Elsevier

Hydrometallurgy

Volume 203, August 2021, 105626
Hydrometallurgy

Selective separation of rare earths from spent Nd-Fe-B magnets using two-stage ammonium sulfate roasting followed by water leaching

https://doi.org/10.1016/j.hydromet.2021.105626Get rights and content

Highlights

  • A flow sheet for selective separation of REEs from spent NdFeB magnets was proposed.

  • 96% of REEs can be selectively separated by ammonium sulfate roasting followed by water leaching.

  • Two-stage roasting process can significantly reduce the amount of ammonium sulfate.

  • The decomposition of NH4RE(SO4)2 and the reaction of Fe2(SO4)3 and RE2O3 together improve the extraction of REEs.

Abstract

The research presents an effective approach to achieve the selective separation of rare earth elements (REEs) from waste Nd-Fe-B magnets. Investigations show that the use of a two-stage roasting process can significantly reduce the amount of ammonium sulfate required and improve the separation efficiencies of REEs. During the first low-temperature roasting stage, almost 80% of REEs can be transformed into RE2(SO4)3 or NH4RE(SO4)2 at 400 °C within 1 h, whilst simultaneously iron and other impurities are converted into insoluble metal ammonium sulfates. These intermediate products can then be subjected to a further roasting procedure at 750 °C for 2 h, leading to an extraction of REEs of up to 96%. In contrast, the extraction of the related impurities: Fe, Al, Cu and Co is only 0.008%, 0.27%, 1.64% and 3.48%, respectively. Through the analysis and characterization of the calcine and leach residue, it was found that the decomposition of NH4RE(SO4)2 and the reaction of Fe2(SO4)3 and RE2O3 together improve the extraction of REEs during the second roasting stage. After separation of REEs, the main phase present in the leach residue is hematite, which can be recycled as a feedstock material for iron or steelmaking processes. Moreover, waste gases from this process – like NH3, SO2, and SO3 - can also be recovered and reused in the preparation of (NH4)2SO4, which significantly reduces the costs of the recycling operations. Overall, this newly developed process has considerable environmental and economic advantages for the recovery of valuable metals from waste Nd-Fe-B magnets.

Introduction

Neodymium - iron - boron (Nd2Fe14B, or Nd-Fe-B for short) permanent magnets - also known as the ‘King of Magnets’ due to their excellent magnetic properties (Li et al., 2016) - typically contain ~1% boron, 60% iron and about 30% rare earth elements (REEs) of which, neodymium accounts for about 90%. The remainder of the REEs normally comprises of praseodymium, dysprosium and terbium that are doped with a small amount of cobalt, aluminum, copper or other elements depending on their intended area of application (Okabe et al., 2003; Zakotnik et al., 2008). The maximum lifetime of a Nd-Fe-B permanent magnet relies on its application and can range from just 2–3 years in the case of consumer electronics to 20–30 years for wind turbines (Du and Graedel, 2011; Yang et al., 2017; Tunsu, 2018). This has led to significant and increasing quantities of Nd-Fe-B magnet waste being generated both as a result of production - approx. 20–30% of the alloy is turned to scrap during processing - and disposal of permanent magnets at the end of their useful service life (Horikawa et al., 2006; Kumari et al., 2018). Annually, almost 22% (> 26,000 tons) of all REEs produced worldwide are utilized in the production of Nd-Fe-B magnets. As a result of the increasing scarcity of rare earth resources, separation and recovery of rare earth elements from Nd-Fe-B scrap materials is becoming increasingly attractive. In addition, the move towards higher levels of recycling not only helps to alleviate the associated environmental burden, but also provides for the reuse of valuable metal resources such as REEs, Co, Fe, Cu and Ti (Wan et al., 2020; Wan et al., 2021). Recently, methods for the efficient recycling of Nd-Fe-B magnet waste have become more prevalent in both academia and industry worldwide. Although serveral pyrometallurgical processes like chemical vapor transport (Murase et al., 1995), glass slag method (Saito et al., 2003), selective chlorination (Itoh et al., 2008), direct reduction-slag‑gold melting method (Deng et al., 2015) and liquid metal extraction (Sun et al., 2015) have been developed to separate REEs from spent Nd-Fe-B magnets, many of these methods are highly energy intensive and only provide low REEs recovery efficiency, often accompanied by secondary pollution. Consequently, recent research has focused on hydrometallurgical methods that allow for high recovery of REEs with less environmental impact. The most widely studied hydrometallurgical process typically involves the use of strong acid (H2SO4, HCl or HNO3) based leaching of spent Nd-Fe-B magnets, purification, sulfate double salt or oxalate precipitation and subsequent extraction of the target metals (e.g. REEs, Co, B) (Lyman and Palmer, 1993; Beltrami et al., 2015; Battsengel et al., 2018). Although almost all the REEs can be extracted with a strong acid leaching system, the main drawback to these methods is that the presence of large amounts of dissolved iron and other impurities make the subsequent purification more complex, resulting in increased costs and low-value products such as Fe(OH)3, and Fe/Co containing REEs oxalates (Bandara et al., 2016; Venkatesan et al., 2018).

Therefore, alternative processes for selective leaching have been developed in order to overcome the disadvantages of complete leaching. For example, the oxidative roasting - low concentration hydrochloric acid leaching process has been widely used in industry such as at the Youli Smelter in China (Koyama et al., 2009; Lee et al., 1998; Hoogerstraete et al., 2014; Kumari et al., 2018). In this process, accurate control of the roasting is essential as insufficient oxidation of the Nd2Fe14B causes some of the iron to dissolve into the solution as ferrous ion, the pH stability of which subsequently inhibits the selective leaching of REEs. On the other hand, excessive roasting leads to the formation of partially insoluble ferrites like NdFeO3 which also challenge the high recovery efficiencies of REEs (Lee et al., 1998; Önal et al., 2015; Yoon et al., 2015; Firdaus et al., 2018; Liu et al., 2019). Consequently, it is very difficult to totally avoid the dissolution of ferrous or ferric irons in industrial production by adoption of a low concentration hydrochloric acid leaching process. In order to decrease the adverse effect of iron on the recovery of REEs, solvent extraction with an extractant like tri-n-octylamine (N235) has usually been adopted in order to remove iron from the leachate, but this makes the recovery process more complex. Additionally, data from industrial production show that the amount of residue from this type of process is large, and the REEs content in the residues is relatively high. According to statistics, the output of iron slag is 2 tons per ton of RE2O3, and the content of rare earth within the iron slag is up to 1%. Hoogerstraete et al. (2014) found that almost all the iron remains in the leach residue if the molar ratio of HCl and REEs is optimized. Nevertheless, the complete separation of REEs from iron could only be achieved after 15 h with a n(HCl)/n(REEs) ratio of 3.5 at 80 °C, which leads to a significantly low production efficiency. Koyama et al. (2009) and Liu et al. (2020) also both suggested that using high pressure leaching process to separate REEs from Nd-Fe-B waste, and over 99% REEs can be extracted. However, the high capital and operational costs of pressure acid leaching is not conducive to industrial production.

As a result, in order to increase REEs recovery, some novel methods have been designed to simplify the REEs recovery process by preferentially transforming REEs from its poorly soluble oxides to soluble metal salts. A related technology, sulfidation - roasting - water leaching for the preferential recovery of REEs from Nd-Fe-B scraps has been developed and is based on roasting between 750 and 800 °C with sulfuric acid to transform rare earth elements - or their oxides - into soluble RE2(SO4)3, whereas the impurities become insoluble metal oxides (Önal et al., 2015; Önal et al., 2017a). Following roasting, a water leaching treatment can be undertaken, which results in 95–100% of the REEs being extracted from the magnet scrap. Although this process has been demonstrated to be efficient, potentially environmentally friendly and applicable to all types of magnets, the roasting temperature and required chemical inputs are both relatively high. Since most nitrates readily decompose at low temperatures, Önal et al. (2017b) also developed a similar nitrate roasting - water leaching process to recover REEs. In this case, a similar level (95–100%) of REEs (Nd, Dy, Pr and Gd) can be extracted after roasting at 200 °C for 2 h, however, the amount of nitric acid needed is still high due to conversion of all the metals (both target and impurities) to their salts by nitric acid. The comparison of different metallurgical methods mentioned above for REEs recovery from Nd-Fe-B magnet is presented in Table 1.

In our previous work, it was found that Fe2(SO4)3 can also induce the sulfate transformation of rare earth oxides, which significantly reduces the amount of sulfuric acid required (0.44 g H2SO4/g Nd-Fe-B scraps). In this case, almost 98% of REE oxides are transformed to their respective soluble sulfates by the combined action of H2SO4 and Fe2(SO4)3 during roasting (Wang et al., 2020). When the strong corrosion effects of inorganic acid on the process infrastructure is taken into account, sulfate-based roasting processes have been further developed and improved by adoption of ammonium sulfate roasting as a less aggressive alternative. For example, the ammonium sulfate roasting process has been widely utilized to achieve separation of Ni-Fe in laterite nickel ores and separation of Zn-Fe in zinc leach residues due to its low corrosivity, ease of operation and more environmental aspects (Liu et al., 2015; Zhang et al., 2017). In order to further reduce production costs and improve the extraction of REEs, a novel ammonium sulfate roasting process concentrating on the different migration and transformation behaviors of REEs and impurities from spent Nd-Fe-B magnets are investigated in this study.

Section snippets

Experimental procedure

A company in Ganzhou working in the recycling business of Nd-Fe-B magnets scraps provided the input materials used in this research. The spent Nd-Fe-B magnets, originally manufactured by sintering, were cleaned prior to demagnetization at 500 °C for 2 h under atmospheric air, before being sequentially comminuted by a jaw crusher (MN 931; Wedage, Germany) and then a ring mill (Pulverisette 9; Fritsch, Germany). After 60–75 s of ring milling, the particles were processed by a sieving machine (AS

Materials and characterization

The REEs, Fe, B and other main element contents of the pretreated Nd-Fe-B scrap are shown in Table 3. Results indicate that REEs - including Nd, Ce, Pr and Gd - within the pretreated Nd-Fe-B scraps accounted for 21.6% of total amount of elements, whereas the content of iron in pretreated Nd-Fe-B scraps was almost 50%. The main mineral phases of REEs and Fe within the samples were identified by XRD. As shown in Fig. 1, the main mineral phases present in the Nd-Fe-B scraps were comprised of Fe2O3

Conclusions

  • (1)

    Use of a two-stage ammonium sulfate roasting process can significantly reduce the ammonium sulfate consumption and improve and improve the selective extraction of REEs. Leaching of REEs can reach 96% under optimal conditions, whereas the leaching of impurities like Fe, Al, Cu and Co are substantially less.

  • (2)

    During the first roasting stage, REEs are either predominantly converted into soluble REE sulfates or partly into insoluble REEs ammonium sulfates, resulting in 78% extraction of REE materials.

Author statement

We hereby submit an original research article entitled “Selective separation of rare earths from spent Nd-Fe-B magnets using two-stage ammonium sulfate roasting followed by water leaching” for reconsideration to be published in Hydrometallurgy. We confirm that all the figures and tables are the original work and have not been published elsewhere.

In this paper, we proposed an innovative route for selective rare earth separation via two-stage ammonium roasting and water leaching. > 97% of REEs

Declaration of Competing Interest

We hereby submit an original research article entitled “Selective separation of rare earths from spent Nd-Fe-B magnets using two-stage ammonium sulfate roasting followed by water leaching” for consideration to be published in Hydrometallurgy. We confirm that all the figures and tables are the original work and have not been published elsewhere.

All of the authors have approved the contents of this paper and have agreed to the submission policies of Hydrometallurgy. All authors have made

Acknowledgements

This paper has been financially supported by Natural Science Foundation of Jiangxi Province (No. 20202BABL204030), Science and Technology Project of the Education Department of Jiangxi Province, China Postdoctoral Science Foundation (No. 2019 M662269), Jiangxi Postdoctoral Science Foundation (No. 2019 KY07), Postdoctoral Innovative Talent Support Program of Shandong Province and Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology (No. JXUSTQJYX2019006).

References (38)

  • M. Sun et al.

    On the production of Mg-Nd master alloy from NdFeB magnet scraps

    J. Mater. Proc.

    (2015)
  • Tunsu

    Hydrometallurgy in the recycling of spent NdFeB permanent magnets

    Waste Electric. Electron. Equipm. Recycl.

    (2018)
  • P. Venkatesan et al.

    An environmentally friendly electro-oxidative approach to recover valuable elements from NdFeB magnet waste

    Sep. Purif. Technol.

    (2018)
  • X. Wan et al.

    A potential industrial waste–waste co-treatment process of utilizing waste SO2 gas and residue heat to recover Co, Ni, and cu from copper smelting slag

    J. Hazard. Mater.

    (2021)
  • M. Zakotnik et al.

    Possible methods of recycling NdFeB-type sintered magnets using the HD/degassing process

    J. Alloys Compd.

    (2008)
  • H.M.D. Bandara et al.

    Rare earth recovery from end-of-life motors employing green chemistry design principles

    Green Chem.

    (2016)
  • Y. Deng et al.

    Recovery of NdFeB scrap by direct reduction-slag-gold melting method

    Rare Earth

    (2015)
  • X.Y. Du et al.

    Global in-use stocks of the rare earth elements: a first estimate

    Environ. Sci. Technol.

    (2011)
  • W.D. Halstead

    Thermal decomposition of ammonium sulphate

    J. Appl. Chem.

    (1970)
  • Cited by (20)

    • Evaluating organic acids as alternative leaching reagents for rare earth elements recovery from NdFeB magnets

      2023, Journal of Rare Earths
      Citation Excerpt :

      Various methods have been studied in order to recover REEs from NdFeB magnets. These methods can be classified into direct reuse8, hydrogen decrepitation9, pyrometallurgical methods10, hydrometallurgical methods11–14 and, combined pyro- and hydrometallurgical methods15–17. All these options come with pros and cons.

    • Extraction of valuable metals from minerals and industrial solid wastes via the ammonium sulfate roasting process: A systematic review

      2023, Chemical Engineering Journal
      Citation Excerpt :

      The results showed that the leaching efficiency of rare earth was 92 %, while the leaching efficiencies of Fe and Al were 3 % and 65 %, respectively under optimum conditions. Another study reported a two-stage ammonium sulfate roasting process for the selective extraction of rare earth from spent Nd-Fe-B magnets [111]. The results showed that nearly 80 % of rare earth elements could be converted to RE2(SO4)3 or NH4RE(SO4)2 at 400 ℃, while iron and other impurities were converted to ammonium sulfate salts.

    View all citing articles on Scopus
    View full text