Contactless electrochemical reduction of titanium (II) chloride by aluminum
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I. INTRODUCTION
TITANIUM and its alloys have attractive mechanical properties such as strength-to-weight and rigidity-to-weight ratios and excellent corrosion resistance. Nevertheless, the application of titanium alloys in use is limited compared to other structural materials (e.g., steels and aluminum alloys) because of the high cost of production. Titanium is produced by metallothermic reduction of TiCl4 by magnesium (Kroll process).[1] The process is batch type and labor intensive and uses an expensive reductant, magnesium. Recently, Sadoway and Okabe[2,3] have demonstrated the importance of electrochemical reactions in the sodiothermic reduction of K2TaF7. They demonstrated that the reduction can proceed without direct physical contact between reactants and euphemistically characterized it as electronically mediated reaction (EMR). Following this, studies of EMR in the metallothermic reduction of titanium chloride were carried out by our group.[4,5,6] The importance of electrochemical reactions during magnesiothermic reduction of titanium chlorides was experimentally demonstrated, and some applications were proposed. These studies indicate that the location of titanium deposition can be controlled using long-range EMR (LR-EMR).[7] Titanium powder can also be produced by magnesiothermic reduction using a reaction mediator (e.g., Dy21/Dy31) in the molten salt.[8,9,10] In principle, short-range EMR (SR-EMR) can be used to design a continuous process for the production of titanium powder. Reported in this article is an exploratory study of the use of aluminum, instead of magnesium, as a reductant for titanium chloride. Aluminum and aluminum scrap are lower T. UDA, formerly Graduate Student, Tohoku University, is Research Associate, Institute for Advanced Materials Processing, Tohoku University. T.H. OKABE, Research Associate, and Y. WASEDA, Professor and Director, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai, 980-8577, Japan. K.T. JACOB, formerly Visiting Professor, Institute for Advanced Materials Processing, Tohoku University, is Professor, Department of Metallurgy, Indian Institute of Science, Bangalore, India. Manuscript submitted January 26, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
priced reductants compared to magnesium. However, when attempts were made in the past to use aluminum as a reductant, only titanium alloys containing a large amount of aluminum were obtained because of the strong affinity between the two metals.[11] An attempt was made in this study to minimize aluminum contamination of titanium by avoiding physical contact between reactants; an electrical circuit links anodic and cathodic reactions at different locations and provides a path for rapid electron transfer. II. BACKGROUND A. Thermodynamics of the System Ti-Al-Cl The results of earlier studies on the aluminothermic reduction of titanium compounds reported in the literature are summarized in Table I.[12–16] Pure titanium was not obtained during the conventional aluminothermic reduction; alu
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