Add time:07/13/2019         Source:sciencedirect.com

A comparative study of the electrochemical oxidation of iron(II) and copper(I) ions on selected sulfide minerals in concentrated chloride solutions has been carried out as part of a broader study of the kinetics of the leaching of chalcopyrite, covellite, enargite and pyrite under chloride heap leaching conditions. Mixed potential, cyclic voltammetric and potentiostatic measurements have been made using rotating disk electrodes of massive mineral samples. For comparative purposes, arsenopyrite and platinum electrodes have also been used under the same conditions. The mixed potentials of the various minerals in solutions containing 4 mol/L sodium chloride, 0.1 mol/L hydrochloric acid and 0.05 mol/L iron(II) or 0.05 mol/L copper(II) ions at 25 °C are all governed by the potentials of the redox couples with the exception of chalcopyrite in solutions of iron(II) for which the mixed potential is lower than the equilibrium potential of the iron(III)/iron(II) couple.Cyclic voltammetry conducted at potentials positive to the mixed potentials at slow sweep rates has shown variable reactivity of the minerals for oxidation of both iron(II) and copper(I) ions. Oxidation of copper(I) occurs readily on all minerals with the limiting current density observed on all with the exception of chalcopyrite for which the limiting current is not reached even at relatively large overpotentials. Partial passivation of the oxidation of copper(I) is observed at potentials above about 0.7 V on covellite. Oxidation of iron(II) is slower than that of copper(I) on all minerals and the limiting current density is only observed on pyrite. Partial passivation of the oxidation of iron(II) is observed with all other minerals at potentials above 0.65 to 0.75 V.Quantitative measurements of the rates of oxidation at the mixed potentials have been obtained from linear polarization measurements and the results compared with previously published data on the cathodic reduction of iron(III) and copper(II) on these mineral surfaces. The rate constant varies by about an order of magnitude within the mineral group for both iron(II) and copper(I) oxidation and the rates of oxidation on platinum are higher for both couples than for the mineral electrodes. The ratio of the rate of copper(II) reduction to iron(III) reduction is significantly greater for the minerals containing copper than for those without copper.The effect of illumination with light of wavelength 405 nm on the rate of anodic oxidation of iron(II) on chalcopyrite has been evaluated and no positive effects that can be attributed to semi-conducting effects have been observed.

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