Solubility of sodium tungstate in molten sodium hydroxide

Abstract

The solubility of sodium tungstate (Na2WO4) in a molten sodium hydroxide (NaOH) bath and its dependence on the partial pressure of water vapor were measured. This was done as part of an ongoing study of a new tungsten (W)- recycling process using molten NaOH. First, the nature of the chemical compound in the equilibrium solid phase in a molten NaOH bath was confirmed by adding excess amount of tungsten oxide as a tungstate ion source into the bath. X-ray diffraction analysis indicated that anhydrous Na2WO4 is the solid phase in equilibrium with the liquid phase of molten NaOH saturated with Na2WO4. Then, the solubility of the anhydrous Na2WO4 was measured under various partial pressures of water vapor. The results revealed that the solubility of anhydrous Na2WO4 drastically increased with the partial pressure of water vapor and the bath temperature from a molar fraction of 0.06-0.17. This value was much higher than those previously reported for molten nitrate and nitrites.