Lanthanothermic Reduction of TiO 2
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INTRODUCTION
THE high cost of the titanium (Ti) smelting process is one of the major factors of the high cost of Ti. Currently, industrial Ti smelting is mainly performed with the Kroll process (Figure 1), which is a multistage process involving the chlorination of Ti ore containing a large amount of Ti oxide to produce pure TiCl4 and the reduction of the purified TiCl4 using magnesium (Mg). It is a batch process that requires a long time for the reduction of TiCl4 and the separation of Mg and MgCl2 from Ti. Therefore, the process has low productivity and high costs.[1,2] Accordingly, the development of a new smelting process is highly desired to decrease the cost of Ti. To this end, the direct reduction of TiO2 has been investigated extensively. Such a direct reduction process is expected to simplify and speed up the Ti smelting process; this is because Ti oxide, which is the main component of Ti ore, can be treated directly.
TAKARA TANAKA is with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, 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 and TORU H. OKABE are with the Institute of Industrial Science, The University of Tokyo. Contact e-mail: [email protected] Manuscript submitted November 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Because of the strong affinity of Ti with O, reductants that can directly reduce TiO2 to metallic Ti with the industrially acceptable oxygen concentration of approximately 500 mass ppm O are thermodynamically limited to calcium (Ca) and some rare earth metals (REs).[3–5] In the past, many reduction processes using Ca as the reductant have been investigated, including the calciothermic reduction process,[6–16] and combinations of calciothermic and electrochemical reduction methods, such as the FFC[17–20] and OS processes.[21–24] However, none of these methods is currently applied in large-scale industry settings because of many technical problems, such as the inefficient regeneration of Ca and contamination with impurities, such as carbon. A number of studies have been conducted on the TiO2 reduction using some metal reductants, such as Mg and Al, which are more efficiently produced and recycled than Ca.[25–30] However, metal (or alloy) reductants have an insufficient reducing ability and cannot stably produce Ti with oxygen concentrations of approximately 500 mass ppm O. However, it is known that some REs can thermodynamically deoxidize metallic Ti to low-oxygen-concentration Ti.[31] Muthmann et al.[32,33] produced some refractory metals, such as tantalum (Ta) and tungsten (W), using a misch metal (i.e., an alloy of several REs, including lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd)) as the reductant. Based on this method, Vogel et al.[34] tried to produce metallic Ti by direct reaction between a misch metal and TiO2.
Fig. 1—The process flow of the Kroll process.
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