Titanium metal and its alloys are popular structural and functional materials due to their excellent properties. Even though titanium element is abundant in the earth, its extraction is very difficult because of its strong chemical affinity to other metals and interstitial elements including O, N, H, C, especially oxygen. The plasticity of titanium metal is sensitive to even trace oxygen content. It has been well acknowledged that it is extremely challenging to produce titanium metal with low enough oxygen. The currently commercialized method to produce Ti metal is the Kroll process, which has been optimized for years and highly matured, but it is costly and energy-intensive. In order to reduce the product cost of titanium metal and thus widen its applications, many new methods have been proposed in recent decades, which can be categorized into two main groups, thermochemical methods and electrochemical methods. This article made a mini-review of the developed thermochemical methods from the aspects of technical features and research status, including the Kroll process, the Hunter process, the ADMA process, the TiRO process, the gas reduction process, the CSIR-Ti process, the ITP-Armstrong process, and the ARC process which using TiCl4 as the precursor, the preform reduction process (PRP), molten salt facilitated Ca reduction process, electronically mediated reaction (EMR) process, Mg reduction?Ca deoxygenation two-step process, and the hydrogen assisted magnesiothermic reduction (HAMR) process which using TiO2 as the precursor, and thermochemical reduction of fluorotitanate. The common reductants are reactive metals and their alloys, including Ca, Mg, Al, and Na. These processes are still at the laboratory or pilot-plant stage, and it is recognized that the potential of industrialization of these methods is different, thus an uncertain time is needed to verify if these methods have advantages over the Kroll process on quality and production cost.
[1] Lütjering G , Williams J C. Titanium[M]. Springer Berlin Heidelberg, 2007.
[2] Lu K. The future of metals[J]. Science, 2010. 328(5976): 319-320.
[3] Reed T B. Free energy of formation of binary compounds: an atlas of charts for high-temperature chemical calculations[J]. Joural of the Electrochemical Society, 1971, 119(12).
[4] Kroll W. The production of ductile titanium[J]. Tr.electrochem.soc, 1940, 78(1Pt1): L175.
[5] Hunter M A. Metallic titanium[J]. J.am.chem.soc, 1910, 32(3):330-336.
[6] Mo W. Titanium metallurgy. 1998: Metallurgical Industry Press.
[7] Seetharaman S. Treatise on process metallurgy[M], Volume 3: Industrial Processes. Vol. 3. 2013: Newnes.
[8] Kasparov S A, Klevtsov A G, Cheprasov A I, Moxson V S, Duz V A. Semi-continuous magnesium-hydrogen reduction process for manufacturing of hydrogenated, purified titanium powder: US 8007562 B2[P], 2011-08-30.
[9] Doblin C, Chryss A, Monch A. Titanium powder from the TiROTM process[J]. Key Engineering Materials, 2012, 520: 95-100.
[10] Hansen D A, Gerdemann S J. Producing titanium powder by continuous vapor-phase reduction[J]. JOM, 1998, 50(11): 56-58.
[11] Vuuren D V, Oosthuizen S J, Heydenrych M D. Titanium production via metallothermic reduction of TiCl4 in molten salt: problems and products[J]. The journal of the southern african instiute of mining and metallurgy, 2011, 111: 141-148.
[12] Crowley G. How to extract low-cost titanium: a new process for titanium extraction and production promises to cut costs and expand applications[J]. Advanced materials & processes, 2003, 161(11): 25-27.
[13] Chen W, Yamamoto Y, Peter W H. Investigation of pressing and sintering processes of CP-Ti powder made by armstrong process[J]. Key Engineering Materials, 2010, 436: 123-130.
[14] Gerdemann S J, Oden L L, White J C. Continuous production of titanium powder[J], in Proc. 1997 Materials Week 'Titanium extraction and processing', ed. by B. Mishra, G.J. Kipouros (TMS, Indianapolis, IN, 1997), pp. 49-54.
[15] Bordbar H, Yousefi A A, Abedini H. Production of titanium tetrachloride (TiCl4) from titanium ores: a review. Polyolefins Journal, 2017. 4(2): 149-173.
[16] Coughlin J P. Contributions to the data on theoretical metallurgy. XII. Heats and free energies of formation of inorganic oxides[J]. US Government Printing Office, 1954, 542.
[17] Mah A D, et al. Thermodynamic properties of titanium-oxygen solutions and compounds. 1955, Bureau of Mines.
[18] Okabe T H, Oda T, Mitsuda Y. Titanium powder production by preform reduction process (PRP)[J]. Journal of Alloys & Compounds, 2004, 364(1): 156-163.
[19] Suzuki R O, Inoue S. Calciothermic reduction of titanium oxide in molten CaCl2[J]. Metallurgical & Materials Transactions B, 2003, 34(3): 277-285.
[20] Park I, Abiko T, Okabe T H. Production of titanium powder directly from TiO2 in CaCl2 through an electronically mediated reaction (EMR)[J]. Journal of Physics and Chemistry of Solids, 2005, 66(2): 410-413.
[21] Henrie T A, Henry D, Kleespies E K. Magnesium reduction of titanium oxides in a hydrogen atmosphere: US 3140170 A[P], 1964-07-07.
[22] Nersisyan H H, Lee J H, Won C W. Combustion of TiO2-Mg and TiO2-Mg-C systems in the presence of NaCl to synthesize nanocrystalline Ti and TiC powders[J]. Materials Research Bulletin, 2003, 38(7): 1135-1146.
[23] Eshed M, Irzh A, Gedanken A. Reduction of titanium dioxide to metallic titanium conducted under the autogenic pressure of the reactants[J]. Inorganic Chemistry, 2009, 48(15): 7066.
[24] Won C W, Nersisyan H H, Won H I. Titanium powder prepared by a rapid exothermic reaction[J]. Chemical Engineering Journal, 2010, 157(1): 270-275.
[25] Zhang Y, Fang Z Z, Xia Y, et al. A novel chemical pathway for energy efficient production of Ti metal from upgraded titanium slag[J]. Chemical Engineering Journal, 2016, 286: 517-527.
[26] Fisher R L. Deoxidation of titanium and similar metals using a deoxidant in a molten metal carrier: US 4923531[P], 1990- 05-08.
[27] Suzuki R O, Saguchi A, Takahashi W, et al. Recycling and high performance waste processing. Recycling of rare earth magnet scraps: Part II. Oxygen removal by calcium[J]. Materials Transactions Jim, 2001, 42(12): 2492-2498.
[28] Okabe T H, Oishi T, Ono K. Preparation and characterization of extra-low-oxygen titanium[J]. Journal of Alloys & Compounds, 1992, 184(1): 43–56.
[29] Oh J M, Lee B K, Suh C Y, et al. Deoxidation of Ti powder and preparation of Ti ingot with low oxygen concentration[J]. Materials Transactions, 2012, 53(6): 1075-1077.
[30] Xia Y, Fang Z Z, Sun P, et al. The effect of molten salt on oxygen removal from titanium and its alloys using calcium[J]. Journal of Materials Science, 2017, 52(7): 4120-4128.
[31] Zhang Y, Fang Z Z, Sun P, et al. Thermodynamic destabilization of Ti-O Solid Solution by H2 and deoxygenation of Ti using Mg[J]. Journal of the American Chemical Society, 2016, 138(22): 6916-6919.
[32] Xia Y, Fang Z Z, Zhang Y, et al. Hydrogen assisted magnesiothermic reduction (HAMR) of commercial TiO2 to produce titanium powder with controlled morphology and particle size[J]. Materials Transactions, 2017, 58(3).
[33] Zhang Y, Fang Z Z, Sun P, et al. Kinetically enhanced metallothermic redox of TiO2 by Mg in molten salt[J]. Chemical Engineering Journal, 2017, 327: 169-182.
[34] Jonas K, New York, N. Y. Cyclic process for the manufacture of titanium-aluminum alloys and regeneration of intermediates thereof: US 8532555A[P], 1958-06-03.
[35] 王武育, 氟盐铝热还原法制取海绵钛的研究[J]. 稀有金属, 1996(3):169-171.
Wang W, Study on extraction of titanium sponge by thermal-reduction method of fluorine salts [J]. Rare metals, 1996(3):169-171.
[36] 冯乃祥，赵坤，王耀武，等. 两段铝热还原制取钛或钛铝合金并副产无钛冰晶石的方法:WO 2017012185 A1[P].2017-01-26.
Feng N, Zhao K, Wang Y, et al. Preparation of titanium or titanium alloy by thermal-reduction in two-stage aluminum :WO 2017012185 A1[P].2017-01-26
[37] Zhao K, Feng N, Wang Y. Fabrication of Ti-Al intermetallics by a two-stage aluminothermic reduction process using Na2TiF6[J]. Intermetallics, 2017, 85: 56-162.
