A comparative study of the oxidative and reductive dissolution of magnetite in acidified CuSO4-acetonitrile-H2O and CuCl2?NaCl?H2O leach solutions

Abstract

The oxidative and reductive dissolution of magnetite in acidified aqueous acetonitrile and acidified aqueous NaCl solutions have been investigated by electrochemical methods and correlated with the proton activity in these fundamentally different solution systems. The results are compared with the rates of magnetite dissolution in the presence of copper(II) and copper(I). Under anodic or oxidizing leach conditions magnetite forms a passive Fe2O3 film which inhibits reaction, but under cathodic or reducing leach conditions in the presence of copper(I), magnetite is believed to dissolve via (FeOH)+ intermediate and reacts up to 600 times faster. The rate of dissolution of magnetite depends on its potential and the activity of the proton. The rate follows a Butler-Volmer relationship with a Tafel slope of 120–130mV per decade and exhibits a proton reaction order between 0.85 and 1.0. After allowing for changes in proton activity there is no significant difference between the rate of dissolution of magnetite in the sulphate and chloride leach solutions and no discernable effect of the presence of acetonitrile in the mixed solvent system. Electrochemical studies and leaching studies in which the potential is controlled by the Cu(II)−Cu(I) couple are in good agreement and lead to a fundamental understanding of the optimum conditions required to leach copper selectively from a Cu−Fe3O4 calcine derived from the segregation roasting of chalcopyrite concentrates.