Review of High-Temperature Recovery of Rare Earth (Nd/Dy) from Magnet Waste
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Rare-earth metals, particularly neodymium, dysprosium, and praseodymium are becoming increasingly important in the transition to a green economy due to their essential role in permanent magnet applications such as in electric motors and generators. With the increasingly limited rare-earth supply and complexity of processing Nd, Dy, and Pr from primary ores, recycling of rare-earth based magnets has become a necessary option to manage supply and demand. Depending on the form of the starting material (sludge or scrap), there are different routes that can be used to recover neodymium from secondary sources, ranging from hydrometallurgical (based on its primary production process), electrochemical to pyrometallurgical. Pyrometallurgical routes provide solution in cases where water is scarce and generation of waste is to be limited. This paper presents a systematic review of previous studies on the high-temperature (pyrometallurgical) recovery of rare earths from magnets. The features and conditions at which the recycling processes had been studied are mapped and evaluated technically. The review also highlights the reaction mechanisms, behaviors of the rare-earth elements, and the formation of intermediate compounds in high-temperature recycling processes. Recommendations for further scientific research to enable the development of recovery of the rare-earth and magnet recycling are also presented.
KeywordsSpent magnet recycling High temperature Rare earth Pyrometallurgy
Conversion electron Mössbauer spectroscopy
Chemical vapor transport
Energy-dispersive X-ray spectroscopy
External oxidation zone
Hard disk drive
Internal oxidation zone
Liquid metal extraction
Rare-earth permanent magnet
Waste electronics and electrical equipment
Voice coil motor
There is a great interest in rare-earth (RE) recycling due to the increasing demand for REs and the complexity of processing REs from primary ores. Recycling can potentially reduce dependence on virgin production while altering the geographic distribution of RE element (REE) supply. Rare-earth permanent magnets (REPM) accounted for 26.3 % of worldwide consumption of RE in 2008, and most of the current stock of redundant material is present in electronic goods such as loudspeakers, mobile phones, and hard disk drives . The main potential sources of REPMs for recycling are residues generated at the final finishing stage of manufacturing process (referred to as swarf); small magnets in end-of-life (EoL) consumer products; and large magnets in hybrid and electric vehicles and in wind turbines.
EoL consumer products in the form of computer hard disk drives (HDDs) are probably the most important source of REE scrap. HDDs are the single largest user of NdFeB in electronic goods with around 600 million pieces manufactured annually. With 10–20 g of NdFeB in each HDD, this equates to 6000–12,000 tons of neodymium–iron–boron alloy. HDDs are not only easy to identify, but are also often routinely removed from waste electronics and electrical equipment (WEEE) and have a rapid turnover (~5 years) . They contain several permanent magnets inside the chassis. The dominant magnets are the permanent magnets of the voice coil motor (VCM) which usually comprise two identical kidney-shaped magnets attached to a metal plate. Other magnets are built into the read/write head and the spindle motor. These are often left out in recycling as they are too small (read/write head) or too difficult to extract (spindle motor magnet).
Generally, metal recycling relies on the application of common metallurgical processes. However, due to the complex design and amount of elements in the REPM waste, adaptations of the available techniques and innovation are required. Numerous papers and conference proceedings have been published on technologies for recovering REs from REPM, and there are several general reviews of the state-of-the-art technologies [1, 2, 3, 4, 5], including the potential, challenges, and possible solutions associated with the recycling of the REs [6, 7, 8]. This paper presents a systematic overview of the high-temperature recycling methodologies for recovery of RE from REPM, and focuses on the scientific (such as the mechanism) and the technical aspects of the material recovery process.
REPM’s Composition, Microstructure, and Coercivity
Chemical composition of REPM waste (mass %)
Coating and additives
The origin of the coercivity in rare-earth–transition-metal permanent magnets is their high easy-axis magnetocrystalline anisotropy. It is strongly related to the interfacial microstructure between the main phase (ϕ phase) and grain boundary phase, with magnetization reversal being the result of the nucleation and growth of reverse magnetic domains [17, 18]. In the case of Nd–Fe–B alloy, the easy axis of magnetization is the c-axis of the complex tetragonal structure of the magnetically hard phase Nd2Fe14B. In the presence of an external magnetizing field, it aligns along the c-axis, becoming capable of being fully magnetized to saturation with a very high coercivity . The crystal structure of Nd2Fe14B is relatively complex, and there are 68 atoms in the unit cell. The tetragonal structure belongs to the space group P42/mnm; it comprises six crystallographically inequivalent Fe sites and two crystallographically inequivalent Nd sites . The homogeneity range of Nd2Fe14B is very small, or even absent; it is effectively a line compound.
REPM Recycling Strategies and Pyrometallurgical Routes
material recycling, in which scrap materials are charged into smelting processes as raw materials;
alloy recycling, in which the materials are regenerated into master alloys for magnet production; and
magnet recycling, in which magnet alloys are reused in their current form.
Pyrometallurgical (high temperature) routes have been developed as an alternative to hydrometallurgical routes. The main disadvantages of hydrometallurgical routes are, first, the large amounts of chemicals and water required, and second, the fact that they require many steps, considerable energy, and relatively longer overall processing times. Much waste water is generated, and not only RE metals but also iron are dissolved by acid solution. Fe and B residues are often considered as industrial wastes, since the recycling cost of Fe does not meet the market price and B is a poisonous element that needs to be controlled environmentally. Pyrometallurgical routes provide a solution in cases where water is considered as a scarce resource and generation of waste is to be limited. Some pyrometallurgical routes also allow remelting of REE alloys or extraction of the REEs from transition metals in the metallic state; other routes can be used for recycling of partly oxidized REE magnet alloys.
Fundamental research on the thermodynamic behavior of REPM materials and on the chemical reactions between the magnet materials and the solvent medium is critical to understand and improve the RE recycling process. The thermodynamic properties of RE metal systems at high temperatures cannot be readily evaluated via traditional methods due to the chemical affinity of RE metals with oxygen or other elements . The thermodynamics of the Nd–Fe–B system have been studied to understand the behavior of the alloy during high-temperature process, and have recently been applied for pyrometallurgical recycling purposes. Most authors used the work of Hallemans et al.  for their thermodynamic evaluation of the Nd–Fe–B system. Van Ende and Jung  recently reviewed and optimized the system. The pseudo-binary system, in which B is held constant, is usually applied when dealing with recycling process, which implies the importance of understanding the Fe–Nd system. Thermodynamic information on intermetallic compounds in Nd–Fe binary system especially at high temperature is very limited. The Fe–Nd system has been reviewed by Zhang et al. , Okamoto , Marazza et al.  and Nagai et al.  and optimized by Van Ende and Jung . The database for the thermodynamic evaluation was developed by Franke and Neuschütz  for Scientific Group Thermodata Europe (SGTE) in 2008. Figure 2 shows the optimized Fe–Nd phase diagram . Thermodynamic data for other REEs such as Dy and Pr in magnetic alloys are much more limited. Nagai et al.  recently evaluated data for the Dy + Fe system required for the design of recycling processes for Dy recovery. Not only the alloying elements but the thermodynamic behaviors of coating elements (Al, Co, Ni and Cu) with Nd are also important to increase the extraction yield of Nd. Hussain et al.  provided critical reviews of the Co–Nd, Cu–Nd, and Nd–Ni systems, while Feng et al.  investigated experimentally the Al–Fe–Nd system at 773 K.
Process conditions, reagents used, and products in different recycling routes for rare-earth permanent magnets
Chemical vapor transport:
1273 K, 6–82 h
NdAl4Cl15 vapor complexes
(2) Vacuum distillation
(1) 1073 K, 12 h
(2) 1273 K, 3 h
Selective extraction + vacuum distillation
1273 K, 12 h
523–623 K, 3–12 h
1573–1823 K, 1–16 h
C, O2, Ar
Nd metal, Nd2O3
(2) Heating in H2
(3) Calciothermic reduction
(1) 1073 K, 1 h
(2) 1253 K, 8 h
(3) 1223–1273 K, 3 h
O2, H2, Ca
1503 K, 12–78 h
(1) 523–573 K, 6 h
(2) 873 k, 4 h
(3) 473 K, 24 h
CO, S (catalyst), H2
(1) Liquid metal extraction
(2) Vacuum distillation
1299 K, 24–74 h
(1) Liquid metal extraction
(2) Oxidation in air
1273–1573 K, 4–74 h
(2) Vacuum desorption
(1) 298–723 K (hydrogen introduced step by step)
(2) 973–1273 K
NdFeB HD powder
553 K, 1–2 h
NdFeB melt spun powder
1123 K, 12 h
Advantages and disadvantages of different pyrometallurgical recycling routes for rare-earth permanent magnets
NdAl4Cl15 vapor complexes
Metallothermic reduction, molten salt electrolysis
High purity of RE chlorides (~98 %); applicable to nonoxidized and oxidized alloys
Require further processing to obtain Nd alloy, consumption of large amounts of chlorine gas, Aluminum chloride is very corrosive
Molten salt electrolysis using Fe to produce Nd–Fe
FeCl recycled back using HCl formed in pyrohydrolysis; high purity (~99 %); applicable to nonoxidized and oxidized alloys
Chlorination requires relatively high temperatures and long reaction times
MgCl can be recycled back as extractant, more than 80 % RE recovery
Long reaction time
Up to 90 % RE recovery
Require further processing to obtain Nd alloy
Nd metal, Nd2O3
Simple process without flux addition,
High energy consumption for oxidation
Easy operation, master alloy can be obtained
CO/CO2 formation; carbon and calcium impurities affect magnetic properties
Molten salt electrolysis
Good separation of RE and iron (good RE recovery), 99 % extraction ratio and 96 % purity
Need further process to separate Nd oxide from Boron oxide
Molten salt electrolysis to recover Nd
Direct recycling of magnet alloy by removing oxide contaminants; low energy consumption; no waste solution generated
Need further process to recover loss Nd in alloy and recovery of Nd from oxides
Molten salt electrolysis
Good separation of RE from iron
Require further processing to obtain Nd alloy
More than 95 % RE recovery;
REEs in metallic state;
Mg rich Nd can be used as master alloy for REPM;
Mg can be recycled
Long reaction time;
Uneconomical if using fresh Mg;
Cannot be applied to oxidized magnets
Molten salt electrolysis
More than 90 % RE recovery;
Mg can be recycled
Long reaction time;
Uneconomical if using fresh Ag
NdFeB HD powder
Less energy input required compared to other routes, no waste generated, especially suited for hard disk drives (little compositional change over the years)
Not applicable to mixed scrap feed, which contains magnets with large compositional variations; not applicable to oxidized magnets;
Decreased magnetic power
NdFeB melt spun powder
Less energy input required than other routes
Only applicable to epoxy resin bonded magnet
Allows for the simultaneous extraction and separation
Not applicable to mixed scrap feed, limited cell design
In the extraction type of recycling, REs are extracted in the form of oxide, halide, fluoride or other metallic compound which can then be reduced to metallic form. These intermediate compounds are important as they can also be used as an extractant (e.g., RE-fluoride in recycling via flux) or as an intermediate product to accommodate the extraction process (e.g., RE-oxide in recycling via glass slag or RE chloride in CVT process). The mechanism, kinetics, and control of formation of these intermediate compounds are emerging topics in REPM pyrometallurgical recycling process development. Refining type processes, such as hydrogen decrepitation (HD), have advantages in terms of their applicability (simple and low energy requirement). However, the product quality is inferior as the processes downgrade the magnetic properties of the magnets. The benefit of the refining type process is that it extends the life of magnets, although the degree of the reduction in magnetic properties needs to be considered.
Recycling or Recovery via Oxidation
Nonmetallic elements such as carbon or oxygen are often considered as contaminants in REPM recycling and the formation of RE oxides or carbides can be detrimental in some pyrometallurgical recycling routes (e.g., in liquid metal extraction route and hydrogen decrepitation process). This is due to the formation of slag during heating/melting which results in a significant loss of REs due to the strong affinity of REs for oxygen. The physical separation of metal from slag is currently a serious technical challenge. These nonmetallic elements, if present in the materials manufactured from the recovered REs, may degrade magnetic properties. Oxides also present many technical problems on melting, partially because the melting temperature is very high (the melting temperatures of pure RE oxides Dy2O3 and Pr2O3 are higher than 2273 K, and Nd2O3 is ~2593 K). The RE oxide (REO) flux prevents sound casting due to its high viscosity and reactivity with other oxides such as alumina. However, due to the stability of REOs and their high occurrence in REPM swarf, the control of oxides is important in the recycling process.
NdFeB’s Oxidation Thermodynamics, Kinetics, and Mechanism
Thermogravimetric analysis (TGA) from early studies at around 573 K showed that rapid surface oxidation with formation of a thin powdered layer on uncoated REPM takes place within the first minute, and this can be substantial as the average particle size decreases [58, 59, 60]. It was suggested that this surface oxide layer does not effectively inhibit further diffusion of oxygen. Energy dispersive X-ray (EDS) analysis showed that iron oxide formed at room temperature in humid air, with a small amount of Nd2O3 also formed but at 423 K the oxidation product became mostly Nd2O3 .
Removal of Oxygen from Oxidized Magnet (Refining Method)
Suzuki et al. and Saguchi et al. [9, 35, 37] studied the removal of carbon and oxygen from magnet scrap. In principle, the technique was based on the calcium halide flux deoxidation technique for RE metals. The scrap was first decarburized by oxidation at 1073 K to convert the carbon to carbon dioxide, and then oxygen was removed in a two stage reduction. First, Fe2O3 was reduced by hydrogen gas at 1253 K then the REOs were reduced metallothermically with calcium at 1223 K. Residual Ca, by-product CaO, and CaCl2 were leached with water and removed. The carbon, calcium, and oxygen contents of the reduced metal were less than 0.001, 0.7 and 0.1 wt%, respectively.
The recycling process, although involving several steps, can be considered as a simple process utilizing basic reduction methods and is easy to operate. The decarburization of the sludge has also been found to be possible under vacuum, with most of the oxygen source coming from the sludge itself . Hydrogenation of the decarburized scraps is done to reduce the Ca consumption in the second stage reduction because iron oxide hinders the subsequently Ca reduction—more Ca is required for reduction of Fe2O3, increasing the recycling cost, and the reduction of Fe2O3 by Ca is exothermic . Although the mechanism of hydrogenation was not clearly described, it is known that the Nd–Fe–B phase decomposes into a mixture of Nd hydrides, α-Fe, and Fe–B phases by hydrogenation as described in Eq. 10 [39, 45, 46, 47]. The hydrogen-reduced scrap consisted mainly of NdFeO3 in the interior, α-Fe at the exterior of the granule and NdBO3 as minor product, with oxygen concentration being around 7–8 mass %.
The second stage reduction process was controlled by the addition of CaCl2. The mechanism of the calcium reduction was explained as follows. Ca attaches to the sample surface, and the layer of CaO by-product surrounds the sample during reduction or deoxidation. This CaO layer is removed by dissolution in CaCl2 in order to enhance the subsequent deoxidation by Ca . Removing residual Ca is important due to its significant effect on the magnetic properties of the recycled product. The addition of CaCl2 enhanced the dissolution of the by-product CaO into the aqueous solution during leaching, but too much addition reduced the concentration of RE elements in the sample. Leaching using distilled water at pH higher than 8 is preferred due to the fact that acid can remove some of the RE as well.
Takeda et al.  investigated a process to refine waste magnets using mixed salt fluoride flux which exploited the high solubility of REOs in molten fluoride to separate the REOs from the magnet alloy. The RE in the mixed salt flux substituted the lost RE from REOs removal. The refined alloys can then be used as master alloys for magnets, and the REOs are regenerated as RE metals using molten salt electrolysis. The oxygen concentration in the magnet alloy waste was reduced from 5000 ppm to less than 200 ppm (exceeding 90 % extraction ratio). Importantly, less energy was consumed by regenerating the magnet alloys without oxidation, and there was much less waste generated.
The process is relatively simple. The flux consisting of LiF-50 mol% NdF3 and LiF-25 mol% NdF3-25 mol% DyF3 is used, and the remelting temperature is about 1503 K, just above the peritectic point of 1454 K. The matrix of the magnet alloy decomposes into γ-Fe and liquid Nd–Fe–B phases due to the peritectic reaction during solidification (Fig. 2). Above the peritectic point, the REOs in the alloy are liberated. The solubility of REOs in the fluoride flux at this temperature was estimated to be around 7.4 wt%. REOs (mainly Nd2O3) were extracted in the form of the oxyfluoride Nd4O3F6 from the reaction with NdF3 in the flux. This approach can be considered highly practical for industrial use. It is likely to develop as a favored recycling process with the advantage of having a short recycling path that does not require wastes to be fed back into the smelting process. The dissolution rates and behaviors of elements in the melts under such condition are key aspects that might be considered in further studies of this process. The dissolution rate can be increased for practical application by employing a device to give strong agitation of the melts.
Separation of RE as REOs from Oxidized Magnet Waste
Nakamoto et al.  efficiently removed iron from highly oxidized magnet alloy sludge as a molten Fe–C alloy by adding carbon to decrease the melting temperature of the iron alloy. The separation of RE elements (Nd, Dy, and Pr) and Fe is based on the difference in the nature of oxidation between the REEs and Fe. The region of coexistence of Fe and REO (Nd2O3) extends over a wide range of oxygen partial pressures as depicted in Fig. 9. To create this oxygen partial pressure, carbon was selected as a contact material. The oxygen partial pressures are approximately 10−17 atm under the experimental conditions and were within the coexistence zone between the REOs and metallic Fe (Fig. 9). From Fig. 7, it is evident that carbon can readily reduce iron oxide at 1273 K and higher. The separation temperature of 1823 K was selected because one phase must be liquid for separation. The melting temperature of the iron was chosen as it is lower. The molten magnet alloy separated into liquid metal and oxide phases at this temperature. The liquid metal phase is then separated by magnetic separation. By this route, the concentration of REs (neodymium, dysprosium, and praseodymium) in the metallic phase was able to be reduced to less than 0.01 wt%. The fluidity of the melts is important for effective separation. The temperature and holding time need to be controlled to reduce B concentration in the oxide phase. Increasing the holding time reduces B concentration in the oxide phase but results in the reduction in RE recovery from the metal phase. Changing temperature affects viscosity and fluidity of the melts which may result in reduced separation efficiency. Nakamoto et al.  also found that B2O3 addition contributes to an efficient separation at reduced temperature of 1623 K by reducing the melting temperature of the oxide phase and increasing the viscosity.
Saito et al.  developed a method using a glass slag whereby magnet scrap is melted and brought into contact with a molten flux exploiting the strong affinity of REs with the slag. During undercooling, a reaction between molten alloy and flux takes place to selectively dissolve the REs from the alloys and supercool to a glass. In this method, RE contained in magnet alloys were first oxidized and extracted into B2O3 flux leaving behind α-Fe and Fe2B phases. The alloy was oxidized through slow heating and melting, up to superheated temperature of 1650 K, then cooled to room temperature in an argon atmosphere. During undercooling, the Nd2Fe14B phase reacts with B2O3 to form α-Fe and Fe2B phases together with Nd oxide. The REs are separated and concentrated through the formation of RExOy–B2O3 melt, which contains 50 wt% RExOy and B2O3 melt (immiscibility gap). The concentrations of REs (neodymium, dysprosium, and praseodymium) in the iron alloy were lowered to less than 0.01 mass %, and almost all REs were extracted into the slag phase. The REs could be recovered from the slag by dissolving the slag in sulfuric acid followed by selective precipitation of the REs as a sulfate double salt or hydroxide . The oxides could also be leached with hydrochloric acid and precipitated as an oxalate. Although this method is suitable for the large-scale treatment of magnet wastes containing iron, it generates a lot of inorganic waste. Bian et al.  improved this method by using FeO–B2O3 fluxes to increase the selectivity of neodymium oxidation and separation of RE with Fe. The FeO selectively reacts with neodymium and will be reduced in molten flux due to its larger chemical potential compared to B2O3. At 1673–1823 K, almost all REEs in the magnet scraps were extracted to the oxide phase, and the FeO in the flux was reduced entirely to metal. Only boron was distributed in both oxide and metal phases. However, the purity of the RE oxides extracted fluctuated and was rather low. The main concern was the formation of REBO3 in the oxide phase (and the potential loss of REs) which reduces the purity of the REOs. The extraction ratios of all REEs were more than 99.5 mass %, and the purity of the REOs was greater than 96 mass % when using 2FeO·B2O3 flux. The purity of the RE improved apparently with the increase of reaction time.
Miura et al.  effectively recovered Fe from Nd–Fe–B sintered magnet powder scrap as Fe(CO)5 via the carbonylation reaction using chalcogen catalysts and leaving compounds containing RE as REOs and/or REH2. The carbonylation reaction has previously been applied to extract or purify transition metals. The process is similar to the production of highly pure nickel from the crude nickel by the Mond process. Iron reacts directly with carbon monoxide gas under high-pressure and temperature conditions to form Fe(CO)5. The reaction conditions were 573 K under 30 MPa for 24 h. The rate of carbonylation is considerably controlled by the presence of S. Fe needs to be disassociated from the Nd2Fe14B phase for the reaction to proceed because the carbonylation reaction proceeds via a FeS–CO intermediate complex and the S have low reactivity with Fe in the Nd–Fe–B lattice. Oxidation and/or hydrogenation is performed before carbonylation to precipitate α-Fe because Nd2Fe14B is stable at the reaction temperature. Miura et al.  found that the oxidized powder gave low iron conversion of 58 % compared to 90 % using hydrogenation. This was due to the fact that FeO and Fe2O3 were formed during oxidation and are much more stable than FeS; thus, the FeS–CO intermediate is difficult to form on FeO or Fe2O3 through the carbonylation reaction. Furthermore, the RE products require further processing to remove unwanted impurities such as Fe2B.
Recycling or Recovery via Chlorination
Recovery of RE from REPM by the formation of RE chlorides has been developed to overcome the difficulties of dealing with oxides due to their high melting point and difficulty to separate. Chlorination is a relatively low cost simpler process which produces less effluent requiring treatment. However, its disadvantage is that the RE elements are recovered as chlorides, and these chlorides have limited application after recovery [55, 56]. Both molten salt extraction and gas phase reactions can be used. Figure 12 shows the various chlorination routes. These are based on the selective reactions between REEs and extractants such as molten FeCl2, NH4Cl, Cl2, and AlCl3. Gas phase chlorination and/or carbochlorination have mostly been used to recover metals such as vanadium, tantalum, niobium, molybdenum, nickel, and cobalt from scrap. Extraction of REs as chlorides in the gas phase was first applied to REPM by Murase et al. [29, 30] based on Adachi et al.  findings. The process was modified by Uda et al. [31, 32] (Route Nos. 1 and 2 in Tables 2 and 3), Mochizuki et al. [55, 56], and several others to increase the separation efficiency. The extraction of RE using molten salt (Route Nos. 2, 3, and 4 in Tables 2 and 3) was first introduced by Uda et al. [31, 32], in combination with distillation, and then by Okabe et al.  following their results in recycling using pure magnesium . Itoh et al.  used the same method but using a different salt. Hua et al.  adapted the process by combining several salts to increase selectivity.
NdFeB Chlorination’s Thermodynamics, Kinetics, and Mechanism
Separation of RE from NdFeB by Solid–Gas Chlorination
Extraction of RE from NdFeB Magnet Using Molten Salt
In a second step, the anhydrous RE trichlorides are hydrated and transformed into the corresponding RE oxides by pyrohydrolysis. HCl is released and can be used for the conversion of Fe to FeCl2. The main advantage of this flowsheet is that only water and carbon are consumed. Moreover, the by-products (carbon dioxide, hydrogen gas, and iron alloy) have low environmental loads. The REOs produced can be reduced by electrolysis in molten fluoride.
Based on their findings of Mg affinity with REEs in liquid metal extraction , Shirayama and Okabe  studied recycling neodymium magnet scrap using molten MgCl2. REEs in NdFeB scrap selectively reacted with MgCl2 to form RECl3, leaving solid Fe–B alloy and impurity elements behind. Approximately 80 % of Nd and Dy could be efficiently extracted as chloride following 12 h of reaction. The technique was modified by Hua et al.  by using the binary system MgCl2–KCl, which exhibits a lower melting point and viscosity, as well as lower volatility compared with pure MgCl2.
Itoh et al.  investigated a process based on the same approach but using NH4Cl as the chlorination agent. Nd and Nd2O3 were easily chlorinated to form NdCl3 with high conversion rate. Both α-Fe and Fe2O3 were chlorinated to generate FeCl2, but with conversion rates of around 30 and 90 %, respectively, due to the higher reactivity of Fe2O3 with NH4Cl. The FeCl2 further reacted with Nd metal to form metallic Fe and NdCl3. The resultant RE chlorides could be consequently recovered by leaching the reacted solids with water. Up to 90 % of the REEs were recovered from low oxygen REPM powder scrap using this technique. The advantage of this approach is that the resin compacts prepared from the α-Fe powder by-product possess high coercivity of around 0.04 T with a saturation magnetization value of 140 emu/g and provide good electromagnetic wave absorption ability in the SHF band. This approach could be appropriate for treating large volumes of scrap such that all elements can be reused as a starting source or a functional material.
Liquid Metal Extraction (LME)
Na et al.  investigated the effects of NdFeB magnet scrap size on extraction behavior and found that the amount of Nd extracted increased with increasing holding time and decreasing scrap size at 1073 K for 10–50 min. They found that reducing scarp size for less than 5 mm increased oxidation and reduced extracted Nd. The maximum contents of Nd in Mg were about 24.2 mass % for conditions of the 5 mm sized scrap heated for 50 min.
Sun et al.  proposed a different approach. A magnesium melt was stirred for 15 min along with the slow addition of REPM powders then held for a time to allow the heavy Fe-rich compounds to settle and also provide sufficient time for the diffusion of Nd from the magnet to the magnesium melt. After a specified holding time about 2/3 of the melt (top melt) was poured into a preheated permanent metallic mold. This is called the first recycle. In the second-recycle the bottom melt, which was rich in Fe compounds and inclusions, was further treated with more pure Mg and then poured into the mold as well. The residual melt at the very bottom was slag-like and was poured out to form a bulk Fe waste, They found that 30 min holding time was sufficient for the compounds to settle to the bottom of the crucible due to the higher density (∼7.9 g/cm3) than the Mg (∼1.6 g/cm3) melt. The recovery of Nd varied with the temperature and Nd addition ratio (in the form of NdFeB magnet). The optimal temperature was 1000 K, at which temperature no Nd2O3 formed. The process is promising because the product can be used as raw material for Mg alloy casting, with acceptable impurity levels of Fe and B, instead of using pure Nd from primary production. In another approach, also introduced by Takeda et al. , silver (melting point 1235 K) was substituted for Mg for the direct extraction of neodymium from NdFeB. Silver dissolves neodymium, but not iron or boron. The neodymium can be separated from the Ag–Nd alloy by oxidation to Nd2O3, which is insoluble in molten silver. The process is an interesting option for an industrial approach because of the recyclability of the silver. However, the end product is a rare-earth oxide, as opposed to a rare-earth metal for the magnesium solvent method .
Recycling via Hydrogenation
This process has the advantage of reducing the REPM lifecycle but is not applicable to RE recovery. The drawbacks are the reduction in magnetic properties of the recycled product and its inability to treat highly oxidized waste. Separation efficiencies of around 95 % have been reported on small scale trials. Further physical processing techniques can then be applied which reduce the Ni content to <325 ppm. After hydrogen processing, the scrap is tumbled in a porous drum to liberate the hydrided NdFeB powder and Ni flakes. The success of this technology depends on sufficient access for hydrogen and an exit route for the hydrided NdFeB powder.
Electrolysis Using Molten Salt and Ionic Liquids
Due to the increasing demand for REEs and their increasing application in green technology, recycling and recovering these elements from waste is a critical issue. High-temperature processing is one alternative that can be used to avoid large consumption of water and production of hazardous waste in the recovery process. A number of techniques are available for recovering REs from magnet waste by high-temperature processing, but all are still at the research stage, and currently none has been applied in a commercial scale in industry. More options in terms of recycling product are available compared to hydrometallurgical approach as both alloying metal and/or RE metal can be produced depending on which technique is used. Most of the available techniques are multistage in which at least two or three steps are required to recover the REEs from the waste. Although the future is promising for high-temperature recycling, there are evident barriers or challenges that need to be overcome including (i) the different mix of wastes produced; (ii) the effects of contaminants on the recycling process; (iii) optimization for mutual separation of REs (Nd, Dy, Pr); and (iv) feasibility (economics and lifecycle). It is also understandable that combinations of methods may be required to completely recover REs from magnetic waste. Thus, further fundamental study on the thermodynamics and kinetics behaviors of the magnets (including the behavior of each individual REE for mutual separation) during high-temperature processes is required to optimize the available techniques and to analyze the best option. It should be noted that it is also important to consider ways to integrate these techniques into primary processing and/or wider e-waste processing.
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