During the calcination process of cement clinker, the position of the kiln mouth is very special, and it rotates rapidly with the kiln body, and is affected by the rapid change of temperature and heat.
The working conditions of the kiln mouth of the precalciner kiln: the temperature of the clinker exiting the kiln is around 1350℃, and the castable is eroded, and the air temperature entering the kiln from the grate cooler is above 1000℃, and the change is irregular. The castable is flushed. The kiln rotates rapidly at a speed of about 4.0r/min, the back iron at the kiln mouth is extruded by the high temperature expansion, and the flame at the kiln head as high as about 1700 ℃ produces huge heat radiation to the castable. The ovality deformation of the kiln mouth cylinder increases the possibility of the castable leaving the kiln body. When the production is abnormal, the castable at the kiln mouth has to undergo the rapid cooling and rapid thermal shock of starting and stopping the kiln. Generally, the castables at the kiln mouth fall off, and the kiln will be stopped immediately.
Factors Affecting the Life of Castables at the Kiln Mouth
1. The size of the negative pressure at the kiln head
The negative pressure of the kiln head reflects the balance between the air used in the kiln and the amount of hot air entering the kiln head cover. Generally, the exhaust air volume of the kiln head exhaust fan increases, and the negative pressure of the kiln head increases, and vice versa. The hazards of positive pressure at the kiln head are mainly manifested in: the flying sand and hot flue gas from the kiln head, the kiln head seal is easy to burn out, the degree of damage to the kiln door and the wear and tear of the kiln head cover are aggravated. Burn down the kiln head cover; the positive pressure can easily cause damage to the peripheral equipment, the kiln head cannot see the fire, and the splashing high-temperature materials cause personal injury. As mentioned above, the kiln head pressure is affected by the discharge volume of the kiln head and kiln tail exhaust fans. In the same production line, the ventilation volume of the kiln is relatively constant, and the clinker needs to be cooled to a reasonable temperature. That is to say, when the system is designed and matched, the capacity of the exhaust fan at the kiln head should not be too small, otherwise the remaining cooling air volume will have nowhere to go. Row.
2. Kiln system operation rate
The comprehensive operation rate of the kiln system indicates whether the production is normal, whether the management is fine and stable, and the frequency of failure and shutdown. Under normal circumstances, if the comprehensive operation rate of the kiln system reaches 85%, it is considered to be basically normal, and if it exceeds 90%, it is considered to be normal production, which indicates that there are few unplanned maintenance of the production line. The refractory material of the kiln system is most damaged by the opening and closing of the kiln, and the kiln mouth castable is the most serious part of the kiln opening and closing. The higher the comprehensive operation rate of the kiln, the longer the service life. In case of accidents such as shrinkage and blockage of the calciner, the surface temperature of the kiln lining will drop from about 1400 °C to 100~200 °C in more than 10 hours. If the heat is repeated repeatedly, the internal structure of the castable at the kiln mouth is damaged, and it is easy to break and fall off, which has a great negative effect on the service life.
3. Kiln mouth back iron structure
The kiln mouth structure is generally composed of cylinder, back iron and castable. The function of setting the kiln mouth guard iron is to effectively protect the kiln mouth cylinder section made of ordinary steel and avoid high temperature flue gas burning. The two are connected and fixed together by 4 bolts to form a whole. The back iron relies on the protection of the castable to reduce the working temperature and abrasion and ensure the stability of the structure. The large vibration caused by the deformation of the kiln head cylinder and the looseness of the back iron bolts are the important reasons for the castable falling off at the kiln mouth.
4. Influence of castable quality and pouring construction quality
As mentioned above, due to the particularity of the working conditions at the kiln mouth, ordinary castables cannot adapt to such a harsh working environment. The castables here should have the characteristics of high temperature resistance, high strength and good thermal vibration performance. At present, there are many domestic manufacturers of kiln mouth castables, the price is basically around 2000~10000 yuan, and the service life of kiln mouth castables is basically proportional to the price.
There are many problems in construction quality control. The structure of the anchoring nail is unreasonable, the welding at the intersection is not standardized, and it is easy to be melted at high temperature; the anchoring nail is single-angle, the welding surface is less, and it is not easy to weld firmly; the layout of the anchoring nail is unscientific, and there are many blind spots, especially the guard plate and the block ring The castable above cannot be pulled. Insufficient understanding of the importance of expansion joint retention during pouring, too little or no retention in actual construction, resulting in kiln mouth castables being squeezed and bursting. In order to save time and not support the formwork in order to catch up with the construction period, the operation point was moved from 7 o’clock or 9 o’clock in the circumferential direction to the 6 o’clock position for construction, resulting in the poured castables being loose and not compact. The mixing is not standardized, and the dry material is not fully mixed before adding water. Considering the influence of construction convenience and construction time, it is adopted to increase the amount of water for mixing and pouring with flowing material while stirring. The mixing and molding time are not well coordinated, resulting in the mixed castable not being used in time. The pouring and rapping are uneven, and the phenomenon of prolonged vibration or revival is prominent. Affected by the urgency of production, the damage to the castable is also very large if the curing time is too short.
5. The influence of the block ring at the kiln mouth
The service life of the kiln mouth castable and the integrity of the kiln mouth block ring are the biggest factors among many factors. The burning loss of the block ring at the kiln mouth forms an inverted triangle, and the kiln bricks will be directly squeezed to the castable at the kiln mouth, which will cause the castable at the kiln mouth to be squeezed off in a large area, and the kiln will be forced to stop. Combined with the many times of kiln mouth guard iron castable falling off accidents in our company in the past one and a half years, the first to bear the brunt is that once the kiln mouth retaining brick ring is burnt, it must be replaced as soon as possible.
solution
1. Optimize the design of the kiln mouth structure
The structure of the kiln mouth was improved. The number of back iron remains unchanged, and the connection method of back iron is changed from the original single block connection to the box connection, that is, it is composed of an inner back iron and an outer box, and the kiln shell is in between, and is connected with countersunk head bolts. As a whole, the outer box is filled with castables to protect the fixing bolts and the cylinder from high temperature burning and deformation. The thickness of the castables of the straight wall below the kiln mouth should be appropriately thickened to ensure that the end face of the kiln mouth is shrunk inside the straight wall without affecting the clinker falling into the grate cooler, so as to avoid the kiln mouth being directly washed and burned by the high temperature hot flue gas of the grate cooler. damage.
2. Strictly control the quality of castable construction, and do corresponding improvement work
(1) In order to increase the wear resistance, the effective thickness of the firing material above the kiln mouth guard plate is increased from 160mm to 220mm.
(2) Reasonably arrange the anchoring nails, and use the materials with a relatively large welding area for the anchoring nail structure.
(3) When pouring the castables at the kiln mouth, the principle of first supporting the formwork and then constructing is strictly adopted, and all pouring must be controlled at 7 o’clock or 9 o’clock in the circumferential direction.
(4) The stirring of the castable must be uniform, and the water is controlled at the minimum value, and a special person is responsible for stirring.
(5) Expansion joints are set. Based on practical experience, 3mm plywood is used to set expansion joints for every 600mm length. All anchoring nails should be coated with asphalt paint and wrapped with black tape to compensate for their expansion space.
(6) The castable material is changed to the special castable for the kiln mouth to improve the high temperature resistance and vibration resistance.
(7) Abandon the original maintenance thinking, give priority to the construction of the castable at the kiln mouth under the conditions of each maintenance, and prolong the curing time of the castable as much as possible.
3. Improve the ventilation of the kiln system to ensure a slight negative pressure on the kiln head cover
(1) Improvement of kiln tail slope ventilation. The total thickness of the original slope refractory material was adjusted from 250mm to 200mm AZM1680 silicon molybdenum brick masonry.
(2) Improvement of the kiln head system. In view of the actual situation that the processing capacity of the kiln head exhaust fan is relatively small, the air leakage of the kiln head system is minimized, so as to improve the air volume processing capacity of the kiln head and basically meet the production requirements.
