Screening is a method of sorting according to the particle size of a material by using a sieve surface equipped with a certain size of sieve holes and a certain movement form. At present, there are many types of screening equipment on the market. In the process design and process transformation, the correct selection of screening equipment in the cleaning process is the key to ensuring the cleaning effect.
1 sieve surface and sieve hole selection
Common cleaning screens: punched screen and woven screen. Punching screen surface, high strength, screen hole is not easy to deform, easy to install, but the screen surface opening rate is low. Under normal circumstances, the impact force is large due to the large flow rate of the cleaning equipment. Punching screen surface is easy to reach the rigidity requirement, generally more choices. The screen surface is woven, the opening ratio of the screen surface is high, and the screening efficiency is high, but the screen hole is easy to be deformed, the installation is difficult, the installation is easy, the strength and the durability are poor, and thus the cleaning process is less used.
Common mesh shapes are: circular mesh holes, oblong mesh holes and rectangular mesh holes. The circular sieve holes are separated according to the width of the wheat and the impurities, such as a vibrating screen. The oblong mesh is separated according to the thickness of wheat and impurities, such as a flat rotary screen.
Mesh hole shape and size: First, according to experience, refer to the commonly used sieve holes. The second is determined by the screening curve of wheat and impurities. The screening curve is a set of test sieves composed of the same hole shape and different sieve size sieve faces, and the sample is screened by a certain sieve movement form, and the sieve hole size of each sieve face is the abscissa, The percentage of the sieve on the sieve surface to the total weight of the sample is plotted on the ordinate and plotted. A circular sieve hole of different sizes is selected to form a set of sieve faces, and the sample is sieved.
The use of a 2.4mm diameter mesh screen can screen out most of the small impurities, and the 4.0mm diameter mesh can screen out most of the large impurities. The circular sieve holes are graded according to the width, and it is difficult to distinguish the overlapping parts of the particle size. It is considered to use the long mesh holes, classify the thickness, make a particle size curve, compare and select the most suitable mesh shape and size. For the overlap of the granularity, other methods can be considered to continue cleaning.
In actual production, due to the large output and many influencing factors, when selecting the mesh size according to the sieving curve, it should be enlarged by 0.1-0.2mm than the size in the sifting curve.
2 screening equipment movement form
The movement modes of the screen surface are: longitudinal reciprocating motion, lateral reciprocating motion, vertical plane inner circular motion, up and down reciprocating motion, horizontal plane inner circular motion and rotational motion. The commonly used forms of wheat cleaning screens are: longitudinal reciprocating motion (such as vibrating screen), circular motion in the horizontal plane (such as plane rotary screen), and rotary motion (such as cylindrical primary screening). The movements of common materials on the screen surface include: spiral sliding motion (such as plane rotary screen), vertical linear reciprocating sliding motion (such as vibrating screen), and a special form of motion. The feeding end is a spiral sliding motion. As the discharge end gradually decreases with the radius of the transverse helix, it gradually evolves into a longitudinal linear reciprocating sliding motion (such as a plane rotary vibrating screen).
3 selection of screening equipment and its characteristics
Commonly used wheat cleaning and screening equipment are: cylinder primary clear sieve, vibrating screen, plane rotary screen, plane rotary vibrating screen.
3.1 cylinder primary cleaning screen
The impurity-removing object is an extra large impurity. Such as large stones, ropes, straws, etc. in wheat. The screen cylinder is used for rotary motion and is generally used in the receiving work tower of raw materials. It is used to protect various types of equipment before the wheat enters the warehouse and enters the warehouse. The cylinder clearing structure is simple, the self-discharging capacity is good, the sieve hole is large, and the processing capacity is large.
3.2 vibrating screen
Can clean large and small impurities can also be used with the air selector to remove light impurities. The screen surface of the vibrating screen is longitudinally reciprocating, and the material is longitudinally reciprocating and sliding on the screen surface. Compared with the plane rotary screen, the moving line is short, and the automatic classification performance is better than that of the flat rotary screen, but the material is in the sieve. The average speed of the surface movement is small, and the material is easy to perforate. This feature determines the vibrating screen to remove large impurities. It is generally used as the first screening equipment in the processing stage of wheat, mainly used to remove large impurities and some small impurities. Amplifying the mesh hole can also be used as a primary clearing screen to remove extra large impurities, but it is not as good as the primary clearing filter of the cylinder. For example, straw, hemp rope, etc., it is easy to block the screen surface and is not easy to discharge by itself.
3.3 plane rotary screen
It can be used in addition to large and small impurities, and can also be used with air selectors to remove light impurities. The plane rotary screen surface is used for the rotary motion, and the material is spirally moved on the screen surface. Therefore, the material moving on the screen surface is long, and it is easy to form the body movement classification, and the material has many opportunities for perforation, so the effect is small except for small impurities. . It is generally used in the wheat processing stage and the light wheat processing stage, and is mainly used in addition to small impurities after the wheat-making equipment.
3.4 plane rotary vibrating screen
In addition to the ability of large and small impurities are relatively strong, can also be used with the air selector, to remove light impurities. The device has the advantages of a vibrating screen and a flat rotary screen. The main feature is that the balance wheel that produces the rotation is placed at the front of the screen body, the motor drives the balance wheel and the screen body to rotate together, the rotation center is offset to the front part, and the rear part has a set of compression spring devices for limiting lateral vibration, the sieve body The unique spiral movement mode in the front section leads to the longest movement path of the feeding end to the screen surface, which makes the material form a good automatic classification. The small particle size material fully contacts the screen surface, which is beneficial to the rapid perforation of more small particle size materials; The rear section gradually evolves from a spiral to a reciprocating motion. The average speed of the material is reduced, and the chance of pause relative to the screen surface is increased, which is beneficial to the sufficient perforation of the small-grained material, thereby improving the screening efficiency. Planar rotary vibrating screen has better ability to remove small impurities than vibrating screen, and is better than flat rotary screen; except for large miscellaneous capacity, it is slightly worse than vibrating screen, but stronger than flat rotary screen. Generally used as two screens and three screens, the effect is better than the plane rotary screen.
4 Reasonable selection and arrangement of screening equipment
In actual production, the cleaning process of “three screens, two dozens, two stones, one selection, one water, three magnetic selections†is generally used. The reasonable combination of “three screens†is: the head screen is a vibrating screen (such as TQLZ type), the second and third screens are preferably flat vibrating screens (such as TQLMZ type), or flat rotary screens (such as TQLM type). .
Reasonable selection and arrangement of screening equipment is the prerequisite for obtaining good cleaning results, and is an important guarantee for the production of qualified products. It directly affects the economic benefits of enterprises. This is a problem worthy of attention.