Grading problem in pharmaceutical crushing
 The pulverized material is difficult to achieve the required particle size requirement by mechanical pulverization at one time, and is often in a large particle size distribution range. In the process, only a part of the products meet the particle size requirements, while the other products do not meet the particle size requirements. If these products have not been separated in time, and they are crushed together with the products that do not meet the requirements, It will cause energy waste and excessive crushing of some products [1] . To this end, a comminution grading technique is produced.
The ordinary powder can be classified by sieving method. At present, the pore size of the finest sieve is only about 20 μm (600 mesh). Considering the clogging problem of the screening process, the actual screening of more than 325 mesh has no industrial significance [1] . In order to meet people's requirements for crushing particle size, especially the pharmaceutical industry has further requirements for powder particle size. As a result, more grading techniques have entered the crushing city.
According to the type of equipment used, it can be divided into: dry mechanical classification (such as impeller type, turbine type), cyclone type classification, dish type classification, horizontal spiral classification, electrostatic field classification, supercritical classification, and the like. The two most common ratings are described below.
1. Dry mechanical classification
Dry mechanical grading is usually done with dry air as a medium. The starting point is to improve the classification efficiency and reduce the classification particle size by various mechanical means. Usually, various mechanical motion devices are introduced into the classification equipment at various levels to increase the centrifugal force of the particles in the classifier, thereby improving the separation factor, increasing the classification speed, and improving the classification accuracy.
1.1, disc type classifier
The principle: in the work, the graded powder enters the center surface of the high-speed rotating disc. Under the action of centrifugal force, the coarse particles are thrown to the periphery of the disc, the fine particles are at the center of the disc, and then the coarse and fine particles pass through different channels respectively. discharge.
1.2, MS impeller classifier
The principle: the graded powder is carried by the airflow and moves into the grading chamber by moving from bottom to top. When rising, the powder is subjected to the secondary air, so that the fine powder contained in the coarse powder is separated, so that the fine powder continues to rise with the gas flow. At the distribution cone, due to its high speed rotation, the ascending powder is dispersed and evenly distributed to move around. When the powder reaches the impeller classification zone, due to the strong centrifugal force field generated by the high-speed rotation of the impeller, the powder is subjected to the centripetal action of the upward airflow and the rear exhaust of the classifier, and is subjected to the centrifugal force generated by the rotation of the impeller. effect. At this time, the coarse particles are subjected to the centrifugal force greater than the centripetal force, and are then raked toward the barrel wall and moved downward along the wall, and discharged through the coarse grain outlet. The fine particles are discharged from the impeller slit through the fine particle outlet due to the centripetal force greater than the centrifugal force, and are collected by the post-process collector.
The MS impeller classifier was developed by Japan's Hosokawa Co., Ltd., and the company further improved the MS-H type, MS-N type, MSS type and other classifiers.
1.3, ATP type classifier
The ATP type classifier is an impeller rotor type classifier, and its principle (as shown in Fig. 3): the pulverized indoor powder is flying in a fluidized state to the grading area of ​​the impeller rotor under the action of the negative pressure air flow. Under the centrifugal force generated by the high-speed rotation of the impeller, the suction generated by the negative pressure airflow, the gravity of the particles and the lift generated by the rising airflow, the coarse-grained material falls into the pulverizing chamber and is pulverized again, and then re-graded with the airflow. The fine particles are sucked away by the airflow from the gap between the impellers, and then collected by components such as a cyclone.
2, swirling classification
The cyclone grading technology is suitable for both dry grading and wet grading. Here we mainly talk about dry grading.
Dry cyclone grading is the use of gas as a medium and as a power source, using a combination of centrifugal force field and gravity field for grading. During the grading process, the high velocity gas stream carries the classified powder tangentially from the separator into the separator, and the gas stream rotates at a high speed along the inner wall of the circular separator. Under the action of strong centrifugal force, the coarse particles in the powder are rotated downward along the inner wall of the cone of the separator to sink to the lower discharge port; the fine particles are concentrated toward the center of the separator due to the centripetal force and rise with the airflow, and are discharged from the upper outlet. In order to achieve the purpose of grading coarse and fine particles.