The role of Thiobacillus ferrooxidans in the bacterial leaching of zinc sulphide

Abstract

The role of Thiobacillus ferrooxidans in the bacterial leaching of mineral sulphides is a point of controversy in the literature. Much of the controversy is due to the fact that the solution conditions change during experimentation, especially the concentrations of ferric and ferrous ions. The role of the bacteria would be more easily discernible if the concentrations of ferric and ferrous ions are maintained at a set value throughout the experimental period. This paper reports the results obtained using a constant redox potential apparatus described previously (1). This apparatus is designed to control the redox potential in the leaching compartment of an electrolytic cell by the reduction or oxidation of dissolved iron. By controlling the redox potential and the pH the concentrations of ferrous and ferric ions are maintained at their initial values. Leaching experiments have been conducted in the presence of T. ferrooxidans and in sterile conditions. At high concentrations of ferric ions, the conversions of zinc sulphide in the absence and presence of the bacteria are the same. However, at high concentrations of ferrous ions, the conversion of sphalerite in the presence of bacteria is higher than in their absence. The results show that at higher concentrations of ferrous ions, diffusion of ferrous ions through the sulphur product layer becomes important in leaching under sterile conditions. However, bacteria oxidise the sulphur layer, which removes this diffusional resistance, and thereby increase the rate of leaching.