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There are two main types of refractory bricks commonly used: clay refractory bricks and high alumina bricks. These two types of refractory bricks are very different in appearance from physical and chemical indicators, and their uses are also very different. So how to distinguish these two types of refractory bricks? Today I will briefly introduce it to you in an article.
1. Clay refractory bricks
Clay refractory bricks are the earliest refractory building materials. They use high-alumina vanadium clay as the main raw material and are used for lining high-temperature kilns to provide thermal insulation.
Clay refractory bricks are the main varieties of silicon-alumina series products. It is a refractory product using clay clinker as aggregate and an Al2O3 content of 30-48%. Clay bricks are divided into ordinary clay bricks and low-creep clay bricks according to their Al2O3 content. Ordinary clay bricks are divided into RN-42 and RN-40. Low creep clay is divided into DRN-45\DRN-42\DRN-40, etc.
The mineral composition of clay bricks is mainly kaolinite (Al2O3·2SiO2·2H2O) and 6% to 7% impurities (potassium, sodium, calcium, titanium, iron oxides). The firing process of clay bricks is mainly a process in which kaolinite continuously loses water and decomposes to form mullite (3Al2O3·2SiO2) crystals. SiO2 and Al2O3 in clay bricks form eutectic low melting point silicates with impurities during the firing process, surrounding the mullite crystals.
2. High alumina refractory bricks
Due to different resources, the standards of high alumina refractory bricks in various countries are not completely consistent. For example, European countries stipulate that the lower limit of Al2O3 content for high-aluminum refractory materials is 42%. In China, high alumina bricks are usually divided into four grades according to the Al2O3 content: special grade: Al2O3 content >80%; first-grade high aluminum – Al2O3 content >75%; second-grade high aluminum – Al2O3 content is 60-75%; third grade: Al2O3 content > 80%; Grade high aluminum – Al2O3 content is 48~60%. There are certainly corundum products with higher alumina content.
The alumina content of high alumina brick is more than 48% and it is a neutral refractory material. It is formed and calcined from alumina or other raw materials with high alumina content. High thermal stability, refractoriness above 1770°C, and good slag resistance. Because high-aluminum products have high Al2O3, low impurities, and less fusible glass, the softening temperature under load is higher than that of clay bricks. Among high-aluminum bricks, the main mineral phases are mullite and corundum, and mullite (3Al2O3·2SiO2) is the only stable compound in the Al2O3-SiO2 system. However, because the mullite crystal does not form a network structure, the load-softening temperature is still not as high as that of silica brick.
The main crystal phase of first-grade high alumina bricks is columnar or granular corundum, with a mass fraction of more than 70%; a small amount of mullite, with a mass fraction of 10% to 20%. This indicates that direct bonding between crystalline phases plays a dominant role. The glass phase with a mass fraction of 5% to 10% is chaotically distributed in it.
The microstructural feature of the secondary high alumina brick is that the mullite crystals with a high bonding rate form a network structure, and the glass phase is distributed in the gaps of the network structure. Therefore, it has high high temperature strength and creep resistance. Although a small amount of liquid phase exists at high temperatures, it only fills the network structure.
In the third-grade high alumina brick, the mass fraction of the glass phase is as high as 20%. The mullite crystals are only embedded in the glass phase aggregate, and the glass phase controls the deformation to a large extent. At low temperatures, the filling crystalline phase helps increase its strength. As the temperature increases, the fluidity of the glass phase increases, the filling and reinforcing effects of mullite are significantly reduced, the deformation rate is significantly accelerated, and the high-temperature mechanical properties are the worst.
3. The difference between clay refractory bricks and high alumina bricks
The refractoriness of clay bricks is roughly the same as that of silica bricks, which can reach 1690 ~ 1730℃, but the load softening temperature is more than 200℃ lower than that of silica bricks. In addition to high refractory mullite crystals, clay bricks also contain nearly half of the amorphous glass phase with a low melting point. In the temperature range of 0 ~ 1000℃, the volume of clay bricks expands uniformly as the temperature increases. The linear expansion curve is approximately a straight line, and the linear expansion rate is 0.6%-0.7%, which is only about half of that of silica bricks. When the temperature reaches 1200°C and continues to rise, the volume begins to shrink from the maximum expansion value. The residual shrinkage of clay bricks leads to the loosening of masonry mortar joints, which is a major disadvantage of clay bricks. When the temperature exceeds 1200°C, the low melting point in the clay bricks gradually melts, and the particles get very close to each other under the action of surface tension, causing volume shrinkage.
The refractoriness and load softening temperature of high-alumina bricks are higher than those of clay bricks, and their slag corrosion resistance (especially against acidic slag) is better. These properties increase with the increase of Al2O3 content, but their thermal stability is not as good as clay bricks. High alumina bricks have high density, low porosity, high mechanical strength, and wear resistance. The burner head of the coke oven combustion chamber and the bottom brick of the carbonization chamber are built with high alumina bricks, which has better results; however, it is not suitable to be used on the walls of the carbonization chamber because high alumina bricks are prone to curling and warping at high temperatures.
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