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Mechanism and kinetics of chalcopyrite passivation and depassivation during ferric and microbial leaching Tshilombo, Alain Fuamba

Abstract

Chalcopyrite is known to be recalcitrant to conventional hydrometallurgical and biohydrometallurgical processes. Formation of passive layers on the chalcopyrite surface results in slow and incomplete leaching. The nature of how these passive layers are formed is the subject of much controversy. The most likely explanation is the formation of polysulphide compounds or copper-rich intermediate products on the chalcopyrite surface. The formation of these products depends mainly on temperature and solution potential. Based on these observations, electrochemical techniques were used to study the behaviour of chalcopyrite under a variety of conditions similar to ferric and microbial leaching. Electrochemical techniques have the advantage over other techniques of measuring properties at the solid-liquid interface. Slow chalcopyrite leaching was mainly observed under the following conditions: (1) low temperature (25°C) and low potential (0.45 to 0.6 V SCE); (2) high temperature (65°C) and high potential (above 0.6 V SCE). Leaching was accelerated at high temperature (65°C) under mildly oxidizing conditions (0.45 to 0.55 V SCE). The study also indicated that a polarized chalcopyrite surface inhibits ferric reduction and that the presence of pyrite during chalcopyrite leaching can be beneficial. The electrochemical study was validated in leaching tests carried out in a stirred-tank reactor with fine chalcopyrite particles. Leaching was retarded at low temperatures due to the presence of an induction period. The duration of the induction period decreased with increasing temperature. The addition of pyrite significantly increased both the rate and the extent of chalcopyrite leaching. Complete conversion of chalcopyrite was obtained within 16 hours at 65°C at a pyrite: chalcopyrite mass ratio of 2:1. An electrochemical model that takes into consideration the galvanic interaction with pyrite and the "passivation" of chalcopyrite was proposed. The addition of microorganisms to the leaching system was investigated. Chalcopyrite was leached almost to completion (95%) within 30 days in the presence of thermophilic bacteria at low potentials and high temperatures. The bioleaching rate of chalcopyrite was further increased with the addition of pyrite. Finally, atmospheric leaching of chalcopyrite was carried out at 80°C under a range of conditions. Complete copper extraction was attained in 4 hours at a pyrite: chalcopyrite ratio of 4:1. The present study has shown that chalcopyrite passivation can be prevented at low solution potentials, high temperatures and in the presence of moderate amounts of pyrite.

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