2.4.1. Total Dissolution of the NdFeB Magnets

To evaluate the elemental composition of ground magnets, four dissolution techniques were used: (1) closed-vessel acid digestion (CVAD, SCP Science, Baie d'Urfée, QC, Canada); (2) microwave (MWD, CEM corporation, Matthews, NC, USA); (3) focused infrared digestion (FID, Coldblock, Niagara Falls, ON, Canada), and (4) alkaline fusion (AF, Malvern Panalytical, Québec, QC, Canada). Table 3 presents the conditions used for each type of dissolution technique. Note that for FID and AF, the temperature ramping was extremely rapid, and boiling and melting temperatures, respectively, were obtained in less than a minute in both cases. The choice of a mixture of concentrated HCl and HNO3 in an 8-to-2 ratio for acid digestion was based on the operating conditions reported by Berghof [20] for the dissolution of permanent magnets. The choice of 3 M of HNO3 as a dissolution medium for alkaline fusion was based on the procedure published by Milliard et al. [21] concerning the dissolution of refractory species in environmental matrices.

#### 2.4.2. Leaching Experiments

Experiments on the leaching of elements from the NdFeB powder were conducted with 1 g of ground magnetic material by FID using the design of experiments (DOE) performed using JMP Pro (Version 14.3, SAS Institute, Cary, NC, USA). The parameters used for this optimization process on the FID are presented in Table 4.


**Table 3.** Operating conditions used for the complete dissolution of ground magnets.

<sup>a</sup> Fusion was performed using a 20:1 (~2 g) ratio of LiT/LiB/LiBr 49.5/49.5/1% flux to sample (Malvern Panalytical, Québec, QC, Canada), which was subsequently dissolved under the acidic conditions presented.

> **Table 4.** Values used during DOE optimization process of the focused infrared digestion of ground magnets.


After optimization, the following optimal leaching methodology was used for FID: either 20 mL (1.6 N) or 10 mL (3.2 N) of HCl or H2SO4, per gram of ground NdFeB, is necessary to quantitatively (>99.9%) solubilize the rare earth elements in 5 min with a lamp power of 100%. For the trials on unaltered magnets, 7.5 mL of 3.2 N of H2SO4 was used for about 0.75 g of magnetic material.
