The screening test can determine the yield of various granular grades in coal and various quality indicators such as ash, moisture, sulfur and calorific value. The screening test data is the main reference when determining the screening level and its quality indicators. However, the screening test value is only the theoretical value, and the yield and quality of the actual screening grade are different. This is not only the inconsistency between the sieved coal sample and the actual coal particle size characteristics, but also the screening efficiency problem.
Factors affecting screening:
(1) The moisture of coal
The external moisture of the feed coal is the main factor affecting the screening. Generally, the external moisture is below 6%, and the dry screening can be performed well; if the external moisture exceeds 6%, the screening is difficult. Wet screening of flushing often results in better screening efficiency. Dry screening is used in general screening plants, and wet screening can be applied to coal preparation products. In the external moisture of 6% or less, the sieve hole can be dry sieved by 13mm using a general vibrating screen to obtain better effects. For example, the sieve hole is 6mm for dry sieve, and the general sieve can not obtain good effect.
(ii) Particle size composition of coal
If the content of the coal is close to the size of the sieve hole, the screening efficiency is higher; if the content of the particle size is large, the efficiency of the sieve is affected. However, the sieve holes are often classified according to national uniform regulations such as 50, 25, 13, and 6 mm, and the screening characteristics of the coal itself are rarely considered. This particle size, which is similar to the size of the mesh, is referred to as "difficult sieving" and is generally calculated as the % yield of coal within ±25% of the mesh size.
(iii) The shape of the sieve plates and sieve holes
The sieve size is above 25mm. Generally, punching or drilling sieve plates are used. Most of the holes are round holes and arranged in a diamond shape. A woven screen can be used below 25 mm, and the woven screen is a square hole. For 25mm sieve holes, punching sieve plates or woven mesh screens can be used. The mesh screen should be prevented from sliding and the mesh holes being deformed. For sieves below 1mm (including de-sludge, dewatering, de-intermediation), it is possible to use a slotted sieve plate (with a screw threaded on a spiral or welded) or a nylon mesh. Whether it is a sieve or screen, it is very important to be tight and fastened to the screen box. It can extend the life of sieve plates, screens and screen boxes, improve the screening efficiency, and reduce noise.
For sieve plates of 50mm or more, it is often made of steel bars or light rails. In particular, the use of old rails for fixed screens is more appropriate. The rod-shaped sieve strips are all made of wedge-shaped, and the upper width is narrower to facilitate the passage of materials.
The circular sieve hole, which indicates the size of the sieve hole by the diameter of the circle, can ensure that the size of the passage is smaller than the size of the sieve hole, and the product under the sieve has substantially no larger than the size of the sieve hole, but the diagonal of the skystone is The side length is 1.414 times. Some data suggest that the largest particle passing through the square hole is equivalent to 1.23 times the maximum particle size through the circular hole. The rectangular mesh has a rectangular short side as the nominal size of the mesh, in which case the particle size exceeding the mesh size, in particular the flat particles, will be sifted along the long side of the mesh. There are also some irregularly shaped mesh holes, such as some woven screens. The sieve holes of the probability sieve are often larger than the separation particle size and are calculated using the projection of the sieve holes. In any case, the particle size of the screened product is a measure of the particle size standard.
(iv) Length and width of the screen surface
The width of the sieve surface determines the processing capacity of the screening machine. If the width of the sieve material is large, the length of the sieve surface determines the screening efficiency of the screening machine. The longer the sieve surface, the longer the sieve material is sieved, and the more thorough the screening. However, the excessively long sieve face is not significant in improving the screening efficiency, but only excessively lengthens the size of the screening machine.
The sieve length of the existing screening machine in China is 3.5~4m for coarse-grain grade, 5.5~5.6m for medium-fine-grain grade, and 6.5m for dehydration and de-intermediation.
(5) Dip angle of the screen surface
The size of the screen inclination angle affects the moving speed of the material on the screen. The angle of inclination is large, the material moves quickly, and the processing capacity is high. The angle of inclination of the screen surface and the structural form of the screen are related to the quality requirements of the screened product. Generally, the sieve holes are above 50mm. For pre-screening, circular motion screening machines, such as inertial vibrating screens and self-centering vibrating screens, have an inclination angle of 15°~20°. The linear motion screening machine is generally installed horizontally with a dip angle of zero degrees. The material moves on the screen surface and relies on the ejection force of the material against the sieve. This screening machine is generally used for dehydration, de-sludge and de-intermediation of coal.
(vi) Amplitude and frequency
The amplitude refers to half of the stroke of the screen box, and the frequency refers to the number of reciprocating vibrations of the screen box. In addition to the screen angle, the screen box must have sufficient speed to advance the material on the screen surface. The experimental results show that the acceleration of the coal vibrating screen box does not exceed 70~80m/s2, the amplitude is about 2~5mm, and the speed is 800~1500r/min.
In order to measure the amplitude of the linear motion screening machine, a triangle for measuring the amplitude is generally drawn on the screen of the linear vibrating screen, and a set of parallel lines parallel to the baseline is drawn on the measuring triangle, and the scale is marked on the top to indicate the width of the corresponding cutting of the triangle. . When the screen box vibrates and the person stays visually, two triangles will be seen. The intersection of the hypotenuse points is the stroke of the screen (twice the amplitude).
(7) Projection angle
The throwing angle is the angle formed by the moving direction of the screen box and the screen surface. If the throwing angle is larger, it is beneficial to the material to be permeable, but the processing amount is small. The linear motion screen mainly pushes the material forward by the projectile action and allows the fine particles to pass through the sieve. The throwing angle of the linear motion screen is generally between 30° and 55°. In China, the 45° circular motion screening machine also has a projectile effect, but its projection angle is not fixed and is related to the frequency and amplitude of the screening machine.
(8) Processing volume
Too large to increase the throughput (unit area or unit width of treatment), severely affect the screening efficiency, so that the amount of sieve on the sieve is smaller than the sieve size.
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