A Novel Process Of Preparation Refractory Metal And Alloy Drectly From Metal Oxide

The existing pyrometallurgical process to prepare refractory metals and their alloys is complex, expensive, energy intensive and probably polluting. Therefore, it is common desirable to develop a simple, low-cost and environmentally sound process to prepare metal. The paper reviewed and summarized the methods for preparation of Ta, Cr, Ti and Ti alloy and analyzed their shortages. As a result, extracting of refractory metals and alloys from their respective oxides directly using SOM (Solid Oxide Membrane) process have been investigated.The electrolysis reduction of oxides cathode is performed by SOM process in the selective molten salt, which liquid copper, saturated with graphite powders and encased in a one-end-closed solid oxide membrane tube, acted as the anode. The anode and the molten are separated by SOM which conducts oxygen ion only. As a result, only oxygen ions are oxidezed at the anode. The desired metal cations are reduced selectively at the cathode through control the electrode potential. Therefore, only metallic oxides take part in electrolytic reaction.The property of the fluxes, the stability of solid oxide membrane, the formation of cathode pellets, the parameter of electrolysis and the electrolytic mechanism are studied in this paper respectively. And the electrochemical reduction mechanism of SOM process was depicted by using the three-phase interlines react mechanism. The study for property of the fluxes showed that the volatility, viscosity and conductivity of the molten 55.5MgF2-44.5CaF2(wt%) are less than 5.4×10-7g/cm2·s, 11.8mPa·s and 5.2 S·cm-1, and of the molten CaCl2 are 1.9×10-6g/cm2·s, 3.4mPa·s and 3.6S·cm-1, respectively. The two fluxes have high conductivity, low viscosity and volatility, which can satisfy the demand of experiment.The study for membrane tube with self made at different doping showed that the 8%mol Y2O3 stabilized ZrO2 tube has compact microstructure, good smooth finish and electro properties. The conductivity is between 0.18 and 0.47S?cm-1 at the temperature of 1000℃to 1300℃. The electrochemical property of the cell system kept stability in the course of experiment.The Ta2O5 and Cr2O3 pellets formed at 4 MPa and sintered at 1100~1150℃for 2 hours exhibited good electrochemical performance and the oxygen content of the products were low after electrolyzing for 2 hours at 1100℃. The metallic particles could increase obviously with electrolytic time. However, the rate of reaction declined rapidly, when the electrolytic temperature was lower than 1100℃. When the sintering temperature increase to 1300℃, the compact core has been shown in electrolyzed Ta2O5 samples, but the compact crust was occurred in the Cr2O3 samples. Under the premise of electrolysis samples with strength, it is suggested that rational range of the porosity and particle size of Ta2O5 cathode should be 30 to 40% and 0.6 to 1.0μm, and of Cr2O3 cathode should be 40 to 50% and less than 1.0μm, respectively.Ti and Ti alloy directly made from Titaniferous residue (Zunyi titanium plant) in the molten CaCl2 (1100℃) has been investigated by SOM process. The results showed that the alloys of TiFe and TiFe2 were obtained from Ti-Fe residue, the pure Ti was obtained from titanium-rich residue after electrolyzed for 6 hours respectively.The ionic migration action and the electrolytic mechanism during the operating of SOM process are investigated. The results showed that the electrochemical reduction of Ta2O5 and Cr2O3 pellets in molten CaCl2 are achieved via two steps. The reaction with Ca participated in is beneficial to reduction of cathode pellets, which the intermediate products are conductive with very short life. The distributing of three-phase boundary inside of pellets and the influence of three-phase boundary on current are dicussed. Which proved the electrochemical reduction of the SOM process obeys the three-phase interlines (3PIs) reaction mechanism at the metal/oxide/electrolyte interface.SOM process is an emerging short-flow technology for extraction of metals directly from their oxides, which is environmentally friendly, less energy cost, and operating continuously. The development of SOM process as a new metallurgy technology has significance for exploitation and utilization with different composite ore resource in China.