The effects of sulphate ions and temperature on the leaching of pyrite. 2. Dissolution rates

Abstract

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 estimates of the rate of leaching of pyrite based on a mixed potential model for the kinetics as a function of the solution composition and temperature. The results of the calculated rates of dissolution are compared with those obtained in a limited study of the dissolution of a milled pyrite sample at controlled potentials.It is demonstrated that the rate of dissolution calculated from the anodic current density in a solution without iron(III) when the potential is held at the mixed potential measured in the presence of iron(III) compared well with the rate predicted by the linear polarization method and that obtained from the change in the solution potential with time in a batch dissolution of a pyrite electrode in solutions containing iron(III).The dissolution rate calculated from the steady-state currents at the mixed potential in the absence of iron(III) decreased with increasing sulphate ion concentration, increased with increasing temperature and also with increasing rotation speed of the pyrite electrode.The results of leaching experiments with milled pyrite under conditions of controlled solution potential has shown that the rate of dissolution is significantly higher at a potential of 0.85V than at 0.80V. Approximately 90% of the sulphide associated with pyrite is oxidised to elemental sulphur. The results of the leach experiments quantitatively agree well with those derived from the electrochemical measurements. The detrimental effect of high sulphate concentrations on the rate of dissolution has been confirmed by the leach tests.