Thermal Isolation of Rare Earth Oxides from NdâFeâB Magnets Using

Research Article pubs.acs.org/journal/ascecg

Thermal Isolation of Rare Earth Oxides from Nd−Fe−B Magnets Using Carbon from Waste Tyres Samane Maroufi,* Rasoul Khayyam Nekouei, and Veena Sahajwalla Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

ABSTRACT: In this study, a novel oxidation−reduction process for the recovery of rare earth elements (REEs; i.e., Nd, Pr, and Dy) from Nd−Fe−B permanent magnets is verified. Nd−Fe−B permanent magnets collected from e-waste were subjected to an oxidation process at 1000 °C for 60 min followed by carbothermal reduction at 1450 °C for 90 min using waste tire rubberderived carbon (WTR-DC) as a reducing agent. Fe-based metal and rare earth oxide (REO) phases were successfully separated from the original magnets. The distribution of elements (i.e., Nd, Dy, Pr, Fe, B, Al, and C) between the Fe-based metal and oxide phases was investigated via Inductively Coupled Plasma (ICP) and Energy-Dispersive X-ray Spectroscopy EDS/Electron Probe Microanalysis (EPMA) elemental mapping. REEs were confirmed as the main components of the oxide phase, and it was shown that the REEs did not remain in the Fe-based metal phase. Given the growing global demand for REEs, critical supply issues, high costs, and extremely low recycling rates worldwide, new recycling options are urgently needed. This new approach to extracting REEs from Nd−Fe−B magnets, using a problematic waste (WTR) as a reductant, promises to simultaneously deliver environmental benefits. KEYWORDS: Rare earth oxides, Nd-Fe-B magnets, Recycling, Waste tire rubber, Thermal isolation

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INTRODUCTION Due to their strong magnetic flux, Nd−Fe−B permanent magnets are now the most widely used type of rare-earth magnets worldwide. They are particularly useful in enhancing the efficiency of motors and drives and are employed in many clean energy, transport, and high tech applications such as computer and laptop hard drives, hybrid and electrical vehicles, and electric generators in wind and water turbines.1,2 Given the growing demands for lightweight products with high magnetic strength to support the miniaturization of equipment in many existing and emerging applications, demand for Nd−Fe−B magnets is, likewise, expected to continue to rise, particular for the clean energy/transport sectors.3 Industry forecasts estimate global sales will increase to some U.S. $12.7 billion by 2019.4 The global supply of rare earth elements (REEs), however, is under considerable strain. Both the European Union (EU Critical Materials list 2010, 2014) and the U.S. Department of Energy (energy critical list, 2010) classify REEs as being at the greatest risk of supply shortages of all the materials needed for clean energy technologies.5 More than 90% of all REEs are © 2017 American Chemical Society

currently extracted and processed in China. Given the increasing domestic demand from its own industries, China has gradually tightened export quotas, resulting in reduced supplies of REEs on the world market and corresponding price increases. In addition, up to 30% of raw materials, including REEs, are lost during magnet manufacturing process itself (i.e., cutting and grinding) and end up as useless sludge and scraps.6 These serious supply and cost challenges, as well as the inefficiencies in REEs ore miningthe extraction of neodymium, for example, results in an excess of the more abundant elements, lanthanum and ceriummean other options for securing supplies of REEs are urgently needed. As many countries have no suitable ore deposits within their territories, and deposits elsewhere are both limited and finite, the recovery of REEs through recycling is essential.7 Currently, less than 1% of REEs are recovered via recycling, while growing stockpiles of Received: April 12, 2017 Revised: May 5, 2017 Published: May 31, 2017 6201

DOI: 10.1021/acssuschemeng.7b01133 ACS Sustainable Chem. Eng. 2017, 5, 6201−6208

Research Article

ACS Sustainable Chemistry & Engineering

reduction of the oxides.22 Other work by the same group23 achieved the recovery of REEs in three steps: First, vacuum induction melting, in which Nd−Fe−B magnets were melted in a vacuum in a graphite crucible, produced rare earth carbides. Second, the hydrolysis of the rare earth carbides was used to form rare earth hydroxides (REH) and iron-based metal. Finally, magnetic separation was used to remove iron residues from the REHs. Bian et al. also utilized Fe2O3, B2O3, and FeOB2O3 flux and were able to separate REEs from Fe-based metals at temperatures of 1400, 1500, and 1600 °C. The purity of REOs produced by 2FeO-B2O3 was 98.4%, and extraction ratios were higher than 99.5% after the FeO-B2O3 fluxes treatment.24 The use of waste materials to achieve the costeffective extraction of REEs from Nd−Fe−B permanent magnets may be an attractive alternative which no study has, to the best of our knowledge, investigated or achieved to date. Of the tens of millions of tonnes of waste tires dumped every year, about 70% is currently landfilled. Waste tires are a ubiquitous, problematic, and complex waste stream that, like waste magnets, is expected to continue to grow. The global car fleet is projected to increase to some 1.7 billion cars by 2030, from about 1 billion today, generating billions of waste tires every year.25 Due to their composition and high calorific valuea typical tire has a carbon content of ∼84.4 wt %26 and a hydrogen content of ∼7.2%waste tires have long been of interest for resource and/or energy recovery. Yet, waste tires are also a complex mix of materials and potential contaminants, and there are currently few ideal processes for their costeffective recycling. Although many nations are seeking to boost waste tire recycling, efforts are falling short. In the U.S., for example, some 40% of the approximately 300 million tires discarded last year were recycled,27 a significant increase over previous years, but still a shortfall of some 180 million waste tires over only 12 months. New options for diverting waste tires away from landfills are also urgently needed. This study addresses the multiple challenges of achieving the cost-effective, environmentally friendly recovery of REEs from Nd−Fe−B permanent magnets via a two-step oxidation− reduction approach, using waste tire rubber-derived carbon (WTR-DC) as a reducing agent. In this work, REEs (i.e., Nd, Pr, and Dy) were extracted from Nd−Fe−B permanent magnets collected from electronic waste (e-waste). Both graphite and WTR-DC were used as reducing agents in the reduction process, and their respective separation efficiency was compared. The process of reduction using WTR-DC proved cleaner than with pure graphite but was similarly efficient in the extraction of REEs. Consequently, the novel process reported here opens up a sustainable, new, cost-effective approach for the thermal isolation of REOs from Nd−Fe−B permanent magnets while, at the same time, potentially reducing the volumes of globally significant waste discarded into the environment.

Nd−Fe−B magnets, mostly from obsolete hard disk drives (HDD), remain untapped. As today’s electric vehicles and wind turbines reach their end of life, on average in 10 and 25 years, respectively, from production, the volume of Nd−Fe−B magnets within the global waste stream is expected to increase significantly. While considerable research effort has been invested in mainly lab scale processes for the recovery of REEs, commercial operations are rare and face technical challenges. For the recovery of REEs from Nd−Fe−B magnets, several techniques have been suggested. Conventionally, hydrometallurgical processes8−10 are used, but these consume a large volume of acids (HCl, H2SiO4) and generate significant amounts of contaminated wastewater, significant barriers to their practical application. In 1993, Lyman and Palmer11 used magnetic and leaching procedures to separate the metallic Fe and rare-earth oxides (REOs) within waste magnets. They reported that the best separation of rare earths from bulk Nd−Fe−B magnet scrap was obtained using magnet dissolution in H2SiO4 followed by the precipitation of recyclable rare-earth salts. Saito et al.12 carried out the hydrometallurgical treatment of magnetic sludge using leaching and precipitation techniques. They used HCL and HNO3 as leaching reagents and extracted the neodymium as either sodium neodymium sulfate hydrate or neodymium hydroxide. Using FeCl2 as a chlorination agent to selectively chlorinate rare earths and activated carbon to retain Fe in its metallic state, Uda13 was able to separate rare earth chlorides from Fe alloys. He also confirmed that the rare earth chlorides could be converted to the corresponding oxides via a pyrohydrolysis reaction using HCL gas formation. Murase et al.14 used chlorine and aluminum chloride as chlorinating and transporting agents, respectively, and recovered REEs through a chemical vapor transport approach based on the differences in thermal stability of the vapor compounds. Okabe et al.15 developed a Nd extraction apparatus which was able to circulate magnesium (Mg) as an extraction medium and maintain a temperature difference within the reaction vessel. They carried out continuous extraction of metal Nd from scrap and further re-extraction of Mg from Mg−Nd alloys, which resulted in the recovery of pure Nd metal with 97.7% purity. Mg and Ag were also used as extraction mediums by Takeda et al.,16,17 which resulted in the recovery of Nd−Mg alloys and Nd−Ag alloys, respectively. From the resulting Nd−Mg alloys, Mg was evaporated. However, the Nd−Ag alloy was oxidized in the air, and Nd2O3 was recovered. In another study, Itoh et al.18 used NH4Cl as a chlorination agent to selectively chlorinate REEs from the Nd−Fe−B magnets. Xu et al.19 have proposed a new dry recycling process for Nd−Fe−B magnets which involves using molten magnesium metal to recover Nd. In recent years, the different oxygen affinities of the REEs and transition metals have received attention from numerous researchers,20−24 as this property can be used as a driving force in the separation of REEs in the form of oxides from transition metals at high temperatures. This method is considered a more environmentally friendly process than other options because it avoids the use of acids and the generation of large volumes of waste solutions. The separation of REEs and Fe-based metals based on the difference in their oxygen affinities via high temperature pyro-metallurgical processing was first investigated by Nakamoto et al.1 Later, Bian et al. carried out hightemperature pyrometallurgical processing for the extraction of REEs from Nd−Fe−B magnets via two steps: first, the oxidization of the magnet particles and, then, the selective

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MATERIALS AND EXPERIMENTAL PROCEDURE

Nd−Fe−B magnets were sourced by dismantling randomly selected obsolete hard drives from laptops collected from the Reverse E-waste Company, Sydney, Australia. The intact magnets were heated up to 300 °C under argon purge (1 L min−1) for demagnetization and the removal of organic constituents prior to grinding, using a ring mill. After pulverizing, the resulting fine powder was analyzed using the Inductively Coupled Plasma (ICP) technique. The crushed magnet powder was heated in a muffle furnace in the air at 1000 °C for 1 h to oxidize all elements including the REEs. Waste tire rubber (WTR) shreds, supplied by Onesteel Australia, were 6202


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Figure 1. (a) XRD, (b) Raman, (c) FE-SEM images of WTR-DC powder produced after 10 min of pyrolysis in argon.

Table 1. Composition of Nd−Fe−B Magnet Powder elements wt. %

B 0.99

Dy 2.06

Fe 64.1

Nd 21.0

Pr 3.81

Co 1.41

Ni 3.15

Al 0.28

Figure 2. XRD spectra of NdFeB magnet (a) as received and (b) after oxidation at 1000 °C for 1 h. separately thermally processed in a high-temperature tubular furnace at 1550 °C for 10 min under argon purge (1 L min−1). The resulting black residue was subsequently ground using a mortar grinder. Waste rubber derived carbon (WTR-DC) was used as a reducing agent in the stoichiometric ratio for the reduction of the iron, cobalt, and nickel oxides in the magnet powder. A layer of WTR-DC powder was first placed on the bottom of a graphite crucible, and then a layer of oxide powder was put on top of the WTR-DC bed. The resulting mixture was heated up to 1450 °C in a hot tubular furnace (100 cm length × 5 cm diameter) under argon purge (1 L min−1) for 90 min. After heat treatment, the sample was kept in the cold zone in an argon atmosphere for 10 min. The composition and element distribution of the resulting phases were further examined by X’pert PRO multipurpose XRD (MPD system), ICP, Scanning Electron Microscopy (SEM)−Energy-Dispersive X-ray Spectroscopy (EDS) and Electron Probe Microanalysis (EPMA).

The morphology of the WTR-DC powder was characterized by FE-SEM analysis. Interestingly, the FE-SEM image of the WTR-DC, Figure 1c, showed spherical carbon nanoparticles (CNPs) with a diameter of approximately 30 to 40 nm. Oxidation of Nd−Fe−B Magnet. The chemical composition of the crushed Nd−Fe−B permanent magnets is shown in Table 1. This indicates the Nd−Fe−B permanent magnet contains not only Nd, Fe, and B but also other useful elements such as Dy, Pr, Co, Ni, and Al. The partial substitution of Dy and Pr for Nd and also the addition of Al, Co, and Ni in this type of magnet are to control the Curie temperature, anisotropy field, and intrinsic coercivity force of the alloy.28−31 The Nd−Fe−B magnet fine powder before and after oxidation in the air at 1000 °C was subjected to XRD analysis. The sample before oxidation consists of three basic phases: the major Nd2Fe14B matrix grain phase, a lesser Nd-rich grainboundary region, and a minor Nd1.1Fe4B4 phase.32 The peaks of the Nd2Fe14B phase can be seen in the XRD pattern of the asreceived sample shown in Figure 2. However, the peaks corresponding to the Nd-rich and Nd1.1Fe4B4 phases are difficult to identify since they overlap those of the Nd2Fe14B phase and constitute only a small fraction of the microstructure.33 The XRD pattern of the Nd−Fe−B magnet fine powder after oxidation in the air at 1000 °C, in Figure 2, shows the strong diffraction peaks that belong to oxide phases. In fact, after oxidation at 1000 °C for 1 h, the XRD pattern shows diffraction peaks corresponding to Fe2O3, FeNdO3, and Nd2O3. The presence of these compounds indicates that the Fe and Nd in the magnet sample were converted to their oxide forms.

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RESULTS AND DISCUSSIONS Characterization of WTR-DC. The WTR-DC was characterized using XRD, Raman, and FE-SEM. In the X-ray diffraction pattern of the WTR-DC powder (Figure 1a), a sharp peak is observed at around 25° corresponding to the diffraction from the (002) plane of graphitic carbon. Another small peak can be observed at around 43° which is assigned to the (001) graphite plane. The Raman spectra of the WTR-DC powder, Figure 1b, shows typical Raman peaks of graphitic carbon, the G band at 1585 cm−1 and the D band at 1360 cm−1. The higher intensity of the G band relative to the D band indicates that the obtained graphitic structure has a high degree of graphitization with a reasonable level of structural defects and disorder. 6203


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ACS Sustainable Chemistry & Engineering The formation of Nd2O3, Fe3O4, and Fe2O3 requires the initial dissociation of the Nd2Fe14B intermetallic phase and then the formation of oxides from their elements. It has previously been established8 that the Nd2Fe14B phase is first decomposed into Nd2O3 and Fe, followed by oxidation of Fe and the formation of Fe2O3. Assuming that Nd2O3, FeNdO3, B2O3, and Fe2O3 are their final oxide forms in the sample powder mixture, during high temperature oxidation, several reactions take place. From the beginning of the treatment, the majority of Nd-rich regions (NdOx) transform into Nd2O3 according to the following reaction: Nd‐rich → NdOx → Nd 2O3

(1)

Then, the Nd2Fe14B phase dissociates into Fe and Nd2O3: 9 1 Nd 2Fe14B + O2 → 14Fe + Nd 2O3 + B2O3 4 2

Figure 3. (a) Generated gas concentration in terms of CO and CO2 as a result of carbothermal reduction of oxides of Nd−Fe−B. (b) Filtration paper for graphite. (c) Filtration paper for WTR-DC.

(2)

After dissociation, the Fe phase is itself oxidized to form primarily Fe2O3: 2Fe +

3 O2 → Fe2O3 2

then FeO (reaction 6). As can be seen from Figure 3, in the first 100 s of reaction time, the concentration of CO and CO2 gases surged, indicating that the Boudouard reaction 7 started to occur. When CO is generated from the Boudouard reaction, the gaseous reduction of Fe2O3 in reaction 6 increases, producing more CO2 gas. When graphite was used as a reductant, after 200 s of reaction time, the concentration of CO2 in the off-gases sharply soared, which can be attributed to reduction of wustite (FeO) by CO (reaction 8) and further generation of CO2. The same trend was observed using WTR-DC as a reductant, except for a 100 s delay in the sharp increase in the concentration of CO2 in the off-gases and the completion of the reduction. This indicates that the rate of reduction by graphite was slightly faster. For both reductants, the reduction of iron oxide was complete in less than 10 min, implying that the rate of reduction was relatively fast for both graphite and WTR-DC.

(3)

FeNdO3 oxide forms due to a reaction between Fe2O3 and the nanocrystallite of neodymium oxides resulting from the dissociation of Nd2Fe14B: Fe2O3 + Nd 2O3 → 2FeNdO3

(4)

32

It has also been established that Nd1.1 Fe4B4 remains unaffected by oxidation at lower temperatures; however, at high temperatures (>611 °C) it starts transforming into oxide forms. Given that the content of B in the original sample is limited, its corresponding oxide phase is below the detection limit of XRD and thus is not detectable from the XRD spectrum of the oxidized form of the sample. It is also worth noting that the high temperature oxidation of the magnet powder is governed by the dissociation of the Nd2Fe14B.32−34 Carbothermal Reduction of Nd−Fe−B Magnet Oxide Powder. For the final recovery of REEs, the carbothermal reduction of the resulting oxide powder at temperatures in the range of 1350 to 1450 °C was carried out. The separation of the REEs (Nd, Dy, and Pr) from Fe via this technique is based on the difference in the nature of oxidation processes between REEs and Fe. According to the Ellingham diagram, carbonaceous materials can reduce iron oxide at temperatures higher than 700 °C. However, oxides of Nd, Pr, and Dy remain unaffected at this temperature; it is not feasible to reduce REOs into metallic elements at such temperatures. Additionally, carbon can be used to efficiently separate the Fe from the REEs. Owing to the diffusion of carbon into metallic iron, the melting point of iron decreases (to its eutectic point of 1153 °C), leading to an improvement in the fluidity of iron which is required for the separation process. WTR-DC and graphite were selected as carbonaceous materials for reactions with the resulting oxidized Nd−Fe−B magnet powder. The concentrations of gases liberated from the carbothermal reduction of the oxidized Nd−Fe−B by graphite and WTR-DC at 1450 °C were monitored using an IR-gas analyzer. Figure 3 shows the volume percentage of CO and CO2 from off-gas measurements over time. The initial contact between solid carbon and Fe2O3 results in the formation of Fe3O4 and CO gas (reaction 5). The produced CO gas in turn reacts with Fe2O3 and reduces it to Fe3O4 and

3Fe2O3 (s) + C (s) ↔ 2Fe3O4 (s) + CO (g)

(5)

Fe2O3 (s) + CO (g) ↔ 2FeO (s) + CO2 (g)

(6)

C (s) + CO2 (g) ↔ 2CO (g)

(7)

FeO (s) + CO (g) ↔ Fe (s) + CO2 (g)

(8)

Figure 3b represents the two filter papers which were placed perpendicular to the off-gas flow coming from the IR-gas analyzer in order to separate dust and pollutants. It can be seen that the filter paper utilized for the case of graphite was completely black, indicating that many fine particles escaped and were caught in the bulk of the filter paper. By contrast, the surface of the filter paper used for carbothermal reduction of oxides of Nd−Fe−B by WTR-DC was almost clean. Such differences in the behavior of the reducing agents can be explained by their density. As graphite has a relatively low density compared to WTR-DC, carbon particles escaped more easily from the crucible and were, therefore, entrapped by the filter paper. In addition, compared to WTR-DC, graphite is highly crystallized with a compact crystalline structure of carbon atoms, which tends to be more stable than chain-like polymers. Such a difference in their structures leads to a faster gasification of WTR-DC in comparison with graphite. Faster gasification (reaction 7) means rapid conversion of carbon to CO gas with less residual carbon remaining behind, and therefore less chance for carbon particles to escape from the crucible. 6204


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Figure 4. (a) Photographs of the sample after reduction at 1450 °C. (b) SEM image of iron based metal and (c) SEM image of REO phases after separation.

3.4. Characterization of Fe-based Metal and Oxide Phases. Although the reduction of iron oxide was complete in 10 min, the sample was kept inside the furnace for 90 min to ensure the holding time was sufficient to achieve the full separation of metal and oxide phases. Figure 4a shows the photographs of Fe-based metal and oxide phases which were separated with a clear interface as a result of carbothermal reduction of Nd−Fe−B using WTR-DC at 1450 °C for 90 min. Both the Fe-based metal and oxide phases after separation were further analyzed using different techniques. Figure 4b and c show SEM images of the cross sections of Fe-based metal and oxide phases, respectively. In the SEM image of the Fe-based metal phase, spherical zones of carbon with homogeneous distribution can be observed, implying that the Fe-based metal phase is saturated with carbon. To investigate the distribution of the elements (i.e., Nd, Pr, Dy, Fe, Ni, Co, Al, B, and C) between the Fe-based metal and oxide phases, the sample was further examined by EDS distribution element mapping and line scanning, as displayed in Figure 5. The concentration of REEs (i.e., Nd, Dy, Pr) shows an apparent decrease from the oxide phase to the metal phase; however, the concentration of Fe displays a clear increase. The REEs such as Nd, Pr, and Dy were nearly undetectable in the Fe-based metal phase. In contrast Nd, Dy, and Pr were present as the main components in the oxide phase with a small amount of Al and B. This is also confirmed by the distribution mappings of the elements shown in Figure 5. It can be concluded that REEs are concentrated in the oxide phase. The cross section of the oxide phase illustrated in Figure 4c indicates three different phases in the oxide phase (i.e., white, bright and dark gray). To investigate the elements’ distribution between these phases, an enlarged part of the sample was further measured by EPMA mapping. The EPMA elemental mapping of the oxide phase, Figure 6, shows that the bright gray phase in Figure 4c contains some percentage of Al compared to the white zone, and the dark gray area has boron. However, the most dominant is the bright gray area that is rich in Nd, Dy, and Pr, meaning that these elements are almost evenly distributed throughout the bright gray area. While oxygen is dispersed throughout all zones, there is more concentration of oxygen in the dark gray area. According to the results of EPMA elemental mapping, the average weight percentages of Nd, Pr, Dy, and O in selected areas of the oxide phase were 45, 10, 4, and 15 wt %, respectively. The contents of Al and B as minor components were 0.5 and 2.4 wt %, respectively.

Table 2 shows the analyzed compositions of the Fe-based metal and oxide phases which were separated out from powdered Nd−Fe−B permanent magnets via carbothermal reduction using graphite and WTR-DC as reducing agents at 1450 °C for a holding time of 90 min. Most of the iron in the Nd−Fe−B oxide phase was reduced to metallic iron and separated from the oxide phase. While the oxide phase mainly consists of Nd, Pr, and Dy, minor impurities of B and Al were found in the oxide phase. The impurity of B can be eliminated by increasing the holding time. The WTR-DC sample had an efficiency similar to that of pure graphite in terms of the separation of the Fe-based metal and REO phases. As mentioned earlier, the process of carbothermal reduction of Nd−Fe−B oxides by WTR-DC is a relatively clean route when compared to the graphite. Given the growing global burden of WTR, and the clean process involved in the separation of REOs from Fe-based metal, the use of WTR-DC could simultaneously demonstrate a novel, low cost means of extracting REOs from Nd−Fe−B permanent magnets and the benefits of transforming waste to value. In addition, this proposed sustainable solution for the recovery of REOs is a two-step process of oxidation and reduction that would enable efficient REO recycling with reduced reaction times and at lower temperatures than currently available options. To develop a better understanding of the mechanical and physical properties of both the iron-based metal and REO phases separated out from Nd−Fe−B magnet powder, the following tests were carried out. Microhardness testing was performed using the Vickers hardness technique applying 30 s of load. The results of hardness tests for Fe-based metal and REOs in Vickers Pyramid Number (HV01) are listed in Table 3. With increases in the reaction time from 1 to 5 h, hardness increased in “as-cast” iron separated from Nd−Fe−B magnets. Various levels of hardness shown in Table 3 are an indication of the different types of cast iron.33 Depending on their hardness, the different classes of “as-cast” iron can be used for a wide range of applications such as automotive, agricultural, piping, and any application in which good machinability is required.8 As can be also seen from Table 3, the oxide phase of the REEs separated from Nd−Fe−B magnet possesses a high level of hardness (580−620 HV), making the REEs promising candidates for abrasion and polishing applications.35,36 It is worth mentioning that the use of REEs in polishing powders represents about 13.3% of global demand,37 which implies that 6205


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Figure 5. BSE image and EDS element mappings and line scan of the Fe-based metal and oxides phases separated after heat treatment at 1450 °C using WTR-DC for 90 min.

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the REO phase separated in this work can be a suitable solution for this sector of the market. The REO phase which was separated from Nd−Fe−B permanent magnets can also be modified in terms of its morphology to coat SiC nanowires/fibers synthesized in previous work38,39 and make it a promising material for many photocatalysis and heterocatalysis applications. The resulting REO can be also further purified via selective dissolution using acids to remove minor impurities of Al2O3 and B2O3 and make that suitable for other appropriate applications.

CONCLUSIONS

REEs were extracted from Nd−Fe−B permanent magnets via a novel, sustainable process of oxidation−reduction in which WTR-DC was utilized as a reducing agent. Nd−Fe−B magnets were first oxidized in the air at 1000 °C for 60 min and then in reaction with WTR-DC at 1450 °C for 90 min. Oxides of iron, cobalt, and nickel were reduced and separated from the oxide phase, which led to the formation of Fe-based metal and oxide phases with a clear interface. The resulting Fe-based metal and oxide phases were further examined for their elemental distribution. The oxide phase mainly contained REEs (i.e., 6206


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Figure 6. EPMA elemental mapping of the REO phase.

Table 2. Composition of Iron-based Metal and Oxide Phase metal (G) 1450 °C, 90 min oxide (G) 1450 °C, 90 min metal (WR-DC) 1450 °C, 90 min oxide (WR-DC) 1450 °C, 90 min

Al

B

Co

Fe

Ni

Nd

Pr

Dy

0.00 1.38 0.00 1.35

0.03 2.37 0.03 2.41

1.33 0.01 1.07 0.01

76.1 1.16 76.4 0.60

3.69 0.01 2.98 0.01

0.01 66.4 0.03 64.2

0.04 12.2 0.02 11.9

0.01 1.88 0.00 1.97

and B2O3 which can be removed using a selective solution in acid. The proposed sustainable process could simultaneously demonstrate a new low cost means of recovering REEs from Nd−Fe−B permanent magnets and the benefits of transforming waste to value.

Table 3. Vickers Hardness Test for the Iron-based Metal and REOs condition

90 min−1450 °C

300 min−1450 °C

hardness

176 ± 7.4

194 ± 8.6

REOs (in all cases) 580−620

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Nd, Dy, and Pr) and a minor amount of B and Al. The Febased metal was saturated with carbon, and REEs were not present in the metal phase. As a comparison, pure graphite was also used, and it was shown that the process of reduction using WTR-DC resulted in lower generation of solid carbon residual compared to graphite. WTR-DC had a similar efficiency to the pure graphite in extracting REEs. The recovered REOs exhibited a high level of hardness (580−620 HV), making them promising candidates for abrasion and polishing applications. By modifying the morphology of the REO phase separated from Nd−Fe−B permanent magnets, it can be used as a photocatalyst for polymer photodegradation and water treatment. The REO phase contains minor impurities of Al2O3

AUTHOR INFORMATION

Corresponding Author

*Tel.: +61(2)9385 4471. E-mail: s.maroufi@unsw.edu.au. ORCID

Samane Maroufi: 0000-0001-5553-8519 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support for this research was provided by the Australian Research Council through Laureate Fellowship FL140100215. 6207


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