The nucleation number density of Nd and Dy nuclei from
Abstract
The nucleation behavior of [Nd(III)(TFSA)5]2- and [Dy(III)(TFSA)5]2- in the TFSA-based ionic liquid (IL), triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide, [P2225][TFSA], was investigated in this study. The initial process of Nd and Dy electrodeposition was evaluated by chronoamperometry, indicating that the initial nucleation and the growth of Nd and Dy on the electrode surface occurred via instantaneous nucleation at -3.40 and -3.60 V, respectively. As the overpotential induced more negative, the nucleation mechanism altered from instantaneous to progressive. The number density of Nd and Dy nuclei tended to decrease as the overpotential gradually increased in this system. Moreover, the potentiostatic electrodeposition of Nd and Dy metals was examined at 393 K. The surface morphology of the electrodeposits was consistent with the chronoamperometric results. From the EDX and the XPS analyses, we ascertained that the main electrodeposits were rare earth metals with a small quantity of light elements. The series of results enabled us to conclude that the greater part of the electrodeposited Nd and Dy metals was obtained from TFSA-based IL bath by potentiostatic electrodeposition with elevating temperatures, and the control of the water content of the electrolyte was an important factor for the recovery of metallic Nd and Dy with high purity.
Keywords
- Electrodeposition
- ionic liquids
- nucleation
- rare earths
1. Introduction
Rare earth (RE) elements have been practically applied in various high-tech industries as magnetic and electronic materials [1-3] owing to their unique characteristics deriving from the 4f electron structure. Neodymium metal in particular is an especially important element as it is the main raw material for permanent Nd-Fe-B magnets used in hard disk drives, mobile phones, and automotive motors [4-7]. A large number of Nd-Fe-B magnets have been produced in the world in recent years, and a significant number of spent Nd-Fe-B magnets will accumulate in the near future. It is therefore necessary to recover the RE metals from these secondary waste magnets. Electrowinning methods with high-temperature molten salts have been employed as a conventional recovery process for RE metals because the reduction potential of RE metals is extremely negative. The high-temperature systems needed for these methods for the last several decades. Although the pyrometallurgical process [8-10] of RE metals has actually developed so far, it is difficult to establish a practical recovery system. This kind of pyrometallurgical process using high-temperature media consumed extraordinary amounts of thermal energy, so that it would not be appropriate as an environmental-friendly process in the near future. Therefore, the development of an environmentally conscious technology for the recovery of RE metals is desirable.
From the standpoint of environmental harmonization, the electrodeposition using ionic liquids (ILs) is attractive as the effective recovery method of Nd and Dy metals. ILs have many original properties, such as nonvolatility, noncombustible, and a wide electrochemical window [11, 12]. In particular, the ILs consisting of bis(trifluoromethyl-sulfonyl)amide anion, [TFSA-], are easily dehydrated compared with other ILs that are miscible with water. There are some reports about the electrochemical behavior and electrodeposition of RE elements such as La, Sm, Eu, and Yb [13-17] in ILs. We have already demonstrated that the recovery process of Nd [18-21] and Dy [22, 23] metals by electrodeposition using quaternary phosphonium and ammonium-based ILs, such as triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide, [P2225][TFSA], and 2-hydroxy ethyl-trimethyl-ammonium bis(trifluoromethyl-sulfonyl)amide, [N1112OH][TFSA]. In addition to the electrodeposition process using ILs, the wet separation process has been recently indispensable for the recycling of spent Nd-Fe-B magnets. In particular, solvent extraction and precipitation separation are effective wet separation processes. Therefore, we have recently proposed the combination process constituting a wet separation, such as solvent extraction [24, 25] or precipitation separation [26, 27], with electrodeposition [18-23].
In this study, we investigated the nucleation behavior for the initial electrodeposition for Nd and Dy metals in TFSA-based ILs. In addition, the nucleation and the growth mechanisms of Nd and Dy nuclei were analyzed by chronoamperometry (CA). Furthermore, the potentiostatic electrodeposition of Nd and Dy metals from TFSA-based ILs was performed, and the obtained electrodeposits were characterized via scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDX), and X-ray photoelectron spectroscopy (XPS).
2. Experimental
2.1. Preparation
Nd(TFSA)3 and Dy(TFSA)3 were prepared from the reaction for Nd2O3 (Wako Pure Chemical Industries, Ltd., >99.9%) or Dy2O3 (Wako Pure Chemical Industries, Ltd., >99.9%) with 1, 1, 1-trifluoro-
The phosphonium type of IL, [P2225][TFSA], was synthesized based on the previous procedure described by Tsunashima [28]. The metathesis reaction between [P2225]Br (Nippon Chemical Industrial Co., Ltd., >99.5%) and Li[TFSA] (Kanto Chemical Co., Inc., 99.7%) proceeded to obtain [P2225][TFSA]. The crude IL was purified by cleaning several times with distilled water. All bromide anions were perfectly removed from the purified IL, as confirmed by AgNO3 titration. [P2225][TFSA] prepared from this synthesis was dried under high vacuum (<−0.1 MPa) for 72 h at 393 K. The average yield of IL based on the synthesis was 96.9%. A solution of Nd(TFSA)3 and Dy(TFSA)3 in [P2225][TFSA] was used as the electrolyte for the ensuing experiment and was prepared by adding an appropriate amount of Nd(TFSA)3 or Dy(TFSA)3 to [P2225][TFSA] inside a glove box (MIWA MFG Co., Ltd., DBO-2LKP-YUM02) filled with Ar (water and oxygen contents <1.0 ppm). These solutions were dried at 373 K in a vacuum chamber (<-0.1 MPa) for more than 72 h.
2.2. Electrochemical analysis
CA was carried out using a three-electrode cell with an electrochemical analyzer (BAS Inc., ALS760D) inside a glove box. The Pt disk electrode (BAS Inc.,
The electrodeposition of Nd and Dy metals from 0.1 mol dm-3 Nd(TFSA)3 and Dy(TFSA)3 in [P2225][TFSA] was carried out using a three-electrode system inside a cylindrical cell at 393 K under an Ar atmosphere (H2O, O2 < 1.0 ppm) in a glove box. A copper substrate and a prismatic Nd-Fe-B rod were employed as a cathode and an anode, respectively. A Vycor glass filter was covered with a soda lime tube at the bottom of anode in order to prevent the diffusion of dissolution components from the anode into the electrolyte. The electrode surface was polished by the suitable fine-grade abrasive papers before experiments. The Pt wire (
3. Results and discussions
3.1. Nucleation behaviors of Nd and Dy nuclei
According to the chronoamperometric analysis, the initial nucleation and the subsequent growth behaviors of Nd and Dy nuclei were investigated in [P2225][TFSA]. A distinctive peak was observed on the nucleation process after the current extinction corresponding to the double layer charging. This peak was characteristic of the three-dimensional growth of the Nd and Dy nuclei; there was an increase tendency of faradaic current based on the nucleation and the subsequent growth of Nd and Dy nuclei. Eventually, a maximum current density,
For the chronoamperogram, the charging current of neat [P2225][TFSA] was deduced from the raw data for [P2225][TFSA] containing 0.1 mol dm-3 [Nd(III)(TFSA)5]2- in order to analyze the faradaic current only in the reduction reaction of [Nd(III)(TFSA)5]2-. Several models have been developed to describe the
where
The relationship between the squared dimensionless current density and the dimensionless time of [Nd(III)(TFSA)5]2- in [P2225][TFSA] is shown in Fig. 2. In the case of the overpotential at -3.40 V, the current tendency was consistent with the instantaneous nucleation process. Conversely, when the overpotentials were induced at -3.50 and -3.60 V, the current curves shifted from that of the instantaneous nucleation process to the progressive nucleation process. A series of experimental results demonstrated that the nucleation process influenced on the induced overpotentials. Thus, the nucleation and the growth process would be correlated with the number density of the active sites from the current transients [35] as discussed below. This transition from instantaneous to progressive nucleation was consistent with the nucleation process for Dy nuclei in our investigations [22]. In the case of Dy nuclei in [P2225][TFSA] system, when the overpotential of -3.60 V was induced, the nucleation behavior obeyed the instantaneous nucleation process. On the other hand, the nucleation curve at -3.80 V was followed by the progressive nucleation process.
The number density was analyzed based on the following procedure: the nucleation number density,
where
Applied potential ( |
-3.40 | -3.50 | -3.60 |
Nucleation number density (1011
|
2.0 | 1.7 | 1.7 |
Applied potential ( |
-3.40 | -3.60 | -3.80 |
Nucleation number density (1011
|
6.3 | 5.5 | 2.6 |
3.2. Electrodeposition of Nd and Dy metals
Electrodeposition at -3.40 V produced blackish electrodeposits on a Cu substrate. The total transported charge was 105.8 C. The SEM image of the electrodeposits was shown in Fig. 3.
The surface morphology of the electrodeposits was heterogeneous orbicular with a nonuniform size distribution. This behavior would be generated from the initial nucleation and the subsequent growth occurred according to the progressive nucleation process in which the rate of the nucleation was faster than that of the crystal growth. From above-mentioned nucleation behaviors, at -3.40 V, the
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-3.40 | 1.26 | 0.94 | 20.36 | 2.83 | 0.89 | 0.23 | 73.49 |
-3.60 | 1.15 | 0.78 | 23.69 | 3.26 | 1.21 | 0.38 | 69.53 |
The EDX data display electrodeposits at -3.40 and -3.60 V, which mainly consist of Nd metal. However, a relatively large amount of oxygen was also detected at the top surface in the electrodeposits, indicating that a part of Nd metal would be oxidized due to the active metal. Thus, the oxidizing state of Nd metal in the electrodeposits was evaluated from XPS with Ar etching. Fe element was scarcely detected in the electrodeposits at both overpotentials. As a result, the separated anode structure from the main electrolyte by the Vycor glass filter was confirmed to be an effective strategy. The decomposition of the ILs was prevented by this separated anode structure. Thus, this separated anode structure and the dissolution of anode compartment enabled the acquisition of Nd metal in the electrodeposits while preventing bonding with the light elements. The Nd 3d5/2 XPS spectra of the electrodeposits at -3.40 and -3.60 V were shown in Figs. 5(a) and 5(b), respectively. The electrodeposited surfaces were sputtered by an Ar ion beam to facilitate in-depth analysis of the middle layer, that is, (a) under 0.75 μm and (b) the top surface of the electrodeposits. The binding energy of Nd 3d5/2 for Nd metal and oxides should be theoretically positioned at 980.5-981.0 and 981.7-982.3 eV, respectively [39]. The Nd 3d5/2 peak maxima acquired for the middle layers (a) under 0.75 μm and (b) the top surface were detected at 981.57 and 982.25 eV, respectively. Hence, the electrodeposits were identified as Nd metal and oxide mixtures. It is conjectured that metallic Nd should initially be electrodeposited on a Cu substrate and subsequently oxidized by the oxygen in ILs. It has reported that the water content of the ILs did not significantly affect the oxidation sate of the electrodeposits but would influence the coordination number of the Nd complex [18]. Therefore, the sufficient dehydration and control of the water content of the ILs, including metallic components, are important factors for achieving the recovery of metallic Nd with high purity.
As for the electrodeposition of Dy metal, the potentiostatic electrodeposition at -3.80 V in [P2225][TFSA], including 0.1 mol dm-3 [Dy(III)(TFSA)5]2-, was performed at 393 K, according to the above-mentioned nucleation analyses. The potentiostatic electrolysis was conducted in a glove box with stirring at 500 rpm to increase the current density by intermittently supplying the electroactive species [Dy(III)(TFSA)5]2- to the reactive electrode surface. The total transported charge under Dy electrodeposition was 300 C for 43.6 h. There was no decomposition reaction of IL under Dy electrodeposition since there was no alternation on the light-yellow electrolyte before and after electrodeposition. The blackish-brown electrodeposits with a strong adhesion were observed on a Cu cathode substrate. A microanalytical SEM/EDX method was applied to the analysis of the surface morphology and the element content of the electrodeposits. The EDX results with SEM image are shown in Figs. 6(a) and 6(b), respectively.
From Fig. 6(b), the round particles on a Cu substrate were confirmed to be Dy metal and other related compounds. A quantitative analysis of the electrodeposits is tabulated in Table 3.
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-3.80 | 10.83 | 1.15 | 11.82 | 3.87 | 0.86 | 1.87 | 9.27 | 60.33 |
Most parts of the electrodeposits were composed of the Dy metallic state because almost no oxygen was detected on the Dy metals from the EDX analysis. The amounts of carbon and oxygen were derived from the surface of the Cu cathode substrate. The Dy metals with ca. 0.8-0.9 μm particles were highly distributed in the electrodeposits. This electrodeposition enabled us to conclude that Dy electrodeposits contained a smaller amount of the oxide state and a large amount of the metallic state on the electrode top surface. We also investigated the depth components of the electrodeposits by XPS with the Al-Kα radiation. The Dy 3d5/2 spectra for the top surface and the middle layer at the depth of 0.5 μm of the Dy electrodeposits are represented in Figs. 7(a) and 7(b), respectively. The XPS analysis with Ar sputtering at the rate of 27.2 nm min-1 was performed on the Dy electrodeposits. The Dy 3d5/2 spectra for Dy metal and oxides were theoretically positioned at 1295.8 and 1289.0 eV, respectively [30]. The Dy 3d5/2 peaks of the electrodeposits were assigned at 1297.2 eV and were hardly shifted by Ar etching treatment. The Dy 3d5/2 spectra of the middle layer and the top surface showed a good agreement with the theoretical value. The XRD result of the Dy electrodeposits is shown in Fig. 8. The Dy metallic crystal planes at a low angle were in good accordance with the ASTM card. Consequently, a series of electrodeposition investigations of Nd and Dy electrodeposits allowed us to conclude that most of the metallic state of Nd and Dy can be recovered from a TFSA-based IL by the potentiostatic electrodeposition with elevating temperatures.
Acknowledgments
This study was partially supported by the Grant-in-Aid for Scientific Research (No. 15H02848) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References
- 1.
D. Yuan, Y. Liu, Mater. Chem. Phys., 96 (2006) 79-83. - 2.
H. Yoshida, S. Kato, K. Hirano, J. Nishimoto, T. Hattori, Chem. Lett., 36(3) (2007) 430-431. - 3.
A.A. Dakhel, Mater. Chem. Phys., 102 (2007) 266-270. - 4.
N. Oono, M. Sagawa, R. Kasada, H. Matsui, A. Kimura, J. Magn. Magn. Mater., 323 (2011) 297-300. - 5.
Y. Matsuura, J. Magn. Magn. Mater., 303 (2006) 344-347. - 6.
O. Takeda, T. Okabe, Y. Umetsu, J. Alloys Compd., 408-412 (2006) 387-390. - 7.
G. Yan, P.J. McGuiness, J.P.G. Farr, I.R. Harris, J. Alloys Compd., 478 (2009) 188-192. - 8.
S. Kobayashi, K. Kobayashi, T. Nohira, R. Hagiwara, T. Oishi, H. Konishi, J. Electrochem. Soc., 158 (2011) E142-E146. - 9.
T. Uda, Mater. Trans., 43(1) (2002) 55-62. - 10.
O. Takeda, T.H. Okabe, Y. Umetsu, J. Alloys Compd., 408-412 (2006) 387-390. - 11.
D.R. MacFarlane, M. Forsyth, P.C. Howlett, J.M. Pringle, J. Sun, G. Annat, W. Neil, E.I. Izgorodina, Acc. Chem. Res., 40 (2007) 1165-1173. - 12.
W. Simka, D. Puszczyk, G. Nawrat, Electrochim. Acta, 54 (2009) 5307-5319. - 13.
S. Legeai, S. Diliberto, N. Stein, C. Boulanger, J. Estager, N. Papaiconomou, M. Draye, Electrochem. Commun., 10 (2008) 1661-1664. - 14.
M. Yamagata, Y. Katayama, Y. Miura, J. Electrochem. Soc., 153 (2006) E5-E9. - 15.
A. I. Bhatt, I. May, V. A. Volkovich, D. Collison, M. Helliwell, I. B. Polovov, R. G. Lewin, Inorg. Chem., 44(14) (2005) 4934-4940. - 16.
K. Binne, Chem. Rev., 107 (2007) 2592-2614. - 17.
Y. Katayama, Electrochemical Aspects of Ionic Liquids, (2005) 111-131. - 18.
H. Kondo, M. Matsumiya, K. Tsunashima, S. Kodama, Electrochim. Acta, 66 (2012) 313-319. - 19.
H. Kondo, M. Matsumiya, K. Tsunashima, S. Kodama, ECS Trans., 50(11) (2012) 529-538. - 20.
M. Ishii, M. Matsumiya, S. Kawakami, ECS Trans., 50(11) (2012) 549-560. - 21.
M. Matsumiya, M. Ishii, R. Kazama, S. Kawakami, Electrochim. Acta, 146 (2014) 371-377. - 22.
R. Kazama, M. Matsumiya, N. Tsuda, K. Tsunashima, Electrochim. Acta, 113 (2013) 269-279. - 23.
A. Kurachi, M. Matsumiya, K. Tsunashima, S. Kodama, J. Appl. Electrochem., 42 (2012) 961-968. - 24.
M. Matsumiya, Y. Kikuchi, T. Yamada, S. Kawakami, Sep. Purif. Technol., 130 (2014) 91-101. - 25.
Y. Kikuchi, M. Matsumiya, S. Kawakami, Solv. Extr. Res. Dev., 21(2) (2014) 137-145. - 26.
K. Ishioka, M. Matsumiya, M. Ishii, S. Kawakami, Hydrometallurgy, 144-145 (2014) 186-194. - 27.
M. Matsumiya, K. Ishioka, T. Yamada, M. Ishii, S. Kawakami, Inter. J. Miner. Process., 126 (2014) 62-69. - 28.
K. Tsunashima, M. Sugiya, Electrochem. Commun., 9 (2007) 2353-2358. - 29.
B. Scharifker, G. Hills, Electrochim. Acta, 28 (1983) 879-889. - 30.
Y. Bando, Y. Katayama, T. Miura, Electrochim. Acta, 53 (2007) 87-91. - 31.
N. Tachikawa, N. Serizawa, Y. Katayama, T. Miura, Electrochim. Acta, 53 (2008) 6530-6534. - 32.
Y.-L. Zhu, Y. Kozuma, Y. Katayama, T. Miura, Electrochim. Acta, 54 (2009) 7502-7506. - 33.
A. Basile, A.I. Bhatt, A.P. O’Mullane, S.K. Bhargava, Electrochim. Acta, 56 (2011) 2895-2905. - 34.
R. Fukui, Y. Katayama, T. Miura, J. Electrochem. Soc., 158(9) (2011) D567-D572. - 35.
J. Mostany, J. Mozota, B.R. Scharifker, J. Electroanal. Chem., 177 (1984) 25-37. - 36.
G.A. Gunawardena, G.J. Hills, I. Montenegro, Electrochim. Acta, 23 (1978) 693-697. - 37.
M.-H. Yang, I.-W. Sun, J. Appl. Electrochem., 33 (2003) 1077-1084. - 38.
D. Grujicic, B. Pesic, Electrochim. Acta, 50 (2005) 4426-4443. - 39.
J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN (1992).