Process for preparing a dry-pressable, easily shaped ferrite powder
By controlling the rotary kiln pre-firing and pulverizing processes, combined with air jet milling and surface modifiers, the problems of poor magnetic powder flowability and agglomeration leading to molding difficulties were solved, thus improving the moldability and yield of dry-pressed ferrite magnetic powder.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- ANTE MAGNETIC MATERIAL CO LTD
- Filing Date
- 2025-10-16
- Publication Date
- 2026-07-10
Smart Images
Figure CN121439488B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of permanent magnet materials, and more particularly to a method for preparing easily formable dry-pressed ferrite magnetic powder. Background Technology
[0002] The forming process of sintered permanent magnet ferrites is divided into two main categories: dry pressing and wet pressing. In existing technologies, dry pressing, due to its ease of pressing and lack of need for drainage, is mostly used for pressing multipole magnetic pillars, magnetic tiles, or irregularly shaped magnets. Currently, to achieve good magnetic properties in dry-pressed permanent magnet ferrites, the magnetic powder needs to be ground as finely as possible. However, excessively fine powder leads to problems such as poor powder flowability, difficulty in forming, agglomeration causing a decrease in magnetic properties, and microcracks after forming and sintering, resulting in a lower yield. In actual production, to balance performance and forming, the magnetic powder is ball-milled to an average particle size of 0.75-0.85μm. However, this still cannot completely eliminate the generation of microcracks. The downstream yield is generally around 80-85%. Very few companies with the capability can increase the yield of rare-earth-free dry-pressed permanent magnet ferrites to over 90% or even 95% by introducing high-end forming, sintering, and grinding equipment, and to over 88% for rare-earth-based ferrites.
[0003] In existing technologies, the following methods are commonly used to optimize molding effects:
[0004] (1) Selecting a binder to improve powder flowability: For example, CN102976767A discloses a method for selecting a binder for dry-pressed strontium permanent magnet ferrite without losing coercivity, including the following steps: coarse grinding - preliminary fine grinding - secondary formulation - secondary fine grinding - discharge and water filtration - dry pressing magnetic field forming - finished product sintering. This scheme uses magnesium stearate instead of calcium stearate as the binder for dry-pressed permanent magnet ferrite, which not only plays a good role in lubrication and bonding, but also hardly reduces the coercivity of the magnet; CN112530654B discloses a sintered permanent magnet ferrite and its forming method, including: Step 1: weighing raw materials and binder; Step 2: pre-firing the raw materials to obtain pre-firing material; Step 3: vibrating and highly dispersing the pre-firing material to obtain raw material A; Step 4: using a dry pressing forming device to dry press raw material A and binder A to obtain permanent magnet raw material; Step 5: sintering the permanent magnet raw material to obtain sintered permanent magnet. The solution includes a PVA binder, which is divided into two parts, A and B, during use. Part A is added to the sintered permanent magnet raw material to improve the flowability of the raw material, enhance its bulk density during dry pressing, and facilitate the expulsion of air from the raw material. Part B is sprayed onto the inner wall of the molding die through a dry pressing device, thereby reducing the friction between the raw material and the molding die during dry pressing and facilitating the filling of the molding die by the raw material. CN101205137A discloses a method for manufacturing dry-pressed sintered permanent magnet ferrite, which includes: a pulverizing process; a dry-pressed magnetic powder preparation process; a dry pressing process; and a sintering process. The organic dispersant added in the pulverizing process is one or more of polyethylene glycol, calcium stearate, and calcium gluconate, and the content of the organic dispersant in the pulverizing process is 0.1~1.5wt%. The binder added in the dry-pressed magnetic powder preparation process is one or more of polyethylene alcohol, polyethylene glycol, camphor, and stearate, and the content of the binder in the dry-pressed magnetic powder preparation process is 0.1~2wt%.
[0005] (2) Improved molding equipment: For example, CN200950391Y discloses a middle mold for wet pressing magnetic field molding of permanent magnet ferrite tiles, including a middle mold body made of weakly magnetic material, a convex arc cavity on the middle mold body, and at least one injection port at the bottom of the convex arc cavity. When the length of the cavity shaft height is greater than 40mm, injection ports are opened on both sides of the convex arc cavity shaft height. When the arc of the convex arc cavity is greater than 120 degrees, two injection ports are opened at the bottom of the convex arc cavity; and the two injection ports are symmetrically arranged.
[0006] (3) Abrasive improvement: For example, CN104496452B discloses a ferrite preparation method and the ferrite prepared therefrom, which includes the following steps: preparing ferrite pre-sintered material and performing wet crushing treatment to obtain ferrite pre-sintered material slurry; drying the ferrite pre-sintered material slurry; using a vibratory mill to vibrate the dried ferrite pre-sintered material powder for 20 to 40 minutes; performing high-speed dispersion treatment on the vibratory milled powder; uniformly mixing the high-speed dispersed ferrite pre-sintered material powder with a binder and placing it in a magnetic field for dry pressing to obtain shaped ferrite; sintering the shaped ferrite to obtain high-performance dry-shaped sintered permanent magnet ferrite.
[0007] The above solutions can all solve the problems of poor magnetic powder flowability, difficult molding, agglomeration leading to decreased magnetic properties, and microcracks after molding and sintering, which lead to a decrease in yield. However, methods (1) and (2) are remedial measures under the condition that the powder properties are fixed, and often sacrifice some magnetic properties. Although method (3) improves from the perspective of powder, the operation of the vibratory mill is cumbersome, and manual operation is required for loading and unloading, which disrupts the continuity of production. In addition, the secondary batching is unevenly distributed, resulting in poor product uniformity. Therefore, it is rarely used in sintered permanent magnet ferrite. Summary of the Invention
[0008] To address the aforementioned technical problems, this invention provides a method for preparing easily formable dry-pressed ferrite magnetic powder. The method of this invention is simple to operate and highly feasible. While ensuring the existing properties of the magnetic powder, it can significantly improve powder flowability, thereby increasing forming efficiency and yield.
[0009] The specific technical solution of the present invention includes: a method for preparing easily formable dry-pressed ferrite magnetic powder, which includes the following steps:
[0010] 1) Ingredient preparation and mixing.
[0011] 2) Pre-firing: The material is fed into a rotary kiln for pre-firing. The final pre-firing temperature is 1300±50℃. The residence time of the material within 100℃ below the final temperature is 1.5-2.5h. The oxygen concentration at the feed end is ≥16vol. The portion of the pre-firing pellets with a diameter of 8-12mm is ≥80wt%.
[0012] 3) Crush and sieve to obtain coarse powder with a span value ≤ 3.
[0013] 4) Secondary grinding to obtain an average particle size of 0.83-0.9μm, D 50 The thickness is 1.2-1.3 μm, D 90 It is a fine powder with a particle size of 2.8-3.0 μm and a span value of 1.8-2.
[0014] 5) Drying to obtain easily moldable dry-pressed ferrite magnetic powder.
[0015] The purpose of this invention is to fundamentally solve a series of problems from the perspective of powder, such as poor flowability of magnetic powder, difficulty in molding, agglomeration leading to decreased magnetic properties, and microcracks after molding and sintering causing a decrease in yield. The main idea of this invention is to control grain growth and powder morphology. Currently, ferrite is mainly pre-fired in a rotary kiln. To ensure flowability within the kiln, dry mixing is followed by pelletizing; wet mixing involves centrifugal dehydration to control moisture content, and finally, the powder is self-rolled into pellets within the kiln. In other words, the final pre-fired stage in the production process targets spherical particles. To ensure uniform grain nucleation and growth, this invention requires strict control of the diameter of the pellets within the kiln to ensure uniform heating. Since most rotary kilns in existing production equipment are integrated kilns, the temperature distribution within the kiln is not uniform. From the feed to the discharge, the temperature first slowly rises to the final temperature (referring to the highest temperature), and then slowly cools to the discharge temperature, which is approximately 900°C. Due to the use of natural gas as the heating medium, the temperature near the flame jet is high, and the final temperature position can be determined using infrared detection. This invention allows for controlling the kiln speed to maintain the material's residence time within 1.5-2.5 hours from the final temperature of 100°C during pre-calcination, ensuring uniform nucleation, complete carbonate decomposition, chloride ion volatilization, and ferrous ion oxidation. The width of the final temperature range (from the final temperature to -100°C) can also be controlled by adjusting the flame state. The limitation on oxygen concentration ensures sufficient oxidation of ferrous ions in the material. The ultimate goal of all these process controls is to obtain uniformly sized pre-calcined magnetic powder grains, laying the foundation for particle size control in the subsequent pulverization process.
[0016] After pre-firing, this invention specifically designed a combined crushing and sieving-secondary crushing process. It is important to emphasize that in this combined process, the essence of the two crushing processes is the exfoliation between grain boundaries rather than the breakage of grains. If the material remains within 100°C of the final temperature during pre-firing for too long, some grains may grow excessively large, making it impossible to effectively reduce the actual grain size through two crushing processes. This results in a wider span value, meaning a large difference in morphology between fine and coarse magnetic powder particles, which is detrimental to performance. Conversely, if the residence time is too short, some grains will be small, resulting in small grain size of the fine powder obtained through secondary crushing. During the molding process, the green gas content is high, leading to poor magnetic powder flowability, poor molding, and microcracks. In addition, particle size control during pulverization and sieving affects the efficiency of secondary pulverization. If the pulverization and sieving process is not well controlled and the particle size distribution is not uniform, it will be very difficult to obtain powder with the above particle size characteristics during secondary pulverization. If the span during pulverization and sieving is greater than 3.0, that is, the difference between large-diameter particles and small-diameter particles in the powder is large, it will be difficult to obtain uniform powder particles through simple processes during secondary pulverization.
[0017] Preferably, in step 1), the ingredients include an iron source and a strontium source with an iron-strontium molar ratio of 5.5-6.2:1, and 0-0.5 wt% silicon dioxide, which is a total amount of iron source and strontium source, mixed evenly.
[0018] Preferably, in 1), the chloride ion content in the iron source is ≤0.4wt%.
[0019] The iron source is usually iron oxide red, iron scale, or concentrate powder, while the strontium source is usually strontium carbonate. Since iron oxide red is a product of steel plant pickling, it inevitably contains chloride ions, which often lead to a deterioration in the magnetic and mechanical properties of sintered permanent magnet ferrites.
[0020] Preferably, in 1), the mixing process includes first performing dry mixing, and then undergoing compaction and pelletizing to obtain pellets; or the mixing process includes first performing wet mixing, and then undergoing centrifugation or pressure filtration to dehydrate and obtain a cake with a moisture content of 18-30 wt%.
[0021] Preferably, in step 2), the length of the rotary kiln is 30-50m, the temperature at the feed end is 10-50℃, the temperature at the discharge end is 700-1100℃, and the total residence time of the material in the rotary kiln is 8-12h.
[0022] Preferably, in step 3), the average particle size of the coarse powder is 4-8 μm, and the particle size distribution satisfies D. 50 2-4μm, D 90 ≤10μm.
[0023] Preferably, in step 3), the crushing and sieving is performed by dry ball milling or ultrafine grinding.
[0024] Preferably, in step 4), the secondary grinding includes: mixing 100 parts of coarse powder with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide and 0.01-4 parts of lanthanum oxide by mass, and then performing wet ball milling.
[0025] Sintered permanent magnet ferrites achieve performance adjustments through the addition of secondary additives: calcium carbonate lowers the melting point, which can be used to adjust remanence, sintering density, etc.; silicon dioxide can form a grain boundary glass phase, which can be used to adjust grain size, sintering shrinkage rate and intrinsic properties; boric acid can affect the pH of the ball milling media to a certain extent, thereby improving overall performance; alumina and chromium oxide are used to adjust intrinsic properties; and the combined addition of lanthanum oxide and cobalt oxide can significantly improve the overall magnetic properties.
[0026] As a further preferred embodiment, in 4), the secondary pulverization includes: first, air-jet milling the coarse powder, and then, by weight, mixing 100 parts of the air-jet milled powder with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide and 0.01-4 parts of lanthanum oxide, and then performing wet ball milling.
[0027] The two secondary grinding schemes mentioned above each have their own characteristics: In the first scheme, the wet ball milling process is mature, but the particle morphology is slightly inferior, and the powder is prone to agglomeration. As a fine grinding technology, this scheme mainly combines the pre-calcination and pulverization and sieving processes to better control the grain size and distribution of coarse powder particles. Therefore, even with wet ball milling, the particle size distribution can be well and accurately controlled through secondary grinding. In the second scheme, an air jet milling process is added before wet ball milling. The powder after air jet milling has better particle size distribution uniformity and also has the characteristics of good particle shape uniformity and fewer sharp edges. Therefore, it is easier to obtain a high green density, form a dense magnet, and ensure performance and molding. However, it should be noted that if an air jet mill is used, it is not suitable to air jet mill the magnetic powder and the powder from the secondary batching together, because the difference in specific gravity is too large, which can easily cause segregation.
[0028] Furthermore, in step 4), the air jet mill comprises: mixing 100 parts by mass of coarse powder with 0.5-1 parts by mass of a surface modifier, with the air jet medium being dry compressed air, the water vapor content being less than 0.5 vol%, and the gas-solid ratio being 2.5-3.5 m. 3 / kg, under gas flow rate of 350-450m / s, pulverized to an average particle size of 0.85-0.95μm, and D 50 The thickness is 1.2-1.4 μm, D 90 The thickness ranges from 2.8 to 3.2 μm, and the span ranges from 1.8 to 2.2 μm.
[0029] The air jet milling process has a significant impact on the particle size distribution of magnetic powder. For example, a low gas-to-solid ratio results in insufficient grinding efficiency, affecting particle size distribution and formability; excessive water vapor content leads to powder agglomeration due to moisture, causing particle size control to fail. Ultimately, this invention found that the best results are achieved within the aforementioned process parameter range.
[0030] Furthermore, in 4), the surface modifier is stearic acid and / or a silane coupling agent.
[0031] The role of stearic acid is to reduce agglomeration and promote pulverization during the air jet milling process. When a silane coupling agent is used, since the ferrite powder itself is alkaline, the moisture in the air promotes micro-hydrolysis on the surface of the magnetic powder during the mixing process of adding the silane coupling agent. After entering the air jet mill, the newly formed grain boundaries have high surface energy and adsorb the silane coupling agent during the collision process, forming a coating effect. In the subsequent wet ball milling process, the silane coupling agent is further hydrolyzed in alkaline water to prevent agglomeration between magnetic powder particles.
[0032] Furthermore, in 4), the calcium carbonate, silicon dioxide, strontium carbonate, aluminum oxide, chromium oxide, cobalt oxide and lanthanum oxide mixed with the coarse powder after air jet milling satisfy an average particle size ≤2μm.
[0033] Preferably, in step 5), the drying temperature is 600-900℃.
[0034] Sufficient temperature is required to ensure that the surface modifier decomposes fully, reducing its impact on magnetic properties.
[0035] Preferably, in step 5), the moisture content of the dried magnetic powder is ≤0.6wt%.
[0036] Compared with the prior art, the beneficial effects of the present invention are:
[0037] (1) Based on the existing production support, the present invention controls the particle size distribution through processes such as pre-firing, crushing and sieving, and secondary crushing, thereby improving the uniformity of particle size distribution and achieving better formability and improving the yield rate of dry-pressed ferrite magnetic powder at the current performance level.
[0038] (2) The present invention further utilizes air jet milling technology, by controlling the air jet milling process and selecting and adding auxiliary surface modification reagents, to further prevent the agglomeration of magnetic powder particles, thereby ensuring the performance level of magnetic powder. Attached Figure Description
[0039] Figure 1 This is a particle size distribution diagram of the coarse powder obtained in Example 1.
[0040] Figure 2 This is a particle size distribution diagram of the fine powder obtained in Example 1. Detailed Implementation
[0041] The present invention will be further described below with reference to embodiments.
[0042] General Implementation Examples
[0043] A method for preparing easily formable dry-pressed ferrite magnetic powder, comprising the following steps:
[0044] 1) Ingredient preparation and mixing.
[0045] In some preferred embodiments, the ingredients include an iron source and a strontium source with an iron-strontium molar ratio of 5.5-6.2:1, and 0-0.5 wt% silica, which is a homogeneous mixture of the total iron and strontium sources.
[0046] In some preferred embodiments, the chloride ion content in the iron source is ≤0.4wt%.
[0047] In some preferred embodiments, the mixing process includes first performing dry mixing, and then undergoing densification and pelletizing to obtain pellets; or the mixing process includes first performing wet mixing, and then undergoing centrifugation or pressure filtration to dehydrate and obtain a cake with a moisture content of 18-30 wt%.
[0048] 2) Pre-firing: The material is fed into a rotary kiln for pre-firing. The final pre-firing temperature is 1300±50℃. The residence time of the material within 100℃ below the final temperature is 1.5-2.5h. The oxygen concentration at the feed end is ≥16vol. The portion of the pre-firing pellets with a diameter of 8-12mm is ≥80wt%.
[0049] In some preferred embodiments, the length of the rotary kiln is 30-50m, the feed end temperature is 10-50℃, the discharge end temperature is 700-1100℃, and the total residence time of the material in the rotary kiln is 8-12h.
[0050] 3) Crush and sieve to obtain coarse powder with a span value ≤ 3.
[0051] In some preferred embodiments, the coarse powder has an average particle size of 4-8 μm, and the particle size distribution satisfies D. 50 2-4μm, D 90 ≤10μm.
[0052] In some preferred embodiments, the crushing and sieving is performed by dry ball milling or ultrafine grinding.
[0053] 4) Secondary grinding to obtain an average particle size of 0.83-0.9μm, D 50 The thickness is 1.2-1.3 μm, D 90 It is a fine powder with a particle size of 2.8-3.0 μm and a span value of 1.8-2.
[0054] In some preferred embodiments, the secondary grinding includes: mixing 100 parts by weight of coarse powder with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide and 0.01-4 parts of lanthanum oxide, and then performing wet ball milling.
[0055] In some more preferred embodiments, the secondary pulverization includes: first, subjecting the coarse powder to an air jet mill, and then, by weight, mixing 100 parts of the air jet milled powder with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide, and 0.01-4 parts of lanthanum oxide, and then performing wet ball milling.
[0056] Further, the air jet mill comprises: mixing 100 parts by mass of coarse powder with 0.5-1 parts by mass of a surface modifier, with the airflow medium being dry compressed air, the water vapor content being less than 0.5 vol%, and the gas-solid ratio being 2.5-3.5 m. 3 / kg, under gas flow rate of 350-450m / s, pulverized to an average particle size of 0.85-0.95μm, and D 50 The thickness is 1.2-1.4 μm, D 90 The thickness ranges from 2.8 to 3.2 μm, and the span ranges from 1.8 to 2.2 μm.
[0057] Furthermore, in 4), the surface modifier is stearic acid and / or a silane coupling agent.
[0058] Furthermore, in 4), the calcium carbonate, silicon dioxide, strontium carbonate, aluminum oxide, chromium oxide, cobalt oxide and lanthanum oxide mixed with the coarse powder after air jet milling satisfy an average particle size ≤2μm.
[0059] 5) Drying to obtain easily moldable dry-pressed ferrite magnetic powder.
[0060] In some preferred embodiments, the drying temperature is 600-900°C.
[0061] In some preferred embodiments, the moisture content of the dried magnetic powder is ≤0.6wt%.
[0062] Specific embodiments and comparative examples
[0063] Unless otherwise specified, the calcium carbonate, silicon dioxide, strontium carbonate, aluminum oxide, chromium oxide, cobalt oxide and lanthanum oxide used in the secondary grinding process all meet the requirement of an average particle size ≤2μm;
[0064] Since products made from iron oxide red raw materials have better performance and do not require further oxidation steps (iron scale and concentrate powder require oxidation), iron oxide red is used as the iron source in the following cases unless otherwise specified, and the indicators are shown in Table 1; the indicators of strontium carbonate used in the ingredients are shown in Table 2, where the component values are determined by XRF, chloride is determined by titration, moisture is determined by a moisture meter, and particle size is determined by an average particle size analyzer.
[0065] Table 1
[0066]
[0067] Table 2
[0068] Example
[0069] 1) Iron oxide red and strontium carbonate with a molar ratio of 5.95:1 and a total mass percentage of 0.3% silica are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 12 minutes, and then densified for 60 minutes. The mixture is then pelletized with a green pellet diameter of 9-14 mm.
[0070] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1300℃. Within the range of -100℃, the material residence time is 2.0h, and the oxygen volume concentration at the kiln tail (i.e. the feed end) is 19.2%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 81wt%.
[0071] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain a coarse powder with an average particle size of 5.8 μm, and the particle size distribution satisfies D. 50 It is 3.2μm, D 90 It has a thickness of 9.0 μm and a span value of 2.49. See Figure 1 As shown.
[0072] 4) By mass, 1 unit of coarse powder is uniformly mixed with 0.5% silane coupling agent KH570 and 0.3% calcium stearate, at a gas-solid ratio of 3.0 m. 3 / kg, under the conditions of a gas flow rate of 380m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.90μm with a particle size distribution satisfying D 50 It is 1.35μm, D 90 The particle size was 3.0 μm, and the span was 2.15. Then, 1 unit of the mass of the magnetic powder pulverized by an air jet mill was mixed with 1.3% calcium carbonate, 0.3% silicon dioxide, 0.1% strontium carbonate, 0.15% boric acid, 0.4% alumina, 0.3% chromium oxide, 0.5% cobalt oxide, and 1.1% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.85 μm, and the particle size distribution met the D... 50 It is 1.23 μm, D 90 Fine powder with a particle size of 2.86 μm and a span of 1.98 was dehydrated by pressure filtration. See [the original text]. Figure 2 As shown.
[0073] 5) After drying at 800℃, the water content is 0.33%, resulting in easily moldable dry-pressed ferrite magnetic powder.
[0074] Example 2
[0075] 1)-3) Same as Example 1.
[0076] 4) By mass, 1 unit of coarse powder is mixed with 1.3% calcium carbonate, 0.3% silicon dioxide, 0.1% strontium carbonate, 0.15% boric acid, 0.4% aluminum oxide, 0.3% chromium oxide, 0.1% cobalt oxide, and 0.3% lanthanum oxide. This mixture is then added to a wet ball mill for secondary grinding until the average particle size is 0.83 μm, and the particle size distribution meets the requirements of D. 50 It is 1.25μm, D 90 Fine powder with a particle size of 2.92 μm and a span of 1.96 was dehydrated by pressure filtration.
[0077] 5) The water content after drying at 800℃ is 0.35%, which yields easily moldable dry-pressed ferrite magnetic powder.
[0078] Example 3
[0079] 1) Iron oxide red and strontium carbonate with a molar ratio of 5.8:1 and a total mass percentage of 0.1% silica are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 10 minutes, and then densified for 30 minutes. The mixture is then pelletized with a green pellet diameter of 9-14 mm.
[0080] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1250℃. Within the range of -100℃, the material residence time is 1.5h, and the oxygen volume concentration at the kiln tail is not less than 16%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 81wt%.
[0081] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain a coarse powder with an average particle size of 4 μm and a particle size distribution that satisfies D. 50 It is 2.73 μm, D 90 It has a thickness of 8.6 μm and a span value of 2.36.
[0082] 4) Mix 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate evenly at a gas-solid ratio of 2.5m. 3 / kg, under the conditions of a gas flow rate of 350m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.85μm with a particle size distribution satisfying D 50 It is 1.2μm, D 90 The magnetic powder, pulverized by an air jet mill, was then mixed with 1.3% calcium carbonate, 0.3% silicon dioxide, 0.1% strontium carbonate, 0.15% boric acid, 0.4% aluminum oxide, 0.3% chromium oxide, 0.3% cobalt oxide, and 0.6% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.84 μm, and the particle size distribution met the requirements of D50 of 1.2 μm, D90 of 2.8 μm, and span of 1.8. The mixture was then dehydrated by pressure filtration.
[0083] 5) After drying at 600℃, the water content is 0.3% by mass, thus obtaining easily moldable dry-pressed ferrite magnetic powder.
[0084] Example 4
[0085] 1) Iron oxide red and strontium carbonate with a molar ratio of 6.0:1 and a total mass percentage of 0.3% silicon dioxide are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 12 minutes, and then densified for 75 minutes. The mixture is then pelletized with a green pellet diameter of 9-14 mm.
[0086] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1300℃. Within the range of -100℃, the material residence time is 2.0h, and the oxygen volume concentration at the kiln tail is not less than 16%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 83wt%.
[0087] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain a coarse powder with an average particle size of 6 μm and a particle size distribution that satisfies D. 50 It is 2.27 μm, D 90 It has a thickness of 8.93 μm and a span value of 2.55.
[0088] 4) Mix 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate evenly at a gas-solid ratio of 3.0 m. 3 / kg, under the conditions of a gas flow rate of 400m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.90μm with a particle size distribution satisfying D 50 It is 1.3μm, D 90 The particle size was 3.0 μm, and the span was 2.0. Then, 1 unit of the mass of the magnetic powder pulverized by an air jet mill was mixed with 0.7% calcium carbonate, 0.2% silicon dioxide, 0.1% strontium carbonate, 0.2% boric acid, 0.25% alumina, 0.2% chromium oxide, 1.0% cobalt oxide, and 2.0% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.85 μm, and the particle size distribution met the D... 50 It is 1.25μm, D 90 Fine powder with a particle size of 2.9 μm and a span of 1.9 was dehydrated by pressure filtration.
[0089] 5) The water content after drying at 750℃ is 0.4%, which yields easily moldable dry-pressed ferrite magnetic powder.
[0090] Example 5
[0091] 1) Iron oxide red and strontium carbonate with a molar ratio of 6.2:1 and a total mass percentage of 0.5% silica are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 15 minutes, and then densified for 120 minutes. The resulting pellets have a diameter of 9-14 mm.
[0092] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1330℃. Within the range of -100℃, the material residence time is 2.5h, and the oxygen volume concentration at the kiln tail is not less than 16%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 81wt%.
[0093] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain coarse powder with an average particle size of 8 μm and a particle size distribution that satisfies D. 50 It is 2.61 μm, D 90 It has a thickness of 9.5 μm and a span value of 2.47.
[0094] 4) Mix 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate evenly at a gas-solid ratio of 3.5m. 3 / kg, under the conditions of a gas flow rate of 450m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.95μm with a particle size distribution satisfying D 50 It is 1.4μm, D 90 The particle size was 3.2 μm, and the span was 2.2. Then, 1 unit of the mass of the magnetic powder pulverized by an air jet mill was mixed with 1.3% calcium carbonate, 0.32% silicon dioxide, 0.2% strontium carbonate, 0.4% boric acid, 0.5% aluminum oxide, 0.5% chromium oxide, 2.0% cobalt oxide, and 4.0% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.9 μm, and the particle size distribution met the D... 50 It is 1.3μm, D 90 Fine powder with a particle size of 3.0 μm and a span of 2.0 was dehydrated by pressure filtration.
[0095] 5) The water content after drying at 900℃ is 0.5%, which yields easily moldable dry-pressed ferrite magnetic powder.
[0096] Example 6
[0097] 1) Iron oxide red and strontium carbonate with a molar ratio of 5.9:1 and a total mass percentage of 0.2% silica are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 11 minutes, and then densified for 50 minutes. The mixture is then pelletized with a green pellet diameter of 9-14 mm.
[0098] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1270℃. Within the range of -100℃, the material residence time is 1.8h, and the oxygen volume concentration at the kiln tail is not less than 16%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 80wt%.
[0099] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain coarse powder with an average particle size of 5 μm and a particle size distribution that satisfies D. 50It is 3.1 μm, D 90 It has a thickness of 9.3 μm and a span value of 2.79.
[0100] 4) Mix 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate evenly at a gas-solid ratio of 2.8m. 3 / kg, under the conditions of a gas flow rate of 380m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.88μm with a particle size distribution satisfying D 50 It is 1.25μm, D 90 The particle size was 2.9 μm, and the span was 1.9. Then, 1 unit of the mass of the magnetic powder pulverized by an air jet mill was mixed with 0.6% calcium carbonate, 0.15% silicon dioxide, 0.05% strontium carbonate, 0.15% boric acid, 0.15% alumina, 0.15% chromium oxide, 0.5% cobalt oxide, and 1.0% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.88 μm, and the particle size distribution met the D... 50 It is 1.22 μm, D 90 Fine powder with a particle size of 2.82 μm and a span of 1.82 was dehydrated by pressure filtration.
[0101] 5) The water content after drying at 650℃ is 0.35%, which yields easily moldable dry-pressed ferrite magnetic powder.
[0102] Example 7
[0103] 1) Iron oxide red and strontium carbonate with a molar ratio of 6.1:1 and a total mass percentage of 0.4% silica are mixed evenly. The mixture is prepared by dry batching, mixed in a strong mixer for 14 minutes, and then densified for 100 minutes. The mixture is then pelletized with a green pellet diameter of 9-14 mm.
[0104] 2) The material is fed into a rotary kiln for pre-firing. The final temperature of the pre-firing is 1310℃. Within the range of -100℃, the material residence time is 2.2h, and the oxygen volume concentration at the kiln tail is not less than 16%. The pre-firing material is spheres, and the portion with a firing diameter of 8-12mm is 80wt%.
[0105] 3) After cooling, the powder is pulverized and sieved using a dry ball mill to obtain coarse powder with an average particle size of 7 μm, and the particle size distribution satisfies D. 50 It is 2.71 μm, D 90 It has a thickness of 9.0 μm and a span value of 2.52.
[0106] 4) Mix 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate evenly at a gas-solid ratio of 3.2m. 3 / kg, under the conditions of a gas flow rate of 420m / s, the gas medium is dry compressed air with a water vapor volume ratio of less than 0.5%, and the particles are pulverized to an average particle size of 0.92μm with a particle size distribution satisfying D 50 It is 1.35μm, D 90 The particle size was 3.1 μm, and the span was 2.1. Then, 1 unit of the mass of magnetic powder pulverized by an air jet mill was mixed with 1.0% calcium carbonate, 0.3% silicon dioxide, 0.15% strontium carbonate, 0.3% boric acid, 0.35% aluminum oxide, 0.35% chromium oxide, 1.5% cobalt oxide, and 3.0% lanthanum oxide. This mixture was then added to a wet ball mill and ball-milled until the average particle size was 0.88 μm, and the particle size distribution met the D... 50 It is 1.28 μm, D 90 Fine powder with a particle size of 2.92 μm and a span of 1.95 was dehydrated by pressure filtration.
[0107] 5) The water content after drying at 800℃ is 0.45%, which yields easily moldable dry-pressed ferrite magnetic powder.
[0108] Example 8
[0109] 1) Iron oxide red and strontium carbonate with a molar ratio of 5.95:1 and a total mass percentage of 0.3% of the total mass of iron oxide red and strontium carbonate silica were added to a mixing tank containing 3.5 times the total mass of the materials to form a slurry. The mixture was wet-milled for 15 minutes and centrifuged to dehydrate to obtain a cake with a moisture content of 20%.
[0110] 2)-4) Same as Example 1.
[0111] Comparative Example 1
[0112] The difference from Example 1 is that dry-pressed ferrite magnetic powder is prepared using conventional processes. In this comparative example, the formulas for the pre-calcination process and the secondary pulverization process are the same as in Example 1. The difference lies in:
[0113] 3) After cooling and dry ball milling, the average particle size of the coarse powder was 5.8 μm, but it was not sieved or classified. 50 It is 2.6μm, D 90 It has a thickness of 10.2 μm and a span value of 3.12.
[0114] 4) The secondary grinding process uses conventional wet ball milling, with an average particle size of 0.85 μm. 50 It is 1.12 μm, D 90 It is a fine powder with a particle size of 3.14 μm and a span of 2.20.
[0115] Comparative Example 2
[0116] The difference from Example 1 is that, 2) during the pre-calcination process, the material residence time is 1 hour within the final temperature range of -100℃, and the final temperature is 1300℃.
[0117] Comparative Example 3
[0118] The difference from Example 1 is that in the pre-calcination process in 2), the material residence time is 3 hours within the final temperature range of -100℃, and the final temperature is 1300℃.
[0119] Comparative Example 4
[0120] The difference from Example 1 is that in the pre-firing process of 2), there are small-diameter spherical particles with a diameter of less than 8 mm, accounting for 13%, and large-diameter spherical particles with a diameter of more than 12 mm, accounting for 8%.
[0121] Comparative Example 5
[0122] The difference from Example 1 is that, 2) during the pre-firing process, there are small-diameter spherical particles with a diameter of less than 8 mm, accounting for 9%, and large-diameter spherical particles with a diameter of more than 12 mm, accounting for 14%.
[0123] Comparative Example 6
[0124] The difference from Example 1 is that, 2) during the pre-burning process, the oxygen volume concentration is 15.5%.
[0125] Comparative Example 7
[0126] The difference from Example 1 is that, in 4), no surface modifier was added during the air jet milling process: the air jet milling process includes, by mass, mixing 1 unit of coarse powder at an air-to-solid ratio of 3.0 m³ / s. 3 / kg, under the condition of gas flow rate of 380m / s, the gas flow medium is dry compressed air, the water vapor volume ratio is less than 0.5%, and the average particle size is 0.85μm.
[0127] Comparative Example 8
[0128] The difference from Example 1 is that, in 4), the gas-solid ratio is lower: the air jet milling process includes uniformly mixing, by mass, 1 unit of coarse powder with 0.5% silane coupling agent KH570 and 0.3% calcium stearate, at a gas-solid ratio of 2.0 m... 3 / kg, under the conditions of a gas flow rate of 340m / s, the gas flow medium is dry compressed air, the water vapor volume ratio is less than 0.5%, and the average particle size is 0.85μm.
[0129] Comparative Example 9
[0130] The difference from Example 1 is that in 3), coarse powder with a span value of 3.15 was obtained by crushing and sieving, and fine powder with a span value of 2.08 was obtained in 4).
[0131] Comparative Example 10
[0132] The difference from Example 1 is that in 4), the particles were directly ground to an average particle size of 0.85 μm using an air jet mill, without wet fine grinding, and the particle size distribution met the D... 50 It is 1.25μm, D 90 The value is 2.92 μm, and the span is 1.98.
[0133] Comparative Example 11
[0134] The difference from Example 1 is that in 4), the particles are ground and pulverized to an average particle size of 1.0 μm using an air jet mill, and the particle size distribution satisfies D. 50 It is 1.42 μm, D 90 The particle size was 3.28 μm and the span was 2.25 μm. The particles were then wet-milled to an average particle size of 0.85 μm.
[0135] Comparative Example 12
[0136] The difference from Example 1 is that, 4) the water vapor content in the air jet mill is controlled at 0.6%.
[0137] Comparative Example 13
[0138] The difference from Example 1 is that, 4) after the air jet mill pulverization, dry ball milling is used for mixing, without wet ball milling, and the mixing time is 30 minutes.
[0139] Comparative Example 14
[0140] The difference from Example 1 is that, 4) the secondary formulation ingredients are directly added during air jet milling, without wet ball milling, and the average particle size obtained after air jet milling is 0.85 μm, and the particle size distribution satisfies D. 50 It is 1.21 μm, D 90 It is a fine powder with a particle size of 2.90 μm and a span of 1.98.
[0141] Performance testing
[0142] Sample preparation for testing: 102 round cakes were pressed for each sample. 400g of magnetic powder was mixed with 1.5g of camphor and 0.5g of calcium stearate using a high-speed sample-making machine (i.e., 34 mixed samples were prepared for each sample). Then, 45g of the mixture was pressed into green blanks with a diameter of 35.1mm and a height of approximately 15.5±0.30mm under a pressure of 8000Gs and 35t. These green blanks were then sintered in a muffle furnace to obtain the fired samples. The sintering temperature for the examples and comparative examples was 1230℃.
[0143] Particle size detection: The average particle size analyzer was used for testing. The porosity was 0.61 and the weight was 5g.
[0144] Performance testing: After the sintered samples are ground by a double-end grinder, the diameter is measured with a vernier caliper and the magnetic properties are tested with a BH magnetometer at 20°C. The average value and standard deviation of the performance are taken for each group of samples (the sintered samples without breakage or missing corners are taken for performance testing).
[0145] The blank group formulation was the same as in Example 1, using conventional sample preparation. The average particle size of the coarse powder was 5.8 μm, and the span was 3.2. Particle size distribution was not controlled. The secondary grinding was performed using conventional wet ball milling, controlling the average particle size to be 0.8 μm. 50 It is 1.35μm, D 90 The value is 3.52 μm, and the span is 2.2.
[0146] Yield: Observe the surface damage of the green blank, record the number of green blanks without breakage or missing corners, and calculate the pressing and forming rate; after the fired samples are ground, record the firing rate without breakage or missing corners; after the magnetization test, record the number of test samples without breakage or missing corners (final yield).
[0147] The magnetic properties results are shown in Table 3, and the yield statistics are shown in Table 4.
[0148] Table 3
[0149]
[0150] Table 4
[0151]
[0152] This invention aims to control the particle size distribution of magnetic powder by optimizing the pre-firing, pulverizing and sieving, and secondary pulverizing processes, thereby fundamentally solving the problem of low yield caused by poor flowability, difficult molding, agglomeration, and microcracks in dry-pressed ferrite magnetic powder. Tables 3-4 show that Examples 1-8 demonstrate that under strict control of pre-firing conditions (such as final temperature residence time, oxygen concentration, and pellet size) and pulverizing processes (such as coarse powder span value ≤3, fine powder span value 1.8-2), the magnetic powder exhibits a uniform particle size distribution and good grain morphology. This not only improves the stability of magnetic properties (e.g., higher Br, Hcb, Hcj, and (BH)max values with smaller standard deviations) but also significantly improves molding efficiency and yield (the final yield exceeds 94%). Its advantages lie in the fact that sufficient residence time and oxygen concentration during the pre-firing process ensure uniform grain growth, full decomposition of carbonates and oxidation of ferrous ions. The combination of crushing and sieving and secondary crushing process achieves precise control of particle size distribution through grain boundary peeling rather than grain breakage, reducing powder agglomeration, thus resulting in high green density and fewer microcracks during dry pressing.
[0153] Specifically: Compared with Example 1, Comparative Example 1 suffered from uneven particle size distribution after secondary grinding due to uncontrolled coarse powder span value, resulting in decreased yield and magnetic properties; Comparative Examples 2 and 3 had excessively short or long pre-calcination residence times, affecting grain uniformity and leading to performance fluctuations and reduced yield; Comparative Examples 4 and 5 had unreasonable ball size distribution, causing uneven pre-calcination heating and poor performance consistency; Comparative Example 6 had insufficient oxygen concentration, resulting in incomplete oxidation and impaired magnetic properties and formability; Comparative Example 7 did not add surface modifiers, leading to severe powder agglomeration in the air jet mill and poor particle size control; Comparative Example 8 had an excessively low gas-solid ratio, resulting in insufficient grinding efficiency and affecting... Particle size distribution and formability: Comparative Example 9 had a high span value for coarse powder, and even after secondary grinding, the initial non-uniformity still resulted in a low yield. Comparative Example 10 used only air jet milling without wet ball milling, resulting in poor powder morphology and slightly poor formability. Comparative Example 11 had a high span value after air jet milling, and the initial non-uniformity affected the final performance. Comparative Example 12 had excessive water vapor, causing the powder to become damp and agglomerate, leading to the failure of particle size control. Comparative Example 13 used ball milling instead of wet ball milling, resulting in uneven mixing and a significant decrease in performance. Comparative Example 14 directly added secondary small materials in the air jet mill, causing segregation due to differences in specific gravity, resulting in uneven mixing and poor performance. These deviations in the comparative examples all confirm the necessity of the key parameters of this invention. Only through the coordinated control of the overall process can the formability and magnetic properties of magnetic powder be improved simultaneously.
[0154] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.
[0155] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.
Claims
1. A method for preparing easily formable dry-pressed ferrite magnetic powder, characterized in that... include: 1) Mixing materials; 2) The material is fed into a rotary kiln for pre-firing. The final pre-firing temperature is 1300±50℃. The residence time of the material within 100℃ below the final temperature is 1.5-2.5h. The oxygen concentration at the feed end is ≥16 vol%. The portion of the pre-firing pellets with a diameter of 8-12mm is ≥80 wt%. 3) Grind and sieve to obtain an average particle size of 4-8 μm, and the particle size distribution satisfies D. 50 2-4 μm, D 90 Coarse powder with a particle size ≤10μm and a span value ≤3; 4) Secondary grinding to obtain an average particle size of 0.83-0.9μm, D 50 The thickness is 1.2-1.3 μm, D 90 Fine powder with a particle size of 2.8-3.0 μm and a span value of 1.8-2; 5) Drying.
2. The preparation method according to claim 1, characterized in that: 1) In, The mixture comprises uniformly mixing an iron source and a strontium source with an iron-strontium molar ratio of 5.5-6.2:1, and 0-0.5 wt% silica, which accounts for the total amount of the iron source and the strontium source. The chloride ion content in the iron source is ≤0.4wt%.
3. The preparation method according to claim 1 or 2, characterized in that: 1) In, The mixing process includes first dry mixing, then densification and pelletizing to obtain pellets; or The mixing process involves wet mixing followed by centrifugation or pressure filtration to obtain a cake with a moisture content of 18-30 wt%.
4. The preparation method according to claim 1, characterized in that: In step 2), the length of the rotary kiln is 30-50m, the feed end temperature is 10-50℃, the discharge end temperature is 700-1100℃, and the total residence time of the material in the rotary kiln is 8-12h.
5. The preparation method according to claim 1, characterized in that: In step 4), the secondary pulverization includes: mixing 100 parts of coarse powder with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide and 0.01-4 parts of lanthanum oxide by mass, and then performing wet ball milling.
6. The preparation method according to claim 1, characterized in that: In step 4), the secondary pulverization includes: first, air-jet milling the coarse powder, and then, by mass, mixing 100 parts of the powder after air-jet milling with 0-1.4 parts of calcium carbonate, 0-0.4 parts of silicon dioxide, 0-0.2 parts of strontium carbonate, 0-0.4 parts of boric acid, 0-0.5 parts of aluminum oxide, 0-0.5 parts of chromium oxide, 0-2 parts of cobalt oxide and 0.01-4 parts of lanthanum oxide, and then performing wet ball milling.
7. The preparation method according to claim 6, characterized in that: 4) The air jet mill comprises: mixing 100 parts by mass of coarse powder with 0.5-1 parts by mass of surface modifier, and mixing the mixture uniformly in a dry compressed air medium with a water vapor content of less than 0.5 vol% and a gas-solid ratio of 2.5-3.5 m. 3 / kg, under gas flow rate of 350-450m / s, pulverized to an average particle size of 0.83-0.9μm, and D 50 The thickness is 1.2-1.3 μm, D 90 The thickness is 2.8-3.0 μm, and the span is 1.8-2.0 μm.
8. The preparation method according to claim 7, characterized in that: 4) In, The surface modifier is stearic acid and / or a silane coupling agent; The calcium carbonate, silica, strontium carbonate, alumina, chromium oxide, cobalt oxide, and lanthanum oxide mixed with the powder after air jet milling meet the requirement of an average particle size ≤2μm.
9. The preparation method according to claim 1, characterized in that: In step 5), the drying temperature is 600-900℃.
10. The preparation method according to claim 1, characterized in that: 5) The moisture content of the dried magnetic powder is ≤0.6wt%.