Comparative Variants of Microbial Pretreatment and Subsequent Chemical Leaching of a Gold-Bearing Sulphide Concentrate

A sulphide concentrate containing 31.4% total sulphur (in which 30.7% was sulphidic), 31.8% iron, 8.0% arsenic, 18.5 g/t gold and 32.9 g/t silver finely disseminated in pyrite and arsenopyrite as the most essential components was subjected to microbial pretreatment to expose the precious metals from the sulphide matrix. Three different types of microbial cultures were tested for their ability to oxidize the sulphides and to expose the precious metals for the subsequent chemical leaching: mesophilic chemolithotrophic bacteria with a temperature optimum for their growth and activity at about 32 – 37°C, moderate thermophilic bacteria with the relevant optimum at 53 – 59°C, and hyperthermophilic archaea at 75 – 84°C. The microbial pretreatment of the concentrate was carried out in reactors with mechanical stirring and enhanced aeration by air enriched in CO2. It was found that sulphide oxidation of about 40 – 45 % was sufficient for achieving gold extraction higher than 90% during the subsequent leaching of the pretreated concentrate by different gold-solubilizing reagent (cyanide, thiourea, thiosulphate). The highest rate of sulphide oxidation was achieved by a mixed culture of hyperthermophilic archaea at 82°C but at relatively low pulp densities (within 7 – 9%). At higher pulp densities (within 10 – 25%) the highest rates were achieved by means of moderate thermophilic bacteria at 57 – 59°C.