Transient Surface Analysis of Dissolving Chalcopyrite in Cupric Chloride Solution

Abstract

This study presents an investigation of chalcopyrite surfaces dissolving in concentrated cupric chloride solutions similar to those used in the HydroCopper® process, [NaCl] = 280 g/L, [Cu2+] = 30 g/L and T = 90 °C. The leaching of chalcopyrite and the parameters of the reaction product layer formed on the mineral surface were studied at the open circuit potential as a function of time (0.5 – 22 hours) and at pHs in the range 1 - 3. The electrochemical impedance spectroscopy (EIS) data indicated that there were two or three time constants present. These were suggested to be due to the double layer and to elemental sulphur and FeOOH. With pH 1 at OCP the reaction product layer was a single-phase layer of elemental sulphur (t = 0.5 – 9 h) or two-phase layer with elemental sulphur and FeOOH (t = 22 h). The apparent charge transfer resistance was higher (>25 Ωcm2) at the beginning of leaching (t ≤ 4 h), but decreased to about 4 Ωcm2 with increasing time to 22 hours. The reaction product layer resistance did not change markedly. It is likely that the apparent charge transfer resistance reflects the resistance of the reaction product layer at pH 1, showing apparent changes in the calculated charge transfer resistance values. At pH 2, the reaction product layer was an elemental sulphur layer at t ≤ 2 h, becoming a two-phase layer of elemental sulphur and FeOOH at t = 3 – 22 h. The apparent charge transfer resistance was <8 Ωcm2 at all times, whereas the reaction product layer resistance decreased with time from 30 Ωcm2 to about 4 Ωcm2. The two-phase layer at pH 2 was electrically less resistive than the one-phase layer at pH 1. At pH 3 the reaction product layer was a two-phase layer at all times, consisting of goethite and S8. This layer allowed more rapid dissolution at the beginning, but with time a reaction product layer grew, increasing the electrical resistance as well as decreasing the dissolution rate of chalcopyrite. The apparent charge transfer resistance at pH 3 was constant at all times.