A comparison of the electrochemical oxidative dissolution of pyrite and chalcopyrite in diluted nitric acid solution

Understanding the oxidation of sulfidic minerals, especially those of pyrite and chalcopyrite, under acidic conditions has important outcomes, such as exposing any encapsulated gold not recovered by traditional cyanidation processes and providing insight into acid mine drainage. This investigation studied the electrochemical oxidation of pyrite and chalcopyrite in a nitric acid solution (0.5 M) using electrochemical techniques. Cyclic voltammetry (CV) was performed using electrodes of each mineral to detect the possible redox reactions and hence suggest products formed from each mineral. Electrochemical impedance spectroscopy (EIS) was also run on each mineral at specific potentials corresponding to regions for anodic reactions detected to gain further information about the reaction mechanism and possible products. The partial oxidation of pyrite at low anodic potentials (0.4 to 0.6 V) produces Fe 1-x S 2 and Fe(OH) 3 , with a sulfur-rich layer forming S 0 which accumulates on the electrode’s surface, leading to a diffusion controlled dissolution process. As the potential increases above 0.7 V, there is sufficient driving force to fully oxidise pyrite and the species already formed on the electrode’s surface to soluble species by producing SO 4 2– . This eradicates the diffusion barrier and cause extensive oxidation of pyrite at high potentials (0.9 V). A similar process occurred for chalcopyrite with preferential oxidation of iron at low potentials (0.3 to 0.5 V), forming mainly iron and copper deficient sulfides (like Cu 1-x Fe 1-y S 2-z , CuS 2 , CuS) and S 0 which partially covers the chalcopyrite electrode’s surface, causing a diffusion barrier. Increasing the potential to beyond 0.7 V leads to these previously formed layers converting to soluble species. EIS provides evidence supporting the reactions and the formed products at the investigated potentials.