The mechanism and kinetics of the replacement of chalcopyrite by covellite under mild-hydrothermal conditions (120–200 °C)

Abstract

In this work, the replacement of chalcopyrite by covellite was investigated under hydrothermal conditions and the kinetic data was extrapolated to lower temperatures. Chalcopyrite was successfully replaced by covellite in the temperature range of 140–200 °C under acidic conditions (pH 0.7–4) via a dissolution-reprecipitation reaction mechanism. Covellite was examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The replacement reaction was controlled by solution chemistry (pH), temperature and dissolved oxygen (provided by H₂O₂). At 120 °C, covellite formed prior to 18 days, and then completely transformed into elemental sulfur. X-ray photoelectron spectroscopy (XPS) analysis was carried out to help understand the reaction mechanism of the chalcopyrite dissolution step. The XPS results confirmed that the formation of elemental sulfur as an intermediate reaction product occurred in the early stage of chalcopyrite dissolution. Hydrothermal experimental results showed that chalcopyrite was replaced by covellite in the presence of H₂O₂ at pH 0.7, 1, 1.5 and 2 and in the temperature range of 140–200 °C. The overall activation energies (E_a) of the chalcopyrite replacement reactions were calculated, using the Avrami-Arrhenius method, to be 90 ± 21 kJ/mol, 86 ± 17 kJ/mol and 27 ± 7 kJ/mol at pH 0.7, 1 and 1.5, respectively. The increasing E_a with decreasing pH suggests that the reaction mechanism may change from interface-controlled to diffusion-controlled reactions.