Deoxidation of Titanium Using Mg as Deoxidant in MgCl 2 -YCl 3 Flux
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UCTION
TITANIUM (Ti) and its alloys are well known for their high specific strength and corrosion resistance. Ti is the ninth most abundant element in the earth’s crust, with a reserve of 0.7 pct, and is thus almost inexhaustible.[1] Ti is also the fourth most abundant metallic element, following Al, Fe, and Mg, and is used as a structural material. Ti is not more widely used as a structural metal because of the high cost of production of metallic Ti from its ores. If a new low-cost smelting process for Ti (compared with that of steel) is developed, the production of metallic Ti would significantly increase. The current process for the production of metallic Ti is called the Kroll process,[2] in which titanium tetrachloride (TiCl4) is reduced by Mg in a reaction retort made of steel, at 1073 K to 1273 K (800 C to 1000 C).
CHENYI ZHENG and AKIHIRO IIZUKA are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan and also with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. TAKANARI OUCHI, YU-KI TANINOUCHI, and TORU H. OKABE are with the Institute of Industrial Science, The University of Tokyo. Contact e-mail: [email protected] Manuscript submitted July 31, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Metallic Ti with a sponge-like structure (Ti sponge) is produced in the reduction process, and this Ti sponge is subsequently melted and used to fabricate wrought Ti products. One of the advantages of the Kroll process is that it produces highly pure metallic Ti. However, the reduction step for the production of Ti sponge is a slow, batch-type process. Currently, no commercial smelting process can replace the Kroll process. Ti has a strong affinity for both Fe and O, making the Kroll process irreplaceable, because the removal of O and Fe from metallic Ti is difficult. In the past, even when metallic Ti was produced, the contamination of O and Fe in Ti was serious. In fact, because of the inability to remove O impurities from metallic Ti, it took about 120 years (from the discovery of Ti in 1791) to achieve the production of highly pure metallic Ti.[3] To develop a new smelting process for Ti, it is essential to avoid impurities such as O and Fe, and shorten the processing time. Therefore, we developed a refining process to remove the O impurities from Ti.[4–13] In this study, we developed a new deoxidation process that utilizes Mg as the deoxidant in a mixture of magnesium chloride (MgCl2) and yttrium chloride (YCl3) at 1300 K (1027 C). The activity of MgO (aMgO) in the system effectively decreased and remained at a low level due to the formation of yttrium oxychloride (YOCl). There have been many reports on the deoxidation of rare-earth metals (M) under equilibrium
conditions with rare-earth metals, rare-earth halides (MCl3 or MF3), and rare-earth oxyhalides (MOCl or MOF).[14–18] However, aside from our previous study,[5] there have been no reports concer
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