The phase equilibria of ternary systems Li2SO4 + Na2SO4 + H2O and Na2SO4 + K2SO4 + H2O at 303.2 K were investigated by using the isothermal dissolution method. The solubility, density, and refractive index of the systems were measured. There are two ternary invariant points and three crystallization regions corresponding to Li2SO4·3Na2SO4·12H2O, Li2SO4·H2O, and Na2SO4·10H2O in the system Li2SO4 + Na2SO4 + H2O at 303.2 K. There are two ternary invariant points and three crystallization regions corresponding to Na2SO4·10H2O, Na2SO4·3K2SO4, and K2SO4 in the system Na2SO4 + K2SO4 + H2O at 303.2 K. By comparing the phase diagrams of the ternary system Li2SO4 + Na2SO4 + H2O at different temperatures, one can find that the existence form of double salt changes from Li2SO4·3Na2SO4·12H2O to Li2SO4·Na2SO4. By comparing the phase diagrams of the ternary system Na2SO4 + K2SO4 + H2O at different temperatures, the double salt Na2SO4·3K2SO4 was not found at 273.2 K, while it was formed at 303.2 and 313.2 K. Meanwhile, the thermodynamic data of these two systems at 303.2 K were fitted by the Pitzer–Simonson–Clegg model, and the calculated values agree well with the experimental values.

The phase equilibria of aqueous ternary systems NH4Cl + CaCl2 + H2O and NH4Cl + MgCl2 + H2O at 308.2 K were measured by the isothermal dissolution method, and the solubility, density, and refractive index were determined experimentally. The stable phase diagrams and the diagrams of density/refractive index versus composition of these two ternary systems are plotted. The results show that both these systems at 308.2 K are complex systems, with the formation of double salts 2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O, respectively. The multitemperature phase diagrams of the ternary systems NH4Cl + CaCl2 + H2O at (273.2, 298.2, 308.2, 323.2, and 348.2 K) and NH4Cl + MgCl2 + H2O at (273.2, 298.2, 308.2, and 323.2 K) are compared, respectively. The results show that within the temperature range discussed, the double salt NH4Cl·MgCl2·6H2O is formed, and the double salt 2NH4Cl·CaCl2·3H2O is only formed at 308.2, 323.2, and 348.2 K. The crystallization areas of 2NH4Cl·CaCl2·3H2O and NH4Cl·MgCl2·6H2O gradually increase with increasing temperature. Thus, the increasing temperature is conducive to the separation of calcium or magnesium from these two systems in the form of 2NH4Cl·CaCl2·3H2O or NH4Cl·MgCl2·6H2O. The interaction relationship between the ions of the system NH4Cl + CaCl2 + H2O becomes simpler at 273.2–298.2 K, which is more beneficial to the precipitation of single salts. When the temperature reaches 323.2 K, NH4Cl·MgCl2·6H2O precipitates into a more stable salt of commensurate type. Meanwhile, the thermodynamic calculations of these two systems at 308.2 K were fitted by the Pitzer–Simonson–Clegg model, and the predicted values agree well with the experimental values.

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