The effect of sulphate ions and temperature on the leaching of pyrite. III. Bioleaching

The effect of the total sulphate concentration on the rate of pyrite dissolution was investigated in batch, bioleach tests on a high-grade concentrate at 35 degrees C, 50 degrees C and 65 degrees C. Good microbial activity was achieved for concentrations up to 42 g/L sulphate. However, higher concentrations resulted in decreased ferrous oxidation kinetics, with the solution potential profiles showing "lag periods" at the highest sulphate concentration. The bioleach results showed that the rate of pyrite dissolution decreased markedly above an average sulphate concentration of 55 g/L for all three temperatures, with the adverse effect on the dissolution kinetics being more pronounced at 50 degrees C and 65 degrees C. As a result of this, increased temperature has no beneficial effect on the rate of dissolution at concentrations of sulphate above 55 g/L. High extents of pyrite dissolution (90% to 98%) were achieved at all three temperatures irrespective of sulphate concentration, with almost identical extents of sulphide oxidation. Rates of pyrite dissolution under bioleaching conditions agree well with those derived under similar conditions from abiotic dissolution experiments and electrochemical measurements. This important observation confirms the so-called indirect mechanism for the bioleaching process in the case of pyrite and also that the mixed potential model can be used to quantitatively describe both abiotic dissolution and biotic dissolution of pyrite in acidic solutions. (C) 2013 Elsevier B.V. All rights reserved.