Vertical roller mill

1. What is the working principle of the vertical mill?

The vertical mill was developed by Loesche in Germany and is a device used for grinding raw materials and clinker in the cement industry. It can significantly reduce power consumption (saving 30%-50% more energy than traditional ball mills). Its working principle is that the material is ground by the pressure of the grinding roller and the rotation of the grinding disc. The fine powder ground is collected by the powder selector and then returned to the grinding disc for further grinding. The vertical mill can produce 30 to 250 tons per hour, and the fineness of the material can be controlled between 80-600 mesh. The vertical mill is also suitable for grinding industrial wastes such as slag, steel slag, and coal powder, and the ground products can be used as cement additives.

2. What is the difference between a vertical mill and a ball mill?

The vertical mill adopts the principle of material bed grinding. The material is crushed by the pressure of the grinding roller on the rotating grinding disc. The advantages are energy saving and high grinding efficiency. It is generally suitable for large-scale grinding of cement, slag, non-metallic minerals, etc. The energy consumption of the vertical mill can be saved by about 30%-40% compared with the ball mill. The ball mill crushes the material through the impact and grinding of steel balls. It is suitable for a wider range of ore and raw material grinding, but its energy consumption is relatively high, usually 20%-30% higher than the vertical mill. The maintenance of the vertical mill is more complicated because its structure is relatively compact and the precision of accessories is high; while the ball mill is relatively easy to maintain due to its simple structure, but it wears faster. In terms of performance, the vertical mill has a more uniform particle size and the product fineness is easy to adjust. It is suitable for the production of high-quality powder materials, while the ball mill is more suitable for coarse grinding operations.

3. What are the advantages of the vertical mill in cement production?

The vertical mill has advantages over the ball mill in terms of efficiency and energy consumption in cement production. Compared with the traditional ball mill, the vertical mill improves the grinding efficiency by rolling and air separation of the material. Its grinding efficiency can reach more than 85%, and the energy consumption is reduced by 20%-30%. The power consumption per ton of cement clinker is only 15-25 kWh. The vertical mill also integrates a drying function. When processing high-moisture materials, no additional drying equipment is required, which reduces equipment investment. Vertical mills are particularly good in cement raw material grinding and slag grinding. They have uniform particle size and stable product quality, ensuring that the fineness and specific surface area of ​​cement products are within the range of 300-350m²/kg. The equipment is stable and easy to maintain, reducing the downtime of the production line.

4. What are the common causes and solutions for vertical mill vibration?

1. Grinding pressure is too high or too low: TONGLI mill can clearly feel the impact of the increase in grinding pressure on mill vibration. When the feed volume is constant and the pressure is too high, the grinding capacity will be greater than the capacity required for the material to be turned into a finished product, causing the mill to vibrate. On the contrary, too low pressure causes too much material in the mill, resulting in large vibration.

2. The speed of the powder selector is too high: In the early stage of trial production, the rotor speed is generally not set to the maximum. Because the feed volume and air volume are very small at this time, if the speed of the powder selector is too high, it is easy to produce over-grinding, which increases the fine powder in the mill. Excessive fine powder cannot form a solid material bed, and the grinding roller “eats” the material deeply, which is easy to vibrate. Therefore, when the mill does not reach the designed output, the particles of the material entering the mill are required to be larger and easy to operate.

3. The inlet temperature is too high or too low: When the outlet temperature of the high-temperature fan changes, the working conditions of the mill will change. If it is too high, it will be difficult to form a material bed on the grinding disc. If it is too low, the material cannot be dried, causing the nozzle ring to be blocked, etc. The thickening of the material bed causes abnormal vibration of the mill. In this case, adjust the water spray in the mill, the water spray in the humidifying tower, or add cold air, circulating air, stabilize the temperature of the mill inlet and outlet, and stabilize the working condition of the mill.

5. What are the reasons for the coarsening of the finished product fineness of the vertical mill?

1.  Improper adjustment of the speed of the powder selection device: Adjusting the rotor speed of the powder selection device is the most common method to adjust the fineness, and it is also the most important method. Usually change the fineness: the first thing that comes to mind is to adjust its speed, and increase the speed when it runs coarse.

2. Too much ventilation: In the trial production, when using a hot air furnace or less material, the internal ventilation of the mill is small, and the speed of the powder selection machine is set to 70-80%, and the fineness can basically be achieved. With the increase of hot air at the end of the kiln, the ventilation in the mill increases, the EP fan baffle increases, and the fineness will increase. At this time, the speed of the powder selection machine should be gradually increased to ensure that the fineness is qualified.

3. Temperature influence: The outlet temperature of the mill rises rapidly, or maintains a high temperature, and the material out of the mill may also run coarse, because the temperature rises, the fluid velocity in the mill and the internal energy of the material in the mill are changed, and the Brownian motion of the fine material increases, and the larger material is pulled out of the mill body. This may be caused by changes in the wind temperature and air volume at the kiln tail or the moisture content of the material entering the mill. At this time, the water spray in the mill can be adjusted to solve the problem. The process line in which the mill is connected in series with the humidifying tower can adjust the water spray of the humidifying tower and add circulating air when the air volume allows.

6. What are the reasons for the blockage of materials entering the mill and how to deal with it?

The blockage of materials entering the mill often seriously affects production efficiency, and the cleaning process is dangerous and laborious. The main reasons and treatment methods are as follows: High viscosity and moisture are the main causes of blockage. The moisture and viscosity in the raw materials vary. In particular, the limestone mined in the early stage of the mine may contain more non-limestone substances on the surface. These substances have small particles and strong viscosity. When the moisture content is high, it is easy to cause material accumulation. Another reason is that foreign matter is stuck. For example, the rotary valve of the ZJTL mill may be stuck by large stones, causing the rotary valve body to fail to work properly. At this time, the special function of the sliding coupling will cause the motor to trip, but the belt entering the mill continues to run, further aggravating the blockage problem. The solution to these problems is to reduce the large pieces of foreign matter in the material entering the mill. At the same time, although the blockage caused by mechanical failure of the air lock valve is rare, its possibility should not be ignored. The air lock valve should be maintained and inspected regularly to detect and deal with potential problems in a timely manner.