Solid–Liquid Phase Equilibria Determination of the Quaternary System Na⁺, K⁺, Ca²⁺//Cl––H₂O at 348.2 and 363.2 K

To enrich the thermodynamic data and exploit valuable components in polyhalite, we utilized the isothermal dissolution method to examine the solid–liquid phase equilibria of the quaternary Na+, K+, Ca2+//Cl––H2O at 348.2 and 363.2 K. The compositions of the equilibrium solid phases were analyzed utilizing X-ray particle diffraction (XRD). The formation of the double salt KCl·CaCl2 occurred at temperatures of 348.2 and 363.2 K. The phase diagram illustrates this relationship with two invariant points, five univariate curves, and four crystal regions. By examining the phase diagrams of the quaternary system at 308.2, 348.2, and 363.2 K, it can be concluded that there are three crystal regions corresponding to a hydrate salt (CaCl2·4H2O) and two single salts (NaCl, KCl) in the phase diagram at 308.2 K. As the temperature fluctuates, CaCl2·4H2O (308.2 K) transitions to CaCl2·2H2O (348.2–368.2 K). Additionally, with the increase in temperature, the crystal regions of NaCl, CaCl2·2H2O, and KCl·CaCl2 gradually become larger and the crystal region of KCl decreases.