- Time:Jun 05, 2023
Why we should study the influencing factors of firebrick life: The length of life of firebrick used in kiln is not only related to the normal operation of production, but also increases the production cost, so improving the service life of firebrick is the main direction of the development of refractory materials.
First, mechanical force
During the operation of rotary kiln, the lining of kiln is subjected to the comprehensive action of mechanical stress such as pressure, tension, torsion and shear, so that the brick is subjected to dynamic load, the weight of brick and kiln skin and the thermal expansion of brick itself, so that the brick bears static load. In addition, the relative motion between brick lining and kiln cylinder, and between brick lining and brick lining will make the brick lining withstand various mechanical stresses. When the sum of all the stresses exceeds the structural strength of the brick, the brick cracks and fails. For bricks that are close to the retaining ring, most of the damage is due to extrusion pressure. Mechanical stress accounts for 37% of resistant material damage.
Mechanical forces affect the dislocation of firebricks
Two, chemical erosion
With the extensive use of coal instead of oil, especially alternative fuels, the volatile components mainly alkali sulfate and alkali chloride increase in the content of kiln gas and kiln materials, and alkali salt penetrates deeply into the brick layer. Chemical erosion is one of the main factors causing the damage of refractory bricks in the transition zone of cement rotary kiln. The gas circulation enrichment in large rotary kiln is more serious. Higher alkali-sulfur content will accelerate the penetration and cause rapid damage of refractory materials.
Three, clinker melt infiltration
The clinker melt is mainly derived from kiln materials and fuels, and the infiltrated phases are mainly C2S (dicalcium silicate) and C4AF (tetracalcium ferroaluminate). The C2S and C4AF penetrating into the metamorphic layer will strongly dissolve the cubic magnesite and other parts in the alkaline brick, precipitate silicate minerals, and sometimes even precipitate potassium nepheline. The melt will fill the pores in the lining of brick, making the brick layer dense and embrittlement. Combined with thermal stress and mechanical stress, the brick is easy to crack and spalling. Since C2S and C4AF begin to form when the temperature of the materials in the pre-decomposition kiln is above 550℃, the clinker melt penetrates through the whole pre-decomposition kiln, that is, the clinker melt has a certain infiltration and erosion effect on the lining of each belt in the pre-decomposition kiln. In addition to suitable refractories and strength, the refractories used in this part should also have excellent corrosion resistance.
clinker melt infiltration
Four, thermal shock phenomenon
When the kiln operation is not normal or the kiln skin is unstable, alkaline brick is easy to be damaged by thermal shock. The sudden collapse of the kiln skin causes a sudden rise in the temperature of the brick surface (even up to thousands of degrees), which causes great thermal force in the brick. In addition, the frequent opening and stopping of the kiln causes frequent alternating thermal stress in the brick. When the thermal stress exceeds the structural strength of the brick lining, the brick begins to crack, and increases and deepens along its structural weakness, and finally makes the brick crumble. Thermal shock phenomenon is easy to occur in the transition zone near the kiln tail.
Five. Thermal stress
In the process of oven temperature rise of kiln lining, due to the thermal expansion of refractory materials generated by high temperature, the adjacent refractory materials squeeze each other. When the extrusion stress is greater than the strength of refractory brick, it will lead to the hot surface of refractory brick spalling off. Thermal stress is one of the important reasons for the spalling and damage of refractory brick in kiln. Taking magnesia iron brick or magnesia aluminum spinel brick as an example, the expansion rate calculated by 1.6% at 1400℃, the expansion of 198mm long firebrick can reach 3.16mm, so it is necessary to properly reserve the ring joint (expansion joint), too large reservation may lead to the phenomenon of drawing lots and dropping bricks, and the thermal stress caused by too small reservation is not released. It will lead to spalling phenomenon of refractory brick, which will seriously shorten the service life of refractory brick.
Six, the operation process is too frequent to stop the kiln
For example, the first-line kiln has been stopped for 36 times from January to August, and the number of kiln stops increases. As a result, the kiln is opened and stopped more times. In the process of heating, cooling and heat preservation, the kiln skin falls off frequently, which damages the firebrick very much. Sometimes the heating and cooling speed of the kiln is too fast, which leads to the cooling speed of the refractory brick and causes the fracture of the refractory brick.
the operation process is too frequent to stop the kiln
Seven. Quality of fuel
If the service life of firebrick lining can be guaranteed and the solid kiln skin effect can be maintained, the thermal stability is the key. But it is difficult to maintain thermal stability because of the uncertainty of the raw fuel. Generally, the raw fuel of cement kiln is coal. Due to the difficulty of coal supply, the ash content of coal will vary from 32% to 45%. The fluctuation of coal quality will affect the sticking of kiln skin, and the kiln skin is easy to peel off along with the brick layer. Especially in the case of kiln with frequent opening and stopping, the protection of kiln skin is lost, and the lining of firebrick is vulnerable to chemical erosion, which reduces the thermal fatigue resistance and shortens the life.
Viii. Quality of masonry construction
Due to construction factors, the masonry of firebrick is too loose and distorted. During the operation of rotary kiln, the kiln cylinder and firebrick have relative motion, and the excessive thrust of brick lining will lead to the torsional dislocation of firebrick, brick layer crack, corner drop, and even brick drop.
Attention needs to be paid to masonry: brick lining close to the kiln shell, brick and brick should be strict, brick seam should be straight, intersection circle should be accurate, lock brick should be firm, good position, not sagging, not empty. In short, to ensure that the firebrick and kiln body in the cellar operation has a reliable concentricity, brick lining stress should be evenly distributed throughout the kiln lining and each brick.
Nine, the burner flame
In the clinker exercise in rotary kiln, the reasonable distribution of the appropriate flame temperature in the kiln plays a very important role in the improvement of clinker quality, the proper thickness and length of kiln skin, the extension of kiln lining life and the amount of fuel consumption.
In the combustion process of rotary kiln burner, the stable flame is an important indicator of the burner, among which the swirl number mainly controls the shape of the flame, so it is usually called the flame shape coefficient. Increasing the swirl number can make the flame coarser and shorter, thus enhancing the flame thermal radiation. However, if the number of swirls is too large, the double-peak flame will appear, resulting in overheating and peeling of local kiln skin. The flame shape can make the whole firing zone have strong and uniform heat radiation, which is conducive to clinker granulation and prolong the service life of firebrick.
