For a long time, the vertical roller mill has been standard grinding equipment for raw material, coal, and volcanic ash. Now, it is more and more applied in the finish grinding of cement clinker and slag. The economic reason has driven demand for low costs, and the VRM in the cement plant is a more energy-saving solution.
The electric power consumption and total cost of the VRM cement plant are both low. In the production of cement, the vertical roller mill has outstanding advantages. To maximize productivity while consuming less energy, it is crucial to operate the vertical roller mill correctly.
Working principle of vertical cement mill
The vertical cement mill is designed for grinding block-shaped or granular raw materials. The main motor drives the grinding table to rotate through the reducer, raw materials are charged to the grinding table by a wind-blocking feeding device, and they are thrown towards the edge because of centrifugal force. As it passes through the grinding area between the grinding disc and the grinding rollers, the grinding rollers, which are pressurized by the hydraulic system, tightly crush and pulverize the material. At the same time, the high-temperature gas (usually from the exhaust gas at the kiln tail) introduced from the inlet is directly in contact with the suspended powder-like material multiple times within the mill, completing the drying process instantaneously.
Key points of operating a vertical roller mill
The normal and safe operation of a vertical roller mill is affected by several factors, such as the thickness of the material layer, machine vibration, water spray, retention ring, etc. We will examine each of them and seek out the right way of vertical roller mill operation.
Vibration Control
Vibration value is a key indicator in the operation of VRM. Excessive vibration may lead to damage to the grinding table and grinding rollers, auxiliary equipment, and measuring meter.
Reasons for the phenomenon: uneven material bed, large metallic objects, excessive grinding pressure, damage to wear parts, unequal pressure of the accumulator, and inadequate ventilation.
So, it is critical to control the vibration value in the allowable range to ensure the stable operation of the vertical roller mill.
Manage the thickness of the material layer
The thickness of the material layer is another key factor for the stable operation of VRM, because it directly affects the air volume, air pressure, and feeding of material.
Theoretically, the thickness of the material bed should be controlled within 2% ± 20 mm. In an actual situation, you’d better decide it based on water content, particle size, particle distribution, grindability, etc., factors.
It is advised to determine the thickness of the material layer by adjusting the height of the retention ring. Making the feed particle size and grade suitable is also important.
Pressure differential management
Pressure difference at the wind ring is also a key value for its prominent influence on vertical roller mill operation.
The operators can judge the material condition by pressure loss changes. Of course, this needs careful watching and the accumulation of experience. Then, one may notice the changes in the amount of material, the volume of air, and grinding efficiency and deal with it properly.
Over low pressure difference usually indicates that the amount of material fed into the mill is insufficient to meet the output requirements, resulting in a decrease in the circulation load and a gradual thinning of the material bed. At this point, the feeding rate should be increased immediately, and the water spraying volume should also be increased accordingly to form a stable material layer.
Over-high pressure difference usually suggests that the amount of material entering the mill has consistently been greater than the amount of material leaving the mill, and the circulation load has been continuously increasing. At this point, measures should be taken to reduce the amount of material, increase the air volume, and appropriately increase the water spraying volume. Additionally, the rotational speed of the cement separator can be temporarily reduced to allow the fine powder accumulated in the mill to be discharged outside the mill. Once the pressure difference returns to normal, gradually restore all parameters.
Air volume and temperature control
The temperature at the VRM outlet needs to be appropriately controlled, which should be consistent with the feed quantity.
If the temperature is too low, the fluidity of the material is poor, resulting in qualified finished products that cannot be withdrawn in time. If the temperature is too high, then damage to the vertical roller mill may happen.
To ensure the stable operation, precisely controlling the temperature at the outlet should be carried out.
Grinding pressure control
Grinding pressure is also a primary factor that affects productivity, grinding efficiency, and mill power.
When grinding pressure increases, the output of the vertical roller mill rises. However, when it reaches a critical value, the output will not continue to rise. Instead, power consumption increases if the grinding pressure continues to increase.
To balance the output and power consumption, a suitable grinding pressure is needed. Pay attention to material property, particle size, and feed quantity.
It is essential to find out the appropriate grinding pressure and reasonable air speed to form good internal recycle at trial operation, which makes a stable material layer and high grinding efficiency.
Deal with abnormal cases
Reason for the excessive amount of slag discharging
A large amount of discharged slag from the vertical roller mill sometimes occurs during operation. This may increase the operating resistance and may also lead to abnormal power of the main motor.
The possible reasons:
- Abnormal increase in the power of the main motor.
- The differential pressure of the mill increases, and the negative pressure at the inlet decreases.
- The vibration of VRM remains at a relatively high level.
- The inlet temperature of the VRM gradually decreases.
The cause of the discharge port blockage problem
The blockage problem at the discharge port may result from the stuck iron pieces, the excessive water spraying, or a broken water spray pipe, which causes the discharged slag to get damp and block the discharge port.
Measures to solve the blockage problem:
Operators should reduce the material fed into the mill immediately, and the mill can generally return to normal. If not, stop feeding materials, and restart until the blocked material is out.
To sum up, the key to vertical roller mill operation lies in balancing the air volume, material, and power to reach a stable, safe, and highly efficient grinding system. Make timely adjustments based on various indicators of the mill material, such as moisture, particle size, grindability, etc., as well as the mill vibration, current, outlet temperature, and system air volume. This includes reasonable adjustment of the mill feed rate, grinding pressure, separator speed, and each baffle opening to ensure the smooth operation of the vertical mill and improve the grinding efficiency.
