New Smelting Process for Titanium: Magnesiothermic Reduction of TiCl 4 into Liquid Bi and Subsequent Refining by Vacuum
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RECENTLY, we reported the electrochemical reduction of TiO2 into a liquid Bi cathode to produce Bi-Ti liquid alloys in molten CaCl2.[1] This process has the potential to improve the productivity of Ti because the reduction and refining steps can be performed continuously using the liquid alloys. However, in this process, Ca co-deposition into liquid Bi easily occurs during the reduction step because of the very low activity coefficient of Ca in Bi, which inhibits the formation of the Bi-Ti alloy. In contrast, electrolysis of TiCl2 in CaCl2 produces Bi-Ti alloys with a relatively high concentration of Ti and little Ca contamination. Thermodynamic considerations based on potential-pO2 (=log aO2 ) diagrams indicate that it is important to maintain a high pO2, i.e., a low concentration of O2 in the melt. However, a high pO2 near the TiO2 cathode is difficult to achieve because O2 ions are formed at the TiO2 cathode during reduction. In the present study, we propose an alternative process involving Bi-Ti liquid alloys; this process is based on the Kroll process used in current industry. The remarkable advantage of this process is a cooling effect of liquid Bi during the reduction step of the Kroll process, which leads to an increased Ti production rate. YUYA KADO, AKIHIRO KISHIMOTO,and TETSUYA UDAare with the Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Yoshida Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan. YUYA KADO is with the Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. Contact e-mail: [email protected] Manuscript submitted April 9, 2014. Article published online August 26, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
Similar processes involving Ti-Zn liquid alloys have been examined by Gleave et al. and Sato et al.[2,3] According to the Ti-Zn phase diagram,[4] the solubility of Ti in Zn at 1173 K (900 C) (13 mol pct) may be sufficiently large for this phase to function as a liquid alloy. Although high vapor pressure of Zn (9.5 9 101 atm at 1173 K (900 C)[5]) is advantageous for vacuum distillation, we believe that the high vapor pressure is an obstacle during reduction to realize a practical process. In comparison to Zn, Bi has a lower vapor pressure of 1.9 9 103 atm and a larger solubility of Ti [i.e., 30 at pct at 1173 K (900 C)].[5,6] Sb is also a potential solvent for Ti because its vapor pressure and solubility of Ti are 2.3 9 102 atm and 16 at pct at 1173 K (900 C), respectively.[5,7] For vacuum distillation, Zn is the superior solvent, followed by Sb and Bi. However, for reduction, Bi is the most appropriate solvent because it has the greatest solubility of Ti and the lowest melting point [545 K (272 C)] among Bi, Sb, and Zn. A low melting point is an important property for a material to function as a cooling agent and is a key to improving the feed rate of TiCl4 with removing the large amount of heat generated in the reduction step. In this
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