Solid–Liquid Phase Equilibrium for the Ternary System NaVO3–KVO3–H2O at 298.15 K

The stable phase equilibrium of the ternary system NaVO₃–KVO₃–H₂O at 298.15 K was investigated using the isothermal solution equilibrium method and the Schreinemaker wet residue method. The equilibrium liquid phase solubility data and physical property data [density (ρ), refractive index (n_D), pH] were determined, and X-ray diffraction was utilized to verify the material composition of the equilibrium solid phase. Phase diagrams and physical property–composition diagrams were constructed based on the equilibrium solubility data and physical property data. The results show that the ternary system has a complex salt generation process with two invariant points, three univariate curves, and three crystalline phase regions corresponding to NaVO₃·2H₂O, KVO₃, and Na_2mK_n(VO₃)_2m+n, respectively. The solid phase precipitated from a saturated NaVO₃ solution at 298.15 K is NaVO₃·2H₂O, as confirmed by a comparison of experimental results and the literature. The chemical composition of the complex salt generated from the NaVO₃–KVO₃–H₂O ternary system follows the general formula of the Na_2mK_n(VO₃)_2m+n complex salt proposed by Druzynski’s group, thereby validating the reliability of the present experiment. This study provides new experimental data for the preparation and separation of metavanadate.