Method for recycling neodymium (Nd), iron (Fe), boron (B) (NdFeB) type magnets, anisotropic powder obtained from the recycling, and method for producing permanent magnets from said powder

Abstract

The present invention relates to a method for recycling NdFeB magnets, comprising the following steps: a) recovering waste containing solid NdFeB magnets to be recycled; b) preheating the waste to a preheat temperature of 300°C-500°C under inert atmosphere in a first chamber, the temperature of which is increased to the preheat temperature in a first station equipped with heating means; and c) decrepitation using hydrogen in a second chamber, located in a second station different from the first station and equipped with a hydrogen source and pumping means, this operation being applied to the hot waste from step b), the decrepitation being carried out at a temperature of 200°C-500°C, said temperature in the second chamber being maintained in this temperature range as a result of the exothermic nature of the reaction of hydrogenating the NdFeB magnets, and step c) is carried out by hydrogenating the NdFe 14 and a decrepitation step resulting in the formation of a first powder having grains containing a primary phase consisting of B and / or grain boundary secondary phases and having a grain size of 5 mm or less.

Description

[Technical Field]

[0001] The present invention relates to the field of recycling rare earth permanent magnets. The invention particularly relates to powders obtained from the recycling of neodymium magnets (of the NdFeB type) and to a recycling method for obtaining said powders. The invention also relates to a method for producing NdFeB permanent magnets from said powders. [Background technology]

[0002] Resources of rare earth metals (especially neodymium (Nd)) for the production of permanent magnets with high remanence and coercivity are limited, and demand is expected to increase significantly in the coming years, as these magnets are useful in many electrical and electronic devices, as well as in emerging areas such as hybrid or electric vehicles (motors) or wind turbines (generators).

[0003] NdFeB magnets are the most commonly used rare earth permanent magnets. They are primarily manufactured by sintering (solid magnets) or by injection or compression molding processes, the latter of which include a polymer binder between particles of NdFeB material (bonded magnets). Additive manufacturing techniques, whether solid or based on a polymer binder, are also beginning to be implemented to produce NdFeB magnets with strong flexibility in the shape of the magnetic object.

[0004] To address the problem of rare earth resources, especially neodymium, several methods have been proposed for recycling NdFeB permanent solid magnets at the end of their life. In particular, EP 2646584 can be mentioned, which describes a method consisting in exposing an assembly containing NdFeB magnets to a decrepitation process known to decompose said magnets, and then separating the resulting NdFeB powder from the remainder of the assembly by sieving the assembly through a rotating porous container (the powder, but not the whole heavy part, is expelled by the pores).

[0005] Starting from this type of method, it remains important to improve the recycling method in order to streamline it (simplify the process, limit its duration, achieve a robust, reliable and safe infrastructure) while maintaining its environmental compatibility. It is also considered necessary to optimize the properties of the powder obtained at the end of the recycling process to facilitate the production of new permanent NdFeB magnets from this recycled powder and to promote their performance. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention addresses the above-mentioned problems and particularly relates to a reliable method for recycling NdFeB solid magnets, as well as the anisotropic and coercive powder obtained from this method, which is particularly suitable for the production of new, powerful NdFeB magnets by additive manufacturing methods. [Means for solving the problem]

[0007] The present invention relates to a method for producing a NdFeB magnet, which includes the following steps: a) recovering waste containing NdFeB solid magnets to be recycled, wherein the NdFeB magnets are NdFe 14 a B magnetic main phase and a non-magnetic grain boundary secondary phase; b) preheating the waste material to a preheat temperature of 300°C to 500°C under an inert atmosphere, the waste material being contained in a first chamber whose temperature has been raised to the preheat temperature in a first station equipped with a heating means; c) a step of hydrogen decrepitation applied to the hot waste obtained from step b) under a hydrogen partial or total pressure of 0.1 bar to 10 bar, said hot waste being contained in a second chamber arranged in a second station different from the first station, equipped with a hydrogen source and pumping means, and said decrepitation is carried out at a temperature of 200°C to 500°C, said temperature in the second chamber being maintained within this temperature range due to the exothermic nature of the hydrogenation reaction of NdFeB magnets; 14 A step results in the formation of a first powder having particles having a size of 5 mm or less, comprising a B major phase and / or a grain boundary secondary phase.

[0008] According to other advantageous, non-limiting characteristics of the present invention, taken alone or in any technically feasible combination, there are: the first chamber and the second chamber are formed by a single chamber that is moved from the first station to the second station between steps b) and c) and is connectable to a hydrogen source and pumping means at the second station; the waste material recovered in step a) comprises metal parts integral with the NdFeB magnets to be recycled, the method comprising, during or after step c), a sieving step d) on a sieve size of 1 cm, in order to separate said parts from the first powder, the method comprises, during or after step c), a step d) of sieving the first powder, the method comprises, after step c) or during or after the sieving step d), a step e) of grinding the first powder or a fraction of the first powder to obtain a second powder having particles with a size of 500 microns or less, particles consisting of at least one compound enriched in rare earth(s) are added to the first powder during step e) to enrich the second powder with grain boundary secondary phases, In step e), the grinding is carried out in a ball mill or by means of a ring mill or by means of a gas jet mill, At least one compound of a fresh alloy material of the TRFeB type, in particular NdFeB (TR=Nd, Pr, Dy, Tb) and / or TRFe 14 At least one magnetic phase compound of type B, in particular NdFe 14 B is added to the first powder or to the second powder during the recycling method, At least one non-magnetic metal compound (rare earth-free) with a low melting point (typically below the melting point of the grain boundary secondary phase) is added to the first powder or the second powder during the recycling process, said compound intended to promote bonding between the NdFeB particles during the production of new magnets. The method comprises a step f) of dehydrogenating the first powder or the second powder.

[0009] The present invention further provides a method for producing NdFe nanoparticles, the method comprising: 14 The present invention relates to a powder obtained from the above-mentioned method for recycling NdFeB magnets, which contains particles composed of B magnetic main phase metal crystal grains, and the metal crystal grains of the same particles have a common crystal orientation that generates magnetic anisotropy.

[0010] Advantageously, the powder comprises: a first number of occupied particles centered on a first size; -A second occupied particle number centered on a second size, in which the first size is 1.5 to 10 times larger than the second size, and the second occupied particle number has a dual particle size distribution.

[0011] Advantageously, the powder comprises between 1% and 50% by volume of grain boundary secondary phase, and even more preferentially, the powder comprises between 10% and 30% by volume of grain boundary secondary phase.

[0012] Advantageously, the powder comprises between 1% and 50% by volume of non-magnetic metal compounds (rare earth-free), and even more preferentially, the powder comprises between 1% and 20% by volume, or between 1% and 10% by volume of non-magnetic metal compounds.

[0013] Finally, the present invention provides a method for producing a NdFeB permanent magnet from the aforementioned powder, comprising: -sintering technology, or - injection or compression techniques to produce plastic bonded magnets, or - injection molding techniques including debinding and sintering, or - A method for implementing additive manufacturing techniques.

[0014] According to a preferred embodiment, the manufacturing method is based on an additive manufacturing technique which involves the melting of at least one phase of a powder, the melting being NdFe 14 This is done at a temperature below the melting point of the Nd2Fe 14 Instead of the B main phase, the grain boundary secondary phase or, if present, the non-magnetic metal compound not containing rare earth elements is melted in whole or in part.

[0015] In this preferred embodiment, the powder particles comprising the main phase are advantageously oriented before or during consolidation of the NdFeB magnet in the form of a printed object in order to impart magnetic anisotropy to said magnet.

[0016] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing steps of a recycling method according to the present invention. FIG. [Figure 2] 1 shows an image made by scanning electron microscopy (SEM) of the particles of the first powder and a graph showing a typical particle size distribution of the particles of the first powder measured by laser diffraction at the end of step c) of the recycling method. [Figure 3a]3a-3c show images made by scanning electron microscopy (SEM) of the particles of the second powder after step e) of the recycling method according to the invention for different grinding conditions. Similarly to Figures 3a-3c, graphs showing the particle size distribution of the particles of the second powder obtained for the different grinding conditions are shown, but the particle size is measured by laser diffraction (Figures 3a and 3b) and image analysis (Figure 3c). [Figure 3b] 3a-3c show images made by scanning electron microscopy (SEM) of the particles of the second powder after step e) of the recycling method according to the invention for different grinding conditions. Similarly to Figures 3a-3c, graphs showing the particle size distribution of the particles of the second powder obtained for the different grinding conditions are shown, but the particle size is measured by laser diffraction (Figures 3a and 3b) and image analysis (Figure 3c). [Figure 3c] 3a-3c show images made by scanning electron microscopy (SEM) of the particles of the second powder after step e) of the recycling method according to the invention for different grinding conditions. Similarly to Figures 3a-3c, graphs showing the particle size distribution of the particles of the second powder obtained for the different grinding conditions are shown, but the particle size is measured by laser diffraction (Figures 3a and 3b) and image analysis (Figure 3c). [Figure 4] 1 shows a scanning electron microscope image of particles of the second powder after the addition of particles of rare earth-rich grain boundary phase compounds. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a method for recycling solid magnets of the NdFeB type, which, as mentioned in the introduction, are metallic only, i.e., do not contain a polymer binder in addition to the NdFeB alloy. Therefore, in the present invention, the term "solid magnet" excludes polymer-based composites, such as, for example, plastic-bonded magnets. Furthermore, although NdFeB magnets are described throughout this specification, the term encompasses any NdFeB magnet, i.e., NdFeB magnets that may contain various additives and / or other rare earth metals, such as neodymium (e.g., dysprosium). By analogy, although they may contain several types of rare earth metals and additives, the main magnetic phase of these magnets is NdFe 14 This is called phase B.

[0019] The first step a) of the method involves recovering waste materials containing NdFeB solid magnets to be recycled. Waste materials are understood to mean solid NdFeB magnets rejected from production, and also to mean any type of mechanical or electronic components that have reached the end of their life or become obsolete during the manufacturing process, including NdFeB magnets whose materials are recyclable. As an example, the waste materials may consist of NdFeB magnets from hard disks, NdFeB magnets from electric motors, etc., fixed to steel tabs. Due to their magnetic properties, these magnets are often very firmly attached to the mechanical (metallic) components associated with their previous use. Nevertheless, it is possible to obtain waste materials consisting solely of NdFeB magnets, i.e., separated from the mechanical components of their previous use.

[0020] Generally, waste-derived NdFeB magnets may be bare or may be typically covered with a metallic (eg nickel or zinc based) or polymer (epoxy) protective layer.

[0021] Recycled bulk NdFeB magnets typically contain 85% (+ / - 10% by volume) of NdFe 14The alloy contains a B magnetic main phase. The melting point of this main phase is about 1180°C. The alloy also contains Nd2Fe 14 B contains non-magnetic secondary phases, hence the grain boundary secondary phases, which serve to magnetically separate (isolate) the main phase metal grains. The secondary phases consist of several rare earth-rich phases with melting temperatures between 500°C and 800°C, depending on the alloy composition. The grain boundary secondary phases typically represent 15% (+ / - 10%) of the alloy by volume.

[0022] The recycling method then includes a step b) of preheating the waste material 100 in a first chamber 10 to a preheat temperature of 300°C to 500°C, preferentially 350°C to 450°C, in particular about 400°C (FIG. 1(b)). For this purpose, the waste material 100 is introduced into the first chamber 10, which is arranged in a first station 1 equipped with heating means 11. The first station 1 may, for example, consist of an annealing furnace in which the first chamber 10 filled with the waste material 100 is arranged. The first station 1 advantageously comprises a gas circuit 12, 13, including at least one neutral gas source 12 and a gas discharge 13, which are connected to the first enclosure 10 and can regulate its internal atmosphere. The atmosphere in the first chamber 10 is therefore advantageously selected to be inert, for example based on argon. The first chamber 10 is preferably completely sealed and is suitable for pressures of several bars and temperatures typically up to 600°C.

[0023] This pre-heating of the waste material 100 is advantageous in that it facilitates demagnetization of the various materials contained in the waste material, thus facilitating physical separation of portions potentially secured by magnetic attraction.

[0024] However, the inventors will now show that this preheating step b) has other important advantages with regard to the reliability and efficiency of the recycling process.

[0025] The following step c) of the recycling method corresponds to a hydrogen decrepitation applied to the hot waste 101 at a temperature between 200°C and 500°C under a hydrogen partial or total pressure between 0.1 bar and 10 bar, preferentially between 1 and 4 bar. The hot waste 101 is accommodated in a second chamber 20 arranged in a second station 2 different from the first station 1 and equipped with a hydrogen source 21 and pumping means 22 (Figure 1(c)).

[0026] According to a preferred embodiment, the first chamber 10 and the second chamber 20 are formed by a single chamber, which is moved from the first station 1 to the second station 2 between steps b) and c) and is connectable to the hydrogen source 21 and the pumping means 22 of the second station 2. In this case, the enclosure 10 is disconnected from the gas circuit 12, 13 of the first station 1 at the end of step b) and then connected to the hydrogen source 21 and the pumping means 22 when the enclosure 10 is placed in or on the second station 2.

[0027] According to another possible embodiment, the hot waste 101 is transferred from the first enclosure 10 into the second chamber 20 between steps b) and c). Care is taken to limit the temperature drop of the waste 101 during this transfer so that at the start of step c) the temperature of the waste 101 remains above 250°C.

[0028] Regardless of the embodiment, when the waste material is in the second chamber 20, air is evacuated therefrom by pumping means 22 before the chamber 20 is supplied with hydrogen by the hydrogen source 21. The second chamber 20 is completely sealed and is suitable for pressures ranging up to at least 10 bar, and temperatures typically ranging up to 600°C. Also, because hydrogen is absorbed by the NdFeB alloy of the waste material 100, it is advantageous to control the distribution of hydrogen by the hydrogen source so as to maintain the hydrogen pressure in the second chamber 20 above a predetermined threshold, for example, a threshold of 25% or less of the initial pressure, throughout the duration of decrepitation.

[0029] Since the hydrogen decrepitation in step c) is an exothermic reaction, the temperature in the second chamber 20 is self-sustaining without the need for any heating means in the second station 2. The NdFeB magnets contained in the high-temperature waste 101 (i.e., at a temperature above 250°C) immediately begin the hydrogenation reaction and decrepitation phenomenon upon contact with the hydrogen atmosphere. Advantageously, the decrepitation temperature is close to the preheating temperature, which can be in practice between 200°C and 500°C. Automatic control of the amount of hydrogen introduced limits the temperature rise and makes it possible to maintain it below 500°C, the temperature at which the NdFeB alloy decomposes.

[0030] Performing steps b) and c) in two separate stations 1, 2, respectively, significantly simplifies the equipment and infrastructure for implementing the recycling method by de-correlating the heating demand (first station 1) from the hydrogen input (second station 2) and ensuring operation.

[0031] Step c) has a typical duration of a few minutes to 12 hours. The end of the hydrogenation reaction is detected by a decrease in the temperature of the second chamber 20 (measured via a temperature sensor immersed in the chamber 20) or by measuring the pressure in the second chamber 20, which tends to remain constant since the NdFeB magnets of the waste 100 no longer absorb hydrogen.

[0032] In the temperature range of 200-500°C, more specifically 300-500°C, it is essentially the grain boundary secondary phases of the NdFeB alloy that are hydrogenated. 14 The B major phase is not or only very slightly hydrogenated because the associated hydrides are not stable above 200° C. In fact, the major phase absorbs hydrogen and forms stable hydrides at lower temperatures, typically between room temperature and 150° C.

[0033] The advantage of this hydrogen decrepitation step c), performed at temperatures between 200°C and 500°C, preferentially between 300°C and 500°C, or between 350°C and 450°C, is that it reduces the amount of hydrogen (H2) required for hydrogenation because only the grain boundary secondary phase is targeted, which generally represents less than one-quarter of the alloy's volume. This typically reduces from 4 g of H2 per kg of hydrogenated magnet at room temperature to 1.25 g of H2 per kg of hydrogenated magnet at 400°C. Another advantage comes from the fact that the magnetic main phase metal grains are largely unaffected or unmodified by this process and therefore retain their initial properties and structure (i.e., the ones they had in the recycled NdFeB magnet). In particular, the overall magnetic orientation of the metal grains remains the same within the same powder particle.

[0034] The second station 2 advantageously comprises an agitation system 23 capable of transmitting motion to the second chamber 20 during step c). The agitation system may consist of a vibration system (with a vibration amplitude of typically 0.5 mm to 3 mm) or a wave vibration system. The agitation in the second chamber 20 promotes the separation of the NdFeB magnets from the other parts and the fragmentation of the NdFeB alloy by hydrogen decrepitation. Thus, the NdFeB magnets of the waste 100 treated in step c) are transformed into a powder (hereinafter referred to as the first powder 110) as this step progresses, the particles of which are NdFe 14 B contains a main phase and / or a grain boundary secondary phase, and has a size of 5 mm or less, or 1 mm or less.

[0035] Preferably, the recycling method comprises a step d) of sieving the first powder 110, performed during or after the decrepitation step c) (FIG. 1(d)).

[0036] To perform the sieving during step c), it is necessary to incorporate at least one sieve 31 (not shown) into the second chamber 20. The hot waste 101 is placed on said sieve 31, and as the NdFeB alloy is hydrogenated, particles of said alloy that are smaller in size than the mesh of the sieve 31 fall into the section of the enclosure below the screen. The fall of the alloy particles from the sieve 31 is facilitated by the agitation applied to the second chamber 20 located in the second station 2.

[0037] Alternatively, the NdFeB alloy first powder obtained at the end of step c) and other waste residues (if any) are transferred from the second chamber 20 to a sieving device 30, for example by means of a hatch associated with a valve (not shown) arranged in the lower part of the second chamber 20. The sieving device 30 may comprise one or more sieves 31. It may optionally be arranged in the second station 2 and, like the second chamber 20, may be subjected to the action of the agitation system 23 or may comprise its own agitation system 33 (FIG. 1(d)).

[0038] The mesh of the screen(s) 31 used in sieving step d) may be 1 cm, 5 mm, 1 mm, 500 microns, 300 microns, 150 microns, 100 microns, 50 microns, 10 microns or smaller. If desired, balls of a larger size (e.g., 10 times larger) than the particles of first powder 110 can be added onto sieve 31 to pre-grind and facilitate sieving of said powder.

[0039] If the waste collected in step a) contains metal parts integral with the NdFeB magnets, at least the sieving step d) is carried out using a 1 cm mesh to separate said parts from the first powder 110. If the metal parts or other chips (e.g. chips of the coating layer of the NdFeB alloy) generated during decrepitation have a smaller size, it is possible to consider sieving with a finer mesh, for example, 1 mm, 500 microns, 150 microns, 100 microns or even smaller mesh.

[0040] Optionally, step d) may comprise sieving the first powder 110 obtained from step c) through a screen size between 800 microns and 100 microns, thereby separating a batch of fine particle size powder 112 that can be used directly in the subsequent manufacture of new magnets, and a batch of coarse particle size powder 111 intended to undergo a subsequent grinding step that may be carried out in the recycling method according to the invention.

[0041] This separation in batches makes it possible to limit the process steps to a fraction of the volume of the recycled NdFeB magnet. Beyond the economic benefits, applying fewer processing steps to the alloy allows it to maintain as much of its initial quality as possible.

[0042] It should also be noted that it is also conceivable to sieve the first powder 110 through an ultrafine sieve, typically having a mesh size of less than 10 microns, or even less than 5 microns, e.g., about 1 micron. If the ultrafine metal particles formed essentially from grain boundary phase at this stage have deteriorated (e.g., by excessive oxidation) or no longer have the appropriate composition, this ultrafine sieving allows them to be separated from the rest of the first recycled powder. They may be replaced, if desired, by grain boundary phase particles of better quality or optimized composition (recycled or new) to improve the properties of new magnets produced from the recycled powder.

[0043] The particles of the first powder 110 have a mostly angular shape (FIG. 2), and a typical example of the particle size distribution after sieving step d) at 800 microns is shown in FIG. 2. The particle size referred to here is the "Sauter mean diameter", which is the diameter of spheres that behave identically during particle size measurement by a defined technique.

[0044] This first powder 110 can be adapted to a particular method for manufacturing new magnets. For example, the fraction of particles smaller than 50 microns in size is compatible with conventional sintering techniques for manufacturing new magnets. The fraction of particles between 100 microns and 500 microns is compatible with conventional techniques for manufacturing plastic bonded magnets by injection or compression.

[0045] Finally, for implementing additive manufacturing techniques, typically NdFeB alloy particles of less than 300 microns, or even less than 100 microns in size are preferred, but preferably they are less angular in shape.

[0046] In order to adjust the particle size distribution of the recycled alloy particles and also soften their shape, the recycling method may comprise a step e) of grinding the first powder 110 (or a fraction thereof) after step c) and / or during and / or after step d), thereby obtaining a second powder in which the particles have a more rounded shape and have a size of 500 microns or less, 300 microns or less, or 100 microns or less, or 50 microns or less.

[0047] Any known grinding means may be used in step e), in particular ball milling, ring milling or gas jet milling, the latter techniques being well known and commonly used to grind powders.

[0048] Ring milling is performed by vibrating the rings and cores circularly and horizontally on a vibrating plate. The sample is then fragmented under the forces of pressure, impact, and friction to a size that can be less than 20 microns.

[0049] The following description particularly relates to ball milling, a preferred embodiment of the present invention. A ball mill comprises rotating blades arranged in a rotating or stationary drum, in which all or part of the first powder 110 is placed, alone or with balls. Thus, the first powder 110 or a thickness thereof intended to undergo the milling process is transferred from the second station 2 to the third milling station.

[0050] If they are used, these balls may be selected from stainless steel and advantageously have a diameter of 1 mm to 30 mm, preferably 1 mm to 10 mm, including for example 5 mm. The mass ratio between the first powder and the balls is preferably selected to be between 0.5 and 3.

[0051] The milling in step e) with or without balls is advantageously carried out by applying a stirring sequence for 10 seconds to 1 hour, for example 10 minutes, at a drum rotation speed of 100 rpm to 800 rpm, for example 450 rpm, followed by a rest sequence for a few seconds (typically 5 seconds) to 40 seconds, for example 20 seconds. The stirring and rest sequences may be repeated 2 to 500 times in succession until the defined particle size of the second powder is obtained.

[0052] As an example, Figures 3a, 3b, and 3c correspond to SEM images of the particles of the second powder at the end of milling step e) for different iterations of the aforementioned stirring and resting sequences: 10, 30, and 300 iterations, respectively. In these examples, milling was carried out with balls. It is possible to observe the change in particle size distribution as well as their shape (Figures 3a, 3b, and 3c).

[0053] The recycling method finally comprises a dehydrogenation step f) that may be applied to the first powder 110 (or a sieved fraction thereof) or to the second powder obtained from the grinding step e) in order to extract the hydrogen present in the alloy particles. The dehydrogenation is carried out by heating the powder to about 800°C in a secondary vacuum in a fourth chamber located in a fourth station equipped with heating and pumping means.

[0054] Dehydrogenation is the process where hydrogen is transferred to Nd2Fe 14 It consists of two successive steps: release from the B-main phase (which may be slightly hydrogenated during step c) if desired) to 200°C, and then release from the grain boundary secondary phase to 350-800°C (depending on the composition, temperature conditions, and vacuum dynamics).

[0055] As mentioned above, the alloy particles contain a low hydrogen content since they are essentially grain boundary secondary phases that are hydrogenated during the decrepitation step c), and therefore the dehydrogenation step f) is faster than following the decrepitation steps typically performed in the prior art.

[0056] The alloy powder (first powder or second powder) obtained by the recycling method according to the present invention is NdFe 14 The alloy powders of the present invention contain particles composed of one or more metallic grains of the B magnetic primary phase, and when several metallic grains are present, they are separated from each other by a non-magnetic, rare earth-rich grain boundary secondary phase. A strong advantage of the alloy powders of the present invention is that the metallic grains contained in the same particle share a common crystallographic orientation that generates magnetic anisotropy. Therefore, these multi-metallic grain particles can be used as anisotropic mono-metallic grain particles in the production of new NdFeB magnets.

[0057] Anisotropic powders are usually obtained by expensive and complicated processes such as the HDDR (Hydrogenation-Disproportionation-Desorption-Recombination) method, and the recycling method according to the present invention makes it possible to obtain good quality anisotropic powders simply and economically.

[0058] The first powder 110 and the second powder obtained from the recycling method according to the present invention contain 1% to 50% by volume, preferably 10% to 30% by volume, of the grain boundary secondary phase. These powders can be directly used for the production of NdFeB permanent magnets.

[0059] According to an advantageous variant of the recycling method, particles consisting of rare earth-rich compounds may be added to the second powder to adjust its grain boundary secondary phase content or modify its composition, if necessary.

[0060] This addition may be made, for example, from neodymium (Nd) or other rare earth compounds containing elements such as Nd, Dy, Tb, Pr, Co, Fe, Cu, Al, Nb, Zr, Ti, etc. The following method may be used to introduce grain boundary secondary phase compounds, particularly those rich in rare earth(s). - hydrogenating the compound, for example at a temperature of 150°C and a pressure of 8 bar, to form the hydride, - grinding the hydride, for example according to grinding step e) of the recycling method, which grinding makes it easy to obtain very fine grain sizes of the hydride, i.e. typically particles having a size of less than 10 microns, less than 2 microns or even less than 1 micron, which grinding comprises mixing the hydrided compound with the first powder 110, It should be noted that if the grinding of the hydrogenated compound is not carried out during step e), it can be carried out during grinding step e) of the recycling method described above by adding a powder of the compound to the second powder during a small co-grinding in a volume fraction of a few tenths to several volume percents (typically 1% to 10%).

[0061] The dehydrogenation is carried out either during step f) of the recycling process or separately on the rare earth compound and the powder which are mixed at the end of the dehydrogenation.

[0062] The SEM image in Figure 4 gives an overview of the mixture obtained after this addition of particles of the grain boundary phase compound to the second powder. The finest particles (<1 micron) correspond to the added grain boundary phase rich in rare earths. The largest particles are part of the second powder.

[0063] According to another variant, in order to improve the performance of new magnets formed from recycled powder, it is possible to add compounds close to the composition of the initial magnet (fresh material of TRFeB), typically in the range of 1% to 90%, preferentially 1% to 40%, or preferentially 2% to 15%. Alternatively, TRFe 14 B type, especially Nd2Fe 14 It is possible to add only the compound consisting of the magnetic phase of B.

[0064] According to yet another variant that can be combined with the previous one, a non-magnetic metal compound (rare earth-free) with a low melting point (typically below the melting point of the grain boundary phase) can be added, the particles of which may have a particle size corresponding to that of the particles of the second powder, for example. Such a compound is intended in particular to promote bonding between the NdFeB particles during the creation of new magnets by additive manufacturing.

[0065] In the different variants mentioned, the compounds added are either added to the waste and simultaneously treated according to the recycling method described above, or are treated and modified in powder form and may then be added to the first or second powder according to the invention at any step of the method before or after the dehydrogenation step f).

[0066] According to a first aspect, the present invention relates to a method for producing a NdFeB permanent magnet from a first powder 110 (described above) or from a second powder according to the invention, implementing known techniques as follows: -Sintering technology, -Injection or compression technology to produce plastic bonded magnets; - Powder injection molding techniques, including debinding and sintering, or -Additive manufacturing methods involving a polymer matrix.

[0067] According to a second aspect, the invention relates to a method for manufacturing a NdFeB magnet, preferably from a second powder, implementing the metal additive manufacturing technique.

[0068] Among the well-known techniques in the additive manufacturing of metals, mention may be made in particular of powder bed melting (Selective Laser Melting - SLM™ or "Laser Beam Powder Bed Fusion - BLF"), Electron Beam Additive Manufacturing (EBAM), Metal Binder Jetting (MBJ), Directed Energy Deposition (DED), and Cold Spray Additive Manufacturing (CSAM). In the first two cited techniques, a light beam (laser or electron) is directed point-by-point to weld together free powder particles on the bed to form an object. In the third technique, the particles of the bed are welded together by droplets of a binder. In the last two examples, it is the spraying of powder onto a support, which allows their collective adhesion by supplying energy. These various techniques subject the metal powder to various flow, stress and processing regimes, and at least the first two cited techniques are based on melting the alloy powder and then consolidating said alloy in the form of the object to be printed. Non-optimized input powder properties directly lead to inconsistent finished part (NdFeB magnet) properties and the potential for defects.

[0069] Two important properties of powder for quality-limited printed parts are compactness (layer density) and flowability. Powders that are sufficiently compacted to provide high density are associated with the production of parts with consistent quality and fewer defects. Flowability is more closely related to the efficiency of the process for that part. The ability to distribute evenly on the bed and form a uniform thin layer without vacuum is essential for powder bed fusion processes, while consistent flowability as aerated powder flows is necessary for spraying techniques. These requirements intensify as additive manufacturing speeds increase.

[0070] Both bulk density and flowability are directly affected by the size and shape of powder particles. Generally, regularly shaped, smooth particles flow more easily than particles with rough surfaces and / or irregular shapes. Rougher surfaces result in increased interparticle friction, while irregularly shaped particles are more susceptible to mechanical agglomeration. These two effects reduce flowability. Similarly, spherical particles tend to compact more effectively than irregular ones, resulting in a higher apparent density. Sphericity is widely recognized in industry, which explains why most powders commonly used in additive manufacturing are produced by gas atomization.

[0071] In terms of particle size, metal powders are necessarily fine, for example, to meet the requirements for forming powder beds several tens of microns thick. However, because the attractive forces between particles increase with decreasing particle size, finer powders generally flow less freely than larger powders. The greatest compactness is achieved with a distribution containing both coarse and fine particles, with the finer particles increasing density by filling the gaps left by the largest ones.

[0072] Thus, the second powder according to the present invention may prove particularly suitable when it has a dual particle size distribution, i.e., a first population of particles centered around a first size and a second population of particles centered around a second size, where the first size is 1.5 to 10 times larger than the second size. Typically, the first size may be 15 to 90 microns, and the second size may be 1 to 15 microns. In the example shown in FIG. 3c, the first size is approximately 15 microns, and the second size is approximately 9 microns. Such a dual distribution may be obtained, in particular, by applying multiple grinding iterations, as described above in the recycling method.

[0073] The presence of rare earth-free non-magnetic metal compound particles and / or rare earth-rich grain boundary phase compound particles reduces the interaction between magnetic particles and significantly improves the injectability of the powder. This eliminates particle shape restrictions and provides outstanding flowability even for angular particles.

[0074] Conventionally, the above-mentioned additive manufacturing techniques provide a large amount of energy that allows the particles of the NdFeB alloy powder to be completely melted. In the manufacturing method according to the present invention, the melting is carried out by melting the NdFe 14 The melting temperature is about 1180°C, but can vary substantially depending on the composition of the NdFeB alloy.

[0075] Therefore, the melting temperature in the manufacturing process is typically less than 1180°C, preferably less than 1000°C, or even less than 800°C, or even less than 600°C. Therefore, the compaction between the powder particles is achieved by partially melting the NdFeB alloy. This has the advantage of not affecting the metallic crystal grains of the magnetic main phase in terms of size, shape, or composition. Therefore, the inherent magnetic properties of the alloy of the second powder can be better maintained.

[0076] For efficient particle compaction, it is important that a sufficient amount of the grain boundary secondary phase is present in the second powder, and therefore the second powder preferably contains 10 to 30 volume % of this phase.

[0077] In the aforementioned variant in which a low-melting non-magnetic metal compound (rare earth-free) is added to the second powder, said compound may be formed from one or more elements selected from Al, Zn, Sn, In, Li, Bi, Cd, Pb, alloys thereof, or other alloys without rare earths (e.g., Ag-Cu). This compound typically has a melting point of 800°C or less.

[0078] Preferentially, the powder contains 1% to 50% by volume, 1% to 20% by volume, or 1% to 10% by volume of said non-magnetic metal compound.

[0079] Other specific properties for the NdFeB magnet powder are necessary for the finished magnetic part to be an efficient solid magnet.

[0080] Magnets are primarily characterized by three main values: coercive force (denoted by Hc and expressed in kA / m), remanence (or residual magnetization, denoted by Br and expressed in Tesla), and maximum energy product (denoted by BHmax and expressed in MGOe). Coercive force corresponds to the resistance of a magnet to demagnetization when exposed to an environment that is either demagnetizing or high temperatures (above 100°C). Therefore, the stronger the coercive force, the better the temperature resistance during operation. Remanence indicates the magnetization and therefore the magnetic force that the magnet can provide to a system. Maximum energy product is characteristic of the total energy that a magnet can provide at its operating point.

[0081] The second powder obtained from the recycling method according to the present invention typically has a coercive force of 500 kA / m to 2400 kA / m and a residual magnetization of 0.5 T to 1.4 T, magnetic properties that are very favorable for making new permanent magnets.

[0082] In the manufacturing method according to the present invention, the remanence can be optimized during additive manufacturing by the magnetic or mechanical orientation system of the particles before compaction due to the crystalline orientation of the magnetic main phase metal grains. The second powder is anisotropic particles (i.e., magnetic phase metal grains NdFe with a common crystalline orientation that generates magnetic anisotropy). 14 B), so that magnets or electromagnets (magnetic orientation systems) carefully placed around or near the object to be printed make it possible to orient each particle comprising the main phase just before its consolidation.

[0083] Alternatively, the as-printed layer can be subjected to a forging operation in a direction perpendicular to the plane of the layer at a deformation rate of at least 8 / sec, thereby mechanically texture- ing each powder particle and / or magnetic phase metal grain and thus providing anisotropic magnetic orientation of the layer. This mechanical orientation system can be combined with the magnetic orientation system described above, if desired.

[0084] Another type of mechanical operation may optionally replace the forging operation (e.g., using rolls to simulate rolling or vibration systems to obtain compaction) to perform anisotropic magnetic orientation and / or to densify the powder bed (especially to avoid porosity defects or cracks in the printed object).

[0085] The coercive force becomes increasingly larger overall as the magnetic main phase metal grains are uniformly surrounded by grain boundary secondary phases in the printed object due to the effective magnetic separation of the metal grains. Coercive force occurs if the quality of the inert atmosphere (oxygen content less than 0.1%) is maintained in the printing chamber during consolidation.

[0086] In the manufacturing method according to the present invention, the fact that the temperature is kept below the melting point of the magnetic main phase is a key factor in the production of NdFe in the recycled initial magnet. 14This allows the magnetic quality (good remanence) of the B-phase metal grains to be maintained, simplifying the process for producing new magnets. Furthermore, the simple melting of the grain boundary secondary phases (and / or non-magnetic metal compounds, if present, that do not contain rare earths) facilitates the coating of the magnetic main phase metal grains, thus enabling the production of new printed magnets with high coercivity.

[0087] The method for producing permanent NdFeB solid magnets by additive manufacturing according to the present invention can find applications in multiple areas that require permanent magnets that are efficient in the great variety of object shapes that are particularly accessible by 3D printing, such as, for example, electronics, automobiles, computers, etc.

[0088] Naturally, the invention is not limited to the described embodiments and examples, but variant embodiments may be provided thereto without departing from the scope of the invention as defined by the claims.

Claims

1. 1. A method for recycling NdFeB magnets, comprising: a) recovering waste (100) containing NdFeB solid magnets to be recycled, said NdFeB magnets being 2 Fe 14 having a B magnetic primary phase and a non-magnetic grain boundary secondary phase; b) pre-heating the waste material (100) to a pre-heat temperature of 300°C to 500°C under an inert atmosphere, the waste material being contained in a first chamber (10) in a first station (1) equipped with heating means (11) and heated to the pre-heat temperature; c) hydrogen decrepitation applied to the hot waste (101) obtained from step b) under a hydrogen partial or total pressure of 0.1 bar to 10 bar, said hot waste (101) being contained in a second chamber (20) arranged in a second station (2) different from said first station (1), equipped with a hydrogen source (21) and pumping means (22), said decrepitation being carried out at a temperature between 200° C. and 500° C., said temperature in said second chamber (20) being maintained within this temperature range due to the exothermic nature of the hydridation reaction of the NdFeB magnets, step c) comprising the steps of: 2 Fe 14 causing the formation of a first powder (110) comprising a B-phase primary and / or grain boundary secondary phase and having grains with a size of 5 mm or less; A method comprising:

2. 2. The recycling method according to claim 1, wherein the first chamber (10) and the second chamber (20) are formed by a single chamber, which is movable from the first station (1) to the second station (2) between steps b) and c) and is connectable to the hydrogen source (21) and to the pump means (22) of the second station (2).

3. 2. The method of claim 1, further comprising a step d) of sieving the first powder (110) during or after step c).

4. 2. The recycling method according to claim 1, comprising a step e) after step c) or during or after the sieving step d), of grinding the first powder (110) or a fraction of the first powder to obtain a second powder having particles with a size of 500 microns or less.

5. 5. The method of claim 4, wherein particles of at least one rare earth rich compound are added to the first powder (110) during step e) to enrich the second powder with grain boundary secondary phases.

6. 5. The recycling method according to claim 4, wherein in step e) the grinding is carried out in a ball mill or by means of a ring mill or by means of a gas jet mill.

7. At least one compound of a fresh alloy of the TRFeB type (where TR is Nd, Pr, Dy and / or Tb), in particular NdFeB, and / or TR 2 Fe 14 B-type magnetic phase compounds, especially Nd 2 Fe 14 5. The recycling method of claim 4, wherein B is added to the first powder (110) or the second powder during the recycling method.

8. 5. The recycling method according to claim 4, wherein at least one rare earth-free non-magnetic metal compound having a melting point below the melting point of the grain boundary secondary phase is added to the first powder or the second powder during the recycling method, said compound intended to promote bonding between NdFeB particles during the manufacture of new magnets.

9. 5. The method of claim 4, further comprising a step f) of dehydridation of said first powder (110) or said second powder.

10. A powder obtained from the method for recycling NdFeB magnets according to claim 1, comprising several NdFeB magnets separated from each other by the grain boundary secondary phase. 2 Fe 14 A powder comprising particles composed of B magnetic main phase metal crystal grains, the metal crystal grains within the same particle having a common crystal orientation that generates magnetic anisotropy.

11. a first number of occupied particles centered on a first size; a second number of particles centered around a second size, said first size being between 1.5 and 10 times larger than said second size; and 11. The powder of claim 10 comprising a dual particle size distribution of:

12. 11. The powder according to claim 10, comprising 1% to 50% by volume of grain boundary secondary phases and / or 1% to 50% by volume of rare earth-free non-magnetic metal compounds.

13. From the powder according to claim 10 - sintering technology, or - Injection or compression techniques resulting in plastic bonded magnets, or - injection moulding techniques including debinding and sintering, or - Additive manufacturing technology A method for producing a NdFeB permanent magnet comprising:

14. Based on an additive manufacturing technique comprising melting at least one phase of the powder, the melting comprising: 2 Fe 14 This is carried out at a temperature below the melting temperature of the B main phase, thereby 2 Fe 14 14. The method according to claim 13, wherein the grain boundary secondary phase or, if present, the rare earth-free non-magnetic metal compound is melted in whole or in part, rather than the B main phase.

15. The method of claim 13 , wherein the powder particles including the primary phase are oriented prior to or during consolidation of the NdFeB magnet in the form of a printed object to impart magnetic anisotropy to the magnet.

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