Scientific Properties of Cyanide (Inorganic) Compounds
Background and properties
Cyanide (inorganic) compounds refer to a group of substances containing the cyanide ion (CN-). CN- forms strong
complexes with metals such as cobalt, gold, iron and silver, and weaker complexes with metals such as zinc. It is because
of these properties that cyanide is used extensively within precious and base metal extraction and concentration.
CN- complexes are found in nature in some plants, blue green algae, bacteria and fungi.
Ferrocyanides (cyanide–iron complexes) and their salts can be found in table salt, Prussian blue pigments and as bleaching
agents used in photography. They are also used in the production of citric acid and wine. Other cyanide salts are used in
the production of organic chemicals, fumigation chemicals, insecticides and rodenticides. Sodium cyanide (NaCN) is used
in the extraction and concentration of precious metals, particularly gold leaching and adsorption.
Cyanide is also found in the atmosphere as hydrogen cyanide (HCN) and is generally thought to result from forest fires, the
manufacture of plastics and fossil fuel combustion.
Cyanide in the environment
Exposure to minor levels of cyanide is normal. Unlike many other pollutants cyanide does not accumulate in the body and
its effect is generally short term. However cyanide (in most forms) is toxic to humans and exposure to large doses can be
fatal.
Cyanide is emitted to air as hydrogen cyanide (HCN) gas or in various forms associated with particulate matter. It may also
be emitted in water in “free” or “complex” states. By far the largest amount of cyanide emitted is as HCN gas.
HCN, the dominant form of cyanide in the atmosphere, is lighter than air (having a lower molecular weight) and tends to rise
rapidly into the upper atmosphere where it is quickly dispersed.
Cyanide found in water and land is naturally degraded, generally by oxidation.
Most animals including humans are able to detoxify and eliminate small amounts of cyanide, converting it to the less harmful
compound thiocyanate (CNO).
Cyanide emissions from mining
Hydrogen cyanide emissions to air
Cyanide emissions to air in the mining industry mainly result from the use of sodium cyanide in precious and base metals
extraction. Cyanide may be released from process tanks and storage areas or from tailings waste streams and storage
areas. Emitted cyanide is almost exclusively in the form of hydrogen cyanide gas (HCN).
The rate of HCN release (volatilisation) from cyanide solutions is largely governed by the pH of the solution. The lower the
pH, the greater the rate of HCN evolution. The loss of cyanide is actively minimised through regulation of pH and the
monitoring of process streams.
The environmental hazards posed by emissions of HCN to air from mineral extraction are considered to be low for the
following reasons:
1
•
Cyanide is not persistent in the environment but is degraded via oxidation to elemental nitrogen, carbon monoxide
and oxides of nitrogen.
•
Ambient HCN levels around tailings storage facilities (TSFs) and above leach and adsorption tanks are monitored
regularly and are typically less than 1 part per million (ppm) at distances of 1 meter or more from the source.
These levels are far below the occupational exposure standard1 or threshold limit value (TLV) of 10 ppm (the
maximum concentration personnel may be exposed to over an 8 hour shift). Toxicity does not normally begin until
an exposure level of 20 to 40 ppm.
•
HCN is lighter than air so it tends to disperse rapidly into the upper atmosphere.
See http://www.safeworkaustralia.gov.au/sites/SWA/about/Publications/Documents/26/CyanidePoisoning_1993_PDF.pdf
Studies2 have indicated that there has been no real discernible increase in concentrations of atmospheric HCN as a result of
the recent use of cyanide in metallic ore processing in mining.
Cyanide in seepage
Seepage of tailings solutions from active tailings storage areas represents the most common risk of release of cyanide
(inorganic) compounds to land from mining facilities. Tailings are waste slurries produced from mineral extraction
processing. These may contain residual amounts of reagents, including cyanide, used in the extraction process.
Concentrations of cyanide in seepage water are generally low, usually only a few parts per million. Analysis of seepage
water compared with input solution typically show far lower concentrations in seepage water reflecting strong retention of
cyanide within the tailings.
Emissions of cyanide to land in seepage water are reported as total cyanide but in reality it is only the weak acid dissociable
(WAD) and free forms of cyanide that have any real environmental consequence. These can often make up only a small
percentage of total cyanide.
Environmental regulations require seepage solutions to be monitored for various substances, including cyanide, and
recovered via collection trenches and drains and water extraction bores.
Cyanide recycling, destruction and neutralisation are employed to ensure that cyanide levels in tailings are minimised.
2
US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry,Toxicological Profile for Cyanide, July 2006. Cicerone, R.J. and Zellner, R., The Atmospheric Chemistry of Hydrogen Cyanide (HCN), Journal of Geophysical Research, vol. 88:C15, Dec. 20, 1983.