# Vibrating screen working principle & screening efficiency

## Vibrating screen working principle

Screening is the process of separating a mixture of different particle sizes of materials into several different particle size levels by means of a single or multi-layer screen. The screening process of loose materials consists of two stages.

1. The particles that can easily pass through the holes of the screen reach the screen surface through the material layer composed of particles that cannot pass through the holes of the screen.
2. The particles that can easily pass through the screen holes pass through the screen holes.

To make these two stages can be achieved, the materials on the screen surface should have appropriate movement, on the one hand, the materials on the screen surface should be in a loose state, the material layer will produce separation (stratification by particle size), large particles are located in the upper layer, small particles are located in the lower layer, easy to reach the screen surface, and pass through the screen hole. On the other hand, the material and the movement of the screen promote the blocked particles separate from the sieve surface, which is conducive to the particles pass through the sieve hole.

Practice shows that the particle size of the material is less than three-quarters of the sieve hole, it is easy to pass the gap formed by the coarse material, to reach the sieve surface, it is quickly pass through the sieve hole. This particle is called “easy to sieve particles”. The particle size of the material is greater than three-quarters of the sieve hole, is difficult to pass through the gap composed of coarse particles, the closer the diameter of such particles to the size of the sieve hole, the mmineral difficult it is pass through the sieve hole, therefmineral, this particle is called “difficult to sieve particles”.

The possibility of mineral particles passing through the sieve hole is called sieving probability. Generally speaking, the probability of mineral particles passing through the sieve hole is affected by the following factors
1. the size of the sieve hole
2. the relative size of the mineral particles and the sieve hole
3. the effective area of the sieve
4. the angle made between the direction of movement of the mineral particles and the sieve surface
5. the moisture content and mud content of the mineral material.

In the loose materials, those particle size is much smaller than the screen hole size will fall down the screen easily after the start of sieving. The particle size which is closer to the sieve hole size takes longer time to pass through the sieve hole. Therefore, the speed of the material pass through the sieve hole in the screening process depends on the ratio of the particle diameter to the size of the sieve hole.

## Screening efficiency

When using a sieve, it is required both to have a large handling capacity and to sieve as much as possible of the fine-grained material smaller than the sieve hole into the product under the sieve. Therefore, the sieve has two important process indicators: one is its processing capacity, that is, the sieve hole size of a certain number of tons of material per square meter of screen surface area per hour (tons / m ².hour), it is an indicator of the number of sieving work. The other is the screening efficiency, which is the quality indicator indicating the screening work.

In the screening process, it is reasonable to say that the fine particles that are smaller than the size of the screen hole should all pass through the screen hole, but in fact it is not so, it depends on the performance and operation of the screening machinery and the material moisture content, mud content, etc.. Therefore, there is always a part of the fine sizes can not pass through the screen hole to become the product under the screen, but discharge together with the product on the screen. Over sieve products, the more amount of fine particles do not pass through the screen hole, the worse the effect of sieving, in order to assess the complete degree of sieving from the number, it needs to use the index of sieving efficiency.

The so-called sieving efficiency refers to the ratio of actual under size materials weight and the weight of the material with a particle size smaller than the size of the screen hole contained in the screen material.

### Factors affecting sieving efficiency

The first type of influencing factors – the nature of the material

#### 1. Particle size characteristics of the material

The composition of sieved material particle size has a decisive impact for the sieving process, it can be seen from the sieving practice that the smaller the particles less than the sieve hole, the easier they can pass through the sieve hole. Particles as large as three-quarters of the sieve hole, although smaller than the size of the sieve hole, it is difficult to penetrate the sieve. Diameter of the materials which are slightly larger than the sieve hole, often cover the sieve wire, preventing fine particles pass through. Materials diameter in 1-1.5 times of the sieve hole, they form the material layer which is not easy to let the “difficult to sieve particles” pass through. However, the diameter of the particles in 1.5 times of the sieve hole form the material layer, they are not very influential to the “easy to sieve particles” or “difficult to sieve particles” pass through. Therefore, there are three particle size limits in the material worth noting.

-Particles smaller than three-quarters of the sieve size are called “easy to sieve particles”.
-Less than the size of the sieve hole but more than three-quarters of the size of the sieve hole is called “difficult to sieve particles”.
-The particle size of 1-1.5 times the size of the sieve hole is called “obstructed particles”.

Obviously, the more materials containing “easy to sieve particles” the easier to sieve. Therefore, when increasing the “easy to sieve” content of the material, the productivity of the sieve increases rapidly, or, in the case of maintaining a certain productivity, it can get a higher screening efficiency. The reason is that when the “easy to sieve grain” content of the material are sent to the sieve, the fine particles will be easy to pass through the holes quickly. The amount of material left on the sieve is reduced, even if there are some “difficult to sieve grain” on the sieve, because the degree of influence of other particles on them is relatively reduced, so it is also conducive to part of the “difficult to sieve particles” to pass through the screen hole. On the contrary, when the original material contains more “difficult to sieve particles” and “obstructing particles”, because they prevent the fine particles and sieve surface contact for pass through the screen hole, so the screening efficiency is reduced.

When the content of fine particles in raw materials is small, and the sieve itself is too coarse, its particle size greatly exceeds the size of the sieve hole, it can take the method of increasing the auxiliary screening, with the sieve hole size larger auxiliary sieve, pre-discharge the coarse particles product on the sieve, and then sieve containing a large number of smaller materials, relatively speaking, it is completely reasonable. In the conditions of two stages of screening, it can improve the screening efficiency and extend the screen wire mesh life.

The maximum allowable size of the material particles and the size of the screen hole between a certain proportion of the relationship is not clearly defined, it is generally believed that the maximum particle size should not be greater than 2.5-4 times the size of the screen hole.

When accurately calculating the productivity of a vibrating screen, it is necessary to determine the content of particles in the feed ore that are smaller than one-half of the screen hole size and the content of particles larger than the screen hole size. This is because they affect both the productivity and the screening efficiency.

#### 2. Water content and mud content of the sieved material

There are two kinds of moisture contained in the material, one is called external moisture, which is on the surface of the particles; the other is called internal moisture, which is in the pores and cracks of the material. The latter has no effect on the sieving process.

The surface moisture contained in the material increases to a certain extent, and the stickiness increases accordingly. The surface moisture of the material can make the fine particles bond to each other and attach to the large particles, and the sticky material will also block the sieve hole. These reasons make the sieving process more difficult and the sieving efficiency will be greatly reduced.

Screening the same material with the same moisture content but different sieve holes, the effect of moisture on the screening efficiency is different. The larger the size of the sieve hole, the smaller the effect of moisture. This is because the larger the size of the screen hole, the smaller the possibility of blocking the screen hole. In addition, the more important reason is because the distribution of moisture in each particle size is not uniform, the smaller particle size, the higher the moisture content. Therefore, when the sieve hole is large, it can quickly sieve out the high moisture content of the fine particles, the sieve material moisture is greatly reduced, so that it does not affect the sieving process. Therefore, when the material moisture content is high and seriously affect the sieving process, it can consider using the appropriate method to increase the sieve hole to improve the screening efficiency.

If the moisture content of the material reaches a certain range, the screening efficiency decreases sharply. This range depends on the nature of the material and sieve hole size. After the moisture contained in the material exceeds this range, the activity of the particle is re-improved, and the viscosity of the material disappears instead, at this time, the moisture has the role of promoting the material through the sieve hole, and gradually reach the conditions of wet screening.

If the material contains sticky materials that are easy to agglomerate (such as clay, etc.), even when there is little moisture, it will be bonded into lumps, so that the fine mud mixed into the product on the screen; in addition, it will also quickly block the screen hole.

When screening cohesive ore, effective measures must be taken to strengthen the screening process, such as wet screening, or pre-washing before screening to remove the mud.

#### 3. The shape of the material particles

If the material particles are round, it is easier to pass through the square and round holes. If the crushed products are mostly polygonal, passing through the square hole and round hole is not as easy as passing through the rectangular hole. Strip, plate, sheet material is difficult to pass through the square hole and round hole, but easier to pass through the rectangular hole.

### The second type of influencing factors – the type of screen surface and working parameters

#### 1. Types of sieve mesh

There are usually three kinds of sieve mesh, those made of steel rods, steel wire woven and steel plate perforated. Their influence on the sieving efficiency is mainly related to their effective area.

The larger the effective area of the screen surface, the more the area occupied by the screen holes, the easier for the ore particles pass through the screen hole, screening efficiency is higher, but the life is shorter. What kind of sieve surface is chosen, should be considered with the actual situation. When the wear and tear is serious and becomes the main conflict, it should use wear-resistant bar sieve or steel plate perforated sieve; when the need for fine screening occasions, it is necessary to use woven wire sieve.

#### 2. Sieve hole shape

The choice of sieve hole shape depends on the requirements for the size of the screen product and the capacity of the sieve. Round sieve hole compared with other shapes of sieve hole, in the case of the same nominal size, pass through this sieve hole under the sieve product particle size is smaller. It is generally accepted that the maximum particle size of particles pass through a round aperture is actually only 80-85% of the particle size through a square screen aperture of the same size on average.

The screen surface of rectangular sieve hole has a larger effective area and higher production capacity; when dealing with materials containing more water, it can reduce the blockage phenomenon of the screen surface. Its disadvantage is easy to make the strip and flake particles through the screen hole, so that the product under the screen is not uniform, therefore, when the requirements of the sieve does not contain fine powder, the sieve allows the strip and flake particles, the material is wet and sticky easy to cause blockage, and expect that the product under the screen is more, etc., the use of rectangular screen hole is more favorable.

When choosing the type of sieve hole, it is best to be suitable with the shape of the material, such as dealing with lumpy materials should use square sieve hole, dealing with plate materials should use rectangular sieve hole.

#### 3. Sieve hole size

The larger the sieve hole, the higher the productivity of the unit screen area, screening efficiency is also better, but the size of the sieve hole depends on the purpose and requirements of the use of sieving. If expect the over sieve product contains less fines, it should use a larger sieve hole; on the contrary, if require the product under the sieve with smaller size, the sieve hole should not be too large, with the specified particle size as the limit of the sieve hole width.

#### 4. The movement condition of the sieve

Although the quality of sieving is first determined by the nature of the sieved material, the same material with different types of sieve sieving can also get different results. Practical experience points out that the fixed sieve, its sieving efficiency is very low, as for the movable sieve, its sieving efficiency and the sieve body movement. If the sieve body is vibrating, the particles are shaken on the sieve surface in the direction close to the vertical sieve hole, and the vibration frequency is high, so the best sieving efficiency. On the shaking screen surface, the ore particles are mainly sliding along the screen surface, and the frequency of shaking is smaller than the frequency of vibration, so the effect is poorer than that of vibrating screen, rotating cylindrical sieve, the sieve hole is easy to block, and the sieving efficiency is not high.

Even if the same nature of the movement of the sieve, its sieving efficiency is different according to different movement intensity. The movement of the sieve can make the material in the sieve surface spread out, which is conducive to fine particles pass through the material layer through the sieve hole, sieving efficiency is therefore improved. But the material along the sieve surface movement speed, and sieve body movement intensity, the sieve surface movement intensity is too large, the material on the faster movement, the particles through the sieve hole less chance, the effect is poor. If the strength of the movement of the screen surface is too small, the material on it can not be dispersed, but also unfavorable to the fine particles through the screen hole.

#### 5. Length and width of the sieve

In production practice can be experienced: for a certain material, productivity depends mainly on the width of the screen surface, sieving efficiency depends mainly on the length of the screen surface. The longer the sieve surface, the longer the material is sieved on the sieve, the higher the sieving efficiency. Initially, a slight increase in sieving time, there are many “easy to sieve particles” through the sieve hole, sieving efficiency will soon increase. Later, most of the easy to sieve particles were sieved, leaving some difficult to sieve particles, although the time to grow, they are sieved down not much, sieving efficiency is not increased. Therefore, the sieving time is too long is also unreasonable. Because when the screen inclination is certain, to increase the screening time, only to increase the length of the screen surface. The screen surface is too long is not good, wasting plant space, the sieve construction is bulky, and the screening efficiency is not increased much, so the sieve length must be appropriate. Only in the high load work sieve, in order to ensure a higher screening efficiency, if the configuration conditions permit, appropriate increase in sieve length, sometimes is beneficial.

The width of the screen surface must also be appropriate, and must maintain a proportional relationship with the screen length. When the sieve load is equal, the sieve width is small and the length is large, the material layer on the screen surface is thick, and the fines are difficult to approach the screen surface and through the screen hole. On the contrary, when the sieve width is large and length is small, the thickness of the material layer is certainly reduced, the fines are easy to approach the sieve surface, but due to the short stay of particles on the sieve surface, the material through the sieve hole opportunity is less, sieving efficiency is bound to be reduced. It is generally believed that the width of the sieve and the length of the ratio of 1:2.5 to 1:3.

#### 6. Inclination of sieve surface

In general, the sieve is installed at an angle to facilitate the discharge of materials on the screen, but the angle of inclination should be appropriate. Angle is too small, can not achieve this purpose; angle is too large, the material is discharged too quickly, the material is shortened by the sieving time, screening efficiency is low. The larger the angle of inclination of the screen surface, the greater the obstruction to the passage of the ore particles. Therefore, the angle of inclination of the screen surface should be appropriate. In fact, the inclination angle of the common vibrating screen in mineral processing plant is generally 0 degrees to 25 degrees. The inclination angle of fixed bar screen is generally 40 degrees to 45 degrees.

### The third category of influencing factors – operating conditions

#### 1. Uniform and continuous feeding

Uniformly and continuously feed the material into the sieve, so that the material is filled into a thin layer along the whole width of the sieve, which not only makes full use of the sieve surface, but also facilitates the fine particles to pass through the sieve holes, so it can ensure to get high productivity and sieving efficiency.

#### 2. Feeding quantity

Feeding volume increases, the production capacity increases, but the screening efficiency will gradually reduce, because the sieve overload. When the sieve is overloaded, it becomes a chute, which actually only plays the role of transporting materials. Therefore, for the screening operation, both high screening efficiency and large capacity are required, and the one-sided pursuit of one side, while the other side is greatly reduced.