The vertical roller mill (VRM) is a type of grinding machine for raw material processing and cement grinding in the cement manufacturing process. In recent years, the VRM cement mill has been equipped in more and more cement plants around the world because of its features like high energy efficiency, low pollutant generation, small floor area, etc.
The VRM cement mill has a more complex structure than other types of cement mills, so we need to ensure the following aspects are normal during the operation, or the machine may not be able to function well.
1. The Material Layer of the Mill
A stable material layer with proper thickness is indispensable for the well functioning of the vertical roller mill.
Too thick material layer will reduce the grinding efficiency of the vertical roller mill. When the pressure difference of the mill reaches the limit, the material layer will collapse and affect the operation of the main motor and the discharge system.
Too thin material layer will cause the mill to generate stronger driving force and damage the rollers, millstone, and hydraulic system of the mill.
2. The Vibration of the Mill
Strong vibration of the mill body may cause to mechanical failure and has a bad effect on the product quality.
There are many factors that may make mill body vibrate, including the grinding pressure, material layer thickness, air volume and temperature, accumulator pressure, wear condition of the roller and the grinding plate, etc.
3. The Grinding Pressure of the Mill
The grinding pressure is one of the key factors that have a great impact on the yield and quality of products.
The grinding pressure of the mill should be adjusted according to the fed quantity, the particle size, and the grindability of the material. It must be well controlled to maintain the thickness of the material layer, reduce the vibration of the mill body, and ensure that the stable operation of the mill.
Increasing the grinding pressure will improve the grinding capacity of the mill, but the grinding capacity will stop increasing when the grinding pressure reaches a certain critical point. If the set pressure of the hydraulic cylinder is too high, it will only increase the driving force and accelerate the wear of parts, but not improve the grinding capacity.
Too low grinding pressure will lead to the increase of the material layer thickness, the main motor current, the pressure difference in the mill, and also the vibration amplitude of the mill.
On the other hand, too high grinding pressure will cause the thickness of the material layer to decrease, which further leads to the increase of the vibrating speed of the mill and accelerates the wear speed of the parts.
4. The Gas Temperature at the Discharging Port
If the gas temperature at the discharging port of the mill is too low, the fluidity of the material will become poor, and the qualified products cannot be discharged from the mill in time.
The gas temperature at the discharging port can be increased by increasing the opening of the hot air door and reducing the opening of the circulating air door. However, when the gas temperature exceeds 130 ℃, the equipment will also be damaged. For example, the dividing wheel at the lower part of the cyclone will expand and get stuck, and the lubricating grease of the grinding roller will dry and crack, which is also unfavorable to the tail dust collection bag.
Each mill has its own adaptive temperature, of which the operators must be aware during the operation.
5. The Air Volume in the Mill
In vertical roller mills, the air volume in the mill is determined by the material feed rate. The air volume can be controlled by adjusting the power of the circulating fan of the mill or the opening of the exhaust fan at the kiln tail.
If the air volume is too large, the pressure difference in the mill and the current of the main motor will decrease, the thickness of the material layer will become thin, and the vibration degree of the mill body will become strong.
If the air volume of the system is too small, the thickness of the material layer, the pressure difference in the mill, and the current of the main motor will increase, and so will the vibration speed of the mill.
6. The Hot Gas from the Kiln
In the raw material grinding vertical mill, the heat source for drying the raw materials is the hot gas exhausted from the rotary kiln.
When operating the vertical roller mill, the operator must control the inlet and outlet air valves to make the air volume of the system in a balanced state, so as not to affect the air pressure at the back end of the rotary kiln.
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