A vertical negative pressure airflow screening device and a screening method thereof

Abstract

The application discloses a vertical negative pressure airflow screening device, which comprises a shell, a screen arranged in the shell, and a cavity and a cover body. The cover body is provided with a powder inlet at the front end and a coarse powder outlet at the lower end. The cavity is provided with a screen frame connected with an ultrasonic generator at the front end. The screen is arranged vertically through the screen frame. The cavity is provided with a fine powder outlet with negative pressure extraction. A rotating shaft is horizontally arranged in the cavity. The rotating shaft is sequentially provided with a first air knife and a second air knife capable of rotating with the rotating shaft, which are used for blowing the screen surface and the inner wall of the cavity, respectively. The application discloses a screening method of the vertical negative pressure airflow screening device. The screening cavity adopts a vertical configuration design. The material screening process is optimized based on the gravity driving principle. The material accumulation problem is effectively avoided. The screening efficiency is significantly improved.

Description

Technical Field

[0001] This invention pertains to screening equipment, specifically relating to a vertical negative pressure airflow screening device and its screening method. Background Technology

[0002] In industrial production, powder screening is a crucial process. Especially for screening micron-sized ultrafine powders, extremely high precision and efficiency are required from the equipment. Currently, commonly used screening equipment mainly includes vibrating screens and air classifiers.

[0003] Vibrating screens use mechanical vibration to force materials through a screen mesh for sieving. However, they face significant technical bottlenecks when processing micron-sized (13-15μm) powders. Their sieving principle, based on mechanical vibration, cannot overcome the van der Waals forces and agglomeration effects between fine particles, resulting in sieving accuracy generally below ±3μm, failing to meet the separation requirements of high-precision powders. In actual operation, screen clogging is particularly prominent: when the material particle size approaches the screen aperture size, material jamming and agglomeration easily occur, requiring 3-5 shutdowns for cleaning during a single sieving process, severely impacting production continuity. Simultaneously, high-frequency mechanical vibration leads to screen fatigue and breakage, with an average service life of only about 500-800 hours, resulting in high replacement costs. Furthermore, the noise generated during operation exceeds 85 decibels, energy consumption reaches 3-5 kW·h / t, and requires real-time manual monitoring and adjustment, making it unsuitable for modern unmanned production scenarios.

[0004] Air classifiers classify powders based on the differences in their trajectories within an airflow. However, their sieving accuracy for micron-sized powders is low, limited by the application boundaries of Stokes' law. Their sieving accuracy for 13-15μm powders is generally below ±5μm, making it difficult to achieve sharp sieving of monodisperse systems. Actual measurements show that when processing calcium carbonate micron powder (D50 = 15μm), the product particle size distribution ranges from D10 = 8μm to D90 = 22μm, resulting in poor product consistency. The equipment typically requires a centrifugal fan of 22kW or higher, with unit energy consumption as high as 8-10kW·h / t, leading to high operating costs.

[0005] Ordinary horizontal negative pressure airflow sieves use a negative pressure environment to sieve powder through a screen. However, these devices lack auxiliary vibration facilities, failing to effectively break up the agglomeration structure of fine particles, leading to easy screen clogging and low sieving efficiency. Furthermore, their air knife structures often employ a direct-blowing design, resulting in poor airflow uniformity and ineffective removal of coarse powder adhering to the screen. The airflow's removal efficiency for coarse powder is less than 70%, leaving a residual coarse powder thickness of 1-2 mm. Moreover, they typically only allow for a single-layer screen, making multi-level precise grading (D10, D50, D90) impossible. The screens, using conventional weaving technology, have a lifespan of only about 600 hours under the combined effects of negative pressure adsorption and airflow scouring. In addition, the equipment casing suffers from severe powder residue due to electrostatic adsorption, requiring disassembly of more than 12 parts for cleaning, with each maintenance session exceeding 2 hours. The traditional electromagnetic drive operates at a noise level exceeding 80 decibels, impacting the comfort of the working environment. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a vertical negative pressure airflow screening device and screening method, which can effectively suppress screen clogging and significantly improve screening efficiency.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] On one hand, a vertical negative pressure airflow sieving device includes a shell and a screen disposed inside the shell. The shell is characterized by being a vertical structure, comprising a cavity and a cover at the front end of the cavity. The front end of the cover has a powder inlet, and the lower end of the cover has a coarse powder outlet. The front end of the cavity has a mesh frame connected to an ultrasonic generator. The screen is vertically arranged via the mesh frame. The cavity has a fine powder outlet with negative pressure. A rotating shaft is also horizontally disposed within the cavity. The shaft has a first and a second air knife that rotate with it. Both the first and second air knives have air ducts connected to an air source. The first air knife has coarse powder blowing holes that blow towards the screen surface, and the second air knife has fine powder blowing holes that blow towards the circumferential inner wall of the cavity.

[0009] The number of cavities is at least one, and two or more cavities are arranged in a front-to-back sequence and connected to each other. Each cavity is provided with a screen in front to form a multi-stage screening structure, and the mesh size of the screen increases from front to back. The rotating shaft passes through each cavity in sequence, and each cavity is provided with a set of first and second air knives.

[0010] The rotating shaft is axially provided with a first ventilation pipe, one end of which is connected to a first connector located at the end of the rotating shaft and connected to an air source through the first connector, and the other end is connected to the air duct of the first air knife.

[0011] The rotating shaft is also provided with a bearing housing, and a second air vent is provided inside the bearing housing. One end of the air vent is connected to a second connector located on the outer wall of the bearing housing and connected to an air source through the second connector. The other end is connected to the air duct of the second air knife.

[0012] The cover body is equipped with a feeder that communicates with the powder inlet. The feeder is funnel-shaped with a diameter that gradually increases from front to back, and multiple circular through holes are arranged in an array on the feeder.

[0013] The rotating shaft is connected to its drive motor via a rubber belt and a tapered pulley.

[0014] On the other hand, a screening method for a vertical negative pressure airflow screening device includes the following steps:

[0015] A. The powder is fed into the housing through the powder inlet, and the powder is evenly spread on the screen by the distributor;

[0016] B. Negative pressure is drawn through the fine powder outlet, and the powder is dispersed with the airflow. The fine powder is pushed through the screen by the airflow and is extracted through the fine powder outlet.

[0017] C. The screen is made to vibrate at high frequency and low amplitude by an ultrasonic generator through the screen frame. At the same time, the screen surface is swept by the rotating first air knife, and the coarse powder is swept to the edge of the screen and discharged through the coarse powder discharge port under the action of gravity.

[0018] D. The fine powder adhering to the circumferential inner wall of the cavity is blown away by the rotating second air knife, and then extracted from the fine powder outlet.

[0019] If the screening equipment has a multi-stage screening structure, negative pressure is drawn through the fine powder outlet, and the powder passes through the screens one by one under the propulsion of the airflow, and is screened stage by stage. At the same time, ultrasonic vibration is performed on each stage of the screen, and the corresponding first and second air knives are used to blow the screen surface and the circumferential inner wall of the cavity.

[0020] The purging of the first and second air knives is controlled independently. When the proportion of fine powder in the powder is high, the air pressure of the first air knife is controlled to be lower than that of the second air knife; when the proportion of coarse powder in the powder is high, the air pressure of the first air knife is controlled to be higher than that of the second air knife.

[0021] Another aspect is a vertical negative pressure airflow screening system, including a screw feeder, a vertical negative pressure airflow screening device, a cyclone separator, a pulse dust collector, and an induced draft fan. The output end of the screw feeder is connected to the material distributor via a powder inlet through a flexible connection structure. The fine powder outlet of the vertical negative pressure airflow screening device is connected tangentially to the inlet of the cyclone separator through a pipe. The outlet of the cyclone separator is connected sequentially to the dust collector and the induced draft fan through a pipe. The dust collector contains a filter cartridge.

[0022] The vertical negative pressure airflow screening device and screening method of the present invention have the following advantages:

[0023] 1. Its screening chamber adopts a vertical configuration design, which optimizes the material screening process based on the gravity drive principle, effectively avoids the problem of material accumulation, and significantly improves screening efficiency.

[0024] 2. The use of a trumpet-shaped feeder with arrayed holes can evenly spread powder materials on the screen surface, forming a material layer of uniform thickness, eliminating local accumulation, and laying a good foundation for subsequent screening processes.

[0025] 3. By transmitting ultrasonic waves through the mesh frame, the screen can generate high-frequency micro-amplitude oscillations, which can effectively prevent screen clogging and enhance the movement intensity of materials on the screen surface. It is especially suitable for high-efficiency screening of fine powder and sticky materials. At the same time, the rotating first air knife generates a high-speed and uniform blowing airflow to blow the screen surface in a directional manner, so as to quickly remove the adhering coarse powder.

[0026] 4. The rotating second air knife can blow away fine powder materials that are easy to stick to the circumferential surface of the cavity and discharge them under negative pressure through the fine powder outlet.

[0027] 5. The cavity, screen, and air knife can be flexibly configured into a multi-stage structure, which can perform multi-stage screening of materials and complete particle size classification. Attached Figure Description

[0028] The invention will now be described in detail with reference to the accompanying drawings and specific embodiments:

[0029] Figure 1 is a three-dimensional schematic diagram of the vertical negative pressure airflow screening device of the present invention;

[0030] Figure 2 is a perspective view of the screening device of the present invention;

[0031] Figure 3 This is a side view of the screening equipment of the present invention;

[0032] Figure 4 This is a schematic diagram of the installation structure of the first and second air knives of the present invention;

[0033] Figure 5 This is a schematic diagram of the structure of the first air knife of the present invention;

[0034] Figure 6 For along Figure 5 A sectional view of line AA in the diagram;

[0035] Figure 7 For the present invention Figure 4 Enlarged diagram of section A in the middle;

[0036] Figure 8This is a schematic diagram of the structure of the screening equipment with a multi-stage screening structure according to the present invention;

[0037] Figure 9 This is a schematic diagram of the distributor of the present invention;

[0038] Figure 10 This is a schematic diagram of the vertical negative pressure airflow screening system of the present invention;

[0039] Figure 11 for Figure 10 Top view. Detailed Implementation

[0040] A vertical negative pressure airflow screening device of the present invention is shown in Figure 1- Figure 9 As shown, it includes a housing and a screen 1 disposed within the housing. Unlike existing technologies, the housing has a vertical structure. The housing includes a cavity 2 and a cover 3 disposed at the front end of the cavity 2.

[0041] The cover 3 is conical, with a powder inlet 4 at the front end and a rear end whose diameter matches and is connected to the cavity 2. A coarse powder outlet 5 is located at the lower end of the cover 3. A feeder 6, communicating with the powder inlet 4, is also located inside the cover 3. This feeder 6 is located after the powder inlet 4, and is shaped like a trumpet with its diameter gradually increasing from front to back. Multiple circular through holes 7 are arranged in an array on the feeder 6.

[0042] The cavity 2 has a vertically circular cross-section. Its front end is equipped with a mesh frame connected to the ultrasonic generator 8. This mesh frame is made of high-strength aluminum alloy and is flexibly connected to the side wall of the cavity 2 via shock-absorbing rubber pads. The screen 1 is vertically installed via the mesh frame and is made of high-strength stainless steel woven material, possessing high mechanical strength, excellent corrosion resistance, and high-precision screening performance. The ultrasonic generator 8 emits high-frequency vibrations in the 30-40kHz frequency band, which are transmitted through the mesh frame to drive the screen 1 to generate high-frequency micro-oscillations, effectively preventing screen 1 from clogging and enhancing the movement intensity of materials on the screen surface. This is particularly suitable for efficient screening of fine powders and sticky materials. The cavity 2 has a fine powder outlet 23 with negative pressure. A blower 9 or similar device can create negative pressure inside the cavity 2 for material screening. A rotatable shaft 10 is also horizontally installed inside the cavity 2. The shaft 10 is connected to its drive motor 12 via a rubber belt 11 and a conical pulley. This pulley structure facilitates disassembly and maintenance and effectively compensates for belt wear. The drive motor 12 can be a three-phase asynchronous motor.

[0043] The rotating shaft 10 is equipped with an air knife structure, including a first air knife 13 and a second air knife 14, which are sequentially arranged on the rotating shaft 10 and can rotate with it. The air knives can be made of aluminum alloy. The air knives are designed based on the Venturi effect principle. The first air knife 13 has a straight rod-shaped structure with a hollow interior to form an air duct that can be connected to an air source. On the front surface of the first air knife 13 facing the screen 1, there are long strip-shaped coarse powder blowing holes 131 arranged along the length of the straight rod, which are used to blow the screen surface of the vibrating screen 1. The second air knife 14 is a hollow rod folded into several impeller-shaped structures, and there are multiple small holes on the arc end of the impeller (i.e., the part facing the inner wall of the cavity 2) to serve as fine powder blowing holes 141. A first air passage 15 is axially arranged inside the rotating shaft 10. One end is connected to a first connector 16 located at the end of the rotating shaft 10 and connected to an air source through the first connector 16. The other end is connected to the air duct of the first air knife 13. A bearing housing 17 is also provided outside the rotating shaft 10. A second air duct 18 is provided inside the bearing housing 17. One end is connected to a second connector 19 located on the outer wall of the bearing housing 17, and the second connector 19 is connected to an air source. The other end is connected to the air duct of the second air knife 14. The air source is provided by an independent high-pressure blower. The blowing pressure can be controlled in the range of 0.3-0.8MPa through a pressure regulating valve to ensure the cleaning effect while avoiding damage to the screen 1.

[0044] The number of cavities 2 is at least one. If there are two or more cavities 2, the cavities 2 are arranged in a front-to-back sequence and are connected. Each cavity 2 is provided with a screen 1 as described above to form a multi-stage screening structure, and the mesh size of the screen 1 increases progressively from front to back. The rotating shaft 10 passes through each cavity 2 in sequence, and each cavity 2 is provided with a set of first and second air knives 13 and 14.

[0045] The screening system employing the screening equipment of the present invention is as follows: Figure 9 , Figure 10 As shown, the system includes a screw feeder 20, a vertical negative pressure airflow screening device, a cyclone separator 21, a pulse dust collector 22, and an induced draft fan 9. The output end of the screw feeder 20 is connected to the distributor 6 via a flexible connection structure through the powder inlet 4. The fine powder outlet 23 of the vertical negative pressure airflow screening device is connected to the discharge pipe through a flexible rubber hose. This connection method combines sealing and flexibility, effectively absorbing the impact of vibration. The discharge pipe is tangentially connected to the inlet of the cyclone separator 21, causing the dust-laden airflow to form a high-speed vortex within the separator. Utilizing the centrifugal separation principle, it achieves efficient separation of fine powder and airflow, with a separation efficiency exceeding 95%. The outlet of the cyclone separator 21 is connected sequentially to the dust collector and the induced draft fan 9 via pipes. The dust collector contains a high-efficiency filter cartridge. The induced draft fan 9 provides a stable negative pressure environment for the system. Its airflow and negative pressure parameters can be dynamically adjusted via a frequency converter to adapt to different material characteristics and screening process requirements.

[0046] The working principle of the screening system of this screening equipment is as follows:

[0047] Material is fed into the system through a dust-free feeding station, where the dust filter cartridges perform primary filtration and purification of the dust generated during the feeding process. Subsequently, the material is conveyed at a stable flow rate to the distributor 6 at the powder inlet 4 of the housing by a screw feeder 20 driven by a variable frequency motor. The distributor 6 evenly spreads the material onto the subsequent screen 1. Under the negative pressure environment created by the induced draft fan 9, the material is dispersed by the airflow. Fine powder with the required particle size is pushed through the screen 1 by the airflow and conveyed to the cyclone separator 21 through the fine powder outlet 23 and the discharge pipe. Inside the cyclone separator 21, the dust-laden airflow rotates at high speed, and the fine powder is separated and collected under centrifugal force. Coarse powder that fails to pass through the screen adheres to the surface of the screen 1 under the negative pressure of the flow channel. At this time, the first air knife 13 corresponding to the screen 1 sprays out a high-speed airflow, sweeping the coarse powder to the edge of the screen 1. The coarse powder is discharged from the equipment or enters the collection device under gravity along the inclined product outlet. Meanwhile, the ultrasonic generator 8 generates a high-frequency vibration signal, which is transmitted to the screen 1 through the mesh frame, driving the screen 1 to generate high-frequency micro-amplitude vibration, effectively suppressing the clogging of the screen 1 and further improving the screening efficiency. Furthermore, the second air knife 14 can also blow away fine powder materials that easily adhere to the circumferential surface of the cavity 2, and discharge them under negative pressure through the fine powder outlet 23. The blowing of the first and second air knives 13 and 14 is independently controlled. Typically, particle size analysis is performed after powder production. When the proportion of fine powder in the powder is high, the air pressure of the first air knife is controlled to be lower than that of the second air knife to prevent fine powder from being blown back into the front cover cavity and to facilitate the discharge of fine powder. Conversely, when the proportion of coarse powder in the powder is high, the air pressure of the first air knife is controlled to be higher than that of the second air knife.

[0048] When a multi-stage screening configuration is adopted, the screens 1 are arranged in descending order of screen aperture size, and the material passes through each layer of screens 1 in sequence to achieve multi-stage screening and grading. The coarse powder retained by each layer of screens 1 is discharged through its respective coarse powder discharge port 5 under the blowing action of the corresponding first air knife 13, which can achieve precise separation of materials with different particle sizes and fully meet diverse production needs.

[0049] Example 1:

[0050] The vertical negative pressure airflow screening equipment in this embodiment adopts a double-layer screen grading structure. The first layer is equipped with a 1000-mesh stainless steel woven screen, and the second layer is equipped with a 2000-mesh high-precision stainless steel woven screen. After each screen layer, first and second air knives are installed. The air knives are made of aerospace-grade aluminum alloy and processed with an anodized surface treatment. The air pressure of the air knives is set to 0.6MPa, which can generate a uniform and stable flat airflow jet, effectively removing materials adhering to the screen. The screen frame is equipped with a 33kHz high-frequency vibration system, using intelligent frequency conversion control technology to achieve precise amplitude control within the range of 0-20μm.

[0051] The operation process is as follows:

[0052] Material conveying stage: The powder to be screened enters the system through a closed dust-free feeding station and is conveyed to the top distributor 6 at a constant rate of 100 kg / h by a high-precision screw feeder 20. After being dispersed by the umbrella-shaped distributor 6, the material is evenly spread.

[0053] Grading and screening stage: Under the negative pressure environment of -5kPa provided by the induced draft fan 9, the upper 1000-mesh screen 1 intercepts coarse particles, while the fine powder that meets the requirements passes through screen 1 and enters the lower 2000-mesh screen 1 for secondary screening. Finally, the ultrafine powder with a particle size of 10-15μm enters the cyclone separator 21 through the discharge pipe, where centrifugal force is used to achieve gas-solid separation and complete collection.

[0054] Discharge and cleaning stage: Coarse powder that fails to pass through the upper screen 1 is discharged from the coarse material outlet along the 45° inclined discharge channel under the blowing action of the corresponding air knife; the intermediate particle size material intercepted by the second screen 1 is cleaned to the corresponding outlet by its air knife. During equipment operation, the screen frame continuously generates micro-vibration at a frequency of 33kHz, which works in conjunction with the air knife cleaning function to construct a composite screen cleaning mode of "vibration dispersion + airflow blowing". After 8 hours of continuous operation testing, the screening efficiency of the equipment is stably maintained above 95%, and the thickness of residual material on the surface of screen 1 is less than 0.1mm, reducing wear by 60% compared to traditional screening equipment.

[0055] Example 2:

[0056] The difference between this embodiment and Embodiment 1 is that a three-stage gradient screening structure is adopted, with the screen mesh size successively set to 500 mesh, 1500 mesh, and 2000 mesh, constructing a three-stage screening system of coarse screen, medium screen, and fine screen. The pressure of each layer of air knife can be independently adjusted, with a set pressure value of 0.75 MPa. The vibration frequency of the ultrasonic mesh frame is increased to 37 kHz. The negative pressure of the induced draft fan 9 is enhanced to -8 kPa, and the conveying capacity of the screw feeder 20 is increased to 150 kg / h.

[0057] The operation process is as follows:

[0058] Multi-stage screening mechanism: The material passes through three-stage screen 1 in sequence to complete particle size classification. The coarse powder retained by each screen 1 is discharged from its corresponding outlet through the inclined discharge channel, which can effectively avoid the mixing of materials of different particle sizes.

[0059] High-efficiency separation system: After initial collection by cyclone separator 21, fine powder enters the post-pulse bag filter for secondary filtration to ensure that the dust content of the exhaust gas is below 5mg / m³. 3 .

[0060] Performance Optimization: After 12 hours of continuous operation testing, the equipment stably achieved a screening efficiency of over 94%, with operating noise controlled below 60 decibels, meeting the occupational exposure limit standards of GBZ 2.2-2007 "Occupational Exposure Limits for Hazardous Factors in the Workplace Part 2: Physical Factors". The equipment adopts a modular cavity design, allowing for quick disassembly of the screen assembly during maintenance, reducing single cleaning and maintenance time to 15 minutes; the equipment shell is treated with electrostatic spraying, providing excellent anti-dust adhesion performance.

[0061] However, those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Any changes or modifications to the above embodiments that are within the essential spirit of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A vertical negative pressure airflow screening device, comprising a shell and a screen disposed within the shell, characterized in that: The housing has a vertical structure, including a cavity and a cover at the front end of the cavity. The front end of the cover has a powder inlet, and the lower end of the cover has a coarse powder outlet. The front end of the cavity has a mesh frame connected to an ultrasonic generator. The screen is vertically arranged through the mesh frame. The cavity has a fine powder outlet with negative pressure. A rotating shaft is also horizontally arranged inside the cavity. The rotating shaft has a first and a second air knife that can rotate with it. The first and second air knives each have air ducts connected to an air source inside. The first air knife has a straight rod structure and faces the screen in front of it. The front surface of the air knife has long, slit-shaped coarse powder blowing holes arranged along the length of the straight rod. The second air knife is formed by folding a hollow rod into several impeller-shaped structures, and multiple small holes are opened on the arc end of the impeller as fine powder blowing holes. There is at least one cavity, and two or more cavities are arranged in a front-to-back sequence and connected to each other. Each cavity is provided with a screen to form a multi-stage screening structure, and the mesh size of the screen increases from front to back. The rotating shaft passes through each cavity in sequence, and each cavity is provided with a set of first and second air knives.

2. The vertical negative pressure airflow screening device according to claim 1, characterized in that: The rotating shaft is axially provided with a first ventilation pipe, one end of which is connected to a first connector located at the end of the rotating shaft and connected to an air source through the first connector, and the other end is connected to the air duct of the first air knife.

3. The vertical negative pressure airflow screening device according to claim 1, characterized in that: The rotating shaft is also provided with a bearing housing, and a second air vent is provided inside the bearing housing. One end of the air vent is connected to a second connector located on the outer wall of the bearing housing and connected to an air source through the second connector. The other end is connected to the air duct of the second air knife.

4. The vertical negative pressure airflow screening device according to claim 1, characterized in that: The cover body is equipped with a feeder that communicates with the powder inlet. The feeder is funnel-shaped with a diameter that gradually increases from front to back, and multiple circular through holes are arranged in an array on the feeder.

5. A vertical negative pressure airflow screening device according to claim 1, characterized in that: The rotating shaft is connected to its drive motor via a rubber belt and a tapered pulley.

6. A screening method for a vertical negative pressure airflow screening device according to any one of claims 1-5, characterized in that, Includes the following steps: A. The powder is fed into the housing through the powder inlet, and the powder is evenly spread on the screen by the distributor; B. Negative pressure is drawn through the fine powder outlet, and the powder is dispersed with the airflow. The fine powder is pushed through the screen by the airflow and is extracted through the fine powder outlet. C. The ultrasonic generator causes the screen to vibrate at a high frequency and a small amplitude through the screen frame. At the same time, the rotating first air knife blows the screen surface, blowing the coarse powder to the edge of the screen and discharging it through the coarse powder discharge port under the action of gravity. D. The fine powder adhering to the circumferential inner wall of the cavity is blown away by the rotating second air knife and extracted from the fine powder outlet; If the screening equipment has a multi-stage screening structure, negative pressure is drawn through the fine powder outlet, and the powder passes through the screens one by one under the propulsion of the airflow, and is screened stage by stage. At the same time, ultrasonic vibration is performed on each stage of the screen, and the corresponding first and second air knives are used to blow the screen surface and the circumferential inner wall of the cavity.

7. The screening method of a vertical negative pressure airflow screening device according to claim 6, characterized in that, The purging of the first and second air knives is controlled independently. When the proportion of fine powder in the powder is high, the air pressure of the first air knife is controlled to be lower than that of the second air knife; when the proportion of coarse powder in the powder is high, the air pressure of the first air knife is controlled to be higher than that of the second air knife.

8. A system comprising a vertical negative pressure airflow screening device according to any one of claims 1-5, characterized in that: It includes a screw feeder, a vertical negative pressure airflow screening device, a cyclone separator, a pulse dust collector, and an induced draft fan. The output end of the screw feeder is connected to the material distributor through a flexible connection structure via a powder inlet. The fine powder outlet of the vertical negative pressure airflow screening device is connected tangentially to the inlet of the cyclone separator via a pipeline. The outlet of the cyclone separator is connected to the dust collector and the induced draft fan in sequence via pipelines. The dust collector contains a filter cartridge.

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