The ion-association-interaction approach as applied to aqueous H2SO4-Al2(SO4)3-MgSO4 solutions at 250 °C

A hybrid ion-association-interaction approach is implemented to describe the chemistry and thermodynamics of aqueous H2SO4-Al2(SO4)3-MgSO4 solutions at 250 °C. These solutions are relevant to the sulfuric acid pressure leaching of nickeliferous laterites. Strong complexes in solution are handled via the ion-association approach. Nonidealities, including weak ion pair formations, are treated through the Pitzer ion-interaction theory. The existing complexes in solution and the Pitzer ion-interaction parameters were identified through processing solubility data in the binary (H2SO4-Al2(SO4)3 and H2SO4-MgSO4) as well as the ternary (H2SO4-Al2(SO4)3-MgSO4) electrolyte solutions at or near 250 °C. The existing aqueous aluminum-bearing species identified were Al3+, Al(SO4)+, and Al2(SO4)30, with Al2(SO4)30 as the dominant species at moderate to high H2SO4 concentrations. The existing aqueous magnesium-bearing species found were Mg2+ and MgSO40, with Mg2+ being dominant except at low concentrations of H2SO4. The dominant species identified for Al and Mg explain why a higher H2SO4 concentration in solution is required during the processing of high-magnesium laterites.