Oxidative Dissolution Behavior of Synthetic Chalcopyrite with Non-Stoichiometric Compositions
Authors: Komorita Katsuya , Taninouchi Yu-ki , Nakano Hiroaki 
Published: December 2024

Chalcopyrite (CuFeS2) is the most important Cu mineral, accounting for 70% of copper reserves in the world. Cu metal is primarily produced from chalcopyrite-containing ores by an extraction process involving flotation, pyrometallurgy, and electrorefining. Owing to the ongoing depletion of high-grade ores, hydrometallurgical processes involving copper leaching from chalcopyrite-containing ores using acidic ferric sulfate solutions have been intensively studied. However, efficient copper leaching remains a challenge, and the details of the dissolution mechanism of chalcopyrite in natural resources remain unclear.
Investigation using synthetic minerals with controlled compositions and morphologies is an effective approach for understanding the dissolution mechanism. In this study, we prepared dense pellets of synthetic chalcopyrite with stoichiometric and non-stoichiometric compositions and compared their oxidative dissolution behaviors in an acidic ferric sulfate solution.
The nominal compositions of the prepared samples are Cu0.250Fe0.250S0.500 (stoichiometric composition), Cu0.240Fe0.260S0.500, Cu0.245Fe0.260S0.495, and Cu0.265Fe0.245S0.490. The powder samples were synthesized via a high-temperature reaction using Cu wire, Fe wire, and S powder as raw materials. The dense pellets (diameter 10 mm and thickness of approximately 3 mm) were obtained by pulse current pressure sintering and subsequent annealing at 500 ℃. Density measurements and scanning electron microscopy observations revealed that the morphologies such as grain size and density of the obtained pellets were almost constant, regardless of the nominal composition. In X-ray diffraction analyses, the diffraction peaks of the obtained pellets were consistent with the chalcopyrite phase (CuFeS2, PDF #00-037-0471). The scanning electron microscopy with energy dispersive spectrometry analysis confirmed that no other phase was detected in the Cu0.250Fe0.250S0.500, Cu0.240Fe0.260S0.500, and Cu0.265Fe0.245S0.490 pellets.
To enable electrochemical measurements, dense pellets were cut and embedded in resin after connecting the copper leads. Obtained electrode samples were immersed in 0.1 M Fe(SO4)1.5 – 10−4 M FeSO4 – 0.18 M H2SO4 at 70 ℃ for 24 h. The concentration of Cu ions in the solution was determined by inductivity coupled plasma atomic emission spectrometry and the dissolution rates of the samples were evaluated. The corrosion potentials of the synthesized chalcopyrite samples were also measured. shows the measured dissolution rates and corrosion potentials. The Cu dissolution rates for the chalcopyrite samples with non-stoichiometric compositions (Cu0.240Fe0.260S0.500, Cu0.245Fe0.260S0.495, and Cu0.265Fe0.245S0.490) were over one order of magnitude than those for the samples with stoichiometric composition (Cu0.250Fe0.250S0.500). This result demonstrates that a small difference in composition significantly affects the dissolution rate in the ferric sulfate solution. The corrosion potentials of the electrodes with non-stoichiometric compositions were less noble than those of the stoichiometric composition sample. At the corrosion potential, the rate of anodic dissolution of the chalcopyrite phase was equal to that of the cathodic reaction of the oxidant in the solution (mainly the reduction rate of the Fe(III) species). Considering the shift in the corrosion potential, increasing the dissolution rate is attributed to the enhancement of the anodic dissolution reaction of the chalcopyrite phase by the change in its composition from stoichiometry.

Figure 1