Vertical negative pressure airflow screening equipment and screening method thereof

Abstract

The vertical negative pressure airflow screening equipment comprises a shell and a screen arranged in the shell, the shell is of a vertical structure and comprises a cavity and a cover, a powder inlet is formed in the front end of the cover, a coarse powder outlet is formed in the lower end of the cover, a net frame connected with an ultrasonic generator is arranged at the front end of the cavity, and the screen is vertically arranged through the net frame. A fine powder outlet with negative pressure is formed in the cavity, a rotating shaft is further transversely arranged in the cavity, and a first air knife and a second air knife which can rotate along with the rotating shaft are sequentially arranged on the rotating shaft and used for blowing the screen surface of the screen and the circumferential inner wall of the cavity respectively. The invention discloses a screening method of vertical negative pressure airflow screening equipment. The screening cavity is designed in a vertical configuration mode, the material screening process is optimized based on the gravity driving principle, the material accumulation problem is effectively avoided, and the screening efficiency is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of screening equipment, and particularly relates to a vertical negative pressure airflow screening equipment and a screening method thereof. BACKGROUND

[0002] In industrial production, the screening of powder materials is one of important processes. Especially for the screening of micron-level ultrafine powder, the precision and efficiency of the equipment are extremely high. At present, commonly used screening equipment mainly includes a vibrating screen and an air classifier.

[0003] The vibrating screen realizes screening by allowing materials to pass through a screen through mechanical vibration. However, there is a significant technical bottleneck when processing micron-level (13-15 μ) powder. The screening principle is based on mechanical vibration, which cannot overcome the van der Waals force and agglomeration effect between fine particles, resulting in a screening precision generally lower than ±3 μ, which is difficult to meet the separation requirements of high-precision powder. In actual working conditions, the screen clogging problem is particularly prominent: when the particle size of the material is close to the size of the screen hole, it is easy to cause material blocking and agglomeration accumulation, and the single screening process needs to be stopped for cleaning 3-5 times, which seriously affects the production continuity. At the same time, the high-frequency mechanical vibration leads to fatigue and damage of the screen, and the average service life is only about 500-800 hours, with high replacement cost. In addition, the noise generated during the operation of the equipment exceeds 85 decibels, the energy consumption reaches 3-5 kW·h / t, and real-time monitoring and adjustment are required, which cannot adapt to modern unmanned production scenes.

[0004] The air classifier classifies by using the difference in the movement track of powder of different particle sizes in airflow, but its screening precision for micron-level powder is low, which is limited by the application boundary of Stokes' law, and the classification precision for 13-15 μ powder is generally lower than ±5 μ, which is difficult to realize sharp screening of monodisperse system. According to actual measurement, when processing calcium carbonate micro-powder (D50=15 μ), the product particle size distribution span reaches D10=8 μ to D90=22 μ, resulting in poor product consistency. The equipment usually needs to be equipped with a centrifugal fan of 22 kW or more, with a unit energy consumption of 8-10 kW·h / t, and the operating cost is high.

[0005] The common horizontal negative pressure airflow screen screens powder through a screen mesh in a negative pressure environment, but the device lacks auxiliary vibration facilities and cannot effectively break the agglomerated structure of fine particles, the screen mesh is easy to be blocked, and the screening efficiency is low; meanwhile, the air knife structure of the device is mostly designed in a straight blowing type, airflow uniformity is poor, and coarse powder adhered to the screen mesh cannot be effectively removed, the stripping efficiency of the air to the coarse powder is less than 70%, and the residual coarse powder has a thickness of 1-2 mm. Moreover, only a single layer of screen mesh can be arranged, and multi-stage accurate grading of D10, D50 and D90 cannot be realized. The screen mesh is arranged by using a conventional weaving process, and the service life is only about 600 hours under the double action of negative pressure adsorption and airflow scouring. In addition, powder residues are serious due to the electrostatic adsorption effect of the device shell, more than 12 parts need to be disassembled for cleaning, and the time consumed for single maintenance is more than 2 hours; the running noise of the traditional electromagnetic driver is more than 80 decibels, which affects the comfort of the working environment. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a vertical negative pressure airflow screening device and a screening method thereof, which can effectively inhibit the screen mesh blocking phenomenon and significantly improve the screening efficiency.

[0007] In order to solve the above technical problems, the present application adopts the following technical solutions:

[0008] On the one hand, a vertical negative pressure airflow screening device comprises a shell, a screen mesh arranged in the shell, and is characterized in that: the shell is a vertical structure, comprising a cavity and a cover arranged at the front end of the cavity, the cover has a powder inlet at the front end, the cover has a coarse powder discharge port at the lower end, the cavity has a mesh frame connected with an ultrasonic generator at the front end, the screen mesh is arranged vertically through the mesh frame, the cavity is provided with a fine powder outlet with negative pressure extraction, and the cavity is further provided with a rotating shaft arranged transversely and rotatably, the rotating shaft is sequentially provided with first and second air knives rotatable therewith, the first and second air knives each have an air duct connected with an air source, the first air knife is provided with coarse powder blowing holes blowing towards the screen surface, and the second air knife is provided with fine powder blowing holes blowing towards the circumferential inner wall of the cavity.

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

[0010] A first air pipe is arranged axially in the rotating shaft, one end of the first air pipe is communicated to a first connector arranged at the end of the rotating shaft and connected to the air source through the first connector, and the other end of the first air pipe is communicated with the air duct of the first air knife.

[0011] The shaft is further provided with a bearing seat, and a second air duct is arranged in the bearing seat, one end of the second air duct is communicated with a second joint arranged on the outer wall of the bearing seat, and the other end of the second air duct is communicated with the air duct of the second air knife.

[0012] The cover is provided with a distributor communicated with the powder inlet, and the distributor is in a horn shape with a gradually increasing diameter from front to back.

[0013] The shaft is connected with the driving motor through a rubber belt and a taper sleeve pulley.

[0014] In another aspect, a screening method of a vertical negative pressure airflow screening device comprises the following steps:

[0015] A. The powder is transported into the shell through the powder inlet, and the powder is uniformly spread on the screen by the distributor;

[0016] B. Negative pressure is drawn through the fine powder outlet, the powder is dispersed by airflow, the fine powder penetrates the screen under the action of airflow and is drawn out through the fine powder outlet;

[0017] C. The screen is vibrated at high frequency by the ultrasonic generator through the screen frame, and the first air knife is rotated to blow the screen surface, so that the coarse powder is blown to the edge of the screen and is discharged through the coarse powder discharge outlet under the action of gravity;

[0018] D. The second air knife is rotated to blow the fine powder adhered to the circumferential inner wall of the cavity, so that the fine powder is blown away and drawn out through the fine powder outlet.

[0019] If the screening device has a multi-stage screening structure, negative pressure is drawn through the fine powder outlet, the powder penetrates the screen under the action of airflow, and the powder is screened step by step, and the screen is vibrated at high frequency and the first and second air knives blow the screen surface and the circumferential inner wall of the cavity.

[0020] The blowing of the first and second air knives is independently controlled, when the proportion of fine powder in the powder is large, the air pressure of the first air knife is controlled to be less than that of the second air knife; when the proportion of coarse powder in the powder is large, the air pressure of the first air knife is controlled to be greater than that of the second air knife.

[0021] In another aspect, a vertical negative pressure airflow screening system comprises a screw feeder, a vertical negative pressure airflow screening device, a cyclone separator, a pulse dust removal tank and a induced draft fan, the output end of the screw feeder is connected with the distributor through a flexible connection structure through the powder inlet, the fine powder outlet of the vertical negative pressure airflow screening device is connected with the inlet of the cyclone separator through a pipeline in a tangential manner, the outlet of the cyclone separator is connected with the dust removal tank and the induced draft fan in sequence through a pipeline, and the dust removal tank is provided with a filter cartridge.

[0022] The vertical negative pressure airflow screening equipment and the screening method have the following advantages:

[0023] 1. The screening cavity adopts a vertical configuration design, optimizes the material screening process based on the gravity driving principle, effectively avoids the material accumulation problem, and significantly improves the screening efficiency.

[0024] 2. The distributor with a horn shape and array holes can uniformly spread the powder material on the screen surface, form a material layer with uniform thickness, and eliminate the local accumulation phenomenon, laying a good foundation for the subsequent screening process.

[0025] 3. The screen can produce high-frequency micro-oscillation by transmitting ultrasonic waves through the net frame, which can effectively prevent the screen from being blocked, and at the same time, enhance the movement intensity of the material on the screen, especially suitable for efficient screening of fine powder and viscous materials; at the same time, the high-speed and uniform blowing airflow generated by the rotating first air knife is used to direct the blowing of the screen surface to realize the rapid removal of adhered coarse powder.

[0026] 4. The fine powder material easily adhered to the circumferential surface of the cavity can be blown down and discharged by negative pressure through the second air knife.

[0027] 5. The cavity, screen and air knife can be flexibly configured into a multi-stage structure, which can perform multi-stage screening on the material to complete the particle size classification. BRIEF DESCRIPTION OF DRAWINGS

[0028] The invention will be described in detail below in conjunction with the drawings and specific embodiments:

[0029] Figure 1 is a perspective view of the vertical negative pressure airflow screening equipment of the present invention;

[0030] Figure 2 is another perspective view of the screening equipment of the present invention;

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

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

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

[0034] Figure 6 is a cross-sectional view along the A-A line in Figure 5 ;

[0035] Figure 7 is an enlarged schematic view of part A in Figure 4 ;

[0036] Figure 8 Structure diagram of the screening device with multi-stage screening structure of the present application;

[0037] Figure 9 Structure diagram of the distributor of the present application;

[0038] Figure 10 Structure diagram of the vertical negative pressure airflow screening system of the present application;

[0039] Figure 11 is a top view of Figure 10 .DETAILED DESCRIPTION

[0040] A vertical negative pressure airflow screening device of the present application is shown in Figures 1-9 , which comprises a shell, a screen 1 arranged in the shell. Different from the prior art, the shell is a vertical structure. The shell comprises a cavity 2 and a cover 3 arranged at the front end of the cavity 2.

[0041] The cover 3 is a conical cover, has a powder inlet 4 at the front end, and the rear end has a diameter matching the cavity 2 and is connected to the cavity 2. The cover 3 is provided with a coarse powder discharge port 5 at the lower end. The cover 3 is further provided with a distributor 6 communicating with the powder inlet 4. The distributor 6 is located behind the powder inlet 4 and has a horn shape with a gradually increasing diameter from front to back, and a plurality of circular holes 7 are arranged in an array on the distributor 6.

[0042] The cavity 2 is a cavity with a circular vertical cross section, and the front end is provided with a screen frame connected to an ultrasonic generator 8. The screen frame is made of high-strength aluminum alloy material and is flexibly connected to the side wall of the cavity 2 through a shock-absorbing rubber pad. The screen 1 is vertically installed through the screen frame, and the screen 1 is made of high-strength stainless steel woven material, which has high mechanical strength, excellent corrosion resistance and high-precision screening performance. Through the ultrasonic generator 8, high-frequency vibration of 30-40 kHz frequency band can be transmitted through the screen frame to drive the screen 1 to produce high-frequency micro-oscillation, effectively preventing the screen 1 from being blocked, and at the same time enhancing the movement intensity of the material on the screen surface, especially suitable for efficient screening operation of fine powder and viscous material. The cavity 2 is provided with a fine powder outlet 23 with negative pressure, and a suction fan 9 or the like can be used to extract negative pressure in the cavity 2 for screening of the material. A rotating shaft 10 is further horizontally arranged in the cavity 2. The rotating shaft 10 can be connected to a driving motor 12 through a rubber belt 11 and a conical sleeve type belt pulley. The belt pulley structure is convenient for disassembly and maintenance, and can effectively compensate for belt wear. The driving motor 12 can be a three-phase asynchronous motor.

[0043] The rotating shaft 10 is provided with a wind knife structure, which comprises first and second wind knives 13 and 14 arranged on the rotating shaft 10 in sequence and capable of rotating together with the rotating shaft 10. The wind knives can be made of aluminum alloy. The wind knives are designed based on the Venturi effect principle. The first wind knife 13 is in a straight rod structure, hollow inside to form an air duct connectable to an air source, and has long strip-shaped coarse powder blowing holes 131 arranged along the length direction of the straight rod and formed on the front surface of the first wind knife 13 facing the front of the screen 1, for blowing and cleaning the screen surface of the vibrating screen 1. The second wind knife 14 is folded into several impeller structures from a hollow rod, and has a plurality of small holes on the arc surface end (i.e. the part facing the circumferential inner wall of the cavity 2) of the impeller as fine powder blowing holes 141. The rotating shaft 10 is provided with a first air supply pipeline 15 axially inside, one end of which is connected to a first connector 16 arranged at the end of the rotating shaft 10 and connected to the air source through the first connector 16, and the other end is connected to the air duct of the first wind knife 13. The rotating shaft 10 is further provided with a bearing seat 17, and the bearing seat 17 is provided with a second air supply pipeline 18 inside, one end of which is connected to a second connector 19 arranged on the outer wall of the bearing seat 17 and connected to the air source through the second connector 19, and the other end is connected to the air duct of the second wind knife 14. The air source is provided by an independent high-pressure fan, and the blowing pressure can be controlled in the range of 0.3-0.8 MPa 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 more than two cavities 2, the cavities 2 are arranged in front of and connected to each other. Each cavity 2 is provided with an above-mentioned screen 1 in front, to form a multi-stage screening structure, and the mesh number of the screen 1 increases gradually 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 wind knives 13 and 14.

[0045] The screening system of the screening device adopting the present application is shown in Figure 9 , Figure 10 , which comprises a spiral feeder 20, a vertical negative pressure airflow screening device, a cyclone separator 21, a pulse dust removal tank 22 and an induced draft fan 9. The output end of the spiral feeder 20 is connected to the distributor 6 through a flexible connection structure via the powder inlet 4. The fine powder outlet 23 of the vertical negative pressure airflow screening device is connected to the discharge pipeline through a rubber hose, which takes into account the sealing and flexibility, and can effectively absorb the vibration effect. The discharge pipeline is connected to the inlet of the cyclone separator 21 in a tangential manner, so as to form a high-speed rotational flow of dust-containing airflow in the separator, and realize efficient separation of fine powder and airflow by using the centrifugal separation principle, with a separation efficiency of more than 95%. The outlet of the cyclone separator 21 is connected to the dust removal tank and the induced draft fan 9 in sequence through a pipeline, and the dust removal tank is provided with a high-efficiency filter cartridge. The induced draft fan 9 provides a stable negative pressure environment for the system, and the air volume and negative pressure parameters can be dynamically adjusted through a frequency converter to adapt to different material characteristics and screening process requirements.

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

[0047] The material is fed into the system through the dust-free feeding station, and the dust filter element in the station performs primary filtration and purification on the dust generated during the feeding process. Subsequently, the material is conveyed to the distributor 6 of the housing powder inlet 4 under the action of the frequency conversion motor driven screw feeder 20 at a stable flow rate. The distributor 6 uniformly spreads the material on the subsequent screen 1. Under the negative pressure environment constructed by the induced draft fan 9, the material is dispersed with the airflow, and the fine powder with a particle size meeting the requirements penetrates the screen 1 under the push of the airflow, is conveyed to the cyclone separator 21 through the fine powder outlet 23 and the discharge pipeline. In the cyclone separator 21, the dust-containing airflow rotates at high speed, and the fine powder is separated and collected under the action of centrifugal force; the coarse powder that fails to pass through the screen 1 adheres to the surface of the screen 1 under the action of the flow channel negative pressure, at which time the first air knife 13 corresponding to the screen 1 sprays high-speed airflow to blow the coarse powder to the edge of the screen 1, and the coarse powder is discharged from the device or enters the collecting device under the action of gravity along the inclined product outlet. At the same time, the ultrasonic generator 8 generates a high-frequency vibration signal, which is transmitted to the screen 1 through the mesh frame to drive the screen 1 to produce high-frequency micro-amplitude vibration, effectively inhibiting the screen 1 clogging phenomenon and further improving the screening efficiency. Moreover, through the blowing of the second air knife 14, the fine powder material easily adhered to the circumferential surface of the cavity 2 can also be blown down and discharged by negative pressure through the fine powder outlet 23. The blowing of the first and second air knives 13 and 14 is independently controlled. Generally, after the powder is made, particle size analysis is performed, when the proportion of fine powder in the powder is large, the air pressure of the first air knife is controlled to be less than that of the second air knife, so as to avoid blowing the fine powder into the front cover cavity, and facilitate the discharge of the fine powder; when the proportion of coarse powder in the powder is large, the air pressure of the first air knife is controlled to be greater than that of the second air knife.

[0048] When a multi-stage screening configuration is adopted, the screens 1 are arranged in order from large to small screen hole size, and the material passes through each layer of screen 1 in turn to realize multi-stage screening classification. The coarse powder intercepted by each layer of screen 1 is blown by the corresponding first air knife 13 and discharged through the respective coarse powder discharge outlet 5, so as to realize accurate separation of different particle size materials and fully meet the diversified production needs.

[0049] Example 1:

[0050] The vertical negative pressure airflow screening device of the present embodiment adopts a double-layer screen 1 grading structure, the first layer is configured with a 1000-mesh stainless steel woven screen 1, and the second layer is equipped with a 2000-mesh high-precision stainless steel woven screen 1. First and second air knives are installed after each layer of screen 1, the air knives are made of aviation aluminum alloy material and are processed by anodic oxidation surface treatment process. The air knife blowing pressure is set to 0.6 MPa, which can produce uniform and stable flat airflow beams, which can effectively remove the material adhered to the screen 1. The mesh frame is equipped with a 33 kHz high-frequency vibration system, which adopts intelligent variable frequency control technology to realize accurate control of the amplitude in the range of 0-20 μm.

[0051] The operation process is as follows:

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

[0053] Classification and screening stage: under the condition of-5 kPa negative pressure provided by the induced draft fan 9, the coarse particle material is intercepted by the upper 1000-mesh screen 1, and the fine powder meeting the requirements passes through the screen 1 to the lower 2000-mesh screen 1 for secondary screening. Finally, the ultra-fine powder with a particle size of 10-15 μm enters the cyclone separator 21 through the discharge pipeline, realizes gas-solid separation by centrifugal force, and completes collection.

[0054] Discharge and cleaning stage: the coarse powder that does not pass through the upper screen 1 is discharged from the coarse material discharge outlet along the inclined 45° discharge channel under the blowing action of the corresponding air knife; the intermediate particle size material intercepted by the second layer screen 1 is cleaned by the air knife to the corresponding outlet. During the operation of the equipment, the screen frame continuously generates a micro-amplitude vibration at a frequency of 33 kHz, which cooperates with the air knife cleaning function to build a composite screen cleaning mode of “vibration dispersion + air flow blowing”. After 8 hours of continuous operation test verification, the screening efficiency of the equipment is stably maintained at more than 95%, and the residual material thickness on the surface of the screen 1 is less than 0.1 mm, which reduces the wear by 60% compared with the traditional screening equipment.

[0055] Example 2:

[0056] The difference between this example and Example 1 is that a three-stage gradient screening structure is adopted, the screen 1 is set to 500 meshes, 1500 meshes and 2000 meshes in turn, and a three-stage screening system of coarse screen-middle screen-fine screen is built. The pressure of each layer of air knife can be independently adjusted, and the set pressure value is 0.75 MPa. The ultrasonic screen frame vibration frequency is increased to 37 kHz. The induced draft fan 9 negative pressure is increased 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 the three-stage screen 1 to complete particle size classification, and the coarse powder intercepted by each layer of screen 1 is discharged from the corresponding outlet through the inclined discharge channel, which can effectively avoid the mixing of materials with different particle sizes.

[0059] High-efficiency separation system: after the fine powder is preliminarily collected by the cyclone separator 21, it enters the rear pulse bag dust collector for secondary filtration to ensure that the dust content of the discharged gas is less than 5 mg / m 3 .

[0060] Performance optimization: after 12 hours of continuous operation test, the equipment realizes more than 94% screening efficiency, the operation noise is controlled below 60 decibels, which meets the occupational exposure limit standard of GBZ 2.2-2007 "Occupational Exposure Limit of Hazardous Factors in Workplace Part 2: Physical Factors". The equipment adopts modular cavity design, and the screen assembly can be quickly disassembled during maintenance, so that the single cleaning and maintenance time is shortened to 15 minutes; the equipment shell is treated by electrostatic spraying process, and has good anti-dust adhesion performance.

[0061] However, those skilled in the art should recognize that the above embodiments are only used to illustrate the present application, and are not used as a limitation to the present application, as long as the changes and modifications of the above described embodiments are within the spirit and principles of the present application, they will fall within the scope of the claims of the present application.

Claims

1. A vertical negative pressure air flow screening apparatus comprising a housing, a screen disposed within the housing, characterized in that: The shell is a vertical structure, comprising a cavity and a cover arranged at the front end of the cavity, the cover has a powder inlet at the front end, and a coarse powder outlet is arranged at the lower end of the cover, the front end of the cavity is provided with a mesh frame connected with an ultrasonic generator, the mesh screen is arranged vertically through the mesh frame, a fine powder outlet with negative pressure is arranged on the cavity, and a rotating shaft is transversely 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, the first air knife and the second air knife are both internally provided with an air duct connected with an air source, the first air knife is provided with coarse powder blowing holes arranged to blow towards the mesh surface, and the second air knife is provided with fine powder blowing holes arranged to blow towards the inner wall of the cavity.

2. A vertical negative pressure airflow screening apparatus according to claim 1, wherein: The number of the cavities is at least one, and two or more cavities are arranged in sequence and are communicated, each cavity is provided with a mesh screen in front of the cavity to form a multi-stage screening structure, and the mesh number of the mesh screen gradually increases from front to back; the rotating shaft sequentially passes through each cavity, and each cavity is provided with a group of first and second air knives.

3. A vertical negative pressure airflow screening apparatus according to claim 1, wherein: The first air duct is arranged in the rotating shaft in the axial direction, one end of the first air duct is communicated to a first connector arranged at the end of the rotating shaft, and the other end of the first air duct is communicated to the air duct of the first air knife.

4. A vertical negative pressure airflow screening apparatus according to claim 1, wherein: The rotating shaft is further provided with a bearing seat, the second air duct is arranged in the bearing seat, one end of the second air duct is communicated to a second connector arranged on the outer wall of the bearing seat, and the other end of the second air duct is communicated to the air duct of the second air knife.

5. A vertical negative pressure airflow screening apparatus according to claim 1, wherein: The cover is provided with a distributor communicated with the powder inlet, the distributor is in the shape of a horn with a gradually increasing diameter from front to back, and a plurality of circular through holes are arranged in an array on the distributor.

6. A vertical negative pressure airflow screening apparatus according to claim 1, wherein: The rotating shaft is connected with a driving motor through a rubber belt and a taper sleeve type belt pulley.

7. A method of screening in a vertical negative pressure air flow screening apparatus according to any one of claims 1-6, characterized in that, The method comprises the following steps: A. The powder is transported into the shell through the powder inlet, and the powder is uniformly spread on the mesh screen by the distributor; B. Negative pressure is drawn through the fine powder outlet, the powder is dispersed by the airflow, the fine powder penetrates the mesh screen under the action of the airflow, and is drawn out through the fine powder outlet; C. The mesh screen is vibrated at high frequency by the ultrasonic generator through the mesh frame, and the first air knife is rotated to blow the coarse powder to the edge of the mesh screen and discharge the coarse powder through the coarse powder outlet under the action of gravity; D. The second air knife is rotated to blow the fine powder adhered to the inner wall of the cavity, and the fine powder is blown away and drawn out through the fine powder outlet.

8. A method of screening according to claim 7, wherein, If the screening device has a multi-stage screening structure, negative pressure is drawn through the fine powder outlet, the powder penetrates the mesh screen under the action of the airflow, and the powder is screened, the mesh screen is vibrated at high frequency, and the first and second air knives blow the mesh surface and the inner wall of the cavity.

9. A method of screening according to claim 7, wherein, The blowing of the first and second air knives is independently controlled, when the proportion of fine powder in the powder is large, the air pressure of the first air knife is controlled to be smaller than that of the second air knife; when the proportion of coarse powder in the powder is large, the air pressure of the first air knife is controlled to be larger than that of the second air knife.

10. A vertical negative pressure airflow screening system according to any one of claims 1-6, characterized in that: The device comprises a screw feeder, a vertical negative pressure airflow screening device, a cyclone separator, a pulse dust removal tank and an induced draft fan, the output end of the screw feeder is connected with a distributor through a flexible connection structure via a powder inlet, the fine powder outlet of the vertical negative pressure airflow screening device is connected with the inlet of the cyclone separator in a tangential mode via a pipeline, the outlet of the cyclone separator is connected with the dust removal tank and the induced draft fan in sequence via a pipeline, and the dust removal tank is provided with a filter cartridge.

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