Operation mechanism and selection of vertical mill

Date: 2019-07-08 Views:

At present, the new dry method technology of cement is developing vigorously, and low-energy vertical mill is widely used in the grinding of raw materials, pulverized coal and slag.

I. basic form of vertical mill

The grinding process of the vertical mill is completed by a pair of grinding device, and the material is ground into powder between the roller and the grinding disc.The motion of the grinding device is rotated by the grinding disc and correspondingly drives the roller to rotate. The pressure required for grinding is mainly pressurized by a set of hydraulic devices in addition to the gravity of the roller itself.After milling materials because of the presence of a large number of coarse powder, so must be selected powder.The grinding process is completed by the matching powder separator. After the material leaves the mill, it is carried into the powder separator by air flow. In the process of air flow rising, most of the coarse grains have fallen to the mill, and the rest of the finished products are selected by the powder separator.

II.Mechanism of vertical grinding

1. Movement track of materials on the grinding disc

Materials fed to the center of the disc are thrown out by centrifugal force during the rotation of the disc, and the materials are crushed by grinding roller during the movement from the center to the edge of the disc.Therefore, the movement path and time of materials from the center to the edge of the plate are very important.The material passes through the grinding disc quickly, and there is an appropriate residence time on the grinding disc. It has both an appropriate passing quantity and an appropriate fine powder quantity, so as to achieve the ideal grinding effect.Therefore, it is necessary to set the corresponding and appropriate rotary speed of the grinding disc according to the requirements of the form of grinding device, so as to obtain an appropriate material residence time.

The residence time of the material on the grinding disc is determined by three factors.One is the speed of grinding disc which affects the size of centrifugal force;Second, the friction force between the material and the grinding disc and the friction force inside the material;The third is to decide the structure and form of the grinding disc and the grinding roller of the running stroke mode and the cooperation between them, which is a very important factor.

2. Stability of vertical abrasive bed

The grinding of the vertical mill mainly depends on the crushing between the materials of the extrusion bed, and whether the extrusion can be realized depends on whether the extrusion pressure of the grinding roller can be effectively transferred to the material.Stable bed is the first important factor.How to form a stable bed, there are many factors at work.

(1) internal friction of materials.

In fact, it is the cohesive force between the particles, compared with the roller press, because the vertical mill has no surrounding walls, so there must be enough cohesive force, when the external force overstock, the material particles do not send out to the surrounding, destroy the material bed.Therefore, the vertical mill can withstand much lower pressure than the roller press.Determine the size of the friction force in the material, depends on the physical properties of the material, particle size composition, moisture and so on.

(2) physical properties of materials.

The physical properties of materials are determined by their mineral composition, and the physical properties play a decisive role in the magnitude of internal friction. Due to different physical properties, the adaptive range of vibration varies greatly due to the change of bed thickness.In the allowable vibration intensity, the slag has a very narrow bed thickness range, while the raw material and coal have a wide bed thickness range, while the raw material and coal have a wide bed thickness range, but they are not exactly the same.Therefore, the vertical mill used for pulverizing bituminous coal is completely different when it is changed to pulverizing anthracite.This difference is due to the difference of internal friction between them, which is also one of the reasons why the test mill must be used to determine the performance of the mill.

(3) particle size composition.

The particle size composition is very important to the stability of the material layer.According to the test results, when the material layer contains 10%-15% 75um particles, the material layer has good stability.When the fineness is not enough (too coarse), the mill shows high frequency vibration;When the material layer is too fine, if the particles below 75um account for more than 20%, the "wave effect" will appear. The material layer becomes very easy to flow and cannot support the grinding roller, which starts to slide and plough the material, without grinding effect and strong vibration.In order to ensure the stability of the material bed, the vertical grinding requires a certain number of large particles to be fed into the material, the size of which is related to the diameter of the roller. Under normal circumstances, the diameter of the feeding material can be between 0-4% and the diameter of the roller, and the maximum allowable diameter is less than 5% of the diameter of the roller.This is completely different from ball mill. The finer the material particle, the higher the ball mill yield.However, for hard materials, the vertical mill also does not want too large particle size, so for hard sandstone, even very hard limestone are required to feed into the mill with relatively small particles.In addition to the fine powder on the bed produced in the direct grinding, there is a separator of the feed.Therefore, in order to avoid too much fine powder on the mill, it is necessary to use an efficient separator to reduce the feed.

(4) moisture content of materials.

Material moisture has a great impact on the internal friction of materials, water is also an important means to regulate the internal friction of materials, some mills need to spray water in front of the roller to increase the internal friction of materials.Dry material internal friction is relatively low, not conducive to the stability of the bed, so do not want to like the mill hot air temperature is too high, need to strictly control the temperature of the grinding gas at 80-100℃, in order to avoid drying too fast on the grinding disc, causing violent vibration of the mill.

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