ENERGY-EFFICIENT PURIFICATION AND ENHANCED RECOVERY OF TUNGSTEN FROM ORES, TAILINGS, AND WASTE THROUGH METHANOL- AND ETHANOL-INDUCED SUPERSATURATION CRYSTALLIZATION

Tungsten is a refractory metal used as the native metal, in alloys, and as tungsten carbide. Because tungsten is extracted from the rare minerals scheelite and wolframite, methods for better recovery of secondary minerals, tailings, and industrial wastes are needed. Sodium tungstate is an intermediate product between tungstate mineral extraction and production of tungsten metal, as well as an intermediate in recovery processes from scrap. The high solubility of sodium tungstate in neutral to basic aqueous solutions likely aids in dissolution and mobility from primary ores. This solubility enables purification processes through dissolution and recrystallization processes, thereby avoiding the very high melting temperature of the native metal. In contrast, sodium tungstate is negligibly soluble in water-miscible alcohols; this property can be used to adjust solubility of tungsten in water-alcohol solutions.

I present new data on Na₂WO₄ ∙ 2H₂O solubility in water-methanol and water-ethanol solutions of varying alcohol ratios at 22, 27, and 40°C. Each system can be fitted with a quadratic polynomial where R² > 0.99 and shows marked decrease in solubility with increasing alcohol content. An addition of concentrated alcohol to tungstate-bearing solutions and slurries can be used to purify and concentrate tungsten. Distillation for alcohol recovery can be used for closed-system purification at a fraction of the cost required for refining through melting. These processes can potentially make low-grade deposits, like soils and finely crystalline deposits, economical.

An excess of measured Na beyond an expected 2:1 Na:W molar ratio in most samples likely indicates at least partial polymerization of tungsten from tungstate into polyoxyanions (e.g. Na₁₀(H₂W₁₂O₄₂) ∙ 4H₂O) due to high molarity at pH 7. The Na:W ratio reaches >5:1 in 50% alcohol solutions, suggesting a more complete reaction to polyoxytungstates in the solid phase. This process reaches an equilibrium state before 48 hours at constant temperature between excess solid Na₂WO₄ ∙ 2H₂O, polyoxytungstates, and solution as evidenced by stable density measurements. Further work is needed to assess viability, but this process may help in producing inexpensive “heavy liquids” for mineral separation and with removing excess alkali from purified tungstates.