Direct Recycling of Nd–Fe–B Magnets Based on the Recovery of Nd2Fe14B Grains by Acid‐free Electrochemical Etching

Abstract Recycling of end‐of‐life Nd–Fe–B magnets is an important strategy for reducing the environmental dangers associated with rare‐earth mining and overcoming the supply risks associated with the rare‐earth elements. In this study, a novel concept for recycling of sintered Nd–Fe–B magnets by directly recovering the matrix Nd2Fe14B grains is presented. The procedure is based on the anodic etching of sintered Nd–Fe–B magnets in a nonaqueous dimethylformamide (DMF)/0.3 mol L−1 FeCl2 bath. Selective recovery of Nd2Fe14B grains was realized within the applied current density <5 mA cm−2 based on the etching priority of phases: metallic Nd > intergranular NdFe4B4 > matrix Nd2Fe14B. The total energy consumption of the proposed recycling route is estimated to be 2.99 kWh kg−1, which is comparable to the state‐of‐the‐art methods. However, the proposed recycling route is currently the only procedure that enables repeated recycling of sintered Nd–Fe–B magnets in a closed‐loop system.

The recycling of Nd-Fe-B PMs can be classified into:i )direct re-use methods, ii)pyrometallurgical processing, and iii)hydrometallurgical separation and recovery. [1,7] In terms of new magnets production by using recycled EoL products,p yrometallurgical processing working at high temperature is energy-intensive, whereas hydrometallurgical routes require multi-processing steps with al arge amount of chemical consumption and wastewater generation.I nc ontrast, direct re-use methods such as resintering, [8] and hydrogenation disproportionation and desorptiona nd recombination (HDDR) [9] of EoL sintered Nd-Fe-B PMs are generally regarded as the most economical and ecological ways because they provides hort processing steps with zero waste generation. However,t he high oxygen content (typically2 000-5000 ppm) in the REE-rich grain boundary phases of Nd-Fe-B EoL magnets severelyl imits their recycling potential. [1,10] These REEo xides( mainly Nd 2 O 3 )c annotb ee xtracted, resultingi nr eprocessed sintered magnets, lacking full density and exhibiting poor magnetic properties. Therefore, extra REE hydrides are generallya dded to compensate for the existing REE oxides. [8b, 11] This then represents only ap artial circular economyf or the magnets. [12] Additionally,t he REE-rich phases,f or example, REE oxides, are nonferromagnetic. [13] With the repeated recycling by direct re-use methods,t he total volumeo ft he nonferromagnetic phases increases owing to the addition of REE hydrides, which then reduces the saturation magnetization andt herefore the remanence of sintered Nd-Fe-B magnets. Consequently,s interedN d-Fe-B magnets produced from the repeated recyclingo fm aterials by direct re-use methods tend to have poorer magnetic properties as the number of cycles increases.
Sintered Nd-Fe-B PMs consist of REE-rich grain boundaries, representing about1 0-12 %o ft he magnet, and the Nd 2 Fe 14 B grains, which is practically oxygen-free, accounting for 85-87% of the magnet. [14] Thus, direct recovery of the Nd 2 Fe 14 Bg rains, leaving REE oxides behinda sastarting point would provide a sustainable recyclingr oute for fresh Nd-Fe-B PMs production with high magnetic properties.
Herein, we describe an electrochemical process to recover the Nd 2 Fe 14 Bm atrix grains from sintered Nd-Fe-B magnets based on the etchingp riority of different phases in the magnets. As ar esult, the Nd 2 Fe 14 Bm atrix grains and the REEo xides were disconnectedf rom each other after electrochemical etching, which allowed magnetic separation of the matrix Nd 2 Fe 14 B grains.
To initiate the etching study,t he microstructure and the crystal phases of the initial sinteredN d-Fe-B magnets were first investigated (see the Supporting Information, Figure S1). The initial sinteredN d-Fe-B magnet exhibited at ypical microstructuret hat consists of the (Nd 1Àx Dy x ) 2 Fe 14 Bm atrix phase, labeled as "Nd 2 Fe 14 B" fors implicity, surrounded by the REE-rich grain boundary phases, which mostly consists of metallicN d and am ixture of different Nd-based oxides. [15] The NdFe 4 B 4 and am ixture of Nd 2 O 3 and Dy 2 O 3 phases sitting in some of the triple points are also observed. The electrochemical etching preference of different phases in the Nd-Fe-B magnetw as then studied by linear sweep voltammetry (LSV, Figure 1).
All the possible anodicr eactions at the Nd-Fe-B magnet anode are given by Equations (1)-(4): Nd 0 À 3e À ! Nd 3þ ; 0 Nd 3þ =Nd 0 ¼ À2:32 Vvs: SHE ð2Þ When using Pt as the working electrode (black curve), the current density started to increase at approximately 0.15 Va long the BC line owing to the onset of the oxidationo fF e 2 + (reaction 1), which includes also the oxidation of the [FeCl 3 (DMF)] À [16] complex and might explain the mild current peak at approximately 0.55 Vand the peak current (P 1 )attributed to [FeCl 4 ] 2À [16] oxidationa tt he potentialo f0 .75 V. When the Nd-Fe-B magnet was used as the workinge lectrode, the current density startedt oi ncrease at the potential of À0.40 V, shown by the red curve.T he peak (P 2 )o f5mA cm À2 recorded at À0.02 Vw as related to the oxidation of metallicN di nt he grain boundaries (reaction2)o wing to itsv ery negative electrode potential (À2.32 Vv s. standard hydrogen electrode, SHE). [17] The peak (P 3 )a t0 .30 Vi sl ikely the result of the combined oxidation of the NdFe 4 B 4 phase (reaction 3) with oxidation of Fe 2 + (reaction 1). From pointCo n, the current density regularly increases along CD on the red curve, which is the response of the oxidation of all the Nd-containing phases together with the Fe 2 + oxidation (reactions 1-4). Accordingly,the etchingp riority of the phases inside the magnet is as follows: metallicN d( in the grain boundary) > NdFe 4 B 4 > Nd 2 Fe 14 B (magnetic phase). This is in good agreement with previous reports. [18] Based on the etching priority,s elective etching of the metallic Nd from the grainb oundary could be realized by applying ap otentialo f< À0.02 V( corresponding to < 5mAcm À2 )o n the anode, whereas applying potentials of highert han 0.40 V (corresponding to > 9.9 mA cm À2 )w ould lead to the nonselective etchingo fa ll the phases present, for example, the metallic Nd phase, the NdFe 4 B 4 ,and Nd 2 Fe 14 B( Figure S2).
To   The sintered Nd-Fe-B magnet was electrochemically etched with an applied current of 10 mA (current density of 2mAcm À2 )f or 360 min to recover the Nd 2 Fe 14 Bg rains. The magnetically collectedp articles shown in Figure 3a are individual particles, confirming that Nd 2 Fe 14 Bg rains can be extracted through selectivee tching. X-ray diffraction( XRD;F igure S3) confirmst hat these magnetic particles maintain the Nd 2 Fe 14 B crystal structure, which can be re-used for making new PMs.
For the 1.61 go ft he sintered Nd-Fe-B magnet treated at 10 mA (2 mA cm À2 )f or 40 h, 1.08 go fN d 2 Fe 14 Bg rains were obtained. Accordingly,6 7.2 %o ft he Nd-Fe-Bm agnet was recovered in the form of Nd 2 Fe 14 Bg rains and the energy consumption per kilogram of the obtained Nd 2 Fe 14 Bg rains was calculated to be 0.58 kWh. Around 20 %o ft he Nd 2 Fe 14 Bg rains was etcheda nd dissolved into the electrolyte (assumingt hat the initial Nd-Fe-Bm agnet contained 87 %N d 2 Fe 14 Bg rains). [14] This is caused by i) the decreasing over-potential for etching the metallic Nd during the etching process, which forces the etchingo ft he Nd 2 Fe 14 Bg rains according to the etchingm echanism ( Figure S4) and ii)untimely removal of the Nd 2 Fe 14 B grains from the magnet anode after the complete etching of the surrounding grain boundaries. However,t he recovery of the Nd 2 Fe 14 Bg rains can be further improved by usinga nu ltrasonic bath during the electrochemical etchingp rocess to removet he Nd 2 Fe 14 Bg rains from the magnet anode in time. The nonmagnetic particlesc ollected by filteringt he electrolyte after electrolytic etching are presented in Figure 3b.T he round particlesc onsist of Nd 2 O 3 and Dy 2 O 3 phases, whereas the elongated ribbed particlesc onsist of Nd 2 O 3 ,D y 2 O 3 ,N d, and NdB 4 phases,a sc onfirmed by the energy-dispersive X-ray spectroscopy (EDS) and XRD analysis( Figure S5).
In parallel, pure Fe metal wasd eposited on the cathode with the current efficiency of 99.6 % ( Figure S6 a). As Fe 2 + was consumed (deposited) on the cathode, while Fe 2 + and REE ions (REE 3 + ), for example, Nd 3 + were generated from the partly etchedm agnet anode, the concentration of Fe 2 + ,a sa whole, decreasedi nt he electrolyte with increasing etching time. In contrast, the concentrations of REE 3 + in the electrolyte increased linearly with the increasing etching time (Figure S6 b). Therefore, the whole electrolysis process, including the magnet etchingo nt he anode and the Fe deposition on the cathode, ends up with the Nd 2 Fe 14 Bg rains, REE-containing electrolyte and REE-based particles, and pure Fe metal as the final products with only the consumptionofFeCl 2 and electricity.
Ar ecycling route for EoL Nd-Fe-B magnets is proposed based on the electrochemical etching ( Figure 4). The obtained Nd 2 Fe 14 Bg rains are used as raw materials for making new magnets. The REE-containing electrolyte and REE-based particles can be further treated by the conventional hydrometallurgical process for ah igh purity of > 99 %R EE recovery [1, 7b] followed by moltens alt electrolysis [20] for making RE metals/ alloys,w hich can be used together with the obtained Nd 2 Fe 14 B grains to make new Nd-Fe-B magnets. DMF can be recovered by distillation and re-used in ac losed-loop with minimized safety risk and environmental impact. Based on that, the overall REE mass balance from the initial magnet is 100 %p reserved, which forms ac ircular economy. The total energyc onsumption of the magnet-manufacturing processu sing the proposed electrochemical recycling route is estimated to be 2.99 kWh kg À1 ,w hich is directly comparable to the re-use methods (Table 1), if we considert he conventional additive of the Nd-Pr hydride (4 wt %). However,t he additive can be replaced by other alloys, such as Nd-Cu [21] and Ce, [22] which could lead to am uch lower energy footprint. In summary,w ea re proposing af acile and cost-effective electrochemical recycling process that selectively recovers the Nd 2 Fe 14 Bg rains from sintered Nd-Fe-B magnets at room temperature. The anodic etchingm echanism is based on finetuning of the applied current density < 5mAcm À2 to exploit the etching priority series of the phases present in the pristine Nd-Fe-B magnet:m etallic Nd > intergranular NdFe 4 B 4 > matrix Nd 2 Fe 14 B, which allows the preferential etching of their surrounding REE-rich grain boundaries, leaving the individual Nd 2 Fe 14 Bg rains behindf or magnetic separation. The total energy consumption of the proposed electrochemical recycling route is estimated to be 2.99 kWh kg À1 ,w hich is, in the long term, expected to be economically more feasible while offering considerably more flexibility.