The role of the iodide/iodine couple as a redox mediator in the dissolution of chalcopyrite in sulfate media. An electrochemical study

A detailed study has been made of the effect of the addition of iodine/iodide on the electrochemistry of the dissolution of chalcopyrite in acidic sulfate media containing iron(III). Mixed potential measurements have revealed that the potential is higher with iodine as an oxidant compared to iron(III) although the solution po-tentials are similar. Potentiostatic measurements in the absence of the oxidants at the measured mixed potentials have shown that chalcopyrite should dissolve some five times faster with iodine as the oxidant. Linear sweep voltammetry has confirmed the much higher reversibility of the iodine/iodide couple on a chalcopyrite surface than that of the iron(III)/iron(II) couple. Limited dissolution tests have confirmed that chalcopyrite dissolves more rapidly with iodine as the oxidant at rates similar to those predicted from the electrochemical measurements.A preliminary spectrophotometric study of the kinetics of the oxidation of iodide by iron(III) in sulfate media has shown that the rate is significantly slower than previously assumed and that the reaction occurs in two stages with tri-iodide as the initial product that is slowly oxidised to iodine. At the low concentrations necessary for economic application, iodide ions do not effect the rate of oxidation of iron(II) by dissolved oxygen in chloride solutions.It is suggested that the use of iodide as a redox mediator during heap leaching of chalcopyrite ores by acidic ferric sulfate lixiviants is unlikely to change current problems with the slow rate of dissolution under ambient conditions.