# A hydro/solvothermal process for recycling Nd-Fe-B magnets

Marwa Kchaw, Virginie Nachbaur, Samuel Jouen, Jean-Marie Le Breton

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Marwa Kchaw, Virginie Nachbaur, Samuel Jouen, Jean-Marie Le Breton. A hydro/solvothermal process for recycling Nd-Fe-B magnets. Sustainable Materials and Technologies, 2025, 46, pp.e01738. 10.1016/j.susmat.2025.e01738. hal-05469396

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# A hydro/solvothermal process for recycling Nd-Fe-B magnets

Marwa Kchaw, Virginie Nachbaur, Samuel Jouen, Jean-Marie Le Breton*

Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Groupe de Physique des Matériaux, UMR 6634, F-76000 Rouen, France

## ARTICLE INFO

### Keywords:

- Permanent magnets

- Nd-Fe-B

- Recycling

- Hydrothermal process

- Solvothermal process

## ABSTRACT

Sintered Nd-Fe-B permanent magnets were subjected to a hydro/solvothermal pulverization process developed for recycling rare-earth permanent magnets. This process, with a low environmental footprint, consists of heating the magnets in a closed reactor in the presence of water (hydrothermal treatment) or ethanol (solvothermal treatment). Different parameters such as preliminary grinding, processing time and nature of the solvent have been investigated. The pulverization process is confirmed by the degradation of the intergranular Nd-rich phase. The magnetic phase is hydrogenated and then decomposed by the reaction of the rare earth with oxygen and hydrogen to form oxides and hydroxides. In order to stop the degradation of the hard magnetic phase and to produce recycled powders with good magnetic properties, the influence of the hydro/solvothermal process parameters (solvent composition and treatment time) was investigated. The results show an important lowering of the hard magnetic phase decomposition for powders subjected to preliminary mechanical grinding, and then treated in pure ethanol for short durations.

## 1. Introduction

Advanced permanent magnetic materials play a crucial role in modern society [1-4]. Neodymium iron boron (Nd-Fe-B) magnets, known for their exceptionally high energy density, are currently recognized as the strongest permanent magnets available [1-4]. They find widespread application in various sectors, including hard disk drives, wind turbines, industrial motors, acoustic transducers, and electric vehicles [1,3]. The extensive use of Nd-Fe-B sintered magnets has led to a substantial market demand and rapid output growth, consequently resulting in a significant increase in waste generated during the production process. This production waste constitutes around 30 wt% to 40 wt% of the total mass of raw materials, of which about 30 wt% consists of rare earth elements, indicating substantial potential for recycling [5]. Consequently, recycling Nd-Fe-B magnet waste has become a central focus in the field of resource recovery [5-11]. On the other hand, huge amounts of Nd-Fe-B magnets are available in WEEE (Waste from Electrical and Electronic Equipment). Thus, recycling Nd-Fe-B magnets from end-of-life consumer goods would reduce material losses and provide secondary raw materials.

Several innovative processes have emerged, such as hydrometallurgy, pyrometallurgy, and Hydrogen Decrepitation (HD) process. Hydro-metallurgy involves a liquid process for treating metals [12]. However, using a lot of chemicals and non-recyclable materials makes the

recycling process less effective. On the other hand, pyrometallurgy is a method that treats materials at temperatures ranging from 250 °C up to 1450 °C [10], inducing physical and chemical transformations that facilitate the recovery of precious metals. As demonstrated by Firdaus et al. [10], this approach serves as a viable alternative to hydrometallurgy. In fact, it can be used to avoid large water consumption and waste production in the recovery process. The HD process is performed at temperatures up to 1000 °C, resulting in a very coarse particle distribution after treatments. Zakotnik et al. confirmed that an anisotropic powder was produced through high-temperature vacuum degassing [13]. However, because of the high values of treatment temperatures, this process is very energy-intensive.

We developed previously an environmentally friendly hydrothermal treatment for recycling Nd-Fe-B permanent magnets [11]. According to this process, sintered magnets are heated at 250 °C for up to 18 h in a closed reactor, in water mixed with sodium chloride. This hydrothermal treatment, which causes the decomposition of the Nd-rich and Nd₂Fe₁₄B phases into Nd(OH)₃ and Fe₃O₄, was shown to be an easy way to extract rare earths from the resulting powders by magnetic separation. Based on these results, the present study presents a modified hydrothermal way, applied for a shorter treatment time, in order to obtain powders composed of Nd₂Fe₁₄B particles. Such a process should allow obtaining Nd₂Fe₁₄B powders suitable for the production of new Nd-Fe-B sintered magnets.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jean-marie.lebreton@univ-rouen.fr (J.-M. Le Breton).</footnote>
The motivation of this work is thus to improve and develop the key parameters of a green and sustainable process aiming at remanufacturing new Nd-Fe-B magnets from end-of-life materials. This approach aims to reduce environmental impact, energy consumption, and chemical waste, while promoting a circular economy in the field of rare earth permanent magnets.

Thus, compared with the other recycling methods mentioned above, the recycling method explored in this investigation would offer significant advantages in terms of energy consumption and production costs, namely:

- low energy consumption thanks to the use of moderate temperatures and short processing times,

- less environmental pollution thanks to the use of a closed system,

- reduced liquid waste thanks to the use of reduced quantities of solvents

- reuse of the product (Nd₂Fe₁₄B phase) as a precursor for remanufacturing.

## 2. Experimental

The magnets used for the recycling process are in the form of sintered arc segments, coated with a protective layer consisting of Ni-Cu, in order to protect them against oxidation. These magnets were extracted from computer hard disk drives (HDD) collected as WEEE. The nickel magnet coating has been removed using a scalpel. Then, the magnets were fragmented using a crusher. A very important step is the sieving process, where a manual sieve has been used to collect particles smaller than 2 mm in size. These specific steps are designed to accelerate the pulverization process of the magnets.

For each treatment, a mass of 2 g of powder was placed into a crystallization bomb containing 30 mL of solvent and heated to 225 °C for different treatment times. At the end of the hydro/solvothermal reaction, the precipitate has been separated by centrifugation, and the powder was finally dried at 50 °C.

The different stages of the process are illustrated in Fig. 1.

The samples were analysed by X-ray diffraction (XRD) analysis using a Bruker D8 Advance X-ray diffractometer using a cobalt anticathode (wavelength: λ(Kα₁) = 0.178897 nm).

The morphology and elemental distribution of the samples were investigated by JEOL 7900F Scanning Electron Microscope (SEM). The elemental composition of the samples has been analysed by Energy-dispersive X-ray spectroscopy (EDS or EDX).

The magnetic properties of the powders were measured using a Superconducting Quantum Interference Device (SQUID) MPMS XL-5 magnetometer from Quantum Design. For the magnetic measurements, the powders fixed in epoxy resin and placed in capsules. The powders were not magnetically aligned.

⁵⁷Fe Mössbauer spectrometry analyses were performed at room temperature with a ⁵⁷Co source in a Rh matrix. The hyperfine parameters, including isomer shift (relative to metallic α-Fe at room temperature), quadrupolar splitting, quadrupolar shift, and hyperfine field, are denoted as δ, Δ, ε, and Bhf, respectively. The estimated errors associated with the hyperfine parameters are derived from statistical errors provided by the fitting program [14].

## 3. Results and discussion

### 3.1. Hydrothermal treatment

#### 3.1.1. Mechanism of the magnet decomposition under hydrothermal conditions

As it has been shown previously [11], under hydrothermal conditions the magnet pulverization starts with the reaction of water with the intergranular Nd-rich phase according to the following equation:

$$2 \mathrm{Nd} + 6 \mathrm{H}_2\mathrm{O} \rightarrow 2 \mathrm{Nd}(\mathrm{OH})_3 + 3 \mathrm{H}_2 \quad (1)$$

Then, hydrogen released by this reaction leads to hydrogenation of the Nd₂Fe₁₄B phase, similarly to the reaction observed in H₂ gas and also during humid corrosion of magnets [15]:

$$\mathrm{Nd}_2\mathrm{Fe}_{14}\mathrm{B} + \frac{x}{2}\mathrm{H}_2 \rightarrow \mathrm{Nd}_2\mathrm{Fe}_{14}\mathrm{BH}_x \quad (2)$$

The decomposition of the Nd₂Fe₁₄BHx hydried phase is the next step. In our previous study [11], this step was supposed to be the same than the reaction taking place in presence of hydrogen and leading to the formation of Fe and Fe₂B [16].

Nevertheless, in order to identify the hydried phase decomposition

<figure><img src="image_3.png" /><figcaption>Fig. 1. - The different steps of the hydro/solvothermal process for recycling Nd-Fe-B magnets.</figcaption></figure>
reaction under hydrothermal conditions, a long-term treatment has been applied to an uncoated Nd-Fe-B commercial magnet (Supermagnete, 20 mm × 10 mm × 5 mm, magnetisation N45). The process has been performed in a Teflon-lined acid digestion vessel (Parr 4744), by adding 30 mL of a sodium chloride (NaCl) solution with a concentration of 29 g. L⁻¹, at 225 °C during 87 h. At the end of the process, the magnet is totally pulverized and the resulting powder has been characterized by XRD and Mössbauer spectrometry.

The XRD pattern (Fig. 2a) clearly demonstrates the presence of Fe₃O₄ (ICDD file n° 19–0629), Nd(OH)₃ (ICDD file n° 70–0215) and NdBO₃ (ICDD file n° 62–0772) phases. As the concentrations of Pr and Dy are low compared to this of Nd, and given that the rare-earth (RE) hydroxides (RE = Nd, Pr, Dy) and RE borate structures are identical with close lattice parameters, Nd(OH)₃ and NdBO₃ are probably mixed RE hydroxide and mixed RE borate. A broad peak, centered on the position of α-Fe (ICDD file n° 06–0696) can also be detected, which is usually observed during the corrosion of Nd-Fe-B magnets [17].

The Mössbauer spectrum (Fig. 2b) has been fitted with the contributions of α-Fe and Fe₃O₄, in agreement with the results of the XRD analysis. A doublet corresponding to the non-magnetic phase Nd₁.₁Fe₄B₄ [18] has also been fitted. This minor phase, which was not evidenced by XRD, is known to be preserved during magnet oxidation or corrosion, even when the Nd₂Fe₁₄B phase is decomposed [17,19]. The hyperfine parameters of the contributions used to fit the spectrum are reported in Table 1.

The degradation mechanism of Nd₂Fe₁₄BHₓ in hydrothermal conditions is then a mix of the reactions observed during humid corrosion of magnets [17], hydrogen deprecitation [16] and oxidation [19]. Oxygen (due to the presence of air when the reactor is closed) and water react simultaneously with the hydried phase to form Nd(OH)₃ and Fe, but also NdBO₃ which is known to be obtained under hydrothermal

<caption>**Table 1**</caption>

Fitted hyperfine parameters of the Mössbauer spectrum of the uncoated Nd-Fe-B commercial magnet after 87 h of hydrothermal treatment.

<table><thead><tr><th>Contribution</th><th>δ<br>± 0.01<br>(mm.<br>s<sup>-1</sup>)</th><th>2ε or Δ<br>± 0.05<br>(mm.<br>s<sup>-1</sup>)</th><th>B<sub>hf</sub><br>±<br>(T)</th><th>Γ<sub>1/2</sub><br>(mm.<br>s<sup>-1</sup>)</th><th>Relative intensity<br>(%)</th></tr></thead><tbody><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>0.07</td><td>0.59</td><td>–</td><td>0.26</td><td>3</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub></td><td>A 0.27<br>B 0.67</td><td>–0.01<br>0.01</td><td>48.9<br>45.7</td><td>0.12<br>0.24</td><td>14<br>31</td></tr><tr><td>α-Fe</td><td>0.03</td><td>0.01</td><td>33.0</td><td>0.33</td><td>52</td></tr></tbody></table>

conditions at around 200 °C [20], and the reaction can be expressed as follows:

$$Nd_2Fe_{14}BH_x + \frac{3-x}{2}H_2O + \frac{9+x}{4}O_2 \rightarrow Nd(OH)_3 + 14 Fe + NdBO_3 \quad (3)$$

Furthermore, iron is known to react with hot compressed water according to the following equilibrium [21]:

$$3 \text{Fe(s)} + 4 \text{H}_2\text{O(l)} \rightleftharpoons \text{Fe}_3\text{O}_{4(\text{s})} + 4 \text{H}_2\text{(g)} \quad (4)$$

Following this succession of reactions, the hydrothermal treatment of the Nd₂Fe₁₄BHₓ hydried phase finally leads to the formation of Fe, Fe₃O₄, Nd(OH)₃ and NdBO₃.

### 3.1.2. Hydrothermal process applied to spent magnets

Nd-Fe-B magnets recovered from hard disk drives without their Ni-Cu coating were first fragmented into particles with a size ranging between 2 and 5 mm (Fig. 3). Some powders were sieved to less than 2 mm. All the particles were then submitted to the hydrothermal treatment.

According to previous studies [11], a first series of experiments were

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** XRD pattern (a) and room temperature Mössbauer spectrum (b) of the uncoated Nd-Fe-B commercial magnet after 87 h of hydrothermal treatment.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Initial hard disk magnet with (a) and without (b) its Ni-Cu protective coating. Fragments of the initial Nd-Fe-B magnet with a size between 2 and 5 mm (c), and fragments smaller than 2 mm (d).</figcaption></figure>

conducted in water, at a fixed temperature of 225 °C, with a sodium chloride NaCl concentration of 29 g.L⁻¹, during 2 h. The treatment temperature and the treatment time were chosen in order to avoid the decomposition of the hydrogenated phase. They are thus lower that the temperature and time that lead to the decomposition of the Nd₂Fe₁₄B phase into Nd(OH)₃ and Fe₃O₄, that is 250 °C and 18 h. After this hydrothermal treatment, we observe a partial pulverization for the fragments with a size between 2 and 5 mm, and a total pulverization for the fragments sieved to less than 2 mm (Fig. 4).

The chemical composition of the initial magnets measured by inductively coupled plasma spectrometry analysis is: Nd₂Pr₅Fe₇₁B₁ (in weight %). Thus, the composition of the hard magnetic phase in the initial magnet is (Nd,Pr)₂Fe₁₄B, and the intergranular phase is a (Nd,Pr)-rich phase. However, for sake of clarity, these two phases will be denoted as Nd₂Fe₁₄B and Nd-rich throughout the manuscript.

SEM images and EDX mapping of the initial magnet and of the pulverized 2 mm sieved powder are shown in Figs. 5 and 6. In the initial magnet, the Nd₂Fe₁₄B particles are interconnected by the Nd-rich intergranular phase (Fig. 5), in agreement with observations reported in the literature [22]. After 2 h of treatment, one can observe new particle geometries compared with the initial magnet. Fig. 6 shows the presence of large grains, octahedral particles, and needle-shaped crystals. EDX analysis reveals that the large grains (position 2) are composed of Nd, Pr and Fe and thus correspond to the Nd₂Fe₁₄B phase. The octahedral particles (position 1) contain Fe and O elements. The needle-shaped crystals (position 3) contain Nd, Pr and O elements. In agreement with previous observations [11], the octahedral particles are identified as magnetite Fe₃O₄ and the needle-shaped crystals are identified as (Nd, Pr)(OH)₃, hereafter denoted as Nd(OH)₃. These observations indicate that the crushed magnet has undergone degradation through the hydrothermal process.

The XRD pattern of the pulverized 2 mm sieved powder is shown in Fig. 7, and compared with that of the initial magnet. The XRD pattern of the initial powder shows peaks of the hard magnetic Nd₂Fe₁₄B phase and peaks of the Nd-rich phase. After 2 h of hydrothermal treatment, the X-ray pattern of the pulverized 2 mm sieved powder reveal three phases: Nd₂Fe₁₄B, Fe₃O₄, and Nd(OH)₃. This result, which is consistent with the SEM observations and EDX analysis, shows that upon the hydrothermal process, the hard magnetic phase has been partly degraded, in agreement with previous observations, while the Nd-rich phase has been transformed into Nd(OH)₃ [11]. It is to be noted that, after the hydrothermal treatment, the peaks of the Nd₂Fe₁₄B phase have been shifted

towards low angles. This is attributed to the volume expansion of the Nd₂Fe₁₄B phase, resulting from the insertion of H atoms. This hydrogenation of the Nd₂Fe₁₄B phase into a Nd₂Fe₁₄BHₓ phase is in agreement with previous investigations [11,23].

The room temperature Mössbauer spectra of the initial magnet and of the pulverized 2 mm sieved powder after 2 h of hydrothermal treatment are shown in Fig. 8.

The spectrum of the initial magnet has been fitted with the contributions of the Nd₂Fe₁₄B and Nd₁.₁Fe₄B₄ phases. The contribution of the Nd₂Fe₁₄B magnetic phase has been fitted with six sextets with relative intensities fixed to the proportions 16:16:8:8:4:4 to account for the occupancy of the six sites of the crystal structure (k₁, k₂, j₁, j₂, c, e). The hyperfine parameters of the six sextets were fitted consistently with the values reported in the literature [24]. The minor contribution of the non-magnetic Nd₁.₁Fe₄B₄ phase has been fitted with a paramagnetic doublet. The hyperfine parameters of the contributions used to fit the spectrum of the initial magnet are reported in Table 2.

The spectrum of the pulverized 2 mm sieved powder after 2 h of hydrothermal treatment has been fitted with:

- the magnetic contribution of the Nd₂Fe₁₄BHₓ phase (six sextets)

- the paramagnetic contribution of the Nd₁.₁Fe₄B₄ phase (one doublet)

- the magnetic contribution of Fe₃O₄ (two sextets)

- the magnetic contribution of α-Fe (one sextet).

The hyperfine parameters of these contributions are reported in Table 3.

The presence of Fe₃O₄ is consistent with the results of the SEM, EDX and XRD analyses. The presence of α-Fe indicates that a part of the Nd₂Fe₁₄B phase has been degraded, in agreement with the fact that α-Fe is formed when the Nd₂Fe₁₄B is decomposed [19,25,26], according to Eq. (3).

The comparison of the hyperfine parameters of the Nd₂Fe₁₄B contribution before and after the hydrothermal treatment reveals an increase of the hyperfine field of the six sextets. The mean hyperfine field of the Nd₂Fe₁₄B contribution increases from (29.1 ± 0.2) T in the initial magnet up to (30.8 ± 0.2) T in the powder after 2 h of hydrothermal treatment. Such an increase can be related to the volume expansion of the R₂Fe₁₄B structure as a consequence of the introduction of H atoms [27–29].

These results show that the hydrothermal treatment leads to the total pulverization of the 2 mm sieved Nd-Fe-B powder. Simultaneously, the magnetic Nd₂Fe₁₄B phase is hydrogenated into Nd₂Fe₁₄B Hₓ. However, the formation of α-Fe and Fe₃O₄ phases indicates that the Nd₂Fe₁₄B phase has been partially degraded during the treatment. In order to prevent the Nd₂Fe₁₄B phase from degradation, water has been progressively replaced by ethanol.

## 3.2. Solvothermal treatment

### 3.2.1. Influence of the solvent composition

The experiments have been conducted on crushed magnets with a size smaller than 2 mm. The solution is a mixture of distilled water and ethanol in different volume fractions (70/30, 50/50, and 0/100). A total

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Fragment of magnets with size between 2 and 5 mm (a) and fragments with a size less than 2 mm (b) after hydrothermal treatment for 2 h.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. SEM image, EDX mappings of Nd, Fe and B elements, and EDX analysis of the initial fragment of a hard disk magnet.</figcaption></figure>

volume of 30 mL of solution is introduced into the reactor, along with the fragments of the Nd-Fe-B magnet. The reaction temperature and time are fixed to 225 °C and 5 h. Regardless of the water/ethanol ratio, a complete pulverization of the crushed magnets is achieved at the end of the reaction.

Fig. 9 shows SEM images of the powders after treatments with different volume fractions of ethanol. The images of the samples treated with 30 % and 50 % ethanol reveal morphologies similar to the 2 mm sieved powder treated with the hydrothermal process. In the SEM images of the powder treated with 100 % ethanol, octahedral Fe₃O₄ particles and needle-shaped Nd(OH)₃ crystals are not observed. The morphology of the powders treated in pure ethanol is thus different from the morphology of the powders treated in water or in a water/ethanol mixture. Ethanol thus appears to have a positive influence on the pulverization process of Nd-Fe-B magnets. In particular, the absence of octahedral particles in powders treated in pure ethanol suggests that Fe₃O₄ is not formed.

The XRD patterns of the pulverized 2 mm sieved powders after 5 h of hydro/solvothermal treatment are shown in Fig. 10. The XRD patterns of the sample treated with 30 % and 50 % ethanol are similar, and reveal the presence of Nd₂Fe₁₄BHₓ, Fe₃O₄ and Nd(OH)₃ phases. In the XRD pattern of the sample treated with 100 % ethanol, the Nd(OH)₃ peaks are observed with a reduced intensity, and the Fe₃O₄ peaks are not observed. These results are in agreement with SEM observations, and confirm that the solvothermal treatment for 5 h in pure ethanol suppresses the formation of Fe₃O₄. In the patterns of the treated powders, the presence of a broad peak with low intensity is observed at the angular position around 2θ = 51°. This broad peak corresponds to the main peak of the α-Fe phase, which is present in the powders as dispersed precipitates that result from the degradation of the Nd₂Fe₁₄B phase [19,25,26].

The enlargement over the 43–47° range of the pattern evidences a shift of the Nd₂Fe₁₄BHₓ peaks towards low angles, indicating an increase of the unit cell volume of the Nd₂Fe₁₄B structure due to the
<figure><img src="image_8.png" /><figcaption>Fig. 6. SEM image and EDX analysis of the 2 mm sieved powder after 2 h of hydrothermal treatment. The C peaks are due to the sample holder.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. X-ray diffraction patterns of the 2 mm sieved powder after 2 h of hydrothermal treatment (up) and of the initial Nd₂Fe₁₄B magnet (down). The non-indexed peaks correspond to the Nd₂Fe₁₄BHₓ phase.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. Room temperature Mössbauer spectra of the 2 mm sieved powder after 2 h of hydrothermal treatment (a), and of the initial magnet (b).</figcaption></figure>

<caption>Table 2</caption>

Fitted hyperfine parameters of the Mössbauer spectrum of the initial powder. The mean hyperfine field of the Nd₂Fe₁₄B contribution is: <Bhf> = 29.1 T.

<table><caption>Table 3</caption><thead><tr><th>Contribution</th><th>δ<br>± 0.01<br>(mm.<br>s<sup>-1</sup>)</th><th>2ε or Δ<br>± 0.05<br>(mm.<br>s<sup>-1</sup>)</th><th>B<sub>hf</sub><br>±<br>0.2<br>(T)</th><th>Γ<sub>1/2</sub><br>(mm.<br>s<sup>-1</sup>)</th><th>Relative<br>intensity (%)</th></tr></thead><tbody><tr><td>16k<sub>1</sub></td><td>- 0.02</td><td>0.23</td><td>28.1</td><td>0.16</td><td>28</td></tr><tr><td>16k<sub>2</sub></td><td>- 0.07</td><td>0.07</td><td>30.3</td><td>0.16</td><td>28</td></tr><tr><td>8j<sub>1</sub></td><td>- 0.03</td><td>0.08</td><td>26.1</td><td>0.16</td><td>14</td></tr><tr><td>8j<sub>2</sub></td><td>- 0.21</td><td>0.67</td><td>34.5</td><td>0.16</td><td>14</td></tr><tr><td>4e</td><td>0.01</td><td>- 0.23</td><td>27.6</td><td>0.16</td><td>7</td></tr><tr><td>4c</td><td>0.11</td><td>0.23</td><td>24.6</td><td>0.16</td><td>7</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>0.12</td><td>0.60</td><td>–</td><td>0.18</td><td>2</td></tr></tbody></table>

Fitted hyperfine parameters of the Mössbauer spectrum of the 2 mm sieved powder after 2 h of hydrothermal treatment. The mean hyperfine field of the Nd₂Fe₁₄BHₓ contribution is: <Bhf> = 30.8 T.

<table><thead><tr><th>Contribution</th><th>δ<br>± 0.01<br>(mm.<br>s<sup>-1</sup>)</th><th>2ε or Δ<br>± 0.05<br>(mm.<br>s<sup>-1</sup>)</th><th>B<sub>hf</sub><br>±<br>0.2<br>(T)</th><th>Γ<sub>1/2</sub><br>(mm.<br>s<sup>-1</sup>)</th><th>Relative<br>intensity (%)</th></tr></thead><tbody><tr><td>16k<sub>1</sub></td><td>- 0.04</td><td>0.18</td><td>30.1</td><td>0.16</td><td>20</td></tr><tr><td>16k<sub>2</sub></td><td>- 0.07</td><td>0.10</td><td>32.7</td><td>0.16</td><td>20</td></tr><tr><td>8j<sub>1</sub></td><td>- 0.05</td><td>0.11</td><td>28.1</td><td>0.16</td><td>10</td></tr><tr><td>8j<sub>2</sub></td><td>0.15</td><td>0.70</td><td>34.3</td><td>0.16</td><td>10</td></tr><tr><td>4e</td><td>- 0.42</td><td>- 0.23</td><td>29.5</td><td>0.16</td><td>4</td></tr><tr><td>4c</td><td>- 0.04</td><td>0.23</td><td>25.8</td><td>0.16</td><td>4</td></tr><tr><td>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub></td><td>0.11</td><td>0.59</td><td>–</td><td>0.16</td><td>2</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub></td><td>A</td><td>0.10</td><td>0</td><td>49.3</td><td>0.24</td><td>5</td></tr><tr><td></td><td>B</td><td>0.30</td><td>0</td><td>46.3</td><td>0.24</td><td>5</td></tr><tr><td>α-Fe</td><td></td><td>0</td><td>0</td><td>33.0</td><td>0.30</td><td>20</td></tr></tbody></table>

hydrogenation of the Nd₂Fe₁₄B phase. The hydrogen content of the hydride phase has been determined using data reported by Isnard et al. [23]. According to this data, the hydrogen content in the Nd₂Fe₁₄BHₓ phase can be estimated from the volume of the unit cell obtained by XRD analysis. The unit cell parameters and volume of the Nd₂Fe₁₄B phase and the estimated values of the hydrogen content x in the Nd₂Fe₁₄BHₓ phase for the treated powders are reported in Table 4.

We observe that the hydrogenation of the hard magnetic phase Nd₂Fe₁₄B after treatments with different water/ethanol ratios can result
<figure><img src="image_11.png" /><figcaption>Fig. 9. SEM images of 2 mm sieved Nd-Fe-B powders after 5 h of hydro/solvothermal treatment for the indicated volume fractions of ethanol.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. (a, left) X-ray diffraction patterns of the initial Nd₂Fe₁₄B magnet (reference), and of the 2 mm sieved powders after 5 h of hydro/solvothermal treatment for the indicated volume fractions of ethanol. The non-indexed peaks correspond to the Nd₂Fe₁₄BHₓ phase. (b, right) Enlargement over the 43–47° range, showing the shift of the Nd₂Fe₁₄B peaks down to low angles, in relation with the increase of the lattice parameters due to the insertion of H atoms.</figcaption></figure>

Table 4

Unit cell parameters, volume and estimated values of the hydrogen content x in the Nd₂Fe₁₄BHₓ phase for the treated powders. The values for the initial magnet (reference) are given for comparison.

<table><thead><tr><th></th><th>a (nm)<br>± 0.001</th><th>c (nm)<br>± 0.001</th><th>volume (nm³)</th><th>Hydrogen<br>content x</th></tr></thead><tbody><tr><td>Reference</td><td>0.879</td><td>1.223</td><td>0.945</td><td>0</td></tr><tr><td>30 % ethanol / 5 h</td><td>0.884</td><td>1.228</td><td>0.960</td><td>1.5</td></tr><tr><td>50 % ethanol / 5 h</td><td>0.885</td><td>1.229</td><td>0.962</td><td>1.6</td></tr><tr><td>100 % ethanol / 5 h</td><td>0.886</td><td>1.229</td><td>0.965</td><td>1.9</td></tr></tbody></table>

in the formation of a hydrogenated Nd₂Fe₁₄BHₓ phase with a hydrogen content x that depends on the treatment conditions. The volume expansion of the Nd₂Fe₁₄B cell from 0.960 nm³ to 0.965 nm³ is

accompanied by the increase of the hydrogen content x from 1.5 to 1.9.

To determine the proportions of the iron-containing phases in the treated powders, all samples were analysed by Mössbauer spectrometry. The corresponding spectra are shown in Fig. 11. They were fitted with the same contributions as those used to fit the spectrum of the 2 mm sieved powder after 2 h of hydrothermal treatment (Table 2). The mean hyperfine field of the Nd₂Fe₁₄B contribution and the relative intensities of all the contributions to the Mössbauer spectra are given in Table 5.

For all samples, the mean hyperfine field of the Nd₂Fe₁₄BHₓ contribution is higher than that of the Nd₂Fe₁₄B contribution (29.1 ± 0.2) T, confirming the hydrogenation of the Nd₂Fe₁₄B phase during the solvothermal treatment with ethanol, in agreement with the results of the XRD analysis. It is worth mentioning that the relative intensity of the Nd₂Fe₁₄BHₓ contribution increases with the proportion of ethanol. The atomic percentage of decomposition of the Nd₂Fe₁₄B phase upon the
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Room temperature Mössbauer spectra of the 2 mm sieved powder after 5 h of solvothermal treatment with 30 %, 50 % and 100 % ethanol.</figcaption></figure>

**Table 5**

Mean hyperfine field of the Nd₂Fe₁₄B phase and relative intensities of the contributions to the Mössbauer spectra of the 2 mm sieved powder after 5 h of solvothermal treatment with 30 %, 50 % and 100 % ethanol.

<table><thead><tr><th>Sample</th><th>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub><br/>&lt;B<sub>hf</sub> &gt; (T)</th><th>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub><br/>(%)</th><th>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub><br/>(%)</th><th>α-Fe<br/>(%)</th><th>Fe<sub>3</sub>O<sub>4</sub><br/>(%)</th></tr></thead><tbody><tr><td>30 % ethanol</td><td>30.5</td><td>58</td><td>2</td><td>29</td><td>11</td></tr><tr><td>50 % ethanol</td><td>30.4</td><td>62</td><td>2</td><td>28</td><td>8</td></tr><tr><td>100 % ethanol</td><td>30.5</td><td>66</td><td>2</td><td>32</td><td>-</td></tr></tbody></table>

solvothermal treatment has been estimated on the basis of the relative intensities of the contributions to the Mössbauer spectra reported in Table 5. We found that 41 %, 37 % and 33 % of the hard magnetic phase

has been dissociated in the samples treated with 30 %, 50 % and 100 % ethanol respectively. These results indicate that the use of ethanol is favourable to reduce the dissociation of the Nd₂Fe₁₄B phase during the solvothermal treatment.

The hysteresis loops at 295 K of the 2 mm sieved powder after 5 h of solvothermal treatment with 30 %, 50 % and 100 % ethanol are shown in Fig. 12, where they are compared with the hysteresis loop of the initial powder. The values of saturation magnetization (denoted as M₀) and coercive field (μ₀H₀) are reported in Table 6.

The values of the coercive field are very low, indicating that the powders are non-coercitive. The saturation magnetization values of the treated powders are comparable, around 160 emu/g, being higher than the value of the initial magnet. This is due phase to the presence of α-Fe in the treated powders, the saturation magnetization of α-Fe being higher than that of the Nd₂Fe₁₄B phase. The saturation magnetization was calculated for each sample on the basis of the Mössbauer relative intensities of the contributions of the magnetic phases reported in Table 5. The calculated values reported in Table 6 are consistent with the measured ones, thus confirming that the presence of α-Fe in the treated powders is responsible for the higher values of M₀ in the treated powders than in the initial powder.

Although the use of ethanol is favourable to reduce the dissociation of the Nd₂Fe₁₄B phase during the solvothermal treatment, the presence of α-Fe in the treated powders is detrimental to obtaining recycled magnets with acceptable magnetic properties. With the aim at reducing the dissociation of the hard magnetic Nd₂Fe₁₄B phase upon solvothermal treatment, the influence of the treatment time has been investigated.

**Table 6**

Magnetic properties measured at 295 K the 2 mm sieved powder after 5 h of solvothermal treatment with 30 %, 50 % and 100 % ethanol. The values obtained for the initial powder (reference) are given for comparison.

<table><thead><tr><th>Sample</th><th>Initial powder<br/>(reference)</th><th>30 %<br/>ethanol,<br/>5 h</th><th>50 %<br/>ethanol,<br/>5 h</th><th>100 %<br/>ethanol,<br/>5 h</th></tr></thead><tbody><tr><td>μ₀H₀ (T)</td><td>0.15 ± 0.01</td><td>0.05 ±<br/>0.01</td><td>0.03 ±<br/>0.01</td><td>0.02 ±<br/>0.01</td></tr><tr><td>M₀ (Am²/kg),<br/>measured</td><td>140 ± 10</td><td>159 ± 10</td><td>155 ± 10</td><td>163 ± 10</td></tr><tr><td>M₀ (Am²/kg),<br/>calculated</td><td>140</td><td>154</td><td>153</td><td>158</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Hysteresis loops at 295 K of the initial powder (a) and the 2 mm sieved powder after 5 h of solvothermal treatment with 30 %, 50 % and 100 % ethanol (b).</figcaption></figure>
### 3.2.2. Influence of the treatment time in pure ethanol

A series of experiments were conducted on initially crushed magnets with a size smaller than 2 mm for times ranging from 2 to 15 h using 100 % ethanol, under the same conditions: solvent volume of 30 mL and heating temperature 225 °C. After each treatment, a complete pulverization of the initial fragments is observed.

SEM images of these powders are shown in Fig. 13. For all treatment times, the images show the presence of particles with a size about 10 µm corresponding to the grains of the magnetic Nd₂Fe₁₄B phase in the initial magnet.

The XRD patterns of the powders after solvothermal treatment in pure ethanol are shown in Fig. 14, where the XRD pattern of the initial powder (reference) is shown for comparison. The XRD patterns appear similar, revealing the presence of Nd₂Fe₁₄BHₓ, Nd(OH)₃ and α-Fe phases, while the peaks of Fe₃O₄ are not observed. As the treatment time increases, a shift of the Nd₂Fe₁₄B peaks towards smaller angles is observed, in relation with the hydrogenation of the hard magnetic Nd₂Fe₁₄B phase. Simultaneously, the intensity of the main peak of α-Fe increases with the treatment time, in relation with the decomposition of the Nd₂Fe₁₄BHₓ phase.

The unit cell parameters and volume of the Nd₂Fe₁₄BHₓ phase for the treated powders, and the estimated values of the hydrogen content x in the Nd₂Fe₁₄BHₓ phase are reported in Table 7. These results indicate that the Nd₂Fe₁₄B magnetic phase is hydrogenated for all the treatment times in pure ethanol, with a hydrogen content that increases with the treatment time.

The Mössbauer spectra of the samples treated in pure ethanol are shown in Fig. 15. They were fitted with the contributions of the Nd₂Fe₁₄BHₓ, Nd₁,₁Fe₄B₄ and α-Fe phases. The mean hyperfine field of the Nd₂Fe₁₄B contribution and the relative intensities of all the contributions to the Mössbauer spectra are given in Table 8.

The results show that decreasing the treatment time leads to a decrease of the proportion of the α-Fe phase, which is in agreement with XRD analysis. The relative intensity of the α-Fe phase decreases from 40 % to 18 % as the treatment time decreases from 15 h to 2 h. As a consequence, the percentage of decomposition of the Nd₂Fe₁₄BHₓ phase decreases with the treatment time from 41 % to 18 %, which is a significant decrease. These results suggest that a further reduction in the treatment time would avoid excessive degradation of the magnetic Nd₂Fe₁₄BHₓ phase.

A shown in Fig. 16, the hysteresis loops at 295 K of the 2 mm sieved powder after solvothermal treatment in pure ethanol during 2, 5 and 15 h reveal a similar behavior. The values of saturation magnetization (M<sub>S</sub>) and coercive field (μ<sub>0</sub>H<sub>C</sub>), reported in Table 9, are comparable. The values of μ<sub>0</sub>H<sub>C</sub> are very low, indicating that the powders are non-coercitive. The value of M<sub>S</sub> increases with the treatment time, in agreement with the increase of the proportion of α-Fe. This is confirmed by the fact that the calculated values of M<sub>S</sub> for each sample on the basis of the Mössbauer relative intensities of the magnetic phases (Table 8) are consistent with the measured ones.

## 4. Discussion

Both hydrothermal and solvothermal processes lead to the pulverization of the initial Nd-Fe-B magnet. For the same processing conditions (2 g of initial material, 30 ml of solvent, heating at 225 °C for 2 h, separation by centrifugation, then drying at 50 °C), the final products derived from the proposed processes are as follows.

For the hydrothermal process:

- Nd(OH)₃ which results from the reaction of the Nd-rich and Nd₂Fe₁₄BHₓ phases with water

- Nd₂Fe₁₄BHₓ which results from the hydrogenation of the Nd₂Fe₁₄B magnetic phase

- α-Fe and Fe₃O₄ which result from the reaction of the Nd₂Fe₁₄BHₓ phase with water

- Nd₁,₁Fe₄B₄ undegraded

- according to the Mössbauer analysis, about 69 % of the initial Nd₂Fe₁₄B magnetic phase is preserved, being hydrogenated into Nd₂Fe₁₄BHₓ.

For the solvothermal process:

- Nd(OH)₃ which results from the reaction of the Nd-rich and Nd₂Fe₁₄BHₓ phases with ethanol

- Nd₂Fe₁₄BHₓ which results from the hydrogenation of the Nd₂Fe₁₄B magnetic phase

- α-Fe which results from the reaction of the Nd₂Fe₁₄BHₓ phase with ethanol

- Nd₁,₁Fe₄B₄ undegraded

- according to the Mössbauer analysis, about 82 % of the initial Nd₂Fe₁₄B magnetic phase is preserved, being hydrogenated into Nd₂Fe₁₄BHₓ.

- the solvothermal treatment does not lead to the formation of Fe₃O₄

These results indicate that the solvothermal process is more effective than the hydrothermal process in preserving the integrity of the Nd₂Fe₁₄B phase. However, the degradation of the magnetic phase during the solvothermal treatment is not avoided under the experimental conditions considered. The experimental treatment conditions could be optimized to supress the degradation of the Nd₂Fe₁₄B phase. Indeed, the proportion of α-Fe resulting from the degradation of the hydrogenated Nd₂Fe₁₄B phase is reduced when the treatment time is reduced. Moreover, the treatment temperature and the solvent volume could also be optimized.

Compared to conventional recycling techniques, the solvothermal process developed in this work offers significant advantages in terms of energy efficiency and production cost. The reaction occurs at moderate temperatures (180–250 °C), which are significantly lower than those required in pyrometallurgical processes (>1200 °C), leading to reduced energy consumption [30]. Moreover, the processing time could be limited to less than 2 h, unlike multi-step hydrometallurgical methods

<figure><img src="image_15.png" /><figcaption>Fig. 13. SEM images of 2 mm sieved Nd-Fe-B powders after solvothermal treatment in pure ethanol for 2 h, 5 h and 15 h.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 14.** (a, left) X-ray diffraction patterns of the 2 mm sieved powders after solvothermal treatment in pure ethanol for the indicated times and of the initial $Nd_2Fe_{14}B$ magnet (reference). The non-indexed peaks correspond to the $Nd_2Fe_{14}BH_x$ phase. (b, right) Enlargement over the 43–47° range, showing the shift of the $Nd_2Fe_{14}B$ peaks down to low angles, in relation with the increase of the lattice parameters due to the insertion of H atoms.</figcaption></figure>

<caption>**Table 7**</caption>

Unit cell parameters, volume and estimated values of the hydrogen content x in the $Nd_2Fe_{14}BH_x$ phase for the treated powders in pure ethanol. The values for the initial magnet (reference) are given for comparison.

<table><thead><tr><th></th><th>a (nm)<br>± 0.001</th><th>c (nm)<br>± 0.001</th><th>volume (nm³)</th><th>Hydrogen<br>content x</th></tr></thead><tbody><tr><td>Reference</td><td>0.879</td><td>1.223</td><td>0.945</td><td>0</td></tr><tr><td>2 h</td><td>0.885</td><td>1.228</td><td>0.962</td><td>1.5</td></tr><tr><td>5 h</td><td>0.886</td><td>1.229</td><td>0.965</td><td>1.9</td></tr><tr><td>15 h</td><td>0.887</td><td>1.230</td><td>0.967</td><td>2.0</td></tr></tbody></table>

that can take several hours [31]. Furthermore, the use of ethanol as a solvent is both cost-effective and environmentally friendly, unlike strong acids and toxic solvents used in traditional wet chemistry approaches [32].

Several approaches are currently being evaluated for the recycling of Nd-Fe-B magnets. Xu et al. [33] investigated electrochemical processes that offer key advantages such as a significant reduction in the use of corrosive chemicals and improved selectivity in rare earth elements recovery. However, these methods remain limited by their technical complexity, high energy demand, and lack of industrial maturity.

Meanwhile, direct recycling, or “magnet-to-magnet” processing based on the reuse of magnetic material without complete elemental separation has been presented as a promising solution by Habibzadeh et al. [34], notably due to its low energy requirements and its ability to preserve magnetic properties when the magnets are not heavily oxidized. However, this method is not suitable for highly degraded or contaminated magnets. Kaya [35], on the other hand, provides a comprehensive analysis of pyrometallurgical routes, capable of processing large volumes and less sensitive to the state of the input material. These advantages come at the cost of high energy consumption, potential losses of rare earths through volatilization, and the need for subsequent hydrometallurgical purification steps.

Finally, hydrometallurgical processes, as discussed by Zhang et al. [36], achieve high rare earth recovery rates (> 90 %) even from oxidized magnets. However, they generate large volumes of liquid effluents and rely heavily on chemical reagents. Emerging alternatives such as bio-hydrometallurgy may offer more environmentally friendly options, but

<figure><img src="image_17.png" /><figcaption>**Fig. 15.** Room temperature Mössbauer spectra of the 2 mm sieved powder after solvothermal treatment in pure ethanol for the indicated times.</figcaption></figure>
<caption>**Table 8**</caption>

Mean hyperfine field of the Nd₂Fe₁₄B contribution and relative intensities of the 2 mm sieved powder after solvothermal treatment in pure ethanol for the indicated times.

<table><thead><tr><th>Sample</th><th>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub><br/>&lt;B<sub>hf</sub> &gt; (T)</th><th>Nd<sub>2</sub>Fe<sub>14</sub>BH<sub>x</sub><br/>(%)</th><th>Nd<sub>1.1</sub>Fe<sub>4</sub>B<sub>4</sub><br/>(%)</th><th>α-Fe<br/>(%)</th></tr></thead><tbody><tr><td>2 h</td><td>30.4</td><td>80</td><td>2</td><td>18</td></tr><tr><td>5 h</td><td>30.5</td><td>66</td><td>2</td><td>32</td></tr><tr><td>15 h</td><td>31.3</td><td>58</td><td>2</td><td>40</td></tr></tbody></table>

<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Hysteresis loops at 295 K of the 2 mm sieved powder after solvothermal treatment in pure ethanol for the indicated times.</figcaption></figure>

<caption>**Table 9**</caption>

Magnetic properties measured at 295 K the 2 mm sieved powder after solvothermal treatment in pure ethanol for the indicated times. The values obtained for the initial powder (reference) are given for comparison.

<table><thead><tr><th>Sample</th><th>Initial powder<br>(reference)</th><th>pure<br>ethanol,<br>2 h</th><th>pure<br>ethanol,<br>5 h</th><th>pure<br>ethanol,<br>15 h</th></tr></thead><tbody><tr><td>μ<sub>0</sub>H<sub>c</sub> (T)</td><td>0.15 ± 0.01</td><td>0.03 ±<br>0.01</td><td>0.02 ±<br>0.01</td><td>0.02 ±<br>0.01</td></tr><tr><td>M<sub>s</sub> (Am<sup>2</sup>/kg),<br>measured</td><td>140 ± 10</td><td>162 ± 10</td><td>163 ± 10</td><td>167 ± 10</td></tr><tr><td>M<sub>s</sub> (Am<sup>2</sup>/kg),<br>calculated</td><td>140</td><td>145</td><td>158</td><td>165</td></tr></tbody></table>

these are currently limited by slow treatment rates and low technological maturity. In summary, despite considerable progress, no method has proven universally applicable. The choice of an appropriate recycling route strongly depends on the condition of the used magnets, environmentally friendly, this method operates under mild conditions without the use of hazardous reagents. Furthermore, it could enable the direct reuse of recycled powders for the manufacture of high-performance sintered magnets, thus eliminating complex purification steps.

In this context, the solvothermal process developed in this study offers a promising alternative. Simple, cost-effective, and environmentally friendly, this method operates under mild conditions without the use of hazardous reagents. Furthermore, it could enable the direct reuse of recycled powders for the manufacture of high-performance sintered magnets, thus eliminating complex purification steps.

## 5. Conclusion

A hydro/solvothermal pulverization process including preliminary grinding of the initial magnet was applied to sintered Nd-Fe-B magnets. During the hydrothermal treatment, preliminary grinding in magnet fragments smaller than 2 mm leads to an acceleration of the

pulverization process of the initial magnet. XRD characterization of the powder resulting from the hydrothermal treatment shows the hydrogenation of the Nd₂Fe₁₄B hard magnetic phase into Nd₂Fe₁₄BHₓ with variable stoichiometry. On the other hand, a partial degradation of the magnetic Nd₂Fe₁₄B phase was observed, as indicated by the presence of both α-Fe and Fe₃O₄ phases.

The influence of the solvent composition on the pulverization of the hard magnetic phase has been investigated by mixing water with ethanol. Increasing the proportion of ethanol up to 100 % leads to the progressive disappearance of the Fe₃O₄ phase, showing that the degradation of the magnetic phase is reduced by replacing water with ethanol. The proportion of α-Fe in the treated powders, quantified by Mössbauer spectrometry, was found to decrease with the increase of the proportion of ethanol in the solvent, in agreement with magnetic measurements.

Ethanol thus acts as a mild reducing agent limiting the formation of the undesirable α-Fe phase and helping to reduce the formation of iron oxides. Optimizing key process parameters, such as temperature, solvent volume, and treatment time, would allow the initial magnet to be pulverized into Nd₂Fe₁₄B particles without degrading the Nd₂Fe₁₄B magnetic phase. This would eliminate the formation of α-Fe.

These results show that controlling the parameters of the hydro/solvothermal process is crucial to avoid the degradation of the hard magnetic phase, and thus increase the yield and recovery efficiency of the process.

## CRediT authorship contribution statement

**Marwa Kchaw:** Writing – review & editing, Writing – original draft, Visualization, Validation, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Virginie Nachbaur:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization. **Samuel Jouen:** Writing – review & editing, Visualization, Validation, Formal analysis. **Jean-Marie Le Breton:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Methodology, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

Funding for this work was provided by the Agence Nationale de la Recherche (ANR) in the frame of the “Nouveau procédé de Recyclage des Aïmants Permanents” - RAP project (ANR-17-CE08-0050).

## Data availability

Data will be made available on request.

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