Phase Equilibria and Phase Diagram of the Quaternary NaOH-Na₃VO₄-Na₂WO₄-H₂O System at 278.15 K to 353.15 K

The solid-liquid phase equilibria and phase diagrams for the quaternary NaOH-Na₃VO₄-Na₂WO₄-H₂O system from 278.15 K to 353.15 K were investigated, and the compositions in the liquid phase and densities were measured experimentally with the isothermal equilibrium method. Na₃VO₄ solubility is significantly affected by the temperature and NaOH concentration, and Na₂WO₄ solubility is significantly affected by the NaOH concentration. The dry-salt phase diagram of the system consists of three crystallization zones (for which the solid phases are Na₃VO₄·12H₂O, Na₂WO₄·2H₂O, and NaOH·H₂O at 278.15-298.15 K; Na₃VO₄·3H₂O, Na₂WO₄·2H₂O, and NaOH·H₂O at 298.15 K to 313.15 K; and Na₃VO₄·3H₂O, Na₂WO_4, and NaOH·H₂O at 313.15 K to 353.15 K). The solubilities of Na₃VO₄ and Na₂WO₄ were compared against those of their ternary subsystems of NaOH-Na₃VO₄-H₂O and NaOH-Na₂WO₄-H₂O, respectively. Na₂WO₄ has a strong salting-out effect on Na₃VO₄ solubility when the NaOH concentration was approximately below 30 wt %. The addition of Na₃VO₄ has almost no salting-out effect on Na₂WO₄ solubility, especially when the NaOH concentration exceeded 20 wt %. The phase equilibrium study of the quaternary system of NaOH-Na₃VO₄-Na₂WO₄-H₂O will provide a theoretical basis for promoting the development of vanadium and tungsten products and may guide the comprehensive utilization of spent selective catalytic reduction catalysts.