Measurement and Theoretical Calculation of Phase Equilibrium in the Quaternary System KCl-NaCl-MgCl₂-H₂O at 303.15 and 308.15 K

The phase equilibrium of the quaternary system KCl-NaCl-MgCl2-H2O and its subsystems was measured at 303.15 and 308.15 K by the isothermal dissolution equilibrium method. The presence of KCl·MgCl2·6H2O, a type of double salt, was observed in this quaternary system. The phase diagrams at both temperatures include two invariant points, five univariant curves, and four crystallization regions. Due to the strong salting-out effect of MgCl2, both KCl and NaCl exhibit lower solubilities. In addition, the solubility of KCl increases with temperature rising from 273.15 to 323.15 K. High MgCl2 concentration and low temperature are advantageous for enhancing KCl yield. The determination of solubility data for the quaternary system KCl-NaCl-MgCl2-H2O at 303.15 and 308.15 K contributes to the completion of the thermodynamic database and facilitates its application in real industrial processes at temperatures close to those studied. Moreover, the applicability of the Pitzer model and relevant parameters in solubility prediction of this quaternary system at 303.15 and 308.15 K were systematically verified. No significant discrepancies were detected with increasing ionic strength in concentrated solutions. The Pitzer model and existing parameters are suitable for the rapid solubility prediction in this system and demonstrate high prediction accuracy.