Reverse-polarity direct current plasma-driven electro-reduction of refractory metals in molten oxide melts

Abstract

This work is concerned with the extraction of refractory metals using an electro-reduction process in which a molten oxide melt, instead of halide, is used as the electrolyte. The process may provide an alternative method for producing refractory metals from their ores through the use of a reverse-polarity direct current (DC) plasma arc heating process. In such a process, the plasma torch itself acts as an anode, and the resulting liquid or solid product metals work as a cathode, whereas the molten oxide melt plays the role of an electron transfer layer. The focus of this study is the electro-reduction of scheelite, vanadium pentoxide and chromite. For vanadium pentoxide, the proposed melt is mainly composed of CaO-V2O5/Li2O-B2O3-V2O5 orFeO-CaO-SiO2-V2O5-Li2O with the addition of other alkali and alkaline metal oxides. In the case of scheelite ore, the SiO2-CaO-Li2O-WO3 system is used as melt and metal aluminum or nickel could be used as a collecting media. As for the reduction of chromite ore, SiO2-CaO-Cr2O3-Li2O system with a small amount of CaF2 is used as the melt. As a demonstration, chromium was successfully produced on a laboratory scale through the plasma-driven electro-reduction process. One of the studied slags had a composition of 13.8% CaO, 4.9% CaF2, 36.4% Cr2O3, 6.9% Li2O and 38.0% SiO2. Two observations are significant in this laboratory-scale study. The first was that the amount of Cr2O3 and FeO, which were predominantly electronic conductors in the oxide melt, greatly affected the conducting mechanism of the melt. With high concentrations of Cr2O3 and FeO in the melt, the current efficiency was fairly low. In this case, chromium was found to be slightly reduced due to electronic conducting mechanism. For example, with an initial slag of 60% Cr2O3, no chromium was found reduced. With low concentrations of Cr2O3 and FeO in the studied melt, typically no more than 30% Cr2O3, it was observed that chromium was readily reduced due to the dominant ionic conducting mechanism. The second observation was that the addition of SiO2 to the melt helped to make the oxide melt more ionic, which was apparently desirable to the molten oxide electrolysis. However, a high content of silica had a negative effect on the fluidity of the melt. This was resolved by adding trace amounts of CaF2 to the melt. Therefore, by judicious selection of oxide melt, refractory metal oxides might be dissolved into it, and yet the melt itself retains the required ionicity. A reverse-polarity DC-plasma-driven molten oxide electrolysis may prove to be a viable alternative route for the extractive metallurgy of refractory metals.