Ultra-fine powder classification technology and its typical equipment

In the pulverization process, only a part of the powder reaches the particle size requirement. If the product that has reached the required requirements is not separated in time, and is pulverized together with the product that does not meet the particle size requirement, energy waste and excessive pulverization of some products may occur. .

In addition, after the particles are refined to a certain extent, pulverization and agglomeration occur, and the pulverization process is deteriorated even due to large agglomeration of the particles. For this reason, in the preparation process of the ultrafine powder, the product should be classified, on the one hand, the particle size of the product is controlled to be in a desired distribution range, and on the other hand, the product having the particle size of the mixture in the mixture is timely separated, so that the coarse particles are returned. It is pulverized to increase the pulverization efficiency and reduce energy consumption.

With the improvement of the required fineness of the powder and the increase of the yield, the difficulty of the classification technology is getting higher and higher. The problem of powder classification has become the key to the development of the powder technology, and is the most important basic technology in the powder technology. one. Therefore, research on ultrafine powder grading technology and equipment is necessary.

 

Large horizontal micrometer

Image source: A well-known powder equipment manufacturer

The generalized classification is to divide the particles into different parts by utilizing the differences in particle size, density, color, shape, chemical composition, magnetic properties, and radioactivity. The narrow grading is based on the different particle size particles in the medium (usually using air and water) by centrifugal force, gravity, inertial force, etc., to produce different trajectories, so as to achieve grading of particles of different sizes.

According to the medium used, it can be divided into dry classification (medium air) and wet classification (medium water or other liquid). Dry grading is characterized by the use of air as a fluid, which is low in cost and convenient, but it has two disadvantages, one is easy to cause air pollution, and the other is that the accuracy of grading is not high. The wet classification liquid uses the liquid as the classification medium, and there are many post-treatment problems, that is, the classified powder needs dehydration, drying, dispersion, wastewater treatment, etc., but it has the characteristics of high classification accuracy and no explosive dust.

According to whether there are moving parts, it can be divided into two categories:

There are no moving parts in the classifier, such as gravity classifier, inertial classifier, cyclone separator, spiral air classifier and jet classifier. This type of classifier is simple in construction, requires no power, and has low operating costs. It is convenient to operate and maintain, but the classification accuracy is not high and it is not suitable for precision grading.

The classifier has moving parts, mainly referring to various turbine classifiers. This kind of classifier has complex structure, requires power, and has high energy consumption, but the classification accuracy is high, and the classification particle size is convenient to adjust. As long as the rotation speed of the impeller is adjusted, the cutting particle size of the classifier can be changed, which is suitable for precise classification.

High performance fine classifier

Image source: A well-known powder equipment manufacturer

For any grading method, the key to achieving a better grading effect is how to improve the dispersion of the grading materials and select the appropriate grading force field.

After the material is ultra-thinned, it exhibits different properties from the original material. Firstly, the surface area increases and the surface energy increases. Secondly, the ratio of surface atoms or ions increases, the surface activity increases, and the gravitational force between the particles increases. It is easy to aggregate due to the action of foreign impurities such as moisture; ultrafine particles are also easily accumulated on large particles due to electrostatic force and the like during collision or pulverization, and are easily formed in air or in liquid phase. The secondary particles with larger particle size make the classification of ultrafine products more difficult than the classification of ordinary products.

Therefore, the primary task of grading is to disperse the particles in a monodisperse state, thereby improving the dispersion of the powder, that is, the basis of the ultrafine powder is the dispersion of the powder particles. It can be said that sufficient dispersion can make the grading process more effective.

After solving the dispersion of powder particles, another big problem is how to design a stable, adjustable force field. The ideal graded force field should have the characteristics of strong classification force, stable flow field and rapid classification. Since the powder particles behave differently in different media and different force fields, it is necessary to understand their physical properties and motion characteristics, and design an efficient and reasonable fractional force field. At present, the force fields used by the classifier are mainly gravity field, inertial force field and centrifugal position.

The wet grading of ultrafine powders is mainly divided into gravity type and centrifugal type from the current market situation.

Hydrocyclone. The classification process of the hydrocyclone is: the material rotates at a high speed inside, generating a large centrifugal force. Under the action of centrifugal force and gravity, the coarser particles are thrown toward the wall, spiral downward movement, and finally discharged from the bottom flow port. The finer particles and most of the water form a swirl, rising along the center, to the overflow pipe. discharge.

There are three types of commonly used hydrocyclones, small diameter hydrocyclones, water-sealed cyclones and ultra-fine hydrocyclones. The advantages of the hydrocyclone are simple structure, no moving parts, low cost, small floor space, and large processing capacity; the disadvantage is that the classification accuracy is low.

 

Horizontal spiral centrifugal classifier. The suspension to be classified is fed into the propeller tank of the auger by the central feeding tube, and is accelerated into the drum by the feed hole on the spiral. Under the action of centrifugal force, the suspension entering the drum is quickly divided into two layers. The coarser or heavier particles are deposited on the inner wall to form a sediment layer, while the liquid phase containing finer or lighter particles forms an inner ring separation. Liquid layer. The separation liquid is discharged by overflow or centripetal pump, and the sediment (solid particles) is pushed by the screw ejector to the cone end of the drum. After further dehydration, the drum is pulled out from the slag outlet, and the general classification granularity is 2 to 5 μm.

Most of the dry classifiers use the centrifugal force field and the inertial force field to classify the powders. They are important fine grading equipments that are currently developing rapidly. The following are several representative devices.

Conical centrifugal air classifier. The conical centrifugal air classifier realizes the separation of coarse powder and fine powder under the action of centrifugal force. The finished product has the finest particle size of about 0.95μm and the classification accuracy of d75/d25 can reach 1.16. The device does not have any moving parts, and its deflector angle can be adjusted between 7 and 15 degrees. The device has compact structure, high classification efficiency and safe and reliable operation.

MS (Micron Separator) impeller classifier. The material to be classified and the primary air flow enter the machine through the feeding pipe and the adjustable pipe, enter the classification zone through the airflow distribution cone, the shaft drives the classifying impeller to rotate, and the fine-grained material is rotated by the grading wheel at high speed in the grading zone to generate strong centrifugal force. Under the double action of the centripetal force generated by the field and the rear induced draft fan, the centripetal force is greater than the centrifugal force, and the gas is discharged upward through the fine particle discharge port through the gap between the blades. The coarse-grained material is discharged from the coarse-grain discharge port at the lower part of the body through the annular body due to the large centrifugal force.

MSS super-segmentation machine. This machine is a modified version of the MS machine. The utility model is characterized in that a tangential airflow injection hole is added to the cylindrical shell wall of the impeller section, and the function thereof is to inject airflow from the hole into the machine, so that the impeller is thrown into the coarse particles of the cylinder wall by the centrifugal force. Fine particles can be completely separated from them.

ATP type classifier. The ATP type classifier is a classifier developed and produced in Germany and is an impeller rotor type. The equipment grading wheel is horizontally installed on the top of the classifier, and is used in combination with a fluidized bed airflow mill, a wheel mill, and the like. At the same time, in order to overcome the problem that the impeller rotation speed is too high and the production capacity is lowered, a plurality of small-diameter staged impellers can be installed in parallel on the top of the classifier to increase the production capacity and ensure a finer product.

Inertial classifier. The inertial classifier uses the particle motion because its mass is different, and the inertial force is also different, thus forming different motion trajectories, thereby achieving grading of the particles. Another type of inertial classifier, jet grading, utilizes the ejection effect of the jet on the powder while the material is subjected to airflow in different directions, resulting in grading of the particles. It is found that when using inertial force field grading, the flow field has many interference factors and is difficult to control.

KSF type new super-segmentation machine. The machine is produced in Japan. The graded rotor is of the impeller type or the cage type, and the rotor speed is high, and the graded particle size is determined by the centrifugal force generated by the rotor and the centripetal force of the air flow. It is reported that the machine is suitable for the classification of various materials such as metal ore, non-metallic minerals, plastics, ceramics, etc., and can obtain ultra-fine products of 0.3 μm.

LHB turbine type super-segmentation machine. The classifier is characterized by low speed (1200 ~ 1 900r / min), graded fineness d97 of 5 ~ 30μm, with the advantages of classification accuracy, high classification efficiency, low energy consumption.

DS type classifier. The DS type classifier is a rotorless semi-free vortex classifier, which has a two-phase flow of fine particles and enters the classifier under the action of negative pressure. After being separated by the rotation of the upper cylinder wall, part of the air and fine powder exit the classifier through the insertion tube; the remaining part of the material enters the classification zone through the center cone, and is divided into coarse powder and fine powder under the action of centrifugal force. The secondary air enters the classification chamber through the angle-adjustable vanes to sufficiently disperse the particles and improve their classification efficiency. The coarse powder enters the discharge bin through the annular passage, and the fine powder is discharged from the lower part of the center cone. The adjustment of the grading fineness is also done by adjusting the height of the center cone and the amount of secondary air. The DS type classifier has a cutting particle size of 1 to 300 μm and a treatment amount of 10 to 4000 kg/h.

 

ACUCUT type classifier. The classifier is developed and produced by the United States. Its structure and principle are as follows: the middle part is a graded rotor, the outer side of the rotor is a fixed wall, and the upper and lower covers seal the classification chamber. The hollow shaft in the upper part of the rotating shaft is the fine powder outlet, and the solid shaft in the lower part is rotated by the motor. The rotor is composed of upper and lower turntables and blades, and the rotor rotates to form a centrifugal force field. At the same time, a negative pressure zone is generated in the hollow portion, so that the airflow rotates along the rotor and flows radially to the hollow shaft portion, thereby constituting a fluid flow in which the centrifugal force field interacts with the pressure field. The granules in the grading chamber are subjected to fluid entrainment. If the radial entrainment force of the granules is greater than the centrifugal force, the granules are discharged through the fine powder outlet, and the coarse powder is discharged from the tangential outlet. The machine is characterized by high classification accuracy, the cutting particle size can be less than 1μm, and the grading fineness can be adjusted only by adjusting the rotor speed.

Coanda cross jet classifier. The Coanda cross-jet classifier is also known as the cross-bend airflow machine. It utilizes the Coanda effect of a high-speed jet while dividing the material into three grades of different fineness. The particles enter the classifier by the feeding nozzle under the entrainment of compressed air. The particle trajectory in the flow is determined by the air resistance and the particle inertia. When passing through the wall surface, the flow deflection is formed due to the Coanda effect. Each particle has different inertia due to its different size. The small particle has a strong wall-attaching effect and flows against the surface of the attached block. The large particles are entrained farther by the air due to the large inertia. The particles will form a sector track according to the size, and under the adjustment of the secondary air flow and pressure, a variety of products can be obtained from different angles.

Multi-rotor micro powder classifier. The classifier is a large throughput classifier consisting of an upper staged chamber (consisting of multiple rotors) and a bottom dispersion unit. The feedstock is fluidized and dispersed at the bottom of the classifier and then carried by the ascending gas stream into the classification zone. The fine powder is lifted through the rotor blades and raised into the collector. The coarse powder and agglomerated particles are disintegrated after encountering the tangentially introduced secondary gas stream during the falling process, and then discharged through the outlet valve at the bottom. This machine is specially designed for the classification process of high fineness and large throughput.

Effective Impact Classifier (Virtual Impactor). It consists of a cylinder made of two upper and lower wear-resistant materials. The cylinder has an aerosol-like particle flow. The inside and outside of the granular layer is a clean airflow. Collision occurs at the entrance into the straight cylinder, making the material easy to disperse, less than graded. Particles of a particle size are discharged from the fine particle outlet. It is characterized by rapid grading, convenient material discharge, simple structure, durability and easy maintenance.

MC type micro powder classifier. The MC type Micron Classification has no moving parts and is classified by the centrifugal force field generated by the two-phase flow along the rotating flow of the wall. The principle is that the gas-solid two-phase flow entrained with dispersed particles enters the upper vortex chamber under the suction of the negative pressure, enters the classification chamber with a stable concentration under the guidance of the guiding cone, and is separated into two by the centrifugal force. Kind of particles. The fine powder passes through the central passage in the upper part of the grading cone, and is discharged from the outlet under the entrainment of secondary air entering through the inlet; the coarse powder falls into the coarse powder chamber along the grading cone. The machine's graded cutting particle size range is 5 ~ 50μm, can be adjusted by changing the gap between the guiding cone and the graded cone, the amount of secondary air and the pressure of different areas, the processing capacity is 0.5 ~ 1000kg / h.

O-Sepa classifier. The main body of the classifier is a volute-shaped cyclone cylinder, which is equipped with a cage rotor, and a vertical guide vane is arranged in the volute of the classifier. The material enters the spreading disc from the two feeding inlets at the top, and the rotating tray throws the powder to the outer retaining ring, and the direction is changed after the impact is dispersed, and the material is formed in the narrow space between the rotating cage and the guiding vane. screen. Under the combined action of the guide vane and the rotation of the rotating cage, the gas in the machine forms a strong horizontal swirling flow. The particles have both the inertia of linear motion along the tangent line and the entrainment of the centripetal airflow. After the wall collision, the fine particles enter the cage with the airflow, and finally the integrated product is sucked out from the central air duct.

(1) Strengthen theoretical research, pay attention to the intersection of disciplines, and actively learn from other disciplines.

(2) Pay attention to the research on the dispersion technology and corresponding equipment of ultrafine powder in various media, study the agglomeration mechanism of ultrafine powder, and explore effective ways to eliminate agglomeration.

(3) Strengthen the research of special equipment and develop high-efficiency, low-consumption, high-precision and large-processing grading technologies and equipment.

(4) Develop artificial intelligence technology based on existing equipment and technology, automatically optimize production process configuration and operating parameters according to raw material characteristics and product fineness requirements, to achieve high efficiency, low consumption and stable product quality.

(5) Improve the stability and reliability of the product, and pay attention to the organic combination of crushing and grading.

(6) Further improve the matching of equipment, and strengthen the research on online testing, monitoring and corresponding monitoring equipment of super-segmentation level.