Abstract: The system for metal solvent extraction is a complicated one, which deals with electrolyte solution, non-electrolyte solution, phase equilibrium and chemical reaction simultaneously. It is interesting and important to establish an extraction thermodynamic model in hydrometaliutgy based on the modern theories of solution. In recent years the extraction thermodynamic model on one-metal system has been presented by authors[1-5].
The two-metal extraction system in which cobalt and copper, are co-extracted from sulfuric acid solution by 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (EHEHPA) has been investigated in this paper.
The aqueous phase contains H_2SO_4, CoSO_4 and CuSO_4, mixed electrolytes with dissociation equilibrium. The activity coefficients for all ions are calculated by computer based on the Pitzer′s electrolyte theory with some assumptions and approximations made. The mean relative deviation between the experimental and calculated pH values is less than 0.02. The calculated results show that the simplified Pitzer′s equtions work very well for the aqueous phase with dissociation equilibrium.
Form the experimental equilibrium data in both phases and the activity coefficient of diluent determined by the gas-chromatography, the activity coefficients for HEHPA, Co(HR_2)_2, CU(HR_2)_2 and the thermodynamic extraction equilibrium constants K_(Co) and K_(Co) are obtained through “working reference state” proposed by the authors. The values of K_(Co) and K_(Co) keep constant respectively in the region of 0.6168-1.079 mol L~(-1) (the initial concentrations of extractant). Combining the improved Scatchard-Hildebrand theoretical model with the “working reference state”, the calculated values of activity coefficient for all components in the organic phase are obtained which are consistent with the experimental data. These results show that the “working reference state” is very useful for the two-metal extraction system and it seems to be possible to establish theoretical model for two-metal extraction equilibria in place of empirical ones.