Effect of mechanical activation on biooxidation and gold extraction of a high-grade flotation concentrate using mesophilic and moderately thermophilic microorganisms

The effect of mechanical activation and biooxidation on the gold extraction from a high-grade (168 g/t) gold sulfide concentrate was investigated using a mixed culture of iron- and sulfur-oxidizing microorganisms. The influence of mechanical activation pretreatment, the nutrient medium type (Norris and 9K), initial pH, and microbial culture type was investigated on the biooxidation ability of mixed cultures of mesophilic and moderately thermophilic microorganisms. Results showed that gold extraction from the non-activated non-biooxidized concentrate was 83.9%, while after biooxidation, it reached 98.8%. The gold recovery from the mechanically activated non-biooxidized concentrate was 77.3%, which reached 97.6% after biooxidation. Therefore, biooxidation was found to have a significant positive effect on gold extraction, but mechanical activation had a negative effect on it in both abiotic and biotic media. The characterization of solid residues was performed by SEM/EDS and chemical analyses. Jarosite and gypsum were the main precipitates formed during biooxidation, especially in 9K nutrient medium. It was found that mechanical activation reduced the efficiency of biooxidation process mainly as a result of overcoming negative effects such as weak bacteria-mineral attachment, the release of toxic species, increasing pulp viscosity, and the coating of gold grains by less soluble solids on the positive effects such as the enhancing of surface area and the disordering of the crystal structure of refractory sulfide phases.