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A cement rotary kiln is generally mainly composed of a tropical zone, decomposition zone, transition zone, firing zone, cooling zone, kiln mouth, and other parts. The refractory materials selected for the construction of these parts are also different depending on the temperature and purpose used. Today’s editor let me give you a brief understanding of how they choose refractory materials.
1. Preheating zone and decomposition zone
The temperatures in these two places are relatively low, requiring the brick lining to have a small thermal conductivity and good wear resistance. In this area, alkali sulfate and alkali chloride from raw materials and fuels begin to volatilize, condense, and concentrate in the kiln, and penetrate the interior of the bricks. Ordinary clay bricks react with alkali to form potassium nepheline and leucite, which makes the brick surface brittle and causes the brick body to expand, causing cracking and peeling. Al2O3 25% ~ 28% and SiO2 65% ~ When about 70% of alkali-resistant bricks or alkali-resistant insulation bricks react with alkali at a certain temperature, a high-viscosity glaze layer will immediately form on the surface of the brick to prevent peeling, but this Bricks cannot withstand service temperatures above 1200 degrees Celsius. Therefore, phosphate and high alumina bricks, spalling-resistant high alumina bricks, or alkali-resistant bricks are generally used in preheating zones.
Decomposition zones generally use high-alumina bricks with good spalling resistance. Silica molybdenum bricks are superior in performance to high alumina bricks with good spalling resistance. Its service life is about twice that of spall-resistant high-alumina bricks, but the price is higher. The feed inlet at the end of the kiln should be made of silicon carbide castable that resists crusting.
2. Transition zone and firing zone
The kiln skin in the transition zone is unstable, and the kiln lining is required to have good resistance to atmosphere changes, good thermal shock ability, low thermal conductivity, and wear resistance. Magnesia-alumina spinel bricks are recommended abroad, but the bricks have large thermal conductivity, high cylinder temperature, and large relative heat consumption, which is not conducive to reducing energy consumption. Domestic silica molybdenum bricks have small thermal conductivity, are wear-resistant, and their performance is comparable to imported materials to a certain extent.
The temperature in the firing zone is high and the chemical reaction is strong, which requires the brick lining to resist clinker erosion and have strong resistance to SO3 and CO2. Magnesia-alumina spinel bricks are generally used abroad, but they are difficult to hang on the kiln skin, while dolomite bricks have poor thermal shock resistance and are easy to hydrate; foreign magnesia-alumina spinel bricks have better effects on the kiln skin, but the cost too high. Low chromium magnesia composite spinel bricks have been well used in China.
3. Cooling zone and kiln mouth
The temperature in the cooling zone and the kiln mouth is as high as about 1,400 degrees Celsius, with large temperature fluctuations. The grinding of clinker and the erosion of airflow are very serious. The thermal conductivity of the brick lining is required to be small, have good wear resistance, and good thermal shock resistance; basic bricks with good thermal shock resistance such as spinel bricks or high alumina bricks are suitable for cooling zones. Spinel bricks are generally recommended abroad, but spinel bricks have large thermal conductivity and poor wear resistance. In recent years, silicon grinding bricks and wear-resistant bricks with good spalling resistance have been widely used in China.
The kiln mouth part mostly uses castables with good thermal shock resistance. For example, wear-resistant and thermal-shock-resistant high-alumina bricks or steel fiber-toughened castables and low-cement high-alumina castables, but ordinary or steel fiber-toughened corundum castables are used for large kilns with extremely high temperatures at the kiln mouth.
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