Experimental Investigation and Thermodynamic Modeling Assessment of the NaCl-NaI-MgCl₂-MgI₂ Quaternary System

The thermochemistry of the quaternary molten salt system NaCl-NaI-MgCl₂-MgI₂ has been studied using an experimental and thermodynamic modeling approach. The binary subsystems NaCl-NaI and NaCl-MgCl₂ were reassessed based on existing data in the literature. The binary subsystem NaI-MgI₂ was subjected to a renewed experimental investigation, to complement and revisit the data in the literature. The subsystem MgCl₂-MgI₂ was investigated for the first time in this work using Differential Scanning Calorimetry (DSC). Furthermore, the phase equilibria in the pseudobinary phase diagrams of NaCl-MgI₂ and NaI-MgCl₂ in the quaternary system were investigated by DSC, while the condensed phases in the quaternary system were investigated using X-ray diffraction (XRD). A thermodynamic model of the quaternary system was developed using the CALPHAD (CALculation of PHase Diagrams) method with the quadruplet approximation in the modified quasichemical model for the liquid phase, and two-sublattice polynomial models for the solid solution phases. With this model, the liquidus surface of the NaCl-NaI-MgCl₂-MgI₂ quaternary system has been described for the first time.