Mechanisms of Hydrogen-Assisted Magnesiothermic Reduction of TiO 2
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TITANIUM (Ti) metal has long been valued as a highly useful metal for its high specific strength, high-temperature capability, corrosion resistance, and biocompatibility.[1,2] Although it is one of the more abundant elements in the earth’s crust, its application in most industries has been limited because of the high energy and high cost associated with refining it to pure metallic form.[3] This is primarily because of its high chemical affinity for oxygen. For decades, researchers have sought to find a less-expensive route for the production of Ti metal to replace the Kroll process,[4–6] where TiCl4 (made by chlorinating concentrated titania minerals) is reduced by molten magnesium (Mg). For a scale of a few tons of titanium per batch, the reduction process typically takes several days, and the energy-intensive vacuum distillation step takes several more days to minimize residual Mg and Cl contents in the Ti-sponge product.[7] Although many approaches have been investigated, so far none have been able to replace the Kroll process. A succinct review on the
HYRUM LEFLER, Z. ZAK FANG, PEI SUN, and YANG XIA are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] YING ZHANG is with the Department of Metallurgical Engineering, University of Utah and also with the Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. Contact e-mail: [email protected] Manuscript submitted December 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
characteristics of those different approaches was recently given by Zhang et al.[8] One method worth noting here is the FFC process,[9–11] where TiO2 (or other titania precursors) instead of TiCl4 is used as the feedstock material and is directly reduced via electrolysis in a molten calcium chloride electrolyte to produce Ti powder.[12–15] And another is the Armstrong process,[7,16] where TiCl4 is continuously reduced by Na to form a sponge-like Ti powder.[17,18] Each of these technologies, among many other R&D processes, has shown advantages and disadvantages, but none of these approaches to date have been able to compete with the Kroll process commercially with respect to both quality and cost.[19] Another general approach that has long held promise for lower-cost Ti metal production is to thermochemically reduce TiO2 directly using calcium (Ca) or Mg.[6,20] Direct TiO2 reduction involves many intermediate phases, as the Ti-O system is quite complex.[21] The use of Ca in this approach has been studied extensively and reported in literature.[22–26] And yet, to date, none of the calciothermic processes have been fully developed which is at least partially attributable to the high cost of Ca. Compared to Ca, Mg is more cost-effective, but is thermodynamically unable to produce Ti with sufficiently low oxygen content to meet industry standards,[25,27–29] even with significantly excess Mg.[30] The thermodynamic limit in oxygen removal by Mg ranges from 2 wt pct (at approx. 800 C) to 1 w
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