Biological processes for thiocyanate and cyanide degradation

A common problem for bacterial oxidation plants that treat refractory gold concentrates is the low tolerance of the bacteria to thiocyanate and cyanide species. Thiocyanate forms from a reaction between cyanide and a sulphur species during cyanide leaching of the bioleached concentrate. The final plant effluent streams contain thiocyanate and cyanide, and generally report to a single tailings dam. Consequently the tailings dam return water cannot be utilised in the bacterial oxidation plant. Environmental legislation and water balance considerations, such as dry season water shortages and wet season water surplus, require a water stream that can be recycled to the bacterial oxidation plant. Such water must be free of thiocyanate and cyanide. A culture of micro-organisms, isolated from a tailings water containment at Fairview Gold Mine, was shown to degrade both cyanide and thiocyanate. Two different thiocyanate destruction processes have been tested on a pilot scale utilising these micro-organisms. The first process piloted was an attached growth process and the second an activated sludge process. This paper gives some results of the laboratory and pilot plant testwork.