Production of Titanium Powder by Sodiothermic Reduction in CaCl 2 Molten Salts
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INTRODUCTION
TITANIUM has been a subject of research interests because of its properties such as low density, large mechanical strength, and good resistance to high corrosion. The kroll process produces metallic titanium commercially in industry.[1] It consists of the conversion from TiO2 to TiCl4, the subsequent reduction of TiCl4 to Ti by Mg liquid, and the electrolysis of the by-product MgCl2. It takes 2 to 5 days in reduction route via TiCl4, and it is still difficult to recover the waste heat from these individual steps.[2] As titanium ores exist as the oxide, a simpler and more compact process directly from TiO2 has been desired to get higher productivity and energy saving. Currently, extensive research has been conducted for the development of new production processes of metal from oxide. The researchers at H.C. Starck Inc. have developed a magnesiothermic reduction process of Nb2O5.[3,4] In this process, Nb2O5 powder is reduced to Nb by magnesium (Mg) vapor, which is physically isolated from Nb2O5. Okabe et al. have also conducted the magnesiothermic reduction process of oxide called the preform reduction process (PRP) to minimize the contamination of the resulting powder.[5–7] In the PRP, The feed preform is fabricated from slurry, which is made by mixing TiO2 powder, flux (e.g., CaCl2), and binder. Nb2O5 in the preform is reduced by Mg vapor, and Nb powder with a purity of more than 99.5 mass pct is successfully obtained by the PRP. These processes CHAO DU and ZHENG WANG, Students, and JUNGANG HOU, SHUQIANG JIAO, and HONGMIN ZHU, Professors, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing100083, P.R. China.Contact e-mail: [email protected] Manuscript submitted November 12, 2013. Article published online September 12, 2014. 1750—VOLUME 45B, OCTOBER 2014
are found to be suitable for producing homogeneous fine powders when the reaction condition is controlled. Ono and Suzuki are also developing a titanium production process called the OS process.[8–11] The reduction system consists of a single cell, where both the reduction reaction of TiO2 and the electrolytic reaction for recovery of reducing agent coexist in the same molten CaCl2 bath. Because the molten CaCl2 has solubility for Ca, TiO2 powder is reduced to Ti by dissolved Ca in the melt. Meanwhile, as the molten CaCl2 has a large solubility for CaO, the by-product CaO dissolves in the melt and is reduced to Ca by electrolysis. Fray et al. have investigated the direct electrochemical reduction of TiO2 to Ti metal in molten CaCl2, called the FFC process, in which TiO2 decomposes to Ti and O2 by electrolysis when a TiO2 pellet in the conductive CaCl2 melt is used as the cathode.[12–15] Because CaCl2 melt can dissolve a large amount of CaO, particularly O2–,in the FFC process, the dissolved O2– diffuses to the carbon anode and releases as O2 gas. In general, CaCl2 melt is indispensable as solvent in both the OS and FFC processes because of its property for dissolving a few mol pct Ca[16
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