Gold recovery enhancement from a refractory flotation concentrate by sequential bioleaching and thiourea leach

In the present work, the applicability of pre-biooxidation using mixed cultures containing Thiobacillus ferrooxidans and Leptospirillum ferrooxidans from a refractory flotation sulfide concentrate was studied. The flotation concentrate consisted mainly of arsenopyrite and pyrite. Parameters for adapting the bacteria to arsenic(III) and catalyzing oxidation of sulfides with silver ion were also studied. The optimum conditions for efficient biooxidation were 10% culture inoculation, 5% pulp density, and a 10 mg/L silver ion catalyst in a pH=2 iron-free 9K suspension at 30 °C for 6 days, using the tamed mixed cultures. Under optimal bioleaching conditions, the results demonstrated that the oxidation times were shortened by almost 9 days (from 15 to 6 days at a 95% arsenic and iron oxidation level) compared to the sterilized control. After biooxidation, the gold extraction from the above biooxidation residue was studied. When sodium sulfite powder of 2.52 g/kg concentrate was added into the pH=2 sulfuric acidic–thiourea system, the consumption of thiourea significantly decreased from 12.0 to 3.0 g/kg concentrate and the gold extraction time was remarkably shortened from 6 to 1 h.