FIELD OF INVENTION

The present invention relates to a method for recovery of Nd2Fe14B grains from bulk sintered Nd—Fe—B magnets and/or magnet scraps.

Neodymium iron boron (Nd—Fe—B) magnets are widely used in many applications such as hard disk drives, wind turbines, industrial motors, acoustic transducers and electric vehicles due to their high energy density [1]. Since they incorporate 20-30 wt % of rare earth elements (REEs) that include mainly Nd with small addition of Dy and/or Tb, they represent an important secondary REEs resource [2]. In 2014, 22% of rare-earth oxides (REOs) were consumed for the production of Nd—Fe—B magnets globally which makes them one of the largest applications among all REEs [3]. The demand for REEs was estimated to grow by >8% per annum until 2020 [4]. Since primary mining of REEs leads to a large environmental footprint associated with chemical usage and harmful emissions [5] and the REEs are considered the most critical based on their economic importance and supply risk [6]. Despite their criticality, currently, less than 1% of REEs are being recycled from end of life products [7]. Up to 30% of the starting REE alloy can be lost during the magnet manufacturing process [8]. Therefore, the recycling of the end-of-life Nd—Fe—B magnets or other magnet scraps has been considered as an important strategy to reduce the environmental risks related to rare earth mining and overcome the supply risk of REEs [9, 10].

BACKGROUND OF INVENTION AND PRIOR ART

Diverse methodologies hitherto for recycling Nd—Fe—B magnets have been summarized in detail by many authors [1, 8, 11]. The recycling approaches can be broadly classified into physical/mechanical processing, pyrometallurgical and hydrometallurgical seperation & recovery. Physical/mechanical processing, including resintering [12, 13], hydrogenation disproportionation desorption and recombination (HDDR) [14-16], recasting and melt spinning [17] of sintered Nd—Fe—B magnet scrap will typically have a smaller environmental footprint compared to recycling routes, which rely on stripping of the REEs. However, the potential disadvantages arise about the high oxygen content (typically 2000-5000 ppm oxygen) entrained in the REE-rich grain boundary phase [18]. The oxide could not be extracted out, because of which grain boundary modification is required by adding REE metals or REEs-containing alloys [13, 19]. The pyrometallurgical routes can be used to remelt the REE alloys and extract the different REE in the form of oxide, halide, fluoride or other metallic compound which can then be reduced to metallic form [20-25]. However, these pyrometallurgical processes operate at a temperature of around 750-950° C. and are thus energy intensive. Hydrometallurgical recycling processes designed for Nd—Fe—B magnets are promising due to the mild operating temperature, relatively simple equipment and the continuous separation ability [4, 9]. In hydrometallurgical processes, however, Nd—Fe—B magnets are completely dissolved with an acid. The roasting pretreatment at 900° C. is generally required. Iron, which is the major component of Nd—Fe—B magnets (60-70%) consumes large amount of acid, alkali and other precipitation agents that cannot be recycled in the whole process [8, 9]. REEs are concentrated by solvent extraction and then are precipitated with either oxalic or carbonic acid. The precipitate is further calcined at 950° C. to form REOs, which can then be returned to the initial manufacturing process for Nd—Fe—B magnets [26]. To summarize, the magnet-REEs-magnet recycling requires large amounts of chemicals and considerable energy with harmful emissions to the environment. Generally, sintered Nd—Fe—B magnets contain 87-92% Nd2Fe14B phase which is barely oxidized due to the protection of the grain boundary (typically Nd-rich phase) [18, 27]. Therefore, in stead of recovering REEs from the end-of-life Nd—Fe—B magnets or magnet scraps, direct recovery of the Nd2Fe14B phase for fresh NdFeB magnets making would dramatically reduce the chemical usage and energy consumption. However, up to date a facile, energy efficient and environmentally friendly route for recovery of the Nd2Fe14B phase is yet to be presented.

The various approaches for recycling Nd—Fe—B magnets have been investigated worldwide. One of these approaches is to bulk-recycle all the materials in an Nd—Fe—B magnet without separation. This has been termed “magnet-to-magnet recycling”. For instance, the hydrogenated Nd—Fe—B powder from voice coil motors (VCMs) was used to produce sintered magnets. After a brief milling treatment, the powder was aligned, pressed and re-sintered and this procedure was repeated four times with a progressive fall in the density (7.56-6.70 g cm−3) and in the magnetic properties (remanence: 1180-1053 mT, coercivity: 870-343 kA m−1). The chemical analysis indicated that this was due to the progressive oxidation of the Nd-rich material (oxygen content: 1.84-4.12 at. %) and to some Nd loss by evaporation (neodymium content: 13.78-13.02 at. %). On resintering, the grain boundary phase no longer melts due to the higher melting point of the oxide and therefore full density cannot be achieved in the resintered magnets. Therefore, extra REE metals/alloys has to be added to aid liquid phase in the grain boundary during sintering, in order to achieve full density. [Zakotnik, M., I. Harris, and A. Williams,Multiple recycling of NdFeB-type sintered magnets. Journal of Alloys and Compounds,2009. 469(1-2): p. 314-321 and Zakotnik, M. and C. Tudor,Commercial-scale recycling of NdFeB-type magnets with grain boundary modification yields products with ‘designer properties’ that exceed those of starting materials. Waste management,2015. 44: p, 48-54].

Other bulk-recycling approaches such as resintering, hydrogenation disproportionation desorption and recombination (HDDR), recasting and melt spinning of sintered Nd—Fe—B magnet scrap have been investigated as well. Since the bulk-recycling of Nd—Fe—B magnets leaves the REOs phase to remain in the grain boundary without seperation, higher REOs content with the repeated recycling could not be avoided which significantly degrades the magnetic properties. The addition of extra extra REE metals/alloys would results in higher cost for recycled magnets making. [Lalana, E. H., et al.Recycling of Rare Earth Magnets by Hydrogen Processing and Re-Sintering. in European Congress and Exhibition on Powder Metallurgy. European PM Conference Proceedings.2016. The European Powder Metallurgy Association. Lixandru, A., et al.,A systematic study of HDDR processing conditions for the recycling of end-of-life Nd—Fe—B magnets. Journal of Alloys and Compounds,2017. 724: p. 51-61. Itoh, M., et al.,Recycling of rare earth sintered magnets as isotropic bonded magnets by melt-spinning. Journal of alloys and compounds,2004. 374(1-2): p. 393-396].

Extraction of all the REEs from NdFeB magnets for new magnet production is an alternative way for recycling. This could be termed “magnet-REEs-magnet recycling”. High-temperature processing of Nd—Fe—B magnet is to selectively convert REEs in the magnet into another phase (i.e., from metallic RE to RE chlorides) which separates the main non-REE components. The separated REEs in the other phase can be used for RE metal production through molten salt electrolysis or metallothermic reduction. Nd—Fe—B magnet scrap can be selectively extracted using molten MgCl2—KCl salts at temperatures of 600-1200° C. REs in the scrap are transformed to RECl3(2RE+3MgCl2=2RECl3+3Mg) where the overall extraction efficiency for REs can reach >90%, [Hua, Z., Wang, J., Wang, L., Zhao, Z., Li, X., Xiao, Y and Yang, Y.,Selective extraction of rare earth elements from NdFeB scrap by molten chlorides. ACS Sustainable Chemistry&Engineering,2014. 2(11): p. 2536-2543].

Other selective extraction of REEs by liquid metal extraction, molten slag extraction, direct melting and direct electrochemical refining can also realize high REEs recovery efficiency. [Moore, M., et al.,A route for recycling Nd from Nd—Fe—B magnets using Cu melts. Journal of Alloys and Compounds,2015. 647: p. 997-1006. Yang, Y., S. Abrahami, and Y. Xiao.Recovery of rare earth elements from EOL permanent magnets with slag extraction. in Proceedings of the3rd International Slag Valorisation Symposium,2013. Bian, Y, et al.,Recovery of rare earth elements from NdFeB magnet by VIM-HMS method. ACS Sustainable Chemistry&Engineering,2016. 4(3): p. 810-818. Kamimoto, Y., et al.,Recovery of rare-earth elements from neodymium magnets using molten salt electrolysis. Journal of Material Cycles and Waste Management,2016: p. 1-5].

High-temperature processing of Nd—Fe—B magnet generally operates at a temperature of around 750-950° C. and is thus energy intensive. Hydrometallurgical recycling processes operated at a mild temperature to recover REEs are alternative methods for Nd—Fe—B magnets recycling. Conventionally, Nd—Fe—B magnet scraps are milled with/without roasting (annealing in air). The obtained powders are completely dissolved in concentrated HCl/H2SO4solutions. Subsequently, REEs can be precipitated from leach solutions by adding oxalic acid or hydrogen fluoride to form oxides or fluorides. The recovery of REEs can reach over 90%. [Bandara, H. D., K. D. Field, and M. H. Emmert,Rare earth recovery from end-of-life motors employing green chemistry design principles. Green Chemistry,2016. 18(3): p. 753-759. Peelman, S., J. Sietsma, and Y Yang,Recovery of Neodymium as(Na, Nd)(SO4)2from the Ferrous Fraction of a General WEEE Shredder Stream. Journal of Sustainable Metallurgy,2018: p. 1-12].

Recently, studies on selective leaching of the REEs have been reported. However, selective leaching of REEs in the magnet scrap is challenging due to the presence of large amount of Fe in the magnet alloy and in the scrap. Quite often a high temperature treatment is needed to completely convert all the metallic phases to oxides which increases the selectivity of REEs over Fe and other transition metal elements. [Lee, C.-H., et al.,Selective leaching process for neodymium recovery from scrap Nd—Fe—B magnet. Metallurgical and Materials Transactions A,2013. 44(13): p. 5825-5833. Yoon, H.-S., et al.,The effect of grinding and roasting conditions on the selective leaching of Nd and Dy from NdFeB magnet scraps. Metals,2015. 5(3): p. 1306-1314. Vander Hoogerstraete, T, et al.,From NdFeB magnets towards the rare-earth oxides: a recycling process consuming only oxalic acid. RSC Advances,2014. 4(109): p. 64099-64111].

In recent patents on recycling of spent or scrapped Nd—Fe—B magnets, efforts were paid on 1) resintering of milled waste materials mixed with raw sintered Nd—Fe—B powder/REE metal (alloys) and 2) selective recovery of REEs from completely leached magnet solution or high temperature molten salts. Selective recovery of Nd2Fe14B grains from waste Nd—Fe—B magnets has not yet reported up to date. [Sun, B. and Y Duan,High-performance NdFeB permanent magnet produced with NdFeB scraps and production method thereof.2017, Google Patents. Berggren, M., et al.,Rare Earth Materials Recovery System.2018, Google Patents. Harris, I. R., A. Walton, and J. D. Speight,Magnet recycling.2017, Google Patents. Taylor, P. R. and B. N. Carlson,EXTRACTION OF RARE EARTH METALS FROM NdFeB USING SELECTIVE SULFATION ROASTING.2016, Google Patents. Harris, I. R., et al., Magnet recycling. 2014, Google Patents].

In summary, the “magnet-to-magnet recycling” of Nd—Fe—B magnets leaves the REOs phase to stay in the grain boundary without any seperation, higher REOs content with the repeated recycling could not be avoided which significantly degrades the magnetic properties. The addition of extra extra REE metals/alloys would results in higher cost for recycled magnets making. While the proposed “magnet-REEs-magnet recycling” of Nd—Fe—B magnets requires either high energy input or multiple process steps with consumption of large amount of chemicals which generates large amount of waste and effluents.

DESCRIPTION OF THE INVENTION

In view of the problems associated with the recycling of EOL (end-of-life) Nd—Fe—B magnets/Nd—Fe—B magnet scraps, it is an object of the present invention to provide a more environmental friendly and cost-effective way for Nd—Fe—B magnet recycling.

The object is achieved with the method according to claim1. Advantageous embodiments of the method are subject of the dependent claims or can be deduced from the subsequent portions of the description.

In the proposed method Nd2Fe14B grains are recovered from bulk sintered Nd—Fe—B magnets and/or magnet scraps. The Nd—Fe—B magnets are typically magnet scraps, but the method is not limited to magnet scraps. The EOF Nd—Fe—B magnets contain up to 90% alloy phases which are suitable for electrochemical recycling. Nd—Fe—B magnet scraps are arising from cutting block magnets into shape or off-quality magnets, which can be suitable as an input for direct recycling processes (e.g. remelting). The wet swarf from magnet finishing processes is only suitable for indirect recycling processes, since the material is contaminated and partly oxidized.

The Nd2Fe14B grains are recovered by electrochemical etching of the bulk sintered Nd—Fe—B magnets or magnet scraps using an anodic oxidation process. In this process the metallic Nd-rich phase (Nd(0)) in the grain boundaries is oxidized to Nd3+as ions on the anode. The liquid electrolyte used in this process is formed of a non-aqueous solvent in order to prevent the Nd2Fe14B grains from oxidation. This allows direct reuse of the collected Nd2Fe14B grains for new magnet making. Generally, the grain boundaries of the Nd—Fe—B magnets and/or magnet scraps contain metallic Nd and Nd-oxides (NdOx) [28, 29]. The metallic Nd in the Nd-rich grain boundary phase is preferentially etched/oxidized and dissolved in the non-aqueous solvent. Due to this anodic oxidation the Nd2Fe14B grains in the Nd—Fe—B magnets and/or magnet scraps are released. The released magnetic Nd2Fe14B grains are then collected, preferably by using an external magnetic field gradient, provided via e.g., commercial Nd—Fe—B magnet. The non-magnetic particles that mainly consist of Nd-rich metal and oxides are preferably recovered by filtration. The dissolved Nd-rich phase in the electrolyte can be further separated and recovered in the form of REOs, the technique of which has been well known publically.

The applications of interest include the recovery of Nd2Fe14B grains ready for new magnet making and recovery of REEs ready for RE metal production by a facile, energy efficient and environmentally friendly route. Preferably the electrochemical etching of the Nd—Fe—B magnets and/or magnet scraps is performed in a three-electrode cell—having an anode, a cathode and a reference electrode—where one or several scrapped or EOL bulk sintered Nd—Fe—B magnets (after the coating is removed) serve(s) as the anode, preferably a Cu foil is used as the cathode and a Pt wire is applied as the reference electrode. In a preferred embodiment dehydrated FeCl2(to improve the electrolyte conductivity) is dissolved in the liquid electrolyte, e.g. in dimethylformamide (DMF) as the electrolyte. The Nd2Fe14B grains can be collected on the anodic part magnetically by a Nd—Fe—B magnet after applying the anodic current density (2˜48 mA·cm−2).

On an industrial level, a conductive netting mesh which is commonly used in the industry can be used together with the collected end-of-life bulk magnets or the magnet scraps to form the anode. Such a netting mesh typically holds around 10 kg Nd—Fe—B magnets.

This proposed anodic etching process of Nd—Fe—B magnets is acid-free and operating at room temperature. Generally, the temperatures can range from 0° C. to about 90° C. On the cathode part, simultaneously, the Fe deposition occurs with a high current efficiency, due to the application of a non-aqueous solvent (e.g. DMF). The collected Nd2Fe14B grains are not oxidized and are ready for a direct new magnet production.

The obtained metallic Fe on the cathode coming from electrodeposition can be used as a raw material for different applications.

