Effect of fluoride ions on coordination structure of titanium in molten NaCl–KCl

Abstract: 

The effects of fluoride ions (F⁻ ) on the electrochemical behavior and coordination properties of titanium ions (Tiⁿ⁺ ) were studied in this work, by combining electrochemical and mathematical analysis as well as spectral techniques. The α was taken as a factor to indicate the molar concentration ratio of F⁻ and Tiⁿ⁺ . Cyclic voltammetry (CV), square wave voltammetry (SWV), and open circuit potential method (OCP) were used to study the electrochemical behavior of titanium ions under conditions of various α, and in-situ sampler was used to prepare molten salt samples when α equal to 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 8.0. And then, samples were analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed that F⁻ in molten salt can reduce the reduction steps of titanium ions and greatly affects the proportion of valence titanium ions which making the high-valence titanium content increased and more stable. Ti²⁺ cannot be detected in the molten salt when α is higher than 3.0 and finally transferred to titanium ions with higher valence state. Investigation revealed that the mechanism behind those phenomenon is the coordination compounds（TiCl_jF ^m- ） forming.

Introduction ：

High purity titanium has excellent properties, such as light weight, high corrosion resistance, high melting point, low resistivity [1–3]. It is mainly used in large-scale integrated circuit manufacturing of high-end new titanium alloy and other industries [4–6]. It is a necessary strategic critical material for electrical and electronics, and aerospace. Among many preparation technologies of high purity titanium, molten salt electrolysis, as a simple process and easy to realize continuous process, has a very broad application prospect and attracts much attention. Considering the electro-refining process, firstly, it is necessary to prepare molten salt containing low valence titanium ions which uses TiCl₄ and high purity titanium [7], which is used as titanium ion source to add eutectic salt in electro-refining cell. Secondly, sponge titanium (95wt%–99wt%) is taken as anode and high purity titanium plate as cathode, and then the electrochemical dissolution of titanium and electrochemical deposition of high purity titanium occur at anode and at cathode, respectively. Finally, high purity titanium products with different morphology and quality are obtained under various electrolysis parameters.

Due to sundry oxidation states of titanium ions, the process of electrolytic production of titanium was complex and involves multi-step process, resulting in low current efficiency [8–11]. There were mainly three possible valence states of titanium ion in chloride melt, namely Ti²⁺, Ti³⁺, and Ti⁴⁺. It has been demonstrated by several researchers that equilibriums exist between metallic titanium and three ions states (Ti²⁺, Ti³⁺, and Ti⁴⁺) [11–15]: 

3Ti²⁺ ⇔ Ti+2Ti³⁺ (1) 

4Ti³⁺ ⇔ Ti+3Ti ⁴⁺ (2)  

The element of electrolyte has an important effect on the equilibrium of titanium ions in molten salt and the electrode-position process of titanium. All kinds of molten salt electrolytes including chlorine, fluoride, and chloro-fluoride have been studied in detail [16–17]. In molten chlorides, the reduction of Ti(IV) follows the steps of Ti(IV)→Ti(III), Ti(III)→Ti(II), and Ti(II)→Ti. However, there are some differences in electrochemical behavior in various chloride systems [18–20]. In the fluoride based molten salt, there is usually no intermediate reduction step involving Ti(II) ions. It means that the presence of fluoride causes Ti(III) to stabilize in the form of . Ti(III) is supposed to be reduced directly to the metal when the concentration of fluoride is high enough, and it will also involve an intermediate reduction step with a lower fluoride concentration [21–24]. In full-fluoride melts, the deposition of titanium may occur directly from trivalent species, and a two-step reduction for Ti(IV) to titanium metal: Ti(IV)→Ti(III) and Ti(III)→Ti [25]. 

Titanium free ions exist in Ti²⁺, Ti³⁺, and Ti⁴⁺ states in different molten salts, specifically, in anodic dissolution, and anions such as Cl− or F− will combined with the dissolved high-valence titanium ions to form a stable coordination compounds, which made the high-valence titanium ions exist stably, so as to affect the existence of different valence titanium ions in the molten salt. 

It has been studied that titanium ions exist in form of complexions in molten chloride salts, TiCl_i^m-. When a certain amount of fluoride ion is added to the electrolyte, titanium ions in intermediate valence states will undergo disproportionation reactions as shown in reactions (1) and (2), and the equilibrium of the reaction will be broken down. Fluoride ion has a smaller ionic radius compared with chloride, and it was more likely to form coordination relationship with cation in molten salt [26–30]. 

Therefore, titanium ions and fluoride ions will form co-ordination compounds: TiF_i^m-. The formation of these compounds changes the equilibrium of above disproportionation reactions. However, the specific forms and reaction mechanisms of these coordination compounds is not clear. In this research, we introduced fluoride ions into NaCl–KCl molten salts to in-depth study of the coordination mechanism of Ti–F ions. It is of great significance for revealing the mechanism of reduction process and electrolytes selecting.