Trans. Mat. Res. Soc. Japan 40[4] 343-346 (2015)

Leaching of Rare Earth Elements from using Electrochemical method

Yuki Kamimoto1, Genki Yoshimura2, Takashi Itoh2, Kensuke Kuroda3, Ryoichi Ichino1, 3 1 Green Mobility Collaborative Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan 2 Graduated school of Engineering , Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan 3 EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan * Corresponding author: Fax: 81-52-747-6595, and/or e-mail:[email protected]

Neodymium contain rare earth elements. In this study, rare earth elements were recovered from neodymium magnets using the molten salt electrolysis process, thereby making the existing recycling processes for neodymium magnets obsolete. Using anodic polarization, rare earth elements were leached by controlled potential electrolysis, without leaching and other elements. The composition of rare earth elements in the molten salt was more than 99 mass% from the controlled potential electrolysis process, not accounting for the molten salt component. Rare earth elements were leached from the boundary phase first. After electrolysis, the boundary phase disappeared from the residual, and rare earth elements were not detected in the residual. The Nd2Fe14B was converted to other materials. The neodymium magnet decomposed with the leaching of the boundary phase and became brittle. It was shown that rare earth elements were leached in preference to other elements using controlled potential electrolysis.Key words: molten salt electrolysis, neodymium magnet, rare earth element, recycling

1. INTRODUCTION was measured to define the optimal conditions for The ore of rare earth elements contains radioactive leaching, and observed the behavior of leaching of the elements, which are difficult to treat in rare earth rare earth elements. element refinery processes. Because the rare earth elements have similar chemical characteristics, it is 2. EXPERIMENTAL difficult to separate them using chemical separation 2.1 Experimental equipment methods, and they have to be separated by multistep The experimental equipment is shown in Fig. 1. The cycles of solvent extraction for isolation of each element. electrolysis potential was controlled using a potentiostat Neodymium magnets contain large amounts of rare earth (HZ-5000; Hokuto Denko). The reactor was produced elements, including neodymium, praseodymium and using borosilicate glass, and was purged with an Ar . Dysprosium is more expensive than atmosphere. Ar gas was purified by molecular sieve to neodymium and praseodymium, and it is added to dehydrate. The electrolysis bath used a eutectic salt neodymium magnets to increase heat resistance. mixture of 59 mol% LiCl and 41 mol% KCl (melting Neodymium magnets are used for many home electrical point: 626 K), which was melted at 723 K. The cathode applications and electric and hybrid-power electrode was a glassy carbon rod or molybdenum plate. automobiles. In recent years, many neodymium magnets The anode electrode was a neodymium magnet, have been wasted with disposal of these products. Thus, neodymium rod, praseodymium rod, dysprosium rod, recovery processes for rare earth elements from iron wire, wire, or aluminum wire. The neodymium magnets have been developed, such as composition of the neodymium magnet is shown in hydrometallurgical treatment [1–3], hydrothermal Table I. The reference electrode was Ag/AgCl (0.1 N) in processing [4], molten metal extraction [5–8], molten a eutectic composition of LiCl–KCl and was placed in a salt extraction [9], chloride volatility processing [10], mullite tube. The electrolysis bath was kept at 473 K for and glass slag processing [11]. The rare earth oxides are 24 h under vacuum to dehydrate. collected from these processes, and these oxides have to be reduced to rare earth metal using molten salt 2.2 Analytical method electrolysis. The composition of the neodymium magnet, the Our previous research reported that rare earth electrolysis bath, cathodic deposit, and the anodic elements were leached from a LaNi5 alloy by controlled residual was analyzed by ICP-AES. Surface morphology potential electrolysis [12]. The Nd−Fe alloy consists of was observed by scanning electron microscope (SEM; and rare earth element as a LaNi5 and JSM-6330F, JEOL) equipped with an energy-dispersive can be synthesized by molten salt electrolysis. X-ray (EDX) analyzer. The electrodeposited products In this study, we focused on the selective leaching of were identified using an X-ray diffractometer (XRD; rare earth elements from neodymium magnets using XRD-6100, Shimadzu). controlled potential electrolysis. The anode polarization The current efficiency (Ceff) was calculated from the

343 344 Leaching of Rare Earth Elements from Neodymium magnet using Electrochemical method

composition change of the neodymium magnet, with constructed element of neodymium magnets. It is earth elements. The oxidized potential of dysprosium Table III Change in composition of the boundary and detected by inductively coupled plasma atomic emission suggested that leaching of iron was caused by localized was similar to neodymium and praseodymium. The main phase during the controlled potential electrolysis spectrometry (ICP-AES; Optima-3300DV, PerkinElmer) cell action in the neodymium magnet. content of rare earth elements in the leaching component process using the following equation: was 99.5 mass% for molten salt and the electrochemical Nd Dy Fe ( Z Nd nNd F  Z Pr nPr F  Z Dy nDy F ) 3.3 Leaching behavior of rare earth elements from deposition on the cathode contained 99.2 mass% of rare (A) 20.99 0.00 79.01 Ceff  neodymium magnet earth elements. The neodymium magnet was Q Main (B) 24.94 0.00 75.06 It was shown above that molten salt electrolysis is an decomposed after electrolysis. The rare earth elements phase (C) 19.75 0.00 80.25 where ZX represents the charge of each element; nX is effective recovery process for rare earth elements from were leached from the boundary phase first, and the the molar amount obtained from the ICP analysis of (D) 1.98 0.00 98.02 neodymium magnet. The neodymium magnet was leaching rate of rare earth elements from the main phase (A) 49.00 46.63 4.37 each element; F is the Faraday constant; Q is the delaminated and decomposed during the electrolysis was slower than that from the boundary phase. quantity of electricity; and X represents neodymium, Boundary (B) 43.66 36.93 19.51 process. The current value of the controlled potential phase (C) 18.91 5.54 75.54 praseodymium, or dysprosium. electrolysis changed with time, with the trend in the (D) 1.92 0.00 98.08 The molar amounts were calculated using the following current change showing a steady pattern. The change in Acknowledgement

reaction formula. current over time is shown in Fig. 3 at −1.0 V. Changes This work was supported by Environment Research 3  Nd  Nd  3e in the SEM image and XRD pattern are shown in Figs. 4 and Technology Development Fund of the Ministry of

3  and 5, respectively. The elemental content of the main the Environment, Japan 3K143005, JSPS KAKENHI Reference Pr  Pr  3e phase (Nd Fe B phase) and the boundary phase (rare Grant Number 24656457, Japan Oil, Gas and Metals 3  2 14 electrode Dy  Dy  3e earth phase) analyzed by SEM-EDX is shown in Table National Corporation, the Hori Sciences and Arts

III. In Figs. 3–5 and Table III, symbols (A)–(D), (I)–(III) Foundation, and the Tokai Foundation for Technology. Container Cathode Anode represent the following phases of the process: (A) is the 3. RESULTS AND DISCUSSION virgin material, (B)–(D) shows the time that the total 3.1 Anodic polarization -1 electric quantities for electrolysis were 50, 250, and References Furnace The scan rate of anodic polarization was 5 mV s . 1200 C, respectively. Symbols (I) shows the initial [1] J. P. Rabatho, W. Tongamp, Y. Takasaki, K. Haga, The anodic polarization curves of the elements in the electric stage, where the current value was decreased A. Shibayama, J. Mater. Cycles Waste Manag., 15, Reactor neodymium magnet are shown in Fig. 2. The oxidation over a short time, (II) shows the second electric stage, 171-178 (2013). potentials of neodymium and dysprosium were about where the current value fluctuated with the increase in [2] T. V. Hoogerstraete, S. Wellens, K. Verachtert, K. –2.2 V and were the lowest of the elements in the current, and (III) shows the third electric stage, where Binnemans, Green Chem., 15, 919-927 (2013). neodymium magnet, while the oxidation potential of Fig. 1 Experimental system the current decreased over time. At the end of stage (III), [3] Y. Kikuchi, M. Matsumiya, S. Kawakami: Solv. Extr. iron was the highest at −0.7 V. Iron is the main the current was not flowing. Res. Dev. Jpn., 21, 137-145 (2014). component of the neodymium magnet, and leaching of iron must be avoided to enable economic recycling of The of the virgin magnet was not clear [4] T. Itakura, R. Sasai, H. Itoh, J. Alloy Compd., (1) (2) (4) (6) (3) (5) from the SEM image, although it became clear with 408-412, 1382-1385 (2006). 0.4 neodymium magnets. It is suggested that rare earth electrolysis time. After electrolysis, cracks were [5] O. Takeda, T. H. Okabe, Y. Umetsu, J. Alloy Compd., elements could be leached from the neodymium magnet using controlled potential electrolysis. The potential observed in the boundary phase. The boundary phase 408-412, 387-390 (2006). 0.3 consists of concentrated dysprosium and neodymium. In [6] T. H. Okabe, O. Takeda, K. Fukuda, Y. Umetsu, ranged from −2.2 to −0.8 V. However, the oxidation / A /

period (B), the rare earth content decreased in the Mater. Trans., 44, 798-801 (2003). I potential of aluminum was between that of the rare earth boundary phase and increased in the main phase. It is [7] H. Sekimoto, T. Kubo, K. Yamaguchi, J. MMIJ., 130, 0.2 elements and iron, and it was −1.3 V. The neodymium magnet was coated with nickel film for corrosion suggested that neodymium was carried from the inner 494-500 (2014).

Current, part of the main phase to the surface by the electrolysis [8] H. Hoshi, Y. Miyamoto, K. Furusawa, J. Japan Inst. protection. The oxidation potential of nickel was higher process. In the XRD analysis, the intensity of the Met. Mater., 78, 258-266 (2014). 0.1 than that of iron, so the nickel film had to be removed Nd2Fe14B alloy decreased in period (B) compared with [9] O. Takeda, K. Nakano, Y. Sato, Mater. Trans., 55, from the neodymium magnet for this process to proceed. the virgin magnet and the sharpness increased. 334-341 (2014). 0.0 In period (C), there was a little rare earth content in [10] M. Itoh, K. Miura, K. Machida, J. Alloy Compd., 3.2 Potentiostatic electrolysis -2.5 -2.0 -1.5 -1.0 -0.5 0.0 The rare earth elements were leached by potentiostatic the boundary phase and it was shown that rare earth 477, 484-487 (2009). + Potential, E/ V vs. Ag/ Ag (10mol%) elements were leached from boundary phase. The XRD [11] T. Saito, H. Sato, S. Ozawa, J. Yu, T. Motegi, J. electrolysis from the neodymium magnet. The anode and Fig. 2 Anodic polarization curves of elements contained peak pattern of rare earth elements was not observed. Alloy Compd., 35, 3189-193 (2003). cathode electrodes were neodymium magnet and carbon in the neodymium magnet: (1) neodymium, (2) The crack of the boundary phase was expanded. The [12] H. Yamamoto, K. Kuroda, R. Ichino, M. Okido, rod, respectively. Since tantalum is the stabilizing dysprosium, (3) iron, (4) aluminum, (5) nickel, (6) residual of period (C) was decomposed with recovery Denki Kagaku, 68, 591-595 (2000). material under this experimental condition, the neodymium magnet from the electrolyte. It is suggested that the noise in the neodymium magnet was inserted into a bucket current value was caused by the change in surface area constructed from tantalum plate to avoid diffusion of the with decomposition of the neodymium magnet. In period (A) (B) (C) (D) magnet residual from the electrolyte. The magnet was (I) (II) (III) (D), the current value decreased smoothly. The rare Table I Composition of the neodymium magnet 0.5 cut to a size of 0.5 by 0.5 by 0.2 mm. The tantalum earth element content of the electrolyzed magnet Nd Pr Dy Fe B Al bucket carried 1 g of neodymium magnet. decreased to less than 2 mass%. As the surface 0.4 The electrolysis potential was −1.0 V, and the quality Magnet 18.0 3.9 9.8 67.0 0.8 0.5 morphology was not decomposed and changed, the XRD of electricity was 1300 C. Table II shows the elemental

content of the residual electrolyzed neodymium magnet pattern of the Fe-B alloys was observed. The leaching / A Table II Composition of the neodymium magnet, I 0.3 rate was slower in period (D) than in the other periods. It and the molten salt. residual, molten salt, and cathode following The residual of iron content increased and that of rare is suggested that rare earth elements were mainly electrochemical deposition 0.2 Col 1 vs Col 2

leached by diffusion reactions in the solid. Current, earth element decreased. Rare earth elements were Nd Pr Dy Fe B Al

leached from the neodymium magnet into the molten Magnet 18.0 3.9 9.8 67.0 0.8 0.5 4. Conclusion 0.1 salt, and the total rare earth content in the molten salt Residual 1.7 1.0 0.4 95.1 1.3 0.5 Rare earth elements were dissolved from neodymium was 99.5 mass%. The cathode was coated with a Molten magnet using electrolysis in a molten eutectic mixture of 56.4 31.0 12.1 0.1 0.2 0.3 metallic film and the rare earth content of the film was salt 0.0 LiCl and KCl. The electrolysis potential of the more than 99.2 mass%. The film also contained iron, as Cathode 56.8 30.4 12.0 0.4 0.1 0.3 1 10 100 1000 10000 the reduction potential of iron was highest in these neodymium magnet was −1.0 V for recovery of rare Time, t/ s Yuki Kamimoto et al. Trans. Mat. Res. Soc. Japan 40[4] 343-346 (2015) 345

earth elements. The oxidized potential of dysprosium Table III Change in composition of the boundary and was similar to neodymium and praseodymium. The main phase during the controlled potential electrolysis content of rare earth elements in the leaching component process was 99.5 mass% for molten salt and the electrochemical Nd Dy Fe deposition on the cathode contained 99.2 mass% of rare (A) 20.99 0.00 79.01 earth elements. The neodymium magnet was Main (B) 24.94 0.00 75.06 decomposed after electrolysis. The rare earth elements phase (C) 19.75 0.00 80.25 were leached from the boundary phase first, and the (D) 1.98 0.00 98.02 leaching rate of rare earth elements from the main phase (A) 49.00 46.63 4.37 was slower than that from the boundary phase. Boundary (B) 43.66 36.93 19.51 phase (C) 18.91 5.54 75.54 (D) 1.92 0.00 98.08 Acknowledgement

This work was supported by Environment Research and Technology Development Fund of the Ministry of the Environment, Japan 3K143005, JSPS KAKENHI Reference

Grant Number 24656457, Japan Oil, Gas and Metals electrode National Corporation, the Hori Sciences and Arts

Foundation, and the Tokai Foundation for Technology. Container Cathode Anode

References Furnace

[1] J. P. Rabatho, W. Tongamp, Y. Takasaki, K. Haga,

A. Shibayama, J. Mater. Cycles Waste Manag., 15, Reactor

171-178 (2013).

[2] T. V. Hoogerstraete, S. Wellens, K. Verachtert, K.

Binnemans, Green Chem., 15, 919-927 (2013). Fig. 1 Experimental system [3] Y. Kikuchi, M. Matsumiya, S. Kawakami: Solv. Extr.

Res. Dev. Jpn., 21, 137-145 (2014).

[4] T. Itakura, R. Sasai, H. Itoh, J. Alloy Compd., (1) (2) (4) (6) (3) (5) 408-412, 1382-1385 (2006). 0.4

[5] O. Takeda, T. H. Okabe, Y. Umetsu, J. Alloy Compd.,

408-412, 387-390 (2006). 0.3 [6] T. H. Okabe, O. Takeda, K. Fukuda, Y. Umetsu,

/ A /

Mater. Trans., 44, 798-801 (2003). I

[7] H. Sekimoto, T. Kubo, K. Yamaguchi, J. MMIJ., 130, 0.2

494-500 (2014).

Current, [8] H. Hoshi, Y. Miyamoto, K. Furusawa, J. Japan Inst.

Met. Mater., 78, 258-266 (2014). 0.1

[9] O. Takeda, K. Nakano, Y. Sato, Mater. Trans., 55,

334-341 (2014). 0.0 [10] M. Itoh, K. Miura, K. Machida, J. Alloy Compd., -2.5 -2.0 -1.5 -1.0 -0.5 0.0 477, 484-487 (2009). + Potential, E/ V vs. Ag/ Ag (10mol%) [11] T. Saito, H. Sato, S. Ozawa, J. Yu, T. Motegi, J. Fig. 2 Anodic polarization curves of elements contained Alloy Compd., 35, 3189-193 (2003). in the neodymium magnet: (1) neodymium, (2) [12] H. Yamamoto, K. Kuroda, R. Ichino, M. Okido, dysprosium, (3) iron, (4) aluminum, (5) nickel, (6) Denki Kagaku, 68, 591-595 (2000). neodymium magnet

(A) (B) (C) (D) (I) (II) (III) Table I Composition of the neodymium magnet 0.5

Nd Pr Dy Fe B Al

0.4 Magnet 18.0 3.9 9.8 67.0 0.8 0.5

/ A Table II Composition of the neodymium magnet, I 0.3 residual, molten salt, and cathode following electrochemical deposition 0.2 Col 1 vs Col 2 Nd Pr Dy Fe B Al Current, Magnet 18.0 3.9 9.8 67.0 0.8 0.5 0.1 Residual 1.7 1.0 0.4 95.1 1.3 0.5 Molten 56.4 31.0 12.1 0.1 0.2 0.3 salt 0.0 Cathode 56.8 30.4 12.0 0.4 0.1 0.3 1 10 100 1000 10000 Time, t/ s 346 Leaching of Rare Earth Elements from Neodymium magnet using Electrochemical method

Fig. 3 Time-dependent change in current during the controlled potential electrolysis process: (A) High Density Formation of FePt Alloy Nanodots Induced by Remote Hydrogen unelectrolysis magnet, (B) 50 C, (C) 250 C, (D) 1200 C Plasma and Characterization of their Magnetic Properties

A B Ryo Fukuoka1, Katsunori Makihara1*, Hai Zhang1, Akio Ohta2, Takeshi Kato1, Satoshi Iwata1 Mitsuhisa Ikeda3, and Seiichi Miyazaki1 1 Graduate School of Engineering, Nagoya University, Furo-cho Chikusa-ku, Nagoya, 464-8603, Japan 10µm 10µm 2 Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan C D 3 Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8530, Japan * Corresponding author: Fax: +81-52-789-2727, and/or e-mail: [email protected]

10µm 10µm We demonstrated formation of magnetic nanodots (NDs) made of FePt alloy by exposing a metal bi-layer stack to remote H2 plasma and characterized their properties. Pt/Fe Fig. 4 Time-dependent change in the SEM image of the bi-layer stacked structures formed on SiO2 were exposed to remote H2 plasma generated by neodymium magnet during the controlled potential inductive coupling with an external single-turn antenna connected to a 60-MHz generator. electrolysis process: (A) unelectrolysis magnet, (B) 50 C, After the remote H2 plasma exposure, the formation of electrically isolated FePt-NDs with an (C) 250 C, (D) 1200 C areal density of ~1011 cm-2 was confirmed. These results imply that surface migration and agglomeration of Fe and Pt atoms induced by remote H2 plasma is promoted simultaneously with the alloying reaction. The FePt-alloy NDs exhibited a large perpendicular anisotropy with an Fe B Nd Dy 3 out-of-plane of ~4.8 kOe, while the in-plane and out-of-plane of the Pt/Fe Nd2Fe14B Nd4.5Fe82.5B12.5 Fe0.9B0.08 500 (A) bi-layer were almost zero, reflecting the small -crystalline anisotropy of the Fe layer. 400 Key words: FePt, Nanodots, Remote H2 Plasma 300 200 100 1. INTRODUCTION 1:1 and 3:1. Subsequently, the Pt/Fe bi-layer stack Intensity (CPS) 0 The application of metal-based nanodots (NDs) to the as-prepared was exposed simply to a H2-RP without 100 (B) 80 floating gate in MOS memories has been attracting external heating [5]. The plasma was generated by 60 much attention from the viewpoint of improvement inductive coupling with an external single-turn antenna 40 charge retention characteristics [1-5]. One of the major connected to a 60-MHz generator through a matching 20 issues for the floating gate application of metal-based circuit. During the H2-RP exposure, gas pressure and (CPS) Intensity 0 nanodots is to minimize metal diffusion into the gate VHF power were maintained at 13.3 Pa and 500 W, 120 100 (C) dielectric layer [6], which is responsible for the respectively. The areal dot density was evaluated by 80 degradation of oxide reliability and a large variation in atomic force microscopy (AFM). Also, electron 60 40 the memory window. So far, we reported the injection to and extraction from NDs were carried out by 20 formation of Ni and Co-NDs on SiO2 with an areal scanning the sample surface with an electrically biased Intensity (CPS) 0 density as high as 1011cm-2 by exposing ultrathin Ni and AFM probe tip in a tapping mode at room temperature in 140 120 (D) Pt films on SiO2 to remote H2 plasma without any clean room air, where a Rh-coated Si cantilever with a 100 80 external heating [7-9]. More recently, ferromagnetic radius of tip apex of ~10nm and a resonance 60 materials with a large perpendicular frequency of 27 kHz was used. Before and after 40 20 such as CoPt and FePt have also been attracting electron injection or extraction, topographic and Intensity (CPS) 0 considerable interest as magnetic storage media because corresponding surface potential images were 30354045505560 magnetic anisotropy should be large enough to ensure simultaneously taken with a non-contact Kelvin-probe 2degree, Cu K) practical thermal stability for the nonvolatile memory mode. Magnetization properties were characterized by Fig. 5 Time-dependent change in the XRD pattern of the application [10-14]. In this work, we extended our magnetic force microscopy (MFM) by using a Si AFM neodymium magnet during the controlled potential research to the formation of FePt-alloy NDs by exposing tip coated by CoPtCr with a magnetization of 220 Oe electrolysis process: (A) unelectrolysis magnet, (B) 50 C, a Pt/Fe bi-layer stack to H2-RP and characterized their and alternation gradient magnetometer (AGM) at room (C) 250 C, (D) 1200 C magnetization properties. temperature.

(Received January 31, 2015; Accepted April 29, 2015) 2. EXPERIMENTAL 3. RESULTS AND DISCUSSION After conventional wet-chemical cleaning steps of Atomic force microscope (AFM) images for an p-type Si(100) wafers, a ~5.0-nm-thick SiO2 layer was as-evaporated ~3.6-nm-thick Pt layer on Fe/SiO2 grown at 1000˚C. Fe layers were first deposited showed a fairly smooth surface morphology with a uniformly on the SiO2 layer by electron beam root-mean-square (RMS) roughness as small as ~0.24 evaporation and then covered uniformly with Pt layers nm, being almost identical to that for the as-grown SiO2 without air exposure. We fabricated bi-layers with surface, as shown in Figs. 1(a) and (b). The result different thicknesses by keeping a total thickness. confirms uniform surface coverage with the ultrathin Each thickness of the Pt/Fe bi-layers were ~4.4 nm/~3.6 Pt/Fe bi-layer. By exposing the ~4.4-nm-thick nm and ~2.4 nm/~5.6 nm, where the film thicknesses Pt/~3.6-nm-thick Fe bi-layer stack structures on SiO2 to were designed with stoichiometric ratios of Fe to Pt of the H2-RP for 10 min, the RMS roughness was increased