Research progress of NdFeB waste recycling and utilization

China produces a lot of NdFeB wastes every year, and these wastes contain a lot of valuable elements such as rare earths. The recycling of NdFeB waste materials will help improve China’s rare earth resource shortage, environmental pollution and resource waste. The green recycling of NdFeB waste has a broad prospect, so it is necessary to do a more comprehensive and systematic study on the recycling and utilization of NdFeB waste. In this paper, some traditional NdFeB waste recycling processes and new methods of NdFeB waste recycling are reviewed, and the characteristics of these methods are summarized, in order to provide guidance and assistance in the research of efficient recycling of NdFeB waste materials.

NdFeB permanent magnets have excellent performance, and high-performance NdFeB permanent magnet materials are widely used in high-tech fields such as computers, motors, and nuclear magnetic resonance imaging [1-2]. In 2017, China’s sintered NdFeB market output was 104,000 tons, and the global sintered market output was 120,000 tons[3]. WTGs predict that the average annual growth rate of NdFeB demand will increase by 10% during 2020-2030 [4]. These data show that the market demand for NdFeB permanent magnet materials is very high, and China’s NdFeB production occupies a high share, which promotes the development of the NdFeB permanent magnet industry. The production process of NdFeB permanent magnet materials is relatively mature, but there are still some problems. In the production process of NdFeB, due to process and equipment reasons, about 25% of the raw materials are generated in the production process, and the mass fraction of rare earth components is about 33% [5-6]. Therefore, the comprehensive recycling and utilization of NdFeB waste has great potential value, and the recycling of rare earth in NdFeB waste is also getting more and more attention. Rare earth resources are non-renewable. The use of economical and effective methods to recycle the valuable substances in NdFeB waste can create certain economic value, save resources and reduce environmental pollution. Figure 1 shows the global output of sintered NdFeB in recent years.

Fig. 1 Global annual production of sintered Nb-Fe-B magnets

Generation of NdFeB waste

The production methods of NdFeB permanent magnet materials mainly include sintering method and bonding method. Among them, sintered NdFeB occupies a larger proportion, so the waste mostly comes from the sintering process. The technological process of sintering NdFeB [7]: mixing → smelting ingot → crushing and powder making → magnetic field orientation forming → sintering heat treatment → post-processing magnetization. The generation of NdFeB scrap [8] mainly comes from the loss during machining and the unqualified products during surface treatment. Due to different production processes, the morphology of NdFeB wastes is also very different. There are powder, granular, block, mud and other forms, with different water content, and the structure and texture of different NdFeB wastes are also different[ 9], this also brings a certain degree of difficulty to the treatment of NdFeB waste.

Research progress of NdFeB waste recycling

NdFeB waste has great recycling value, which has promoted the majority of scientific researchers to carry out related research on the recovery of rare earth and other valuable components from NdFeB waste [10]. Looking at the relevant literature, it is found that the most research is to use hydrochloric acid, sulfuric acid, etc. for acid leaching to decompose the NdFeB waste, and then further recover the rare earth [11]. This kind of method has simple operation, high leaching rate, and mature process, but it will produce a large amount of acidic wastewater, pollute the environment, and have high treatment costs. Moreover, environmental protection requirements are becoming more and more stringent, and it is necessary to find a more environmentally friendly and effective method for processing NdFeB waste. According to the related literatures reviewed, some related researches on traditional acid leaching and new processes for recovering rare earths from NdFeB waste are summarized.

Traditional method to recover rare earths from NdFeB waste

Recovery of rare earths in NdFeB waste by hydrochloric acid solution

In the hydrochloric acid excellent solution method, the rare earth is preferentially dissolved under the conditions of strict control of pH and rare earth concentration, and then the rare earth is separated, and finally the rare earth oxide is obtained. The main processes are oxidation roasting, impurity removal, extraction separation, precipitation, and burning. Figure 2 shows a schematic diagram of the hydrochloric acid solution process. Liu Mingqing [12] studied the treatment of NdFeB waste with hydrochloric acid and oxalic acid to precipitate rare earths. The W_REO in the slag was controlled to be less than 0.6%, the rare earth recovery rate was greater than 95%, and the purity of rare earth products was greater than 99%. Wang Yijun et al. [13] studied the separation and recovery of neodymium dysprosium from neodymium iron boron waste by hydrochloric acid optimization, using ammonium bicarbonate to precipitate rare earths, controlling the W_REO in the slag to be less than 0.6%, and finally obtaining the recovery rate of rare earths greater than 92%, and the dysprosium oxide produced The absolute purity is greater than 99%. Wu Jiping [14] studied the separation and extraction of rare earths from NdFeB waste by oxidation roasting-hydrochloric acid decomposition, and explored the influence of various factors on the oxidation rate of iron and the leaching rate of rare earths. It is concluded that when roasting at 700 ℃ for 1.5 h, the iron oxidation rate reaches 99.3%; when the hydrochloric acid concentration is 4 mol/L, the leaching time is 1.5 h, the liquid-to-solid ratio is 5:1, and the temperature is 90 ℃, the rare earth leaching rate reaches 99.33% . Jiang Zezuo et al. [15] studied the sodium chlorate oxidation method to remove iron in the hydrochloric acid solution. Generally, hydrogen peroxide is used to remove iron from industrial oxidation. However, due to the high temperature of the system, the hydrogen peroxide will decompose in a large amount, leading to waste of reagents and incomplete Fe²⁺ oxidation. Using the theoretical amount of sodium chlorate to react for 2 hours, the Fe²⁺ in the hydrochloric acid solution can be reduced to 0.001 g/L, the Fe²⁺ oxidation rate reaches 99.99%, and the product quality is stable and qualified.

Fig. 2 Schematic diagram of the process of hydrochloric acid solution

The hydrochloric acid excellent solution method has a simple operation process, and the obtained rare earth oxide has high purity. The key to the excellent solubility of hydrochloric acid is the oxidative roasting of NdFeB waste, which can obtain rare earth oxides and iron trioxide (Fe₂O₃). Low-concentration acids will preferentially dissolve rare earth oxides to achieve selective leaching. However, due to the use of low-concentration acids, the dissolution efficiency of waste materials is not high, and a large amount of waste water is generated.

Recovery of NdFeB waste by all-solvent extraction

The total solvent extraction method uses hydrochloric acid to dissolve all the rare earths, iron, etc., and then gradually separates and purifies the rare earths, iron and cobalt, and finally obtains various products. Figure 3 shows a schematic diagram of the all-solvent extraction process. Chen Yunjin [16] proposed the all-solvent extraction method to recover NdFeB waste. He uses hydrochloric acid to dissolve all the waste, oxidizes Fe²⁺ to Fe³⁺ with hydrogen peroxide, extracts iron with N503, obtains the aqueous solution of rare earth and cobalt, extracts the rare earth with P507, and then separates the rare earth with hydrochloric acid of different acidity in stages, and precipitates oxalic acid. After sintering, 99% Nd₂O₃ and 98% Dy₂O₃ are obtained. The raffinate is precipitated with sodium carbonate to obtain 99% cobalt carbonate.

Fig.3 Schematic diagram of the process of total solvent extraction

The all-solvent extraction method has a high recovery rate, and can recover the rare earth in the waste material and separate the cobalt in the form of cobalt carbonate. However, the total solution method consumes a large amount of acid, and the solution causes environmental pollution. Moreover, the iron content in the solution is very high, the purity of the product is not high, and the removal of iron also consumes a lot of reagents.
The separation and recovery of rare earths by solvent extraction is a relatively common method. The key to the extraction and separation of hydrochloric acid is the choice of extractant. A suitable extractant can separate the rare earths completely to obtain high-purity products. With the deepening of research, more and more reagents have been used to extract rare earths. For example: NaCyanex 302[17], Cyanex 923[18], D2EHPA[19], TBP[20], dinonylphenylphosphoric acid[21] and other extractants have achieved good results in the application of rare earth solvent extraction .

Recovery of NdFeB waste by sulfuric acid double salt method

Liu Su [22] elaborated on the process flow of the sulfate double salt method: the sulfate double salt method uses sulfuric acid to dissolve the oxidized and roasted NdFeB waste, and then adds sodium sulfate at a higher temperature to form a rare earth double salt precipitation. Alkali conversion, hydrochloric acid leaching, and then precipitation of rare earths with oxalic acid, and finally burning to obtain rare earth oxides. Under the optimal process conditions, the direct yield of neodymium oxide reaches 95%. Xu Tao et al. [5] studied the recovery of neodymium, dysprosium and cobalt in neodymium iron boron waste, using sulfuric acid dissolution, double salt precipitation of rare earth, alkali conversion, hydrochloric acid dissolution, double salt precipitation of iron, P507-hydrochloric acid system extraction, precipitation, burning and other operations can fully separate the neodymium, dysprosium, and cobalt in the neodymium iron boron waste. Figure 4 shows a schematic diagram of the sulfate double salt process. Wei Chengfu et al. [23] studied the treatment of molten iron in the recovery of NdFeB waste by the sulfate double salt method. After the formation of double salt precipitation, iron exists in the form of ferrous sulfate (FeSO₄), and high-purity ferrous sulfate is obtained through processes such as evaporation and concentration, cooling and crystallization, drying, and recrystallization.

Fig.4 Schematic diagram of the process of double salt of sulfuric acid

The key to the sulfuric acid double salt method is the formation of rare earth double salt. This process requires higher temperature and high energy consumption. In addition, the acid in the solution must be neutralized when alkali is added for conversion, which consumes a large amount of alkali, and a large amount of acid is consumed in the second acid leaching. The whole process is relatively complicated and the reagent consumption is large.

Recovery of NdFeB waste by slag-gold melting separation method

At present, fire methods are also used to recover rare earths from NdFeB waste materials, among which the slag-gold melting technology has achieved certain results in the recovery of NdFeB waste materials. Deng Yongchun et al. [24] used a direct reduction-slag gold melting method to recover ferroalloy and rare earth oxide slag from rare earth NdFeB waste. Sponge iron is obtained by direct reduction with reducing agent semi-coke in the reaction tank, and then metal iron alloy and rare earth oxide slag are obtained by slag gold melting. Among them, iron and cobalt exist in the form of elemental alloys. REO-SiO₂-Al₂O₃ slag is formed by rare earth oxides and gangue components in iron concentrates. The rare earth content in the slag reaches 48.42%, which has great recycling value. Lu Xiaoneng et al. [25] used slag-gold melting separation method to recover rare earth and iron in NdFeB ultrafine powder waste. The effects of crucible material, slag-forming agent ratio, melting temperature and reaction time on the slag gold melting effect were studied. It was found that after 4 hours of reaction at 1550 ℃, the fraction of rare earth oxides in the slag reached 82.72%; using graphite crucibles and adding CaO and SiO₂ as slagging agents and controlling the alkalinity of the slag system can obtain a slag phase enriched with rare earth oxides and iron In the base metal phase, the separation of slag and gold is obvious.
The slag-gold melting method belongs to the fire method, which can effectively separate metals and rare earths, but to obtain single metals or rare earth oxides, further processing is required, and slagging is required. The temperature of the experiment is relatively high, and there are certain requirements for the experimental equipment .

Recovery of NdFeB waste by short-process remanufacturing method

Recycling of NdFeB waste by wet method or fire method is to destroy the crystal structure of NdFeB waste and separate and purify rare earth and other valuable elements to achieve resource recovery and reuse. The short-process remanufacturing method is to remanufacture magnets through special treatment, such as adding alloy NdDyCoCuFe[26-28], etc., to improve the performance of magnets to meet the requirements of commercial products.
Zhou Toujun et al. [29] processed the recovered magnets to remove surface oxidation, mechanical crushing, hydrogen explosion and air milling to prepare magnetic powder with an average particle size of 3 μm. The sintered neodymium iron boron magnet is prepared by mixing magnetic powder and neodymium praseodymium powder and processing. Experiments show that the coercivity of the magnet after adding 2% PrNd is restored to 102% compared with the primary product, the remanence and the magnetic energy product are respectively 95% and 90% of the primary product, and the squareness is reduced. Li Chencheng et al. [30] used the grain boundary diffusion method to add Dy₂O₃ to the sintered waste NdFeB powder at 100 mm level. Experiments show that adding a small amount of Dy₂O₃ can significantly increase the B_r of the magnet, the maximum H_cj  can reach 1 310 kA/m, and the maximum (BH) can reach 204 kJ/m³. Liu Weiqiang et al. [31] added DyH₃ nanoparticles to waste NdFeB powder to prepare regenerated magnets. Compared with the initial waste sintered magnet, the best regenerated magnet prepared contains 1.0% (referring to the mass fraction) of DyH₃ nanoparticles. The maximum values of H_cj, Br and (BH) were 101.7%, 95.4% and 88.58%, respectively, and the recovery was good. The volume fraction of Nd-rich phase increases with the increase of DyH₃ content.
The recovery of NdFeB waste by using waste magnets to regenerate magnets has shorter process flow, lower energy consumption and less environmental pollution than the method of chemical purification and recovery. However, the performance of regenerated magnets is difficult to reach directly produced magnets.

Using new technology to extract rare earth from NdFeB waste

Recovery of NdFeB waste by selective electrochemical leaching

On the basis of hydrochloric acid leaching, the selective electrochemical leaching method uses electric energy and hydrochloric acid leaching to recover rare earths from neodymium iron boron waste. Prakash Venkatesan et al. [32] studied the recovery of rare earths from NdFeB waste by electrochemical leaching at room temperature. First, the NdFeB waste is partially leached with hydrochloric acid (HCl) and sodium chloride (NaCl) as the additive salt. Then the leaching solution and the remaining NdFeB waste are used as the anolyte, and the catholyte is a low-concentration sodium chloride solution separated by an anion exchange membrane. During electrolysis, iron is oxidized and precipitated in the form of Fe（OH）₃, and ≥95% of rare earth elements and cobalt are extracted into the leaching solution. After the electrolysis is completed, the iron is removed by filtration, the rare earth is precipitated with oxalic acid, and the rare earth oxide with a purity of ≥99% is obtained by burning. The whole process can completely remove iron, and only consumes sodium chloride, oxalic acid and electricity.
The hydrochloric acid in this selective electrochemical leaching process can be recycled, which can solve the problem of large amounts of acidic wastewater generated in the general acid leaching process. The key lies in the choice of membrane and the control of the concentration of sodium chloride in the bipolar electrolyte.
Regarding the electrochemical treatment and recycling of NdFeB waste, Prakash Venkatesan and others have also done extended research. Prakash Venkatesan et al. [33] used hydrochloric acid to completely leached the NdFeB magnet waste, and then electrochemically oxidized it to selectively oxidize the iron (II) in the leachate to iron (III). Finally, oxalic acid is directly added to the electro-oxidation leachate to selectively precipitate more than 98% of rare earth elements as rare earth oxalates. The purity of mixed rare earth oxide produced by calcination of rare earth oxalate is 99.2%, and the by-product is Fe(Ⅲ) solution. Prakash Venkatesan et al. [34] also studied another electrochemical process for selective extraction of rare earths. They used an electrolytic pretreatment method to convert the elements in the NdFeB magnet waste into corresponding hydroxides. The dual anode system is adopted, and the NdFeB magnet waste and the inert anode are used as the anode in the electrochemical reactor. The use of inert anodes ensures that the iron in the magnet scrap is converted into iron (Ⅲ). The mixed hydroxide is leached with hydrochloric acid to obtain more than 97% of the rare earth and cobalt leaching solution, while the iron remains in the slag. Then, oxalic acid is used to selectively precipitate rare earth oxalates, and oxalic acid regenerates hydrochloric acid, forming a cyclic process. The calcination of rare earth oxalate produces rare earth oxides with a purity of 99.2%, which can be directly used in the production of rare earth metals.

Hydrochloric acid combined with modifier to recover NdFeB waste

A large amount of iron will dissolve in the process of acid dissolution in hydrochloric acid, which has always been a problem affecting the extraction of rare earths in the subsequent purification of impurities. Although the hydrochloric acid excellent solution method can selectively extract rare earths, this method uses a low acid concentration, a long leaching time, and oxidative roasting of NdFeB waste. Tian Yilan et al. [35] added tartaric acid or hexamethylene tetramine (HMTA) as a chelating agent during the leaching process with hydrochloric acid. Tartaric acid and hexamethylene tetramine (HMTA) can form stable complexes with iron and interact with rare earths. The formation of unstable complexes can prevent iron leaching. The complexation reaction of HMTA and tartaric acid with iron:

Tian Yilan et al. [35] studied the influence of hydrochloric acid concentration, chelating agent concentration and temperature on the leaching process, and the extraction rate of rare earths was 99.27% under optimal conditions. After the leaching is completed, oxalic acid is used to precipitate rare earths, and then roasted to obtain rare earth oxides. The purity of the rare earth oxides obtained by roasting is over 95.83%, and the recovery rate is 90.18%.
The addition of hexamethylenetetramine (HMTA) and tartaric acid during hydrochloric acid leaching can reduce the acid consumption during the leaching process. The leaching rate is very high, but the purity and recovery rate of the rare earth oxides obtained by roasting are not very high. Hexamethylenetetramine (HMTA) and tartaric acid form a small amount of unstable complex, which reduces the recovery rate and purity of the product when oxalic acid is precipitated.

Recovery of NdFeB waste with carbon (C) as extractant

In recent years, some progress has been made in the study of carbon (C) as an extractant. Liu Bowen et al. [36] used waste sawdust biochar as an extractant and proposed a new carbonization/hydrohydrolysis process for recovering rare earth elements from waste NdFeB magnets. The waste sawdust is pulverized and then made into biochar at high temperature. The NdFeB waste is demagnetized and then pulverized with a grinder. Then, the biochar and NdFeB waste are roasted in different stacking methods, and protected by argon. It is found that placing the biochar at the bottom of the crucible and the roasting method of the biochar with magnet powder on the top has a higher purity and recovery rate of rare earths. The NbFeB-C/H alloy (RECs and REHs) is obtained by roasting, and water is added to form rare earth hydroxides (REOHs). The purity of the recovered rare earth hydroxides (REOHs) under better conditions reaches 99.43%, and the rare earth recovery rate is 88.4%. The final roasting Obtain rare earth oxides. BianYuyang et al. [37] used a vacuum smelting method to melt the NdFeB waste in a vacuum graphite crucible, and the carbon reacted with rare earth to obtain a rare earth carbide alloy. Then, the as-cast NbFeBCsat alloy was mechanically crushed and hydrolyzed in water to obtain rare earth hydroxides, and magnetic separation was used to remove iron slag from the rare earth hydroxides. The better recovery rate of the method reaches 93%, the purity of the rare earth hydroxide reaches 99.7%, and the rare earth oxide obtained after roasting can be used for electrolytic production of rare earth metals.
Using biochar, graphite carbon and other carbon (C) sources as extractants can effectively recover rare earths from NdFeB waste. Some researchers have done research on the recovery of rare earths from NdFeB waste with other extractants, such as FeO－B₂O₃[ 38] etc. However, the experimental conditions of this kind of method are strict and must be carried out under argon protection or vacuum conditions. The requirements for NdFeB waste are relatively high, if it is a NdFeB magnet that is not polluted, oxidized or slightly oxidized. Moreover, the result is a mixture of rare earth oxides, and it is difficult to obtain a single high-purity rare earth product.

Hydrofluoric acid electrodeposition method to recover ultrafine NdFeB waste

Hydrofluoric acid electrodeposition method to recover ultrafine NdFeB waste is a method proposed by Yang YuSheng et al. [39] to recover rare earth from ultrafine NdFeB waste. Ultra-fine NdFeB waste (particle size less than 1 μm), their crystal structure is broken during the production process, can not be used to produce magnets, rare earth exists in the form of rare earth hydroxide. Hydrofluoric acid is used to convert rare earths into rare earth fluorides, and the rare earth fluorides are directly recovered from ultrafine neodymium iron boron waste materials by taking advantage of their insoluble properties in hydrofluoric acid. Iron is mainly present in hydrofluoric acid in the form of ligands. The recovered rare earth fluoride is used for electrolysis to produce rare earth metals, and the iron dissolved in hydrofluoric acid is recovered by electrodeposition.
The hydrofluoric acid electrodeposition method to recover ultrafine NdFeB waste is different from the process of adjusting the pH of iron to recover rare earths. The selective precipitation of rare earths can greatly reduce the loss of rare earths in the recovery, and the waste generated in the whole process Less, it is an environmentally friendly craft. However, this method is only suitable for ultrafine NdFeB waste in the form of rare earth hydroxide, and the raw material is single, which has certain limitations compared with the traditional wet recycling of NdFeB waste.

Development trends and prospects

The method of using wet acid leaching (hydrochloric acid, sulfuric acid, etc.) plus oxalic acid and other precipitating agents to recover rare earths in neodymium iron boron waste is a common method for recovering rare earth-containing wastes. Among them, the hydrochloric acid excellent solution method is simple in process and low in cost, but requires strict control of the process conditions, making industrialization difficult; although the hydrochloric acid total solution method is easier to achieve industrial production, it consumes a large amount of acid and requires the separation of rare earth and iron. ; Sulfuric acid double salt process is more complicated, it needs a lot of acid and alkali, and the cost is higher. The above-mentioned pyrometallurgical methods are relatively environmentally friendly, but have higher requirements for waste materials, and the waste materials that can be processed are relatively single, and the benefits of industrialized production are low or temporarily unable to industrialized production. The electrochemical method has less acid consumption and generated waste liquid than the wet method, and has less pollution to the environment, but the production cost and strict process conditions are the main problems that limit its industrialization. For example, the selective electrochemical leaching method requires the selection of a suitable anion exchange membrane, and the development and replacement of the membrane will increase the production cost. Therefore, in order to efficiently and comprehensively recycle neodymium iron boron waste materials and reduce environmental pollution while obtaining economic benefits, the following three aspects can be studied:

• 1) The research on the recycling and treatment of NdFeB waste should be carried out in the direction of reducing the amount of hydrochloric acid and oxalic acid. The use of acid can be reduced by adding modifiers, and more economical and effective modifiers can be selected under the premise of ensuring product quality. Or choose a more green and economical precipitation agent to replace oxalic acid and ammonium bicarbonate.
• 2) NdFeB waste containing a large amount of sludge is not suitable for direct wet treatment, it is better to roast at high temperature. In addition, wet leaching waste slag contains a large amount of iron, which can be used as raw material for ironmaking in steel plants after treatment.
• 3) Optimize the process conditions and equipment of pyrometallurgy, improve the adaptability to neodymium iron boron waste, enable it to be industrialized, and form an economical and environmentally friendly production process.

Author: FU Liwen, WANG Jinliang, LEI Xiang, WANG Houqing. Research progress on the recycling and utilization of Nd-Fe-B wastes. Nonferrous Metals Science and Engineering, 2020, 39(1): 92-97.DOI: 10.13264/j.cnki.ysjskx.2020.01.015.

Source: China Permanent Magnet Manufacturer – www.rizinia.com

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