Calciothermic reduction of titanium oxide in molten CaCl 2
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I.
INTRODUCTION
THE Kroll process is connnercially used at present in industry to produce metallic titanium. [11 It consists of twostep operation; the conversion from TiO2 to TIC14 and the subsequent reduction of TIC14 to sponge Ti by Mg liquid. This reduction route via TIC14 takes 2 to 5 days. A simpler and compacter process in a single step directly from TiO2 is desired to get the higher productivity. Alkaline earth metals such as Ca and Mg and some rare earth elements can be selected as the candidates for direct reductant of TiO2, both because they do not dissolve into Ti and because their oxides are thermodynamically more stable than the lowest titanium oxide, TiO. [21 The rare earth elements are expensive and not suitable as economic reductants. The Mg equilibrates thermodynamically with Ti containing about 2.5 mass pct oxygen, [3'41and the acid leaching of MgO is too slow for practical application. [51 Residual oxygen in Ti has been measured as 300 to 730 mass ppm O when Ca and CaO coexisted in equilibrium at 1173 to 1373 K , [3'4'6-101 This low oxygen level is applicable for industrial standards. The solubility of Ca in/~-Ti is also as low as 50 to 200 ppm Ca at 1155 to 1600 K , [111 Alexander proposed the reduction of TiO2 using Ca first in 1936, [121 however, Kroll reported that the ductile titanium could not be produced. [51It was because the by-product CaO becomes the obstacle for further reaction with Ca, and at least a few thousand ppm O remains in Ti mainly as CAO.[4,6 81 Molten CaC12 can dissolve about 20 mol pct C a O , [13'14'151 and the halide flux deoxidation of Ti showed that the residual oxygen in Ti could be extracted as CaO into GAG12 .[16'17'181 This technique has been successfully applied also to the deoxidation of N b , [19'201 g[211 and many rare earth metals. [22~23'241 When we apply tiffs dissolution mechanism to Ca reduction from TiO2, not from metallic bulk, it is expected that the by-product CaO can be removed in situ from the reaction place, and that tiffs dissolution enhances the reduction and RYOSUKE O. SUZUKI, Associate Professor, is with the Department of Energy Science and Technology, Kyoto University, Kyoto 606 8501, Japan. Contact e mail: [email protected] u.acdp SHUICHI INOUE, formerly Graduate Student in Master Com~e, Department of Energy Science and Technology, Kyoto University, is with the Osaka Gas Com pany, Osaka, 541 0046 Japan. Manuscript submitted May 10, 2002.
METALLURGICALAND MATERIALS TRANSACTIONS B
subsequent deoxidation more effectively. [2'15'2~2sl Considering the recycling of the by-product CaO to the reductant Ca, we proposed the process, as shown in Figure 1, that the dissolved CaO decomposes into Ca and CO2 gas by the molten salt electrolysis, and that this Ca returns to the reduction. [251 Both the electrolysis and reduction will be done simultaneously in the same reaction bath. [2,26,271This single unit operation can compensate the heat evolution of Ca reduction and the heat absorption of electrolysis. We believe that this process can save the energ
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