Working principle of HRM vertical mill

HRM vertical mill is a kind of grinding machine that uses the principle of material bed crushing to grind materials. It has been widely used in cement, coal, power and other industries. HRM vertical mill is a full air swept mill. After being extruded, the materials entering the mill are thrown off the disc edge and sunk to the nozzle ring under the action of centrifugal force. It is blown up and dispersed by the high-speed wind at this place, and the metals and heavy ores will settle under the nozzle ring and be discharged. The fine powder is brought to the upper part of the vertical mill, sorted by the separator, and the finished product is collected with the gas into the dust collector, and the coarse powder is recycled back. Coarse powder and particles are thrown up, and with the decrease of wind speed, they lose their support, settle on the disk surface, and enter the grinding track by centrifugal force for a new round of circulation. In multiple cycles, heat transfer between particles and gas makes water evaporate. Therefore, HRM mill integrates many advantages of material grinding, transportation, powder selection, drying, and separation of metal blocks and heavy ores.

About ventilation in mill and temperature control at inlet and outlet

1.1 source and matching of grinding air

Most of the hot air entering the mill uses the exhaust gas of the rotary kiln system, and some process systems use the hot air stove to provide hot air. In order to adjust the temperature of the air and save energy, cold air and circulating air can also be added before entering the mill.

In order to save energy, 20% - 50% circulating air can be added according to the water content of the material. The system that uses the waste gas of precalcining kiln as the hot air source hopes that all the waste gas can be used in the mill. If there is surplus, the waste gas can be directly discharged into the dust collector through the pipeline. If it is not enough for all the waste gas to enter the mill, it can be determined to add some cold air or circulating air according to the temperature of the waste gas entering the mill.

1.2 control of air volume, wind speed and air temperature

(1) Selection principle of air volume

The concentration of dust (finished product) in the grinding gas should be between 550 ~ 750g/m3, generally less than 700g/m3; The wind speed of the outlet pipeline shall be generally >20m/s, and horizontal layout shall be avoided; The wind speed standard at the nozzle ring is 90m/s, and the large fluctuation range is 70% - 105%; When the grindability of materials is poor and the mill output is low, it is often necessary to choose a larger model of vertical mill. Under comparative conditions, when the outlet air volume is appropriate, the air speed of the nozzle ring is low. Iron plate should be used to block the hole of the nozzle ring behind the grinding roll as required, so as to reduce the ventilation area and increase the air speed. How many holes to block shall be determined by wind balance calculation; It is allowed to adjust the air volume within the range of 70% - 105% according to the specific conditions of the vertical mill, but the system in series with the kiln mill shall not affect the flue gas emission of the kiln.

(2) Control principle of air temperature

The temperature of raw mill outlet air shall not exceed 120 ℃. Otherwise, the soft connection will be lost, and the dividing wheel of the cyclone may expand and stop; The outlet air temperature of coal mill depends on the coal quality. If the volatile matter is high, the outlet air temperature of coal mill should be lower, otherwise it can be higher. Generally, it should be controlled below 100 ℃ to avoid burning, explosion and other phenomena of the system.

In the system of heating air with hot blast stove, as long as the moisture of the material out of the mill meets the requirements and the air temperature into the dust collector is more than 16 ℃ higher than the dew point, the air temperature at the inlet and outlet can be appropriately reduced to save energy.

When drying and grinding, the inlet air temperature should not exceed 200 ℃, so as not to deteriorate the lubricating oil in the grinding roll.

1.3 prevent air leakage of the system

System air leakage refers to the air leakage at the vertical mill body, outlet pipe, dust collector, etc. Under the condition of constant total air volume, the air leakage of the system will reduce the wind speed at the nozzle ring, resulting in serious slag spitting. Due to the reduction of outlet wind speed, the discharge of finished products is less, the circulating load is increased, and the differential pressure is increased. Due to the vicious cycle, the total air volume is reduced, which is easy to cause full grinding and vibration shutdown. It will also make the conveying capacity in the mill insufficient and reduce the output. In addition, it can also reduce the air temperature entering the dust collector, which is prone to condensation.

If the ventilation volume is increased in order to maintain the wind speed at the nozzle ring, it will increase the load of the fan and dust collection and waste energy. At the same time, it is also limited by the capacity of fans and dust collectors. Therefore, the air leakage of the system is one of the ways to overcome. According to our national conditions, HRM vertical mill should be designed according to the style of air leakage <10%, so the air leakage of the system must not be >10%.