Method for crushing sintered neodymium-iron-boron magnet through jet milling by utilizing neon

Abstract

The invention discloses a method for crushing a sintered neodymium-iron-boron magnet through jet milling by using neon. Firstly, magnet coarse particles subjected to hydrogen decrepitation treatment are placed in a jet mill grinding chamber, then neon flowing at a high speed is introduced, powder particles are further crushed and enter a sorting device by adjusting the neon flowing speed and the ventilation time, particles larger than the specified size return and continue to be crushed, powder smaller than the specified size is separated, and particles with too small sizes are removed; and finally, powder with the particle size of 2.5-4 microns is obtained. Compared with a traditional airflow milling method using nitrogen, the neon gas is adopted, the powder particle size is successfully controlled within the ideal single domain critical size range, and meanwhile the powder flowability and the filling density are improved. The process compatibility and the forming efficiency of the magnetic powder in the subsequent magnetic field orientation and pressing forming process are remarkably improved, the nitridation reaction of rare earth elements is avoided due to the chemical inertness of neon, the nitrogen content of the magnetic powder is remarkably reduced, and a foundation is laid for obtaining a magnet with better performance.

Description

Technical Field

[0001] This invention belongs to the field of rare earth magnetic materials technology, specifically relating to a method for crushing and sintering NdFeB magnets using a gas flow mill with neon gas. Background Technology

[0002] Sintered NdFeB magnets, due to their excellent magnetic properties such as high remanence (Br), high coercivity (Hcj), and high energy product ((BH)max), are widely used in strategic fields such as new energy vehicle drive motors, wind power generation, and high-end electroacoustic devices. Their manufacturing process mainly includes key steps such as melting, hydrogen breaking, air jet milling, magnetic field orientation, molding, sintering, and tempering. Among these, the air jet milling process plays a crucial role in determining the particle size and fundamental performance of the magnetic powder.

[0003] Currently, the industrial process widely employs a combination of hydrogen pulverization and air jet milling to break coarse-grained magnets into fine powder with a particle size of approximately 3 μm. This particle size is close to the critical size of a single magnetic domain in NdFeB magnets, effectively improving coercivity. Therefore, the selection of the process gas is crucial in the air jet milling process. Nitrogen is currently the primary working gas due to its low cost, small molecular weight, and ability to achieve fine particle size control. However, during high-energy air jet milling, nitrogen reacts with the rare earth elements (such as Pr and Nd) abundant in the magnetic powder, generating rare earth nitrides. This consumes active rare earth elements, reducing the effective rare earth content in the sintered magnet. This reaction is particularly detrimental in the preparation of high-performance NdFeB magnets. As sintered NdFeB magnets demand higher performance consistency and coercivity, the total rare earth content in modern formulations is approaching the theoretical saturation value (approximately 26.7%). Any additional losses can lead to a decline in magnet performance, specifically insufficient coercivity, poor product consistency, and impact on the reliability of high-end applications.

[0004] Both theory and practice show that the smaller the molecular weight of a gas, the higher the airflow velocity and the greater the kinetic energy it carries, thus being more conducive to achieving efficient particle crushing and refining. Helium (molecular weight 4 g / mol), due to its extremely small molecular weight, can generate extremely high airflow velocities and is often considered the theoretical choice for achieving optimal pulverization effects. However, as a strategic resource, its high price and unstable supply limit its application in large-scale industrial production. Currently, nitrogen is widely used as a working medium in industry, but the presence of nitrogen reduces the effective rare earth content in sintered magnets. Furthermore, while argon (molecular weight 40 g / mol) is an ideal inert gas compared to helium, its larger molecular weight and relatively lower airflow velocity result in insufficient kinetic energy transfer efficiency to particles, placing higher demands on the precise control of powder particle size distribution and single-domain critical size. Therefore, finding an alternative working gas that can provide efficient kinetic energy close to that of light gases, possess good chemical inertness, and is economically feasible is of great significance for improving the preparation level of high-performance sintered NdFeB magnets. Summary of the Invention

[0005] The purpose of this invention is to provide a method for crushing and sintering NdFeB magnets using an air jet mill with neon as the working gas, in order to solve the dilemma of selecting protective gas in existing processes: avoiding rare earth nitriding reactions caused by nitrogen, overcoming the limitation of insufficient kinetic energy transfer efficiency of argon, and seeking a balance between the high performance and high cost of helium.

[0006] To achieve its purpose, the present invention adopts the following technical solution: This invention provides a method for crushing sintered NdFeB magnets using a gas flow mill with neon gas, comprising the following steps: (1) The coarse-grained sintered NdFeB magnets after hydrogen pyrolysis were placed in the grinding chamber of an air jet mill; (2) High-speed flowing neon gas is introduced into the bottom and side wall of the grinding chamber described in step (1) to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed and are broken. (3) The crushed magnet powder is sent to the sorting device, and particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with a particle size <1 μm and collect NdFeB magnetic powder with uniform particle size.

[0007] As a further preferred embodiment of the technical solution of the present invention, in step (1), the amount of coarse-grained NdFeB magnets added to the air jet mill grinding chamber is equivalent to 15%-30% of the grinding chamber volume.

[0008] Further, in step (2), the flow rate of the neon gas is 80-200 m / s, and the gas-solid ratio of neon gas to magnet in the grinding chamber is 6-150 m. 3 / kg.

[0009] Furthermore, in step (2), the neon gas is introduced for 10-20 minutes. Too short a time will result in insufficient particle crushing, which will increase the content of heavy particles and reduce the classification efficiency; too long a time will increase energy consumption, reduce production efficiency, and may exacerbate the risk of powder oxidation.

[0010] Furthermore, in step (3), the sorting device is an adjustable-speed classifying wheel structure with a classifying wheel speed of 2000-6000 rpm. When the classifying wheel speed is too low, the classification accuracy is insufficient, and larger particles are easily mistakenly discharged into the finished product; when the speed is too high, some qualified particles may be mistakenly judged as coarse particles and returned to the grinding zone, resulting in over-grinding, generating excessively fine powder, which affects the powder flowability and subsequent molding performance.

[0011] Furthermore, in step (4), the collected NdFeB magnetic powder has a particle size range of 2.5-4 μm. This particle size is close to the size of a single magnetic domain, which helps to form a single-domain particle structure, thereby effectively improving the coercivity of the magnetic powder and the consistency of magnet performance.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Using neon as the grinding medium of the air jet mill, its molecular weight is smaller than that of commonly used nitrogen and argon, which are also inert gases. This is more conducive to particle acceleration and efficient crushing, and helps to obtain magnetic powder with a particle size close to the critical size of the magnetic single domain of neodymium iron boron. (2) Neon is cheaper than helium, and although slightly more expensive than nitrogen, its cost can be fully covered by high-value rare earth resources given the current rising price of praseodymium and neodymium metals. By rationally controlling the gas-solid ratio of neon and magnetic powder, the flow rate of neon, and the introduction time, efficient crushing of magnets can be effectively achieved, resulting in magnetic powder with an ideal particle size range. This also improves the powder's flowability and filling density, thus providing better process compatibility for subsequent magnetic field orientation and molding processes. While ensuring good compatibility between the key physical properties of the magnetic powder (such as particle size distribution and morphology) and the powder prepared by the nitrogen process, the method significantly reduces the nitrogen content of the magnetic powder because it does not react with rare earth elements, laying the foundation for obtaining magnets with superior magnetic properties. Specifically, this method helps improve the coercivity and product consistency during the subsequent magnetic powder orientation pressing process, significantly enhancing the overall performance and reliability of sintered NdFeB magnets. Attached Figure Description

[0013] Figure 1 This is a flowchart of the process of the present invention; Figure 2 This is a bar chart showing the nitrogen content of Examples 1-4 of the present invention. Detailed Implementation

[0014] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, any changes that fall within the spirit and scope of the present invention as defined and determined by the appended claims are all within the scope of protection of the present invention.

[0015] In the following embodiments of the present invention, the particle size of the coarse-grained sintered NdFeB magnets subjected to hydrogenation crushing treatment is 0.5-5 mm.

[0016] Example 1 A method for crushing sintered NdFeB magnets using a gas flow mill with neon gas, the specific process of which includes the following steps: (1) Weigh 100 g of coarse sintered NdFeB magnets that have undergone hydrogenation crushing treatment and place them evenly into the air jet mill grinding chamber; (2) High-speed neon gas is introduced into the bottom and side wall of the grinding chamber. The neon gas flow rate is adjusted to 80 m / s and the introduction time is 20 min to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed to achieve crushing. (3) The crushed powder is fed into the sorting device. The speed of the sorting device is set to 3000 rpm and the target particle size is set to 4 μm. Particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with too small particle size (<1 μm) and finally obtain NdFeB magnetic powder with an average particle size of 3.666 μm. Table 1 is a simplified table of the particle size distribution of the magnetic powder in Example 1. D50 represents the median diameter, a core indicator characterizing the average particle size of the powder. D50 = 3.666 μm indicates that 50% of the particles in the sample have a particle size less than or equal to 3.666 μm. This value shows that the average particle size of the powder is precisely controlled within the ideal single-domain critical size range (3-4 μm) for NdFeB, providing a crucial particle size basis for obtaining high coercivity. Simultaneously, the relatively small distribution span ((D90-D10) / D50) indicates a concentrated particle size distribution and good uniformity, which is beneficial for improving the compact density and the consistency of magnetic properties.

[0017] Table 1. Simplified Particle Size Distribution of Magnetic Powder in Example 1 (5) The nitrogen content in the magnetic powder sample was determined to be 580 ppm using the inert gas melting-thermal conductivity detection method (IGC-TCD). Figure 2 The reduction in effective rare earth content in sintered magnets is negligible, which is particularly beneficial for the preparation of high-performance NdFeB magnets.

[0018] The specific steps are as follows: Preliminary preparations: 1. Sample preparation: Weigh the sample and prepare it into blocks or chips suitable for feeding into the graphite crucible. 2. Instrument preparation: Ensure a stable supply of high-purity helium (purity ≥99.999%) and check the system's airtightness. Perform high-temperature degassing on the graphite crucible to remove the crucible itself and adsorbed water and air until the blank value drops to a stable and sufficiently low level.

[0019] Analysis Procedure: 1. Before adding the sample, run one analysis cycle to measure the system's blank value (the oxygen and nitrogen content released by the system itself). This value must be subtracted from subsequent sample analysis results. 2. Place the sample into a degassed, helium-protected high-temperature graphite crucible. Apply high power (typically >2000℃) to completely melt the sample within seconds. 3. The released gas mixture (CO, N2, H2) is carried by helium and first passes through a catalytic furnace to convert CO to CO2. Subsequently, the gas mixture is separated according to peak elution time by a gas chromatography column and finally sequentially enters a thermal conductivity detector (TCD). The TCD generates electrical signal peaks proportional to the concentration of each gas.

[0020] Example 2 A method for crushing sintered NdFeB magnets using a gas flow mill with neon gas, the specific process of which includes the following steps: (1) Weigh 120 g of coarse sintered NdFeB magnets that have undergone hydrogenation crushing treatment and place them evenly into the air jet mill grinding chamber; (2) High-speed neon gas is introduced into the bottom and side wall of the grinding chamber. The neon gas flow rate is adjusted to 150 m / s and the introduction time is 20 min to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed to achieve crushing. (3) The crushed powder is fed into the sorting device. The speed of the sorting device is set to 5000 rpm and the target particle size is set to 4 μm. Particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with too small a particle size (<1 μm), and finally neodymium iron boron magnetic powder with an average particle size of 3.736 μm is obtained; data analysis is the same as in Example 1; Table 2 is a simplified table of magnetic powder particle size distribution in Example 2, where D50 = 3.736 μm.

[0021] Table 2. Simplified particle size distribution of magnetic powder in Example 2 (5) The nitrogen content of the magnetic powder sample was determined to be 670 ppm using the inert gas melting-thermal conductivity detection method (IGC-TCD). Figure 2 The specific operation and data analysis are the same as in Example 1.

[0022] Example 3 A method for crushing sintered NdFeB magnets using a gas flow mill with neon gas, the specific process of which includes the following steps: (1) Weigh 120 g of coarse sintered NdFeB magnets that have undergone hydrogenation crushing treatment and place them evenly into the air jet mill grinding chamber; (2) High-speed neon gas is introduced into the bottom and side wall of the grinding chamber. The neon gas flow rate is adjusted to 100 m / s and the introduction time is 15 min to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed to achieve crushing. (3) The crushed powder is fed into the sorting device. The speed of the sorting device is set to 4000 rpm and the target particle size is set to 4 μm. Particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with too small a particle size (<1 μm), and finally neodymium iron boron magnetic powder with an average particle size of 3.712 μm is obtained; data analysis is the same as in Example 1; Table 3 is a simplified table of magnetic powder particle size distribution in Example 3, where D50 = 3.712 μm.

[0023] Table 3. Simplified Particle Size Distribution of Magnetic Powder in Example 3 (5) The nitrogen content of the magnetic powder sample was determined to be 610 ppm using the inert gas melting-thermal conductivity detection method (IGC-TCD). Figure 2 The specific operation and data analysis are the same as in Example 1.

[0024] Example 4 A method for crushing sintered NdFeB magnets using a gas flow mill with neon gas, the specific process of which includes the following steps: (1) Weigh 100 g of coarse sintered NdFeB magnets that have undergone hydrogenation crushing treatment and place them evenly into the air jet mill grinding chamber; (2) High-speed neon gas is introduced into the bottom and side wall of the grinding chamber. The neon gas flow rate is adjusted to 100 m / s for 10 min to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed to achieve crushing. (3) The crushed powder is fed into the sorting device. The speed of the sorting device is set to 6000 rpm and the target particle size is set to 4 μm. Particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with too small a particle size (<1 μm), and finally neodymium iron boron magnetic powder with an average particle size of 3.691 μm is obtained; data analysis is the same as in Example 1; Table 4 is a simplified particle size distribution table for Example 4, where D50 = 3.691 μm.

[0025] Table 4. Simplified Particle Size Distribution Table for Example 4 (5) The nitrogen content of the magnetic powder sample was determined to be 660 ppm using the inert gas melting-thermal conductivity detection method (IGC-TCD). Figure 2 The specific operation, namely data analysis, is the same as in Example 1.

Claims

1. A method of comminuting sintered neodymium-iron-boron magnets by air-jet milling using neon gas, characterized in that, Includes the following steps: (1) The coarse-grained sintered NdFeB magnets after hydrogen pyrolysis were placed in the grinding chamber of an air jet mill; (2) High-speed flowing neon gas is introduced into the bottom and side wall of the grinding chamber described in step (1) to form a rotating airflow field. Under the action of the neon gas flow, the magnetic particles collide at high speed and are broken. (3) The crushed magnet powder is sent to the sorting device, and particles larger than the set particle size are returned to the grinding chamber for further crushing. (4) Powder with qualified particle size is fed into a high-speed cyclone separator to remove particles with a particle size <1 μm and collect NdFeB magnetic powder with uniform particle size.

2. The method of claim 1, wherein the sintered Nd-Fe-B magnet is broken by the jet mill using neon gas. In step (1), the amount of coarse NdFeB magnets added to the air jet mill grinding chamber is equivalent to 15%-30% of the grinding chamber volume.

3. The method of claim 1, wherein the sintered Nd-Fe-B magnet is broken by the jet mill using neon gas. In step (2), the neon gas has a flow rate of 80-200 m / s, and the gas-solid ratio of the neon gas to the magnet in the milling chamber is 6-150 m / kg. 3 / kg.

4. The method for crushing sintered NdFeB magnets using an air jet mill with neon gas as described in claim 3, characterized in that... In step (2), the neon gas is introduced for 10-20 minutes.

5. A method for crushing sintered NdFeB magnets using an air jet mill with neon gas as described in any one of claims 1-4, characterized in that... In step (3), the sorting device is a speed-adjustable grading wheel structure with a grading wheel speed of 2000-6000 rpm.

6. A method for crushing sintered NdFeB magnets using an air jet mill with neon gas as described in any one of claims 1-4, characterized in that, In step (4), the particle size range of the collected NdFeB magnetic powder is 2.5-4 μm.

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