Solubility, correlation and thermodynamic properties of Li3PO4 in Li-Na-K-B-Cl-S system and its application

Abstract The study of dissolution thermodynamics of Li₃PO₄ in Li-Na-K-B-Cl-S system is of significance to direct prepare Li₃PO₄ with phosphates which avoids concentrating the LiCl brine. The solubility of Li₃PO₄ in Li-Na-K-B-Cl-S system and in LiCl brine were obtained for the first time at temperatures ranging from 291.15 to 363.15 K. Results showed that solubility of Li₃PO₄ in Li-Na-K-B-Cl-S is related to temperature and impurity, which decreases with increasing temperature. A small amount of Na₂B₄O₇ has the greatest effect on improving solubility of Li₃PO₄, followed by Na₂SO₄. The NaCl or KCl has a certain effect on increasing solubility at low temperature and concentration. However, ion association structures play a crucial role in decreasing solubility of Li₃PO₄ with increasing NaCl or KCl concentration. The common ion effect of added LiCl has more significant influence on decreasing solubility of Li₃PO₄, than salting out effect of added NaCl or KCl. Furthermore, the Van’t Hoff, Modified Apelblat, Quadratic Polynomials models were used to fit solubility, Quadratic Polynomials was best. The greatest values of RD, RAD, RMSD were 1.62%, 7.05×10⁻ ³, 2.34×10⁻ ⁵. Moreover, analysis of apparent thermodynamic parameters indicate that dissolution of Li₃PO₄ in Li-Na-K-B-Cl-S is unfavorable to exothermal and exentropic. It’s non spontaneous and the process is mainly entropy driven. The salts solution shows a nonlinear enthalpy-entropy compensation relationship, and the driving forces varies with the content of components. This study will provide thermodynamic basis for crystallization process of Li₃PO₄ from Dong-Taijinaier Salt Lake in the industrial production process.