The level of vertical roller mill output not only affects the production schedule and goals, but also affects energy consumption, enterprise costs and benefits. This article discusses the influence on the output of the vertical roller mill from the aspects of the mill structure, scraper, hydraulic system, material and pressure difference.
1. The internal structure of the mill is worn out
The internal parts of the mill that affect the output include: grinding rollers, grinding discs that directly participate in the grinding, baffle rings that control the material layer, and nozzle rings that control the direction of the hot air.
1.1 Wear of grinding roller and grinding disc liner
It can be known from the working principle of the mill that the vertical mill grinds the material by applying pressure from the grinding roller when it is working. After a long time of operation, the grinding rollers and grinding discs become uneven and irregular due to abrasion, the grinding efficiency decreases, and the output decreases.
In this case, the wear of the liner should be checked regularly. When the wear amount of the grinding roller approaches 25mm and the wear amount of the grinding disc approaches 10mm, the size of the grinding roller and grinding disc can be repaired by surfacing welding to ensure the output.
Avoid the increase of vibration due to the wear of the lining plate, which will affect the safety of the main reducer. If damage to the base material of the grinding disc liner is found, the liner should be replaced in time.
1.2 Wear of retaining ring
The function of the stopper ring is to enable the material to be fully ground on the grinding disc. The height of the stopper ring determines the thickness of the material layer.
When the thickness of the material layer is large, the vibration of the mill will increase. The coarse powder enters the outer circulation under the action of centrifugal force without being sufficiently pulverized, so that the total amount of grinding is increased, the output per machine is reduced, and the vibration of the mill is large.
When the retaining ring is too low, it is difficult to ensure a stable material layer thickness. Under the pressure of the grinding roller and the weight of the grinding roller, intermittent rollers and discs will directly collide and cause vibration, which will affect the normal operation of production.
Setting the height of the retaining ring reasonably has a positive effect on increasing the output. At the same time, since the retaining ring of the mill is composed of three steps, the bottom step is 45mm wide. The relative height of the grinding disc is 50mm. In the grinding state, it forms an effective grinding space with the large end of the main roller. The wear of the bottom step of the retaining ring will destroy the formation of effective grinding space. Therefore, in addition to setting the height of the retaining ring reasonably, repairing the size of the retaining ring is also very important.
1.3 Wear of nozzle ring
The main function of the nozzle ring is to change the wind direction through the wind guide blades to ensure the same air flow rotation direction. The inclination angle, height, and wear conditions of the wind guide blades all affect the airflow. The angle of the air deflector determines the time from the separation of the air material to the powder selection area, and the length of time affects the internal working conditions of the mill.
If the wind guide vane is damaged, the air volume passing through the nozzle ring will not be evenly distributed, and a stable upward airflow will not be formed. The airflow is turbulent, which weakens the ability of the gas to carry mineral powder. The material returns to the grinding disc or falls into the scraping cavity at the bottom of the nozzle ring, causing uneven thickness of the material layer. Increase the circulation volume and reduce the hourly output of the mill. The damaged nozzle ring must be repaired after shutdown to avoid disturbing the grinding internal circulation.
2. Add scraper
In order to reduce the accumulation and compaction of the material in the center of the grinding disc, and to improve the control of the auxiliary roller on the material layer, a scraper is added in the center of the grinding disc, and the height of the grinding disc is 20cm. It can scrape the material in the center area of the grinding disc to better stabilize the material layer and ensure that the material is evenly ground. It can also effectively reduce the wear of the mill body.
3. Hydraulic system pressure
When other factors remain unchanged, the grinding pressure is directly proportional to the output. The greater the grinding pressure, the greater the positive pressure acting on the material on the grinding disc, and the better the grinding effect. But too high pressure will increase the probability of causing vibration, and the motor load will increase accordingly. Therefore, the operator must consider the setting value of the grinding pressure according to the grindability of the material, the output and fineness index, the thickness of the material bed and the vibration condition.
Hydraulic system valve failure, pressure switch malfunction or internal leakage will cause pressure drop. The pressure value in the nitrogen cylinder and the accumulator does not meet the requirements, which will affect the output per unit.
Use each downtime to check the pressure of the nitrogen cylinder to ensure that the nitrogen pressure of the piston accumulator is at 8 bar. Insufficient pressure will cause increased vibration and frequent shutdowns. The nitrogen pressure of the bladder accumulator is 40 bar, and insufficient pressure will cause the roller to be overtime and the system will respond slowly.
During production, check the leakage of the positive and negative pressure oil pipes of each hydraulic cylinder, and deal with it during shutdown and maintenance.
The requirements of vertical mills on materials are mainly manifested in grindability and particle size. The higher the hardness of the material, the longer it takes for the material to be milled into a finished product, and the more cycles within the mill. At the same time, it also increases the wear and tear of the equipment.
Moisture-containing slag accumulates and compacts on both sides of the feeding pipe, causing blockage of the feeding pipe of the mill, and frequent shutdown of the rotary feeder due to overload, making the mill unable to produce normally.
An overhaul hole is added under the rotary feeder, and simple dredging is performed every 4 days. Shut down once every 15 days, concentrate manpower to clear the material in the feeding pipe, and use the air cannon to assist in clearing the blockage to ensure the production of the mill.
5. Differential pressure
The pressure difference is composed of two parts: the pressure difference in the mill and the pressure difference of the bag collector. It is a key factor to improve product quality and output.
5.1 Pressure difference in the mill
It is mainly composed of two parts, one is the local ventilation resistance caused by the hot air nozzle ring of the mill, and the other is the fluid resistance of the suspended material from the top of the nozzle ring to the pressure point. The sum of these two resistances constitutes the internal pressure difference of the mill.
The size of the feeding amount and the amount of circulating material in the mill cavity will cause the change of fluid resistance in the mill cavity, leading to the change of the pressure difference. In order to measure pressure changes more accurately, compressed air can be used to clean the measurement pipeline.
5.2 Pressure difference of bag collector
Determined by the fluid resistance of the suspended material in the bag collector, factors such as poppet valve, pulse valve, filter bag permeability, air flow direction in the filter bag, and sealability of the cover can all change the resistance. By repairing damaged poppet valves, pulse valves, regularly replacing filter bags, adjusting the position of the bag cage, repairing damaged shells, etc., the transportation efficiency of materials in the bag collector is guaranteed and the output is increased.