Solution Equilibria for Uranium Ore Processing: The BaSO 4-H 2SO 4-H 2O System and the RaSO 4-H 2SO 4-H 2O System
A radio-tracer determination of the solubility of BaSO 4 in H 2SO 4-H 2O mixtures was undertaken at temperatures of 25°C and 60°C. These were subsequently modelled using the Pitzer model for the activity coefficients, and estimates were made for Ba 2+-SO 42- and Ba 2+-HSO 4 - interaction parameters. The final model yields an excellent description of the solubility of BaSO 4 in H 2SO 4-H 2O mixtures up to 6 mol/kg and at both 25°C and 60°C. The logarithm of the thermodynamic solubility product constant (log Ksp) for barite obtained by regression analysis was -10.02 ± 0.02 at 25°C and -9.68 ± 0.01 at 60°C. The values of the ion association constant derived from these data are 2.49 ± 0.03 at 25°C and 2.55 ± 0.1 at 60°C. The enthalpy of dissolution for BaSO 4 was estimated to be 17.6 ± 0.2 kJ mol -1. Literature solubility data for the RaSO 4-H 2SO 4-H 2O system have also been interpreted, using the Pitzer model for the necessary activity coefficients. The logarithm of the thermodynamic solubility product constant (log Ksp) for radium sulfate obtained was -10.21 ± 0.06 at 25°C, while the value of the ion association constant derived is 2.76 ± 0.05 at 25°C. The high negative value of the Pitzer parameter β 2 obtained by regression analysis of these data suggests that the explicit recognition of a RaSO 4(aq) ion association species is to be preferred in this case.