Solid–Liquid Phase Diagram of the Ternary System NaF–Na₃PO₄–H₂O at 308.15, 323.15, and 348.15 K

Abstract

The solid–liquid phase behavior of the NaF–Na3PO4–H2O ternary system and its binary subsystems NaF–H2O and Na3PO4–H2O at 308.15, 323.15, and 348.15 K has been measured by the isothermal dissolution equilibrium (IDE) method. Compared with the measured solid–liquid phase behavior data, the previously reported results have been evaluated and the unreasonability has been discussed. The phase diagrams at the investigated temperatures for this ternary system are drawn according to the measured solid–liquid phase behavior and the corresponding wet solid and liquid-wet solid–solid lines. From the phase diagrams, it is found that at a given temperature, there are three types of equilibrium solid phases, NaF(s), double salt 2Na3PO4·NaF·19H2O(s), and Na3PO4·12H2O(s) in the ternary system. Accordingly, there are two cosaturated points for NaF(s) and double salt 2Na3PO4·NaF·19H2O(s), double salt 2Na3PO4·NaF·19H2O(s) and Na3PO4·12H2O(s), respectively. The comparison of phase diagrams at different temperatures suggests that, with the rise of temperature, the double salt crystal region became a little bit larger, and correspondingly, the crystal regions of pure salt (NaF(s) and Na3PO4·12H2O(s)) became smaller, which is not beneficial to the separation of fluoride and phosphate. This indicates that the fluoride and phosphate separation process should be at lower temperatures.