Hydrogen reduction of tungsten oxides: Alkali additions, their effect on the metal nucleation process and potassium bronzes under equilibrium conditions

Abstract

The addition of alkali compounds influences W particle growth during the reduction of tungsten oxide using hydrogen. Existing literature does not reveal the mechanism and clear mode of interaction. To improve understanding of the reduction process, lithium, sodium and potassium compounds were added to a highly pure tungsten oxide prior to reduction, and interrupted reduction experiments were carried out at 750°C. The resulting powders were investigated by metallographic and chemical methods: X-ray diffraction, SEM and EDX analysis.The experiments confirmed that intermediately formed tungsten bronzes play an important role in the early stages of reduction. Further experiments showed that bronzes are formed from potassium containing compounds in equilibrium with mixtures of W and WO2 or WO2 and WO2.72. Furthermore it is demonstrated that the kinetics of the reduction sequence differs significantly between doped and undoped tungsten oxide powders, as demonstrated by the on-line measurement of reaction water in the hydrogen process gas.The results help to understand how the presence of alkali compounds effects the reduction of tungsten oxides. Investigations in production scale with intentionally contaminated tungsten oxide showed that bronzes can play a role in industrialised production.