**3. Results and Discussion**

*3.1. Particle Size Distribution of the Ground Demagnetized NdFeB Magnets*

The particle size plays an important role in the dissolution process of ground magnets. The particle size distribution of ground NdFeB magnets obtained from HDD are presented in Table 5.

**Table 5.** Mass percentage composition as a function of the particle size of the ground NdFeB magnet powder obtained from HDD.


The particle size distribution shows that most of the ground magnet powder (76%) was reduced to particles smaller than 151 μm. The three fractions were pooled together and used for the remaining trials.

#### *3.2. Elemental Composition of Ground Demagnetized NdFeB Magnets*

To properly assess various leaching approaches, the elemental composition of the powdered magnet must be known exactly. Four total dissolution approaches were compared: closed-vessel acid digestion (CVAD), microwave digestion (MW), focused infrared digestion (FID), and alkaline fusion (AF). The elemental composition was determined by ICP-OES. The instrument response was validated by analyzing Al, Cu, Dy, Fe, Nb, Nd, Ni,

and Pr in the certified reference material REE-1 (CanmetMINING, Ottawa, ON, Canada). The REE-1 material was prepared for ICP-OES analysis by AF using 0.4 g of the material with 1.2 g of flux. Other parameters were as presented in Table 3. Table 6 presents the elemental composition of powdered NdFeB magnets determined by ICP-OES based on the total dissolution technique used.

**Table 6.** Elemental composition (%) of the powdered NdFeB sample determined by ICP-OES based on the total digestion used.


N.M.—not measurable, due to the addition of borate flux. N.D.—not determined.

The results obtained from four distinct digestion approaches for ground magnets led to a relatively consistent elemental composition except for Nb. This suggests that the sample used was sufficiently homogeneous to be used for leaching comparisons, which is discussed in the next section. As the digestates were filtered prior to ICP-OES analysis, a black residue was noticeable on the filters used for the samples digested by CVAD and FID. Pre-weighed filters were used to determine the mass fraction of the undissolved magnet powder. After filtration, they were washed with water, dried, and weighed using an analytical balance. The filters were then subjected to elemental analysis by XRF (Figure 1). It was determined that the residue represents 0.40 ± 0.08% (σ = 1 SD) of the ground magnet mass used initially.

**Figure 1.** XRF spectrum of the residue present after FID.

The XRF analysis showed that the residue is composed of Nb and Fe. Nb is present in some NdFeB magnets to increase resistance to corrosion and enhance some magnetic properties [22,23]. Its presence in our powdered sample is not unexpected, because of the refractory nature of niobium oxides [24]. We suspect that the presence of Fe is the result of incomplete washings of the filter surface. Except for traces of Nd and Fe, all four digestion techniques were effective to completely dissolve ground demagnetized magnets.

#### *3.3. Leaching of Powdered and Demagnetized Magnets*

While complete dissolution of the magnets is mandatory for comparing the digestion techniques and determining the degree of leaching, it is not necessary from the perspective of developing a hydrometallurgical strategy for the recycling of rare earth elements in magnets. Helmeczi et al. [11] recently reported the rapid dissolution of REEs in mineral and environmental matrices using FID. As FID also demonstrated equivalent dissolution performances to other digestion techniques for the complete digestion, this approach was investigated for leaching purposes through a design of experiments (DOE) approach.

### 3.3.1. Design of Experiments

While the previous digestion procedure, which used a mixture of HCl and HNO3, was certainly effective to completely dissolve powdered magnets, total digestion is not necessary for the recycling of REEs. The cost of nitric acid and its oxidative characteristics have limited its use in the hydrometallurgical separation of REEs in favor of HCl and H2SO4 [25]. Thus, an investigation of the leaching of ground demagnetized magnets was performed in either HCl or H2SO4. Five parameters were assessed through a DOE approach (Table 7). A first assessment of the DOE results showed that no parameter had a statistically significant impact on the dissolution. However, by removing either the acid type, dissolution time, or lamp power factors, the results showed that the concentration of acid, the acid-to-sample ratio, and their two-factor interaction were the only significant factors in the leaching of REEs. This suggests that the number of moles of acid was the main parameter for this leaching optimization. Experimentally, it was also determined that the low value used for the dissolution time (i.e., 300 s) set in the DOE was exceedingly sufficient to completely dissolve powdered magnets.


**Table 7.** Statistical importance of the DOE factors according to a 5- and 4-factor analysis.

<sup>a</sup> A log worth value of minimum 2 is required for a factor to be considered significant. <sup>b</sup> Dissolution time (Time) factor removed.

To adequately determine the quantity of acid required per gram of powdered magnet to completely dissolve REEs, tests were performed in HCl and H2SO4 by varying the number of moles of acid used per gram of magnet. The results are presented for Nd as a representative of the REEs in Figure 2.

**Figure 2.** Nd dissolution yield (%) as a function of the amount of acid (HCl or H2SO4, in mmol) for 1 g of powdered magnet after a digestion by FID (300 s).

This test highlights the fact that the quantity of acid available to react with the powder is the limiting factor in the dissolution of REEs using FID. The required quantity of H2SO4 (16 mmol/g of ground magnet) is exactly half of the needed HCl (32 mmol/g), which is consistent with the balanced redox formulas (Equations (1) and (2)) for both acids. As two atoms of REEs are being oxidized to a trivalent oxidation state, six H<sup>+</sup> cations need to be reduced, which can be found in either three molecules of H2SO4 or six of HCl, hence the need for twice as much HCl as H2SO4. A similar logic can be applied to iron, one of the main components of the magnet. Based on the composition of the powdered magnet, it was calculated (from composition obtained by FID) that approximately 34 mmol of H<sup>+</sup> was required to oxidize and dissolve the magnet, which is coherent with the quantity found experimentally.

$$2\text{ REE}\_{\text{(metalic)}} + 6\text{ HCl}\_{\text{(aq)}} \rightarrow 2\text{ REE}^{3+} \text{(aq)} + 6\text{ Cl}^- \text{(aq)} + 3\text{ H}\_2\text{(g)} + \Delta,\tag{1}$$

$$2\text{ REE}\_{\text{(metal)}} + 3\text{ H}\_2\text{SO}\_{4\text{(aq)}} \rightarrow 2\text{ REE}^{3+} \text{(aq)} + 3\text{ SO}\_4^{2-} \text{(aq)} + 3\text{ H}\_2\text{(g)} + \Delta,\tag{2}$$

Based on these observations, the following final leaching methodology can be proposed for FID: either 20 mL of 1.6 N or 10 mL of 3.2 N of HCl or H2SO4 per gram of NdFeB powder will be sufficient to completely solubilize the rare earths.

As stated previously, FID totally leached REEs from magnets, even with the shortest dissolution time tested (5 min). This suggests that the power output associated with the FID is more than sufficient to enable the complete dissolution of REEs. Thus, the Nd dissolution yield was monitored for dissolution times ranging from 60 to 300 s to determine how long, at full lamp power, it would take to achieve complete dissolution. No statistical differences in Nd dissolution yield were noted in the time range selected, except for the trial that ran for only 60 s (94%).

#### 3.3.2. Leaching Performance Comparisons (CVAD, FID)

To determine whether there is a significant advantage in using FID for the leaching of REEs from magnets, the FID approach was compared to closed-vessel acid digestion (CVAD). The digestion of powdered NdFeB magnet was easily achieved in 300 s. As with FID, shorter dissolution times (60 to 300 s) were also investigated with CVAD. However, this technique required longer times—more than 120 s—to achieve complete digestion and quantitative dissolution (84% and 95% for 60 and 120 s, respectively).

Comparisons of the dissolution yields for FID and CVAD are presented in Figure 3. Statistically, both techniques yielded similar results when 10 mL of acid was used. Samples prepared using 20 mL of acid per gram showed lower yields with CVAD than FID. This difference could be explained by the short digestion time used (300 s), which does not allow the larger volume of the solution (20 mL) to be properly heated; this highlights the importance of heat in the rapidity of the digestion process. When performed at room temperature and a contact time of 300 s, the dissolution yields of REEs (i.e., Nd, Pr, Dy, and Tb) reached 80–90% with H2SO4 and 30–45% with HCl. As dissolution at room temperature with H2SO4 was more effective, further trials used H2SO4. However, it should be noted that HCl is also a very suitable acid for such purposes and could be a judicious choice if the subsequent separation scheme is performed in this media.

**Figure 3.** Comparison of FID and CVAD for the digestion of 1 g of magnet powder during 300 s with 10 mL of 3.2 N or 20 mL of 1.6 N of HCl or H2SO4.

3.3.3. Magnet Pieces Leaching

To test the effectiveness of FID vs. conventional CVAD on magnet pieces, cylindrical magnets were cut in half and put in with 7.5 mL of 3.2 N of H2SO4 for 15 min under various conditions (Table 8). The volume of acid and the molarity were adjusted to correlate with the number of moles required for the complete dissolution of the magnet mass used.

**Table 8.** Dissolved magnet mass after 15 min of contact time with 3.2 N of H2SO4 (*n* = 3).


<sup>a</sup> The coating of the magnets is not digested with the proposed methods and represents 1.9 ± 0.2% of the total mass of the magnets. <sup>b</sup> Agitation was performed by the magnets pieces themselves interacting with the magnet field of a stirring plate.

As observed with the ground magnet, once the optimal quantity of acid per gram of magnet is used, the temperature of the acid is a critical parameter for the rapid dissolution of unaltered magnets. These results support the idea that FID, as a powerful heating source, could be an effective method to rapidly dissolve NdFeB magnets for hydrometallurgical recycling.

To determine the required time for the complete dissolution of the magnet pieces by FID and CVAD, the amount of magnet digested was monitored as a function of digestion time (10 to 35 min, Figure 4). The same parameters as previously described were used, but with 15 mL of 3.2 N of H2SO4 to avoid the complete evaporation of the acid due to an incomplete condensation of the acidic vapor inside the part of the digestion vessel surrounded by the Peltier cooling block. The temperature of H2SO4 was monitored using a thermocouple inserted into the solution while the lamps were on. It showed a working temperature of 103 ◦C after 1 min, which is close to the expected boiling point ofa2M solution of H2SO4 (102 ◦C [26]).

**Figure 4.** Undissolved magnet mass percentage after digestion in H2SO4 by FID and CVAD.

When the remaining magnet mass in the digestion vessel was approximately 10% of its initial value, the efficiency of the digestion tended to decrease for the FID approach. This is likely due to the round shape of the magnets used, which resulted in a smaller contact surface as the digestion progressed. Nonetheless, this experiment demonstrates that dissolution of magnet pieces is faster by FID than by CVAD.

#### **4. Conclusions**

The results obtained in this study indicate that focused infrared digestion (FID) could be used as an effective method for the recycling of rare earth magnets, either for the complete dissolution of ground samples (apart from refractory niobium oxides), or for the quantitative dissolution of REEs on magnetized and coarse magnet pieces. The absence of magnetic constituents in the FID unit enables the digestion of the magnets without any prior demagnetization process. The FID method is safer than others because, due to its rapid digestion time, FID does not require crushing of the magnets into fine powders. Skipping the grinding step also facilitates the separation of the undigested Ni-Cu-Ni coating and could potentially help in hydrometallurgical separation later in the recycling process.

**Author Contributions:** Conceptualization, M.B. and D.L.; methodology, M.B. and D.L.; formal analysis, M.B.; writing—original draft preparation, M.B.; writing—review and editing, F.-G.F. and D.L.; supervision, F.-G.F. and D.L.; project administration, D.L.; funding acquisition, F.-G.F. and D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This researchwas funded by FRQNT—Team Research Grant (2021 competition)—"Développement d'une filière hydrométallurgique de recyclage des métaux et des terres rares à partir des déchets de téléphones portables et de tablettes électroniques", grant number 284426.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in this article.

**Acknowledgments:** The authors want to thank Vicky Dodier for her help with the grinding and sieving of the samples and Christa Bedwin for her editorial comments on the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
