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Recovery of rare-earth element from rare-earth permanent magnet waste by electro-refining in molten fluorides
Separation and Purification Technology 233 (2020) 116030
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Recovery of rare-earth element from rare-earth permanent magnet waste by electro-refining in molten fluorides
T
⁎
Yusheng Yanga,b, Chaoqun Lana, Lingyun Guoa, Zhuoqing Ana,b, Zengwu Zhaoa,b, , Baowei Lia a b
Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 014010 Baotou, China Inner Mongolia Autonomous Regional Engineering Technology Research Center of Sustainable Exploitation of Rare Earth Secondary Resources, 014010 Baotou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare-earth permanent magnet waste Separation Recovery Anode channel Molten fluoride
Tens of thousands of tons of rare-earth permanent magnet (REPM) wastes are produced worldwide every year. The recovery of rare-earth elements from the wastes can effectively reduce the exploitation of rare-earth minerals. The recovery of neodymium and praseodymium from a REPM waste is investigated by electrolysis in molten LiF-CaF2 where no anode gas releases, making the process environmentally attractive. Neodymium and praseodymium are selectively oxidized from the REPM waste and transformed into rare-earth ions. To facilitate the oxidation of rare-earth elements inside the REPM waste, we intended to make REPM electrode form channels by oxidation of Nd-Pr and Nd2Fe14B alloys. By analyzing the microstructure of REPM electrode after forming channels, it can be found that the channels provide continuous transport access of the electrolyte to the inside of the REPM electrode. The separation rates of Nd and Pr increase with increasing current, while the rate of oxidation reaction is proportional to current. In addition, the oxidation of Fe initially occurs at the inside of the REPM electrode where rare-earth elements have been first depleted. The separated rare-earth ions are directly prepared as rare-earth metals at the cathode by electrolysis, leaving the porous Fe2B alloy and metallic Fe.
1. Introduction The recovery of rare-earth elements (REEs) from their secondary sources has been paid more attention than the extraction from minerals, owing to the continuous growth of rare-earth solid wastes and the lack of rare-earth minerals [1–4]. In 2012, 53,360 tons of rare-earth solid wastes were recovered in China, including 41,700 tons of REPM wastes that includes about 30 wt% of REEs. In addition, the average annual growth rate forecast for REPM wastes will still increase by 10% in the future decade [5]. Therefore, the recycling of REEs from REPM wastes is of significantly importance for the sustainable rare-earth industry. According to the source of waste, REPM waste includes industrial wastes generated during production and end-of-life magnets. The block wastes such as end-of-life wind turbines are more easily recovered and sorted, thus their recycling is easier to industrialize. There are various approaches to separate and recover the REEs from end-of-life REPM wastes, such as hydrometallurgical processing [6,7], liquid metal extraction [8–11] and molten salt electrolysis [12–16]. The hydrometallurgical process of recycling REEs from REPM waste mainly includes calcination of wastes, dissolution of oxidized wastes, separation and extraction of REEs [6]. Although this process maintains high
recovery rate of REEs, there are several drawbacks such as complex flowsheet and the consumption of chemicals which leads to a significant environmental impact [17]. Moreover, rare-earth products, mainly related products of Pr and Nd, are generally necessary to be further converted to rare-earth metals by electrolysis, because their metals can be directly used to produce magnets [18,19]. The method of liquid metal extraction is suitable for recycling metal scraps, but the recycled products are rare-earth alloys including Mg, Ag, and Bi elements from metal extractant. The REEs have to be further separated from these rare-earth alloys. Currently, there are two approaches of recycling end-of-life REPM wastes by using molten salt electrolysis. One method is to use the reaction between REEs and fluxes to separate REEs from REPM wastes, and then REEs are extracted from molten salts by electrolysis. AlCl3 [12] and MgCl2 [13] were used as chlorinating agent to separate and extract REEs from REPM wastes in molten chlorides. In addition, AlF3, ZnF2 and FeF3 were added into fluoride salts as fluxes to extract REEs, which were capable to transform rare-earth metals into rare-earth fluorides [14,15]. The use of these fluxes can effectively separate REEs from other elements, the recycled products, however, are the rare-earth alloys including Mg, Al, Zn and Fe elements from fluxes, owing to more
⁎ Corresponding author at: Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 014010 Baotou, China. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.seppur.2019.116030 Received 8 April 2019; Received in revised form 1 August 2019; Accepted 5 September 2019 Available online 06 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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positive reduction potential of these elements from fluxes than that of rare-earth ions. Therefore, the REEs need to be further separated from these rare-earth alloys for preparing metallic rare-earth products. An alternative method is to use REPM wastes as anode without the halide conversion. The REEs from the REPM electrode are selectively oxidized and dissolved in the form of rare-earth ions that then are deposited at cathode as rare-earth metal. The possibility of recycling REEs from Nd-based alloy was investigated by this method in LiCl-KCl melt where the electrochemical signal of RE ions was detected [16]. The oxidation reaction (NdFeB(s)-3e− → Nd(III) + Fe-B(s)) was observed above 1123 K. Since this temperature exceeds the eutectic temperature of LiCl-KCl mixture by 497 K, there is a volatilization of melts. Moreover, it was reported that the oxidation reaction of iron occurred because of the limited diffusion rate of REEs from the inside of the block REPM waste to its surface. However, it is difficult to make the diffusion rate of REEs in a solid electrode close to their oxidation rate. To facilitate the diffusion of REEs, we supposed that some anode channels can be formed in the REPM electrode by oxidation reaction. These channels could provide continuous transport paths between the electrolyte and the REEs inside the electrode, thereby promoting the oxidation of REEs that do not diffuse to the surface of REPM electrode. Currently, some pores have been observed during the corrosion process of REPM alloy [20,21]. Therefore, it is critical to connect the pores to form anode channels by controlling oxidation reaction. This paper aims to recycle REEs from end-of-life REPM wastes in molten LiF-CaF2 where metallic rare-earth products were directly prepared. To facilitate the oxidation of REEs, we intended to make REPM electrode form anode channels by controlling oxidation reaction process of REEs. These channels provided continuous transport access from the electrolyte to the inside of REPM electrode. The electrochemical characteristic of REPM electrode was investigated by electrochemical technologies to obtain the parameter of forming channels. The recycled wastes and products were characterized by scanning electron microscope and X-ray spectroscopy. The relationship between oxidation reaction rates of REEs and applied currents was studied to optimize the recycling method. The separation rate of REEs from REPM wastes was evaluated by the inductively coupled plasma analyses.
ultrasonically cleaned with ethanol (99.8% purity) prior to use. 2.2. Experimental set-up and procedure The prepared LiF-CaF2 mixture was placed in a pyrolytic boron nitride crucible which was located in a tube furnace. The electrode setup is shown in Fig. S1. Four electrodes were fixed through the water cooled flange to keep the immersion depths of all electrodes. Three electrodes (tungsten wire, REPM and graphite rod) separated each other with distance of 3 cm in an equilateral triangular configuration where platinum wire was placed in the centre. Electrochemical measurements started after the molten salts were stable at the experimental temperature which was measured using Pt/Pt-10%Rh thermocouple. All electrochemical measurements were performed by using an Autolab electrochemical workstation (PGSTAT 302N, Metrohm) with Nova 1.8 software package under argon atmosphere. The rare-earth products were prepared under argon atmosphere (less than 0.1 ppm O2 and H2O) in the combined device of a glove box and a tube furnace (Fig. S2). This device can avoid the oxidation of rareearth products that are relatively active. The rare-earth products were pre-treated and sealed to prepare for other analyses in the glove box. 2.3. Sample analysis X-ray diffraction (XRD, X Pert Powder, PANalytial) was used for structure analyses of samples. The morphologies of samples were observed by scanning electron microscope (SEM, Supra 55VP, Zeiss) with Energy dispersive X-ray Spectroscopy (EDS, XFlash 6160, Bruker). Inductively coupled plasma (ICP, ICAP6300, Thermo) was used to analyze the chemical composition of samples that were thoroughly dissolved in dilute hydrochloric acid in advance. 3. Results and discussion 3.1. Electrochemical study to separate and recycle REEs from the REPM waste Fig. 1a shows the SEM image (back scattered electron mode, BSE) of original REPM waste, where dark and light phases were observed. The microstructure of dark phase is dense, while light phase appears to be embedded in the dark phase. Based on the result of EDS mapping analyses, the light phase is mainly Nd-Pr alloy. The point 1 in light phase and the point 2 in dark phase were further analyzed by EDS. The result (Fig. 1b) shows that both Nd and Pr are present in the light area where Fe is present in a low concentration. Most of Fe elements are distributed in the dark area in the form of Fe-Nd-Pr alloy that was identified as Nd2Fe14B phase by XRD (Fig. 2). Since the quantitative analysis of light element boron is inaccurate by EDS, the concentration of boron was subsequently detected by ICP. Fig. 1 shows a typical microstructure of REPM alloy by comparing with the literature [28]. According to the microstructure observation of the original REPM waste, the Nd-Pr phases are not connected. To form channels on the REPM, the Nd and Pr in the Nd2Fe14B phase must be also oxidized. The electrochemical measurements were performed to study the oxidation reactions of the two phases. Fig. 3 shows two cyclic voltammograms (CV) using REPM and graphite as counter electrode, respectively. Based on the literatures [22–25], the R4/O4 corresponds to the deposition of lithium ions and the dissolution of lithium metal, respectively. Comparing to the curve 1 (graphite as an inert counter electrode), three couples of new redox peaks was recorded on the curve 2 (using REPM as counter electrode), which are clearly observed from the magnified graph. The reduction potentials of Nd(III) and Pr(III) ions at 1123 K were calculated by Factsage 7.1 software (customer ID: 0473). The reduction potential of Nd(III) ions (−4.81 V vs. F2/F−) is more positive than that of Pr(III) ions (−4.89 V vs. F2/F−), which is consistent with the results of the literatures in molten chlorides [29,30]. In addition,
2. Experimental section 2.1. Preparation of molten salts and electrodes In our work, LiF-CaF2 mixture (80:20 mol.%) with the eutectic temperature of 1040 K was used as electrolyte to reduce the volatilization of electrolyte, which can also offer large background limits [22–26] and the solubility for rare-earth oxides [27]. In addition, the fluorides are the common electrolyte for industrial production of rareearth metals. LiF (99.99% metals basis, CAS No. 7789-24-4), CaF2 (99.99% metals basis, CAS No. 7789-75-5) and NdF3 (99.99% metals basis, CAS No. 13709-42-7) were sourced from Aladdin Industrial Corporation. All chemicals were dried before use at 773 K for 24 h in a vacuum to remove excess water. After the vacuum dehydrating, a mixture of LiF-CaF2 was ground using an agate mortar in a glove box under argon atmosphere. Platinum (Pt) (d = 1.0 mm, 99.95% purity) and tungsten (W) wires (d = 1.0 mm, 99.99% purity) were used as quasi-reference electrodes (QRE) and working electrodes (WE), respectively. A spectrally pure graphite rod (d = 6 mm, 99.99% purity) served as the counter electrode (CE). A REPM waste (8 mm × 8 mm × 1 mm, composition in wt%: 27.1Nd, 8.1Pr, 63.5Fe, 1.1B, 0.2others) was used as working electrode or counter electrode for different research purposes, which was connected with electrochemical workstation through an iron wire (d = 0.5 mm, 99.995% purity). The REPM waste came from a used wind turbine provided by Baotou research institute of rare earth. All metallic electrodes were polished thoroughly using SiC papers, then 2
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Fig. 1. (a) BSE image and (b) EDS analyses of original REPM waste.
the reduction process of Nd(III) ions was reported to be taken place in two steps in molten fluorides with Nd2O3 [31]. Since the REPM electrode came from an end-of-life wind turbine, the oxygen from the REPM is inevitably brought into the electrolyte. Therefore, peaks R1/O1, R2/ O2 and R3/O3 should correspond to the electrochemical redox of Nd (III)/Nd(II), Nd(II)/Nd(0) and Pr(III)/Pr(0), respectively. Based on the above analysis, the REEs can be oxidized from the REPM anode and then deposited on the cathode. In order to further investigate the electrochemical reactions of Nd and Pr in LiF-CaF2 system, a series of square wave voltammogram (SWV) at different frequencies was obtained on a W electrode. The results are shown in Fig. 4a where the potentials and reactions of four signals (R1, R2, R3 and R4) correspond to those of Fig. 3. Gaussian fitting model was used to analyze the peak R2 that is the most complete one among the four peaks. The results can be mathematically analyzed by the following equation [32]:
W1/2 = 3.52RT / nF
(1)
where W1/2 is the half-peak width (V), T is the temperature (K), R is the ideal gas constant (J mol−1 K−1), n is the number of electrons transferred, and F is the Faraday constant (C mol−1). The validity of Eq. (1) was proved by plotting peak current density versus the square root of frequency [33], as shown in Fig. 4b. The linear relation indicates that Eq. (1) can be applied to calculate the number of electrons transferred in the frequency range. The values of n at different frequencies were calculated by Eq. (1) and listed in the Fig. 4a. The average of n is 1.78, therefore the reaction of peak R2 should involve two-electron transfer. The electrochemical reactions of Nd and Pr were verified by a galvanostatic oxidation and the addition of NdF3. Galvanostatic oxidation was carried out at 0.1A on the REPM working electrode. The measurement was ceased after 360 s, 720 s, 2160 s and 2460 s oxidation, respectively. Then SWV method was used to measure current densities of cathodic peaks on a W working electrode. NdF3 was added into the melts for calibration after the measurement of last oxidation. All SWV curves are shown in Fig. 5 where the current densities of these peaks increased with increasing oxidation time, suggesting more REEs dissolution into the electrolyte with further oxidation. With the addition of 0.1 g NdF3, the current densities of peak R1 and R2 increased obviously. This result demonstrates that the reactions of peak R1 and R2 correspond to the reduction of Nd ions. To investigate the oxidation reaction of Nd2Fe14B phase, REPM was used as the working electrode. Fig. 6a shows two CV curves which were
Fig. 2. XRD pattern of original REPM waste.
Fig. 3. Cyclic voltammograms in molten LiF-CaF2 at 1123 K. working electrode: W; reference electrode: Pt; counter electrodes: graphite (curve 1) and REPM (curve 2); sweep rate: 0.1 V s−1.
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Fig. 4. (a) Square wave voltammograms at different frequencies in molten LiF-CaF2 after the dissolution of Pr and Nd from REPM. Temperature: 1123 K; pulse height: 25 mV; potential step: 1 mV; working electrode: W; reference electrode: Pt; counter electrode: graphite. (b) Variation of the peak current density versus the square root of the frequency in the Fig. 4a.
measured at the different limiting potentials. Comparing to the CV curve on the W electrode, two new reduction signals at −1.03 V and −1.34 V (vs. Pt) were observed on the REPM electrode, which should be corresponding to the deposition of rare-earth ions on the active REPM electrode. In addition, only oxidation signal O1′ was detected when the sweep potential did not reach the potential at which the Nd and Pr ions were reduced to their metals (see the grey curve). This indicates that this electrochemical signal O1′ was owing to the oxidation reaction of metallic Nd and/or Pr from the Nd2Fe14B phase rather than Nd-Pr phase. Only that the sweep potential was more negative than the reduction potentials of Nd and Pr ions (black curve), the oxidation peaks of Nd and Pr in Nd-Pr alloy can be observed in reverse sweep. Linear sweep voltammetry with a slow sweep rate was used to obtain the current of oxidation reactions. Fig. 6b shows a linear sweep voltammogram (LSV) on a REPM electrode with the sweep potential from −1.4 V to 0 V (vs. Pt), where the oxidation currents increased with increasing potentials. A potential plateau on the REPM electrode was observed when the current reached 0.30A, which is corresponding to the oxidation reaction of Fe. In addition, the current of the oxidation reaction O1′ of REEs in Nd2Fe14B alloy was obtained to be 0.06A. Table 1 summarizes above oxidation reactions and potentials at W and REPM working electrodes. The potential difference of oxidation reaction of Nd(II) ions is resulted from the concentration polarization of Nd(II) ions near the REPM electrode in which Nd is sufficient. Based on the analyses of this section, the oxidation sequence of Nd2Fe14B phase is lower than that of Nd-Pr phase. Therefore, when the current of the oxidation reaction of REPM electrode reaches 0.06A, the micropores on
Fig. 5. Square wave voltammograms after galvanostatic oxidation (0.1 A) for different time and the addition of NdF3. Temperature: 1123 K; frequency: 20 Hz; pulse height: 25 mV; potential step: 1 mV; working electrode: W; reference electrode: Pt; counter electrode: graphite.
Table 1 Oxidation reactions and potentials at W and REPM electrodes (graphite as the counter electrode). Working electrode
Oxidation potential/V vs. Pt
Corresponding reaction
W
−1.85 −1.62 −1.46 −1.85 −1.60 −1.28 −0.82
O3: NdPr(alloy)-3e → Pr(III) + Nd(0) O2: Nd(0)-2e → Nd(II) O1: Nd(II)-e → Nd(III) O4′: NdPr(alloy)-3e → Pr(III) + Nd(0) O3′: Nd(0)-2e → Nd(II) O2′: Nd(II)-e → Nd(III) O1′: REFeB(alloy)-3e → RE(III) + FeB(alloy)
REPM
Fig. 6. (a) Cyclic voltammograms and (b) linear sweep voltammogram in molten LiF-CaF2 at 1123 K. Working electrode: REPM; reference electrode: Pt; counter electrode: graphite.
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observed during the corrosion process of REPM alloy [20,21]. As seen from Fig. 8a, these pores with different sizes connected to form the dark channels, like various lakes join together to become a river. Based on the result of EDS mapping analyses, Fe, Nd and Pr are present in the light areas, while Ca element is distributed in the dark channels, suggesting CaF2 melts entering into the channels. The EDS element analysis for the whole area is shown in Fig. 8b. Comparing with the EDS result of the original REPM (Fig. 1b), most of REEs have been removed from the REPM waste. To further study the microstructure inside oxidized REPM electrodes, the BSE images of different positions on the section of oxidized REPM were observed by SEM. Fig. 9a shows a top view of four electrodes. Three electrodes (W electrode, REPM and graphite rod) separated each other with distance of 3 cm like an equilateral triangular and the platinum wire was placed in the center. A coordinate system was given to further understand the selected positions. The BSE images at the positions of x = 1, 3, 5 and 7 are shown in Fig. 9b where the light areas are also separated into many areas by dark channels like the surface micromorphology of Fig. 8a. The second-phase constituent content of these images was determined by automatic image analysis according to ASTM Standards E1245-03. Fig. 9c shows the analysis results of dark phases, where the phases are continuous across the oxidized REPM electrode. This demonstrates that the channels have provided continuous transport paths between the electrolyte and the REEs inside the REPM electrode. Fig. 9d shows the EDS line scanning analyses at the position of x = 3. As seen from the EDS results, the distribution of Nd and Pr changes very little throughout the electrode and their counts are much lower than those of Fe and Ca elements. This indicates that most of REEs were evenly oxidized, wherever the REEs were located. In addition, the counts of Fe elements decrease at the positions of channels where the counts of Ca elements increase accordingly, suggesting that these channels in the oxidized REPM electrode were filled with LiF-CaF2 melts. The area in Fig. 9d was magnified and analyzed by EDS mapping, as shown in Fig. 9e. The cooled fluoride crystal can be observed at the channel from the magnified SEM image. In addition, CaF2 salts are present in the channel according to the EDS mapping analyses. These evidences indicate that electrolytes flowed into the channels of oxidized REPM electrode. There are still many micropores that have not yet been connected into channels on the light phase where unreacted Nd and Pr are suggested to be left. As seen from Figs. 8 and 9, LiF-CaF2 melts flowed into channels of oxidized electrodes, therefore the actual area of oxidized electrodes cannot be accurately measured by the common methods of measuring area of porous material. The area fraction of second-phase was used to quantify the variation of channel areas with positions and currents. The BSE images of oxidized electrodes obtained at different currents (Figs. 9 and S3) were equally divided into 10 parts along y-axis. Then the area fraction of second-phase of each part was plotted versus its position, as shown in Fig. 10. The area fractions are a little fluctuation, because the distribution and size of Nd-Pr alloy in the waste are not exactly same in different regions, as seen from the BSE image of original waste in Fig. 1. However, it can be considered that the area fractions of second-phase were relatively uniform in most regions, except for the closest positions to working electrodes and around the positions of y = 0.5. The oxidation reactions at the two positions seem to be more obvious, especially around the positions of y = 0.5 where the area fractions of secondphase all reached the maximum at different currents. The components of elements in the regions were further analyzed by EDS. Fig. 11 shows no REEs were found from these regions. During the oxidation reaction, the content of metallic Nd in the REPM electrode was decreasing, resulting in the variation of the partial reduction potential of each individual component in alloy. The relationship between the partial reduction potential of an individual component in an alloy and the molar fraction of the component is described as the following equation [34].
Fig. 7. (a) Cyclic voltammogram at the end of oxidation reaction of REPM electrode in molten fluorides at 1123 K. Work electrode: W; counter electrode: REPM; reference electrode: Pt; sweep rate: 0.1 V s−1. (b) Comparation of polarization curves of original and oxidized REPM electrodes in molten LiF-CaF2 at 1123 K. Working electrode: original REPM (hollow square) and oxidized REPM (solid sphere); counter electrode: graphite; reference electrode: Pt; sweep rate: 1 mV s−1.
the Nd2Fe14B phase could be formed. In addition, the oxidation current has to be lower than 0.30A to avoid the oxidation reaction of Fe, which has been observed when higher currents were applied [16]. 3.2. Electrochemical preparation and characteristics of anode channels 3.2.1. Electrochemical preparation of anode channels The experiments of making anode channels were completed by the alternate operation of galvanostatic electrolysis for 60 s on the REPM working electrode and a measurement of CV on a W working electrode. When the oxidation signal O6 of Fe was detected (Fig. 7a), the separation of REEs from the REPM electrode was considered to have been completed. In addition, the oxidation peak O5 of REEs from deposited RE-Fe alloy was also observed, and its potential was consistent with the potential obtained at the REPM working electrode. Both signals O5 and O6 show that Fe ions were present in the electrolyte. The less obvious reduction signals of Fe ions are due to the low concentration of Fe ions at the beginning of the CV measurement. With the potential sweep towards more negative direction, more metallic Fe at REPM electrode was oxidized. These newly generated iron ions were reduced at W electrode, resulting in the more obvious oxidation peaks. Meanwhile, the open circuit potential of the oxidized REPM electrode was −0.29 V (vs. Pt) as seen from Fig. 7b, which agree well with the potential plateau of the oxidation reaction of Fe in Fig. 6b. Comparing to the polarization curve of the original REPM, the open circuit potential of REPM electrodes moved from −1.25 V to −0.29 V (vs. Pt). This open circuit potential indicated the end of recycling REEs from the REPM wastes. 3.2.2. Characteristics of anode channels According to the results of electrochemical analyses, four currents (0.096A, 0.160A, 0.224A and 0.288A) were applied to oxidize REEs in the REPM electrode, respectively. Fig. 8a shows the BSE image of the surface of the oxidized REPM at the current of 0.096A, where light areas are separated into many areas by dark pores and channels. Since the sizes of these pores are close to those of the Nd-Pr phases (the light phases in Fig. 1a), these pores are suggested to be formed by the oxidation of bulk Nd-Pr metals. In addition, many micropores are also observed on the light areas, suggesting their formation by the dissolution of Nd and Pr from Nd2Fe14B phase. The micropores have been also 5
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Fig. 8. (a) Superficial BSE image and (b) EDS mapping analyses of REPM electrode oxidized at the current of 0.096 A.
RT ⎞ lnNi Ei = −⎛ ⎝ zi F ⎠ ⎜
where Asurf, Achan and Ased-ph are the external surface area of electrode, the area of formed channel and the area of second-phase on the selected region, respectively. Because Achan ≫ Asurf, the electrode area A ≈ ∑Achan. Assume that the electrode is composed of m selected regions, hence
⎟
(2)
where Ei is the difference between the equilibrium electrode potential of the component i in the alloy and the corresponding potential of the same component in the individual phase (V), Ni is the molar fraction of the component i (assuming that the alloy is a perfect solution), and zi is the number of electrons transferred in the reaction. Since R, T, zi and F are constant in this reaction, the value of ENd increases with the decrease of NNd value (NNd less than 1) according to Eq. (2). When the equilibrium electrode potential of Fe equals to that of Nd in the REPM electrode, the oxidation reaction of metallic Fe occurs. According to Eq. (2), Eq. (3) was derived to calculate the molar fraction of Nd in the electrode under Fe oxidation, which was calculated to be 3.77 × 10−24 by the data of references [22,35] and 9.96 × 10−29 by Factsage 7.1 software, respectively. This indicates that metallic Nd in the REPM alloy should have been depleted when the metallic Fe is oxidized, resulting in the additional area fractions of second-phase. The rare-earth metals were oxidized from both sides of REPM electrode to its inside, therefore it is suggested that the oxidation reaction of both sides occurred together at the position of the maximum second-phase area fractions. Based on the calculated results of Eq. (3), the maximum separation rate of REEs from REPM should be theoretically close to 100% by this recycling method, if REEs can be more uniformly oxidized.
ln
NNd zF o o = −⎛ i ⎞ (EFe − ENd ) 1 − NNd ⎝ RT ⎠
Rate = i/ nmπFAsed − ph
Based on the average Ased-ph (Fig. 12a) at the different position of electrodes, the average reaction rate of REEs was calculated by Eq. (7) and then plotted versus currents, as shown in Fig. 12b. As seen from the figures, increasing current can improve both of channel area and reaction rate. When the current distribution on the electrode is uniform (the same m value at each position), the reaction rate at the position of (3, y) is faster than that at other positions, which is considered as an optimal electrode distance. The separation rate of Nd and Pr from the REPM waste were estimated based on the ICP analyses of REPM wastes before and after the lab experiment. Fig. 12c shows the relationship between the separation rate of REEs and current. The separation rate of REEs increases with increasing current, which agree well with the results of channel areas. A better Pr recovery was obtained than Nd due to the prior sequence of Pr oxidation. 3.3. Characteristics of recycled products The recycled products were obtained by the electrolysis experiment in which W wire was used as a working electrode. To obtain lager cathodic deposits that can be collected, an anode basket woven with molybdenum wires was used as the anode, into which some REPM wastes were placed. In addition, liquid rare-earth metals were prepared at a temperature of 1293 K. Fig. 13a shows the XRD patterns of oxidized REPM, where Fe and Fe2B phases were detected after the separation of REEs, indicating that most of the metallic Nd and Pr have been removed from original REPM waste. Fig. S4 shows spherical cathode deposits which were collected at the bottom of electrolyte after the end of electrolysis. Based on the result of XRD analysis (Fig. 13b), the deposit was mainly Nd metal in which Pr metal was dissolved, because the diffraction peak of Nd metal slightly shifted. Fig. 14 shows the effects of applied current on the composition of cathodic deposits and current efficiency. The content of Nd and Pr in deposits changes little with increasing current, but the iron content has a tendency to increase rapidly. This is consistent with the results of the electrochemical research in the Section 3.1. In addition, the higher current provides higher current efficiency, and the calculated anodic current efficiency is higher the calculated cathodic current efficiency. This is resulted from the formation of metallic Li at the beginning of
(3)
The average area fraction of second-phase was used to mathematically evaluate the average rate of oxidation reaction of REEs with currents at different positions of electrodes by following Eq. (4) [32]. (4)
Rate = i/ nFA
In the Eq. (4), i is the applied current (A), n is the number of electrons transferred, F is the Faraday constant (C mol−1) and A is the electrode area (cm2). To calculate the electrode area A, some assumptions are made to simplify this problem. The current distribution on the electrode is considered to be uniform, suggesting that the area of formed channels is same at the same electrode distance. All channels are considered to be cylindrical or be composed of an infinite number of cylinders. The observed second-phase is considered to be the largest area of channel section. Based on these assumptions, the electrode area A is calculated by following Eqs. (5) and (6).
A=
∑ (Asurf
+ Achan )
Achan = πAsed − ph
(7)
(5) (6) 6
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Fig. 9. (a) Top view of four electrodes; (b) Sectional BSE images of REPM electrode oxidized at the current of 0.096A at the position of x = 1, 3, 5 and 7; (c) Secondphase pictures corresponding to Fig. 9(b); (d) EDS line scanning analyses at the position of x = 3; (e) SEM image and EDS mapping analyses of the selected area in Fig. 9(d).
were then deposited at the cathode as rare earth metals. In the early stages of this process, the pores of different sizes formed at the REPM electrode. With increasing the number of pores, some pores connected to form channels some of which extended toward the interior of the alloy. Meanwhile, the electrolyte entered into channels. When the REEs inside the electrode contacted with the electrolyte through the channels, the oxidation reactions of REEs occurred more easily. Finally, the REEs were successfully separated from REPM wastes and deposited on the cathode as rare earth metal. Further work is needed to optimize the process conditions and finally develop an industrial scale flowsheet for
galvanostatic electrolysis when there was no Nd ions in the electrolytes. Since these formed Li was difficult to be collected, the calculated cathodic current efficiency only includes the part of the current efficiency of formed rare-earth metal. In other words, this calculated value is smaller than the actual value. The anodic current efficiency is close to the cathodic current efficiency of industrial rare earth electrolysis, therefore this recycling method is promising for industrial applications. Based on the above results, we can describe the recovery process of REEs from the REPM wastes. The REEs from the REPM electrode were oxidized and dissolved into the electrolyte in the form of RE ions that 7
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Fig. 10. Relationships of second-phase area fraction with detected positions and applied currents.
provided a good access between the electrolyte and REEs inside the REPM electrode, facilitating the diffusion of REEs. The formed channels are critical to develop this recycling method for block REPM wastes in industry. The separation rates and reaction rates of Nd and Pr increased with increasing current, and the rate of oxidation reaction was proportional to current. The oxidation reaction of Fe initially occurred at the inside of the REPM alloy, owing to the depletion of RE metals. This reaction could be avoided by increasing rare-earth content on anode, e.g. the continuous supplementation of REPM wastes in an anode basket.
the REPM recycling. 4. Conclusion This work provides an alternative and promising method to recycle REEs from block REPM wastes. Nd and Pr from REPM wastes were selectively dissolved into molten LiF-CaF2 salts in the form of rare-earth ions. The separated rare-earth ions were directly prepared as rare-earth metals by electrowinning, while other elements was left in the form of Fe and Fe2B. This method is different from common molten fluoride electrolysis in that no gas release at the anode, making the process environmentally friendly. The pores and channels were formed by controlling current to oxidize REEs from Nd-Pr and Nd2Fe14B phases. This microstructure
Acknowledgements We gratefully acknowledge Prof. Muxing Guo (KU Leuven, Belgium)
Fig. 11. EDS analyses for the regions of the area fraction maximum of second-phase at different currents. 8
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Fig. 12. Effects of the applied current on (a) the average area fraction of second-phase at different positions, (b) the average reaction rate at different positions and (c) the separation rate of REEs from REPM wastes.
Fig. 14. Effects of applied currents on the composition of cathodic deposits and current efficiency. Fig. 13. XRD patterns of (a) the oxidized REPM electrode and (b) the cathodic deposit by galvanostatic electrolysis at −0.288 A.
References
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Recovery of rare-earth element from rare-earth permanent magnet waste by electro-refining in molten fluorides
T
⁎
Yusheng Yanga,b, Chaoqun Lana, Lingyun Guoa, Zhuoqing Ana,b, Zengwu Zhaoa,b, , Baowei Lia a b
Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 014010 Baotou, China Inner Mongolia Autonomous Regional Engineering Technology Research Center of Sustainable Exploitation of Rare Earth Secondary Resources, 014010 Baotou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Rare-earth permanent magnet waste Separation Recovery Anode channel Molten fluoride
Tens of thousands of tons of rare-earth permanent magnet (REPM) wastes are produced worldwide every year. The recovery of rare-earth elements from the wastes can effectively reduce the exploitation of rare-earth minerals. The recovery of neodymium and praseodymium from a REPM waste is investigated by electrolysis in molten LiF-CaF2 where no anode gas releases, making the process environmentally attractive. Neodymium and praseodymium are selectively oxidized from the REPM waste and transformed into rare-earth ions. To facilitate the oxidation of rare-earth elements inside the REPM waste, we intended to make REPM electrode form channels by oxidation of Nd-Pr and Nd2Fe14B alloys. By analyzing the microstructure of REPM electrode after forming channels, it can be found that the channels provide continuous transport access of the electrolyte to the inside of the REPM electrode. The separation rates of Nd and Pr increase with increasing current, while the rate of oxidation reaction is proportional to current. In addition, the oxidation of Fe initially occurs at the inside of the REPM electrode where rare-earth elements have been first depleted. The separated rare-earth ions are directly prepared as rare-earth metals at the cathode by electrolysis, leaving the porous Fe2B alloy and metallic Fe.
1. Introduction The recovery of rare-earth elements (REEs) from their secondary sources has been paid more attention than the extraction from minerals, owing to the continuous growth of rare-earth solid wastes and the lack of rare-earth minerals [1–4]. In 2012, 53,360 tons of rare-earth solid wastes were recovered in China, including 41,700 tons of REPM wastes that includes about 30 wt% of REEs. In addition, the average annual growth rate forecast for REPM wastes will still increase by 10% in the future decade [5]. Therefore, the recycling of REEs from REPM wastes is of significantly importance for the sustainable rare-earth industry. According to the source of waste, REPM waste includes industrial wastes generated during production and end-of-life magnets. The block wastes such as end-of-life wind turbines are more easily recovered and sorted, thus their recycling is easier to industrialize. There are various approaches to separate and recover the REEs from end-of-life REPM wastes, such as hydrometallurgical processing [6,7], liquid metal extraction [8–11] and molten salt electrolysis [12–16]. The hydrometallurgical process of recycling REEs from REPM waste mainly includes calcination of wastes, dissolution of oxidized wastes, separation and extraction of REEs [6]. Although this process maintains high
recovery rate of REEs, there are several drawbacks such as complex flowsheet and the consumption of chemicals which leads to a significant environmental impact [17]. Moreover, rare-earth products, mainly related products of Pr and Nd, are generally necessary to be further converted to rare-earth metals by electrolysis, because their metals can be directly used to produce magnets [18,19]. The method of liquid metal extraction is suitable for recycling metal scraps, but the recycled products are rare-earth alloys including Mg, Ag, and Bi elements from metal extractant. The REEs have to be further separated from these rare-earth alloys. Currently, there are two approaches of recycling end-of-life REPM wastes by using molten salt electrolysis. One method is to use the reaction between REEs and fluxes to separate REEs from REPM wastes, and then REEs are extracted from molten salts by electrolysis. AlCl3 [12] and MgCl2 [13] were used as chlorinating agent to separate and extract REEs from REPM wastes in molten chlorides. In addition, AlF3, ZnF2 and FeF3 were added into fluoride salts as fluxes to extract REEs, which were capable to transform rare-earth metals into rare-earth fluorides [14,15]. The use of these fluxes can effectively separate REEs from other elements, the recycled products, however, are the rare-earth alloys including Mg, Al, Zn and Fe elements from fluxes, owing to more
⁎ Corresponding author at: Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 014010 Baotou, China. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.seppur.2019.116030 Received 8 April 2019; Received in revised form 1 August 2019; Accepted 5 September 2019 Available online 06 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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positive reduction potential of these elements from fluxes than that of rare-earth ions. Therefore, the REEs need to be further separated from these rare-earth alloys for preparing metallic rare-earth products. An alternative method is to use REPM wastes as anode without the halide conversion. The REEs from the REPM electrode are selectively oxidized and dissolved in the form of rare-earth ions that then are deposited at cathode as rare-earth metal. The possibility of recycling REEs from Nd-based alloy was investigated by this method in LiCl-KCl melt where the electrochemical signal of RE ions was detected [16]. The oxidation reaction (NdFeB(s)-3e− → Nd(III) + Fe-B(s)) was observed above 1123 K. Since this temperature exceeds the eutectic temperature of LiCl-KCl mixture by 497 K, there is a volatilization of melts. Moreover, it was reported that the oxidation reaction of iron occurred because of the limited diffusion rate of REEs from the inside of the block REPM waste to its surface. However, it is difficult to make the diffusion rate of REEs in a solid electrode close to their oxidation rate. To facilitate the diffusion of REEs, we supposed that some anode channels can be formed in the REPM electrode by oxidation reaction. These channels could provide continuous transport paths between the electrolyte and the REEs inside the electrode, thereby promoting the oxidation of REEs that do not diffuse to the surface of REPM electrode. Currently, some pores have been observed during the corrosion process of REPM alloy [20,21]. Therefore, it is critical to connect the pores to form anode channels by controlling oxidation reaction. This paper aims to recycle REEs from end-of-life REPM wastes in molten LiF-CaF2 where metallic rare-earth products were directly prepared. To facilitate the oxidation of REEs, we intended to make REPM electrode form anode channels by controlling oxidation reaction process of REEs. These channels provided continuous transport access from the electrolyte to the inside of REPM electrode. The electrochemical characteristic of REPM electrode was investigated by electrochemical technologies to obtain the parameter of forming channels. The recycled wastes and products were characterized by scanning electron microscope and X-ray spectroscopy. The relationship between oxidation reaction rates of REEs and applied currents was studied to optimize the recycling method. The separation rate of REEs from REPM wastes was evaluated by the inductively coupled plasma analyses.
ultrasonically cleaned with ethanol (99.8% purity) prior to use. 2.2. Experimental set-up and procedure The prepared LiF-CaF2 mixture was placed in a pyrolytic boron nitride crucible which was located in a tube furnace. The electrode setup is shown in Fig. S1. Four electrodes were fixed through the water cooled flange to keep the immersion depths of all electrodes. Three electrodes (tungsten wire, REPM and graphite rod) separated each other with distance of 3 cm in an equilateral triangular configuration where platinum wire was placed in the centre. Electrochemical measurements started after the molten salts were stable at the experimental temperature which was measured using Pt/Pt-10%Rh thermocouple. All electrochemical measurements were performed by using an Autolab electrochemical workstation (PGSTAT 302N, Metrohm) with Nova 1.8 software package under argon atmosphere. The rare-earth products were prepared under argon atmosphere (less than 0.1 ppm O2 and H2O) in the combined device of a glove box and a tube furnace (Fig. S2). This device can avoid the oxidation of rareearth products that are relatively active. The rare-earth products were pre-treated and sealed to prepare for other analyses in the glove box. 2.3. Sample analysis X-ray diffraction (XRD, X Pert Powder, PANalytial) was used for structure analyses of samples. The morphologies of samples were observed by scanning electron microscope (SEM, Supra 55VP, Zeiss) with Energy dispersive X-ray Spectroscopy (EDS, XFlash 6160, Bruker). Inductively coupled plasma (ICP, ICAP6300, Thermo) was used to analyze the chemical composition of samples that were thoroughly dissolved in dilute hydrochloric acid in advance. 3. Results and discussion 3.1. Electrochemical study to separate and recycle REEs from the REPM waste Fig. 1a shows the SEM image (back scattered electron mode, BSE) of original REPM waste, where dark and light phases were observed. The microstructure of dark phase is dense, while light phase appears to be embedded in the dark phase. Based on the result of EDS mapping analyses, the light phase is mainly Nd-Pr alloy. The point 1 in light phase and the point 2 in dark phase were further analyzed by EDS. The result (Fig. 1b) shows that both Nd and Pr are present in the light area where Fe is present in a low concentration. Most of Fe elements are distributed in the dark area in the form of Fe-Nd-Pr alloy that was identified as Nd2Fe14B phase by XRD (Fig. 2). Since the quantitative analysis of light element boron is inaccurate by EDS, the concentration of boron was subsequently detected by ICP. Fig. 1 shows a typical microstructure of REPM alloy by comparing with the literature [28]. According to the microstructure observation of the original REPM waste, the Nd-Pr phases are not connected. To form channels on the REPM, the Nd and Pr in the Nd2Fe14B phase must be also oxidized. The electrochemical measurements were performed to study the oxidation reactions of the two phases. Fig. 3 shows two cyclic voltammograms (CV) using REPM and graphite as counter electrode, respectively. Based on the literatures [22–25], the R4/O4 corresponds to the deposition of lithium ions and the dissolution of lithium metal, respectively. Comparing to the curve 1 (graphite as an inert counter electrode), three couples of new redox peaks was recorded on the curve 2 (using REPM as counter electrode), which are clearly observed from the magnified graph. The reduction potentials of Nd(III) and Pr(III) ions at 1123 K were calculated by Factsage 7.1 software (customer ID: 0473). The reduction potential of Nd(III) ions (−4.81 V vs. F2/F−) is more positive than that of Pr(III) ions (−4.89 V vs. F2/F−), which is consistent with the results of the literatures in molten chlorides [29,30]. In addition,
2. Experimental section 2.1. Preparation of molten salts and electrodes In our work, LiF-CaF2 mixture (80:20 mol.%) with the eutectic temperature of 1040 K was used as electrolyte to reduce the volatilization of electrolyte, which can also offer large background limits [22–26] and the solubility for rare-earth oxides [27]. In addition, the fluorides are the common electrolyte for industrial production of rareearth metals. LiF (99.99% metals basis, CAS No. 7789-24-4), CaF2 (99.99% metals basis, CAS No. 7789-75-5) and NdF3 (99.99% metals basis, CAS No. 13709-42-7) were sourced from Aladdin Industrial Corporation. All chemicals were dried before use at 773 K for 24 h in a vacuum to remove excess water. After the vacuum dehydrating, a mixture of LiF-CaF2 was ground using an agate mortar in a glove box under argon atmosphere. Platinum (Pt) (d = 1.0 mm, 99.95% purity) and tungsten (W) wires (d = 1.0 mm, 99.99% purity) were used as quasi-reference electrodes (QRE) and working electrodes (WE), respectively. A spectrally pure graphite rod (d = 6 mm, 99.99% purity) served as the counter electrode (CE). A REPM waste (8 mm × 8 mm × 1 mm, composition in wt%: 27.1Nd, 8.1Pr, 63.5Fe, 1.1B, 0.2others) was used as working electrode or counter electrode for different research purposes, which was connected with electrochemical workstation through an iron wire (d = 0.5 mm, 99.995% purity). The REPM waste came from a used wind turbine provided by Baotou research institute of rare earth. All metallic electrodes were polished thoroughly using SiC papers, then 2
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Fig. 1. (a) BSE image and (b) EDS analyses of original REPM waste.
the reduction process of Nd(III) ions was reported to be taken place in two steps in molten fluorides with Nd2O3 [31]. Since the REPM electrode came from an end-of-life wind turbine, the oxygen from the REPM is inevitably brought into the electrolyte. Therefore, peaks R1/O1, R2/ O2 and R3/O3 should correspond to the electrochemical redox of Nd (III)/Nd(II), Nd(II)/Nd(0) and Pr(III)/Pr(0), respectively. Based on the above analysis, the REEs can be oxidized from the REPM anode and then deposited on the cathode. In order to further investigate the electrochemical reactions of Nd and Pr in LiF-CaF2 system, a series of square wave voltammogram (SWV) at different frequencies was obtained on a W electrode. The results are shown in Fig. 4a where the potentials and reactions of four signals (R1, R2, R3 and R4) correspond to those of Fig. 3. Gaussian fitting model was used to analyze the peak R2 that is the most complete one among the four peaks. The results can be mathematically analyzed by the following equation [32]:
W1/2 = 3.52RT / nF
(1)
where W1/2 is the half-peak width (V), T is the temperature (K), R is the ideal gas constant (J mol−1 K−1), n is the number of electrons transferred, and F is the Faraday constant (C mol−1). The validity of Eq. (1) was proved by plotting peak current density versus the square root of frequency [33], as shown in Fig. 4b. The linear relation indicates that Eq. (1) can be applied to calculate the number of electrons transferred in the frequency range. The values of n at different frequencies were calculated by Eq. (1) and listed in the Fig. 4a. The average of n is 1.78, therefore the reaction of peak R2 should involve two-electron transfer. The electrochemical reactions of Nd and Pr were verified by a galvanostatic oxidation and the addition of NdF3. Galvanostatic oxidation was carried out at 0.1A on the REPM working electrode. The measurement was ceased after 360 s, 720 s, 2160 s and 2460 s oxidation, respectively. Then SWV method was used to measure current densities of cathodic peaks on a W working electrode. NdF3 was added into the melts for calibration after the measurement of last oxidation. All SWV curves are shown in Fig. 5 where the current densities of these peaks increased with increasing oxidation time, suggesting more REEs dissolution into the electrolyte with further oxidation. With the addition of 0.1 g NdF3, the current densities of peak R1 and R2 increased obviously. This result demonstrates that the reactions of peak R1 and R2 correspond to the reduction of Nd ions. To investigate the oxidation reaction of Nd2Fe14B phase, REPM was used as the working electrode. Fig. 6a shows two CV curves which were
Fig. 2. XRD pattern of original REPM waste.
Fig. 3. Cyclic voltammograms in molten LiF-CaF2 at 1123 K. working electrode: W; reference electrode: Pt; counter electrodes: graphite (curve 1) and REPM (curve 2); sweep rate: 0.1 V s−1.
3
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Fig. 4. (a) Square wave voltammograms at different frequencies in molten LiF-CaF2 after the dissolution of Pr and Nd from REPM. Temperature: 1123 K; pulse height: 25 mV; potential step: 1 mV; working electrode: W; reference electrode: Pt; counter electrode: graphite. (b) Variation of the peak current density versus the square root of the frequency in the Fig. 4a.
measured at the different limiting potentials. Comparing to the CV curve on the W electrode, two new reduction signals at −1.03 V and −1.34 V (vs. Pt) were observed on the REPM electrode, which should be corresponding to the deposition of rare-earth ions on the active REPM electrode. In addition, only oxidation signal O1′ was detected when the sweep potential did not reach the potential at which the Nd and Pr ions were reduced to their metals (see the grey curve). This indicates that this electrochemical signal O1′ was owing to the oxidation reaction of metallic Nd and/or Pr from the Nd2Fe14B phase rather than Nd-Pr phase. Only that the sweep potential was more negative than the reduction potentials of Nd and Pr ions (black curve), the oxidation peaks of Nd and Pr in Nd-Pr alloy can be observed in reverse sweep. Linear sweep voltammetry with a slow sweep rate was used to obtain the current of oxidation reactions. Fig. 6b shows a linear sweep voltammogram (LSV) on a REPM electrode with the sweep potential from −1.4 V to 0 V (vs. Pt), where the oxidation currents increased with increasing potentials. A potential plateau on the REPM electrode was observed when the current reached 0.30A, which is corresponding to the oxidation reaction of Fe. In addition, the current of the oxidation reaction O1′ of REEs in Nd2Fe14B alloy was obtained to be 0.06A. Table 1 summarizes above oxidation reactions and potentials at W and REPM working electrodes. The potential difference of oxidation reaction of Nd(II) ions is resulted from the concentration polarization of Nd(II) ions near the REPM electrode in which Nd is sufficient. Based on the analyses of this section, the oxidation sequence of Nd2Fe14B phase is lower than that of Nd-Pr phase. Therefore, when the current of the oxidation reaction of REPM electrode reaches 0.06A, the micropores on
Fig. 5. Square wave voltammograms after galvanostatic oxidation (0.1 A) for different time and the addition of NdF3. Temperature: 1123 K; frequency: 20 Hz; pulse height: 25 mV; potential step: 1 mV; working electrode: W; reference electrode: Pt; counter electrode: graphite.
Table 1 Oxidation reactions and potentials at W and REPM electrodes (graphite as the counter electrode). Working electrode
Oxidation potential/V vs. Pt
Corresponding reaction
W
−1.85 −1.62 −1.46 −1.85 −1.60 −1.28 −0.82
O3: NdPr(alloy)-3e → Pr(III) + Nd(0) O2: Nd(0)-2e → Nd(II) O1: Nd(II)-e → Nd(III) O4′: NdPr(alloy)-3e → Pr(III) + Nd(0) O3′: Nd(0)-2e → Nd(II) O2′: Nd(II)-e → Nd(III) O1′: REFeB(alloy)-3e → RE(III) + FeB(alloy)
REPM
Fig. 6. (a) Cyclic voltammograms and (b) linear sweep voltammogram in molten LiF-CaF2 at 1123 K. Working electrode: REPM; reference electrode: Pt; counter electrode: graphite.
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observed during the corrosion process of REPM alloy [20,21]. As seen from Fig. 8a, these pores with different sizes connected to form the dark channels, like various lakes join together to become a river. Based on the result of EDS mapping analyses, Fe, Nd and Pr are present in the light areas, while Ca element is distributed in the dark channels, suggesting CaF2 melts entering into the channels. The EDS element analysis for the whole area is shown in Fig. 8b. Comparing with the EDS result of the original REPM (Fig. 1b), most of REEs have been removed from the REPM waste. To further study the microstructure inside oxidized REPM electrodes, the BSE images of different positions on the section of oxidized REPM were observed by SEM. Fig. 9a shows a top view of four electrodes. Three electrodes (W electrode, REPM and graphite rod) separated each other with distance of 3 cm like an equilateral triangular and the platinum wire was placed in the center. A coordinate system was given to further understand the selected positions. The BSE images at the positions of x = 1, 3, 5 and 7 are shown in Fig. 9b where the light areas are also separated into many areas by dark channels like the surface micromorphology of Fig. 8a. The second-phase constituent content of these images was determined by automatic image analysis according to ASTM Standards E1245-03. Fig. 9c shows the analysis results of dark phases, where the phases are continuous across the oxidized REPM electrode. This demonstrates that the channels have provided continuous transport paths between the electrolyte and the REEs inside the REPM electrode. Fig. 9d shows the EDS line scanning analyses at the position of x = 3. As seen from the EDS results, the distribution of Nd and Pr changes very little throughout the electrode and their counts are much lower than those of Fe and Ca elements. This indicates that most of REEs were evenly oxidized, wherever the REEs were located. In addition, the counts of Fe elements decrease at the positions of channels where the counts of Ca elements increase accordingly, suggesting that these channels in the oxidized REPM electrode were filled with LiF-CaF2 melts. The area in Fig. 9d was magnified and analyzed by EDS mapping, as shown in Fig. 9e. The cooled fluoride crystal can be observed at the channel from the magnified SEM image. In addition, CaF2 salts are present in the channel according to the EDS mapping analyses. These evidences indicate that electrolytes flowed into the channels of oxidized REPM electrode. There are still many micropores that have not yet been connected into channels on the light phase where unreacted Nd and Pr are suggested to be left. As seen from Figs. 8 and 9, LiF-CaF2 melts flowed into channels of oxidized electrodes, therefore the actual area of oxidized electrodes cannot be accurately measured by the common methods of measuring area of porous material. The area fraction of second-phase was used to quantify the variation of channel areas with positions and currents. The BSE images of oxidized electrodes obtained at different currents (Figs. 9 and S3) were equally divided into 10 parts along y-axis. Then the area fraction of second-phase of each part was plotted versus its position, as shown in Fig. 10. The area fractions are a little fluctuation, because the distribution and size of Nd-Pr alloy in the waste are not exactly same in different regions, as seen from the BSE image of original waste in Fig. 1. However, it can be considered that the area fractions of second-phase were relatively uniform in most regions, except for the closest positions to working electrodes and around the positions of y = 0.5. The oxidation reactions at the two positions seem to be more obvious, especially around the positions of y = 0.5 where the area fractions of secondphase all reached the maximum at different currents. The components of elements in the regions were further analyzed by EDS. Fig. 11 shows no REEs were found from these regions. During the oxidation reaction, the content of metallic Nd in the REPM electrode was decreasing, resulting in the variation of the partial reduction potential of each individual component in alloy. The relationship between the partial reduction potential of an individual component in an alloy and the molar fraction of the component is described as the following equation [34].
Fig. 7. (a) Cyclic voltammogram at the end of oxidation reaction of REPM electrode in molten fluorides at 1123 K. Work electrode: W; counter electrode: REPM; reference electrode: Pt; sweep rate: 0.1 V s−1. (b) Comparation of polarization curves of original and oxidized REPM electrodes in molten LiF-CaF2 at 1123 K. Working electrode: original REPM (hollow square) and oxidized REPM (solid sphere); counter electrode: graphite; reference electrode: Pt; sweep rate: 1 mV s−1.
the Nd2Fe14B phase could be formed. In addition, the oxidation current has to be lower than 0.30A to avoid the oxidation reaction of Fe, which has been observed when higher currents were applied [16]. 3.2. Electrochemical preparation and characteristics of anode channels 3.2.1. Electrochemical preparation of anode channels The experiments of making anode channels were completed by the alternate operation of galvanostatic electrolysis for 60 s on the REPM working electrode and a measurement of CV on a W working electrode. When the oxidation signal O6 of Fe was detected (Fig. 7a), the separation of REEs from the REPM electrode was considered to have been completed. In addition, the oxidation peak O5 of REEs from deposited RE-Fe alloy was also observed, and its potential was consistent with the potential obtained at the REPM working electrode. Both signals O5 and O6 show that Fe ions were present in the electrolyte. The less obvious reduction signals of Fe ions are due to the low concentration of Fe ions at the beginning of the CV measurement. With the potential sweep towards more negative direction, more metallic Fe at REPM electrode was oxidized. These newly generated iron ions were reduced at W electrode, resulting in the more obvious oxidation peaks. Meanwhile, the open circuit potential of the oxidized REPM electrode was −0.29 V (vs. Pt) as seen from Fig. 7b, which agree well with the potential plateau of the oxidation reaction of Fe in Fig. 6b. Comparing to the polarization curve of the original REPM, the open circuit potential of REPM electrodes moved from −1.25 V to −0.29 V (vs. Pt). This open circuit potential indicated the end of recycling REEs from the REPM wastes. 3.2.2. Characteristics of anode channels According to the results of electrochemical analyses, four currents (0.096A, 0.160A, 0.224A and 0.288A) were applied to oxidize REEs in the REPM electrode, respectively. Fig. 8a shows the BSE image of the surface of the oxidized REPM at the current of 0.096A, where light areas are separated into many areas by dark pores and channels. Since the sizes of these pores are close to those of the Nd-Pr phases (the light phases in Fig. 1a), these pores are suggested to be formed by the oxidation of bulk Nd-Pr metals. In addition, many micropores are also observed on the light areas, suggesting their formation by the dissolution of Nd and Pr from Nd2Fe14B phase. The micropores have been also 5
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Fig. 8. (a) Superficial BSE image and (b) EDS mapping analyses of REPM electrode oxidized at the current of 0.096 A.
RT ⎞ lnNi Ei = −⎛ ⎝ zi F ⎠ ⎜
where Asurf, Achan and Ased-ph are the external surface area of electrode, the area of formed channel and the area of second-phase on the selected region, respectively. Because Achan ≫ Asurf, the electrode area A ≈ ∑Achan. Assume that the electrode is composed of m selected regions, hence
⎟
(2)
where Ei is the difference between the equilibrium electrode potential of the component i in the alloy and the corresponding potential of the same component in the individual phase (V), Ni is the molar fraction of the component i (assuming that the alloy is a perfect solution), and zi is the number of electrons transferred in the reaction. Since R, T, zi and F are constant in this reaction, the value of ENd increases with the decrease of NNd value (NNd less than 1) according to Eq. (2). When the equilibrium electrode potential of Fe equals to that of Nd in the REPM electrode, the oxidation reaction of metallic Fe occurs. According to Eq. (2), Eq. (3) was derived to calculate the molar fraction of Nd in the electrode under Fe oxidation, which was calculated to be 3.77 × 10−24 by the data of references [22,35] and 9.96 × 10−29 by Factsage 7.1 software, respectively. This indicates that metallic Nd in the REPM alloy should have been depleted when the metallic Fe is oxidized, resulting in the additional area fractions of second-phase. The rare-earth metals were oxidized from both sides of REPM electrode to its inside, therefore it is suggested that the oxidation reaction of both sides occurred together at the position of the maximum second-phase area fractions. Based on the calculated results of Eq. (3), the maximum separation rate of REEs from REPM should be theoretically close to 100% by this recycling method, if REEs can be more uniformly oxidized.
ln
NNd zF o o = −⎛ i ⎞ (EFe − ENd ) 1 − NNd ⎝ RT ⎠
Rate = i/ nmπFAsed − ph
Based on the average Ased-ph (Fig. 12a) at the different position of electrodes, the average reaction rate of REEs was calculated by Eq. (7) and then plotted versus currents, as shown in Fig. 12b. As seen from the figures, increasing current can improve both of channel area and reaction rate. When the current distribution on the electrode is uniform (the same m value at each position), the reaction rate at the position of (3, y) is faster than that at other positions, which is considered as an optimal electrode distance. The separation rate of Nd and Pr from the REPM waste were estimated based on the ICP analyses of REPM wastes before and after the lab experiment. Fig. 12c shows the relationship between the separation rate of REEs and current. The separation rate of REEs increases with increasing current, which agree well with the results of channel areas. A better Pr recovery was obtained than Nd due to the prior sequence of Pr oxidation. 3.3. Characteristics of recycled products The recycled products were obtained by the electrolysis experiment in which W wire was used as a working electrode. To obtain lager cathodic deposits that can be collected, an anode basket woven with molybdenum wires was used as the anode, into which some REPM wastes were placed. In addition, liquid rare-earth metals were prepared at a temperature of 1293 K. Fig. 13a shows the XRD patterns of oxidized REPM, where Fe and Fe2B phases were detected after the separation of REEs, indicating that most of the metallic Nd and Pr have been removed from original REPM waste. Fig. S4 shows spherical cathode deposits which were collected at the bottom of electrolyte after the end of electrolysis. Based on the result of XRD analysis (Fig. 13b), the deposit was mainly Nd metal in which Pr metal was dissolved, because the diffraction peak of Nd metal slightly shifted. Fig. 14 shows the effects of applied current on the composition of cathodic deposits and current efficiency. The content of Nd and Pr in deposits changes little with increasing current, but the iron content has a tendency to increase rapidly. This is consistent with the results of the electrochemical research in the Section 3.1. In addition, the higher current provides higher current efficiency, and the calculated anodic current efficiency is higher the calculated cathodic current efficiency. This is resulted from the formation of metallic Li at the beginning of
(3)
The average area fraction of second-phase was used to mathematically evaluate the average rate of oxidation reaction of REEs with currents at different positions of electrodes by following Eq. (4) [32]. (4)
Rate = i/ nFA
In the Eq. (4), i is the applied current (A), n is the number of electrons transferred, F is the Faraday constant (C mol−1) and A is the electrode area (cm2). To calculate the electrode area A, some assumptions are made to simplify this problem. The current distribution on the electrode is considered to be uniform, suggesting that the area of formed channels is same at the same electrode distance. All channels are considered to be cylindrical or be composed of an infinite number of cylinders. The observed second-phase is considered to be the largest area of channel section. Based on these assumptions, the electrode area A is calculated by following Eqs. (5) and (6).
A=
∑ (Asurf
+ Achan )
Achan = πAsed − ph
(7)
(5) (6) 6
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Fig. 9. (a) Top view of four electrodes; (b) Sectional BSE images of REPM electrode oxidized at the current of 0.096A at the position of x = 1, 3, 5 and 7; (c) Secondphase pictures corresponding to Fig. 9(b); (d) EDS line scanning analyses at the position of x = 3; (e) SEM image and EDS mapping analyses of the selected area in Fig. 9(d).
were then deposited at the cathode as rare earth metals. In the early stages of this process, the pores of different sizes formed at the REPM electrode. With increasing the number of pores, some pores connected to form channels some of which extended toward the interior of the alloy. Meanwhile, the electrolyte entered into channels. When the REEs inside the electrode contacted with the electrolyte through the channels, the oxidation reactions of REEs occurred more easily. Finally, the REEs were successfully separated from REPM wastes and deposited on the cathode as rare earth metal. Further work is needed to optimize the process conditions and finally develop an industrial scale flowsheet for
galvanostatic electrolysis when there was no Nd ions in the electrolytes. Since these formed Li was difficult to be collected, the calculated cathodic current efficiency only includes the part of the current efficiency of formed rare-earth metal. In other words, this calculated value is smaller than the actual value. The anodic current efficiency is close to the cathodic current efficiency of industrial rare earth electrolysis, therefore this recycling method is promising for industrial applications. Based on the above results, we can describe the recovery process of REEs from the REPM wastes. The REEs from the REPM electrode were oxidized and dissolved into the electrolyte in the form of RE ions that 7
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Fig. 10. Relationships of second-phase area fraction with detected positions and applied currents.
provided a good access between the electrolyte and REEs inside the REPM electrode, facilitating the diffusion of REEs. The formed channels are critical to develop this recycling method for block REPM wastes in industry. The separation rates and reaction rates of Nd and Pr increased with increasing current, and the rate of oxidation reaction was proportional to current. The oxidation reaction of Fe initially occurred at the inside of the REPM alloy, owing to the depletion of RE metals. This reaction could be avoided by increasing rare-earth content on anode, e.g. the continuous supplementation of REPM wastes in an anode basket.
the REPM recycling. 4. Conclusion This work provides an alternative and promising method to recycle REEs from block REPM wastes. Nd and Pr from REPM wastes were selectively dissolved into molten LiF-CaF2 salts in the form of rare-earth ions. The separated rare-earth ions were directly prepared as rare-earth metals by electrowinning, while other elements was left in the form of Fe and Fe2B. This method is different from common molten fluoride electrolysis in that no gas release at the anode, making the process environmentally friendly. The pores and channels were formed by controlling current to oxidize REEs from Nd-Pr and Nd2Fe14B phases. This microstructure
Acknowledgements We gratefully acknowledge Prof. Muxing Guo (KU Leuven, Belgium)
Fig. 11. EDS analyses for the regions of the area fraction maximum of second-phase at different currents. 8
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Fig. 12. Effects of the applied current on (a) the average area fraction of second-phase at different positions, (b) the average reaction rate at different positions and (c) the separation rate of REEs from REPM wastes.
Fig. 14. Effects of applied currents on the composition of cathodic deposits and current efficiency. Fig. 13. XRD patterns of (a) the oxidized REPM electrode and (b) the cathodic deposit by galvanostatic electrolysis at −0.288 A.
References
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