Speciation of the sulfuric acid-ferric sulfate-ferrous sulfate-water system and its application to chalcopyrite leaching kinetics up to 150 ºC

Hydrometallurgical oxidation methods are increasingly being considered for the extraction of Cu from chalcopyrite. However, the kinetics of cathodic ferric ion reduction are poorly understood. This thesis investigates the kinetics of cathodic ferric ion reduction on chalcopyrite and its influence on the leaching process in acidic iron sulfate solution, with an emphasis first placed on the development of a speciation model for the H₂SO₄-Fe₂(SO₄)₃-FeSO₄-H₂O system from 25°C-150°C. Speciation results show that most Fe(III) is distributed as complexes or precipitates and the free Fe³+ accounts for only a minor percentage (up to 5.2% of total ferric) whereas a large amount of Fe(II) exists in the form of free Fe²+. The Nernst equation was used to study the redox potential of Fe³+/Fe²+ couple. The speciation model explains the change of redox potential with temperature for all nominal Fe³+/Fe²+ ratios. This model was validated by reliable prediction of measured redox potential, comparison of previously published results of ferric solubility, together with an analysis of the calculated pH and ionic strength. A novel expression was also developed to predict the redox potential of the ferric/ferrous couple. It seems that the redox potential can be easily and accurately determined only based on the variables of temperature and nominal Fe³+/Fe²+ ratio. The calculated free Fe³+ concentration allowed for a detailed investigation of the reduction kinetics of ferric ion on chalcopyrite by cathodic potentiodynamic polarization. The exchange current densities of the Fe³+/Fe²+ couple are on the order of 10-⁷–10-⁵ A/cm² in the range of 25–150°C, substantially less than that on platinum. Calculated rate constants can be well described by the Arrhenius equation. The transfer coefficient increases linearly with temperature (rather than being constant). The importance of the cathodic ferric ion reduction reaction on the overall leaching process is progressively increased when increasing the nominal Fe³+/Fe²+ ratio and temperature. Leaching is under anodic control when the nominal Fe³+/Fe²+ ratio is around 1:1, whereas at higher nominal Fe³+/Fe²+ ratios and temperatures it is under mixed control. These findings provide the basis for mechanistic analyses and attendant optimization studies of industrial leaching processes of chalcopyrite and other sulfide minerals.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat