phosphonium ionic liquids and the feasibility of recovery by direct electrodeposition

Separation and Purification Technology 130 (2014) 91–101

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Extraction of rare earth ions by tri-n-butylphosphate/phosphonium ionic liquids and the feasibility of recovery by direct electrodeposition Masahiko Matsumiya a,⇑, Yuya Kikuchi a, Takahiro Yamada a, Satoshi Kawakami b a b

Graduate School of Environment and Information Sciences, Yokohama National University, 79-2 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Environmental Protection Laboratory, DOWA ECO-SYSTEM Co., Ltd., 65-1 Omoriyama-shita, Hanaoka, Odate, Akita 017-0005, Japan

a r t i c l e

i n f o

Article history: Received 29 November 2013 Received in revised form 27 March 2014 Accepted 6 April 2014 Available online 19 April 2014 Keywords: Electrodeposition Ionic liquids Rare earths Solvent extraction Tri-n-butyl phosphate

a b s t r a c t As the preliminary experiments for development of the recycling process to recovery of rare earths (REs) composed of Nd–Fe–B magnets by direct electrodeposition from an organic phase after extraction procedures, solvent extraction of RE such as Pr, Nd and Dy by tri-n-butylphosphate (TBP) with ionic liquids (ILs) were performed. Triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]), which is beneficial as an electrolyte for electrodeposition of electrochemically negative metals such as REs were employed as the IL. It was revealed that the extraction performance of RE(III) was enhanced significantly in TBP with [P2225][TFSA] when extraction of RE ions was carried out from an aqueous phase including TFSA anions. Considering that the extraction mechanism was supposed to the cationexchange of the IL with solvating the RE3+ cations by TFSA anions, the mechanism was investigated by variation of concentrations of TBP, TFSA anions in the aqueous phase and [P2225][TFSA]. As the results, the stoichiometry of both TBP and TFSA anions in this IL extraction system with RE(III) was determined to be 3:1. According to UV–Vis–NIR spectroscopic analysis, the complexation of RE(III) extracted by TBP with [P2225][TFSA] was altered due to the coordination of TBP to RE(III). Estimation of the number of the TFSA anion solvated to the centered Nd3+ cation in TBP with [P2225][TFSA] by Raman spectroscopy resulted in the centered Nd3+ cation solvated by three TFSA anions and this result was consistent with the extraction mechanism analysis. From the electrochemical analysis, the extracted [Nd(TBP)3]3+ complex in TBP/[P2225][TFSA] was reduced to the following reaction: [Nd(TBP)3]3+ + 3e ? Nd(0) + 3TBP at 2.4 V. The direct electrodeposition of the extracted [Nd(TBP)3]3+ in TBP/[P2225][TFSA] was carried out using a three-electrode system at 373 K. The chemical bonding energy of Nd electrodeposits was investigated by XPS and the middle layer of electrodeposits was the metallic state, while a part of the top surface was the oxidation state. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction RE elements belong to rare metal, which is essential to current industries. RE metals have been widely applied to various important products such as magnets [1,2], catalysts [3], and glasses [4]. Hence, the demand of REs has drastically increased in recent and the import price of REs has increased drastically by occasional supply restrictions from the main exporting countries [5]. That is one of the reasons why the concern with techniques to recover RE metals from wastes in the ‘‘urban mining’’, which means that there are concentrated metal resources included in the manufactured products at the end of their lives, have been growing for the last several years. It is important to develop the recycling system of

⇑ Corresponding author. Tel./fax: +81 45 339 3464. E-mail address: [email protected] (M. Matsumiya). http://dx.doi.org/10.1016/j.seppur.2014.04.021 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

REs such as Pr, Nd, and Dy included in spent Nd–Fe–B permanent magnets since especially it is anticipated that over the next 25 years the demand for Nd and Dy will rise by 700% and 2600%, respectively [6]. The Nd–Fe–B magnet is loaded in high technology products such as voice coil motors (VCMs) in hard disk drive, magnetic generators for magnetic resonance imaging, driving motors for hybrid electric vehicles, and so on, due to their superior magnetic properties to Alnico or ferrite magnets [7]. Consequently, the magnets occupy the largest share in the hard magnet market and will continue to be produced around the world. Solvent extraction is the effective method to enrich metal ions and the study on extraction of Pr(III) [8], Nd(III) [9] and Dy(III) [10] by an extractant in conventional organic solvents has been advanced. It has been reported that the extractability of lanthanide ion in ILs showed higher than that in organic solvents [11]. The ILs have been employed as profitable alternatives to conventional

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solvents such as volatile organic compounds (VOCs) in electrochemical, synthetic, and separation processes owing to their unique physicochemical properties such as wide electrochemical window, hard thermal stability, non-inflammability, and hard volatility [12–14]. Thus, the IL based extraction is more benign and safe for environment compared to the VOC based extraction. For example, Goto et al. studied the extraction behavior of lanthanide ions with a proton dissociation type extractant, di(2ethylhexyl)phosphonic acid (PC-88A), using ILs, 1-octyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)amide ([C8mim][TFSA]) and 1-dodecyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) amide ([C12mim][TFSA]) as the extracting phase [15]. Dai et al. investigated the separation of RE elements by di(2-ethylhexyl) phosphate-based ionic extractants in 1-hexyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)amide ([C6mim][TFSA]) [16]. The extraction behavior of Eu(III) from a nitric acid medium by a solution of n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and TBP in the IL, 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)amide ([C4mim][TFSA]), was studied by Strinivasan et al. [17]. There are many beneficial studies on the extraction of RE ions from nitric acid medium by various extractants in imidazolium-based ILs. We have already demonstrated that Nd [18–20] and Dy [21,22] metals were recovered by electrodeposition by using [P2225][TFSA] as an electrolyte. The application of [P2225][TFSA] which has the wide electrochemical window leads to electrodeposition of more negative elements such as REs although it is impossible to electrodeposite these elements in imidazolium-based ILs due to the narrow cathodic limit in the electrochemical window. In order to develop our efficient recovery process, we have proposed the recovery of RE metals by direct electrodeposition from an IL phase after the extraction process. In the case of recovering RE metals from the wastes such as spent Nd–Fe–B magnets, the combination process for extraction and electrodeposition is expected to purify RE metals owing to separation of traces and simplify the total recycling process. For instance of studies on direct electrodeposition from an IL phase, Vasudeva Rao electrodeposited uranium (IV) oxides from TBP with [C4mim][TFSA] [23]. TBP is a proper neutral extractant for direct electrodeposition because of the high solubility in IL. In the present study, the extraction mechanism of RE(III) (RE = Pr, Nd, and Dy), which was derived from the synthesized salts (RE(TFSA)3) and the waste of Nd–Fe–B magnets, in TBP with [P2225] [TFSA] was evaluated from the slope analysis. The investigation of the mechanism would contribute to efficient extraction of REs. Metal ions are generally extracted from an aqueous phase including nitrate ions by solvating mechanism using a neutral extractant in the conventional solvent [24] or by cation-exchange mechanism in the imidazolium-based IL extraction system [25]. RE(III) was extracted from a neutral aqueous phase containing TFSA anions by TBP with [P2225][TFSA] in our study and it was revealed that the extractability was enhanced in comparison with extraction of metal ions from the aqueous phase containing nitrate ions. The extraction mechanism involving TFSA anions existing in the aqueous phase has not been revealed so far. Moreover, the complexation of RE(III) in TBP with [P2225][TFSA] were also investigated by spectroscopic studies such as UV–Vis–NIR and Raman spectroscopy so as to support the extraction mechanism.

2. Experimental 2.1. Synthesis of ILs and RE metallic salts The phosphonium ILs were prepared according to the procedure described elsewhere [26,27]. [P2225][TFSA] was synthesized by the

(A) C2H5

TFSA-

(B)

C2H5

CH3

P+ CnH2n+1

TFSACnH2n+1 N

+

N

C2H5 n=5, 8, 12

TFSA-=(CF3SO2)2N-

n=2, 6

Fig. 1. Molecular structure of (A) phosphonium and (B) imidazolium-based ionic liquids.

metathesis reaction of P2225Br (Nippon Chemical Industrial Co., Ltd. >99.5%) with LiTFSA (Kanto Chemical Co., Inc. >99.7%). The solution containing Pþ 2225 precursor was allowed to react with the LiTFSA in distilled water, and then, an organic phase composed of the [P2225][TFSA] was spontaneously separated from a solution phase including lithium and bromide ions. The separated [P2225][TFSA] was purified by stirring with distilled water until no residual halide ion was detected by a AgNO3 titration method. The solution phase was evaporated at 373 K. In this study, all the cationic precursors, e.g., triethyl-octyl-phosphonium (Pþ 2228 ), triethyl-dodecyl-phosphonium (Pþ 222ð12Þ ) (Nippon Chemical Industrial Co., Ltd. >99.5%), 1-ethyl-3-methyl-imidazolium (C2mim+) and 1-hexyl-3-methylimidazolium (C6mim+) (Tokyo Chemical Industrial Co., Ltd. >97.0%) were used as supplied and the corresponding ILs were prepared by similar methods. The molecular structure of these ILs was shown in Fig. 1. There are reports that the solubilities of TFSAbased ILs in water are the magnitude of 103–102 M at 298 K [28,29]. The solubilities of synthesized TFSA-based ILs in this work would be close to the same order. Thermal decomposition temperature of the phosphonium ILs is approximately 673 K [26]. In the case of these ILs, the potential window lies between approximately 3.2 and +3.0 V vs. ferrocene (Fc)/ferrocenium (Fc+), showing quite high electrochemical stability [26]. Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 were prepared by the reaction with 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (HTFSA, Kanto Chemical Co., Inc. >99.0%) and each oxide, i.e., Pr2O3 (Sigma–Aldrich Chemical Co. >99.9%), Nd2O3 (Wako Pure Chemical Industries, Ltd., >99.9%) and Dy2O3 (Wako Pure Chemical Industries, Ltd., >99.5%). The suspension solution was clearly altered to the transparent characteristic color with each RE component after the reaction between RE metallic components and HTFSA at 373 K. The solution dissolving RE components was evaporated at 423 K and each RE metallic salts with fine powder was obtained. The synthesized RE(TFSA)3 salts were dried under vacuum at 393 K for 48 h.

2.2. Preparation of an aqueous phase including metal ions derived from VCM In this study, VCM was applied as the waste of Nd–Fe–B magnets to the extraction and electrochemical processes [30]. In demagnetization process, Nd–Fe–B permanent magnets were heated in an electric muffle furnace (FUW220PA, ADVANTEC MFS, INC.) at 90 K/h up to Curie temperature: 583 K. The magnetic flux density was measured by a digital TESLA meter (TM-701, KANETEC Co., Ltd.) after demagnetizing treatment. It was observed that the residual magnetic force field on VCM sample was almost zero. Ni–Cu–Ni coated in the surface of the VCM sample was removed by grinding using a whetstone. After elimination of Ni– Cu–Ni layers, the VCM fragments were crumbled by an automatic grinder (A11BS1, IKA-GMBH & Co.KG). The obtained powder was oxidized in an electric muffle furnace at 100 K/h up to 1133 K. The average of the particle diameter of the oxide powder was 25 lm. Metal ions were leached from 2.0 g of the pretreated

M. Matsumiya et al. / Separation and Purification Technology 130 (2014) 91–101

sample into 200 mL of 2.0 M or 0.2 M HTFSA at 343 K for 20 h with magnetic stirring. After leaching, the residual wastes were separated by filtering through a 5.0 lm filter. The prepared solution was applied to an aqueous phase when extraction procedure.

All extraction studies were carried out by equilibration of equal volumes of aqueous and organic phases at 298 K. The aqueous phase was distilled water dissolving 0.1 M Pr(TFSA)3, Nd(TFSA)3 or Dy(TFSA)3. The organic phase was IL dissolving TBP (Wako Pure Chemical Industries, Ltd., >98.0%). The solutions were placed in a test tube and shaken for 5 min with a vortex mixer. The concentrations of metal ion in the aqueous phase before and after the equilibration were measured by ICP-AES (ICPE-9000, Shimadzu Co.). The concentration of metal ion in the organic phase after the equilibration was calculated using a mass balance equation. The distribution ratio D and extraction efficiency E are defined as follows:

½Maq;i  ½Maq;f ½Maq;f

Eð%Þ ¼

½Maq;i  ½Maq;f  100 ½Maq;i

a deuterium and a halogen lamp, respectively. In this spectroscopic measurement, a quartz cell with 1.0 cm optical path length was applied for the wavelength region of from 250 to 1400 nm. 2.5. Raman spectroscopic analysis

2.3. Extraction mechanism analysis

D¼

93

ð1Þ

ð2Þ

where [M]i and [M]f represent the initial and final concentrations of metal ion in an aqueous phase, respectively. The subscript (aq) denotes an aqueous phase. In order to investigate the effect of TBP concentration, the organic phase was equilibrated with the above aqueous phase. In addition, the effect of TFSA concentration in the aqueous phase on the extraction of RE(III) such as Pr(III), Nd(III) and Dy(III) was studied by the equilibration of TBP/[P2225][TFSA] with the above aqueous phase and the concentration was adjusted by adding KTFSA (Kanto Chemical Co., Inc. >99.8%) into aqueous phase. Moreover, the effect of [P2225][TFSA] concentration in the organic phase was researched by the extraction of RE(III) from the above aqueous phase. [P2225][TFSA] concentration in the organic phase was varied by adding dichloromethane (Wako Pure Chemical Industries, Ltd., >98.0%) into the IL. Furthermore, the extraction of RE(III) was performed by TBP with [P2225][TFSA] adding P2225Br into the above aqueous phase in order to support the extraction mechanism by cation-exchange in [P2225][TFSA] based extraction system. Five types of ILs, [P2225][TFSA], [P2228][TFSA], [P222(12)][TFSA], [C2mim][TFSA] and [C6mim][TFSA] were applied for the cation effect of the IL on RE(III) extraction from the above aqueous phase. Ultimately, the effect of HTFSA concentration for extraction of Nd(III) as a representative RE(III) ion in this system was studied. The slope analysis for concentrations of TBP, TFSA in the aqueous phase and [P2225][TFSA] was also carried out for the extraction system which HTFSA including RE(III) ions composed of VCM was applied as the aqueous phase. 2.4. UV–Vis–NIR spectroscopic analysis In the present study, the spectroscopic properties of RE(III) such as Pr(III), Nd(III) and Dy(III) were investigated in order to confirm the structure of the RE(III) solvated by TBP with [P2225][TFSA]. The absorption spectra of 0.1 M Pr(TFSA)3, Nd(TFSA)3 or Dy(TFSA)3 dissolved in [P2225][TFSA] and RE(III) extracted by 1.8 M TBP with [P2225][TFSA] from the above aqueous phase were measured by UV–Vis–NIR spectrometer (Lambda 750, Perkin Elmer) after all solutions were dried in a vacuum chamber at 373 K for 24 h. The spectra were compensated for the baseline to remove the slight absorption in the UV region related [P2225][TFSA] with no RE complexes. The light sources for the UV and Vis–NIR regions were

As for the complexation of RE cations and TFSA anions, the number of the TFSA anion solvated to the centered RE3+ cation in TBP with [P2225][TFSA] was evaluated by Raman spectroscopy. The sample solutions for Raman spectroscopic measurements were prepared by dissolving Nd(TFSA)3 into 1.8 M TBP with [P2225][TFSA] (0.3–0.6 M). All solutions were dried in a vacuum chamber at 373 K for 24 h before the measurement. Raman spectra were measured at room temperature using 532 nm laser of Raman spectrometer (Renishaw inVia Reflex Raman Microscope). Spectral data were accumulated 512 times to obtain data of a sufficiently high signal-to-noise ratio. The overlapped Raman bands were deconvoluted to extract each signal Raman band with pseudoVoigt function. The solvation number of the RE ions in the IL was estimated by the similar analytical method suggested by Umebayashi et al. [31,32]. The integrated intensity of the deconvoluted Raman band of the free TFSA in the bulk IL is represented as If = Jfcf, where Jf and cf stand for the molar Raman scattering coefficient and the concentration of the free TFSA in the bulk, respectively. The cf is given as cf = cT  cb = cf  ncM, where cT and cb denote the concentrations of total and bound TFSA (solvated to the metal ion), respectively, and cM and n denote the concentration and the solvation number of the metal ion, respectively. By inserting the equation into If = Jfcf, the following relationship was obtained:

  If cT ¼ Jf n cM cM

ð3Þ

If the solvation number of the RE ion in the IL is kept unchanged under the examined experimental conditions, the plots of If/cM against cT/cM would become a straight line; thus, the value of n is obtained as n = b/a from a slope a = Jf and an intercept b = Jf n. 2.6. Electrochemical analysis Cyclic voltammetry (CV) was carried out using a three-electrode system with a cylindrical cell. The Pt disk electrode with 1.6 mm inside diameter was employed as a working electrode and the electrode surface was polished with an alumina and a diamond paste before use. Pt wires with 0.5 mm inside diameter were used as a counter and a quasi-reference electrode (QRE), because the potential using a Pt QRE was stable and exhibited a good reproducibility at mid-range temperatures. The potential was compensated for the IL standard using the Fc/Fc+ redox couple. The CV measurement was carried out 373 K with an electrochemical analyzer (ALS760E, BAS, Inc.). The electrolyte for the CV was the extracted Nd(III) dissolved in TBP/[P2225][TFSA] and was dried under vacuum at 373 K for 48 h. The water content of the extracted and dried IL was less than 250 ppm measured by a Karl-Fischer moisture titrator (MKC-610-NT, Kyoto Electronics Manufacturing Co., Ltd.). The electrochemical analysis was conducted dry Ar atmosphere (H2O < 1.0 ppm) in a glovebox (DBO-1KP-YUM01, MIWA, Inc.). The electrodeposition of the Nd metal from the extracted [P2225][TFSA] was carried out using a three-electrode system inside a cylindrical cell at 373 K under an Ar atmosphere in a glovebox. In this study, a prismatic Nd–Fe–B rod and a Cu substrate were employed as an anode and a cathode, respectively. The anode was surrounded by a soda lime tube with a Vycor glass filter at the bottom in order to prevent the diffusion of dissolution components from the anode into the electrolyte. The surface of each electrode was polished by some kinds of suitable fine-grade

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waterproof abrasive papers. The applied QRE was composed of a Pt wire with 0.5 mm inside diameter. On the potentiostatic electrodeposition, the overpotential applied to the cathode was 3.2 V against the extracted IL bath at 373 K. The metallic states of Nd in the electrodeposits were evaluated by XPS (Quantera SXM, ULVAC-PHI, Inc.).

Considering that [P2225][TFSA]IL is partitioned into ½Pþ 2225 IL and [TFSA]IL in Eq. (6), D and extraction equilibrium constant (K) would be expressed by Eqs. (7) and (8), respectively.

h

 REðTBPÞ3þ m ðTFSA Þxþy h i D¼ RE3þ

i IL

ð7Þ

aq

3. Results and discussions

h

i  REðTBPÞ3þ m ðTFSA Þxþy IL i K¼h  x  y RE3þ ½TBPm ½ TFSA  ½ TFSA aq IL IL

Anions in an aqueous phase play an important role in the extraction of metal ions into neutral extractant like TBP and CMPO. In cases where a conventional organic solvent such as kerosene and n-alkane was used as the diluent, the mass transfer of metal ions takes place through anions such as nitrate ion assisted the complexation mechanism. The extraction mechanism of metal ions related to nitrate ion in solvent systems containing the conventional solvent as the diluents is represented by the following equilibrium:

þ

nNO3;aq

! þ mLorg   ½MðNO3 Þn  mLorg

ð4Þ

where M, L, the subscript (aq) and (org) denote metal ion such as lanthanide ion [24], a neutral extractant, an aqueous phase and an organic phase, respectively. For this type of extraction mechanism, increasing the feed nitrate ion concentration causes an increase in the extraction of the metal ions. The nitrate ion favors formation of neutral nitrate complex of the metal ions. On the other hand, it has been reported that the mechanism of transfer of metal ion into the IL extraction system is different generally [25]. The mechanism is cation-exchange mechanism which metal ions transfer into an IL phase combining with anions of the IL and cations of the IL release into an aqueous phase. In the case of [Cnmim][TFSA] based extraction system, the mechanism is given below:

Mnþ aq þ mLIL þ x½Cn mim½TFSAIL  þ nþ    !  MðLÞm ðTFSA Þx IL þ x½Cn mim aq



þ mTBPIL þ x½P2225 ½TFSAIL þ y½TFSA aq h i   3þ   !  REðTBPÞm ðTFSA Þxþy þ x Pþ2225 aq IL

K¼

D  x ½ TFSA IL ½TFSA yaq ½TBPm IL

ð6Þ

[P2225][TFSA] has no extraction ability because no metal ion was extracted by [P2225][TFSA] neat solution. The cation possesses weak donor to complex metal ion due to short chain unlike trihexyl-tetradecyl-phosphonium chloride (P666(14)Cl) [33]. It was considered that Pþ 2225 was released into an aqueous phase by cation-exchange mechanism as well as [Cnmim][TFSA] based extraction system. The possibility was suggested as described later. The extraction experiments and analysis were carried out so as to evaluate the extraction mechanism expressed by Eq. (6).

ð9Þ

The logarithm of D is following by Eq. (10) according to Eq. (9).

log D ¼ log K þ m log ½TBPIL þ x log ½TFSA IL þ y log ½TFSA aq ð10Þ [TFSA]IL corresponds to the concentration of TFSA anion based on [P2225][TFSA]IL. Each stoichiometry related with the complex allowed us to determine by the slope analysis according to Eq. (10). The variation of E and D for extraction of RE(III) as a function of TBP concentration in [P2225][TFSA] was shown in Fig. 2. It was seen that extracted RE(III) increased with an increase of the TBP

100

(A) 80

60

40

ð5Þ

where the subscript (IL) denotes an IL phase. This equation means anions present in an aqueous phase is independent of the extraction of metal ions. The extractability of the IL based extraction system is not enhanced even if anions included in the aqueous phase are increased. However, an increase in the extraction of metal ions was observed in the TFSA-based IL extraction systems with an increase of TFSA anions in an aqueous phase in the present study. It is guessed that this mechanism is just like Eq. (4) combined with Eq. (5) because anions in an aqueous phase were related to the extraction and the extraction of metal ions was conducted in the IL based system. In the present study, extraction of RE(III) such as Pr(III), Nd(III) and Dy(III) was performed in TBP with [P2225] [TFSA] as follows:

RE3þ aq

aq

20

Pr Nd Dy

0

0.5

1.0

1.5

2.0

[TBP]IL (M) 2.0

(B) 1.0

log D

Mnþ aq

ð8Þ

Eq. (9) is obtained by substituting Eq. (7) for Eq. (8).

E (%)

3.1. Extraction mechanism

0

-1.0 Pr (slope=2.6) Nd (slope=2.8) Dy (slope=3.0)

-2.0 -1.0

-0.5

0

0.5

log [TBP] IL Fig. 2. Effect of TBP concentration on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 in distilled water, respectively. Feed solution: pH = 4.5.

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(A) 80

60

40

20

Pr Nd Dy

0

0.5

1.0

1.5

2.0

-

[TFSA ]IL (M) 0.8

(B)



þ 3TBPIL þ ½P2225 ½TFSAIL þ 2½TFSA aq

ð11Þ

3þ  þ  !  ½REðTBPÞ3 ðTFSA Þ3 IL þ ½P2225 aq

This equation conceivably means that the metal ion is extracted with TBP and TFSA anions from aqueous and IL phases are co-extracted because of electroneutrality. TFSA anions from the

100

(A)

0.4

log D

RE3þ aq

100

E (%)

concentrarion. The linear relationship between log D and log [TBP]IL was obtained with a slope of approximate 3. The slope indicated the stoichiometry of TBP with RE(III) was 3:1. As represented in Fig. 3, the effect of TFSA concentration in an aqueous phase on extraction of RE(III) was studied. It was indicated that as the TFSA concentration increased, the extracted RE(III) increased in this condition. A linear slope of approximate 2 in log D-log [TFSA]aq plot revealed the stoichiometry of [TFSA]aq with RE(III) was 2:1. Dichloromethane was employed as inactive and soluble diluents for IL to reveal the effect of [P2225][TFSA], i.e., [TFSA]IL on RE(III) extraction in order to determine the stoichiometry of [TFSA-]IL with RE(III). From the correlation between log D and log [TFSA]IL as seen in Fig. 4, the stoichiometry of [TFSA]IL with RE(III) was 1:1 according to the linear slope of approximate 1. Thus, the stoichiometry of TFSA anions from aqueous and IL phases with RE(III) was supposed to be estimated approximate 3:1. As the above results, the extraction equation of TBP/[P2225][TFSA] system for RE(III) from the aqueous phase containing TFSA anions would be represented by the following equation:

0

-0.4

Pr (slope=0.9) Nd (slope=1.1) Dy (slope=1.3)

E (%)

80 -0.6

60

Pr Nd Dy

0.3

0.4

0.5

0.6

-

(B)

1.0

log D

0.2

0.4

Fig. 4. Effect of [TFSA]IL concentration in dichloromethane on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 in distilled water, respectively. Feed solution: pH = 4.5; extracting phase: [TBP] = 1.1 M.

[TFSA ]aq (M)

0.8

0.6 Pr (slope=1.8) Nd (slope=1.6) Dy (slope=1.5)

-0.6

0 -

20

1.2

-0.2

log [TFSA ]IL

40

0

-0.4

-0.5

-0.4

-0.3

-0.2

-0.1

-

log [TFSA ]aq Fig. 3. Effect of [TFSA]aq concentration from KTFSA on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 in distilled water, respectively. Feed solution: pH = 4.5; extracting phase: [TBP] = 1.1 M.

aqueous phase would influence effectively extraction of metal ion compared to that from the IL phase. In order to confirm this anion effect, we attempted to extract Nd (III) from 0.1 M Nd(TFSA)3 or 0.1 M Nd(NO3)3 in distilled water by 1.8 M TBP with [P2225] [TFSA]. Contrary to our expectations, a large amount of Nd (III) in the aqueous phase derived from Nd(TFSA)3 was extracted in [P2225][TFSA]. On the other hand, the Nd (III) in the aqueous phase derived from Nd(NO3)3 was hardly extracted in [P2225][TFSA]. As a result, each extraction efficiency of Nd(III) from Nd(TFSA)3 and Nd(NO)3 was 96.5% and 3.9%, respectively. Hence, this results enabled us to indicate that the TFSA anions in the aqueous phase were significantly contributed with the extraction of RE(III) ions. Consequently, RE ions in the aqueous phase including TFSA anions are found to be effectively extracted in the TFSA-based IL extraction system in comparison with the aqueous phase including other anions. Moreover, in order to support the cation-exchange mechanism in [P2225][TFSA] based extraction system, the extraction experiment was conducted changing Pþ 2225 concentration in the aqueous phase as displayed in Fig. 5. As seen in Fig. 5, the extraction efficiency of RE(III) was diminished with increasing the Pþ 2225 concentrarion. This trend would mean transfer of the equibilium as expressed in Eq. (11) owing to increase of Pþ 2225 concentration in the aqueous phase disturbed the formation of complex in the IL phase. Therefore, this result indicated that Pþ 2225 was released into the aqueous phase by cation-exchange mechanism.

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100

100

80

80

60

E (%)

E (%)

Nd

Pr Nd Dy

60

40

40

20

20

0

0.1

0.2

0

0.3

0.5

1.0

+

1.5

2.0

[HTFSA] aq (M)

[P2225 ]aq (M) Fig. 5. Effect of Pþ 2225 concentration on the extraction efficiency of Pr(III), Nd(III) and Dy(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 in distilled water, respectively. Feed solution: pH = 4.5; extracting phase: [TBP] = 1.1 M.



The distribution of RE(III) from TFSA anions present in the aqueous phase to the IL phase with various types of ILs was investigated in order to compare the properties of [P2225][TFSA] as a diluent for TBP to that of other ILs. The extraction efficiency of RE(III) by 1.8 M TBP with various ILs was listed in Table 1. The extraction mechanism as described above was found to be functioned in all IL extraction systems in the present study. Although a high extractability was observed in any IL systems, the extraction efficiency decreased slightly with increasing the alkyl chain length of the ILs on phosphonium-based ILs and imidazolium-based ILs. Each IL with the longer chain became relatively more hydrophilic [11]. This result suggested that the extractability would be dependent on a difference of the degree of hydrophobicity for each IL. Considering from Eq. (11), the extractability might be slightly higher in the IL extraction system which cations of IL are released easily into the aqueous phase. By comparison with the phosphonium and imidazolium cations, the extraction efficiency on phosphonium-based IL with long chain corresponds approximately to that on imidazolium-based IL with short chain. It is guessed that the structure of the phosphonium cation is released easily compared to imidazolium cation. As the above result and electrochemical properties of [P2225][TFSA], [P2225][TFSA] is the most beneficial media as an electrolyte and a diluent for TBP of the three phosphonium-based ILs investigated in the present study. The effect of HTFSA for the extraction of Nd(III) was investigated as indicated in Fig. 6. From the result, as the HTFSA concentration in the aquous phase was increased, the extracted Nd(III) was decresed. TBP might be difficult to solvate to metal ion in the solution concentrated high HTFSA due to extraction of the acid. In order to investigate the extraction behavior of RE(III) ions in the complex system of real situation, RE(III) ions composed of VCM from the prepared solution in Section 2.2. were extracted by TBP with [P2225] [TFSA]. The metal components of the solution were listed in Table 2. The above slope analysis for concentrations of TBP, TFSA in the

Fig. 6. Effect of HTFSA concentration on the extraction efficiency of Nd(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3. Extracting phase: [TBP] = 1.8 M.

Table 2 Composition of leaching metal ions in 2.0 M HTFSA using VCM sample (wt.%), pH = 0.1.

a

B

Fe

Pr

Nd

Dy

Tracesa

2.6

24.8

7.1

57.6

1.2

6.7

Traces are Al, Co and Ga.

aqueous phase and [P2225][TFSA] was conducted applying the solution as the aqueous phase. Although RE (III) ions were extracted from the feed solution prepared by 2.0 M HTFSA after adding KTFSA into the solution, the extraction efficiency was not enhanced in spite of an increase of [TFSA]aq. Presumably, the effect of TFSA anion was limited because TFSA anions were excessively present in the solution. Thus, the feed solution in which metal ions were leached by 0.2 M HTFSA was applied as the aqueous phase adjusted TFSA anion so as to observe the effect of the anion. As the results in Figs. 7–9, the extraction efficiency of RE(III) was increased with an increase of the concentrations such as [TBP]IL, [TFSA]IL and [TFSA]aq. Moreover, m, x, and y in Eq. (6) were approximately 3, 1, and 2, respectively. The determined stoichiometry of TBP and TFSA anions from aqueous and IL phases with RE(III) was corresponded to cases of the above simple mixture. Accordingly, regardless of the real system which present elements and the concentrations in the solution were complex, RE(III) ions were considered to be distributed into the IL phase according to Eq. (11). 3.2. UV–Vis–NIR spectroscopic analysis The UV–Vis–NIR absorption spectra of [P2225][TFSA] containing 0.1 M Pr(TFSA)3, Nd(TFSA)3 or Dy(TFSA)3 and RE(III) extracted by 1.8 M TBP with [P2225][TFSA] from the aqueous phase described in Section 2.3. are shown in Fig. 10. Each ground state of Pr, Nd and Dy is 3H4, 4I9/2 and 6H15/2, respectively. In the UV–Vis absorption spectra several intraconfigurational transitions within

Table 1 Effect of various ILs as diluents for TBP on extraction efficiency E (%) of Pr(III), Nd(III) and Dy(III) from 0.1 M Pr(TFSA)3, Nd(TFSA)3 and Dy(TFSA)3 in distilled water: [TBP] = 1.8 M, pH = 4.5.

Pr(III) Nd(III) Dy(III)

[P2225][TFSA]

[P2228][TFSA]

[P222(12)][TFSA]

[C2mim][TFSA]

[C6mim][TFSA]

96.7 97.6 96.3

95.0 96.7 95.4

92.5 93.6 91.9

94.8 97.9 95.0

90.1 93.5 91.0

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M. Matsumiya et al. / Separation and Purification Technology 130 (2014) 91–101

100

(A)

80

80

60

60

E (%)

E (%)

100

(A)

40

40

20

20

Pr Nd Dy

0

1.5

2.0

2.5

0

3.0

Pr Nd Dy 0.2

0.3

[TBP]IL (M) 2.0

0.4

0.5

-

[TFSA ]aq (M) 1.4

(B)

(B)

1.2

1.0

0

log D

log D

1.0 0.8 0.6

-1.0 0.4 Pr (slope=3.0) Nd (slope=2.7) Dy (slope=2.7)

-2.0

0

0

0.1

0.2

0.3

0.4

0.5

Pr (slope=1.8) Nd (slope=1.7) Dy (slope=1.7)

0.2

0.6

log [TBP] IL Fig. 7. Effect of TBP concentration on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) leached in 2.0 M HTFSA from 2.0 g VCM. Feed solution (ppm): Pr: 230, Nd: 1890, Dy: 40, pH = 0.1.

4f shell of RE(III) were visible. The NIR absorption spectra of Dy(III) was based on the interaction between the Dy(III) and the surrounding ligands. The UV–Vis–NIR spectra of RE(III) dissolved in [P2225][TFSA] and RE(III) extracted by TBP with [P2225][TFSA] were also observed several peaks at similar wavelength. It was reported that a hypersensitive peak was sensitive to the surrounding ligands in solvent components [34]. The hypersensitive peaks of RE(III) such as Pr(III), Nd(III) and Dy(III) dissolved in [P2225][TFSA] appeared at approximately 444 (3H4 ? 3P2), 577 (4I9/2 ? 2G2/7), and 1288 nm (6H15/2 ? 6F11/2), respectively. On the other hand, those of RE (III) extracted by TBP with [P2225][TFSA] were slightly shifted from the absorption wavelength. In addition, each absorbance of the hypersensitive peaks on RE(III) coordinated by TBP with [P2225][TFSA] was smaller than that on RE(III) in [P2225] [TFSA]. The alternation of the spectra would be caused by changing the solvation structure by coordination of TBP ligands with the centered RE3+ cations in IL phase. The slight shift of the peak indicated that the O atom of TBP ligands would preferentially coordinate with the centered RE3+ as a function of extractant because the strength of O atom in TBP was expected to be larger than that of TFSA anion. It was also observed that the hypersensitive peak was shifted and the intensity was decreased in the absorption spectrum of RE(III) in H2O solvent compared with the spectrum of RE(III) in [P2225][TFSA] solvent [18,21]. This result indicated that the coordinate strength of H2O molecule for the centered RE3+ cat-

-0.7

-0.6

-0.5

-0.4

-0.3

-

log [TFSA ]aq Fig. 8. Effect of [TFSA]aq concentration from KTFSA on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) leached in 0.2 M HTFSA from 2.0 g VCM. Feed solution (ppm): Pr: 24, Nd: 190, Dy: 4.0, pH = 1.0; extracting phase: [TBP] = 1.0 M.

ion was stronger than that of TFSA anion. Therefore, according to HSAB theory, the coulombic interaction between the centered RE3+ cation and TFSA ligands was weaker than that between RE3+ cation and H2O or TBP ligands. Considering from the extraction mechanism described as above, three TBP molecules solvated with 3þ  RE3+ cation ð½REðTBPÞ anions combined with h  3 IL Þ and three iTFSA 3þ  from aqueous and IL the complex REðTBPÞ3 ðTFSA Þ3 IL phases in an electrostatic interaction due to electroneutrality at the extraction equilibrium. It was confirmed that a peak observed at approximately 1200 nm in the spectrum of Dy(III) extracted by TBP with [P2225][TFSA] was derived from free TBP molecules. This peak was also obtained in the spectra of Pr(III) and Nd(III). 3.3. Raman spectroscopic analysis A Raman spectrum of Nd(III) in 1.8 M TBP with [P2225][TFSA] was measured in the frequency range of 200–1800 cm1 as shown in Fig. 11. It was known that the Raman spectrum of ILs based on the TFSA anion with an intense band was assigned to the CF3 bending vibration ds (CF3) coupled with the S-N stretching vibration ms (SNS) at around 740 cm1 [30,31]. Hence, the intense band at around 740 cm1 observed in the Raman spectrum of the IL would be attributed to be the bending vibration ds (CF3) and the stretching vibration ms (SNS) of the TFSA anion. The Raman

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100

(A) Absorbance A (a.u.)

E (%)

80

60

40

20

0

*

P2

3

P1

3

P0

Pr Nd Dy 0.2

0.4

0.6

0.8

1.0

20

21

Absorbance A (a.u.)

log D -0.8

Pr (slope=1.3) Nd (slope=1.2) Dy (slope=1.2)

-1.2

-0.6

-0.4

-0.2

2

*

G7/2

4

F5/2 4

F7/2

12

13

0

14

15

16

17

18

Wavenumber ν (10 cm ) 3

log [TFSA ]IL 

6

* F11/2

7

-1

Ground term:6H15/2 *:Hypersensitive peak

Dy

Absorbance A (a.u.)

spectra in the frequency range of 720–770 cm1 for 1.8 M TBP with [P2225][TFSA] containing 0.3–0.6 M Nd(TFSA)3 were deconvoluted into two components at around 740 and 750 cm1, as shown in Fig. 12. The band at 750 cm1 was ascribed to the TFSA anion bound to the metal ion, and the vibrational mode of the band at 740 cm1 corresponds to the free TFSA anion. As can be seen from Fig. 12, a new band was observed as a shoulder at the higher frequency side (750 cm1) of the intense band (740 cm1), and this new band was intensified with an increase in the Nd(TFSA)3 concentration. Similar results at the Raman spectra of [P2225][TFSA] containing Nd(TFSA)3 [20] and Dy(TFSA)3 [22] have been reported. Therefore, the band of around 750 cm1 in the present system was also considered to be due to the TFSA anions bound to the centered Nd3+ cation. According to the procedures described in Section 2, the number of the TFSA anion solvated to metal ion in the IL can be evaluated from the If/cf vs. cT/cM plots. Fig. 13 showed a plot of If/cf against cT/ cM, which was found to give a good liner relationship. Hence, the estimated n value was evaluated as 3.2. This result enabled us to suggest that the centered Nd3+ cation in TBP with [P2225][TFSA]  was solvated by three TFSA anions, ½NdðTBPÞ3þ 3 ðTFSA Þ3 IL complexation as described in the above extraction mechanism analysis. It was also indicated that the number of the TFSA anion solvated to the centered Pr3+ and Dy3+ cations in the IL can be evaluated as approximately 3.0, respectively. Moreover, we have already

24

-1

Nd 4 Ground term: I9/2 *:Hypersensitive peak

-

Fig. 9. Effect of [TFSA ]IL concentration in dichloromethane on (A) the extraction efficiency and (B) the distribution ratio of Pr(III), Nd(III) and Dy(III) leached in 2.0 M HTFSA from 2.0 g VCM. Feed solution (ppm): Pr: 230, Nd: 1890, Dy: 40, pH = 0.1; extracting phase: [TBP] = 1.8 M.

23

Wavenumber ν (10 cm )

(B)

-0.4

22 3

[TFSA-]IL (M) 0

3

Pr 3 Ground term: H4 *:Hypersensitive peak

6

F7/2

6

F9/2

8

9

10

11

12

Wavenumber ν (10 cm ) 3

-1

Fig. 10. The UV–Vis–NIR spectra of 0.1 M RE(TFSA)3 such as Pr, Nd and Dy in [P2225][TFSA] (solid line) and RE(III) extracted by 1.8 M TBP with [P2225][TFSA] from 0.1 M RE(TFSA)3 in distilled water (broken line). Each ground term of Pr, Nd and Dy is 3H4, 4I9/2 and 6H15/2, respectively. The superscript (*) denotes hypersensitive peak.

demonstrated that five TFSA anions acted as a bidentate ligand with its two O atoms and coordinated with the centered Nd3+ and Dy3+ cations in the TFSA based ILs, i.e., the complex was [RE(TFSA)5]2, according to Raman spectroscopy [20,22]. Consequently, this result was allowed us to conclude that this difference of the number of the TFSA anion solvated to RE3+ cation between in neat [P2225][TFSA] and in TBP/[P2225][TFSA] was supposed to be resulted from changing the solvation structures by coordination of TBP.

99

If / cM

Intensity (a.u.)

M. Matsumiya et al. / Separation and Purification Technology 130 (2014) 91–101

500

750

1000

1250

1500

0

2

4

Wavenumber (cm -1)

6

8

10

cT / cM

Fig. 11. Raman spectrum of Nd(III) in 1.8 M TBP with [P2225][TFSA].

Fig. 13. Relationship between If/cM and cT/cM for 1.8 M TBP with [P2225][TFSA] including Nd(III).

(A) (d) (a) 0.1 mA

Current i/mA

Intensity (a.u.)

(c)

(b)

(b)

(c)

(a)

[Nd(T BP)3]

720

730

740

750

760

3+

-

+ 3e

Nd(0) + 3T BP

770 -4.0

Wavenumber (cm-1)

-2.0

0

2.0

Potential E/V vs. Fc/Fc+ Fig. 14. Cyclic voltammogram of (a) [Nd(TBP)3]3+ in TBP/[C2mim][TFSA], (b) no rare earth complex in TBP/[P2225][TFSA] and (c) [Nd(TBP)3]3+ in TBP/[P2225][TFSA] at the scan rate of 10 mV s1 at 373 K.

(B)

Intensity (a.u.)

(d) (c)

(b)

(a)

720

730

740

750

760

770

-1

Wavenumber (cm ) Fig. 12. Raman spectra (A) and the corresponding deconvoluted spectra (B) of Nd(III) in 1.8 M TBP with [P2225][TFSA] containing (a) 0.3, (b) 0.4, (c) 0.5, and (d) 0.6 M Nd(TFSA)3.

in [C2mim][TFSA] and the cathodic limit of [C2mim][TFSA] was more positive than the reduction reaction of [Nd(TBP)3]3+. This result allowed us to reveal that it was impossible to obtain the Nd metal by direct electrodeposition of the extracted Nd(III) in TBP/[C2mim][TFSA] although Nd(III) was easily extracted in TBP/ [C2mim][TFSA]. On the other hand, the cyclic voltammograms of no Nd complexes and [Nd(TBP)3]3+ in [P2225][TFSA] at 373 K were shown in Fig. 14(b) and (c), respectively. For comparison with these electrochemical results, the remarkable cathodic peak related with [Nd(TBP)3]3+ was appeared approximately at 2.4 V in TBP/[P2225][TFSA], because the cathodic limit in TBP/[P2225] [TFSA] was more negative than TBP/[C2mim][TFSA]. According to our recent electrochemical investigations of Nd(III) complex in [P2225][TFSA] [18,19], this cathodic reaction would be assigned to the following reaction.

½NdðTBPÞ3 

3þ

þ 3e ! Ndð0Þ þ 3TBP

ð12Þ 3+

3.4. Electrochemical analysis The cyclic voltammogram of [Nd(TBP3]3+ in [C2mim][TFSA] after extraction at 373 K was shown in Fig. 14(a) and there was no cathodic peak related with [Nd(TBP)3]3+ in this kind of IL, because the reduction of C2mim+ cation occurred approximately at 2.0 V

This result indicated that the extracted [Nd(TBP)3] present in IL phase can also be reducted and deposited as Nd metal electrochemically, provided the IL phase is a good electrical conductor and has wide electrochemical window. Based on the above fundamental electrochemical analysis, the electrodepostion of Nd metal was conducted against the extracted sample in TBP/[P2225][TFSA] using a cathodic Cu substrate, an

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M. Matsumiya et al. / Separation and Purification Technology 130 (2014) 91–101

Nd3d5/2

Intensity / a.u.

981.7-982.3eV (Nd2O3)

980.5-981eV (Nd metal)

(b)

(a)

990

985

980

975

Binding energy / eV Fig. 15. XPS spectra for Nd3d5/2 region of electrodeposits with different depths, (a) top surface (981.5 eV) and (b) middle surface at 1.70 lm (980.5 eV).

anodic Nd–Fe–B rod and a platinum QRE. The applied overpotential on the cathode for the potentiostatic electrolysis was set at 3.2 V with stirring the electrolyte at 500 rpm in order to increase the current density under the electrodeposition by continuously supplying the electroactive species [Nd(TBP)3]3+ to the electrode surface. The average current density and the total transported charge under electrodeposition were 15.7 mA cm2 and 28.4 C, respectively. As for the cathode, we confirmed that the blackishbrown electrodeposits had a relatively strong adhesion on the Cu substrate. The electrodeposited Nd was investigated by XPS with Al Ka radiation and the Nd3d5/2 spectrum for the top surface and the middle layer (under 1.70 lm) of the electrodeposits was shown in Fig. 15(a) and (b), respectively. Theoretically, the Nd3d5/2 peaks for the Nd metal and oxides were positioned at 980.5–981.0 eV and 981.7–982.3 eV, respectively [35]. As shown in Fig. 15, the peak height in the Nd3d5/2 spectrum upper surface of the electrodeposits was detected at 981.5 eV. Through the Ar etching, the observed Nd3d5/2 peak shifted to lower binding energy of 980.7 eV, which was in accordance with the theoretical binding energy of Nd metal. The carbon and the oxygen bonding peaks in the electrodeposits were detected on the top surface of the electrodeposits and the residual carbon and oxygen contents in the electrodeposits were gradually decreased with increasing the depth. This result enabled us to conclude that the top surface of the electrodeposits obtained by direct electrodeposition of the extracted Nd(III) in TBP/[P2225] [TFSA] was partially non-metallic Nd state and the inner region of the electrodeposits was found to be Nd metallic state. 4. Conclusion In the present study, the fundamental solvent extraction of REs was conducted for establishment of the recycling process. RE(III) such as Pr(III), Nd(III) and Dy(III) was extracted from an aqueous phase including TFSA anions compared to that including other anions by TBP in [P2225][TFSA]. This extraction mechanism was based on ion-pair mechanism combined with cation-exchange mechanism. In the extraction experiments, distilled water dissolving RE(TFSA)3 and HTFSA leaching metal ions from VCM were applied as the aqueous phase. According to the slope analysis, RE(III) was extracted with TBP and TFSA anions from aqueous and IL phases as a 1:3:3 complex in cases of both aqueous phases. The release of Pþ 2225 into the aqueous phase by the cation-exchange mechanism was proceeded in [P2225][TFSA] extraction system because a decrease of extraction efficiency of metal ion was observed with an increase of Pþ 2225 in the aqueous phase by adding P2225Br. The extractability of extraction system using [P2225][TFSA]

with short chain cation was the highest of the phosphonium-based ILs selected in the present study. The extraction performance using phosphonium-based IL system was corresponded to that using imidazolium-based IL system. The extraction performance of Nd(III) was diminished in the extraction system with an increase of the HTFSA concentration in the aquous phase was increased. The UV–Vis–NIR spectra indicated that the complex of RE(III) extracted by TBP with [P2225][TFSA] was different from that of RE(III) dissolved in [P2225][TFSA] since the hypersensitive peak was shifted and the absorbance became lower because of solvation of TBP to RE(III). The solvation structure of Nd(III) as a representative of trivalent RE metal species in TBP/[P2225][TFSA] solutions containing Nd(TFSA)3 was examined using Raman spectroscopy. From this study, the number of TFSA anion solvated to the centered Nd3+ cation was evaluated to be about 3, therefore, the Nd(III) species in TBP/[P2225][TFSA] solutions existed as  ½NdðTBPÞ3þ 3 ðTFSA Þ3 IL complexation. This result was also consistent with the extraction mechanism analysis. These studies enabled us to conclude that main factor which affected the extraction mechanism was revealed when RE(III) ions were extracted by TBP with [P2225][TFSA]. From the electrochemical investigations, the extracted [Nd(TBP)3]3+ complex in TBP/[P2225][TFSA] was cathodically proceed to the following reaction: [Nd(TBP)3]3+ + 3e ? Nd(0) + 3TBP. The potentiostatic electrodeposition of the extracted [Nd(TBP)3]3+ in TBP/[P2225][TFSA] was executed using a three-electrode system at 373 K. The metallic state of Nd electrodeposits was analyzed by XPS. The middle layer and the top surface of Nd electrodeposits were confirmed as the metallic state and a part of the oxidation state, respectively. This study enabled us to conclude that Nd metal was able to obtain by the direct electrodeposition of the extracted [Nd(TBP)3]3+ in TBP/[P2225][TFSA] system. Acknowledgment This research was partly supported by the Environment Research and Technology Development Fund (3K123018) of the Ministry of the Environment, Japan. This work was partially supported by the Grant-in-Aid for Scientific Research (No. 26550075) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] L. Peng, Q. Yang, H. Zhang, G. Xu, M. Zhang, J. Wang, Rare earth permanent magnets Sm2(Co, Fe, Cu, Zr)17 for high temperature applications, J. Rare Earths 26 (3) (2008) 378–382. [2] O. Takeda, T.H. Okabe, Y. Umetsu, Recovery of neodymium from a mixture of magnet scrap and other scrap, J. Alloys Compd. 408–412 (2006) 387–390. [3] S. Zeng, D. Du, F. Bai, H. Su, Bridging complexes of rare earth and cobalt cluster as catalyst precursor for Fischer–Tropsch synthesis, J. Rare Earths 29 (4) (2011) 349–353. [4] L. Kokou, J. Du, Rare earth ion clustering behavior in europium doped silicate glasses: simulation size and glass structure effect, J. Non-Cryst. Solids 358 (2012) 3408–3417. [5] X. Du, T.E. Graedel, Global in-use stocks of the rare earth elements: a first estimate, Environ. Sci. Technol. 45 (2011) 4096–4101. [6] K. Binnemans, P.T. Jones, B. Blanpain, T.V. Gerven, Y. Yang, A. Walton, M. Buchert, Recycling of rare earths: a critical review, J. Clean Prod. 51 (2013) 1–22. [7] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, New material for permanent magnets on a base of Nd and Fe, J. Appl. Phys. 55 (1984) 2083– 2087. [8] Y.A. El-Nadi, Effect of diluents on the extraction of praseodymium and samarium by Cyanex 923 from acidic nitrate medium, J. Rare Earths 28 (2) (2010) 215–220. [9] M.-S. Lee, J.-Y. Lee, J.-S. Kim, G.-S. Lee, Solvent extraction of neodymium ions from hydrochloric acid solution using PC88A and sponified PC88A, Sep. Purif. Technol. 46 (2005) 72–78. [10] H. Ying, T. Mikiya, Solvent extraction equilibrium of dysprosium(III) from nitric acid solutions with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester, Trans. Nonferrous Met. Soc. China 20 (2010) 707–711. [11] S.J. Yoon, J.G. Lee, H. Tajima, A. Yamasaki, F. Kiyono, T. Nakazato, H. Tao, Extraction of lanthanide ions from aqueous solution by bis

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