Preparation method for micron-sized high-purity α"-fe 16n 2 magnetic powder

Abstract

Disclosed in the present invention is a preparation method for a micron-sized high-purity α"-Fe16N2 magnetic powder. The preparation method comprises the following steps: 1) sequentially performing high-pressure roller milling, high-energy grinding, and drying treatment on an iron ore concentrate to obtain a micron-sized fine particulate iron oxide powder; 2) performing hydrogen reduction on the micron-sized fine particulate iron oxide powder to obtain a porous active reduced iron powder, wherein the temperature for the hydrogen reduction is controlled within a range of 50ºC below the critical temperature of a reduction transformation pathway; and 3) performing nitriding on the porous active reduced iron powder in an ammonia gas atmosphere to obtain α"-Fe16N2 iron nitride. The method uses an inexpensive iron ore concentrate as a raw material, has a low cost and a simple process, can achieve mass production, and can obtain a micron-sized high-purity α"-Fe16N2 iron nitride powder, which can be then formed by means of a conventional additive manufacturing method to produce a high-performance magnet.

Description

A micron-sized high-purity α″-Fe 16 Preparation method of N2 magnetic powder Technical Field

[0001] This invention relates to an α″-Fe 16 A method for preparing N2 iron nitride magnetic powder, particularly involving a method for preparing large-size α″-Fe through synergistic activation by high-pressure roller milling and high-energy grinding. 16 The method for preparing N2 iron nitride powder belongs to the field of magnetic powder material preparation technology. Background Technology

[0002] Permanent magnet materials have a wide range of applications and are essential basic materials for economic development. Currently, high-quality permanent magnet materials are based on rare-earth permanent magnets, such as Nd-Fe-B, which possess high coercivity. However, the maximum energy product (BH)max is limited by the low Ms value of the rare-earth compounds themselves, hindering further breakthroughs, particularly limiting their application in high-tech industries such as electric vehicles and wind power generation facilities. Furthermore, the rare-earth metals required for rare-earth permanent magnets are valuable strategic resources, expensive and facing the risk of depletion; their mining volume is also insufficient to meet the needs of the rapidly developing new energy sector. Therefore, developing new zero-rare-earth high-performance permanent magnet materials is undoubtedly a crucial guarantee for solving future energy problems. Currently, iron nitride is the most likely material to replace rare-earth permanent magnets on a large scale. Iron nitride includes various compounds such as FeN, Fe2N, Fe3N, and Fe4N, among which α″-Fe... 16 Nitrogen compounds are the materials with the highest saturation magnetic induction intensity discovered to date, and also possess characteristics such as relatively high magnetocrystalline anisotropy, almost meeting all the material performance requirements of high-performance magnets. α″-Fe 16 N2 is composed of Fe and N elements, and is abundant and inexpensive. However, it has poor stability at high temperatures and is easily decomposed above 200°C. Currently, when using methods such as magnetron sputtering, there are significant limitations in terms of production yield, purity, and cost. The main difficulty in preparing it in the reduction nitriding method lies in how to achieve iron nitriding in the extremely low temperature range below the decomposition temperature. Technical issues

[0003] In existing technologies, the nitriding process mainly promotes the reaction by refining the particle size of iron powder, and is further enhanced by reaction pressure or external fields. For example, Chinese Patent (Publication No.: CN105861978A) discloses a mechanical ball milling method for preparing nitrided ferromagnetic powder, and Chinese Patent (Publication No.: CN114730662A) discloses Fe as a permanent magnet. 16Methods for preparing N2 compounds include a low-temperature plasma nitriding method for preparing ferronitride powder, as disclosed in Chinese patent (publication number: CN106086776A), and others. Generally speaking, these existing technologies prepare α″-Fe... 16 Nitrogen (N2) materials are mostly in the form of ultrathin coatings or nano-sized particles, making them unsuitable for forming high-performance magnets through pressing or bonding, especially when preparing large-particle products where purity cannot be guaranteed. Therefore, existing α″-Fe... 16 The purity and particle size of N2 substances urgently need to be improved. Technical solutions

[0004] The existing technology struggles to prepare high-purity α″-Fe with micron-sized particles. 16 To address technical issues such as N2, the objective of this invention is to provide a high-purity α″-Fe with micron-sized N2. 16 Preparation method of N2 iron nitride magnetic powder.

[0005] The micron-level high-purity α″-Fe of this invention 16 The N2 magnetic powder is micron-sized, preferably 2~40μm, more preferably 10~30μm. "High purity" refers to a pure phase iron nitride content of 97% or more, preferably 98% or more.

[0006] To achieve the above technical objectives, the present invention provides a micron-sized high-purity α″-Fe 16 A method for preparing N2 magnetic powder includes the following steps:

[0007] 1) Iron concentrate is sequentially subjected to high-pressure roller milling, high-energy grinding and drying to obtain micron-sized fine iron oxide powder;

[0008] 2) Reduce micron-sized fine iron oxide powder with hydrogen to obtain porous active reduced iron powder; the temperature of hydrogen reduction is controlled to be within 50°C below the critical temperature of the reduction conversion path, wherein the critical temperature of the reduction conversion path refers to the temperature at the intersection of the hydrogen reduction fork curve of iron oxide.

[0009] 3) The porous activated reduced iron powder was nitrided in an ammonia atmosphere to obtain α″-Fe. 16 N2 iron nitride.

[0010] The technical solution of this invention can obtain large-size, high-purity α″-Fe. 16 The key to N2 ferronitride magnetic powder lies in controlling the reduction process of iron concentrate to obtain micron-sized iron particles with porous structure, high specific area, and high activity. These iron particles are more conducive to low-temperature nitriding and nitriding reactions, thereby obtaining high-purity, large-size α″-Fe. 16N2 ferrite nitride magnetic powder. More specifically, the technical solution of this invention first uses high-pressure roller milling and high-energy grinding to mechanically activate iron concentrate in order to prepare highly active micron-sized iron oxide powder. The high-pressure roller milling process hardly changes the particle size of the iron concentrate; it mainly causes surface cracks, reduces its mechanical strength, and increases its reactivity. Its activation mechanism for the iron concentrate mainly comes from the extrusion and shearing action under high pressure between the rollers, which can change the surface properties of the iron concentrate. High-energy grinding, on the other hand, activates the iron concentrate by impacting and grinding it on a high-speed rotating grinding disc. The high-energy grinding process significantly changes the particle size of the iron concentrate. Therefore, the activation mechanisms of high-energy grinding and high-pressure roller milling are opposite and complementary. This invention combines high-pressure roller milling and high-energy grinding to activate the iron concentrate, giving it significant activation energy storage and ensuring that the iron concentrate has a micron-sized particle size. Based on this, by strictly controlling the hydrogen reduction temperature to ensure the hydrogen reduction process follows the transformation pathway of Fe3O4→Fe, porous active reduced iron powder can be obtained. This is beneficial for improving the reactivity and rate of the low-temperature nitriding process, and thus enabling the rapid reaction to obtain micron-sized, high-purity α″-Fe. 16 N2 iron nitride.

[0011] As a preferred embodiment, the total iron content of the iron concentrate is above 71.5%. The iron concentrate is a pure iron concentrate, mainly comprising deep beneficiation concentrate and iron phosphorus and iron slag from metallurgical processing, characterized by a total iron content of above 71.5%, with the remaining impurities being silicon, manganese, aluminum, and their oxides. According to existing beneficiation methods, iron concentrate with a total iron content of above 71.5% is readily available and represents a relatively inexpensive iron raw material.

[0012] As a preferred embodiment, the conditions for the high-pressure roller mill are as follows: the feed particle size of the iron concentrate is 40~100µm, the roller spacing is 100~150µm, the roller pressure is 10~20MPa, and the number of high-pressure roller cycles is 1~5. The high-pressure roller mill has a wide adaptability to iron concentrate feed particle sizes, and can directly use iron concentrate that has been processed to a particle size of 40~100µm in beneficiation or primary processing. The roller spacing is mainly selected and set according to the feed particle size of the iron concentrate. The key in the high-pressure roller mill process is the setting of the roller pressure, which has a significant impact on the grinding and activation effect. A further preferred roller pressure range is 15~20MPa. Due to the defect of large edge effects in high-pressure roller mills, 2~4 grinding cycles are set for the iron concentrate to achieve a better roller pressure level.

[0013] As a preferred embodiment, the high-energy grinding conditions are: grinding bead diameter of 0.05mm~0.5mm, grinding speed of not less than 2000r / min, grinding time of not more than 30min, grinding medium of water, and solid-liquid mass-volume ratio of 1g:0.5~5mL. High-energy grinding can impart abundant mechanical energy and active sites to iron concentrate. A further preferred grinding bead diameter range is 0.05~0.2mm. A further preferred grinding speed range is 2500~3000r / min. Since the iron concentrate after high-pressure roller milling has low strength, it is easily over-grinded under the high-speed grinding action of high-energy grinding. Therefore, activation needs to be completed within a very short grinding time. The grinding time should be controlled within 30min to avoid excessive particle refinement. A further preferred grinding time is 10~20min. Furthermore, since high-pressure roller milling changes the hydrophilicity of the iron concentrate particle surface, water can be used as the medium during high-energy grinding, eliminating the need for grinding aids, dispersants, or other additives, thus minimizing the introduction of new impurities. This invention uses mismatched small grinding beads to grind large particles of material and combines this with roller milling to activate the material, enabling iron concentrate to be activated in a short time while retaining micron-level particle size characteristics.

[0014] As a preferred embodiment, the drying conditions are: vacuum drying at a pressure of 90-100 kPa and a temperature of 60-100°C. Drying and dehydration reduce the water content of the iron concentrate, which is beneficial for the subsequent hydrogenation process. Low-temperature vacuum drying helps prevent particle agglomeration, and the dried material often exhibits a porous and loose structure, which is conducive to subsequent gas-solid reactions. If the iron concentrate material clumps after drying, it can be broken up using conventional methods, such as ultrasonication or grinding.

[0015] As a preferred embodiment, the micron-sized fine iron oxide powder has a particle size of 1~30µm and an activated lattice size of 150~800Å. Under the synergistic activation effect of high-pressure roller milling and high-energy grinding, the particle size of the iron concentrate did not undergo significant refinement, but the mechanical activation effect was quite prominent. XRD analysis showed that the activated lattice size was refined to 150~800Å, indicating that the reactivity of the iron concentrate was greatly improved, which is beneficial to the subsequent hydrogen reduction process.

[0016] As a preferred embodiment, the hydrogen reduction conditions are: a temperature 20-50°C below the critical temperature of the reduction conversion path, and a time of 2-5 hours. Generally, in the process of preparing iron powder from iron concentrate by hydrogen reduction, the hydrogen reduction temperature is controlled above the critical temperature of the reduction conversion path to ensure the reduction efficiency of the iron concentrate. The key to the technical solution of this invention is to control the hydrogen reduction temperature within the range of 20-50°C below the critical temperature of the reduction conversion path. For example, the critical temperature of the reduction conversion path for a certain micron-sized iron concentrate is 500°C, while the technical solution of this invention controls the hydrogen reduction temperature below the critical temperature of the reduction conversion path (preferably at a temperature of 450-480°C), so that the reduction path follows the Fe3O4→Fe reduction path, rather than the Fe3O4→FeO→Fe reduction path. The Fe3O4→Fe reduction path allows the iron concentrate particles to expand during the reduction process, increasing the particle size. After reduction, the iron powder particles are approximately 2-40µm in size, and the particles exhibit an embedded porous structure with well-developed pores extending deep into the particle interior, with a specific surface area of ​​not less than 10m². 2 / g, with a preferred specific surface area range of 20~30 m² 2 / g. However, the reduction temperature cannot be too low, otherwise it will be difficult to completely reduce the iron oxide within a suitable time. During hydrogen reduction, micron-sized fine iron oxide powder is loosely distributed on the reactor, with a typical layer thickness of 1~5mm. When the iron concentrate particle size is between 1~10µm, due to its large surface energy, to avoid iron powder agglomeration during the reduction process, a more preferable reduction temperature is 40~50℃ lower than the critical temperature of the reduction conversion path.

[0017] As a preferred embodiment, the critical temperature for the reduction path transformation of the micron-sized fine iron oxide powder relative to iron concentrate is reduced to 350-520℃ based on the activation effect of high-pressure roller milling and high-energy grinding. The method for determining the critical temperature for the reduction path transformation in this invention is as follows: 1) Theoretical calculation method: Based on mechanical activation theory and XRD detection, the mechanical energy stored during activation is calculated, and the temperature point relative to the iron oxide energy storage curve is the critical temperature. 2) Experimental detection method: The composition of the phases involved in the reduction process is tested at different reduction temperatures. The temperature point at which the FeO phase does not appear below a certain temperature is the critical temperature. According to actual grinding and energy storage conditions, this temperature point is between 350-520℃, which is approximately 20-220℃ lower than the critical temperature of 570℃ for the reduction path of conventional iron concentrate.

[0018] As a preferred embodiment, the nitriding conditions are: ammonia gas pressure not exceeding 0.5 MPa, nitriding temperature of 170–195 °C, and time of 8–15 h. Based on micron-sized iron particles with porous structure, high specific area, and high activity, the nitriding process becomes easier, producing α″-Fe. 16The purity of N2 iron nitride is above 97%, preferably above 98%. The ammonia atmosphere is further preferably pure ammonia. During the nitriding process, the particle size essentially maintains the original particle size of the reduced iron powder, thus ultimately preparing α″-Fe. 16 The particle size of N2 magnetic powder is 2~40µm. Beneficial effects

[0019] Compared with existing technologies, the beneficial technical effects of the present invention are as follows:

[0020] 1) The α″-Fe prepared in this invention 16 N2 ferronitride magnetic powder is the most promising material for large-scale replacement of rare earth permanent magnets. Compared with existing rare earth permanent magnets, the product has advantages such as abundant raw material reserves, low price, sufficient supply, large-scale preparation, and green production process, without the need to worry about rare earth element quota supply or resource depletion.

[0021] 2) This invention prepares α″-Fe with a pure phase and high amorphousness. 16 N2 iron nitride magnetic powder has higher coercivity than comparable products and exhibits the "giant magnetic moment" phenomenon, resulting in optimal saturation magnetization. Its improved overall magnetic performance may even surpass that of rare earth permanent magnet materials, making it suitable for fields with higher requirements for magnet performance, such as electric vehicles and wind power generation facilities.

[0022] 3) This invention prepares pure phase α″-Fe with a larger (micrometer-scale) size. 16 N2 magnetic powder particles have reached the processing and forming particle size range of rare earth magnetic powders with existing technology. Compared with the currently prepared micro-iron nitride, it is more suitable for the forming process of magnetic powder pressing and bonding, and has higher practical value. Attached Figure Description

[0023] Figure 1 is a thermodynamic calculation diagram of the 6~15µm iron concentrate obtained by high-pressure roller milling and high-energy grinding in Example 1 after mechanical energy storage. As can be seen from the figure, due to the presence of activated energy storage, the conversion temperature of the reduction path changes from 571℃ to 465℃.

[0024] Figure 2 shows the appearance of the micron-sized iron concentrate obtained by high-pressure roller milling and high-energy grinding in Example 1 after hydrogen reduction; it can be seen from the figure that the iron powder has a porous structure, which is extremely beneficial to gas permeation in the subsequent solid-gas reaction process.

[0025] Figure 3 shows the α″-Fe prepared in Example 2. 16 XRD pattern of N2; shown in the figure as α″-Fe 16 N2 is a pure phase product with almost no other impurities, indicating a high degree of nitriding.

[0026] Figure 4 is a scanning electron microscope image of the iron powder prepared in Comparative Example 1. As can be seen from the figure, the surface of the iron powder is sintered, with a high degree of density and low reactivity, which is not conducive to the subsequent solid-gas reaction process.

[0027] Figure 5 shows the α″-Fe prepared in Comparative Example 1. 16 XRD pattern of N2 material; The figure shows that due to insufficient activity of iron powder, the nitriding reaction was still insufficient after a long period of nitriding, and the material was mainly Fe phase.

[0028] Figure 6 shows the α″-Fe prepared in Comparative Example 2. 16 XRD pattern of N2; the figure shows α″-Fe 16 The low purity of N2 indicates that the activation degree of a single activation method is not high, and the purity of the product prepared at the same level is low. Embodiments of the present invention

[0029] To better understand this invention, specific embodiments are now used to more clearly illustrate and analyze its content. However, the claims of this invention are not limited to the following examples and conditions. Other examples obtained by those skilled in the art without creative effort are all within the protection scope of this invention. Example 1

[0030] The raw material is high-quality iron scale from a steel rolling mill, with an iron grade of 72.1% and a total content of other impurities (acid-insoluble matter) of only 0.3%. Testing revealed that the main component is high-purity iron oxide, differing from iron concentrate in that it contains a greater amount of Fe2O3. 3， After testing, the particle size of the ore beneficiated minerals was found to be over 96% within the range of -200 mesh (under a 200 mesh sieve) to +325 mesh (over a 325 mesh sieve). A high-pressure roller mill was set with a roller spacing of 150µm and an inter-roller pressure of 12MPa. A suitable feed rate was established, and the milling process was repeated 5 times. The resulting particle size D was determined after testing. 50 The particle size was 54µm. The material was then finely ground in a high-energy grinder. A high-efficiency rod-pin type high-energy grinder was selected, with a rotation speed of 2000 r / min, 0.1mm zirconia balls as grinding beads, water as solvent, and a solid-liquid ratio of 1g:3mL. After grinding for 10 minutes, a particle size D was obtained. 50 The ultrafine particle slurry with a particle size of 30µm was dried under low temperature vacuum (95KPa, 80℃) for 3 hours to obtain powder.

[0031] The powder lattice size was determined to be approximately 420 Å based on the detection of its lattice characteristics and other key parameters. The theoretical calculations, combined with mechanical activation energy and other key parameters, show the changes in the hydrogen reduction thermodynamic diagram after activation, as illustrated in Figure 1. Experiments verified that the critical temperature for the reduction transformation pathway is approximately 465℃.

[0032] The reduction temperature was controlled at 430℃ during the powder reduction stage. The agglomerates were loosely distributed on the reduction steel strip to a height of 2mm. Pure hydrogen was used as the reducing gas, and reduction lasted for 3 hours. After reduction, the iron powder showed slight agglomeration. Subsequent dispersal and testing revealed a purity of 99%. The powder was then placed in a tubular reactor, and pure ammonia gas was introduced at a controlled pressure of approximately 0.2MPa. Nitriding was carried out at 175℃ for 15 hours to obtain a pure phase sample. Quantitative XRD calculations showed that the obtained pure phase was α″-Fe. 16 The purity of N2 magnetic powder is 98.6%. Example 2

[0033] Super iron concentrate obtained through deep beneficiation was selected as raw material, with an iron grade of 72.1% and a total impurity content (acid-insoluble matter) of only 0.3%. The material had a small particle size after deep beneficiation; tests showed that over 96% of the particles were between -200 mesh and +325 mesh. A high-pressure roller mill was used, with a roller spacing of 100µm and a roller pressure of 18MPa. A suitable feed rate was set, and the material was ground once. Testing showed that the particle size D after grinding was... 50 The particle size was 45µm. The material was then finely ground in a high-energy grinding mill. A high-efficiency rod-pin type high-energy grinding mill was selected with a rotation speed of 2500r / min. The grinding beads were 0.05mm zirconia balls, water was used as the solvent, and the solid-liquid ratio was 1g:0.5mL. After grinding for half an hour, an ultrafine particle slurry with a particle size D50=20µm was obtained. After vacuum low temperature (93KPa, 70℃) drying for 5 hours, powder was obtained.

[0034] The powder lattice size was determined to be 154 Å by detecting the characteristics and other properties of the powder lattice. Combined with other lattice parameters, the calculation of its lattice energy storage by mechanical activation theory and the verification by experimental data showed that the critical temperature of the reduction transformation path is about 350℃.

[0035] In the powder reduction stage, the reduction temperature was controlled at 300℃. The agglomerates were loosely distributed on the reduction steel belt, with a layer height of 1 mm. Pure hydrogen was used as the reducing gas, and reduction lasted for 5 hours. After reduction, the iron powder showed slight agglomeration. Subsequent dispersal and testing revealed a purity of 99.15% and a particle size of approximately 25 µm. The morphology of the iron powder is shown in Figure 2. Similarly, the powder was arranged in a belt reactor with a layer thickness of 3 mm. The reaction temperature was controlled at 195℃, and nitriding under pure ammonia at atmospheric pressure for 8 hours yielded a pure phase sample. The XRD pattern of the sample is shown in Figure 3, indicating almost no other impurities. Quantitative calculations using XRD revealed that the obtained pure phase α″-Fe... 16 The purity of N2 magnetic powder is 99.2%. Example 3

[0036] The same super iron concentrate as in Example 2 was used as raw material. The raw material was first subjected to high-pressure roller milling with a roller spacing of 130µm, a roller pressure of 20MPa, and a suitable feed rate. The material was milled 3 times. Then, the material was finely ground in a high-energy grinding mill. A high-efficiency rod-pin type high-energy grinding mill was selected with a rotation speed of 3000r / min, 0.2mm zirconia balls as grinding beads, water as solvent, and a solid-liquid ratio of 1g:1mL. After grinding for 15min, an ultrafine particle slurry with a particle size D50=22µm was obtained. The slurry was then dried under vacuum at low temperature (93KPa, 100℃) for 2 hours to obtain powder.

[0037] The powder lattice size was determined to be 621 Å by detecting the characteristics and other properties of the powder lattice. Combined with other lattice parameters, the lattice energy storage was calculated by mechanical activation theory and corroborated by experimental data. The intersection temperature of the fork curve of its hydrogen reduction thermodynamic diagram after activation was 495℃.

[0038] In the powder reduction stage, the reduction temperature was controlled at 450℃. The agglomerates were loosely distributed on the reduction steel belt to a height of 3mm. Pure hydrogen was used as the reducing gas, and reduction lasted for 3 hours. After reduction, the iron powder showed slight agglomeration. Subsequent dispersal and testing revealed a purity of 98.85% and a particle size of approximately 25µm. The powder was also arranged in a belt reactor with a thickness of 2mm. The reaction temperature was controlled at 195℃, and nitriding was performed under pure ammonia at atmospheric pressure for 11 hours to obtain a pure phase sample. The sample showed almost no other impurities. Quantitative XRD calculations revealed that the obtained pure phase α″-Fe... 16 The purity of N2 magnetic powder is 98.4%. Example 4

[0039] The same super iron concentrate as in Example 2 was used as raw material. The raw material was first subjected to high-pressure roller milling with a roller spacing of 130µm, a roller pressure of 15MPa, and a suitable feed rate. The material was milled 4 times. Then, the material was finely ground in a high-energy grinding mill. A high-efficiency rod-pin type high-energy grinding mill was selected with a rotation speed of 2700r / min, 0.5mm zirconia balls as grinding beads, water as solvent, and a solid-liquid ratio of 1g:2mL. After grinding for 30min, an ultrafine particle slurry with a particle size D50=27µm was obtained. The slurry was then dried under vacuum at low temperature (93KPa, 100℃) for 2 hours to obtain powder.

[0040] The powder lattice size was determined to be 800 Å by detecting the characteristics and other properties of the powder lattice. Combined with other lattice parameters, the lattice energy storage was calculated by mechanical activation theory and corroborated by experimental data. The intersection temperature of the fork curve of its hydrogen reduction thermodynamic diagram after activation was 520℃.

[0041] In the powder reduction stage, the reduction temperature was controlled at 500℃. The agglomerates were loosely distributed on the reduction steel belt to a height of 5mm, and reduction lasted for 5 hours. After reduction, the iron powder showed slight agglomeration. Subsequent dispersal and testing revealed a purity of 98.65% and a particle size of approximately 30µm. The powder was also arranged in a belt reactor with a thickness of 1mm, and the reaction temperature was controlled at 195℃. Ammonia gas was introduced at a controlled pressure of 0.5MPa, and nitriding with pure ammonia gas for 13 hours yielded a pure phase sample. The sample showed almost no other impurities. Quantitative XRD calculations revealed that the obtained pure phase α″-Fe... 16 The purity of N2 magnetic powder is 97.8%. Example 5

[0042] The same super iron concentrate as in Example 2 was used as raw material. The raw material was first subjected to a high-pressure roller mill with a roller spacing of 120µm and an inter-roller pressure of 15MPa. A suitable feed rate was set, and the material was ground once. Then, the material was finely ground in a high-energy grinding mill. A high-efficiency rod-pin type high-energy grinding mill was also used, but grinding parameters favoring finer particle size were set, such as a rotation speed of 2000 r / min, 0.2mm zirconia balls as grinding beads, water as solvent, and a solid-liquid ratio of 1g:5mL. After grinding for 30 minutes, a particle size D was obtained. 50 The ultrafine particle slurry with a particle size of 1µm was dried under vacuum at low temperature (90KPa, 60℃) for 3 hours to obtain powder.

[0043] The powder lattice size was determined to be 480 Å by analyzing its lattice characteristics and other properties. Combined with calculations of lattice energy storage using mechanical activation theory and experimental data, the crossover point temperature of the hydrogen reduction thermodynamic curve after activation was 470℃. During the powder reduction stage, the reduction temperature was controlled at 420℃. The dried powder was loosely distributed on a reduction steel belt to a height of 2 mm, and reduction was carried out for 4 hours. After reduction, the iron powder showed slight agglomeration. Subsequent dispersal and testing revealed a purity of 98.7% and a particle size of approximately 2 µm. The powder was then again arranged in a belt reactor with a thickness of 1 mm, and the reaction temperature was controlled at 185℃. Nitriding under pure ammonia at atmospheric pressure for 10 hours yielded a pure phase sample with almost no other impurities. Quantitative XRD calculations confirmed that the obtained pure phase α″-Fe... 16 The purity of the N2 magnetic powder was 98.8%. This example also yielded a high-purity product with micron-sized particles, α″-Fe. 16 N2 magnetic powder product particle size D 50 The particle size is 2µm, and some products also exist in the submicron range. These products are more suitable as auxiliary fillers for use with larger micron-sized α″-Fe. 16 When used in conjunction with N2 products, it fills the pores during the molding process.

[0044] Compare with Example 1

[0045] This comparative example is mainly used to illustrate the difference between controlling the hydrogen reduction temperature within and outside the critical temperature range of the hydrogen reduction pathway.

[0046] The only difference compared to Example 1 is that the hydrogen reduction temperature was set to 500℃, which is higher than the critical temperature of the reduction conversion path and not within the required range of 20~50℃ below the critical temperature of the reduction conversion path. The morphology image of the iron powder after hydrogen reduction is shown in Figure 4. The iron powder surface has no deep pores, which will limit the ammonia permeation reaction. After nitriding under the same conditions as in Example 1, α″-Fe was obtained. 16 N2 sample, α″-Fe 16 The XRD pattern of the N2 sample is shown in Figure 5, indicating that the product purity is low.

[0047] Compare with Example 2

[0048] This comparative example is mainly used to illustrate that it is difficult to fully activate iron concentrate using a single high-pressure roller mill or nano-grinding.

[0049] Compared with Example 1, the only difference is that only high-pressure roller milling is used. The roller milling material is not subjected to high-energy grinding, but is directly reduced by hydrogen. The results show that the activation purity of high-pressure roller milling alone is not enough. It can only be rapidly reduced at a temperature above 700°C. Moreover, the resulting iron powder agglomerates and has a small specific surface area, making it difficult to complete the subsequent nitriding process.

[0050] Compare with Example 3

[0051] This comparative example is mainly used to illustrate that it is difficult to fully activate iron concentrate using a single high-pressure roller mill or nano-grinding.

[0052] Compared with Example 1, the only difference is that only high-energy grinding is used, and the iron concentrate is not subjected to high-pressure roller milling. After high-energy grinding, hydrogen reduction is performed. The results show that the activation degree of single impact and grinding is insufficient. Under the same grinding conditions without the assistance of high-pressure roller milling, the critical temperature can be achieved at around 480°C. There are fewer activation features such as particle cracks. The XRD images of the products prepared after reduction and nitriding under the same conditions are shown in Figure 6.

Claims

1. A microscale high purity a"-Fe 16 A method for producing a N2 magnetic powder, characterized by: Includes the following steps: 1) Iron concentrate is sequentially subjected to high-pressure roller milling, high-energy grinding and drying to obtain micron-sized fine iron oxide powder; 2) Reduce micron-sized fine iron oxide powder with hydrogen to obtain porous active reduced iron powder; the hydrogen reduction temperature is controlled within 50°C below the critical temperature of the reduction conversion path, wherein the critical temperature of the reduction conversion path refers to the temperature at the intersection of the hydrogen reduction fork curve of iron oxide. 3) Nitriding of porous active reduced iron powder in ammonia atmosphere to obtain α"-Fe 16 N2 nitrided iron.

2. A micro-sized high purity a"-Fe according to claim 1 16 A method for producing N2 magnetic powder, characterized by: The total iron content of the iron concentrate is above 71.5%.

3. A micro-sized high purity a"-Fe according to claim 1 16 A method for producing N2 magnetic powder, characterized by: The conditions for the high-pressure roller mill are as follows: the feed particle size of the iron concentrate is 40~100µm, the roller spacing is 100~150µm, the roller pressure is 10~20MPa, and the number of cycles of the high-pressure roller is 1~5 times.

4. A micro-sized high purity a"-Fe according to claim 1 16 A method for producing N2 magnetic powder, characterized by: The conditions for high-energy grinding are as follows: the diameter of the grinding beads is 0.05mm to 0.5mm, the grinding speed is not less than 2000r / min, the grinding time is not more than 30min, the grinding medium is water, and the solid-liquid mass-volume ratio is 1g:0.5~5mL.

5. A micro-sized high purity a"-Fe according to claim 1 16 A method for producing N2 magnetic powder, characterized by: The drying conditions are as follows: vacuum drying is used, and the air pressure is 90~100. KPa, temperature 60~100℃.

6. A micro-sized high purity a"-Fe according to any one of claims 1 to 5 16 A method for producing N2 magnetic powder, characterized by: The micron-sized fine iron oxide powder has a particle size of 1~30µm and an activated lattice size of 150~800Å.

7. A micro-sized high purity a"-Fe according to any one of claims 1 to 5 16 A method for producing N2 magnetic powder, characterized by: The conditions for hydrogen reduction are: a temperature 20-50°C below the critical temperature of the reduction conversion path, and a time of 2-5 hours.

8. A micro-sized high purity a"-Fe according to any one of claims 1 to 5 16 A method for producing N2 magnetic powder, characterized by: The critical temperature for the reduction path transformation of the micron-sized fine iron oxide powder relative to iron concentrate is reduced to 350~520℃ based on the activation effect of high-pressure roller milling and high-energy grinding.

9. A micro-sized high purity a"-Fe according to any one of claims 1 to 5 16 A method for producing N2 magnetic powder, characterized by: The nitriding conditions are as follows: ammonia gas pressure not higher than 0.5 MPa, nitriding temperature 170~195℃, and time 8~15h.

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