REVERSE-POLARITY DC PLASMA-DRIVEN ELECTRO-REDUCTION OF REFRACTORY METALS IN MOLTEN OXIDE MELTS

摘要

This work is concerned with the extraction of refractory metalsusing an electro-reduction process, in which a molten oxide melt,instead of halide, is used as the electrolyte. The process may providean alternative method of producing refractory metals from their oresthrough use of a reverse-polarity direct current (DC) plasma archeating process. In such a process, the plasma torch itself acts as ananode, and resulting liquid or solid product metals work as a cathode,whereas the molten oxide melt plays the role of an electron transferlayer.The focus of this study is: the electro-reduction of scheelite;vanadium pentoxide; and chromite. For the vanadium pentoxide, theproposed melt is mainly composed of CaO-V_2O_5/Li_2O-B_2O_3-V_2O_5or FeO-CaO-SiO_2-V_2O_5-Li_2O with the addition of other alkali andalkaline metal oxides. In the case of scheelite ore, the SiO_2-CaOLi_2O-WO3 system is used as melt and metal aluminum or nickel couldbe used as a collecting media. As for the reduction of chromite ore,SiO_2-CaO-Cr_2O_3-Li_2O system with trace amount of CaF2 is used asthe melt.As a demonstration, chromium was successfully produced at alaboratory 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% Cr_2O_3; 6.9% Li_2O; and 38.0% SiO_2. Two observationsare significant in this laboratory scale study. The first one was that theamount of Cr_2O_3 and FeO, which were predominantly electronicconductors in the oxide melt, greatly affected the conductingmechanism of the melt. With high concentration of Cr_2O_3 and FeO inthe melt, the current efficiency was fairly low. In this case, chromiumwas found to be barely reduced because of electronic conductingmechanism. For example, with initial slag of 60% Cr_2O_3, nochromium was found reduced.With low concentration of Cr_2O_3 and FeO in the studied melt,typically no more than 30% Cr_2O_3, it was observed that chromiumwas readily reduced due to the dominant ionic conductingmechanism. The second observation was that the addition of SiO_2 tothe melt helped to make the oxide melt more ionic, which wasapparently desirable to the molten oxide electrolysis. However, highcontent of silica had a negative effect on the fluidity of the melt. Thiswas resolved by adding trace amounts of CaF2 to the melt. Therefore,by judicious selection of oxide melt, refractory metal oxides might bedissolved into it and yet the melt itself retains the required ionicity. Areverse-polarity DC plasma driven molten oxide electrolysis mayprove to be a viable alternative route for the extractive metallurgy ofrefractory metals.