A method for recycling waste NdFeB magnets

Abstract

The present invention discloses a method for recycling waste NdFeB magnets, comprising the following steps: (1) demagnetizing, grinding, cleaning, and drying waste NdFeB magnets of different grades and different coercive forces; (2) subjecting the dried waste NdFeB magnets to hydrogen explosion crushing and airflow milling to obtain magnet powder particles; (3) electroplating the surfaces of the magnet powder particles with a low-melting-point metal; (4) calculating the demagnetization field distribution of a target new magnet using micromagnetic simulation software, and programming a powder filling control program in a mold based on the demagnetization field distribution; (5) filling the electroplated magnet powder particles into a mold using the filling control program, with high-coercive force magnet powder being filled into the region corresponding to the large demagnetization field, and obtaining a blank after magnet orientation and pressure treatment; (6) hot-pressing and sintering the blank to obtain a new NdFeB magnet. The present invention can achieve efficient recycling of waste NdFeB magnets of different grades and produce new high-performance NdFeB magnets.

Description

Technical Field

[0001] The invention relates to a waste recycling method, in particular to an efficient recycling method for waste NdFeB magnets. Background Art

[0002] Recovering rare earth elements from scrapped NdFeB magnets for reuse has been a research hotspot in recent years. For example, electric vehicles typically have a lifespan of 10-15 years, and the number of permanent magnet motors in these scrapped vehicles will be substantial. Electronic products like mobile phones and headphones typically have a lifespan of 3-5 years before becoming electronic waste, containing large quantities of NdFeB magnets. However, the extraction of rare earth elements from scrapped NdFeB magnets currently typically involves chemical extraction, which is costly and has significant environmental impacts. The ability to directly crush and pulverize scrap NdFeB magnets without separating the rare earth elements would improve the recycling efficiency of scrap NdFeB magnets. Invention patents (such as CN202010421916.X, CN201710245829.1, and CN201410258657.8) all utilize a method of directly crushing waste NdFeB magnets into powder, then re-pressing and sintering them to produce NdFeB magnets. However, due to the wide variety of grades of waste NdFeB magnets, different grades have significantly different coercive forces due to varying rare earth element content and heavy rare earth element Dy / Tb content. Directly crushing and recycling waste NdFeB magnets of varying coercive force is not conducive to efficient utilization of waste NdFeB magnets. Summary of the Invention

[0003] Purpose of the invention: In order to solve the technical problems existing in the prior art, the present invention aims to provide an efficient recycling method for preparing new high-performance NdFeB magnets from waste NdFeB magnets.

[0004] Technical solution: The method for recycling waste NdFeB magnets of the present invention comprises the following steps:

[0005] (1) Demagnetize, polish, clean and dry the waste NdFeB magnets of different brands and coercive forces;

[0006] (2) hydrogen explosion crushing the dried waste NdFeB magnets to obtain magnet hydrogen explosion powder, and then grinding the hydrogen explosion powder into magnet powder particles using a jet mill;

[0007] (3) electroplating the surface of the magnet powder particles with a low melting point metal to obtain electroplated magnet powder particles;

[0008] (4) Using micromagnetic simulation software to calculate the demagnetization field distribution of the target new magnet, and writing a powder filling control program in the mold based on the demagnetization field distribution;

[0009] (5) filling the electroplated magnet powder particles obtained in step (3) into a mold under the filling control program of step (4), filling the high coercive force magnet powder into the area corresponding to the large demagnetization field, orienting the filled electroplated magnet powder particles in the magnetic field, and applying pressure to press the oriented powder into a blank;

[0010] (6) The blank obtained in step (5) is subjected to spark plasma hot pressing sintering to prepare a new NdFeB magnet.

[0011] Furthermore, in the step (1), the grades of the waste NdFeB magnets are 3-5 of N40, N52, 45M, 48H, 45H, 42SH, 45SH, 50M, and 42UH, and the average grain size is 3-10 μm. The waste NdFeB magnets are heated to a temperature of less than 1×10 - 2 Pa vacuum furnace for demagnetization at a temperature of 320-350 °C.

[0012] Furthermore, in the step (2), the size of the magnet hydrogen explosion powder is 0.5-1 mm, and the size of the magnet powder particles is 50-100 μm.

[0013] Furthermore, in step (3), the low melting point metal is Zn, Al or Cu, the electroplating aqueous solution is a sulfate aqueous solution or a chloride aqueous solution, and the coating is 0.1-1 μm.

[0014] Furthermore, in step (4), the micromagnetic simulation software is open source OOMMF software.

[0015] Furthermore, in step (5), the intensity of the magnetic field orientation is 1-2T, the applied pressure is 5-10MPa, and the direction of the pressure is perpendicular to the direction of the magnetic field orientation.

[0016] Furthermore, in step (6), the sintering temperature is 400-800° C., the sintering time is 5-20 min, and the applied pressure is 20-50 MPa.

[0017] Principle of the Invention: This invention considers the differences in composition and coercivity of different brands of scrap NdFeB magnets. Using micromagnetic simulation, the demagnetization field distribution of the target new magnet is calculated. Magnetic powders of varying coercivity are then electroplated and placed in corresponding positions in the magnet preparation mold according to the demagnetization field distribution of the new magnet to be prepared. High-coercivity magnet powder is placed in areas with large demagnetization fields. After orientation pressing, the new NdFeB magnet is prepared using spark plasma sintering technology. The NdFeB magnet with uneven coercivity distribution prepared by this invention exhibits high coercivity in areas with large demagnetization fields, preventing demagnetization at lower demagnetization fields and maintaining a high overall coercivity. Because the magnet powder particles are electroplated with a low-melting-point metal, this invention can be sintered using spark plasma sintering technology at a lower temperature (400-800°C) and a shorter time (5-20 minutes). Conventional sintered NdFeB magnets require higher sintering temperatures (around 1050°C) and longer sintering times (around 5 hours). At the same time, the crushed NdFeB powder used in the present invention has a size range of 50-100 μm, which is much larger than the 2-14-1 main phase grain size (3-10 μm) inside the powder. It can protect the original thin layer of rare earth-rich phase at the edge of the 2-14-1 main phase grain of the waste magnet to the maximum extent from being destroyed during the secondary processing process, and can ensure that the new magnet after the oriented sintering of the powder particles of this size has good orientation and high density.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the recycling method adopted by the present invention has a simple process, low sintering temperature, short sintering time, and can efficiently utilize waste NdFeB magnets. The coercive force of the obtained new NdFeB magnets presents an uneven distribution characteristic, and the coercive force distribution gradient is consistent with the decomposition distribution law. It can maximize the coercive force advantages of each waste NdFeB magnet and save heavy rare earth resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a flow chart of the recycling method of the present invention;

[0020] Figure 2 This is a backscattered electron image of the new NdFeB magnet prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The present invention is further described below with reference to specific embodiments, comparative examples and accompanying drawings.

[0022] Example 1: The method for recycling waste NdFeB magnets of the present invention comprises the following steps:

[0023] (1) Used NdFeB magnets with the grades of N40, 45M and 48H were placed in a vacuum of 1×10 -2Pa vacuum furnace to 350 ° C for demagnetization, then grind off the surface coating of the magnet, and then degrease and clean the surface and dry it;

[0024] (2) The dried waste NdFeB magnets were subjected to hydrogen explosion crushing to obtain 1 mm magnet hydrogen explosion powder, which was then ground into magnet powder particles with a size of 100 μm using a jet mill;

[0025] (3) electroplating the magnet powder particles in a zinc sulfate solution to obtain a Zn coating with a thickness of 0.1 μm on the surface;

[0026] (4) Use the micromagnetic simulation software OOMMF to calculate the demagnetization field distribution of the target new magnet with a size of 10 mm × 10 mm × 20 mm, and write the filling control program of the powder in the mold based on the demagnetization field distribution;

[0027] (5) filling the electroplated magnet powder particles obtained in step (3) into a mold under the filling control program of step (4), and filling the high coercive force magnet powder into the area corresponding to the large demagnetization field. The proportions of three grades of N40, 45M, and 48H electroplated magnet powder are 30%, 30%, and 40%, respectively. Orienting the filled powder in a pulsed magnetic field of 1T, and applying a pressure of 5MPa to press the oriented powder into a blank;

[0028] (6) The blank obtained in step (5) was subjected to spark plasma hot pressing sintering at 400°C and a pressure of 20 MPa for 5 minutes to prepare a new NdFeB magnet. The flow chart is shown in FIG. Figure 1 .

[0029] Comparative Example 1: The difference from Example 1 is that step (4) is not included, and in step (5), the three magnet powders are mixed evenly and then filled into the mold without considering the uneven distribution characteristics of the magnet demagnetization field.

[0030] Performance tests were conducted on three brands of waste NdFeB magnets, as well as the new NdFeB magnets prepared in Example 1 and Comparative Example 1. The test results are shown in Table 1. Table 1 shows that the coercivity of the new magnet prepared in Example 1 using the three brands of waste NdFeB magnets is much greater than that of the new magnet prepared in Comparative Example 1 and the average coercivity of the three waste magnets. Furthermore, the density of the new magnet fully meets the density standard for commercial sintered NdFeB magnets. This is primarily because the magnet powders of varying coercivity are evenly distributed within the new magnet prepared in Comparative Example 1. During the demagnetization process of the NdFeB magnet, the low-coercivity magnet powder in the area with the large demagnetization field demagnetizes first, triggering the demagnetization of the adjacent high-coercivity powder particles coupled to it, resulting in a relatively low overall coercivity of the magnet. In contrast, the coercivity of the magnet powder in Example 1 is high in the area with the large demagnetization field, which prevents the NdFeB magnet from demagnetizing under lower demagnetization fields, resulting in a high overall coercivity of the magnet.

[0031] Table 1 Magnetic properties and magnet density of three brands of waste NdFeB magnets, new NdFeB magnets prepared in Example 1 and Comparative Example 1

[0032] magnet Coercive force (kOe) Remanence (T) <![CDATA[Density (g / cm 3 )]]> N40 12 1.25 7.56 45M 14 1.32 7.55 48H 17 1.36 7.60 Example 1 16 1.32 7.52 Comparative Example 1 14 1.31 7.51

[0033] It can be seen from the backscattered electron image of Example 1 that ( Figure 2 ), the magnet prepared by the process of the present invention has a uniform grain size distribution, a good distribution of rare earth-rich phases at the grain boundaries, and a dense interface between powder particles, which meets the microstructural characteristic requirements of high-performance NdFeB magnets.

[0034] Example 2: The recycling method of waste NdFeB magnets of the present invention comprises the following steps:

[0035] (1) Used NdFeB magnets with the grades of N52, 48M, 45H and 42SH were placed in a vacuum of 8×10 -3 Pa vacuum furnace to 340 ℃ for demagnetization, then grind off the surface coating of the magnet, and then degrease and clean the surface and dry it;

[0036] (2) The dried waste NdFeB magnets were subjected to hydrogen explosion crushing to obtain 0.5 mm magnet hydrogen explosion powder, which was then ground into magnet powder particles with a size of 70 μm using a jet mill;

[0037] (3) electroplating the magnet powder particles in a zinc sulfate solution to obtain an Al coating with a thickness of 0.5 μm on the surface;

[0038] (4) Use the micromagnetic simulation software OOMMF to calculate the demagnetization field distribution of the target new magnet with a size of 10 mm × 10 mm × 20 mm, and write the filling control program of the powder in the mold based on the demagnetization field distribution;

[0039] (5) filling the electroplated magnet powder particles obtained in step (3) into a mold under the filling control program of step (4), and filling the high coercive force magnet powder into the area corresponding to the large demagnetization field, with the four brands of electroplated magnet powder accounting for 25% each. Orienting the filled powder in a pulsed magnetic field of 1.5 T, and applying a pressure of 7 MPa to press the oriented powder into a blank;

[0040] (6) The blank obtained in step (5) is subjected to spark plasma hot pressing sintering at 600° C. and a pressure of 35 MPa for 10 min to prepare a new NdFeB magnet.

[0041] Comparative Example 2: The difference from Example 2 is that step (4) is not included, and in step (5), the four magnet powders are mixed evenly and then filled into the mold without considering the uneven distribution characteristics of the magnet demagnetization field.

[0042] Performance tests were conducted on four brands of scrap NdFeB magnets, as well as the new NdFeB magnets prepared in Example 2 and Comparative Example 2. The test results are shown in Table 2. Table 2 shows that the coercive force of the new magnets prepared in Example 2 using the four brands of scrap NdFeB magnets is much greater than the coercive force of the new magnets prepared in Comparative Example 2 and the average coercive force of the four scrap magnets. In addition, the density of the new magnets fully meets the density standards for commercial sintered NdFeB magnets.

[0043] Table 2 Magnetic properties and magnet density of four brands of waste NdFeB magnets, new NdFeB magnets prepared in Example 2 and Comparative Example 2

[0044]

[0045]

[0046] Example 3: The method for recycling waste NdFeB magnets of the present invention comprises the following steps:

[0047] (1) Used NdFeB magnets with the grades of N52, 50M, 48H, 45SH and 42UH were placed in a vacuum of 6×10 -3 Pa vacuum furnace heated to 320 ℃ for demagnetization, then polished off the surface coating of the magnet, and then degreased and dried;

[0048] (2) The dried waste NdFeB magnets were subjected to hydrogen explosion crushing to obtain 0.7 mm magnet hydrogen explosion powder, which was then ground into magnet powder particles with a size of 100 μm using a jet mill;

[0049] (3) electroplating the magnet powder particles in a zinc sulfate solution to obtain a Cu coating with a thickness of 1 μm on the surface;

[0050] (4) Using the micromagnetic simulation software OOMMF to calculate the demagnetization field distribution of the target new magnet with a size of 20 mm × 20 mm × 20 mm, a powder filling control program in the mold was written based on the demagnetization field distribution;

[0051] (5) filling the electroplated magnet powder particles obtained in step (3) into a mold under the filling control program of step (4), and filling the high coercive force magnet powder into the area corresponding to the large demagnetization field, with the five brands of electroplated magnet powder each accounting for 20%, and orienting the filled powder in a 2T pulse magnetic field, and applying a pressure of 10 MPa to press the oriented powder into a blank;

[0052] (6) The blank obtained in step (5) is subjected to spark plasma hot pressing sintering at 800° C. and a pressure of 50 MPa for 20 min to prepare a new NdFeB magnet.

[0053] Comparative Example 3: The difference from Example 3 is that step (4) is not included, and in step (5), the five types of magnet powders are mixed evenly and then filled into the mold without considering the uneven distribution characteristics of the magnet demagnetization field.

[0054] Performance tests were conducted on five brands of scrap NdFeB magnets, as well as the new NdFeB magnets prepared in Example 3 and Comparative Example 3. The test results are shown in Table 3. Table 3 shows that the coercive force of the new magnets prepared in Example 3 using the five brands of scrap NdFeB magnets is much greater than that of the new magnets prepared in Comparative Example 3 and the average coercive force of the five scrap magnets. In addition, the density of the new magnets fully meets the density standards for commercial sintered NdFeB magnets.

[0055] Table 3 Magnetic properties and magnet density of five brands of waste NdFeB magnets, new NdFeB magnets prepared in Example 3 and Comparative Example 3

[0056]

[0057]

[0058] Since there are many brands of commercial sintered NdFeB magnets, the present invention only selects some brands of waste NdFeB magnets as research objects to verify the effect of the present invention.

[0059] It can be seen from the above embodiments and corresponding comparative examples that, based on the demagnetization field distribution characteristics of the target magnet, the present invention uses spark plasma sintering technology to prepare NdFeB magnets with uneven coercive force distribution from electroplated waste NdFeB magnet powders with different coercive forces. The coercive force of the prepared new magnets is significantly higher than that of the comparative example and the average coercive force of the selected waste magnets.

[0060] The present invention provides a new method for the efficient recycling of waste NdFeB magnets. The promotion and application of this technology will effectively promote the efficient recycling of waste NdFeB magnets and reduce the pressure of the production of high coercive force NdFeB magnets on the country's rare earth resources.

Claims

1. A method for recycling waste NdFeB magnets, characterized in that: The following steps are involved: (1) Demagnetizing, grinding, cleaning and drying waste NdFeB magnets of different brands and different coercive forces; the brands of the waste NdFeB magnets are 3-5 of N40, N52, 45M, 48H, 45H, 42SH, 45SH, 50M and 42UH, and the average grain size is 3-10 μm; (2) The dried waste NdFeB magnets are subjected to hydrogen explosion crushing to obtain magnet hydrogen explosion powder, and then the hydrogen explosion powder is ground into magnet powder particles using a jet mill; the size of the magnet hydrogen explosion powder is 0.5-1 mm; the size of the magnet powder particles is 50-100 μm; (3) electroplating the surface of the magnet powder particles with a low melting point metal to obtain electroplated magnet powder particles; (4) Use micromagnetic simulation software to calculate the demagnetization field distribution of the target new magnet, and write a powder filling control program in the mold based on the demagnetization field distribution; (5) filling the electroplated magnet powder particles obtained in step (3) into the mold under the filling control program of step (4), filling the high coercive force magnet powder into the area corresponding to the large demagnetization field, orienting the filled electroplated magnet powder particles in the magnetic field, and applying pressure to press the oriented powder into a blank; (6) The blank obtained in step (5) is subjected to spark plasma hot pressing sintering to prepare a new NdFeB magnet; the sintering temperature is 400-800°C, the sintering time is 5-20 minutes, and the applied pressure is 20-50 MPa.

2. The method for recycling waste NdFeB magnets according to claim 1, wherein: In the step (1), the waste NdFeB magnets are placed in a vacuum of ≤1×10 -2 Pa vacuum furnace for demagnetization at a temperature of 320-350 °C.

3. The method for recycling waste NdFeB magnets according to claim 1, wherein: In the step (3), the low melting point metal is Zn, Al or Cu.

4. The method for recycling waste NdFeB magnets according to claim 1, wherein: In the step (3), the electroplating aqueous solution is a sulfate aqueous solution or a chloride aqueous solution, and the plating layer is 0.1-1 μm.

5. The method for recycling waste NdFeB magnets according to claim 1, wherein: In the step (4), the micromagnetic simulation software is the open source OOMMF software.

6. The method for recycling waste NdFeB magnets according to claim 1, wherein: In the step (5), the intensity of the magnetic field orientation is 1-2 T, the applied pressure is 5-10 MPa, and the direction of the pressure is perpendicular to the direction of the magnetic field orientation.

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