Effects of solid particles on thermophilic bioleaching of sulphide minerals

The effect of mineral particle size and pulp density on the bioleaching of pyrite by the acidophilic thermophile Sulfolobus metallicus was investigated in a batch bioreactor. Decreasing the particle size from a mean diameter of 159 micron (size fraction:150-180 micron) to a mean diameter of 29.5 micron (size fraction:25-45 micron) enhanced the bioleaching rate from 0.05 to 0.098 kg m(-3) h(-1). The particle size distribution of the mineral in this range did not influence the morphology and growth kinetics of the cells. Decreasing the particle size of the mineral to a mean diameter of 0.2 micron (size fraction< 25 micron) severely damaged the structure of the cells, resulting in their inability to oxidise pyrite. The rate of bioleaching of pyrite was not influenced by the mineral pulp density in the range 6 to 12%. With a pulp density of 6%, complete oxidation of pyrite (based on the concentration of released iron) was achieved, while with the pulp densities of 10 and 12%, the extent of pyrite oxidation was 67%. The bioleaching rate of pyrite at a pulp density of 15% was relatively slower than those observed in the presence of 6 to 12% pyrite and the extent of pyrite oxidation was only 30%.