Study of Kinetics and Mechanism of Pyrite Oxidation in Sulfuric Acid Solution

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Li, Lin
Pyrite; ferric-ferrous couple redox reaction; kinetics; mixed potential theory; chronoamperometry; cyclic voltammetry; Electrochemical impedance spectroscopy
There is a significant interest in studying pyrite oxidation in terms of refractory gold ore processing and acid mine drainage (AMD) treatment. However, the mechanism of pyrite oxidation has not been well understood yet. Most of the kinetics studies of pyrite oxidation focus on the corrosion process of pyrite, neglecting the kinetics of Fe3+/Fe2+ redox reaction on the surface of pyrite. In this work, based on the mixed potential theory, the chronoamperometry and cyclic voltammetry methods were used to isolate the Fe3+/Fe2+ reaction from other interfering reactions to study pyrite oxidation kinetics. As part of the kinetics study, the effects of the pyrite origin, the Fe3+ and Fe2+ ions concentrations, and the operating temperature on the Fe3+/Fe2+ reaction on the pyrite surface were investigated. The exchange current densities of the Fe3+/Fe2+ redox reaction in this study had the same magnitude of 10-5 Acm-2 for all the experiments; increasing iron concentration by an order of magnitude or the test temperature from 22 to 65 °C did not have significant effects on the Fe3+/Fe2+ couple redox reaction exchange current density nor on the value of the cathodic transfer coefficients of the reaction. Nevertheless, the anodic transfer coefficients had increased substantially with increasing the total Fe concentration and the test temperature. The activation energy for Fe2+ oxidation and Fe3+ reduction on the pyrite surfaces were calculated to be 102.0 and 32.7 kJ mol-1, respectively. In order to study about the effect of Ag on the pyrite electrodissolution process kinetics, Ag-doped pyrite was fabricated by hydrothermal method. The electrochemical dissolution kinetics of the Ag-doped pyrite were studied in different sulfuric acid electrolytes, with or without the addition of Fe and Cu cations. The experimental results showed a reduced pyrite electrochemical reactivity with increasing the Ag content of the Ag-doped pyrites. However, the anodic dissolution of the Ag-doped pyrite electrode showed a substantial increase in the Cu and Fe-containing sulfuric acid solution.
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