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An investigation of the Cu-Fe-S-H₂O system at 200⁰C Baratin, François


The goal of this study was to extend Pourbaix's work to the Cu-Fe-S-H₂0 system at 200°C, and more specifically, to collect the relevant free energies of formation G° at 200°C, to compute solid-aqueous equilibria and display the results on thermodynamic diagrams, and to discuss these diagrams with rate data available in the literature. Two main difficulties had to be overcome to achieve such a study. 1) Eh-pH diagrams plotted for the Cu-Fe-S-H₂0 system by the available computing methods were difficult to interpret. 2) A number of G° data were missing, and had to be generated. A method was developed to compute and plot diagrams which semi-quantitatively represent the complex C₁--CM-H₂0 systems used in hydrometallurgy, even those involving several metals and ligands. The fundamentals are similar to Pourbaix's, and for simple systems, the diagrams previously published may be duplicated. However the method presented here is also well adapted tomulticomponent systems. The diagrams are calculated at constant temperature and water activity. Conditions can be imposed on the aqueous phase, in the form of constant activities of solutes or constant concentrations of one or several components C[sub i]'s. These conditions are automatically converted into the appropriate chemical potentials for the solutes in each region of the diagram. The equations corresponding to the presence of solids, and those corresponding to the conditions on the aqueous phase are generated independently. This independence makes the method extremely flexible for generating the large variety of different classes of diagrams needed to reliably describe a complex system. The method was implemented as a computer program. The programs published previously by other authors determine a specific type of composite diagram for systems containing one metal along with ligands and water by strictly applying Pourbaix's method. Since the program described in this study computes and plots different classes of diagrams, even for very complex systems, it is appropriate' to identify it as a new generation program. Two approaches have been followed to generate the data missing from the literature. First, an extension of the lever-arm method to ternary systems has been developed, which allows the retrieval of information contained in ternary phase diagrams. A set of inequalities is thereby generated, providing a range within which the missing free energy data are consistent with both the available data and the ternary phase diagram. It has been applied to the Yund and Kullerud.Cu-Fe-S phase diagram at 200°C. Data were provided in this way for the minerals idaite, cubanite, digenite, monoclinic pyrrhotite and for iron rich boundaries of the digenite and chalcocite phases. Secondly, solubility experiments were carried out in order to determine the aqueous phase composition in equilibrium with several solid assemblages at 200°C. The resulting data were interpreted in terms of energies. New data were provided for basic cupric sulfate, basic ferric sulfate, monohydrated ferrous sulfate, and for the ion pairs CUSO₄ (aq) 1 FESO₄(aq) and FeSO₄⁺. The G°'s which account for the experimental results are substantially lower than those predicted by classical extrapolation methods from low temperature data. Selected thermodynamic diagrams have been presented and discussed. They point out the important effects of ion pair formation at 200°C, and account for the main features of chalcopyrite leaching at elevated temperature.

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