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Part I of this series described the results of a study of the electrochemistry of pyrite under conditions similar to those encountered in acidic bioleaching of the mineral. In this paper, the electrochemical data obtained in the previous paper is used to derive The detrimental effect of high sulfate concentrations on the rate of dissolution has been confirmed by the leach tests.

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°C, 50°C and 65°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°C and 65°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 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTfor the bioleaching process in the case of pyrite and also that the mixed potential model can be used to quantitatively describe both abiotic and biotic dissolution of pyrite in acidic solutions.

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate-chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3-0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5-0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.

(2013) The effects of sulphate ions and temperature on the leaching of pyrite. 1. Electrochemistry. Hydrometallurgy, http://researchrepository.murdoch.edu.a/25114/ This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Studies of the cathodic reduction of iron(III) and dissolved oxygen have shown that the rate of the former is several orders of magnitude more reversible(rapid) than that of dissolved oxygen which only exhibits measurable reactivity at potentials well below the mixed potentials in the presence of iron(III). The reduction of iron(III) is inhibited by sulfate ions due to the formation of electrochemically less reactive sulfate complexes. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT