01 Existing problems

The 5000t/d clinker production line of a company adopts ZGM-113N vertical coal mill, which was put into production in March 2008. After May 2017, the vibration of the mill was too large, the grinding pressure could not be increased (controlled at 7.0 ~ 8.0MPa), and the fineness was too coarse (up to 15% ~ 18%), which had a great impact on the safe operation of the coal mill system, the pulverized coal combustion in the rotary kiln, the thermal system of the kiln system and the production and quality.

(1) Large amount of slag spitting. Before the reconstruction, the average slag spitting in each shift is about 30 small rows of cars, 150kg for each car, and the slag spitting is nearly 14t per day. Due to the high output of rotary kiln, up to 5900t/d and large coal consumption, the pulverized coal silo does not increase silo and keeps low position for a long time. Encounter easy grinding poor coal or mill has abnormal stop, to protect the warehouse is forced to improve the output of the stage, vomit slag more, cause a vicious circle, coal consumption rises substantially, the labor intensity of workers on the spot is big, affect the environment at the same time.

(2) Vibration of mill. In the normal production process, the vibration value of the mill has been fluctuating in the range of 2 ~ 12mm/s, and the upper and lower limits of the fluctuation are obviously too large, and the vibration can be obviously felt in the field. The analysis is due to the impact of raw coal granularity and pulverized coal fineness. Several coal from different producing areas have been replaced, but there is no obvious improvement. Moreover, there was a tie rod fracture accident on June 5.

(3) Fineness is coarse. Because of the low grinding pressure and large vibration, the grinding efficiency decreases, the amount of slag is large, and the fineness of the grinding coal cannot be controlled. After increasing the speed of the separator, the fine powder in the mill is too much, which leads to the aggravation of the vibration of the mill. Coarser pulverized coal has a great impact on the process operation of rotary kiln. After pulverized coal is burned at the kiln head, it is not burned sufficiently in the calciner, the temperature in the smoke chamber is high and the crust is more, and thick kiln skin is easy to appear at the end of the kiln, which affects the ventilation in the kiln and results in the decline of production and quality.

02 Analysis of the causes and improvement measures

2.1 Cause analysis

(1) The roller skin and the lining plate of the grinding disc are seriously worn. According to the use and replacement cycle of the roller skin and lining board, the maintenance period was reached in January 2017, but the replacement was not done because the spare parts were not in place.

(2) The wear of nozzle ring is heavy, as shown in Figure 1. The moving ring and the roller lining are the same, which also exceed the replacement cycle and cause serious wear. The clearance between the moving ring and the rolling ring has reached nearly 50mm, which is far from the 8-15mm design requirements of the manufacturer.

Fig. 1 Heavy wear state of nozzle ring before modification

2.2 Improvement measures

(1) Measures taken for the first time. During maintenance in July, the lining plate of the grinding disc and the sleeve of the grinding roller should be surfaced online to restore their wear-resistant layer and improve the grinding efficiency.

The iron plate with thickness of 20mm and height of 200mm is rolled into an arc and welded on the static ring, 100mm higher than the horizontal height of the nozzle ring; The three cyclone nozzles were evenly blocked on the moving ring, and the ventilation area decreased from 1.21m² to 0.95m², as shown in Figure 2.

Fig. 2 Schematic diagram of the first transformation

After starting the mill, the grinding condition was improved, and the slag emission was reduced by more than half. The output and vibration of the mill were improved, but not reached a better state. At the same time, due to the small welding surface of the iron plate on the static ring, three pieces fell off successively in the later period. The pressure air plate on the nozzle is also quickly worn, and the wear condition gradually gets worse, which is more serious than before maintenance.

(2) Measures taken for the second time. By measurement, the ventilation area at the nozzle ring is calculated to be 1.02m². It is proposed to reduce the ventilation area at the nozzle, improve the wind speed at the nozzle, reduce the subsidence of coal cinder, and improve the cyclone effect at the nozzle. Welding main measures as follows: the static ring width of 60 mm, 8 mm thickness of steel plate, with triangle reinforcement reinforcement on static ring, and with the mill shell with long steel reinforcement again, will cover all the clearance of dynamic and static ring vents with 60 mm wide vents, welding steel plate with ring control the vertical distance of 15 mm from top to bottom, to prevent the operation of friction, as shown in figure 3. The ventilation area is reduced from 1.0m² to 0.56m² by calculation.

Fig. 3 Schematic diagram of the second transformation

03 Achievements

The grinding fineness is controlled at 8% ~ 10%, the vibration of the mill is restored to normal value, the grinding pressure is raised to 11 ~ 12MPa, the mill runs stably and the working condition is good.

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