The Experience of Producing Calcium Carbonate Micro Powder by Vertical Grinding Mill

Since the vertical grinding mill was put into production, and after many-times technical transformations and optimization of vertical grinding mill parameters, the vertical grinding mill has been running smoothly, the production capacity has been steadily improved, and the production plan for stable production of superfine/micro calcium carbonate powder has been realized.

The crushed calcium carbonate is measured by a belt scale, and then sent to the vertical grinding mill through the rotary air lock by a belt conveyor, and falls into the center of the mill via a chute. The grinding table is driven by the main motor through a reducer to rotate at a constant speed. Under the centrifugal force, the calcium carbonate on the grinding table simultaneously completes the circumferential movement around the center of the grinding table and moves radially outward along the grinding table into the grinding area. The edge of the grinding table is equipped with a retaining ring, so that the calcium carbonate forms a grinding zone on the grinding table, and the material is ground between the roller and grinding table. The ground calcium carbonate is carried by the high-speed hot air flow into the classifier through the upper part of the mill for classifying.

The fine calcium carbonate powder enters the vertical grinding mill dust collector with the air flow and is collected, and is sent to the calcium carbonate micro powder storage by the air conveying chute and bucket elevator for storage; the coarse particles are returned to the grinding table along the inner wall of the powder classifier for being re-ground. Iron slag and hard-to-grind materials are discharged out of the vertical grinding mill by the iron eliminator.

Vertical Grinding Mill Air leakage control

When the total air volume keeps same, the vertical grinding mill air leakage will reduce the wind speed, which will influence the grinding effect, and more calcium carbonate will be discharged out of the grinding table; the reduction of the outlet wind speed will also reduce the output of the vertical grinding mill, increase loading, and roller pressure will increase, such a vicious circle causes frequent occurrence of breakdown such as full grinding and vibration shutdown, which seriously affects the vertical grinding mill operation rate and production capacity. In addition, the air leakage of the system will inevitably lower the temperature of the air in the mill and the pipeline, which will affect the drying effect of the vertical grinding mill. In severe cases, it will cause condensation in the bag filter, which will make the vertical grinding mill unable to operate normally and increase wearing rate. If the ventilation rate is increased in order to maintain the wind speed at the air ring, the load of the fan and the dust collector will be increased, which will increase the power consumption of the vertical grinding mill. At the same time, the increase of the ventilation rate will be restricted by the capacity of the dust collector and the exhaust fan. Therefore, air leakage in the vertical grinding mill is a must-check in the routine maintenance.

Vertical Grinding Mill Pressure Difference Control:

The pressure difference of the vertical grinding mill includes the air resistance at the air inlet and the fluid resistance of the suspended material from the grinding table to the lower part of the classifier. When the vertical grinding mill is running normally, the vertical mill air volume is kept within a reasonable range. Therefore, the local resistance at the air inlet does not change too much. The fluid resistance change in the mill house mainly reflects the change in the amount of suspended materials in the vertical grinding mill. The fluid resistance depends on the feeding rate and the amount of circulating materials in the mill house. The feeding rate can be  controlled and is relatively stable under normal conditions. Therefore, the change of pressure difference directly reflects the amount of circulating material in the grinding house. Under normal working conditions, the pressure difference of the vertical grinding mill is stable, which shows that the amount of grinding material and the amount of grinding material have reached a dynamic balance, and the loading is stable. Once this balance is broken, the circulating loading will change, and the pressure difference will change accordingly. If the change of pressure difference cannot be effectively controlled in time, it will inevitably bring undesirable consequences. The common situations are:

(1) The pressure difference is reduced. It shows that the amount of feeding-in is less than the amount of feeding-out, the cycle loading is reduced, the thickness of the material in the grinding table gradually becomes thinner, and when it reaches a certain limit, the vertical grinding mill will vibrate and stop work.

(2) The pressure difference continues to increase. It shows that the amount of feeding-in is more than the amount of feeding-out, and the circulating loading continues to increase, which will eventually lead to instability of the material bed, resulting in full grinding and vibration shutdown. This phenomenon is generally not caused by uncontrolled feeding, but is often caused by unreasonable process parameters in different part of the vertical grinding mill, resulting in a reduction in the amount of grinding material.

The factors causing the fluctuation of the pressure difference of the vertical grinding mill include: system ventilation, changes in roller pressure, fluctuations in material characteristics, system air leakage, changes in powder classifying efficiency, and operating conditions of the dust collector. Therefore, during the operation of the equipment, it is necessary to closely observe the changes of the monitoring instruments, analyze the causes of fluctuations according to the changes in the pressure difference in time, and take timely adjustment measures to ensure the stability of the pressure difference. In actual production, the feeding rate should be stabilized firstly, and proper and stable ventilation in the vertical grinding mill should be maintained to stabilize the pressure difference of the vertical grinding mill, so as to realize the smooth operation.

vertical grinding mill

Vertical Grinding Mill Roller Pressure Control:

Control of grinding roller pressure The grinding roller pressure (mainly the pressure of the main roller) is an important factor for stabilizing the operation of the mill, as well as the main factor affecting the power, output and grinding efficiency of the mill. If the pressure is too low, the output of the vertical grinding mill will be low, increasing the thickness of the material bed and the cycle load; if the pressure value is too high, the main motor current will increase, the vibration value will increase, and the wear will increase significantly, and the grinding capacity will not be obtained. Increase accordingly. The pressure of the grinding roller should be adjusted according to the feed amount of the vertical grinding mill.

Vertical Grinding Mill Classifier Rotation Speed Control:

When the speed of the classifier is constant, the more the air volume and the higher the wind speed, the coarser the product fineness; when the wind speed is constant, the faster the speed of the classifier, the finer the product fineness. Generally speaking, the vertical grinding mill air volume is stable, and the wind speed is basically stable, so the speed of the classifier becomes the key to the fineness.

Since the operating parameters of the vertical grinding mill system influence and restrict each other, the change of a certain parameter will also affect other parameters, and attention should be paid to the coordination of various operating parameters adjustment. Through continuous strengthening of technical management, and on the basis of good system operation, the system’s process parameters have been well optimized, and the operating procedures have been standardized, so the output also increased.

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