**1. Introduction**

As electronic devices play an increasingly more important role in our lives, society needs to develop strategies to recover valuable metals from end-of-life electronic products. This need is driven by critical metal supply concerns, and by environmental issues with the rapid generation of electronic waste (e-waste) worldwide [1]. The annual global production of e-waste was approximately 53.6 million metric tons (Mt) in 2019 and is expected to increase to 74 Mt by 2030 [1]. Because e-waste contains up to 69 elements from base to precious metals [1], e-waste mining has been proposed as a promising and cost-effective alternative to conventional mining [2]. However, it is estimated that less than 20% of the discarded e-waste is recycled at this time. This low recycling rate is partly attributed to the lack of proper recycling methods for most metals [1]. Therefore, there is a sustainable need to propose new and alternative strategies for e-waste recycling.

Besides precious and base metals, e-wastes also contain rare earth elements (REEs), which are increasingly used in high technology and clean energy applications [3]. REEs are present, for example, in common electronic components such as speakers, hard disk drives, and vibrators [4]. While REEs have relatively low concentrations in most bulk e-waste, the volume of end-of-life electronic devices discarded annually represents a great recycling opportunity. The presence of REEs within these devices comes mostly from neodymium (NdFeB) magnets [5]. NdFeB magnets are typically composed of up to 31 wt% REE [6]. Apart from Nd, which is the main REE, Dy, Pr, and Tb are also added to the magnet in various proportions depending on the application and quality of the magnets required [7]. The possibility of digesting NdFeB magnets into their isolated REE constituents is therefore a critical aspect of an effective recycling strategy.

While the recycling of REEs derived from NdFeB magnets in end-of-life electronic devices is not yet performed commercially, Rademaker et al. [7] emphasized that it would be technically feasible if efficient physical dismantling, separation, hydrometallurgical, and refining methods were available in the future. One challenge associated with REEs

**Citation:** Bonin, M.; Fontaine, F.-G.; Larivière, D. Comparative Studies of Digestion Techniques for the Dissolution of Neodymium-Based Magnets. *Metals* **2021**, *11*, 1149. https://doi.org/10.3390/met11081149

Academic Editor: Bernd Friedrich

Received: 30 June 2021 Accepted: 14 July 2021 Published: 21 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

is the difficulty of isolating individual REEs from their neighbouring elements in the periodic table [8]. However, most separation techniques for REEs require the dissolution of solid matrices, and NdFeB magnets are no exception. Therefore, digestion of such magnets in a rapid, efficient, and cost-effective manner is an important aspect of a REEs recycling strategy.

Numerous researchers have investigated the dissolution of REEs from NdFeB magnets via various hydrometallurgical approaches (Table 1). Most procedures used variable acid types and concentrations and a two-step sample pre-treatment prior to the acid dissolution. This sample pre-treatment was driven by the need to demagnetize and pulverize the magnet to facilitate its manipulation and dissolution. These steps are time-consuming and potentially hazardous from a chemical and health perspective. For example, as the grinding of magnets leads to small particles, they can be inhaled and deposited within the respiratory system. The exposed magnet surface is increased for smaller particulates which can facilitate the ignition of metallic powders. Hoogerstraete et al. [9] reported a fire after opening a grinding mill containing NdFeB magnets. Sometimes, the heat generated during the grinding process can create a vacuum upon cooling, rendering the opening of the grinding mill challenging. Moreover, even though rare-earth magnets are brittle because they consist of agglomerated particulates (pressed and/or sintered), they are still a hard material and are sufficiently abrasive to damage steel, which can lead to premature wear of the equipment used for e-waste recycling. Grinding NdFeB magnets also requires powerful and resistant grinding equipment [10]. There is therefore a significant interest in assessing the dissolution performances of analytical procedures that do not require grinding and demagnetization processes.

Most dissolution approaches reported in the literature (Table 1) use elevated temperatures to accelerate NdFeB dissolution, usually heated with conventional heat sources such as hotplates and ovens. Recently, Helmeczi et al. [11] reported a rapid digestion of REEs in phosphoric acid by short-wavelength focused infrared radiation (FIR). They reported excellent recoveries and reproducibilities for REEs in various certified reference materials such as OREAS-465 (carbonatite supergene REE-Nb ore), OKA-2 (rare earths and thorium ore), and REE-1 (rare earths, zirconium, and niobium ore). These results suggest that FIR digestion could be a potential alternative to other heat sources for magnet dissolution and, potentially, alleviate previously mentioned sample pretreatments.


**Table 1.** Hydrometallurgical techniques published on the recycling of REEs from NdFeB magnets.


**Table 1.** *Cont*.

<sup>a</sup> Presented as "Nd" (or mixed REEs) or "Nd, Dy" percentages.

In this article, we compared the dissolution of intact and pulverized magnets (both magnetized and demagnetized) by focused infrared digestion (FID) to other, more conventional dissolution techniques (microwave digestion, hot plate, and alkaline fusion).

## **2. Materials and Methods**

#### *2.1. Materials and Reagents*

Two different samples were used in this study. The first sample, for method comparison and optimization, was prepared from magnets obtained from hard disk drives (HDD) collected in electronic waste bins located on the main campus at Laval University (Quebec City, QC, Canada) and separated from their brackets. The second sample, for the study concerning the dissolution of unaltered magnets, consisted of cylindrical magnets of 0.751 ± 0.006 g (6 mm in diameter and 2 mm in height) manufactured as one single batch, purchased from MagnetsShop (Culver City, CA, USA). These magnets were physically cleaved into similar fractions to enable the acid to penetrate beyond the protective Ni-Cu-Ni coating.

Nanopure water (18.2 MΩ·cm at 25 ◦C) obtained using a Milli-Q system (Millipore, Bedford, MA, USA) was used to dilute the solutions. Standard solutions (1 g·L<sup>−</sup>1) of Al, B, Co, Cu, Dy, Fe, Nb, Nd, Ni, Pr, Rh (ISTD), and Tb, purchased from PlasmaCal (SCP Science, Baie d'Urfée, QC, Canada), were used to prepare calibration standards. Concentrated ACS-grade H2SO4 (Fisher, Ottawa, ON, Canada), and trace metal grade HCl and HNO3 (VWR, Mississauga, ON, Canada) were used for sample digestion. Unless stated otherwise, all the confidence intervals represent a confidence level of 95%.

#### *2.2. Demagnetisation and Grinding*

Hard disk drive magnets (237 g) were demagnetized by thermal demagnetization based on the procedure proposed by Tanvar et al. [16]; the intact magnets were heated in a muffle furnace at 350 ◦C in a porcelain crucible for 1 h. Then, the demagnetized magnets were placed in a steel dish with grinding rings and ground with an 8500 Shatterbox mill (SPEX SamplePrep, Metuchen, NJ, USA) during successive cycles lasting 70, 50, and 40 s. Between each cycle, the ground sample was sifted through a 250 μm (mesh #60) brass sieve. Pieces larger than 250 μm were reintroduced to the steel dish for the next cycle. The resulting powder had a final mass of 216 g, representing 91% of the original mass. The difference between the initial and final masses can be explained by the presence of

unground particles (13 g) that were larger than 250 μm after the last grinding cycle, and losses during grinding and sieving operations.

The particle size of the ground magnets was analyzed with 53 and 150 μm (mesh #270 and #100) sieves with a W.S. Tyler RX-29 Ro-Tap Sieve shaker (Laval Lab, Laval, QC, Canada) for 15 min, and each collected fraction was weighed. The mass loss from sieving represents less than 0.5% of the total mass used. For all the grinding and sieving steps, the sieves used were W.S. Tyler 12 inch (30.5 cm) brass sieves (Laval Lab, ibid.) with a PM4000 balance (Mettler Toledo, Mississauga, ON, Canada).
