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NdFeB magnet scrap is an alternative source of neodymium that could have a significantly lower impact on the environment than current mining and extraction processes. Neodymium can be readily oxidized in the presence of oxygen, which makes it easy to recover neodymium in oxide form. Thermochemical data and phase diagrams for neodymium oxide contain...
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... However, Fe 2 O 3 and Nd 2 O 3 react to produce NdFeO 3 (Reaction (6)). This creation of mixed oxides poses issues as these compounds are insoluble under mild conditions, leading to reduced rates of REEs leaching (Jakobsson et al., 2017). ...
Increasing concerns over climate change have led to global decarbonisation efforts, in the form of new legislation , to phase out the sale of internal combustion engine (ICE) vehicles. As a result, the transition to electrified powertrain vehicles as the mode of transportation has never been greater. Electric motors (EMs), serving as the pivotal component of e-mobility, have gained much attention by policy makers and economic experts concerning the supply chain of raw materials needed for manufacturing. Permanent magnets (PMs), including rare earth elements (REEs), account for 40 to 60 % of the total EM cost. Given the importance of these materials to the e-mobility efforts, there has been a great impetus by leading economies to mitigate supply chain instability and mining operation constraints, by identifying and establishing a sustainable supply source through circular economy strategies. Although extensive studies on REEs recovery, via various techniques, have been undertaken by the research community, there remain underlying concerns over feed source, quality and technical challenges surrounding the retrieval of PM via a functional disassembly approach, all of which are yet to be elucidated. The present study serves to highlight state-of-the-art recycling of EMs from EoL electrified vehicles using a circular economy approach.
... 34 The formation of such mixed oxides is problematic since these phases are not soluble under mild conditions and can cause a decrease of REEs leaching rates. 35,36 2NdO þ1/2O 2 (g) / Nd 2 O 3 ...
This study proposes an advanced leaching method using organic acids to recover rare earth elements (REEs) from NdFeB permanent magnets from end-of-life computers hard disk drives (HDDs). The end-of-life HDDs were first dismantled in order to recover NdFeB magnets, which were then thermally demagnetized at 350 °C during 30 min before crushing in a ball mill under inert atmosphere. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) analyses performed on the NdFeB magnets show the heterogeneous structure containing the major matric phase Nd2Fe14B and the rich-REEs phase containing Nd and Pr oxides. Additionally, X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) analyses on the ground NdFeB magnet show that grinding NdFeB magnets under inert atmosphere helps to minimize its oxidation. Chemical analysis shows that the composition of the ground sample is Nd: 22.8 wt%, Pr: 3.3 wt%, Dy: 1.2 wt%, Fe: 62.6 wt%, Co: 1.5 wt%, B: 0.9 wt%, Ni: 0.6 wt%. Diagrams of speciation and equilibrium phases (Eh vs. pH) were calculated to determine the predominance of the formed species in the REEs–organic acids systems. The influence of the organic acid type (acetic acid, formic acid, citric acid and tartaric acid), the acid concentration (10 vol%, up to saturation), and the solid/liquid (S/L) ratio (0.5%–10%) on NdFeB magnets leaching was investigated employing an optimal experimental design conceived by the statistical software JMP. Acetic acid (CH₃COOH) shows the highest leaching performance of REEs, allowing leaching yields over 90% for Nd, Dy and Pr in the acid concentration (mol/L) range of 1.6 – 10 and the S/L ratio (%) range of 0.5 – 5 using a temperature of 60 °C. The results presented in this investigation suggest that REEs can be recovered from magnets of end-of-life HDDs using an eco-friendly method assisted by organic acids.
... After milling the powder was roasted in a muffle furnace for 6-48 h at 850 • C in a ceramic crucible. Roasting temperature was selected to attain reasonably high oxidation rate and to avoid slag formation reported by Jakobsson et al. [28]. According to thermogravimetric analysis (not shown here), 48 h at 850 • C was sufficient for complete oxidation and at that point the weight increase was 27%. ...
... Fig. 2 shows the XRD profiles of milled and milled/roasted samples and peaks of NdFeO 3 are clearly present, while those of Nd 2 O 3 can be detected only tentatively. According to the calculations of Jakobsson et al. [28], Fe 2 O 3 and NdFeO 3 are the predominant phases obtained at the conditions of this study (T = 850 • C, p O2 = 0.21 bar). Formation of NdFeO 3 during roasting of NdFeB magnets at 800 • C was discussed also by Xin et al. [29]. ...
Acid leaching of Nd and Fe from roasted NdFeB magnet powders was studied in stirred-tank and packed-bed reactors. The experimental data at sulfuric acid concentrations of 0.02-1.0 mol/L and at 80 oC were correlated using a diffusion-reaction model assuming homogeneous distribution of the constituents in the magnet. The results show that the experimental data can be correlated well when formation of NdFeO3 during roasting is taken into account. Decomposition of NdFeO3 to Nd³⁺(aq) and Fe³⁺(aq) is the main route in neodymium dissolution from roasted NdFeB. The model parameters derived from stirred-tank data predict reasonable well leaching in packed-bed reactor. In both systems, re-precipitaion of iron due to acid depletion allows selective dissolution of Nd and other rare earth elements.
... The following compound and solution databases were used: FactPS (pure substance), FToxid (oxide compound), and SGTEa (intermetallic compound). Additional Nd-and Fe-oxide' thermodynamic data from [34][35][36] were inserted manually as a private database in the FactSage to assist with the thermodynamic calculations. ...
The oxidation behaviour of rare earth permanent magnets contributes significantly to the understanding and efficiency of their high temperature recycling processes. The current paper evaluated the high temperature oxidation kinetics of end-of-life Ni/Cu/Ni coated NdFeB rare earth permanent magnets at 973-1473 K under ambient air conditions. The general microstructure observations and kinetics measurements showed that the oxidation was controlled by a diffusion mechanism that followed the Ginstling–Brounshtein model. The overall activation energy was calculated to be 168 kJ/mol. Results suggest that the presence of a Ni/Cu/Ni coating has a negative effect, reducing the overall kinetic rates by ten times.
... The inclusions, on the other hand, are actually the remnants of the formation of these complex oxides, which were initially formed step-wise from simpler stable oxides like, for example, the Nd-B-O system is the most stable with -10,395 eV/unit cell [34]. With an increasing time of heat treatment (10 h at 850 °C and 1 h at 500 °C) the concentration of oxygen decreases, moving a complex heterogeneous equilibrium of multicomponent oxides towards the oxygen-lean side of the FeO-Fe 2 O 3 -Nd 2 O 3 -B 2 O 3 system, leading to the disappearance of the FeB 4 O 7 and Nd 4 B 2 O 9 oxides[34]. In summary, the oxygen-rich phaseequilibrium system, when combined with Nd, Fe and B, predicts certain end-member equilibrium phases, which suggests that the proposed exchange mechanism favours the formation of a cascade of oxides. ...
We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.
... This is in contrast to most iron alloys, which have detailed solution models developed specifically by and for the steel industry. Although building a CALPHAD model for the rare-earth magnet system is ongoing, [15][16][17][18][19][20][21] so far published models are limited to specific cases, such as Fe-Pr-B or Fe-Nd-B, with limited integration of multiple rare-earth elements alongside iron and boron. The case is further complicated by a lack of a solution model able to accommodate both additives (e.g. ...
... The "other metallic" phase was modeled using the FactPS database in FactSage, 23) with the exception of R-B compounds, R-Fe compounds, and non 2-14 Fe-R-B compounds. The oxide phase was modeled using FactPS and Fe-R-B-O compounds optimized by Jakobsson et al. 21) The 2-14 phase has been reported to initially oxidize less than other magnet components. 24,25) This limited oxidation did not appear to be accounted for in published thermodynamic models, [16][17][18] and so herein is assumed to be of a kinetic origin. ...
Recycling rare-earth magnets poses a metallurgical challenge due to their high reactivity and the difficulty in separating individual rare-earth elements. These challenges are compounded when considering magnet machining sludge, which is more heavily oxidized and contains more contaminants than typical end-of-life magnets. If recycled, these materials are sent back to the primary smelter, where they are separated and purified to make new feedstocks which are often re-mixed into a new magnet. Here, a thermodynamic study is presented, assessing the oxidation behavior of rare-earth magnets. The theoretical minimum energy to reduce the whole magnet sludge, without separation and purification, is also presented. A comprehensive model including 25 elements is provided, using a hybrid CALPHAD-classical method. Oxygen distribution in a rare-earth magnet, with a total O content ranging between 0.09% to 5.4 wt%, is assessed. The results predict a final distribution of 40 wt% rare-earth in the oxide phase, with 60 wt% still remaining in the metallic phase. The model performance with respect to published experimental data is used to shed light into the possible processing methods for recycling.
... Roasting of Nd-Fe-B permanent magnets is known to improve the selectivity and the dissolution rate of the consequent leaching step [9][10][11]. However, when roasting is carried out in air at temperatures above 500 °C, neodymium is converted to a ternary neodymium-iron oxide phase, NdFeO 3 , and not to the binary neodymium oxide phase, Nd 2 O 3 [12,13]. If properly carried out, roasting can convert neodymium to Nd 2 O 3 and separate the rare-earth elements from the less valuable iron. ...
... The dissolution of NdFeO 3 can take up to 72 h [12,17,18]. The phase stability of the products from the roasting of Nd-Fe-B permanent magnets depends on the composition of the magnet, the partial oxygen pressure p O 2 and the roasting temperature [13,19]. The p O 2 at equilibrium can be controlled by the C/CO equilibrium, where CO is used as the oxidizing agent and carbon as the reducing agent [13]. ...
... The phase stability of the products from the roasting of Nd-Fe-B permanent magnets depends on the composition of the magnet, the partial oxygen pressure p O 2 and the roasting temperature [13,19]. The p O 2 at equilibrium can be controlled by the C/CO equilibrium, where CO is used as the oxidizing agent and carbon as the reducing agent [13]. In particular, the conditions favorable to avoid the formation of NdFeO 3 are (1) temperatures between 1350 °C and 2000 °C; and (2) a p O 2 between 10 −15 and 10 −25 atm. ...
Oxidative roasting of Nd–Fe‒B permanent magnets prior to leaching improves the selectivity in the recovery of rare-earth elements over iron. However, the dissolution rate of oxidatively roasted Nd–Fe‒B permanent magnets in acidic solutions is very slow, often longer than 24 h. Upon roasting in air at temperatures above 500 °C, the neodymium metal is not converted to Nd2O3, but rather to the ternary NdFeO3 phase. NdFeO3 is much more difficult to dissolve than Nd2O3. In this work, the formation of NdFeO3 was avoided by roasting Nd–Fe‒B permanent magnet production scrap in argon atmosphere, having an oxygen content of \( p_{{{\text{O}}_{2} }} \, \le \,10^{ - 20} \;{\text{atm}}, \) with the addition of 5 wt% of carbon as an iron reducing agent. For all the non-oxidizing iron roasting conditions investigated, the iron in the Nd–Fe‒B scrap formed a cobalt-containing metallic phase, clearly distinct from the rare-earth phase at microscopic level. The thermal treatment was optimized to obtain a clear phase separation of metallic iron and rare-earth phase also at the macroscopic level, to enable easy mechanical removal of iron prior to the leaching step. The sample roasted at the optimum conditions (i.e., 5 wt% carbon, no flux, no quenching step, roasting temperature of 1400 °C and roasting time of 2 h) was leached in the water-containing ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N]. A leaching time of only 20 min was sufficient to completely dissolve the rare-earth elements. The rare-earth elements/iron ratio in the leachate was about 50 times higher than the initial rare-earth elements/iron ratio in the Nd–Fe‒B scrap. Therefore, roasting in argon with addition of a small amount of carbon is an efficient process step to avoid the formation of NdFeO3 and to separate the rare-earth elements from the iron, resulting in selective leaching for the recovery of rare-earth elements from Nd–Fe‒B permanent magnets.
... Phase equilibria data from Rollet, 239, 240 Polyakova & Tokareva, 241 243 than the data measured by Fujiwara et al. 244 As expected, discrepancy also exists between database calculations of solid and liquid FeO activity with the activity data of Fujiwara et al. 244 as the authors used the activity measurements as a basis for the liquidus boundary points. As noted by Jakobsson et al. 245 in a previous assessment of the FeO-B2O3 system, experimental data is limited, and thus obtaining good agreement with phase equilibrium data from one of the two available studies is considered sufficient. ...
High concentrations of Na₂O and Al₂O₃ in the liquid high-level radioactive waste (HLW) stored at the Hanford Site can cause nepheline (NaAlSiO₄) to precipitate in a vitrified monolithic waste form upon cooling. Nepheline phase formation removes glass-former SiO₂ and -modifier Al₂O₃ from the immobilization matrix in greater proportion to alkalis, which can reduce glass durability and consequently increase the leach rate of radionuclides into the surrounding environment.
Current uncertainty in defining the HLW glass composition region prone to precipitating nepheline necessitates targeting a conservative waste loading, which raises operational costs by extending the liquid radioactive waste disposal mission and increases the required permanent repository storage capacity. An accurate thermochemical representation of HLW glass compositions is necessary to obtain a comprehensive understanding of the composition-temperature space for nepheline formation, which can facilitate the development of a phase field model of the mesoscale microstructural evolution of nepheline crystallization in HLW glass. As such an understanding of nepheline nucleation and grain growth kinetic behavior may lead to significant improvements in the production efficiency of durable HLW glass, generating thermochemical descriptions of the constituent phases is of primary importance.
Thus, a database consisting of the oxides of the nepheline-forming Na₂O-Al₂O₃-SiO₂ system and HLW glass nepheline solutes B₂O₃, K₂O, CaO, Li₂O, MgO, Fe₂O₃, and FeO has been developed to yield a thermochemical model capable of characterizing nepheline precipitation in HLW glass at equilibrium. Due to their high molar concentrations within vitrified glass, Na₂O, Al₂O₃, B₂O₃, and SiO₂ were considered major oxides whereas more dilute B₂O₃, K₂O, CaO, Li₂O, MgO, Fe₂O₃, and FeO were treated as minor constituents. All pseudo-binary systems composed of the major as well as major-minor oxide systems were thermodynamically assessed according to the CALculation of PHAse Diagrams (CALPHAD) methodology.
Additionally, all pseudo-ternary systems consisting of the major oxides were assessed due to the increased probability of interactions between these higher concentration oxides. Gibbs energies of solid solution phases and the oxide liquid were modeled using the compound energy formalism (CEF) and two-sublattice partially ionic liquid (TSPIL) model, respectively.
Accuracy of the thermodynamic database was validated by comparing model calculations to HLW glass experimental data. Both annealed and canister centerline cooled (CCC) glass sample data were considered. Additionally, nepheline phase compositional data was included for comparison with database computations. Results of these comparisons indicate that the database-derived calculations agree well with HLW glass experimental data. As phase precipitation in a CCC glass sample is dependent on kinetics, however, a phase field or similar model will need to be utilized to obtain a non-equilibrium description of CCC HLW glass behavior, which in turn often require accurate Gibbs energies of phases.
Hollandite has been studied as a candidate ceramic waste form for the disposal of HLW due to its inherent leach resistance and ability to immobilize alkaline-earth metals such as Cs and Ba at defined lattice sites in the crystallographic structure. The chemical and structural complexity of hollandite-type phases with a large number of potential additives and compositional ranges for high-level waste immobilization would require impractical systematic experimental exploration. Modeling the equilibrium behavior of the complex hollandite-forming oxide waste system would aid in the design and processing of hollandite waste forms by predicting their thermodynamic stability. Thus, a BaO-Cs₂O-TiO₂-Cr₂O₃-Al₂O₃-Fe₂O₃-FeO-Ga₂O₃ thermodynamic database was developed according to the CALPHAD methodology. The CEF was used to model solid solutions such as hollandite while the TSPIL model characterized the oxide melt. The database was validated by experimental hollandite compositional data, and an isothermal BaO-Cs₂O-TiO₂ pseudo-ternary diagram with added hollandite solutes was generated to extrapolate phase equilibrium behavior to regions not experimentally explored.
... It has been suggested from the thermodynamic modelling/phase diagram study by Parida et al. [21] and Jung et al. [22] that at equilibrium in air (at pO 2 = 0.21) at 700-1300 K, the stable phases will Fig. 10. ...
The microstructure evolution and mechanism of oxidation of (Nd,Pr)FeB magnet in the range 700-1500 K have been investigated. Bulk (10 × 5 × 5 mm) and powder samples (-150 μm) were oxidized under isothermal and non-isothermal conditions. DSC, XRD, high-temperature XRD, SEM and EPMA characterization techniques were utilized to investigate the general oxidation reactions, microstructure evolutions and phase transformations. Three different microstructure evolutions were observed in the temperature ranges 700-1000 K, 1000-1300 K and 1300-1500 K. The different product microstructures formed during the oxidation have implications for devising strategies for the recycling/recovery process of (Nd,Pr)FeB magnets.
Molten oxide electrolysis is a promising pathway to decarbonize primary metal production. The oxide electrolyte is less hazardous and eco‐toxic than halides (molten salt electrolysis) and, with the use of renewably generated electricity and an inert anode, oxygen is produced as a by‐product instead of CO2. Building fundamental understanding of electrolytic operating windows requires starting with simple chemistries. Here we investigate a simplified binary oxide system as an electrolyte for titanium oxide reduction to metal. The TiO2−Na2O binary system forms a eutectic, reducing the liquidus temperature over a wide composition range. FactSage 8.1 predictions suggested Ti reduction would become favorable over Na reduction for compositions greater than 0.49 mole fraction TiO2. The prediction was validated by the detection of metallic Ti after electrolysis experiments. However, the reduction efficiency was too low (0.24±0.08 % at −0.1 V vs Ti reference electrode) for the TiO2−Na2O system to be a viable industrial electrolyte for Ti production. Scoping for other binary oxide systems was performed for electrolytic production of two critical metals, tantalum and neodymium. Based on TiO2−Na2O system's predicted behavior, candidate binary oxide systems were identified that contained congruently melting line compounds flanked by eutectic reactions from the ACerS‐NIST Phase Equilibria diagrams database.
