Introduction

Rare earth sintered magnet scrap (Nd-Fe-B scrap) was generated in the production process of magnet, luminescent materials and catalysis1,2. The scrap contained approximately 30% of rare earths, including neodymium (Nd), praseodymium (Pr), dysprosium (Dy) and terbium (Tb)3, and 50%–65% of impurity Fe. The content of rare earths in the scrap was higher than in monazite (25 wt.%)4, amphibole (5 wt.%)5, and phosphorus (1.8–2.0 wt.%)6 ores. Thus, the scrap was an important resource for rare earth recovery.

Many approaches have been developed for rare earth recovery, which was generally initiated by dissolving the scrap in acids, such as sulfuric, hydrochloric and nitric acids. After dissolution, rare earths in acids were recycled in two ways. One was extraction by solvents, e.g. N,N-dioctyldiglycolamic acid7,8, together with complexes of D2EHPA9, PC88A10 and Cynaex57211. The extraction agents have high selectivity to rare earth and can effectively recycle rare earths from acids via tedious stratification. Following the dissolving of impurity Fe in scrap, the generated Fe2+/Fe3+, an active cation, reacts with the extraction reagent. This process leads to the accumulation of Fe2+/Fe3+ with repeated use of the extraction reagent, thereby reducing the efficiency of rare earth extraction and increases the cost11. The other way was precipitation of rare earths or Fe2+/Fe3+ by adjusting pH and/or adding a precipitant, such as oxalic acid12 and sodium sulfite13. Vander et al. reported that rare earths were precipitated by adding oxalic acid at the pH range of 2–2.5 after the dissolution of scrap in hydrochloric acid12, but Fe2+ reacted with oxalate acid to form precipitates of Fe2+ oxalate, which added impurity into the rare earth precipitates. Moreover, Fe3+ precipitated spontaneously with pH >2, and when the pH was increased to 4, about 99% of the dissolved Fe3+ from scrap was removed from the nitric acid solution14. During Fe3+ precipitation, it was hydrolysed rapidly to Fe3+ oxyhydroxide, in which one Fe atom coordinated with six hydrogen groups15,16,17. Therefore, the formed Fe3+ oxyhydroxide generated abundant hydrogen groups, in which rare earths could be coordinated, thereby resulting in low levels of dissolved rare earths in the solution.

When Fe3+ oxyhydroxide was converted to the well-crystallised Fe oxides, the two adjacent Fe-OH bonds on Fe3+ oxyhydroxide were dehydrated to form the Fe-O-Fe bond18,19, and the average number of coordination sites on Fe3+ oxyhydroxide decreased20,21, thereby subsequently reducing the precipitation of rare earths. He et al. reported that 90.7% of Fe3+ was eliminated as hematite when the Fe3+/Zn2+-bearing sulfuric acid solution was hydrothermally treated at 210 °C for 2 h with the addition of H2O222. Despite the effective removal of Fe3+, the Fe3+ residual was still high (nearly 1,500 mg/L)23 and needed to be removed before rare earth extraction.

In this study, an integrated acid dissolution and hematite precipitation method was developed for the effective removal of the impurity Fe from scrap. After the scrap’s dissolution in nitric acid, 99.7% of total Fe was hydrothermally converted to hematite with the addition of glucose. Meanwhile, more than 97.1% of rare earths remained. This is the first report on the effective removal of impurity Fe from a rare earth-bearing solution with high rare earth retention.

Results and Discussion

After the scrap was dissolved in the nitric, hydrochloric and sulfuric acids, the generated acidic solutions were designated as Nitric-A, Chloric-A and Sulfuric-A, respectively. The concentrations of rare earths and total Fe (including Fe2+ and Fe3+) were similar in the three acids, as shown in Fig. 1(a,b). However, in Nitric-A, Fe2+ was only 54.9 mg/L, whereas Fe3+ was about 10,038 mg/L, as shown in Fig. 1(b), thereby indicating that Fe3+ predominated in the total Fe in Nitric-A. In comparison with Nitric-A, Fe2+ was approximately 10,000 mg/L in both Chloric-A and Sulfuric-A, as shown in Fig. 1(b), thereby suggesting that Fe2+ was rich in Chloric-A and Sulfuric-A due to the lack of oxidising agent (e.g. nitrate).

Figure 1
figure 1

The concentrations of (a) rare earths, (b) total Fe and Fe2+ in the three acids.

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After hydrothermal treatment, the concentrations of rare earths were almost unchanged in the three acids, as shown in Fig. 2(a). However, in Nitric-A, the total Fe concentration decreased from 10,093 mg/L to 2,257 mg/L, corresponding to 77.6% of the total Fe removal rate, as shown in Fig. 2(b). Meanwhile, the solution pH slightly decreased from 0.38 to 0.19, as shown in Fig. 2(c), due to the generation of H+ from the hydrolysis of Fe3+. The hydrolysed Fe3+ was in irregular form with the uniform distribution of element Fe and sparse distributions of Nd, Pr and Dy (Fig. 3), demonstrating that element Fe was dominant in the generated particles. Moreover, only indicative peaks of hematite (JSCPDS 33-0664) were observed in the curve of the generated particles (Fig. 4), indicating that Fe3+ was hydrolysed in the form of well crystallised hematite. Compared with Nitric-A, the total Fe concentrations in Chloric-A and Sulfuric-A were constant, as shown in Fig. 2(b), suggesting that the oxidation and hydrolysis of Fe2+ did not occur.

Figure 2
figure 2

(a) Retention rate of Nd, Pr and Dy, (b) removal rate of total Fe after hydrothermal treatment, and (c) pH value of the three acids before and after hydrothermal treatment.

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Figure 3
figure 3

SEM image and EDS mapping of the Fe-bearing particles generated in nitric acid after scrap dissolution.

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Figure 4
figure 4

XRD pattern of the Fe-bearing particles generated in nitric acid after scrap dissolution.

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To further remove the total Fe from Nitric-A, glucose was introduced. Glucose’s efficiency to remove Fe is shown in Fig. 5. After hydrothermal treatment, the retention rates of rare earths were 98.4% for Nd, 97.5% for Pr and 97.1% for Dy, as shown in Fig. 5(a), and these rates were similar to those obtained without glucose (Fig. 2(a)). However, the removal rate of total Fe increased to 99.6% (Fig. 5(b)), much higher than that without glucose (Fig. 2(b)), indicating that glucose was important factor for total Fe removal without losing rare earths. With glucose, the total Fe was removed as hematite particles (Fig. 6(a)), similar to the product generated without glucose (Fig. 4), but with the average diameter of 80–100 nm, as shown in Fig. 7(a). During the process, the pH increased slightly from 0.24 to 0.71 (Fig. 5(c)), whereas total organic carbon dramatically decreased from 3,465 mg/L to 39.2 mg/L (Fig. 5(d)). Moreover, the nitrate concentration considerably decreased from 80.9 g/L to 8.08 g/L (Fig. 5(e)). These findings demonstrated that redox reaction occurred between glucose and nitrate, in which abundant glucose was oxidised by nitrate to CO2 and H2O with the consumption of H+.

Figure 5
figure 5

(a) Retention rates of Nd, Pr and Dy and (b) removal rate of total Fe after hydrothermal treatment with the addition of glucose; the variation of (c) pH and (d) total organic carbon before and after reaction; and (e) nitrate concentration in Nitric-A.

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Figure 6
figure 6

XRD patterns of the precipitates in the (a) nitric acid, (b) hydrochloric acid and (c) sulfuric acid after hydrothermal treatment with the addition of glucose.

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Figure 7
figure 7

SEM images and EDS mappings of the precipitates generated in the (a) nitric acid, (b) hydrochloric acid and (c) sulfuric acid after hydrothermal treatment with the addition of glucose.

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In comparison with Nitric-A, no apparent change in total Fe concentration was observed in Chloric-A and Sulfuric-A after hydrothermal treatment with glucose, and only a few spherical particles with average diameters of 3–5 μm (Fig. 7(b,c)) were precipitated. The spherical particles showed an extremely broad XRD peak at 2θ = 23.2° (Fig. 6(b,c)) that probably belonged to the carbon sphere generated from the dehydration and polymerisation of glucose, similar to the hydrothermal product of glucose reported by Mi et al.24.

This approach exhibited a high removal rate (99.6%) of total Fe in the recycling of rare metals from the rare earth-bearing scrap. This rate was higher than that obtained through other reported processes, such as the complex leaching and electrolysis process with addition of H2SO4 and MnO225, the extraction processwithtri-n-butyl phosphate(TBP)and tricaprylmethylammonium nitrate ([A336][NO3])26, the mechano-chemical treatment with HCl and (COOH)227 and selective leaching with nitric acid2, sulfuric acid28 and ascorbic acid29 (Table 1).

Table 1 Comparison of the removal rate of total Fe and the retention rate of rare earths.
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When the scrap was dissolved in hydrochloric and sulfuric acids separately, Fe2+-bearing solutions were generated. Fe2+ was stable in the two acids and not hydrolysed during the hydrothermal process, thereby resulting in low removal rates of total Fe. However, with nitric acid dissolution, Fe3+ was generated from the oxidation of impurity Fe in scrap by nitrate. The generated Fe3+ was further hydrothermally hydrolysed to hematite with generation of nitric acid when the temperature increased to 160 °C (Eq. (1))30,31. Hematite was well crystallised and had a protonated surface at pH <132 in which the net surface charge on its surface was positive and blocked the adsorption of metal ions, such as Nd, Pr and Dy.

$$2F{e}^{3+}+6N{O}_{3}^{-}+3{H}_{2}O=F{e}_{2}{O}_{3}+6HN{O}_{3},$$
(1)

As the reaction continued, Fe3+ was hydrolysed to produce a large amount of nitric acid. An increase in nitric acid concentration decreased the solution pH from 0.38 to 0.19 (Fig. 2(c)) and shifted the hydrolysis equilibrium to the left (Eq. (1))30, resulting in a decrease of Fe3+ hydrolysis. Therefore, residual total Fe at a concentration of 2,257 mg/L was left in the solution (Fig. 2(b)).

Glucose was hydrothermally oxidised by nitrate to generate levulinic acid and 5-hydroxymethylfurfural, which were further oxidised to CO2 and H2O via Eq. (2). With the oxidisation of glucose, nitrate was hydrothermally reduced to N2, and its concentration apparently decreased from 80.9 g/L to 8.08 g/L (Fig. 5(e)), thereby promoting Fe3+ hydrolysis. Moreover, H+ was also involved in the redox reaction between glucose and nitrate (Eq. (2)). Thus, the solution pH increased from 0.19 to 0.71 (Fig. 5(c)), which further accelerated the formation of hematite33. During glucose oxidisation, the generated intermediates levulinic acid and 5-hydroxymethyl furfural were electrostatic adsorbed on the positively charged surface of hematite particles, thereby inhibiting the aggregation and crystal growth of hematite particles34 and resulting in smaller hematite particle sizes than without glucose.

$$\frac{5}{3}{C}_{6}{H}_{12}{O}_{6}+8N{O}_{3}^{-}+8{H}^{+}\to 10C{O}_{2}+4{N}_{2}+14{H}_{2}O,$$
(2)

Materials and Methods

Nd-Fe-B scrap

Nd-Fe-B scrap was acquired from the calcinator of a local alumina refinery in Jilin, China. The scrap was ground to pass through a sieve with the mesh aperture of 1 mm and then dried overnight in a vacuum drying oven. The ground scrap was characterised by X-Ray fluorescence (XRF, XRF-1800, Shimadzu, Japan) and thermogravimetric analysis (TGA, 409PC, NetzschSTA, USA), and its major component was Fe (63.4%), Nd (21.6%), Pr (8.1%), Dy (3.9%) (Fig. 8(a)) and water (less than 4%, (Fig. 8(b)).

Figure 8
figure 8

(a) Composition and (b) TGA plot of the Nd-Fe-B scrap.

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Separation of Fe from Nd-Fe-B scrap

Dissolution of Nd-Fe-B scrap

The scrap was dissolved in acids as follows. Scrap (5 g) was dispersed in 250 mL of 3 M nitric acid under constantly stirring at 150 rpm overnight. A yellowish solution was generated and denoted as Nitric-A. The control experiments were also performed using 3 M hydrochloric and sulfuric acids, and the generated acidic solutions were designated as Chloric-A and Sulfuric-A, respectively.

Separation of Fe

Impurity Fe was separated from the acids via a one-step hydrothermal method. Nitric-A at 20 mL was transferred to a 50 mL Teflon kettle, hydrothermally treated at 160 °C for 10 h and cooled down to room temperature. The reddish particles generated at the bottom of kettle, were collected and vacuum-dried at 50 °C for 20 h before characterisation. The Chloric-A and Sulfuric-A were also hydrothermally treated, respectively, but no deposit was generated.

To further remove total Fe in Nitric-A, glucose was added at the glucose/total Fe molar ratio of 0.7. After agitating at 150 rpm for 5 min, a brownish suspension was generated. The suspension was hydrothermally treated according to the procedure described above. The obtained particles were collected for characterisation. Glucose was added to Chloric-A and Sulfuric-A with the same treatment as that used on Nitric-A, and the generated particles were collected and characterised separately.

Each experiment was performed thrice, and the averaged date was reported.

Characterisation

Total Fe and rare earths in the acids were analysed by inductively coupled plasma optical emission spectrometry (ICP-OES, Avio-200, Perkinelmer, USA). Fe2+ and Fe3+ in the acids were determined through the standard method35. Nitrate in the nitric acid was determined by ion chromatograph (881 pro, Metrohm, Switzerland). Total organic carbon and pH were measured by organic matter analyser (TOC 500, Shimadzu, Japan) and pH meter (S210-S, Mettler Toledo, USA). The crystallisation and morphology of the obtained particles were recorded by X-ray diffractometer (XRD, Rigaku, Rint2200, Japan) and scanning electron microscope (SEM, JSM-6400, JEOL, Japan), respectively.

Conclusions

The scrap was dissolved in nitric, hydrochloric and sulfuric acids. Among these acids, nitric acid was optimal for dissolving the scrap, and nearly all the impurity Fe in the scrap was converted into Fe3+. In the following hydrothermal treatment, 77.6% of the total Fe in nitric acid was removed as hematite particles. By adding glucose, the total Fe removal rate was further increased from 77.6% to 99.6%, whereas over 97.1% of the rare earths in nitric acid remained. In nitric acid dissolution, the generated Fe3+ was hydrothermally hydrolysed to form hematite, which was promoted by the reduction of nitrate with glucose. With this method, total Fe was efficiently separated from nitric acid while retaining a high concentration of rare earths. This method has great potential for use in recovering rare earths from Nd-Fe-B scrap.