Measurement and Correlation of the Solubility of Potassium Hexafluorosilicate (K₂SiF₆) in MgCl₂ and FeCl₃ Solutions at the Temperature Range of 20–70 °C

The solubility of K2SiF6 in MgCl2 and FeCl3 solutions within a temperature range between 20 and 70 °C was experimentally determined by the static method under atmospheric pressure. The equilibrium solids obtained under various conditions were analyzed by XRD, SEM, and FTIR to confirm the stability of corresponding solid phases. The solubility of K2SiF6 in MgCl2 and FeCl3 solutions increased with increasing temperature and concentration, and K2SiF6 remained stable in MgCl2 and FeCl3 solutions. Moreover, the experimental data were successfully correlated with the Apelblat empirical equation. The thermodynamic properties of the solution process, including the Gibbs energy, enthalpy, and entropy, were calculated by the van’t Hoff analysis and the Gibbs equation. The dissolution process of K2SiF6 in the measured solutions was found to be endothermic and nonspontaneous. The findings above can provide fundamental data and theoretical support for optimizing the crystallization of K2SiF6 in acid leaching solution from phosphorus–potassium associated ore and preventing scale formation of K2SiF6 in wet-process phosphoric acid (WPA) vacuum filtration.