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                   unpublished literature and through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.

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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

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  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

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  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
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EPA/600/S2-88/002
                                       0001961    HWER

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                                       230  S  OEAR80RN  ST
                                       CHICAGO
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