rock or soil mass to reduce water flow
and strengthen the formation. Cement is the most commonly used material for
80
-------
grouting applications; however, clay and chemical polymer grouts also are
widely used. Chemical grouts can be used to seal porous materials and cracks
that are too small to accept a water-cement grout.
Grout curtains are another type of subsurface barrier created in uncon-
solidated materials by pressure injection. Grout curtains may be much more
expensive than slurry walls, however, and achieving low permeabilities in
unconsolidated materials may be difficult as a result of gaps that are left
in the curtain because of nonpenetration of the grout.
Whereas grout curtains are used to create subsurface barriers around an
operation, rock grouting is used to seal fractures, fissures, and other voids
in rock. This technique has been used at copper heap and dump leaching sites
primarily to seal fractures, fissures, and other voids in rock around dams to
reduce seepage. It has also been used in mines to stabilize and strengthen
porous and fissured rock.
The effectiveness of grouting depends on accurately locating the water-
bearing voids or zones. Complex ground-water flow in fractured and fissured
bedrock (such as may occur under copper heap and leach dump sites) can make
rock grouting very difficult. The overall permeability of a grout barrier
can be significantly reduced if even minor gaps are left in the barrier.
Consequently, a thorough site characterization must be conducted during the
hydrogeologic evaluation to determine if a site is groutable and the type of
31
grout that should be used.
Cost Analysis of Ground-Water Management Systems
Many of the same conditions that affect the installation cost of moni-
toring wells will also affect the cost of ground-water pumping systems. In
addition to those costs, however, sites installing pumping systems must also
absorb the capital cost of the pumps and accessories as well as the operating
costs related to the period and duration of required pumping. Table 17
31
presents some representative costs for selected pumps and accessories.
A method for estimating the total capital and operating costs for well
31
systems has been developed based on the use of existing hydraulic models.
This method has been applied to a number of assumed aquifer and plume charac-
teristics to demonstrate their effect on the cost of well systems. Table 18
31
summarizes the results of this analysis.
81
-------
TABLE 17. 1986 COSTS FOR SELECTED
PUMPS AND ACCESSORIES
Pump/accessory Cost, $
Jet pumps
Shallow well
(<7.6 m) 200-490
Deep wel1
(<97.5 m) 240-630
Jets and valves 25-100
Seals 15-40
Foot valves 10-50
Air volume controls 10-30
Submersible pumps
4-inch pump
(depth <274 m) 415-1500
Control box 70-1500
Magnetic starters 160-240
Check valves 15-410
Well seals 20-120
Vacuum pumps
Diesel motors 13,000-49,000
Electric motors 8,800-34,000
aSource: Reference 31 (modified).
82
-------
TABLE 18. SUMMARY OF SEVEN RECOVERY SYSTEM COST SCENARIOS3
(lOOO's $, 1986)
Aquifer and plume characteristics
design parameters
*
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As with other management practices used at copper leaching operations,
the costs of installation and materials for subsurface drains can vary widely
with site conditions. Installation costs will be affected by the depth of
excavation, ground-water flow rates, and the characteristics of the soil or
rock in which the drain is to be located. Material costs can include pipes,
gravel, pumps, and other accessories. Material and installation unit costs
are shown in Table 19.
The costs of a slurry wall will be affected primarily by the type of
backfill and, to a lesser extent, by the depth and ease of excavation. Table
20 presents the average cost for soil-bentonite and cement-bentonite slurry
31
walls based on depth and type of material excavated. The higher costs for
cement-bentonite walls is due primarily to the cost of Portland cement.
The cost of drilling holes and injecting them with grout is shown in
31
Tables 21 and 22. 1 As shown, the cost of grout will have the most signifi-
cant impact on the barrier's cost. The difficulty encountered in drilling
will also affect the cost. The practical depth limit is about 25 meters.
SURFACE WATER MANAGEMENT SYSTEMS
Diversion Systems
Surface-water diversion systems are generally constructed to prevent
uncontaminated offsite runoff and potentially contaminated onsite runoff from
mixing. Offsite water is prevented from entering the mine site and causing
erosion and flooding. Onsite storm runoff is intercepted for transport to an
evaporation pond or a contaminant treatment system. Diversion systems can
also help to recover supernatant for recycling.
Combinations of drainage ditches, diversion berms, and collection dams
are common methods of controlling surface-water movement at copper heap and
27
dump leaching operations. The selection and cost of a specific type of
diversion method or combination of methods at a particular site will depend
on the characteristics of that site.
Drainage ditches are the most common diversion technique used at copper
leaching sites. Drainage ditches are typically designed to accommodate flows
84
-------
TABLE 19. 1986 COSTS OF MATERIALS AND
INSTALLATION FOR SUBSURFACE DRAINS
Item
Unit cost, $
Remarks
Trench Excavation
Trencher, ladder-type
470-630
1.5-2.5 m deep; 20-40 cm wide
Backhoe, hydraulic
1.80-2.80/m3
1.2 m wide trench, damp sandy loam
soi1, 3.6-6 m deep
Dragline
2.50- 3.80/m3
27-50 m3/h capacity
Clamshell
4.10-6.20/m3
15-27 m3/h capacity
Wall Stabilization
Sheet piling
75-80/m
Includes pull and salvage
4.5-12 m excavation
Wooden shoring
60-70/m2
4.3-6 m excavation
Dewatering
.
Sumphole
25-50/m3
Includes excavation and gravel with
30-60 cm pipe
Opening pumping
350-420/day
5-15 cm diaphragm pump
Submersible centri-
fugal pump
200-420 each
Bronze without installation;
20-90 gpm
Diaphragm pump
300-1150 each
Cast iron starter and level control,
without installation, 2 in.
discharge; 40-600 1pm
(continued)
85
-------
TABLE 19 (continued)
Item
Unit cost, $
Remarks
Drain Pipe
PVC perforated
underdrain
6.90-18.6/m
30 m length; 10.2-30.5 cm diameter
Corrugated steel or
aluminum
14.8-25.6/m
15-25 cm diameter
Porous wall concrete
13.2-28.0/m
15-25 cm diameter
Envelope
Gravel
11.80-13.50/m3
Backfill
No compaction
1.40/m3
Air tamped
1.40/m3
Compacted
2.00-2.15/m3
15-20 cm lifts
a Source: Reference 31 (modified).
86
-------
TABLE 20. 1986 COSTS OF INSTALLING A SLURRY WALL*
Slurry trench, Unreinforced slurry wall,
soil-bentonite backfill, cement-bentonite backfill,
$/m2 $/m
Depth Depth Depth Depth Depth Depth
Medium <9 m 9-23 m 23-37 m <18 m 18-46 m .>46 rr
Soft-medium soil 30-60 65-120 120-150 220-300 300-440 440-1100
Hard soil 65-110 75-150 150-300 370-440 440-580 580-140C
Occasional boulders 65-120 75-120 120-370 300-440 440-580 580-1250
Soft-nedium rock,
sandstone, shale 95-180 150-300 300-730 730-880 880-1250 1250-2570
Boulder strata 220-370 220-370 730-11,000 440-580 900-1400 1400-3090
Hard rock (granite, - - - 1400-2050 2050-2570 2570-3450
gneiss, schist)
a Source: Reference 31 (modified).
87
-------
TABLE 21. 1986 COSTS OF COMMON GROUTS3
Grout type Cost, $/m3
Portland cement 350
Bentonite 460
Si 1icate
20% 460
30% 770
40% 1,000
Epoxy 11,000
Acrylamiae 2,400
Urea formaldehyde 2,100
aSource: Reference 31 (modified).
TABLE 22. 1986 COSTS FOR GROUND BARRIER IN R0CKa
Unit operation Unit cost, $
Injection hole drilling 45/m
Grout pipe 27/m
Grout injection 7/m3
lSource: Reference 31 (modified).
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resulting from rainfall events with a frequency of between 2 to 100 years,
and they should be constructed to intercept and convey the resulting flows at
29
nonerosive velocities. The failure of such systems often results from
insufficient capacity and excessive velocity. When operating improperly,
such systems can actually increase seepage. Although wider, shallower ditches
are generally used to reduce the potential for erosion, site conditions at
most copper leaching sites often necessitate the use of narrower and deeper
channels that may require stabilization or frequent retrenching.^*
Drainage ditches are generally installed during the active phase of the
operation; however, they can also be used as part of a comprehensive closure
plan. When the dump leaching operation at the Copper Cities mine site was
closed in 1982, Pinto Valley Copper Corporation constructed a system of
diversion trenches to channel overflows from the leach pile collection sumps
into the tailings pond for evaporation. The trench system was designed to
handle flows resulting from a 100-year storm event and was lined with riprap
to prevent erosion.
Diversion berms are usually constructed to prevent excessive erosion by
diverting surface flow and reducing slope length. Because they generally
consist of compacted earthen ridges designed to direct water away from an
area needing protection, they eliminate the need for excavation. Ideally,
berms are constructed of erosion-resistant, low-permeability, clayey mate-
rials or waste rock.
Regrading is another relatively inexpensive diversion technique that can
be used when suitable cover materials are available on site or close to the
mine. This technique is effective, however, only after the termination of
active leaching operations at a site. A properly sealed and graded surface
will reduce ponding, which will in turn minimize infiltration of precipita-
tion and reduce subsequent differential settling and subsurface leachate
27
formation. Surface grading can also reduce runoff velocities, reduce
erosion, and roughen and loosen soils in preparation for revegetation.
Certain disadvantages are associated with grading the surface of a
copper leach site. Large quantities of a cover material may be difficult to
obtain. Haulage costs may be prohibitive if suitable quantities of cover
material cannot be located on or near the mine site. Moreover, periodic
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regrading and future site maintenance may be necessary to eliminate depres-
sions formed through differential settlement and compaction or to repair
slopes that have slumped or eroded. In some cases, regrading may actually
increase the permeability of a leach pile by breaking up deposits of iron
salts that have precipitated onto the surface of the pile.
Containment Systems
At leach operations, containment systems are used in conjunction with
surface-water diversion systems to collect onsite stormwater or dike seepage
for the treatment necessary for final disposal of the waste or to prepare the
waste for recycling. Secondary containment systems also may be used to
intercept offsite runoff during and after major storms and equipment malfunc-
tions to prevent liquids from escaping the primary recirculating leaching
system.
Most of the containment systems used at copper heap and dump leaching
sites are built in existing valleys or natural drainage basins. A concrete
or earthen dike is constructed between the valley walls and across the valley
floor to form a holding pond.
A typical secondary containment system has been installed at ASARCO's
Silver Bell mine for the pregnant liquor and barren solution collection
ponds. This system includes several catchment basins located in a dry wash
and enclosed by a dam downgradient from the ponds. These basins were designed
to handle liquid flows resulting from a 20-year storm event, and they increased
the capacity of each of the holding ponds by a factor of approximately 10.
As is typical of many mitigative systems installed around leaching
operations, the construction work was performed by mine personnel and equip-
ment. The vegetation and existing sandy dirt and rock were removed and
hauled to a site on the mine property. The sides and bottom of each of the
basins were then lined with .dirt and clayey material obtained near the mine.
The dam was also constructed of earth and grouted to the bedrock on the
bottom and sides of the basin.
Cost Analysis of Surface-Water Management Systems
A major portion of the expense incurred in the construction of a surface-
water management system is the capital cost of excavation. This cost depends
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primarily on the type of drainage area and the consequent size of the required
31
diversion and/or containment system. The cost of excavation will also be
affected by the soil and rock conditions at the site, the return period of
design storms, and the expected velocity of the resulting runoff.
Most mining operations can supply the necessary equipment and personnel
for removal and disposal of excavated material. If special equipment must be
acquired or a subcontractor must be hired to dispose of this material, the
costs will increase significantly. Generally, areas are available at most
mining sites where the excavated soil and/or rock can be disposed of.
Consequently, the haulage distance and resulting cost of disposal will be
minimal.
Fill material is used to line diversion ditches and containment areas
and to construct berms and dams. The cost of acquiring, hauling, and placing
the fill material will depend on the type and availability of the material
used, the amount of fill required, and the topography of the site. Gabions
and rock riprap are typically used to stabilize ditches against erosion.
Fine-grained soils and clay are used to line containment areas. The cost of
such materials will depend on the required thickness and screening, the type
31
of equipment needed, and whether grouting is required.
All cost estimates must be made on a site-specific basis. In an esti-
mate of the cost for the construction of a surface-water management system,
the following factors should be considered.
° Source and required amount of fill material
° Type and amount of other material required
0 Cost of transportation, installation, and/or placement of the
materials
0 Cost of stabilization
° Maintenance and repair costs
Table 23 presents typical costs associated with the establishment of surface-
31
water control systems.
RECLAMATION AND CLOSURE SYSTEMS
The use of cover systems is generally considered one of the most effec-
tive reclamation and closure activities. The proper installation of such
systems at a copper heap or dump leaching operation controls surface-water
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TABLE 23. 1986 COSTS FOR ESTABLISHING
SURFACE WATER CONTROLS
Operation
Output
Unit cost, $
General excavation
Front-end loader
Bulldozer
35-140 m3/h
40-120 m3/h
0.80-1.40/m3
1.20-1.10/m3
Ditch excavation
0.9 m deep
1.2 m deep
75-100 m/day
50-70 m/day
4.78-7.00/m
6.80-10.20/m
Building embankments;
spreading, shaping,
compacting
Material delivered by
scraper
-
0.55-1.10/m3
Material delivered by
backdump
-
1.10-1.70/m3
Ditch stabilization
Riprap
Gunite (with 5-cm mesh,
2.5 cm thick)
47 m3/day
21.7-25.8/m3
Hauling, spreading of
gravel (purchase off
site)
790 m3/day
7.00-8.00/m3
a Source: Reference 33 (modified).
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infiltration, promotes proper drainage, and creates an area that is more
aesthetically pleasing. Because of the size and construction of the leach
pile, lack of suitable cover materials, climatology of the area, and the
effect of leachates within the piles, the installation of cover systems may
be difficult to accomplish or inappropriate at copper leaching operations,
however.
Cover systems generally include some type of capping system overlaid
with a material capable of supporting revegetation of the area. The systems
are discussed in greater detail in the following subsections.
Capping
Capping a leach dump could reduce the infiltration of onsite surface
water. Capping is inappropriate for use at active leaching operations be-
cause of the ongoing nature of the disposal process. Capping may be prohibi-
tively expensive to install and maintain at inactive and abandoned sites.
Various site-specific factors influence the design of a cap and the
selection of capping materials for a particular application. These include:
° Size and configuration of the operation
° Type of ore and waste rock in the leach pile
° Local climate and hydrogeology
° Local availability and cost of cover materials
0 Potential for ground-water contamination
Capping entails the placement of a layer of material composed of natural
soils and rock, admixed soils, a synthetic liner, or a combination of these
materials over the leach pile. Multilayered caps consisting of a vegetative
layer, a drainage layer, and a low permeability layer are the most common.
Single-layer caps, however, may be effective in many of the mining areas of
the Southwest because the climate is arid to semiarid and the average annual
precipitation is less than 50 cm.
Mixing of coarse and fine-grained overburden or crushed waste rock
obtained from the mining site is probably the most cost-effective method of
creating a stronger and less porous cover material. Chemical stabilizers and
cements can be added to relatively small amounts of onsite soils to create
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stronger and less permeable surface sealants. Soils also may be treated with
31
lime, fly ash, bottom ash, and furnace slag.
The capping of copper leach piles has several disadvantages. For exam-
ple, the cost of preparing, transporting, and applying adequate capping and
drainage materials to a leach pile may be prohibitive. As noted earlier,
copper leach piles typically cover hundreds of hectares and are constructed
in lifts totaling a hundred meters or more. Adequate sealing of the top
surface of these piles requires that an enormous amount of material be hauled
to the site, spread, and compacted. Also, because most of the side slopes
are inaccessible, a large proportion of the ore and waste rock would remain
exposed.
Leach dumps tend to form a natural low-permeability cap as a result of
the saturation of the pile with acid solutions and the exposure of the waste
rock to air. The dissolution of the copper and other minerals in the ore
forms a fine, clayey material on the surface of the dump, which decreases the
surface permeability of the pile. In addition, ores having a high percentage
of pyrite and other iron-bearing minerals will precipitate iron oxide, which
further decreases surface permeability. Some of this iron oxide and clayey
material also will be deposited in the rocky layers beneath the surface
during the downward percolation of the leaching solution, which plugs voids
and prevents liquids from reacting with portions of the pile.
Revegetation
Because widely varying climatological factors and soil conditions affect
growing conditions, the level of effort required to revegetate leach areas
successfully will also vary. A great deal of work would be required at a
Southwestern copper facility where a combination of poor soils (high in salts
and sulfides and low in nutrients) and arid climate requiring managers to
introduce nonnative plant species, to install irrigation systems, and to
provide constant maintenance to develop and maintain vegetative cover.
Revegetation also requires extra effort at sites in mountainous terrain,
where erosion rates are often high, growing seasons are short, and winters
are long and severe.^
Most revegetation efforts have been directed at tailings ponds in an
effort to prevetn blowing dust. For example, Pinto Valley attempted to
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revegetate the Solitude Tailings Pond near Miami, Arizona, beginning in 1959.
These efforts have involved spreading native soil over the top of the 4-hec-
tare tailings pond and planting native plants. The soil was obtained from
the surrounding hills to minimize haulage costs. The area surrounding the
tailings pond was stripped, and vegetation and the surficial layer of dirt
was excavated. A layer of this dirt approximately 25 cm deep was spread over
the entire surface of the tailings pond. Available mining equipment was used
to remove, haul, and spread the soil. The surface of the tailings pond and
the area from which the dirt had been obtained were then seeded with a 10-seed
mixture of native plants. The project required approximately 20 months to
complete.
Cost Analysis of Reclamation and Closure Systems
The costs of capping and revegetaton measures can vary significantly.
Table 24 sets forth typical costs associated with the construction of a cover
34
system. The costs of soil conditioners, fertilizers, and plant species at
27
a typical mining site have been estimated to be approximately $2500/hectare.
TABLE 24. TYPICAL COSTS FOR CAPPING AND REVEGETATION0
Unit cost, $/m3
Material available on site
Clay Sand Soil
Excavation
Loading
Hauling
Spreading and compacting
1.86 1.00 1.17
1.58 0.84 0.98
3.39 3.39 3.39
2.52 0.55 2.52
Material purchased off site
Purchase .
Transportation
Spreading
10.9 7.88
7.35 7.35
2.52 c
13.7
7.35
c
a Source: Reference 34.
k Transportation approximately 32.2 km.
c Included in purchase cost.
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OTHER ALTERNATIVE MANAGEMENT PRACTICES
Process Modifications
Sulfuric acid leaching is by far the most economical and commonly used
hydrometallurgical process for the extraction of copper. Sulfuric acid is
used primarily because it is formed naturally by the oxidation of sulfide
minerals. In addition, large quantities of this acid are produced as a
byproduct of the copper smelting operation. In 1985, the copper industry
produced approximately 729 thousand metric tons of sulfuric acid. Although
the market for sulfuric acid has been increasing, most smelting operations
produce more than can be conveniently sold, and leaching has become a benefi-
cial means of disposing of the acid. As a result, even though the commercial
use of sulfuric acid has been increasing, its average price has remained
around 54tf/lb ($1•20/kg). Despite the fact that many of the mining companies
have reduced their mining and processing activities because of the low price
of copper, leaching operations have remained profitable, in part because of
this cheap supply of sulfuric acid.
Various alternative hydrometallurgical processes for the recovery of
copper have been investigated. The impetus for most of this research, how-
ever, has been the identification and development of technologies that pro-
duce less sulfur dioxide to be able to meet air pollution control standards
while remaining economically competitive with current pyrometallurgical
techniques. Consequently, most of this research has focused on the leaching
of concentrates. Very little research has been done to identify techniques
for reducing the potential for acid generation and/or ground-water contamina-
tion from sulfuric acid leaching.
Ferric chloride has been proposed as a leaching agent for copper concen-
trates and, less seriously, for low-grade ore. The stoichiometry of the
leaching reaction of ferric chloride in copper ore has been found to be:
CuFeS2 + 3FeCl3 - 4FeCl2 + CuCl + 2S° (Eq. 20)
As this reaction indicates, sulfur is not substantially oxidized to form
sulfates as it is when sulfuric acid is used as the leaching reagent. As a
result, the acid generation capacity of the leach pile is greatly reduced.
In addition, the rate of dissolution of copper in ferric chloride is much
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faster than in sulfuric acid. Ferric chloride, however, is considerably more
expensive than sulfuric acid, and corrosion problems inherent in this type of
reagent have limited its use. Furthermore, the nature and impact of the
leach liquor's constituents on the environment have not been investigated.^
Another concept that has been proposed is the leaching of native copper
with a cupric ammonium carbonate solution. Native copper (which accounts for
a very small percentage of the Nation's copper supply) is readily soluble in
cupric ammonium carbonate solutions, as shown by the following reaction:
Cu° + Cu(NH3)2C03 + 2NH40H * [Cu(NH3)2]C03 + H20 (Eq. 21)
The cuprous solution formed in this reaction is reoxidized by contact with
air. Copper is recovered as a mixed oxide by boiling the pregnant solution,
and the ammonia is recovered for recycle. Various studies have been conduc-
ted to investigate the characteristics and feasibility for extracting copper
from native copper in a leaching system. This process has not been used-on a
commercial scale, however, and its potential impact on the environment is
unknown.
Enhancements of the biological activity that occurs naturally in most
leach dumps are also being studied. New strains of microorganisms that would
selectively attack the copper sulfide components in the leach dumps while
leaving the iron-containing minerals relatively unaffected are being dis-
cussed. The use of bacteria to convert sulfates in the ore into elemental
sulfur rather than sulfuric acid is also being investigated. These systems
would considerably reduce the acid generation capacity of copper leach dumps
and also produce potentially salable sulfur. These enhancements will require
considerable research, however, and their commercial exploitation is many,
many years away.
Security Systems
Security systems prevent entry into the mining operations and ban access
of animals and unauthorized persons to the leaching ponds. The major objec-
tive of installing this type of system is to protect the general public and
34
prevent activities that might damage onsite control systems.
Mining sites currently use a variety of security systems, from simply
posting "No Trespassing" signs to a comprehensive system of fences, locked
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gates, and security guards. Fencing equipped with noise-making devices is
generally used to limit access by wildlife. Many of the copper heap and dump
leaching operations are located in sparsely populated areas, and the mine
operators at these sites do not employ extensive security measures. Typi-
cally, these sites just limit access to mine service roads, fence easily
34
accessible routes, and post the mine property.
Water Balances
Solution losses through seepage, runoff, and other release mechanisms
may be assessed by maintaining a water balance for the leaching operation.
Maintaining a water balance involves a total accounting of water entering the
leach system and water leaving the system. Water is introduced directly into
the system in the leach solution. It also may enter as precipitation, surface-
water runoff, or ground-water infiltration. Water may leave the system
through process losses, evaporation, transpiration, seepage, or precipitation
27
of hydrated metal salts (e.g., gypsum, CaSO^f^O). The accuracy of a water
balance is limited, however, because the amount of water contained"in the
dump can only be estimated.
To be effective, a water balance must be kept current to assure the
efficient use of water and to identify any potential water losses and treat-
ment requirements. Initially, the development of a water balance will require
data from a site characterization and monitoring study. This may entail
additional engineering time, instrumentation, and outside consultation.
After the initial water balance has been completed, the operating parameters
of the leaching system, such as flow volumes and direction, need to be updated
and verified by continuous field monitoring. Frequently, the operating
expense of these activities will be offset by more efficient management of
the systems solutions.
Postclosure Monitoring and Maintenance Activities
After copper leaching operations have ceased, long-term monitoring and
maintenance activities will be necessary to identify and limit water qua'ity
degradation at the site. These activities may include taking periodic sam-
ples from ground-water monitoring wells around the site or the continuation
of certain inspection and maintenance activities routinely performed during
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the active life of the leaching operation. Management practices initiated
during the closure period also may require inspection and maintenance to
assure their continued integrity and effectiveness.
The purpose of the ground-water monitoring program is to determine the
long-term impact of leachates generated at the leach site on the surface and
subsurface conditions of the surrounding area. The location of the well
sites, the sampling frequency, and the scope of the data analysis should be
selected to define contaminant migration and dilution and to evaluate the
overall effectiveness of the mitigative systems at the site. Where contin-
uing ground-water impacts are identified, further actions, such as ground-
water cleanup, placement of subsurface barriers, or plume treatment, may be
required.
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SECTION 6
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
The conclusions drawn from the information gathered during the study of
copper leaching operations are summarized under four major groupings parallel-
ing the organization of this report.
General Characteristics
1) Although the number of active mines in the United States has de-
creased in recent years, the percentage of primary copper produced
by leaching has increased.
Low copper prices have resulted in the closing of many mining,
milling, and smelting operations. Although many of the leaching
operations associated with these sites also have been closed, a
significant number are still active because of the relatively low
operating costs associated with dump leaching. The result has been
an increase in the percentage of copper produced by leaching opera-
tions in the United States. Estimates indicate that by 1990 approxi-
mately 30 percent of the copper produced in this country will be
recovered by some type of leaching process.
2) The areas in which copper leaching is practiced are similar in
general characteristics.
Most of the active U.S. copper dump and heap leaching sites are in
the Southwest. The climate in these areas ranges from arid to
semiarid. The average annual precipitation is generally less than
50 cm, and the average annual temperature ranges from about 10° to
30°C.
The topography of these areas varies from gently rolling hills to
mountainous terrain. Vegetation is sparse. The active leaching
operations in Utah and northern and eastern Arizona tend to be
located in more mountainous terrain than those in southern Arizona
and western New Mexico. The land surrounding many of the active
leaching operations consists primarily of sparsely populated and
undisturbed vacant land. Some of the active leaching operations,
however, are relatively close to residential and urban areas.
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Very little surface water is found near active leaching sites in
the Southwest. Ground water, which is the principal source of
water at most of these mines, tends to be very deep. In the more
mountainous environments, however, the amounts of surface-water
runoff and ground water are greater because of winter snow accumu-
lation.
Operating Practices
1) Copper leach dumps typically cover hundreds of hectares, are more
than a hundred meters high, and contain millions of metric tons of
leach material.
Copper leach piles as small as 8 hectares (Cyprus Johnson) and as
large as 850 hectares (Bingham Canyon) were observed during this
study. Estimates indicate that more than 5.5 billion metric tons
of leach material now exists in copper leach dumps scattered around
the United States and in excess of 40 million metric tons of new
material is being added to these dumps annually.
2) Dump leaching and heap leaching are distinguished by the use of
liners.
Dump leaching refers to the leaching of low-grade ore that has been
deposited directly on the existing topography. The pregnant leach
solution is typically collected in unlined natural drainage basins.
In contrast, heap leaching refers to the leaching of ore that has
been deposited on specially prepared pads. Lined collection systems
are used more frequently in heap leaching.
3) Leaching operations are always constructed in the immediate vicinity
of the mine site.
Leach sites are selected to minimize haulage costs and to utilize
the natural drainage patterns of the native terrain for collection
of the pregnant liquor solutions.
4) Leaching of copper from massive dumps of sulfide ore is accomplished
by bacterial activity and, often, by the addition of sulfuric acid.
Ferric sulfate, the major lixiviant, forms in the presence of
oxygen and bacterial activity. The bacteria generate acid in situ,
which provides acid for acid-consuming reactions, including oxygen
reduction. Frequently, only makeup water is needed in copper dump
operations because the oxidation of the sulfide minerals generates
sufficient acid to dissolve the copper and maintain an active
bacteria population. More effective leaching reagents have been
identified, but they are generally more expensive and their impact
on the environment is uncertain.
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5) Copper is recovered from pregnant leach liquors either by cementa-
tion or by solvent extraction/electrowinning.
These processes remove copper from solution and allow other dis-
solved substances to accumulate. The recovery process itself may
add other substances to the leach solutions. The cementation
process uses scrap iron to precipitate copper from the pregnant
solution. The iron replaces copper in solution, and this iron-rich
solution is subsequently recycled to the top of the leach pile.
Upon exposure to the atmosphere, the dissolved iron oxidizes to
form insoluble salts, which precipitate on the surface of the pile
and restrict the flow of solution.
Solvent extraction uses a complexation mechanism whereby copper is
coordinated by an organic compound; the copper is then stripped
from the organic phase by a strong acid solution. Kerosene is a
common carrier used in most solvent extraction operations, and it
may appear in small quantities in the raffinate recirculated to the
dump.
Environmental Impact
1) Seepage from leach dumps and solution collection systems is the
most significant potential mechanism for the release of contamina-
tion into the ground water.
One of the primary criteria in siting leaching operations is prox-
imity to the mine. In dump leach operations, the ground surface is
neither lined nor treated in any manner to reduce seepage. Because
the leaching solutions are in direct contact with the earth, some
continuous solution loss results. Releases can also result from
pipe and dam failures, equipment malfunctions, and overflows due to
severe storm events.
2) The solutions generated in copper dump and heap leaching operations
usually have a lower pH and higher concentrations of metals and
total dissolved solids than the natural waters surrounding the
s i te.
Most leach materials, particularly those found in copper dumps,
contain pyrites and other naturally occurring metal sulfides that
oxidize to generate a low-pH solution when exposed to air and
microbial activity. The solvent extraction process also reduces
the pH of the solution by ion exchange before it is distributed on
the leach dump. Generally, acidic solutions increase the solubility
and bioavailability of heavy metals contained in the leach material
and rock surrounding the dump.
3) The water quality around several active copper dump leaching opera-
tions has been affected by leachates that have seeped into the
ground water.
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The available ground-water monitoring data indicate that some
degradation of the ground water has occurred around several copper
dump leaching operations. Some seepage of leachates into the ground
beneath copper leach dumps is inevitable. The amount of seepage
and its impact depend on site-specific factors.
Management Practices
1) Some active copper leaching operations have implemented management
practices that include one or more mitigative measures (e.g., pond
and trench liners) designed to minimize solution losses.
Historically, such management practices were implemented solely for
economic reasons (to improve copper recoveries). As the potential
for ground-water contamination problems associated with leaching
became apparent, these practices were implemented for environmental
reasons as well. The measures used at a particular site depend on
various site-specific factors, the most significant of which are
the geology, hydrogeology, topography, and meteorology of the site.
The land use and population density of the area surrounding the
operation are also considered, as is the cost of constructing
and/or installing each potential mitigative measure.
2) The application and efficiency of standard waste management prac-
tices at copper leaching operations are frequently limited by the
size and environmental characteristics of the site.
Copper leaching operations are massive; thus, management practices
required for adequate control of potential ground-water contamina-
tion from leaching operations also must be on a large scale. The
geologic and hydrogeologic evaluation required to design and imple-
ment an effective surface-water and ground-water control system
will be extremely complex, and the required control systems may
cover several hundred hectares. The environment of the site may
necessitate a system to divert surface water resulting from the
torrential rains periodically experienced in the region, but annual
precipitation may not be adequate to sustain revegetation efforts.
The size of the leaching operation and its surrounding environment
often combine to make both lining and capping economically imprac-
tical .
3) The cost of implementing and maintaining an effective system of
management practices to minimize solution loss and reduce potential
ground-water contamination depends on site-specific factors.
Traditional management practices tend to be very expensive to
implement at copper leaching operations because of the size of the
oeprations and the natural characteristics of the site. Neverthe-
less, most of these practices have been implemented economically at
or around one or more leaching operations. Proper planning and
design procedures are required to select the most appropriate
management practices and to minimize costs.
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RECOMMENDATIONS
The conclusions of this study have pointed up the need for additional
investigation. The following recommendations indicate the areas where fur-
ther study would be of value.
1) The potential environmental impact of active and abandoned copper
leaching operations on surface and ground waters should be eval-
uated more extensively.
Currently available surface-water and ground-water monitoring data
from active leaching operations are sparse. Only two of the sites
visited during the course of this study (Tyrone and Bingham Canyon)
were routinely monitoring ground water. New ground-water protec-
tion regulations recently implemented in Arizona (where the majority
of the active sites are located) should substantially increase the
availability of such data.
A comprehensive investigation of abandoned copper leaching opera-
tions, including site preparation used, also should be conducted
to evaluate the environmental impact of these sites. A recent
study of water-quality problems in the Globe-Miami mining district
in Arizona identified more than 200 active and abandoned mines, but
it did not specify the number and size of the leaching operations
at these sites. Such determinations are needed for an adequate
assessment of the impact of abandoned leaching operations on surface-
water and ground-water quality.
2) The design and operating characteristics of in situ leaching opera-
tions should be investigated further.
This method is increasing in prevalence. Commercial operations at
the Lakeshore and Miami mines and an experimental operation at San
Manuel were toured during this study. This method of leaching
significantly reduces operating costs because it eliminates the
costs of excavation and haulage of ore. The potential environ-
mental impact of these operations, however, may be greater because
the leaching solution is injected directly into the ground. Addi-
tional information should be developed concerning the distribution
of the solution through the rock, the impact of the local geology
and hydrogeology on solution and ground-water flow patterns, and
the impact of ore fracturing on the surrounding rock.
3) State ground-water protection programs pertaining to copper leach-
ing operations should be reviewed and evaluated*
New Mexico, for example, requires each mine to develop a discharge
plan for each new leaching area on a site. The regulations generally
call for extensive geotechnical, geochemical, and modeling studies
of the proposed leach site. All discharge plans also must contain
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monitoring provisions that require sampling, analysis, and report-
ing of ground-water and leachate quality. Numerical standards for
ground water have also been established. Relatively few regulatory
requirements have been imposed on copper leaching operations in the
State of Arizona prior to this year. Arizona began implementing an
extensive new ground-water protection program in August 1986 that
may have a significant impact on both active and inactive leaching
operations in that State. Each of these programs should be reviewed,
along with those of other States, to evaluate their effectiveness
with regard to copper dump and heap leaching operations and to
determine the need for additional regulations at the Federal level.
State and/or Federal ground-water protection regulations should
identify the criteria for determining water-quality degradation as
they apply to copper leaching operations. The criteria for deter-
mining whether water quality degradation from dump leaching has
occurred should be identified and evaluated.
The first criterion for determining degradation of water quality
concerns the application of water-quality standards. New Mexico,
for example, has established one standard for all ground water,
regardless of the mine's location or the nature of water use in the
area. Many of the leaching operations in this country, however,
are located in relatively sparsely populated areas where ground
water is used primarily by the mining operation. Less stringent
standards may be more appropriate for such sites.
The second criterion for determining degradation of water quality
concerns the point of compliance. If ground-water quality is
measured at the property boundary, which may be several miles
distant from the site of leaching operations, considerable environ-
mental degradation may occur before the point of compliance is
reached. Under such circumstances, effective mitigative measures
may be difficult to implement, and an earlier detection system may
be required. On the other hand, if ground-water quality is measured
at the boundary of the leach pile, there would be no opportunity
for natural attenuation processes to take place.
The economic impact of additional regulatory controls and guide-
lines on the copper mining industry should be studied.
In its July 3, 1986, regulatory determination, EPA concluded, in
part, that the cost of various alternative management practices
must be one of the factors considered in determining its regulatory
approach to mining wastes. Most U.S. mining operations are opera-
ting at less than full capacity, an-1 employment has declined consid-
erably. Foreign competition and large inventories of refined copper,
among other factors, have severely depressed the price of copper.
As a result, many mining, milling, and smelting operations have
been closed. Although many of the leaching operations have remained
-------
active, the imposition of some types of new environmental control
measures or management practices could increase production costs
and result in additional closures. On the other hand, the cost of
some control measures and management practices may be offset by an
increase in the amount of solution recovered, and the efficiency of
the operation may be improved.
106
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Yu, P. H., et. al. 1973. Kinetic Study of the Leaching of Chalcopyrite at
Elevated Temperature. INT. SYMP. HYDROMETALLURGY, 2ND 1973 (Pub. 1973),
375-402 (Eng). C.A. 81, 80891b.
Yurko, W. J. 1966. Refining Copper by Acid Leaching and Hydro Metallurgy.
Chem. Eng. 73(18):64-66.
Zimmeriey, S. R., et. al. 1968. Cyclic Leaching Process Employing Iron
Oxidizing Bacterial. U.S.P. 2,829,964, April 8, 1968. (to Kermecott
Copper Corp.)
Acid Leaching of Oxidized Copper Ores by Downward Percolation. 1960. U.S.
Bureau of Mines Rept. of Inv. 5629.
Chemistry of Leaching Bornite. 1931. U.S. Bureau of Mines Tech. Paper 486.
Chemistry of Leaching Chalcocite. 1930. U.S. Bureau of Mines Tech. Paper
473.
Chemistry of Leaching Covellite. 1930. U.S. Bureau of Mines Tech. Paper
487.
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Economics Provide Motive for Growth of Bacteria Leaching. 1966. Eng. Mining
Hour., Vol. 167.
Flotation and Leaching at Kennecott. 1928. Eng. and Min. J.
126(26):1008-1015.
How Bacteria Leaches Low-Grade Ores. 1958.- Eng. Mining Hour. 159(6):89-91.
How New Leach-Float Plant Handles Greater Butte's Ore. 1953. Eng. and Min.
J. 154(6):90-93.
Innovations in Copper Leaching Employing Ferric Sulfate-Sulfuric Acid. 1930.
U.S. Bureau of Mines bull. 321.
Leaching Copper Ores: Advantages of Wet Charging. 1930. U.S. Bureau of
Mines Rept. of Inv. 3050.
Leaching Copper Sulfide Minerals with Selected Autotrophic Bacteria. 1964.
U.S. Bureau of Mines Rept. of Inv. 6423.
Leaching Mixed Copper Ores with Ferric Sulfate; Inspiration Copper Company.
1926. Trans, AIME, Vol. 73, pp. 58-83.
Method of Leaching Copper Sulfide Material with Ammoniacal Leach Solution.
1955. U.S. Pat. 2,727,818, Dec. 20, 1955.
Microbial Leaching of Copper Minerals. 1963. Min. Eng. 15(6):37-40.
New Technology of Leaching Waste Dumps. 1962. Min. Cong. Journal.
48(11):82-85.
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APPENDIX A
TRIP REPORTS
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TRIP REPORT
CYPRUS BAGDAD MINING COMPANY
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 2, 1986, a site visit was conducted at the mining operations of
the Cyprus Bagdad Copper Company located in Bagdad, Arizona. The objectives
of the visit and tour were to gain familiarity with the Bagdad operation and
to discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
Manford F. Swain - Cyprus Leaching Superintendent
James A. Sturgess - Cyprus Environmental Coordinator,
Development
F.S. Mooney, Vice President and General Manager of Cyprus Bagdad Copper
Company also participated in a portion of the meeting.
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Cyprus personnel provided a
description of the facility's operations during the meeting and, afterward,
conducted a tour of the operation. During the tour, additional, more
detailed information about the leaching operation was provided. Photographs
of the facility were taken by PEI and EPA during the tour.
General
The Bagdad mine is located approximately 110 miles northwest of Phoenix
in western Yvapai County, Arizona. Initial open pit mining began at the site
in 1956. The dump leaching operation began in 1960. Approximately 50,000
tons per day of ore is currently mined at the site, producing about 14
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million pounds of copper annually. Between 8 and 10% of this copper is
recovered from the dump leaching operation. The mine currently employs
approximately 550 people.
Site Characteristics
The area surrounding the mine is relatively arid and consists of low
rolling hills with minimal vegetation. The average seasonal temperatures
range from 35° F in the winter to 95° F in the summer. The average annual
precipitation is approximately 10 inches per year.
Western Yvapai County is relatively sparsely populated. The town of
Bagdad is located adjacent to the mining operations and is comprised
primarily of residences rented by the company to employees. Approximately
500 people currently live in Bagdad.
The ore body at Bagdad contains a chalcocite-enriched zone in a
monzonite porphyry. Copper minerals mined at the site are largely
chrysocolla, malachite and azurite with a little chalcocite enrichment.
Cyprus is currently operating 4 leach dumps, including the Alum Creek,
Mineral Creek, Copper Creek and Niagara Creek dumps. These dumps contain
approximately 600 million tons of ore and it is estimated that the leaching
system has the capacity to hold an additional 300 million tons of ore.
Design and Management Practices
Low-grade, mine-run ore is used in the leach dumps. Ore having a copper
content of at least 0.25% is generally deposited on the leach dumps. The
dumps have been built directly upon the existing topography, utilizing the
natural drainage created by the contours of several canyons located on the
property to divert and collect the pregnant leach solution. There was no
prior surface preparation of the dump sites.
Haulage trucks carry the ore from the pit to a leaching area where it
is dumped and spread by a bulldozer. Lift heights range from 40 to 300 feet
depending upon the particular topography of the land. After a lift is
completed, the surface is ripped to a depth of about 5 feet and the solution
distribution system is installed.
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The leaching solution is distributed by a wiggler type sprinkling
system. The solution consists of dilute H2S04 (containing 8 gpl of H2S04)
having a pH of approximately 1.0. The flow rate from the sprinklers is
about 3200 gpm. Initially, each lift is leached until the surface begin to
pond due to a buildup of iron salt precipitates. After this period, the
dumps are allowed to rest. The ratio between the leach period and the rest
period is approximately 3:1. The pregnant solution is collected at the base
of the each pile in an unlined reservoir. Pregnant solution from the Allum
Creek reservoir is pumped to the top of the Copper Creek dump through which
it is allowed to percolate. The pregnant solutions from the leach piles are
then combined in an unlined pond. The pregnant solution from the pond is
metered out through Niagara Dam into a trench and a collection reservoir The
dam is made of concrete, and keyed into the bedrock of the surrounding
hillside. Both the trench and the collection pond have been lined with 100
mil polyethylene liner. The pregnant solution collected in the reservoir is
then pumped to the solvent extraction electrowinning (SX-EW) plant.
After the copper has been recovered in the SX-EW plant, the barren
solution is recycled to the leach dumps. Approximately 100 tons per day of
acid is added to this solution to reduce the pH. Mine water is used as
makeup water.
Environmental Impact
The land upon which the dumps have been built was described as hard,
impermeable rock. The overburden is post-mineralization alluvium, exhibiting
relatively low permeability.
The depth of the groundwater was not known. However, between 500-800
gpm of water is produced in the mine pit. The mine water collected in the pit
is used only in the mining operations as makeup water and is not discharged
off the property.
The natural contours of the land divert the runoff from the surrounding
hills around the mining and leaching operations. Precipitation falling
within the mine area itself will be collected in either the pregnant solution
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collection ponds or the mine pit. An overflow floodplain reservoir has been
constructed to protect against a hundred year flood event. Runoff collected
in the floodplain reservoir is pumped into the pregnant solution collection
reservoir and used in the leaching circuit.
There was no available groundwater monitoring information.
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TRIP REPORT
NORANDA LAKESHORE MINES, INC.
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 3, 1986, a site visit was conducted at the mining operations of
the Noranda Lakeshore Mines, Inc. The objectives of the visit and tour were
to gain familiarity with the Lakeshore operation and to discuss the current
copper leaching project being conducted by PEI for the EPA. The following
personnel participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
John T. Kline - Chief Metallurgist, Noranda Lakeshore
Brent C. Bailey - Manager of Environmental Service,
Noranda Lakeshore
An initial meeting was held to discuss the EPA1s mine waste program in
general and the current project in detail. Lakeshore personnel also gave an
overview of the operations after which a tour was conducted . During the
tour, additional, more detailed information about the leaching operations was
provided by Lakeshore personnel. Several documents dealing with the history
and operations at the Lakeshore property were provided. In addition, a 308
study (308-FY86-009) recently conducted at the facility by EPA Region IX was
cited. Portions of this report are taken from the information provided in
those documents as well as subsequent conversations with Lakeshore personnel.
Photographs of the facility were also taken by PEI and EPA during the tour.
General
The Noranda Lakeshore Mine is located in the Slate Mountains
approximately 70 miles south of Phoenix and 60 miles northwest of Tucson.
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The property consists of approximately 10,500 acres leased from the Papago
Indian Tribe. The only activities currently being conducted at the site are
an in situ mining operation and a solvent extraction-electrowinning (SX-EW)
copper recovery plant. Approximately 3600 tons of copper are being produced
annually by this operation.
Block caving operations began at the site in early 1970 and continued
until 1977 when low copper prices resulted in a shut down. In 1979, Noranda
Lakeshore Mines, Inc. acquired the property and full production was resumed.
However, when low copper prices again caused a shutdown of the underground
mining operations in 1983, the current in-situ operation was started.
Site Characteristics
The area surrounding the mine is typical Sonoran Desert climate and
terrain. The average seasonal temperatures range from 53°F and 87°F. The
average annual precipitation is 8 inches.
The property is located on a relatively sparsely populated indian
reservation of the Tohono O'Odham Nation. The nearest community is North
Komelik which is located 2.5 miles from the mine site and has a population of
125 people. The reservation upon which the mine is located contains about
973 people within a radius of approximately 12.5 miles.
The property contains three copper bearing bodies; three sulfide and one
oxide. The initial block caving operations began in both the oxide and
sulfide ore bodies. The ore deposit is covered with a thick, dense layer of
fanglomerate consisting of silt, sand, and boulders and is bounded by two
faults, the Lakeshore fault on the southeast and the 'C' fault on the west.
Design and Management Practices
The in situ leaching operation is being conducted in the existing tlock
caved underground mine. The block caving operation has created a large
subsidence area on the surface bounded on three sides by an 80° enscarpment.
The east side has been structurally controlled by the Lakeshore fault giving
a scarp angle of 20°. Test have indicated that the subsiding process has
resulted in significant ore crushing, allowing better access to the leaching
solution.
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Considerable surface preparation was required to create an area
accessible for drilling injection wells and the construction of well heads.
Forty-four holes were drilled. All holes were sampled at five foot
intervals. The casing installed in each of the holes is 1.5 inches diameter
schedule 80 PVC pipe. Casing perforations start 20 feet below the
fanglomerate.
Dams were placed across the No. 4, 5 and 6 haulage drifts at the 1100
level to contain the pregnant liquor solutions flowing from the extraction
drifts. These dams were connected to the main dam at the shaft using 15
inch plastic irrigation pipe. Pregnant liquor solutions collected at the 900
level are pumped up to the 1100 level. The main pumping station is located
at the 1100 level. Stainless steel pipes are used to pump the pregnant leach
solution to the surface where the transition to polybutylene lines is made.
The polybutylene pipes lay on the surface and run to the SX-EW plant.
Currently, the leach solution contains about 15 gpl acid and has a pH of
about 1.3. Generally, the solution is pumped into the mine continuously.
The pregnant leach solution recovered from the mine has a pH of about 1.95
and contains approximately 1.05 gpl copper.
Environmental Impact
The fanglomerate surrounding the ore deposit is a high acid consuming
material and, therefore, any movement of the acid or heavy metals released
by the acid should be restricted.
The depth of the groundwater ranges from 110 to 638 feet. Because of
the depth of the block-caved area, the mine tends to act as a sump collecting
water from the surrounding area. In addition, because of the size and extent
of the subsidence, water runoff flowing into the area will tend to be
collected in the haulage areas of the mined and collected with the pregnant
liquor solution. There are no other runon/runoff controls.
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TRIP REPORT
SILVER BELL MINE - ASARCO, INCORPORATED
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 3, 1986, a site visit was conducted at the mining operations of
the Silver Bell unit of ASARCO, Incorporated. The objectives of the visit
and tour were to gain familiarity with the Silver Bell operation and to
discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
Scott L. Burrill - Director of Technology and Environment,
ASARCO
Verle C. Martz - Environmental Engineer, ASARCO
David J. Duncan - Mill Superintendent, ASARCO
David F. Skidmore - Assistant to General Manager, ASARCO
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Silver Bell's personnel provided
a description of the facility's operations after which a tour was conducted.
During the tour, additional, more detailed information about the leaching
operations was provided. Several documents outlining the operations at the
facility were provided including a flow diagram, site plan and topographical
map. Portions of this report are taken from the information provided in
those documents as well as subsequent conversations with ASARCO personnel.
Photographs of the facility were taken by PEI and EPA during the tour.
General
The Silver Bell mine is located approximately 40 miles northwest of
Tucson in Pima County, Arizona. ASARCO began to acquire the properties in
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1915 and underground operations were conducted at the site until 1921, The
property was then idle until 1951, when preparations for the open pit
operations began. Dump leaching began in 1960. Approximately 150 acres of
the mine property are covered by leach dumps. Approximately 9,012,000 tons
of copper are produced annually at the facility. Active mining of the site
was suspended in August, 1984. The mine currently employs 48 people.
ASARC0 is currently operating two leach dumps at Silver Bell.
Approximately 21 million tons of low grade ore is currently being leached in
the Oxide Leach Dump. This leach dump also includes an area containing 5.0
million tons of ore which will be leached in the future. The El Tiro Leach
Dump currently contains approximately 40 million tons of ore.
Site Characteristics
Silver Bell lies to the west of the Avra Valley. The average seasonal
temperatures range from 95°F in the summer to 55°F in the winter. The
average annual precipitation is approximately 4 inches.
The property is located in a relatively sparsely populated area.
Marana, with a population of approximately 1700, is located 25 miles east of
the mine.
The rock surrounding the ore deposit at Silver Bell consists of
monzonite, dacite porphyry and alaskite which have been hydrothermically
altered, exhibiting the entire range of alteration features from propylitic
through phyllic to potassic.
Copper mineralization in the enriched zone occurs primarily as
chalcocite with lesser amounts of chalcopyrite along with minor covellite,
cuprite, malacite, azurite and chrysocolla. Ore in the primary zone consists
principally of chalcopyrite, with lesser amounts of bornite. Minor amounts
of molybdenite, galena and sphalerite also occur in various parts of the ore
zone.
Design and Management Practices
The ore deposited in the leach dumps contains between 0.3% and 0.45E
total copper while the acid soluble copper is in the range of 0.15% to 0.30%.
Since ore is not currently being mined from either of the pits, operation of
the dump leach circuit and the recovery of copper in
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cementation cells are the only activities being conducted at the facility.
Periodically, the surface of each of the dump areas is ripped to a depth of
approximately 6 feet and the ore is mounded to form troughs and ponds into
which the leaching solution can be pumped. The leach solution is then
applied to the dump utilizing the ponding method for a period of 6 to 12
months until the precipitation of iron salts prevent the infiltration of
leaching solution into the pile. At the end of that period, the dump is
allowed to "rest" for a period of 5 to 12 months after which the cycle is
begun again. Only about 40% of the leach dumps are being leached at any one
time.
The dumps are leached with a solution of dilute HgSO^. The solution
contains about 0.15 gpl of acid and has a pH of approximately 2.8. Between
0.75 and 0.80 lb. sulfuric acid is used for each pound of recovered copper.
Operational and seasonal variations result in solution flow rates ranging
from 2000 gpir to 2500 gpm.
The pregnant liquor is collected at the toe of the dumps in unlined
holding ponds situated on bedrock. An example of the characteristics of the
pregnant leach solution are as follows:
The pregnant solution is pumped from the holding ponds to a collection
reservoir . The leach dumps produce approximate 2200 gpm of pregnant liquor.
The combined solutions are then pumped through epoxy lined pipes to the
cementation operation.
The pH of the barren solution from the cementation operation is
approximately 3.5. Makeup acid is added to the barren solution before it is
pumped into an unlined holding pond. Although the barren solution from the
cementation cells is clear, the iron salts that have precipitated from
solution and line the sides and bottom of the pond gives the resulting brown
appearance. The solution is pumped, as needed, from this holding pond to the
leach dumps as feed solution.
Copper
h2so4
Ferrous iron
Ferric iron
0.80 gpl
0.50 gpl
0.01 gpl
0.60 gpl
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Environmental Impact
The leach dumps are sited on the rocky hillsides. There was no special
surface preparation prior to building the dumps . All retaining dams are
constructed of concrete and have been keyed into the bedrock to prevent
seepage.
There is no aquifer underlying the site.
Runoff and seepage is contained in overflow and catchment dams which
have been constructed downgradient of the leach solution and barren solution
holding ponds and leach dumps. Solution volume in the circuit is controlled
to address varying weather conditions, so as to absorb rain that may fall
within the localized water shed.
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TRIP REPORT
INSPIRATION CONSOLIDATED COPPER COMPANY
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 4, 1986, a site visit was conducted at the mining operations of
the Inspiration Consolidated Copper Company. The objectives of the visit
and tour were to gain familiarity with the Inspiration operation and to
discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in both the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
Jack Castner - Senior Environmental Engineer, Inspiration
Tom B. Larsen - Manager of Environmental Affairs, Inspiration
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Inspiration personnel provided a
description of the facility's operations during the meeting after which a
tour of the operation was conducted. During the tour, additional, more
detailed information about the leaching operation was provided. Inspiration
also provided PEI with several articles describing the operations at the
facility. In addition, photographs of the facility were taken by PEI and EPA
during the tour. This trip report will include information contained in
those articles as well as information obtained in subsequent conversations
with Inspiration personnel.
General
The Inspiration mine is located between the towns of Claypool and Miami,
about 75 miles east of Phoenix, in Gila County, Arizona. The mine was
originally a block cave operation, but is presently active only as an open
pit mine. Dump leaching was introduced at the Inspiration mine in 1955.
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Inspiration's mining operations include the Barney, Thornton, Joe Bush,
Live Oak, Upper and Lower Ox Hide, and the old Bluebird pits. Of these, only
the Bluebird pit is active. The Bluebird pit was acquired from Ranchers
Exploration and Development Corporation in July, 1984. Approximately 80,000
tpd of ore is being rrined at the Bluebird pit. The Live Oak pit is being
dewatered.
Site Characteristics
The mine is located 1n the Mescal Mountains at an altitude of
approximately 4000 feet. The average seasonal temperatures range from 95°F
in the summer to 50°F in the winter. The average annual rainfall is
approximately 20 inches per year.
The towns of Claypool and Miami, with a total population of
approximately 5500, are located across U.S. Highway 60-70. Water for these
residences is supplied from wells operated by the Arizona Water Company
located approximately 3-5 miles from the site.
The host rock for the ore is granite schist. The principal copper
minerals mined at the site are malachite, azurite and chrysocolla with minor
amounts of chalcocite and chalcopyrite.
Design and Management Practices
Inspiration operates two separate leach circuits: a conventional dump
leaching operation and a ferric cure leaching operation. Ore containing
above 0.3% copper as chalcocite and oxides is delivered to the ferric cure
circuit while ore containing less than the 0.3% copper cutoff is delivered
to the conventional leaching circuit. These circuits are operated 1n series,
i.e. the pregnant leach solution recovered from the conventional operation is
used is the leaching solution for the ferric cure operation.
The leach dumps in the old Inspiration property were deposited on the
existing topography. The underlying surface was cleared of existing
vegetation and graded to channel the pregnant leach liquor into the
collection ponds located at the toe of the pile. The underlying surface of
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the old Bluebird leach dumps was also cleared of vegetation and dressed after
which the soil was cemented and covered with dilute tar for curing and
sealing.
New lifts of leach material are built on previously leached dump piles.
Prior to the placement of a new lift, the surface of the dump is ripped to a
depth of approximately six feet. The ore is then hauled to the pad by trucks
and spread with bulldozers. After the lift has been completed, the surface
of the lift is ripped and the solution distribution piping is laid.
The distribution system consists of 2 inch piping perforated with 1/8
inch holes to allow for distribution of the leaching solution. The leaching
solution contains approximately 5-15 gpl H2S04 and has a pH of 1.0. It is
applied to each lift for a period of up to 125 days at varying flow rates. A
flow rate of approximately 15,000 gpm is maintained for the entire system.
The leaching techniques used in the ferric cure operations are unique in
that the leach pads are carefully constructed in uniform dimensions. The
leach pads are generally rectangular, measuring approximately 250 feet wide x
600 feet long. A pad is stacked to a height of approximately 30 feet.
After completion of the pad, the pile is cured. The cure solution contains
200 gm/liter H2S04 and 2-3 gm/liter ferric iron. Sufficient cure solution is
applied to saturate the pad in two separate applications. The pad is then
allowed to "cure" or rest for 15 days, after which it is rinsed with
conventional leach solution for up to 120 days. It is estimated that at the
end of the leaching cycle, approximately 70% of the copper has been
recovered.
The leach solutions from each of the leaching circuits are collected in
ponds at the base of each dump. All of the retaining dams used to hold the
pregnant solution are made of concrete with either clay or concrete cores.
All of the dams have been keyed into the bedrock in the existing hillsides to
prevent leakage.
The pregnant solution collected in the ponds is pumped to a solvent
extraction/electrowinning plant for copper recovery. The SX-EW plant
currently receives and processes approximately 4500 gpm of pregnant liquor.
The barren solution or raffinate produced by the SX-EW plant is then recycled
into the conventional leaching circuit.
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Environmental Impact
The majority of the leach dumps have been built upon the existing
topography. In addition, most of the collection reservoirs are unlined. The
surface upon which the dumps and collection ponds have been constructed was
described, however, as a tight formation of bedrock and, therefore,
relatively impermeable.
Diversion ditches have been dug around some of the dumps to divert
runoff from the piles into collection ponds. In addition, diversion ditches
have also been dug to divert surface runoff from outside the property away
from the dumps.
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TRIP REPORT
PINTO VALLEY COPPER CORPORATION
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 4, 1986, a site visit was conducted at the mining operations of
the Pinto Valley Copper Corporation, a subsidiary of the Newmont Mining
Corporation. The objectives of the visit and tour were to gain familiarity
with the Pinto Valley operation and to discuss the current copper leaching
project being conducted by PEI for the EPA. The following personnel
participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
Robert G. Ingersoll - Environmental Engineer, Pinto Valley
Norm Greenwald - Chief Environmental Engineer, Newmont
Gene Santellanes - Leaching General Foreman, Pinto Valley
Chris Erskine - Senior Hydrologist, Pinto Valley
An initial meeting was held to discuss the EPA1s mine waste program in
general and the current project in detail. During the meeting, Pinto Valley
personnel gave an overview discussion of the operations. A tour of the
operation was then conducted. During the tour, additional, more detailed
information about the leaching operation was provided. Pinto Valley
provided PEI with an article describing the operations at the facility. In
addition, photographs of the facility were taken by PEI and EPA during the
tour. This trip report will include information contained in that article as
well as information from subsequent conversations with Pinto Valley
personnel.
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General
The mining operations of the Pinto Valley Copper Corporation are located
about 70 miles east of Phoenix in Gila County, Arizona. The property
includes the Castle Dome* Miami, and Copper Cities mines which were acquired
from Cities Services Company in 1983.
Initial open pit mining began at the Pinto Valley site around 1972.
Active dump leaching operation began in 1981 when construction of the solvent
extraction plan was completed. The operation currently covers an area of
approximately 6570 acres of which 470 acres are cover by leach dumps.
Approximately 85,000 tons of copper are produced annually from the Pinto
Valley operation, 15% of which is produced from the leaching operation.
Leaching at the Pinto Valley site consists of seven waste dumps. The
dumps currently contain approximately 297 million tons of Teachable waste
ore. About 28 million tons of leachable waste are being added to the dumps
each year.
Conventional mining at the Miami mine ended in 1959. In situ leaching
began on a small scale in 1942 with full-scale leaching beginning when the
underground mine was closed in 1959.
The Copper Cities unit consisted of an open pit operation and
concentrator that were active between 1954 and 1975. All mining and milling
operations ceased in late 1975. Dump leaching began in 1962 and continued
until June, 1982.
Site Characteristics
The mines are located at an altitude of approximately 4000 feet above
sea level. The average seasonal temperatures range from 95 degrees in the
summer to 50 degrees in the winter. The average annual precipitation is
approximately 20 inches per year.
The towns of Claypool and Miami, with a total population of
approximately 5500, are located adjacent to the Miami property across U.S.
Highway 60-70 and within ten miles of the Pinto Valley and Copper Cities
sites. Water for these residences is supplied from wells operated by the
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Arizona Water Company located approximately 3-4 miles from the Miami unit and
10-12 miles from the Pinto Valley operation. There is no identifiable
aquifer under the property.
The host rock for the ore at the Pinto Valley site is quartz monzonite
and granite porphyry. The principal copper minerals mined at the site are
chalcopyrite and chalcocite with minor amounts of covellite, cuprite,
azurite, and malachite.
The host rock for the deposit at the Miami mine is Precambrian Pinal
Schist, which is partially covered by the Gila Conglomerate. The area is
highly faulted and fractured. The principal copper mineral is chalcocite
with minor amounts of chalcopyrite,bornite, covellite, mamachite, azurite,
chrysocolla, cuprite and native copper.
The host rock for the Copper Cities mine ore deposit is quartz
monzonite. The principal copper minerals are chalcocite and chalcopyrite
with minor amounts of covellite, turquoise, malachite and azurite.
Design and Management Practices
The leach dumps at the Pinto Valley site have been constructed on
existing topography with no prior subsurface preparation. Currently, only
about 70 acres of the dumps are being leached at the Pinto Valley site.
Trucks haul the material from the mine pit to the leach dump. After each
lift is completed, the surface is ripped to a depth of approximately 3 to 4
feet using a cat ripper and the distribution system is installed. The
distribution system consists of 2 inch perforated Drisco pipe spread over the
dump.
The leach solution applied to the Pinto Valley dumps contains
approximately 2.25 gpl H^SO^ and has a pH of around 1.7 to 1.8. It is
applied continuously until the surface of the dump begins to pond, indicating
excess precipitation of iron salts. The pregnant leach liquor contains about
0.95 gpl H2$04 and has a pH of about 2.0 to 2.1 and is collected in the
drainage below the dumps. Pumps lift the solution through one mile of pipe
to the solvent extraction-electrowinning (SX-EW) plant.
The ore body at the Miami site is leached in place, using the old
underground mining works. The leach solution is percolated through the
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caved area by underground injection and surface spraying. The pregnant leach
liquor is collected at the 1000 ft. haulage level, and pumped to the surface.
The operation produces approximately 2900 gpm of pregnant leach solution.
The pregnant leach solution contains 0.57 gpl of H^SO^ and has a pH of
2.2. The raffinate from the solvent extraction plant contains 1.6 gpl H^SO^
and has a pH of 1.7 to 1.8. The raffinate is recycled back to the caved area
for distribution as part of the leach solution.
Env i ronmenta1 Impact
The leach dumps have been built on the existing topography and the
collection reservoirs are unlined. The subsurface area upon which the
leaching operation is conducted consists of bedrock according to company
personnel.
Pregnant liquor from the leach dumps at the Pinto Valley site is
collected in an unlined reservoir behind Gold Gultch Dam #1. An overflow
catchment dam, Gold Gultch Dam #2, has been constructed down the valley to
retain any flows that may result from an upset condition. Both dams have a
rock shell with a clay core and are key cut grouted to bedrock.
The Miami mine's in situ operation has a positive water balance
indicating that the underground mine is acting as a sump, collecting water
from surrounding areas and, at least in part, preventing the migrating of
leachate away from the mined area.
Diversion ditches and collection ponds have been constructed around the
entire Copper Cities leach pile to catch any run-off and leachates. Overflow
catchment dams have been constructed to retain any flow from these
containment areas during any upset conditions. Solutions collected in the
ponds and catchment areas are diverted to the inactive tailings ponds where
the liquid is evaporated.
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TRIP REPORT
RAY MINES DIVISION
KENNECOTT COPPER CORPORATION
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 5, 1986, a site visit was conducted at the mining operations of
the Ray Mines Division of Kennecott Copper Corporation. The objectives of
the visit and tour were to gain familiarity with the Kennecott operation and
to discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
C.S. Fitch - Director of Safety & Environmental Control,
Ray Mines Division
Gerald Schurtz - Kennecott Copper Corporation
Neil Gamble - Acting Mining Manager, Ray Mines Division
Bobby Armenta- Safety and Environmental Control Supervisor,
Ray Mines Division
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Kennecott personnel provided a
description of the facility's operations during the meeting and then
conducted a tour of the operations. During the tour, additional, more
detailed information about the leaching operations was provided. Kennecott
provided PEI with several documents describing the operations at the
facility. In addition, photographs of the facility were taken by PEI and EPA
during the tour. This trip report will include information contained in that
article as well as information from subsequent conversations with Kennecott
personnel.
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General
The mining operation of Ray Mines Division is located in east central
Arizona approximately 75 miles southeast of Phoenix, and 70 miles north of
Tucson in the Mineral Creek Mining district of Pinal County. It lies in the
Mineral Creek valley approximately five miles north of the Gila River.
Underground mining activity at the site began around 1880 and continued
sporadically until 1948 when it was decided that the Ray ore body could
better be mined by open pit methods. The transition from underground to open
pit mining was completed in 1955. Mining activity is currently being
conducted in the West Pit and the Pearl Handle Pit. There are currently
five active low grade copper sulfide ore leach process areas and one active
copper silicate ore leaching area.
Site Characteristics
The Ray Mine and associated ore leaching operations are constructed on
the west side of Mineral Creek Valley, in a surface water flow channel
restricted by bedrock. The average seasonal temperatures range from 85-95°F
in the summer to 50-60°F in the winter. Average annual precipitation in the
area is about 17.5 inches.
The Ray mine is underlain by bedrock, primarily by the Precambrian Pinal
Schist. The Pits and surrounding bedrock are relatively dry from a
hydrogeologic perspective. No alluvial aquifers exist. Water 1s present at
depth in isolated fracture zones, but none of the bedrock formations are
capable of supplying significant or sustained yield.
The Gila River receives all drainage in the area and flows southwest to
the Ashurst-Hayden diversion dam near Florence, approximately 15 miles below
Kelvin, where the river is totally diverted for use as agricultrual
irrigation water. Mineral Creek, which was the original drainage course
through the Ray Mine, meets the Gila River at the town of Kelvin. In order
to prevent contamination of Mineral Creek and the Gila River, Kennecott has
constructed a large flood control and diversion dam north of the mine site
which diverts the flow of the Mineral Creek into a 3.4 mile concrete tunnel
which conveys the flow of the Mineral Creek around the mine site and
discharges the flow back into the creek below the mine.
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Chalcocite has been the main copper mineral in ore from the Ray deposit.
Minor amounts to covellite are also present. Chrysocolla and other copper
silicates are also relatively abundant. Pyrite is ubiquitous throughout the
ore.
Design and Management Practices
Approximately 1100 acres are available for the low grade copper sulfide
ore leaching processes; only 10-15 percent of which is being flushed with
water at any one time. The remaining area is at rest under oxidizing
conditions. The dump leach piles are located directly on the existing
topography. There was no special surface preparation prior to the deposition
of dump material.
Mine-run ore is hauled to the leach piles by truck and spread with
bulldozers. After each lift is completed, the surface of the pile is ripped
to depth of approximately 5 feet and, depending upon the solution
distribution method, trenched. The leach solution is distributed by either
sprinkling or border irrigation. The choice of distribution method depends
upon the pump capacity available for the particular leach area.
The leach solution is applied to a pile until it begins to pond due to
the precipitation of iron salts on the surface. This usually takes
approximately 10 weeks.
The solution applied to the dumps has a pH of 3.5. It is delivered to
the dumps at 8700 gpm and applied through a series of flapper sprinklers.
The pregnant liquor is collected in unlined ponds from which it is pumped to
either the North or South precipitation plants. The pregnant liquor
generally contains approximately .42 gpl acid and has a pH of 2.8. The tail
water from the precipitation plants is redistributed onto the leach surfaces.
The copper silicate ore leaching operation is used to recover copper
from :opper silicate mineralized ores. Prior to building the 70-foot
heap, the previous heap is ripped. Mine-run ore is delivered by haulage
trucks to the primary crusher which reduces the ore to minus 8-inch size.
The crushed ore is then conveyed to an open air coarse ore stockpile. The
crushed ore is then conveyed to a secondary/tertiary crushing facility which
reduces the ore to minus 7/16 inch. This fine crushed product is then
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conveyed to a fine ore building, which has a capacity of approximately 35,000
tons.
The crushed ore is fed from fine ore storage onto a series of conveyors
which move the ore to an area adjacent to the copper leaching area. The ore
is also prewet with a solution of water and 18-19 gpl HgSO^. Trucks then
load and transport the ore to the heap site. Each heap contains
approximately 40,000 tons of crushed ore.
The leaching solution, containing 18 gpl H^SO^, is delivered to the heap
leach site at 3000 gpm and applied to the ore heaps through a series of
sprinklers. Each lift is leached for 42 days. The pregnant liquors, which
contain approximately 4.5 gpl acid and 3.6 copper, are collected in unlined
ponds and pumped to the solvent extraction-electrowinning (SX-EW) plant.
Environmental Impact
The entire Ray Mine area is underlain with bedrock. All solution
recovery dams are keyed into bedrock to ensure containment of pregnant
solutions. Dams and associated pipelines which lie above gradient are
designed to flow into pit containment areas during any upset condition. Dams
lying down gradient of the headwater reservoirs are equipped with primary and
backup pumping capability. In the event this capability is lost or is
insufficient for incoming flows, each dam is designed to overflow into the
plastic lined Big Dome reservoir, an 18 million gallon capacity pond.
The pregnant leaching solutions from the leach dumps is retained by a
dam constructed across the down-gradient side of the drainage channel.
Waters which might overflow the leach dams are collected in Big Dome
reservoir. Process water spills and runoff from process areas would also be
contained in this pond. This water is either pumped back to the leach dumps
or treated at the lime neutralization/precipitation facility.
All natural surface and groundwater drainage from the area would be via
Mineral Creek and its subflow and would be confined by the narrow bedrock
boundaries of Mineral Creek. Diversion ditches have been constructed around
the sulfide ore leach dumps located west of the open pit workings to minimize
the amount of surface water entering the process water system.
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An estimated 1 billion gallons of water are in storage in the mine pits.
Both pits are confined by bedrock and are located well below the elevation of
Mineral Creek.
Mine overburden is separated into barren and copper-bearing portions.
Only barren material is placed on those dumpsites on the northeast side of
the mine to prevent pollution of Mineral Creek from dump drainage.
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TRIP REPORT
SAN MANUEL MINE - MAGMA COPPER COMPANY
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On June 5, 1986, a site visit was conducted at the mining operations of
the San Manuel Mine owned by the Magma Copper Company. The objectives of the
visit and tour were to gain familiarity with the San Manuel operation and to
discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in the meeting and tour:
Jack Hubbard - U.S. EPA Project Officer
Robert Hearn - PEI Project Manager
Dave Baker - Environmental Engineer, Newmont
Marcel F. DeGuire - Director of Environmental Affairs, Newmont
Harry Smith - Mine Supervisor, Newmont
Charles 0'Coyne - Assistant Superintendent
Chris Burt - Project Engineer (In situ operation). Magma
Newmont personnel provided a tour of the facilities during which
information about the leaching operation was provided. Photographs of the
facility were taken by PEI and EPA during the tour. This trip report
includes information provided during the tour as well as information from
subsequent conversations with Newmont personnel.
General
The San Manuel mining operation is located in southeast Arizona
approximately 40 miles north of Tucson. Undergrojnd mining of the site began
in June, 1956. An open pit operation was begun in 1985. Leaching operations
began in April, 1986.
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Site Characteristics
The area surrounding the mine is relatively arid with low rolling hills
dissected by erosions gullies and covered with minimal vegetation. The
average seasonal temperatures range from 95° F in the winter to 50° F in the
summer. The average annual precipitation is 10 inches per year. The Water
Table for the area is between 1200 and 2500 feet below the surface.
Approximately 99% of the ore mineralization in the open pit operation is
chrysocolla. All of this ore will be leaching at a site which will
eventually contain a total of 55 million tons of low-grade ore.
Design and Management Practices
The area selected for the leach dump area is within the cone of
depression created by the underground mining activity. The area was first
stripped of all vegetation and graded to eliminate steep hills and drain into
ditches which would divert the solution collection pond* The surface of the
area consisted of Gila Conglomerate which was used as the subgrade for a
synthetic liner. This material was compacted using rollers and compactors to
crate a smooth even surface. Internal dams where also built onto the terrain
using the Gila Conglomerate which, after placement of the liner, would
isolate any failures in the liner and prevent excess solution loss. French
drains were also keyed into the natural terrain to channel solution out of
the dump area to a collection pond or into the subsidence area of the
underground mine.
The liner is made of 60 or 100 mil of of high density polyethylene.
This was selected because of its tensile strength and flexibility. The 60
mil material was used over ridges of the dump while the 100 mill liner was
used in the solution collection areas. After the liner has been installed,
the seams were sealed using a heat gun and "extrusion machine.' This seam
was then vacuum tested and splices were taken every hundred feet for
additional testing in a laboratory. A walking inspection was also made of
the liner to identify any punctures or large rocks under the liner which
might cause it to tear. Any problems areas located were fixed.
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After the liner was completed, a collection system of pipes was layed in
the collection trenches. This system consisted of 4 inch performated HDPE
pipes connected to main collection pipes of 18 to 24 inch HDPE. A mixture
of sand and graded stone was then placed over the pad to a depth of 18 inches
to protect it from damage by earth moving equipment, to provide a permeable
drainage blanket and to reduce excessive pressure points on the liner. The
size of the gravel was between approximately +1 inch and - 3 inches.
Mine-run ore from the open pit operation is hauled and dumped on the pad
by trucks and spread with a bulldozer. The ore is piled in 20 foot lifts .
Each lift is substantially rectangular in shape. After a lift is completed,
it is ripped to a depth of approximately 9 feet and the solution distribution
system is then installed.
The leaching solution is distributed through 3 inch pipes connected to
wobbler sprinklers. The solution consists of dilute H2S04 (containing 10
gpl of H2S04) having a pH of not greater than 2.0. The flow rate from the
sprinklers is adjusted to be approximately 1.5 gallons of leaching solution
for each 100 sq. ft. Initially, each lift will be leached for a period of
60 days. After this period, each lift will be allow to sit for a- period
before the surface is ripped and another lift is added. The maximum height of
each dump pile is anticipated to be approximately 280 feet.
The pregnant solution is expected to contain approximately 1.6 gpl free
acid and 1.3 gpl copper. It will be collected in the perforated pipes under
each of the dumps which will direct the flow into a lined reservoir. The
pregnant solution collected in the reservoir is then pumped to the solvent
extraction-electrowinning (SX-EW) plant.
After the copper has been recovered in the SX-EW plant, the copper
content of the barren solution or raffinate is about 0.10 gpl or less. The
raffinate also contains about 7 gpl acid. After adding acid, the solution is
recycled to the leach dumps.
Environmental Impact
As long as the integrity of the liner remains intact, the escape of the
pregnant liquors and runoff from the piles into the groundwater should be
minimal. If there should be a power failure or a major upset event, the
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collection reservoir will overflow into the subsidence area created by the
underground mining operation. Since the lowest level of the underground
mining activity is at least 100 feet below the water table, the mine acts as
a sump drawing water from the surrounding area and therefore preventing the
spread of any subsurface contamination.
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TRIP REPORT
TYRONE MINE
PHELPS DODGE CORPORATION
EPA Contract No. 68-02-3995
PN 3650-25
Prepared by
PEI Associates, Inc.
On August 13, 1986, a site visit was conducted at the mining operations
of the Tyrone Branch of Phelps Dodge Corporation located near Silver City,
New Mexico. The objectives of the visit and tour were to gain familiarity
with the Tyrone operation and to discuss the current copper leaching project
being conducted by PEI for the U.S. Environmental Protection Agency. The
following personnel participated in the meeting and tour:
o Robert Hearn - PEI Project Manager
o Oudy McArdle - PEI Environmental Engineer
o Michael Koranda
o David Kimbal
o David Horton
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Tyrone personnel provided a
description of the facility's operations during the meeting and conducted a
tour of the operation. During the tour, additional information about the
leaching operation was provided. Several documents dealing with the leaching
operation were also provided. Portions of the report are taken from the
information provided in those documents as well as subsequent conversations
with Tyrone personnel. Photographs of the facility were taken by PEI during
the tour.
General
The Tyrone mine is located approximately 10 miles south of Silver City
in Grant County, New Mexico. The operation includes three open pits covering
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approximately 730 acres, a mill, leach dumps, precipitation plant and solvent
extraction/electrowinning plant. Operations at the site began in 1969.
Leaching of the first dump began in 1971.
Site Characteristics
The mine is located in the southern Rocky Mountains. The area is
characterized by low mountain ranges with adjacent, flat-floored valleys.
The area is semi-arid with penion, oak and juniper vegetation. The
elevations at the site range from 6450 feet at the dump sites to 5150 feet in
the Mangas Valley at No. 3 tailing pond. The average annual rainfall is
approximately 20 inches and seasonal temperatures range from 67°F to 41°F.
Chalcocite is the most important mineral in the ore body. Significant
amounts of pyrite, chacopyrite and sphalerite also occur in the sulfide zone
of the ore body. Chrysocolla is the most abundant mineral in the oxidized
zone.
There is an aquifer in the bedrock in the area of the leach dumps. This
aquifer is of generally low yield with porosity the result of fracturing and
faulting. The Burro Chief Fault in the mine area acts as a barrier to ground
flow to the west. The depth of the groundwater generally ranges between 50
and 500 feet.
Design and Management Practices
The leach dumps are generally located along the perimeter of the mine.
Three dumps are currently in operation (1, 1-A, 2), two are under
construction (1-B, 3) and two dumps are proposed (2-A, 1-C). The size of
these dumps are set forth below:
1
140
acres
1-A
120
acres
1-B
212
acres
1-C
285
acres
2
807
acres
2-A
509
acres
3
267
acres
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Most of the existing dumps are located on the existing topography.
Dumps 1 and 3, however, have a partial clay pad to prevent seepage and
minimize potential groundwater contamination. Generally, the clay liner
under these dumps are placed only in areas having slopes less than 5:1. The
pregnant liquor (PLS) ponds for Dumps 1, 1A, IB and 2 are also lined with
clay. The proposed PLS pond for Dump 3 will be lined as well.
As previously noted, No* 3 dump is 267 acres. Approximately 29 acres
are located directly on bedrock having a conductivity of 1.8 x 10^ ft/sec.
o
109 acres were scarified and recompacted to a conductivity of 3.8 x 10
ft/sec., and 37 acres were lined with clay and aluminum from the valley floor
g
to achieve a conductivity of 2.2 x 10 ft/sec. Approximately 92 acres
remained untreated. The clay liner consisted of 18 inches of compacted soil
placed in 6 inch lifts. The minimum lines content of the soil was 12% and
the moisture content was +22 of optimum. The soil was compacted to 98% of
its maximum dry density. Lose on-site soil was placed on top of the liner to
a depth of 1.5 feet to protect it and keep it from drying out.
Generally, the ore is hauled to the dumps by truck. Approximately
42,000 tons of leach material is hauled to the dumps each day. This material
has an average copper content of 0.31%. After the ore has been dumped, it is
spread by bulldozer and, after completion of each lift, is ripped to a depth
of approximately 5 feet to improve the permeability of the surface.
The leaching solution is applied to the piles by both spraying and
ponding. The application rate for spraying is 300 gpm/acre while the rate
for ponding is 900 gpm/acre. The leaching reagent is water derived from the
raffinate generated by the solvent extraction process plus make-up water, if
necessary, from wells. The pH of the leaching reagent averages approximately
2.4 and contains 4-5 gpl of sulfuric acid.
The pregnant liquor solutions from Dumps 2, 2A and 3 are collected in
ponds and pumped to a solvent extraction/electrowinning (SX-EW) plant. The
SX-EW plant is currently handling approximately 6600 gpm of solution.
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Environmental Impact
Each of the dumps, except Dump 1, are permit under the New Mexico Water
Quality Control Regulations. Consequently, each of the operating dumps is
currently monitored by wells as follows:
Dump Monitoring Wells
1A 3 wells (normally dry)
IB 7 wells (1 deep well
2 aquifer monitor wells
4 neutron access tubes)
2 12 wells
3 10 wells
Effluent quality is sampled by these wells and report and reported as
part of each dumps discharge plan requirement.
Each of the PLS ponds have overflow ponds (unlined) to collect any
overflow from the ponds to a rainfall event or equipment malfunction.
Portions of Dumps 1, 2 and 3 are lined with clay and most of the PLS
ponds are also lined.
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TRIP REPORT
CYPRUS JOHNSON COPPER COMPANY
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On August 13, 1986, a site visit was conducted at the mining operations
of the Cyprus Johnson Copper Company near Benson, Arizona. The objectives of
the visit and tour were to gain familiarity with the Cyprus Johnson operation
and to discuss the current copper leaching project being conducted by PEI for
the EPA. The following personnel participated in the meeting and tour:
Robert Hearn - PEI Project Manager
Judy McArdle - PEI Environmental Engineer
Rana Medhi - Cyprus Johnson Resident Manager
Bill Rudy - Cyprus Johnson SX/EW Plant Superintendent
Tony Gomez - Cyprus Minerals Co. Environmental Coordinator
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Cyprus Johnson personnel provided
a verbal description of and written handouts pertaining to the facility's
operations after which a tour was conducted. During the tour, additional,
more detailed information about the leaching operations was provided.
Photographs of the facility were taken by PEI during the tour.
General
Cyprus Johnson's open pit mine and heap leaching operation are located
approximately 15 miles northeast of Benson, Arizona, in Cochise County.
Mining of the predominantly oxide ore began in 1975 and was discontinued in
January 1984. Although no new ore is being added to the piles, copper
continues to be recovered from the existing material by sulfuric acid
leaching and solvent extraction/electrowinning. Annual capacity of the SX/EW
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plant is rated at 10 million pounds of cathode copper. The plant, which
employs 17 people, is currently operating at 50 percent capacity and is
scheduled to shut down permanently in December 1986. No facilities for
processing sulfide ore by conventional milling, concentrating, and smelting
have ever existed at this location.
Copper is leached from two ore heaps located immediately southwest of
the mine. The No. 1 pile was started in 1975, and the No. 2 pile was started
in 1980. Combined, these heaps contain an estimated 15.2 million tons of
leach material and have a total surface area of approximately 42 acres. The
mine waste dump is located just northeast of the pit and occupies an area of
80 acres.
Site Characteristics
The Cyprus Johnson mine site is located at the base of the Little
Dragoon Mountains (elevation 5030 feet). The average seasonal temperatures
range from 48°F in the winter to 80°F in the summer. The average annual
precipitation is approximately 12 inches.
The area in the vicinity of the mine is sparsely populated. The town of
Benson (pop. 4200) is located approximately 15 miles southwest of the site,
and the town of Willcox (pop. 3200) is located some 15 miles in the opposite
direction.
The copper mineralization occurs primarily as fracture filling in tilted
paleozoic sediments (Lower Abrigo shale). The most abundant mineral is
chrysocolla with lesser amounts of other secondary oxides of copper,
including tenorite, malachite, azurite, cuprite, and rarer amounts of
dioptase. There is no aquifer present beneath the leach areas.
Design and Management Practices
During the active life of the mine, ore containing greater than 0.4
percent total copper was hauled by truck from the pit to the No. 1 and No. 2
leach piles and spread by bulldozer. The two leach heaps, which are located
in natural drainages, were built in successive lifts of 4 to 5 feet. The
Number 1 heap ranges in height from 145 to 250 feet; the Number 2 heap
averages 63 feet in height.
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Leaching of the two ore heaps proceeds 1n stages (see Figure 1).
Approximately 2100 gpm of a dilute sulfuric acid solution (0.5 to 1.5 percent
H2S0^) is pumped to the top and sides of the No. 1 heap and distributed by
means of 3/4-inch-diameter HDPE drip tubing. The pregnant liquor (0.5 gpl
Cu; pH 2.3) is collected at the toe of the pile in the 4,400,000-gallon PLS
pond. The pond is constructed over quartzite bedrock, and the face of the
earther dam is lined with 30-mil hypalon. From the PLS pond, the copper-
bearing solution is delivered to the top and sides of the No. 2 heap. The
pregnant liquor (0.75 gpl Cu; pH 2.5) is collected in the 8,000,000-gallon
middle pond, which is located between the two leach heaps. The middle pond
is also constructed over quartzite bedrock. The pregnant liquor from the
middle pond is fed to the SX plant, where approximately 93 percent of the
copper is extracted from solution. The barren solution is then directed to
the 2,500,000-gallon raffinate pond, where makeup water and acid are added at
a rate of 125 gpm and 6 gpm, respectively. The raffinate pond is constructed
on quartzite bedrock and lined with a 4-inch-thick layer of acid-resistant
gunite. The raffinate is recirculated from the pond to the No. 1 heap, and
the leach cycle is repeated.
Environmental Impact
The No. 1 and No. 2 leach heaps were constructed in natural drainages
over "tight" quartizite bedrock. The PLS pond, middle pond, and raffinate
pond were also constructed over bedrock. The raffinate pond is lined with 4
inches of gunite; the other two ponds are not lined. There is no ground-
water aquifer beneath the leaching or mining operations.
Storm water runoff is collected in two catchment ponds (earthen dams
with total capacity of 9,640,000 gallons), located upstream of the heap leach
area, as well as in the PLS, middle, and raffinate ponds. Overflow from the
catchment ponds, the raffinate pond, and the middle pond is diverted around
the ore heaps to the next downstream pond. Overflow from the PLS pond runs
into the overflow catchment pond (earthen dam constructed over bedrock with a
capacity of 2,800,000 gallons). The system of storm water runoff ponds has
been designed to completely contain the 10-year, 24-hour storm. Facility
personnel state that the overflow catchment pond has been filled to 50
percent only twice since 1975.
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TRIP REPORT
BINGHAM. CANYON MINE - KENNECOTT COPPER CORP.
EPA Contract No. 68-02-3995
PN-3650-25
Prepared by
PEI Associates, Inc.
On August 15, 1986, a site visit was conducted at Kennecott Copper
Corp.'s Bingham Canyon Mine in Bingham Canyon, Utah. The objectives of the
visit and tour were to gain familiarity with the Bingham Canypn operation and
to discuss the current copper leaching project being conducted by PEI for the
EPA. The following personnel participated in the meeting and tour:
Robert Hearn - PEI Project Manager
Judy McArdle - PEI Environmental Engineer
Gerrald Schurtz - Kennecott Manager of Environmental Health
Steven Taylor - Kennecott Manager of Environmental Engineering
Gary Jungenberg - Kennecott Precipitation Plant Superintendent
An initial meeting was held to discuss the EPA's mine waste program in
general and the current project in detail. Kennecott personnel provided an
overview of the facility's operations and a helicopter tour of the area.
During the tour, additional, more detailed information about the leaching
operations was provided. Aerial photographs of the facility were taken by
PEI during the tour.
General
Kennecott's Bingham Canyon mine is located in Bingham Canyon, Utah,
approximately 20 miles southwest of Salt Lake City. Open-pit mining of the
copper porphyry ore body began in 1904 and has continued for.more than 80
years. (Mining was temporarily suspended in March 1985 for economic reasons
and resumed in October 1986. The Bingham Canyon mine has the distinction of
being the largest open-pit mine in the world--it covers an
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area of about 1400 acres (2.2 square miles) and is more than 0.5 mile deep.
When active, approximately 106,000 tons of ore, 360,000 tons of leach material,
and 20,000 tons of waste rock are removed from the pit each day.
Dump leaching and cementation operations at the Bingham Canyon mine were
initiated in 1923. The leach dumps (east and west) currently occupy approxi-
mately 2110 acres (3.3 square miles) and contain an estimated 1500 million
tons of material. Annual precipitate production in 1985 was 17,000 tons.
Currently, only the east dumps are being leached. Leaching of the west
dumps was suspended indefinitely in 1984. The carbonaceous material in the
south dumps is not leached.
In addition to the mine, leach dumps, and precipitation plant, Kennecott
operates a crushing plant, two concentrators, a smelter, an electrolytic
refinery, and a tailings pond. These operations are located 15 miles north
of the mine site.
Site Characteristics
The Bingham Canyon mine is located in the Oquirrh Mountains in north
central Utah. The average seasonal temperatures range from 31°F in the
winter to 70°F in the summer. The average annual precipitation is 16 inches.
The area has snow cover for about 5 months of the year.
Land use in the immediate vicinity of the mine is rural. The town of
Magna (pop. 8600) is located 15 miles north of the site. Salt Lake City, a
major metropolitan area, is located 20 miles northeast of the site.
The Bingham Canyon ore body is a typical porphyry or disseminated copper
deposit that is centered in and around a complex monzonitic stock. Chalcopyrite
is the principal copper mineral, although bornite also is common in the
primary, instrusive ore zone and covellite, chalcocite, and other nonsulfide
copper minerals are present in the zone of secondary enrichment. Surrounding
the intrusive granite and the granite porphyry is a halo of mineralized
quartzite that is characterized by a very high pyrite content.
The mine pit and leach and waste dumps border on two surface water
drainages: Bingham Canyon and Butterfield Canyon. Surface runoff from these
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drainages flows east to the Jordan River, which feeds Utah Lake. The major
water supply for the mine is from two wells in the intervening valley, about
3 miles east of the mine. The depth to ground water in these wells is 600 to
800 feet.
Design and Management Practices*
Under normal operations, low-grade ore (containing less than 0.4 percent
recoverable copper) and barren waste rock are hauled from the pit by truck
and deposited in segregated dumps constructed on bedrock. The low-grade ore
is leached with a dilute solution of sulfuric acid, which is introduced to
the dump surface by spraying (rainbird sprinklers). The pregnant liquor is
collected at the base of the dumps in clay-lined ponds. The ponds were
created by constructing concrete cutoff walls across natural drainages; these
walls are keyed into bedrock to prevent subsurface losses. From the ponds,
the pregnant liquor is conveyed via a main collection canal, which is
constructed of epoxy-lined concrete, to the precipitate plant surge pond.
The precipitate plant (largest in the world) contains 26 cones and operates
on a continuous basis. After the copper has been recovered, the barren
solution from the cones flows to a sump in the central pump station, from
which it is pumped back onto the piles. The pH of this solution ranges from
2.5 to 3.0, hence makeup acid is not required.
Approximately 10 percent of the total area of the east dumps is leached
at one time. A typical leach cycle is 60 days leach and 60 days rest. To
minimize the buildup of iron precipitates on the surfaces of the dumps, the
top 4 or 5 feet of material is ripped by a bulldozer after each rest cycle.
After about two cycles, the top layer is scraped off and pushed over the edge
of the dump.
~
Because only the east side dumps are active, design and management practices
relating to these dumps are described; see Figure 1.
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Environmental Impact
Kennecott's east side collection system 1s "state-of-the-art." In
addition to the main collection canal, a second, emergency overflow canal
(also constructed of epoxy-lined concrete) collects excess stormwater runoff
and conveys it to a 500-mi11 Ion-gallon overflow pond. This pond is partially
lined with clay (i.e., the face of the dam and the bottom of the pond
extending away from the dam for several feet are lined). This excess
stormwater is treated with lime and discharged to a series of evaporation
ponds. Site personnel have stated that this collection system does not
contribute to existing ground-water contamination problems at the site.
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