Common problems and treatment of vertical mill during operation

Compared with the ball mill, the vertical mill has many important outstanding advantages, such as high grinding efficiency, simple process flow, small footprint, and the advantages of drying, grinding and powder selection are more and more widely used in the new dry cement production line, which is a large new dry cement production line ideal and effective grinding equipment. According to statistics, in the new dry process production lines that have been built or newly built, the vertical grinding of raw coal and raw materials accounts for more than 90%. How to carry out fine management, optimize process parameters, stable operation, increase output, reduce energy consumption, improve operation rate, and continuously improve economic benefits is the central issue of maintenance and management of vertical mill. Gansu Yongdeng Qilianshan Cement Co., LTD. Raw material preparation system adopts ATOX37.5 vertical mill provided by Denmark Smith (FLS) company. Since the mill has been put into operation, the actual operation situation is not ideal, but in the process of use, we continue to explore and improve, constantly optimize the operation plan and implement fine management, and solve all kinds of problems since the operation has been put into operation, so that the system operation is increasingly perfect and stable, and the operation rate is as high as 90%. This article introduces the equipment and operation.
1 Equipment
1.1 Adjustment of center support deviation The purpose of roller position adjustment is to make the combined center of the roller and center support located in the center of the mill. Due to long-term wear and vibration, the gap between various connection parts becomes larger, the torque rod buffer pad is aging and hardening and loses the buffering effect, and the impact thickness becomes thinner (the original thickness of each piece is 125mm, but now it is only 120mm), so that the activity of the roller becomes larger, and the center of the roller position is offset (if the center of the support is larger). It can be seen from the gap of the roller air seal that the gap of the air seal on both sides will not be the same); At the same time, due to vibration and wear, the position of the torsion rod and the tensile rod is offset, which not only makes the torsion rod lose its protective effect, but also forms the effect of torsion bending on the tensile rod and produces torque on the tensile rod, which is the cause of frequent fracture of the tensile rod and bolts. When checking the position of the roller, the roller must be placed on the grinding disc, and no grinding pressure is applied, the reducer lubrication station is in a normal working state, and the grinding disc is rotated by manually turning the shaft connecting device between the reducer and the motor. After manual rotation of the grinding disc for a week, measure the distance between the block at the end of the lining plate of the grinding roller and the edge of the stopper ring of the grinding disc. The difference is within ±5mm, and it can be considered that the position of the grinding roller is correct. Otherwise, it needs to be adjusted until the position is correct.

1.2 Maintenance of tensioning hydraulic system The hydraulic system is one of the most important equipment systems in vertical mill, and its operation reliability directly affects the normal operation of the mill. The huge grinding pressure exerted by the mill roller on the material is provided by the hydraulic system, but the action of the tensile rod caused by the failure of the hydraulic system is inconsistent. When rolling down and lifting the three rollers are not synchronized, it can cause huge vibration of the mill. If the accumulator pressure is insufficient or the nitrogen air bag and check valve are damaged, the roller will lose its buffering effect, so the hydraulic system plays a key role in the stable operation of the opposite mill.
(1) Sealing and inspection of hydraulic cylinder and oil circuit. In the early stage of vertical mill operation of Qilian Mountain Cement Co., LTD., Gansu Yongdeng, due to the top of the hydraulic cylinder, the fine particle material is squeezed between the hydraulic cylinder and the piston rod, and the chrome layer on the surface of the piston rod produces longitudinal tensile marks due to wear, resulting in oil seal damage and oil leakage. At the same time, due to the break of the valve handle of the accumulator check valve, the bolts and washers enter the hydraulic cylinder, resulting in longitudinal tensile marks in the cylinder, and the piston ring is also pulled, and it is replaced by the maintenance opportunity and sent to the professional manufacturer for plating treatment. The hydraulic oil in the hydraulic system has high cleanliness requirements. In order to keep the hydraulic oil clean, the surrounding environment must be clean and clean when the cylinder block is checked, the pipeline is cleaned, and the nitrogen air bag and hydraulic oil are replaced, so as to prevent micro-particle objects from being brought into the hydraulic oil, causing the cylinder block to be scratched and the nitrogen air bag to be damaged. In order to prevent fine particles from entering the cylinder, a soft connection sheath can be made on the outside to protect it.
(2) Inspection and maintenance of the accumulator. The motion of the vertical grinding roller makes the oil enter the cylinder from the actuator, and the nitrogen in the accumulator air bag works like a gas spring. A small change in pressure will affect the flow of oil, which means that a large amount of nitrogen is equivalent to a large correction pressure, which will cause elastic buffer. The accumulator must correct the appropriate pressure, and the pressure of the nitrogen air bag in the accumulator is 50%-70% of the normal grinding pressure. Gansu Yongdeng Qilianshan Cement Co., Ltd. set the grinding pressure between 100-115mbar, so the nitrogen air bag inflation pressure between 55-60mbar. The accumulator should be checked regularly, if there is damage or low pressure to find the cause, timely replacement and supplement. During the inspection, if the temperature of the accumulator is found to be high, and the bottom of the accumulator is accompanied by a knocking sound, then the check valve of the accumulator may have been damaged, easy to damage the nitrogen air bag, the accumulator will not play the role of buffering shock absorption, and can not absorb the impact of the hydraulic system due to the change of the thickness of the grinder material layer, which is easy to cause the tensile rod and bolt fracture. During installation, hydraulic oil should be applied to the outer wall of the nitrogen air bag and the inner wall of the shell to reduce the friction between the nitrogen air bag and the inner wall of the shell and protect the nitrogen air bag. During the normal production process, the nitrogen of the accumulator is checked once a week.
(3) Control valve adjustment. The balance of the pressure of the hydraulic station system, that is, the balance between the given pressure and the feedback pressure, is adjusted by the hydraulic valve. Before the control valve of the hydraulic system is adjusted, the hydraulic system must be firstly vented, the hydraulic pump should be opened to make the hydraulic oil system full of hydraulic oil, and the gauge pressure should be detected when the grinding roller rises to the highest level. If the gauge pressure is higher than the selected pressure, the adjusting bolt of the control valve should be adjusted counterclockwise. And vice versa. Repeated adjustment three times, the meter pressure is consistent, the adjustment process is complete.

1.3 Maintenance of roller lubrication system ATOX37.5 vertical mill roller adopts thin oil circulation lubrication system, each roller has its own oil supply pipe, return pipe and balance pipe. The vacuum degree of the roller return tubing determines the oil level in the roller chamber. In order to keep the oil level in the roller relatively stable, the adjustment of the vacuum pressure must be careful, which is directly related to the lubrication state of the roller bearing. The oil pressure is too high, the oil level in the roller rises, the oil amount is too high, easy to cause the roller oil leakage, and even damage the oil seal; The oil pressure is too low, the oil content in the roller is low or lacking, and the lack of lubrication is easy to damage the roller bearing. The general vacuum degree adjustment range is between -0.10 and -0.35bar. And to add grease to the roller seal ring regularly, the general adding cycle is one month.

1.4 Mastering the wear and replacement cycle of the mill roll and the liner of the mill disc Due to the rotation of the mill disc, the material is affected by the centrifugal force, and large materials are concentrated in the outside area of the mill disc. In the production process, the wear of the outside of the mill roll and the liner of the mill disc is large. In addition, in the production of low alkali raw materials, the addition of siliceous SLATE is large, the hardness is high, and the grindability is poor. It accelerates the wear of the lining plate, and sometimes the phenomenon of falling blocks occurs. When the material layer changes greatly or the material layer is thin, this unbalanced wear will cause the impact of the inside of the grinding roll and the grinding disc, resulting in vibration.

1.5 Attention during the replacement of the connecting bolt of the stretching rod and the joint head Due to the fracture of the connecting bolt of the stretching rod and frequent replacement of the bolt, the surface of the flange of the stretching rod is uneven, and the surface of the press head of the hydraulic plate is not smooth enough. In this way, the force of the bolt is not uniform in the process of pressure, and the bolt may also be stretched and bent, and the fastened bolt is stressed unevenly. Bolts with large forces are easy to break during operation. Therefore, the connection flange should be replaced in time when it is not smooth, and the hardness and strength of its material are relatively high, and the use of the hydraulic wrench should be correct when tightening the bolt. The tightening sequence of the tensile rod bolt should be divided into three times, the first pressure to 540bar, the second pressure to 720bar, the third pressure to 960bar, not one pressure in place.

In normal operation, the air seal is washed by materials and air flow, the gap between the inner and outer rings becomes larger, and the air pressure between the inner and outer rings becomes smaller, so that the powder material enters the joint bearing, and the adjustment function of the bearing fails, which will cause the fracture of the tensile rod bolt. When the gap between the inner and outer rings of the air seal becomes large, it is necessary to adjust it in time, add a cushion under the outer ring or replace it at the same time, ensure that the gap between the inner and outer rings is about 0.5mm, and ensure the normal operation of the sealing fan, and the wind pressure shall not be less than 15bar, and it shall be kept above 20bar under normal circumstances. In order to ensure that the incoming positive pressure air is clean, a filter cloth should be installed at the entrance of the fan to prevent micro-particle materials from entering the roller sealing air chamber, damaging the roller sealing ring and oil seal, resulting in oil leakage of the roller. The lubricating oil should be kept clean, and the replacement interval of the oil filter should be used to determine whether to replace the lubricating oil.

1.7 Adjusting the height of the stopper ring The output of the vertical mill is directly related to the height of the stopper ring. The height of the stopper ring is not a fixed value. When the feeding amount and grinding pressure are certain, the thickness of the material layer is mainly adjusted by the height of the stopper ring. Different material characteristics and product fineness, there are different stopper ring height, so in the specific operation and adjustment, it is necessary to take into account the stopper ring height and grinding pressure, with the wear of the roller liner to reduce the stopper ring height in time.

2 Central control operation
2.1 Control of pressure difference and materials in the mill During operation, the stability of pressure difference is crucial to the normal operation of the mill, and the change of pressure difference mainly depends on the feeding amount and the particle size of the material. The stability of pressure difference indicates that the amount of material in the mill and the amount of material out of the mill tend to balance, and the mill runs normally. When the pressure difference rises, the amount of material in the mill is greater than the amount of material out of the mill, the amount of internal circulation increases, the ventilation resistance of the whole system increases, and the amount of external circulation also increases, at this time, the appropriate material reduction operation can be considered. When the pressure difference decreases, it indicates that the amount of material entering the mill is less than the amount of material leaving the mill, the amount of internal circulation decreases, and the thickness of the material layer becomes thinner. When the particle size of the material is too small, there are too many powdered materials, such as fly ash feeding fluctuation increases, due to the poor adhesion of the fine particles, good fluidity, the mill roller is not easy to effectively control the normal grinding of the material, so that the dust concentration in the mill increases, the pressure difference increases, the ventilation resistance increases, the normal trajectory of the air flow is destroyed, resulting in the mill vibration intensification, the vertical mill will suddenly stop. Controlling the stable pressure difference is conducive to maintaining the stable operation of the mill.

2.2 Control of the thickness of the material layer The material layer is actually a cushion between the roller and the grinding disc. Under normal circumstances, the extrusion of the roller and the grinding disc to the material is the extrusion of the material in the material layer. When the material layer is too thin, its buffering effect will be weakened and the vibration will be increased. Gansu Yongdeng Qilianshan Cement Co., LTD. ATOX37.5 vertical mill material layer thickness is generally controlled in 30mm ~ 45mm, when the material layer thickness is thin, the measures can be taken are:
① Increase the amount of water in the mill;
② Reduce grinding pressure;
(3) Appropriately increase the feeding amount, and generally the fluctuation of the feeding amount shall not be greater than 5%;
④ Increase the speed of the separator and increase the amount of material circulation in the mill. If the thickness of the material layer is thicker, the reverse is true. Stabilizing material layer can reduce vibration of vertical mill.

2.3 Control of temperature inside the mill Too much change in gas temperature at the outlet of the vertical mill will lead to unstable operation of the mill and cause vibration, the temperature inside the mill is too high, the material becomes very loose, not only the material layer becomes thin, but also not easy to roll, which will produce violent vibration. The outlet temperature of 2 ATOX37.5 vertical mills of Qilianshan Cement Co., Ltd. in Yongdeng, Gansu Province is controlled between 70 and 80℃, and the exit grinding temperature is controlled mainly by the temperature of inlet grinding gas and the amount of water spraying. The function of water spraying is to stabilize the material layer of the mill and reduce the grinding temperature. Too much water spraying will form a cake, resulting in low temperature in the mill. The amount of water spraying is too little, the material layer is unstable, and the vibration will be intensified. The water spraying amount can be adjusted according to the temperature. When the feeding amount and the wind temperature and air volume are certain, the water spraying amount can be controlled to the lowest amount.

2.4 Matching of system air volume, wind speed, material volume and grinding pressure such as low system air volume, resulting in more slag discharge, the gas can not blow the medium particle material back to the mill, resulting in thinning of the material layer; Too much lapping will also cause the material layer to become thinner and cause vibration. In actual operation, it is necessary to control the two key parameters of air volume and wind speed. The size of the wind speed on the one hand controls the size of the dropped particles, on the other hand controls the circulation amount and the thickness of the bed. Reasonable wind speed can form a good internal circulation conducive to the classification of materials, so that the thickness of the material layer on the disk is appropriate, and the grinding efficiency is high. ATOX37.5 vertical mill wind speed is generally controlled at 50 ~ 80m/s. In the production process, when the air ring area has been determined, the wind speed is determined by the air volume. From the perspective of stable mill operation, matching the air volume with the output is conducive to the relative stability of the mill load, such as large feeding volume, the air volume should be large; The feeding amount is small, and the air volume should be reduced. The low material wind leads to the increase of circulation, the increase of pressure difference in the mill and the vibration. When the material is small and the wind is large, the pressure difference drops, and the material layer on the mill is too thin, which also affects the normal operation of the equipment. Under normal circumstances, the ventilation volume in the mill can be adjusted by controlling the current of the circulating fan, and the inlet pressure of the mill can be stabilized.

2.5 Mastering the timing of lifting roll and dropping roll When the grinding lift roll is stopped late, it is likely to cause the material layer is thin or there is no material layer and the grinding roll is still on the grinding disc; When the opening roll is early, there is no material or less material on the grinding disc, the grinding roll falls. In both cases, the roller and the disk will directly impact, causing vibration.

3 Conclusion By solving the above problems, summarized the use and maintenance of vertical mill in the following aspects of the key:
(1) Vertical mill is an efficient and energy-saving grinding system with high precision and requirements, and maintenance, operation and overhaul must be performed in accordance with the specified requirements.
(2) Do daily spot inspection and professional spot inspection, find problems early, solve them in time, do not expand the problem, and maintain good performance for a long time.
(3) The damage of the hydraulic system, the failure of the accumulator, the wear of the roller and the lining plate of the grinding disc are the main reasons for the fracture of the tie rod bolt, and improper operation will accelerate the frequency of the fracture of the tensile rod bolt.
(4) Maintenance must be strictly implemented in the "Equipment maintenance Code", each step must be accurate as required, and its performance must be fully restored after maintenance.