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

The role of Thiobacillus ferrooxidans in the bacterial leaching of mineral sulphides is a point of controversy in the literature. Much of the controve rsy is due to the fact that the solution conditions change during experimen tation, especially the concentrations of ferric and ferrous ions. The role of the bacteria would be more easily discernible if the concentrations of f erric and ferrous ions are maintained at a set value throughout the experim ental period This paper reports the results obtained using a constant redox potential apparatus described previously (1). This apparatus is designed t o control the redox potential in the leaching compartment of an electrolyti c cell by the reduction or oxidation of dissolved iron. By controlling the redox potential and the pH the concentrations of ferrous and ferric ions ar e maintained at their initial values. Leaching experiments have been conduc ted in the presence of T. ferrooxidans and in sterile conditions. At high c oncentrations of ferric ions, the conversions of zinc sulphide in the absen ce and presence of the bacteria are the same. However, at high concentratio ns of ferrous ions, the conversion of sphalerite in the presence of bacteri a is higher than in their absence. The results show that at higher concentr ations of ferrous ions, diffusion of ferrous ions through the sulphur produ ct layer becomes important in leaching under sterile conditions. However, b acteria oxidise the sulphur layer, which removes this diffusional resistanc e, and thereby increase the rate of leaching.