Since the kiln mouth is not protected by the kiln skin, it is exposed to the flame radiation of 1700℃. This area faces multiple challenges, including the scouring of high-temperature clinker from the rotary kiln and the impact of 1200℃ secondary air and airflow on the powdered clinker. In addition, factors such as cylinder deformation caused by mechanical force, loosening of iron guards, welding of brick retaining rings, and extremely cold and hot cycles caused by frequent kiln opening and closing make the environmental conditions in this area extremely harsh. These factors are interrelated and together lead to the damage of kiln mouth refractory castables.
1. Selection of castables
High-quality castables are an important guarantee for extending the service life. With the continuous increase in the diameter of cement kilns and the increase in kiln mouth outlet materials, this not only increases the mechanical force, but also greatly increases the thermal strength in the kiln, the damage of thermal stress is more obvious, and the heat load per unit area of the castables also increases accordingly. The kiln mouth castables need to meet sufficient refractoriness, mechanical strength, thermal shock stability and high wear resistance.
The kiln mouth is determined to use corundum castables or corundum silicon carbide high temperature castables. In view of the weak characteristics of the kiln mouth, it is made of high-quality corundum, silicon carbide, imported andalusite and binder. It has the advantages of good volume stability, thermal shock resistance and good wear resistance. In normal operation, the life of the kiln mouth special castable can reach more than 12 months.
2. Increase the length of the kiln mouth castable
Take the 4.8×72m 4500d/t cement kiln as an example. When the length of the kiln mouth refractory castable is 0.6m, the length is too short, which will cause the castable to be weak in the longitudinal direction of the kiln and cannot effectively resist the scouring force of the clinker. However, if the castable length is increased to 0.8m, its longitudinal integrity can be improved, the ability to resist scouring can be enhanced, and the damage of the castable can be reduced.
3. Adding a brick retaining ring
The 72m long cement kiln was originally equipped with three brick retaining rings, located at 0.8m, 50.4m and 64.6m from the kiln mouth, to share the downward pressure of more than 670 tons of refractory bricks in the kiln. Every time the refractory bricks and castables were replaced during maintenance, it was found that the brick retaining ring at 0.8m was squeezed out, indicating that the brick retaining ring was under greater pressure. Moreover, the fall of the brick retaining ring caused the downward force of the refractory bricks to act directly on the kiln mouth, resulting in damage to the castables at the kiln mouth. In view of the above factors, a brick retaining ring was added at 28m of the kiln. The four brick retaining rings jointly bear the pressure of the forward movement of the refractory bricks, reducing the pressure on the brick retaining ring at 0.8m and reducing the damage to the castables at the kiln mouth.
4. Changing from single nails to combined nails
The single nails are directly welded to the kiln mouth guard iron or kiln barrel, cannot be moved, and have a small welding area. When the castable expands due to heat, the single nails are easy to fall off. In contrast, the combined nail adopts a square base and is directly welded to the guard iron or kiln body, which has the advantages of large welding area and strength, and is not easy to fall off.
5. Change the construction method of the kiln mouth castable
The construction of the kiln mouth refractory castable is changed from installing the guard iron at the kiln mouth first, and then pouring the castable as a whole, to first pouring the castable of each guard iron end face separately under the kiln, then installing the guard iron on the kiln mouth, and then pouring the castable of the inner arc surface. In this way, the castable of each guard iron end face is separated from the inner arc surface, and the castable of each guard iron is also separated, overcoming the mechanical stress on the castable during the kiln rotation process and reducing the damage to the castable by the kiln rotation.
6. Modify the smoke chamber at the end of the kiln
Observing the pressure of the smoke chamber at the kiln head and the end of the kiln, it was found that the kiln head often had positive pressure, while the negative pressure of the smoke chamber was high (-400~-700 Pa). Analysis showed that the diameter a of the upper shrinkage of the smoke chamber and the distance b from the lower material tongue to the upper arch were too small, resulting in less air volume in the kiln, causing positive pressure to be generated by tempering.
Increasing a from 2300 mm to 2400 mm and b from 1850 mm to 2000 mm improved the airflow environment in the kiln, reduced the positive pressure at the kiln mouth, and reduced the negative pressure of the smoke chamber to -200~-400 Pa, thereby improving the temperature condition at the kiln mouth and reducing the damage to the kiln mouth refractory castable.
