A preliminary study of electro-reduction pretreatment preceding chalcopyrite leaching in ammonium chloride media in the presence of copper ions

Chalcopyrite has notoriously slow dissolution kinetics in oxidative leaching media. Pre-conditioning using electro-reduction is a potential alternative, aimed at the surface reduction of the chalcopyrite mineral. This process has been well explored in acid systems, where the concomitant dissolution of iron significantly interferes with the electro-reduction process. Chalcopyrite leaching is feasible also in alkaline ammonia solutions, and hence the present study explores chalcopyrite electro-reduction in a 2 M ammonium chloride solution in the presence of cupric ions. A series of electrochemical tests with a chalcopyrite rotating electrode was run to investigate this system. Cathodic chronopotentiometry tests were done to evaluate the feasibility of the electro-reduction in ammonia solution. Cyclic voltammetry tests were then conducted to evaluate the feasibility and kinetics of this process at different copper concentrations and rotation rates. Chronoamperometry experiments were then conducted to simulate (electro-) leaching, and the process was finally tested in a small-scale test apparatus using a real chalcopyrite concentrate. It was found that cupric ions were reduced to cuprous ions on the chalcopyrite surface to form secondary copper sulfides, depending on the prevailing potentials. Analysis of charge transfer at anodic dissolution of the pre-reduced electrode at -350 mV (SHE) confirmed that the reduced phase formed was chalcocite. The cathodic reduction is enhanced by increasing copper concentration and electrode rotation speed, which support that cupric in solution is reduced at the chalcopyrite surface to form chalcocite. Current densities and lengths of time enhance the extent of the pre-reduction reaction, but to a diminishing extent as the reduced product builds up. The pre-reduced (2.5 h at 180 A/m 2 ) chalcopyrite electrode dissolved nearly 18 times faster anodically compared to when there is no pre-conditioning. Alternating pre-reduction and anodic leaching showed that all reduction products were fully removed, and the chalcopyrite surface could be reused repeatedly. The small-scale tests demonstrated that electro-reduction pre-treatment substantially increases the subsequent copper extraction rate in a 10 g/L chalcopyrite concentrate slurry from 30 % with no pre-treatment up to 54 % with pre-treatment at 210 A/m 2 over 5 h. • Electro-reduction in ammonia solutions rapidly transforms the surface of chalcopyrite to secondary copper sulfides. • Presence of the diamine cuprous complex in solution increases the reduction kinetics of the electro-reduction. • Electro-reduced phases can easily be leached in a cuprous ammonia solution. • Electro-reduction pretreatment of a chalcopyrite concentrate slurry results in substantially enhanced leaching compared to untreated concentrate.