Phase Equilibria of Aqueous Ternary Systems NH4Cl + CaCl2 + H2O and NH4Cl + MgCl2 + H2O at 308.2 K: Measurement and Calculation

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.