Electrochemically depositing titanium(III) ions at liquid tin in a NaCl–KCl melt

Abstract

The electrochemical behavior of titanium ions at a liquid tin cathode has been investigated by cyclic voltammetry and square wave voltammetry in a NaCl–KCl melt at 1023 K. The results show that the deposition potentials of alkali metals and titanium at liquid tin are more positive than those at a solid tungsten cathode. Meanwhile, the results prove that titanium(III) ions can be reduced at liquid tin with a one-step reduction, Ti³⁺ + 3e = Ti, which is a quasi-reversible process with diffusion-controlled mass transfer. The diffusion coefficient of titanium(III) ions is 1.05 × 10⁻⁵ cm² s⁻¹. Additionally, galvanostatic electrolysis has been carried out to clarify the effect of the current density on the cathodic products. The result demonstrates that a greater depth of titanium will be diffused into the liquid tin cathode during electrolysis with a lower current density.

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1. Introduction

Molten salt electrolysis has been widely used to extract most types of metals (for example, part of alkali,^1,2 alkali earth,^3,4 rate earth^5–8 and refractory metals^9,10) and alloys¹¹ because of its wide potential window and excellent ionic conductivity. Normally, a solid cathode, as a substrate for nucleating, growing and coarsening of atoms, is employed to deposit the metals or alloys in such processes. Besides, liquid cathodes are also used to realize the continuous production of metal (aluminum¹²) or alloys (Ca–Mg alloy¹³). Actually, the use of the liquid cathode is the most important technology in electrowinning or electrorefining of rare earth and actinide metals.^14–18 The investigations show that the deposition potentials of those elements at liquid cathodes are more positive than those at solid cathodes.¹⁴ Additionally, an alloy with uniform composition could be synthesized¹⁹ at liquid cathodes.

Generally, refractory metals, such as titanium, zirconium, hafnium, etc., are commercially produced by thermal reduction.²⁰ Recently, molten salt electrolysis has received more attention as it is more environmentally friendly and energy saving compared to thermal reduction.^21–23 Most of the electrolytic processes for titanium extraction apply a solid metal cathode as the substrate to deposit titanium.^22,23 However, some of the processes, including the FFC-Cambridge process,²¹ the OS process²⁴ and the SOM process,²⁵ utilize the raw material of titanium dioxide as the cathode to electrochemically produce metallic titanium.

The purpose of the present work is to investigate the cathodic process of titanium(III) ions at a liquid tin electrode in a NaCl–KCl melt, and to disclose the nature of the deposition of titanium at such a liquid cathode.

2. Experimental

The electrolyte consisted of an equimolar NaCl–KCl (reagent grade, Aladdin Industrial Corporation) melt. The electrolyte was pre-dried in an alumina crucible under vacuum by heating it from ambient temperature to 573 K, and then maintaining it at this temperature for 24 h. Afterwards, the cooled and pre-dried electrolyte was placed in a glove box and 45 mM TiCl₃ prepared beforehand was added, for electrochemical testing. The working temperature was measured by a thermocouple (protected by an alumina tube) inserted into the melt. All experiments were carried out in a sealed vessel under a dried argon atmosphere at 1023 K.

A three-electrode setup was applied to investigate the electrochemical behavior of the titanium ions. The working electrode, as shown in Fig. 1, was liquid tin stored in a quartz U-bent tube with a 4 mm inner diameter. For comparison, a solid tungsten wire and glass carbon with a 1 mm diameter were used as working electrodes as well. The counter electrode was a high purity graphite rod (99.9995%, Alfa Aesar) with a 6 mm diameter. The reference electrode was an Ag/AgCl electrode, attached to a silver wire with a 1 mm diameter contained in a Mullite tube and dipped into a solution of AgCl (4 wt%) in a NaCl–KCl melt. The reference electrode was calibrated with respect to the Cl₂/Cl⁻ electrode and all experiment potentials were recorded versus the Cl₂/Cl⁻ electrode potential.

Fig. 1 The photograph of the liquid tin working electrode.

The electroanalytical techniques, including linear polarization, cyclic voltammetry and square wave voltammetry, were performed under the same conditions (at 1023 K, under dried argon atmosphere) using a PAR Model 263 potentiostat/galvanostat communicated with a Powersuite software.

Galvanostatic electrolysis was carried out to deposit titanium onto the liquid cathode. The electrolyte used in the cathodic deposition consisted of an equimolar pre-dried NaCl–KCl melt with titanium(III) ions. The liquid tin (45 g) was placed at the bottom of the graphite crucible, with an internal lining of alumina tube (32 mm in diameter) as the cathode. Subsequently, the NaCl–KCl salt with titanium ions was put into the graphite crucible serving as the electrolyte. A titanium plate (28 mm in diameter) was employed as the anode. The distance between the titanium anode and the liquid tin cathode remained within 20 mm. Galvanostatic electrolysis was used to deposit the titanium at different cathodic current density (0.2 A cm⁻², 0.1 A cm⁻² and 0.05 A cm⁻²) at 1023 K. Characterization of the deposits was analyzed by Scanning Electron Microscopy (JSM-6701F) equipped with an EDS probe (Thermo NS7). X-ray diffraction (Model MAC, M21XVHF22) was adopted to explore the crystalline structure of the cathodic products.

3. Results and discussion

Electrochemical tests tend to require a high purity electrolytic cell. The dissolved tin ions at any case, from the electrode, could affect further electrochemical tests. Therefore, it is necessary to confirm the dissolution potential of the liquid tin. Fig. 2 demonstrates the linear polarization curves of the liquid tin, solid tungsten and glass carbon working electrodes in the NaCl–KCl melt without the addition of titanium ions. The results show that the electrochemical dissolution of liquid tin occurs at about −1.3 V (vs. Cl₂/Cl⁻), solid tungsten at about −0.2 V (vs. Cl₂/Cl⁻) and glass carbon at about 0 V (vs. Cl₂/Cl⁻). That indicates that the electrochemical dissolution of the liquid tin cathode is more negative than the solid electrodes. In order to prevent the dissolution of the liquid tin, the scan range of cyclic voltammetry hereinafter should be less than −1.3 V (vs. Cl₂/Cl⁻).

Fig. 2 The linear polarization curves of the liquid tin, solid tungsten wire and glass carbon working electrodes in a NaCl–KCl melt at 1023 K, scan rate: 100 mV s⁻¹.

The potential window of a blank NaCl–KCl melt has been obtained by a potential scan at the liquid tin and solid tungsten electrodes. The corresponding voltammograms are given in Fig. 3. The results show that the deposition potential of sodium at the solid tungsten electrode is about −3.0 V (vs. Cl₂/Cl⁻). It is clear that the deposition potential at the liquid tin electrode is about −2.0 V (vs. Cl₂/Cl⁻), which is far more positive than that at the solid tungsten electrode. It results from lowering of the sodium activity at the liquid tin cathode. The deposition potential is related to the activity of the element. Compared to the solid cathode, the metal element can be dissolved in the liquid cathode, which causes a change of the activity and deposition potential of the metal element at liquid cathode.

Fig. 3 The potential windows of the cyclic voltammogram at the liquid tin and solid tungsten electrodes in a blank NaCl–KCl melt, scan rate: 200 mV s⁻¹.

Fig. 4a shows the cyclic voltammogram at the solid tungsten electrode in the NaCl–KCl melt with 45 mM TiCl₃. It is noticed that there are two cathodic peaks (R₁ and R₂) which demonstrate a two-step reduction of Ti³⁺ to Ti²⁺ and then to Ti.²⁶ In comparison, the cyclic voltammogram obtained at the liquid tin electrode shown in Fig. 4(b) has only one cathodic peak (about −1.6 V vs. Cl₂/Cl⁻). This cathodic peak is likely to be the reduction of Ti³⁺ to Ti. The result indicates that the titanium(III) ions are liable to be directly reduced to titanium at the liquid cathode. Importantly, the disproportionation reaction of the Ti²⁺ at the cathode would be eliminated by using the liquid tin cathode. And that could be significant to improve the cathodic current efficiency of titanium deposition.²⁷

Fig. 4 The cyclic voltammograms at the solid tungsten electrode (a) and at the liquid tin electrode (b) in a black NaCl–KCl melt before and after dissolving 45 mM TiCl₃ , scan rate: 200 mV s⁻¹.

Fig. 5 is the cyclic voltammogram at the liquid tin and at the solid tungsten electrode in the NaCl–KCl melt with 45 mM TiCl₃ dissolved. It is clear that the deposition potential of the Ti³⁺ at the liquid electrode shifts to the positive side. This phenomenon can be explained by the changing of the activity of titanium ions and by the forming of the cathodic alloy, which is confirmed by the appearance of the alloy phase (Ti₆Sn₅) shown in the subsequent XRD pattern.

Fig. 5 Cyclic voltammograms at the liquid tin and solid tungsten electrodes in a NaCl–KCl melt with 45 mM TiCl₃ dissolved, scan rate: 200 mV s⁻¹.

In order to obtain the diffusion coefficient of the titanium(III) ions, cyclic voltammograms with different scan rates were performed and Fig. 6a shows the corresponding results. The peak potentials shift slowly to the negative side with an increasing of the scan rate, which demonstrates the characteristic of the quasi-reversible process. In such case, the peak current density (i_p) is related to the square root of the scan rate by eqn (1).^28,29

where i_p is the peak current density (A cm⁻²), n the number of exchange electrons, F the Faraday constant, C the bulk concentration of Ti³⁺ (mol cm⁻³), R the ideal gas constant, T the absolute temperature, D the diffusion coefficient (cm² s⁻¹) and ν the scan rate (V s⁻¹).

Fig. 6 (a) The cyclic voltammograms with different scan rates (100, 200, 300, 400 and 500 mV s⁻¹) at the liquid tin electrode in a NaCl–KCl melt with 45 mM TiCl₃ dissolved. The inset is the cyclic voltammograms with a different scan range. (b) The linear relationship between cathodic current density and square root of the scan rate.

Fig. 6(b) is the linear relationship between cathodic current density and square root of the scan rate. It is obvious that there is a good linear relationship, which indicates that the electrochemical process has been controlled by the diffusion of titanium(III) ions. According to eqn (1), the diffusion coefficient of the titanium(III) ions is calculated to be 1.05 × 10⁻⁵ cm² s⁻¹. This result is in the same order as the diffusion coefficient of titanium(II) ions reported by other literature.²⁸

Furthermore, an accurate square wave voltammetry has been performed to clear the cathodic process of the titanium ions at the liquid tin cathode. The corresponding results are given in Fig. 7(a), which confirms the one-step process taking place during Ti³⁺ reduction at the liquid cathode. Afterwards, the cathodic peaks on the square wave voltammograms obtained at different frequencies (shown in Fig. 7(a)) shift slowly to the negative side, which is consistent with the result of the cyclic voltammograms. Fig. 7(b) shows the number of electrons at different frequencies. It is clear that the exchanging electron number is around 3 even with varying frequency. It further indicates that this cathodic peak is characteristic of the Ti³⁺ one-step reduction to Ti.

Fig. 7 (a) Square wave voltammograms recorded at different frequencies (2, 5, 10 and 20 Hz) in a NaCl–KCl melt with 45 mM TiCl₃ dissolved and (b) the number of electrons exchanged that are calculated at different frequencies.

Galvanostatic electrolysis was used to deposit titanium onto the liquid tin cathode. Fig. 8 gives the cell potentials of the electrolytic cell recorded under different cathodic current densities. The results indicate that the deposition process could last the full 5 h period at the performed current densities (0.2, 0.1, 0.05 A cm⁻²). The theoretical bulk concentrations of Ti in the cathodic tin can be calculated by a Faraday reaction, which are about 9.62%, 5.04% and 2.60% at 0.2, 0.1 and 0.05 A cm⁻², respectively. It is also found that the voltage profile tested at 0.2 A cm⁻² is much different during 2.5–5.0 h, rising up to 2.0 V. It implies that the overpotential could be increased while the cathode is solidified due to a large amount of deposit on the surface at the large current density to form the intermetallics with high melting point. Fig. 9 shows the XRD patterns at the surface of the cathode obtained by galvanostatic electrolysis under different current densities for 5 h. It is clear that the main deposit is the intermetallic of the Ti₆Sn₅. However, some of the pure titanium could be found as well. It should be mentioned that the amount of pure titanium could be increased with an increase of the current density. Indeed, the product surface obtained at 0.2 A cm⁻² has been highlighted in Fig. 10 through EDS mapping and pointing tests. From elemental mapping results shown in Fig. 10a, it can found that the molar ratio of the titanium and tin at the surface of the product deposited at 0.2 A cm⁻² is 58.62 : 36.38, which demonstrates that there is a large amount of titanium deposited on the surface of the cathodic ingot. The pointing test of the EDS shown in Fig. 10b further indicates that there is pure titanium on the surface of the deposit. To disclose the nature of the phenomenon obtained by galvanostatic electrolysis, the phase diagram of the Ti–Sn system is given in Fig. 11. The phase diagram reveals that the solubility of titanium in liquid tin is about 16 mol% at a temperature of 1023 K. Therefore, it tends to form the high melting point alloy (Ti₆Sn₅) or even the elemental titanium when the concentration of the titanium in the liquid tin cathode is above 16 mol%.

Fig. 8 Cell potentials of the electrolytic cell recorded under different current densities.

Fig. 9 XRD patterns of the surface material deposited at the liquid tin electrode under different cathodic current densities (0.2, 0.1, 0.05 A cm⁻²) for 5 h.

Fig. 10 (a) SEM image, and elemental mapping of titanium and tin, (b) SEM image and an EDS scan at a point of the surface of the cathodic ingot under 0.2 A cm⁻².

Fig. 11 The phase diagram of the Ti–Sn system.

Fig. 12 shows the EDS scans of the longitudinal section of the cathodic tin ingot under different cathodic current density, the insets are SEM images of the cathodic tin ingot. It is clear that the lower the current density, the greater the depth at which the titanium will be diffused into the liquid tin cathode. Thus, the cathodic process of the titanium at the liquid tin can be assumed as follows. First, in the electric field, the titanium ions are transferred to the surface of the liquid tin electrode. Then, the titanium ions are reduced and dissolved into the liquid tin or they unite with the tin to form the compound (Ti₆Sn₅). Finally, the dissolved titanium or the compound of titanium and tin are transferred to the depths of the liquid tin. It is found that the last step is controlling, which has been clarified by the XRD (Fig. 9) and EDS results (Fig. 12). The diffusion rate of the titanium in the tin is less than the reduction rate of the titanium ions to titanium at high current density (0.2 A cm⁻²), which leads to the appearance of the higher melting point alloy, or even of the titanium, which acts as a solid shell, and prevents titanium from entering the liquid tin. Therefore, there are a large amount of Ti and Ti₆Sn₅ phases on the surface of the cathodic tin ingot at the current density of 0.2 A cm⁻².

Fig. 12 EDS line scans of the longitudinal section of the cathodic tin ingot under different cathodic current densities (0.2, 0.1, 0.05 A cm⁻²), the inset shows SEM images of those longitudinal sections.

4. Conclusions

In summary, some electroanalytical techniques, including linear polarization, cyclic voltammetry and square wave voltammetry, have been employed to study the electrochemical behavior of the titanium(III) ions at a liquid tin cathode in a NaCl–KCl melt. The results show that the deposition potentials of alkali metals or of titanium at a liquid tin cathode are more positive than those at a solid tungsten cathode. Meanwhile, the results prove that the cathodic process of titanium(III) ions at a liquid tin cathode is a one-step reduction of Ti³⁺ + 3e = Ti, which is a quasi-reversible process with diffusion-controlled mass transfer. The diffusion coefficient of titanium(III) ions is 1.05 × 10⁻⁵ cm² s⁻¹. In addition, galvanostatic electrolysis was carried out to investigate the effect of current density on cathodic deposition. The results indicate that the less the current density, the greater the depth at which the titanium will be diffused into the liquid tin cathode.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No.51322402) and the National High Technology Research and Development Program of China (863 Program, No. 2012AA062302).

References

1. M. E. Weeks and H. M. Leicester, J. Chem. Educ., 1956, 820–823.
2. T. Wallace, Chem. Ind., 1953, 34, 876–882.
3. K. J. Cathro, R. L. Deutscher and R. A. Sharma, J. Appl. Electrochem., 1997, 27, 404–413.
4. J. Zhang, C. Yan and F. Wang, Appl. Surf. Sci., 2009, 255, 4926–4932.
5. I. V. Zubeck, R. S. Feigelson, R. A. Huggins and P. A. Pettit, J. Cryst. Growth, 1976, 34, 85–91.
6. Y. Castrillejo, P. Fernández, J. Medina, P. Hernández and E. Barrado, Electrochim. Acta, 2011, 56, 8638–8644.
7. E. Stefanidaki, C. Hasiotis and C. Kontoyannis, Electrochim. Acta, 2001, 46, 2665–2670.
8. M. Iizuka, J. Electrochem. Soc., 1998, 145, 84–88.
9. G. Z. Chen, D. J. Fray and T. W. Farthing, Nature, 2000, 407, 361–364.
10. A. Girginov, T. Z. Tzvetkoff and M. Bojinov, J. Appl. Electrochem., 1995, 25, 993–1003.
11. R. D. Srivastava and R. C. Mukerjee, J. Appl. Electrochem., 1976, 6, 321–331.
12. F. C. Frary, J. Electrochem. Soc., 1948, 94, 31–40.
13. K. G. Claus, D. W. Schoppe and M. R. Earlam, U.S. Pat. 5,024,737, 1991, pp. 6–18.
14. M. Iizuka, K. Uozumi, T. Inoue, T. Iwai, O. Shirai and Y. Arai, J. Nucl. Mater., 2001, 299, 32–42.
15. K. Uozumi, M. Iizuka, T. Kato, T. Inoue, O. Shirai, T. Iwai and Y. Arai, J. Nucl. Mater., 2004, 325, 34–43.
16. O. Shirai, M. Iizuka, T. Iwai, Y. Suzuki and Y. Arai, J. Electroanal. Chem., 2000, 490, 31–36.
17. M. Matsumiya, R. Takagi and R. Fujita, J. Nucl. Sci. Technol., 1998, 35, 137–147.
18. Y. Castrillejo, M. R. Bermejo, P. D. Arocas, A. M. Martínez and E. Barrado, J. Electroanal. Chem., 2005, 579, 343–358.
19. T. Kato, T. Inoue, T. Iwai and Y. Arai, J. Nucl. Mater., 2006, 357, 105–114.
20. W. Kroll, Trans. Electrochem. Soc., 1940, 78, 35–47.
21. G. Z. Chen, D. J. Fray and T. W. Farthing, Nature, 2000, 407, 361–364.
22. S. Jiao and H. Zhu, J. Mater. Res., 2006, 21, 2172–2175.
23. J. R. Myron and P. Palmerton, U.S. Pat., No. 2 939 823, 1960.
24. R. O. Suzuki, J. Phys. Chem. Solids, 2005, 66, 461–465.
25. Z. Z. L. X. D. Weizhong and Z. Guozhi, Shanghai Met., 2005, 2, 011.
26. Q. Wang, Y. Li, S. Jiao and H. Zhu, Electrochem. Commun., 2013, 35, 135–138.
27. Q. Wang, J. Song, G. Hu, X. Zhu, J. Hou, S. Jiao and H. Zhu, Metall. Mater. Trans. B, 2013, 44, 906–913.
28. X. Ning, H. Åsheim, H. Ren, S. Jiao and H. Zhu, Metall. Mater. Trans. B, 2011, 42, 1181–1187.
29. T. Berzins and P. Delahay, J. Am. Chem. Soc., 1953, 75, 555–559.

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