Calciothermic reduction of titanium oxide and in-situ electrolysis in molten CaCl 2
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Calciothermic Reduction of Titanium Oxide and in-situ Electrolysis in Molten CaCl2 RYOSUKE O. SUZUKI, KOH TERANUMA, and KATSUTOSHI ONO A concept for calciothermic direct reduction of titanium dioxide in molten CaCl2 is proposed and experimentally tested. This production process consists of a single cell, where both the thermochemical reaction of the calciothermic reduction and the electrochemical reaction for recovery of the reducing agent, Ca, coexist in the same molten CaCl2 bath. A few molar percentages of Ca dissolve in the melt, which gives the media a strong reducing power. Using a carbon anode and a Ti basket-type cathode in which anatase-type TiO2 powder was filled, a metallic titanium sponge containing 2000 ppm oxygen was produced after 10.8 ks at 1173 K in the CaCl2 bath. The optimum concentration of CaO in the molten CaCl2 was 0.5 to 1 mol pct, to shorten the operating time and to achieve a lower oxygen content in Ti.
I. INTRODUCTION
THE Kroll process produces metallic titanium industrially. It consists of a three-step operation: the conversion from TiO2 to TiCl4, the subsequent reduction of TiCl4 to sponge Ti by liquid Mg, and the electrochemical recycling of MgCl2 into metallic Mg.[1] It takes 2 to 5 days in this reduction route via TiCl4, and the batch operation makes it difficult to save thermal energy. A simpler, more rapid, and compact process in a single step directly from TiO2 has been desired to achieve higher productivity and energy savings.[2–8] Recently, we proposed briefly the possible routes to go from TiO2 directly to metallic Ti, using calcium as a reductant and CaCl2 as the supporting media.[4–8] The purpose of this article is to report our concept in detail and to show some experimental verification. II. CONCEPT A. Thermodynamic Requirements The possible elements that are candidates as reductants (R) of TiO2 should meet several requirements. First, the oxides of the reductants (RO) should be thermodynamically more stable than the lowest oxide of titanium, TiO. TiO + R = Ti + RO
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The third criterion is the deoxidation capacity. For example, Mg equilibrates thermodynamically with Ti containing about 2 to 3 mass pct oxygen,[11,12] and a subsequent deoxidation procedure is needed after reduction with Mg. In addition, the acid leaching of MgO was too slow for practical mass production.[13] Rare-earth elements are not suitable as reductants if they cannot be recycled inside the operating system. Radioactive or noxious elements are excluded as TiO2 reductants. Ca was selected as being applicable both for direct reduction of TiO2 and for deoxidation of Ti. The residual oxygen in Ti has been measured as 300 to 730 mass ppm, when Ca and CaO coexisted in equilibrium at 1173 to 1373 K.[11,12,14–18] This oxygen level is suitable for industrial purposes. The solubility of Ca in β-Ti is also as low as 50 to 200 ppm at 1155 to 1600 K.[19] Alexander first patented the reduction of TiO2 using Ca in 1936.[20] However, ductile titanium could not be produced,[13] because a few thousand parts per mill
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