New information on the pyrite bioleaching mechanism at low and high temperature

The mechanism of pyrite bioleaching has been studied through both the indirect action of bacteria by recycling the leaching agent (Fe3+) and the direct attack of microbial cells attached to the mineral surface. Causes of the decrease of the dissolution rate and the ultimate levelling off of the process are also identified. The study shows that pyrite bioleaching is markedly affected by the oxidation state of the dissolved iron. The ferrous/ferric ratio controls the relative rate of the oxidation reactions involved in the process. Additionally, the pyrite dissolution is controlled by the competitive chemisorption between Fe2+ and Fe3+ on the mineral surface. The accumulation of ferrous ions on the surface results in a diffusion barrier to the ferric attack, producing an increase of the ferric ion concentration in the bulk solution that inhibits the microbial oxidation capacity for concentrations higher than 16 g/L. There is a relationship between the rate of cell attachment to mineral surfaces and the mineral dissolution rate, since the initial attachment accounts for much of the dissolution rate during the bioleaching process. This is explained by the release of Fe2+ from the mineral attack by contact bioleaching and the rapid oxidation of Fe2+ accumulated on the surface by attached microorganisms. The results point to a cooperative bioleaching strategy for pyrite dissolution, whereby contact bioleaching and indirect bioleaching of the mineral possibly occur simultaneously via a thiosulphate mechanism. (C) 2003 Elsevier B.V. All rights reserved.