How does the pressure setting of roller press affect the grinding material?

The grinding process of vertical mill is completed by a pair of grinding devices (including grinding disc, two pairs of large rollers and two pairs of small rollers). The transmission device drives the grinding disc to rotate, and correspondingly drives the grinding roller to rotate, and materials are ground and broken between the grinding roller and the grinding disc. In addition to the gravity of the grinding roller, the pressure on the material mainly depends on the hydraulic device to pressurize the grinding roller. Because the grinding pressure of vertical mill mainly comes from hydraulic device. Usually, the grinding pressure is related to the material characteristics of the grinding disc and the thickness of the material layer, because the vertical grinding is the crushing of the material bed, and the extrusion force is transmitted between particles. When the grinding pressure exceeds the strength of the material, the greater the grinding pressure, the higher the degree of crushing. Therefore, the harder the material, the higher the grinding force. At the same time, the thicker the material layer, the greater the pressure required, otherwise the effect is not good.

For materials with good grindability, excessive grinding pressure is a waste, and thin material layer often causes vibration; But for the materials with poor grindability, the grinding pressure is high and the material layer is thin, which will achieve better grinding effect. Another important basis for selecting grinding pressure is the main current of the grinding machine. Under normal working conditions, the main current of the mill is not allowed to exceed the rated current. Otherwise, the grinding pressure should be reduced.

Because blast furnace water-quenched slag is a kind of refractory material, only when the specific surface area reaches 465 1.5m2/kg can the early strength of slag be fully exerted. The direct factor affecting the specific surface area of mineral powder is the grinding pressure of the main roller. At the initial stage of commissioning, when the feed rate is 90t/h, the grinding pressure is set at 65bar. With the adjustment of hourly output, the grinding pressure gradually increases. Under the condition of normal equipment and other process parameters, the grinding pressure of the main roller is finally determined to be about 70bar. The mill runs stably and can meet the control requirements.

Second, the system pressure difference

Vertical mill pressure difference refers to the static pressure difference between the lower mill cavity of the separator and the hot flue gas inlet during operation. This pressure difference mainly consists of two parts: one part is the local resistance caused by hot air entering the nozzle ring, which is about 2000 ~ 3000Pa under normal circumstances, and the other part is the fluid resistance of suspended materials from above the nozzle ring to the lower part of the classifier. The sum of these two resistances constitutes the pressure difference of the mill system.

During normal operation, the air volume of the mill is kept within a reasonable range, so the local resistance of the nozzle ring changes little, and the change of the pressure difference of the mill depends on the change of the fluid resistance in the grinding cavity. The change of fluid resistance in the mill chamber mainly reflects the change of suspended material quantity in the fluid, which depends on the feeding quantity and the circulating material quantity in the mill chamber. The feed quantity is a controlled parameter, which is stable under normal conditions, so the change of pressure difference directly reflects the circulating material quantity (circulating load) in the grinding chamber.

Under normal working conditions, the pressure difference inside the mill is stable, which indicates that the amount of materials entering the mill and the amount of materials leaving the mill are in dynamic balance and the circulating load is stable. Once this balance is broken, the cycle load changes, and the pressure difference will also change. If the change of pressure difference cannot be effectively controlled in time, it will inevitably affect the stable operation of the vertical mill. Common situations are as follows:

1, the pressure difference decreases, indicating that the amount of material entering the mill is less than that leaving the mill, the circulating load decreases, and the bed thickness gradually becomes thinner. When it reaches a certain limit, it will vibrate and stop grinding.

2. The pressure difference is getting bigger and bigger, indicating that the feed rate is greater than the discharge rate, and the circulating load is getting bigger and bigger, which will eventually lead to unstable feed bed or serious slag spraying, resulting in vibration shutdown of the mill.

The direct reason for the increase of pressure difference is that the amount of material entering the mill is greater than that leaving the mill. This phenomenon is generally not caused by uncontrolled feeding, but is often caused by unreasonable process links, resulting in a decrease in the amount of abrasive.

The negative pressure at the inlet of the vertical mill in our company had better be controlled at 0.5Kpa. After the pressure difference between the inlet and outlet is stabilized at 3.8Kpa, the vibration value of the mill is generally stabilized at 3mm/s, the vertical mill system runs smoothly, and the output is steadily increased from 90t/h to130 t/h..

Of course, the fluctuation of material characteristics, air leakage of the system, the fluctuation of hot blast stove working condition, the change of powder separation efficiency and the operation of dust collector will all cause the fluctuation of vertical grinding system, thus affecting the output and quality of mineral powder. Therefore, in the process of equipment operation, we should closely observe the changes of monitoring instruments, analyze the fluctuation reasons in time according to the pressure difference changes, take timely adjustment measures, and reasonably control the process parameters to ensure the balanced and stable operation of equipment.