United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-88/002 Mar. 1988
vvEPA Project Summary
Gold/Silver Heap Leaching and
Management Practices that
Minimize the Potential for
Cyanide Releases
Robert L. Hoye
This report presents a description
of the magnitude and distribution of
gold/silver heap leaching, the design
and operation 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unpublished literature and through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 heap
leaching Industry. Six leaching
operations, were visited to acquire
firsthand knowledge and site-
specific information.
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
Heap leaching refers to percolation
leaching of low grade (approximately
0.05 oz/ton) gold and silver ores that
have been stacked on prepared surface
(pads). These heaps range from less
than 1 to about 50 acres and 15 to over
100 feet in height. The leaching cycle
covers a period from several weeks to
over a year. The percentage of gold and
silver produced by leaching operations
has increased over recent years and this
trend is expected to continue. An alkaline
cyanide solution is used as the lixiviant at
all heap leach operations. Currently,
there are 78 commercially active gold
and silver leaching operations in the
United States. Forty-seven of these
sites are in Nevada. Additionally, there
are numerous 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 cyanide associated with
heap leaching. The EPA subsequently
issued a regulatory determination on July
3, 1986, that expressed continued
concern about mining wastes containing
cyanide. 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. This report addresses
these issues with regard to the
development, operation, and closure
activities associated with precious metals
heap leaching operations.
-------
Industry Characteristics
The application of heap leaching has
increased in recent years because of the
relatively low capital investments and fast
payouts involved. These techniques
allow recovery of low-grade resources
that otherwise could not be profitably
extracted. The mining industry first
became interested in the U.S. Bureau of
Mines' developments in gold/silver heap
leaching technology in the late 1960s,
and the first commercial cyanide heap
leaching process was used at the Carlin
Gold Mine Company in northern Nevada
on mine cutoff material. Since the early
1970s, interest in heap leaching has
continued to grow primarily in response
to the high prices of gold and silver.
Low-grade (e.g., 0.05 oz/ton) gold
deposits previously considered
uneconomical to recover are now being
exploited at a profit. Currently, 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, nine in Utah, two in
South Carolina, and one in South Dakota.
In 1984, 525,000 troy ounces of gold was
recovered from 19,860,000 tons of ore
treated by cyanide heap leaching. The
application of cyanide heap leaching has
grown in recent years and this trend is
expected to continue.
Operating Practices.
Heap leach operations involve the use
ot liners and specially constructed leach
pads and solution ponds. The basic
design and operational layout of heap
leach projects are very similar at all
facilities. Low-grade ore (typically from
a surface mine) is stacked 15 to 50 +
feet high in engineered heaps on sloped
(1 to 6%), relatively impermeable pads,
and a weak alkaline cyanide solution is
sprayed over the ore. The solution
percolates through the heap and
dissolves finely disseminated free metal
particles (gold and/or silver). Care is
taken during the construction of heaps to
ensure that the material is uniformly
permeable.
The design, engineering and
construction of liners in this industry
have reached a high level of
sophistication. Pads, 1/4 to 50 acres, are
constructed of native or modified clays,
synthetic liners (e.g., HOPE, PVC, or
Hypalon), or asphalt. This helps ensure
that product and reagents are not lost
through seepage. The pads must be
capable of providing structural support
without suffering damage from deflection
due to the weight of the ore or
equipment traffic. Selection of pad
materials and specifications is
determined by site-specific parameters
such as availability of local materials,
slope, geotechnical properties of the
sub-base, temperature variations, and
operational considerations (i.e., single-
or multiple-use pads).
The pregnant solution flows over the
pad to a lined collection ditch. The ditch
carries the gold-bearing cyanide
solution to a lined pregnant solution
pond. Pregnant solution is then pumped
to a recovery plant, where the metal
product is removed by carbon adsorption
followed by elution 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).
Sodium cyanide is the only commercially
proven lixiviant. It is added to maintain a
concentration in the barren solution of
~0.5 Ib/ton of solution (250 ppm CN).
The optimal pH for the gold dissolution is
between 10 and 11. 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 zero discharge facilities.
The leaching cycle is relatively short
(e.g., 20 to 90 days) but may last a year
or more. At completion of leaching
operations, the leach ore is rinsed with
fresh water to remove residual cyanide.
With few exceptions, heap leach residue
(the barren ore remaining after precious
metal values have been extracted) is left
in place on the pad. At a very few
operations it is excavated, hauled by
truck, and disposed of in an on-site
disposal area (load-unload operations).
Although the basic process just
described is similar at all operations,
each site is unique, and several
alternative approaches exist. Specific
leaching times, 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.
Environmental Concerns
Because cyanide is the lixiviant used
in heap leaching of precious metals,
there is concern over the potential for
release of toxic cyanides into the
environment. Because an alkaline pH is1
maintained in the solution, most of the
cyanide is present as free cyanide, as
required in the leaching reaction. The
barren solution pond typically holds
hundreds of thousands of gallons of this
solution. The pregnant solution pond
contains lesser concentrations of free
cyanides because of the destruction and
complexation that occur in the heap;
however, a significant concentration of
free cyanides may be present. The
solution in these 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.
Cyanide in leach residue occurs in
combinations of various metallo-cyanide
complexes, free cyanides, and cyanates.
Cyanide complexes vary from strongly
bound forms to others that dissociate
more readily. The complexes in a given
heap are determined by the mineralogy
on the ore. Essentially no data are
available on the content and fate of
cyanides or cyanates in leach residue.
There are no reports of cyanide
contamination or migration from properly
constructed and operated heap leach
operations. However, there have been a
few reported incidents involving pond
failure or overtopping and contamination
resulting from clandestine operations that
did not use typical operational practices.
The principal transport mechanism is
reported to be volatilization of HCN to the
atmosphere. Although the toxicity of HCN
is well documented, no problems with
these atmospheric releases have been
documented.
Management Practices
A limited number of alternative
management practices can be applied to
minimize the potential for cyanide
contamination from heap leach
operations. These include alternative liner
construction, oxidation of cyanide during
post-leach flush-out, and use of
reagents other than cyanide. Most heap
leach operations are relatively small, their
only sources of potential contamination
are the heaps themselves and the two
process solution ponds. After cessation
of operations, only the heap leach
residue remains as a potential source of
contamination, as the ponds must be
emptied during closure. Additionally,
most obvious controls, such as pond and
leach pad liners, surface water
diversions, and post-leach rinsing, are
-------
eady standard practice in the industry.
.•though, the need for controls beyond
those currently in use has not been
demonstrated, the concerns related to
potential releases of cyanide may
warrant additional controls or overdesign
of existing controls. The management
practices that were evaluated are listed
below.
Most of the controls listed have been
incorporated into the design and
operation of at least one existing heap
leach facility. The feasibility and cost to
use these controls at other locations
would have to be determined on a site-
by-site basis. It would depend on
differences in mineralogy, topography,
geology, hydrogeology, climate, and
design and operational characteristics.
The use of double liners in solution
ponds is both technologically feasible
and is a demonstrated practice at 'some
heap leach sites. A doubleliner system
consisting of two layers of 40- mil
HOPE separated by a leachate detection
and collection system was evaluated.
The pond was assumed to be 300 ft by
150 ft (approximately 1 acre). For the
purpose of comparison, the costs
associated with a single 40-mil HOPE
(High Density Polyethylene) liner system,
Believed to be common in the industry,
rere also estimated. The cost
comparison indicates the double-liner
system increased the cost of the pond
by a factor of at least two. The cost of
constructing the solution ponds at a site
can represent a significant percentage of
the total capital cost of the operation.
Cyanide is the only lixiviant currently
used at commercial heap leach facilities.
Because of the toxicity associated with
cyanide, the question of the availability of
suitable substitutes for cyanide is raised.
The development of alternative lixiviants
(e.g., thiosulfate, malononitrile, and
thiourea) is still in the laboratory or
pilot-scale testing stage, however. If
alternative lixiviants are developed, the
environmental impacts associated with
their use must be fully evaluated. While
thiourea can rapidly leach gold from
leach ore, it requires a very acidic
medium (pH 1) that would be an
environmental concern. Additionally,
reagent consumption and cost are high
and the toxicity and mobility of its
degradation products have not been
assessed.
The type and sophistication of
ground-water monitoring systems vary
considerably in this industry. The
requirements for these systems are
specified on a site-specific basis by
State regulatory personnel. The cost for
installing a detection monitoring system
will vary greatly from site to site. The
primary factors that influence costs are
the size of the operation and the
complexity of local hydrology. The
principal factors are the diameter, depth,
and components of the wells, the drilling
specifications, the geologic material, the
sampling and analytical requirements,
and site access. Estimates made for an
example site indicate that the costs of
installing a system of 10 to 13 wells to
depths of 25 to 300 feet would range
between $12,500 and $195,000.
Consultant fees for a qualified
hydrogeologist could be expected to
range from $6,000 to $50,000. Analytical
costs would amount to $12,000 to
$16,000 annually plus reporting and
recordkeeptng. These costs point up the
great variability due to site-specific
conditions.
During post-closure period, the heap
leach residue is the only potential source
of cyanide contamination. Current
practice is to rinse the leach residue with
fresh water for a predetermined time or
until some preset cyanide concentration
(e.g., 0.2 mg/liter) or pH (e.g., pH 8) in
the rinse water is achieved. An additional
control option could be the addition of a
cyanicide, a strong oxidant, to the rinse
water. Alkaline chlorination is a proven
technology for cyanide destruction and is
the most highly developed of the
available methods in terms of experience,
simplicity, control, availability of
equipment, and engineering expertise.
This process destroys most cyanide
except iron cyanide and the more stable
metallo-cyanide complexes. Treatment
of heap leach residue by alkaline
chlorination has been carried out at a few
operations. When this system is used
during the operational period, the facility
must incorporate at least one additional
pond, a neutralization pond, in its solution
management system. If it is used only at
closure, the existing process solution
ponds would be adequate.
Application of a clay or synthetic cap
over leach residue could prohibit
infiltration and run-on and thereby
preclude formation of leachate. However,
it would hinder the natural degradation of
Operational
phase
Management practice
Pre-operation Installation of French drains beneath pads and pond liners
Use of RCRA double-liner systems with leak detection in ponds
Operational Use of alternative lixiviants
More extensive ground-water monitoring
Closure Flush heaps with cyanicide
Recontour and cap heaps
Post-closure Long-term maintenance of heaps and monitoring systems and site security
-------
cyanide by limiting volatilization and
photodecomposition. Forty-seven of the
72 heap leach operations are located in
Nevada in arid climates where capping
may provide even fewer marginal
benefits. In order to place a cap, the side
slopes of the heap would have to be
reduced to at least 3:1 or more from the
1:1 slopes existing duration operations.
Assuming a suitable source of cap
material exists near the site, recontouring
and capping a 1-acre, 15-foot-high
heap would cost about $40,000 and a
50-acre, 100-foot-high heap would
cost about $2 million.
Conclusion
The low production costs, relatively
short startup time, and relative simplicity
of heap leaching have lead to increased
use of this method to recover precious
metals that are otherwise not
economically recoverable. Current
state-of-the-art design, construction,
and operation of precious metals heap
leach facilities incorporates obvious
controls including relatively impervious
leach pads, lined collection trenches and
process ponds, and closed loop zero
discharge solution management.
Depending on site-specific
considerations, it may be beneficial to
incorporate redundancies and
overdesigns into these systems.
However, the need for additional controls
is not currently documented.
Additionally, research to determine the
presence, fate, and toxicity of cyanide
and cyanate in heap leach residue is just
beginning.
Robert L Hoye is with PEI Associates, Inc.. Cincinnati, OH 45246.
S. Jackson Hubbard is the EPA Project Officer (see below).
The complete report, entitled "Gold/Silver Heap Leaching and Management
Practices That Minimize the Potential for Cyanide Releases," (Order No. PB
88-154 281/AS; 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:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PA
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-88/002
0001961 HWER
REGION V
230 S OEAR80RN ST
CHICAGO
60604
------- |