Understanding the Magnesiothermic Reduction Mechanism of TiO 2 to Produce Ti
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INTRODUCTION
METALLOTHERMIC reduction exploits the direct oxygen anion exchange between an oxide feedstock and reducing agent under a thermally activated environment.[1] Several special and widely used metallic elements and alloys including Nb, Mn, Cr, V, Zr, Nd, and FeV is obtained from metallothermic reduction.[2] The exothermic energy provided by the reaction allows for a self-reducing system once sufficient activation energy is supplied for the initial reaction to occur. Typical reducing agents used in the metallothermic process include Ca, Al, Mg, and Si or a combination of these elements. In particular, Mg has a very low solubility in Ti, which makes this reductant a candidate when implementing a metallothermic reduction process.[3,4] With respect to the production of the highly valued Ti, several novel methods have been proposed to replace the energy-consuming and unfavorable productivity of the existing Kroll process.[5,6] FFC (Fray-FarthingChen) Cambridge process,[7,8] molten oxide electrolysis,[9–12] and TiRO processes[13] are representative examples. Nevertheless, the process dynamics are difficult to control and the by-products used as medium for cation and anion exchange during the reduction require KYUNSUK CHOI and IL SOHN are with the Materials Science and Engineering, Yonsei University, Seoul, 120-749, Korea. Contact e-mail: [email protected] HANSHIN CHOI is with the Advanced Materials and Processing R&D Group, Korea Institute of Industrial Technology, Incheon, 406-840, Korea. Manuscript submitted October 16, 2016. Article published online January 18, 2017. 922—VOLUME 48B, APRIL 2017
separate disposal. In addition, the typical electrodes used within these processes are costly and difficult to obtain. According to the thermodynamic assessment of magnesiothermic reduction of TiO2 using existing thermochemical software FactSage 7.0[14] with the FToxid and FTlite databases, the overall forward reaction expressed as Eq. [1] is spontaneous and favorable. The melting point of Mg is above 923 K (650 C) and process temperatures above this temperature indicate that reduction to Ti is spontaneous. On the other hand, the intermediate oxide phases such as Ti6O, Ti2O, MgTi2O4, which are known to form during reduction, are not predicted due to a lack of thermodynamic data. Empirical experiments through lab-scale verifications have shown that metallic Ti can be obtained.[15] Yet, the soluble oxygen existing in the reduced Ti via the magnesiothermic reduction was not low enough for industrial applications.[16,17] According to the Ti-O binary phase diagram,[18,19] the oxygen content in Ti as a result of the Mg/MgO equilibrium is approximately 2.6 mass pct, and limiting the oxygen content in Ti below 1 mass pct would be difficult at higher MgO activity. It may be possible to lower the activity if the activity coefficient of MgO could be lowered with additional fluxes, but this may also impact the Mg activity and inhibit the forward reaction of the magnesiothermic reduction of TiO2. Thus, the current limitations of recently pro
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