COBALT-CYANIDE COMPLEXING IN ORE PROCESSING EFFLUENTS AT GOLD MINES: SIGNIFICANCE FOR CYANIDE PERSISTENCE AND COBALT MOBILITY

The cyanide anion (CN^-) is a key hydrometallurgical agent that removes gold from crushed ores as the soluble Au(CN)₂ complex. Where ore processing solutions are discharged, cyanide is of environmental concern because it can be toxic to aquatic life. The primary focus is on free cyanide (CN^- or HCN_aq), but inevitably various cyanometallic complexes are also present, the environmental significance of which is debated. To a large extent, the debate continues because the standard analytical methods for cyanide give incomplete, and sometimes inaccurate, information on speciation.

To improve the understanding of cyanide persistence, we investigated cyanide speciation in waters from four remediated leach operations using an improved distillation method for total cyanide that overcomes a well-known deficiency of the EPA-approved method, incomplete yield for the cobalt cyanocomplex. Cyanide and metal analyses of heap effluent, pond waters, and monitoring well waters indicate that the cyanide contained in strong cyanometallic complexes (total cyanide minus weak-acid-dissociable cyanide) commonly exceeds what can be accommodated in the Fe(CN)₆ complex alone, which is widely thought to be the dominant strong cyanocomplex. On the other hand, strongly-complexed cyanide correlates with cobalt, and cobalt is abundant enough in most samples to account for the balance of the strongly-complexed cyanide. We infer that the cobalt complex accounts for a substantial fraction of the cyanide that persists at these operations. This conclusion is supported by positive identification of the Co(CN)₆ complex via ion chromatography in a representative sample from one site.

Our finding that Co(CN)₆ may be the most long-lived cyanometallic complex at four different leach operations with diverse ore types across three western states suggests that this phenomenon may be general. Cobalt-complexed cyanide will not be evident from total cyanide analyses performed for regulatory compliance because the standard analytical methods severely underreport this species. On the other hand, the presence of Co(CN)₆ may be signaled by mobile cobalt. Important outstanding questions are the toxicity of Co(CN)₆, and the conditions under which the complex will dissociate to give highly toxic free cyanide.