Thermodynamics on the Bi-Fe-Ti System and the Gibbs Energy of Bi 9 Ti 8
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TRODUCTION
A. New Smelting Process of Titanium
TITANIUM has been produced by the Kroll process[1] for more than 60 years. However, the productivity of the Kroll process is low, and a new smelting process of Ti is desired. Recently, we proposed a new process of Ti via Bi-Ti liquid alloy for fast titanium production.[2–4] At the first step, titanium tetrachloride (TiCl4) is reduced to Bi-Ti liquid alloy by Bi-Mg alloy at 900 °C. At the second step, pure Ti is obtained by electrorefining using the alloy for an anode at 500 °C to 700 °C,[4] or segregation of Ti and vacuum distillation.[3] In previous works, Bi-Ti alloy was produced by magnesiothermic reduction of TiCl4,[2,3] and Ti powder (Bi content: < 180 ppm) was obtained from Bi-Ti alloy by electrorefining in equimolar NaCl-KCl at 700 °C.[4] Alternatively, the Bi-Ti alloy was distilled in vacuum at 1000 °C.[2] In the present work, we investigated vessel material for the new process. Iron is one of the strong candidates because of its low solubility in liquid Bi, low cost, and good workability. Partial isothermal sections of the Bi-Fe-Ti system at 700 °C and 900 °C, therefore, were constructed to investigate the reactivity between Fe and Bi-Ti alloy. In addition, electromotive force (emf)
AKIHIRO KISHIMOTO and TETSUYA UDA are with the Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan. Contact email: [email protected] Manuscript submitted October 11, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
between Bi-20 mol pct Ti alloy and pure Ti in molten chlorides was measured to estimate the Gibbs energies of Bi-Ti liquid alloy, Bi9Ti8, and Bi2FeTi4. B. Information on Bi-Fe, Bi-Ti, Fe-Ti, and Bi-Fe-Ti Systems in the Literature The solubility of Fe in liquid Bi was measured at 602 °C to 902 °C (e.g., 127 ppm at 700 °C and 612 ppm at 900 °C),[5,6] and there is no intermetallic compound in the Bi-Fe system.[7] Based on the information, Boa et al. determined a Bi-Fe phase diagram by the CALPHAD method, as shown in Figure 1.[8] In the Bi-Ti system, the solubility of Ti in liquid Bi was measured below 700 °C by Weeks,[9] and a few Bi-Ti phase diagrams were proposed.[10–13] Recently, Maruyama et al. determined invariant reaction temperatures and the liquidus between 710 °C and 1123 °C by thermal analysis, as shown in Figure 2.[14] According to Figure 2, there are some intermetallic compounds such as Bi2Ti,[11,14–16] Bi9Ti8,[17] Bi2Ti3,[14] BiTi2,[18] and BiTi3.[19] Huang et al. estimated Gibbs energies of intermetallic compounds and solution phases by the CALPHAD method.[20] Alger tried to measure the activity of Ti in Bi-Ti alloys at 856 °C and 873 °C by emf measurement using pure Ti and Bi-Ti alloys (Ti content: 1.0 to 10.7 mol pct) held in alumina (Al2O3) tube.[21] However, large uncertainty was confirmed due to the contamination of alloys by Al2O3 and the fluctuations of potential of electrodes. The Fe-Ti system includes Fe2Ti and FeTi, and the enthalpies of formation of these compounds w
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