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Abrasive micro-powder refers to abrasive particles with a particle size of 63um or finer. Micro-powder is an indispensable raw material for precision processing, and is widely used in machinery manufacturing, optical instruments, semiconductor material processing and aviation and other fields. The manufacturing level of abrasive micro-powder has become an important symbol to measure the level of industrial development of a country. In recent years, significant progress has been made in the introduction and absorption of international advanced production equipment and industrial technology, etc., and the products are gradually approaching the international advanced level.
Domestic micro-powder products are mainly white corundum micro-powder and green silicon carbide micro-powder, and a small amount of cubic silicon carbide micro-powder and boron carbide micro-powder.
Manufacturing process of micro-powder abrasives
The production of micro-powder abrasives is different from the manufacturing of abrasive grains. The production of micro-powder should use high-quality raw materials, white corundum usually use F36-F100 abrasive as raw materials, green silicon carbide mostly use F24-F60 finished products as raw materials, because this particle size range of the abrasive material with the least impurities.
The biggest difference between micro-powder manufacturing and abrasive manufacturing is the method of particle size classification. Because of the fine particles of micro-powder, it is difficult to classify by screening, so micro-powder is mostly classified by hydrodynamic classification or airflow classification. Typical production processes such as white corundum micro-powder, green silicon carbide micro-powder.
1.Raw material crushing
According to the crushing equipment used, micro-powder raw material crushing process has intermittent ball milling, air mill crushing method and vibration mill crushing method.
(1) Intermittent ball milling method: characterized by simple equipment, but low processing efficiency. Due to wear and tear of steel balls mixed into the raw material more iron. Ball milling method of crushing dry and wet two types of operations, the production of micro-powder wet is preferred.
(2) Airflow crushing method: airflow crushing belongs to the media-free crushing. Compressed air, so that the crushed material particles collide with each other friction, so as to achieve the purpose of ultra-fine pulverization. The introduction of this method is not long, but the development is very fast. Because of the advantages of high production efficiency and not contaminated by the crushed material (almost no introduction of foreign impurities), is the technical innovation of the production of abrasive micro-powder provides a way.
(3) Vibration mill crushing method: vibration mill, also known as vibration ball mill, there are dry and wet method. Characterized by light machine quality, small footprint, large production capacity, low power consumption, good grinding effect, can be ultra-fine grinding. But the mechanical requirements are relatively high, some parts (such as springs and bearings) easy to damage.
2.Magnetic separation and acid washing
White corundum micro-powder with finished abrasive as raw material, the production is no longer magnetic separation, but after crushing to be acid washing. Acid washing can remove the iron, part of the calcium oxide and magnesium oxide impurities. Acid washing is carried out directly after the raw material is crushed.
Green silicon carbide crushed by wet ball milling method, in order to improve the purity of the general magnetic separation and acid washing. If the raw material is more pure, there is no acid and alkali washing, but the quality of the micro-powder produced is poor. Green silicon carbide should be crushed and magnetic separation and then acid and alkali washing, chemical treatment order is first alkali washing and then acid washing, acid and alkali washing and then hydraulic classification.
Classification of micro-powder abrasive grains
Due to the fine particles of micro-powder, it is extremely difficult to use the sieving method of grading, commonly used hydraulic grading. Commonly used hydraulic classification methods are sedimentation method, overflow method and cyclone method.
Sedimentation method and overflow method are two commonly used classification methods. The principle is based on the same specific gravity of the particles in the water settling speed due to differences in particle size, large particles of the static speed, small particles of the static speed of slow, so as to carry out particle size grading.
Micro-powder in water classification needs to add a certain amount of dispersant in the slurry, so that the micro-powder particles are dispersed to prevent unity.
Temperature has a negligible effect on the speed of settling micronized grinding particles. In addition, there are many factors that affect the settling speed of the mill grains, such as the shape of the particles, the concentration of the suspension, and the collision between the particles.
Cyclone method of hydraulic classification is to feed a certain concentration of slurry into the cone in a tangential direction. High-speed rotation generates centrifugal force, centrifugal force to the fixed particles thrown to the wall. Due to the size of the centrifugal force and particle thickness of the spinning until related to the particles of coarse centrifugal force, see all the center of the spinning centrifugal force, so the finer particles near the center of the position of the centrifugal force and kinetic centripetal force to achieve equilibrium, while the coarser particles in the position of the wall to achieve equilibrium.
The advantage of this method is that no dispersant is needed, the centrifugal force of the cyclone can make the aggregated particles scattered, and the sorted slurry is dispersed very quickly. Equipment structure is simple, high production efficiency, small footprint, but the accuracy is not stable enough, the particle size changes, it is difficult to adapt to the narrow level of sorting, mostly used in the grouping of micro-powder.
The concentration of slurry will affect the accuracy of grading particle size, so before cyclone grading, the concentration of slurry must be blended well.
3. Airflow classification
Airflow classification mainly has gravity, inertial force, centrifugal force in three forms, the use of centrifugal force field classification is the more common form.
The main factors affecting airflow classification are air inlet, rotating speed of classification wheel and feed rate.
Air intake: Its size has an effect on the particle size distribution of the graded product. For the coarser principle of using a larger air volume, is conducive to the dispersion and grading, and smaller air inlet is beneficial to the sorting of fine-grained micro-powder, to avoid the possibility of large particles into the fine micro-particle.
Grading wheel speed: Adjusting the rotating speed of the grading wheel can change the particle size composition of the discharged material, so as to achieve the purpose of separating out products with different particle sizes. When the rotating speed is high, the particle size of micro-powder is fine, and vice versa, the particle size is coarse.
Feeding rate: affects the efficiency of the classifier. Coarse particle size sorting, the feed rate will be larger, fine particle size will need to reduce the speed, so as not to enter the coarse material.
In actual production, many factors affecting the airflow classification are influenced by each other, and the adjustment program needs to be explored and summarized in practice.
Micro-powder Drying
Drying is a process in which materials gain energy and liquid is vaporized and discharged. The drying process should prevent the mixing of all particle size, the upper layer of fine particles, the lower layer of coarse particles. Drying temperature is 80-250℃.
The use of convection hot air drying is characterized by simpler equipment, cheaper energy prices, the disadvantage is that the heat consumption is large, the equipment volume is large, difficult to automate. The characteristics of electric drying are just the opposite of hot air drying.