The management of cyanide

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Cyanide Use in Gold Production​
O​
ne of the reasons for the high value placed on gold is its resistance to attack by
most chemicals. One exception is cyanide, or more specifically, a cyanidecontaining
solution, which dissolves the precious metal.
Cyanide is used in mining to extract gold (and silver) from ores, particularly low-grade
ores and ores that cannot be readily treated through simple physical processes such as
crushing and gravity separation.

FIGURE 2. Gold Production​
7​
Cyanide Use in Gold Production​
Courtesy of WMC Limited​
The Process​
The use of water-based solutions to extract and recover metals such as gold is called
“hydrometallurgy.” Gold mining operations use very dilute solutions of sodium cyanide
(NaCN), typically in the range of 0.01% and 0.05% cyanide (100 to 500 parts per million).
The process of metal dissolution is called leaching. The sodium cyanide dissolves in
water where, under mildly oxidizing conditions, it dissolves the gold contained in the
ore. The resultant gold-bearing solution is called “pregnant solution.” Either zinc metal
or activated carbon is
then added to the pregnant
solution to recover
the gold by removing it
from the solution. The
residual or “barren” solution
(i.e. barren of gold)
may be re-circulated to
extract more gold or routed
to a waste treatment
facility. Approaches to
treating this waste solution
of cyanide are discussed
in Section 7.
There are two general
approaches to leaching
gold from mined ore
using cyanide: tank leaching
and heap leaching.
Tank leaching is the conventional
method, in
which gold ore is
crushed and ground to a
size of less than one millimetre in diameter. In some cases, a portion of the gold can be
recovered from this finely ground material as discrete particles of gold using gravity-separation
techniques. In most cases, the finely ground ore is directly leached in tanks to
dissolve the gold in a cyanide solution. When gold is recovered in a conventional plant
with leaching in tanks, the barren solution will be collected along with the solid wastes
(tailings) in a tailings impoundment system. There, part of the solution will remain within
the pores of the settled tailings and part will decant and collect in a pond on top of the
tailings, from which it is recycled back to the plant. In most plants, because impurities​
8​
The Management of Cyanide in Gold Extraction​
Gold recovery from cyanide solution using activated carbon
(charcoal).​
Photo courtesy of Minorco​
build up in these solutions, some of the cyanide-bearing solutions must be pumped to a
treatment system for disposal (see Section 7).
Recent technical advances enable the heap-leaching of some gold ores. With this
method, the ore is crushed to less than a few centimetres in diameter and placed in large
piles or heaps. A solution of cyanide is trickled through these heaps to dissolve the gold.
When heap-leaching technology is used to extract gold, the barren solution is collected
in a pond, from which it is commonly recharged with cyanide and recycled back into the
leaching system.
The modern gold industry uses cyanide almost exclusively as the leaching agent for gold.
Other complexing agents such as thiourea, chlorides and other halides have been used
to extract gold from ores, but these are not generally cost-effective and present their own
environmental and health concerns. Cyanide complexes are more stable and effective,
and do not require additional aggressive chemicals to effect gold recovery. Cyanide has​
9​
Cyanide Use in Gold Production​
Construction of a leach pad at Pikes Peak, Colorado, USA.​
Photo courtesy of Minorco​
been used in mining for over a century​
(see box). An older technique for gold recovery,
which is no longer used in modern gold plants, is amalgamation with liquid mercury. In
some developing countries, artisanal miners still use liquid mercury as a means of complexing
gold from small mine workings. This practice is discouraged, however, as poor
management of both liquid mercury and the vapour arising from volatilizing mercury
contributes to serious health problems among artisanal miners.

10​
The Management of Cyanide in Gold Extraction​
Box 1. History of Cyanide Use in Mining​
While environmental concerns over the use of cyanide in mining have become more
public only in the last few years, there actually is a very long history of cyanide use
in metallurgical and related processes all around the world. Dippel and Diesbach discovered
“Prussian blue” (iron ferrocyanide) in 1704. The earliest well-documented
work was Scheele’s studies of solubility of gold in cyanide solutions dating from 1783
in Sweden. Gold-cyanide chemistry was studied actively in the mid-19th century in
England (Faraday), Germany (Elsner), and Russia (Elkington and Bagration). By
1840, Elkington held a patent for the use of potassium cyanide solutions for electroplating
gold and silver. Elsner led the evaluation of the role of oxygen in gold dissolution
using cyanide solutions, and “Elsner’s Equation” describing the extraction of
gold from ores by cyanide was known by 1846.
Patents formalized by McArthur and the Forrest brothers in 1887 and 1888 effectively
established the current cyanidation process, the use of cyanide dissolution and
precipitation using zinc. However, there were still earlier patents in the USA for
cyanide leaching (Rae in 1869) and recovery from chlorinated solutions using charcoal
(Davis in 1880). The first commercial-scale cyanidation plant began operating at
the Crown Mine in New Zealand in 1889, and by 1904 cyanidation processes were
also in place in South Africa, Australia, United States, Mexico and France. Therefore,
by the turn of the century, the use of cyanide to extract gold from low-grade ores was​
a fully established metallurgical technology.:28:
 

مواضيع مماثلة

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Natural Occurrences of Cyanide​
C​
arbon and nitrogen, the two elements that make up cyanide, are present all
around us. Together they make up almost 80% of the air we breathe, and both are
present in the organic molecules that are the basis of all life forms. Hydrogen
cyanide was formed in the earliest stages of the development of our planet as a precursor
to amino acids, from which life on Earth evolved. Cyanide is formed naturally. It is
produced and used by plants and animals as a protective mechanism that makes them
an unattractive food source. Many organisms may either adapt to the presence of cyanide
or detoxify it.
A natural source of hydrogen cyanide (HCN) is a sugar-like compound called amygdalin,
which exists in many fruits, vegetables, seeds and nuts, including apricots, bean sprouts,
cashews, cherries, chestnuts, corn, kidney beans, lentils, nectarines, peaches, peanuts,
pecans, pistachios, potatoes, soybeans and walnuts. In the kernel of bitter almond, there
is about 1 mg of HCN as amygdalin. Table 1 presents data on the amount of cyanide
present in a variety of other foodstuffs.

3​
Natural Occurences of Cyanide​
TABLE 1. Cyanide Concentrations in Selected Plants​
Plant Species Concentration (mg.kg​
-1)

Cassava (sweet varieties)
leaves 377–500
roots 138
dried roots 46–<100
mash 81
Bamboo tip Max. 8,000
Lima bean (Burma) 2,100
Almond (Bitter) 280–2,500
Sorghum (young plant, whole) Max. 2,500​
Source: Excerpted from Eisler, 1991​
Cyanide compounds are produced in thousands of plant species and in other life forms.
In some plants, cyanide occurs in concentrations that would be judged “hazardous” if
they were associated with manufactured sources. Plants such as alfalfa, sorghum and
cassava are known sources of cyanide poisoning to livestock and humans.
In addition to these naturally occurring forms of cyanide, cyanide compounds are also
present in such everyday anthropogenic sources as automobile exhaust, cigarette smoke,
and even road and table salt.​
4​
The
 

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Production and Handling
of Cyanide​
C​
yanide is produced industrially in one of two ways: as a by-product of the manufacture
of acrylic fibres and certain plastics, or by combining natural gas and
ammonia at high temperatures and pressures to produce hydrogen cyanide (HCN)
gas. Subsequently, hydrogen cyanide gas can be combined with sodium hydroxide
(NaOH) to produce sodium cyanide (NaCN) and water (H
2O). The water is then removed
by drying and filtering, and the sodium cyanide is formed into solid, white briquettes that
are about 10 centimetres square.
The solid sodium cyanide briquettes are maintained under controlled temperature and
moisture. At the manufacturing location, the briquettes are packaged in labelled, sealed
containers to protect the briquettes from both crushing and moisture. The containers
may be disposable plywood boxes with non-returnable liners, non-returnable steel
drums, or re-useable steel bins. In some circumstances, the briquettes are dissolved and
the cyanide solution is transported as a liquid in specially designed tanker trucks.
All shipments of sodium cyanide are accompanied by Material Safety Data Sheets
(MSDSs) that provide the chemistry and toxicity of sodium cyanide, instructions in case
of accidents, emergency telephone numbers for assistance and additional information
from the manufacturer. All shipments are inventoried as material leaves the producer,
and the inventory is checked against delivery records to ensure proper surveillance at all
times.
There are three primary producers of solid, liquid and gaseous cyanide in the world:
Dupont, in the United States, ICI, in England, and Degussa Corporation, in Germany.
Annual worldwide production is approximately 1.4 million tonnes of HCN.
1 As mentioned
earlier, 20% of the total HCN production is used to produce sodium cyanide (NaCN) and
the remaining 80% is used in numerous other industrial activities such as the production
of chemicals. Sodium cyanide is also produced in the USA by FMC Corporation.
The three primary producers are major international chemical manufacturers that
understand their responsibility for their products. For example, formal corporate policies

11​
Production and Handling of Cyanide​
1 1996 amounts. Usage in mining has remained essentially constant for the last decade.​
ensure that cyanide is sold only to companies that have the ability and commitment to
protect workers, the public and the environment. The manufacturers contract only with
selected carriers that have records of transportation safety consistent with the manufacturers’
internal standards. The manufacturers maintain a staff of safety and transportation
specialists to work with purchasers and others in the areas of training, facility design
and related safety measures.
Mining companies store sodium cyanide in secure areas that are kept dry, cool, dark and
ventilated. In the storage area, cyanide packages are placed on pallets in their original
containers above watertight floors, usually made of concrete, with proper containment
in the unlikely event of spillage. Regardless of the container type, empty containers are
washed and the rinse water is used in the site’s gold recovery plant (to take advantage of​
12​
The Management of Cyanide in Gold Extraction​
Storage of drums containing sodium cyanide.​
Photo courtesy of DuPont​
the small amounts of cyanide that could be
present) or is processed through the
wastewater treatment system prior to being
discharged under controlled and permitted
conditions.
Mining companies hold special training programs
for all employees who work with or
around cyanide. They also have materials
handling and safety plans prepared by qualified
industrial hygienists and supervised by
project safety officers. These health and safety
plans assign employee responsibilities and
control the handling and use of sodium
cyanide from its arrival at the mine site
through to the metallurgical process. Area
gas monitors, proper protective clothing,
self-contained breathing apparatus and firstaid
stations equipped with eyewash and
shower facilities are utilized by cyanide-handling
operations at mines. Companies’
industrial hygiene programs include annual
training, access to all MSDSs and air monitoring
to ensure worker safety, as well as
procedures for documenting all health and
safety information and incidents at mine
sites.
Modern industrial hygiene programs at gold mining operations have been effective at
minimizing accidental cyanide poisoning at mine sites. Indeed, a search of industrial
accident records in Australia, Canada, New Zealand and the United States has revealed
only three accidental deaths in which cyanide was implicated at gold mine sites in the
past 100 years. The first was not directly related to gold recovery, the second involved
entry into an enclosed space—a fatal mistake, and the third was not conclusively attributed
to cyanide.​
2

13​
Production and Handling of Cyanide​
2 Both incidents were found in the 107-year fatality database of the Ontario Minister of Labour. In 1952, a
blacksmith at the MacLeod-Cockshutt Gold Mines died due to cyanide poisoning following an explosion of
molten cyanide; he had been preparing a bath of melted sodium cyanide to case-harden a wrench. In 1961,
a worker at the Hallnor Mines Mill died of poisoning from hydrocyanic gas after climbing into an agitator
tank to retrieve flake cyanide he had inadvertently thrown into the tank. In 1982, a labourer at an Arizona
gold recovery operation collapsed at work and died five days later. Cyanide was suspected, but the evidence
as to how the worker became exposed to cyanide was inconclusive.​
Photo courtesy of Degussa Corporation​
On-site assistance and safety training
are provided to gold mines by cyanide
producers.​
SECTION 6​
Cyanide in Solutions​
A​
fter gold is extracted via the hydrometallurgical processes, three principal types of
cyanide compounds may be present in wastewater or process solutions: free
cyanide, weakly complexed cyanide and strongly complexed cyanide. Together,
the three cyanide compounds constitute “total cyanide.” An understanding of the chemistry
of these three types of cyanide provides insights into their behaviour with respect to
safety and the environment.

Free Cyanide​
“Free cyanide” is the term used to describe both the cyanide ion (CN-) that is dissolved
in the process water and any hydrogen cyanide (HCN) that is formed in solution. The
solid sodium cyanide briquettes dissolve in water to form sodium ion and the cyanide
anion (CN-). The cyanide
anion then combines with
hydrogen ion to form molecular
HCN. The concentration of
hydrogen ion in the process
water is expressed by the
familiar parameter pH.​
3

Nearly all free cyanide is present
as HCN when there is
ample hydrogen ion present,
(i.e. at a pH value of 8 or less).
This HCN can then volatilize
and be dispersed into the air.
When the pH is greater than
10.5, there is little hydrogen
ion present and nearly all of
the free cyanide is present as
CN-. Under normal conditions
of temperature and pressure,
the concentrations of HCN and
CN- are equal at a pH value of
approximately 9.4.​
15​
Cyanide in Solutions​
3 When the pH of a solution is 7, the solution is said to be neutral. Solutions with pH less than 7 are said to be
acidic, whereas those with pH greater than 7 are said to be alkaline.​
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
11 10 9 8 7 6​
% CN​
CN-
HCN

pH​
% HCN​
FIGURE 3. CN-/HCN Equilibrium with pH​
Source: Scott and Ingles, 1981​
.

These forms of free cyanide are important because they are considered to be the most
toxic cyanides. However, they also happen to be the forms that are readily removed from
solutions through both engineered treatment processes and natural attenuation mechanisms.
The biological, chemical and physical processes that affect cyanide concentrations
in water, soil and air have been extensively studied during the last two decades, so
that their behaviour in the environment is well understood.
One of the most important reactions affecting free cyanide concentration is the volatilization
of HCN, which, like most gases, will separate from water and escape into the air.
Free cyanide is not persistent in most surface waters because the pH of such waters is
usually about 8, so that HCN volatilizes and disperses. Hydrogen cyanide’s volatility and
subsequent transformation to benign compounds in air are important because they act
as a natural mechanism for controlling free cyanide concentrations in waste and process
waters at mines.
Natural processes alone can reduce the free
cyanide concentration from solutions in
areas open to the atmosphere in the gold
production facilities, such as process ponds
and tailings impoundments, to very low values—
often to levels below regulatory concern
or even the limits of detection.
In the gold plant, however, operators maintain
the solution pH at values near 10.5 in
order to prevent volatilization. This preserves
cyanide in the gold extraction system
where it is needed and at the same time
limits the risk of worker inhalation exposure
to high concentrations of HCN gas in a
confined space.​
Cyanide Complexes​
While cyanide-bearing solutions are used in mining because they react with gold, they
also react with other metals. Gold ores almost always contain other metals, including
iron, copper, zinc, nickel and silver as well as other elements such as arsenic. In most
ore bodies, the concentrations of other metals typically exceed the concentration of gold
by several orders of magnitude. For example, a low-grade gold ore suitable for cyanide
leaching might contain 0.5 to 1 gram of gold per tonne (0.5 to 1 part per million [ppm]
gold); in contrast, the iron concentration of average crustal rocks is about 3.5% (35,000
ppm). Metals such as copper, zinc and nickel may be present in concentrations ranging​
16​
The Management of Cyanide in Gold Extraction​
Control centre for gold recovery plant
(cyanidation).​
Barrick Gold Corporation​
from tens to thousands of parts per million. Table 2 shows that significant amounts of
other metals may be dissolved when ores containing them are leached with cyanide
solutions.
Chemical analyses of process solutions and wastewater derived from the processing
indicate that most of the cyanide in solution is chemically linked with metals other than
the small amounts of gold or silver. When chemical elements combine in solution to
form soluble species, chemists refer to them as “complexes.” There is a wide range of
chemical and physical interactions between the components of complexes. Some complexes
are very stable, whereas others are easily destroyed. Analytical chemists are able
to define the relative stability of cyanide complexes of different metals with great precision.
The evaluation of the quantity and types of cyanide is important to all aspects of
cyanide use. It is particularly important to be able to distinguish both accurately and precisely
between the various cyanide compounds to ensure the selection of an effective
detoxification methodology.​
17​
Cyanide in Solutions​
CONCENTRATION RANGE
milligrams per litre​
5 (mg.L-1)

Total Cyanide 50–2000
Arsenic 0–115
Copper 0.1–300
Iron 0.1–100
Lead 0–0.1
Molybdenum 0–4.7
Nickel 0.3–35
Zinc 13–740​
4 Scott, J. S.,​
Status of Gold Mill Waste Effluent Treatment, Report to CANMET, Natural Resources Canada,
March 1993.
5 In environmental studies, concentrations of cyanide and other solutes in solutions are ordinarily presented
in terms of mass per unit volume, or sometimes as the dimensionless unit “part per million” (ppm).
Concentrations in milligrams per litre (mg.L
-1) are the same as concentrations in grams per cubic metre
(g.m
-3), and both of these are essentially identical to concentrations in ppm (because the density of solutions
is usually very close to 1 kilogram per litre [kg.L
-1]).

TABLE 2. Analyses of Barren Solutions​
4

Weak and Strong Cyanide Complexes​
Conventionally, cyanide chemists distinguish “weak” from “strong” cyanide complexes.
The weak cyanide complexes, often referred to as “weak acid dissociable” or WAD
cyanide, can dissociate in solution to produce environmentally significant concentrations
of free cyanide. The weak complexes include cyanide complexes of cadmium, copper,
nickel, silver and zinc. The degree to which these complexes dissociate is dependent
largely on the pH of the solution.
Strong cyanide complexes, on the other hand, degrade much more slowly than WAD
cyanide under normal chemical and physical conditions. Complexes of cyanide with
gold, cobalt and iron are strong and stable in solution. This stability of the gold–cyanide
complex is a key factor in the use of cyanide for the extraction of gold from ores. Once
gold enters into solution tied to the cyanide, it remains complexed with the cyanide until
process conditions are changed in order to remove it from solution. Cobalt is present
only in trace amounts but iron is virtually ubiquitous in geological materials. For most
mining situations, the strong complexes of cyanide are predominantly iron cyanides.
The rate at which complexes dissociate and release free cyanide into solution depends
on several other factors, including the initial concentration of the cyanide complex,
the temperature, the pH of the solution, and the intensity of light, especially ultraviolet
radiation.​
Analysing and Monitoring Cyanide​
Cyanide is generally measured by one of two analytical methods: total cyanide analysis
or WAD cyanide analysis. The first is used to determine total cyanide in solutions, including
free cyanide and metal-bound cyanides, such as the more stable, non-toxic iron
cyanides. The analytical procedure for determining WAD cyanide is used for free and
complexed forms of cyanide, except iron cyanide. An older but still used alternative
method to that of WAD cyanide analysis is called “cyanide amenable to chlorination.”
Cyanide analyses are needed for operational control, regulatory compliance and toxicity
evaluation, as well as for public information about the handling of hazardous materials.
Monitoring cyanide both during and after the gold recovery process is essential to
good operating practice and the protection of both health and the environment. Rigorous
sampling protocols and analytical procedures are required to ensure the quality of information
available for decision making. This requires excellent planning and performance
from trained personnel working with well-designed and well-managed systems.​
18​
The Management of Cyanide in Gold Extraction​
SECTION 7​
Attenuation of Cyanide
Concentrations in the Environment​
A​
s explained in Section 4, once gold has been recovered, the solution becomes barren
of gold but still contains cyanide. The processes that decrease the concentration
of cyanide in solution, whether in the natural environment or in engineered
facilities, are called “attenuation.” Volatilization of HCN, which reduces the concentration
of free cyanide in solution, is the prominent natural attenuation process. Figure 4
provides a schematic representation of the relationships between forms of cyanide and
the processes controlling them.
Over the past two decades, the chemical and mining industries have made major
advances in handling waste cyanide solutions so that they will not harm public health
or the environment. Two technologies are used, often in combination: treatment and
recycling.

Cyanide Solution Treatment and Re-use​
Treatment:​
Four general forms of cyanide solution treatment are in use:
• Natural degradation
• Chemical oxidation
• Precipitation
• Biodegradation
In addition, several technologies enable the re-use of cyanide through recycling.

Natural degradation:​
The principal natural degradation mechanism is volatilization
with subsequent atmospheric transformations to less toxic chemical substances. Other
factors such as biological oxidation, precipitation and the effects of sunlight also contribute
to cyanide degradation.
Cyanide species may be adsorbed on the surfaces of minerals or organic carbon debris
in the soils of a pond embankment, in a clay liner, or along a groundwater flow path. In
soils, bacteria assimilate the cyanide through a variety of aerobic and anaerobic reactions.
In some instances, the combination of these processes of natural degradation are
sufficient to meet regulatory requirements for discharge of cyanide-containing solutions.

19​
Attenuation of Cyanide Concentrations in the Environment​
In tailings impoundments, the large surface area enables decomposition of WAD
cyanide. Figure 5 illustrates a typical situation in which half of the total cyanide (CN​
T)
degraded naturally in less than three weeks from the initial concentration of 20 milligrams
per litre. The CN
T disappeared almost completely within about 100 days.
Actual degradation rates need to be determined through test work on a site-specific basis
using conditions that mimic, as closely as possible, the types of solution and the natural
processes that are likely to occur at that location.
Table 3 compiles data from natural degradation systems at a number of gold mines
around the world. The values in this table demonstrate the ability of natural degradation
to reduce the cyanide concentration of solutions.

Chemical oxidation​
processes for cyanide treatment include the SO2/Air process (developed
by the Canadian mining company INCO) and the H
2O2 (hydrogen peroxide)

20​
The Management of Cyanide in Gold Extraction​
FIGURE 4. The Cyanide Cycle​
anaerobic biological
activity within
sediments
adsorption/desorption processes
all complexes subject
to biological oxidation
HSCN
2H​
20
3H
2

HCN/CN​

S​
°

H​
2O
HCOONH
4

CH​
4 + NH3

NH​
3 + CO2

NH​
3 + H2S + CO2

H​
2 CH4
+

CO​
2

SEDIMENTS​
CN​
Fe(CN)5
2
uv light
Fe(CN
)6
3

O​
2

Fe​
3+

NaFe [Fe(CN)​
6]°

Fe complexes subject
to partial biological
oxidation
CN​
Fe(CN)5
3
uv light
Fe(CN)
6
4

dimers,
trimers etc
polymerisation
Fe​
2+

CN​

biological oxidation
SCN HS​
– –

Ni​
2+
Cu
+

Zn​
2+

Ni(CN)​
4
2

Cu(CN)​
2

–​
Zn(CN)​
4
2

O​
2

HCO​
3

–​
+ NH3

NH​
3 + HCO3

–​
+ HSO4

–​
NO​
2

–​
+ NO3

–​
biological
oxidation
biological
oxidation
HCOO​
+NH4
+
TAILINGS POND

approaches background
concentrations
diffusion/dispersion
photolysis/oxidation
HCN(g)
moisture
hydrolysis
biological
oxidation
to soil or
surface water
far below
detection limits
HCOO​
+ NH4
+

HCN/CN​

(very dilute)
hydrolysis
biological
oxidation
plant nutrient
animal
metabolism​
AIR​
NH​
3 + CO2

HCN
sunlight
uv​
••
••​
Fe​
3+ hydrolysis
Prussian Blue
biological
nitrification

Source: Smith and Mudder, 1991.​
Courtesy of Environment Australia

treatment process (pioneered by Degussa). An older chemical oxidation alternative, the
Alkaline Chlorination Process, is rarely used in the mining industry today.
In the SO​
2/Air process, free and WAD cyanide are oxidized, and iron cyanide is precipitated
as an insoluble solid. The process can be applied to either solutions or slurries, and
reaction is rapid. Potential limitations are the need to obtain a licence to use the process,

21​
Attenuation of Cyanide Concentrations in the Environment​
22
20
18
16
14
12
10
8
6
4
2
0
1 5 10 15 20 25​
1980 April 30 June 4 July 9 August 13 September 12​
Concentration of CN​
T mg/L

TABLE 3. Natural Degradation of Cyanide in Tailings Impoundments​
Sources: a) Scott, 1993; b) Smith et al., 1985; c) Smith, 1987; d) Smith, 1994​
FIGURE 5. Example of Cyanide Degradation in a Shallow Pond​
Source: adapted from Schmidt et al., 1981.​
MINE CN entering CN discharging
the tailings from the tailings
system (mg.L​
-1) system (mg.L-1)

Lupin, NWT, Canada​
(a) 184 0.17
Holt McDermott, Ontario, Canada
(a) 74.8 0.02
Cannon, Washington, USA
(b) 284 <0.05
Ridgeway, South Carolina, USA
(c) 480 0.09
Golden Cross, New Zealand
(d) 6.8 (WAD CN) 0.33 (WAD CN)
the cost of building a processing plant, the need for empirical testing to optimize the system,
and the inability of the process to oxidize intermediate by-products of cyanide.
Hydrogen peroxide, a strong oxidant, oxidizes free and WAD cyanide to ammonium and
carbonate. Iron cyanides are not oxidized by peroxide, but precipitate as insoluble and
stable solids. Sometimes it is necessary to add chemicals to control the copper concentration
of solutions to meet environmental regulations. The peroxide system is not as
well suited to the treatment of slurries because of irregular hydrogen peroxide requirements
when solids are present.
Both methods of chemical oxidation are capable of producing residual concentrations of
cyanide that can meet stringent discharge standards. Both processes require testing on
representative samples of site-specific materials prior to the final plant design. Caro’s
acid, which combines sulphuric acid with hydrogen peroxide to form H
2SO5, is also used
as an oxidation agent to decompose cyanide in solution.

Precipitation​
of stable cyanides can be achieved by the deliberate addition of complexing
agents such as iron. This reduces the free cyanide concentration and is also effective
in controlling elevated levels of other metals that may be present. Iron cyanides may
react with other chemicals in solution and produce solid precipitates, which may contain
a dozen insoluble cyanide salts, thereby removing cyanide from solution. Some of the
cyanide in process solutions will react with other chemical components within the system
to form much less toxic concentrations of compounds such as ammonia, nitrate and
carbon dioxide.

Biodegradation​
of cyanide is the basis for industrial wastewater treatment systems such
as those used by Homestake Mining Company in the United States and ICI Bioproducts
in the United Kingdom. A biological process has been used to treat cyanide to meet environmental
discharge criteria for more than a decade at the Homestake Mine in Lead,
South Dakota. Aerobic conditions are much more favourable to cyanide degradation than
are anaerobic conditions, although anaerobic organisms can be effective in treating
cyanide at concentrations of up to several milligrams per litre. Both active and passive
biological treatment systems have been built—these systems remove cyanide using
either aerobic or anaerobic micro-organisms.
At Homestake, the gold-mill barren solution is channelled through reaction vessels containing
bacteria. These use oxygen from air to decompose cyanide compounds into
nitrates, bicarbonates and sulfates. This microbial process is capable of oxidizing metal
cyanide complexes, the metal ions from the WAD cyanide species and intermediate byproducts
of cyanide oxidation.

22​
The Management of Cyanide in Gold Extraction​
Advantages of the biological treatment process are its simple design and operational
process control, low chemical costs and capacity of treating all forms of cyanide and its
by-products. Potential limitations of biological treatment systems include reduced performance
at cold temperatures and at very high cyanide concentrations.​
Recycling:​
While the technologies for cyanide management have centred on cyanide
destruction in single-pass systems, it is possible to recover and re-use cyanide, thus minimizing
the total amount of cyanide used and reducing operational costs for some mines.
Recycling lowers cyanide concentrations in waste solutions and decreases the cost of
cyanide destruction.
Cyanide recovery and recycling has been used since the 1930s, notably at Flin Flon
(Manitoba, Canada), Pachuca (Hidalgo, Mexico) and Golcanda Minerals (Tasmania,
Australia). The basic process involves three steps: pH control, volatilization under highly
controlled conditions, and capture of the cyanide that has been released. Recent engineering
advances have made it a much more attractive prospect than was the case
formerly, and cyanide recovery has been adapted in the last decade to treatment of slurries
in a patented, commercial process called Cyanisorb. The process is being applied at
the Golden Cross Mine (Waikato, New Zealand) and at the Delamar Silver Mine (Idaho,
USA). Two additional Cyanisorb plants have recently been started up in Brazil and
Argentina.
Research into cyanide recovery continues, including the testing of a treatment approach
that separates cyanide complexes from solutions and absorbs them onto polystyreneresin
beads called Vitrokele (the Cyanosave process). Modifications of this process can
be applied to either solutions or slurries, and both cyanide and metals can be recovered.
The recovered cyanide is then recycled for use in the gold plant. While there have been
successful tests of the process at mines in Canada, Australia and the USA, no commercial
plant yet exists, and development continues.

23
 

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