Preg-robbing of gold from cyanide and non-cyanide complexes: Effect of fungi pretreatment of carbonaceous matter

Cyanide has been the universal reagent for gold leaching on counts of higher stability, lower cost and better understood chemistry. However, its high toxicity and slow leaching kinetics have called for research into alternative leaching reagents, of which thiosulfate and thiourea appear to be promising. Unlike the cyanide system, interaction of thiourea and thiosulfate systems with carbonaceous matter (CM) has not received much research attention. As non-cyanide reagents become important alternatives for gold leaching, it is imperative to assess the degree of gold adsorption (preg-robbing) in these systems and find ways of reducing this effect.This paper presents the results of a study on reduction in gold uptake by CM in cyanide and non-cyanide systems due to fungal-treatment. Comparison is made between cyanide and non-cyanide systems on the bases of changes in gold adsorption by surrogate CM and CM in refractory gold ores. The results indicate that gold adsorption by as-received surrogates is in the order of cyanide > thiourea > thiosulfate, whereas after fungal-transformation, reduction in gold adsorption is more pronounced in cyanide than in thiourea and thiosulfate solutions. The decrease in gold adsorption on the CM is about 70–97% in cyanide, 60–92% in thiourea and 50–91% in thiosulfate. In general, the results exhibit a potential technique for reducing preg-robbing in the processing of carbonaceous gold ores with cyanide, thiourea and thiosulfate solutions.

► The toxicity of cyanide has necessitated research into other leaching reagents.
► Preg-robbing in the thiourea and thiosulfate systems requires more research attention.
► Fungal-treatment reduces preg-robbing in the cyanide and non-cyanide systems.
► Preg-robbing reduction is more pronounced in the cyanide system than in the others.