Cyanide (CN-) is a common chemical used to extract metals, especially gold and silver.
In addition to gold, recycled leach solutions in the barren pond can contain arsenic, copper, iron, lead, molybdenum, nickel, and zinc (Table 1) (Logsdon et al., 1999).
Table 1. Analyses of barren solutions. From Logsdon et al., 1999.
Cyanide in solution after gold extraction is either in free cyanide, weakly complexed cyanide, or strongly complexed cyanide. Insert Table.
Free cyanide refers to both CN- and HCN- which are the most toxic forms of cyanide. The formation of HCN is pH dependant and occurs when ample hydrogen ions are present at a pH below 8 (Logsdon et al., 1999) (Figure 2). At a pH greater than 10.5, all free cyanide is in the CN- form. HCN is the form of cyanide that readily volatilizes. Volatilized HCN is toxic, so caution must be taken to protect worker safety. Weakly complexed cyanide or weak acid dissociable (WAD) consists of cyanide complexes of cadmium, copper, nickel, silver and zinc (Logsdon, 1999). WAD cyanide can easily dissociate, resulting in the release of appreciable amounts of free cyanide into the environment. Cyanide strongly complexes with gold, cobalt, and iron (Logsdon, 1999). Strong cyanide complexes are more stable than WAD complexes and degrade slowly. The rate at which complexes dissociate and release free cyanide into solution depends on several other factors, including temperature, solution pH, and light intensity (Logsdon, 1999).
Unlike other organic toxic chemicals, cyanide is not persistent in the environment. Cyanide concentrations can be decreased by the following processes: 1) abiotic degradation, 2) chemical oxidation, 3) precipitation, and 4) biodegradation (Logsdon et al., 1999).
References:
Logsdon, M.J., Hagelstein, K., and Muddler T.I., 1999, The Management of Cyanide
in
Gold Extraction, International Council on Metals and the Environment, 40 pp.
Scott, J.S.,
and Ingles J.C., 1981. Removal of cyanide from gold mill effluents, In
Proceedings of the 13th Annual Meeting of the Canadian Mineral Processors, Ottawa,
Ontario, Canada, January 20-22, pp. 380-418.
Acid Drainage Water Source Control And Treatment Waste Source Control and Treatment Cyanide Treatment Waste Disposal Physical Stability Topographic Reconstruction Revegetation Air Prediction and Monitoring
University
of Nevada