The influence of physicochemical parameters on the bioleaching of zinc sulfide concentrates using a mixed culture of moderately thermophilic microorganisms

This work investigates the extraction of zinc from Bama zinc sulfide concentrate (Isfahan, Iran) using a mixed culture of moderately thermophilic iron- and sulfur-oxidizing microorganisms including Leptospirillum ferriphilum, Acidithiobacillus caldus, Sulfobacillus sp. and Ferroplasma sp. The effects of some physicochemical parameters, namely pH, nutrient medium type, temperature, and the external addition of ferrous iron on the zinc extraction, redox potential and iron mobilization were investigated in shake flasks. Experiments were carried out at the pulp density of 5% (w/v) and a stirring rate of 150 rpm. The highest zinc extraction (91%) was achieved at 45 °C, leaching time of 18 days, initial pH of 1.8 and Norris nutrient medium. The zinc extraction was 3.7-fold higher in the bioleaching compared to chemical leaching. The leaching rate of the sphalerite concentrate tended to decrease with decreasing pH from 1.8 to 1.2. It was also found that the increasing of ferrous iron addition from 0.02 to 0.05 M led to a decrease both in the zinc extraction and bacterial activity indicating the inhibitory effect of increased iron on the microorganisms. SEM-EDX analyses revealed both poorly crystallized iron hydroxyl sulfates and well-crystallized jarosites on the surface of the solid residues.