Recovery of Rare-Earth Elements from Neodymium Magnet Waste Using Glycolic, Maleic, and Ascorbic Acids Followed by Solvent Extraction
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Rare-earth elements (REEs) play a key role in modern societies as their usage keeps increasing in new technologies and green energy sources. However, they are also considered the most critical raw materials in the EU and the USA in terms of supply. There is an increased global interest in the recycling of REEs from end-of-life products and industrial waste. Some REEs, such as Nd, Pr, and Dy, can be recovered from neodymium magnets. These magnets are of special interest since they are present in various technological wastes, such as hard disk drives, electric generators for wind turbines, electric motors, etc. Separation of REEs from other magnet components, such as Fe, which is the main part of the alloy, and further reprocessing of REEs, is the main goal of this work. In this work, neodymium magnet powder was successfully leached using the fully combustible organic lixiviants maleic, glycolic, and ascorbic acids, in order to potentially decrease the usage of strong mineral acids in the hydrometallurgical recovery of REEs. Subsequently, the REEs were selectively extracted from these leachates. For this separation step, several phosphate extractants (TBP, D2EHPA, Cyanex 272, and 923) were investigated, alongside TODGA, which follows the CHON principle and is fully combustible, with no ash or acidic gases being produced. The influences of various diluents (1-octanol, cyclohexanone, hexane, pentane, and dodecane) on the extraction were also studied since the diluents can play an important role in the extraction process and increase selectivity between the extraction of REEs and other impurities. Leaching was shown to be more efficient with maleic and glycolic acids than with ascorbic acid, even at room temperature. Values above 95% were reached for REEs with 1 M concentration and 1/80 solid/liquid ratio. For ascorbic acid, heating the leaching system to 70 °C allowed similar values to be reached. D2EHPA has shown good extraction properties for the recovery of REEs from both glycolic and maleic leachate, mostly with nonpolar diluents such as pentane and hexane, without extracting transition metals (distribution ratios under 0.1). TODGA also showed good extraction of REEs and selectivity between elements, but only in the maleic leachate. As expected, the increasing concentration of the D2EHPA led to the increased distribution ratios. Thus, glycolic, maleic, and ascorbic organic acids, which have not been used before for leaching of neodymium magnet waste, showed good potential for the recovery of REEs from neodymium magnets and for the further development of large-scale recovery processes for REEs.
KeywordsLeaching Solvent extraction Organic acids Neodymium magnets Sustainability
Rare-earth elements (REEs) are metallic elements that are used in a vast range of technological applications, especially in domains linked to green energy . Their properties make some of them essential for wind turbines, electric car batteries, computer hard disk drives and even phosphorous lamps. Due to their supply risk, they have been categorized as the most critical elements in the EU . With China providing almost 90–95% of the world’s demand, exports can tighten according to domestic demand and political interest.
Since their procurement is essential for a various range of applications, instead of simply trying to find new stocks, many studies have proposed ways to efficiently recycle the REE wastes produced by industry [3, 4] or recover REEs from domestic wastes. One such industrial waste is neodymium (NdFeB) magnets. Their demand is increasing rapidly, especially due to the development of hybrid/electric cars, which need specific rechargeable batteries, and wind turbines . The magnets are composed mainly of Fe, Nd, and B, but up to four other REEs can be found; Dy, Pr, Gd, and Tb. For instance, Dy is added to improve high-temperature performances and intrinsic coercivity . The amount of neodymium magnet waste is expected to grow in the future, and it is seen as a feasible waste stream for the recovery of REEs. This would not entirely replace mining, but it would help stabilize the markets and lower the prices for countries that can only rely on recycling of industrial technological wastes . The recycling of REEs is currently far from being feasible on an industrial scale and is still in its early stages. In 2011, despite research already containing numerous possible processes, only 1% of REEs end-of-life wastes was recycled using various existing techniques, such as gas-phase extraction, pyrometallurgical, or hydrometallurgical methods . As of today, only 7% of the light REEs and 6% of the heavy REEs are produced by recycling from secondary sources to meet the EU demand for REEs .
Hydrometallurgical methods of recycling were developed because of their low energy consumption in contrast to pyrometallurgical methods, where high temperatures are needed to melt the alloys. However, hydrometallurgical methods can also have some drawbacks, such as large waste generation and usage of nonenvironmentally friendly chemicals. The methods provide excellent extraction properties and good separation factors between the REEs and other impurities, if the correct chemicals are selected [3, 7]. Numerous studies have been performed to determine the ideal reagents and experimental conditions for the treatment of most REEs [3, 5, 7]. Hydrometallurgical processes normally consist of two steps: dissolution of the material/waste and separation of the elements using solvent extraction or ion-exchange. Dissolution is usually performed by leaching the metals using acids of varying concentrations. The acids most used and developed for magnet recycling are HCl, H2SO4, and HNO3 [8, 9, 10, 11, 12]. These have been examined for decades, meaning the criteria affecting the efficiency of the leaching, such as acid concentration, solid/liquid (S/L) ratio, temperature, or even the use of the ultrasonic bath, have been studied in detail . Despite their excellent leaching properties, using strong inorganic acids can have some drawbacks, such as poisonous gas evolution during leaching, adverse impact on the environment, soil acidification (in cases of release), and challenges during handling. Leaching has already been studied using some organic acids, such as acetic acid . Acetic acid has the advantage of being a green solvent, it is easily degradable via aerobic and anaerobic conditions, and has possible regeneration capacities . The literature on leaching of neodymium magnets using organic acids is, however, very scarce and should be studied further in order to determine their potential use for industrial-scale REEs reprocessing.
If a solvent extraction step is performed afterward, several extracting agents have been developed industrially, i.e. organic phosphorous extractants, such as phosphoric acids. Cyanex 923 and D2EHPA (di-(2-ethylhexyl)phosphoric acid) are two examples [19, 20], but they sometimes suffer from various drawbacks, such as poor selectivity, poor stripping of the metals from the organic phase, and a low rate of extraction. Due to the need to develop greener and totally combustible products for the process, it would be beneficial to use extractants that follow the CHON principle, such as amides or carboxylic acids, such as tetraoctyl diglycolamide (TODGA) . This specific extractant has shown great specificity for lanthanides and other bloc-f elements, extracting only divalent and trivalent ions with large ionic radii over 80 pm . Furthermore, the diluent in which the extracting agent is dissolved to form the organic phase (solvent) has great impact on the efficiency and selectivity of the extraction [10, 21, 22, 23]. Other criteria that affect the extraction process are also of great importance, such as acidic concentration in the aqueous phase, as this provides counter ions for the exchange mechanisms and complex formations.
For this work, several extractants and diluents were tested. To ascertain the extraction feasibility on an industrial scale, the following common phosphate extractants were tested: TBP (tributyl phosphate), D2EHPA, Cyanex 923 (mixture of trialkyl-phosphine oxides), and Cyanex 272 (di(2,4,4-trimethylpentyl) phosphinic acid), as well as TODGA (to determine whether a greener procedure could be feasible). Several diluents, such as alcohols (1-octanol), ketones (cyclohexanone), and alkanes (pentane, hexane, dodecane, and solvent 70), were also tested. The first goal was to achieve separation of the REEs from other elements, then to ascertain if they could be separated from each other, and finally to explore the potential efficiency of greener lixiviants for the leaching steps of magnet recycling for REE separation using the hydrometallurgical route. Due to their mostly harmless properties, these lixiviants could replace mineral acids and are much easier to recycle. If the leaching process is effective, some conditions would be mitigated, such as temperature or solid-to-liquid ratios, which were kept constant in the first tests. The leachates could then be used to test different extraction procedures, with different extractants and diluents, to ascertain the feasibility of a separation step with these specific leachates.
Fine NdFeB magnet powder was procured for the experiments from the University of Birmingham . Pretreatment heating was carried out by roasting the powder at 400 °C for 1.5 h in an Entech muffle furnace (LF2) in a ceramic crucible. After roasting, the powder was sieved to a particle size of < 355 μm using a Retsch AS 200 vibratory sieve shaker. To determine the chemical composition of the magnet powder, 0.5 g of powder was dissolved in 20 mL of aqua regia at 70 ± 1 °C, heated for one hour on a heating plate, and the samples were analyzed after total dissolution with ICP-OES (ppm scale) to determine the weight quantities of Nd, Pr, Dy, Fe, Co, and B in the magnet. Experiments were done in triplicates.
Aqua regia was prepared by mixing concentrated HNO3 (65%, suprapur©, Merck) and hydrochloric acids (37%, ACS reagent, Sigma-Aldrich), in 1:3 volume ratio, respectively. Samples were diluted with 1 M HNO3 before the ICP-OES measurement. The same method was used throughout the experiments, with a calibration curve prepared using standard solutions.
Leaching experiments were carried out using glycolic acid (Sigma-Aldrich, > 99.5%), maleic acid (Sigma-Aldrich, > 99.5%), and L-ascorbic acid (Sigma-Aldrich, > 99.5%), in 50 mL polypropylene bottles, with constant agitation using magnetic stirrers. Leaching agents were prepared by diluting the concentrated acids with MilliQ water (Merck Millipore Q-POD©). The volume of the different acids remained constant, 25 mL, and changes were made to: (1) acid concentration, (2) S/L ratios, (3) temperature, and (4) stirring speed. The influence of acid concentration was investigated for each acid with the S/L ratio fixed at 1/50 g/mL, temperature set at 25 ± 1 °C and concentration ranging from 0.6 to 1 M. S/L ratio was then varied from 1/30 to 1/80 g/mL using 1 M of each acid. Furthermore, the temperature was varied between 25 ± 1 °C and 70 ± 1 °C. Finally, rotation speed was varied between 400 and 1000 rpm.
For the kinetic studies, 0.1 mL samples were collected after 100, 200, 300, 400 min, and after 24 h, with the agitation stopped so that particles could deposit. The samples were diluted with 1 M HNO3 and analyzed by ICP-OES. Experiments were conducted in triplicate to ascertain reproducibility. The heating and stirring plate used was an IKA©RT15. Powder was weighed on a Fisher MH-214 analytical scale. The pH values of the acids before and after leaching were measured using a MeterLab™ PHM 240 pH/ion Meter pH electrode.
Liquid–Liquid Extraction Experiments
Five organic extractants were investigated: TBP (Sigma-Aldrich, 97%), D2EHPA (Sigma-Aldrich, 97%), TODGA, Cyanex 272 (Cytec, 85%), and Cyanex 923 (Cytec, 93%). TODGA was synthesized at the Nuclear Chemistry Department of Chalmers Institute of Technology and 98% purity was ascertained by NMR. Solutions of 1 M concentration were prepared by dilution, using 6 mL for the TODGA and 20 mL for all others. For other experiments, 0.2, 0.4, 0.6, and 0.8 M solutions of the best extractant were then prepared to determine the influence of concentration in the organic phase on extraction efficiency. The first diluent chosen was Solvent 70 (hydrocarbons C11–C14, < aromatics, Statoil, Sweden). Furthermore, the influence of the diluent was tested using 1-octanol (Sigma-Aldrich, Cylcohexanone (> 99%, ACS reagent, Sigma-Aldrich), pentane (Sigma-Aldrich, Hexane (95%, anhydrous, Sigma-Aldrich) and n-dodecane (99%, Sigma-Aldrich) and compared to Solvent 70. Leachates were prepared using 100 mL of glycolic acid and 100 mL of maleic acid, 1/80 S/L ratio, at room temperature and with a stirring speed of 400 rpm, for 400 min. Extractions were performed by putting 1 mL of aqueous leachate in contact with 1 mL of organic phase. The shaking vials were shaken for 10 min at 25 ± 1 °C, using an IKA VIBRAX VXR Basic at 1500 vibrations per minute, and then centrifuged for 1 min at 5000 rpm in a Heraeus Labofuge 200 centrifuge. Samples of the remnant aqueous phase were collected for ICP-OES analysis.
Results and Discussion
Magnet Powder Characterization
Composition of the magnet powder leached with aqua regia. The leachate was diluted with 1 M HNO3 and measurement was performed with ICP-OES iCAP 6500, Thermo Fischer (ppm scale)
20.4 ± 0.2
2.36 ± 0.01
0.74 ± 0.01
52.7 ± 0.7
1.18 ± 0.01
0.94 ± 0.01
Leaching Kinetics and Concentration Effect
Lastly, the rotation speed of the magnetic stirrer was investigated with 1 M acids, 1/80 g/mL S/L ratio, at 25 ± 1 °C. At low rotation speeds leaching can be restricted by matter transfer, so speeds were varied between 400, 700 and 1000 rpm to see if such limitations could occur. If the stirring is too slow, magnets particles tend to aggregate on the stirrer, which diminishes the surface available for the acid to reach. The results show that 400 rpm is sufficient for leaching to take place normally, as no significant changes were seen between samples.
Treatment of Leached Solutions
Some color changes were observed during the experiments. At first the aqueous phase was yellow, and this remained the same for some experiments (for instance with TODGA), whereas in other experiments, such as Cyanex 923 with maleic leachate, the inverse was seen, and the organic phase became yellow. For D2EHPA, both phases became transparent. In the case of Cyanex 923 with glycolic leachate, a third phase was observed, appearing to be an emulsion and insoluble in both aqueous and organic media. This phenomenon can also appear during industrial processes, such as the PUREX process, and should be avoided .
Diluent Effect on Solvent Extraction
The values of the dielectric constant for cyclohexanone, 1-octanol, dodecane, hexane, pentane, and Solvent 70 
Stripping of the REEs from D2EHPA-containing organic phases can be easily performed using acids of various strengths (e.g., 2 M HCl) to tailor to the efficiency of stripping as shown in previous research [7, 22].
This work was aimed at providing both a leaching method and a separation process for the recycling of REEs from magnets using organic acids. The main goal was to minimize the use of mineral acids and optimize experimental conditions in the solvent extraction step to find the most suitable and optimal design for industrial use. Glycolic, maleic, and ascorbic acids had not been used before for the leaching of neodymium magnet waste. It was shown that these could be a feasible replacement for mineral acids in the leaching step.
Leaching was more efficient in glycolic and maleic acids than in ascorbic acid, dissolving the studied elements almost entirely and reaching equilibrium in 400 min. As expected, increasing the concentration of the acids and decreasing the S/L ratio enhanced the leaching efficiency for all metals tested, giving results comparable to those where mineral acids were used. Temperature increase was beneficial for the leaching in ascorbic acid, allowing the leaching to reach values above 95% extraction at 70 °C for REEs. These results make the organic acids as efficient as inorganic acids at the conditions tested, and these could prove to be a promising leaching alternative.
Extraction was tested using glycolic and maleic leachates. Low distribution ratios and poor separation factors between REEs and other metals were determined for TBP, Cyanex 272, and 923, using Solvent 70 as a diluent. However, for TODGA with the maleic leachate, and for D2EHPA with both leachates, D values showed that REEs were extracted preferentially from the aqueous phase, contrary to Fe, Co, and B. This shows that these types of leachate have potential future use on a large scale in terms of selectively extracting REEs from other impurities. Nonpolar aliphatic diluents, such as pentane and hexane, showed the best efficiency.
The research leading to these results has received funding from the European Community’s Seventh Framework Programme ([FP7/2007-2013]) under Grant No. 607411 (MC-ITN EREAN: European Rare Earth Magnet Recycling Network). This publication reflects only the authors’ views, exempting the Community from any liability. Project website: http://www.erean.eu. The authors would like to acknowledge with thanks for the contribution with additional funding toward the ENVIREE project (Environmentally friendly and efficient methods for extraction of rare-earth elements (REE) from secondary sources—ERA-MIN Project 2015–2017).
Compliance with Ethical Standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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