Synergism in the oxidation of covellite and pyrite by iron(III) and copper(II) ions in chloride solutions

An electrochemical study of the cathodic reduction of copper(II) and iron(III) ions on covellite and pyrite electrode surfaces has been carried out using rotating disk electrodes in acidic chloride solutions. The results of linear sweep voltammetric measurements have revealed that reduction of copper(II) ions on both mineral surfaces can be enhanced by a typical EC'(electrochemical/chemical) or catalytic mechanism in which the product copper(I) ions, are reoxidised in a rapid chemical reaction with iron(III) ions. Fe(III) + Cu(I) ⇆ Fe(II) + Cu(II) In the case of covellite, the catalytic effect is substantial with a significant positive shift in the mixed potential in the presence of both ions. An estimate of the magnitude of the catalytic effect using conventional EC theory for a rotating disk and published data for the rate of oxidation of copper(I) by iron(III) is consistent with the observed effect. It is suggested that the effect on the leaching rate of covellite in such solutions at ambient temperatures will not be significantly affected by the presence of this catalytic process given that the mixed potentials occur in a potential region in which the rate of anodic dissolution of the mineral is largely independent of potential. The effect is less substantial in the case of pyrite and a more extensive study of the effects of varying concentrations of chloride, copper(II) and iron(III) ions has given results which are also qualitatively consistent with those predicted from the theory in that the concentration of copper(II) has a greater effect than that of iron(III) on the catalytic currents. The practical significance is expected to be minimal in the case of pyrite for which higher potentials are required than those at which copper(II) is electrochemically active.