Electrochemical Behavior Of Titanium Ions In Alkali Chlorides

The fused salt electrochemistry is now used for producing and electrorefinning titanium. In contrast to other metals prepared by electrolysis at high temperatures such as alkali metals, magnesium or aluminum, the titanium ion exists in a variety of oxidation states. Therefore, it is complexes for the equilibrium research of titanium ions in the alkali chlorides. Literatures disclosed that the composition of the solvent have dramatic influence on the equilibrium of titanium ions in fused electrolytes. The stability of the various oxidation states depends on the bath composition. There are two steps for the electrodeposition:from Ti3+to Ti2+and Ti+to Ti. However, there is one step for the electrodeposition in the alkali chlorides that containing high concentration of fluoride ion. Beside the fluoride ions, alkali cations also have influence on the equilibrium of titanium ions. By now, there are still lots of work need be done on the quantitative research of titanium ions in alkali chlorides. The present paper is devoted to the study of the electrochemistry properties of solutions of titanium ions in fused alkali chlorides.The core work includes four parts:firstly, the behavior of titanium ion equilibrium in different alkali chlorides were studied which obtained precise equilibrium constant; Secondly, the influence of fluoride ion on the equilibrium of titanium ions were studied in NaCl-KCl melt and the complexation equilibrium constant were obtained by the best-fitting method; Thirdly, the electrochemical behavior of titanium ions were studied in the molten salt by the use of electrochemical tests in different alkali chlorides; and fourthly, the electrochemical behavior of titanium ions in different concentrations of fluoride ions were studied by electrochemical test methods.Firstly, the equilibrium constant, Kc, of the reaction3Ti2+=2Ti3++Ti was studied in the molten binary mixtures of LiCl. The accurate values of the Kc were calculated by the best-fitting method under consideration of the TiOCl(s) dissolution at temperature1023K. The influence of the cation radius of the melt on the equilibrium constant was studied in molten binary mixtures of LiCl on the basis of the reliable method for obtaining accurate value of Kc. It was found that the Kc values decreased with the concentration increasing of LiCl. More importantly, results disclosed that the constant values of polarizing power corresponds with Kc no matter in CsCl-LiCl, KCl-LiCl or NaCl-LiCl melt.Secondly, a series of experiments were employed to explore the influence of fluoride on the equilibrium of3Ti2+=2Ti3+-Ti. The balance transfers to the right direction when fluoride added in the NaCl-KCl melt. The reasons are mainly because of the coordination compounds formation (TiCl63"+iF-=TiCl(6-i)Fi3-+6Cl-). The formation equilibrium constants of the complexes TiCl(6-i)Fi3-were evaluated by best-fitting method. Furthermore, the values of coordination compound formation equilibrium constant for the reaction TiCl62-+6F-=TiF62-+6Cl-was obtained, then the Kc2for the reaction4Ti3+=3Ti4++Ti.Thirdly, the redox potentials were determined from dynamical electrochemical techniques such as cyclic voltammetry, square wave voltammetry and chronopotentiometry in alkali chlorides mixtures, LiCl, KCl-LiCl, NaCl-CsCl and CsCl. Our aim here is to use transient electrochemical techniques to clarify the reaction scheme associated with the reduction of TiCl3in a fused mixture. Results disclosed that the the redox potentials (ETi4+/Ti3+, ETi3+/Ti2+and ETi2+/Ti transfer to the negative position when the cation radius of the melt increased. There are two steps for TiCl3reducing in LiCI melt (Polaring power1.67) which are Ti3+to Ti2+and Ti2+to Ti. However, There is one step for TiCl3reducing in CsCl melt (Polaring power0.36) which is Ti3+to Ti directly.Fourthly, the electrochemical behaviour of titanium ions in NaCl-KCl melt containing different concentration of fluoride ions. The redox potentials were determined from dynamical electrochemical techniques such as cyclic voltammetry square wave voltammetry and chronopotentiometry. The redox potentials changed to the negative direction when the the [F’]/[Tin+] ratio increased. The reduction potential dimmed for Ti3+/Ti2+when the [F-/[Tin+] ratio is2.0and even disappeared when [F’]/[Tin+] ratio is3.0. There is one step for TiCl3electrodepostion when [F-]/[Tin+] ratio is4.0in NaCl-KCl melt.