Kinetics of leaching of chalcocite in acid ferric sulfate media : chemical and bacterial leaching
Creators: name: Bolorunduro, Samuel Adewale
Publisher: name: University of British Columbia
Year: 2009

description: The lack of a clear understanding of the rate of chalcocite (Cu₂S) and covellite (CuS) leaching in the presence and absence of bacteria has been a limitation on the optimization of hydrometallurgical processes for the recovery of copper from these minerals. In order to enhance the performance of heaps and other leaching processes for these minerals, there is a need to examine the conditions required to improve the rate of leaching. Such an investigation will produce a particle scale model (for the intrinsic rate of leaching), which can be combined with a heap scale model to form a comprehensive heap leaching model, or with a leaching macro-model to form a tank leaching model. The robustness of such a model will be its ability to determine the effects of changing parameters on the rate limiting steps. Chalcocite oxidation was investigated by leaching high grade natural minerals in acidic ferric/ferrous sulfate solutions. The temperature was varied between 35° and 75° C. The ferric concentration varied between 0.015 and 0.232 mol/L, and the ferrous concentration varied between 0.001 and 0.233 mol/L. The redox potential of the solution (at 25°C) was varied between 450 and 651 mV (vs. Ag/AgCl) to determine the effect of this parameter on the leaching kinetics. The two well known, significant stages of leaching were observed and characterized by mineralogical studies. The first stage leach was characterized by 50% copper extraction and the conversion of chalcocite, ultimately to second stage covellite (CuS). Some non-stoichiometric copper sulfides were formed prior to the formation of the second stage covellite. The first stage leaching reaction was rapid at all temperatures. The redox potential had no effect on the rate of this reaction. The second stage leach was characterized by the conversion of the second stage covellite (which was the by-product of the first stage) to copper, elemental sulfur and sulfate. At higher temperatures, sulfur formation was predominant and the reaction was fast. At lower temperatures, sulfur formation was predominant up to about 70% copper extraction. Subsequently, sulfate formation occurred. The effect of the solution redox potential on the kinetics of second stage leaching was significant. During bacterial leaching, it was observed that the principal role of the ferrous oxidizing bacteria (Thiobacillus ferrooxidans) was to maintain the required high redox potential at the surface of the minerals. A mathematical model was formulated to explain the ferric leaching kinetics of chalcocite. The first stage kinetics can be explained in terms of a mixed diffusion/chemical reaction model, in which the rate of reaction is simultaneously limited by the diffusion of ferric ions to the mineral surface and by the chemical reaction. Though the partially oxidized particles disintegrate before the commencement of the second stage leach, each of the particles leaches as a discrete grain and the second stage kinetics are controlled by the chemical reaction, which is one-half order dependent on the ferric concentration. The leaching process can be described by an electrochemical mechanism in which the rate-limiting step of the first stage is electron transfer in the cathodic reaction, and the rate-limiting step of the second stage is electron transfer in the anodic reaction.