Electrochemical and spectroscopic studies of the leaching mechanisms of representative refractory copper sulfides: chalcopyrite and enargite
Creators: name: MA, Yalong
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401103
subject: Global and planetary environmental engineering
classificationCode: 401903
subject: Hydrometallurgy

description: Chalcopyrite, the most abundant copper sulfide on the earth's crust, has become one of the most important copper sources with the extractive method transforming from traditional pyrometallurgy to hydrometallurgical methods due to the gradual depletion of high-grade ores and the need for processing low-grade and complex copper ores. Meanwhile, many of these low-grade copper ores contain arsenic-bearing sulfide phases, for example, enargite, which is commonly found in the copper deposits of some mining countries. Hydrometallurgical processing has been regarded as a competitive option to deal with the low-grade and arsenic-bearing copper concentrates due to its harmony with the environment, moderate capital investment and economic operating costs. Unfortunately, both chalcopyrite and enargite are refractory copper sulfides with slow leaching kinetics under ambient conditions, which limit their commercial hydrometallurgical processing. In the present study, various electrochemical techniques combined with surface spectroscopic analysis have been applied to study the dissolution mechanism of enargite and chalcopyrite with the hope to provide valuable information about their surface reactivity under different leaching conditions, which could further help to improve the copper recovery rate from these refractory copper minerals.

Enargite leaching studies

The electrochemical dissolution study of enargite in sulfuric acid solution demonstrated an active-passive transition, during which enargite surface was passivated when the applied potential was above 450 mV (Ag/AgCl. sat). The arsenic-sulfide surface layer was found on the passivated electrode surface, which might account for the decrease in current density in the passive potential region. A trans-passive region with a small potential window between 750 mV and 900 mV was observed which was related to the partial dissolution of the passive surface film with the formation of elemental sulfur. Based on the above electrochemical studies, enhancement of the copper extraction rate from enargite by bioleaching with the addition of pyrite was proposed and validated. Bioleaching experiment showed that the addition of pyrite significantly promoted enargite bioleaching rate by increasing the final copper extraction rate from 5 % (without pyrite) to 47 % (with a pyrite-to-enargite mass ratio of 3:1). The dissolution rate of enargite without pyrite was controlled by the diffusion process, confirming a passivation layer formed around the enargite surface. The presence of pyrite changed the rate-limiting factor of enargite dissolution to the surface chemical control, indicating that the addition of pyrite eliminated the passive effect. The galvanic effect of the pyrite on the dissolution of enargite was further investigated by the electrochemical techniques, and the results indicated that the cathodic reduction controlled the dissolution of enargite electrode and galvanically coupled pyrite continued to act as a cathode to facilitate the oxidation of enargite by oxygen and ferric ions even after enargite was passivated.
 
Chalcopyrite leaching studies

The passivation film formation on chalcopyrite surface during chemical leaching in ferric sulfate acid solutions and electrochemical dissolution process was investigated by Scanning Electrochemical Microscopy (SECM). The effect of various leaching solution redox potentials of 420, 500 and 580 mV on the passivation film formation of chalcopyrite was studied by SECM with feedback mode, which showed the passivation film formed quickly under the high redox potential condition of 580 mV. The approach curves study further confirmed that a passivation film formed under high redox potential conditions. A similar phenomenon was observed after the chalcopyrite electrode was electrochemically oxidized with the applied potential in the passivation region of chalcopyrite. With the intrinsic spatial resolution of the SECM technique, it was found a relationship between the crystallographic plane orientation of the chalcopyrite grains and their surface reactivity. The results revealed that the surface crystal plane with &lt;111&gt; orientations showed a less preferential property to form the passivation film than the other surface crystal planes of &lt;001&gt; and &lt;101&gt;. These results demonstrated that the SECM is a promising technique for the investigation of the surface reactivity of sulfide minerals.

Furthermore, the oxidative dissolution mechanism of chalcopyrite in ammonium chloride solutions under room temperature and pH 8.5 was studied by evaluating the surface chemistry and leaching products. Both batch leaching experiments and electrochemical studies showed that Cu(II) ions were more efficient than dissolved oxygen for chalcopyrite oxidation in ammonium chloride solutions. Thiosulfate rather than sulfate was confirmed as the primary soluble sulfur species produced during chalcopyrite dissolution by investigating the chemical compositions of both the leaching solution and residues. Decomposition of thiosulfate in the presence of both dissolved oxygen and Cu(II) resulted in the final stable product, sulfate. Amorphous hematite and six-line ferrihydrite were the major components residing on the chalcopyrite surface. Based on the previous studies and the results obtained in this study, a model for the chalcopyrite dissolution mechanism in alkaline ammonium chloride solutions under ambient conditions has been proposed.