Improving gold recovery from refractory gold ores through biooxidation using iron-sulfur-oxidizing/sulfur-oxidizing mixotrophic bacteria

The effect of biooxidation (BIOX) pre-treatment of refractory gold concentrates using mixotrophic and chemolithotrophic bacteria on the gold extraction during cyanidation of the concentrates at neutral pH was studied. A series of biooxidation and cyanidation of the biooxidized concentrates was carried out using three strains of bacteria; two mixotrophic bacteria of SKC1 and SKC2 and a chemolithotrophic bacteria Acidithiobacillus ferrooxidans (AC). Two distinct types of refractory gold concentrates with high sulfur content (i.e. S > 20%) and low sulfur content (i.e. S < 5%) were used. The experimental results showed that biooxidation with any of the three bacteria generally showed positive effect on the gold extraction. The highest gold extraction (91.4%) from high sulfur-concentrate was achieved by BIOX with SKC2 for 14 days. It was 18% higher than the extraction level of direct cyanidation of the untreated concentrate. Biooxidation with AC, on the other hand, resulted in only slight increase of gold extraction from both concentrates due to the high solution pH of > 5.0 which is not a suitable living environment for the bacteria. Cyanide consumption for cyanidation of the biooxidized concentrates with both mixotrophic bacteria was significantly increased due to reactions of sulfur and iron species that were precipitated during the biooxidation step with cyanides to form thiocyanate and ferrocyanide ions.