Experimental study and thermodynamic assessment of the MgSO4-CaSO4 system

Abstract

This work presents a comprehensive experimental investigation into the thermal properties of the binary system MgSO₄-CaSO₄, alongside thermodynamic modelling of its thermodynamic properties, with a focus on enhancing its application in thermal energy storage. The phase diagram and thermodynamic properties of the pure sulphates and intermediate compounds were determined using Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). The DSC results led to the refinement of the enthalpy values for phase transitions of MgSO₄, with updated values of 14.6 kJ/mol and 43.4 kJ/mol for the solid-solid and solid-liquid transitions, respectively. High-Temperature X-ray Diffraction (HTXRD) was employed to study the intermediate compounds, leading to the identification of CaMg₂(SO₄)₃. For the first time, the melting temperature and the enthalpy of fusion for this compound were experimentally determined, yielding a value of 145.5 kJ/mol at 1213 ± 5 °C. A novel phase with the composition CaMg(SO₄)₂ was identified using DTA, HTXRD, and Scanning Electron Microscopy (SEM). This phase exhibits a melting temperature of 1309 ± 5 °C, as determined by DTA, and demonstrates thermal stability within a high-temperature range of 1020–1308 °C. These experimental data were used to update the thermodynamic database for the system MgSO₄-CaSO₄ for more accurate thermochemical calculations and predictions.