Preparation process of permanent magnetic ferrite capable of inhibiting invisible cracks

Abstract

The invention discloses a preparation technology of permanent magnetic ferrite capable of inhibiting invisible cracks. The preparation technology comprises the steps of raw material pretreatment, surface corona treatment, magnetic control wet grinding, slurry treatment, forming, sintering and surface grinding. A nano rough structure is formed on the surface of raw material particles through surface corona treatment, and the uniformity of the blank is improved by combining the synergistic effect of magnetic zirconium oxide beads and an alternating magnetic field in magnetic control wet grinding. According to the process, the particle size distribution span of the blank is reduced to 0.8-1.0, the number of invisible cracks is reduced by about 60%, the problems of cracks and precision in a traditional process are effectively solved, and the process is suitable for the high-end field.

Description

Technical Field

[0001] This article relates to a preparation process of permanent magnetic ferrite that can suppress invisible cracks. Background Art

[0002] In the production of permanent ferrite magnets, the initiation and propagation of invisible cracks is a key challenge that hinders product quality. Uneven mixing of raw materials during pretreatment can lead to variations in the ferrite's internal composition and microstructure, which can cause internal stress concentration during subsequent processing. For example, inadequate mixing of raw material particles can lead to varying reaction rates and shrinkage during sintering, generating localized stresses that foster the initiation of invisible cracks.

[0003] In existing technologies, the "pre-sintering-crushing-wet grinding" process is a promising approach for improving product performance. This allows for a thorough solid-phase reaction of the raw materials to form permanent ferrite, initially establishing a stable crystal structure that reduces grinding agglomeration and improves efficiency.

[0004] However, as market requirements for permanent ferrite magnets gradually increase, especially those for stability, this process has become difficult to meet these growing market demands. Furthermore, it only improves the activity of the raw materials to a limited extent. The double sintering process not only requires high space requirements and consumes a lot of energy, but can also lead to excessive grain growth due to localized excessive temperatures or prolonged holding times during pre-sintering, disrupting magnetic domain uniformity and causing a 10%-20% decrease in coercivity.

[0005] At the same time, this process fails to effectively address the problem of irrational internal stress distribution caused by external stress or temperature changes during processing, making it difficult to suppress the generation of invisible cracks and unable to meet the stringent requirements of high-end fields for product stability and dimensional accuracy. Therefore, developing a preparation process that can effectively suppress invisible cracks and accurately control magnet dimensions is of great significance to improving product quality and market competitiveness. Summary of the Invention

[0006] The purpose of the present invention is to provide a preparation process for permanent ferrite that can suppress invisible cracks. By combining the synergistic effect of surface corona treatment and magnetron wet grinding, combined with innovative processes such as vacuum dehydration and composite sintering and heating, the uniformity of the blank is improved, the internal stress is dispersed, and the dimensional accuracy is controlled, thereby effectively suppressing the generation of invisible cracks. The specific preparation process is as follows:

[0007] A preparation process of a permanent magnetic ferrite capable of suppressing invisible cracks comprises the following steps:

[0008] S1. Raw material pretreatment

[0009] According to the ingredient requirements, CaO, SrO, La2O3, Fe2O3, CoO, and silicon carbide whisker powders are weighed and placed into a double planetary mixer. Mixing is performed at a low speed of 200-300 rpm for 5-10 minutes. During this low-speed mixing process, an ultrasonic oscillator is activated at a frequency of 20-40 kHz and a power of 200-400 W. This cavitation effect breaks down agglomerated raw material particles, resulting in a uniformly mixed raw material powder.

[0010] S2. Surface corona treatment

[0011] The mixed raw material powders undergo surface corona treatment, placing the powders in a high-voltage electric field of the corona treatment equipment at a field strength of 15-25 kV / cm for 3-8 minutes. After treatment, the raw material particles develop a nanoscale roughened surface structure, significantly increasing their surface energy. The surface activity of the different materials is enhanced, transforming the large relative differences between the low-activity powder types into smaller differences at high activity. Compared with traditional pre-sintering, this method not only further enhances the surface activity of the particles but also shortens the treatment time, laying the foundation for uniform dispersion during subsequent wet grinding.

[0012] S3. Wet grinding

[0013] After corona treatment, deionized water is added at a solid-liquid ratio of 1:1.2-1:1.5, and a composite dispersant (formed by ammonium polyacrylate and sodium hexametaphosphate at a mass ratio of 3:2) accounting for 0.5-1.2% of the powder mass is added for wet grinding.

[0014] In traditional wet grinding, the movement of beads is dominated by mechanical agitation forces, resulting in a highly regular trajectory that can easily lead to uneven force distribution on particles. To improve grinding uniformity, wet grinding can be controlled by an alternating magnetic field. The grinding medium used is magnetic zirconia beads coated with a 50-100nm iron-cobalt alloy film. When the magnetic field is in the same direction, the magnetization of the magnetic zirconia beads creates an attractive force, forming a "magnetic chain" structure that enhances shear force on large particles. When the magnetic field is reversed, the repulsive force between the beads breaks the magnetic chains, resulting in disordered collisions and breaking up particle agglomerates. This periodic convergence and divergence makes the collision energy of the magnetic zirconia beads more evenly distributed, avoiding the localized areas of strong and weak shear caused by mechanical agitation in traditional grinding.

[0015] The particle sizes of magnetic zirconia beads are 0.4mm, 0.8mm, and 1.2mm, respectively, with a mass ratio of 3:2:1, so as to obtain a slurry material of the specified particle size. During the wet grinding process, an alternating magnetic field is applied, which is specifically divided into three stages:

[0016] Coarse grinding stage: Set the sand mill linear speed to 5-7 m / s, the slurry flow rate to 15-20 L / h, and the grinding time to 30-45 min. Turn on the electromagnetic coil and apply a constant magnetic field of 0.1-0.3 T. Under this magnetic field strength, the magnetic zirconia beads are oriented and vibrate slightly, exerting a uniform impact and grinding force on the raw material particles, accelerating the breakup of agglomerates.

[0017] During the intermediate grinding stage, the linear speed is increased to 7-9 m / s, the flow rate is adjusted to 10-15 L / h, the grinding time is 60-90 minutes, and the magnetic field is switched to an alternating magnetic field with a frequency of 5-10 Hz and an intensity of 0.3-0.5 T. The alternating magnetic field causes the magnetic zirconia beads to produce periodic mutual attraction and repulsion, forming a complex flow field in the grinding chamber, promoting particle collision and mixing.

[0018] Fine grinding: The linear speed is maintained at 9-11 m / s, the flow rate is reduced to 5-10 L / h, and an ultrasonic-assisted device (frequency 20-30 kHz, power 200-300 W) is activated. The grinding time is 120-180 minutes, and the magnetic field is adjusted to a gradient magnetic field, with the magnetic field intensity gradually decreasing from 0.5 T at the grinding chamber entrance to 0.1 T at the exit. The gradient magnetic field promotes orderly circular motion of the magnetic zirconia beads, which, combined with the ultrasonic cavitation effect, refines the particles to 0.5-1 μm, with a particle size distribution span of less than 1.2.

[0019] The fine grinding stage utilizes a gradient magnetic field that decreases from 0.5T to 0.1T. The magnetic zirconia beads undergo directional migration due to the difference in magnetic field strength. In the high-magnetic field (entrance), the beads are attracted to form a dense grinding zone, suitable for coarse particle crushing. In the low-magnetic field (exit), the beads disperse, primarily for fine grinding. This gradient distribution allows the material to undergo a graded process within the grinding chamber, progressing from coarse crushing to fine grinding to homogenization, further narrowing the particle size distribution span.

[0020] During the grinding process, the slurry particle size distribution is monitored through sampling every 15 minutes. Magnetic sensors are installed at various locations in the grinding chamber to monitor the magnetic field strength and distribution in real time. Based on a preset particle size target curve and magnetic field control strategy, the PLC control system dynamically adjusts the sand mill's linear speed, flow rate, electromagnetic coil current and frequency, and the power of the ultrasonic assist device to ensure a stable grinding process and optimal stock uniformity.

[0021] S4. Slurry treatment

[0022] The stirred slurry is quickly vacuum-dried. In a vacuum-sealed container, the vacuum degree is maintained at -0.08MPa to -0.1MPa, the temperature is maintained at 60-80°C, and the temperature change does not exceed ±5°C. At the same time, the slurry is turned over at a frequency of 1 time / 5-10 minutes to promote rapid evaporation of the water inside the slurry until the water content of the slurry drops to 10-15%.

[0023] S5, Molding

[0024] The dehydrated slurry is filled into a custom multi-layer mold, where it is pressed through high pressure to create the green body to be sintered. This high-pressure pressing utilizes a variable pressure gradient: 150-200 MPa is applied to the upper layer of the mold, 120-150 MPa to the middle layer, and 100-120 MPa to the lower layer, maintaining pressure for 3-5 minutes. The pressure is then gradually increased to 250-300 MPa for the upper layer, 200-250 MPa for the middle layer, and 180-220 MPa for the lower layer, again maintaining pressure for 5-8 minutes. Following this pressure hold, isostatic pressing is performed to enhance the green body's overall density and uniformity. The green body is placed in a high-pressure vessel and subjected to a pressure of 100-200 MPa for 15-30 minutes.

[0025] S6. Sintering

[0026] The green body is sintered using microwave-infrared composite heating:

[0027] The initial sintering temperature is controlled below 200°C, mainly using infrared heating, with a heating rate of 5-8°C / min and a heat preservation time of 1-2 hours to avoid local overheating;

[0028] Then the temperature is gradually increased to reach the high-temperature sintering stage, and the medium-temperature stage (600-1000℃) is heated by microwave and infrared synergistic heating at a heating rate of 3-5℃ / min;

[0029] The high-temperature sintering stage (1000-1300°C) is mainly heated by microwave (frequency 2.45GHz), with infrared auxiliary temperature control to control the temperature fluctuation within ±3°C and keep warm for 2-4 hours;

[0030] After sintering, cool to 800-900℃ at a rate of 1-2℃ / min, keep warm for 2-4h to release residual stress, and then gradually cool to room temperature.

[0031] S7, surface grinding

[0032] The sintered blanks are finely ground using cryogenic minimal lubrication technology: a vegetable oil-based grinding fluid is mixed with cryogenic compressed air to form a mist lubricant. The grinding fluid flow rate is 5-15 ml / min, and the cryogenic compressed air is controlled at a temperature between -10°C and 0°C before being sprayed onto the grinding area. Real-time dimensional monitoring during the grinding process ensures that the grinding accuracy is within ±0.005 mm, resulting in a final permanent ferrite magnet that meets the design parameters.

[0033] Beneficial effects:

[0034] Synergistically Enhanced Uniformity: Surface corona treatment modifies the surface properties of raw material particles, allowing them to interact more effectively with the grinding media and dispersant during the magnetron wet grinding process. Compared to single processes, the combined effect of these two methods can reduce the particle size distribution span to 0.8-1.0, increase the specific surface area by 50-60%, and significantly improve the uniformity of the raw material.

[0035] Significant crack suppression: Highly uniform billets reduce internal stress concentration caused by uneven composition and particle size. During subsequent forming, sintering, and processing, the number of invisible cracks is expected to be reduced by approximately 60%, effectively improving product quality. Uniform billets provide a good foundation for magnet formation, significantly enhancing the product's market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a performance index comparison chart of Examples 1-3. DETAILED DESCRIPTION

[0037] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to embodiments and drawings. The embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0038] Example 1:

[0039] S1. Raw material pretreatment

[0040] Weigh 0.2 kg of CaO, 0.1 kg of SrO, 0.5 kg of La2O3, 10.4 kg of Fe2O3, 0.25 kg of CoO, and 0.04 kg of silicon carbide whisker powder, put them into a double planetary mixer, and stir them at a low speed of 200 r / min for 8 minutes. During the stirring process, turn on the ultrasonic oscillation device with a frequency of 30 kHz and a power of 300 W to break up the agglomerated particles and obtain a uniform mixture.

[0041] S2. Surface corona treatment

[0042] The mixture was transferred to a corona treatment device, an electric field strength of 18 kV / cm was applied, and the treatment time was 5 minutes. A nano-scale rough structure was formed on the surface of the particles, and the surface activity was significantly improved.

[0043] S3. Magnetic Control Wet Grinding

[0044] Coarse grinding stage: add deionized water at a solid-liquid ratio of 1:1.3, add a composite dispersant accounting for 0.8% of the powder mass, pour into a magnetic controlled horizontal sand mill, set the linear speed at 5.5 m / s, flow rate at 18 L / h, turn on the electromagnetic coil, apply a 0.2 T constant magnetic field, and grind for 40 min;

[0045] Intermediate grinding stage: the linear speed is increased to 8m / s, the flow rate is 12L / h, the magnetic field is switched to an alternating magnetic field with a frequency of 8Hz and an intensity of 0.4T, and the grinding is carried out for 70min;

[0046] Fine grinding: Linear speed 10m / s, flow rate 8L / h, ultrasonic assist (frequency 25kHz, power 250W), magnetic field adjusted to a gradient field of 0.5T at the inlet and 0.1T at the outlet. Grind for 150 minutes to obtain a slurry with a particle size of 0.5-1μm and a particle size distribution span of 0.9. Particle size and magnetic field were monitored every 15 minutes during the grinding process, and the PLC system dynamically adjusted parameters.

[0047] S4. Slurry treatment

[0048] The slurry was transferred into a vacuum sealed container, the vacuum was maintained at -0.09 MPa, the temperature was controlled at 60±5°C, and the container was turned over every 8 minutes until the water content dropped to 12%.

[0049] S5. Molding

[0050] The dehydrated slurry is filled into a multi-layer mold. First, a pressure of 180 MPa is applied to the upper layer of the mold, 140 MPa to the middle layer, and 110 MPa to the lower layer, and the pressure is maintained for 4 minutes. Then, the pressure of the upper layer is increased to 280 MPa, the middle layer to 230 MPa, and the lower layer to 200 MPa, and the pressure is maintained for 6 minutes to prepare the green body. Subsequently, isostatic pressing treatment is performed with a pressure of 150 MPa and the pressure is maintained for 20 minutes.

[0051] S6. Sintering

[0052] The green body was placed in a microwave-infrared composite sintering furnace. Initially, it was heated by infrared heating at a heating rate of 6°C / min to 200°C and kept warm for 1 hour. In the medium temperature stage, the heating rate was increased by 4°C / min to 1280°C and kept warm for 3 hours. After sintering, it was cooled to 850°C at a rate of 1.5°C / min, kept warm for 3 hours, and then cooled to room temperature at a rate of 1°C / min.

[0053] S7. Surface grinding

[0054] Using low-temperature minimal lubrication grinding technology, a grinding fluid flow rate of 10 ml / min, a low-temperature compressed air temperature of -5°C, grinding to a dimensional accuracy of ±0.005 mm, permanent magnet ferrite products were obtained.

[0055] Example 2

[0056] S1. Raw material pretreatment

[0057] Weigh 0.2 kg of CaO, 0.1 kg of SrO, 0.5 kg of La2O3, 10.4 kg of Fe2O3, 0.25 kg of CoO, and 0.04 kg of silicon carbide whisker powder, put them into a double planetary mixer, stir them at a low speed of 250 r / min for 10 min, and use an ultrasonic oscillation device with a frequency of 40 kHz and a power of 400 W to obtain a mixture.

[0058] S2. Surface corona treatment

[0059] The mixture was placed in a corona treatment device with an electric field strength of 22 kV / cm and a treatment time of 6 min to enhance the surface activity of the particles.

[0060] S3. Magnetic Control Wet Grinding

[0061] Coarse grinding stage: solid-liquid ratio 1:1.4, adding 1% of the powder mass of the composite dispersant, sand mill linear speed 6m / s, flow rate 20L / h, constant magnetic field 0.3T, grinding for 45min;

[0062] Intermediate grinding stage: linear speed 9m / s, flow rate 15L / h, alternating magnetic field frequency 10Hz, intensity 0.5T, grinding 90min;

[0063] Fine grinding stage: linear speed 11 m / s, flow rate 10 L / h, ultrasonic assisted frequency 30 kHz, power 300 W, gradient magnetic field inlet 0.5 T, outlet 0.1 T, grinding 180 min.

[0064] S4. Slurry treatment

[0065] The vacuum drying temperature is 70±5℃, the vacuum degree is -0.1MPa, the turning frequency is 1 time / 5min, and the moisture content is reduced to 10%.

[0066] S5. Molding

[0067] The pressure of the upper layer of the mold is 200MPa, the middle layer is 150MPa, and the lower layer is 120MPa. After holding the pressure for 5 minutes, the pressure of the upper layer is increased to 300MPa, the middle layer is 250MPa, and the lower layer is 220MPa, and the pressure is held for 8 minutes; the isostatic pressure is 200MPa, and the pressure is held for 30 minutes.

[0068] S6. Sintering

[0069] The initial heating rate is 8℃ / min to 200℃, and the temperature is kept for 1.5h; the medium temperature stage heating rate is 5℃ / min to 1300℃, and the temperature is kept for 4h; cool to 900℃ and keep for 4h, and then cool to room temperature.

[0070] S7. Surface grinding

[0071] The grinding fluid flow rate is 15 ml / min, the low-temperature compressed air temperature is 0 °C, and the dimensional accuracy is controlled within ±0.005 mm.

[0072] Example 3

[0073] S1. Raw material pretreatment

[0074] Weigh 0.2kg of CaO, 0.1kg of SrO, 0.5kg of La2O3, 10.4kg of Fe2O3, 0.25kg of CoO, and 0.04kg of silicon carbide whisker powder, stir at a mixer speed of 300r / min for 5min, ultrasonic frequency of 20kHz, and power of 200W.

[0075] S2. Surface corona treatment

[0076] The electric field strength was 15 kV / cm, the treatment time was 8 min, and a nano-rough structure was formed on the particle surface.

[0077] S3. Wet grinding

[0078] Coarse grinding stage: linear speed 5m / s, flow rate 15L / h, grinding 30min;

[0079] Intermediate grinding stage: linear speed 7m / s, flow rate 10L / h, grinding 60min;

[0080] Fine grinding stage: linear speed 9m / s, flow rate 5L / h, grinding 120min.

[0081] S4. Slurry treatment

[0082] Vacuum degree -0.08MPa, temperature 80±5℃, moisture content dropped to 15%.

[0083] S5. Molding

[0084] Mold pressure gradient: upper layer 150MPa, middle layer 120MPa, lower layer 100MPa, after holding pressure for 3 minutes, upper layer 250MPa, middle layer 200MPa, lower layer 180MPa, holding pressure for 5 minutes; isostatic pressure 100MPa, holding pressure for 15 minutes.

[0085] S6. Sintering

[0086] The initial heating rate was 5°C / min, the high temperature sintering temperature was 1250°C, kept at this temperature for 2 hours, and then cooled to 800°C and kept at this temperature for 2 hours.

[0087] S7. Surface grinding

[0088] The grinding fluid flow rate is 5ml / min, the low-temperature compressed air temperature is -10℃, and the grinding accuracy is ±0.005mm.

[0089] Comparative Example:

[0090] The same formula components as those in Comparative Example 3 were used, and the mixed materials were ground and then pre-sintered to form the particles. The effect of pre-sintering is similar to that of surface corona treatment, which is to activate the particles.

[0091] After the pre-sintering is completed, conventional wet grinding is performed, and after the wet grinding is completed, the slurry is preliminarily dried, and then sintered and machined in sequence. The sintering and machining parameters are the same as those in Example 3.

[0092] Testing revealed that while the comparative examples also produced products with relatively good performance, their overall pass rates were significantly lower than those of Examples 1-3. Example 1 achieved a pass rate of 98.7% by optimizing the vacuum drying temperature (65°C) and holding time (5 min + 7 min) to eliminate microporosity in the green body. Example 2, leveraging closed-loop magnetic milling control (particle size detection frequency 10 min / time) and precise sintering atmosphere regulation, achieved a pass rate of 99.3%, the highest of the three examples.

[0093] Example 3 strengthens the ultrasonic treatment of the raw materials (power 250 W, time 7 min) and adjusts the grinding parameters. In addition, in Example 3, the effect of the alternating magnetic field on particle dispersion is eliminated, thereby reducing costs and increasing the qualified rate to 98.2%, which can balance cost and performance.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A process for preparing permanent ferrite capable of suppressing invisible cracks, characterized in that: The following steps are involved: S1. Raw material pretreatment: Weigh CaO, SrO, La2O3, Fe2O3, CoO, and silicon carbide whisker powders and put them into a mixer. Stir and mix them at a low speed while breaking up agglomerated particles by ultrasonic vibration to obtain a mixture. S2. Surface corona treatment: placing the mixture in a high voltage electric field with an electric field strength of 15-25 kV / cm for 3-8 minutes to form a nano-scale rough structure on the particle surface; S3, wet grinding: adding deionized water to the mixture after corona treatment and performing wet grinding; S4. Slurry treatment: The ground slurry is quickly vacuum dried, and the temperature change is controlled to ≤±5℃ until the moisture content drops to 10-15%; S5, molding: filling the mold with the dehydrated slurry, and performing high-pressure pressing and auxiliary molding after pressure maintenance; S6, sintering: initial temperature ≤ 200℃, gradually increase the temperature to high temperature for sintering, and gradually cool down after completion; S7. Surface grinding: Finely grind the sintered blank to obtain permanent ferrite.

2. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that The wet grinding method is a wet grinding method developed under magnetic control conditions, which uses magnetic zirconia beads coated with an iron-cobalt alloy film as the grinding medium, and applies an alternating magnetic field for wet grinding. The magnetic field intensity decreases as the particle size decreases; It is divided into coarse grinding, medium grinding and fine grinding stages: Coarse grinding stage: linear speed 5-7m / s, flow rate 15-20L / h, grinding 30-45min, applying 0.1-0.3T constant magnetic field; Intermediate grinding stage: linear speed 7-9m / s, flow rate 10-15L / h, grinding 60-90min, applying an alternating magnetic field with a frequency of 5-10Hz and an intensity of 0.3-0.5T; Fine grinding stage: linear speed 9-11m / s, flow rate 5-10L / h, grinding 120-180min, applying gradient magnetic field, magnetic field intensity decreasing from 0.5T at the inlet to 0.1T at the outlet, and turning on the ultrasonic auxiliary device at the same time, frequency 20-30kHz, power 200-300W.

3. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that During the grinding process, the slurry particle size distribution is sampled and tested every 15 minutes. According to the test results, the grinding equipment linear speed, flow rate, electromagnetic coil parameters and ultrasonic power are adjusted.

4. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that In the sintering step, microwave-infrared composite heating is adopted. In the initial stage, infrared heating is mainly used with a heating rate of 5-8℃ / min. Microwave heating is mainly used in the high-temperature sintering stage, and the temperature fluctuation is ≤±3℃. After sintering, it is cooled to 800-900℃ and kept warm for 2-4h, and then cooled to room temperature at a rate of 1-2℃ / min.

5. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that In the magnetron wet grinding, the grinding media is selected from magnetic zirconia beads of different particle sizes. The magnetic zirconia beads are ordinary zirconia coated with a layer of iron-cobalt alloy film.

6. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 5, characterized in that ,In magnetron wet grinding, the particle sizes of magnetic zirconia beads are 0.4mm, 0.8mm, and 1.2mm, with a mass ratio of 3:2:1, and the thickness of the iron-cobalt alloy coating is 50-100nm.

7. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that In the raw material pretreatment step, the frequency of the ultrasonic oscillation device is 20-40kHz, the power is 200-400W, the stirring speed is 200-300r / min, and the stirring time is 5-10min.

8. The process for preparing a permanent magnetic ferrite capable of suppressing invisible cracks according to claim 1, characterized in that In the slurry treatment, the vacuum degree of vacuum drying is maintained at -0.08MPa to -0.1MPa, the drying temperature is 60-80℃, and the turning frequency is 1 time / 5-10min.

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