1 Overview

　　Under normal circumstances, the vertical grinder operates smoothly with a noise level of no more than 90 decibels. However, if not operated properly, it can cause vibration and generate significant noise. For vertical mills, reasonable vibration is allowed, but if the vibration is too large, it will inevitably cause damage to some components in the vertical mill. Therefore, the vibration of the vertical mill should be strictly controlled during the production process.

　　The grinding principle of 2 vertical mills

　　A vertical mill is composed of hydraulic devices, grinding rollers, grinding discs, transmission devices, and other components. Its main parts are the grinding disc and the rolling grinding rollers on the grinding disc. The material entering the mill forms a circular material bed through the action of centrifugal force. When the grinding roller and the grinding disc roll together, the larger material is first clamped between the grinding disc and the grinding roller, and is crushed into powder through compression. During the crushing process, large pieces of material are subjected to relatively high pressure, which is then transmitted to the second largest material. As the particle size of the material decreases, the grinding pressure gradually decreases. In this way, under the action of rolling, particles of various sizes can be densely packed in a very small space, and the small particles can slide relative to each other to generate shear force, enabling further fine grinding of the fine particles.

　　There is a powder selection device at the upper part of the vertical mill. The hot air introduced into the lower part of the vertical mill overflows from the edge of the grinding disc under the action of centrifugal force, and is lifted by high-speed airflow and sent to the powder selection machine for the next step of powder separation. The coarse powder will return to the grinding disc for secondary grinding. The large pieces of material thrown out from the grinding disc will not be blown by the hot air, but will fall into the scraper bin under the action of gravity, pass through the external circulation system, and then return to the grinding system for further grinding.

　　Analysis of the causes of vertical mill vibration

　　There are many reasons that can cause vertical mill vibration, mainly including control system, equipment, and process issues. Process control is an important factor in causing vertical mill vibration. In actual production operations, many vibrations are caused by improper operation, such as those caused by unstable material layer control.

　　Due to the thin material layer, there will be very little material that can be ground, resulting in a decrease in grinding output. At the same time, a thin material layer can also cause rigid contact between the grinding disc and the grinding roller, which can cause significant vibration, leading to severe wear and increased energy consumption of the vertical grinding equipment. If the material layer is too thick, the grinding ability of the vertical mill will be reduced, and the internal circulating load will increase, causing the grinding roller to jump up and down, which will cause serious damage to the equipment and affect its use. Therefore, the control of the material layer is very important, and there are many reasons that affect the changes in the material layer. Among them, the material characteristics include the moisture content, friction factor, and grinding particle size of the material, as well as the influence of factors such as temperature and pressure difference.

　　3.1 Material Characteristics

　　Material friction factor: directly reflects the magnitude of internal friction generated during the grinding of various materials. It is directly related to whether the material layer can form a stable abrasive layer thickness, and whether the stable material layer thickness can be maintained directly affects the degree of vertical grinding. Materials with low friction factors will be squeezed out of the groove due to the squeezing effect, which will inevitably result in a thin material layer and make it easy to

　　Causing direct contact between the grinding disc and the grinding roller, resulting in vibration and damage to internal equipment; If the friction factor is high, the material will be subjected to minimal crushing and form a cake shape, which is difficult to loosen and causes difficulty in grinding.

　　The particle size of the material entering the grinding disc should match the size of the grinding roller and disc. As the diameter of the grinding roller increases, the peripheral linear velocity of the grinding roller also increases, resulting in an increase in frictional force. Therefore, the grinding roller is more likely to bite into the material, and the material being ground is more easily crushed into a layer. Therefore, the particle size of the material entering the grinding process can be appropriately increased. But when the material particles are too large, it will cause significant impact on the grinding rollers, resulting in vibration caused by the imbalance of the three rollers; Due to the small particle size of the material, it will enter the powder selection machine with the hot air blower and cannot form a material layer on the grinding disc, resulting in direct contact between the grinding disc and the grinding roller and causing vibration. Generally speaking, the particle size entering the vertical mill should be around 5% of the diameter of the grinding roller.

　　Material moisture content: The amount of moisture in the material has a significant impact on the grinding process. If the moisture content is too high, the material is not easy to dry. If the moisture content is too high, the material will be crushed and adhere to other materials, and even wrap around the grinding roller, causing material accumulation on the grinding disc and making it difficult for the grinding roller to press down, leading to vibration and stopping. If the moisture content of the material is too low, the viscosity of the material will be too low, and the friction factor between the materials will decrease, making it difficult for them to stay on the grinding disc for a long enough time, and unable to form a certain thickness of material layer. This will lead to rigid contact between the grinding disc and the grinding roller, intensifying vibration.

　　3.2 Difference between Grinding Pressure and Grinding Pressure

　　The difference between the in grinding pressure and the out grinding pressure is called the pressure difference, which is one of the main reasons affecting the vibration of the vertical mill. The change in pressure difference directly reflects the change in the internal circulation load rate of the vertical mill, which is related to many factors such as material particle size, material characteristics, grinding pressure, ventilation inside the mill, material moisture, and feeding rate of the mill. For example, materials with too small abrasive particles, high grinding pressure, low moisture content, large feeding volume, and insufficient ventilation can all cause an increase in differential pressure and thickening of the material layer; On the contrary, it will lead to a decrease in differential pressure, thinning of the material layer, and vibration of the vertical mill. If not adjusted in time, it will cause the vertical mill to jump and stop.

　　3.3 Temperature of hot air

　　The temperature of the hot air has a significant impact on the vibration of the grinding mill, as the ventilation rate in a fixed vertical mill needs to ensure the drying efficiency of the material. Therefore, controlling the temperature of the hot air entering the mill mainly depends on the moisture content of the materials entering and exiting the mill. However, poor control of air temperature can also affect the grinding efficiency of the vertical mill to a certain extent. If the air temperature is too high, it can achieve good drying and grinding effects, but it can cause the material layer to be too thin. If the air temperature is too low, it will cause difficulties in drying materials, grinding them, and the materials are easily compressed into a cake shape, causing vertical mill vibration. In general, the wind temperature at the inlet should be between 200 ℃ and 250 ℃, and at the outlet, the temperature should be controlled between 80 ℃ and 120 ℃ based on the moisture content of the material and the water spray inside the mill. Under this parameter, the vertical mill runs smoothly without generating significant vibration.

　　3.4 Grinding pressure

　　Grinding pressure is mainly the sum of pressure applied by the center frame, hydraulic system, and the self weight of the grinding roller. Grinding pressure directly affects the compression ratio of the material and the thickness of the material layer. As the grinding pressure increases, the thickness decreases; On the contrary, the pressure decreases and the thickness increases.

　　4. Relevant measures for controlling vertical mill vibration

　　4.1 Feeding operation

　　When the moisture content of the material is high, the outlet temperature should be appropriately increased to raise the pressure difference inside the mill to the ideal value. If there is insufficient feeding or insufficient return, the feeding should be increased to ensure normal pressure difference inside the mill and avoid vibration. If the material layer is too thick, the feeding amount should be reduced in a timely manner and the discharge port should be ensured to be unobstructed; On the contrary, the feeding amount should be increased.

　　4.2 Grinding pressure control

　　If the feedback value of the grinding pressure of the vertical mill is less than the set value, it indicates that the accumulator pressure is too low, and nitrogen should be replenished in a timely manner.

　　4.3 Temperature Control

　　If the temperature is too high, the grinder power can be adjusted appropriately or the cold air valve can be directly opened.

　　4.4 Differential pressure control

　　When the feeding amount is large and the grinding capacity is insufficient, it will cause an increase in pressure difference inside the mill. At this time, the feeding amount should be appropriately reduced according to the mill power. If the product is too fine, it will increase the internal circulation load and cause the pressure difference to rise, leading to vibration. Therefore, it is necessary to appropriately reduce the speed of the powder selector.

　　5 Conclusion

　　In the actual production process, we need to carry out appropriate operations on the grinding machine according to the specific situation. In addition to summarizing the above reasons, we also strengthen equipment maintenance and inspection in production to ensure a smooth operating environment.