Solid–Liquid Equilibrium of the Quaternary System Ca2+, Mg2+//SO32–, SO42–-H2O at 298.15 K

Solid–liquid equilibria of the quaternary system (Ca2+, Mg2+//SO32–, SO42–-H2O) and its ternary systems (Ca2+//SO32–, SO42–-H2O; Mg2+//SO32–, SO42–-H2O and Ca2+, Mg2+//SO32–-H2O) at 298.15 K were investigated via isothermal solution saturation method. Furthermore, the phase diagrams of these systems were constructed. The results show that the quaternary diagram contains three invariant points, seven univariant curves, and five crystallization fields corresponding to MgSO4·7H2O, MgSO3·6H2O, CaSO3·0.5H2O, Ca(SO3)0.89(SO4)0.11·0.5H2O, and CaSO4·2H2O. The crystallization field of Ca(SO3)0.89(SO4)0.11·0.5H2O is the largest compared with the others. In addition, crystal complex Ca(SO3)0.89(SO4)0.11·0.5H2O also appears in the ternary system (Ca2+//SO32–, SO42–-H2O), which contains two invariant points, three invariant curves, and three crystallization fields. Both of the other two ternary diagrams contain one invariant point, two invariant curves, and two crystallization fields. The Pitzer model was applied to calculate salt solubilities, and the results agree well with the experimental data. Binary and ternary ionic interaction parameters (θSO4,SO3, ψMg,SO4,SO3, ψCa,SO4,SO3, and ψCa,Mg,SO3) and ion pair parameters (CaSO3 and MgSO3) are presented in this work based on the solubilities of these ternary systems.