Coal screening test is the basic method to determine the particle size composition and the quality of each particle fraction of coal. Through the screening test, we can know the lump coal yield of each production coal seam and the quality characteristics of coal with different particle sizes. The obtained screening data is an important basis for rational utilization of coal resources and formulation of quality standards for coal products. Screening is the most important operation link in coal screening test, and it is a method to classify coal samples according to particle size by using sieves with different apertures. Theoretically, all coal samples larger than the mesh size should be left on the screen surface; All coal samples smaller than the mesh size pass through the mesh, thus achieving the purpose of classification. However, in actual operation, due to various factors, some particles smaller than the mesh size can not pass through the screen normally, while others larger than the mesh size enter the lower part of the screen through the screen. This will adversely affect the screening effect and reduce the accuracy of detection. The screening effect is influenced by many factors, which can be summarized as follows. That is, the particle size composition of coal sample, the external moisture and mud content of coal sample, the performance of screen body and the skills of operators.

1 particle size composition of coal sample

In coal samples, particles with a diameter less than 3/4 mesh easily penetrate through the screen, which we call "easy-to-screen particles"; Particles with a particle size smaller than the mesh size, but larger than 3/4 pore size are not easy to penetrate through the mesh, which is called "hard-to-sieve particles"; Particles with a diameter of 1~ 1.5 times the mesh size often form a layer of material, which is close to the screen surface, making it difficult for "hard-to-screen particles" to penetrate, and is called "screen blocking particles"; However, the gap between particles with a diameter larger than 1.5 times the mesh size is large, so the material layer formed by them has little influence on the "easy-to-screen particles" and "difficult-to-screen particles" approaching the screen surface through it. Obviously, when there are a large number of "easy-to-sieve particles" and particles with a diameter larger than 1.5 times the sieve hole in the coal sample, the sieving speed is fast and the effect is good. However, when the contents of "particles difficult to screen" and "particles resistant to screen" increase, the screening efficiency will be reduced and the screening accuracy will be affected. In this case, the feeding amount of each screen can be reduced, so that most particles (especially those difficult to screen) have a chance to fully contact with the screen surface to obtain good classification effect. Classification can also be achieved by increasing the residence time of coal particles on the screen surface and enhancing the vibration frequency of the screen plate (suitable for vibrating screen machine).

2 External moisture and mud content of coal samples

In coal samples, the water content of fine coal is generally higher than that of large coal. When the external moisture increases, the viscosity of coal also increases, which makes fine particles adhere to larger particles or make fine particles adhere to each other, resulting in some coal particles smaller than the mesh size can not pass through the screen. Some coal samples contain sticky minerals that are easy to agglomerate, which tend to stick to coal particles or screen, making the screen smaller and reducing the screening efficiency. If the above situation occurs, the coal sample can be dried in advance before screening to reduce the viscosity of the coal sample, or the feeding amount can be reduced each time during screening, and a vibrating screen can be used to make the coal sample difficult to stick and block holes.

3 screening equipment

Screening equipment mainly includes punching screen, screen, standard screen and electric vibrating screen. Its technological and mechanical properties directly affect the screening effect.

(1) Type and shape of screen surface

The type of screen surface is divided into mesh screen and punching screen, and the shape of screen hole is divided into round hole screen and square hole screen. Round hole screens are all perforated screens, which have high screening accuracy, long service life and are not easy to wear. However, the opening rate of the screen surface is low, the effective screening area is small, the screening speed is slow and the output is low. The screens are all square-hole screens, which have high opening rate, large effective screening area, high screening speed and high yield. However, the service life of the screens is short and they are easy to deform, and the shapes and sizes of some screen holes will change after deformation, thus affecting the screening accuracy. However, the performance of square hole perforated screen is somewhere between the two. Therefore, when the particle size of coal sample is large and it is easy to damage the screen surface, the round hole screen is often used, while when the particle size of coal sample is small, the square hole screen is used for screening.

(2) Tilting and moving of the screen body

When manual screening, the surface of coal sample moves left and right, which is slow and labor-intensive. However, the operator can extend or shorten the screening time according to the actual situation, and the screening effect is easy to master.

When the vibrating screen machine is used, the coal sample vibrates in the direction perpendicular to the sieve plate and slides down the inclined plane of the sieve surface. In this process, coal samples were screened. Because of high vibration frequency, high screening speed and high efficiency. When using the vibrating screen separator, the motion intensity and inclination angle of the screen body should be adjusted reasonably. If the vibration frequency of the sieve body is too strong, the movement speed of the coal sample will be accelerated, the chance of passing through the sieve will be reduced, and the separation will be incomplete. The vibration of the sieve body is too weak, and the coal sample can not be fully dispersed, which is not conducive to screening. The inclination angle of the screen body is too large, the discharging speed is fast, and the processing capacity is strong, but the screening is not thorough; The inclination angle is too small, and the screening is complete, but the discharging is slow and the processing capacity is reduced. Due to the increase of coal sample screening time, some particles will be broken, which will affect the authenticity of the yield of each particle fraction in the screening test. Generally, it is appropriate to adjust the inclination angle of vibrating screen to 12.5 ~ 17.5. If necessary, the following methods can be used to check whether the screen is clean, so as to reasonably determine the main parameters (inclination angle, etc.). ) mechanical screen.

The method is as follows: after the coal sample is sieved in the required sieve, part of the oversize is re-sieved. If it meets the requirements in the following table, it is considered clean. If the sieve size is greater than the requirements in the table below, all coal samples of this size should be re-screened.

Coal sample screening parameters:

Mesh size (mm) 50 25 13 6 3 0.5

Feeding amount (kg/m2) 10 10 5 5 5 5

Shake times (times) 2 3 6 6 10 20

Screen size (%) < 3 <; 3 & lt3 & lt2 & lt2 & lt 1.5

4 operator skills

The technical quality of operators plays a key role in the screening test. Operators should have solid theoretical knowledge and rich practical experience, skillfully operate correctly according to national standards, and formulate corresponding test plans according to different production conditions, so as to make efficient and reasonable use of various production resources and minimize various factors that adversely affect the test.

Coal screening test is a complicated operation process. The number of coal samples tested varies from 1~2t to10t, and it will take many people several days to complete. Screening is the core work of testing. Only by grasping the screening process can we accurately reflect the particle size composition of coal and the quality characteristics of coal samples with different particle sizes, and make the test results accurate and reliable.

(Excerpted from Northeast Asia Coal Trading Center)