There are many factors that affect the service life of refractory linings in rotary kiln , and they are not just problems with refractory products. They include thermal stress, chemical stress, mechanical stress generated by cement production operations, the quality of calcined raw materials, the design and selection of refractory materials, the quality of refractory materials, storage, construction, masonry and other factors.
The impact of mechanical stress on the service life of refractory bricks in kilns
Mechanical stress is the internal force that interacts between the various parts of an object when it is deformed due to external factors, in order to resist the action of such external factors and try to restore the object from the deformed position to the position before deformation. When the mechanical stress on the refractories bricks in the kiln exceeds its own strength, the bricks will be partially or completely damaged under the action of stress.
The main factors that cause mechanical stress are the following two reasons:
(1) Elliptical deformation. Due to the combined factors of the lining bricks, kiln materials and the weight of the cylinder itself, the kiln cylinder is deformed. Under the action of gravity and heat load, the circular cross-section of the cylinder becomes elliptical. When the kiln is running, the ellipse causes mechanical stress on the refractory bricks, and the greater the ellipse, the greater the mechanical stress. The shear stress generated by the change in ellipse acts on the tangent direction of each ring of bricks, causing the bricks to peel off in a ring shape. Under normal circumstances, the peeling thickness is uniform and hard. (2) The axis of the rotary cement kiln is offset. The rotary kiln is supported by a belt, a supporting wheel, and a roller. Its axis should be the line connecting the center points of the kiln's cross-section circles and on a straight line. However, after the kiln shell is installed and part of the shell is cut and replaced, or after the kiln has been running for a period of time and the thermal system in the kiln is unstable, the axis of the kiln will be offset under the action of heat load and load. After that, after a long period of operation, the belt and roller will wear, the roller will deviate outward and inward, and the load conditions of each support point will change. In particular, when the load of the support point is too large, it is easy to cause the roller bearing to burn out, the belt and roller surface to peel off abnormally or crack, which will further aggravate the axis offset of the kiln body; as a result, the refractory bricks will be squeezed and deformed, causing damage or falling off. The shape of damaged fire bricks varies in depth.
The impact of thermal stress on the service life of refractory firebricks in the kiln
The thermal stress refers to the stress generated when the object cannot expand and contract completely freely due to external constraints and mutual constraints between internal parts when the temperature changes. High-temperature thermal expansion can easily cause axial expansion and compression stress in refractory firebricks, which is one of the important reasons for the spalling and breakage of fire bricks in the kiln. Taking magnesia-chromium or spinel bricks as an example, the expansion rate at 1400℃ can be calculated as 1.6%. The expansion of a fire brick with a length of 198mm can reach 3.17mm. Such a large expansion, if the annular gap is not properly left, will cause the refractories bricks to draw, fall off, or spall, seriously shortening the service life of the refractory bricks.
The impact of rotary kiln production operation on the service life of refractory materials
The impact mechanism of production operation on the service life of refractory fire bricks is complex and there are many factors. It is mainly analyzed from the following two aspects.
(1) Brick damage caused by excessively high calcination temperature. The flame temperature within the new dry precalciner kiln can reach over 1700°C. Furthermore, the operating temperatures of the transition zone, firing zone, cooling zone, kiln hood, cooler throat, high-temperature zone, and nozzle exterior are significantly higher than those of conventional kilns. Even with high-quality refractory materials, the lining life of the transition, firing, and cooling zones of large rotary kilns is typically 0.5 to 1 year, with some experiencing only 3 to 5 months. The kiln mouth and nozzle linings typically have a lifespan of only 0.5 to 1 year, or even less. The kiln hood and cooler throat linings have a lifespan of approximately 2 years. During the pilot production phase, rotary kilns typically operate at only 70% to 75% or even lower, with rare instances reaching 85% to 90%. If this is coupled with poor preheater and precalciner operation, the decomposition rate of the incoming material can be highly unstable, leading to frequent shifts in the positions of the various process zones within the kiln. This can lead to unstable kiln operation and accelerated lining damage. For example, excessively high firing temperatures can damage the kiln's refractory bricks, as shown in the circled area in Figure 5. (2) Brick damage caused by high kiln speed. The rotation speed of the new dry pre-decomposition kiln is often 3-3.7 r/min, or even as high as 4 r/min or more, and the linear speed of the rotary kiln shell reaches more than 1 m/s. In the new dry kiln with high rotation speed, large diameter and high temperature, the combined destructive effect of thermal stress, mechanical stress and chemical erosion on the kiln lining is much greater than that of traditional kilns. This requires that the kiln lining of the new dry kiln must have sufficient strength and stability whether in cold or hot conditions.
