THE NATURE OF THE PASSIVATION FILM ON COVELLITE EXPOSED TO OXYGEN

Abstract

Evidence has been presented by Corrans et al. [1] that in the synthetic leaching of synthetic covellite, a direct mechanism operates by preventing the accumulation of a protective layer of sulphur and by the depolarisation of a cathodic reaction. The process of covellite passivation has been further studied, using sensitive methods for measurement of the consumption of oxygen and acid, and for the production of cupric and sulphate ions. The results indicate that passivation arises from the accumulation of approximately 30 micromole of elemental sulphur per square metre of covellite surface. The oxygen consumed during depassivation by various strains of Thiobacillus ferrooxidans supported this conclusion. Assuming an even distribution of sulphur over the surface, the passivation film has been calculated to be one or two atoms thick.