Thermodynamic assessment of industrial pyrometallurgical extraction routes for refractory metals

Abstract

Industrial production of refractory metals is governed primarily by thermodynamic constraints arising from the strong affinity of these metals toward carbon and oxygen. In the present work, a unified thermodynamic assessment of representative refractory metals (V, Nb, Ta, Mo, W, and Re) was performed to establish the thermodynamic basis for selecting pyrometallurgical extraction routes. Standard Gibbs free energy calculations, Ellingham diagrams, and HSC Chemistry equilibrium calculations were employed to compare oxide stability, carbide formation tendency, hydrogen reduction, metallothermic reduction, and chlorination-based extraction routes. The calculations show that Nb₂O₅ and Ta₂O₅ possess the highest thermodynamic stability and simultaneously exhibit the strongest tendency toward carbide formation, explaining why chlorination followed by metallothermic reduction is preferred industrially. In contrast, MoO₃ and WO₃ are sufficiently reducible by hydrogen, while equilibrium calculations demonstrate the important influence of hydrogen excess on the final metallization degree. Metallothermic reduction of V₂O₅ with Al, Mg, and Ca is thermodynamically feasible over the investigated temperature range, whereas industrial preference for aluminothermy is mainly determined by technological and economic factors. Thermodynamic analysis of chlorination and carbochlorination further demonstrates that direct chlorination of Nb₂O₅ and Ta₂O₅ is unfavorable, while carbochlorination provides a strong driving force for pentachloride formation. The results provide a consistent thermodynamic explanation for the selection of industrial extraction routes for refractory metals.