Development of Hydrometallurgical Process for Recovery of Rare Earth Metals (Nd, Pr, and Dy) from Nd-Fe-B Magnets

Abstract

Non-availability of rich primary resources of rare earth metals (REMs) and the generation of huge amounts of discarded magnets containing REMs, compelled the researchers to explore the possibilities for the recovery of REMs from discarded magnets. Therefore, the present paper reports the recovery of REMs (Nd, Pr, and Dy) from discarded Nd-Fe-B magnets. The process consists of demagnetization, pre-treatment, and hydrometallurgical processing to recover REMs as salt. Leaching studies indicate that 95.5% Nd, 99.9% Pr, and 99.9% Dy were found to be dissolved at the optimized experimental condition i.e., acid concentration 2 M H₂SO₄, temperature 75 °C, pulp density 100 g/L, and mixing time 60 min. Solvent extraction technique was tried for the selective extraction/separation of REMs and Fe. The result indicates that 99.1% (24.42 g/L) of Nd along with 90% (1.08 g/L) of Pr and total Fe were co-extracted using 35% Cyanex 272 at organic to aqueous (O/A) ratio 1/1, eq. pH 3.5 in 10 min of mixing time. It requires multistage separation and therefore, not feasible in view of economics. Thus, direct precipitation of REMs salt and iron oxide as pigment was studied using two stages of precipitation at different pH. The obtained precipitate of REMs and Fe hydroxides were dried separately to remove the moisture and further treated at elevated temperature to get pure REMs oxide and red oxide.

1. Introduction

Due to rapid industrialization and technological advancement, old electronic devices are replaced with new ones, resulting in the generation of massive amounts of discarded electronic scraps such as personal computers, mobile phones, televisions, etc. [1]. It has been reported that 53.5 million tons (Mt) of e-waste were generated in 2019 and it has been expected that 74.7 Mt of e-waste will be generated by 2030 [2]. The discarded e-waste contains a variety of metals such as base metals (Cu, Ni, Sn, Pb, etc.), rare earth metals (Nd, Pr, Dy, etc.) including hazardous metals that may cause a serious environmental threat if disposed into the landfilling without treatment, as well as the loss of valuables. Therefore, it is needed to develop a sustainable process for the extraction of metals from e-waste, which may also minimize the dependency on primary natural resources.

Various studies have been made to recover rare earth metals (REMs) using pyro-/hydro-metallurgy and hybrid processes. Onal et al. [3] reported the nitration of Nd-Fe-B magnet at 200 °C to convert the refractory magnet into soluble species of REMs. Further, calcined magnets were leached in water at room temperature to dissolve more than 95% REMs while maintaining a pulp density of 60 g/L. In addition, a magnet was roasted at 800 °C to transform the refractory magnet into soluble species of REMs. The roasted product was pressure leached in 0.6 M hydrochloric acid at 180 °C to leach 99% REMs [4]. The mechano-chemical studies have also been made with the addition of ferric sulfate to convert the refractory magnet into their sulfate salt to make the subsequent leaching step easier. The pre-treated product was further leached in water at elevated temperature and subsequently, REMs were precipitated with oxalic acid at pH 1.9 [5].

In addition, oxidative roasting followed by organic acid leaching has been used to recover the REMs from Nd-Fe-B magnet. The refractory Nd-Fe-B magnet was oxidized into the acid susceptible species by heating at 900 °C for 480 min. The oxidized product was dissolved in a mixture of malic and citric acids at 90 °C to leach more than 90% REMs [6]. Kumari et al. [1] reported the sulfuric acid leaching and precipitation process for recovery of Nd, almost ~99.99% REMs (Nd, Dy and Pr) were leached in 1 M sulfuric acid at room temperature. Further, Nd was precipitated with ~98% purity by using ammonia to get Nd salt at pH 1.65; however, minor co-precipitation of Dy and Pr also occurred. In a subsequent study, electrochemical leaching was conducted in a mixture of 0.1 M H₂SO₄ and 0.05 M H₂C₂O₄ at the current density of 20 A/dm² to enhance the leaching efficiency of REMs [7]. The obtained leach liquor was subjected to separation and purification studies to extract the REMs. Pavon et al. [8] extracted 99.9% Nd from the leach liquor of Nd-Fe-B magnet at eq. pH 0.8 to 1.5 with 0.3 M Cyanex 572. Further, 58.62% Nd, 98.46% Dy, and 85.59% Pr were extracted at pH 2 with 1 M D2EHPA and stripped with 2 M nitric acid [9].

Besides the solvent extraction, precipitation studies have also been investigated for the extraction of REMs from leach liquor of Nd-Fe-B magnet. Lee et al. [10] used sodium hydroxide to precipitate Nd as Nd(OH)₃ at pH 0.6. Rabatho et al. [11] leached 98% of Nd and 81% of Dy in 1 M HNO₃ at 25 °C in presence of 0.3 M H₂O₂, but Fe remained in the residue. Further, 81.8% Dy and 91.5% Nd were recovered by precipitation using oxalic acid (H₂C₂O₄) in a range of pH 8 at room temperature [11]. A lot of research on this topic is reported, but due to various factors it is still not applied for commercial exploitation in industries. Another aspect of this research is recycling, which will play an important role in the circular economy as well as in the conservation of natural resources. The important findings in the developed process are closed-loop, which means all the materials will be recycled in the production flow with no waste, i.e., zero waste generation.

Keeping in view of the above, the present paper reports the novel process for selective recovery of REMs oxides (Nd, Pr, and Dy) from discarded magnets of the hard disk considering the cost of the process. The magnets were separated by being dismantled, demagnetized by heat treatment, crushed to reduce the particle size, leached to dissolve the REMs, and selective extraction of REMs using solvent extraction/precipitation to produce REMs oxides. Compared to previous studies, the developed process reports less energy for demagnetization of discarded Nd-Fe-B magnets as well as a smaller number of stages to get REMs like rare earth oxides.

2. Materials and Methods

2.1. Pre-Treatment of Nd-Fe-B Magnets

At first, a discarded hard disk of CPUs was dismantled to separate the Nd-Fe-B magnets for recovery of rare earth metals including iron. Further, Nd-Fe-B magnets were demagnetized by heating at 300 °C for 3 h. The demagnetized material was crushed into small sizes (2 × 2 mm) using the mortar pestle to make the particle surface area higher, which will enable effective leaching. The process flow-sheet for the pre-treatment of Nd-Fe-B magnet is presented as Figure 1. The composition of Nd-Fe-B magnet was determined by the chemical analysis method and presented in

Table 1. Composition of metals present in Nd-Fe-B magnet.

REMs	Contents (wt %)	Non-REMs	Contents (wt %)
Nd	29.5	Fe	59.5
Dy	2.5	Ni	4.1
Pr	1.3	Co	3.1

.

2.2. Leaching Procedure

Dissolution experiments were carried out in a leaching reactor of capacity: 100 mL (Borosil, Mumbai, India) equipped with a condenser facility to avoid the loss of liquid through evaporation due to heating. Leaching studies were carried out using a hot plate fitted with a temperature controller and sensor. To optimize the process parameters for the effective dissolution of REMs, the concentration of sulfuric acid leachant and temperature were varied between 0.5 to 2.5 M and 25 to 90 °C, respectively. To study the dissolution behavior of REMs along with other metals, the samples of slurry/leach liquor were collected at regular intervals of time during the leaching experiment. The leach liquor was separated from the leached residue using filtration method. The metals present in the leached residue and leach liquor were checked to see the material balance. Satisfactory material balance was obtained for each set of leaching experiments by calculating the weight of the sample before leaching, which is equal to the amount of metals concentration in solution plus the weight of leached residue. Further, the leach liquor was processed for REMs extraction using the solvent extraction technique.

2.3. Solvent Extraction Procedure

Solvent extraction studies were carried out in a conical flask using a magnetic stirrer (Borosil, Mumbai, India) at room temperature. The equal volume of leach liquor and extractant Cyanex 272 (Figure 2) (50 mL/50 mL) were put in the conical flask for proper mixing. Ammonium hydroxide was used to maintain the eq. pH in the range of 1 to 3.5. When the solution attained the equilibrium, the metal-loaded organic extractant and raffinate were separated using a separating funnel. The metals-loaded organic extractant was stripped using dil. sulfuric acid. The concentration of metal ions in the loaded extractant and raffinate were analyzed by using ICP-OES (VISTA–PMX, CCD Simultaneous, Make: Australia). The material balance was checked by calculating the metal ions present in the strip solution obtained from loaded extractant, raffinate, and the head sample.

2.4. Precipitation Procedure

Precipitation experiments were conducted in a beaker (capacity: 100 mL) under the constant stirring speed (300 rpm) at standard temperature using ammonium hydroxide as a precipitant. The solution pH was varied in the range of 1.0 to 2.5 by the addition of ammonium hydroxide (25.96 M) for the precipitation of REMs. During the precipitation reaction, samples were collected at regular intervals of time to observe the precipitation behavior of metals at various pH and time.

2.5. Characterization and Analysis of Samples

Non-ferrous metals were analyzed using Atomic Absorption Spectrophotometer, AAS, Analyst 200 (Perkin Elmer, Waltham, MA, USA), and REMs were analyzed using Inductively Coupled Plasma-Optical Emission Spectrophotometer, ICP-OES, VISTA-PMX, CCD Simultaneous (Australia). X-ray diffraction (XRD, Bruker AXS D8 instrument) (Bruker, Wisconsin, WI, USA) and scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS, JXA-8230 Electron Probe Micro Analyzer, JEOL) (JEOL, Tokyo, Japan) were carried to analyze the phases as well as morphological appearances of the beneficiated magnet. XRD pattern shows that Nd-Fe-B magnet mainly contains the peak of Nd₂Fe₁₄B (Figure 3) and was also confirmed by EDS analysis (Figure 4). The Eh- pH of the solution was analyzed (Eh: 0.55 to 1.05 V) using Eutech pH 700 (Thermo Fisher Scientific, 7 Gul Circle, level 2M, Keppel Logistic Building, Singapore). Analytical grade (AR) ammonium hydroxide, sulfuric acid, hydrochloric acid supplied by Rankem, India, were used for experimental purposes.

3. Results and Discussion

In order to recover the REMs from discarded Nd-Fe-B magnets, leaching, solvent extraction, and precipitation experiments were carried out. At first, Nd-Fe-B magnets were demagnetized by heat treatment. The demagnetized material was crushed to get fine powder for making it suitable for the effective leaching of REMs. The leaching studies were conducted varying different experimental parameters to get suitable conditions for metals dissolution. Further, from the leach liquor, solvent extraction studies were made to extract the REMs, particularly, Nd from the leach liquor. The loaded organic was stripped to get Nd enriched solution. Finally, the precipitation studies were carried out to recover the Nd as precipitate with other elements as minor/trace. The obtained results are discussed below.

3.1. Leaching Study

For the effective dissolution of metals from waste Nd-Fe-B magnets, the various experimental parameters, i.e., acid concentration, reaction time, temperature, pulp density, etc. were studied to leach the REMs effectively from the demagnetized and crushed powder of magnets.

3.1.1. Effect of Acid Concentration

Leaching experiments were performed using different acid concentrations of leachant varied from 0.5 to 2.5 M at temp. 60 °C and mixing time 60 min maintaining pulp density 100 g/L. Results (Figure 5) show that the dissolution of REMs (Nd, Pr and Dy) was found to be increased with increasing acid concentration due to the increase in the acidic strength of the leachant. But, above the acid concentration of 2 M, the leaching efficiency of REMs remained the same. This indicates that 2 M H₂SO₄ has enough acidity to leach the REMs. Hence, 2 M H₂SO₄ was chosen as the optimum concentration of acid for the dissolution of REMs from spent Nd-Fe-B magnets. The chemical reaction involved during the leaching of REMs is presented in Equation (1):

2Nd₂Fe₁₄B + 37H₂SO₄ → 2Nd₂(SO₄)₃ + 28FeSO₄ + B₂(SO₄)₃ + 18.5H₂

(1)

3.1.2. Effect of Temperature

To optimize the temperature for leaching of REMs, experiments were performed at various experimental temperatures varying in the range of 60 to 90 °C at a pulp density of 100 g/L. Figure 6 showed that leaching efficiency of Nd, Pr, and Dy enhanced with the increase in solution temperature due to the increase in the rate of reaction. A total of ~95.5% Nd was found to be leached at 75 °C and a further increase in solution temperature above 75 °C had no significant enhancement on the dissolution of REMs. Therefore, 75 °C was chosen as the optimum temperature for leaching of REMs for further sets of leaching experiments.

3.1.3. Effect of Pulp Density

For the optimization of solid to liquid ratio, experiments were performed in a range of pulp density variations from 25 to 200 g/L at 60 °C in 60 min. It was found that leaching of REMs remains the same in the range of pulp density 25 to 100 g/L (Figure 7), but leaching efficiency of REMs was found to be decreased at the pulp density above 100 g/L. This indicates that the number of moles of metallic constitutes (Nd, Pr, and Dy) of REMs became higher than that of leachant molecules, resulting in the decrease in the leaching efficiency of REMs above the pulp density of 100 g/L. Therefore, 100 g/L pulp density was chosen as the optimum condition for the dissolution of REMs from the discarded magnet.

3.2. Solvent Extraction of Nd and Pr

Selective extraction and stripping of REMs/Fe using solvent extraction technique were tried. Experiments were carried out to recover Nd from the obtained leach liquor containing 24.45 g/L Nd, 1.2 g/L Pr, 2.49 g/L Dy, and 49.76 g/L Fe using Cyanex 272. It was found that extraction of Nd increases with the increase in eq. pH, while Pr and Fe also co-extracted (Figure 8). About 15.59 g/L Nd were extracted using 35% Cyanex 272 at eq. pH 0.75 in 10 min mixing time, while complete extraction of Nd occurred at eq. pH 3. Further increase in eq. pH had no significant effect on the extraction of Nd. Further increase in eq. pH had no significant effect on the extraction of Nd. Hence, eq. pH 3 was considered as optimum pH for the extraction of Nd from leach liquor. Further, extraction co-efficient (Kd) (Equation (2)) and separation factor (β_Nd/Pr) (Equation (3)) of Nd were calculated and found to be 1.5 and 4.42, respectively. The low value of separation factor and extraction co-efficient for Nd indicates that the separation can be possible using multistage extraction. The pH was adjusted during the solvent extraction experiment. The pH was not adjusted separately for aqueous feed, therefore, no precipitation occurred. As per the Eh-pH diagram, the REMs (Nd, Pr) were precipitated as their hydroxides at or above pH 1.5. However, at the same pH, REMs form their complexes and had a tendency to be transferred into the organic phase during solvent extraction. The solvent extraction reaction is very fast in comparison to the slow precipitation reaction. Hence, no precipitation was observed during the solvent extraction of REMs under the studied conditions.

$$Kd=\frac{Conc. of Nd in organic phase}{Conc. of Nd in aqueous phase}$$

(2)

$$\beta =\frac{Extraction coefficient of metal Nd}{Extraction coefficient of metal Pr}$$

(3)

The multistage solvent extraction process for the separation of REMs and Fe will increase the production cost. Therefore, to make the process feasible and economical, direct precipitation studies were carried out.

3.3. Recovery of REMs from Leach Liquor by Precipitation

Further, the precipitation studies were carried out to recover REMs from the obtained leach liquor 24.45 g/L Nd, 1.2 g/L Pr, 2.49 g/L Dy and 49.76 g/L Fe using the precipitation technique. The pH of the solution was varied in the range of 0.5 and 2.0 using ammonium hydroxide to precipitate selectively the REMs at room temperature, where REEs represents the rare earth elements (Equations (4) and (5)):

REE₂(SO₄)₃ + 6NH₄OH → 2REE(OH)₃ + 3(NH₄)₂SO₄

(4)

2REE(OH)₃ → REE₂O₃ + 3H₂O

(5)

The precipitation of Nd, Pr, and Dy (Figure 9) was found to be increased with the increase in pH from 0.5 to 2.0 due to the low solubility of REMs at eq. pH 2. Eh-pH diagram drawn using the HSC chemistry version 6.0, which shows that Nd exists as Nd(OH)₃ above pH 0.5 (Figure 10), while iron hydroxide formed at pH above 1.5 (Figure 11). Therefore, the pH of the solution was kept between 0.5 to 2 to avoid the co-precipitation of iron during the recovery of REMs hydroxides. The precipitated product was characterized by SEM-EDS (Figure 12), which shows that a strong peak of Nd appeared for the precipitated products. The composition of the precipitated product is shown in

Table 2. Composition (EDS) of precipitated salts of REMs.

Element	Wt %	At %
O K	26.1	62.9
S K	16.8	20.2
Pr L	6.10	1.67
Nd L	41.6	11.1
Fe K	1.02	0.70
Dy L	5.03	1.20
Co K	0.91	0.65
Ni K	0.47	0.23

. It confirmed the formation of Nd product in the precipitated sample leaving iron in the solution.

After the separation of REMs, iron (49.7 g) was completely removed by maintaining the pH above 3 using ammonium hydroxide as a precipitant at 60 °C as shown in Equations (6) and (7). It has been assumed that iron was precipitated as Fe³⁺ due to the high stability of ferric ion. The complete process flow sheet has been developed for the dissolution/extraction of REMs from waste Nd-Fe-B magnets of hard disks as shown in Figure 13. Further, comparative data for extraction of REMs from discarded Nd-Fe-B magnets are summarized in

Table 3. Literature review on the extraction of rare earth elements from discarded Nd-Fe-B magnets.

Demagnetization and Roasting Conditions	Experimental Conditions (Leaching and Precipitation/Evaporation)	Salient Results	References
Demagnetization and roasting not done	Leachant: 2 M H2SO4, Temp.: 25 °C, Time: 24 h, Evaporation: Temp.: 110 °C	~95% REEs were extracted	Yingnakorn et al. [12]
Demagnetization: 500 °C, Time: 60 min	Not done	Demagnetized	Lee et al. [10]
Demagnetization: 400 °C, Time: 120 min	Not done	Demagnetized	Feng et al. [13]
Demagnetization: 350, Time: 60 min, Roasting: 800 °C, Time: 120 min	Temp.: 180 °C, Time: 120 min; Acid concentration: 0.6 M HCl, Pulp density: 100 g/L Precipitation: Temp.: 50 °C, Initial pH: 2.2, Time: 30 min	99% of REEs were extracted	Liu et al. [4]
Demagnetization: 350 °C, Roasting: 900 °C	Leachant: 1 M Citric/Maleic Acid, Temp.: 90 °C, Pulp density: 50 g/L	99% recovery of Nd occurred	Reisdorfer et al. [6]
Roasting: 950 °C	Leachant: 1.8 M HCl + 3.5 M NH4Cl, Pulp density: 100 g/ L, Temp.: 100 °C, Time: 5 days Precipitant: Oxalic acid	90% REMs were dissolved	Hoogerstraete et al. [14]
Roasting: 950 °C	Leachant: 5 M HNO3, Pulp density: 1000 g/L, Temp.: 80 °C, Time: 72 h Precipitant: Ammonium nitrate, Eq. pH: 2, Time: 60 min, Temp.: 70 °C	Nd and Dy recovered as Nd2O3 and Dy2O3 with purity 99.6% and 99.8%, respectively	Riano and Binnemans [15]
Roasting: 750 °C	Leachant: Water, Pulp density: 20 g/L, Time:1 h, Temp.: 25 °C	95–100% REMs were extracted	Onal et al. [16]

, which reflects that high temperature is required for pre-treatment (demagnetization, roasting) of magnets prior to hydrometallurgical extraction of REMs. However, demagnetization temperature is comparatively low in the present study, which makes the process feasible from an energy point of view.

Fe₂(SO₄)₃ + 6NH₄OH → 2Fe(OH)₃ + 3(NH₄)₂SO₄

(6)

2Fe(OH)₃ → Fe₂O₃ + 3H₂O

(7)

4. Conclusions

Based on the laboratory scale studies to recover REMs from discarded Nd-Fe-B magnet of hard disks, the following conclusions have been made and are discussed below.

• It was found that the magnetic strength of Nd-Fe-B hard disk magnet was lost by heat treatment at 300 °C in 2 h.
• The complete leaching of Nd, Pr, Dy, and Fe occurred from the demagnetized magnet using 2 M H₂SO₄ at 75 °C in 60 min and pulp density 100 g/L.
• It was found that 99.1% Nd, 94% Pr, and 99.9% Fe got extracted from leach liquor using 35% Cyanex 272 at O/A ratio 1/1 in 10 min. The selective extraction of REMs can only be achieved by using multistage solvent extraction. Thus, direct precipitation studies were carried out at different eq. pH for the selective extraction of REMs and Fe.
• Further, ~93.15% Nd, ~85% Pr, and ~68% Dy were precipitated from the leach liquor of discarded magnet at pH 1.75 at room temperature in 15 min. The precipitated hydroxide of REMs was converted to their oxides by heating at 120 °C for 2 h.
• Finally, 96.5% Fe was precipitated as ferric ion through the air sparging between eq. pH 3.5 to 4 at 60 °C.