A method for preparing permanent ferrite using microwave pretreatment

Abstract

The present invention relates to the field of permanent magnetic materials and discloses a method for preparing permanent magnetic ferrites by using microwave pretreatment, comprising: 1) crushing iron scales and then subjecting them to microwave pretreatment under oxygen supply conditions to obtain iron scales with oil film removed and pre-oxidized; 2) mixing the iron scales with strontium carbonate or barium carbonate, wet ball milling, and dehydrating to obtain a dehydrated material; 3) transferring the dehydrated material to an agglomeration and pre-firing integrated kiln for agglomeration and pre-firing to obtain a pre-firing material; 4) preparing the pre-firing material into permanent magnetic ferrites. On the one hand, the present invention uses microwave pretreatment to remove oil and oxidize the iron scale raw material, which can avoid the generation of foam due to the high oil content of the iron scale during ball milling. The use of microwave pretreatment for oxidation has lower energy consumption and higher efficiency. On the other hand, the agglomeration and pre-firing integrated kiln of the present invention can realize that the oxidized dehydrated material automatically agglomerates in the agglomeration and pre-firing integrated kiln without the need for separate ball making. Therefore, the production continuity of the process of the present invention is better and the production efficiency can be significantly improved.

Description

Technical Field

[0001] The present invention relates to the field of permanent magnetic materials, and in particular to a method for preparing permanent magnetic ferrite by using microwave pretreatment. Background Art

[0002] During steel forging and hot rolling, the reaction between the steel surface and oxygen in the air forms a layer of iron oxides, known in the industry as iron scale. Its primary components are elemental iron, ferrous iron, and ferric iron. Iron scale is brittle and easily falls off. To maximize resource utilization, existing technologies have used it as a raw material for permanent ferrite magnets.

[0003] To obtain high-performance permanent ferrite magnets, it's necessary to oxidize the low-valent iron in the scale to high-valent iron as much as possible. Therefore, a series of pretreatments are usually required before sintering the scale. Currently, the most common method involves wet grinding, water removal, pelletizing, drying, pre-oxidation, and pre-sintering the scale.

[0004] The pre-oxidation temperature is typically 800-1000°C, with a holding time of 30-90 minutes. This process consumes a significant amount of energy. To address this, patent CN1031622A combines the pre-oxidation and pre-firing stages to achieve energy savings. However, this combination places extremely high demands on the oxidizing atmosphere and stricter control of the kiln's internal conditions. Therefore, this process is only suitable for small-batch trials and is difficult to implement in actual production.

[0005] On the other hand, since iron scale itself is a surface material such as steel ingots, it is covered with an oil film before becoming iron scale. The applicant discovered that wet grinding of iron scale without removing the oil film, if the oil film content is high, will produce a large amount of foam, affecting the mixing ratio; the oxidation process also consumes more heat, and the oil film oxidation easily competes with the low-valent iron elements in the iron scale for oxygen, thus affecting material properties. However, the applicant's search found that no prior art has addressed this issue. No literature, including CN1031622A, has previously reported pre-degreasing of iron scale.

[0006] Furthermore, in traditional processes, the materials must be pelletized (usually using a dry pelletizing process) before drying, oxidation, and pre-calcining to ensure better flowability during subsequent kiln processing. However, this cumbersome process severely disrupts the flow of the entire production process and significantly reduces production efficiency.

[0007] In summary, in the process of preparing permanent ferrite using iron scale as raw material, it is necessary to propose a method that is more suitable for industrial production to reduce energy consumption and improve production efficiency, while removing the oil film on the surface of the iron scale, thereby further improving the performance of the permanent ferrite. Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a method for preparing permanent magnet ferrite using microwave pretreatment. On the one hand, the present invention uses microwave pretreatment to deoil and pre-oxidize the iron scale raw material, which can avoid the generation of a large amount of foam due to the high oil content of the iron scale during ball milling, and the microwave pretreatment for pre-oxidation has lower energy consumption and higher efficiency, which is conducive to obtaining high-performance permanent magnet ferrite. On the other hand, the use of the agglomeration and pre-firing integrated kiln of the present invention can realize the automatic agglomeration and pre-firing of the dehydrated material after pre-oxidation in the agglomeration and pre-firing integrated kiln, without the need for separate ball making. Therefore, the production continuity of the process of the present invention is better, and the production efficiency can be significantly improved.

[0009] The specific technical solution of the present invention is: a method for preparing permanent ferrite using microwave pretreatment, comprising the following steps:

[0010] 1) The iron scale is crushed and sieved, and then subjected to microwave pretreatment under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0011] 2) The iron scale obtained in step 1) is mixed with strontium carbonate or barium carbonate, and wet-milled to an average particle size of 3-5 μm. After dehydration, a dehydrated material with a moisture content of 8-12 wt% is obtained.

[0012] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. The dehydrated material is agglomerated and pre-calcined in the agglomeration and pre-calcination kiln during the tumbling and discharging process to obtain a pre-calcined material with a particle size of 2-6 mm.

[0013] 4) The pre-burned material is crushed, and then secondary batching, fine grinding, pressing, and secondary sintering are performed to obtain permanent ferrite.

[0014] The present invention abandons the traditional method of crushing-ball milling-balling-drying-oxidation-pre-burning the iron scale raw material in sequence. Specifically, the traditional balling and oxidation processes are eliminated. Instead, microwave pretreatment is performed directly after the iron scale is crushed. The microwave pretreatment of the present invention can simultaneously remove oil and pre-oxidize the iron scale raw material. Among them: (a) In terms of degreasing: the present invention finds that under microwave radiation, the iron scale can act as a "hot spot" and can more efficiently remove oil stains on the surface of the iron scale at the same heating temperature, thereby facilitating the subsequent oxidation of the iron scale (the presence of the oil film will compete with the low-valent iron element in the iron scale for oxygen during the oxidation process, thereby improving the material properties) and avoiding the generation of a large amount of foam due to the high oil content during subsequent ball milling, thereby affecting the material properties; (b) In terms of oxidation, the iron scale contains elemental iron. These surface points can strongly interact with microwave energy, thereby quickly converting microwave energy into heat energy, so that these surface points are selectively and quickly heated to a higher temperature. Therefore, the iron scale can be quickly oxidized under aerobic conditions to form iron oxide. Compared with oxidation in a traditional oxidation kiln, the microwave pretreatment of the present invention not only has lower energy consumption, but also has higher efficiency (lower temperature, shorter time), more complete oxidation, and is therefore more conducive to obtaining high-performance permanent magnet ferrite.

[0015] Preferably, in step 1), the oil content of the iron scale is ≤0.5wt%.

[0016] Since the invention performs microwave pretreatment on the iron scale, it is applicable to the iron scale with higher oil content, while the conventional process is only applicable to the iron scale with oil content ≤0.1wt%.

[0017] Preferably, in step 1), the sieving is through a 50-mesh sieve.

[0018] Preferably, in step 1), the microwave pretreatment is performed at a temperature of 100-200° C., a frequency of 500-5000 MHz, and a time of 10-20 min.

[0019] Compared with conventional oxidation processes, the microwave pretreatment process of the present invention has lower temperature (the conventional oxidation process temperature is usually 800-1000° C.) and shorter time (the conventional oxidation process time is usually 30-90 minutes), and therefore has lower energy consumption.

[0020] Preferably, in step 2), the molar ratio of the iron scale to strontium carbonate or barium carbonate is 5.2-5.8:1, calculated based on the iron element in the iron scale.

[0021] Preferably, in step 3), the agglomeration and pre-firing integrated kiln comprises:

[0022] Rotating bearings;

[0023] The rolling cylindrical kiln body is tilted and mounted on a rotating bearing in a manner that is higher in the front and lower in the back. It is divided into an agglomeration section and a pre-burning section according to the direction of material feeding and discharging, and the inner diameter of the agglomeration section is smaller than that of the pre-burning section. A number of axial auxiliary agglomeration plates perpendicular to the inner wall are provided on the inner wall of the agglomeration section along the circumferential direction. A number of through holes with a diameter of 6 mm are provided on the auxiliary agglomeration plates. The cross-section of each auxiliary agglomeration plate on the same cross-section is bent in two sections, and the bending direction is toward the rolling direction. The discharge end of the rolling cylindrical kiln body is provided with a fuel spray gun facing the feeding direction (the fuel is a mixture of natural gas and compressed air).

[0024] Prior art usually requires pelletizing the raw materials using pelletizing equipment before drying, oxidizing, and pre-firing them, which disrupts the smoothness of the entire production process and significantly reduces production efficiency. To this end, the present invention, through process improvements and the use of a self-developed integrated agglomeration and pre-firing kiln, not only replaces the oxidation stage with microwave pretreatment, but also enables the dehydrated material after pre-oxidation to be automatically agglomerated and pre-fired in the integrated agglomeration and pre-firing kiln, eliminating the need for pelletizing equipment such as discs to separately pelletize the material. Therefore, the process of the present invention has better production continuity and can significantly improve production efficiency.

[0025] The agglomeration principle of the present invention is as follows: dehydrated material is added to a rolling agglomeration and pre-calcination integrated kiln. The dehydrated material first tumbles continuously in the agglomeration section. Under the combined effects of gravity, its own viscosity, and the agglomeration auxiliary plate, it automatically forms pellets and solidifies at high temperature (also achieving more complete oxidation). The agglomerated pellets are then pushed toward the pre-calcination section by the rolling force. In this section, the iron scale in the pellets reacts with strontium carbonate or barium carbonate in the following reaction (using strontium carbonate as an example): 6Fe2O3 + SrCO3 → SrO·6Fe2O3 + CO2, thus obtaining the pre-calcined material.

[0026] The key points in the above process are:

[0027] (1) Agglomeration auxiliary plate: The present invention is provided with an agglomeration auxiliary plate on the inner wall of the agglomeration section. The existence of the agglomeration auxiliary plate can enhance the tumbling effect of the dehydrated material under the drive of the rolling cylindrical kiln body, accelerate the evaporation of water, thereby promoting solidification into agglomerates, and fully oxidize the iron scales. Furthermore, the plate surface of the agglomeration auxiliary plate is designed to be a hollow structure with through holes, which can avoid the formation of large lumps in the early stage of agglomeration due to the excessive viscosity of the dehydrated material locally and the inability to disperse into small pellets, thereby playing an auxiliary agglomeration effect. In addition, the present invention designs part of the agglomeration auxiliary plate into a two-stage bending shape, which can further enhance the tumbling effect of the pellets on the one hand; on the other hand, the second bending section can play a certain buffering and protective role in the falling process of the pellets after being lifted up, thereby preventing the pellets from breaking.

[0028] (2) The present invention designs the inner diameter of the agglomeration section to be smaller than that of the pre-burning section. This design can avoid the return of the pellets on the one hand. On the other hand, more importantly, since the pellets in the agglomeration section have not yet been completely solidified, they should not be subjected to excessive tumbling forces. Therefore, reducing the inner diameter of the agglomeration section can reduce the linear speed of the pellets (the smaller the inner diameter, the smaller the linear speed at the same rotation speed), thereby protecting the pellets. At the same time, since the inner diameter of the pre-burning section is larger, the internal space is also larger, which can improve the pre-burning efficiency of the material; in addition, the wind speed in the agglomeration section increases, which can take away moisture faster, reduce the temperature at the kiln tail, and achieve energy-saving effects.

[0029] (3) Particle size of agglomerates: Currently, the particle size of permanent magnet ferrites is usually large (>5mm), and the present invention has found that the particle size of agglomerates in the agglomeration and pre-firing integrated kiln of the present invention cannot be too large, otherwise they are easily crushed during the tumbling process, affecting the pre-firing effect, and some of the crushed materials will become dust in the kiln. These dusts will run around in the kiln, which may easily lead to abnormal batching and ultimately unstable performance. On the contrary, if the particle size of the agglomerates is too small, the fluidity of the material will deteriorate, making it impossible to efficiently advance in the kiln. Ultimately, the present invention has found that it is ideal to control the particle size of the agglomerates to 2-6mm. In order to obtain agglomerates in the above-mentioned specific particle size range, it is necessary to control the particle size and moisture content of the dehydrated material to an average particle size of 3-5μm and 8-12wt%, respectively.

[0030] Preferably, the rolling cylindrical kiln body has an inclination angle of 3-5° to the horizontal plane.

[0031] Preferably, the feature is that: the length ratio of the agglomeration section to the pre-burning section is 1:5-1:8; the inner diameter ratio of the agglomeration section to the pre-burning section is 0.8-0.95:1; and the linear speed of the inner wall of the pre-burning section is 0.3-0.6 m / s.

[0032] Preferably, the included angle of the two-stage bend is 150-170°.

[0033] Preferably, in step 3), the temperature of the feeding end of the agglomeration and pre-calcining integrated kiln is 150-250°C, and the temperature of the discharging end is 1250-1350°C.

[0034] Preferably, in step 4), the secondary ingredients are added to the crushed pre-calcined material: 0.6-1.3wt% CaCO3, 0.1-0.6wt% SiO2, 0-0.2wt% HBO3, 0-0.3wt% SrCO3 and 0-0.3wt% Al2O3;

[0035] Preferably, in step 4), the average particle size of the material after fine grinding is 0.75-0.85 μm.

[0036] Preferably, in step 4), the pressing is wet pressing or dry pressing.

[0037] Preferably, in step 4), the secondary sintering is carried out at 1230-1260° C. for 1.5-3 hours.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The present invention uses microwave pretreatment to replace the oxidation process in the traditional permanent magnet ferrite preparation process. The microwave pretreatment of the present invention can simultaneously remove oil and pre-oxidize the iron scale raw material. Compared with direct wet ball milling of the iron scale, the microwave pretreatment of the present invention can avoid the generation of a large amount of foam during ball milling due to the high oil content of the iron scale, resulting in poor oxidation effect and thus affecting material properties. Compared with oxidation in a traditional oxidation kiln, the microwave pretreatment of the present invention not only has lower energy consumption and higher efficiency than conventional oxidation processes, but is also more conducive to obtaining high-performance permanent magnet ferrite.

[0040] (2) The agglomeration and pre-firing integrated kiln of the present invention can realize the automatic agglomeration of the dehydrated material after oxidation in the agglomeration and pre-firing integrated kiln, without the need to use special pelletizing equipment to pelletize the material separately. Therefore, the production continuity of the process of the present invention is better and the production efficiency can be significantly improved.

[0041] (3) The present invention controls the particle size and moisture content of the dehydrated material, which is conducive to its agglomeration into pellets with a particle size of 2-6 mm in the agglomeration section of the agglomeration and pre-firing integrated kiln. The pellets with this particle size not only have good fluidity but are also not easy to break during the rolling and impact process.

[0042] (4) The present invention is provided with an agglomeration auxiliary plate on the inner wall of the agglomeration section, which can enhance the tumbling effect of the dehydrated material under the drive of the rolling cylindrical kiln body, accelerate the evaporation of water, and thus promote solidification into agglomerates. Furthermore, the agglomeration auxiliary plate is designed with through holes, which can avoid the formation of large agglomerates in the early stage of agglomeration due to the excessive viscosity of the dehydrated material locally and the inability to disperse into small agglomerates, thereby playing an auxiliary agglomeration effect. The present invention designs part of the agglomeration auxiliary plate into a two-section bending shape, which can further enhance the tumbling effect of the agglomerates on the one hand; on the other hand, the second section of the bending can play a certain buffering and protective role in the process of the agglomerates falling after being lifted up, preventing the agglomerates from breaking.

[0043] (5) The present invention designs the inner diameter of the agglomeration section to be smaller than that of the pre-burning section, which can avoid the return of the agglomerates on the one hand and protect the agglomerates on the other hand. At the same time, since the inner diameter of the pre-burning section is larger, the internal space is also larger, which can improve the pre-burning efficiency of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 A structural schematic diagram of the agglomeration and pre-firing integrated kiln in Example 1;

[0045] Figure 2A schematic diagram of the distribution of the agglomeration auxiliary plates in Example 1;

[0046] Figure 3 This is a structural schematic diagram of the agglomeration auxiliary plate in Example 1;

[0047] Figure 4 This is a photo of the material after wet ball milling and before dehydration in Example 1;

[0048] Figure 5 This is a photo of the material after wet ball milling and before dehydration in Comparative Example 1;

[0049] Figure 6 This is a photo of the material after wet ball milling and before dehydration in Comparative Example 3.

[0050] The reference numerals are: rotating bearing 1, agglomerating section 2, pre-burning section 3, auxiliary agglomerating plate 4, through hole 5, fuel spray gun 6. DETAILED DESCRIPTION

[0051] The present invention will be further described below with reference to the embodiments.

[0052] A method for preparing permanent ferrite using microwave pretreatment comprises the following steps:

[0053] 1) Crushing iron scale with an oil content of ≤0.5wt% and passing it through a 50-mesh sieve, and subjecting it to microwave pretreatment (100-200°C, 500-5000MHz for 10-20min) under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0054] 2) The iron scale obtained in step 1) is mixed with strontium carbonate or barium carbonate in a molar ratio of 5.2-5.8:1 based on the iron element in the iron scale, wet-milled to an average particle size of 3-5 μm, and dehydrated to obtain a dehydrated material with a moisture content of 8-12 wt%.

[0055] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. The dehydrated material is agglomerated and pre-calcined in the agglomeration and pre-calcination kiln during the tumbling and discharging process to obtain a pre-calcined material with a particle size of 2-6 mm.

[0056] 4) The pre-sintered material is crushed and then secondary batched (0.6-1.3wt% CaCO3, 0.1-0.6wt% SiO2, 0-0.2wt% HBO3, 0-0.3wt% SrCO3 and 0-0.3wt% Al2O3 are added), finely ground to an average particle size of 0.75-0.85μm, pressed (wet pressed or dry pressed), and sintered again (1230-1260℃ for 1.5-3h) to obtain permanent magnet ferrite.

[0057] As a preference, Figure 1 As shown, the agglomeration pre-firing integrated kiln includes:

[0058] Rotating bearing 1;

[0059] The rolling cylindrical kiln body is mounted on a rotating bearing at an angle of 3-5 degrees with the front higher and the back lower. It is divided into an agglomeration section 2 and a pre-burning section 3 according to the direction of material feeding and discharging. The inner diameter of the agglomeration section is smaller than that of the pre-burning section (the length ratio of the agglomeration section to the pre-burning section is 1:5-1:8; the inner diameter ratio is 0.8-0.95:1, and the rotation speed of the inner wall of the pre-burning section is 0.3-0.6m / s); a plurality of axial auxiliary agglomeration plates 4 are provided on the inner wall of the agglomeration section along the circumferential direction; Figure 3 As shown, the auxiliary agglomeration plate is provided with a plurality of through holes 5 with a diameter of 6 mm; Figure 2 As shown, the auxiliary agglomeration plates located on the same cross section have a cross section of each alternate auxiliary agglomeration plate bent in two sections (with an angle of 150-170°), and the bending direction is toward the rolling direction; the discharge end of the rolling cylindrical kiln body is provided with a fuel spray gun 6 (the fuel is a mixture of natural gas and compressed air) facing the feeding direction.

[0060] During operation, the temperature at the feed end of the agglomeration pre-calcining integrated kiln is 150-250℃, and the temperature at the discharge end is 1250-1350℃.

[0061] Example 1

[0062] 1) Crushing iron scale with an oil content of 0.3 wt% and passing it through a 50-mesh sieve, and subjecting it to microwave pretreatment (150° C., 2500 MHz, 15 min) under oxygen supply conditions to obtain iron scale with the oil film removed and pre-oxidized.

[0063] 2) The iron scale obtained in step 1) (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.5:1 based on the iron in the iron scale, and wet ball milled to an average particle size of 4.4 μm. After dehydration, a dehydrated material with a moisture content of 11 wt% was obtained.

[0064] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. During operation, the temperature at the feed end of the agglomeration and pre-calcination kiln is 200°C, and the temperature at the discharge end is 1300°C. The dehydrated material is agglomerated and pre-calcined in the process of tumbling and discharging in the agglomeration and pre-calcination kiln, and a pre-calcined material with a particle size of 2-6 mm is obtained.

[0065] 4) The pre-sintered material was crushed and then secondary batched (adding 0.9wt% CaCO3, 0.3wt% SiO2, 0.1wt% HBO3, 0.15wt% SrCO3 and 0.15wt% Al2O3), finely ground to an average particle size of 0.8μm, pressed (dry pressed), and sintered twice (1245℃ for 2.5h) to obtain permanent ferrite.

[0066] Among them, Figure 1As shown, the agglomeration and pre-burning integrated kiln includes: a rotating bearing 1 and a rolling cylindrical kiln body tilted at a 4° angle on the rotating bearing in a front-high and rear-low manner. The rolling cylindrical kiln body is divided into an agglomeration section 2 and a pre-burning section 3 according to the direction of material feeding and discharging, and the inner diameter of the agglomeration section is smaller than that of the pre-burning section (the agglomeration section is 6 meters long and has an inner diameter of 1.6m; the pre-burning section is 42 meters long and has an inner diameter of 2m, and the rotational linear speed of the inner wall of the pre-burning section is 0.45m / s). Three circles of axial auxiliary agglomeration plates 4 are provided on the inner wall of the agglomeration section in the circumferential direction, and each circle includes 4 auxiliary agglomeration plates distributed at equal angles. Through holes 5 with an aperture of 6mm are distributed on the surface of the auxiliary agglomeration plates, and the cross-section of each auxiliary agglomeration plate located on the same cross section is bent in two sections (angle 160°), and the bending direction is toward the rolling direction. The discharge end of the rolling cylindrical kiln body is provided with a fuel spray gun 6 (the fuel is a mixture of natural gas and compressed air) facing the feeding direction.

[0067] Example 2

[0068] The same agglomeration and pre-firing integrated kiln as in Example 1 was used:

[0069] 1) Crushing iron scale with an oil content of 0.3 wt% and passing it through a 50-mesh sieve, and subjecting it to microwave pretreatment (200° C., 3000 MHz, 12 min) under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0070] 2) The iron scale obtained in step 1) (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.8:1 based on the iron in the iron scale, and wet ball milled to an average particle size of 4.2 μm. After dehydration, a dehydrated material with a moisture content of 12 wt% was obtained.

[0071] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. During operation, the temperature at the feed end of the agglomeration and pre-calcination kiln is 180°C, and the temperature at the discharge end is 1280°C. The dehydrated material is agglomerated and pre-calcined in the process of tumbling and discharging in the agglomeration and pre-calcination kiln, and a pre-calcined material with a particle size of 2-6 mm is obtained.

[0072] 4) The pre-sintered material was crushed and then secondary batched (1.2 wt% CaCO3, 0.3 wt% SiO2, and 0.15 wt% HBO3 were added), finely ground to an average particle size of 0.8 μm, pressed (dry pressed), and sintered twice (at 1245°C for 2.5 h) to obtain permanent ferrite magnets.

[0073] Example 3

[0074] The same agglomeration and pre-firing integrated kiln as in Example 1 was used:

[0075] 1) Iron scale with an oil content of 0.3 wt% was crushed and passed through a 50-mesh sieve, and then subjected to microwave pretreatment (200° C., 5000 MHz, 10 min) under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0076] 2) The iron scale obtained in step 1) (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.2:1 based on the iron in the iron scale, and wet ball milled to an average particle size of 4.5 μm. After dehydration, a dehydrated material having a moisture content of 10.5 wt% was obtained.

[0077] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. During operation, the temperature at the feed end of the agglomeration and pre-calcination kiln is 220°C, and the temperature at the discharge end is 1320°C. The dehydrated material is agglomerated and pre-calcined in the process of tumbling and discharging in the integrated agglomeration and pre-calcination kiln, and a pre-calcined material with a particle size of 2-6 mm is obtained.

[0078] 4) The pre-sintered material was crushed and then secondary batched (1.2wt% CaCO3, 0.3wt% SiO2, 0.1wt% HBO3, 0.1wt% SrCO3 and 0.05wt% Al2O3 were added), finely ground to an average particle size of 0.8μm, pressed (dry pressed), and sintered twice (1245℃ for 2.5h) to obtain permanent ferrite.

[0079] Comparative Example 1 (using traditional crushing-ball milling-ball making-drying-oxidation-pre-sintering-secondary sintering process)

[0080] 1) The iron scale was crushed and passed through a 50-mesh sieve. The iron scale (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.5:1 based on the iron element in the iron scale. The mixture was wet-ball milled and dehydrated to obtain a dehydrated material (average particle size of 4.5 μm and water content of 3 wt%).

[0081] 2) The dehydrated material is granulated, and the average particle size of the pellets is about 10 mm.

[0082] 3) The spherical material is transferred to a conventional oxidation kiln and a conventional pre-firing kiln in sequence for dry oxidation (1000° C.) and pre-firing (1330° C.) to obtain a pre-firing material.

[0083] 4) Same as Example 1.

[0084] Comparative Example 2

[0085] The same agglomeration pre-sintering integrated kiln as in Example 1 was used; the difference between the preparation methods of Comparative Example 2 and Example 1 was only that the microwave pretreatment conditions in step 1) were different:

[0086] 1) Iron scale with an oil content of 0.3 wt% was crushed and passed through a 50-mesh sieve, and then subjected to microwave pretreatment (300° C., 6000 MHz, 10 min) under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0087] 2) The iron scale obtained in step 1) (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.5:1 based on the iron in the iron scale, and wet ball milled to an average particle size of 4.6 μm. After dehydration, a dehydrated material with a moisture content of 11 wt% was obtained.

[0088] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. During operation, the temperature at the feed end of the agglomeration and pre-calcination kiln is 200°C, and the temperature at the discharge end is 1300°C. The dehydrated material is agglomerated and pre-calcined in the process of tumbling and discharging in the agglomeration and pre-calcination kiln, and a pre-calcined material with a particle size of 2-6 mm is obtained.

[0089] 4) The pre-sintered material was crushed and then secondary batched (adding 0.9wt% CaCO3, 0.3wt% SiO2, 0.1wt% HBO3, 0.15wt% SrCO3 and 0.15wt% Al2O3), finely ground to an average particle size of 0.8μm, pressed (dry pressed), and sintered twice (1245℃ for 2.5h) to obtain permanent ferrite.

[0090] Comparative Example 3

[0091] The same agglomeration pre-sintering integrated kiln as in Example 1 was used; the difference between the preparation methods of Comparative Example 3 and Example 1 was only that the microwave pretreatment conditions in step 1) were different:

[0092] 1) Iron scale with an oil content of 0.3 wt% was crushed and passed through a 50-mesh sieve, and then subjected to microwave pretreatment (300° C., 300 MHz, 10 min) under oxygen supply conditions to obtain iron scale with oil film removed and pre-oxidized.

[0093] 2) The iron scale obtained in step 1) (containing 73% iron) was mixed with strontium carbonate in a molar ratio of 5.5:1 based on the iron in the iron scale, and wet ball milled to an average particle size of 4.2 μm. After dehydration, a dehydrated material with a moisture content of 11 wt% was obtained.

[0094] 3) The dehydrated material is transferred to an integrated agglomeration and pre-calcination kiln. During operation, the temperature at the feed end of the agglomeration and pre-calcination kiln is 200°C, and the temperature at the discharge end is 1300°C. The dehydrated material is agglomerated and pre-calcined in the process of tumbling and discharging in the agglomeration and pre-calcination kiln, and a pre-calcined material with a particle size of 2-6 mm is obtained.

[0095] 4) The pre-sintered material was crushed and then secondary batched (adding 0.9wt% CaCO3, 0.3wt% SiO2, 0.1wt% HBO3, 0.15wt% SrCO3 and 0.15wt% Al2O3), finely ground to an average particle size of 0.8μm, pressed (dry pressed), and sintered twice (1245℃ for 2.5h) to obtain permanent ferrite.

[0096] Comparative Example 4

[0097] The same agglomeration and pre-firing integrated kiln as in Example 1 was used; the only difference between Comparative Example 4 and Example 1 was that the moisture content of the dehydrated material in step 2) was about 3 wt %. It was found that the particle size of the pellets obtained by subsequent agglomeration was too small (mainly between 0.5 and 1.5 mm), resulting in poor fluidity of the pellets and too slow advancement speed in the agglomeration and pre-firing integrated kiln, which greatly reduced production efficiency.

[0098] Comparative Example 5

[0099] The same agglomeration and pre-firing integrated kiln as in Example 1 was used; the only difference between Comparative Example 5 and Example 1 was that the moisture content of the dehydrated material in step 2) was about 18 wt %. It was found that the particle size of the pellets obtained by subsequent agglomeration was too large (mainly between 5-9 mm), resulting in low strength of the pellets. A large amount of breakage occurred during tumbling in the agglomeration and pre-firing integrated kiln, and a large amount of dust was generated, which reduced the raw material utilization rate and affected the performance of the material.

[0100] Sintered magnet performance test

[0101] The permanent ferrites of Examples 1-3 and Comparative Examples 1-3 were subjected to magnetic property tests, and the results were as follows:

[0102] Case Br(Gs) Hcj(Oe) Shrinkage Example 1 4210 2804 1.141 Example 2 4217 2776 1.140 Example 3 4235 2902 1.143 Comparative Example 1 4198 2736 1.140 Comparative Example 2 4186 2771 1.142 Comparative Example 3 4124 2744 1.139

[0103] Analyzing the data in the above table, we can see that:

[0104] The performance of the permanent ferrite obtained in Examples 1-3 is Br>4200Gs, Hcj>2770Oe; the above data all meet the expected performance requirements. It can be seen that compared with the prior art (Comparative Example 1), the present invention uses microwave pretreatment instead of the conventional oxidation process in an oxidation kiln, which can effectively reduce energy consumption (lower oxidation temperature). At the same time. Figure 4 and Figure 5By comparison, Example 1 effectively removes the oil film through microwave pretreatment, resulting in virtually no foam after wet ball milling. However, Comparative Example 1, which did not undergo degreasing, exhibits a thick layer of foam on the surface of the slurry after wet ball milling. Therefore, microwave pretreatment can reduce the negative impact of the oil film on iron scale during oxidation, thereby producing a permanent ferrite with improved performance. Furthermore, Comparative Example 1 requires separate pelletizing of the dehydrated material prior to drying, oxidation, and pre-calcining. However, the integrated agglomeration and pre-calcining kiln of the present invention eliminates the need for separate pelletizing, resulting in higher production efficiency.

[0105] The difference between Comparative Example 2 and Example 1 lies in the different microwave pretreatment processes. Although the performance of the obtained permanent magnet ferrite is only slightly worse than that of Example 1, it is found that due to the excessively high microwave temperature and microwave frequency, the particles agglomerate, resulting in poor uniformity of the subsequent ball milling particle size and a longer time required to grind to the target particle size.

[0106] The difference between Comparative Example 3 and Example 1 is that the microwave pretreatment temperature is higher and the frequency is lower. The results show that the microwave pretreatment cannot fully remove the oil film. Figure 4 and Figure 6 As shown in the comparison, there is almost no foam in the material after wet ball milling in Example 1, while there is more foam in the material after continued wet ball milling in Comparative Example 3, and strontium carbonate accounts for a large proportion in the foam, resulting in a deviation in the raw material ratio during the dehydration process, affecting the subsequent pre-firing performance.

[0107] Unless otherwise specified, the raw materials and equipment used in the present invention are commonly used in the art; the methods used in the present invention are conventional methods in the art unless otherwise specified.

[0108] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing permanent ferrite by microwave pretreatment, characterized in that The following steps are involved: 1) Crushing the iron scale and screening it, and then subjecting it to microwave pretreatment under oxygen supply conditions to obtain iron scale that has been degreased and pre-oxidized; the microwave pretreatment conditions are: temperature 100-200°C, frequency 500-5000 MHz, and time 10-20 minutes; 2) mixing the iron scale obtained in step 1) with strontium carbonate or barium carbonate, wet-milling the mixture to an average particle size of 3-5 μm, and dehydrating the mixture to obtain a dehydrated material having a moisture content of 8-12 wt%; 3) The dehydrated material is transferred to the agglomeration and pre-calcination integrated kiln. The dehydrated material is agglomerated and pre-calcined in the agglomeration and pre-calcination integrated kiln during the tumbling and discharging process, and the pre-calcined material with a particle size of 2-6 mm is obtained; 4) After the pre-burned material is crushed, it is secondary batched, finely ground, pressed, and sintered for the second time to obtain permanent magnet ferrite.

2. The method according to claim 1, wherein: In step 1), The oil content of the iron scale is ≤0.5wt%; The sieving is through a 50-mesh sieve.

3. The method according to claim 1, wherein: In step 3), the dehydrated material enters the agglomeration section of the rolling cylindrical kiln body of the agglomeration and pre-calcining integrated kiln, and the inner wall of the agglomeration section is provided with a plurality of axial auxiliary agglomeration plates perpendicular to the inner wall along the circumferential direction; the auxiliary agglomeration plates are provided with a plurality of through holes, and the cross-section of each auxiliary agglomeration plate located on the same cross section is bent in two sections, and the bending direction is toward the rolling direction; under the action of the rolling cylindrical kiln body and the auxiliary agglomeration plates, the dehydrated material is agglomerated, and continues to enter the pre-calcining section with an inner diameter larger than that of the agglomeration section for pre-calcination, and is discharged after pre-calcination to obtain pre-calcined material.

4. The method according to claim 3, wherein: The inclination angle of the rolling cylindrical kiln body to the horizontal plane is 3-5 degrees.

5. The method according to claim 3, wherein: The length ratio of the agglomeration section to the pre-burning section is 1:5-1:8; The inner diameter ratio of the agglomeration section and the pre-burning section is 0.8-0.95:1; The linear velocity of the inner wall of the pre-burning section is 0.3-0.6 m / s.

6. The method according to claim 3, wherein: The included angle of the two-stage bend is 150-170°.

7. The method according to any one of claims 1 to 6, characterized in that: In step 3), the temperature of the feeding end of the agglomeration and pre-calcining integrated kiln is 150-250°C, and the temperature of the discharging end is 1250-1350°C.

8. The method according to claim 1, wherein: In step 4), The secondary ingredients are 0.6-1.3wt% of CaCO3, 0.1-0.6wt% of SiO2, 0-0.2wt% of HBO3, 0-0.3wt% of SrCO3 and 0-0.3wt% of Al2O3 added to the crushed pre-calcined material; The average particle size of the material after fine grinding is 0.75-0.85μm; The pressing is wet pressing or dry pressing.

9. The method according to claim 1, wherein: In step 4), the secondary sintering is carried out at 1230-1260° C. for 1.5-3 hours.

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