United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-88/005 Mar. 1988
c/EPA Project Summary
Copper Dump Leaching and
Management Practices That
Minimize the Potential for
Environmental Releases
Robert L. Hearn and Robert Hoye
The full report presents a
description of the magnitude and
distribution of copper dump
leaching, the design and operation of
leaching facilities, the potential for
environmental impact, and
management practices that can be
used, to minimize environmental
releases. The information contained
in the report was obtained through
searches of published and
unpublished literature and through
contact with knowledgeable
individuals involved in the dump
leaching industry. Ten leaching
operations were visited to acquire
firsthand knowledge and site-
specific information.
Seepage frqm leach dumps and
process solution collection systems
is the most significant potential
mechanism for the release of
contaminants. These solutions have
low pH and high concentrations of
metals and total dissolved solids
(TDS). Ground-water impacts have
been documented. The application
and efficiency of standard waste
management practices at dump
leach operations are site specific
and are limited by the magnitude of
these facilities.
This Project Summary was
developed by EPA's Hazardous Waste
Engineering Research Laboratory,
Cincinnati, OH, to announce key
findings of the research project that
is fully documented in a separate
report of the same title (see Project
Report ordering Information at back).
Introduction
Dump leaching refers to percolation
leaching of copper from run-of-mine
low-grade ores that have been piled on
native ground. These dumps typically
cover hundreds of acres and contain
millions of tons of low-grade ore. The
leaching cycle covers a span of many
years. The percentage of copper
produced by leaching operations has
increased and this trend is expected to
continue. Currently, there are 18
commercially active copper leaching
operations in the United States with a
total production capacity of 280,000
metric tons of copper per year. Fourteen
of these sites are in Arizona. There are
about 23 inactive and abandoned
leaching sites.
Sections 8002(f) and (p) of the
Resource Conservation and Recovery
Act (RCRA) and its amendments require
the U.S. Environmental Protection
Agency (EPA) to conduct studies on the
"adverse effects on human health and
the environment of the disposal and
utilization of solid wastes from the
extraction, beneficiation, and processing
of ores and minerals." The EPA
submitted a report to Congress on
December 31, 1985, that indicated
concern with the low pH and the
potentially high concentration of metals
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found in leachates and leach material
associated with copper dump leaching.
The EPA subsequently issued a
regulatory determination on July 3, 1986,
that expressed continued concern about
mining wastes having high acid-
generation potential. Also in this
determination, the EPA indicated that it
would develop a regulatory program for
mining wastes under Subtitle D of RCRA
and collect additional information on the
nature of mining wastes and
management practices and the potential
for exposure to these wastes. The full
report addresses these issues with
regard to the development, operation,
and closure activities associated with
copper dump leaching operations.
General Characteristics
Although the number of active mines
in the United States has decreased in
recent years, the percentage of primary
copper produced by leaching has
increased. Low copper prices have
resulted in the closing of many mining,
milling, and smelting operations.
Although many of the leaching
operations associated with these sites
also have been closed, a significant
number are still active because of the
relatively low operating costs associated
with dump leaching. The result has been
an increase in the percentage of copper
produced by leaching operations in the
United States. Estimates indicate that by
1990 approximately 30 percent of the
copper produced in this country will be
recovered by some type of leaching
process.
The areas in which copper leaching is
practiced are similar in general
characteristics. Most of the active U.S.
copper dump and heap leaching sites
are in the Southwest. The climate in
these areas ranges from arid to semi-
arid. The topography of these areas
varies from gently rolling hills to
mountainous terrain. Vegetation is
sparse. The active leaching operations in
Utah and northern and eastern Arizona
tend to be located in more mountainous
terrain than those in southern Arizona
and western New Mexico. The land
surrounding many of the active leaching
operations consists primarily of sparsely
populated and undisturbed vacant land.
Some of the active leaching operations,
however, are relatively close to
residential and urban areas.
Very little surface water is found near
active leaching sites in the Southwest.
The depth to ground water, which is the
principal source of water most of these
mines, varies. In the more mountainous
environments, however, the amounts of
surface-water runoff and ground water
are greater because of winter snow
accumulation.
Operating Practices
Copper leach dumps typically cover
hundreds of hectares, are more than a
hundred meters high, and contain
millions of metric tons of leach ore.
Copper leach piles as small as 8
hectares (Cyprus Johnson) and as large
as 850 hectares (Bingham Canyon) were
observed during this study. Estimates
indicate that more than 5.5 billion metric
tons of leach ore now exist in copper
leach dumps scattered around the United
States and in excess of 40 million metric
tons of new material is being added to
these dumps annually.
Dump leaching and heap leaching are
distinguished by the use of liners. Dump
leaching refers to the leaching of low-
grade ore that has been deposited
directly on the ground. The pregnant
leach solution is typically collected in
unlined natural drainage basins. In
contrast, heap leaching refers to the
leaching of ore that has been deposited
on specially prepared pads. Dump
leaching operations are always
constructed in the immediate vicinity of
the mine site. Leach sites are selected to
minimize haulage costs and to utilize the
natural drainage patterns of the native
terrain for collection of the pregnant
liquor solutions.
Leaching of copper from massive
dumps of sulfide ore is accomplished
bacterial activity and, often, by the
addition of sulfuric acid. Ferric sulfate,
the major, lixiviant, forms in the presence
of oxygen and bacterial activity. The
bacteria generate acid in situ, which
provides acid for acid-consuming
reactions, including oxygen reduction.
Frequently, only makeup water is needed
in copper dump operations because the
oxidation of the sulfide minerals
generates sufficient acid to dissolve the
copper and maintain an active bacterial
population. More effective leaching
reagents have been identified, but they
are generally much more expensive and
their impact on the environment is
uncertain.
Copper is recovered from pregnant
leach liquors either by cementation or
by solvent extraction/electrowinning.
These processes remove copper from
solution and allow other dissolved
substances to accumulate. The recovery
process itself may add other substances
to the leach solutions. The cementation
process uses scrap iron to precipital
copper from the pregnant solution. Th
iron replaces copper in solution, and thi
iron-rich solution is subsequent!
recycled to the top of the leach dum|
Upon exposure to the atmosphere, th
dissolved iron oxidizes to form insolubl
salts, which precipitate on the surface c
the dump and restrict the flow of solutiot
Solvent extraction uses a complexatio
mechanism whereby copper is chelatec
the copper is then stripped from th
organic carrier by a strong acid solutior
Kerosene is a common carrier used i
most solvent extraction operations, and
may appear in small quantities in th
raffinate recirculated the dump.
Environmental Impact
Seepage from leach dumps ani
solution collection systems is the moj
significant potential mechanism for thi
release of contamination into the groum
water. One of the primary criteria in sitini
leaching operations is proximity to thi
mine. In dump leach operations, thi
ground surface is neither lined no
treated in any manner to reduci
seepage. Because the leaching solution:
are in direct contact with the earth, somi
continuous solution loss results. Release;
can also result from pipe and dan
failures equipment malfunctions, an<
overflows due to severe storm events.
The solutions generated in coppe
dump and heap leaching operation:
usually have a lower pH and highe
concentrations of metals and tola
dissolved solids than the natural water:
surrounding the site. Leach ores contair
pyrites and other naturally occurring
metal sulfides that oxidize to generate <
low-pH solution when exposed to aii
and microbial activity. The solven
extraction process also reduces the pH o
the solution by ion exchange before it i;
distributed on the leach dump. Generally
acidic solutions increase the solubility
and bioavailability of heavy metal:
contained in the leach material and rock
surrounding the dump.
The water quality around severa
active copper dump leaching operations
has been affected by leachates that have
seeped into the ground water. The
available ground-water monitoring data
indicate that some degradation of the
ground water has occurred around
several copper dump leaching
operations. Some seepage of leachates
into the ground beneath copper leach
dumps is inevitable. The amount ol
seepage and its impact depend on site-
specific factors.
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Management Practices
Most active copper leaching
operations have implemented a system
of management practices that includes
one or more mitigative measures
designed to minimize solution losses.
Historically, such management practices
were implemented solely for economic
reasons- (to improve copper recoveries).
As the potential for ground-water
contamination problems associated with
leaching became apparent, these
practices were implemented for
environmental reasons as well. The
measures used at a particular site
depend on various site-specific factors,
the most significant of which are the
geology, hydrogeology, topography, and
meteorology of the site. The land use
and population density of the area
surrounding the operation are also
considered, as is the cost of constructing
and/or installing each potential mitigative
measure.
The application and efficiency of
standard waste management practices at
copper leaching operations are
frequently limited by the size of the
dump and environmental characteristics
of the site. Copper leaching operations
are massive; thus, management
practices required for adequate control of
potential ground-water contamination
from leaching operations also must be on
a very large scale. The geologic and
hydrogeologic evaluation required to
design and implement an effective
surface-water and ground-water
control system is complex, and the
required control systems must cover
several hundred hectares. The
environment of the site may necessitate
a system to divert surface water resulting
from the torrential rains periodically
experienced in the region, but annual
precipitation may not be adequate to
sustain revegetation efforts. The size of
the leaching operation and its
surrounding environment often combine
to make both lining of new facilities and
capping economically impractical.
The cost of implementing and
maintaining an effective system of
management practices to minimize
solution loss and reduce potential
ground-water contamination depends
on site-specific factors. Traditional
management practices tend to be very
expensive to implement at copper
leaching operations because of the size
of the operations and the natural
characteristic of the site. Nevertheless,
several of these practices have been
implemented economically at or around
one or more leaching operation. Proper
planning and design procedures are
required to select the most appropriate
management practices and to minimize
costs.
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Robert L Hearn and Robert Hoye are with PEI Associates, Inc., Cincinnati, OH
45246.
S. Jackson Hubbard is the EPA Project Officer (see below).
The complete report, entitled "Copper Dump Leaching and Management
Practices That Minimize the Potential for Environmental Releases" (Order No.
PB 88 J55 114IAS; Cost: $19.95, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
EPA'1''7
PERMITyNo. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-88/005
* 0000329 f»S
•ft- U.S. GOVERNMENT PRINTING OFFICE: 1988—648-013/8701
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