钛在氟氯熔盐中的电化学研究

0021-891X/88 $03.00 + .12 9 1988 Chapman and Hall Ltd.
ELECTROCHEMICAL STUDIES OF TITANIUM IN FLUORIDE-CHLORIDE MOLTEN SALTS
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JOURNAL OF APPLIED ELECTROCHEMISTRY 18 (1988) 80-85
Electrochemical studies of titanium in fluoride-chloride molten salts
CHEN GUANG-SEN, MASAZUMI OKIDO, TAKEO OKI
Department of Metallurgy and Iron and Steel Engineering, Faculty of Engineering, Nagoya University, Nagoya, Japan
Received 6 May 1987; revised 7 July 1987
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Fig. 2. Effects of K F concentration on the weight changes of Ti specimens immersed in K C I - N a C I - K F melts (30g) with K2TiF6: (5) 3 wt %; (2) 10 wt % for 3 h. The dotted lines present the theoretical Ti weight changes deduced from reactions: Ti 4+ + Ti = 2Ti 2+ [(3) 3 w t % K2TiF6]; 3Ti4+ + Ti = 4Ti 3+ [(I) 10wt% K2TiF6; (4)3 wt % K2TiF6].
1. Introduction The electroreduction mechanism of Ti 3+ ions in molten alkali chlorides has previously been studied by several workers [1-6]. Two reduction steps, i.e. Ti 3+ + e = Ti 2+ and Ti 2+ + 2e = Ti, have been observed. The behaviour of Ti4+(K2TiF6) in fluoride or chloride-fluoride molten salts has also been investigated in the literature [7-11]. It has been proposed that the reduction of Ti 4+ ions in the molten salts proceeded in a two step process: i.e. TP + + e = Ti 3+ , Ti 3+ + 3e = Ti. No existence of the intermediate reaction involving Ti 2+ was observed. In our previous paper [12], the reaction between titanium metal and Ti ions in equimolar KC1-NaC1 melts with TiCI3 or K2TiF6 at 973 K was investigated. It was found that the reaction in the molten melts with TiC13 proceeded according to 2Ti 3+ + Ti = 3Ti 2+. However, the reaction was more complex in the bath with K2TiF6 than that in the case of TIC13. The reaction was Ti 4+ § Ti = 2Ti 2+, in KC1-NaC1 melts with the concentration of less than 3 wt % K2TiF6 and 3Ti4+ § Ti = 4Ti 3+ in the case of the concentration being higher than 9wt %. The reaction of Ti in chloride-fluoride melts was affected by fluoride ions. In this paper, we describe the effects of fluoride ions on the reaction of Ti and the electroreduction behaviour of Ti 4+ (K2TiF6) with addition of K F to KC1-NaC13 wt % or 10 wt % KzTiF6 melts by means of electrochemical and physical measurements. 2. Experimental details The same experimental cell as in our previous paper 80
3.1. The 77 weight changes associated with reaction between Ti and Ti 4+ ions in the molten salts
Fig. 1 presents the time dependence of the weight changes of Ti plates in KC1-NaC1-3wt% (0.9g) K2TiF6 molten salts with K F (0 or 23.3 wt %) at 973 K in an Ar protective atmosphere. The dashed lines represented the theoretical Ti weight changes according to various possible reactions. From this figure, it can be seen that the Ti weight changes increased with time initially. After about 3h, a steady state was
[12] was used. All experiments were performed in a protective purified Ar atmosphere at 973 K. The cathode used in these experiments was a Ti or a Pt electrode with a diameter of 1.6mm or 0.5mm. The counter electrode was a Pt wire with a much larger surface area. The reference electrode was Ag/AgCI(0.1 N) in an equimolar KC1-NaC1 melt which was placed in a very thin mullite tube (~4 ram). This kind of reference electrode gave good stability and reproducible behaviour. The Ti plates of dimensions 10 x 10 x 2 m m were immersed in the molten salts for Ti weight change measurements. An equimolar KC1-NaC1 mixture was heated to its melting point, then cooled under a highly pure argon atmosphere to eliminate the moisture. The equimolar KC1-NaC1 salt prepared in this way was chosen as the solvent. High purity K2TiF6 was used for the solute. K F was added to the bath to study the effects of fluoride ions on the reaction between Ti metal and Ti 4+ (K2TiF6) and the electroreduction mechanism of Ti 4+ in the molten salts. 3. Results and discussion
T h e i n t e r a c t i o n b e t w e e n t i t a n i u m a n d Ti 4+ ions (K2TiF6), the e l e c t r o r e d u c t i o n r e a c t i o n o f Ti 4+ ions a n d the a n o d i c r e a c t i o n o f Ti in K C 1 - N a C 1 - K F melts with K2TiF6 at 973 K were studied b y m e a n s o f e l e c t r o c h e m i c a l a n d physical m e a s u r e m e n t s . It was f o u n d t h a t the fluoride ions p l a y e d a very i m p o r t a n t role in these r e a c t i o n s In K C 1 - N a C 1 - 3 wt % K2TiF6 m o l t e n salts with less t h a n 3 wt % K F , the i n t e r a c t i o n r e a c t i o n was c o n s i d e r e d to p r o c e e d as Ti 4+ + Ti = 2Ti 2+. I f the b a t h c o n t a i n e d m o r e t h a n 1 0 w t % K F , the r e a c t i o n 3Ti 4+ + Ti -- 4Ti 3+ o c c u r r e d . T h e e l e c t r o c h e m i c a l r e d u c t i o n o f Ti 4+ (KzTiF6) ions in the m o l t e n salts with tess fluoride ions was o b s e r v e d to p r o c e e d a c c o r d i n g to three r e a c t i o n steps, i.e. Ti 4+ + e = Ti 3+, Ti 3+ + e = Ti 2+, Ti 2+ § 2e = Ti. In the case o f the fluoride ion c o n c e n t r a t i o n being higher, two r e d u c t i o n steps, i.e. Ti 4+ + e = Ti 3+, Ti 3+ + 3e = Ti were suggested.