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Basic concept of wind grading
The air grading with air as the medium is also called dry grading. The treated materials must be dry, the moisture content is less than 5~6%, and the effect of wind grading is generally worse than the hydraulic grading, but the product is dry, not Dehydration is economical in some cases. Some can not be a wet process or wet process uneconomical materials such as talc, aluminum, alumina, apatite, coal powder, kaolin; certain chemicals, building materials, metallurgical and other raw materials, grinding in dry Dry processing in processing , such as dry magnetic separation, electric selection, wind beneficiation, screening, etc., is often used in conjunction with wind classification. In addition, wind classification is often applied to dust removal and dust collection of dusty exhaust gas, flue gas and various gases.
Wind classification is mainly used to process finer materials, with a graded particle size of between 1.5 and 0.005 mm, and coarse materials with a sieve.
The principle of wind classification is basically the same as the hydraulic classification, and the settlement theory described above can also be used for wind classification. However, in wind classification, because the density and viscosity of air are much smaller than water, the same particles settle much more in the air than in water. Moreover, the density and viscosity of air vary with temperature and pressure.
Wind classification is carried out in a moving air stream. In order to cause air flow, wind grading equipment generally needs to be equipped with appropriate fans (exhaust or blast).
When the wind is graded, the fines are generally carried away by the gas stream. In some cases, the feedstock is fed to the classifying equipment along with the gas stream. It is known that the load of airborne solid particles under normal pressure does not exceed 500 g/ m3 of air, and when it reaches 1 kg/ m3 , the particles fall from the gas stream. The amount of load is related to the density and particle size of the particles. When handling fine-grained materials, the load is generally not more than 200~300 g/ m3 of air. According to this calculation, the volume concentration of solids in the gas stream is much less than 0.03. Therefore, when the wind is graded, in most cases, it belongs to free settlement.
Different air flow required to move particulate material in the horizontal direction and the vertical speed, the desired speed in the horizontal direction is relatively large, and with increasing particle density is increased, e.g., to a density of 1.5 g / cm3 pf The minimum air velocity is 2 m/s; for powder materials having a density of 2.5 g/cm, the speed is not less than 3.5 m/sec; the powder having a density of 4 g/cm 3 has a minimum speed of 4 m/sec. Due to the complexity of the situation, the appropriate airflow rate in each specific case should be determined by experiment.
In an updraft, the velocity of the gas stream required to suspend particles of a certain density and particle size is greater than the calculated value of the free sedimentation due to the uneven distribution of the velocity of the gas stream along the section of the classification chamber.