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, Na2WO4, 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.