An investigation into the ferric leaching of chalcopyrite : a sub-process in the bioleaching of chalcopyrite
The current focus of research in the UCT Minerals Bioprocessing Research Unit is to develop an understanding of the chalcopyrite bioleaching sub-processes. This thesis forms part of the greater study on bioleaching, investigating the ferric leaching subprocess. The objective of this thesis was two-folds. Firstly, a detailed literature review was undertaken to develop a better understanding of the ferric leaching of chalcopyrite and the cause of passivation during chalcopyrite leaching. Secondly, leach experiments on pyrite were used to establish the applicability and reproducibility of measuring the rate of ferric leaching at a constant redox potential, using the methodology developed by Kametani and Aoki (1985). Following this, chalcopyrite ferric leach experiments were conducted to obtain the redox potential range where chalcopyrite leaching occurs. The rate of chalcopyrite leaching in a sulfate media decreases with time due to the formation of a passivating layer. This has been described by many researchers as parabolic kinetics (Dutrizac, 1982; Beckstead et al., 1976; Munoz et al., 1979 and Dutrizac and Mac Donald, 1974). The nature of the passivating layer is still under considerable debate. Current theories of passivation include either the formation of jarosite, ferric hyroxy sulfate, sulfur or iron deficient polysulfide like covellite (Klauber et al. , 2001 , Parker et al., 1981 , Munoz et al, 1976, Warren et al., 1985 and Parker et al., 1981). Recent research has suggested that different passivating layers are formed during the various stages of chalcopyrite leaching. These include a ferric hydroxy sulfate layer followed by jarosite over extended period of time (Klauber etal., 2001 and Parker et al., 2001 ). Current investigations are underway to establish whether semiconductor properties of the mineral affects the type of passivating layer formed. To date, most of the work has been performed at temperatures higher than those at which chalcopyrite bioleaching occurs and with varying pH and redox potentials. There has been little consideration of the potential difference between the suspended particles surface and the solution. This problem can be overcome by conducting experiments at a constant solution redox potential. Kametani and Aoki (1985) first presented a method of maintaining a constant redox potential by using potassium permanganate as an oxidant to continuously re-oxidise ferrous-iron to ferric-iron. This method was then adopted by our research group to determine the initial rates of chalcopyrite leaching. Preliminary rates of chalcopyrite leaching were presented in the thesis by Furamera (2000). Further work was required to firmly establish the rates of chalcopyrite leaching and the optimal redox potential range within which chalcopyrite leach occurs. This involved optimising the redox potential control system to obtain better redox potential control during the chalcopyrite leach experiment.
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