Visualization of TiO 2 Reduction Behavior in Molten Salt Electrolysis
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the refining industries of the oxide-stable metallic elements, the electrolysis of the molten chloride is indispensable. Thus, and efficient electrolysis has been developed, for example, for use in the Kroll process for titanium production. In the Kroll process, TiO2 is first converted to TiCl4 by Cl2 gas. Then, liquid Mg reduces the TiCl4 such that high-purity metallic sponge Ti is obtained. The liquid Mg and gaseous Cl2 are regenerated by electrolysis of the byproduct MgCl2 and recycled. To avoid some complicated steps in the Kroll process, the direct electrochemical decomposition of TiO2 in
SHUNGO NATSUI is with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo 060-8628, Japan and also with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Contact e-mail: [email protected] TAKUYA SUDO, RYOTA SHIBUYA, TATSUYA KIKUCHI, and RYOSUKE O. SUZUKI are with the Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University. HIROSHI NOGAMI is with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. Manuscript submitted May 12, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
molten CaCl2 has been proposed. In the FFC Cambridge process, the oxide anion cathodically transfers from the solid TiO2 pellet to the anode in a molten salt bath.[1] Because the Ti-O binary system contains many suboxides, oxygen in the higher oxide is removed to form a lower oxide upon receiving an electrical charge from the cathode. Another promising method, the OS process, has been proposed, in which the oxide anion transfer in CaCl2 is better utilized because as much as 20 mol pct CaO that acts as an electrolyte can dissolve into the molten CaCl2 at 1173 K.[2–5] The electrochemically deposited, liquid Ca at the cathode also dissolves into the molten CaCl2, and the dissolved Ca works effectively to reduce the titanium oxide powder. Similarly, LiCl and its binary chloride systems can dissolve oxygen anions at lower temperatures,[6–8] and KCl is sometimes added to lower the temperature further.[9] Electrochemically deposited liquid Li in molten LiCl-KCl has been observed to form droplets on an attached cathode.[10] Despite its importance, there is only limited knowledge about the dynamic reducing behavior of TiO2 by liquid Li. Recently, black, film-like, colloidal Li (in the form of a metal fog) was observed on an electrodeposited thin Li metal in molten LiCl-KCl.[11] The detailed behavior of reducing TiO2,however, has yet to be clarified. A detailed understanding of the dynamic behavior of TiO2 reduction is necessary to control and optimize the electrolysis. Besides improving the FFC and OS processes, such knowledge can be applied immediately to the current molten salt electrolysis processes and would bring large energy savings due to increased thermal efficiency in the metal-refining industries. Cyclic voltammograms (CVs) of the ox
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