The effects of sulphate ions and temperature on the leaching of pyrite. 1. Electrochemistry

A detailed study of the electrochemical behavior of pyrite under typical bio-leaching conditions has been carried out with the focus on the effect of the concentration of sulphate ions on the mixed potential, cyclic voltammetric and potentiostatic measurements.The mixed potential decreases with increasing sulphate ion concentration and the addition of iron(II) and increases as expected with increasing iron(III) concentration and agitation at constant pH. Temperature, pH and the concentration of dissolved oxygen have minimal effects on the mixed potential in the presence of iron(III).Cyclic voltammetric and potentiostatic data confirm that the rate of anodic oxidation of pyrite decreases with increasing sulphate ion concentration and increases with increasing temperature. The pH and dissolved oxygen concentration have little effect while addition of chloride ions inhibits the anodic oxidation of pyrite.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 sulphate ions due to the formation of electrochemically less reactive sulphate complexes.

► The dissolution of pyrite conforms to classical mixed-potential theory.
► The rate of anodic oxidation of pyrite decreases with increasing sulphate ion concentration.
► The pH and dissolved oxygen concentration have little effect.
► Addition of chloride ions inhibits the anodic oxidation of pyrite.
► Reduction of both iron(III) and dissolved oxygen is inhibited by high sulphate concentrations.