Various recovery techniques for rare earths.
This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) ionic liquids (ILs) for the recovery of rare earth (RE) metals from spent Nd-Fe-B magnets. The hydrometallurgical treatments were carried out at bench scale to produce RE amide salts of high purity. In the leaching process employing 1.7 kg of oxidized Nd-Fe-B fine powder and 14.2 L of an acid medium of 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (H[TFSA]), selective leaching of RE ions (85.7±5.8% Nd) was performed at bench scale. Then, Fe (<99.9%) was successfully separated from RE ions in the deironization process. The total amount of the recovered amide salts through the evaporation treatment using a spray dryer was 3.57 kg. From the CV/EQCM measurements for Nd(III) at 373 K, a clear cathodic peak with the mass increased, and the ηρ decreased was observed at −2.79 V. Considering our previous investigations, the reduction of Nd(III)/Nd(0) was indicated as [Nd(III)(TFSA)5]2− + 3e− → Nd(0) + 5[TFSA]−. In addition, the Mapp value in the range of −2.49 V ~ −2.94 V was 46.8 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol−1. Moreover, the electrodeposition of Nd(0) was carried out under the condition of −3.20 V versus Fc/Fc+ at 373 K. The electrodeposits were identified with the metallic Nd in the middle layer investigated by X-ray diffraction and X-ray photoelectron spectroscopy. Finally, we demonstrated that the novel recovery process consisted of hydrometallurgy and electrodeposition using ILs was effective by calculating material flow.
- ionic liquids
- neodymium metal
Rare earth (RE) elements are currently regarded to be the most critical elements necessary for sustainable applications, and their RE groups play an important role in the development of future high-tech industries. Significant price fluctuations and high demand have raised their potential recovery from end-of-life products . It is important to recover them from urban mining and secondary products containing permanent magnets.
Hydrometallurgical treatment is widely applied as an effective method for extracting RE components from primary sources and is potential in reclaiming these elements . As listed in Table 1, the various techniques effective for the recovery of RE elements, such as chemical vapor transport , solvent extraction , electrolytic method , and hydrometallurgical processes [6, 7], have been listed, although a number of recovery methods for RE elements on the laboratory scale have not been so widespread. There was almost no information about the plants and processing paths. From the above situation, this study focused on a bench-scale hydrometallurgy to separate and recover RE components from spent Nd-Fe-B magnet. The preliminary research in our procedure is reported in the previous publications [8, 9].
|Chemical vapor transport||59% Nd and 68% Dy were recovered from|
scrap of RE intermetallic materials.
|Solvent extraction and|
|The selective permeation of Nd and Dy|
by IL based supported liquid-membrane
|Electrolysis||Polarization on Pt electrode led to|
recover Nd and Dy metals in fused
Na2SO4 from Nd and Dy oxides
|Electrochemical process||Mass ratio between Nd and Dy was|
indicated as 121 from RE-Ni alloys
|Hydrometallurgical process||The recovery efficiencies were indicated as|
69.7% Nd and 51% Dy from magnetic waste sludge
Pyrometallurgical treatment using high-temperature molten salts (HTMSs) is generally known as a conventional method for the recovery of RE metals. However, the HTMSs such as fluorides [10, 11] consumes a large amount of thermal energy owing to the high melting points of molten salts; thus, the recovery method of RE metals from HTMS baths is inappropriate as a next-generation technique. From the standpoint of saving energy, the development of a recovery process for RE metals with reduced energy consumption is hopeful in the near future. For this purpose, we have proposed an electrowinning-recovery method of the REs using ILs, which have unique physicochemical properties, such as high ionic conductivity, a wide electrochemical window, low vapor pressure, and incombustibility . The electrochemical behaviors and the electrodepositions of RE metals such as La, Sm, Eu, and Yb in ILs are reported [13, 14]. We have already demonstrated the electrochemical behaviors and the electrodepositions of Nd [15–18] and Dy metals [19, 20] using hydrophobic ILs, such as triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl) amide [P2225][TFSA] and 2-hydroxyethyl-trimethyl-ammonium bis(trifluoromethyl-sulfonyl) amide [N1112OH][TFSA]. In addition, we have developed the wet separation processes such as the solvent extraction using the hydrophobic ILs [21–23] and the precipitation separation [8, 9]. The wet separation process was combined with the electrodeposition for the recovery of the Nd and Dy metals from the practical wastes of Nd-Fe-B magnets.
For the purpose of the analysis of reduction behavior of Nd(III) in ILs, in situ investigation using an electrochemical quartz crystal microbalance (EQCM) was conducted in this study. The EQCM technique is based on the piezoelectric properties of a quartz crystal and can detect a nano-level mass change in a quartz crystal electrode during electrochemical experiments from the resonance frequency of the quartz . Although a conventional oscillator technique, namely self-excited or active technique EQCM, was inoperative in some ILs due to their high viscosity, the possibility of applying EQCM measurements with the use of separately excited or passive technique in highly viscous fluids including ILs was recently demonstrated and the cases of successful application have been reported [25–30]. On the EQCM measurement, the resonance resistance can also be measured and reflects a product of the viscosity and the density of the media near the quartz crystal electrode  and the viscoelasticity of the electrodeposits [26, 27, 31, 32]. Thus, the change in the mass and the viscoelasticity of the electrodeposits on the electrode and the product of the viscosity and the density of the electrolyte near the electrode, relating to the concentration of a soluble species particularly in the case of ILs [25, 26] and corresponding to the electrochemical behavior, can simultaneously be observed by using the EQCM measurements, and very useful information for the specific examination of the electrode reaction can be acquired.
In this study as a new attempt, the electrochemical behavior of Nd(III) in ILs was analyzed by EQCM measurement at elevated temperatures because it is desirable to decrease the overpotential of the electrodeposition and to increase the diffusion rate of the metallic species by lowering the viscosity by means of elevating the temperature. There were a few reports about the theoretical equations: the Sauerbrey equation  and Kanazawa-Gordon . However, these equations were not applied at elevated temperatures [25, 34]; therefore, we have demonstrated that the applicability of EQCM method in a medium temperature range around 373 K was revealed in the previous study . In addition, we discuss the electroreduction behavior of Nd(III) in ILs on the potentiostatic condition in this study.
Considering the fundamental electrochemical investigations, the electrodeposition on the condition of constant potential was carried out on a relatively large scale. The Nd metal recovered by electrodeposition is applicable in the production of new Nd-Fe-B magnets because Nd metal of high purity is obtained by polishing the oxide layer after electrodeposition using ILs. Finally, we demonstrated the effectiveness of hydrometallurgy and electrodeposition process through the material flow.
2.1. Pretreatment process
The spent Nd-Fe-B magnets were recovered from voice coil motors (VCMs) that were heated in an electric furnace at 623 K for 3 h for the demagnetization treatment. After demagnetization, the magnetic flux density was measured using a digital TESLA meter. The residual magnetic force field of this sample was almost zero, and the percentage of demagnetization was >99.9%. Then, the Ni-Cu-Ni triple layer on the Nd-Fe-B sample was removed by a grinding machine. After the stripping of the layer, fragments of Nd-Fe-B sample were pulverized using a stamp mill. The fine powders obtained were sieved to less than 150 μm and heated at 90 K h−1 to 1133 K, which was kept for 3 h in an electric furnace in order to oxidize the Nd-Fe-B components. After the roasting process, these fine powders were reground again by the automatic grinder. The surface area and the particle size (
2.2. Bench-scale leaching
The fine powders of oxidized Nd-Fe-B sample (1.7 kg) were leached in 14.2 L of a 1.0 M aqueous solution of 1,1,1-trifluoro-
2.3. Bench-scale deironization and preparation of RE salts
Dried oxygen gas was introduced into the leaching solution with a flow rate of 5.0 L min−1 after the leaching process. The oxidizing agent for Fe2+ was effective in the leaching solution at pH > 3.2. Some kinds of alkali metal hydroxides [8, 9] were acted as precipitation agents in the previous study, and the perfect removal of the iron components was successfully carried out at laboratory scale. The deironization treatment without precipitation agents is important as a further development, because the additive materials contaminated the final RE salts. Considering the leaching process, reuse of the oxidized Nd-Fe-B powder as a precipitation agent is desirable, because RE2O3 in the oxidized Nd-Fe-B sample was selectively leached in the H[TFSA] solution. The oxidized Nd-Fe-B fine powder was also available for sediment formation of [Fe(OH)
After the deironization treatment, a turquoise filtrate was obtained; the color of the solution was based on the RE components. The evaporation treatment of the filtrate was carried out by a spray dryer (SD-1000, EYELA Co., Ltd.). In the operation of the spray dryer, the inlet temperature was maintained at 473 ± 1.6 K and dried nitrogen gas was introduced through the evaporation part at 105 ± 5.0 kPa. Then, the H[TFSA] filtrate was introduced using a roller pump at 150 mL h−1. In order to recover the dried M(TFSA)
2.4. Electrochemical analysis
The resistance of a quartz oscillator and resonance frequency were observed using an EQCM system, (Seiko EG&G, QCA922) applying AT-cut platinum-coated [9 MHz,
The voltammetric measurements were carried out using an electrochemical analyzer (ALS-440A, BAS Inc.,) with the EQCM system employing the Pt-coated quartz oscillator as a working electrode. Two Pt wires with 0.5 mm inside diameter were used as a counter and a quasi-reference electrode (QRE). The counter electrode was surrounded by a Vycor glass filter at the bottom in order to prevent the diffusion of decomposition components from the anode into the electrolyte. The Pt QRE showed a high stability and a good reproducibility of the potential at elevated temperatures. The potential was compensated for the IL standard using a ferrocene (Fc)/ferrocenium (Fc+) redox couple. Before all the electrochemical measurements, the dissolved oxygen was removed from the electrolytes by bubbling Ar gas for 30 min, and the measurements were conducted under flowing Ar gas in the cell with a rate of 20 ml min−1. Cyclic voltammetry (CV) of 0.01 M Fc in [P2225][TFSA] was carried out at 298 K with a sweep rate of 1.0 mV s−1 after
A schematic illustration of the electrodeposition cell is shown in Figure 2. In the electrodeposition with the three-electrode system, a copper substrate with a surface area of 2.0 × 10−2 m2 and a platinum were employed as the working electrode and quasi-reference electrode, respectively. Fe rod was employed as a counter electrode and was surrounded with a glass tube via a Vycor glass filter at the bottom to prevent the diffusion of dissolved [Fe(TFSA)3]− complexes from the anode into the electrolyte. The electrolytic bath was stirred at 500 rpm, because the current during electrodeposition decreased immediately to the limiting current when the bath was not stirred. The overpotential was constant at −3.20 V versus Fc/Fc+ at 373 K during potentiostatic electrodeposition in a glovebox. After electrodeposition, the electrodeposits were leached into a super-dehydrated acetone (>99.5%, Wako Pure Chemical Industries, Ltd., water content <10 ppm) in a glovebox to remove the electrolyte thoroughly. The surface morphology of the electrodeposits was observed by scanning electron microscopy (SEM) and the composition of the electrodeposits was analyzed by energy dispersive X-ray analysis (EDX) (JSM-6510LA, JED-2300, JEOL, Ltd.). The metallic state and the crystallinity of the electrodeposits were evaluated by X-ray photoelectron spectroscopy (XPS) (Quantera SXM, ULVAC-PHI, Inc) and X-ray diffraction (XRD) (RINT-2500, Rigaku Co.), respectively.
3. Results and discussion
3.1. Leaching behavior at bench scale
The leaching reactions of the oxidized Nd-Fe-B sample were represented as follows:
The leaching behavior in H[TFSA] solution using the oxidized Nd-Fe-B fine powder as a precipitation agent is shown in Figure 3. The leaching percentage of Nd and Fe for 66h in the H[TFSA] solution were 85.7±5.8% and 5.8±0.1%, respectively. A drastic increase in the pH value was observed at the initial stage of the leaching process, which indicates that leaching reaction Eq. (1) mainly occurred in this system. The leaching behavior accounted for the potential (
3.2. Deironization and purification of RE amide salts at bench scale
The effective treatment for deironization is precipitation separation through the oxidation from Fe2+ to Fe3+ by oxygen bubbling, because [Fe(OH)
|Run no.||Flow rate /mL |
|Total: 3571.5 g||Ave.: 91.7%|
After the solid-liquid separation, the evaporation treatment of filtrate was performed using an improved spray dryer to effectively recover dried M(TFSA)
|Run no.||Pr||Nd||Dy||RE*||Fe||B||Molecular weight|
A frequency shift (Δ
The relationship between Δ
Although a frequency shift by the aqueous solution contacting the quartz is small, in the case of employing ILs as the electrolyte, it is necessary to consider the influence of Δ
The theoretical value of
3.4. Electrochemical analysis
For the investigation of the reduction behavior for Nd(III) in [P2225][TFSA], cyclic voltammetry with EQCM measurements (CV/EQCM) were carried out at elevated temperatures. The EQCM behavior was confirmed in advance by CV/EQCM at 298 K measuring Fc/Fc+ redox couple in [P2225][TFSA]. The potential difference, Δ
The CV/EQCM results for Nd(III) in [P2225][TFSA] at 373 K were shown in Figure 5. A clear cathodic peak with the mass increased and the
The controlled potential electrolysis with EQCM measurements (CPE/EQCM) at −3.20 V was conducted with 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K. The parameters of Δ
Considering the above fundamental electrochemical behavior of Nd(III), the electrodeposition of Nd(0) was carried out, and the condition was listed in Table 4. The electrodeposition was smoothly performed on the high anodic current efficiency. The current slowly decreased to the limiting current during electrodeposition. After the electrodeposition, the blackish electrodeposits were obtained on the Cu substrate. The electrodeposits observed by SEM had a granular morphology with a nonuniform size distribution. This morphology is considered to be explained from the fact that the initial stage of nucleation and growth occurred according to the progressive nucleation model . The quantitative analysis using EDX for the electrodeposits obtained from the electrodeposition at −3.20 V versus Fc/Fc+ is summarized in Table 5. The results indicated that the electrodeposits on Cu substrate comprised mainly Nd component. However, a small amount of O component was also included in the electrodeposits, suggesting that the oxidizing Nd metal with O atoms would occur. In order to investigate the chemical bond state of Nd, XPS analysis with Al-Kα was carried out on the electrodeposits. The metallic and oxide components for Nd correspond to the binding energies of Nd3d5/2 at 980.5–981.0 eV and 981.7–982.3 eV, respectively, in the case of monochromated Al-Kα line . The Nd3d5/2 spectra for the middle layer and top surface of electrodeposits are shown in Figure 7. For an in-depth analysis of the middle layer, the oxide layer of the electrodeposits was perfectly removed with an Ar ion beam, that is, (a) 0.42 μm under the electrodeposits. The peak maximum in the Nd3d5/2 spectrum acquired for the layers below 0.42 μm was at 980.77 eV. Hence, the electrodeposits obtained through electrodeposition using [P2225][TFSA] with M(TFSA)3 were identified as Nd metal and partial oxide mixtures. This result indicated that metallic Nd would have been electrodeposited on the Cu substrate and subsequently oxidized by O in the electrolyte, that is, residual water or dissolved oxygen. The XRD profile of the electrodeposits is shown in Figure 8 with the profile from Ref.  for Nd metal. The position of 2
3.6. Material flow of VCM recycling
As described earlier, it is worthwhile to evaluate the material flow from a series of processes such as pretreatment, hydrometallurgy, and electrodeposition using [P2225][TFSA] melts. The whole material flow is shown in Figure 9
Hydrometallurgical process based on leaching, deironization, and purification of rare earth (RE) amide salts were carried out at bench scale. In the leaching process using 1.7 kg of oxidized Nd-Fe-B sample and 14.2 L of an aqueous solution of 1,1,1-trifluoro-
The cyclic voltammetry (CV) with electrochemical quartz crystal microbalance (EQCM) was applied at elevated temperatures in this study. CV/EQCM measurements for the investigation of the reduction behavior related to Nd(III) in triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) were conducted at 373 K. At the potential of −2.79 V, the objective electrodeposition, Nd(III)/Nd(0) was confirmed because a clear cathodic peak was observed and the apparent molar mass,
This study was partly supported by the Grant-in-Aid for Scientific Research (No. 15H02848) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.
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