The reduction of niobium and tantalum pentoxides by silicon in vacuum

The use of silicon as a reducing agent for Nb2O5 and Ta2O5 to obtain the metals under pyrovacuum conditions has been investigated. Reduction occurs due to the removal of oxygen by evaporation of volatile silicon monoxide, SiO(v). Pourbaix–Ellingham diagrams for Nb–Si–O and Ta–Si–O systems were constructed using the published thermodynamic data. The sequences of reduction, as indicated by these diagrams, were also observed by vacuum thermogravimetry experiments and X-ray diffraction (XRD) analysis of products. The reduction starts at around 1150°C for both niobium and tantalum systems and proceeds smoothly as the temperature is raised to 1600°C under vacuum. The extents of reduction were found to be more than that observed in carbothermic reductions at comparable levels of temperature, pressure and durations. The product was an Nb–Si–O or Ta–Si–O alloy, which is refineable to the pure metal by pyrovacuum treatment at a higher temperature.