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Jun 24, 2015 - Molycorp Magnequench, Greenwood Village, Colorado 80111, United States. •S Supporting Information. ABSTRACT: The rare earth elements ...
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Selective Extraction of Rare Earth Elements from Permanent Magnet Scraps with Membrane Solvent Extraction Daejin Kim,† Lawrence E. Powell,† Lætitia H. Delmau,‡ Eric S. Peterson,§ Jim Herchenroeder,∥ and Ramesh R. Bhave* †

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Nuclear Materials Processing Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Center for Advanced Energy Studies, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States ∥ Molycorp Magnequench, Greenwood Village, Colorado 80111, United States

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‡

S Supporting Information *

ABSTRACT: The rare earth elements (REEs) such as neodymium, praseodymium, and dysprosium were successfully recovered from commercial NdFeB magnets and industrial scrap magnets via membrane assisted solvent extraction (MSX). A hollow fiber membrane system was evaluated to extract REEs in a single step with the feed and strip solutions circulating continuously through the MSX system. The effects of several experimental variables on REE extraction such as flow rate, concentration of REEs in the feed solution, membrane configuration, and composition of acids were investigated with the MSX system. A multimembrane module configuration with REEs dissolved in aqueous nitric acid solutions showed high selectivity for REE extraction with no coextraction of non-REEs, whereas the use of aqueous hydrochloric acid solution resulted in coextraction of non-REEs due to the formation of chloroanions of non-REEs. The REE oxides were recovered from the strip solution through precipitation, drying, and annealing steps. The resulting REE oxides were characterized with XRD, SEM-EDX, and ICP-OES, demonstrating that the membrane assisted solvent extraction is capable of selectively recovering pure REEs from the industrial scrap magnets.

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INTRODUCTION The world today has hugely benefited from the application of rare earth elements (REEs) to most high-tech devices such as cell phones, rechargeable batteries, computers, TV, and fluorescent lighting.1−3 These REEs are the 15 lanthanides in the periodic table along with scandium and yttrium. Over 90% of REEs worldwide are currently being produced in China which until very recently had restricted REE exports. The rising REE demand in high-tech devices over the past two decades resulted in skyrocketing prices of REEs in just a few years.4−7 This motivated the U.S. Department of Energy to launch the Critical Materials Institute in 2013 in order to tackle the REE supply challenge, focusing on three different areas such as diversifying REE supplies, developing REE substitutes, and recycling REE wastes.8 Among those approaches, this study focuses on the recovery and recycling of end-of-life REE products. When compared to REE mining processes, recycling of REEs from postconsumer products provides significant environmental benefits with regard to air emissions, groundwater contamination, soil acidification, eutrophication, and climate change.2 The recoverable REEs from end-of-life products are (1) neodymium (Nd), dysprosium (Dy), and praseodymium (Pr) © 2015 American Chemical Society

from NdFeB permanent magnets used in automobiles, mobile phones, hard disk drives, computers, tablets, electric motors, and hybrid electric vehicles; (2) europium (Eu), terbium (Tb), and yttrium (Y) from phosphors in fluorescent lamps, LEDs, LCD backlights, and plasma screens; and (3) lanthanum (La), cerium (Ce), Nd, and Pr from nickel metal hydride batteries in rechargeable batteries, and hybrid electric vehicle batteries.9 Even though there are a variety of end-of-life products available for REE recovery, the actual REE recovery is currently estimated to be less than 1% due to low efficiencies and limitations in recycling processes.10 Therefore, it is pivotal to develop and improve the effective REE recycling process in order to widen the reuse of REE-containing end-of-life products. Even though commercial REE recycling is presently very limited, the REEs can be recovered through several processing technologies such as hydrometallurgy, pyrometallurgy, and gasSpecial Issue: Critical Materials Recovery from Solutions and Wastes Received: Revised: Accepted: Published: 9452

March 25, 2015 June 22, 2015 June 24, 2015 June 24, 2015 DOI: 10.1021/acs.est.5b01306 Environ. Sci. Technol. 2015, 49, 9452−9459

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Environmental Science & Technology

characterized by X-ray diffraction (XRD), energy-dispersive Xray spectroscopy (EDX), and inductively coupled plasmaoptical emission spectroscopy (ICP-OES).

phase extraction. Hydrometallurgy uses strong acid to dissolve REE-containing end-of-life products and then uses basic solutions to selectively precipitate the REEs of interest. This process includes solvent extraction, leaching, and precipitation. However, this process requires high chemical usage and suffers from generation of large amounts of chemical waste and low selectivity due to coextraction of non-REEs.9 The pyrometallurgical process utilizes high temperatures to chemically convert feed materials into the valuable REEs. This process requires fewer processing steps than the hydrometallurgical route, but it requires larger energy input and generates larger amounts of solid waste.9,11 The REE recovery by gas phase extraction route is based on volatility differences, involving chlorination and carbochlorination, but corrosive aluminum chloride and hydrogen chloride gas generated from this process pose serious environmental and safety hazards.12,13 Membrane assisted solvent extraction (MSX) can provide another alternative to recover REEs, eliminating the disadvantages of traditional equilibrium-based solvent extraction processes such as loading, flooding, and third phase formation.14,15 In the continuous operation mode, separation in MSX is enhanced under nonequilibrium conditions due to the maintenance of high driving forces over extended periods of time compared to equilibrium limited conventional solvent extraction. While the solvent extraction process carries out extraction and stripping in two separate steps, MSX combines both processes in a single step, circulating the feed and strip solutions continuously without dispersion of different phases.16,17 Utilizing hollow fiber membrane modules in the MSX system provides high contact surface area per unit volume to achieve a high REE extraction rate. In the MSX system, an organic phase consisting of extractant and organic solvent is immobilized in the pores of hollow fiber membranes, and the aqueous feed and strip solutions flow through the lumen and shell side of hollow fiber modules, respectively, as shown in Figure S1 (Supporting Information). The organic solvent is immiscible with very low solubility in aqueous solution which minimizes extractant and solvent losses. Acidic extractants are widely used in current industrial practice, but they suffer from relatively low REE selectivity due to simultaneous coextraction of other cations such as Fe, Cu, and Ni.18,19 On the contrary, neutral extractants such as tetraoctyl diglycol amide (TODGA) and trialkyl phosphine oxides (Cyanex 923) have shown the capability of selectively extracting several lanthanides and actinides such as cerium, europium, uranium, and cesium.20−22 Therefore, we have utilized neutral extractants, TODGA and Cyanex 923 in this work. They were evaluated to selectively extract REEs from permanent magnets in the presence of non-REEs such as Fe, B and other transition metals (Cu, Ni) used in the outer coating. The distribution coefficient (D) is the ratio of REE concentration in the organic phase to that in the aqueous phase at equilibrium. D values of neodymium extracted by TODGA and Cyanex 923 were first measured to determine their relative efficiencies prior to incorporating them in the membrane supports. The effects of the concentration and flow rate of the feed solution, membrane configuration, and acid composition on REE extraction were then systematically investigated using commercial NdFeB magnets. The MSX system was also utilized to extract REEs from industrial scrap magnets. The REEs in the scrap magnets were recovered from the strip solution in the MSX system by precipitating, filtration, drying, and annealing steps. The resulting REE oxides were

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EXPERIMENTAL SECTION The distribution coefficient of neodymium was determined by mixing organic phase with the REE-containing aqueous solution for 2 h, followed by separating aqueous phase from organic phase. The organic phase is composed of extractants (TODGA (Marshallton Research) or Cyanex 923 (CYTEC Industries Inc.)), tributyl phosphate (TBP, Stauffer Chemical Company) as another extractant, and Isopar L (Isoparaffin, ExxonMobil Chemical) as solvent. The aqueous phase was prepared by dissolving permanent magnets, neodymium(Nd)iron(Fe)-boron(B) magnets (K&J Magnetics, grade D42) in 1−6 M HNO3 and 3−6 M HCl. The concentration of REEs in the organic phase was back-calculated from the REE concentration in the aqueous phase measured by ICP-OES (IRIS Intrepid II XSP, Thermo Electron Corporation). Hydrophobic membranes are desirable for the extraction of REEs because hydrophobicity prevents wetting of the membrane by aqueous feed solution and also prevents the displacement of organic phase immobilized in the pores into the strip solution. For this reason, hydrophobic polypropylene membrane modules (MicroModule, Membrana, area 100 cm2, ID: 0.25 mm, number of hollow fibers: 700) were used as membrane supports. The REE sources for the feed solution were chosen from three different NdFeB magnets; commercial NdFeB magnets from (1) K&J Magnetics (Grade D42) and industrial scrap magnets from (2) Molycorp Magnequench (Harrodsburg, KY) and (3) Daido Electronics Co. Ltd. (Japan). The composition of the magnets is shown in Table S1 (Supporting Information). These magnets were dissolved in concentrated nitric acid or hydrochloric acid for 24 h. Using the commercial NdFeB magnets, the membrane assisted solvent extraction was carried out with 2 L feed solution containing approximately 1000 and 2000 ppm of Nd and the strip solution of 0.23 or 0.5 L. The molar concenstration of HNO3 in the feed solution was 6 M and the strip solution was 0.2 M HNO3. The pores of hollow fibers in the membrane modules were filled with organic phase consisting of the TODGA, Isopar L, and TBP in the ratio of 3:4:3, respectively. The shell side of hollow fibers was supplied with the aqueous strip solution, and the lumen side of the hollow fibers was fed with the feed solution containing REEs. Both the feed and strip solutions were simultaneously circulated throughout the MSX system with peristaltic pumps. The feed solution was thoroughly mixed with a mixer (Talboys, TROEMNER, LLC) to maintain uniform concentration. The pressure on the feed side was maintained at 15 psig and the strip side was kept at atmospheric pressure. The flow rate of the feed solution varied from 35 to 105 mL/min to investigate the effect of flow rate on REE recovery. For ICP analysis, ∼5 mL samples were collected from both feed and strip solutions over the duration of the experiment. The effect of membrane configuration on REE extraction was studied by connecting 8membrane modules in series and parallel (Figure S2, Supporting Information). Experiments with 3 and 6 M hydrochloric acid were also conducted to study the effect of acid composition on REE extraction. 0.2 M HCl solution was used as the strip on shell side of the module. Upon the optimization of experimental conditions with the commercial NdFeB magnets, the two industrial scrap magnets 9453

DOI: 10.1021/acs.est.5b01306 Environ. Sci. Technol. 2015, 49, 9452−9459

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Figure 1. Distribution coefficient of Nd from NdFeB magnets (D42) with TODGA and Cyanex 923 in (a) HNO3 and (b) HCl.

Figure 2. (a) The elemental concentration of NdFeB magnets in the (a) feed and (b) strip solutions while running the 8-module system in parallel, (c) the ratio of REEs in the strip to the feed and (d) REE extraction rate (feed: 6 M HNO3, ∼1000 ppm of Nd, strip: 0.2 M HNO3, 0.23 L).

concentration on the distribution coefficient was only considered in this study.23 Figure 1 shows distribution coefficients of neodymium from NdFeB magnets (D42) obtained with TODGA and Cyanex 923 in aqueous HNO3 and HCl solutions. As shown in Figure 1, distribution coefficient of Nd extracted by TODGA was substantially greater than that obtained with Cyanex 923 at several different representative acid concentrations. High D values with TODGA can be attributed to strong oxygen donor atoms in TODGA, whereas low D values on Cyanex 923 are likely due to the low adsorption ability of Cyanex 923 for the REEs.24,25 This result confirms that TODGA is more suitable than Cyanex 923 for extraction of REEs from NdFeB magnets. Thus, TODGA was chosen as the preferred extractant in the MSX system to extract REEs from the permanent magnets in this research. There are several process variables that can have an effect on REE recovery in the MSX system, such as REE concentration, molar concentration of acid, and membrane area. These variables were investigated in a prior study.26 The molar concentration of HNO3 in the feed and strip was chosen as 6 M and 0.2 M, respectively. This was based on favorable values of distribution coefficient at high acid concentration. The 8-

procured from Molycorp Magnequench and Daido Electronics were utilized to recover REEs with the MSX system. The extracted REEs in the strip solution from those scrap magnets were precipitated out with oxalic acid, followed by filtration, drying at room temperature, and annealing at 750 °C for 2 h. The resulting REE oxides were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX, HITACHI S-4800). The recovered REEs were redissolved in nitric acids to evaluate the purity of REE oxides with ICP-OES (IRIS Intrepid II XSP, Thermo Electron Corporation).

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RESULTS AND DISCUSSION The critical requirement in the extraction of REEs with the MSX system is the incorporation of a selective extractant in organic phase by impregnation in the pores of hollow fiber support. We have evaluated two different neutral extractants, TODGA and Cyanex 923. These were first evaluated to determine the distribution coefficient of neodymium at various acid concentrations. Although the distribution coefficient is a function of temperature, pH, metal ion concentration, extractant and solvent concentration, the effect of molar acid 9454

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Figure 3. Elemental concentration of NdFeB magnets in the (a) feed and (b) strip solutions, and (c) the ratio of REEs in the strip to the feed and (d) REE extraction rate while running the 8-module system in series for 96 h (feed: 6 M HNO3, 2L; strip: 0.2 M HNO3, 0.5 L).

accumulation of particulate matter. However, in order to maintain the desired concentration of extractants in the membrane pores, the organic phase might need to be supplemented periodically to achieve long-term stability (>1000 h). The expected frequency of replenishing extractants would be once every couple of months. Figures S4(a) and (b) (Supporting Information) show the elemental concentration of NdFeB magnets in the feed and strip solutions when the feed concentration was doubled from ∼1000 ppm to ∼2000 ppm of Nd, while maintaining all other conditions as shown in Figure 2. It can be seen from Supporting Information Figure S4(b) that Nd, Dy and Pr were selectively recovered with no coextraction of non-REEs such as Fe and B. The other non-REEs such as Cu and Ni used in the outer coating of the magnets were also undetectable by ICP-OES. This high selectivity of REEs over non-REEs could be attributed to the nonequilibrium separation process of membrane assisted solvent extraction. In this case, REEs are continuously extracted at high driving forces without approaching equilibrium where coextraction can occur. However, as shown in Figure S4(c) (Supporting Information), the recovery of Nd and Pr with higher concentration of feed solution (∼2000 ppm of Nd) decreased by about 50%, indicating that REE extraction through the membrane is not limited by concentration gradient, but by the amount of available extractants in the hollow fiber pores. Figure S5 (Supporting Information) shows the extraction results while running the 8-membrane module system for 96 h with increased strip volume (0.5 L). All other conditions were identical to those reported for Supporting Information Figure S4. The behavior of the Nd, Pr, Dy recovery over time with the higher strip volume was very similar to that of REEs with the strip volume of 0.23 L (see Figure S4, Supporting Information). A small increase in the extraction rate was observed with the higher strip volume in Supporting Information Figure S5(d)

membrane module system was used to extract REEs. Another experimental variable that can impact REE extraction is the feed flow rate. The flow rate of the feed side in the 8-membrane modules was varied from 35 to 105 mL/min, while keeping all other experimental conditions constant. Figure S3 (Supporting Information) shows the effect of flow rate of the feed solution on REE recovery in the MSX system. As can be seen in Supporting Information Figure S3, the ratio of REE concentration in the strip to the feed only slightly decreased as the feed flow rate increased, suggesting that the extraction rate is not significantly impacted by the feed flow rate. Therefore, a feed flow rate of 35 mL/min was selected in the MSX system for the rest of this study. The 8-membrane modules were connected in parallel to enable the feed solution to be evenly distributed into each membrane module, as shown in the schematic of Figure S2 (a) (Supporting Information). Figure 2 shows the elemental composition of (a) the feed (commercial NdFeB magnets (K&J Magnetics, grade D42)) and (b) the strip solutions while operating the 8-membrane module system for 55 h. The molar concentration of nitric acid in the feed and the strip was 6 M and 0.2 M, respectively. At around 24 h from the start, Nd concentration in the strip was about 900 ppm and then decreased slowly as operating time increased, whereas Pr concentration kept increasing over 55 h. As shown in Figure 2 (c), the Nd concentration ratio in the strip to the feed was >1.0 after a run time of 20 h, and the concentration of Pr in the strip increased up to 95% of the initial Pr concentration in the feed. Figure 2 (d) shows extraction rates of Nd and Pr while running the 8-membrane module system. In both cases, the REE extraction rates rapidly declined at the beginning of the run even at high REE concentration, indicating that REE extraction may be significantly limited by the amount of extractants available in the pores of hollow fibers. Typically, MSX system does not experience any fouling issue often caused by the 9455

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Figure 4. Elemental concentration of NdFeB magnets in the (a) feed and (b) strip solutions, and (c) the ratio of REEs in the strip to the feed and (d) REE extraction rate while running the 8-module system in series for 96 h (feed: 6 M HCl, 2L; strip: 0.2 M HCl, 0.5 L).

Figure 5. Elemental concentration of scrap magnets (Molycorp Magnequench) in the (a) feed and (b) strip solutions, and (c) the ratio of REEs in the strip to the feed and (d) REE extraction rate while running the 8-module system in series for 100 h (feed: 6 M HNO3, 2L; strip: 0.2 M HNO3, 0.5 L).

compared to the lower strip volume (0.23 L) in Supporting Information Figure S4(d). In order to evaluate the influence of membrane module configuration on REE extraction, the 8-membrane modules were connected in series as shown in Supporting Information Figure S2(b). All other extraction conditions were identical to those reported in Supporting Information Figure S5. Figure

3(a) and (b) show the concentration of NdFeB magnet elements in the (a) feed and (b) strip solutions while running the 8-module system in series for 96 h. The ratio of REE concentrations in the strip to the feed solution and REE extraction rates with the 8-modules in series configuration are shown in Figure 3(c) and (d). The Nd and Pr recoveries with the 8-membrane modules connected in series were about 20% 9456

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Figure 6. SEM-EDX analysis of REEs recovered from Magnequench scrap magnets by membrane assisted solvent extraction.

practice, it would be more desirable to use HNO3 in the MSX system to recover high purity REEs from the Nd-based magnets, based on the results obtained in this study. However, utilizing ionic liquids, such as trihexyl(tetradecyl)phosphonium chloride, which has shown high distribution ratio for iron over REEs in HCl, may be an option to post-treat the HCl based strip solution to recover pure REEs.28 Upon the optimization of the MSX system performance with commercial NdFeB magnets, the REE recovery from industrial scrap magnets with membrane assisted solvent extraction was investigated. The end-of-life scrap magnets were procured from Molycorp Magnequench, which contains about 30 wt % REEs (see Supporting Information Table S1). Figure 5 (a) shows the elemental concentration of the Magnequench scrap magnet dissolved in 6 M HNO3. The elemental concentrations in the strip solution and the ratio of REEs in the strip to the feed while operating the 8-membrane module system are shown in Figure 5(b) and (c), respectively. The results show that only REEs such as Nd, Pr, and Dy were selectively extracted from the scrap magnets into the strip solution with the MSX system, without any coextraction of non-REEs such as Fe and B. Figure S7 (Supporting Information) shows the REE extraction results obtained with the industrial scrap magnets procured from Daido Electronics. As shown in Supporting Information Figure S7(b) and (c), only REEs were selectively recovered from Daido Electronics scrap magnets with the MSX system, and there was no coextraction of non-REEs in the strip solution over the 120 h run. These results clearly demonstrate that membrane assisted solvent extraction is capable of selectively recovering REEs from industrial scrap magnets. The REEs extracted into the strip solution from the scrap magnets were then precipitated with oxalic acid, followed by filtration, drying, and annealing steps. Figure S8 (Supporting Information) shows the XRD patterns of resulting REE oxides recovered from the scrap magnets procured from Molycorp Magnequench. The majority of peaks shown in the XRD analysis correspond to the peaks for REE oxides such as Nd2O3 (JCPDS 21−0579) and PrO1.83 (JCPDS 06−0329), indicating that REE oxides were successfully recovered from industrial scrap magnets via the MSX system. The purity of recovered REEs was examined by SEM-EDX analysis, as shown in Figure 6. There is no indication of the presence of iron in the recovered REE oxides based on the EDX analysis. In addition,

higher than that with the 8- modules in parallel. In addition, the extraction rate for the first 24 h run time was almost double to that observed for the 8-modules connected in parallel. These results confirm that the membrane configuration in series shows better REE extraction performance than the configuration where modules are connected in parallel. The use of hydrochloric acid instead of nitric acid for REE extraction was evaluated in the MSX system as it is generally the preferred acid in many traditional precipitation and extraction technologies for REE recovery. Figure 4(a) and (b) show the elemental concentration in the feed and strip solutions while operating the 8-membrane module system connected in series with 6 M HCl in the feed and 0.2 M HCl in the strip. Figure 4(c) and (d) show the ratio of REE concentration in the strip to the feed and REE extraction rate, respectively. As shown in Figure 4(b) and (c), significant amounts of Fe and B were coextracted with REEs into the strip solution. The ratio of Fe concentration in the strip to the feed in 48 h was about 1.6 which is much higher than that of Nd and Pr in Figure 4(c). The coextraction of REEs could be attributed to the formation of tetrachloroanions of non-REEs in the presence of HCl as described below.27 This mechanism is not applicable as nitrate anions are not formed that can be coextracted when nitric acid is used for REE extraction. 2Fe + 6HCl → 2FeCl3 + 3H 2 TODGA + FeCl3 + HCl ↔ TODGA·H+·FeCl−4 3TODGA·H+·FeCl−4 + Nd3 + + 3Cl− ↔ [(TODGA)3 Nd]3 + (FeCl−4 )3 + 3HCl

Figure S6(a) and (b) (Supporting Information) show the elemental concentration in the feed and strip when 3 M HCl was used in the feed solution with the 8-membrane modules to extract REEs from permanent Nd-based magnets. As shown in Figure S6(b) and (c) (Supporting Information), even though the ratio of Nd and Pr in the strip to the feed was significantly higher compared to that with 6 M HCl in the feed, the coextraction of Fe was still observed resulting in high concentration of Fe in the strip, unlike when HNO3 was used in the aqueous solutions. As can be seen from these results, although HCl is widely used in current industrial 9457

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(3) Alonso, E.; Sherman, A. M.; Wallington, T. J.; Everson, M. P.; Field, F. R.; Roth, R.; Kirchain, R. E. Evaluation rare earth element availability: A case with revolutionary demand from clean technologies. Environ. Sci. Technol. 2012, 46, 3406−3414. (4) Stone, R. As china’s rare earth R&D becomes ever more rarefied, others tremble. Science 2009, 325, 1336−1337. (5) Service, R. F. Nations move to head off shortages of rare earths. Science 2010, 327, 1596−1597. (6) Humphries, M. Rare Earth Elements: The Global Supply Chain; Congressional Research Service Report for Congress, 2013; https:// www.fas.org/sgp/crs/natsec/R41347.pdf. (7) Du, X.; Graedel, T. E. Global in-use stocks of the rare earth elements: a first estimate. Environ. Sci. Technol. 2011, 45, 4096−4101. (8) Cho, A. Hubs aim to reinvent DOE research culture. Science 2013, 340, 914−918. (9) Binnemans, K.; Jones, P. T.; Blanpain, B.; Gerven, T. V.; Yang, Y.; Walton, A.; Buchert, M. Recycling of rare earths: A critical review. J. clean. Prod. 2013, 51, 1−22. (10) United Nations Environment Programme. Recycling rates of metals: A status report. 2011; http://www.unep.org/resourcepanel/ Portals/24102/PDFs/Metals_Recycling_Rates_110412-1.pdf. (11) Huang, K.; Li, J.; Xu, Z. A novel process for recovering valuable metals from waste nickel-cadmium batteries. Environ. Sci. Technol. 2009, 43, 8974−8978. (12) Uda, T. Recovery of rare earths from magnet sludge by FeCl2. Mater. Trans. 2002, 43, 55−62. (13) Itoh, M.; Miura, K.; Machida, K. Novel rare earth recovery process on Nd-Fe-B magnet scrap by selective chlorination using NH4Cl. J. Alloys Compd. 2009, 477, 484−487. (14) Lee, L. T. C.; Ho, W.-S.; Liu, K.-J. Membrane solvent extraction. U.S. Patent, 3,956,112, 1976. (15) Ho, W. S. W.; Poddar, T. K. New membrane technology for removal and recovery of chromium from waste waters. Environ. Prog. 2001, 20 (1), 44−52. (16) Yun, C. H.; Prasad, R.; Sirkar, K. K. Membrane solvent extraction removal of priority organic pollutants from aqueous waste streams. Ind. Eng. Chem. Res. 1992, 31 (7), 1709−1717. (17) Gaikwad, A. G.; Chitra, K. R.; Surender, G. D.; Damodaran, A. D. Membrane solvent extraction of some rare earth elements. Chem. Biochem. Eng. Q. 2003, 17 (3), 191−199. (18) Lee, M.-S.; Lee, J.-Y.; Kim, J.-S.; Lee, G.-S. Solvent extraction of neodymium ions from hydrochloric acid solution using PC88A and saponified PC88A. Sep. Purif. Technol. 2005, 46 (1−2), 72−78. (19) Li, W.; Wang, X.; Meng, S.; Li, D.; Xiong, Y. Extraction and separation of yttrium from the rare earths with sec-octylphenoxy acetic acid in chloride media. Sep. Purif. Technol. 2007, 54 (2), 164−169. (20) Horwitz, E. P.; McAlister, D. R.; Thakkar, A. H. Synergistic enhancement of the extraction of trivalent lanthanides and actinides by tetra-(n-octyl)diglycolamide from chloride media. Solvent Extr. Ion Exch. 2008, 26 (1), 12−24. (21) Shimojo, K.; Kurahashi, K.; Naganawa, H. Extraction behavior of lanthanides using a diglycolamide derivative TODGA in ionic liquids. Dalton Trans. 2008, 37, 5083−5088. (22) Larsson, K.; Ekberg, C.; Odegaard-Jensen, A. Using Cyanex 923 for selective extraction in a high concentration chloride medium on nickel metal hydride battery waste. Hydrometallurgy 2012, 129, 35−42. (23) Baker, R. W. Membrane Technology and Applications, 3rd ed.; John Wiley and Sons Ltd.: West Sussex, U.K., 2012. (24) Lee, G. S.; Uchikoshi, M.; Mimura, K.; Isshiki, M. Distribution coefficients of La, Ce, Pr, Nd, and Sm on Cyanex 923-, D2EHPA-, and PC88A-impregnated resins. Sep. Purif. Technol. 2009, 67, 79−85. (25) Sasaki, Y.; Tsubata, Y.; Kitatsuji, Y.; Sugo, Y.; Shirasu, N.; Morita, Y.; Kimura, T. Extraction behavior of metal ions by TODGA, DOODA, MIDOA, and NTAamide extractants from HNO3 to ndodecane. Solvent Extr. Ion Exch. 2013, 31, 401−415. (26) Kim, D.; Powell, L.; Delmau, L. H.; Peterson, E. S.; Herchenroeder, J.; Bhave, R. R. Membrane solvent extraction for the selective recovery of rare earth elements from neodymium-based permanent magnets. Sep. Sci. Technol. 2015, submitted.

when the recovered REE oxides were analyzed by the ICP-OES after being redissolved in the nitric acid solution, only REEs such as Nd, Pr, and Dy were detected with nondetectable concentration of non-REEs in the sample. The XRD patterns for recovered REE oxides from Daido scrap magnets are shown in Figure S9 (Supporting Information) and their SEM-EDX analysis are presented in Figure S10 (Supporting Information). Supporting Information Figures S9 and S10 also demonstrate that nearly pure REEs were successfully recovered from different scrap magnets with the MSX system. These results confirm that the membrane assisted solvent extraction technology is capable of selectively extracting REEs from industrial scrap magnets. This is the first comprehensive research showing the successful demonstration of applying membrane assisted solvent extraction to REE recovery from Nd-based scrap magnets. In comparison, the traditional approaches of utilizing precipitation result in low recovery and the recovered REE may contain significant levels of non-REEs such as Fe and Co.29,30 It is also difficult to dispose of hazardous wastes. The use of equilibrium-limited solvent extraction would require a large number of stages to produce highly pure REEs suitable for reuse. The significant possibility for solvent and extractant losses due to solubility in highly acidic solutions may also hinder its application for REE recycling and reuse. The recovery of REEs in highly pure form is essential to potentially enable its direct use in REE recycling. There would be a huge cost advantage if the REE oxides are recovered in nearly pure form as it would eliminate the need for further purification and processing prior to reuse. Thus, the application of membrane assisted solvent extraction to the REE recovery from the scrap magnets would result in a more environmentally friendly and cost-effective process compared to the conventional routes such as precipitation and solvent extraction.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1, Figures S1−S10.The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01306.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (865)574-6025; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research/work is supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. We thank Dr. Huseyin Ucar at ORNL for his assistance in XRD analysis.

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REFERENCES

(1) Critical Materials Strategy, DOE/PI-0009; U.S. Department of Energy, December 2011. (2) Schüler, D.; Buchert, M.; Liu, R.; Dittrich, S.; Merz, C. Study on Rare Earths and Their Recycling, Final Report for The Greens/EFA Group in the European Parliament; Ö ko-Institut e.V., 2011; www. oeko.de/oekodoc/1112/2011-003-en.pdf. 9458

DOI: 10.1021/acs.est.5b01306 Environ. Sci. Technol. 2015, 49, 9452−9459

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DOI: 10.1021/acs.est.5b01306 Environ. Sci. Technol. 2015, 49, 9452−9459