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During the operation of the cement rotary kiln, faults such as egg accumulation, flying sand, snowman making, and ring formation often occur. These failures not only affect the operation of the kiln but also affect the service life of the lining refractory material. To solve these problems, it is necessary to clarify the formation mechanism of these faults and destroy the formation conditions of the faults, so as to achieve the purpose of high yield, high quality, and low consumption.
Today we will talk about the ring formation problem of cement rotary kilns. The former and the latter are divided into three types. Among them, the most difficult to deal with is the rear ring.
Rear Ring Problem
The latter is the ring formed at the junction of the launch zone and the transition zone. The kiln back ring has a close relationship with the low melting point material in the kiln. It is also closely related to coal ash. Therefore, the latter ring is also called the pulverized coal ring.
When materials in the transition zone enter the firing zone, they will be blocked by the kiln skin. Some materials stay for a certain period of time before entering the kiln belt. Due to the increase in the temperature of the kiln material or the existence of some low-melting point materials, this part of the material may adhere to the front of the kiln skin to form a ring-shaped root. After the rear ring is formed, its protruding part can hinder the flow of gas in the kiln and adhere to the coal powder in the kiln gas to form a coal powder ring. If the strength of the rear ring is low, when the length of the rear ring is large, the heat insulation effect will be enhanced, the temperature of the kiln body near the rear ring will be reduced, and the shrinkage of the kiln body will cause pressure on the inner lining, and the back ring may be damaged due to pressure. If there is a large amount of unstable C2S in the posterior ring, the β-C2S-γ-C2S transformation will occur when the temperature is lowered to a suitable value, leading to pulverization and collapse. However, if the ring is not damaged for various reasons, it can develop into an intractable failure.
The latter is related to coal, especially its ash content and ash melting point. According to Zhang Yanwei’s research, when using Dengfeng No. 2 coal with an ash flow temperature of 1388°C, the cement kiln often forms a loop. If Jincheng coal whose flow temperature is higher than 1500℃ is used, the formation of the back ring can be avoided.
If the back knot doesn’t self-destruct, it will be longer and taller. If it develops to a certain extent, manual treatment is required. Ring formation after treatment is usually accomplished by hot firing and cold flashing. According to Zhang Gaofei’s experience, when burning in a far circle, it is mainly cold, and when burning in a near circle, it is mainly hot.
① When the kiln skin in the kiln is long and thick or has a slight ring root, the coal injection pipe should be pulled out to move the firing zone forward, reduce the solid temperature, change the coal ash settlement position, and make the thick and long kiln skin gradually collapsed. At the same time, adjust the air-coal combination to accelerate the combustion of pulverized coal, so that the low-temperature parts at both ends of the high-temperature zone will not be lengthened to prevent the growth of ring roots.
② When the thick and long kiln skin in the kiln is not handled properly or not in time, it will lead to a rapid increase in the kiln skin cycle, making the thick kiln skin develop into a back ring. First, determine the position and thickness of the ring and the accumulated material behind the ring, then reduce the amount of feed, increase the flame temperature, strengthen pre-firing, gradually increase the kiln speed, keep the kiln rotating rapidly, and discharge part of the accumulated material behind the ring. At this time, it is not appropriate to reduce the temperature of the coal entering the coal circle, so as to reduce the temperature of the flame. After burning for 4-5 hours, pull out the coal injection pipe and burn repeatedly, so that the ring body collapses due to temperature changes. In order to ensure the complete combustion of pulverized coal and prevent the development of ring formation, it is necessary to appropriately change the composition of raw materials, reduce the liquid phase content in the material, appropriately change the coal quality, and use coal with high volatile content and low ash content.
In general, it is difficult to form rings after sintering. Sometimes the ring body is very hard, if the ring burning time is too long, it will burn the kiln skin and lining, or the kiln skin is long and thick in the transition zone, and then a second ring is formed after the ring body. Therefore, be careful when treating it. Under the premise of protecting the kiln skin, it is necessary to take into account the front and rear, and operate carefully to avoid excessive concentration of flames, avoid burning the kiln skin and lining materials, and cause greater losses.
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