Kinetics of Ferric Ion Reduction on Chalcopyrite and its Influence on Leaching up to 150 °C

The reduction kinetics of ferric ion on chalcopyrite in the quaternary H2SO4-Fe2(SO4)3-FeSO4-H2O system were systematically studied by cathodic potentiodynamic polarization (0.5 mV/s) through a wide range of solution compositions and temperatures (25-150 °C). Results reveal that all of the pseudo-polarization plots exhibit reproducible well-defined linear Tafel regions and thus the temperature dependence of various kinetic parameters, such as the exchange current density, transfer coefficient and rate constant, has been determined. The exchange current densities of the Fe3+/Fe2+ couple obtained by extrapolating the cathodic linear Tafel line to the reversible potential are on the order of 10−7-10−5 A/cm2 in the range of 25-150 °C, substantially less than that on platinum due to the formation of a passive film. In general, at the same nominal Fe3+/Fe2+ ratio, increasing the temperature from 25 to 150 °C was observed to give rise to an increase in i0, especially when a larger amount of free ferrous existed in the solution. Based on the obtained exchange current densities, the rate constants for ferric ion reduction were calculated, and further analysis indicates that the rate constants can be well described by the Arrhenius equation. Experimental evidence clearly shows an anomalous independent variation of Tafel slope with temperature and a corresponding unexpected increased linear variation of transfer coefficient with respect to temperature (rather than being constant throughout the investigated temperature range). Analysis by mixed potential theory indicates that in the acidic iron sulfate leaching system the control scenario highly depends on both the solution composition and temperature. The importance of the cathodic ferric ion reduction reaction on the overall leaching process is progressively increased when increasing the nominal Fe3+/Fe2+ ratio and temperature. The leaching reaction is under anodic control when the nominal Fe3+/Fe2+ ratio is around 1:1, whereas at higher nominal Fe3+/Fe2+ ratios and temperatures the reaction is under mixed control, by both anodic oxidation and cathodic reduction reactions.