Environmental Engineering Reference
In-Depth Information
with oxygen (in air) and small amounts of copper sulphate catalyst, to oxidize the cyanide to
cyanate. Free cyanide is oxidized to cyanate in accordance with the following reaction:
CN
free
CNO
2
SO
2
O
2
(6.13)
H
2
O
→
H
2
SO
4
Metal (Me) cyanide complexes are also decomposed, according to reactions such as the
following:
Me(CN)
2
CNO
Me
2
4O
2
4H
2
SO
4
(6.14)
4
4SO
2
4H
2
O
→
These processes take place in a single reaction tank, fitted with an agitator, air sparging
system and pH control involving lime addition.
Various environmental and occupational health concerns are associated with the use of cya-
nide. At high concentration cyanide is acutely toxic. At mines, however, cyanide is used in
low concentrations in water, typically 0.01% to 0.05% cyanide. Its use is tightly controlled and
monitored. Mining employees are trained to handle cyanide with care, while the manufac-
ture, transport, storage, use, and disposal of cyanide are strictly regulated; it has been used
safely for decades (The International Cyanide Management Code 2005). At gold mines, all
tanks, pipes, ponds, and other areas that contain cyanide are required by law to have appropri-
ately designed and engineered containment facilities (USEPA 1981). This means that if there
is any problem, there is a back-up system to contain the material. Cyanide in mining solutions
is collected either to be recycled or effectively destroyed to permitted levels on-site after gold is
removed. Common well-proven detoxification technologies are the above mentioned INCO
SO
2
Air System or the use of hydrogen peroxide (H
2
O
2
). Cyanide can be destroyed quickly
when required using several commonly available chemicals kept at all mines.
In the natural environment, cyanide is naturally decomposed, generally by oxidation
(see also Chapter Eighteen). It is not persistent in the environment, neither does it bio-
accumulate. It is not carcinogenic, or mutagenic. However it does present three potential
environmental hazards: (1) cyanide-containing ponds and ditches can represent an acute
hazard to wildlife and birds. Tailings ponds may pose similar hazards, although cyanide
concentrations are typically much lower; (2) spills can result in cyanide reaching surface
water or groundwater and causing short-term (e.g. fish kills) or long-term (e.g. contami-
nation of drinking water) impacts; (3) cyanide in active heaps and ponds and in mining
wastes (e.g. heaps and dumps of spent ore, tailings impoundments) may be released and
present a hazard in surface water or groundwater. Cyanide may also increase the potential
for metals to go into solution and, therefore, be transported to other locations.
Some basic knowledge of the different forms of cyanide is necessary to understand regula-
tory standards. Cyanide concentrations are generally measured as one of the following forms:
●
In the natural environment,
cyanide is naturally decomposed,
generally by oxidation.
Free Cyanide - the sum of the free cyanide (CN
) and hydrocyanic acid (HCN) and
includes cyanide-bonded sodium, potassium, calcium, or magnesium. Free cyanide is
very difficult to measure except at high concentrations and its results are often unreli-
able, difficult to duplicate, or inaccurate.
●
Titratable Cyanide - the cyanide concentration measured by titration with silver
nitrate (AgNO
3
); may include cyanide from dissociation of some complex forms in
addition to free cyanide.
●
Simple cyanides - containing only one type of metal ion which dissociates in water to
release free cyanide.
●
Complex cyanides - contain more than one metal and dissociate in water to release a
metal ion and a cyanide-metal ion complex, which may subsequently dissociate further
to release free cyanide.
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