Solid–Liquid Phase Equilibria in the Ternary Systems NH ₄ Cl–LiCl–H ₂ O and NH ₄ Cl–CaCl ₂ –H ₂ O at 258.15 K

Based on the characteristics of oilfield waters in the Nanyi Mountain area of western Qaidam basin at subzero temperature, the solid–liquid phase equilibria of ternary systems NH4Cl–LiCl–H2O and NH4Cl–CaCl2–H2O at 258.15 K were investigated by using an isothermal solution equilibrium method. The phase diagrams of ternary systems NH4Cl–LiCl–H2O and NH4Cl–CaCl2–H2O at 258.15 K were plotted, respectively. The results show that the equilibrium phase diagram of the ternary system NH4Cl–LiCl–H2O at 258.15 K has one invariant point, two univariate curves, and two solid-phase crystallization zones (NH4Cl and LiCl·2H2O), belonging to hydrate type I. In the equilibrium phase diagram of the ternary system NH4Cl–CaCl2–H2O at 258.15 K, there are one invariant point, two univariate curves, and two solid-phase crystallization zones (NH4Cl and CaCl2·6H2O). In addition, the unreported Pitzer parameters were obtained based on the solubility data of the ternary systems. Using the Pitzer model, the solubilities of salts in the ternary systems NH4Cl–LiCl–H2O and NH4Cl–CaCl2–H2O at 258.15 K were modeled in detail. The modeling solubilities are in great agreement with the experimental results, indicating that the Pitzer model can be well applied to study the thermodynamic phase equilibria at subzero temperature.