REE Recovery from End-of-Life NdFeB Permanent Magnet Scrap: A Critical Review

Properties and Production Methods of Rare-Earth Permanent Magnet (REPM)

Today, key drivers for the development of new permanent magnet materials are the growing demands for industrial motors and automation, consumer electronics, e-mobility, and the shift toward renewable energies, most importantly wind power [29]. As a consequence of the 2010/2011 Rare Earth Crisis, research has also been driven by the quest for permanent magnet materials without [30] or with a reduced amount of critical rare-earth metals [31]. There is a paradigm shift in how this research problem is approached: while so far the impressive advances in permanent magnet research have almost exclusively been pushed by experimental materials science research, today, the modeling of new materials systems gains more importance [32].

Making a permanent magnet is governed by following two major principles: first, choosing the right starting material that comprises good intrinsic properties, that is, saturation magnetization, Curie temperature, and magnetocrystalline anisotropy; second, translating these intrinsic properties into functional extrinsic properties, most importantly a high remanent magnetization and coercivity (hysteresis). This is done by processing the materials appropriately and adjusting respective microstructures [33]; a schematic of main processing routes can be found in Fig. 4. In the case of Nd–Fe–B, the market is dominated by sintered, the so-called nucleation-type magnets. In this case, a Nd–Fe–B starting alloy is produced by a strip caster, which is used to cast the alloy on a water-cooled, rotating wheel that rapidly quenches the alloy. The great advantage of this technique is that the resulting alloy comprises a very homogenous microstructure with Nd–Fe–B grains in the microscale; the segregation of α-Fe is largely avoided. In a second step, the material is pulverized by hydrogen decrepitation (HD) and then jet-milled down to a grain size of typically 3–5 µm [34]. The powder is then aligned in a magnetic field and pressed to make a textured green compact, which is then sintered. The overall REE content of the magnet is considerably higher as suggested by the stoichiometry of the main phase: the extra REE portion makes it possible to run a liquid-phase sintering process. The sintered body is then finally annealed to improve the magnetic properties, most importantly the coercivity. The REE-rich, intergranular low-melting eutectic phases are the key to enhance coercivity, that is, the ability of the magnet to withstand demagnetization: the idea is to magnetically decouple the grains in order to avoid the movement of transverse domains as much as possible. The magnets are mechanically worked (sawing, grinding, polishing) to get a final product. In terms of (raw materials) cost, this final production step should not be underestimated: many tons of REE-rich grinding slurry are accumulated each year, a valuable resource, in respect of which an appropriate, cost-effective, and clean recycling process is still a challenge.

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Magnetization reversal, i.e., the nucleation of domains after the magnet has first been saturated and mounted and then been exposed to an opposing magnetic field in the application, happens by nucleation at “imperfect” interfaces of individual Nd–Fe–B grains (“nucleation-type magnet”)—“imperfect” in the sense of magnetic, structural, chemical, or geometrical imperfections [35]. This observation led to the development of the so-called grain boundary diffusion process, in which HREEs, most importantly Dy and Tb, are diffused into the microstructure of the magnet along the grain boundaries, magnetically hardening particularly these areas of the grains (Dy and Tb comprise higher magnetocrystalline anisotropies than Nd). Apart from the cost-wise benefits, the reduced addition of HREEs increases the remanence of highly coercive Nd–Fe–B, thus the energy density, (BH)_max, due to the lower amounts of HREE 4f spins that couple antiparallel with the 3d electrons. Interestingly, this kind of advanced processing experienced its industrial breakthrough in the aftermath of the 2010/2011 global Rare Earth Crisis, when prices particularly for HREE oxides were multiplying by several orders of magnitude.

The second major class of Nd–Fe–B permanent magnets are nanocrystalline, polymer-bonded magnets. The main steps of the production route adopted are (1) rapidly quenching a Nd–Fe–B alloy by melt spinning, that is, at much higher solidification speeds than those achieved by a strip caster, to produce polycrystalline flakes with grains in the nanoscale; (2) milling these flakes to a standardized particle size; and (3) mixing the flakes with a polymer and injecting or pressing this compound into a magnet. The major advantage of this approach is the near-net-shaped production of magnets with more complex geometries. Energy densities of these kinds of magnets are, of course, much lower than those of sintered magnets: the permanent magnet material is “diluted” by the polymer, and the standard bonded magnets are isotropic, which means that the Nd–Fe–B grains in the polycrystalline particles are not textured.

There are two ways to induce texture in a polymer-bonded magnet: (1) using melt-spun flake materials that underwent a hot deformation process (see below); or (2) using materials processed in a dynamic hydrogenation disproportionation desorption and recombination (d-HDDR) process [36]. In the d-HDDR process, larger single-grain particles obtained by strip casting and hydrogen decrepitation (see above) are disproportionated at 800–900 °C hydrogen pressures of up to 100 kPa to form anisotropic, polycrystalline particles in the nanoscale. The dynamic control of hydrogen pressure and temperature during the process makes it possible to preserve a high degree of texture within the individual grains—the so-called texture memory effect [37]. Finally, it is important to note that HD and HDDR processing constitutes an important recycling pathway for NdFeB magnets [38, 39, 40, 41].

Nanocrystalline, melt-spun Nd–Fe–B can also be hot pressed and hot deformed to make isotropic and anisotropic permanent magnets, respectively [42]. By backward extrusion of the compacts, ring-shaped magnets or those with a high degree of texture and radial magnetization can be produced [43]. The great advantage of nanocrystallinity are the high values of coercivity that can be achieved, since grain size correlates negatively with coercivity until the sizes of the grains scale-up to match with those of the critical single-domain size of Nd–Fe–B, that is, at around 300 nm. In addition, nanocrystalline materials are more corrosion resistant than sintered magnets [44]. The disadvantages of hot pressing and compaction are the relatively high processing costs, caused by the noncontinuous nature of the batch-process and expensive pressing tool that have to be changed regularly. In addition, energy densities of hot-deformed magnets are lower than those of sintered magnets, due to a lower degree of texture that can be achieved.

Applications of NdFeB Magnets

According to Constantinides [45], more than 20 types of applications could be identified for rare-earth magnets. Different ways of classifications for REE magnet applications can be found. Figure 5 shows the relative share of REE magnet applications, predicted for the2012 year end by Shaw and Constantinides [23]. HDDs and CD/DVDs used to be the number 1 applications of the REE permanent magnets, and as is seen from the data, motors and generators are, however, becoming the most important applications in recent years. Although these figures are relatively old, and there are quite some discrepancies from different sources and for different years, it is nevertheless possible to observe some general trends. The classification and grouping of applications are sometimes different, and will lead to data inconsistencies. It is now a commonly accepted fact that the market for HDD and CD/DVD drives is shrinking, and that the use of the magnets in the transport sector, in particular EV/HEV and E-bikes, is rapidly growing.

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Conventional Shredding Operation for ELVs and Household Appliances

As is discussed earlier, most of the permanent magnets go to shredders together with the bonded EOL products. A recent study by Bandara et al. [46] gives a good overview about the destinations and concentrations of the permanent magnets and the associated REEs. Their study for shredding ELVs and household appliances indicates that the ferrous scrap of the shredder products contains about 130–290 g of Nd/ton, or 130–290 ppm. Figure 7 illustrates the shredder feed compositions and the destination of the REE magnets for a mixed shredding of ELVs, household appliances, and other sources. This concentration is even much lower than the REEs content in some mine tailings which can be up to 1000–1500 ppm. Without pre-concentration and novel technologies, the low REE content in the scrap is currently very difficult to recover in the metal industry. Technologies for pre-shredder separation are needed for feasible metallurgical recovery.

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Magnet Scrap From WEEE Shredding

At the moment, most of the collected WEEEs are shredded without pre-dismantling for the magnets or magnet-containing components such as HDDs from PCs and laptops or speakers from TVs or mobile phones. In principle, the collected WEEEs are directly shredded as a whole after removing batteries or screens, or any toxic/dangerous components and materials. The permanent magnets are normally reported to the ferrous fraction due to its strong magnetics or stick to the shredder systems without separate collection. In this operation mode, the REEs in the permanent magnets have very low concentration and is very difficult to be up-concentrated for economic recovery. Some recycling companies are stockpiling the manually dismantled REE magnet-containing components such as computer HDDs for future REE recovery, since there is no mature or economically feasible REE recover technologies. Shredding of manually dismantled HDDs is normally practiced for data security of data centers and network drives, and the shredded HDDs are sold as ferrous and nonferrous scrap, and thus REEs in the permanent magnet are lost in the subsequent metal recovery in smelters. This will be further discussed in “Shredding and Physical Upgrading of Pre-dismantled Computer HDDs” section.

Habib et al. [53] studied the flow of resources in electronic waste—with a case of EOL computer hard disk drives. Their results demonstrate the complete loss of REEs in the existing shredding-based WEEE treatment processes. Dismantling and separate processing of NdFeB magnets from their end-use products can be a more preferred option over shredding. However, it remains a technological and logistic challenge for the existing system.

There have been several joint industry–academic efforts in developing separate treatment technologies for computer HDDs and compressors of air conditioners, and this will be described in the “Separation of Magnets Prior to Shredding of HDDs and Air Conditioning Units” section following this. Furthermore, within an EU FP7 project REEcover (http://www.reecover.eu), great efforts are being made to develop economically feasible technologies to recover low concentration REEs in the NdFeB permanent magnets from various standard and specific WEEE shredder products. Industrial operation outputs of various product streams (ferrous, nonferrous, and dust) are sampled and characterized systematically, and further upgrading of REE-rich streams (Nd, Pr, and Dy) originating from NdFeB permanent magnets were intensively investigated using various combination of size reduction and physical separation technologies, including thermal demagnetization, screening, cryo-grinding, and sieving. Quite promising results were obtained and reported in a recent conference paper in 2015 [54]. More detailed publications are expected in the near future, according to the project management.

In order to separate the magnets prior to shredding, several problems have to be overcome. First, the magnets are often held in position with glues and the components held in the electronics with screws. Then the magnets are also often coated with Ni or Ni–Cu–Ni, which has to be removed prior to reprocessing. and finally the magnets have to be demagnetized. Ueberschaar et al. [55] suggested that standardized alloy mixtures for NdFeB magnets, their coatings, and alloy types would help to simplify the recovery of the magnet material for a subsequent use. Several solutions have been proposed, and they are outlined below in Separation of Magnets Prior to Shredding of HDDs and Air Conditioning Units” section.

Hydrogen decrepitation of Pre-dismantled Computer HDDs

The University of Birmingham (UK) has developed a hydrogen-processing method to extract sintered NdFeB magnets from computer hard disk drives [13, 41]. During this process, either the voice coil assemblies from the hard disk drive or the cropped corner from the drive is exposed to hydrogen at atmospheric pressure and room temperature. The sintered magnets react with the hydrogen forming NdH_2.7 at the grain boundaries. The grain boundary reaction is exothermic and results in the Nd₂Fe₁₄B matrix grains forming an interstitial hydride. The overall reaction in hydrogen results in about 5 % volume expansion, and as the magnets are brittle, they break apart resulting in friable hydrogenated powder. The coercivity of the powder becomes so low that the hydrogenated material is essentially soft magnetic and is no longer magnetically attracted to ferrous components in the scrap. This process has been named Hydrogen Processing of Magnet Scrap (HPMS) [41]. During the HPMS process, when applied to hard disk drives, the nickel coating peels away from the surface of the magnets as thin sheets. After the HDD components are processed in hydrogen, they are rotated in a porous drum in order to liberate the hydrogenated powder from the electronics. At this stage, the powder still contains small nickel particles. The nickel contamination is subsequently removed by mechanically sieving the powder mixture. Figure 8 shows the pilot facilities developed at the University of Birmingham.

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As the hydrogenated powder is much more friable than the nickel flakes, and hence, it mills faster than the nickel, and when passed through a 90-μm sieve, the nickel contamination is reduced from around 2–3 wt% in the starting magnet to below 400 ppm in the final hydrogenated NdFeB powder. This is equivalent to the nickel content of the base alloy, and thus it has entirely been removed. The NdFeB alloy powder which is generated can be used directly to form new NdFeB magnets using a number of routes which are outlined in “Direct Alloy Recycling Routes” section.

Mechanical Separation of Magnets From HDDs and Air Conditioners

The Hitachi Group (Japan) developed a mechanical dismantling and separation technique for NdFeB magnets in HDDs and air conditioners, using a rotational dram. Through vibration and impact by tumbling, the screws fixing the HDD casing become loose, and the magnets can be separated out of the casing in about 30 min. The apparatus can process 100 HDDs per batch and 200 HDDs per hour, much faster than manual dismantling at a rate of 12 units per hour [9, 56, 57]. Hitachi and Mitsubishi Materials have also developed similar technology to recover the NdFeB magnets from the compressors in air conditioners. After cutting off the casing, the rotors in the compressor can be removed and treated by either nonthermal demagnetization using resonance damping demagnetization at room temperature [57] or thermal demagnetization at 400–500 °C. Hitachi may use the similar rotating drum for disintegration of HDDs to loosen the magnets from the air conditioners, and Mitsubishi uses other equipment (e.g., drills). Furthermore, Mitsubishi Materials is also trying to separate the NdFeB magnets from direct drive (DD) motors in washing machines through thermal demagnetization and removing the resin bounding [9]. Figure 9 illustrates the commercial process to separate REE magnets from HDDs and other scraps to automatically separate fragments of precious metals or other valuables [57]. According to Baba et al. [57], trials were to be conducted on a commercial scale. However, no more recent update about the status of commercialization and operation has been reported to the public.

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Leaching of NdFeB Magnet Scrap

Leaching is always the first step to dissolve the REEs in the magnet or magnet scrap, preferably prior to other metals in the magnet and magnet scrap. Depending on the complexity level, different dissolution methods could be used. Dissolution of magnet scrap can be performed in three different ways: (1) complete dissolution of the NdFeB magnet (with or without a prior roasting step), (2) roasting followed by selective leaching of the REEs, and (3) selective conversion of REEs in solid magnet or magnet scrap directly to a new solid phase. Alternatively, transformation of REEs in the magnet scrap into REE compound precipitates could be realized, based on solubility of REE salt at different temperatures or under hydrothermal conditions.

Complete leaching (with/without roasting) H₂SO₄ leaching at room temperature has been developed by Layman and Palmer in 1993 [17] of US Department of the Interior Bureau of Mines in 1993 for relatively clean production scrap. All components in NdFeB magnets will be dissolved. Nd can be precipitated as double sulfate salt which could be easily converted to NdF₃ or Nd₂O₃. However, large amount of Fe dissolved in the solution needs to be properly disposed (jarosite precipitation). Recently Abrahami et al. [59] successfully applied a similar H₂SO₄ leaching for the treatment of complex EOL NdFeB magnet scrap from an HDD shredder. Very recently Bandara et al. [76] have demonstrated a HCl leaching process to selectively dissolve NdFeB magnet from shredded electric motors—a mixture of REE magnet, (stainless) steel and copper. The thermally demagnetized magnet particles were completed dissolved in 4 M HCl solution for 24 h at room temperature, while the (stainless) steel and copper particles remained completely unreacted. The leach solution was subsequently treated with oxalic acid to precipitate REEs (Nd, Pr, and Dy) as REE oxalate, and the effluent containing Fe and other dissolved transition metals was suggested conceptually to go through a pyrohydrolysis process to generated iron oxide and HCl back to leaching. A total of 82 % REE recovery has been reached from the total processing from shredding to oxalic precipitation with a product purity of 99.8 % of REE oxalate.

Yoon et al. [77] used an oxidative roasting at 500 °C for sintered scrap and at 700 °C for bonded scrap followed by H₂SO₄ (2 mol/L) leaching at 50 °C for 2 h and double salt precipitation. Dissolution of Nd₂O₃ and Fe₂O₃ seem to be much faster than direct leaching of the magnet, but there is no selectivity of leaching Nd₂O₃ over Fe₂O₃. The achieved recovery of Nd yield was over 99.9 %, but about 95 % of iron was also dissolved similar to Layman and Palmer [17]. HCl could also be used for complete magnet dissolution [78]. Subsequently, REEs can be precipitated from leach solutions by adding oxalic acid or hydrogen fluoride to form oxides or fluorides.

Selective leaching (with roasting) Selective Nd leaching could be obtained by first roasting of the magnet in air at 700 °C followed by H₂SO₄ (4 mol/L) of Nd₂O₃ by controlling pH at 70 °C for 3 h with pulp density of 100 kg/m³ [79]. However, it reached only 70 % Nd leaching recovery. Formation of NdFeO3 at higher roasting temperatures may hinder the dissolution of Nd. Selective leaching after oxidative roasting at 900 °C for 6 h was also reported by subsequent dissolution with HCl (0.02 mol/L) at 180 °C for 2 h [80, 81]. More than 99 % REE and less than 5 % Fe recovery have been reached.

Hydrothermal conversion Itakura et al. [82] investigated the hydrothermal treatment of Nd–Fe–B sintered magnet using a mixture of hydrochloric and oxalic acids. Hydrothermal treatment is an in situ conversion method to change one mineral to another under hydrothermal conditions (high temperature and high pressure). They used a mixture of HCl (3 mol/L) and oxalic acid (0.2 mol/L) to recover Nd as oxalate. A Nd conversion of over 99 % was achieved at 110 °C for 6 h in the magnet recovered as solid precipitate of Nd₂(C₂O₄)₃ with the purity of 99.8 %.

Electrochemical dissolution At TU Delft, a new method using anodic dissolution of NdFeB permanent magnet scrap is under investigation to dissolve REE species electrochemically [83], separating non-REEs in the scrap into anode slimes or precipitated onto the cathode, through accurate control of electrode and cell potentials, current density on the electrodes, and electrolyte chemistry. It is proved technical feasible to dissolve the REEs from the magnet anode, and simultaneously dissolve ferrous metals and reprecipitate them as oxides/hydroxides in the solution or as metals on the cathode.

Leaching methods offer the flexibility of magnet and magnet scrap dissolution. However, selective leaching of REEs in the magnet scrap is challenging due to the presence of large amount of Fe in the magnet alloy and in the scrap. Quite often a high temperature treatment is needed to increase the selectivity of REEs over Fe and other transition metals. For more detailed description and more leaching methods applied to NdFeB magnet scrap, please see the recent reviews by Binnemans et al. [8] and Tanaka et al. [9].

REE Separation Through Solvent Extraction

Solvent-extraction separation of ions from an aqueous solution is based on the formation of complexes between ligand molecules soluble in an organic phase and ions present in an aqueous phase and the transfer of these complexes to the immiscible organic phase. The distribution ratio (D) represents the distribution of the ion of interest between the two immiscible phases. It is defined as the ratio of the total concentration of the ion/element A in the organic phase and the total concentration of A in the aqueous phase (D_A = [A]_org/[A]_aq) The separation factor (SF), defined as SF_A/B = D_A/D_B, is used to show the degree of separation between two solutes A and B in an extraction system. It is important to understand that the distribution ratio of a metal depends both on the chemical equilibria between the different forms of the metal in both phases and on the Gibbs free energy change which occurs when a metal containing species is transferred from one phase to the other. The efficiency of solvent-extraction processes depends on the chemical properties of the ions to be separated, the properties of the ligands and the solvent that they are present in (often called the diluent), the pH of the aqueous phase, the temperature, and other parameters, such as the efficiency of mixing between the aqueous phase and the organic phase and the time of contact between these phases.

The body of literature on specific solvent-extraction processes and extractant systems for separation of REEs is very large. Separation of lanthanides from actinides has been of major interest in the nuclear industry where it is necessary to separate the lanthanides from the actinides. This has inspired to a very large research effort since the 1950s. One of the difficulties in separating the REEs, or lanthanides, by methods like solvent extraction is caused by the fact that they are closely related to each other based on their chemical properties [84]. They are located in the f-block in the periodic table and due to the consecutive filling of 4f-orbitals going from Ce toward Lu, while having the same number of 6 s electrons, the properties of these elements are very similar even though the shielding of the increased positive charge from the nucleus by the electrons create differences in atom radii (“the lanthanide contraction”). The oxidation state +3 is the most common one, both in solid compounds and in solutions. One result of the similarity is that the lanthanides occur together in ores. The similarities in thermodynamic properties influences the formation of complexes in aqueous and organic phases and thus the efficiency of solvent-extraction separation systems. Small differences in the shielding of the charge of the nucleus by 4f electrons between lanthanide ions have to be used to create systems for separation of these ions from each other or in groups [85, 86]. In the production of pure REEs from virgin ores containing mixtures of them with additional elements this generally means that the solvent-extraction separation process must include many mixer settler steps in order to give a sufficient purity of the products. Recycling of REEs from spent products, such as permanent magnets, by leaching and solvent extraction presents an interesting opportunity since the REEs present in the system have been used as a group and can be recovered and reused as a similar group. However, if the recycling is supposed to produce pure REEs full separation needs to be done.

In industrial solvent-extraction separation of REEs the most common ligands are organophosphorous compounds, such as the di(2-ethylhexyl)-phosphoric acid called D2EHPA [87]. Typical separation factors between adjacent REEs using DEHPA is around 2.5 [88]. Separations of REEs from solutions based on different acids by DEHPA have been investigated by several groups showing that aqueous phases based on hydrochloric and sulfuric acids give better results than those based on nitric acid [89, 90, 91]. Other organophosphorous ligands that have attracted a lot of interest for separation of lanthanides are saponified 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (EHEPA or PC88A) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) [92]. EHEPA was recently compared with D2EHPA for separation of Nd, Dy, and Y from hydrochloric acid solution by Mohammadi and co-workers [G]. The results showed that both ligands separately and in mixtures extracted the metals in the order of preference Y(III) > Dy(III) > Nd(III). Both ligands are acidic, which means that H⁺-ions are released in the formation of REE-ligand complexes. This introduces the pH of the aqueous phase as a factor that needs to be controlled when controlling a process. Depending on the concentration of REE ions in the solution pure D2EHPA or a mixture of D2EHPA and EHEPA gave the best extraction efficiencies. Different complex stoichiometry dominates depending on the relation between REE and ligand concentrations in the respective phases. The best separation between Nd and Dy was achieved using pure D2EHPA.

A rather detailed review about alternative solvent-extraction routes for separation of REEs from permanent magnets has recently been published by Yoon and co-workers [92]. The group of extractants most used in industry are the organophosphorous compounds di-(2-ethylhexyl)phosphoric acid (D2EHPA), saponified 2-ethyl-hexyl phosphonic mono-2-ethyl-hexyl ester (PC88A) and bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272). It has, for example, been reported that the extraction capability for REEs by organophosphorus extractants decreases in the order D2EHPA > PC88A (EHEPA) > Cyanex 272. However, the separation factors between adjacent REEs have been shown to vary according to Cyanex 272 > PC88A > DEHPA [93]. Although Cyanex 272 gives the highest separation factor between Pr and Nd of the three extractants compared by Banda and co-workers, the total extraction percentage of REEs by Cyanex 272 was low. Therefore, in order to obtain high separation factor together with high extraction efficiency, solvent-extraction experiments separating Nd and Pr from chloride solution containing both metals in a mixture have been performed by Liu and co-workers using mixtures of Cyanex 272 and tertiary amines [94]. A combination of Cyanex 272 and Alamine 336 gave the best synergy. Although the separation factors of the two metals were not increased, the extraction of both metals was enhanced. Organophosphorous extractants and amines have been compared as single extractants by for example Abreu and Morais [95] showing that the organophosphorous compounds were the most efficient. The conclusion from the review of Yoon and co-workers [92] was that DEHPA and PC88A are the most effective and promising extractants for the separations needed for recycling of magnet REEs with PC88A seen as the most promising. In both cases, kerosene can preferably be used as diluent. A hydrometallurgical process for the recycling of NdFeB magnet waste with focus on the separation of Tb and Dy from Pr and Nd by solvent extraction with PC-88A was recently presented by a group in Germany [96]. Results from tests in a laboratory size mixer–settler plant showed that twelve stages are necessary for nearly complete separation (4 N purity, rare-earth element basis). The authors state that although further optimization can be done, the process already shows significant advantages over the primary production route used in industry. The fact that the recent laboratory investigation of these ligands by Mohammadi and co-workers came to the conclusion that DEHPA is superior to PC88A [89] shows that these systems are complex and that many factors influence the results.

One example of an amide compound that has been shown to be valuable as a ligand or co-extractant in sovent-extraction systems for REE separation is N,N,N′,N′-tetraoctyl-diglycolamide, also called TODGA. TODGA is a solvating extractant, which creates strong tridentate complexes with metal ions and has shown particularly good extraction properties for REEs ions in terms of selectivity in comparison to other ions in the aqueous solution [97]. This extractant has shown good stability at room temperature and miscibility with the commonly used diluents. A disadvantage for large-scale industrial use is currently the price of this reagent.

The recycling of metals from spent products presents new challenges in the form of dilute flows of REEs mixed with other elements that are not present in the ores used for primary production. Therefore, new separation technologies such as novel extractants for solvent-extraction separations are needed to extract REE species from low concentration leaching solutions and to produce REEs in pure compounds. As stated by Tekda and Okabe in a review on the current status of resource and recycling technologies for rare earths from 2014: “It is (also) important to establish a recycling-based society from the perspective of the conservation of finite and valuable mineral resources and the reduction of the environmental load associated with mining and smelting” [10]. In that context, the chemistry of the ligands used in the solvent-extraction processes for REE production and recycling needs to be taken into account. If possible, it would be beneficial to use ligands based on combustible (leaving no ash products) ligands such as those designed according to the CHON-principle (ligands containing only the elements C, H, O and N) instead of phosphorous or sulfur containing ones. For some time a considerable effort has been put into design and synthesis of efficient reagents complying with the ‘CHON’ principle for the solvent extraction of trivalent f-block elements [98]. An important advance has been the development of malonamides as extractants for lanthanides and actinides. The comparatively easy synthesis of the malonamides, their selectivity for trivalent f-block metals and the fact that they are built according to the ‘CHON’ principle has contributed to, for example, the DIAMEX process [99]. One disadvantage of the malonamides, however, is that the distribution coefficients (ratios) of f-block metal ions are not very high [100]. By increasing the distribution ratio during the extraction part of a purification process, the number of stages of countercurrent extraction required can be reduced thus reducing the size and cost of the plant. Novel extractants for magnet REE solvent-extraction separation are presently being developed at Chalmers University in Gothenburg (Sweden) as a part of the efforts toward better lanthanide recycling options. Three polyamides (multicoordinate, neutral ligands) bearing tetrabutylmalonamide functional groups have been synthesized and evaluated for use as reagents for the solvent extraction of rare-earth elements from nitrate media. These extractants are based on the CHON chemistry, avoiding phosphorous and other ash forming elements which makes it easier to combust the extractant and the diluent once it can no longer be recycled into the process. The results showed good effects of structural modifications of tetrabutylmalonamide on the extraction of 152Eu, which was used as a model for lanthanides due to that it allows for precise radio-analytical (radiotracer) determination. The best distribution ratios for Eu were obtained with the tetraamide 2,2′-(1,2-phenylenebis(methylene))bis(N,N,N′,N′-tetrabutylmalonamide). As for other malonamides a diluent with quite high polarity is favorable for solvent extraction of rare-earth metals [101]. It is important to note that oxidation in supercritical water has been shown to be able to process tributyl phosphate, so in the event that a CHON-extraction system for the recycling of the lanthanides cannot be found then an industrial scale recycling process will still be able to operate without recourse to the incineration of phosphorus rich organic wastes. However the absence of phosphorus and sulfur in the organic waste produced by a solvent-extraction plant would increase the value of such a waste as a fuel.

In some cases, it may be beneficial to have a stationary phase containing the complexation ligands. One example is presented in the work by Kim et al. [102], which reported in 2015, an application of membrane-assisted solvent extraction (MSX) to recover Nd, Pr, and Dy from commercial NdFeB magnets and industrial scrap magnets. A hollow hydrophobic polypropylene fiber membrane module system was used to extract REEs in a single step with the feed and strip solutions circulating continuously through the MSX system. The pores of hollow fibers in the membrane modules were filled with organic phase consisting of the TODGA, Isopar L (An alphatic diluent), and TBP in the ratio of 3:4:3, respectively. The shell side of hollow fibers was supplied with the aqueous strip solution (0.2 M HNO₃), and the lumen side of the hollow fibers was fed with the REE-containing solution (1000–2000 ppm) in 6 M HNO₃. They found that a multimembrane module configuration with REEs dissolved in HNO₃ solutions showed high selectivity for REE extraction with no co-extraction of non-REEs, while the use of HCl solution resulted in co-extraction of non-REEs. REE oxides in highly pure form were produced from the strip solutions. It seems that 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 with the conventional routes such as precipitation and solvent extraction.

REE Separation by Ionic Liquids

The replacement of the organic phase in solvent-extraction processes is a novel approach in hydrometallurgy. Ionic liquids are solvents that consist entirely of ions. They are nonvolatile and nonflammable so that they can be safer alternatives for molecular organic solvents in extraction processes, including processes for the separation of rare earths [103, 104, 105, 106, 107]. The very high concentration of extractants in the case of using undiluted ionic liquids that act as basic extractants (anion exchangers) can lead to very high loadings of metals in the ionic liquid phase and thus to process intensification (less solvent has to be used). The mechanism of solvent extraction with ionic liquids can be different from solvent extraction with molecular solvents, and this offers an opportunity for the development of new more selective separation processes. There are several challenges to deal with when designing ionic-liquid-based solvent-extraction processes: (1) the high viscosity of the ionic liquids phases which hampers mass transport; (2) the tendency of the ionic liquid to extract via an ion exchange mechanism, resulting in losses of the ionic liquid components to the aqueous phase; (3) the hydrolytic instability of some fluorinated anions such as hexafluorophosphate; (4) the recyclability of ionic liquids; (5) (cyto)toxicity issues; (6) the high price of most types of ionic liquids. Examples of REE separation systems relevant to magnets recycling making use of combinations of ionic liquids and extractants are (1) 1-methyl-3-octyl imidazolium bis(trifluoromethylsulfonyl)imide [C₈mim][Tf₂N] in combination with N,N-dioctyldiglycolamid acid (DODGAA) [105] and (2) 1-alkyl-3-methylimidazolium hexafluorophosphate [C_nmim][PF₆] (n = 2,4) in combination with D2EHPA, which is one of the most frequently used extractants for conventional solvent extraction of REEs [108]. DODGAA is a modification of the extractant TODGA which has proven to have a high affinity to lanthanides. In systems with the ionizable DODGAA the REE ions are extracted as 1:3 complexes and the metal ions can be completely stripped by acid solution. This is a significant improvement since the stripping has been a troublesome issue for many systems based on ionic liquids, such as TODGA [109, 110, 111]. Kikuchi et al. [112] investigated the extraction of the REE ions Pr(III), Nd(III) and Dy(III) with tributylphosphate (TBP) in the ionic liquid tricaprylmethylammonium nitrate [A336][NO₃].

During the recent years, the Binnemans group at KU Leuven (Belgium) has made major research efforts to develop sustainable ionic liquid processes for the recovery of REEs from end-of-life NdFeB magnets. The focus of this work is on the use of hydrophobic ionic liquids with nonfluorinated anions (chloride or nitrate). By increasing the alkyl chain lengths on the cation, the ionic liquids can be made hydrophobic (immiscible with water) without the need of using hydrophobic fluorinated anions. The ionic liquids are typically quaternary ammonium of phosphonium ionic liquids such as Aliquat^® 336 or trihexyl(tetradecyl)phosphonium chloride (Cyphos^® IL 101). In case the metal ions are extracted via an anion exchange mechanism, the loss of chloride or nitrate ions from the ionic liquid phase to the organic phase is not a major issue. The ionic liquids are used in undiluted form, i.e., without addition of molecular diluents, to take full advantage of the ionic liquid properties. The selectivity of the extraction process can be tuned by the choice of the ionic liquid anion. For instance, transition metal ions are extracted from a chloride aqueous solution to a chloride ionic liquid phase leaving the rare-earth ions behind in the aqueous solution [113], while the reverse situation holds for extraction from a nitrate aqueous phase to a nitrate ionic liquid [114]. After removal of the transition metals by extraction with a chloride ionic liquid, the REEs can be recovered from the raffinate by precipitation with oxalic acid, followed by calcination of the oxalates to oxides [115]. An efficient process was developed to extract Nd and Dy from a nitrate solution, leaving Co behind, followed by separation of Nd/Dy by selective stripping with EDTA [116]. Dupont and Binnemans [117] developed a new recycling process for (microwave) roasted NdFeB magnets, based on the carboxyl-functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf₂N]. This ionic liquid shows a temperature-dependent miscibility with water (=thermomorphic behavior): the [Hbet][Tf₂N]–H₂O system is biphasic below 56 °C, but it forms one single-phase below this temperature. Using the thermomorphic properties of the [Hbet][Tf₂N]–H₂O system, a combined leaching/extraction step was designed. The change from a homogeneous system during leaching (80 °C) to a biphasic system at room temperature causes the dissolved metal ions to distribute themselves among the two phases: Fe is reported to be present in the ionic liquid phase, while Nd, Dy, and Co remain in the aqueous phase. By selective stripping and precipitation steps, the REEs could be recovered as a mixture of Nd₂O₃/Dy₂O₃ (to be separated in a separate process), cobalt is recovered as CoO, and the ionic liquid [Hbet][Tf₂N] could be regenerated for reuse.

Roasting

The aim of roasting is to change the form or state of REEs in the NdFeB magnets at high temperatures, so that the roast product could be more selectively dissolved hydrometallurgically for REEs, leaving Fe and other constituents in the magnet and the scrap stream in the solid residue. After roasting, a more effective separation could be reached in the subsequent hydrometallurgical processing.

Chlorination roasting For selective chlorination of REEs in the magnet, various chlorination agents have been investigated mainly by the Japanese researchers: using metal halides such as MgCl₂ to convert the REEs into molten NdCl₃ and DyCl₃ at 1000 °C [118], using FeCl₂ at 800 °C [14], using NH₄Cl at much lower temperature of 300 °C [15], or using Cl₂ gas and AlCl₃ (the so-called chemical vapor transport process) [119]. In most of these cases, the RECl₃ (NdCl₃ and DyCl₃) were distilled at elevated temperatures to separate from the Fe-based residue. For all chlorination approaches, selectivity and reaction rates are key issues, in particular when treating highly contaminated and partially oxidized magnet scrap.

Sulfation roasting During sulfation roasting, the metallic elements are sulfated to form water-soluble sulfates, preferably for REE only, so that the non-REEs such as iron will be restricted at the solid state in oxide form. There are two steps for this process [120]: in the first step, the whole magnetic material is converted into sulfates by digestion in concentrated H₂SO₄ at room temperature; in the second step, the resulting mixture is roasted at a temperature at which the REE sulfates are still thermally stable, but the iron sulfates decompose into iron oxide insoluble in water.

Taking advantage of the difference in temperature stability of REE sulfates and iron sulfates, the process of sulfation roasting, followed by water leaching allows the separation of more than 95 % of REE in the leaching solution with no iron present. Iron remains in the leaching residue forming a marketable hematite-dominated by-product. This hematite (Fe₂O₃) could be used as a red pigment. Impurities such as Co and Ni were also reported to be present in the residue, thus enabling the production of a leaching liquid with at least 98 % REE purity. SO₃ gas released by thermal decomposition of the iron sulfates can be recovered and used for production of H₂SO₄. In this way, a cyclic process can be developed, minimizing emission of SO₃.

Oxidation roasting The objective of oxidation roasting is to convert the REE magnet into oxides for all metallic elements, and the subsequent leaching process of REEs becomes easier or faster, or more selective. The oxidation roasting—REE leaching has been studied by many researchers [77, 78, 79, 80, 81], and it has been discussed in the previous section for hydrometallurgical leaching process. It is important to combine the leaching selectivity and roasting conditions.

Selective Extraction of REEs Through Melt Processing

In this type of method, magnet scrap is treated with either a molten metal, a molten salt, or a molten oxide slag. The REEs in the magnet will be selectively converted or dissolved to another phase, leaving the iron, boron and other contaminating metals as unreacted solid or as a new molten ferrous alloy. This leads to the separation of REEs from the rest of the magnet and magnet scrap.

Liquid metal extraction The Ames Laboratory (USA) has developed a liquid metal-extraction process to selectively recover REEs from NdFeB and other REE-bearing alloys using Mg as extractant [121], and later the process was studied further by other researchers, [16, 122, 123]. In this process, the molten Mg and potentially other alkaline earth metals (Ca or Ba) claimed in Ref. [121] can dissolve selectively Nd and other REEs such as Pr and Dy at about 1000 °C (above the melting point of the extractant metal), leaving Fe and B or other transition metal unreacted in solid state. The formed molten Mg–REE alloy could be separated from the iron and other transition metals (in solid state). The Mg–REE separation can be realized through distillation to evaporate Mg metal due to its high volatility. However, this process is very slow (24–72 h), and it cannot be applied to (partly) oxidized NdFeB scrap. Alternatively, selective extraction of Nd was also reported successfully using silver [124] and copper [125]. Using Ag as the extractant, a Nd-Ag alloy containing 45–50 at % Nd was obtained in the temperature range of 1000–1300 °C, and Nd could be separated as Nd₂O₃ from silver by oxidation of the Nd-Ag melt in the air [124]. According to Moore et al. [125], after a quick arc or induction melting of the magnet–copper mixture (holding time 1–2 min.), a Cu-Nd rich intermetallics phase was formed upon slow cooling 350 K/min.) on the outer shell of the ingot, when a higher fraction of copper was used in the mixture feed (Cu > 21.5 %, in weight percentage unless otherwise stated). The bath temperature of arc melting could not be detected and controlled, but the induction melting temperature was reported to be in the range of 1450–1500 °C. The Nd–Cu-rich shell (nonmagnetic) is brittle and was mechanically fragmented, thus a mechanical separation from the ferromagnetic core (Fe-rich) could be obtained with magnetic separation. However, the separation is not very efficient: ferromagnetic fraction still contains about 10.4 % Nd and 7.8 % Cu, and nonmagnetic fraction contains 11.4 % Fe and 43.9 % Cu. Furthermore, further Nd–Cu separation has not been investigated, and the alloy contains almost equal amount of Nd and Cu of 44 %. This could be a fundamental limitation and challenge by using copper as an extractant.

Molten salt extraction At Anhui University of Technology (China), Hua et al. [126] reported an integrated process using molten chloride salt (MgCl–KCl) to selectively dissolve Nd and other REEs in the magnet scrap at temperatures of 700–1000 °C with an REE recovery yield of approximately 90 %. NdCl₃ in molten the MgCl₂–KCl salt mixture can be subsequently electrolyzed to produce a magnesium-neodymium alloy directly. Tanaka et al. [9, 127] proposed a process based on molten fluoride to remove the oxidized rare earths in the NdFeB magnet scrap. In this process, the molten fluoride reacts with the rare-earth oxides (REOs) and the formed REO–fluoride mixture can be used in molten salt electrolysis for REE metal production. The nonoxidized NdFeB scrap does not react, and it obtained as a cleaned master alloy that can be used for the production of new NdFeB magnets. It should be noted that the molten chloride and molten fluoride processes are complementary: molten chloride salts can be used for treatment of nonoxidized NdFeB magnet scrap only, while molten fluoride salts are suitable for the treatment of oxidized NdFeB magnet scrap only. Hua et al. [128] also reported the REE magnet treatment using a fluoride melt, and REEs in NdFeB magnet scrap have been successfully recovered using AlF₃–NaF melts as the fluorination reagent. Furthermore, active research is going on at TU Delft as part of EU FP7 funded project EREAN (http://www.erean.eu) to investigate the salt extraction of NdFeB magnet scrap for further molten salt electro-deposition of the REEs.

Molten slag extraction Two different slag treatment methods have been reported. In the glass slag method, the REE alloy is brought into contact with molten B₂O₃ that is able to selectively oxidize and dissolve the REEs from the NdFeB alloys to form Nd₂O₃–B₂O₃ slag [19]. The slag can be then leached with acid to dissolve Nd and the Nd can be recovered from the leachate by precipitation as Nd(OH)₃. However, the yield of Nd recovery is relatively poor [129]. The second slag method is the use of CaO–SiO₂–Al₂O₃ or CaO–CaF₂ fluxes to extract the REEs in the complex magnet scrap at about 1500 °C [130]. This method is very effective for EOL magnet scrap such as HDD shredder residues with more than 99 % REE recovery. It can separate completely the REEs in the magnet from the ferrous materials in the NdFeB magnet and the mixed scrap. The REE-rich slag can be used for hydrometallurgical extraction, and the separated iron-rich alloy can be a good source of raw materials for steelmaking. The advantages of molten slag extraction are its capability to effectively separate REEs and iron, and the recovery of both fractions at the same time. According to Tanaka et al. [9], a Japanese group reported an in-plant waste (sludge) separation process where the magnet waste separates into liquid iron and solid rare-earth oxide phases at 1550 °C under argon atmosphere with a low oxygen partial pressure without any flux addition. It was found that all of the REEs in the magnet waste were converted to the oxide phase, and the formed REOs were converted to RE metal through conventional technologies.

Direct melting Recently a research group at Shanghai University reported a new method for the treatment of relatively clean NdFeB magnet (without Ni coating) [131]. They first melted the magnet in a vacuum induction furnace at about 1400 °C in a graphite crucible, after which the NdFeB magnet will form REE carbides in the form of NdFeBC alloy. The solidified carbon-saturated alloy containing REE carbides are milled to fine particles and treated with water at room temperature for selective hydrolysis, and the NdC₂ in the alloy phase was converted to Nd(OH)₃, and the mixed metallic iron and iron carbide are removed by magnet separation. The recovered Nd(OH)₃ is easily calcined to Nd₂O₃ for direct commercial use or as raw materials for production of Nd metal through molten salt electrolysis. The process of vacuum induction melting (VIM) followed by hydrolysis and magnetic separation (HMS) was named as VIM–HMS method by the authors. This is a very clean process, and no toxic chemicals are used and no significant solid residues or effluent are generated. However, the method was developed with very clean magnets, and it is not clear if the method is suitable for EOL magnet scrap with low rare-earth content and highly complex compositions. Contamination of other transition metals in the NdFeB magnet-bearing streams such as copper and nickel may be a challenge.

In the EU FP7-funded project REEcover (http://www.reecover.eu), an alternative pyrometallurgical treatment of a REE-containing ferrous fraction from physically upgraded WEEE shredder products was reported recently [132]. The WEEE shredder product was first thermal demagnetized, and a fine fraction (after screening) was ground and screened to 75 μm [54]. The fraction above 75 μm is used for pyrometallurgical treatment, and the fraction below 75 μm can be treated directly with hydrometallurgical leaching. The upgraded ferrous stream from the shredder product contains about 6500 ppm REEs, originating from NdFeB permanent magnet (Nd, Pr, and Dy). It was melted at 1600 °C under reducing conditions in the graphite crucible for one-hour holding time. A complete separation of REEs from the ferrous fraction was achieved, and all REEs in the scrap were oxidized into a slag phase. The amount of the resulted slag is small, but with a relatively high concentration of REEs (over 33 % in REO form). The REE-bearing slag contains also Al₂O₃, SrO, BaO, SiO₂, and CaO, with a melting temperature in the range of 1320–1400 °C. The REE-concentrated slag is used for REE separation and extraction through hydrometallurgical leaching and further precipitation to produce more pure REOs, and finally for producing REE metal or alloy through molten salt electrolysis. More research is under way to optimize the melting conditions, e.g., using fluxing agent for enhanced REEs slag formation, to achieve more favorable and selective leaching recovery of REEs. However, the resulting concentration of REEs in the slag will be much lower than the above reported figures.

Direct Electrochemical Refining

Electrochemical processes can be applied to the metallic magnet scrap based on electro-refining principles in the molten salt [9, 133]. In a molten salt electro-refining cell, the impure REE magnet scrap is used as starting anode, and the REEs (Nd and/or Dy) can be selectively dissolved in a molten chloride or fluoride electrolyte, whereas on the cathode, only REEs are preferably deposited forming Nd-based alloys. In the cell, a REE—an iron group metal alloy is used as bipolar electrode (diaphragm).

High-temperature processing of NdFeB magnet scrap has also been reviewed by Takeda and Okabe [10], and they described advantages of the so-called dry process over the hydrometallurgical “wet processes.” They predicted that the EOL scrap from products is expected to be generated in the future at a constant volume such as the large magnets in EVs and HEVs. Therefore, efficient mechanisms and processes for recovering Nd and Dy from these types of product scrap should be developed for the future needs. Firdaus et al. [11] indicated a number of challenges for REE recovery from NdFeB magnet scrap: (1) the different mix of wastes produced—the complexity of magnet waste composition; (2) the effects of contaminants on the recycling process; (3) optimization for mutual separation of REEs (Nd, Dy, Pr); and (4) economic feasibility of the recycling and recovery process and lifecycle of the magnet-containing products (i.e., magnet scrap availability).