6 factors affecting vertical grinding operation

Date: 2019-07-10 Views:

Superfine vertical mill


Although the development of vertical mill is relatively late, it has been widely used in the grinding of modern industrial raw materials due to its excellent technical properties. Especially the grinding of cement clinker and slag is more and more vertical mill. . Today we share several factors that affect the operation and operation of vertical mills.

1, the characteristics of the grinding material


The characteristics of the material to be milled mainly refer to the hardness, particle size, water content and wearability (bond function) of the material.

(1) Hardness of grinding material


The hardness of the milled material is usually characterized by Mohs hardness (range 1 to 10), which differs depending on the type and content of inclusions in the ore. Generally speaking, the material has high hardness, poor wearability and high abrasion, so the hardness of the material is directly related to the product output and the service life of the wear parts of the mill.

(2) Grinding material granularity


The particle size of the material can be reasonably controlled according to the fineness of the finished product. In principle, it is “more broken and less grinding”. Vertical mill has a certain range of requirements on the particle size of the raw material. If the particle size is too large or too small, the gradation balance of the upper layer of the grinding disc will be destroyed, which is not conducive to the normal operation of the mill. If the feed particle size is too large, the primary grinding efficiency is reduced, the number of cycles of the material is increased, the grinding power consumption of the mill is increased invisibly, and the vibration of the mill is increased due to the lack of buffering of the fine particle material. In the operation encountered this problem should be appropriate to reduce the material, to stabilize the mill pressure difference and the layer. If the feed particle size is too small, the powdery material will increase. Due to the poor adhesion of the fine particles and the internal gas flow, the fluidization trend of the bed is obvious, so that the vertical mill can not effectively penetrate a large number of particles, resulting in material. The layer is unstable, causing the mill to vibrate.

In the non-metallic mineral powder industry, product fineness is generally relatively high. The control recommendation for the particle size of the feed should be smaller (<30mm), which is more conducive to the ultra-fine grinding characteristics of the vertical mill to maximize the production capacity. Based on the experience of commissioning, the maximum particle size of the milled material should not exceed 3% of the diameter of the mill roll, which can be appropriately controlled.

(3) Moisture content of the grinding material


The control of the moisture content of the milled material is critical to the stable operation of the vertical mill. Usually, the hot air system is not designed in the production line (the raw material moisture content is generally <3%), and the heat energy is mainly generated by the grinding, friction and air circulation of the materials in the mill. During the grinding process, the mill has a large number of finished products and unqualified fine powder that are not taken away by the airflow in time. If the water content of the raw material is too large, the new abrasive material will bond the fine powder on the material bed, resulting in the formation of the material on the grinding disc. The cake, in the case of continuous feeding, will cause the thickness of the grinding disc to be continuously thickened, so that the grinding roller cannot effectively crush and grind the material, and the mill generates vibration or vibration due to excessive load.

(4) Grinding material wearability


The grindability of the milled material is directly related to the mill's output, power consumption and the service life of the liner. The material is easy to grind, easy to be crushed and ground, and easy to produce ultra-fine powder; on the contrary, the material with poor wearability needs multiple grinding and large grinding pressure, increasing the power consumption of the grinding, and accelerating the sleeve and the liner. Wear and reduce its service life. The wearability of raw materials is generally reflected by the wearability index (Bonde work index). The smaller the value, the easier the material is to be ground.

At the same time, the feeding requirements of the vertical mill are stable and continuous, and the adjustment is required to gradually increase or decrease. If there is a large fluctuation in the feed amount, the dynamic balance of the material in the mill will be broken, causing fluctuations in the material layer, causing uneven grinding of the grinding disc material, which may easily cause the mill to vibrate. Generally, the thickness of the layer is 2%±20mm of the diameter of the grinding roller (the empirical rule of the cement raw material industry), and it can be controlled for the non-metallic mineral industry.

2, mill pressure difference


The mill pressure difference is one of the important parameters reflecting the circulating load of the material inside the mill.

The pressure difference of the mill is mainly composed of two parts, one is the local ventilation resistance at the vertical friction ring; the other is the resistance generated when the powder selection machine selects the powder, and the sum of these two resistances constitutes the pressure difference of the mill. Under normal working conditions, the internal pressure difference should be stable, that is, it is in a stable state in a range, which indicates that the amount of raw material entering the vertical mill and the amount of finished grinding products reach a dynamic balance. Once this balance is broken, When the internal circulation load changes, the pressure difference will suddenly increase or decrease. If it is not effectively controlled in time, it will affect the stable operation of the mill. There are many factors affecting the pressure difference of the mill, such as material wearability, feed amount, system air volume, grinding pressure, and mill speed. Almost all factors affecting the smooth running of the mill can be reflected on the pressure difference.

The increase of pressure difference indicates that the amount of raw material to be milled is larger than the amount of finished grinding, and the circulating load of the grinding increases. At this time, the current of the feeding hoist becomes larger and the amount of slag is increased. At this time, the limit device of the mill can judge that the layer is continuously Thicken. In order to ensure the fineness of the product, the separator speed is generally not adjusted (or the speed can be appropriately reduced without affecting the fineness). The usual practice is to reduce the material and temporarily cut the material. The pressure difference of the mill should be restored to a stable value, and then the feeding should be increased to a reasonable value. Or, within the allowable range of the main motor load of the mill, the material should not be reduced or broken, and appropriately increased. Rolling pressure increases the grinding ability, avoiding the “saturated grinding” of the mill, and generating vibration due to excessive load.

The decrease of pressure difference indicates that the amount of raw material entering the mill is less than the amount of finished grinding, and the circulating load in the mill is reduced. At this time, the current of the feeding hoist is reduced, and the amount of slag discharging is reduced. At this time, the grinding machine limit device can judge that the discharging bed is at Gradually thinning. At this time, measures such as feeding, decompressing, or reducing the amount of air can be taken to avoid the occurrence of "empty grinding" of the mill, and the material layer is too thin to generate vibration. According to the debugging experience, under the same working conditions, the finer the fineness of the product, the higher the pressure difference of the mill, and vice versa. The many operational parameters of the practice neutral mill are interrelated, and the dynamic coordination between the parameters should be emphasized during operation.

3, system ventilation


The maximum ventilation of the vertical mill system is generally calculated according to the mill's production capacity. Considering the appropriate excess of the air duct resistance, system leakage and other factors (5% to 10%), the maximum ventilation capacity and reasonable wind pressure of the vertical mill system can be obtained. This is also the basis for the selection of fans.

The SHM vertical mill operates under negative pressure, and a large amount of wind is required for material transportation, drying and grading. The proper internal ventilation volume is a necessary condition for the stable operation of the mill. The amount of ventilation during the operation of the mill directly affects the yield and fineness of the product. The ventilation is large, the wind speed in the mill increases, the ability to dry and convey materials is enhanced, the circulation inside and outside the mill is reduced, the coarse particles on the bed are increased, the material layer is thinner, the mill output is increased, and if the air volume is too large, it may result in The product fineness is unqualified (running coarse) or the product fine powder content is reduced (the number of cycles is small, the grinding time is short), the quality is degraded, the mill will also vibrate due to the thin layer of the material; the ventilation is small, and the wind speed in the mill is reduced. The drying and stripping capacity is weakened, the internal and external circulation of the mill is increased, the material layer is thicker, the grinding mill power consumption is increased, the product fineness is fine, but the mill output is reduced, and the mill will also The vibration or vibration may be caused by too much load on the material layer.

The ventilation of the vertical mill system is mainly adjusted by the opening degree of the fan damper (or the frequency conversion speed regulation of the fan motor). The air leakage of the vertical mill body, the dust collector and the ventilation duct has a great influence on the ventilation inside the mill, which often causes the output to drop and The factors of unstable operation.

4, the working pressure of the grinding roller


The grinding force of the vertical mill comes from the self-weight of the grinding roller and the pressing pressure of the hydraulic station, wherein the hydraulic tensioning device is the main source of the grinding force. The working pressure of the grinding roller should be reasonably given according to the feeding amount, the thickness of the material layer, the fineness of the product, etc. The pressure is too small to achieve effective grinding, the powdering rate is low, and the output is low; if the pressure is too large, the material may be caused. The layer is unstable and may cause unnecessary damage to the reducer.

5, the speed of the classifier


SHM vertical mill is a new type of grinding equipment integrating material grinding, conveying and grading functions. Its top-loading-static high-efficiency classifier can flexibly adjust product fineness and particle gradation distribution, and has large adjustment range and adaptability. it is good. The technical parameters of the classifier mainly include: rotor diameter, rotor height, rotor blade angle, rotor blade number, guide vane (static vane) angle and rotor speed.

The diameter and height of the rotor are designed according to the mill model (appropriate aspect ratio). Generally, the angle of the guide vane is adjusted periodically during the stoppage according to the working conditions. The angle and number of the rotor blades are determined at the time of equipment selection (according to product characteristics and fineness requirements), and the rotor speed can be adjusted in the production process. The fineness of the finished product is mainly determined by the rotor speed and the amount of system ventilation. In the case of a certain amount of ventilation of the system, the rotor speed is high, and the fineness of the material to be ground is high; on the contrary, when the coarse powder is produced, the rotor speed is required to be reduced.

6, other factors

 

(1) Height of the retaining ring


The height of the retaining ring directly affects the stability of the inner layer of the mill and the grinding efficiency. The height of the retaining ring is too high, which is not conducive to the overflow of the material on the grinding machine, which leads to thickening of the material bed. A part of the qualified finished product can not be carried away by the airflow in time on the material bed, causing over-grinding; the height of the retaining ring is too low. The powder overflow rate is accelerated, causing the bed to be too thin, resulting in the roller not being pressurized or causing the mill to vibrate.

According to the debugging experience, in order to ensure the effectiveness of the grinding, the grinding material needs to have a certain residence time on the grinding machine, and the height of the retaining ring is generally set at 30-50 mm.

(2) Wind ring clearance (area)


In actual production, it is often found that the amount of mill return is relatively large, but the vertical mill operation is still stable. At this time, the area of ​​the air ring gap (filled round steel at the outer edge of the retaining ring or the upwind ring) can be appropriately reduced to improve the wind. Wind speed at the ring, increase the capacity of the material, reduce the amount of slag, and play a certain role in improving production.

(3) Wear of grinding rolls and grinding discs


According to the project situation, when the mill runs for a long time, the mill production capacity will decrease to a certain extent, mainly due to the wear of the grinding roller and the grinding disc, resulting in the grinding structure and grinding pressure of the grinding area of ​​the roller disc. Change.

In the first stage, the roller is coarsely ground: due to the centrifugal force, the material falling into the center of the grinding disc is bitten into the lower edge of the grinding roller to complete further crushing and rough grinding;

The second stage, compaction: the coarsely ground material continues to move toward the outer edge of the grinding disc due to the centrifugal force, and is gradually compacted;

The third stage, fine grinding area: the area of ​​the roller disc is the smallest in this area, the compacted material layer is finely ground under high pressure, and the overflow is carried by the air flow at the air ring to the classifier to be selected as qualified products.

After the completion of the three stages, the gap of the roller plate is increased, and the new material is not effectively crushed and coarsely ground. After entering the compaction and fine grinding area, the high pressure of the grinding roller cannot be completely transmitted to the material layer, and the high-efficiency powder is not obtained. Grinding, it is difficult to produce enough fine powder, and because the degree of wear of each area is different, the material layer is unstable, and the operation of the mill is limited.

Roller wear problems For products with high fineness requirements, the sudden drop in capacity is easier to reflect. At this point, it is advisable to adjust the roll cover and replace the new roll cover and disc liner (this measure should be considered when the capacity is reduced by more than 10%).

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