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Reprocessing of spent hydrogen absorbing alloys by using electrochemical techniques in molten salts
Journal of Physics and Chemistry of Solids 66 (2005) 439–442 www.elsevier.com/locate/jpcs
Reprocessing of spent hydrogen absorbing alloys by using electrochemical techniques in molten salts H. Matsuuraa,*, H. Numatab, R. Fujitac, H. Akatsukaa a
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan b Department of Metallurgy and Ceramic Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan c Nuclear Chemical System R & D Department, Power and Industrial Systems R & D Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan Accepted 4 June 2004
Abstract Rare earths separation from hydrogen absorbing alloys by using molten salt media is proposed. The procedure consists of the following three electrochemical techniques.
(1) Rare earths are anodically electro-dissolved into ionic melt bath from spent hydrogen absorbing alloys. (2) Rare earth cations are then concentrated near the anode area into the column by using the countercurrent electromigration method. (3) Rare earths are finally cathodically electro-deposited in metallic form. Several electrochemical measurements were studied in the molten salts containing rare earth chlorides condition, and the electrodeposition was tested to check the feasibility of processes (1) and (3). Rare earths could be separable anodically from hydrogen absorbing LaNi5 alloy by using electrochemical techniques. q 2004 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen absorbing alloys have been utilized as anode activating material in secondary batteries not only for portable electronic devices but also for electric vehicles. Currently, most of the hydrogen absorbing alloys in commercial base is categorized as AB5 (A: rare earths, B: Ni, Fe, etc.) type due to their lower production costs and better charge-discharge characteristics for repeated use. Rare earths are not ‘rare’ elements, however, 70% of these are produced at the localized areas, e.g. China, North America and Russia. For example, Japanese demands for rare earths were over 12,000 tons in 1996, and they still
* Corresponding author. Fax: C81 3 5734 3057. E-mail address: [email protected] (H. Matsuura). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.091
increase in recent years. Because the demands for rare earths will increase rapidly in the world, one should make an effort to recycle the metals from the batteries currently in use. Thus, we proposed one of the optional techniques of rare earths separation from hydrogen absorbing alloys by using molten salt media [1]. The procedure consists of the following three electrochemical techniques, schematically drawn in Fig. 1. (1) Rare earths are anodically electro-dissolved into ionic melt bath from spent hydrogen absorbing alloys. (2) Rare earth cations are then concentrated near the anode area into the column by using the countercurrent electromigration method. (3) Rare earths are finally cathodically electro-deposited in metallic form.
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Fig. 1. A schematic view of the concept: reprocessing of spent hydrogen absorbing alloys.
Depending on the concentration and current density, separation of each rare earth would be possible. The advantages of the suggested procedure are the compact system, simplicity of the process, and the additional possibility of reducing secondary wastes, resulting in reduction of reprocessing cost and energy consumption. The purpose of this study is several electrochemical measurements under different operating conditions have been performed to check the feasibility of processes (1) and (3).
Tables 1 and 2, respectively. The geometry of electrodes and a crucible is very similar to that previously reported [2].
3. Results and discussion
To avoid from contamination of moisture and oxygen, an electrochemical cell were installed in a dry argon circulated glove box (Miwa seisakusho Co. Ltd). Electrochemical measurements have been performed by using a PC controlled electrochemical analyzer (ALS). All anhydrous chlorides have been purchased in ampoules with dry argon atmosphere through AAPL and used for electrochemical measurements without any further treatments. As the first step, several electrochemical measurements have been performed to confirm the difference of electro-deposition potentials among rare earths. After that, electrolysis tests have been carried out by using electrochemical conditions previously optimized. Typical conditions for electrochemical measurement and electrolysis studies are listed in
Cyclic voltammograms (CV) of LaCl3, CeCl3 and NdCl3 in LiCl–KCl eutectic at 793 K are shown in Fig. 2. In all cathodic sweeps, shoulders are observed before electrodeposition of melt bath component. They correspond to electro-deposition of rare earths. Clear peaks are also identified in all anodic sweeps. These peaks are due to anodic dissolution of rare earths. With increasing scan rate, the potential of the cathodic shoulders become negatively larger, while the peak position of the anodic curves do not change much, as shown in previous reports [2,3]. It is conjectured that the cathodic reaction is quasi-reversible, while the anodic reaction is almost reversible. In the cathodic sweeps of CVs containing LaCl3, two shoulders were observed which correspond to absorption and normal peaks [2]. However, in CVs containing CeCl3, only one shoulder was appeared. The cathodic sweeps of CVs containing NdCl3 have more complicated feature due to the possible existence of Nd2C during electrochemical reactions. Electro-deposition and electro-dissolution potentials are clearly determined by differential pulse voltammetry (DPV). DPV obtained from the system containing rare
Table 1 Experimental condition for electrochemical study
Table 2 Experimental condition for electrolysis
Molten salt Temperature Counter electrode Working electrode Composition Scan rate Electrode potential
Molten salt Temperature Anode Cathode Composition Electrolytic pot. Transp. charge
2. Experimental
59 mol%LiCl–41 mol%KCl w250 g 793 K Glassy carbon (3 mmf!4 mm) Molybdenum (1.5 mmf!2 mm) La, Ce, Nd 0.5w/0 0.01–0.5 V/s 0 to K2.2 V vs. AgC(0.1w/0)/Ag
59 mol%LiCl–41 mol%KCl w250 g 793 K LaNi5 ingot (10 mm!w30 mm) Low carbon steel (8 mmf!w20 mm) La 5w/0 K1.90 V vs. AgC(0.1 w/0)/Ag 5300 C
H. Matsuura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 439–442
441
Fig. 2. Cyclic voltammograms for LaCl3(a), CeCl3(b), and NdCl3(c) in LiCl–KCl eutectic at 793 K. (Working electrode: Mo 1.5 mmf).
earths are shown in Fig. 3. These potentials of La and Ce are almost the same as those reported in several published papers [4] as well as listed in reference books [5]. However, the electro-deposition potential of Nd is slightly shifted to negative direction. This fact implies electro-deposition of Nd as metallic form seems to be rather difficult. This is presumably because the disproportionation would be possible in the melts containing neodymium. The disproportionation reaction of Nd have been also confirmed by using UV–Vis spectrophotometric technique [6]. The curves from the system containing both La and Ce show somewhat broader peaks than those of DPV for the single chloride. Selective separation of La and Ce is also possible by controlling the electrochemical condition precisely, e.g. pulsed electrolysis. We have tested the bulk-electrolysis of rare earths by using the prototype of an anode basket under constant voltage condition. The photos of cathode (a) and anode (b) after electrolysis are shown in Fig. 4. Rare earths are clearly electro-dissolved from LaNi 5 ingot alloy and electro-deposited in typical dendritic form at
a cathode. Preliminary chemical analysis on the deposited materials surface by X-ray fluorescence shows mostly lanthanum was deposited. Precise chemical characterization of these samples is going underway. For the next step, we intend to perform practical tests, i.e. by using meshed anode baskets, ionic melts in complicated composition, dirty (oxidized) materials and/or mixture with polymer binder, and the samples in powder form, etc. Furthermore, the optimization of electrodeposition cell construction and the estimation of energy consumption should be required for practical utilization.
4. Conclusion In order to develop the recycle processes of hydrogen absorbing alloys, we have investigated electrochemical behaviour of La, Ce and Nd in LiCl–KCl eutectic melts. In all kinds of tested rare earths, cathodic reactions are quasi-reversible. The possibility of the disproportionation
Fig. 3. Differential pulse voltammograms for LaCl3, CeCl3, NdCl3 and LaCl3CCeCl3 in molten LiCl–KCl eutectic at 793 K (cathodic sweep (a) and anodic sweep (b)).
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H. Matsuura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 439–442
reaction in NdCl3 containing melts is also confirmed. Bulk electrolysis tests have been performed for LaNi5 ingot by using electrodeposition potentials that already optimized at previous electrochemical measurements. Rare earths are separable by electrochemical technique from transition metal alloys such as LaNi5, although there have still several tasks from technical point of view.
Acknowledgements The authors are fully acknowledged to Dr Matsuzaki in technical assistance of our experiments. This study is financially supported by Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References
Fig. 4. Photographs of the cathode (a) and the anode (b) after electrolysis.
[1] H. Matsuura, The report of Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan in FY2000-2001. [2] R. Fujita, Y. Akai, Molten Salts 39 (1996) 112 (in Japanese). [3] R. Fujita, Y. Akai, J. Alloys Comp. 271–273 (1998) 563. [4] F. Lantelme, Y. Berghoute, J. Electrochem. Soc. 146 (1999) 4137. [5] J.A. Plambeck, Encyclopedia of Electrochemistry of the Elements, vol. 10, 1976 (and references therein). [6] T. Higashi, K. Minato, T. Ogawa, S. Miyamoto, Proceedings of 8th Japan–China Bilateral Conference on Molten Salts and Technology, 2000 p. 206.
Reprocessing of spent hydrogen absorbing alloys by using electrochemical techniques in molten salts H. Matsuuraa,*, H. Numatab, R. Fujitac, H. Akatsukaa a
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan b Department of Metallurgy and Ceramic Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan c Nuclear Chemical System R & D Department, Power and Industrial Systems R & D Center, Toshiba Corporation, 4-1, Ukishima-cho, Kawasaki-ku, Kawasaki 210-0862, Japan Accepted 4 June 2004
Abstract Rare earths separation from hydrogen absorbing alloys by using molten salt media is proposed. The procedure consists of the following three electrochemical techniques.
(1) Rare earths are anodically electro-dissolved into ionic melt bath from spent hydrogen absorbing alloys. (2) Rare earth cations are then concentrated near the anode area into the column by using the countercurrent electromigration method. (3) Rare earths are finally cathodically electro-deposited in metallic form. Several electrochemical measurements were studied in the molten salts containing rare earth chlorides condition, and the electrodeposition was tested to check the feasibility of processes (1) and (3). Rare earths could be separable anodically from hydrogen absorbing LaNi5 alloy by using electrochemical techniques. q 2004 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen absorbing alloys have been utilized as anode activating material in secondary batteries not only for portable electronic devices but also for electric vehicles. Currently, most of the hydrogen absorbing alloys in commercial base is categorized as AB5 (A: rare earths, B: Ni, Fe, etc.) type due to their lower production costs and better charge-discharge characteristics for repeated use. Rare earths are not ‘rare’ elements, however, 70% of these are produced at the localized areas, e.g. China, North America and Russia. For example, Japanese demands for rare earths were over 12,000 tons in 1996, and they still
* Corresponding author. Fax: C81 3 5734 3057. E-mail address: [email protected] (H. Matsuura). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.091
increase in recent years. Because the demands for rare earths will increase rapidly in the world, one should make an effort to recycle the metals from the batteries currently in use. Thus, we proposed one of the optional techniques of rare earths separation from hydrogen absorbing alloys by using molten salt media [1]. The procedure consists of the following three electrochemical techniques, schematically drawn in Fig. 1. (1) Rare earths are anodically electro-dissolved into ionic melt bath from spent hydrogen absorbing alloys. (2) Rare earth cations are then concentrated near the anode area into the column by using the countercurrent electromigration method. (3) Rare earths are finally cathodically electro-deposited in metallic form.
440
H. Matsuura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 439–442
Fig. 1. A schematic view of the concept: reprocessing of spent hydrogen absorbing alloys.
Depending on the concentration and current density, separation of each rare earth would be possible. The advantages of the suggested procedure are the compact system, simplicity of the process, and the additional possibility of reducing secondary wastes, resulting in reduction of reprocessing cost and energy consumption. The purpose of this study is several electrochemical measurements under different operating conditions have been performed to check the feasibility of processes (1) and (3).
Tables 1 and 2, respectively. The geometry of electrodes and a crucible is very similar to that previously reported [2].
3. Results and discussion
To avoid from contamination of moisture and oxygen, an electrochemical cell were installed in a dry argon circulated glove box (Miwa seisakusho Co. Ltd). Electrochemical measurements have been performed by using a PC controlled electrochemical analyzer (ALS). All anhydrous chlorides have been purchased in ampoules with dry argon atmosphere through AAPL and used for electrochemical measurements without any further treatments. As the first step, several electrochemical measurements have been performed to confirm the difference of electro-deposition potentials among rare earths. After that, electrolysis tests have been carried out by using electrochemical conditions previously optimized. Typical conditions for electrochemical measurement and electrolysis studies are listed in
Cyclic voltammograms (CV) of LaCl3, CeCl3 and NdCl3 in LiCl–KCl eutectic at 793 K are shown in Fig. 2. In all cathodic sweeps, shoulders are observed before electrodeposition of melt bath component. They correspond to electro-deposition of rare earths. Clear peaks are also identified in all anodic sweeps. These peaks are due to anodic dissolution of rare earths. With increasing scan rate, the potential of the cathodic shoulders become negatively larger, while the peak position of the anodic curves do not change much, as shown in previous reports [2,3]. It is conjectured that the cathodic reaction is quasi-reversible, while the anodic reaction is almost reversible. In the cathodic sweeps of CVs containing LaCl3, two shoulders were observed which correspond to absorption and normal peaks [2]. However, in CVs containing CeCl3, only one shoulder was appeared. The cathodic sweeps of CVs containing NdCl3 have more complicated feature due to the possible existence of Nd2C during electrochemical reactions. Electro-deposition and electro-dissolution potentials are clearly determined by differential pulse voltammetry (DPV). DPV obtained from the system containing rare
Table 1 Experimental condition for electrochemical study
Table 2 Experimental condition for electrolysis
Molten salt Temperature Counter electrode Working electrode Composition Scan rate Electrode potential
Molten salt Temperature Anode Cathode Composition Electrolytic pot. Transp. charge
2. Experimental
59 mol%LiCl–41 mol%KCl w250 g 793 K Glassy carbon (3 mmf!4 mm) Molybdenum (1.5 mmf!2 mm) La, Ce, Nd 0.5w/0 0.01–0.5 V/s 0 to K2.2 V vs. AgC(0.1w/0)/Ag
59 mol%LiCl–41 mol%KCl w250 g 793 K LaNi5 ingot (10 mm!w30 mm) Low carbon steel (8 mmf!w20 mm) La 5w/0 K1.90 V vs. AgC(0.1 w/0)/Ag 5300 C
H. Matsuura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 439–442
441
Fig. 2. Cyclic voltammograms for LaCl3(a), CeCl3(b), and NdCl3(c) in LiCl–KCl eutectic at 793 K. (Working electrode: Mo 1.5 mmf).
earths are shown in Fig. 3. These potentials of La and Ce are almost the same as those reported in several published papers [4] as well as listed in reference books [5]. However, the electro-deposition potential of Nd is slightly shifted to negative direction. This fact implies electro-deposition of Nd as metallic form seems to be rather difficult. This is presumably because the disproportionation would be possible in the melts containing neodymium. The disproportionation reaction of Nd have been also confirmed by using UV–Vis spectrophotometric technique [6]. The curves from the system containing both La and Ce show somewhat broader peaks than those of DPV for the single chloride. Selective separation of La and Ce is also possible by controlling the electrochemical condition precisely, e.g. pulsed electrolysis. We have tested the bulk-electrolysis of rare earths by using the prototype of an anode basket under constant voltage condition. The photos of cathode (a) and anode (b) after electrolysis are shown in Fig. 4. Rare earths are clearly electro-dissolved from LaNi 5 ingot alloy and electro-deposited in typical dendritic form at
a cathode. Preliminary chemical analysis on the deposited materials surface by X-ray fluorescence shows mostly lanthanum was deposited. Precise chemical characterization of these samples is going underway. For the next step, we intend to perform practical tests, i.e. by using meshed anode baskets, ionic melts in complicated composition, dirty (oxidized) materials and/or mixture with polymer binder, and the samples in powder form, etc. Furthermore, the optimization of electrodeposition cell construction and the estimation of energy consumption should be required for practical utilization.
4. Conclusion In order to develop the recycle processes of hydrogen absorbing alloys, we have investigated electrochemical behaviour of La, Ce and Nd in LiCl–KCl eutectic melts. In all kinds of tested rare earths, cathodic reactions are quasi-reversible. The possibility of the disproportionation
Fig. 3. Differential pulse voltammograms for LaCl3, CeCl3, NdCl3 and LaCl3CCeCl3 in molten LiCl–KCl eutectic at 793 K (cathodic sweep (a) and anodic sweep (b)).
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H. Matsuura et al. / Journal of Physics and Chemistry of Solids 66 (2005) 439–442
reaction in NdCl3 containing melts is also confirmed. Bulk electrolysis tests have been performed for LaNi5 ingot by using electrodeposition potentials that already optimized at previous electrochemical measurements. Rare earths are separable by electrochemical technique from transition metal alloys such as LaNi5, although there have still several tasks from technical point of view.
Acknowledgements The authors are fully acknowledged to Dr Matsuzaki in technical assistance of our experiments. This study is financially supported by Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References
Fig. 4. Photographs of the cathode (a) and the anode (b) after electrolysis.
[1] H. Matsuura, The report of Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan in FY2000-2001. [2] R. Fujita, Y. Akai, Molten Salts 39 (1996) 112 (in Japanese). [3] R. Fujita, Y. Akai, J. Alloys Comp. 271–273 (1998) 563. [4] F. Lantelme, Y. Berghoute, J. Electrochem. Soc. 146 (1999) 4137. [5] J.A. Plambeck, Encyclopedia of Electrochemistry of the Elements, vol. 10, 1976 (and references therein). [6] T. Higashi, K. Minato, T. Ogawa, S. Miyamoto, Proceedings of 8th Japan–China Bilateral Conference on Molten Salts and Technology, 2000 p. 206.