The Cathodic Behavior of Ti(III) Ion in a NaCl-2CsCl Melt
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INTRODUCTION
TITANIUM and its alloys have attracted great interests for their desirable properties such as high strength, low density, excellent flexibility, strong springback characteristics, corrosion resistance, and biocompatibility. However, its use has been restricted because of its high processing cost of the traditional Kroll process. To extract the titanium and its alloys in a simpler and lower cost way, attractions have been drawn on molten salt electrolysis process, such as FFC-Cambridge process,[1] OS process,[2,3] molten oxide electrolysis (MOE),[4] and the electrolysis of titanium oxycarbides (the USTB process[5–7]). The electrochemistry of titanium subchlorides in molten salts and the deposition of metallic titanium from titanium chlorides have been the subject for many investigators.[8–12] The electrowinning of titanium metal has been realized by dissolving titanium chlorides (TiCl4, TiCl3) into chlorides, fluorides, and mixtures of chlorides and fluorides. In all-fluoride melts, especially the LiF-NaF-KF eutectic melt, a two-step reduction of Ti(IV) to titanium metal, Ti(IV) fi Ti(III) fi Ti, is known to proceed, which means the presence of fluorides leads to the stabilization of Ti(III), and the deposition of titanium may occur directly from a trivalent species.[13] Many investigations of the electrochemistry have also been involved in pure chloride melts. For example, in molten LiCl-KCl or NaCl-KCl, the reduction of Ti(IV) follows the steps of Ti(IV) fi Ti(III) fi Ti(II) fi Ti,[14–16] which are more complicated. Despite the extensive investigations, as well as the advantages in economics and product quality, the YANG SONG, Postgraduate, SHUQIANG JIAO and ZHANCHENG GUO Professors, and LIWEN HU, PhD Student, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mails: [email protected]; [email protected] Manuscript submitted January 30, 2015. Article published online December 18, 2015. 804—VOLUME 47B, FEBRUARY 2016
electrolysis of titanium from chloride melts has not yet been commercialized as the different valencies of titanium in pure chloride electrolyte may undergo reoxidation and disproportionation reactions, which results in a very low current density. Therefore, a better understanding of the equilibria between the different oxidation states of titanium in the chloride molten salts is of great significance. The previous work has found that Ti(III) is stable in molten CsCl at 973 K (700 C) but the metal deposition takes place via Ti(III) fi Ti(II) fi Ti.[17] However, when the temperature is lowered to 673 K (400 C), the Ti(III) is reduced to titanium directly. Although the preliminary work investigated the effects of temperature for the stability of trivalent titanium ions and indicated that in alkali chloride baths the stability of valence states Ti(IV) and Ti(III) of titanium increases with the radius of the cations in the melts,[18,19] the kinetic parameters for the charge transfer steps and the mechanism of electrodep
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