In the actual screening process, particles smaller than the mesh are required to enter the sieve and are affected by many factors. To sum up can be divided into three areas: the material properties, device performance and screening operations management sieving machine.
(1) The nature of the material that affects the screening process mainly includes: the relative size of the particles and the mesh, the moisture and mud content of the material.
1. Relative size of the granules and the mesh holes Only when the particles are smaller than the mesh holes, the contact with the mesh holes is repeated several times. There is a chance to be screened. The probability of particle sieving depends primarily on the ratio of the projection of the particle cross section to the screen area on the screen plane. It is proved by sieving theory and practice that particles with a particle size smaller than 3/4 of the mesh size are called "easy granules". Particles with a particle size greater than 3/4 of the mesh size are referred to as "difficult sieves". The closer the diameter of the particles is to the mesh size. The difficulty in sifting through it is greater. For the layer of material formed on the sieve surface by 1.5 times the diameter of the sieve hole, the effect of the easy sieve and the difficult sieve on the screen surface is not large, and the diameter is larger than the sieve hole but smaller than the sieve. The coarse particles having a pore size of 1.5 times cover the sieve surface during the sieving process, block the sieve pores, and hinder the movement of the particles toward the sieve surface. The particles of this particle size are generally referred to as "obstruction particles".
Among the raw materials, the higher the content of the granules and the hindering granules, the lower the sieving rate.
2. Moisture material contained in the water can be divided into two types: one is inherent moisture present in the pores of the material; the second is external moisture, attached to the material surface moisture, such as water, raw coal mining, underground water spray off the dust, open-air Rain when it is stacked, etc. The external moisture content is proportional to the surface area of ​​the material. Due to the large specific surface area of ​​the fine-grained material, the fine particles contain a high external moisture. The internal moisture has no effect on the screening process, and the external moisture has a great influence on the screening process.
The influence of external moisture on the screening process can be proved by the following experiments: the former Soviet Union scholars used the conventional vibrating screen and the sieve hole to be 6,13,25mm for the dry coal classification test. The relationship between the original coal moisture and the screening efficiency is shown in the figure. As shown in Figure 1, when the raw coal moisture increased from 4% to 6%, the screening efficiency decreased not significantly, about 3% to 5%; when the moisture exceeded 6%, the screening efficiency decreased sharply; the moisture increased from 6%. At 8.5%, the sieving efficiency is reduced by an average of about 20%. At this time, the sieving efficiency by 6 mm is only 40%.
Figure 1 Relationship between raw coal moisture and screening efficiency
1 – 25 × 25mm mesh holes; 2 – 13 × 13mm mesh holes; 3 – 6 × 6mm mesh holes
The relationship between raw coal moisture and treatment capacity is shown in Figure 2. When the moisture content exceeds 6%, the treatment capacity per unit area is significantly reduced.
According to the statistical study of the former Soviet scholars: the working state of the vibrating screen is unchanged, and the screening efficiency is maintained at 90%~95%. The raw coal moisture starts from 5.5%, and for every 1% increase, the screening machine handles 6 × 6, 10 × 10,13 × 13,25 × 25mm mesh size were reduced by 25%, 12%, 10% and 5%. [next]
From the experimental research and practice, the following conclusions can be drawn: when the water is within a certain range, the moisture increases, and the screening efficiency decreases sharply. This is because the particles on the surface of the material are bound to each other due to cementation or surface tension, or The fine particles stick to the surface of the large particles, or paste on the sieve wire to block the sieve holes.
Figure 2 Relationship between raw coal moisture and unit area treatment capacity
1. The screening efficiency is 85%; 2. The screening efficiency is 95%.
The range of moisture affecting the screening process varies depending on the nature of the material. For example, clay , fertilizers and chemical products allow very low moisture, even if only 1% of water is seriously affected. When the coal is sieved by dry method, when the water content is less than 4%, the impact is small. Generally, the moisture content is in the range of 6% to 13%, and the screening efficiency is most obvious. The higher the viscosity between materials, the more difficult it is to screen. Of course, different coal types have different porosity and the situation is different. When the water exceeds a certain value, the particle activity increases the moisture and promotes the sifting of the particles, and the screening efficiency rises again (Fig. 3).
Figure 3 Effect of material moisture on screening efficiency
1. a material with weak hygroscopicity; 2. a material with strong hygroscopicity
The moisture of the particles has different effects on different mesh sizes. It can be seen from Fig. 1 that when the water content is 8.5%, the screening efficiency of the 6 × 6 mm sieve hole is only 40%; and the screening efficiency of the 25 × 25 mm sieve hole is still above 90%. Moisture has a smaller effect on the size of the mesh, and the effect of humidity is particularly significant. Because the sieve hole is too large to paste the screen, the sieve hole is small, and the sieve hole can be killed in an instant, so that the screening cannot be performed. When the coal is screened by dry method, the ordinary sieving method cannot be used for classification of 13 mm. In recent years, the use of approximate screening such as probability sieves has solved the classification below 13 mm.
The mud content is high, that is, there are many clay-like minerals, and the viscosity of the particles is large, and even if the water is low, the particles are easily bound into a mass and block the mesh. [next]
(II) Influence of screening equipment performance and process parameters on the screening process Although the material properties have a great influence on the screening process, different screening effects can be obtained with different types of screening equipment. This mainly depends on the process parameters of the screening equipment: the screen movement form, the screen length, the width, the screen inclination, the mesh shape and the like.
1. Screen movement form The screen movement form is one of the important factors affecting the screening effect. Practice has proved that when the screen surface is fixed, the screening efficiency is very low. Even the moving screen surface is related to the form of its movement. For example, on the surface of the vibrating screen, the particles are used for the projectile movement, the material is thrown high, the material layer is loose, and the vibration frequency is high, which is favorable for the fine particles to be sieved, so the vibrating sieve has a good screening effect. On the shaking screen surface, the particles mainly slide along the screen surface, and the frequency of the shaking screen is lower than that of the vibrating screen, so the screening effect is worse than that of the vibrating screen. Rotating cylinder screen, the screen hole is easy to block, and the screening efficiency is low. Shake screens and cylinder screens are currently used less.
Studying the form of motion of the sieve surface is one of the important ways to solve the classification of fine-grained hard-to-screen materials such as viscous fine-grained materials or high-moisture coal. For example, when studying the depth screening of the vibration probability sieve, the lowermost sieve surface used as the fine particle classification is not fastened due to looseness, and the sieve surface generates secondary vibration with respect to the sieve box during the movement of the sieve box. At this time, the wet coal was classified by 3 mm, and there were few holes and paste holes in the continuous production for two days. When the sieve surface and the screen box are fastened, the sieve is less than half an hour, and the sieve paste can be instantaneously formed due to the formation of a group of moist materials. It has been proved by experiments that the secondary vibration of the sieve surface is effective for improving the screening efficiency and preventing the plugging and the paste hole. However, the secondary vibration affects the service life of the conventional screen surface. Relaxation sieve is the use of the sieve surface to relax, not only to play the role of projectile material, but also because the sieve hole is continuously relaxed during the screening process, preventing the fine grain bonding and clogging phenomenon, when the moisture in the coal is as high as 10% or more It is also possible to separate 0~6mm grade fine coal from anthracite.
2. Screen Dip Angle The angle between the screen surface and the horizontal plane is called the screen inclination angle. In order to facilitate the discharge of the screen, the screen surface is generally installed obliquely. The size of the dip is closely related to the productivity of the screening equipment and the efficiency of the screening. The dip angle is large, the particle group moves forward fast on the sieve surface, and the production capacity is large; however, the residence time of the material on the sieve surface is shortened, the particle permeability opportunity is reduced, and the screening efficiency is affected.
The size of the screen surface is related to the form of screen movement. The linear motion screen has a throwing force that moves forward along the screen, so this type of screen has a small angle of inclination. For a sieving machine with a circular vibration, the angle of inclination should be larger.
When the screen surface is placed obliquely, the area of ​​the screen through which the particles pass is actually the projected value of the area of ​​the screen on the horizontal plane. As shown in Fig. 4, the sieve opening length is L, and the particle diameter d which can pass through the sieve hole unimpeded is:
Figure 4 Schematic diagram of the particles passing through the inclined sieve mesh
Obviously, the larger the inclination angle of the sieve surface, the narrower the sieve passage of the particles, but the size of the sieve hole is actually constant. Using this principle, when sieving difficult materials, large sieve holes and large inclination angles can be appropriately used to separate the materials at a fine particle size. Probability screening is to use this principle.
3. Width and length of the screen surface Generally speaking, the width of the screen surface directly affects the productivity; and the length of the screen surface directly affects the division efficiency. That is, the sieve surface is long, the material stays on the sieve surface for a long time, and the sieve has many opportunities, so the screening efficiency is high.
In fact, the screen width is for the screening efficiency; the screen length also has an effect on the screening capacity. Generally, the width to length ratio is 1:2~3. As shown in Fig. 5, as the screening time increases, the screening efficiency starts to increase rapidly, and after a certain period of time, the screening efficiency increases are not obvious. [next]
Figure 5 Relationship between screening efficiency and screening time
4. Mesh shape The shape of the mesh is varied. The choice of mesh size depends primarily on the size of the screened product and the requirements for the use of the product under the screen. The circular sieve holes have smaller particle size through the circular sieve holes than the other sieve holes, for example, the maximum particle size of the particles passing through the circular sieve holes is only transmitted through the same 80% to 85% of the size of the square mesh particles.
The particle size through the rectangular mesh is larger than the particle size of the circular and square mesh through the same size.
The sieve surface of the rectangular sieve hole is compared with the square or circular sieve surface, and its advantages are: the effective area of ​​the sieve surface is large, the quality of the sieve surface is light, the production capacity is large, and the water treatment is treated. When the material is used, it can reduce the blockage of the screen surface. The disadvantage is that the size of the product under the sieve is not uniform, and some sheet-like and strip-shaped particles are easily transmitted through the rectangular mesh.
The mesh size of different shapes, the relationship between the maximum particle size of the sieved product and the size of the mesh is:
d max =K•L
Where d max - the maximum particle diameter (mm) in the sieve;
L — mesh size (mm);
K - coefficient, He-shaped hole: take K = 0.7; square hole: K = 0.9; rectangular hole: K = 1.2 ~ 1.7; sheet, strip take a large value.
The shape of the mesh is mainly determined according to the purpose of the screening operation.
(3) Impact of operation management on screening results
Operation management of the screening machine: The feeding material should be cleaned and repaired in a uniform, continuous and timely manner to ensure the normal operation of the screen surface.
The sieving machine requires uniform continuous feeding, and the material is filled into a suitable equal thickness layer along the entire width of the screen surface. This not only makes full use of the screen surface, but also facilitates the screening of the material, thereby achieving higher productivity and screening efficiency. However, if the granules are not uniform and the layer is too thin, the treatment amount is low; the layer is too thick, the fine particles are too late to pass through the sieve, and remain on the sieve surface, affecting the screening efficiency. Even when the load is overloaded, the material is not graded and the screener becomes a transporter.
The thickness of the material layer varies depending on the size of the mesh hole. Generally, the thickness of the material layer does not exceed 4 times the size of the mesh hole.
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