Fe + 4 hno3 → fe(no3)3 + no + 2 h2o

Rare earths, e.g. neodymium (Nd), praseodymium (Pr) và dysprosium (Dy), are abundant in the rare earth sintered magnet scrap (Nd-Fe-B scrap), but their recycling is tedious và costly due to the high content of impurity Fe. Herein, a novel approach was developed lớn effectively recycle rare earths from the scrap via an integrated acid dissolution and hematite precipitation method. The scrap contained 63.4% sắt, 21.6% Nd, 8.1% Pr and 3.9% Dy. It was dissolved in nitric, hydrochloric and sulfuric acids, separately. Nearly all impurity Fe in the scrap was converted to Fe3+ in nitric acid but was converted lớn Fe2+ in hydrochloric & sulfuric acids. After hydrothermal treatment, the rare earths in the three acids were almost unchanged. From nitric acid, 77.6% of total sắt was removed, but total sắt was not from the hydrochloric và sulfuric acids. By adding glucose, the removal of total sắt was further increased lớn 99.7% in nitric acid, và 97% of rare earths remained. The major mechanism underlying total sắt removal in nitric acid was the hydrolysis of Fe3+ inkhổng lồ hematite, which was promoted by the consumption of nitrate during glucose oxidation. This method effectively recycled rare metals from the waste Nd-Fe-B scrap và showed great potential for industrial application.

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Rare earth sintered magnet scrap (Nd-Fe-B scrap) was generated in the production process of magnet, luminescent materials và catalysis1,2. The scrap contained approximately 30% of rare earths, including neodymium (Nd), praseodymium (Pr), dysprosium (Dy) & terbium (Tb)3, and 50%–65% of impurity Fe. The nội dung 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 sầu 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 sầu high selectivity to lớn rare earth và can effectively recycle rare earths from acids via tedious stratification. Following the dissolving of impurity sắt in scrap, the generated Fe2+/Fe3+, an active cation, reacts with the extraction reagent. This process leads to lớn the accumulation of Fe2+/Fe3+ with repeated use of the extraction reagent, thereby reducing the efficiency of rare earth extraction & 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 và 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 lớn form precipitates of Fe2+ oxalate, which added impurity inkhổng lồ the rare earth precipitates. Moreover, Fe3+ precipitated spontaneously with pH >2, and when the pH was increased to lớn 4, about 99% of the dissolved Fe3+ from scrap was removed from the nitric acid solution14. During Fe3+ precipitation, it was hydrolysed rapidly to lớn Fe3+ oxyhydroxide, in which one sắt atom coordinated with six hydroren groups15,16,17. Therefore, the formed Fe3+ oxyhydroxide generated abundant hydroren 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 khổng lồ the well-crystallised Fe oxides, the two adjacent Fe-OH bonds on Fe3+ oxyhydroxide were dehydrated khổng lồ form the Fe-O-sắt bond18,19, và 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 & needed to lớn 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 sắt from scrap. After the scrap’s dissolution in nitric acid, 99.7% of total sắt was hydrothermally converted khổng lồ hematite with the addition of glucose. Meanwhile, more than 97.1% of rare earths remained. This is the first report on the effective sầu removal of impurity Fe from a rare earth-bearing solution with high rare earth retention.

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After the scrap was dissolved in the nitric, hydrochloric and sulfuric acids, the generated acidic solutions were designated as Nitric-A, Chloric-A và 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 khổng lồ the lack of oxidising agent (e.g. nitrate).

Figure 1

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

Full form size image

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 sắt concentration decreased from 10,093 mg/L to 2,257 mg/L, corresponding lớn 77.6% of the total Fe removal rate, as shown in Fig. 2(b). Meanwhile, the solution pH slightly decreased from 0.38 lớn 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 khung with the uniform distribution of element sắt và 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 sầu of the generated particles (Fig. 4), indicating that Fe3+ was hydrolysed in the size of well crystallised hematite. Compared with Nitric-A, the total sắt concentrations in Chloric-A & Sulfuric-A were constant, as shown in Fig. 2(b), suggesting that the oxidation and hydrolysis of Fe2+ did not occur.

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

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

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