Thermodynamic evaluation of phase equilibria in NaNO3-KNO3 system

Abstract

The binary phase diagram of NaNO3-KNO3 was studied by differential scanning calorimetry (DSC) and high-temperature x-ray diffractometry. The solid solutions in the intermediate concentration phase appeared to be a mixture of the NaNO3-based solid solutions and KNO3-based solid solutions. This behavior below the solidus suggests that NaNO3-KNO3 might well be regarded as a system with limited solid solubilities instead of a continuous series of solid solutions. The temperatures for the solidus and liquidus have been determined with consideration of limited terminal solid solutions. Two models, the Henrian solution and regular solution theory for NaNO3-based phase and the KNO3-based phase, respectively, were used to reproduce the solidus and liquidus of the phase diagram. The results are in good agreement with the DSC data. The thermodynamic properties for the NaNO3-based and KNO3-based solid solutions have been derived from an optimization procedure using the experimental data. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent. Thus, the limited solid solution model seems to be more consistent with the phase diagram obtained by DSC than the continuous solid solution model.