Summary of sources of environmental exposure

22.4Summary of sources of environmental exposure

Organic substances containing cyanide are produced naturally by various organisms, including some micro-organisms, plants, algae, fungi and invertebrates. These substances include forms such as cyanogenic glycosides, where free cyanide may be released upon consumption by an animal, potentially causing poisoning if the amounts present exceed the detoxicification capacity of the animal. Burning vegetation can form HCN under some combustion conditions, and biomass burning is considered to be the major source of HCN in the atmosphere.

Cyanide may also be released to the environment from a wide range of activities by humans additional to those using sodium cyanide. As well as industrial uses of HCN and various cyanide salts, these anthropogenic sources include some sources where cyanides are not used directly, such as incomplete combustion of coal, oil and waste materials. Release from fuel consumption in vehicles is considered a major source of release of cyanide to air.

Releases of cyanide associated with sodium cyanide in Australia could potentially occur during its manufacture, storage and transport, during use, and during disposal of residues remaining after use. The major use in Australia is in the mining industry for the beneficiation of ores containing gold and other precious metals. Other uses include ore flotation with base metal mining, uses in metal industries for electroplating, cleaning, and for case hardening and similar processes for steel, and minor uses such as in analytical laboratories.

Gaseous HCN is used to produce sodium cyanide and may subsequently be emitted to the atmosphere from HCN in solution or from certain other reactions. Thus some release of HCN to the atmosphere may occur during the manufacture of sodium cyanide, storage, loading and unloading operations, transport, industrial and mining uses, and from residues remaining after use, such as in tailings storage facilities, heap leach piles, landfills, or sewage effluent. This is generally minimised by the highly alkaline conditions pertaining in manufacture, storage and transport of NaCN in liquid form, and in process streams.

Stack emissions of HCN at manufacturing facilities are monitored to ensure they comply with environmental protection licences, which place strict limits on release. Wastewaters containing free cyanide may be generated during the manufacturing process, and cyanide residues in these are recovered and used, or are treated to destroy free cyanide prior to disposal under environmental protection licence conditions (the discharge concentration limit is 1 mg CN/L, but monitoring shows discharges to sewer/outfall are typically much lower). After material containing cyanide is deposited in tailings storage facilities (TSFs) HCN emission to the atmosphere is expected as a significant means of dissipation.

After use, various substances originating from sodium cyanide may remain in aqueous or solid media, including process wastes from industrial treatments, or tailings and heap leach piles from the treatment of metal ores. As explained in the following chapter, cyanide may be present as free cyanide (HCN/CN^- in solution) or various metal compounds and complexes. Cyanide may also have been altered by processes to destroy cyanide and by natural degradation processes to form cyanate, thiocyanate and other nitrogenous products (e.g. ammonia, nitrite, nitrate). Weak acid digestible cyanide (WAD CN) measurements are used as a measure of forms of cyanide available at the pH of the stomachs of wildlife such as birds.

Gold beneficiation is the primary source of environmental exposure to CN. In mining situations where sodium cyanide is used in tank leach facilities, cyanide residues from process streams are recirculated and/or disposed of in the tailings stream, with or without steps to reduce the amount of remaining free cyanide. As the majority of use in Australia is in tank leach facilities for gold beneficiation, tailings storage facilities are the principal repository and potential source of release to the environment for cyanide residues.

Concentrations of sodium cyanide used in tank leach facilities are typically in the range 100-500 mg CN/L, affected by factors such as the ore and dilution water characteristics. During the leaching process this is contained within large tanks. It is then released to the TSF, in some cases after treatments to recover the cyanide solution for re-use and to reduce loss to the TSF, or to destroy residual cyanide. Concentrations of WAD CN released into TSFs may therefore range widely, from as low as 1 mg WAD CN/L to well above 100 mg WAD CN/L (as high as ~600 mg WAD CN/L) at exceptional sites or under exceptional conditions.

The most exposed use situation is heap leach mining, where NaCN solution - at concentrations again typically ranging from 100-500 mg CN/L - is applied directly to the exposed ore heap. The solution containing gold is then collected at the bottom. However, these situations are engineered to enable control of the flow and storage of NaCN-containing water. Residues remaining after use in ore flotation are also disposed of to tailings storage facilities.

With mining use, cyanide or cyanide degradation products may potentially be emitted through seepage to groundwater or overflow/run-off, or in some situations through planned discharge. Tailings storage facilities (TSFs) and heap leach pads may be substantial structures holding large volumes of material containing heavy metals and other chemical species in addition to cyanide residues. Hence it is important that they have adequate capacity and are structurally sound, and are designed and operated correctly so that any releases to the aquatic or terrestrial environments are managed appropriately. To protect surface and groundwater from release of water containing cyanide residues, surface drains and bores are installed to intercept run-off and leaks and to monitor and if necessary intercept seepage in groundwater. Techniques to destroy cyanide to acceptably low levels are used where outflows are released to downstream aquatic areas.

In industrial situations with metals such as electroplating and case hardening, solid and liquid residues are treated to destroy free cyanide not consumed during the process, before release of the treated residues to landfill or the sewer. Laboratory protocols also indicate that cyanide residues in waste should be destroyed before disposal.

The hazards of cyanide in these use situations are well known and there is an extensive management regime established in Australia to manage them. However, unintentional incidents have occurred resulting in potential situations or actual releases (spills, leaks) of sodium cyanide during storage and transport. Some spills have occurred during road or rail transport in Australia, but only in one incident were wildlife or aquatic life reported to have been affected. Environmental damage was minimised at other incidents where significant spills occurred, due to emergency response measures to contain and recover the spilt material and remove contaminated soil or water. However, some overseas incidents where significant release of NaCN occurred to water led to downstream environmental contamination and harm to aquatic organisms. NICNAS has not investigated the regulatory control measures that were in place in the various overseas jurisdictions at the time of these overseas incidents.

Unintended releases of material containing cyanide have also occurred overseas and in Australia due to incidents such as leaks from pipelines containing slurry, overflows and leaks from TSFs and heap leach operations, and major structural failures. Such release varies widely in scale, from minor leaks that are soon corrected and cause no environmental harm, to major releases that have led to substantial environmental effects due to physical effects and other toxic components, in addition to cyanide residues. These incidents indicate the need for appropriate monitoring and response measures for operations on an ongoing basis, as well as correct design and operation of TSFs and heap leach facilities.