Performance of three chemical models on the high-temperature aqueous Al2(SO4)3-MgSO4-H2SO4-H2O system

This work involves chemical modeling of concentrated aqueous sulfate solutions during an industrial process: pressure acid leaching of laterites at high temperature of interest to the metals and minerals industries. Equilibrium constants of Al and Mg complexes were estimated or extrapolated. Three activity coefficient models were used: the B-dot equation, the Pitzer model, and the Bromley-Zemaitis model. The B-dot and Pitzer models were implemented with EQ3/6 software (version 7.0), and the Bromley-Zemaitis model was implemented through OLI-Systems software (version 6.2). Original databases in all cases were modified in order to include the equilibrium constants of Al and Mg species. Calculated values are in good agreement with the experimental solubility data for MgSO4-H2SO4-H2O and Al-2(SO4)-H2SO4-H2O ternary systems with all three models. However, significant differences were found in the predicted speciation and pH in both ternary and quaternary systems because of the different correlations and assumptions employed in the models. A self-consistent model and new thermodynamic data for high-temperature aqueous processes are highly recommended to handle concentrated solutions of interest to the process industry.