EPA530-R-944J37
NTB PB94-201837
TECHNICAL REPORT
TREATMENT OF CYANIDE HEAP
LEACHES AND TAILINGS
September 1994
U.S. Environmental Protection Agency
Office of Solid Waste
Special Waste Branch
401 M Street, SW
Washington, DC 20460
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ireatment of Cyanide Heap Leaches and Tailings
DISCLAIMER AND ACKNOWLEDGEMENTS
This document was prepared by the U.S. Environmental Protection
Agency (EPA). The mention of company or product names is not to
be considered an endorsement by the U.S. Government or the EPA.
This Technical Resource Document consists of two sections. The first
section is EPA's Profile of the lead-zinc industry; the remaining
section is a Site Visit Report from a site visit conducted by EPA. The
Technical Report was distributed for review to the U.S. Department of
the Interior's Bureau of Mines and Bureau of Land Management, the
U.S. Department of Agriculture's Forest Service, the Western
Governors Association, the Interstate Mining Compact Commission,
and the American Mining Congress. EPA is grateful to all individuals
who took the time to review sections of this Technical Report
Document.
The use of the terms "extraction," "beneficiation," and mineral
processing" in this document is not intended to classify any waste
streams for the purposes of regulatory interpretation or application.
Rather, these terms are used in the context of common industry
terminology.
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Treatment of Cyanide Heap Leaches and Tailings
TABLE OF CONTENTS
Page
TREATMENT OF CYANIDE HEAP LEACHES AND TAILINGS 1
1. INTRODUCTION 1
2. CYANIDE LEACHING 2
2.1 Heap Leaching 2
2.2 Tank Operations (Carbon-in-Pulp and Carbon-in-Leach) 3
3. CYANIDE TREATMENT TECHNOLOGY 5
3.1 Rinsing of Heaps 5
3.2 Sulfur Processes 6
3.3 INCO Process 6
3.4 Noranda Process 9
3.5 Alkaline Chlorination Process 10
3.6 Hydrogen Peroxide Process 12
3.7 AYR Cyanide Recovery Process 14
3.8 Biological Treatment 16
3.8.1 Introduction 16
3.9 Homestake Mine, South Dakota 17
3.10 Pintail's Biotreatment Process 19
3.11 Natural Degradation 20
, 4. OTHER RELATED ISSUES 22
- *• 4.1 Closure and Reclamation Issues 22
«/ 4.1.1 Analytical Methods 22
i.,. 4.1.2 Mobility of Constituents In Heaps and Impoundments 23
' 4.1.3 Reduction of Constituents in Solutions 25
v ! 4.1.4 Rinsing/Treatment Duration 26
^4 4.1.5 Water Balance 26
V) 4.1.6 Percolation of Solution thfough Heaps 27
- 4.1.7 Acid Generation 27
:(> 5. REGULATORY PROGRAMS 28
5.1 Federal Requirements 28
5.1.1 Environmental Protection Agency - NPDES Program 28
5.1.2 Bureau of Land Management 29
5.1.3 U.S. Forest Service 31
5.1.4 National Park Service 31
5.2 State Requirements 32
5.2.1 California 32
5.2.2 Colorado 35
5.2.3 Idaho 36
5.2.4 Montana 37
5.2.5 Nevada 3*
5.2.6 South Carolina -u)
5.2.7 South Dakota •»!
6. CASE STUDIES *)
6.1 Hecla, Yellow Pine, Idaho •>)
6.2 Zortman Mining, Landusky Heaps, Montana 44
6.2.1 McCoy/Cove Mine, Echo Bay Mining Company, Nevada -
INCO process 4*
7. REFERENCES **
U S Environmental Protection Agency
Region 5, Library (PL-12J)
77 woct lar.kson Boulevaro. i^u»
u
Chicago, )L 60604-359^
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Treatment of Cyanide Heap Leaches and Tailings
LIST OF TABLES
Table 1. Relative Stabilities of Cyanide Complexes in Water 24
Table 2. Some Metal-Cyano-Complex Ions and Their Stability Constants 24
Table 3. Summary of State Requirements: Cyanide Heap Leach and
Tailings Impoundment Closure and Reclamation 33
LIST OF FIGURES
Page
Figure 1. INCO Sulfur Dioxide-Air Process 7
Figure 2. Alkaline Chlorination Process 11
Figure 3. Hydrogen Peroxide Process 13
Figure 4. AYR Cyanide Recovery Process 15
Figure 5. Biological Treatment Process 18
111
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Treatment of Cyanide Heap Leaches and Tailings
TREATMENT OF CYANIDE HEAP LEACHES AND TAILINGS
1. INTRODUCTION
The purpose of this report is to provide information on cyanide treatment methods for heap leaches
and tailings activities associated with cyanidation operations, including disposal units that receive
wastes from such operations. Such practices not only prevent environmental degradation but also
prevent costly remedial actions under Superfund and other programs. The Agency has collected this
information for use in regulatory agencies and the mining industry to better understand treatment
options. The Agency has not, however, evaluated the efficiency of the methods discussed.
Cyanidation includes both heap leaching and tank leaching. Spent ore or tailings containing residual
amounts of cyanide are generated as wastes. These wastes are typically treated to neutralize or
destroy cyanide prior to final closure. This report discusses cyanide detoxification or treatment in
terms of chemistry, duration, removal efficiencies, and advantages and limitations. After a discussion
of treatment techniques, the report describes typical closure and reclamation activities for heaps and
tailings impoundments, identifying issues that are still outstanding.
The report also describes Federal and state requirements that apply to cyanide operations. In
addition, selected case studies at active mines are presented. The active sites were selected to reflect
a range of facility types, a large heap operation with several permanent pads, and a site using
biological treatment by bacteria.
The report is based on literature reviews, publicly available documents, and telephone contacts with
Federal and state agencies.
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Treatment of Cyanide Heap Leaches and Tailings
2. CYANIDE LEACHING
Cyanidation uses solutions of sodium or potassium cyanide as lixiviants (leaching agents) to extract
precious metals from ore. Cyanidation techniques used in the gold industry today include heap or
valley fill leaching, agitation leaching followed by carbon-in-pulp (CIP), and agitated carbon-in-leach
(CIL). Cyanidation is best suited to fine-grain gold in disseminated deposits. Heap or valley fill
leaching is generally used to beneficiate ores containing less than 0.04 Troy ounces/ton (oz/t). CIP
and CIL techniques, commonly referred to as tank or vat methods, are generally used to beneficiate
ores containing more than 0.04 oz/t. These cut-off values are dependent on many factors, including
the price of gold and an operation's ability to recover the precious metal (van Zyl et al. 1988). For
the purposes of this report, a brief description of both heap and tank leaching is provided below. A
more detailed description may be found in EPA's Technical Resource Document: Extraction and
Benefication of Ores and Minerals Volume 2: Gold (EPA 1992a, 1994), which can be obtained from
the National Technical Information System or at EPA Regional Libraries.
2.1 Heap Leaching
Since the 1970's and early 1980's, heap leaching has developed into an efficient way to beneficiate a
variety of low-grade, oxidized gold ores. Compared to tank leaching, heap leaching has several
advantages, including simplicity of design, lower capital and operating costs, and shorter startup
times. In many cases, heaps are constructed on lined pads with ore sent directly from the mine (run-
of-mine ore) with little or no preparation. However, at about half of the heap leaching operations,
ore is crushed and agglomerated prior to placement on the heap to increase permeability of the heap
and maintain the high pH needed for leaching to occur (Bureau of Mines 1986). Agglomeration
entails mixing the crushed ore with portland cement, lime, ash, or other materials. In some cases,
after crushing, sulfide ores may be treated by roasting, autoclaving, bio-oxidation, or chlorination
prior to heap leaching.
Two common types of pads used in gold heap leaching include permanent heap construction on a pad
from which the leached ore is not removed and on-off pads, which allow the spent ore to be removed
following the leach cycle and fresh ore to be placed on the pad. Permanent heaps are typically built
in lifts. Each lift is composed of a 5- to 30-foot layer of ore. On-off pads are not commonly used in
the industry and are constructed to allow spent ore to be removed after the leaching cycle and re-use
of the pad (Lopes and Johnston 1988).
The reaction of the cyanide solution with the free gold is oxygen-dependent. Therefore, the solution
is oxygenated prior to application or during spraying. The solution concentration is generally
between 0.5 and 1.0 pounds of sodium cyanide per ton of solution. Cyanide solution is applied using
drip or spray irrigation. The cyanide leachate percolates through the ore and is collected by pipes
located under the pile or carried on the asphalt or plastic liner directly to ditches around the pile
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, Ci«//«<;f« uj Cyanide Heap Leaches and Tailings
(Bureau of Mines 1986; Lopes and Johnston 1988). The pregnant solution is then collected in a lined
pond or tank. (Bureau of Mines 1984).
Leaching occurs according to the following reactions, with most of the gold dissolving in the second
reaction (van Zyl et al. 1988):
• 4Au + 8NaCN + O2 + 2H2O -» 4NaAu(CN)2 + 4NaOH (Elsener's Equation and
Adamson's 1st Equation)
• 2Au + 4NaCN + O2 + 2H2O -* 2NaAu(CN)2 + H2O2 + 2NaOH (Adamson's 2nd
Equation)
Leaching is generally effective at a pH of 9.5 to 11, with the optimum being approximately 10.5.
More acidic conditions may result in the loss of cyanide through hydrolysis, reaction with carbon
dioxide, or reaction with hydrogen to form hydrogen cyanide (HCN). Alternatively, more basic
conditions tend to slow the reaction process (Bureau of Mines 1984). Typically, the recovered
cyanide solution, the pregnant solution, contains between 1 and 3 ppm of gold material (Bureau of
Mines 1986). Irrigation of the heap generally stops when the pregnant solution falls below about
0.005 ounces of gold per ton of solution (Lopes and Johnston 1988). Recovery rates for heap and
valley fill leaching range from 60 to 80 percent. Leaching typically takes from several weeks to
months, depending on the permeability and size of the pile. An "average/normal" leach cycle takes
approximately three months (Lopes and Johnston 1988).
After leaching has been completed, such that no further recovery of gold will occur, the spent ore and
remaining cyanide solution become wastes. There are several approaches to the decommissioning of
cyanide-contaminated ore heaps and neutralizing of cyanide solutions. Typically, the heap is rinsed
with water until the cyanide concentration in the effluent is below a specific standard set by the State
regulatory agency. In some cases, analysis of the heap solids is required. The heap may then be
reclaimed with wastes in place. If the heap is an on/off pad, the spent ore will have been periodically
removed to a permanent disposal area. Solution ponds and other areas are also neutralized and
closed, sometimes with residue or wastes in place, prior to reclamation. Cyanide treatment
technologies are discussed in detail in Section 3. Closure and reclamation are discussed in detail in
Section 4. State regulations and their applicability to closure and reclamation is discussed in Section
5.
2.2 Tank Operations (Carbon-in-Pulp and Carbon-in-Leach)
In Carbon-in-Pulp and Carbon-in-Leach cyanidation methods, primary leaching takes place in a series
of tanks. Finely ground gold ore is slurried with the leaching solution. The resulting gold-cyanide
complex is then adsorbed on activated carbon. CIP conducts the leaching and recovery operations in
two separate series of tanks, while CIL conducts them in a single series. Tank operations have
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Treatment of Cyanide Heap Leaches and Tailings
significantly higher recovery efficiencies than heap leaching facilities, recovering from 85 to 98
percent of the gold contained in the ore. Generally, CIP or CIL methods are used for high grade ore.
Oxide ores are typically beneficiated by grinding to 65 mesh and leaching with 0.05 percent sodium
cyanide over a 4- to 24-hour period with a pulp density of 50 percent solids. Sulfide ores are
typically beneficiated by grinding to 325 mesh and leaching with 0.1 percent sodium cyanide for a 10-
to 72-hour period with a pulp density of 40 percent solids. (Weiss 1985).
In the CIP method, a slurry consisting of ore, water, cyanide, and lime is pumped to the first series
of tanks for agitation and leaching. Gold is leached from the ore in the leach tank train. The slurry
containing leached ore and pregnant solution is pumped to the second series of tanks for gold
recovery. In the second series of CIP tanks, the slurry is introduced into a countercurrent flow with
activated carbon. The slurry enters the first tank in the series containing carbon that is partially
loaded with the gold-cyanide complex. In the suspended slurry, the activated carbon adsorbs gold
material on the available exchange sites. As the carbon material becomes laden with precious metals,
it is pumped forward in the circuit towards the incoming solids and pregnant solution. Thus, in the
last tank, the low-gold percentage solution is exposed to newly activated and relatively gold-free
carbon that is capable of removing almost all of the remaining precious metals in the solution. Fully
loaded carbon is removed at the feed end of the absorption tank train for further beneficiation.
Tailings are discharged to a tailings impoundment. (Bureau of Mines 1978, 1986; Stanford 1987).
The CIL technique differs from CIP in that leaching and recovery of values occur in the same series
of tanks. Activated carbon is mixed with the ore pulp in the agitated leach tanks. A countercurrent
flow is maintained between the pulp (ore and leaching solution) and the activated carbon. In the first
tanks of the series, leaching of the fresh pulp is the primary activity. In later tanks, adsorption onto
activated carbon is dominant as the concentration of gold in solution increases and fresh carbon is
added to the system. The loaded carbon is transferred to a stripping vessel while the spent ore is
pumped as a slurry to the tailings impoundment (Bureau of Mines 1986; Calgon Carbon Corporation,
undated; Stanford 1987). Tailings from tank leaching may be treated prior to discharge to the tailings
impoundment. Treatment standards for tailings prior to discharge to the impoundment are often set
by the State regulatory authority. As the tailings impoundment becomes dewatered, reclamation may
take place.
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Treatment of Cyanide Heap Leaches and Tailings
3. CYANIDE TREATMENT TECHNOLOGY
This section discusses various treatment methods for neutralizing or detoxifying cyanide solutions,
spent leached ores, and tailings. Treatment methods range from rinsing heaps with water to more
complex techniques such as alkaline chlorination and sulfur dioxide processes, which treat both
solutions (spent cyanide solutions and heap rinsate) and slurries (tailings), to recovery of cyanides.
Natural degradation and biological treatment of cyanide is also discussed. Where possible,
information on chemistry, duration, removal efficiencies, and advantages and limitations has been
described, as presented in current literature. Independent field testing or confirmation of these
techniques has not been conducted by EPA.
3.1 Rinsing of Heaps
There are three fundamental approaches to the decommissioning of cyanide-contaminated ore heaps.
The first is to leave the heap alone and allow the cyanide to degrade, perhaps slowly, but without any
human intervention. The second is to dismantle the heap and treat the ore in smaller batches. This
approach may be necessary when sections of the heap have become impermeable or when it is desired
to reclaim the leach pad area for other uses. The third approach is to rinse the heap to flush out
cyanide, with the rinse solution then being treated by any of the methods described below.
Ore heaps may be rinsed with fresh water or with recycled rinse water that has been treated so that it
contains little cyanide. The rinse medium may or may not contain chemicals designed to oxidize the
residual cyanide as it trickles through the heap.
Mines using cyanide heap leaching will already have equipment available to supply rinse solution.
The same system used to apply the cyanide solution can be used for rinsing of the heap. At Echo
Bay's Borealis Mine in Nevada, the heaps were rinsed at a rate of about 0.005 gals/min/ft2 (Schafer
and Associates 1991b) using Rainbird sprinklers. At Brohm Mining's Gilt Edge on-off heap leach
operation in South Dakota, a cyanide neutralization solution containing hydrogen peroxide has been
applied at a rate of 0.0043 gal/min/ft2 (Damon, Smith, and Mudder 1992).
Rinsing also may be accomplished, or enhanced, by natural precipitation; some facilities have
included precipitation as part of their detoxification plans (WGA 1991b). However, many cyanide
heap leach operations are located in arid areas of the western United States where precipitation rates
wouldn't be sufficient to be a source of rinse water.
The duration of rinsing required to reach a specified cyanide level in water leaving the base of the
heap may vary considerably. At the Borealis mine, rinsing continued for several months, with each
section of the pad being rinsed for 10-20 days. At the Gold Fields Operating Company's Mesquite
mine, the heaps were rinsed for five days. At the Snow Caps mine in California, rinsing continued
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••••«••—i^-^—
treatment of Cyanide Heap Leaches and Tailings
for a total of 160 days, with the effluent from the heap being treated to remove cyanide and then
returned to the sprinklers.
The amount of cyanide remaining in the heap and in effluent from it at the end of rinsing program
will depend on the hydraulic behavior of the heap as well as on its chemistry. At the Gold Maple
mine in Montana, for example, it was found that a heap which had been rinsed with calcium
hypochlorite during State-managed remediation activities contained a number of zones with high levels
of residual cyanide. Schafer & Associates (1991) believe that this poor rinsing performance was
partly due to the same hydraulic problems which had resulted in poor gold leaching efficiency and
had made the mine uneconomic.
3.2 Sulfur Processes
In the sulfur processes, cyanide in solution is oxidized to cyanate using sulfur dioxide or ferrous
sulfate and air in the presence of copper ion:
CN + SO2 + O2 + H2O -> CNO- + H2SO4
The sulfuric acid formed in the reaction is neutralized with lime. Cyanate may be less toxic than
cyanide to fish, animals, and humans. Higgs and Associates (1992) report that CNO" is 3,000 - 5,000
times less toxic than CN. The International Nickel Company's (INCO) SO2-air process is one of two
patented sulfur dioxide treatment processes. The other is patented by Noranda Inc. The INCO
process can be applied both to barren solutions and to cyanide-bearing tailings. Reagent requirements
may be higher for tailings. The Noranda Process has been used for wastewater solutions, but may be
applicable to heaps.
3.3 INCO Process
Equipment requirements for the SO2 process are relatively simple. Wastewater to be treated is
introduced into a mixing vessel, where it is reacted with sulfur dioxide or sodium bisulfite (Figure 1).
The theoretical SO2 requirement is 2.64 Ib/lb CN. INCO has reported actual dosages to be 3 - 5 Ib/lb
for clear barren solutions and 4 - 7 Ib/lb for tailings slurries. Air is sparged into the vessel. Copper
sulfate is added as a catalyst at a concentration of approximately 50 mg/1. The pH is controlled by
addition of lime. The optimal pH range is 8-10. While it is possible to feed the lime as a powder,
dry feeding equipment is troublesome and not suited to processes such as this which have short
residence times. A more reliable solution is to make up a slurry of lime and recirculate it through a
ring main. Electronically or pneumatically controlled valves can be opened to obtain long or short
pulses of well-mixed slurry. Similar lime dosing circuits are used in many ore flotation mills.
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UJ
eacfies and Tailings
Wastewater
containing
cyanide
Sulfur dioxide from
tanks or sodium
bisulphite powder
i
Clarifler
Lime slurry
makeup i ring main
system
Treated
wastewater
Metal-bearing
" sludge
Figure 1. INCO Sulfur Dioxide-Air Process
(Source: Adapted from Higgs 1992)
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INCO tests showed that a feed stream could be reduced from 1680 mg/1 CNf to 0.13 mg/1 CNT~
using a retention time of 97 minutes in a one-stage reactor. A feed containing 420 mg/1 CNT" was
treated to 0.11 mg/1 CNT" using two reactors in series with a retention time of 26 minutes in each.
These tests were conducted in a continuous flow apparatus (Ingles and Scott 1987). Total cyanide can
be reduced to 0.5 mg/1 or less in low nickel wastewaters. About 1 mg/1 is achievable in high nickel
wastewaters (Smith and Mudder 1991). Other data from bench-scale tests indicate that CNf < 0.1
mg/1 is achievable.
An INCO SO2-air treatment process was installed at Echo Bay's Cove-McCoy mine in Nevada in
1990. This system is designed to treat tailings pulp (Devuyst 1992). The pulp is 40 wt percent solids
and the flow rate of cyanide to the treatment process is 270 kg CN per hour. The authors do not
v
give a feed concentration, but at a total mill throughput of 8,500 short tons/day this would correspond
to a total cyanide concentration of about 335 mg/1. The addition rate of SO2 is adjusted based on
periodic analyses of the feed stream (feed-forward or anticipatory control). Fine adjustment is
provided by a feed-back or reactive control loop based on measurements of CN and pH in, the
reactors and in the effluent. This system was designed to be capable of reaching 5 mg/1 CNWAD, if
necessary.
The INCO process has also been applied to the detoxification of a heap leach pad at the Snow Caps
mine in California (Vergunst 1991). The existing barren pond was converted for use as a reactor by
adding an air sparging system. Sodium metabisulfite was used as the SO2 source. The pH was
controlled by addition of NaOH rather than lime; manual control of pH was found to be satisfactory.
The system was operated under total recycle, with all of the reactor effluent being returned to the top
of the leach pad, until total cyanide reached acceptable levels. After 130 days of operation, the total
cyanide level in the effluent was < 0.2 mg/1. Fresh water was then used to rinse the heap for an
additional 30 days, after which the heap and spent solution ponds were deemed to have met State
requirements for reclassification as "Group C" waste. This project was carried out using existing
mine equipment, a secondhand blower and some piping. Total costs to the mine operator were under
$ 125,000. A pre-construction estimate had indicated that merely doing nothing and allowing the
cyanide to degrade naturally to the levels specified by the State would have taken three years and cost
$ 1,500,000 for security and maintenance.
Limitations to the SO2 process appear to be that the reaction proceeds more slowly at low
temperatures. A drop in temperature from 25 °C to 5 °C can cause a tenfold decrease in reaction
rate. Correspondingly larger residence times and tank volumes would be required to achieve the
same CN' removal efficiencies at lower temperatures. The SO2 process generally does not remove
thiocyanate, cyanate, or ammonia. Cyanate can be transformed into ammonia by microbial action;
ammonia is toxic to fish. In addition, removal of to*ic mewls may not be sufficient to meet permit
requirements.
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Treatment of Cyanide Heap Leaches and Tailings
3.4 Noranda Process
Noranda, Inc. holds a patent for detoxification of cyanide in tailings effluent (US patent 4840735;
Canadian patent 1321429). The process was tested and is still in use at Noranda's Hemlo Gold
Mines, Inc. Golden Giant site in Northwestern Ontario, Canada. Although similar to the INCO
process, the Noranda Process was developed for the specific site and is well suited to ores with
significant antimony or arsenic concentrations. A representative of Noranda stated that if the cyanide
mine effluents were primarily cyanide and cyanide metals, and did not contain arsenic or antimony,
then the Noranda Process would likely be less economical than other existing methods of cyanide
detoxification. While the Noranda Process has been used to treat cyanide effluents, it may be
adaptable to heap detoxification. (Noranda 1994, Konigsmann et al 1989)
In the initial Hemlo tests, liquid sulfur dioxide and copper sulfate were added to the cyanide-
containing solution to destroy the cyanide, with a typical ratio of sulfur dioxide to total cyanide in the
feed solution of 7 to 1 (weight basis). Total cyanide concentrations were reduced from 47 mg/L to
0.15. Although the results were encouraging, Hemlo had significant safety concerns regarding the
storage of the sulfur dioxide, and developed a ferrous sulfate process. (Konigsmann et al 1989)
In the Noranda Process, copper and ferrous sulfate is added to the cyanide effluent, with the
following reaction:
Cu2+ -I- Fe2+ + 3OH- — > Cu+ + Fe(OH)3
In the presence of hydroxide ions, the ferrous ion is oxidized to ferric oxide while the cupric ion is
simultaneously reduced to cuprous ions. The cuprous ion removes free cyanide as an insoluble
precipitate of cuprous cyanide (Konigsmann et al 1989). The formation of cuprous cyanide creates a
shortage of free cyanide ions in solution, which leads to further removal of cyanide through
d -sociation of soluble metal cyanide complexes of copper, zinc and nickel into simple cyanide and
metal ions. Final removal of cyanide is completed by the addition of hydrogen hydroxide at high pH
in a second stage to oxide the residual simple cyanides. »
Noranda claims the copper requirements are directly related to the amount of cyanide present in the
wastewater, and that a 3 to 1 ratio of copper to cyanide is sufficient for effective cyanide removal.
At the Hemlo site, typical operating results reduced the total cyanide concentrations from 23 to 0.13
mg/L. (Konigsmann et al 1989)
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Treatment of Cyanide Heap Leaches and Tailings
3.5 Alkaline Chlorination Process
The alkaline chlorination process is one of the oldest cyanide destruction methods (Higgs 1992). In
this process, cyanide in solution is oxidized to cyanate using chlorine or hypochlorite in solution:
- + Cl2-> CNC1 + Cl-
CNC1 + 2OH --> CNO + Cl +H2O
Alkaline chlorination can be applied to both clear wastewaters and slurries. Equipment requirements
for the alkaline chlorination process are similar to those for the other two oxidation processes
(hydrogen peroxide, sulfur dioxide). Wastewater to be treated is introduced into a mixing vessel,
where it is reacted with chlorine or hypochlorite (Figure 2). The pH is maintained in the alkaline
range by addition of lime. Precipitated metals are removed in a clarifier before the wastewater is
discharged.
Smith and Mudder (1991) state that the first-stage reaction (cyanide to cyanate) requires
approximately 15 minutes at pH 10.5. Hydrolysis of the cyanate to ammonia and carbonate requires
an additional 1-2 hours.
The Giant Yellowknife mine reported that the process reduced CNT" from 7.8 mg/1 to 0.05 mg/1.
Fifteen days retention in a polishing pond reduced CNT' to 0.02 mg/1. These data were averages for
the 1984 operating year. At the Mosquito Creek mine, total cyanide was reduced from 310 mg/1 to
25 mg/1 and WAD cyanide was reduced from 226 mg/1 to 0.5 mg/1.
Few sites are currently using this technology. The Thunder Mountain Mine in Idaho operated from
1984 to 1991 (Mohr Undated). The mine was operated as an on-off heap leach. Leached ore was
treated by alkaline chlorination. This method was also used to treat wastewater generated by rinsing
of the pads during decommissioning. Effluent from the treatment process was disposed at a
"wastewater land application facility", so that there was no direct NPDES point source discharge.
However, the operator may be required to apply for a NPDES permit to cover storm water discharges
from the site under recently EPA promulgated regulations. Mohr (Undated) does not give any details
regarding cyanide destruction efficiency or operating parameters.
Generally, wastewaters which can be discharged indirectly through natural or artificial wetlands or
land treatment facilities do not need to meet the same requirements as direct discharges, because
natural processes in the wetland lead to additional cyanide destruction and metals removal.
Environment Canada conducted a study of this process at three mills in British Columbia and one m
the Northwest Territories during the period 1981 - 1983. The Giant Yellowknife mine used this
process followed by an arsenic precipitation step and a polishing lagoon to treat a wastewater whi«.h
10
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Treatment of Cyanide Heap Leaches and Tailings
Wastewater
• Sodium hypochlorlte or
Calcium hypochlorite or
Chlorine gas
Lime slurry makeuo &
recirculation system
Metal-Oearinq
sludge
Figure 2. Alkaline Chlorination Process
(Source: Adapted from Higgs 1992)
11
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Treatment of Cyanide Heap Leaches and Tailings
had relatively low CNT values. Reagent costs were very high, about CANS 46.50 per kg CNT" in
1983. This would correspond to approximately US$ 43.50 at 1992 prices. This was partly due to the
additional chlorine loading required to make the arsenic precipitation step operate properly, and partly
due to the very remote location of the mine, which resulted in high transportation costs.
Limitations of this process are that it does not remove iron cyanides, and chloramines and free
chlorine remain in solution; these are toxic to fish.
3.6 Hydrogen Peroxide Process
In the hydrogen peroxide process, cyanide in solution is oxidized to cyanate using hydrogen peroxide
in the presence of copper ion:
CM" + HA -> CNO- + H2O
Cyanate ion hydrolyses to form ammonia and carbonate:
CNO- + 2H2O -> CO32 + NH4+
This process can be applied to wastewaters. Reagent requirements increase when this method is
applied to slurries.
Equipment requirements for the hydrogen peroxide process are similar to those for the INCO process.
Wastewater to be treated is introduced into a mixing vessel, where it is reacted with hydrogen
peroxide (Figure 3). Copper sulfate is added as a catalyst. The pH is controlled by addition of lime.
Hydrogen peroxide is a strong oxidizer, which can give rise to violent explosions and fires if brought
in contact with combustible organic material (wood, old cloth rags). Specially designed storage tanks
and handling equipment must be used.
Griffiths (Degussa 1988) reported that a mine in northern Ontario, Canada was planning to use this
process. Under these cold conditions, batch tests indicated that 27 hours would be required to reduce
total cyanide from 25.7 mg/1 to 0.94 mg/1. Higgs (1992) indicates that retention times should be jn
the range of 45 minutes to 2 hours, but bench scale tests are needed for each individual waste stream.
An example of removal efficiencies is provided by the Annie Creek Mine (McGrew and Thrall, cited
in Brooks 1992). At this mine, effluent from a heap was reduced to 0.57 mg/1 CNf and 0.09 mg/1
CNWAD' after 97 days of rinsing with H2O2 solution.
The hydrogen peroxide process was applied at the Timberline mine in Utah (Brooks 1992). This gold
mine operated as a heap leach operation from 1984-1986. In 1989, the operator declared bankruptcy
and forfeited the bond to Tooele County. The sheriffs department arranged for the placing of 800
12
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Treatment of Cyanide Heap Leaches and Tailings
Hydrogen
peroxide
Wastewater
containing
cyanide
Lime slurry
makeup 8. ring main
system
1 !
'•••••••••••••••••••••••*••••••••••••••••••
Clarlfier
Metal-bearing
sludge
Figure 3. Hydrogen Peroxide Process
(Source: Adapted from Higgs 1992)
13
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leap Leaches and Tailings
pounds of calcium hypochlorite in the solution pond, but this was found not to be sufficient to
neutralize the cyanide leached from the pile, especially after heavy rainfall events. The county, the
State, and the U.S. Bureau of Land Management decided to treat the leached ore in lifts consisting of
layers one foot thick, using 0.01 gallons of H2O2 per ton of ore. After treatment, the ore averaged
6.3 mg/kg CNWAD- and 24 mg/kg CNf. By 1991, the cyanide levels were 2.12 mg/kg CNWAD and
8.46 mg/kg CNT" in the leached ore, and 1.11 mg/1 CN in rinsate samples. The state considers the
heap to be neutralized. This project was completed on a very low budget (the $ 20,000 bond) using
BLM personnel and equipment and volunteers from a nearby mining company.
The limitations of hydrogen peroxide treatment include handling and costs. In particular, hydrogen
peroxide is a hazardous material, and can be expensive. Special equipment for hydrogen peroxide
service may increase the total capital cost. The treatment process generates ammonia, which is toxic
to fish.
3.7 AYR Cyanide Recovery Process
4
In the handling of cyanide solutions, significant efforts are taken to ensure that the pH is always kept
in the alkaline range so that toxic hydrogen cyanide gas will not be released. The Acidification-
Volatilization-Recovery (AYR) process runs directly counter to this principle. The pH of a cyanide
solution is lowered by addition of sulfuric acid so that HCN gas is formed. This gas can then be
absorbed into a NaOH solution:
CN'(aq) + H+(aq) -> HCN(g)
HCN(g) + NaOH(aq) -> NaCN(aq)
The process has generally been applied to barren solutions. However, a system to handle slurries was
designed for the Golden Cross mine in New Zealand (Smith and Mudder 1991).
The Acidification-Volatilization-Neutralization Process for cyanide recovery is illustrated in Figure 4.
In this process, wastewater containing cyanide is mixed with sulfuric acid, liberating HCN gas. The
mixing vessel must be sealed. The liquid stream leaving the reactor is stripped with a current of air
in a packed column. The HCN-laden air is absorbed in a second column containing a downward-
flowing stream of caustic soda, forming sodium cyanide. This can be returned to the leaching
process. Lime is added to the detoxified wastewater to precipitate heavy metals. Based on bench-
scale tests, total cyanide levels were routinely reduced from 330 mg/1 to < 2 mg/1. (Smith and
Mudder 1991). No examples could be found in the literature of an application of this technology in
the United States.
An early version of this technology was operated by Hudson Bay Mining and Smelting, Flin Flon.
Manitoba from 1931-1978. Four stripping towers were used in series. The cyanide content was
14
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Treatment of Cyanide Heap Leaches and Tailings
Sealed vessel CN stripping
column
Ltme slurry
makeup & ring ma"
system
Clartfier
' 3 fudge
Figure 4. AYR Cyankfc Recovery Process
(Source: Adapted from Higgs)
15
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eaches and Tailings
lowered from 560 mg/1 to 44 mg/1 and the resulting effluent, which contained copper cyanide, was
fed to a copper sulfate plant. As an example of duration, the process as applied at Flin Flon,
Manitoba, had a liquid flowrate in the cyanide stripping column of 107 m3/hr and an air flowrate
about 525 times greater. This process was also being used in 1984 at a silver mine in Mexico. A
more modern version of the process was operated from 1985-1987 at the Beaconsfield gold mine in
Tasmania. The system was designed for maximum safety, incorporating an enclosed negative
pressure system. Cyanide recoveries of nearly 95 percent were reported. (Smith and Mudder 1991)
One of the advantages of this technology over the treatment alternative is that cyanide can be
recovered for reuse. The economics may be favorable in very remote locations where the costs of
cyanide threaten the economics of the mining project. In addition, the potential aquatic toxicity of
cyanide oxidation products (cyanate, ammonia, chloramines) does not arise.
The major limitation of this technology is that it is a more complex process than the various treatment
alternatives. Sealed mixing vessels and packed columns are required. All mining operations
involving cyanide are operated under alkaline conditions to avoid the evolution of HCN. This process
may be perceived as too hazardous because HCN is deliberately generated.* It also has not been
conclusively demonstrated that this technology can achieve allowable discharge limits for CNT~ in this
country. The economics may vary depending on the value of the recovered cyanide. OSHA, EPA,
and insurance requirements associated with the handling of free HCN in the U.S. may increase both
capital and operating costs and limit the applicability of cost estimates from other countries. Studies
in New Zealand indicated that the AYR process could generate an operating profit of NZ$ 2.15-3.20
per ton of ore processed. However, cyanide costs in New Zealand are 3-5 times higher than those in
the United States.
3.8 Biological Treatment
3.8.1 Introduction
Microbial action, either naturally occurring or as a cyanide detoxification technique, causes
transformation of cyanide to ammonia. Metal ions released from metal cyanides will be absorbed by
the biomass and thiocyanates are converted to sulfate:
Cu2CN + 2 H2O + 14 O2 -> Cu-biofilm + HCO3 + NH3
SCN- + 2 H2O + 2V4 O2 -> SO42 + HCO3
16
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Treatment of Cyanide Heap Leaches and Tailings
Further microbial action will convert the ammonia to nitrate:
NH4+ + 1V4 O2 -> NO2- + 2 H+ + H2O
NO2 + l/2 O2-> NO3
The aim of biological treatment processes is to greatly increase the rate at which these natural
transformations occur.
Until recently, known applications of this technology were confined to barren solutions. A Homesite
Mine has used biological treatment on cyanide wastewaters. In addition to Homestake, biological
treatment has been applied at the Hecla Yellow Pine heap in Idaho. Hecla's treatment has reduced
cyanide levels to the State detoxification criteria of 0.2 mg/1 WAD cyanide (Idaho 1993). Pintail
Systems Inc. has recently applied biological treatment to heap leaches at the Hecla Yellow Pine Site.
3.9 Homestake Mine, South Dakota
The Homestake gold mine in South Dakota has also implemented a biological treatment system. A
simplified flowsheet of he Homestake plant is shown in Figure 5. Wastewater to be treated is dosed
with phosphoric acid, which acts as a nutrient. The water is then fed to the first set of rotating
biological contactors (RBCs). These units consist of a shaft of circular plastic elements revolving
partly submerged in a contour-bottomed tank. The disks are spaced such that wastewater can enter
between them. When rotated out of the tank, air enters the spaces while the liquid trickles out over
films of biological growth attached to the media. The biomass consumes phosphate and other
nutrients in solution, and also converts cyanide to ammonia. Excess biomass flows out of the unit
with the wastewater. A second set of RBCs is used to convert the ammonia to nitrate. Toxic metals
and biomass can be precipitated using ferric chloride and a polymeric agent. A sludge of metals and
microbes then settles out in the clarifier, and can be removed for disposal. The effluent is polished
using a sand filter before being discharged. (Ingles and Scott 1987; Higgs 1992)
The Homestake mine reduced total cyanide levels from 10 ppm (feed level) to 0.3 ppm cyanide in the
effluent.
The US Bureau of Mines conducted a demonstration project to study the viability of bacterial cyanide
oxidation at USMX's Green Springs mine in Nevada (Lien and Altringer 1993). Staff at the Bureau »
Salt Lake Research Center had isolated a cyanide-degrading bacterium, Pseudomonas
pseudoalcaligenes, from a tailings pond containing 280 mg/1 CM". This bacterium was cultivated ai
the Research Center and introduced into the carbon adsorption tanks at Green Springs. The tanks.
transformed into bioreactors, were used to treat water being recirculated through the ore heaps as part
17
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PNJSDfiortc acid
Sodium nydrOKlde
ting Biological
contactors
lor conversion or cyanide
to ammoni*
Clarlfier
Rotating Biological contactors
for conversion of ammonia
to nitrate
Sand niter
Sludge containing
metals and Diomass
Treated
wastewater
Figure 5. Biological Treatment Process
(Source: Adapted from Higgs 1992)
18
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Treatment of Cyanide Heap Leaches and Tailings
of the closure process. WAD cyanide was reduced from 20 mg/1 to 8.5 mg/1 over a period of 15
weeks.
With regard to duration, performance of RBCs depends in large part on the loading of Biochemical
Oxygen Demand (BOD) per unit area of contactor surface. At the Green Springs mine, residence
times in the five carbon tanks were on the order of 80 minutes.
3.10 Pintail's Biotreatment Process
The Pintail System Inc. approach to biotreatment of heap leach pad cyanide solutions uses bacteria
native to the project environment. The process involves isolating and enhancing native bacteria with
the ability to use or transform cyanide into non-toxic components: carbon dioxide, water and
nitrogen. These "working" bacteria are grown to concentrations capable of supporting effective
biotreatment, while the "non-working" bacteria are selectively eliminated from the microbial
community. Pintail has researched both heterotrophic and autotrophic strains of bacteria for cyanide
detoxification. As long as the biotreatment solutions are heated and the solution is effectively applied
to the heap, cyanide biotreatment in cold weather is possible (Caldwell, 1993).
Pintail Systems Inc. conducted a full scale cyanide detoxification project at a heap leach pad at the
Hecla Mining Company's Yellow Pine Mine near McCall, Idaho (Pay Dirt, 1992). The Yellow Pine
mine, although located at 6,500 ft. above mean sea level, is in a geographic location that experiences
extreme weather conditions similar to an alpine location. This cold temperature environment presents
hurdles to the use of bacteria, since the bacteria live and flourish under warm temperatures.. The
subject of the biotreatment was a heap leach pad 114 feet high, containing 1.3 million tons of material
and cyanide solutions with an average WAD cyanide concentration of 46.6 ppm. Bacteria used for
the treatment were collected at Yellow Pine from the soil and freshwater environments. To take
advantage of the most favorable weather conditions, the detoxification project began in March 1992
when 10,000 gallons of treatment bacteria solution were added to the barren solution pond. In this
manner, the treatment bacteria solution was applied to the spent ore by drip irrigation. By May 1992,
WAD cyanide levels were below 0.2 ppm. The project ended in mid-September of the same year.
Biological treatment does-not require the use of toxic or hazardous chemicals, apart from a small
volume of phosphoric acid, which can be stored as a dilute solution. Rotating biological contactors
were originally developed for treatment of sewage, and are an alternative to the use of the activated-
sludge process with fixed aeration basins and two stages of clarification. Their operating
characteristics are well understood.Limitations include the fact that it may not be possible to treat
wastewaters containing high concentrations of cyanide and the process may be adversely affected by
cold temperatures. Capital costs may also be higher than for the oxidation processes. Capital cost of
the Homestake installation has been reported to be about $ 10 million in 1984. In addition, system
response to a sudden change in cyanide or nutrient concentration may be sluggish. Canadian sources
19
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^ -
lave voiced concerns that such a process would not work at the low temperatures encountered in
many of their mining districts.
3.11 Natural Degradation
Natural degradation is a general term for all of the processes that may reduce the total cyanide
concentration of a waste in the absence of any human intervention. These processes include:
• Microbial generation of cyanate/ammonia in soil:
CN + V4 O2-+ enzyme -> CNO
CNO + H2O -> NH3 + CO2
• Volatilization of cyanide from solution after absorption of CO2 or SO2 from the atmosphere
and consequent formation of acid:
CO2(g) + H2O -> H2CO3(aq) ** HCO3 + H3O+
H30+ + CN--> HCN(g)
• Hydrolysis in soils:
HCN + 2 H2O -> NH4COOH
• Anaerobic biodegradation:
CN- + H2S(aq) -> HSCN + H+
HSCN + 2 H2O -> NH3 + H2S + CO2
• Complexation:
Zn(CN)2 + 2 CN- -> Zn(CN)4
Natural degradation processes, to some extent, occur in barren solution lagoons, tailings
impoundments, and heaps. The efficiency of cyanide destruction may be lower in the interior of
heaps or the bottoms of lagoons. No technology is required. Environment Canada found that total
cyanide in a tailings pond at Dome Mines, Ontario, decreased from 68.7 mg/1 to 0.008 mg/1 over a
15-week period from April 30 to August 6, 1980 (Todd 1986). Rates of removal at other sites may
be much slower than this.
Echo Bay's Lupin mine uses natural degradation as its sole method of treatment. Tailings are
discharged to a series of two surface impoundments with a total retention time of two years. Water
from the second impoundment is discharged once a year. Water can then be transferred into this unit
20
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Treatment of Cyanide Heap Leaches and Tailings
from the first impoundment. The mill tailings had an average total cyanide concentration of 166 mg/1
in 1991. The effluent from the second impoundment had a total cyanide concentration of 0.019 mg/1.
Effluent concentrations ranged from 0.06-0.26 mg/1 CNT" over the period 1985-1990. (Higgs 1992)
It is unclear, however, if natural processes can generally be used to meet Federal or state standards.
One site in Colorado (Battle Mountain Resources, Inc.) planned solely on using natural degradation to
reduce cyanide levels in its tailings slurry, tailings impoundment, and collection pond. Natural
degradation, however, was unsuccessful in reducing cyanide to levels required in the permit (4.4 ppm
total or 3.8 ppm wad cyanide). In 1991 and 1992, elevated cyanide concentrations (up to 260 ppm
total, 240 ppm free and 110 ppm wad) led to a notice of violation and issuance of an administrative
order (Colorado Mined Land Reclamation Board 1992).
While natural degradation does not require capital investment or chemical costs, it may never reduce
cyanide levels to within the limits specified by state agencies. Information on other constituents was
not obtained. It should also be noted that while natural degradation is occurring, the waste may
continue to pose a threat to humans and animals. In addition, security costs for preventing public
access over a period of several years may prove to be very high.
21
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Treatment of Cyanide Heap Leaches and Tailings
4. OTHER RELATED ISSUES
4.1 Closure and Reclamation Issues
»
Cyanide is not the only contaminant that is present in tailings effluents or heaps; numerous other
constituents may be present in the waste material and present potential problems. Nitrate and heavy
metal migration are examples of other problems that can be faced at closure of cyanide operations.
Testing and analysis of cyanide is also an issue because of problems obtaining consistent and reliable
test results. Another significant concern is the generation of acid drainage, often caused by the
presence of sulfides that break down to form sulfuric acid. Issues relating to metals, acid generation,
and operational problems encountered by some facilities are discussed in greater detail below.
4.1.1 Analytical Methods
In developing the national effluent limitation guidelines for the Ore Mining and Dressing Point Source
Category (at 40 CFR Part 440), the Agency established a technology-based standard for all discharges
from mills that use the "cyanidation" process to recover gold and silver, and mills that use cyanide in
froth flotation of copper, lead, zinc, and molybdenum ores. In this process, the Agency considered
several methods (e.g., alkaline chlorination, hydrogen peroxide treatment, etc.) used to reduce
cyanide levels in mill wastewaters. However, the Agency found that the cyanide levels in both .
treated and untreated mill wastewaters were below the 0.4 mg/1 quantification limit for EPA-approved
test methods (i.e., treatment performance could not be evaluated). Therefore, the Agency established
a zero discharge requirement as the national technology-based standard. The Development Document
for the Part 440 guidelines does indicate that EPA was aware of specific sites where laboratory
methods were effectively being used to quantify cyanide removal (but does not describe the methods).
The document further suggests these methods could be used by permit writers to establish cyanide
limits in individual NPDES permits on a site-by-site basis.
Analytical methods used to determine cyanide concentrations in tailings, heap effluents, and pore
water are still being debated. At low concentrations, testing is inaccurate and measurements of
cyanide may not be good predictors of actual cyanide concentrations in the field. (Durkin 1990;
Colorado 1992a; ORD 1993)
Cyanide is generally measured as one of three forms: free, weak acid dissociable (WAD), and total.
Free cyanide refers to the cyanide that is present in solution as CM' or HCN, and includes cyanide-
bonded sodium, potassium, calcium or magnesium. Free cyanide is very difficult to measure. WAD
cyanide is the fraction of cyanide that will volatilize to HCN in a weak acid solution at a pH of 4.5.
WAD cyanide includes free cyanide, simple cyanide, and weak cyanide complexes of zinc, cadmium.
silver, copper, and nickel. Total cyanide measures all of the cyanide present in any form, including
iron, cobalt, gold and platinum complexes. (Colorado 1992a)
22
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Treatment of Cyanide Heap Leaches and Tailings
Many states are continuing to debate over the proper test methods for measuring cyanide (WAD, free,
or total). A South Dakota hydrologist with the State Department of Environment and Natural
Resources (DENR) points out that many of the commonly used test methods for cyanide leaching
yield questionable results for certain parameters. (Durkin 1990)
Mudder & Smith point out that historically, cyanide was regulated as "free" cyanide, but that newer
standards specify weak acid dissociable (WAD) cyanide. "Free" cyanide has been shown to be
analytically inexact at desired regulatory levels and WAD cyanide levels are more easily determined
below one part per million (ppm) and more relevant from an environmental standpoint. (Mudder and
Smith 1992)
t
EPA's Office of Research and Development (ORD) is currently evaluating cyanide test procedures
and methods, and is investigating a proprietary, privately developed, distillation method that appears
to be successful for cyanide analysis. One of ORD's activities includes revising the current methods
for measuring and detecting cyanide fractions and eliminating interferrents. ORD is also reviewing
performance data and problems of 17 currently used methods. Future efforts of ORD will involve
continued evaluation of cyanide species. (ORD 1993)
4.1.2 Mobility of Constituents In Heaps and Impoundments
Because of the great variability among cyanide operations, including ore characteristics and climatic
conditions, adequate characterization of wastes and materials is an important consideration for site
reclamation. Aqueous cyanide (CM') has a negative valence and reacts readily to form more stable
compounds. At a pH below 9 (approximately), cyanide forms hydrogen cyanide (HCN), a volatile
gas that rapidly evaporates at atmospheric pressure. Aqueous cyanide complexes readily with metals
in the ore, forming complexes ranging from readily soluble complexes such as sodium cyanide and
calcium cyanide to strong complexes such as iron-cyanide. The stronger complexes are very stable in
natural aqueous conditions. Tables 1 and 2 provide the solubility of some of the cyanide complexes.
(Colorado 1992a)
Limited information was found on the mobility of cyanide and cyanide complexes in closed and/or
reclaimed heaps and tailings impoundments. However, at several South Dakota sites, nitrate, one of
the degradation products of cyanide, has been detected in areas beyond the heap. Operators have
been able to meet the 0.2 mg/1 cyanide detoxification criteria, but elevated levels of nitrate associated
with cyanide heap leach on/off operations have prevented facilities from meeting other site-specific
state criteria. The nitrate levels in surface runoff from the mine sites have exceeded treatment criteria
and low levels of nitrate have been detected in downgradient wells. (Durkin 1990)
In addition, the chemistry of a spent heap or tailings impoundment may change over time. Although
effluent samples may meet State requirements, the effluent characteristics may be dependent on the
pH. The question of what happens to the heap or impoundment when the pH or moisture content
23
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Treatment of Cyanide Heap Leaches and Tailings
Table 1. Relative Stabilities of Cyanide Complexes in Water
Cyanide Species
Free Cyanide
Simple Compounds
Readily Soluble
Relatively Insoluble
Weak Complexes
Moderately Strong Complexes
Strong Complexes
Examples Present in Gold and Silver
Processing Solutions
CN-, HCN
NaCN, KCN, Ca(CN)2, Hg(CN)2
An(CN)2, CuCN, Ni(CN)2, AgCN
Zn(CN)42-, Cd(CN)3-, Cd(CN)42'
Cu(CN)2-, Cu(CN)32-, Ni(CN)42-, Ag(CN)2'
Fe(CN)6\ CotCN)/-, Au(CN)2'
Source: Colorado 1992a.
Table 2. Some Metal-Cyano-Complex Ions and Their Stability Constants
Metal
Cobalt (III)
Iron (III)
Iron (II)
Nickel (II)
Cadmium (II)
Manganese (III)
Complex Ion
Hexacyanocobaltate
Ferricyanide
Ferrocyanide
Tetracyanonickelate
Tetracyanocadmiate
Hexacyanomanganate
Formula
[Co(CN)6]3-
[Fe(CN)6]3-
[Fe(CN)6r
[Ni(CN)4]2'
[Cd(CN)4f
[Mn(CN)6]3-
Stability Constant
(at 25° C)
IxlO64
IxlO52
IxlO47
IxlO22
7. IxlO16
5xl09
Source: Colorado 1992a.
24
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and ladings
changes is an important consideration for closure and reclamation. Modeling can be performed to
assess the long-term geochemical conditions at the site taking into consideration the chemistry of a
spent heap over time, and be used to design closure and reclamation plans. Factors affecting
chemical changes in a heap or tailings impoundment include pH, moisture, mobility, and geochemical
stability of the material.
4.1.3 Reduction of Constituents in Solutions
According to Mudder and Smith, in addition to high cyanide concentrations, the post-leach solution
(pre-cyanide treatment) is likely to have the following characteristics:
• HighpH, 9.5 to 11
• Moderate to high dissolved species, mainly sodium, calcium (from added lime), and sulfate
• Potentially elevated metals of ionic-forming complexes such as arsenic, molybdenum, and
selenium
• Potentially elevated metals which form soluble metallo-cyanide complexes such as iron,
copper, mercury, cadmium, and zinc. (Mudder and Smith 1992)
One site, the Pegasus Gold Relief Canyon mine, used carbon columns to reduce the levels of soluble
metals contained in its heap rinse solution. Specific metals, their concentrations and ultimate disposal
practices were not obtained. (Logue Undated)
Placer Dome, Inc.'s Campbell gold operation added an arsenic treatment step to its tailings treatment
process to remove arsenic from its tailings. Placer Dome uses an autoclave and two neutralization
stages to precipitate arsenic and heavy metals. The resulting tailings slurry is then disposed of in a
plastic-lined tailings pond. Arsenic concentrations in the tailings slurry have been reduced from
around 1.2 mg/1 to <0.3 mg/1 since the facility added the arsenic treatment as part of its conversion
from a roasting system to a pressurized oxidation process for gold recovery. (Mining Engineering
1992)
A representative from Nevada described a problem that occurred with one mine. While recirculating
the solution during leaching, gold was removed from the pregnant solution but other metals and
constituents continued to accumulate and were not removed from the solution. As a result, during
rinsing, the mercury levels in the rinse water were 4.0 mg/1, three magnitudes higher than the
primary drinking water standard of 0.002 mg/1. The tremendous amount of water required for
consecutive rinses in order to reach the 0.2 mg/1 cyanide standards has also been an issue. (Nevada
1993b)
Furthermore, a research study presented at the Environmental Management for the 1990s symposium
noted that a several mines in five western states have experienced elevated selenium levels (Altringer
25
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Treatment of Cyanide Heap Leaches and Tailings
1991). The US Bureau of Mines is investigating the use of biological and chemical reduction of
selenium in cyanide tailings pond water. Although high costs may make the treatment prohibitive, the
research study was successful in reducing selenium concentrations in the laboratory from up to 30
ppm selenium to 0.02 ppm. Details on the mine sites and specific treatment practices used were not
available in the information reviewed for this study.
4.1.4 Rinsing/Treatment Duration
Section 3 described rinsing/detoxification periods of several months; however, in practice a site may
require several rounds of rinsing in order to meet State or Federal standards. One problem that
frequently has been encountered is that rinsing/treatment is conducted and effluent standards may be
met, but subsequent rinsing or testing reveals increased cyanide and other constituent concentrations.
(Nevada 1993b) Spring snowmelts have caused effluent concentrations to rise. Several States, as
well as BLM, request follow-up effluent sampling after periods of rest or after rainy season/spring
snowmelts prior to approving completion of detoxification. (BLM 1992; Idaho 1993, South Dakota
1993) Although the reasons for incomplete or variable rinsing have not been confirmed, Durkin
(1990) proposes that non-uniform neutralization or dilution may be factors. A number of facilities
have had to switch treatment methods after a chosen method did not reach the desired concentrations.
Thus, in practice, rinsing may take many seasons, or years, to complete.
In addition, climatic conditions effect the amount of time needed for closure and reclamation. Cold
weather effectively shuts down many operations. Natural and biological treatment methods cease
naturally at low temperatures over the winter months. (Schafer and Associates 1990; McGill &
Comba 1990; BLM 1992; Higgs 1992)
Furthermore, according to a Nevada Bureau of Water Quality representative, the cyanide rinsing
standard of 0.2 mg/1 WAD cyanide has been difficult for many operators to achieve, and the mining
community would like to see the standard changed. Agglomerated heaps are more difficult to rinse
because aggregating the material (lime, etc.) keeps the pH elevated, which in turn makes reduction of
pH and detoxification of cyanide more difficult. The Trinity mine near Lovelock, Nevada operated
an agglomerated heap; at closure initial WAD cyanide concentrations were 400 - 500 mg/1. The
facility proposed using natural degradation to reduce the cyanide concentrations, but has had little
success to date at lowering the cyanide levels via natural degradation. A final decision on the Trinity
mine was not available; the amount of time since operations ceased also was not obtained. In several
instances, the State has issued or is considering variances from the rinsing criteria. (Nevada 1993c>
4.1.5 Water Balance
Water balance can be a concern at some sites. In arid regions, with limited water resources, the
amount of water that is necessary to rinse heaps to a required standard may be a significant concern
Conversely, in wet climates like South Carolina, excess water from heavy precipitation can place j
26
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treatment of Cyanide Heap Leaches and Tailings
strain on system operations and may make draining or revegetating a heap or impoundment very
difficult. (ELI 1992) South Carolina has experienced severe problems as a result of weather
conditions, such as heavy and persistent rainfall causing flooding, leaks, dam compromises, etc.,
making closure difficult. South Carolina has also had trouble with revegetation at the Brewer mine
facility heap leach pad; details were not available for this report. (South Carolina 1993). Sudden
snowmelt also can affect operations.
4.1.6 Percolation of Solution through Heaps
The presence, or potential for "blind-offs" in heaps may cause incomplete neutralization or treatment.
Blind-offs are less permeable lenses or isolated areas of a heap that affect percolation and flow
through the heap, leading to preferential paths for fluid migration. Research suggests that preferential
flow paths and blind-offs increase with time and volume of liquid. These preferential flow paths can
limit the effectiveness of treatment and may leave pockets of contaminants behind in a heap during
closure, which then have the potential to leach out after reclamation.
4.1.7 Acid Generation
Acid generation may be a major problem facing many mines. At one time, acid generation at cyanide
sites was not considered to be a potential problem as many mining facilities used only oxide ores (not
sulfide ores). However, cyanide leaching facilities have reported cases of acid generation. Even
tailings that were originally alkaline have subsequently experienced acid generation. Although lime
may be added during cyanide leaching, with residuals existing in tailings or agglomerated heaps, the
lime component eventually washes away through weathering leaving sulfide compounds to form acid
drainage. (Ritcey 1989; California 1993b)
Colorado's Summitville mine, for example, has experienced a number of problems including acid
drainage, water in excess of its calculated water balance, liner failure, and inability to reduce silver
and copper levels to meet surface discharge limits. Following the operator's bankruptcy, the site is
now undergoing costly "removal" action under CERCLA and has been proposed for listing on the
NPL. (Danielson and McNamara 1993)
27
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Treatment of Cyanide Heap Leaches and Tailings
5. REGULATORY PROGRAMS
The following section provides a brief overview of those Federal and state requirements that are
specific and unique to waste management at cyanide leach operations, such as programs related to
cyanide tailings impoundments, spent heaps and pads, and solution wastewater. The section is not a
comprehensive summary of all the regulatory requirements that apply to a cyanide facility but rather,
introduces some of the key Federal and State programs involved in the oversight of cyanide
operations.
5.1 Federal Requirements
5.1.1 Environmental Protection Agency - NPDES Program
The Environmental Protection Agency's (EPA) National Pollution Discharge Elimination System
(NPDES) program requires permits for all point source discharges to surface water. For most
industries, technology-based discharge effluent limits have been established using best available
technology.
The standard established in 40 CFR 440 Subpart J for mills that beneficiate gold or silver by
cyanidation is zero discharge: such mills may not discharge process wastewater unless they are in
areas where net precipitation exceeds net evaporation. In such areas, mills may discharge the
difference between annual precipitation and evaporation, subject to National effluent limitations for
total suspended solids, copper, zinc, lead, mercury, cadmium, and pH, and subject to other standards
established on a case-by-case basis.
Because tailings dams may leak, the point at which the zero discharge limitation is applied may be
downgradient of the impoundment. In such cases, seepage from the impoundment is collected in
ponds and pumped back to the impoundment during the active life of the facility.
It should also be noted that the effluent limitation guideline was developed for mills that use cyanide
and predates the widespread use of heap leaching to recover gold. However, the zero discharge
standard has been universally applied to heap leach operations. Although there are provisions for
permit applicants, on a case-by-case basis, to seek different limits based on "fundamentally different
factors" that apply to their discharges relative to those studied by the Agency in setting the standards,
to date there have been no known requests for different limits.
In many cases, seepage and runoff from impoundments, spent ore piles, and waste rock piles has not
been considered to be a point source discharge and thus has not been subject to NPDES permits.
However, EPA and States are currently in the process of developing general NPDES permits for
currently unpermitted discharges of these types under the storm water program. This program now
28
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^yaniae neap Leaches and Tailings
requires permits for all point source discharges of storm water from active and inactive mine sites,
and general permits began to be promulgated in late 1993.
The Federal drinking water maximum contaminant level (MCL) for cyanide is 0.2 mg/1. In the
absence of an effluent guideline for inactive operations, this level is often used as a measurement for
acceptable effluent quality following closure.
5.1.2 Bureau of Land Management
The Federal Land Policy and Management Act of 1976 (FLPMA) requires the Department of Interior
to prevent unnecessary and undue degradation of the public lands. The Bureau of Land Management
Policy for Surface Management of Operations Utilizing Cyanide or Other Leaching Techniques,
issued in August 1990, describes minimum acceptable design requirements, mandatory waterfowl
death and discharge reporting, and quarterly inspection requirements for facilities located on Federal
lands, (BLM 1990a, GAO 1991) A Cyanide Advisory Committee oversees the cyanide policy at
BLM.
BLM issued the policy in response to the increased use of cyanide heap leach technology on public
lands. The policy outlines general activities and standards to be implemented by the state or district
offices. The policy was issued to ensure that operations that use cyanide or other solutions lethal to
humans, wildlife, or livestock are conducted in a manner that ensures the safety and protection of the
public and the public lands. (BLM 1990a)
According to BLM, the NEPA process will be used to evaluate impacts of proposed cyanide
operations. As with other types of mining, cyanide leaching facilities must file a plan of operation
*s»
with BLM. Training of BLM personnel has and will continue to be conducted, including inspection
and enforcement training, with plans to conduct quarterly inspections of cyanide operations. BLM
has plans for a core group of cyanide management experts.
Facility requirements contained in the policy address use of best practicable technology and measures
to fence active areas including ditches and conveyances containing cyanide. All tanks containing
lethal solutions are to be bermed. Leak detection and recovery systems are required for heaps and
solution containment structures. Facilities must have overflow ponds for cyanide solution containmem
and for runoff from leach pads; containment should be sufficient for the maximum operating water
balance plus runoff from a 100-year 24-hour storm event: Weekly samples of sublethal cyanide
solutions are to be collected from open containment and transfer structures.
On August 14, 1990 BLM issued a Modification of Bonding Policy for Plans of Operation Authorial
by 43 CFR Part 3809. This modification requires operators who use "cyanide/other leachates" to
post a bond equal to 100 percent of estimated closure costs. The bonding policy modification was
29
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Treatment of Cyanide Heap Leaches and Tailings
applicable to leach heaps, pads, and cyanide-bearing tailings impoundments and ponds, but did not
apply to vat leach facilities using cyanide.
Since the original August 1990 cyanide policy and bonding modification, BLM has issued two
additional changes to its cyanide policy. The first change, issued on October 9, 1991, was an
additional modification of the Bonding Policy. It removed the vat leach exemption from the 100
percent estimated closure cost bonding requirement. The second change, issued on October 10, 1991,
recommended rotation of trained BLM personnel in an effort to improve cyanide inspections.
BLM policy requires bonds for the full cost of reclamation, including heap and solution detoxification
and neutralization to State and Federal standards, for all cyanide operations on Federal lands. BLM
requires that cyanide solutions and heaps be neutralized or detoxified prior to solution release to the
environment. Neutralization of cyanide solutions is also required for any prolonged period of
inactivity and for temporary or final closure. Specific concentrations for neutralization or
detoxification levels are not specified in BLM policy. Heaps must be neutralized upon completion of
each heap. Flushing alternatives may be used, but heap materials and/or discharges must meet the
appropriate state and EPA discharge limits. The conditions necessary for release of bond were not
addressed in the BLM policy.
Monitoring of groundwater and surface water through closure and final reclamation is required.
Specific monitoring requirements such as the frequency, location, chemical parameters, and analytical
methods were not outlined in the policy and are left to the discretion of the state and BLM district
offices. Additional details on detoxification, closure, and reclamation of cyanide operations are not
addressed in the BLM policy.
In 1992, BLM issued its Solid Minerals Reclamation Handbook with guidance on reclamation of
mining sites on Federal and Indian lands (BLM 1992). The manual specifically addresses cyanide
heap and vat leach systems and provides general reclamation guidance and approaches. According to
the BLM, the mine reclamation plan should cover cyanide detoxification of residual process solutions,
ore heaps, tailings impoundments, and processing components. BLM strongly encourages laboratory
and pilot test studies of selected/proposed detoxification. Concurrent reclamation during active
mining also is recommended. In the Handbook, BLM does not require any specific metal or cyanide
concentrations that must be achieved. Criteria are established on a site-specific basis reflecting any
special concerns of the area. The Handbook is written as a general "how to" manual as opposed to
setting specific requirements of procedures that must be followed. It discusses the various methods of
treatment available (hydrogen peroxide, natural degradation with fresh water rinse, alkaline
chlorination, etc.) and outlines the various phases of reclamation (treatment of cyanide solutions,
disposal of treated solutions, spent heap and tailings, shaping and revegetation, surface water
diversions, process ponds, and liner disposal).
30
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Treatment of Cyanide Heap Leaches and Tailings
BLM recommends allowing an extended period of time, six months or more, between cessation of
neutralization and evaluation of effluent when determining the success of neutralization or
detoxification. The extended period should cover a spring run-off or substantial precipitation event.
Once this has been done, surface reclamation can begin. (BLM 1992)
BLM recommends breaching the heap pad liner or tailings containment dike after detoxification
criteria have been met, and adding sized rock to promote infiltration. Precipitation is thus allowed to
drain and accumulation of liquids is averted. Accumulation of liquids may generate leachate or
adversely affect heap or tailings structural stability. (BLM 1992)
Reclaimed heaps should be reduced in slope to at least 2h:lv, with bench terracing for slopes greater
than 200 feet in length. Reclaimed tailings should have flatter slopes in order to resist erosion of
fine-grained material (wind or water erosion) and allow for revegetation. (BLM 1992)
BLM (1992) allows for land application of treated rinse solutions and pond water, and mixing of
nonhazardous pond sludges with cement and disposal (burial) onsite.
5.1.3 U.S. Forest Service
The Forest Service has not developed a cyanide policy. Cyanide leaching operations are handled in
the same manner as any other mining operation on Forest Service land, requiring submittal of a plan
of operations to the appropriate district or field office. Each operation plan must address closure and
final reclamation activities but there are no specific cyanide requirements for closure and reclamation.
The Forest Service does not require dilution or detoxification to specific cyanide concentrations; each
plan of operation varies depending on site operations, terrain, distance to surface water, etc. (GAO
1991; USFS 1993)
5.1.4 National Park Service
The National Park Service has published a cyanide handbook (Environmental Handbook for Cyanide
Leaching Projects, June 1986) providing general guidance on the fundamentals of cyanide leaching
and environmental safety and controls. The handbook provides a brief overview of cyanide
decommissioning and reclamation, but specific standards, such as detoxification requirements or
cyanide concentrations, have not been published. The National Park Service appears to defer to
individual states, or other local authority, for specific cyanide guidance and regulatory authority, but
may be involved on a case-by-case basis depending upon site-specific concerns. (National Park
Service 1986)
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Treatment of Cyanide Heap Leaches and Tailings
5.2 State Requirements
In the following paragraphs, a summary of applicable state requirements for cyanide operations are
reviewed. This discussion does not include a complete characterization of all applicable mining
programs. An attempt has been made to highlight those factors that are unique to cyanide operations
that may vary from state to state, such as rinsing criteria. Construction design and operating
standards for heaps and tailings are briefly discussed, as they affect closure and reclamation. The
requirements for California, Colorado, and Montana are up to date as of 1992. The Agency is aware
that substantive changes in the regulations in these states have been made since that time. For more
information, the reader should contact the state.
States do not have prescribed technologies that must be used for detoxification, but rather rely on
performance standards requiring detoxification to specific criteria. If a site is unable to meet criteria,
a state will often issue a variance based on an alternative treatment method or treatment standard.
Alternatives are typically supported by pathway fate and transport analysis or modeling. For
detoxification and closure, monitoring parameters typically include WAD, free and/or total cyanide,
pH, and metals.
Table 3 provides a comparison summary of various state treatment or detoxification criteria for wastes
and highlights unique reclamation and/or bonding requirements. The table illustrates that there is not
a uniform standard in terms of concentration or constituent specie. Many states use these levels as
guidelines and may issue site-specific variances if a facility is unable to meet these levels. Several
states are debating between WAD cyanide and free or total cyanides, thus, the cyanide species/levels
presented in Table 3 may be subject to change. Typically, once detoxification levels are met, and
approval is received from the State, final closure begins, followed by reclamation of the site.
5.2.1 California
The California Regional Water Quality Control Boards (RWQCBs) oversee rinsing and closure
activities. Once the rinsing standards have been met, reclamation is turned over to the County, and if
applicable, to the Federal land management agency. Information on the specific requirements for
release of bonding for mine sites (cyanide operations) in California was not available.
In California, selection of both rinsing/neutralization and detoxification methods is left to facility
discretion. The State requires that for detoxification to be considered complete, the residual cyanide
in the tailings, leach pad, or solution pond must not exceed the limits listed in Table 3 (for liquids:
total cyanide 1.0 mg/1; WAD cyanide 0.2 mg/1)1. The state has separate requirements for solid
samples, which must meet the following levels: soluble WAD cyanide 0.5 mg/1; soluble total cyanide
2.5 mg/1; total cyanide2 10.0 mg/1.
1 Note the current WAD cyanide standard has been changed from the 1987 standard of 0.5 mg/1.
2 Total cyanide after extraction of soluble WAD and soluble total cyanide. Source RWQCB, 1987.
32
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Table 3. Summary of State Requirements:
Cyanide Heap Leach and Tailings Impoundment Closure and Reclamation
State
Treatment Criteria
Reclamation/Bonding
California
Residual cyanide in heaps, tailings or solution ponds must meet
the following prior to discharge of wastes:
Liquid component:
• WAD cyanide: 0.2 mg/1
• total cyanide: 1.0 mg/1
Solid component:
• soluble WAD cyanide: 0.5 mg/1
• soluble total cyanide: 2.5 mg/1
• total cyanide3: 10.0 mg/1
Where feasible State requires
spent ore to be used as backfill
in mine pits for reclamation.
Colorado*
Detoxification standards are determined on a site-specific basis
by permit writer5. They are based on ambient water quality,
characteristics and uses of water in the area, and projected
effluent characteristics; may address cyanide and metals. As of
mid 1994, Colorado is finalizing regulatory requirements for
cyanide operations.
Same as other mining
operations. (However, costs
estimates must reflect cyanide
detoxification.)
Idaho6
Prior to disposal or abandonment of leached ore, concentrations
of WAD or free cyanide and other pollutants in process-
contaminated water draining from the leached ore must be:
• reduced to a level set by the permit writer based on
disposal method, location, and potential for surface
water and groundwater contamination;
or
• pH between 6.5 and 9 (stabilized)7.
Seasonal closures must also address water balance.
Requires permanent closure
plan and possible post-closure
monitoring. No reclamation
requirements specified in
cyanidation regulations.8
Bonding for permanent closure
required in cyanidation
regulations.
Montana
Water Quality Act requires no discharge from cyanide
operations into State waters. Metal Mine Reclamation Act
(includes new standards for small cyanidation facilities):
• Based on permit writer's discretion.
• to levels considered acceptable based on under
applicable water quality standards.
Same as other mining
operations, except cyanide
operations that would normally
qualify for the small miner's
exclusion are subject to
operating and reclamation
requirements.
Total cyanide after extraction of soluble WAD cyanide and soluble total cyanide. (RWQCB, 1987)
California Regional Water Quality Board "Cyanidation Requirements for Cyanidation Process Wastes", April 22, 1987.
The State of Colorado has published "Guidelines for Cyanide Leaching Projects," March 1992, to present an overview
of the considerations necessary in preparation and review of reclamation permits for cyanide leaching operations.
Idaho cyanidation rules became effective January 1, 1988. Facilities existing as of the effective date are not subject to
the requirements.
In Idaho, tailings impoundments that require recycling of process water to prevent a point source discharge may be exempt
by the State.
It is unclear if Idaho subjects cyanidation facilities to the reclamation requirements of the Idaho Surface Mining Act
33
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Treatment of Cyanide Heap Leaches and Tailings
Table 3. Summary of State Requirements:
Cyanide Heap Leach and Tailings Impoundment Closure and Reclamation (Continued)
State
Treatment Criteria
Reclamation/Bonding
Nevada
General Performance Standard that facilities may not degrade
the waters of the State. Surface water quality is set by NRS
445.253. Groundwater is set at Federal or State drinking water
standards and WAD cyanide at 0.2 mg/1.
Heaps: Spent ore must be rinsed until effluent reaches9:
• WAD cyanide: 0.2 mg/1
• pH: between 6 and 9
• Remaining solids tested using Meteoric Water Mobility
• Test
Tailings (vat leaching)10: For impoundments that do not have
underdrainage collection systems, solids must be tested using
Meteoric Water Mobility Test.
Pond sludges, heap solids must
be tested using Meteoric Water
Mobility Test during closure
prior to reclamation.''
Reclamation similar to other
mining operations.
South
Carolina
Based on permit writer's discretion. Current criteria:
• free cyanide: 0.2 mg/112
In practice, level to which constituents (including CN) are
reduced determine how post-closure leachate and wastes may be
managed.
Closure required as part of
reclamation.
Under Mining Act, bond for
"affected area" is
approximately $1000/acre.
However, under Pollution
Control Act, (which does not
specifically provide for
bonding), one mine in the State
was required to post $10
million financial assurance.
South Dakota
Heaps: spent ore can be off loaded when effluent or pore
water meet:
• WAD cyanide less than 0.5 mg/1
• pH of 6.5 to 8.5
• all other parameters must meet existing State standards
or ambient concentrations
• alternative treatment criteria available
• Cyanide based on effluent samples taken at base of
heap. Criteria other than cyanide may be effluent
samples or pore water extracted from solid sample
analysis.
Tailings (vat leach)
• information not obtained
Same as other mining
operations.
In Nevada, variances are available.
10
Cyanidation tailings generated from tank or vat leaching tn not specifically called out in the Nevada regulations. The
treatment standard for cyanidation tailings prior to discharge 10 the tailings impoundments has not been obtained.
1 In Nevada, units that have a history of liner integrity proMeim may be required to test underlying soils using the Meteoric
Water Mobility Test.
12 South Carolina is currently considering changing limit from free cyanide to total cyanide.
34
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Treatment of Cyanide Heap Leaches and Tailings
According to State personnel, experience has shown that particle size influences detoxification of
heaps. When a heap is composed of run-of-mine ore, rinsing can be completed within 4 to 6 days
with cyanide levels dropping to 1.0 mg/1; a long rinse may take as long as 6 months, however.
Crushed and agglomerated ores are more difficult to rinse. (California 1993c)
Mines are also required to determine the acid neutralization versus the acid generation potential of
their ore and waste rock as pan of their permit requirements. (California 1993b)
Monitoring requirements for each site are typically addressed in Water Discharge Requirement
(WDR) permits. These permits are required from the Regional Water Quality Control Boards for all
discharges. Monitoring requirements are site-specific, depending on site conditions.
5.2.2 Colorado
The Colorado guidelines do not require specific methods for rinsing, neutralization or detoxification.
Detoxification standards for cyanide (heaps and ponds) are established on a site-by-site basis between
the applicant and the Division of Minerals and Geology. Colorado is currently drafting regulations
for cyanide operations. Factors taken into consideration during the permit review process include
water balance of process and detoxification solutions, ambient water quality, characteristics and uses
of the water in the area, and projected effluent characteristics. If residual metals are anticipated to be
a problem, then appropriate treatment standards are also developed for those constituents. (Colorado
1992a)
Specific practices for cyanide leaching are addressed in a Colorado-issued guidance manual for State
staff entitled "Guidelines for Cyanide Leaching Projects" (March 1992). The cyanide guidelines
recommend double liners on all surfaces that will potentially come in contact with a cyanide solution,
including all pads, ponds, piping and conveyance systems, and pretreatment areas. Soil layers are
allowed as double liners if the soil meets the "impermeable" standard of 1 x 10"6 cm/sec; however, to
be used as a lower liner in a pond, a soil liner must have a maximum permeability of 1 x 10~7 cm/sec.
Colorado has not set a permeability standard for synthetic liners. (Colorado 1992a; ELI 1992)
The State also requires leak detection systems, installed between the upper and lower liners at all
leach pad areas and pond areas. Although double liners are not specifically required around tanks or
vats, a method of detecting leakage must be provided, such as a permeable zone overlying a low-
permeability layer. (Colorado 1992a)
Colorado has approximately 17 active cyanide leach sites, the majority (12) of which are permanent
heap leach pads. There are three vat leach operations and one each of valley and on/off pads. (WGA
199Ib)
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Treatment of Cyanide Heap Leaches and Tailings
For cyanide operations, Colorado requires surface and groundwater quality monitoring, threerto four
times annually, during reclamation as well as during active operations. (Colorado I992a)
Bonding requirements for cyanide operations are the same as for other mining activities, but
calculation of the costs must address detoxification of the ore and process solutions by including: an
estimation of the volume of solutions in ponds during detoxification, the average residual cyanide in
ore for each rinsing, and calculation of the amount of detoxification agent for each rinse until the
detoxification standard is reached. (Colorado 1992a)
Colorado reclamation standards are broadly defined. Reclamation must be such that "all refuse and
acid-forming or toxic producing materials that have been mined shall be handled and disposed of in a
manner that will control unsightliness and protect the drainage system from pollution." (ELI 1992)
The Colorado guidelines "require" geochemical testing of tailings, including acidification/
neutralization testing (acid/base potential) and recommend kinetic tests such as humidity cell or
column tests. Depending on the test results, appropriate design measures can be determined.
(Colorado 1992a)
Heap reclamation varies depending upon the type of heap used. The Colorado guidelines recommend
free draining and expanding heaps be leveled to blend with the ground surface. The State
recommends grading the top of valley heaps and capping them with an impermeable cover. Drilling
holes through a valley heap's liner system is also suggested to allow "natural passage of water down
the valley". For valley fill leach pad areas on top of bedrock, drilling a horizontal drain through the
impoundment may be done to reestablish drainage. Reconstructed channels may be necessary through
valley areas to reestablish "natural" surface drainage systems. (Colorado 1992a)
For closure and reclamation of ponds, the Colorado guidelines state that "After all the solution in the
pond has been eliminated through spray evaporation or land application, stabilization of the pond
areas can commence." As discussed above, detoxification standards are determined on a site-specific
basis. Pond stabilization measures may include folding synthetic pond liners over any remaining
sludge, or excavating the remaining sludge, followed by grading of the surface contours. Additional
details or guidance concerning spray evaporation and land application is not provided in the state
manual. (Colorado 1992a)
It should be noted that (as of Spring 1993) the Colorado legislature was considering a major revision
to the Mined Land Reclamation Act. In addition, the Mined Land Reclamation Board placed a
moratorium on approval of new cyanide operations in early 1993 (the status of the moratorium was
not determined).
36
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of Cyanide tieap Leaches and Tailings
5.2.3 Idaho
Idaho has a program for permitting the construction, operation, and closure of cyanide operations that
applies to new facilities (effective January 1988). For facilities in existence prior to 1988, the
existing general mining (not cyanide-specific) regulations apply. (Idaho Title 1 1992)
New cyanide operations are required to meet general design standards for cover containment,
impoundments, liner criteria, and storage requirements. Site-specific determinations can be made if
the State determines that some parameters are not applicable based on ore, operations, or other site-
specific factors. Surface impoundments must have "efficient leak detection" and "adequate leak
recovery"; however, tailings structures more than 30 feet high are exempt from the impoundment
requirements (subject to Idaho Code Title 42, Chapter 17). (ELI 1992) Leach pads and
impoundments are required to have a hydraulic liner designed for a maximum permeability coefficient
of 10"7 cm/sec; clay liners are also to have a minimum thickness of 12 inches.
Idaho requires a monitoring strategy in each cyanide operation plan addressing baseline water quality
(surface and groundwater), proposed monitoring, leak detection, and emergency response procedures.
Ground water, and if applicable, surface water, monitoring is required at all cyanide operations.
Duration of monitoring through closure, reclamation and post-reclamation is not identified in the
regulations. (Idaho Title 1 1992)
Proposals for land application or economic reuse of cyanide solutions must be included with the
permit application. Details on land application are not specifically addressed in Idaho's Rules and
Regulations for Ore Processing by Cyanidation.
Several seasonal heap leach facilities operate in Idaho. The State has separate requirements for
seasonal, temporary and permanent closure. Seasonal closure requires an increase in freeboard to
allow for seasonal runoff and snowmelt. Cyanide concentrations are to be reduced and pH controlled
(6.5-9.0) in solution and process waters during seasonal closure. A temporary closure plan is
submitted for temporary closure; it details the procedures and schedule for treatment and drainage
control. Permanent closure activities are to be included in the operation permit application. (ELI
1992)
Prior to disposal or abandonment of the spent ore, process-contaminated water drained from leached
ore must be stabilized at a pH of 6.5 to 9.0, or WAD cyanide levels are to be reduced to 0.02 mg/l.
If WAD cyanide is used as the determining value, then other pollutants must be reduced to an
appropriate level based on disposal criteria. The other pollutants include those addressed by surface.
drinking or other water quality standards that the Stale deems appropriate on a site specific basis.
(Idaho Title 1 1992) Financial assurance for cyanide operations is released when the facility
completes permanent closure in accordance with an approved plan. (Idaho Title 1 1992)
37
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Treatment of Cyanide Heap Leaches and Tailings
5.2.4 Montana
In Montana, all cyanide operations are subject to the general regulations for mining operations
adopted in 1980. Each mine must obtain an operating permit that addresses operations, practices,
closure and reclamation. In addition, Montana has a permitting program for small cyanide mines less
than five acres. The regulations for small mine cyanide operations are more detailed than the general
operating permits standards. (WGA 1991b)
In practice, the Montana Department of State Lands is using the same standards for both small and
full-size cyanide operations. The technical standards promulgated for small cyanide operations are
applied to full-size operations by the permit writer who issues the operating permit. Montana has
approximately eight active, full-scale cyanide operations (requiring permits), three of which are vat
leach and five are heap leach (permanent pads). (WGA 1991b)
Cyanide operations must have a remedial action plan for controlling and mitigating discharges; must
design and construct diversions and sediment impoundments capable of withstanding a 10-year, 24-
hour storm event; must install a leak detection system to monitor WAD cyanide, pH, and electrical
conductivity; must have groundwater monitoring wells; develop a wildlife exclusion plan; and
construct cyanidation facilities to withstand a 50-year, 24-hour storm event. Monthly construction
reports and as-built drawings must be submitted for ponds, tailings disposal units, and other facilities.
(ELI 1992)
5.2.5 Nevada
Nevada is one of the leading gold producing states, with over 100 active cyanide leach operations13.
Nevada's Water Pollution Control Law has cyanide performance standards for groundwater: a facility
may not allow cyanide concentrations in groundwater to exceed 0.2 mg/1 WAD cyanide. (Nevada
1990) Nevada also has a policy of zero discharge to surface waters from cyanide facilities (this is
common to the other States' NPDES programs as well).
Guidelines specific to cyanide operations for closure and reclamation are outlined in the Nevada
Department of Environmental Protection's (DEP) "Evaluations for Closure". All mining permits
require stabilization of tailings and spent ore during closure. For materials that have been
beneficiated by cyanide, both free and WAD cyanide analysis must be conducted.
At the end of active use, tailings and impoundment materials should be sampled and characterized.
Spent ore from cyanide heap leaching methods are to be rinsed until weak acid dissociable (WAD)
cyanide levels in the effluent rinse water are less than 0.2 mg/1; the pH of the effluent rinse water is
Nevada has approximately 90 permanent heap pads, 33 vat leach, 2 on/off pads, and 3 valley leach operations according
to a 1991 summary by the Western Governors Association. (WGA 1991b)
38
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11
1992.
39
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Treatment of Cyanide Heap Leaches and Tailings
Nevada Department of Wildlife frequently accepts dilution or neutralization of free cyanide to below
50 ppm as sufficient to avoid mortalities. (WGA 1991b; ELI 1992)
5.2.6 South Carolina
The Department of Heath and Environmental Control (DHEC) issues construction permits for
industrial wastewater treatment systems, including mining process wastewater ponds and rinse systems
for cyanide leaching operations, and regulates discharges to surface water and groundwater. The
Land Resources and Redevelopment Division (LRRD) issues permits for operational components,
including impoundments and heaps, and requires full reclamation.
During rinsing of heaps, quarterly reports must be submitted to DHEC and the LRRD. Rinsing
standards for leached ore are being considered by the State for regulatory development, and
permeability standards for operating units also may be developed. The current detoxification trigger
is 0.2 mg/1 free cyanide, but the State is considering changing this to total cyanide. Core samples of
the heap must be analyzed before closure can be considered complete. (South Carolina 1993b)
There are no specific regulatory standards for closure of cyanide operations, and each case is handled
on a permit-by-permit basis. DHEC and LRRD approval is needed prior to conducting closure
activities, and a deed record of cyanide unit locations is required. At present, there are four
operating cyanidation units in South Carolina; none has undergone complete closure, although several
have conducted, or are conducting, rinsing. Until firm standards are developed, site-specific
considerations drive most State decision-making. For example, the means by which a facility may
manage post-operational heap leachate (e.g., discharge to surface water, impounded in pits, treated)
depend on the effluent quality achieved.
When the decommissioning (closure) requirements have been met, administrative responsibility for the
mine shifts from the DHEC to the LRRD. Once a heap has met the detoxification and closure
requirements, it must, at a minimum, re-slope to 3:1 and revegetate with perennials. The state is
considering lengthening the time needed to demonstrate revegetation success from two years to some
longer period. (South Carolina 1993a)
Legislation under development (the Solid Waste Management Act) may broaden the State's definition
of "industrial solid waste" to include chemically-altered materials, including wastes from cyanidation
operations, which would then subject mining wastes to solid waste disposal permit requirements. (Joy
1990)
One cyanide operation in South Carolina with on/off pads has a State industrial solid waste permit for
disposal of spent, rinsed ore that is off-loaded but that does not meet the rinsing standards of the site's
wastewater construction permit. A decision on whether other cyanidation operations will require an
40
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Treatment of Cyanide Heap Leaches and Tailings
industrial solid waste disposal permit has not been determined. The State will evaluate the issue
during closure of those other cyanide operations. (ELI 1992)
5.2.7 South Dakota
Concerns over the impacts of surface mining led to a State moratorium on new large-scale gold and
silver mines in South Dakota, effective in 1992. One of the instituted reforms was a requirement for
large-scale cyanide operations to develop contingency plans and obtain sufficient financial assurance to
cover releases to the environment, and submit annual reports listing annual cyanide use. (ELI 1992)
Permitting for a typical heap leach facility would involve pre-submission meetings, submission of the
permit application, a socio-economic study, a completeness review, technical review, a public notice
and a public hearing before the South Dakota Board of Minerals and Environment. (WGA 199 Ib)
South Dakota mining laws, regulations, and permit conditions require that spent ore be adequately
neutralized and designated as suitable for disposal before off-loading from heap leach pads. Heaps
must be rinsed until treatment standards are reached. (Durkin 1990)
Prior to detoxification, effluent or pore water from the ore must be characterized for cyanide, metals,
anions, cations, pH, radioactivity and total dissolved solids. The South Dakota Department of
Environment and Natural Resources (DENR) then uses these results to designate key parameters for
an individual site to monitor throughout the treatment/neutralization cycle. Spent ore is considered
suitable for disposal when the effluent meets the criteria listed in Table 3.
South Dakota DENR requires that neutralization and off-load criteria be based on effluent samples
collected at the toe of the spent ore heap or analysis of leachate extracted from a representative solid
sample (pore water) taken from the spent ore heap. (Durkin 1990)
If detoxification (treatment) of tailings is not successful and the operator wishes to try a new treatment
method, a new treatment plan must be submitted to DENR. In cases where the treatment criteria
cannot be achieved, DENR may develop alternate, site-specific criteria, or tailings may be reclaimed
such that infiltration, percolation, and discharge are minimized, as indicated by appropriate pathway
and fate analysis. (Durkin 1990)
The South Dakota Water Pollution Control Law has set a regulatory groundwater standard for WAD
cyanide at 0.75 mg/1. Concentrations detected at or above this level prompt remedial groundwater
investigations. (ELI 1992)
The State has five leach operations, of which three are on/off pads. The fourth is a permanent heap
pad and the fifth is a vat leach operation (WGA 199Ib). With detoxification of heap leaches, the
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Treatment of Cyanide Heap Leaches and Tailings
State's experience has shown that 2.5 pore volumes are required before effluent concentrations are
within the required standards. (South Dakota 1993)
During reclamation of heap leach operations, the spent ore must be used as backfill in the mine pits
where feasible. Reclamation of in-place tailings piles must not block drainage pathways.
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Treatment of Cyanide Heap Leaches and Tailings
6. CASE STUDIES
Selected case studies at active sites are presented in this section. The active sites selected as case
studies were not selected as either good or bad models, but rather to examine a range of cyanide
treatment techniques. The three active case studies highlight a vat leach operation with INCO
treatment of tailings slurry, a large leach heap operation with several heap pads, and a site using-
biological treatment by bacteria. Material for the case studies came from publicly available
documents, and file materials collected from the respective State regulatory offices. The Agency has
not evaluated the efficiency of any of the methods presented. This section was developed for
informational purposes only.
6.1 Hecla, Yellow Pine, Idaho
Hecla Mining Company completed gold extraction from its heap leach pad at Yellow Pine, Idaho in
1992 (Minerals Today). Currently, the facility is detoxifying the cyanide in the spent ore by applying
a bacterial biological treatment method to the heap.
The bacterial solution is applied to the spent heap using sprinklers to spray the top of the heap. The
solution then percolates through the spent ore with bacteria consuming the cyanide as it progresses.
After the solution passes through the heap it is collected in a process pond and then recycled back
through the heap. At the end of treatment, the solution pond will be allowed to evaporate, materials
(sludges) will be sampled and analyzed, and the unit will be closed in place by folding the pond liner
over any remaining sludge, contouring, and revegetating. Information on the type of bacteria,
additional nutrients required, and the developer of the process used by Hecla have not yet been
obtained. (Idaho 1993)
After treatment, effluent from the heap reached the 0.2 mg/1 state standard for cyanide during the fall
of 1992. The Idaho DEQ wants to review cyanide levels through one wet rainy season prior to
approving the start-up of reclamation activities. The facility planned to collect effluent samples
during spring sampling. Depending on the results of this sampling, the state may approve initiation
of reclamation for Fall 1993. (Idaho 1993)
The detoxified heap may not require capping. The fourth (top) tier of the heap will be removed and
used to regrade and contour the heap. A berm will be constructed around the top of the heap to
prevent runoff erosion from destroying the steep slopes of the heap. The liner beneath the heap will
not be broken with numerous perforations, but approximately three spots will be used to drain any
percolated material to the ground. The state will require 5 to 10 years of groundwater monitoring of
onsite wells (for cyanides and metals). (Idaho 1993)
An Idaho representative (Idaho 1993) stated that the biological cyanide destruction process has
appeared to work well at the Yellow Pine facility. The sue has been able to meet the 0.2 mg/1 WAD
43
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treatment of Cyanide Heap Leaches and Tailings
cyanide standard for heap effluent, although the state is still waiting to see the results after a wet
season, before it approves closure. Metals have not been a problem at the site, although the state
representative credits this to the composition of the ore rather than the result of the biological
treatment process.
6.2 Zortman Mining, Landusky Heaps, Montana
Zortman Mining, Inc. of Zortman, Montana (a subsidiary of Pegasus Gold Corporation) operates
several heap leach pads, a processing plant, and barren and pregnant leachate solution ponds. The
Landusky reclamation plan was approved in 1990 by the BLM and Montana Department of State
Lands. The heaps are low grade run-of-mine ore (neither crushed or agglomerated) that were leached
in 25-foot high lifts. (Fitzpatrick 1992; Schafer and Associates 199la) A Montana state file lists the
site as one of the 13 largest metal mines in Montana, processing 75,000 tons of ore a day. The heaps
and tailings cover 175 acres and contain 60 million tons of material.
Being one of the first mines permitted under the 1974 Montana Metal Mining Reclamation Act and
the first large-scale gold heap leaching operations in the US (operations began in 1979), the
Zortman/Landusky site represents the evolutionary changes in heap leach technology and regulatory
requirements through the past two decades. The first heap pad constructed at the site consisted only
of a 12-inch clay layer, side berms, and a simple reclamation plan. With each successive addition or
modification there came additional permit stipulations. The operators also voluntarily added more
engineered controls. In 1990, the State required the heaps to be neutralized to a cyanide level of 0.22
mg/1 WAD cyanide, and declared that heap slopes greater than 2:1 would no longer be acceptable.
A pilot study of rinsing techniques was begun at the Zortman Landusky heaps in 1982. It was
followed with a cyanide degradation study in 1986. These investigations were conducted as part of
the detoxification and closure program for the three heaps. The rinsing and degradation studies found
that rinsing of the heaps was an effective means of accelerating (compared to natural degradation)
cyanide, metal, and nitrate removal. (Schafer and Associates 199la)
Prior to rinsing of the heaps, the Landusky heaps were allowed to degrade naturally (through natural
processes of volatilization, oxidation, formation of thiocyanate, and biodegradation). Natural
degradation was followed by rinsing of the heap with fresh water to reduce WAD cyanide
concentrations in solution. (Schafer and Associates 199la)
The facility concluded that rinsing with one pore volume removes 50 to 90 percent of cyanide and
metals in pore water and was an effective means of accelerating the rate of cyanide and metal
degradation in heaps. (Schafer and Associates 1991a)
During the rinsing of the Landusky heaps, however, several issues were raised: the amount of
solution retained in the heap, long-term seepage from decommissioned and reclaimed heaps, the long-
44
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Treatment of Cyanide Heap Leaches and Tailings
term degradation of cyanide in heaps, and the attenuation of metals in heaps. During investigations at
the site, evidence suggested that movement of water through the heap was rapid and homogeneous.
(Schafer and Associates 199la)
Periodically, Landusky land applies neutralized solution to a specified parcel of land at the site. In
1987, during one round of land application four million gallons were applied during a five day
period. Cyanide concentrations of the water applied to the ground were above 3 mg/1 (exact
concentrations were not obtained for this report).
6.2.1 McCoy/Cove Mine, Echo Bay Mining Company, Nevada - INCO process
The McCoy/Cove Mine, near Battle Mountain, Nevada, operates a vat leach operation for extraction
of gold. The facility added an INCO cyanide treatment system in 1990 to its operations after
experiencing more than 1000 water-fowl deaths caused by migratory birds drinking out of its cyanide
tailings impoundment (such deaths are in violation of the U.S. Migratory Bird Treaty Act).
At the McCoy mine, the INCO process is used to remove cyanide from the tailings pulp after gold
has been recovered from the milling process. Prior to the use of INCO, the spent tailings were
discharged directly to a tailings pond. The liquid fraction of the tailings was reused as make-up in the
leaching process. The cyanide-containing liquid in the 145 hectare tailings pond attracted the
migratory birds in a desert area with few open bodies of water.
The system treats tailings pulp (thickener) underflow containing 268 kg WAD cyanide/hour in two
parallel reactors (40 percent solids; 8,500 stp mill throughput) to reach a target residual cyanide level
of 25 mg/l WAD cyanide (it can reach 5 mg/1 WAD cyanide, if necessary). Other INCO references
suggest that the McCoy/Cove effluent has total cyanide levels below 10 mg/1. Periodic WAD cyanide
analysis, as well as SO2 feed, slurry flow rate, pH, and percent solids, are monitored. Tailings are
disposed of in a tailings impoundment where WAD cyanide levels are monitored daily, ranging from
4 to 7 ppm. (Devuyst 1992; INCO 1992) The facility has cut its cyanide consumption by reusing
cyanide recovered from the INCO process.
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Treatment of Cyanide Heap Leaches and Tailings
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51
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