SURFACE SULFUR AS PROMOTING AGENT OF PYRITE LEACHING BY THIOBACILLUS-FERROOXIDANS

Methanol extraction conducted with a HPLC-like device and spectroscopi c analysis were used to remove and characterize the sulphur layer (S8) present on freshly ground pyrite surface after dry grinding. Accurate measurements of ferric and sulphate contents in the leachate showed a significant delay in the lag phase and in the first step of oxidation by Thiobacillus ferrooxidans for the so-cleaned pyrite (without sulph ur layer) in comparison to the initial pyrite (with sulphur layer). Vo ltammetric studies (current-potential curves) showed a modification of the anodic behaviour of the initial pyrite, corresponding to a higher chemical oxidability of the uncleaned pyrite. During the bacterial ox idation, the difference in redox potential between a special pyrite el ectrode and a platinum standard electrode both placed in the bioleachi ng reactor was shown to be related to the occurrence of a sulphur laye r. This difference, which is more important in the case of the initial pyrite (with sulphur layer), corresponded to an increase in oxidation kinetics of the pyrite by Thiobacillus ferrooxidans.