Mechanical screening is the main screening technology used in feed mills. The main working component of vibrating screen equipment is the screen surface. Currently, the steel plate punching screen and woven screen are widely used. The users of pig e net will introduce the application of feed screening. Aspects and general calculation methods.
The application of screening technology in feed processing is concentrated in two aspects. One is to clean up the impurities in the raw materials, and the other is to classify the raw materials or products according to the particle size, including raw material impurity cleaning, pulverizing material classification, and powder before granulation. Material impurity cleaning, grading of granulation products. The effect of screening during processing has a significant impact on the quality and yield of feed products.
1 Screening efficiency and its influencing factors
1.1 Screening efficiency
The sieving efficiency includes two aspects: the retention ratio on the sieve which should retain the sieve material (the expected sieve material) and the retention ratio on the sieve which should pass through the sieve material (the expected sieve material). These two indicators affect the removal effect of impurities and the loss of net raw materials in the cleaning operation, affect the particle size and output of the product in the classification operation, and affect the reliability of the classification result in the detection. The former is called the screening rate, the latter is called the erroneous screening rate, which is expressed by the formula:
=w1/w2×100%
=w3/w4×100%
η1—Screening ratio, %; η2—missing rate, %; W1—predicted sieve retention on the sieve, kg/h; W2—the total amount of expected sieve, kg/h; W3—expectation The amount of sieve residue on the sieve, kg / h; W4 - the total amount of sieve under the expected, kg / h.
The above two indicators are used to evaluate the cleaning efficiency. When the sieve is an impurity, η1 is equivalent to the impurity removal rate, and η2 is equivalent to the net material loss rate.
1.2 Factors affecting the screening effect
The maximum material particle diameter through the mesh can be estimated by the following formula:
d=D cos α-e sin α
Where d - the maximum particle diameter through the mesh, mm; D - mesh diameter, mm; e - mesh wire diameter, mm; alpha - screen tilt.
It can be seen from formula (3) that the mesh diameter, the mesh diameter, and the screen inclination angle all affect the maximum particle size of the particles that can pass through the mesh. However, formula (3) can only determine the critical particle size, and whether a particle smaller than the critical particle size can pass through the mesh, depends on other conditions.
1.2.1 Particle and mesh shape
The calculation of formula (3) is based on spherical particles and circular sieve holes. In the actual production of the feed industry, the screening raw materials are mostly cylindrical (granular feed classification) or irregular particles, and the sieve holes are both round and Rectangular, the state of the material particles in contact with the sieve hole has a great influence on the passage of the particles. For example, a particle of 4×10 mm can pass through a sieve hole with a diameter of 5 mm when erected, and the transverse direction cannot. Therefore, the passage of particles has certain contingency and can only be studied by statistical means. Generally, for cylindrical particles, the rectangular mesh has better pass performance; and for irregular particles with small difference in size in each direction, the pass performance of the round hole is better.
1.2.2 Screen opening rate
The larger the opening ratio of the sieve surface, the better the pass performance. In the case of ensuring the strength of the screen surface, the woven screen can obtain a higher opening ratio than the punching screen, and thus the pass performance of the former is superior to the latter.
1.2.3 material layer thickness
When using a flat screen, if the material layer passing through the screen of the vibrating screen is too thick, it is difficult for the small particles in the upper part of the material layer to pass through the sieve hole, which will cause the false screening rate to rise, which will increase the net material loss in the material cleaning, in the particle classification. This will reduce the yield (the upper sieve layer is too thick) and affect the quality of the finished product (the lower sieve layer is too thick). If the layer is too thin, the sieve yield is too low and it is not desirable. The thickness of a suitable layer should be determined by experiment. When the angle of the screen is small and the amplitude of the screen is large, the layer can be slightly thicker. In theory, the thickness of the layer is determined by the yield, but 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 layer to be too thick and affecting the screening effect. Cylindrical sieves and conical sieves have similar problems. When the instantaneous material flow rate is too large, the screening effect is also affected.
1.2.4 Screen movement state
One of the necessary conditions for the sieving process is that there is a suitable relative motion between the screening material and the screen surface. The method for generating this relative motion may be a horizontal reciprocating linear motion (rotation) and a vertical reciprocating linear motion (vibration). Or a combination of the two. The sieve body only has horizontal reciprocating motion or vertical reciprocating motion, and the screening effect is not ideal. In the latter case, due to the lack of material and the relative movement of the screen surface, it is easy to cause uneven thickness of the material layer. Practice has shown that the rotary vibrating screen combining the two kinds of movements has a better effect.
1.2.5 Material characteristics
The particle size, water content, friction characteristics, fluidity, etc. of the material are all related to the screening process. The difference in particle size of the material is a prerequisite for the separation of the material components, and the greater the difference, the easier the screening process. The higher the moisture content of the material, the greater the internal and external friction angle, and the worse the fluidity, the worse the performance of the particles passing through the mesh. Therefore, in actual use, to obtain a good screening effect, different process parameters should be selected according to the specific conditions of the materials.
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