Mechanical screening is the main screening technology used in the feed mill at present, the main working component of the vibrating screen equipment is the screen surface, and the steel plate punching screen and braid screen are widely used at present, here we will introduce the application of feed screening and the general calculation method.
The application of screening technology in feed processing is concentrated in two aspects: one is to clean up impurities in raw materials, and the other is to grade raw materials or products according to particle size, including raw material impurity cleaning, crushing material classification, powder impurity cleaning before pelleting, and the classification of pelleting products. The screening effect in the process of processing has a very important impact on the quality and yield of feed products. Screening efficiency and its influencing factors
1. Screening efficiency. The screening efficiency consists of two aspects: the on-screen retention ratio of the material that should be retained on the screen surface (what is expected to be on the screen) and the on-screen retention ratio of the material that should pass through the screen surface (what is expected to be on the screen). These two indexes affect the removal effect of impurities and the net loss of raw materials in the cleaning operation, affect the particle size and yield of the product in the grading operation, and affect the reliability of the grading results in the detection.
2. Factors affecting the screening effect. The diameter of the screen hole, the diameter of the mesh wire and the Angle of the screen surface all affect the maximum particle size that can pass through the screen hole.
(1) Particle and sieve shape. Based on spherical particles and round sieving, in the production practice of the feed industry, the screening raw materials are mostly cylindrical (granular feed classification) or irregular particles, the sieve has both circular and rectangular, the state of the material particles when they contact the sieve has a great impact on whether the particles can pass through, such as a 04x10 mm particle can pass through a sieve with a diameter of 5 mm when standing upright. Horizontal is not. Therefore, whether the particles pass or not has a certain chance, can only be studied by statistical means. Generally, for cylindrical particles, the rectangular sieve has better passing performance, and for irregular particles with little difference in the size of each direction, the circular hole has better passing performance.
(2) sieve surface opening rate. The larger the opening rate of the screen surface, the better the passing performance. Under the condition of ensuring the strength of the screen surface, the braided screen can obtain higher opening rate than the punched screen, so the passing performance of the former is better than the latter.
(3) Material layer thickness. When using a flat screen, if the material layer is too thick through the vibrating screen, the small particles on the upper part of the material layer are difficult to pass through the screen, it will cause the misscreening rate to rise, which will increase the net raw material loss in the raw material cleaning, and will reduce the output (the upper screen layer is too thick) and affect the quality of the finished product (the lower screen layer is too thick) in the particle classification. If the material layer is too thin, the screening yield is too low and not desirable. The appropriate thickness of the material layer should be determined by test, and the material layer can be slightly thicker when the screen surface inclination is small and the screen body amplitude is large. In theory, the thickness of the material layer is determined by the output. In actual use, due to the uneven feeding of the screen surface, the material may be concentrated on one side of the screen surface, causing the local material layer to be too thick, which affects the screening efficiency
A result. Cylinder screen and cone screen have similar problems, when the instantaneous material flow is too large, the screening effect is also affected.
(4) Motion state of the sieve body. One of the necessary conditions for the screening process is that there is a suitable relative motion between the screening material and the screen surface, which can be generated by the method of horizontal reciprocating linear motion (rotation), vertical reciprocating linear motion (vibration) or a combination of the screen surface. The sieve body only has horizontal reciprocating movement or vertical reciprocating movement, and the screening effect is not ideal. The latter is easy to cause uneven thickness of the material layer due to the lack of horizontal movement of the material and the screen surface. Practice shows that the rotary vibrating screen combined with two kinds of motion has better effect
(5) Material characteristics. The particle size, moisture content, friction characteristics and fluidity of the material are all related to the screening process. The difference of material particle size is the premise of material component screening separation, and the greater the difference, the easier the screening process. The higher the water content of the material, the greater the internal and external friction Angle, and the worse the fluidity, the worse the performance of its particles through the sieve. Therefore, in actual use, in order to obtain a good screening effect, different process parameters should be selected according to the specific situation of the material.
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