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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 the 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
gold/silver heap 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 objectives of this study were to
describe the design and operation of current gold and silver heap leaching
operations, to summarize briefly the issues of toxicity and mobility of
cyanide in the environment as related to heap leaching, to. develop conceptual
alternative management practices for both existing and new facilities, and to
develop cost estimates for these conceptual controls and practices.
The data included in this report were obtained primarily from a literature
search and from visits to several major heap leaching operations in Nevada.
The intent of the literature search was to collect information on heap
leaching practices, recovery technologies, and the environmental impact of
cyanide leaching operations in the gold and silver mining industry segments.
State environmental personnel and experts from organizations such as the
Bureau of Mines were consulted to identify available information dealing with
heap leach practices and control technologies. Six active heap leach
operations were visited to obtain information on industry practices and the
characteristics of the leaching process.
Thomas Mauser, Director
Hazardous Waste Engineering Research Laboratory
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ABSTRACT
This report presents a description of the magnitude and distribution of
gold/silver heap leaching, the design and operation of leaching facilities,
the potential for environmental impact, and management practices that may be
used to minimize potential 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
heap leaching industry. Several leaching operations were visited to acquire
firsthand knowledge and site-specific information.
Currently, there are about 78 active heap leach operations in the United
States. The majority of these sites are located in Nevada. Heap leaching is
percolation leaching of low-grade gold and silver ores that have been stacked
in engineered heaps on specially constructed pads. An alkaline cyanide solu-
tion is the only lixiviant used. Relatively impervious pads (e.g., 10"
cm/s) constructed of synthetic or clay materials and lined (HOPE, PVC, etc.)
ponds and trenches are used at all sites.
There are very few data available on the concentration of cyanide re-
maining in heap leach residue after operations cease. No damage cases were
identified that indicated impact from properly constructed or operated facil-
ities.
Several management practices (i.e., french drains, double pond liners,
alternative lixiviants, cyanide destruction, ground-water monitoring, and
capping) were assessed. Costs of each of these systems were estimated for an
example facility.
IV
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CONTENTS
Foreword i i i
Abstract iv
Figures vii
Tables ix
1. Introduction 1
Background 1
Purpose and scope 3
Report organization 3
2. Overview of the Heap Leaching Industry 5
Overview of heap leaching 5
Industry status 9
3. Design and Operation of Heap Leaching Projects 17
Introduction 17
Mining and ore preparation 17
Pad construction 20
Heap construction ' 28
Solution handling/leach cycle 31
Metal recovery 36
Residue disposal and site closure 37
Site security 39
Valley leach 39
4; Summary of the Toxicity and Mobility of Cyanide 42
Cyanide in process solutions 42
Cyanide in leach residue 43
Toxicity of cyanide 46
Migration of cyanide 47
Summary 48
5. Alternative Management Practices 49
Incorporation of French drains in leach pads 51
Double liners for process solution ponds 55
(continued)
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CONTENTS (continued)
Alternative lixiviants 62
Leachate detection 64
Cyanide destruction 71
Capping 77
Post-closure monitoring and maintenance 80
6. Summary of Findings 81
General characteristics 81
Design and operation 81
Toxicity and mobility 82
Alternative management practices 82
References 85
Appendix A Trip Reports 88
Pinson Mining Company 89
Newmont Gold Company (formerly Carl in Gold Company) 94
Round Mountain Gold Corporation 97
NERCO Metal's Candelaria Mine 99
State of Nevada Department of Conservation and Natural
Resources Division of Environmental Protection 102
VI
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FIGURES
Number Page
1 Conceptual Flow Diagram of a Heap Leach Operation 7
2 Approximate Location of Several Gold/Silver Heap Leach
Operations 15
3 Number of Gold/Silver Heap Leach Operations by County in
Nevada 16
4 Conceptual Flow Diagram of Typical Heap Leach Operation 18
5 Examples of Heap Leach Pad Construction 22
6 Relative Effect of Hydraulic Head on Seepage Through
Pond Liners and Leach Pads 23
7 Conceptual Diagram of Internal Berms on a Heap Leach Pad 27
8 Schematic Flow Diagram of Leach Solutions at a Typical
Heap Leach Operation 32
9 Typical Cross-Section of a Valley Leach Type Heap 40
10 Effect of pH on Dissociation of Hydrogen Cyanide 45
11 Specifications of French Drains Incorporated in the
Pinson Mining Co. Leach Pads 53
12 Design of Leach Pad Leak Detection System at the Superior
Mining Co. Stibnite Operation 54
13 Cross Sections of Single- and Double- Liner Systems Used
to Develop Cost Estimates 58
14 Design of Process Solution Pond With Single 40-mil HOPE
Liner Used for Cost Estimating 59
15 Design of Process Solution Pond With Double 40-mil HOPE
Liner System Leak Detection Used for Cost Estimating 60
16 Conceptual Application of a Detection Monitoring System
at a Small Heap Leach Operation 68
(continued)
vii
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FIGURES (continued)
Number Page
17 Example Site Layout Showing Additional Pond Required for
Cyanide Destruction/Neutralization 76
18 Example Process Flow Diagram of Cyanide Destruction Cir-
cuit 76
19 Capital and Annualized Costs of Cyanide Neutralization
System at a Gold Leaching Facility 78
VTM
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TABLES
Number Page
1 Gold Produced in the United States by Cyanidation 6
2 Agencies Contacted for Listing of Active Heap
Leach Facilities 11
3 Active Precious Metal Heap Leach Operations in the
United States 12
4 Design and Operational Characteristics of Selected
Heap Leaching Operations 29
5 Cost of Constructing a Clay Leach Pad With and Without a
French Drain System 56
6 Comparison of Costs of Process Solution Ponds Constructed
With Single and Double Liners 61
7 1986 Costs for Drilling and Installing 2- to 4-Inch
Diameter Wells 70
8 Cyanide Complexes Likely to be Present in Leach Residue 72
IX
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SECTION 1
INTRODUCTION
BACKGROUND
Since the surge in precious metal prices in the 1970's, gold has con-
tinued to be the bright spot in the domestic metal mining industry. This
trend is expected to continue, and reserves are likely to be enlarged as the
result of increased exploration activity. Because of the low capital invest-
ment and fast payout involved, the production of gold by heap leaching is
1 2
becoming increasingly popular. ' This method permits operators to process
small quantities of low-grade newly mined ores, waste rock, and tailings from
previous mining activities. Currently, the United States has about 78 active
heap leach operations, about 50 of which are located in Nevada. The others
are scattered throughout California, New Mexico, Colorado, South Dakota,
Idaho, and Montana. Only two operations, both in South Carolina, are located
in the eastern United States.
Cyanide is the only commercially proven lixiviant available for the heap
leaching of low-grade gold and silver ores, and the U.S. Environmental Pro-
tection Agency (EPA) is concerned about whether adequate measures are being
taken to ensure protection of the environment. In the EPA's Report to Con-
gress on mining wastes, the Agency indicated its intent to conduct additional
studies on mining wastes containing cyanide. In the recent regulatory
determination regarding mining wastes and RCRA, the Agency indicated that it
will "...focus on identifying environmental problems and setting priorities
for applying controls at mining sites with such potential problems as high
acid-generation potential, radioactivity, asbestos and cyanide wastes."
This report represents one of the EPA's efforts to supplement the data base
on the issue of cyanide and mine waste. A brief discussion of the mining
waste issue is presented in the following paragraphs so that the current
study may be placed in context.
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In 1976, Section 8002(f) of the Resource Conservation and Recovery Act
(RCRA) required the EPA to conduct an investigation of all solid waste manage-
ment 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."
When Congress amended RCRA in 1980, it 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, beneficiation, and process-
ing of ores and minerals." Moreover, it required EPA to make a "regulatory
determination" within 6 months after submitting the study to Congress, stating
either that regulations would be promulgated or that regulations were unwar-
ranted for,such mining wastes. A report, mandated by Sections 8002(f) and
(p), was submitted to Congress on December 31, 1985.
On July 3, 1986, the EPA published the regulatory determination regard-
ing the issue of mining wastes that heretofore had been excluded from regula-
4
tion under RCRA. The determination indicated that RCRA's hazardous waste
management standards "...are likely to be environmentally unnecessary, techni-
cally infeasible, or economically impractical when applied to mining wastes."
The EPA indicated that it plans to develop a special program for mining
wastes under Subtitle D, but acknowledged that, after additional study, some
mining wastes may have to be regulated under Subtitle C. The EPA specifically
expressed continued concern about problems and potential problems associated
with mining wastes having high-acid generation potential, radioactivity,
asbestos, and cyanide. The EPA's current policy, as stated in the regulatory
determination, regarding active heap leach piles and leach solutions is that
these materials are not wastes, but rather are raw materials used in the
A
production process and a product, respectively. Only leach solutions that
escape from the production process and abandoned heap leach piles are wastes.
To develop a mining waste program under Subtitle D, EPA is collecting
additional information on the nature of mining wastes, mining waste manage-
ment practices, and mining waste exposure potential. Toward this end, EPA's
Office of Research and Development contracted PEI Associates, Inc. to conduct
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an evaluation of the gold and silver heap leaching (cyanide leaching) indus-
try. This report characterizes the industry, describes current design and
operational practices, summarizes environmental concerns, and presents con-
ceptual alternative management practices that could mitigate potential im-
pacts which may be caused by cyanide contamination from this industry seg-
ment. Several heap leach operations were visited to obtain information on
current industry and site-specific practices. Practices that could mitigate
actual or potential escape of cyanide were identified and evaluated.
PURPOSE AND SCOPE
The objectives of this study were to describe the design and operation
of current gold and silver heap leaching operations, to summarize briefly the
issues of toxicity and mobility of cyanide in the environment as related to
heap leaching, to develop conceptual alternative management practices for
both existing and new facilities, and to develop cost estimates for these
conceptual controls and practices.
The data included in this report were obtained primarily from a litera-
ture search and from visits to several major heap leaching operations in
Nevada. The intent of the literature search was to collect information on
heap leaching practices, recovery technologies, and the environmental impact
of cyanide leaching operations in the gold and silver mining industry seg-
ments. State environmental personnel and experts from organizations such as
the Bureau of Mines were consulted to identify available information dealing
with heap leach practices and control technologies. Six active heap leach
operations were visited to obtain information on industry practices and the
characteristics of the leaching process.
REPORT ORGANIZATION
Section 2 of this project report contains an overview of the heap leach-
ing industry and provides the current status of the industry and the
geographic distribution of active sites. Section 3 discusses the design and
operation of current heap leaching operations. Section 4 presents a summary
of the toxicity and mobility of cyanide as it is related to heap leaching.
Section 5 discusses conceptual alternative management practices that could
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limit or prohibit actual and potential releases of cyanide from heap leaching
and their application. Cost estimates of example applications are also
presented. Section 6 summarizes the information and presents comments on the
likely effectiveness and applicability of conceptual alternative management
practices and control techniques. Trip reports covering the six visits are
included as Appendix A.
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SECTION 2
OVERVIEW OF THE HEAP LEACHING INDUSTRY
OVERVIEW OF HEAP LEACHING
Heap leaching—percolation leaching with cyanide of relatively coarse,
low-grade gold/silver ore piled on an impervious surface—was first suggested
by the Bureau of Mines in 1969. This processing method is suitable for
processing ores containing free, disseminated, submicron particles of gold
and/or silver in porous host rock. Increases in the price of gold during
the 1970's, stimulated by removal of restraints on private ownership, resulted
in efforts to improve the process and make it more widely applicable. In the
mid-1970's, the Bureau of Mines developed the process of agglomeration, which
permits the leaching of low-grade clayey deposits. Since the early 1970's,
heap leaching facilities have been developed that range in size from small,
one-man intermittent and batch operations to large, well-capitalized opera-
tions capable of the continuous processing of up to 20,000 tons of ore per
day. In 1984, about 525,000 troy ounces of gold was recovered from 20,000,000
o
tons of ore treated by cyanide heap leaching. By comparison, 1,140,000 troy
ounces of gold was recovered from conventional cyanidation extraction in
Q
vats, tanks, and closed containers. The average ore grade treated by heap
leaching operations is about 0.05 ounce of gold/ton and 0.09 ounce of gold/per
ton of ore by conventional cyanidation. As shown in Table 1, the application
of cyanide heap leaching has grown over recent years, and this trend is
expected to continue.
The relatively small capital investment and low operating costs of heap
leaching have made it an increasingly attractive method for recovery of gold
and silver from low-grade resources. The capital costs of heap leaching are
q
20 to 36 percent of those required for conventional cyanidation milling.
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TABLE 1. GOLD PRODUCED IN THE UNITED STATES BY CYANIDATION
a,b
Year
1980
1981
1982
1983
1984
Extraction in vats, tanks,
and closed containers
Ore treated,
short tons
7,869,000
7,024,000
7,616,000
11,317,000
12,064,000
Gold recovered,6
troy ounces
483,000
648,000
711,000
1,086,000
1,136,000
Leaching in open heaps or dumps
Ore treated,
short tons
3,910,000
8,875,000
12,290,000
16,180,000
19,860,000
Gold recovered,
troy ounces
120,000
264,000
391,000
499,000
525,000
Source: Reference 8.
D May include small quantities recovered by leaching with thiourea, by bio-
extraction, and by proprietary processes.
c Includes autoclaves.
May include tailings and waste ore dumps.
B May include small quantities recovered by gravity methods.
Operating costs are 40 to 55 percent of those for the conventional cyanidation
Q
process. Average production costs are about $290/oz., and many operations
are below $200/oz. Heap leaching facilities also entail a shorter startup
period and can be applied on a small scale. The efficiency of heap leaching,
however, is less than that of conventional cyanidation which uses an agitated
leach. The conventional process achieves about 90 percent recovery, while
o
heap leaching recoveries range from 50 to 85 percent. Heap leach operations
that process newly mined run-of-mine ore usually recover about 50 to 60
percent of the metal values, whereas operations that process ores subjected
9
to crushing and agglomeration may recover 75 to 85 percent.
The basic heap leaching process involves spraying an alkaline cyanide
solution (pH 9 to 11) over ore that has been stacked on a sloped, impermeable
pad. Metal values (gold and silver) are dissolved in the solution and flow
off the pad to a lined impoundment. This pregnant solution is pumped from
the impoundment to a metals-recovery process, where the gold and/or silver
are removed. The barren solution with makeup reagents (i.e., sodium cyanide
and lime) is returned to the ore to complete a closed loop. This process is
depicted conceptually in Figure 1. After the leaching is completed, the
leach residue is rinsed with fresh water, drained, and either left on the pad
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T
15 to 100 ft
JL
SOLUTION .
SPRAYS \
\\
HEAP
(GOLD ORE)
IMPERMEABLE PAD
(CLAY, ASPHALT, OR
SYNTHETIC SHEETING)
«— MAKE-UP SOLUTION
(NaCN AND LIME)
BARREN SOLUTION POND
(SYNTHETIC LINER)
GOLD RECOVERY
PROCESS
PREGNANT SOLUTION POND
(SYNTHETIC LINER)
I
GOLD
Figure 1. Conceptual flow diagram of a heap leach operation.
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or excavated and hauled to a disposal area on site. Although the basic
process is constant, site-specific factors dictate specific design and opera-
tional parameters. The design and operation of heap leach facilities are
discussed in Section 4, and examples are presented.
Cyanide is the only commercially proven lixiviant used in heap leaching.
The basic principle involved is that a weak alkaline cyanide solution prefer-
entially dissolves gold and silver contained in the ore. The cyanidation
reaction proceeds in two stages. Most of the gold is dissolved by the
following reaction:
2 Au + 4 CN" + 02 + 2 H20 + 2 Au(CN)2" + H202 + 2 OH" (Eq. 1)
A small but significant portion of the gold is dissolved by the reaction
shown in Equation 2-2.
4 Au + 8 CN" + 02 + 2 H20 * 4 Au(CN)2" + 4 OH" (Eq. 2)
The rate of the cyanidation reaction depends on the concentration of cyanide
and the alkalinity of the solution; a pH between 10 and 11 is usually optimum.
Sodium cyanide (NaCN) is used as the source of cyanide by all heap leach
operations. It typically is purchased in bulk solid form. Sodium cyanide is
available in nonreturnable 100-1b-net and 200-1b-net steel drums and 1-ton
p
boxes and in returnable 3000-lb-net FLO-BIN containers. Used sodium cyanide
drums must be triple rinsed and disposed of appropriately. When returnable
bins are used, the customer does not have to clean or dispose of containers.
Lime (CaO) or caustic soda (NaOH) is used to maintain the alkalinity of the
leach solution in a pH range of 9 to 11. Reagent usage differs from site to
site and depends on ore characteristics. If cyanicides (materials that
destroy cyanide or otherwise inhibit the cyanidation reaction) are present in
the ore, relatively greater quantities of cyanide have to be added to the
leaching solution. Cyanicides include arsenic-bearing minerals such as
realgar (As^S^) and orpiment (As^g), and others, such as stibnite (Sb^S,),
that react rapidly with cyanide and inhibit dissolution of gold; carbonaceous
materials that act as adsorbents for dissolved gold; base metal ions, such as
Fe2+, Fe3+, Ni , Cu , Zn , and Mn +, which retard cyanidation; acids that
hydrolyze cyanide; and organics that consume the dissolved oxygen necessary
for the reaction. The quantity of caustic, lime, or caustic soda required
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is also determined by ore mineralogy. If acid-forming constituents are
present, more caustic will be required to maintain the high-pH protective
alkalinity necessary for the cyanidation reaction.
Precious metal recovery usually is accomplished by either carbon adsorp-
tion followed by eluting and electrowinning or by Merrill-Crowe zinc dust
precipitation. Typically, the dissolved gold and silver are recovered from
the pregnant solution on site; however, small operations may ship loaded
carbon off site for metal recovery. Other possible metal recovery systems
include ion exchange, direct electrowinning, soluble sulfide precipitation,
and aluminum dust precipitation.
INDUSTRY STATUS
The mining industry first became interested in the U.S. Bureau of Mines'
developments in gold/silver heap leaching technology in the late 1960's, and
the first'commercial cyanide heap leaching process was used at the Carlin — 'A"-
Gold Mine Company in northern Nevada on mine cutoff material. Since the
early 1970's, interest in heap leaching has continued to grow primarily in
response to the high prices of gold and silver. Low-grade (e.g., 0.03 oz/ton)
gold deposits previously considered uneconomical to recover are now being
exploited at a profit.
A 1984 survey indicated that more than 80 operations were experimenting
with leaching, actively leaching, or seriously planning to use heap leach-
2
ing. At the time of the survey, 48 facilities were reportedly actively heap
and dump leaching precious metals in Arizona, California, Colorado, Montana,
and Nevada. Twenty-one facilities were planning leaching operations, seven
facilities were inactive, and the status of leaching operations at the remain-
ing sites was unknown. Reportedly, the great number of permits being issued
made it impossible to compile a complete up-to-date list of heap leaching
2
operations. For comparison, a 1981 survey reported 38 active gold and
silver heap and dump leaching operations." This represents an increase of
about 21 percent over a 3-year period. As indicated by the survey data, gold
and silver heap leaching operations were experiencing a dramatic increase in
popularity.
During this project, an effort was made to compile an updated listing of
active gold and silver heap leaching operations. Several sources were used
to generate this listing. First, a literature search was conducted of several
9
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popular mining journals (i.e., Engineering and Mining Journal, Mining Con-
gress Journal, and Mining Engineering) from 1981 to the present. Articles
discussing heap leaching operations were extracted to track operational
developments at individual facilities. The 1984 and 1985 issues of Heap and
Dump Leaching International Newsletter (published quarterly by DHL Company in
Lakewood, Colorado) were also reviewed. The 1985 and 1986 issues of Mineral
Industry Surveys (prepared monthly and quarterly by the U.S. Bureau of Mines)
also were reviewed for recent developments and to determine the operational
status of heap leach facilities. A document entitled "Directory of Nevada
Mine Operations Active During Calendar Year 1985" (compiled by the Department
of Industrial Relations, Division of Mine Inspection in Carson City, Nevada)
and a 1985 (fourth quarter) Mine Safety and Health Administration directory
of metal mines were obtained from the State of Nevada. These directories
were very helpful in the development of a listing of active heap leaching
operations.
In addition to published sources of heap leaching operational informa-
tion, PEI contacted the U.S. Bureau of Mines offices and State agencies
listed in Table 2 to obtain the most current operational status information
and to refine the listing of active heap leaching facilities generated from
the literature search.
Currently, about 78 gold and silver heap leaching operations are active
in the United States. The majority (47) of these operations are in Nevada.
Ten of the active heap leaching operations are in California, nine in Colorado,
two in Idaho, three in Montana, one in New Mexico, two in South Carolina,
three in Utah, and one in South Dakota. Alligator Ridge and Zortman-Landusky
are the largest producers of gold from heap leaching; each facility generates
approximately 70,000 troy ounces per year. Other large gold producers are
Smoky Valley (60,000 oz/yr) and Northumberland (40,000 oz/yr). The largest
U.S. producer of heap leach silver is NERCO Mineral's Candelaria Mine, which
currently produces more than 2 million ounces of silver annually. The next
largest silver producer is Zortman-Landusky, which produces 125,000 ounces
per year as a valuable coproduct. A listing of active heap leach operations
by state is presented in Table 3. The maps shown in Figures 2 and 3 indicate
the approximate locations of these heap leaching operations.
10
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TABLE 2. AGENCIES CONTACTED FOR LISTING OF ACTIVE HEAP LEACH FACILITIES
State
Agency
Contact
Arizona, Colorado,
New Mexico
California, Nevada
Idaho, Montana
Arizona
California
Colorado
Nevada
New Mexico
South Carolina
Utah
U.S. Bureau of Mines
Intermountain Field Operations Center
Denver, Colorado
U.S. Bureau of Mines
Reno Research Center
Reno, Nevada
U.S. Bureau of Mines
Western Field Operations Center
Spokane, Washington
State of Arizona Department of Mines & Minerals Resources
Phoenix, Arizona
State of California Department of Conservation
State Water Resources Control Board
Sacramento, California
State of Colorado Division of Mines
Denver, Colorado
State of Nevada Department of Conservation
and Natural Resources
Department of Industrial Relations
Division of Mine Inspection
State of New Mexico Bureau of Mines and Mineral Resources
Socorro, New Mexico
State of South Carolina Land Resources Conservation Com-
mission
State of Utah Department of Natural Resources
Division of Oil, Gas, and Mining
Salt Lake City, Utah
Dan Witkowsky
(303) 236-0421
Fred Carlllo
(702) 784-5215
Bill Rice
(509) 456-5350
Ken Phillips
(602) 255-3791
Charlene Herbst
(916) 445-3993
Floyd Dooley
(303) 866-3567
Harry Van Drielen
C702) 885-4670
Norton Plckett
(702) 885-5243
Mike Harris
(505) 835-5420
Craig Kennedy
(803) 734-9100
David Wham
(801) 538-5340
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TABLE 3. ACTIVE PRECIOUS METAL HEAP LEACH OPERATIONS
IN THE UNITED STATES3
State/company
Operation
Product metal
Gold Silver
California
Glanris Gold, Ltd.
Carson Hill Mining Co.
Gold Fields Mining Corp.
Royal Gold, Inc.
Rand Mining (formerly Chemgold)
Catus Gold Mines Co.
Shell Mining
Rattle Snake Mining Co.
Castle Mt. Mining
American Girl Gold Mining Corp.
Colorado
Cripple Creek and Victor Gold
Mining Co.
Saratoga Mines, Inc.
Galactic Resources
H&M Joint Venture
Great West
Crystal Hill Mining
Little Pedro Mining
Hull Mining Co.
Idaho
Pioneer Metals Corp.
Coeur d'Alene Mines Corp.
Montana
Zortman Mining Inc. - Pegasus
Explorations, Ltd.
Landusky Mining Inc. - Pegasus
Explorations, Ltd.
Mt. Hagen Mining Co.
Nevadab
Millcreek Mining, Inc.
Windfall Venture
Newmont Gold Co. (formerly
Carl in Gold Co.)
Cominco American, Inc.
Pinson Mining Co.
Cortez Gold Mines
(continued)
Picacho
Carson Hill
Mesquite
Calgom
Randsburg
Catus Gold
Standard Hill
Rattle Snake Mine
Castle Mt.
American Girl
Carlton Operation
Victory Operation
Saratoga
Summitville
Doves Nest
Vulcan Project
Crystal Hill
Jerry Johnson Group
Rubie Operation
Stibnite Mine
Thunder Mountain Mine
Ruby Gulch
August Mine, Gold Bug Mine
Crenshaw Mine
Fondaway Canyon Mine
Windfall Mine
Bootstrap Plant
Gold Quarry Mine
Maggie Creek Plant
Buckhorn Mine
Pinson Mine & Mill
Preble Mine
Cortez Gold Mine & Mill
x
x
x
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
12
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TABLE 3 (continued)
State/company
Operation
Product metal
Gold Silver
Nevada (continued)
NERCO Metals, Inc.
CanAm
Smoky Valley Mining Co.
Cyprus Mines Corporation
Lacana Gold, Inc. (Pegasus)
Amselco Minerals, Inc.
Hi-Tech Corp.
Kenneth C. Jones
Belmont Resources (U.S.), Inc.
Joe Williams
Tonkin Springs Gold Mining Co.
Panhandle Drilling & Blasting,
Inc.
Western Arlington Resources, Inc.
Tomi-Gee Mining Co., Inc.
Dee Gold Mining Co.
Priet Joint Venture
Birco Development & Joe Stock
Blackhawk Mines Corp.
H.C&C Mining, Inc.
Falcon Mining & Exploration Co.
Vector Exploration, Inc.
Western States Minerals Corp.
E.B. King
Silver Coin Mining Co.
The Standard Slag Co.
S.W. Mining
FRM Minerals, Inc.
Bauer Metals, Inc.
Alhambra Mines, Inc.
Alhambra Mines, Inc.
' N-M Recovery Corp.
Centennial Minerals, Inc.
Saga Exploration Co.
Ivy Minerals
Minerals Associates, Inc.
Placer Amex, Inc.
Nevex Gold Co., Inc.
New Dynasty Mines (U.S.), Inc.
New Mexico
Westar Corp.
(continued)
Candelaria Mine
Boreal is Mine
Round Mountain Gold
Northumberland Mine & Heap
Relief Canyon Mine
Alligator Ridge Gold Mine
Hi-Tech Mill
Desert Rat Mine
Wonder Mine
Oasis Mine & Mill
Tonkin Springs Project
Tonkin Springs Project
Wall Street Mine
Tomi-Gee Mine & Mill
Dee Gold Mine
Cornucopia Mine
Bluster Mill Site
Goldfield Tailings Project
H.C&C Mining Mill
Falcon Exploration Mine &
Mill
Goldfield Project
Gold Strike Mine & Mill
Jumbo Mine
Silver Coin Mine
Lewis Mine
Gold Bug Claims 1
through 8
Getchell Project
Bauer Metals Millsite
Dayton Tailings
Dayton Tailings
Stanmore Mine
Aurora Trial Heap Leach
Sterling Mine
Old Sullivan Mine
Rose-Dale Mine
Bald Mountain Project
Santiago-Haywood Mine
Little Bald Mountain
Lordsburg
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
13
-------
TABLE 3 tcontinued)
State/company
Operation
Product metal
Gold
Silver
South Carolina
Peidmont Mining Co.
Westmont Mining Co.
South Dakota
Wharf Resources
Utah
Mercur Hill Gold Property
Timberline Industries, Inc.
Vipont Mines, Ltd.
Haile Gold Mine
Brewer Gold Mine
Annie Creek
Barrick Mercur Gold Mines
Ophir Canyon
Vipont Mine
x
x
x
x
Based on data obtained by PEI Associates, Inc., during telephone communica-
tions with Bureau of Mines State Activity Officers and State regulatory
officials.
Directory of Nevada Mine Operations Active During Calendar Year 1985. Depart-
ment of Industrial Relations, Division of Mine Inspection. State of Nevada.
September 1986.
14
-------
Washington
• •Zortman-Landusky
Thunder Mtn^/ Montana
Oregon ( •stibnite
New Mexico
•Westar
Carson Hill •
C
California
Colorado
•
•Saratoga
•Cripple Creek
•Galaxy
Arizona
Mesquite •• Pichacho
0 220 miles
^ *Ha
N. South Carolina
\>
Approximate Scale
* See Figure 2-3.
Figure 2. Approximate location of several gold/silver heap leach operations.
15
-------
Miles
100
Figure 3. Number of gold/silver heap leach operations by county in Nevada,
(Based on Directory of Nevada Mine Operations Active During Calendar Year 1985.
Department of Industrial Relations, Division of Mine Inspection. State of
Nevada, September 1986)
16
-------
SECTION 3
DESIGN AND OPERATION OF HEAP LEACHING PROJECTS
INTRODUCTION
The basic design and operational layout of heap leach projects are very
similar at all facilities. Low-grade ore is stacked in engineered heaps on
sloped, impermeable pads and a weak alkaline cyanide solution is sprayed over
the ore. The solution percolates through the heap and dissolves the metal
values (gold and/or silver). This pregnant solution then flows over the pad
to a lined collection ditch. The ditch carries this gold-bearing cyanide
solution to a lined pregnant solution pond. The pregnant solution is then
pumped to a recovery plant, where the metal product is removed by carbon
adsorption followed by stripping and electrowinning or by precipitation with
zinc followed by filtration (Merrill-Crowe zinc dust precipitation). The
barren solution is then pumped to a lined holding pond, where it is treated
with additional NaCN and caustic (e.g., lime or caustic soda). From the
barren pond, the solution is again pumped to the heap and sprayed over it to
complete the closed-loop cycle. Heap leach operations are typically zero
discharge facilities.
A conceptual flow diagram of the heap leach operation is presented in
Figure 4. Although the basic process just described is similar at all
operations, each site is unique, and several alternative approaches exist.
Specific leaching times, cyanide concentrations, reagent use, flow rates,
heap dimensions, pad construction, pond capacities, liner materials, and
other design and operational parameters vary from site to site, depending on
the characteristics and quantity of the ore and the climate, topography,
hydrology, and hydrogeology of the site.
MINING AND ORE PREPARATION
Low-grade gold ores (i.e., 0.03 to 0.05 oz/ton) and low-grade silver
ores (i.e., 1 to 4 oz/ton) with finely disseminated free metal particles are
17
-------
BARREN
SOLUTION
POND
(LINED)
NaCN CAUSTIC
ADDITION (LIME)
ADDITION
CO
MAKE-UP
WATER
BARREN
SOLUTION
DISTRIBUTION
SYSTEM
PREGNANT
SOLUTION
POND
(LINED)
CARBON "•**
ADSORPTION
COLUMNS
EMERGENCY
OVERFLOW
BASIN
(UNLINED)
CARBON -
STRIPPING
OPERATION
*• GOLD
PRODUCT
Figure 4. Conceptual flow diagram of typical heap leach operation.
-------
amenable to heap leaching. Heap leaching is applied not only to newly mined
low-grade ore, but also to waste rock dumps and tailings from previous opera-
tions. Some ores (i.e., refractory ores) do not respond to cyanidation.
Refractory ores include carbonaceous ores (carbon prevents much of the gold
from dissolving and adsorbs any dissolved metal before recovery), pyritic f
ores, and complex sulfide ores. Also, copper, cobalt, and zinc in the ore
may preferentially take the place of gold and silver in the leaching reaction
2
and reduce the extraction efficiency. If the metal values are totally
encased by an impervious matrix (e.g., quartz), leach solutions cannot con-
tact the gold. The following major ore types are amenable to heap leaching:
oxidized, disseminated, sulfide ores in which precious metals are not inti-
mately associated with sulfide minerals, and certain load or placer deposits
12
that contain fine particles of gold. Most of the ores subjected to heap
leaching are obtained from surface mines.
Newly mined ore is either placed on the leach pad without crushing and
leached as run-of-mine ore, or subjected to crushing or crushing and ag-
glomeration prior to leaching. The treatment of the ore is a function of
site economics (i.e., tradeoffs between increased recovery and cost of treat-
ment) and ore mineralogy and it is determined through bench- and pilot-scale
tests during the development stage of the operation.
The heap must be built with ore that is uniformly permeable so the leach
solution contacts the available metal values contained therein. The ore must
also be physically strong enough to be placed in heaps and to maintain its
structural stability when wetted. Many resources that previously (pre-1980)
could not be heap-leached because of poor permeability or lack of structural
strength due to the presence of clays or fine particles in the ore can now be
leached after agglomeration. The Bureau of Mines suggested this process in
1979, and it has become a relatively common treatment practice, especially
necessary when old tailings are being leached. Agglomeration involves the
following sequence of operations: crushing ore particles to a minus 1-inch
or finer size, treating the ore with 5 to 10 pounds of portland cement per
ton of ore, wetting it with water or a strong cyanide solution to achieve 8
14
to 16 percent moisture, mechanically tumbling it, and curing it. Agglom-
eration causes the clay and fine particles to adhere to coarser particles
present in the ore and this prevents these fines from segregating in the heap
19
-------
and causing binding (impermeable zones) and solution channeling. In addi-
tion, if cyanide is used during agglomeration, the leaching process is initi-
ated before the ore is stacked on the pad and may reduce leach cycle time and
increase metal recovery. Agglomeration effectively promotes good percolation
characteristics in low-grade resources, particularly tailings, that otherwise
could not be heap-leached. The cement added during agglomeration also aids
in maintaining the alkaline pH necessary for cyanide leaching (optimum pH is
10.3).
Crushing and agglomeration involve a significant capital outlay for
equipment and materials or represent a significant operating cost if done on
a contract basis. For comparison purposes, $2 to $3 per ton of ore repre-
sents the typical mining costs for heap leach operations, whereas crushing
and agglomeration can cost from $1.85 to $4.35 per ton in addition to the
15
mining costs. At a large operation the expenditure for the equipment and
materials .necessary for agglomeration could well be several million dollars.
In summary, mining and ore preparation entails two major considerations:
1) that material to be heap-leached contains gold and/or silver amenable to
cyanidation, and, 2) that the material allows a high rate of percolation
throughout the heap. Crushing can expose more precious metal to the lixivi-
ant, but it adds to the cost. Increases in recoveries after crushing must be
sufficient to offset the added cost. Agglomeration can be used to make some
materials amenable to heap leaching that otherwise could not be treated in
this manner.
PAD CONSTRUCTION
An impermeable pad must be constructed to ensure that product and reagents
are not lost through seepage from the heap. In this case, the goals of the
operator and environmentalist are the same. Leach pads normally are con-
structed on smooth, gently sloping ground (1 to 6 percent slopes); however,
heaps can be constructed in areas of high relief by using a technique called
"valley fill." (This modified version of a heap leach operation is discussed
later in this section.) The leach pad is sloped so that pregnant solution is
directed to a lined collection ditch situated along the one or two downgrade
sides of the pad. Pads are constructed of native clays, modified clays,
asphalt, or synthetic materials such as HOPE (High Density Polyethylene),
20
-------
PVC, or Hypalon. The pads not only must be relatively impermeable (i.e.,
10 cm/s), they also must be capable of providing structural support for the
heap without suffering damage from deflection due to the weight of the ore or
from equipment traffic. Selection of pad specifications and materials is
determined largely by site-specific parameters such as availability of local
materials (i.e., clays), slope of the site, geotechnical properties of the
subbase, temperature variations, and operational considerations (i.e., single-
or multiple-use pads). Single liners are believed to be the norm, although
double and even triple liners are being used. Synthetic liners are often
placed over a layer of compacted native clay soils.
Pad construction involves clearing and grubbing the area, grading and
compacting the subbase (usually with a sheep's-foot or vibratory roller),
placing the liner (clay is typically placed and compacted, with added moisture,
in multiple lifts, each usually 6 inches thick), and placing a layer of
graded ore or gravel over the clay and synthetic liners to provide both a
drainage blanket and to protect them from damage. Examples of some different
types of pad construction are shown in Figure 5. Permeabilities of the
subbase and clay liners are determined through compaction tests and the use
of nuclear densiometers. Design engineering and construction of liners in
this industry have reached a high level of sophistication. The impetus for
the achievement of this competency is the fact that the process solution
contains the product gold. At current prices of over $400 per ounce, loss
prevention is of paramount concern to the operator.
Sizes of leach pads vary greatly. Smaller pads cover less than an acre,
whereas individual pads as large as 50 acres are in use in some places. For
example, Newmont Gold Company has constructed two 50-acre pads lined with
80-mil HOPE at its Gold Quarry Operation near Carlin, Nevada.
Except in valley heaps, (which are discussed later in this section), the
hydraulic head on the pad is kept to a minimum; thus, the driving force
pushing any potential seepage is not as great on a pad as it is, for example,
in a full surface impoundment (Figure 6). Because heaps are constructed to
be highly permeable, are contained only on the base, are on sloped surfaces
to promote drainage, and often have drainage blankets (coarse gravel) or
drain pipes over the pad, buildup of a saturated zone within the heap is
minimized. Also, atmospheric oxygen is required for the efficient extraction
21
-------
RUBBERIZED ASPHALT MEMBRANE
GRADED AND COMPACTED NATIVE SOIL
SMOKY VALLEY COMMON OPERATION
ROUND MOUNTAIN GOLD COMPANY
g^^^^vgg%F I NjE .GRADED .AND ^ ^^J^J^
COMPACTED NATIVE SOIL SUBBASE
GOLD QUARRY OPERATION
CARLIN GOLD COMPANY
LAY COMPACTED
3-6 in. LIFTS \V.
-t N^ >^
x 10* cm/sec)
COMPACTED NATIVE SOIL SUBSASE
COMPACTED CLAY
80 mi 1 HOPE
4 in.
CWKJOJUUA. WINE
HERCO MINERALS
Figure 5. Examples of heap leach pad construction.
22
-------
POND
SURFACE
PONDED
SOLUTION
LINER
SUBSOIL
SEEPAGE
10 ft
TOTAL HYDRAULIC
HEAD, H
ORE HEAP
PHREATIC SURFACE>
ZONE OF SATURATION
LINER
SUBSOIL
0°<^?p00oy°po0r
OAQ
-------
of gold by cyanide; therefore, it is important, from an operational efficiency
standpoint, to prevent inundation. In addition, if the saturated zone over-
lying the pad were thick enough, solution could exit the side of the heap at
some point and cause erosion of the heap. Although measurements were not
available, operators indicated that the zone of saturation over the pad is
typically only inches thick. However, the hydraulic head on some pads, for
example, the Darwin operation, can be as much as 5 feet.
Pads are either single-use, dedicated (sometimes called permanent) pads
or reusable (restackable) pads. The industry norm is single-use clay or
synthetic pads, although a few operations (e.g., Round Mountain Gold/Smoky
Valley Common Operation, Newmont Gold Company/Maggie Creek, Gold Fields
Mining Company/Ortiz and Superior Mining Company/Stibnite Operation) use
reusable asphalt pads. The decision whether to use single- or multiple-use
pads is based on site-specific characteristics. Considerations include the
length of the leaching cycle, which is determined by ore mineralogy, and the
availability of level terrain for the pad area and the spoil (leach residue)
disposal sites. Homogeneous ores exhibiting consistent leach cycle times and
satisfactory target recoveries are amenable to placement on reusable pads.
In these cases, production schedules are set, and heaps are leached for a
predetermined period of time, then rinsed, and excavated. At operations with
variable ore that requires long leach times or multiple leach cycles, single-
use pads are normally used. This allows leaching to continue as long as
metal recoveries justify it. It also permits multiple leach cycles separated
by periods of no solution application (e.g., over winter). This can permit
additional oxidation of gold minerals and result in increased recoveries in
subsequent leach cycles.
An example of the application of multiple-use pads is the Smoky Valley
Common Operation at Round Mountain, Nevada (See Trip Report in Appendix A).
At this site, a single heap covered by 30 individual solution distribution
areas is constructed on two adjacent pads (1.2 million tons of ore on both
pads, total). The total length of the two pads together is 3150 ft; each is
280 ft wide. Crushed ore, stacked to a height of 35 ft, is leached for 50 to
55 days, allowed to drain for 1 day, and rinsed with fresh water for 3 or 4
days. The ore is then excavated by front-end loader and hauled by truck to
the adjacent spoil-disposal area. This operation moves from one end of the
24
-------
pad to the other and creates a 75- to 100-ft slot in the heap. One side of
the slot is being excavated and removed while new ore is being added on the
other. It takes 60 days for this moving slot to traverse the entire pad
length. The production schedule does not allow variation in the leaching
cycle.
An example of single-use pads was found at the Pinson Mining Company's
Pinson Operations near Vlinnemucca, Nevada (see Trip Report in Appendix A).
As many as 60 individual pads were originally planned at this operation. At
a heap height of 20 ft, each 300-ft by 300-ft pad is capable of holding
90,000 tons of ore. The chemistry of the pregnant solution from each pad is
monitored individually. This allows the leach cycle on individual pads to
continue for as long as it is profitable. A total of two or three leach
cycles is applied to each heap over a period of 9 months to a year. Ini-
tially, a 45- to 90-day leach recovers 55 to 60 percent of the gold values.
A second leach is usually conducted after the heap has gone through a winter.
An additional 2 to 5 percent of the total gold is recovered in the second
leach. Depending on production schedules, a third leach cycle may be used.
Although reusable asphalt pads were considered, they were not used because
being able to releach the ore over extended times was desirable and because
they wanted to avoid the cost of moving the ore twice. After a heap has been
leached, a second lift of ore is placed on it and the cycle repeated.
Comprehensive state-of-the-art construction and quality control tech-
niques have been developed and are being used during pad construction at the
larger operations and at small operations of established companies. Com-
petent and qualified managers and engineers direct the design and installa-
tion of these expensive systems. For example, quality control checks include
frequent destruction testing of seams and materials during the installation
of synthetic liners. Some small, undercapitalized operations, however, may
not have adequate ability to construct pads and liners, and the integrity of
these installations may be questionable.
Internal berms are sometimes constructed in leach pads. These berms,
which run from side to side in the direction of the slope of the pad, internally
compartmentalize large heaps. They also allow monitoring of pregnant
solution chemistry (i.e., gold recovery) from different sections of the heap.
25
-------
In addition, should a liner failure occur, only the leach solution applied to
the area defined by the berms could escape. The application of internal
berms was observed at Newmont's Gold Quarry Operation. This application is
shown conceptually in Figure 7.
Selection of liner material may vary among pads at a given site. For
example, Nerco Minerals installed several clay pads at the Candelaria Mine.
More recently, the operator has constructed new pads on which 80-mil HDPE is
placed over a compacted native soil subbase. Another example of different
pad construction at the same site is the Preble Mine operated by Pinson
Mining Company near Winnemucca, Nevada. Most pads at this site are con-
structed of native clays that have been compacted to achieve permeabilities
-7 -8
of 10 to 10 cm/s. Because it is desirable to have a new pad ready to put
in service (ready for heap construction) after the winter season ends, how-
ever, pads with synthetic liners are constructed in the fall. Synthetic
liners (30-mil PVC) with a protective gravel layer can survive the winter
undamaged and be put in immediate service in the spring. Whereas, unloaded
clay pads would be damaged by drying and winter weather if not protected by
ore heaps. Also, clay cannot be worked properly in the colder weather en-
countered during the period in which these pads are constructed. Because
clays are available locally, however, they are the material of choice for
pads constructed during the warmer months of the year.
Clays and ancient lake bottom sediments suitable (with added moisture)
for use in heap leach pad construction are prevalent in the Western States.
On a cost basis, compacted clay liners are usually preferred over synthetics
and asphalt. Clay pads must be kept moist until the heap is constructed
and leaching has been initiated. A clay pad that is allowed to dry will form
cracks large enough for sand to blow into, and the channels so formed will
not reseal themselves. If this occurs, pregnant solution may be lost
through seepage when leaching begins. Treating the surface of a clay pad
with chemical polymer or emulsion surfactants will make it less permeable.
Clay pad construction costs have been reported to be in the range of $0.40 to
$0.60 per square foot. The cost of a clay pad is highly dependent on the
proximity of a source of suitable clay. In contrast to the cost of clay
pads, the 7-inch-thick asphalt pad at the Smoky Valley operation cost
$3.75/ft2.18
26
-------
LEACH ORE
ro
'PROTECTIVE
GRAVEL
COMPACTED SUBBASE
: SYNTHETIC LINER
\ I
INTERNAL BERMS
(CROSS SECTION PERPENDICULAR TO SLOPE OF PAD)
Figure 7. Conceptual diagram of internal berms on a heap leach pad.
-------
For purposes of illustration and comparison, design and operational
characteristics of several selected heap leach operations are presented in
Table 4.
HEAP CONSTRUCTION
Heaps are constructed with standard earth moving equipment, such as
front-end loaders, bulldozers, and haul trucks. Conveyors or radial stackers
also can be used. Ore is placed with a front-end loader to a maximum height
of about 16 ft, pushed up with a dozer, or dumped by truck on top of the
heap. Conveyors or stackers (as used at Ortiz, for example) convey ore to
the heaps and thus avoid compaction due to equipment traffic on the ore. The
slope of the sides of the heaps, which are shaped like truncated pyramids, is
the natural angle of repose of the ore, typically about 1:1. Heights of
heaps vary from 16 ft (limit of front-end loaders) up to 200 ft.
The specific ore placement technique profoundly influences the efficiency
2
of gold recovery from the heap. Care must be taken during ore placement to
avoid compacting the material, which can reduce its permeability significantly.
Heap height is limited by the ability of the foundation and pad to support
the weight of the heap without failure, the structural stability of the ore,
and its permeability. It was once thought that leaching solutions became
oxygen-deficient after percolating through about 10 ft of ore, and that gold
2
extraction could be hindered. Heaps much higher than 10 ft are now in
common use, however, and achieve good recoveries.
After the ore has been leached, it is either spoiled on the pad, exca-
vated, and removed to a spoil-disposal area or another lift of ore is placed
on top of it and leaching is continued. Ideally, when multiple lifts are
leached, the leach solution will percolate through all lifts and dissolve
additional metal values from the lower previously leached lifts. Fine ores
are generally leached in a single lift, whereas coarse material (e.g., run-
of-mine ore) may be leached in multiple lifts. This is because fine ores are
typically depleted after one leach cycle, and coarse ores generally take
2
longer to leach. The application of multiple lifts allows more efficient
use of pad space. Characteristics of the particular ore and land availability
dictate heap height, and the choice between multiple- and single-use pads.
28
-------
TABLE 4. DESIGN AND OPERATIONAL CHARACTERISTICS OF SELECTED HEAP LEACHING OPERATIONS
Company/opera 1 1 on
Amselco Minerals, Inc. /Alligator Ridge
Newmont Gold Co. /Bootstrap Plant
Newmont Gold Co,/Car11n-2
Newmont Gold Co. /Haggle Creek Plant
Cyprus Mines Corp. /Northumberland
Fischer Watt-Pecos Resources/
Tuscarora
NERCO Metals, Inc./Candelarla
Pegasus Explorations, Ltd./
Zortman-Landusky
Plnson Mining Co./P1nson Mine
Plnson Mining Co./Preble Mine
Saratoga Mines, Inc. /Sara toga
Round Mountain Gold Corp. /Smoky Valley
Superior Mining Co./Stlbnlte Mine
Location
White Pine
Co.. NV
Elko, NV
Carl In, NV
Carl In, NV
Austin, NV
Elko Co., NV
Mineral, NV
Zortman, MT
Hlnnemucca, NV
Ulnnemucca. NV
Central City, CO
Round Mountain,
NV
Yellow Pine, 10
Or*
grade,
01 /ton
0.120 Au
0.044 Au
0.03 Au
0.080 Au
0.400 Ag.
0.020 Au
1.75 Ag
Minor Au
3.15 Ag
0.066 Au
0.02-0.03
Au
0.062 Au
0.04-0.10
Au
0.03-0.04
Au
0.05-0.08
Au
Ore
treatment
Crush to -3/4 tn.s
agglomerate to
-1/2 In. mine run
Uncrushed mine run
Crush to -M In.;
agglomerate
Uncrushed nine run
(fines to 6 In.)
Crush to -1 In.;
agglomerate
Uncrushed mine run
Uncrushed mine run
Crush and agglomer-
ate
Crush to -1| In.,
agglomerate
Crush to -1/2 In.
Crush to 1.25 In.
Leach pad
construction
12 In. of compact
clay
Compact clay lake
bed
80-mll HOPE Hner
2-ft. clay base,
5-1n. asphalt layer,
2-in. rubberized
asphalt membrane,
seal coat
Wet, compacted, lake-
bed clays
18-1 n. compacted clay
or 80-m1l HOPE
12 In. of compacted
bentonltls shale
and 30-mll PVC
12 In. of compacted
clay
12 In. of competed
clay or 30-mll PVC
6 In. of compacted
tailings and 30-m1l
PVC
3 In. of rubberized
asphalt, 4 1n. of
asphalt
3 In. of sealed asphal
coarse geotextlle
fabric, 1 ft of grave
30-mll PVC liner
MeUl
recovery
method
CAb
CA
CA
CA
HCC
HC
MC
CA
MC
CA
t, C«
1.
Ore processed,
1000 tons
750
220
4500
100
2000
3640
125
2000
400-500
Production,
1000 oz/yr
70 Au
6 Au
40 Au
70 Ag
1 Au
70 Ag
>2000 Ag
70 Au
125 Ag
3 Au
10 Ag
60 Au
30 Ag
25-30 Au
-------
TABLE 4 (continued)
Company/operation
Tenneco Minerals Co. /Boreal Is
The Anaconda Co. /Darwin
The Standard Slag Co. /Atlanta Mine
Windfall Venture/Windfall
Location
Mineral, NV
Darwin. CA
Lincoln Co., NV
Eureka, NV
Ore
grade,
oza/ton
0.090 Au
0.544 Ag
Minor Au
1.38 Ag
0.028 Au
Minor Ag
Ore
treatment
Crush to -2 In.}
agglomerate
Agglomerate tail-
ings
Crush to -0.25 to
0.50 In.
Uncrushed mine ore
Leach pad
construction
5-1n. of compact
asphalt
18-ln. of compacted
soil and clay
2 In. of compact
asphalt
Metal
recovery
method
MC
MC
MC
CA
Ore processed,
1000 tons
450
120-135
220
Production ,
1000 oz/yr
30
5 Au
*0z • troy ounces throughout table
Carbon adsorption
1-Crowe
oo
O
-------
Construction techniques vary from site to site and are altered at any
given site as better means are developed or if ore characteristics change as
the ore body is developed. For example, Pinson Mining Company originally
planned 60 individual adjacent single-lift heaps, each with a height of about
16 ft. During construction and operation of the heaps, it was determined
that two additional lifts could be placed after the top of the previously-
leached material was scarified. The benefits of this multiple-lift method
are additional gold recoveries from releaching lower lifts and lower pad
construction costs because more ore can be treated on any given pad. As
another example, heap construction at the NERCo Minerals Candelaria Mine has
involved placement of seven contiguous heaps, each about 1200 ft long, 200 ft
wide, and 20 ft high. After leaching is completed on each heap (45 to 60
days), the surface is scarified and another lift is placed on top of it.
Each lift of ore is placed by truck and dozer to a height of 25 ft. The top
5 ft is then pushed off to remove any material compacted by the equipment.
The ultimate height of the heap will be 110 ft. Another example is the
Carlin-2 operation of Newmont Gold Company, where two 50-acre pads have been
constructed. Run-of-mine ore is stacked in a single lift, to a height of 50
ft. Additional lifts may be added in the future, which will result in an
ultimate heap height of 200 ft.
SOLUTION HANDLING/LEACH CYCLE
A schematic flow diagram of the path taken by leach solutions is shown
in Figure 8, and the characteristics of the solutions and the construction of
impoundments are discussed in the paragraphs that follow.
Leach solutions basically consist of sodium cyanide, caustic (i.e.,
1-ime), and water. Antiscaling additives are sometimes used to prevent foul-
ing of the sprinkler heads. Sodium cyanide, the only commercially proven
lixiviant, is added to maintain a concentration in the barren solution of
about 0.5 Ib per ton of solution (approximately 250 ppm CN). Caustic (usual-
ly lime, caustic soda, or sodium hydroxide) is added to maintain the alkaline
pH of the barren solution.
Most operations have a net water loss due to evaporation (as much as 10
percent) and require the addition of fresh water for makeup. Makeup water is
typically obtained from wells installed for that purpose. The solution
31
-------
EVAPORATION
(30 TO 40 gpm/acre)
PRECIPITATION
(220 gpm/acre)
CLEAN WATER RINSE
CO
ro
NET EVAPORATION
PREGNANT
SOLUTION POND
LEGEND
SOIL STORAGE -NET EVAPORATION
(10 TO 25 gal/ton)
PAD AND CONTAINMENT
BERMS
EXTRACTION
PROCESS
NATURAL WATER CIRCUIT
PROCESS CIRCUIT
BALANCING FlOW
REAGENT
BARREN SOLUTION POND
BLEED
L STORAGE
Figure 8. Schematic flow diagram of leach solutions at a typical heap leach operation.
Source: Hutchinson, Ian. Surface Water Control. Chapter 9 in Short Course on Evaluation, Design, and
Operation of Precious Metal Heap Leach Projects. AIME, SME, Albuquerque, New Mexico, October 13-15, 1985.
-------
management systems are designed to be a closed-loop recycle with no discharge
of effluents during operations. Some operations may discharge treated bleed
streams (under NPDES permit), however, if they have a net water gain as a
result of local climatic conditions.
The barren solution is sprayed onto the surface of the heap. Plastic
pipes distribute the solution to impulse (rainbird) or wobbler-type sprinklers.
These sprinklers are favored because they distribute the solution evenly and
produce a large droplet, which minimizes evaporation. Sprinklers are usually
placed on 40-ft centers over the top of the heap. The typical solution
o
application rate is 0.005 gallon per minute/ft . This operation is monitored
to ensure that the sprinklers are functioning, that they do not become stuck
in one position, and that no ponding of solution occurs on the surface.
Ponding indicates the application rate is too great or that a zone of low
permeability exists. In either case, efforts are made to correct the situa-
tion by reducing the solution application rate or by scarifying the top of
the heap.
Moisture content in run-of-mine ore varies, but levels in the 5 to 10
percent by weight range could be considered typical. Solution applied to a
fresh heap percolates through the ore, flowing over ore particles and into
cracks and crevices by capillary action. Sufficient solution must be applied
to saturate the heap and overcome its storage capacity before any solution
can drain from the heap. Storage capacity of the heap is determined by the
porosity and quantity of the ore in the heap. When saturated, the moisture
content is typically in the range of 10 to 15 percent by weight. Solution
equivalent to 10 percent of the weight of the ore may be required to wet the
heap, and an additional 10 percent may be stored in the heap during steady-
19
state leaching. For example, the 20-ft-high heaps at the Pinson operation
contain about 90,000 tons of run-of-mine ore. Initial breakthrough of solu-
tion occurs about 18 hours after solution application begins (application
2
rate is 0.005 gallon per minute/ft ) and a steady-state flow is achieved
after about 72 hours. Based on site measurements, about 250,000 gallons are
required to saturate an individual heap. When heaps are idled over winter at
this site and releached in the spring, the same storage capacity is noted.
The volume of pregnant solution leaving the heap under steady-state
leaching conditions is a function of the area of the top of the heap, the
33
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solution application rate, and the evaporation rate. A small heap having
2
40,000 ft of surface area, for example, would generate a maximum of 200
gallons per minute, assuming no losses (40,000 ft x 0.005 gallon per min-
O
ute/ft ). The 50-acre heap at Carlin's Gold Quarry Operation generates a
flow of 4000 gallons per minute under steady state conditions.
The chemistry of the pregnant solution differs from that of the barren
spray. Concentrations of gold are measured in hundredths of an ounce per ton
of pregnant solution. The pregnant solution at Round Mountain, for example,
contains about 0.04 oz/ton of solution (about 1.2 ppm). The pH of the preg-
nant solution is usually lower than the barren spray because of the neutrali-
zation that occurs in the heap and COp pickup from the atmosphere. In the
case of agglomerated ore, however, the pH of the pregnant solution may be the
same or higher as that of the barren spray because of the alkalinity imparted
by the cement used as the binder.
The free cyanide content is also lower in the pregnant solution because
some cyanide volatilizes at the surface of the heap. Cyanide percolating
through the heap is also tied up in metal complexes (e.g., with iron), is
destroyed by cyanicides present in the ore, is bound by carbon present in the
ore, or is otherwise consumed. To maintain the typical 1 pound of cyanide
per ton of solution concentration in the barren spray that is necessary for
efficient leaching requires the addition of an amount of cyanide equal to its
consumption in the heap. At the sites visited during the project, reagent
usage reportedly varies from 0.1 to 0.3 pound of NaCN per ton of ore (see
Appendix A), but 0.5 Ib/ton may be closer to the industry average. Caustic
in the form of lime is added to the barren liquid at a rate of 0.3 Ib/ton of
ore at two of the sites visited.
The pregnant solution flows down-slope over the leach pad to a lined
collection ditch. Collection ditches are situated along one or two sides of
the pad, depending on the pad's slope. If the pad slopes to one side, the
collection ditch is located along that side. If the pad slopes to a corner,
the ditch is located along both sides joining at that corner. Collection
ditches, like leach pads, are lined to prevent loss of pregnant solution. If
the leach pad is constructed with a synthetic liner, the collection ditch is
lined with a continuation of the same material or a different thickness of
the same material. If the leach pad liner is constructed of compacted clay,
34
-------
the ditch is lined with a synthetic material that is keyed into the pad. In
the case of asphalt pads, the collection ditches are continuous with the pad
and are constructed of asphalt.
The pregnant solution flows by gravity through the lined collection
ditch to a lined surface impoundment, known as the pregnant solution pond.
This impoundment is normally situated adjacent to and immediately downgrade
from the heap. The industry standard is believed to be the use of synthetic
single liners placed over a compacted subbase of native soils or clays.
Observation ports are installed under all leach ponds in Nevada in order to
detect seepage through pond liners. Groundwater monitoring around solution
ponds varies in extent and sophistication. The pregnant solution pond is the
largest of the surface impoundments used in the operation because it must be
able to hold not only the normal flow of pregnant solution, but also any
additional flow due to normal rainfall or unusual (i.e., 100-yr) storm events
and still provide sufficient freeboard to prevent overtopping. Thus, the
dimensions of the pregnant solution pond are a function of the size of the
heap and the climate at the site. These ponds are typically 10 to 20 ft deep
and have side slopes of 3:1 (h:v). For instance, Candelaria's pond has a
capacity of 9 million gallons and 2.4 acres of surface area.
Typically, an emergency overflow basin is situated immediately downgrade
from the pregnant solution pond. This basin may be lined with a synthetic,
as at Candelaria, or it may be constructed of native clays or unlined. Its
function is to provide emergency containment of any overtopping of the
pregnant solution pond.
Pregnant solution is pumped to the precious metal recovery process,
where gold or silver is removed. The barren solution is usually sent to a
barren solution pond, which may be about half the size of the pregnant solu-
tion pond. Some sites (e.g., Mesquite) do not use a barrens pond. Construc-
tion of the barren pond is the same as that of the pregnant pond—synthetic
liners placed over compacted earth. The barren solution is treated with
cyanide and caustic, as discussed previously, as it is pumped back to the
heap. The heap leach system often has a total residence time of more than 3
days.19
Solution application often must be curtailed or halted during winter be-
cause of ice formation. Some sites heat the barren solution to allow leach-
ing to continue during mild winter conditions, which extends the leaching
35
-------
season. Other sites have developed application methods (e.g., subsurface
solution application from distribution pipes buried in the heap and spraying
to produce ice domes), which allow leaching to continue during winter, albeit
at reduced rates. The impact of winter on heap leach operations is more pro-
nounced in northern climates (e.g., Montana) than in the Southwest.
Leaching operations also may be interrupted for other reasons. Some
facilities may operate only intermittently to allow oxidation to occur in the
heap, which permits additional gold values to be recovered. Still others may
operate in this manner because they are undercapitalized or only part-time
business ventures.
METAL RECOVERY
Precious metals in the pregnant solution are recovered either by adsorp-
tion on activated carbon or by Merrill-Crowe zinc dust precipitation. Uncon-
ventional recovery processes, such as ion exchange, solvent extraction, and
direct electrowinning, also may be applicable in special circumstances. The
choice between carbon adsorption and zinc precipitation is usually based on
20
processing cost and on the gold/silver content of the pregnant solution.
The zinc dust precipitation method is usually preferred at large operations
where the silver/gold ratio exceeds two or where the concentration exceeds
-pt
20
20
2 ppm. Carbon adsorption is the method of choice at smaller operations
because it costs less.'
The Merrill-Crowe zinc dust precipitation process involves four major
process steps:
1) Clarification of pregnant solution to achieve efficient precipita-
tion.
2) Vacuum deaeration to remove dissolved oxygen, which causes passiva-
tion of the zinc surface, and carbon dioxide, which can react to
form calcium carbonate that blinds filters.
3) Addition of lead salts to remove sulfides prior to precipitation of
precious metals with powdered zinc.
4) Filtration of precipitates, which are typically dried, fluxed, and
smelted to form a gold or silver bullion. The barren solution is
returned to the heap.
The carbon adsorption process involves three unit operations:
36
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1) Loading the precious metals from the pregnant solution onto the
activated carbon. This is normally accomplished in a series of
countercurrent tanks. The barren solution is returned to the heap.
2) The gold/silver is eluted, usually with a hot caustic cyanide
solution. The stripped carbon is regenerated by steam or thermal
reactivation.
3) The gold/silver is recovered from the concentrated cyanide solution
by electrowinning, followed by fire refining to produce dore bul-
lion. Zinc dust precipitation can also be used.
Assuming a 1 ppm content in the pregnant solution, the carbon adsorption
method would produce an ounce of gold at a cost of about $7.50. The Merrill-
Crowe method would entail production costs of between $6.00 and $8.20 per
ounce.
RESIDUE DISPOSAL AND SITE CLOSURE
Heap leach residue, the barren ore remaining after obtainable gold val-
ues have been extracted, is either left in place (i.e., spoiled on the pad)
or excavated, hauled by truck, and disposed of in an onsite disposal area.
At the majority of sites, the leach residue currently generated is left on
the pads at closure.
At closure, the smallest heap would be less than an acre in size and 16
ft or less in height. Such a heap would be generated by a small-scale,
short-term venture. One of the largest heaps would be 50 or more acres in
size and 100 to 200 ft in height. A large heap such as this would have been
generated by a large-scale operation that processed the ore over a period of
years and added multiple lifts to the heap. Only at those sites using reus-
able pads is the leach residue removed from the pad. At these sites, the
process of removing the leach residue from the pads and disposing of it is
continuous over the active life of the site.
Standard industry practice is to follow the leaching of an individual
heap with a drainage period lasting 1 or more days, during which no solution
is applied. The heap is then rinsed with fresh water for several days. For
example, the State of Nevada requires rinsing until a preset limit (i.e., a
pH of 8.5 and a cyanide content of 0.2 ppm) is achieved in the rinse water.
The barren solution remaining at the time leaching is ceased is allowed to
37
-------
evaporate to dryness (if the climate permits) or is treated with a cyanicide.
Again in Nevada, for example, no discharge of solution is permitted and the
barren solution is lost to evaporation.
As documented in previous reports, essentially no data are available on
21
the quantity of cyanide remaining in heap leach residue. How effective a
short-term rinsing with fresh water is in removing all free cyanide from the
residue has not been demonstrated. The addition of a cyanicide (such as
hypochlorite) to rinse water has been documented at three sites (Annie Creek,
Stibnite, and Darwin). Again, the effectiveness of this treatment is largely
unknown.
Leach residue disposal at sites that have reusable pads is accomplished
by excavating the residue after it has been drained and rinsed with fresh
water, loading it into trucks, and hauling it a short distance to a dump
area. The residue is end-dumped from the top of the disposal pile and cas-
cades over the outer face of the pile. This spreads the residue out in a
thin layer that dries rapidly. This method of disposal was observed at both
the Smoky Valley and Carlin-2 operations. Free cyanide left in the residue
probably would volatilize and escape to the atmosphere.
Operations of one of the larger sites visited (Candelaria) indicated
that at closure, heaps will be rinsed as discussed earlier, and any solution
remaining in the impoundments (pregnant and barren ponds) will be evaporated
to dryness as specified in their permit. Collection ditch liners and other
exposed liner material around the heap will be removed and placed in the
empty impoundment. The impoundment liner will then be folded over on itself
and buried in place.
After closure, the liners beneath heaps would afford little protection
against leachate seepage. Any leachate formed in the heap would flow down-
ward over the liner and run off the end of the liner onto the ground. If
ditch and impoundment liners were left in place, the leachate would collect
in the pregnant pond, where it would evaporate or leak to the ground if the
liner should fail over time. The generation of leachate after closure is a
function of the rates of precipitation and evaporation and the water reten-
tion characteristics of the spoil. (Additional discussion on leachate forma-
tion is presented in Section 5.6.)
38
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SITE SECURITY
Security is provided by guard houses, locked gates, and perimeter
fencing. In particular, pregnant and barren solution ponds are surrounded by
fencing, and signs are posted to warn of the presence of cyanide. In arid
areas, the presence of surface water is a strong attraction to passing
animals, especially when the nearest surface water may be five or more miles
away. Operators often construct fresh water ponds at a location away from
the production area to provide a drinking water source, thereby lessening the
chance that animals would attempt to gain access to and drink the cyanide
solutions in the pregnant or barren solution ponds or collection ditch.
Flags and netting are also used to discourage birds from using solution ponds
and ditches at some sites.
VALLEY LEACH
Heap leaching has been conducted in areas where relatively flat native
terrain was not available by constructing and grading a waste rock or over-
burden fill to form the necessary surface. The Nevex Mine near Carson City,
Nevada, is operating a heap constructed in this manner. More recently, an
approach called "valley leach" has been used for heap leaching operations in
moderate to steep terrain. Valley leach is a modification of heap leaching
that is applicable in steep terrain where typical heaps cannot be constructed.
The system involves construction of a containment dike by using compacted
waste rock at the downhill limit of the heap and placement of a liner over
22
the upstream face of the dike and over the pad area above it. The heap is
then constructed behind the dike. The major difference between a valley
leach and a typical heap leach operation is that a pregnant solution pond is
not constructed in the valley leach system unless the cyanicides in the ore
necessitate the use of an external pond. The pregnant solution is stored
internally in the heap (in the voids of the ore) and contained by the con-
tainment dike and liner. A conceptual diagram of a valley leach system is
shown in Figure 9. A valley leach system requires a durable ore (one that
will not break down upon leaching) for stability. It also requires a high-
integrity liner (usually a double liner, synthetic over clay, for example)
because of the hydraulic head imparted by the internal storage of pregnant
39
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CONTAINMENT
DIKE
BARREN SOLUTION
SPRINKLERS
LINER
PREGNANT SOLUTION
STORED IN HEAP
SURFACE WATER
DIVERSION DIKE
SLOPE OF NATIVE
TERRAIN
Figure 9. Typical cross-section of a valley leach type heap.
Source: Ref. 22.
-------
solution. The strength of the foundation, the clay liner, and the clay/syn-
thetic liner interface must be assessed to determine safe loading and slope
22
limits. Pregnant solution is extracted either by pumping wells or by
gravity through valved pipes passing through the containment dike. Valley
leach systems have been used at the Zortman/Landusky operation of Pegasus
Gold in Montana and the Summitville Mine of Galactic Resource in Colorado.
41
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SECTION 4
SUMMARY OF THE TOXICITY AND MOBILITY OF CYANIDE
This section includes a brief discussion of the available information on
cyanides present in process solutions and leach residues, the toxicity associ-
ated with various forms of cyanide, and the potential for migration of cyanides
from heap leach operations and leach residues. A considerable data base is
available on cyanides from mining operations and the environment. Very
little data are available, however, on the types or quantities of cyanides in
heap leach residue. The following discussion is meant to be generic to allow
the reader, especially anyone unfamiliar with the topic, to put information
presented in previous sections and the conceptual controls discussed in the
following section in context.
CYANIDE IN PROCESS SOLUTIONS
Typically for gold leaching, sodium cyanide is added to the barren
solution to maintain a concentration of about 0.5 pound per ton of solution.
This equates to about 250 ppm. The protective alkalinity is maintained in
the barren solution pond by the addition of lime, and a pH between 9 and 11
is maintained. Under these conditions, the cyanide present is mostly free
cyanide, as required in the leaching reaction. The barren solution pond typ-
ically holds hundreds of thousands of gallons of this solution. The pregnant
solution pond contains lesser concentrations of free cyanides because of the
destruction, losses, and complexation that occur in the heap; however, a
significant concentration of free cyanide is still present. The solution in
these surface impoundments represents the greatest source of free cyanide at
a leach operation. Failure of the containment system, liner failure, or
overtopping of the pond would result in free cyanide in an alkaline solution
being released to the environment.
42
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CYANIDE IN LEACH RESIDUE
Cyanide in leach residue occurs in combinations of various metallo-cya-
nide complexes, cyanates, and free cyanide (HCN and CN" ion). Several metallo-
cyanide complexes can occur in leach heaps. Some of these are strongly bound
and stable, some are moderately bound and dissociate with time, and some
other forms dissociate easily. As the metal!o-cyanide complexes dissociate,
they may form different metal!o-cyanide complexes as well as free cyanide
(HCN and CN" ion). Strong metal!o-comp!exes include those of iron, cobalt,
and gold [Fe(NC)"4, CO(CN)"4 and Au(CN):1].21 Moderately strong complexes
12
include those of copper, nickel, mercury, and silver [Cu(CN)" , Cu(CN)T ,
21 c 6
NI(CN). , and Ag(CN)9 ]. Weak complexes include those of zinc and cadmium
2 1 2
[Zn(CN)^ , Cd(CN)^ , Cd(NC)^ ]. The complexes that are formed in a given
heap are determined by the mineralogy of the ore. As metal!o-cyanide com-
plexes dissociate, they may form other complex ions and free cyanide. Depend-
ing on the p'H, the free cyanides can be released to the atmosphere or leached
with rainwater. If the pH of the heap leach is less than 9t some or most of
the free cyanide will be released to the atmosphere. If the pH is greater
than 9, free cyanide will remain in solution. Low concentrations of soluble
free cyanides are amenable to biodegradation. High concentrations of free
cyanide are not easily biodegraded, however, and could result in cyanide
contamination in runoff or in liquids percolating to underlying soils and
ground water. During gold heap leaching an alkaline pH (9 to 11) is main-
tained, but afterwards the piles are usually rinsed with fresh water and the
pH approaches a more neutral range; thus, much of the free cyanide can be
volatilized. Leach residue near the surface of the heap can become less
alkaline with the absorption of CCL from the atmosphere. If the heap is not
rinsed or rinsing is inadequate, the solution remaining in the interior of
the heap may remain alkaline and be a potential source of free cyanide.
Concentrations of cyanide in heap leach residue vary with the ore composi-
tion, the pH of the residue, treatment of the residue (i.e., water or chlorine
rinses), age of the residue, and environmental factors such as the amount of
rainfall, temperature, and degree of aeration.
Ore mineralogy affects the types of complexes formed, which determines
their long-term stability, as previously discussed. The pH of the residue
affects the rate of release of cyanides from the heap as volatilized HCN,
43
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solubilized free cyanide, or metal!o-cyanide complexes. Few researchers have
described the effects of treatments such as fresh water rinse and treatment
with hypochlorite on cyanide concentrations in the heap; however, alkaline
rinses, water rinses, and rinses with oxidizing agents are some of the meth-
ods that may be used to increase rates of destruction of free cyanide and
thus more quickly reduce the cyanide concentrations remaining in the heap
leach residue. For example, the State of Nevada requires that leach residue
be rinsed with fresh water until the pH of the solution exiting the heap is
8.5 or until the cyanide concentration is below 0.2 mg/liter.
Few data are available on the content and fate of cyanide in heap leach
residue. Available data indicate that cyanide in heap leach residue decreases
over time. As part of a study on the long-term degradation of cyanide in an
inactive heap, Engelhardt measured total and free cyanide concentrations at
23
various depths and locations in a heap over an 18-month period. He found
that free cyanide concentrations varied from about 8 mg/kg to nearly 200
mg/kg in core samples taken 3 months after leaching had been terminated. At
the end of 18 months, free cyanide concentrations were below 40 mg/kg, which
indicated an approximate decrease of 85 percent in free cyanide concentra-
tions over the H-yejar study duration. It was also found that free cyanide
concentrations in heap samples were only slightly less than total concentra-
tions. In two other studies, Schmidt et al. reported 65 percent decreases in
nr ne
total cyanide over a 70-hour residence time in a tailings pond. ' Ely
documented cyanide in leach residue and soil at the American Mine. Contamina-
tion was limited to a depth of less than 24 inches in the soil at this site
24
where liner failure had occurred.
Several environmental factors influence the rate of degradation of
cyanide in heap leach residue. Intermittent rainfall and higher temperatures
are both conducive to increased release of cyanide and degradation within the
heap. Mixing of the material that occurs when leach residue is spoiled
(disposed of) off the pad enhances cyanide degradation. As leach residues
are aerated, they become more neutral and less alkaline through absorption of
carbon dioxide from the atmosphere. As the pH decreases, cyanide is in-
creasingly volatilized as HCN. In solution below pH 7, essentially all the
free cyanide is present as HCN and equilibrium is maintained with atmospheric
HCN vapor. The free cyanide/pH relationship is shown in Figure 10.
44
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Figure 10. Effect of pH on dissociation of hydrogen cyanide.
(Source: Workshop - Cyanide From Mineral Processing. Utah
Mining and Mineral Resources Research Institute.
J. L. Huiatt, ed. 1982.)
45
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TOXICITY OF CYANIDE
The toxicity of cyanide varies widely with the form of the cyanide.
Free cyanide in the form of HCN is the most toxic; many metal!o-cyanide com-
27
plexes are far less toxic. The toxicity of metal!o-cyanide complexes in
the aquatic environment varies with the stability of the complexes. Those
that dissociate readily to release free cyanide, such as zinc and cadmium
cyanide complexes, are highly toxic. Others that exhibit moderate dissocia-
tion are less toxic; these include copper and nickel cyanide complexes. Iron
27
and cobalt complexes are tightly bound and are considered to be nontoxic.
Complexing cyanides in receiving streams has been suggested as a means of
27 —
reducing toxicity due to cyanides. | In 1985, however, Heming and Thurston
'reported that ferrocyanide and ferricyanide were more acutely toxic to rain-
bow trout when tested in light than in the dark because of the tendency of
these iron-cyanide complexes to decompose under sunlight to release free
cyanide. Both ferrocyanide [Fe(CN)g ] and ferricyanide [Fe(CN)g ] are
byproducts of the cyanidation processes for gold extraction.
Cyanide in the form of HCN is a respiratory or cellular asphyxiant that
prevents tissues from utilizing oxygen. Depression of the central nervous
system, the tissue most sensitive to hypoxia (oxygen deficiency), can result
from exposure to HCN.
In humans, cyanide can be absorbed through the skin, mucous membranes,
30
and by inhalation. Alkali salts are toxic only upon ingestion. Inhalation
of cyanide fumes can be rapidly fatal depending on concentration. The
nonvolatile cyanide salts seem to be nontoxic systemically, as long as they
30
are not ingested and the formation of hydrocyanic acid is prevented.
Exposure to small amounts of cyanide compounds over long periods of time may
cause loss of appetite, headache, weakness, nausea, dizziness, and irritation
29
of the upper respiratory tract and eyes.
Free cyanide concentrations of 0.05 to 0.10 mg/liter can be fatal to
many fish, and levels as low as CLOi mg/liter have had adverse effects on
fish. The EPA has established an ambient concentration of 0.005 mg/liter
in surface water as the criterion level to protect fresh-water and marine
21
aquatic life and wildlife. Cyanide toxicity to aquatic organisms is due
mainly to HCN derived from dissociation and hydrolysis of cyanide compounds.
Thus, both the free cyanide (as HCN) concentration, which produces the toxic
46
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effects- :nd any cyanide complexes, which have the potential to release free
21
cyanides as HCN through degradation, are important.
Thiocyanate is a degradation or reaction product that exists in leach
residue. Thiocyanate is much less toxic to humans than free cyanide. However,
thiocyanate has been found to be acutely toxic to trout.
MIGRATION OF CYANIDE
Cyanide from leach residues can migrate through release to air, ground
and surface waters, and to soils. The principal transport process for free
cyanide (HCN and CN~) in mining wastes is through volatilization as HCN to
21
the atmosphere. The alkalinity of leach residues is reduced near the
surface through absorption of carbon dioxide, which decreases the pH of the
residue and increases the volatilization of cyanide in the form of HCN. The
literature review yielded only limited amount of information on free cyanide
concentrations in the atmosphere. Stampfli conducted air monitoring at
distances of 10 to 100 meters around a leach heap that had been deactivated
21
by flushing hypochlorite solution and then water through the residue.
Cyanide concentrations in air ranged from(2 to 259 yg/m , and most samples
showed less than '$2 yg/m . (The TLV for cyanides in air is^lO mg/m .) As
would be expected, Stampfli found that cyanide levels decrease as wind speeds
increase; levels also decreased at greater distances from the heap piles.
Several references cite volatilization as the most significant route of
release of cyanide from leach residues. No reference documents any health
problem related to atmospheric releases of cyanide from leach piles. Docu-
mented sampling data are not available on the release of cyanide to the
atmosphere from heap leach operations and heap leach residue. The remote
location of most heap leach operations, however, would tend to minimize the
likelihood of adverse human health impacts.
The secondary transport process for free cyanide and soluble metal!o-
cyanide complexes is leaching. Zinc and cadmium cyanide complexes are more
amenable to solubilization than most cyanide metallo-complexes. If the pH of
solution retained in the leach spoil remains above 9 (e.g., because of in-
adequate rinsing), free cyanide can remain in solution and potentially could
be transported to surface water or ground water through runoff and/or per-
colation.
47
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Some data indicate that cyanides migrate only short distances in soils
and sediments. The distance of migration varies with the type of soil and
the form of cyanide. McGrew measured total cyanide in clay beneath leach
and found that all the cyanide was confined to the first four inches of
21 —
clay. Ely's study of the American Mine indicated cyanide contamination of
24
soil beneath ruptured liners was limited to the top 24 inches. Free cya-
nides have the potential to migrate through saturated and unsaturated soils,
but transport is limited by complexing between the soils and the cyanide
compounds and by sorption onto clays or organics in the soil. Low levels of
27
cyanides can be metabolized by microbes. In aerobic soil zones, cyanides
can be decreased by microbial nitrification. In anaerobic soil zones, cya-
nide is attenuated by sorption and precipitated as metal!o-cyanide complexes,
with some microbial denitrification.. Transport of cyanides can occur in
27
sandy soils or soils low in organic content. Migration is limited in soils
with a high'clay content; soils with hydrous oxides of iron, manganese,
27
aluminum, and other metals; and soils with high organic content.
SUMMARY
Cyanide can be highly toxic or relatively innocuous depending on its
species. The species of cyanide and cyanate present are determined by the
physical and chemical environment and by the compositional characteristics of
the ore. During active leaching, an alkaline pH is maintained to promote the
existence of free cyanide necessary for the leaching reaction. After leaching
and rinsing, natural mechanisms act to degrade and/or complex the cyanide
remaining in the spoil. Few data are available on the content and fate of
cyanide or cyanates in heap leach spoil. The principal transport mechanism
is reported to be volatilization to the atmosphere as HCN. Although the
acute toxicity of HCN is well documented, no problems with atmospheric releases
of HCN from heap leach residue have been documented. Secondary transport is
via runoff or percolation. Available data indicate that cyanides migrate
only short distances in soils.
48
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SECTION 5
ALTERNATIVE MANAGEMENT PRACTICES
Based on our understanding of the objective of this task and our knowl-
edge of the industry, we have determined that only a limited number of alter-
native management practices could be applied to minimize the potential for
cyanide releases from heap leach operations (both during and after leaching
operations). These include alternative liner construction, oxidation of
cyanide during post-leaching flush-out, and use of reagents other than cyanide.
In the first place, most heap leach operations are relatively small, and
their only discrete sources of potential releases are the heaps themselves
and the two process solution ponds. After cessation of operations, only the
heap remains as a potential source, as the ponds must be emptied during
closure. Secondly, most obvious controls, such as pond and leach pad liners,
surface water diversions, and post-leach rinsing, are already standard prac-
tices in the industry.
Although a relatively small number of potential alternative practices
that are currently in use at some sites could be used more widely in this
industry segment, their application and cost would have to be determined on a
site-by-site basis because of differences in ore mineralogy, topography,
geology, hydrogeology, climate, and design and operational characteristics.
Economic considerations (the cost of the practice and how it affects the
profitability of the operation) also are site specific. Control options
applicable to an operation located in a very arid area situated over deep
ground water and away from surface water or population would differ from
those suitable for a site situated over relatively shallow ground water and
near a population center or surface waters.
The current EPA policy regarding active heap leach piles and leach
liquors is that these materials do not represent wastes, but rather are raw
materials used in the production process and products, respectively (51 Fed.
Reg. 24496). Only the leach liquor that escapes from the production process
49
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and abandoned heap leach piles is considered a waste material. This position
has influenced the definition and evaluation of alternative management prac-
tices.
The applicability of an alternative management practice is determined
primarily by the operational phase during which it will be used. Four opera-
tional phases are listed here:
0 Preoperations - This phase includes site characterizations; deter-
mination of engineering, design, and operational parameters; and
construction of the facility. Potential for environmental impact
is assessed, and appropriate controls are designed to satisfy
permit requirements.
0 Active operations - This phase covers the leaching cycle, which may
vary from 1 or 2 months to 3 or more years.
0 Closure - This phase covers the period immediately following cessa-
tion of leaching, during which the site is brought to the condition
in which it will remain.
0 Post-closure - This phase is the period, usually 30 years for RCRA
programs, following site closure, during which primary activities
are monitoring and maintenance of the site.
Inadequate data are available for a quantitative assessment of the
21
cyanide content in leach residue and releases from heap leach operations.
Information on actual releases is also sketchy. Therefore, the need for
controls beyond those currently in use has not been demonstrated. The goal
of this effort was to evaluate conceptual management practices that could
prevent or mitigate actual or potential seepage from the heaps, leach
residue, or solution ponds. The conceptual management practices evaluated
are categorized in the following list according to the operational phase of a
heap leach facility.
Operational phase Management practice
Preoperations ° Installation of a system of French drains
beneath the pad and solution pond liner to
allow leakage detection.
0 Construction of pregnant and barren solution
ponds with synthetic double-liner systems with
leak detection per RCRA guidance for hazardous
waste.
50
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Operational ° Use of alternative lixiviants (i.e., thiourea)
to eliminate potential for cyanide contamina-
tion.
0 Installation of a system of ground-water moni-
toring wells.
Closure * Flushing of heap with cyanicides (i.e., hypo-
chlorite) to destroy residual cyanide.
0 Recontouring of heap and application of imper-
meable (i.e., compacted clay) cap.
Post-closure ° Long-term (i.e., 30-year) maintenance of heaps,
monitoring of ground and surface waters, and
maintenance of site security.
Many unknowns exist with regard to the type, quantity, and fate of cya-
nide in heap leached material. Only sparse data are available on the amount
of cyanide in and around heap leach operations, especially after closure.
Studies for the development of appropriate test methods (e.g., methods to
determine representative data on cyanide content and speciation in heaps) and
the collection of the needed data are just beginning. The preceding list of
management practices presented was based on the assumption that controls may
be required.
An attempt was made to develop a set of conceptual management practices
defined on an example site basis. The practices are specified in sufficient
detail to permit cost estimates. The diagrams presented describe both the
construction and function of the management practices. Cost estimates are
based on the assumed details of the management practices by using unit costs
from sources such as the current edition of Means Construction Cost Handbook.
Present-day costs in 1986 dollars are presented for direct-cost items (e.g.,
earth moving, compaction, and surveying) and indirect costs (e.g., engineer-
ing design, contingency, insurance, and bonds).
INCORPORATION OF FRENCH DRAINS IN LEACH PADS
The three main sources of potential cyanide release at gold/silver heap
leach operations are the barren solution pond, the pregnant solution pond,
and the ore heap. As discussed in Section 3, current industry practice
51
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includes the use of impermeable pads and liners in the construction of each
of these three facilities. As insurance against potential cyanide releases,
redundancies and overdesign of these liner systems may be appropriate in some
cases. The need for such redundancies, however, must be determined on a
site-specific basis.
In the design and construction of the leach pads, one management prac-
tice considered entails installation of a system of French drains beneath the
leach pad. Pads are currently constructed of compacted clays, asphalt, or
synthetic liners. Placement of a drainage blanket with leachate detection
and collection capabilities between the subbase of native soils and the leach
pad would allow the operator to determine if the integrity of the pads had
been breached.
One active leach operation (at Pinson Mining Company near Winnemucca,
Nevada) has installed a system of French drains beneath their leach pads.
Individual Teach pads at this site measure about 300 ft by 300 ft and are
constructed of compacted clays and silts obtained from nearby borrow pits.
Schematic diagrams of this system are shown in Figure 11. The drain systems
beneath the leach pads are individually monitored to determine, if any pads
are leaking. Ultimately, as many as 60 contiguous pads with French drains
may be constructed at this site. In this particular liner system, a layer
consisting of 12 inches of free-draining gravel is placed between the clay
leach pad and the compacted subbase. This drainage layer is sloped, as is
the pad, at a 5 to 6 percent grade. Slotted, 2-inch, Schedule 40 PVC pipe is
installed in the gravel along the two downgrade sides of the pad. Seepage
through the pad would flow preferentially through the gravel alona the sur-
face of the compacted native soil subbase to the slotted collection pipes and
then to a collection sump accessed by a manhole. The sump is periodically
checked for the presence of seepage. If any seepage is found (none has been
at this site), it can be pumped from the sump to the pregnant collection
pond. If leakage is significant, the pad could be taken out of service and,
if possible, repaired.
Another example is at Superior Mining Company's Stibnite Project, which
is situated in a National Forest, near surface water, and over shallow ground
water. This site incorporates a seepage collection drain pipe sandwiched in
gravel between an upper liner constructed of 3-inch-thick asphalt and a lower
31
liner of 30-^m'l PVC (Figure 12). This perforated drain pipe collects any
52
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ELEV. VIEW
in
to
COMPACTED CLAY OR
SILTY
SLOPE - S-g%
LEACH PAD
DRAINS
PLASTIC PI PC TO MAN-
HOLE, OR rRENCH DP A IN
TO NEXT PAD
COLLECTION
TCH^
• ' ' .•*
FRfHCH DRAIN tO NEXT PAD OH
PLASTIC PIPE -NON SLOTTED- TO
MONITORING STATION
PLASTIC PIPE DETAIL
GRAVEL
COMPACTED SOIL
SECTION B-B
riLTEH FABRIC
PL ASTIC PIPE
GRAVEL
TOP AND
' V/'
5'0f5>|
PLASTIC
'BENTONITIC CLAY OR OTHER
SftLtR
COLLECTION DITCHES
MHHHOLE OR OTHER
COLLECTION SUHP _
(CONCRETE .PLASTIC,
OR STEEL )
Figure 11. Specifications of French drains incorporated in the Pinson Mining Co. leach pads.
-------
Wethtd
Protective
S9 mil P.V.C. Llnvr
OW T«llhl«i
l Oreund
TYPICAL BUILD-UP
Figure 12. Design of leach pad leak detection system at the
Superior Mining Co. Stibnite Operation.
Source: Ref. 31.
54
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solution seeping through the asphalt layer and discharges it to the pregnant
solution pond. A second drain pipe, placed 2 feet below this pipe and be-
neath the PVC liner, collects ground-water flow from beneath the pads and
from adjacent hillsides. This flow is discharged outside the plant area.
Both drains are sampled for free cyanide.
Without a system of drains (i.e., a leakage detection and collection
system), ground-water monitoring or possibly the use of lysimeters beneath
the pad would be the only way to determine leakage through the leach pad.
Depending on the location of the monitoring wells and the hydrogeology of the
site, considerable time could elapse before leakage is detected. By the time
leakage is detected by ground-water monitoring, the ground water and native
soils will already have been impacted. The use of French drains allows imme-
diate detection of leakage through the pad. Such a system could be placed
beneath liners constructed of clay, asphalt, or synthetic materials. The
drain system would have to be constructed so that it could support the weight
of the heap without failing and so that it would not compromise the integrity
of the leach pad as a result of deflection or settlement of the drain blanket
during the loading of the pad with ore (or afterwards). Such a system could
only be placed during the construction of the pad; it could not be retrofit-
ted to heaps during the operational phase. Many sites, however, conduct
leaching on multiple pads that have been constructed at different times over
the life of the operation. For example, French drains could be incorporated
into new leach pads that are constructed at an existing site.
A conceptual system was considered for evaluation of a system incorpo-
rating French drains. Because of its proven design, a system such as that
developed and implemented at the Pinson operation was chosen. The cost
developed for the system was based on standard unit costs for each component.
Preparing the cost estimates in this manner permits a comparison of the costs
of a system with and without the French drains. The specific construction
activities and estimated costs for these systems are shown in Table 5.
DOUBLE LINERS FOR PROCESS SOLUTION PONDS
As indicated in Section 3, the standard practice in the heap leach
industry is the use of single synthetic liners in the pregnant and barren
solution ponds. A layer of compacted clay is often placed beneath the
55
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TABLE 5. COST OF CONSTRUCTING A CLAY
LEACH PAD WITH AND WITHOUT A FRENCH DRAIN SYSTEM8
(1986 dollars)
Cost item
Direct costs
Site preparation - clear and grub
Remove and stockpile 6 in. of topsoil
Remove 12-in. layer of soil
Purchase and place 12 in. of gravel
Install drain pipe
Install 18-in. sump
Level with a blade
Compact base - three roller passes
Excavate and haul clay for 6-in. lift
Place clay layer
Add moisture and compact
Construct 2nd and 3rd lifts
Unit cost,
$/unit
0.29/yd2
1.45/yd3
1.45/yd3
16.16/yd3
1.72/ft
23.05/ft
0.44/yd2
51/h
7.81/yd3
1.34/yd3
1.21/yd3
Subtotal direct costs
Indirect costs
Total cost
Pad cost/ft2
Without
French
drain, $
2,900
2,500
-
-
-
-
4,400
410
13,300
2,300
2,060
35,300
63,170
20,200
83,400
0.93
With
French
drain, $
2,900
2,500
5,000
53,900
930
50
4,400
410
13,300
2,300
2,060
35,300
123,050
39,400
162,000
1.80
3 Pad is 300 ft x 300 ft.
Indirect costs are assumed to be 32 percent of direct costs.
56
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synthetic liner, and some operations have constructed process ponds with
double liners and leak detection. The process solutions themselves are a
product, whereas EPA considers any solution seepage or leakage from the ponds
to be a waste. Because release of leach solutions represents a loss of
valuable product to the operator, the goal of no release is as important from
a production standpoint as it is from an environmental viewpoint. The alka-
line process solutions contain significant concentrations of free cyanide;
therefore, the incorporation of some additional redundancies or overdesigns
in the construction of these solution ponds may be warranted at some sites.
Although the standard practice is to use single liners, some sites
already have double liners in their ponds (usually synthetic liners over
compacted clay). The State of California considers the ponds at leaching
operations to represent "threatening discharges" and therefore requires the
use of double liners. This control, therefore, is both technologically
feasible and a demonstrated practice.
A double-liner system that consists of two layers of 40-mil HOPE sepa-
rated by a leachate detection and collection system was evaluated. In prac-
tice, the bottom liner may be constructed of native or modified clays, if
sufficient quantities of suitable material exists on or near the site. The
synthetic liners, however, were chosen to standardize the system. The use of
synthetic liners also prevents the problems associated with assuring that a
clay liner meets the permeability requirements (i.e., 10" cm/s) at the time
of construction and over the life of the facility. A drainage blanket is
placed between the liners; this can be a layer of free-draining sand or
gravel or a synthetic drainage blanket. The latter was used in the evalua-
tion. A leachate detection/collection system is installed in the bottom of
the pond. This system of perforated pipes in the drainage blanket leads to a
sump that can be accessed to determine if the upper liner has failed. If
failure has occurred, the leachate can be removed from the sump. Details of
the example liner systems are shown in Figures 13, 14, and 15.
Costs of the liner systems are detailed in Table 6. For the purpose of
comparison, costs associated with a single-liner system (believed to be the
industry standard) are also included. Dimensions of the pond are the same in
both systems. The cost comparison indicates the double-liner system increas-
es the cost of the pond by a factor of at least 2. The cost of constructing
57
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40 mil HOPE LINER
6 in. CLEAN FILL
NATIVE SOIL
SINGLE LINER SYSTEM
UPPER 40 mil HOPE LINER
GRAVEL
NATIVE SOIL
LEACHATE COLLECTION LAYER
LOWER 40 mil HOPE LINER
6 1n.
COMPACTED FILL
(951 PROCTOR)
4 in. PERFORATED PIPE
DOUBLE LINER SYSTEM SHOWING LEACHATE COLLECTION
Figure 13. Cross sections of single- and double-
liner systems used to develop cost estimates.
58
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LINER .ANCHOR
TRENCH
TOE OF SLOPE
RADIUS « 3 ft
BACKFILL v \ /LINER X
T
2 ft
1 ft
150 ft
30 ft —H
CROSS SECTION
NOT TO SCALE
Figure 14. Design of process solution pond with single 40-mil
HOPE liner used for cost estimating.
59
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300 ft
LINER ANCHOR
TRENCH
jf
LEACHATE DETECTION
x DRAIN
OAr\ ** h.
-------
TABLE 6. COMPARISON OF COSTS OF PROCESS SOLUTION PONDS CONSTRUCTED
WITH SINGLE AND DOUBLE LINERS
(1986 dollars)
Cost item
Direct costs
Pond excavation
Anchor trench excavation
Backfill of anchor trench
Drain excavation
Placement of 6-in clay bed
Primary liner (40-mil HOPE)
Secondary .liner (40-mil HOPE)
Drainage blanket (HOPE)
2-in. Schedule 40 slotted drain
Sump and 2-in. connector
Gravel backfill for drain
Unit cost,
$/um"t
0.97/yd3
2.99/yd3
0.96/yd3
2.99/yd3
1.17/yd3
0.55/ft2
0.55/ft2
0.25/ft2
1.72/ft
-
9.31/yd3
Subtotal direct costs
Indirect costs
Total cost
Single liner
system, $
12,600
210
70
-
1,060
28,300
_
-
-
-
-
42,200
13,500
55,700
Double liner
system, $
12,600
420
130
240
1,080
28,600
29,300
12,300
410
460
480
86,000
27,500
113,500
a Indirect costs are estimated to be 32 percent of direct cost.
costs include engineering, design, and contingencies.
Indirect
61
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the pregnant and barren solution ponds at a site can represent a significant
percentage of the total capital cost of the operation. The impact of a
double-liner requirement on the viability or profitability of heap leaching
has to be determined on a site-by-site basis. The additional cost of double
liners will definitely increase the cost to produce each ounce of gold. The
amount of such increase depends on processing rates, recovery efficiencies,
ore grade, and other site-specific parameters.
ALTERNATIVE LIXIVIANTS
Cyanide is the only lixiviant currently used at commercial precious met-
al heap leach operations. Because of the actual or assumed environmental
problems associated with cyanide, the question of the availability of suita-
ble substitutes for cyanide is raised. The development of alternative lixiv-
iants that could replace cyanide in heap leach operations is still in the
laboratory or pilot-scale testing stage. Pilot tests of some alternative
lixiviants reportedly have been performed with some success in Australia, New
32
Zealand, South Africa, Taiwan, and the U.S.S.R. Publications were found
33 34
concerning the use of ammoniacal thiosulfate, malononitrile, and thio-
urea ' as alternatives to cyanidation; however, these alternatives are
experimental and require additional developmental research on a commercial
scale before they realistically could be considered. Thiourea, which has re-
ceived the most attention and serious examination, is discussed in more
detail in the following paragraphs.
Thiourea [CS(NH)2]2 is an organic compound derived from urea. Thiourea
crystals dissolve in water to yield an aqueous form that is stable in acidic
32
solutions. In aqueous solution, it reacts with certain transition metal
32
1ons to form stable cationic complexes. Extraction of gold by thiourea
requires that the pH of the leach solution range between 0.5 and 2.0, as j
opposed to the alkaline pH (9 to 11) necessary for cyanide leaching. Ex-
perimental results indicate that the leaching ability of thiourea is signif-
icantly reduced at a pH above 2.0. Sulfuric or nitric acids can be used to
acidify the leach solution. The thiourea reaction with gold also requires
the presence of a condensed-phase oxidant, usually a ferric iron compound.
In cyanide leaching, the oxidant is gaseous atmospheric oxygen. The dissolu-
tion reactions for thiourea and cyanide leaching are as follows:
62
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Cyanide: 2Au + 4NaCN + *02 + H20 «• 2Na[Au(CN)~] + 2NaOH (Eq. 3)
Thiourea: 2Au + 4TU + Fe2(S04)3 - [Au(TU)2]2 S04 + 2FeS04 (Eq. 4)
2Ag + 6TU + Fe2(S04)3 * [AgCTU)*^ S04 + 2FeS04 (Eq. 5)
The effectiveness of thiourea leaching is controlled by the amount of
thiourea in solution, the leaching time, and the amount of trivalent iron
(Fe ) present. One of the main advantages of thiourea is that it can leach
gold from ore in a matter of hours instead of the days required for leaching
with cyanide. Another advantage is that thiourea does not form complexes
32
with cations present in the gangue mineralogy as readily as does cyanide.
One major problem with thiourea leaching is that the reagent cost is
approximately 25 percent more than with cyanide leaching. Another problem
is the amount of reagent consumption through oxidation; oxidation must be
controlled to prevent excessive reagent consumption. Also, the intermediate
product of oxidation of thiourea is formamidine disulfide, which can coat the
ore particles and prevent fresh thiourea from reacting with the available
gold. The addition of sulfur dioxide gas can control the oxidation and
restore thiourea at the expense of formamidine disulfide. Still another
potential problem is that thiourea leaching requires a very acidic environ-
ment; therefore, the heap may' have to be neutralized during closure. Native
sulfur that is produced from the decomposition of thiourea and left in the
leach residue could create acidic leachate during precipitation events,
which, in turn, could mobilize metals in the post closure period.
As an example of a conceptual application of thiourea leaching, NERCO
Minerals estimated that for every ton of ore treated at its Candelaria Mine,
178 Ib of H2S04 and 8 Ib of H2N03 would be required to achieve the acidic pH
necessary to use thiourea as the lixiviant. (Current cyanidation leaching
consumes 2.5 pounds each of sodium cyanide and lime per ton of ore.) They
further estimate that the cost of thiourea reagent alone at the Candelaria
operation (a silver heap leach operation) would be $53 per ounce of silver
recovered. Based on current silver prices of about $5 per ounce, thiourea
obviously is not a viable option at this site.
Pyper published results from experiments performed on comparisons of
•}C
thiourea and cyanide leaching. Variables in the experiments were pH,
temperature, time, and the concentrations of thiourea and ferric iron. The
63
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ore treated was a classic Carlin-type ore (a dolomite viitstone in a quartz
matrix with finely disseminated gold) obtained from the Northumberland mining
area in south-central Nevada. The results of these experiments indicate
that thiourea can leach gold from this type of ore with similar or better
yields than cyanide, but at higher cost (no exact amount was stated). Opti-
mum leaching was obtained when 1) temperature was 20° to 25°C, 2) thiourea
concentration was 50 to 100 kg/ton, 3) ferric sulfate was 5 kg/ton, 4) leach-
ing time was 8 to 24 hours, and 5) pH was 1.0.
Thiourea eventually may become an alternative commercial-scale lixiviant
for some applications in precious metals heap leaching if oxidation of the
thiourea can be controlled to reduce reagent consumption and if the cost of
reagents can be reduced. Sulfur dioxide may be a suitable treatment for pre-
venting the oxidation of thiourea. Thiourea rapidly leaches gold from ore in
pilot and bench tests; this speed of leaching may help to offset the differ-
ence in cost between cyanide and thiourea leaching in some situations. How-
ever, the technology of recovery of precious metals from thiourea complexes
is not well developed. The majority of gold ores in the United States con-
tain carbonates that would require that prohibitive amounts of acid be used.
Before commercial scale applications are made, several environmental concerns
related to thiourea must be addressed fully. For example, extremely acidic
solutions may require additional or different composition liners; the need
for post-leach neutralization must be addressed; the toxicity and mobility of
Thiourea and its degradation products (e.g., formadine disulfide) must be
evaluated; and the possibility of acid formation within the heap after clo-
sure with subsequent mobilization of metals must be studied.
LEACHATE DETECTION
As indicated earlier, gold heap leaching operations incorporate three
primary operational units that may have a direct environmental impact on sur-
rounding soils, geology, and ground-water quality: the barren solution pond,
the heap leaching pad and associated solution collection systems, and the
pregnant solution pond. The primary constituents of concern are cyanide and
metals associated with the leach ore. Although each operation has unique
process characteristics and design features, all operations also have several
common design features, which include synthetic liners in the barren solution
pond, low-permeability heap leaching pad liners and solution collection
64
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systems, and synthetic liners in the pregnant-solution pond. The primary
purposes of these design features are to manage solution losses and fluid
balances as an integral part of the operation and to reduce the potential for
soil and ground-water contamination.
In addition to operational management practices, corrective actions may
have to be implemented to control prior releases or newly discovered releases
of contaminants into the surrounding environment. Corrective actions may be
required at any point in the operational or closure phase of a facility life.
The monitoring of soil and ground water is often fundamental to determining
the need for the implementation and establishment of the design features of
such corrective actions.
Our evaluation of gold heap leaching operations uncovered little infor-
mation on recently implemented facility-specific monitoring systems or cor-
rective actions. Therefore, data are sparse on how effective current opera-
tional techniques are in preventing uncontrolled releases or in protecting
the environment. However, some form of leachate detection currently is
practiced at many sites. For example, the State of Nevada requires that
observation ports be installed beneath the liners of solution ponds to allow
rapid detection of liner failure. Similarly, the State of California re-
quires monitoring of groundwater and the vadose zone. Little data were found
on monitoring systems or corrective actions applied specifically to gold heap
leaching operations. Consequently, several conditions relative to monitoring
system design and corrective actions must be assumed. These assumptions are
reflected in the following discussion on ground-water monitoring.
A key assumption concerns the meteorological and hydrogeological setting
in which the majority of heap operations are located, i.e., an arid environ-
ment with deep ground-water tables. Costs also must be considered in the
planning and design of a ground-water monitoring system and any necessary
corrective actions. Because no actual cost data were available on the design
and implementation of monitoring systems at gold heap leaching facilities, it
was necessary to rely on data obtained from ground-water monitoring systems
installed at land disposal facilities for the development of cost informa-
tion. In the evaluation of these cost data, a range was developed (whenever
possible) to reflect the effects of gold heap leaching operations and typical
environmental and hydrogeologic setting on a ground-water monitoring program.
65
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The effective implementation of any ground-water monitoring program re-
quires a fundamental understanding of the surrounding geology and hydroge-
ology and the operational characteristics of the gold heap leaching facility.
The purpose of the monitoring system is to detect and assess leachate produc-
tion resulting from the gold heap operation (barren solution pond, heap pad,
and pregnant solution pond) and to characterize the pathways for contaminant
transport.
Information on the geologic and hydrogeologic setting of the gold leach-
ing operation should be evaluated on both a regional and site-specific scale.
The data should include:
0 Nature, history, and location of the leaching operation
0 Characteristics of the ore and deposition practices
0 Size and location of the solution ponds and leach pads
0 Design features of the solution ponds and leach pads
0 Current surface- and ground-water characteristics and uses in
proximity to the facility
Historical precipitation records and any existing geologic and topographic
maps also should be compiled.
Data on the materials used in the gold leaching operation help in the
identification of the characteristics of potential contaminants and their
likely routes of migration. The primary constituent of concern is cyanide;
however, heavy metals present in the ore also may be a concern. The poten-
tial for release of these constituents is a function of the effectiveness of
the pad collection system and liners to contain and collect the process
solutions. The extent to which these constituents migrate in the surrounding
soils and geologic units depends on the composition of the soils and the
structural features of the bedrock. Because most gold heap leaching
operations are located in the arid West, the depth to ground water may be
significant, and the effects on the hydrogeologic system may take several
66
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years to detect. Also, the high evapotranspiration rates may retard the
downward transport of released contaminants and cause them to be confined
primarily to the vadose zone. Detection monitoring in the vadose zone is an
emerging science, and is often difficult and expensive to implement and
maintain.
Monitoring wells are generally used to obtain site-specific data on the
geologic and hydro!ogic characteristics of the site and to detect contami-
nants that may have been released to the subsurface environment. Because i
data were obtained on gold heap leaching operations that have actually in-
stalled ground-water monitoring networks around their process units, little
perspective exists to assist in establishing a proven design for a generic
monitoring system. In the establishment of monitoring well locations and
screen depths, however, consideration should be given to identifying back-
ground water quality, transport pathways, environmentally sensitive areas,
local and regional receptors, and the water-bearing zones to monitor. The
approximate number and location of monitoring wells will depend on the number
and complexity of the gold heap leaching process units, their size, the
characteristics of the surrounding surface, and the subsurface environment.
A typical monitoring network consists of a series of nonpumping wells
located downgradient from the solution ponds and heap leaching pad. In addi-
tion, at least one well is located upgradient in an area that has not been
affected by potential contaminant migration. The number and the complexity
of the well network is purely a site-specific determination. Figure 16 shows
an example system for a small operation. Water quality studies around
similar ore-processing operations have entailed the use of both new and
existing wells. These networks have included existing water supply wells,
multidepth well nests, and single large-diameter monitoring wells.
67
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A MONITOR WELLS
BARREN SOLUTION
A POND
1
L
100 ft
J SCALE
CD
SLOPE OF NATIVE
TOPOGRAPHY
5%
A
COLLECTION
DITCH
ORE HEAP
(5 ACRE)
PREGNANT
SOLUTION
POND
I
A
li
A A A—
COMPLIANCE
POINT
Figure 16- Conceptual application of a detection monitoring system
at a small heap leach operation.
-------
A ground-water monitoring program will initially be designed to detect
contamination. If ground-water contamination is detected, additional wells
may then be installed to define dispersion and attenuation of the contami-
nants. If improperly pursued, this process can be expensive and time-
consuming; therefore, such a program should be balanced against the potential
health and environmental impacts of gold heap leaching operations.
The depth of monitoring wells depends on the depth and characteristics
of the underlying aquifer and the vertical spread of potential contaminants.
At gold heap leaching operations, the depth of wells probably will vary
considerably (e.g., from 25 to 300 ft.); however, in the arid regions of the
southwest, such wells are frequently deep.
Sizing of monitoring wells will be a function of the flow rates and
depth of the aquifer and proposed sampling methods. Well diameters may vary
from 2 inches to 6 inches. The most common well is 4 inches in diameter and
placed in a 6- to 8-inch annul us. Drilling methods are determined on the
basis of the geologic formation to be penetrated, the depth and size of the
hole, and the potential for contamination as a result of the drilling itself.
Methods and materials used during well construction should not interfere with
ground-water quality. Methods outlining well construction and materials ap-
plication are provided in EPA's "Draft RCRA Ground-Water Monitoring Technical
Enforcement Guidance Document."
The cost for installing a monitoring well system at a gold heap leaching
operation will vary greatly from site to site. The primary factors that
influence costs are the size of the operation and the complexity of the local
hydrology. The characteristics of the ground water, the extent of contamination,
the availability of supplies and equipment, and local wage rates will also
affect the cost.
69
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Installation costs include the costs of drilling, well materials, crews,
and equipment, all of which will be affected by the conditions under which
the well system must be installed. The principal factors are the diameter,
depth, and components of the well; the drilling specifications; the geologic
material; the sampling requirements; and site access. Table 7 presents some
typical unit costs for drilling and installing well systems.
TABLE 7. 1986 COSTS FOR DRILLING AND INSTALLING
2- TO 4-INCH DIAMETER WELLS3»D
Drilling method
Conventional hydraulic rotary
Reverse circulation hydraulic rotors
Air rotary
Auger (hollow-stem)
Bucket auger
Cable tool
Hole
e puncher (jetting)0
f-jettinqc
Self-jetting
Mobilization
Cost, $/ft
24 to 39
34 to 44
17 to 24
11 to 21
10 to 20
15 to 17
39
21
488 to 586/rig
a Source: U.S. Environmental Protection Agency. Remedial Action at Waste
Disposal Sites (revised). EPA 625/6-85-006, 1985 (modified).
Includes drilling, well material, and installation costs.
Includes rental of all necessary equipment, e.g., well points, pumps, and
headers.
Based on the example site shown in Figure 16, an assumed cost of $50 per
foot for well installation, and our best engineering judgment, the cost of
installing a system of 10 to 13 wells to depths of 25 to 300 feet is
estimated to be $12,500 to $195,000. Consultant fees for a qualified hydro-
geologist could be expected to range from $6,000 to $50,000. Analytical
costs (assuming $150 per sample, semiannual sampling, and quadruplicate sam-
ples) would amount to about $12,000 to $16,000 plus the cost of reporting and
recordkeeping. These costs point up the great variability due to site-
specific conditions. The number of wells required and sampling and analyti-
cal costs will vary significantly from site to site. As pointed out earlier,
70
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if this detection monitoring were to indicate the presence of contamination,
assessment monitoring probably would be required, and installation of such a
system would entail significant additional costs.
CYANIDE DESTRUCTION
During the closure and post-closure periods, the heap leach residue is
the only potential source of cyanide contamination. Current permitting re-
quirements state that process solutions present in the barren and pregnant
solution ponds must be evaporated to dryness, or be treated to destroy cya-
nide and then released if evaporation is not possible. The current practice
at most sites is to rinse leach residue with fresh water. The fresh water
rinse is applied with the same distribution system used during leaching.
2
Application rates are also the same (e.g., 0.005 gpm/ft ). The rinse time
may be predetermined, as would be the case when leach residue is removed from
reusable pads, or it may continue until some preset cyanide concentration
(e.g., 0.2 mg/liter) or pH (e.g., pH 8.5) in the leachate is achieved.
Rinsing typically lasts from 1 day to a week or more; however, no published
information is available on the effectiveness of this practice.
As indicated earlier, very few data are available on the concentration
and fate of cyanide in heap leach residue. The need for post-leaching cya-
nide destruction must be assessed on a site-specific basis. For example,
cyanide destruction has been incorporated into the post-leaching operation of
the Superior Mining Company operation described later in this section.
One of the control options evaluated during this study is the addition
of a cyanicide, a strong oxidant, to the rinse solution at the time of clo-
sure. The addition of a cyanicide would help to control the amount of free
cyanide left in the leach residue that could escape to the environment.
However, some stable cyanides such as the iron complexes may not be destroyed.
A variety of processes have been demonstrated to be effective in destroying
cyanide or in removing it from the solution. These treatments include natural
degradation, evaporation, alkaline chlorination, oxidation with sulfur dioxide
air, biodegradation, oxidation with hydrogen peroxide, adsorption on ferrous
sulfide, acidification-volatilization, ozonation, ion exchange, chemical
precipitation, electrochemistry, reverse osmosis, ion flotation, adsorption
on activated carton, electrodialysis, high-pressure oxidation, photolysis,
71
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38
and polymerization. Some of these methods are applicable to the treatment
of cyanide-containing solutions from conventional cyanidation processes, and
some may be effective in destroying residual cyanide contained in heap-leached
material.
The form of the cyanide present in heap leach residue depends on the
mineralogy of the ore. The most likely types of cyanide species are listed
in Table 8 in the order of increasing stability. The stabilities of the
compounds are important in determining an effective treatment and the quanti-
ty of reagent required. Leach residue probably will also contain significant
quantities of thiocyanate, which requires an additional amount of reagent in
38
most oxidation-type cyanide destruction reactions.
TABLE 8. CYANIDE COMPLEXES LIKELY TO BE PRESENT IN LEACH RESIDUE3
Terms
Forms of cyanide
1. Free cyanide
2. Simple compounds
a) Readily soluble
b) Relatively insoluble
3. Weak complexes
4. Moderately strong complexes
5. Strong complexes
CN", HCN
NaCN, KCN, Ca(CN)2> Hg(CN)2
Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN
Zn(CN)*', Cd(CN)', Cd(CN)*"
", Ag(CN)~
Cu(CN)~,
Fe(CN)*', Co(CN)*
Source: Reference 38.
Natural degradation, evaporation, and alkaline chlorination are the
cyanide reduction and destruction processes that have been associated with
heap leach residue. Natural degradation and evaporation are the most common
methods of destroying any cyanide left in the residue after removal of solu-
tion from leach residue following the water rinse. Natural degradation of
cyanide results from a combination of physical, chemical, and biological
processes. These processes include volatilization, photodecomposition,
72
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chemical oxidation, microbial oxidation, chemical precipitation, hydrolysis,
38
and adsorption on solids. Volatilization is the most important process.
Vlhen exposed to air, HCN vaporizes from cyanide solutions, especially if the
39
pH is below 10.5. As the pH of entrained solution falls from operating
39
levels, cyanide volatilization occurs. The effectiveness of natural degra-
dation depends on the species of cyanide present, pH, temperature (which
affects reaction kinetics), bacterial metabolism, photodegradation, and
38
aeration. If sufficient time is allowed, natural processes will return
heap leach material to acceptable conditions.
Because most heap leach operations are located in arid regions (e.g.,
the ratio of evaporation to precipitation can be as high as 10 to 1), evapo-
ration plays a major role both in removing solution from the heap and in
determining if any leachate will be formed.
The alkaline chlorination process for cyanide destruction is a proven
technology, -and it is the most highly developed of all available methods in
terms of experience, simplicity, control, availability of equipment, and
engineering expertise. This process destroys most cyanides except iron
cyanide and the more stable metal!o-cyanide complexes, which may be the most
prevalent forms in leach residue. Calcium hypochlorite, sodium hypochlorite,
or chlorine gas, plus lime or caustic to maintain an alkaline pH can be used
in solution to treat heap leach residue. The oxidation of cyanide by calcium
hypochlorite occurs in two stages:
2NaCN + Ca(OCl)2 + 2H20 * 2CNC1 + Ca(OH)2 + ZNaOH (Eq. 6)
CNC1 + ZNaOH * NaCNO + NaCl + H20 (Eq. 7)
Chlorine is consumed by other oxidizable substances present in the leach
residue. A dramatic increase in reagent usage will occur if thiocyanate is
38
present, as is usually the case.
As indicated in the preceding equations, a significant quantity of
chloride results from the destruction of cyanide. Chloride is soluble,
highly mobile in the environment, and is conservative; that is, it is not
attenuated or degraded. The potential therefore exists for chloride
contamination in groundwater or surface water as a result of efforts to
destroy cyanide.
73
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Three references found in the literature document the content and fate
of cyanide in heap leach residue and the effectiveness of treatment with
chlorine. Englehardt reported on the natural degradation of cyanide in an
23
inactive leach heap at an Anaconda operation at Darwin. At this facility,
84,000 tons of agglomerated tailings were leached in heaps stacked 15 ft
high. During the 6 months of operation, about 105,000 pounds of NaCN was
23
applied to the heaps. About 70 percent of the NaCN applied was consumed
during leaching by the cyanicides in the ore. Most of the cyanide remaining
was removed by the fresh-water rinse during the post operations. About
12,000 pounds of NaCN remained in the entrained solution in the leached
tailings.
Almost all of the cyanide was present as free cyanides. Over the course
of the 15-month study, the moisture content in the heap decreased from 14.4
23
to 13.1 percent. The water-soluble residual cyanide content decreased from
23
0.58 gram/liter to 0.11 gram/liter. The decrease in cyanide was due to
natural degradation, as no treatments were applied during this period.
As an alternative, treatment of the leach residue with alkaline chlorine
23
was tested. Pilot-plant tests indicated that treating the,leach residue
with a calcium hypochlorite solution (0.5 gram/liter) immediately after
leaching was effective in destroying the cyanide left in the heap. Consump-
23
tion of the hypochlorite was 0.6 pound per ton of leach residue. This
treatment was reported to be quick and effective, but it would be expensive
23
in terms of reagents and manpower.
Another reference to hypochlorite treatment of heap leach residue con-
on
cerns the Annie Creek operation in South Dakota. At this site, a hypochlo-
rite solution was flushed through heap leach residue, and then fresh water
was recirculated through the system. In one month, the cyanide concentration
21
in the solution had decreased from 300 to 3.4 ppm. A year later the cya-
21
nide content ranged from 0.2 to 0.01 ppm. The reported information, however,
does not include data on hypochlorite dosage or consumption, on recirculation
rates, or on the original concentration and speciation of cyanide present in
the residue.
Superior Mining Company has operated a heap leach operation incorporat-
ing post-leach cyanide destruction using hypochlorite. The operating
permit issued by the U.S. Forest Service requires cyanide destruction and a
74
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24-hour average free cyanide concentration not exceeding 0.2 mg/liter in the
rinse solution leaving the heap; no single sample can exceed 1.0 mg/liter.
To meet this limit, Superior treats the leach residue with an alkaline chlo-
rine/calcium hypochlorite solution that is prepared on site. At this site,
newly mined ore is crushed to -1.25 inches and treated with lime (1.5 to 2
pounds/ton) before it is stacked on the five leach pads. The heaps are
leached with alkaline cyanide solution for 20 days and then treated for
cyanide destruction for 6 days before being offloaded to a spoil disposal
area on site. The chlorine concentration of the treatment solution is about
1000 ppm free chlorine. Chlorine is consumed at the rate of 4000 pounds per
day, and 8000 pounds of lime per day is required to maintain the high pH that
31
is necessary. After treatment with chlorine, the leach residue is exca-
vated and hauled to the disposal site where it is spread out in layers 1 to 2
feet thick. This allows further oxidation and volatilization of any remain-
ing cyanide and chlorine. No significant accumulation of chloride or heavy
metal salts has been detected. No information was given on cyanide con-
centrations or accumulation in leach spoil prior to treatment.
When this system is used, the facility must incorporate at least one
additional pond (a neutralization pond) in its solution management system
(Figure 17). This neutralization pond receives the solution that percolates
through a heap during the post-leaching neutralization/cyanide destruction
process. Existing pregnant and barren solution ponds cannot be used for this
purpose because they are needed to manage leach solutions being applied to
the active heaps. The same distribution system that is used during leaching,
however, can be used to spray the chlorine solution over the heap. The
system at the Stibnite mine also incorporates a 30-ton lime bin and an auger
feeder to an 8 ft by 8 ft mixing tank having a 15-hp agitator, which produces
the milk of lime (Figure 18). The milk of lime is injected into the pipe
carrying solution from the chlorine pond by means of a venturi injector.
Chlorine gas, regulated by a chlorinator, is then injected into this line by
using another venturi injector. Chlorine gas is supplied by ten 1-ton chlo-
rine cylinders.
An evaluation was made of the cost of installing and operating an alka-
line chlorination system for cyanide destruction during the post-leaching
rinse. The evaluation assumed a system with the specifications of that oper-
ated at the Stibnite facility. The cost of this system, estimated for a
75
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Figure 17 . Example site layout showing additional pond
required for cyanide destruction/neutralization.
TON CYUHDERS-^
(TYP. OF 10)
MIXING TANK W/
IftHP AQrTATOR
«.,
iNWALT —-
LORWATOR
.K OF LAIC
IM
Mi
^m
•^
••
••
•^
mm
•M
^
»6HP
•OOSTER PUMP.
VENTURI INJECTOR
CUt QAS
4000 LA/DAY
Figure IB. Example process flow diagram
of cyanide destruction circuit.
Source: Ref. 31.
76
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range of processing rates, is shown in Figure 19. The system specified is
sized to produce 200 to 350 gpm of alkaline chlorine solution, a volume
sufficient for application over 0.9 to 1.6 acres at a rate of 0.005 gpm/ft2.
Heaps with much larger surface areas and solution flow rates are common. To
treat a large heap (e.g., one with 20 acres of top surface) would require
that the capacity of the example alkaline chlorination system be increased by
a factor of 20 to allow treatment of the entire heap at one time. As an
alternative, a smaller system could be used to apply the treatment solution
at a slower rate for a longer time or at the same rate in sequential applica-
tions over small portions of the heap.
In the case of multiple-use pads, a system such as that used at Stibnite
would be in constant use over the life of the operation. At sites using
single-use pads, however, such a system may only be applicable at closure.
In addition, at operations having single-use pads, the leach residue would
not be excavated and spread out so that additional oxidation of cyanide and
chlorine could take place. If cyanide destruction occurs only at closure,
the existing process solution ponds, distribution system, and lime-addition
facilities could be used. In this case, only the capability of adding chlo-
rine would have to be provided.
All of the essential elements of the solution-handling system necessary
to effect cyanide destruction by alkaline chlorination of heap leach residue
during closure are already present at each operation. Solution storage res-
ervoirs, pumps, distribution pipes, and lime addition equipment used during
leaching operations could also be used during alkaline chlorination. Calcium
hypochlorite would just replace cyanide in the flow scheme. Thus, the cost
of this treatment represents the cost of the reagents and the manpower to
operate the system.
CAPPING
The application of a clay or synthetic cap over a waste disposal unit is
required in the closure of hazardous waste facilities. The purpose of this
cap is to prohibit infiltration of rainfall or surface water run-on and
thereby to preclude formation of leachate. During the post-closure period at
heap leach operations, only the leach residue remains as a potential source
of cyanide contamination through leachate generation. Capping this residue
77
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1500
o
o
o
10
o
1000
800
600
500
400
300
200
TOO
j_
300 400 500 600 800 1000 1500
FACILITY SIZE, 1000 tons ORE PROCESSED PER YEAR
2000
Figure 19. Capital and annualized costs of cyanide neutralization
system at a gold leaching facility.
78
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pile could reduce this potential. As indicated earlier in this report,
however, the actual need for such control has not been documented because
little is known regarding the content and fate of cyanide in leach residue.
Also, capping would hinder the natural degradation of cyanide remaining in
leach residue by limiting volatilization.
Cyanides remaining in the leach residue could degrade to mobile forms
over time; however, the leachate formation potential is low at many sites.
For example, at the Darwin operation in Southern California, evaporation
(approximately 70 inches/yr) greatly exceeds precipitation (3 to 7 inch-
es/yr). Leachate formation potential is also low in the individual heaps at
the Pinson Mining operation, which typically retain about 250,000 gallons of
solution before their field capacity is reached. The approximate surface
o
area of the top of each of these heaps is about 114^000 ft . For a heap to
reach field capacity and start producing leachate as a result of a precipita-
tion event in the post-closure period would require that more than 3.5 inches
of rain fall on the heap and that no losses to evaporation occur. The 100-
year, 24-hour, storm event for this area is 2.0 inches; thus, it is possible
that such conditions may preclude leachate generation. Sixty of the 79 heap
leach operations in the United States are located in semi arid regions of
Nevada, where capping to reduce infiltration may not be necessary. Some
operations, however, are located in areas that receive considerable rainfall
(South Carolina) and precipitation in the form of snowfall (Montana). At
these locations, capping at closure may be a control option.
Heaps are constructed with steep (1:1) side slopes, which are usually
dictated by the natural angle of repose of the ore. Heaps are stacked as
steeply as possible to maximize the use of pad area. Such steep slopes
cannot be capped. The slopes would have to be reduced to at least 3:1 for
capping to be applicable. The cost of slope modification would vary with the
original size and height of the heap. For example, regrading a 1-acre heap
having 1:1 slopes and a height of 15 ft would require movement of 1700 yd of
material at a cost of about $2500. After slope modification, the surface
area of the heap would be 1.4 acres. Application of a cap consisting of 3
feet of earth would require 6800 yd of cap material. Assuming a suitable
source could be found near the site, the cap could cost about $36,000. A
large heap (e.g., one covering 50 acres and having a height of 100 ft) would
79
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o
require the regrading of 550,000 yd of material. The modified heap would
require capping to be placed over 67 acres at a total cost of $1.8 million.
Although capping can reduce the potential for leachate formation by
limiting infiltration, it also reduces the rate of natural degradation of
cyanide left in the heap leach residue. Because the cap would limit natural
aeration of the heap, it would reduce volatilization of any free cyanides
present.
POST-CLOSURE MONITORING AND MAINTENANCE
After the closure of heap leach operations, the leach residue is the
only potential source of cyanide contamination, assuming that the process
solution ponds have been removed as required. Leach sites in typically arid
climates have less potential for long-term impacts than leach operations
where significant precipitation or surface water flows are present. Monitor-
ing requirements normally also would be less in the dry areas. Monitoring of
ground-water wells installed at the time of closure would be continued
through the post-closure period. The typical RCRA post-closure monitoring
period extends over a period of 30 years. As an alternative, the monitoring
of wells used during the active life of the operation can continue through
the post-closure period if such wells are available. In addition to the
monitoring of ground water, maintenance of the monitoring system, and any
other controls, such as caps or access control fencing, also may be required.
The annual cost of maintaining such systems is usually estimated to be 1 to 5
percent of their capital cost. The cost of monitoring ground water varies
with the number of wells, the required samples, the number of duplicates,
analytical parameters, etc. The cost of this monitoring will vary widely and
depends primarily on the number of wells. For example, the cost to monitor a
relatively small site having eight wells around the heap would be about
$6400/yr for analytical services plus the costs of reporting and
recordkeeping.
80
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SECTION 6
SUMMARY OF FINDINGS
The information gathered and evaluations made during this study of
gold/silver heap leach operations are summarized in the following four sub-
sections paralleling the organization of this report.
GENERAL CHARACTERISTICS
Although other industry segments have experienced closures and produc-
tion cutbacks, the practice of gold heap leaching continues to increase dra-
matically. The low production cost (e.g., $200/ounce), short startup time,
and relative simplicity of heap leaching have made it the method of choice
for recovering low-grade deposits. Also, because the production of gold as a
byproduct of copper mining has declined as a result of cutbacks in that
industry, demand must be met by other sources. In 1984, heap leaching ac-
counted for more than 30 percent of total gold production, and the surge of
activity in this industry segment is expected to continue.
Of the 66 currently active gold heap leach operations that were identi-
fied, 47 are located in Nevada. In fact, Nevada accounts for about half of
the total gold produced in the United States. Heap leach operations are
scattered throughout this sparsely populated, typically arid State. With the
exception of one operation in South Carolina, the remaining operations are
also located in Western States. Exploration activity, however, is occurring
throughout the country.
DESIGN AND OPERATION
The basic design and the operational layout of heap leach projects are
similar at all facilities; however, site-specific conditions (especially ore
mineralogy) control specific practices and applications. An alkaline cyanide
81
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solution is used universally as the lixiviant in heap leaching; however, spe-
cific ore treatment, leaching time, reagent use, flow rates, heap construc-
tion, pad specifications, liner materials, and other site-specific parameters
depend on the ore mineralogy, climate, topography, hydrology, and hydrogeolo-
gy at each site. Heap leach operations are relatively small, discrete opera-
tions compared with other mining operations.
The use of liners to prevent loss of process solutions is standard
industry practice. Leach pads are constructed of asphalt, clay, or synthetic
materials. The two process solution ponds at each site are lined with syn-
thetic material (e.g., HOPE or Hypalon). Some operations have incorporated
redundancies and overdesigns into their liner systems. These include such
items as leak detection (e.g., French drains) in pad construction and ponds
constructed with double liners and leachate detection systems. The degree to
which these features are incorporated depends on the regulator (e.g., Cali-
fornia requires double liners) and the potential for impact (e.g., many
operations in Nevada are very remote, miles away from any surface water and
situated over deep ground water, whereas operations in other states may be
located near surface and ground waters where advanced systems would be appro-
priate).
TOXICITY AND MOBILITY
An extensive data base is available on the toxicity of various cyanides
and cyanates; however, very little information is available on the quantity
and speciation of cyanides and cyanates present in heap leach residue.
Therefore, it is difficult to assess the potential for environmental impact
that may be caused by leach residue.
- The toxicity of cyanides varies from the acutely toxic free cyanide
(HCN) to some nontoxic iron and cobalt metallocyanide complexes. Thiocyanate
has relatively low toxicity. The types of cyanides and cyanates present in
leach residue and process solution depend on the ore mineralogy. Process
solutions contain significant quantities of free cyanide.
ALTERNATIVE MANAGEMENT PRACTICES
French drains may be incorporated into the design and construction of
leach pads and could provide early warnings of process solution leakage
82
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through the pad. This kind of system has been used at a few active opera-
tions. Incorporation of a French drain system roughly doubled the cost of an
example clay pad. The need for and applicability of such a system must be
determined on a site-by-site basis.
Double liners (synthetic or synthetic over compacted clay) with leachate
detection/collection may be incorporated into the design and construction of
the two process solution ponds used at each site. Double-liner systems
represent a proven technology that could be used at gold heap leach opera-
tions. A synthetic double-liner system costs at least 2 times the amount of
a single-liner system, which is the industry standard. Although the solution
in the pond is not a waste, leakage or seepage is, and double liners would
add another level of protection against failure of the impoundment. In some
settings, full double liner systems might be an unnecessary design redundancy
(e.g., locations over thick clay zones and/or away from ground water).
Currently, no alternative lixiviants are available to replace cyanide.
Research is continuing on alternatives (e.g., thiourea); however, their ap-
plication has not been demonstrated on a commercial scale. Although the use
of other reagents may remove real or perceived problems with cyanide, they
could pose other environmental concerns. Thiourea, for example, requires a
very acidic (pH 1) process solution.
Implementation of increased monitoring of ground water or the vadose
(unsaturated) zone beneath leach pads and solution ponds is a possibility.
Heap leach operations entail three potential sources of contamination: the
leach pad and two solution ponds. After closure, only one source, the heap
leach residue, remains. By mining industry standards, these operations are
small and discrete and do not pose the problems typically encountered in
monitoring very large mining operations. The extent of monitoring at active
operations varies depending on regulatory requirements currently in place,
hydrogeological settings, and the extent of the design of containment systems.
Installation of a ground-water monitoring system at a small example site
having a 5-acre leach pad and two process ponds could be expected to cost
between $12,500 and $195,000 plus consultant fees of between $6,000 and
$50,000. Depth to groundwater is the major variable. These estimates assume
10 to 13 wells with depths of 25 to 300 ft. Lysimeters are less desirable
because of the difficulty in obtaining samples and lower reliability-
83
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Current industry practice includes rinsing of heaps with fresh water at
the completion of the leach cycle or during closure. Limits are set by the
State or other regulators on the amount of cyanide (e.g., less than 0.2
ing/liter) that can be present in rinse water and the pH (e.g., pH 8.5) of the
rinse water leaving the heap. At least a few sites have incorporated alka-
line chlorination during the rinse process to enhance degradation of cyanides.
An example alkaline chlorination system capable of treating 300,000 tons of
leach residue annually would cost about $280,000 for installation. The lack
of data on the effectiveness on water or chlorine rinses or on the cyanide
content of leach residue makes it impossible to assess the need for and
benefits of additional treatment.
A cap could be added to the heap at closure to prohibit leachate forma-
tion. Placement of a cap that would limit infiltration of precipitation into
the heap would require recontouring to lessen the slope of the sides. Cap-
ping may not be applicable at many sites because of the typically arid
nature of the area, particularly those in Nevada. At sites in areas that
receive significant precipitation, however, capping may provide environmental
benefits if potentially mobile cyanides are present in the residue. Capping
costs are in the range of $28,000 per acre, assuming a 1-meter-thick earthen
cap and the availability of suitable cap material on site. Recontouring
heaps to a 3:1 slope could cost as little as $2,500 for a small heap (1 acre,
15 ft high) to $800,000 for a large heap (50 acres, 100 ft high). Costs of
placing the earthen cap on these two example heaps would be about $36,000 and
$1.8 million, respectively.
Post-closure monitoring can be implemented to determine if potentially
mobile forms of cyanide in heap leach residue are causing contamination of
ground water, the unsaturated soil zone, or surface water, if present. In
the post-closure period, only the leach residue remains as a potential source
of contamination. The two process solution ponds typically must be removed
during closure as part of the permit requirements. A ground-water monitoring
system could be maintained during the post-closure period to ascertain if
cyanide is migrating from the leach residue. Annual analytical costs could
be about $6,400 for a small (8 well) system. Post-closure monitoring typically
is carried out over a 30-year period.
84
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REFERENCES
1. Bureau of Mines. Mining and Quarrying Trends in the Metal and Nonmetal
Industries. Preprint from the 1984 Bureau of Mines Minerals Yearbook.
2. Chamberlain, P. 6., and M. 6. Pojar, Gold and Silver Leaching Practices
in the United States. Bureau of Mines. IC8969, 1984.
3. U.S. Environmental Protection Agency. Report to Congress - Wastes From
the Extraction and Benificiation of Metallic Ores, Phosphate Rock, As-
bestos, Overburden From Uranium Mining, and Oil Shale. EPA/530-SW-85-
033. December 1985.
4. Federal Register. 40 CFR Part 261. Regulatory Determination for Wastes
From the Extraction and Beneficiation of Ores and Minerals.
51(128):24496-24502, July 3, 1986.
5. Resource Conservation and Recovery Act of 1986. PL94-580. 94th Con-
gress. October 21, 1976.
6. Solid and Hazardous Waste Disposal Act Amendments 8002(P).
7. Heinen, H. J., D. 6. Peterson, and R. E. Lindstrom. Processing Gold
Ores Using Heap Leach-Carbon Adsorption Methods. Bureau of Mines.
IC8770. 1978.
8. Bureau of Mines. Gold. Preprint from the 1984 Bureau of Mines Minerals
Yearbook.
9. Bhappu, R. B. An Updated Review of the Economics of Gold and Silver
Recovery. Forum 85 - Gold and Silver Recovery. Randol International,
Inc. Santa Fe, New Mexico. October 14-15, 1985.
10. Lewis, A. Leaching and Precipitation Technology for Gold and Silver
Ores. Engineering and Mining Journal, June 1983.
11. In Situ Mining Research. Information Circular 8852. U.S. Bureau of
Mines. August 5, 1981.
12. McClelland, G. E. Testing of Ore. Chapter 4 in Short Course on Evalu-
ation, Design, and Operation of Precious Metal Heap Leaching Projects.
AIME, SME, Albuquerque, New Mexico, October 13-15, 1985.
13. Heinen, H. J., G. E. McClelland, and R. E. Lindstrom. Enhancing Perco-
lation Rates in Heap Leaching of Gold-Silver Ores. Bureau of Mines.
RI8388. 1979.
85
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14. McClelland, 6. E. Ore Precipitation: Crushing and Agglomeration.
Chapter 5 in Short Course on Evaluation, Design, and Operation of Pre-
cious Metal Heap Leaching Projects. AIME, SME, Albuquerque, New Mexico,
October 13-15, 1985.
15. Callicutt, W. W. Economic Aspects of Heap Leaching. Chapter 3 in Short
Course on Evaluation, Design, and Operation of Precious Metal Heap
Leaching Projects. AIME, SME, Albuquerque, New Mexico. October 13-15,
1985.
16. Shoemaker, R. S. Economic Aspects of Precious Metals Plant Design.
Mining Congress Journal, August 1981. pp. 52-55.
17. Pizarro, R. S., and W. J. Schlitt. Innovative Technology for Improved
Processing of Gold Ores. Mining Engineering, November 1984. p. 1533.
18. Heap and Dump Leaching. International Newsletter. 2(3), September
1985.
19. Milligan, D. A. Solution Control. Chapter 10 in Short Course on Evalu-
ation, Design, and Operation of Precious Metal Heap Leaching Projects.
AIME, SME, Albuquerque, New Mexico, October 13-15, 1985.
20. Muhtadi, 0. Metal Extraction (Recovery Systems). Chapter 11 in Short
Course on Evaluation, Design and Operation of Precious Metal Heap Leach-
ing Projects. AIME, SME, Albuquerque, New Mexico, October 13-15, 1985.
21. Versar, Inc. Quantities of Cyanide-Bearing and Acid-Generating Wastes
Generated by the Mining and Beneficiating Industries, and the Potentials
for Contaminant Release. Draft report prepared for U.S. Environmental
Protection Agency, Office of Solid Waste, Washington, D.C. June 27,
1986.
22. Harper, T. G. New Approach Overcomes Problem of Heap Leaching on Steep
Terrain. (Newsletter of unknown title or date.)
23. Englehardt, P. R. Long-Term Degradation of Cyanide in an Inactive Leach
Heap. In: Cyanide and the Environment, Proceedings of a Conference
held in Tucson, Arizona, December 11-14, 1984. pp. 539-547.
24. Ely, M. F. Final Report, American Mine Cleanup and Abatement. American
Mine Operations. A.V.S.R. Box V-ll, Apple Valley, California 92307.
December 26, 1985.
25. Schmidt, J. W., L. Simovic, and E.E. Shannon. Development Studies for
Suitable Technologies for the Removal of Cyanide and Heavy Metals From
Gold Milling Effluents. In: Proceedings of 36th Industrial Wastes
Conference, Purdue University, 1981. pp. 831-846.
26. Schmidt, J. W., L. Simovic, E.E. Shannon. Natural Degradation of
Cyanides in Gold Milling Effluents. Paper presented at Cyanide and the
Gold Mining Industry Seminar, Ottawa, Canada, January 22-23, 1981.
86
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27. Fuller, W. H. Cyanides in the Environment with Particular Attention to
the Soil. In: Cyanide and the Environment, Proceedings of a Conference
held in Tucson, Arizona, December 11-14, 1984. pp. 19-46.
28. Heming, T. A., and R. V. Thurston. Physiological and Toxic Effects of
Cyanides on Fishes: A Review and Recent Advances. In Cyanide and the
Environment, Proceedings of a Conference. Tucson, Arizona. December
11-14, 1984. pp. 85-104.
29. Sax, N. I. Dangerous Properties of Industrial Materials. 6th ed. 1984.
pp. 822-823.
30. U.S. Environmental Protection Agency. Quality Criteria for Water,
Cyanide. Office of Water Planning and Standards. 1976.
31. Stotts, W. G. Handling Cyanide at Superior Mining Company's Stibnite
Heap Leaching Operation, in Conference or Cyanide and the Environment,
Tucson, Arizona. December 1984. Published by Colorado State University.
32. Hiskey, J. B. Thiourea as a Lixiviant for Gold and Silver. In Pro-
ceedings of the 110th AIME Meeting, Chicago, February 22-26, 1981.
33. Berezowsky, R. and Sefton, 1979, Recovery of gold and silver from
oxidation leach residues by ammoniacal thiousulfate leaching: 1979
Annual AIME Meeting, New Orleans, LA.
34. Heinen, H., Eisele, J. and Scheiner, B. Malononitrile Extraction of Gold
from Ores: Bureau of Mines RI 7464. 1970.
35. Hiskey, J. B. Thiourea Leaching of Gold and Silver - Technology Update
and Additional Applications. Minerals and Metallurgical Processing
1(3): 173-178 November 1984.
36. Pyper, R. A. Extraction of Gold From a Carl in-Type Ore Using Thiourea.
In: Proceedings of the 110th AIME Meeting. Chicago, IL. February
22-26, 1981.
37. Schulze, R. G. New Aspects in Thiourea Leaching of Precious Metals.
Journal of Metals, June 1984.
38. Scott, J. S. An Overview of Cyanide Treatment Methods for Gold Mill
Effluents. In: Cyanide and the Environment. Proceedings of a Con-
ference organized by the University of Arizona. Ed by Dirk Van Zyl.
December 11-14, 1984.
39. Milligan, D. A. Cyanide Destruction. Chapter 14 in Short Course on
Evaluation, Design, and Operation of Precious Metal Heap Leaching
Projects. AIME, SME, Albuquerque, New Mexico. October 13-15, 1985.
87
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APPENDIX A
TRIP REPORTS
-------
REVISED TRIP REPORT
PINSON MINING COMPANY
EPA Contract No. 68-02-3995
PN3650-24
Prepared by
PEI Associates, Inc.
(Draft Trip Report revised per comments received from John Pekrul July 7,1986)
The Pinson Mining Company operations (Pinson and Preble Mines) near
Winnemucca, NV were visited on May 19,1986. The objectives of the visit and tour were
to gain a familiarity with the Pinson operation and to discuss the current precious metals
heap leach project being conducted by PEI for the EPA. The following personnel
participated in the meetings and tour
Jack Hubbard - U.S. EPA Project Officer
Robert Hoye - PEI Project Manager
Dan Harper - Pinson General Manager
Bruce Thorndycraft - Pinson Mill Superintendent
Keith Belingheri - Pinson Chief Mine Engineer
John Pekrul - Pinson Chemist
Norm Greenwald - Newmont Gold Co. representing
Nevada Mining Association
An initial meeting was held to discuss the EPA's mine waste program in general
and the current project in detail. Pinson personnel gave an overview discussion of the
operations. Pinson personnel then gave detailed discussions of the various operations
during a site tour. A follow-up meeting was held after the tour to further discuss the
operations. Pinson provided PEI with two engineering diagrams of the Pinson heaps.
Additionally, photographs of the facilities were taken by PEI and EPA during the tour.
Discussion of the Pinson and Preble operations are presented separately in this trip report.
This trip report includes some information included in a pamphlet on the Pinson operations
provided by Pinson personnel.
Pinson Mine
General - The Pinson heap leach operation is relatively new having started year round
production in 1982. The Pinson open pit mine feeds both a mill, which is the major source
of gold recovery, and the heaps. The heap leach facility is situated about 1.5 miles from
the pit Currently about 2 million st of material are on the heaps. The heaps currently cover
about 40 acres. The pads are constructed of compacted clays and are permanent (single-
use) facilities. Run-of-mine material grading 0.02 to 0.03 oz. gold per st is leached with an
alkaline sodium cyanide solution. Pregnant solution is collected in lined trenches, flows to
a lined collection pond and then to the carbon adsorption plant. Barren solution is returned
to the heaps. The water table is reported to be about 150 ft below ground level.
Profitability of heap leached gold is enhanced by the necessity to remove this
material from the mine in order to mine mill-run grade ore. Therefore, the only additional
cost incurred is the haulage from the waste dump where it would have gone, to the heaps,
and the heap construction and solution management costs. The ability to leach run-of-mine
ore is also a large contributor to the endeavor. Pinson is able to leach material grading 0.01
to 0.04 oz gold/st that would otherwise have been dumped in the waste rock piles.
89
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Pad Construction - The heap pads are constructed on a naturally sloping (3 to 6 %) valley
floor. Topsoil is first removed and stockpiled. The sub-base of native soils then is
compacted Two 6-inch lifts of local clays (alluvial lake bed material) are then placed and
compacted using a sheepsfoot roller with a vibratory steel drum. The permeability of the
clay liner is determined through compaction tests and the use of a nuclear densiometer. The
sub-base is compacted to achieve permeabilities between 10'5 and 10'6 cm/sec. The pad
liner is compacted to acheive permeabilities between 10~6 and 10"^ cm/sec.
French drains constructed of 2-inch diameter schedule 40 PVC pipe are placed
under the pads to monitor for leakage through the pad. The pads slope to two sides, the
French drains are located along the downstream sides as shown in Figure 1. These drains
are connected to sumps which are monitored to determine if any individual pad is leaking.
To-date one of the twenty pads has shown some weeping. Any solution found in the drain
system is returned to the solution handling system.
Collection ditches are constructed along the two down-slope sides of the pad and
are lined with 37 mil reinforced hypalon that is keyed into the clay pad. Four inch diameter
perforated PVC pipe and clean gravel are placed in these lined ditches.
Pads are nominally 300 ft by 380 ft diamond shaped areas which take advantage of
the natural ground contours. Initally it was planned that over the life of the mine,
approximately 60 pads would be constructed, each capable of holding 90,000 st at 20 ft of
heap height. Current operations, however, involve stacking second and possibly third lifts
on top of leached heaps. The individual heaps butt against each other, giving the
appearance of a single heap. Leaching and stacking are done in a manner that allows four
individual heaps to be leached at the same time without intermingling pregnant sloutions
prior to their introduction to the pregnant solution pond. The flow rate and chemical
characteristics of these individual solutions are monitored.
Permanent asphalt pads were considered, but were not used because it was
desirable to be able to releach the ore over extended periods of time for additional gold
recovery, and because the cost of moving the ore twice was avoided.
Heap Construction - After a pad has been constructed, a layer of 8 to 12 inches of gravel is
placed over the pad to protect it from damage by earth moving equipment, to provide a
permeable drainage blanket, and to protect the clay from erosion. The run-of-mine low
grade ore is placed to a height of about 15 ft using a front loader. Once a heap has been
leached a second 15 ft lift of leach material is placed on top of it and leached.
Leaching - A total of 2 to 3 leach cycles are applied to an individual heap, over a period
lasting from 9 months to a year. Initially the heap is leached for 45 to 90 days. During this
time 55 to 60 % of the gold values are obtained. The second leach of a heap is usually
conducted after the heap has gone through a winter season. This allows oxidation and
weathering of the rock to occur. Additional gold, 2 to 5 % of the total, is recovered during
this leach. Depending on production schedules, a third leach cycle may be used. Because
of their ability to re-leach the heaps and recover more gold, Pinson views the heaps as a
resource and not as waste on the pad.
The volume of solution applied to the heaps is measured using both magnetic and
mechnical flow meters. The volume of solution collected from each heap is measured
using flumes. Solutions from each heap are monitored separately to allow more precise
metalurgical control of the operation.
About 250,000 gal of leach solution are required to saturate an individual heap.
Initial breakthrough of solution is achieved about 18 hours after solution application is
begun and a steady state flow is achieved after about 72 hours. These parameters do not
vary significantly when a second leach cycle is conducted. The initial moisture content of
90
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ELEV. VIEW
I*.' COHPACTtO CLAY OH
5/1 rr 54*01
I' Ct fAN. FRCt DRAINING GRAVtL
t ^COMPACTED HATivf son
SLOPC ' s.e%
LEACH PAD
FHfNCH Off tlHS
PLASTIC PIPf TO MAN
HOlf, OH rPCNCH OKtIN
TO t/Fxr p»o
COLLCCTIOH
OITCH.
e
'r*CHCH QRtIN TO HtXT PAD O*
PLASTIC PI PC -KOH SLOTTfO- TO
MONITOR 116 STtTIOH
PLASTIC PIPE DETAIL
SHAVtL
SLOTTtO TOP *»O SIDfS
PLASTIC PIPf
'tCHTOHITIC CLAY OR OTHC*
SCALfR
COLLfCTIOH DITCHfS
MAHfttOLf O» OTHtH
COLLCCTIOH SUMP _
(CONCfifTC, PLASTIC,
OR STCtL)
TT
-------
the run-of-mine ore is about 5 percent The moisture content of ore under steady state
leaching conditions is 14 to 15 percent.
The barren solution applied to the heaps contains 80 to 100 ppm cyanide and has a
pH of 10.3 to 10.5. The solution is delivered through 8-inch HDPE pipe, then distributed
by 3-inch pipe and wobbler and rainbird sprinklers on 40-foot centers.
The pregnant solution contains 8 to 10 ppm cyanide and has a pH of 8.0 to 9.5.
Reagent usage amounts to 0.3 pounds of NaCN per ton of ore and is very consistent in
practice.
Solution Handling - Solution is collected in the low corner of each pad by a "manhole"
system consisting of three 12-inch diameter steel pipes with slotted caps installed vertically.
Pregnant solution from each of the four heaps being leached flows to a common pregnant
solution pond via 6- and 8-inch HDPE piping. This pond has a volume of 2.2 million gal.
The pond is lined with 40 mil HDPE (heat welded seams) which was placed over 8-inches
of compacted clay. A French drain monitoring system, similar to that described for the
leach pads, is located beneath the liner. Pregnant solution is pumped to the carbon
adsorption plant and barren solution is pumped to a pond situated adjacent to the pregnant
pond. The barren solution pond has the same construction details as the pregnant solution
pond and has a volume of 1.1 million gallons. NaCN and caustic are added to the flow
into the barren pond. Barren solution is then pumped to the heaps.
A clay lined pond with 3 million gallons of capacity provides overflow protection
for the pregnant and barren solution ponds.
Residue Disposal - Pinson uses dedicated single-use pads, leached material is spoiled in-
place. Pinson re-leaches its heaps when additional gold recovery can be obtained and when
it benifits the water balance of the operation.
Preble Mine
General- The Preble mine is a satellite operation of the Pinson mine. Mining and heap
leach operations began at this site in 1984. Most of the ore mined will be leached on-site,
however, some will be hauled to the Pinson mill for processing. The heap leach process
includes crushing and agglomerating prior to leaching. Gold from the pregnant solution is
adsorbed onto carbon which is hauled to the Pinson mill for stripping. Other aspects are
similar to the Pinson operations.
Pad Construction - Pad construction at Preble is the same as at Pinson with two
exceptions: permeability of the Preble clay pads is 10'7 to 10-8; and two pads at Preble are
lined with 30-mil PVC. Plastic liners were used because these pads were constructed in
winter weather. Clay cannot be properly worked in cold weather.
Heap Construction - Run-of-mine ore is crushed and agglomerated with 8 pounds of
Portland No. 2 cement per ton of ore. Fresh water is also added during agglomeration.
Agglomerated ore is stacked on the heaps by a front loader to a height of about 17 ft
Currently, about 400,000 st of ore are on the the Preble heaps.
Leaching - The leaching operation is similar to that at Pinson except that it is conducted
only in warm weather. Like Pinson the barren spray at Preble contains 80 to 100 ppm
cyanide, however, the pregnant solution contains 50 ppm cyanide and has a pH of 10.3
tol 1.0 (the alkalinity of the cement keeps the pH up). The ban-en spray has a pH of 10.3
to 10.5. At Preble, 0.2 pounds of NaCN are consumed per ton of ore leached.
92
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Solution Handling - Pregnant solution is collected in ditches lined with 37-mil reinforced
hypalon. This solution flows to a lined pregnant pond then to carbon adsorption columns
then to a barren pond. The pregnant and barren ponds are lined and monitored for leakage
with the same specifications presented for the Puison ponds.
Residue Disposal - Preble leached ore is spoiled-in-place on the dedicated single-use pads.
This provides for long term management and allows re-leaching if appropriate.
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REVISED TRIP REPORT
NEWMONT GOLD COMPANY
(FORMERLY CARLIN GOLD COMPANY)
EPA Contract No. 68-02-3995
PN 3650-24
Prepared by
PEI Associates, Inc.
(Draft Trip Report was revisiedper verbal comments received from Norm Greenwald on
July 21,1986.)
Carlin Gold Company operations (Carlin-2/Gold Quarry and Maggie Creek
operations) near Carlin, NV were visited on May 20,1986. The objectives of the visit and
tour were to gain a familiarity with the Carlin operations and to discuss the current precious
metals heap leach project being conducted by PEI for the EPA. The following personnel
participated in the meetings and tour
Jack Hubbard - U.S. EPA Project Officer
Robert Hoye - PEI Project Manager
Norm Greenwald - Newmont Services Ltd. and
Nevada Mining Association
Walt Lawrence - Newmont Gold Company,
Manager of Mill Operations
An initial meeting was held to discuss the EPA's mine waste program in general
and the current project in detail. Mr. Lawrence gave an overview discussion of the
operations. Mr. Lawrence and Mr. Greenwald then gave detailed discussions of the
various operations during a tour of the site. Photographs of the facilities were taken by PEI
and EPA during the tour. Discussion of the Carlin-2/Gold Quarry and Maggie Creek
operations are presented separately in this trip report.
Carlin-2/Gold Ouarrv Operation
General - This is a new operation, construction of the heap leach operation was started in
•April 1985 and leaching began in March of 1986. Both heap leaching and conventional
cyanidation milling are conducted on-site along with open-pit mining. Run-of-mine low
grade ore is leached on dedicated single-use pads with HDPE liners. Leaching is planned
to be conducted in three phases. Construction of the first phase, consisting of two 50-acre
pads, is just being completed. Subsequent phases will involve additional pads constructed
near the mine site.
Pad Construction - Phase 1 leaching operations consist of two 50-acre heaps. The first of
these heaps has been constructed and has been leached since March 1986. The pad of the
second heap is currently under construction.
The first step of pad construction was to clear and grubb the area. The sub-base
was then compacted. Internal dikes are constructed in the base to segregate flow of leach
liquor. An 80-mil HDPE liner was then placed over the entire pad including the collection
ditches. Seams of the liner material were heat welded, the integrity of the seams was
verified through a quality assurance testing procedure. A 2-ft layer of graded gravel was
placed over the plastic liner. The gravel serves to protect the liner during heap construction
94
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and to provide a high permeability blanket over the liner to prevent build-up of hydraulic
head during leaching.
Two monitor wells have been installed and are monitored. The wells are located
immediately downgradient of the leaching operation. They intersect groundwater at a depth
of about 70 ft.
Heap Construction - Run-of-mine low grade ore is dumped by haul trucks onto the liner to
form a single 50-ft high lift Detailed geotechnical investigations were conducted of the
foundation and the heaps themselves to ensure stability. The material is spread by dozer
then ripped and cross-ripped using a Cat D9L with a 6-ft ripper. Pebble lime is added to
the ore in the haul trucks at a rate of 3 Ib/st of ore to ensure that the alkalinity is maintained
at a pH of 10.0 to 10.5.
The ultimate height of the heap currently is under evaluation, it may go as high as
200 ft Current production sends 300,000 st of ore to the heaps each month, at full
operation 4.5 million st will be placed on the heaps annually. There now are about 1
million st of ore on the pads.
Surface water is diverted around the area of the heaps chiefly by the tailings pond
and dam situated upgrade of the leaching operation.
Leaching - Barren solution is sprayed over the heap using wobbler sprinklers with
individual pressure regulators. The total flow of the pregnant solution is about 4000
gal/min. If the ultimate height of the heap reaches 200 ft then the pads will be in production
for about 10 years. Figures on reagent usage were not immediately available. Likewise
information on water balance (volume of barren solution and the moisture content at steady
state leaching) were not available. The initial moisture content of the ore is 6 %.
Solution Handling - Pregnant solution is collected in ditches along the two down-slope
sides of the heap. The pH of both pregnant and barren solutions is 11.4. The ditch is lined
with 80-mil HDPE and is actually a continuation of the pad. About 4000 gal/min of
pregnant solution flows to the pregnant solution pond. This pond has a hypalon liner. An
emergency overflow pond is located immediately downgradient of the pregnant pond. This
pond is lined with natural clay. The heap leaching operation is designed as a zero discharge
facility.
Residue Disposal - The heaps at the Carlin-2/Gold Quarry operation are constructed on
dedicated single-use pads. The leached material will be spoiled-in-place on the plastic liner
at completion of leaching and following a rinse with fresh water.
Maggie Creek Operations
General - The Maggie Creek operation is located adjacent to the Carlin-2/Gold Quarry
operation. The Maggie Creek mine pit will eventually be engulfed by the Carlin-2 pit
Unlike Carlin-2 the ore is crushed, agglomerated and leached on restackable asphalt pads at
Maggie Creek. The ore grade is 0.03 oz gold/st.
Pad Construction - A single, restackable asphalt pad is used in the heap leach operations at
Maggie Creek. The pad is nominally 850 by 260 ft The pad slopes 3 percent over the
short dimension and 2 percent over the long side. The pad was constructed by excavating
the sub-base then placing a 2-ft thick engineered clay fill base. Next a 5-inch layer of
asphalt was placed over the pad. A 2-inch thick rubberized asphalt membrane (sealcoat
mixed with ground rubber) was placed over the asphalt. Another 2-inch layer of asphalt
was placed over the rubberized membrane. Finally, a top coating of seal coat was applied.
The pad liner extends to line the pregnant solution collection launders.
95
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Carlin personnel indicated that, in retrospect, they would probably use a dedicated
single-use pad instead of the restackable pad in current use.
Heap Construction - Run-of-mine ore is crushed to a minus 1.5 inch size then
agglomerated with 5 to 6 pounds of portland cement per ton of ore. No cyanide is added
during the agglomeration process. The agglomerated ore is loaded onto the pads by front-
end loader to a height of 16 ft The pad is divided into 5 seperate heaps. Each heap
contains 16,000 to 18,000 st of ore. About 900,000 st of ore are placed on the pads
annually.
Observation wells have been installed at the immediate downgradient comers of the
pads to detect if seepage through the pad is occuring.
Leaching Cvcle - A complete leach cycle takes about 25 days for each heap. This cycle
consists of 2 to 3 days to load the heap, 15 to 18 days of actual leaching, 1 day to drain, 2
to 3 days for a fresh water rinse, and 1 day to remove (spoil) the leached material and place
it in the disposal area (Carlin-2 tailings pond). During this leaching cycle 65 to 70 percent
of the gold is recovered. Studies conducted by Carlin have indicated that longer leaching
cycles do not seem to increase recovery. Leaching is carried out essentially year round
(solution heating is used in the winter).
Reagent usage is 0.3 Ib cyanide /st of ore leached.
At any given time 2 or 3 heaps will be leaching, 1 will be rinsing, and 1 will be
under construction. Barren solution is sprayed over the heaps using wobbler sprinklers.
Solution Handling - Approximately 600 gal/min of pregnant solution is collected from the
heaps. This solution flows through launders along the downstream sides of the pad to the
pregnant solution pond. The launders are of the same construction as the pad and are a
continuation of it The pregnant solution pond is lined with 36-mil scrim reinforced
hypalon. This solution is pumped to carbon adsorption columns for gold recovery. Barren
solution is returned to a lined pond adjacent to the pregnant pond and is then pumped to the
heaps.
Residue Disposal - Spoil from the heap leaching operation is removed using a front end
loader and placed in haul trucks. The spoil is disposed of by end dumping on a spoil pile.
The spoil pile is located in an area that will become part of the Carlin-2/Gold Quarry tailings
impoundment.
96
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REVISED TRIP REPORT
ROUND MOUNTAIN GOLD CORPORATION
EPA Contract No. 68-02-3995
PN 3650-24
Prepared by
PEI Associates, Inc.
(Draft Trip Report revised per comments received from Donald L. Simpson on July
31,1986.)
The Round Mountain Gold Corporation's Smoky Valley Common Operation near
Round Mountain, NV was visited on May 21,1986. The objectives of the visit and tour
were to gain a familiarity with the Smoky Valley operation and to discuss the cuirent
precious metals heap leach 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 Hoye - PEI Project Manager
Otto Walls - Round Mountain Gold Mill Manager
Norm Greenwald - representing Nevada Mining
Association
An initial meeting was held to discuss the EPA's mine waste program in general
and the current project in detail. Mr. Walls gave an overview discussion of the operations.
Mr. Walls gave detailed discussions of the various operations during a site tour.
Photographs of the facilities were taken by PEI and EPA.
General - Smoky Valley is the world's largest gold heap leaching operation. Ore from the
open pit mine is crushed and placed on restackable asphalt pads. After leaching the ore is
hauled to an adjacent spoil pile. Field evaluations of a single-use pad with an 80-mil HDPE
liner are underway. These evaluations will determine the design of future leaching facilities
for Type 2 ore. Type 1 ore, that which has been mined to-date, has been processed as
described below.
Pad Construction - The restackable pad is constructed of a 7 inch thick asphalt layer which
has a rebberized asphalt membrane 2.5 inches from the bottom of the pad. A 2 foot thick
layer of graded (1 inch) ore is placed over the asphalt to protect it and to provide drainage
for the heap. The larger of the two pads is nominally 2500 feet long by 280 feet wide. The
smaller pad is 650 by 280 feet. The pads are sloped at about 4 percent to the collection
ditch.
Hp.?p Construction - Currently about 18,000 st of ore (0.03 to 0.04 oz./st) are placed on
the pads each day. Ore is crushed in an open-circuit, three-section crushing plant. Lime is
added to the crushed ore stream, at 3 Ib/st, before the conveyor discharges into an
automated truck loading bin. The crushed ore is dumped on top of the coarser layer and
pushed up to a height of 35 ft.
The pads are continuous but divided into 30 areas for solution distribution. The
heap is 250 ft. wide at the base and 200 ft. wade on top. Current operations put up one half
of a solution distribution area per day. Total leach pad capacity is 1.2 million st.
97
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Leaching Cycle - Each individual heap is leached for 50 to 55 days, drained for one day,
then rinsed with fresh water for 3 to 4 days. Sodium cyanide solution is added to the leach
solution to maintain a concentration ofl Ib/st of solution; the stripped and barren solution
that is recycled contains a residual cyanide concentration of about 0.7 Ib/st of solution,
cyanide usage is about 0.1 Ib/st of ore.
Leach solution is heated during the winter so that leaching can be continous.
Leach solution is distributed over each individual heap by 30 wobbler sprinklers
placed in staggered arrangement Each wobbler sprays at 4 to 5 gal/min. A total of about
2,700 gal/min are sprinkled on the entire leach pile and about 2,300 gal/min flow out of the
bottom of the pile. The remaining 400 gal/min is absorbed by the ore particles or is lost to
evaporation.
Solution Handling - About 2,300 gal/min of pregnant solution containing 0.04 oz gold/ton
flows out of the heaps and to the pregnant solution sump. Unlike other sites visited, large
pregnant solution and barren solution ponds are not present at this site. Instead, Smoky
Valley uses a system of sumps that allow control of these solutions. One large containment
pond is in place to back-up three sumps.
Residue Disposal - After each heap is leached and rinsed it is excavated by a front-end
loader and hauled by trucks to the spoil disposal area. As the loading operation moves
from one end of the pad to the other, a 75- to 100 ft.-wide slot is cut through the barren
pile. One side of the slot is being loaded, while a new heap is being built at the opposite
side of the slot. It takes 60 days for this moving slot to move through the entire leach pile.
The spoil is end dumped over a face that is about 150 ft high. The spoil spreads
over this face and quickly dries. About 30 million st (an estimate by site personnel) of
spoil have been placed on this pile.
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REVISED TRIP REPORT
NERCO METAL'S CANDELARIA MINE
EPA Contract No. 68-02-3995
PN 3650-24
Prepared by
PEI Associates, Inc.
(Draft Trip Report revised per comments received from Michael Minette July 18,1986)
NERCO Metal's Candelaria Mine, located 135 miles southeast of Reno NY was
visited on May 22,1986. The objectives of the visit and tour were to gain a familiarity
with the Candelana heap leach operation and to discuss the current precious metals bean
leach project being conducted by PEI for the EPA. The following personnel participated in
the meeting and tour: r
Jack Hubbard - U.S. EPA Project Officer
Robert Hoye - PEI Project Manager
Mike Minette - Candelaria Permits and Planning
Engineer
Pat James - Candelaria Safety Engineer
O. W. Lively - NERCO Minerals, Manager
Environmental and Safety Affairs
Norm Greenwald - Newmont Services, Ltd.
representing Nevada Mining Association
An initial meeting was held to discuss the EPA's mine waste program in general
and the current project in detail. Mr. Minette gave an overview discussion of the
Candelaria operations. Mr. Minette then gave detailed discussions of the various
operations during a site tour. Photographs of the facilities were taken by PEI and EPA
during the tour. A pamphlet titled "Candelaria Mine" was provided by site personnel and
provided supplemental information.
General - The Candelaria mine is the largest open-pit silver mine in the country. The mine
was opened in 1980 by Occidental Minerals and closed in 1982 due to depressed silver
prices. NERCO Minerals acquired Occidental and reopened the mine in February of 1983.
All ore is crushed, agglomerated and treated by heap leaching with cyanide. Dedicated
single-use clay or HDPE lined pads are used, with multiple lifts of ore placed and leached.
Ultimate heap height is 120 ft. Leach residue will be spoiled in-place at closure.
Mining operations currently handle 57,000 st of material daily, 10,000 st of which
is ore. Over 2 million oz of silver will be produced this year. The ore grade cutoff is 0.5
oz/st. The silver occurs in oxide ores, sulfides are less than 0.1 percent. Site personnel
point out that oxide ores do not have the acid formation potential of sulfide ores.
Additionally the Candelaria ore does not have any carbonaceous material associated with it.
(Carbonaceous material is a cyanicide and increases reagent usage.)
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The site receives about 8 inches of annual rainfall, of which about 2.5 inches is
snow. Eleven wells have been installed in the area of the pads. Each of these wells are
plumbed quarterly and are dry. The area of the pads has been drilled to a depth of 500 ft
without hitting water. There is no surface water for an 8 mile radius. There are no
residences within 5 miles.
Pad Construction - The Candelaria heap leach system uses 7 adjacent pads. Each pad is
1300 to 1500 ft long and 300 ft wide. The combined pad area is 2100 ft wide by 1300 ft
long on one side and 1500 ft long on the other. Plans include adding 2 more pads to this
system in the near future, ultimately a total of 12 pads could be constructed with the
available area. Five and a half of the 7 existing pads are constructed of clay, the other one
and a half pads are newer and are lined with HDPE.
The clay lined pads were constructed by first clearing and grubbing the area. The
sub-base was then brought up to optimum moisture content and compacted to 95%. Clay
imported from a borrow pit located 5 miles away was placed in three 6-inch lifts. The clay
liner was compacted to 99% of its dry density to achieve a permeabiliiy of lO"7 cm/sec.
The clay liners are keyed to a hypalon lined collection ditch on the downstream side. The
pads are tilted at a slope of 1.5 to 5 % towards this ditch.
The HDPE lined pads were constructed by first clearing and grubbing the area. A
4-inch layer of clay and soils compacted to 95% dry density was then placed over the area.
An 80-mil HDPE liner was then put down. This pad liner was joined to a 100-mil HDPE
collection ditch liner.
Candelaria conducted comprehensive geotechnical studies on several different pad
construction materials. For example, the use of bentonite and montmorillonite as additives
to clay liners was explored. In addition, they have evaluated the effects of UV radiation,
temperature variations, slope, and chemical compatability during the pad selection process.
It has been determined that the existing pads can support heap heights of 140 ft and sub-
base slopes of 6 % without damage. Pad No. 1 is currently at a height of 110 ft. A new
pad can be put into service in about 3 months.
Heap Construction - Run-of-mine ore is crushed to minus one inch in two stages. The
ore is then agglomerated by tumbling and wetting with NaCN solution. The agglomerated
ore is hauled by truck to the leach pads.
Heaps are constructed in 20 ft lifts. Ore is piled to a depth of 25 ft, the top 5 ft are
pushed off and the heap is ripped with an 8 ft ripper on a Cat D9H. The top material is
pushed off and the heap ripped to increase the heaps permeability. The 20 ft lifts are added
to the heaps in a continuous sequence. It takes about 6 months for a lift to be added to a
single pad. There are currently about 11 million st of ore in the heaps.
Leaching Cvcle - Each 20 ft lift is leached for 45 to 60 days. Leaching is conducted year-
round. In the winter, a trickle solution distribution system is buried in the heaps to a 4 ft
depth. During other months, leach solution is sprayed over the heap at a rate of 0.005
gal/min per square foot using wobbler and rainbird sprinklers. Total flow to the heaps is
3000 gal/min.
Currently, 2.5 Ib of caustic and 2.5 Ib of cyanide are added to each ton of leach
solution. The pH of this solution is 11.2, the pH of the pregnant solution is 10.5. The
pregnant solution contains 0.45 oz silver per ton of solution.
The heaps will not be rinsed with fresh water until the time of closure.
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on Handling - The Candelaria heaps arc zero discharge facilities the only outlet is to
evaporation. Pregnant solution flows from the heaps to the HOPE lined collection ditch
which feeds the pregnant solution pond. Two Hypalon lined ponds have capacities of 9.0
million gallons each. Currently, one of the ponds is used as a surge pond and the other is
used for clean pregnant solution. These ponds are designed to contain any spillage in
addition to runoff from a 100 year storm.
Pregnant solution is pumped from the pond to a surge tank, then to clarifiers which
remove solids. The solution then goes to a vacuum tower where oxygen is removed. Zinc
dust is added to precipitate the silver and gold which is recovered as a filter cake in filter
presses. The filter cake is treated in a furnace and the dore bullion product is formed.
Barren solution flows to a lined pond located near the plant. Caustic and sodium
cyanide are added to this solution before it is pumped back to the heaps as leaching
solution. Process water from the clarifier backwash, plant washdown, et. is sent to a
smaller lined evaporation pond located adjacent to the barren solution pond. Clean-out and
any spillages from the agglomerator building are contained in a similar lined pond. When
necessary solid residue from these ponds will be removed and placed on the heaps.
Surface water diversion ditches have been constructed upgradient of the heaps so
that any storm water will not contact the heaps.
Candelaria has constructed some fresh water supplies for animals that live on or
pass through the site. This keeps them from drinking the process waters. Additionally, the
process water ponds are fenced. These arc common practices at the heap leach facilities
visited.
Residue Disposal - Heap leach pads in use at Candelaria are dedicated, single-use pads.
The leached material will be spoiled in-place. At the time of closure, plans call for the
heaps to be rinsed with fresh water. Exposed liner material (i.e. collection ditch liners)
will be taken up and placed in the empty pregnant solution pond. The liner of that pond
will then be folded over on itself and buried.
Alternatives to Cyanide - Candelaria personnel made the following points in addressing
the feasibility of alternatives that they believe EPA may consider based on the Report to
Congress:
Use of Thiourea - Thiourea requires a very acidic environment. Candelaria estimates
that for every ton of ore treated, 178 Ib of H2SO4 and 8 Ib of
H2NO4 would be required. Current reagent usage is 2.5 Ib of
caustic and 2.5 Ib of cyanide. Thiourea would cost an estimated
$53 per oz of silver recovered, just for reagents (current price of
silver is $5/oz)
Cyanide Destruction - Would prohibit recycling of solutions. 1984 costs to operate with
cyanide destruction (instead of recycling) would increase
production costs by $4/oz of silver, it would require 15 million Ib
of added cyanide and would necessitate the use of a tailings pond.
Multiple-Use Pads - Forcing the mine to remove ore from the single use pads would
increase loading and haulage costs by $0.40/ton. Currently there
are 11 million tons on pads thus multiple use pad would cost $2
million per year minimum.
Imperroeable Cap - To place an impermeable cap over the heaps at the end of
operations would cost a minimum of $7.5 million for a 2 ft
compacted clay layer.
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TRIP REPORT
STATE OF NEVADA
PFPARTMENT OF CONSERVATION AND NATURAL RESOURCES
DIVISION OF ENVIRONMENTAL PROTECTION
EPA Contract No. 68-02-3995
PN3650-24
Prepared by
PEI Associates, Inc.
A meeting was held with personnel of Nevada's Division of Environmental
Protection in Carson City, NV on May 23,1986. The objectives of the meeting were to
gain a familiarity with the state's approach to permitting and regulating heap leach facilities
and to discuss the current project. The following personnel participated in the meeting:
Jack Hubbard - EPA Project Officer
Robert Hoye - PEI Project Manager
Harry Van Drielen - Nevada Environmental
Management Specialist
Verne Rosse - Nevada Waste Management
Program Director
An initial meeting was held to discuss the EPA's mine waste program in general
and the current project in detail. Mr. Van Drielen gave an overview of his permitting
activities and perspective on the heap leach industry. It was decided during the meeting that
it would be possible and beneficial to make brief visits to two nearby heap leach operations.
These two operations, Alhambra Mines and Nevex, are smaller operations than we had
seen and are located in populated areas. These tours were then made with Mr. Van Drielen
to get a better perspective of the industry.
The State takes a somewhat flexible approach to permitting and regulating heap
leach operations. The type of pad and pond liner that will be used is agreed upon by the
State and the heap operator. Similarly the operations and closure requirements are specified
on a site-specific basis. Closure requirements for spoil are dictated by pH levels, the state
requires operators to attain a pH of 8.5. Where cyanide has been liberated natural or
background levels must be attained There are 3 known sites in the state with cyanide
contamination- 2 are inactive, historic facilities and the other is the Cortez operation.
One interesting point made by Mr. Van Drielen was that free cyanide occurs in
detectable and measurable levels decades after release to the environment. Current
literature indicates that free cyanides quickly decompose or are complexed to stable forms
in the environment. He gave as an example a leak that occured at a cyanide loading station
near Carlin, NV. A significant volume of cyanide solution was lost. This solution did not
intersect groundwater but remained in the aerated zone in contact with earth rich in iron.
The cyanide is being extracted by wells installed for that purpose, a process that has been
going on for about 5 years. Free cyanide is still present in the solutions.
The biggest problem the State has with heap leach operations is with "sham"
operators and with clandestine operations. "Sham" operators are those who develop a
facility to the extent necessary to solicit investments and then abandon the facility
potentially leaving cyanide contamination. Clandestine operations are typically small
facilities in remote areas that operate without the state's knowledge or approval. These
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operations also often leave cyanide contaimination due to poor management practices and
lac* of controls.
Mr. Van Drielen indicated that the Maggie Creek and Round Mountain operations
are the only multiple-use pads in the state.
The site visits made with Mr. Van Drielen are reported below:
This heap leach operation is entering the closure phase. The facility leached old
tailings from previous amalgamation milling processes. The tailings contain significant
quantities of free mercury and some gold. Homes were built very near, and in some cases
actually on, these tailings Alhambra Mines excavated the tailings and placed them on two
triple- lined pads. The pads are constructed of modified soil and two layers of PVC with
leak detection between the PVC liners. Cyanide solution was sprayed over the heaps and
mercury and gold were recovered from the pregnant solution.
Specific information on sizes and tonnages was not obtained, however, it appeared
that the total area covered by the two heaps was less than 5 acres. The heaps are piled to a
height of about 25 ft. Currently, the heaps are being flushed with water. Occupied houses
arc located within 100 yards of the facility. This operation has reduced a significant health
risk by removing the free mecury from the site.
Nevex - The Nevex site is located near Carson City, NV on a hill- side above a populated
area. A single heap is used to treat ore from an open pit mine located nearby. Photographs
of the site were taken, but because of time constraints the site operator could not be
interviewed With the exception of the proximity to populated areas and its relatively small
size, the Nevex operation appeared similar to others visited.
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