Classification refers to separating material particles with a particle size below 250 um as the dividing point, and the classification below 10 um is superfine. The role of classification is to control the progress of the pulverization process and the final product’s particle size, and the latter plays an essential role in product quality in particular.
Classification is divided into wet and dry classification. Classifying material particles in a liquid medium (usual water) is called wet classification, and the classification in a gas medium (usually air) is called dry classification. Dry grading is the only option in water-scarce and arid areas where water is not allowed. In severe cold regions, the application of dry grading is also not affected. Dry classification saves a lot of water and also eliminates the problem of subsequent dehydration in wet grading, which is an effective energy-saving grading method.
Main Dry Classification Principle
(1) Classification by particle size
The classification according to particle size is mainly fine particle sieving, which is a more accurate classification. There are two ways to control particle size: sieve holes and sieve slits. The sieve hole can control the two-dimensional size of the particles, and the sieve slit can only control the one-dimensional size of the particle so that the sieve hole can control the particle size more accurately. The fine screen mainly uses the screen slot instead of the screen hole to prevent clogging. The minimum screen gap can reach 70-100 um. Fine screens often work in the form of high-frequency vibration to reduce screen clogging.
(2) Gravity settlement classification
Gravity sedimentation classification is achieved by means of the different sedimentation speeds of material particles in the fluid medium (liquid or gas). Downward movement; the sedimentation speed of the particles with finer particle size is slower than the rising speed of the fluid, so they are driven upward by the fluid to achieve classification. Since the particles’ density impacts the sedimentation speed, it also has a certain impact on the classification process. Due to the gas density being Lower than liquids, dry gravity settling classification is suitable for finer particle size classification than wet gravity settling classification.
(3) Inertia classification
When the fluid motion direction changes, the material particles moving with the fluid deviate from the fluid motion direction under the rule of inertia. The larger the particle quality, ie the particle size, the greater the degree of deviation, whereby particles of different particle sizes can be classified. When using this principle for classification, to avoid the particles moving with the fluid, the friction between the fluid and the material particles must be as small as possible, so this principle is more suitable for dry classification.
(4) Centrifugal sedimentation classification
Centrifugal sedimentation classification is the inertial classification of particles in a fluid moving in a curve. And it is the sedimentation classification produced by the centrifugal motion of particles. In the gravity field, the acceleration of gravity limits the classification of material particles, and the classification process is slow, so the efficiency is low. It can greatly improve the speed and efficiency of classification using centrifugal motion.
(5) New dry superfine classification principle
In recent years, with the continuous development of superfine grading technology, new superfine grading principles have emerged, the most prominent ones being rapid grading, decompression grading, and high-voltage electrostatic grading.
In the dry classification process, the closer the particle size is to the classification tangent point, the slower the particle movement speed is. (The coarser the particle size is, the slower the sedimentation is, and the smaller the particle size is, the slower the rise is). Therefore, the longer the residence time in the classification zone is. It causes the aggregation and accumulation of these particles and interferes with the classification process. Thus, it should complete the classification process as soon as possible, called rapid classification. The measures for fast classification are to make the materials highly dispersed and provide high-precision micro-feeding before classification.
In the dry classification with air as the medium, the higher the air pressure in the classification area, the slower the sedimentation speed of the particles, and the larger the particle size of the classification cut point. Therefore, to obtain a finer tangent particle size, it should reduce the air pressure, that is, reduced pressure classification. The difficulty in realizing this principle is that the lower the air pressure, the worse the classification accuracy and the wider the product particle size range.
The principle of high-voltage electrostatic classification is to make ultrafine powder particles charged after dispersion, subject to the combined action of electrostatic force and gravity in the electrostatic field and gravity field, and particles with different particle sizes have different motion trajectories due to various resultant forces of electrostatic force and gravity, so as to achieve classification.
Dry Classification Equipment – Cyclone Separator
A cyclone separator is a standard dry-type centrifugal sedimentation classification equipment. Its main body comprises an upper cylinder and a lower truncated cone. A core tube is inserted along the central axis from top to bottom at the top of the cylinder. There is a crude product outlet at the bottom of the truncated cone. The feed enters tangentially with the gas flow from the upper part of the cylinder near the outer circumference. It forms a swirling motion restricted by the shape of the classification chamber. The material particles generate a radial centrifugal sedimentation motion in the gas flow. The centrifugal settling speed of coarse particles is relatively fast, and they move close to the cylinder wall and then slide down along the cylinder wall to discharge from the bottom. The centrifugal sedimentation speed of fine particles is slow, and they are suspended near the axis and discharged upward with the air flow entering the core pipe. There are many improved products in practical application to adapt to different classification requirements and obtain higher classification performance. The classification particle size of the cyclone separator is related to its specification (cylinder diameter)—the smaller the specification, the finer the classification particle size.
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