The Binary Alkali Nitrate and Chloride Phase Diagrams: NaNO3-KNO3, LiNO3-NaNO3, LiNO3-KNO3, and NaCl-KCl

LiNO3-NaNO3, LiNO3-KNO3, NaNO3-KNO3, and NaCl-KCl phase relations were formulated using the regular solution model to obtain desired compositions for their potential application as a heat transfer fluid. The simultaneous nonlinear equations formed were solved numerically using a contour plot technique in Matlab. An algorithm was developed to determine the real roots of the coupled nonlinear equations and subsequent phase diagrams were constructed. The phase diagrams were compared with those predicted using FactSage thermochemical package and literature data from experimental studies. The calculated NaNO3-KNO3 phase diagram shows that this system has a eutectic point at 0.5 mole fraction of KNO3 with a eutectic temperature of 220.85 °C, while the LiNO3-KNO3 system was found to have a eutectic point at 0.44 mole fraction LiNO3 at 137 °C. The LiNO3-NaNO3 system shows a eutectic at 0.524 mole fraction LiNO3 at 192.8 °C, and finally the NaCl-KCl system has a eutectic point at 0.48 mole fraction NaCl at 634 °C. The liquidus and solidus curves obtained fitted close against values obtained from the quasi-chemical model used in FactSage and experimental values from the literature. This study showed that the regular solution model combined with the contour plots approach can sufficiently describe the liquidus–solidus curves of the alkali binary nitrate and chloride systems and could be a useful method for predicting the phase diagrams for higher order alkali nitrates and chlorides.