Solution equilibria for uranium ore processing:: The BaSO4-H2SO4-H2O system and the RaSO4-H2SO4-H2O system

A radio-tracer determination of the solubility of BaSO4 in H2SO4-H2O mixtures was undertaken at temperatures of 25 degrees C and 60 degrees C. These were subsequently modelled using the Fitter model for the activity coefficients, and estimates were made for Ba2+-SO42- and Ba2+-HSO4- interaction parameters. The final model yields an excellent description of the solubility of BaSO4 in H2SO4-H2O mixtures up to 6-mol/kg and at both 25 degrees C and 60 degrees C. The logarithm of the thermodynamic solubility product constant (logK(sp)) for barite obtained by regression analysis was -10.02 +/- 0.02 at 25 degrees C and -9.68 +/- 0.01 at 60 degrees C. The values of the ion association constant derived from these data are 2.49 +/- 0.03 at 25 degrees C and 2.55 +/- 0.1 at 60 degrees C. The enthalpy of dissolution for BaSO4 was estimated to be 17.6 +/- 0.2 kJ mol(-1). Literature solubility data for the RaSO4-H2SO4-H2O system have also been interpreted, using the Pitzer model for the necessary activity coefficients. The logarithm of the thermodynamic solubility product constant (logK(sp)) for radium sulfate obtained was -10.21 +/- 0.06 at 25 degrees C, while the value of the ion association constant derived is 2.76 +/- 0.05 at 25 degrees C. The high negative value of the Fitter parameter beta(2) obtained by regression analysis of these data suggests that the explicit recognition of a RaSO4(aq) ion association species is to be preferred in this case. Copyright (C) 1998 Elsevier Science Ltd.