Effect of pH reduction and ferric ion addition on the leaching of chalcopyrite at thermophilic temperatures

Up to now various methods including bioleaching with thermophilic archea have been studied to enhance the leaching of chalcopyrite. However, the combined effect of pH and low ORP values at thermophilic temperatures (65–80 °C) has attracted little attention. Therefore, in this work, various chalcopyrites including two concentrates and two ore minerals were leached to study the combined effect of pH and ORP. It was found that a temperature of 65 °C at an initial pH of 1.0 can lead to copper extractions comparable to those obtained in bioleaching with A. brierleyi and S. metallicus. Furthermore, at an initial pH of 1.0, the addition of Fe3+ (200 mg/L) at concentration much lower than the usually employed (e.g. 5000 mg/L), where formation of iron salt precipitates could be avoided and low ORP values (< 450 mV) could be induced, higher leaching yields than in thermophilic bioleaching were obtained. In addition to the low ORP values derived from the presence of low amounts of Fe3+, the notable enhancement is attributed to the generation of sufficient amounts of Fe3+ that resulted from the oxidation of Fe2+ by oxygen at an initial pH of 1.0 and 65–80 °C without the aid of thermophiles. The oxidation of Fe2+ by oxygen only took place under the presence of sulfides such as Cu2S or CuS.