2.2.1. Demagnetization

Nd magnets were obtained by disassembling the actuators in various hard drives. After loading in a ThermoScientific Lindberg/Blue M box furnace, the furnace was programmed to heat up to 500 ◦C at 5 ◦C per minute under ambient air. This was done in order to cause demagnetization and weaken the adhesive used to hold the magne<sup>t</sup> on the steel plate. The temperature was held at 500 ◦C for 60 min. Afterwards, the demagnetized magnets were air-cooled and sorted from the steel plates.

## 2.2.2. Comminution

Comminution was done to liberate the NdFeB part of the demagnetized sample so that ground mass could be easily leached with the lixiviant in view of the increased surface area of the material. Using a disc pulverizer from Bico Inc. (Burbank, CA, USA), the samples were initially comminuted

with a set of 3.1 mm. That product was further comminuted with a set of 0.3 mm. A sieve analysis was performed to determine the size distribution of the comminuted material and is discussed later.

#### *2.3. Hydrometallurgical Processing*

REEs were leached from the comminuted sample in a 2M H2SO4 acid solution with sample to solution ratio of 1g:10mL [4]. This process was done under a fume hood for 2 h with the acid solution being added in small amounts because of the aggressiveness of the reaction as shown in Equation (1).

$$\rm Nd\_2Fe\_{14}B + 45H^+ + 3H\_2O \to 2Nd^{3+} + 14Fe^{3+} + BO\_3^{3-} + 25.5H\_2 \tag{1}$$

Filtration was performed after leaching to separate the REE-acidic pregnan<sup>t</sup> solution from the residue. NH4OH was then added to the filtrate in a ratio of 1 mL NH4OH to 20 mL rare earth rich pregnan<sup>t</sup> solution to adjust the pH to 1.2, as shown in Equation (2).

$$\mathrm{NH\_4OH} + \mathrm{Nd^{3+}} + 2\mathrm{SO\_4^{2-}} + \mathrm{H^+} + 2\mathrm{H\_2O} \rightarrow \mathrm{(NH\_4)Nd(SO\_4)\_2(H\_2O)\_3} \tag{2}$$

Upon addition, the solution was stirred at 90 rpm to completely dissolve back into solution anything that formed when the NH4OH was added. After that, the solution was allowed to sit for 12 h so the REE-rich precipitate could fully form and settle. Finally, filtration was performed so that the REE-rich residue could be collected and allowed to air dry. The dry REE-rich precipitate is then added into a mixture of NH4F·HF and deionized water in a ratio of 1g:1.5g:10g and stirred for 45 min to enhance the formation of REF. The residue obtained after filtration was air dried and analyzed for REF content. The process flow sheet is shown in Figure 1. Various stages are also identified in the figure to facilitate further discussion later in this paper.

**Figure 1.** Flow sheet of Nd magne<sup>t</sup> recycling to produce REF.

Recovery of Fe in the form of ammonium jarosite, as shown in Equation (3), can also be done by the addition of more NH4OH into the Fe-rich solution (as shown in Stage 3). With Equation (3) representing the reaction, the pH of the solution increased until a pH of 2 where maximum ammonium jarosite precipitated [17].

$$\mathrm{NH\_4OH} + 3\mathrm{Fe^{3+}} + 2\mathrm{SO\_4}^{2-} + 5\mathrm{H\_2O} \rightarrow \mathrm{(NH\_4)Fe\_3(OH)\_6(SO\_4)\_2} + 5\mathrm{H^+} \tag{3}$$

#### *2.4. Modelling of REF Recovery*

Stage 1 and Stage 2 of the flow sheet in Figure 1 were identified as less critical compared to Stage 3 of the process. Hence, it was decided to optimize Stage 3 in the present work. To optimize the REF recovery, Response Surface Methodology (RSM) was pragmatic in the analysis of the experiments. RSM is a mathematical and statistical technique that employs empirical models to fit the experimental data with reference to the Design of Experiments (DOE). With the process responses not following a linear model, the Box-Behnken design was employed for designing the experimental matrix to delineate the response surfaces generated by the condition variables [39–44]. This design selects points in the experimental domain for a three-level factorial arrangemen<sup>t</sup> in such a way that permits proficient approximation of the first and second order coe fficients for the mathematical model [42]. The user identifies a high level and a low level of each condition variable and the mid-point is automatically identified for the point selection. Several experiments (usually 3 or 5) are conducted at the mid-point of all variables to estimate the inherent variability associated with the experimental technique.

In this work, the objectives were to maximize the amount of REF recovered from the precipitate along with their purity. This RSM was used at the point in the experiment where the REE-rich precipitate is added to NH4F·HF to produce the REF in Stage 3. The experiments were performed as per the RSM design of experiments developed using the statistical software Design Expert 9 procured from Stat-Ease Inc., Minneapolis, MN, USA [43]. During the scoping tests, several condition variables were identified that a ffected the amount of REF recovered from the precipitate as well as its purity. To limit the number of experiments, the most important three condition variables were chosen, namely, the amount of deionized water (mL), amount of NH4F·HF (g) and degree of stirring (min). Of course, the identified responses were the amount of REF recovered from the precipitate and the purity of the REF. The selected points in the experimental domain and the responses obtained are discussed later, along with the response surfaces, model equations, optimization and the interaction of the condition variables.

#### *2.5. Material Characterization*

After demagnification, the feed materials as well as intermediate and final products from this study were characterized using Scanning Electron Microscopy with Energy Dispersive X-ray (SEM-EDX), X-ray Di ffractometry (XRD), Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES), and a thermal analyzer with thermogravimetric (TG) and di fferential scanning calorimetry (DSC) capabilities.

## 2.5.1. SEM/EDX

The SEM-EDX analyzer was employed to determine the chemical compositions of all phases in the demagnetized material. The SEM-EDX system uses a TESCAN TIMA with a tungsten filament and an EDAX Z2 analyzer (TESCAN ORSAY HOLDING, a.s., Kohoutovice, Czech Republic). Cross-sectioned sides of a representative sample were hand-separated and cold-mounted in epoxy using molds approximately 25 mm in diameter and 10 mm in thickness. Resulting mounts were ground and polished to a smooth finish and then conductively coated with carbon to obtain SEM images by backscattered electron (BSE) detection. EDX analyses helped determine the chemical compositions of all products.
