Sintered Mullite Bricks

Mullite brick is an aluminum-silicon refractory product with mullite as the main crystal phase. The molecular formula of mullite is 3Al2O3, 2SiO2, and its composition is Al2O371.8%, SiO228.2%. Under certain conditions, the Al2O3 content exceeds the above value and no free Al2O3 is found. This is because Al2O3 is dissolved in mullite. At this time, the molecular formula of mullite is 2Al2O3·SiO2·Al2O3, and the content is 77.3%. 3Al2O3·2SiO2 is α-mullite, and 2Al2O3·SiO2 is β-mullite. Some mullite brick products have a large amount of secondary crystalline phase - corundum, which accounts for more than 10% of the phase composition, and are called mullite-corundum products.

Performance of sintered mullite bricks Since mullite is the main crystal phase of bricks, the properties of the products are mainly determined by mullite. Mullite is the only stable compound in the Al2O3-SiO2 system under normal pressure and oxidizing atmosphere. The refractoriness of mullite bricks is around 1850℃. Since the raw material is pure and the needle-shaped and long columnar crystals of mullite form a solid skeleton, the load softening temperature is high and the high-temperature creep rate is low. Mullite has a low expansion coefficient, and mullite bricks have good thermal shock resistance and acid slag erosion resistance. The high-temperature mechanical properties of sintered mullite corundum bricks are better than those of mullite bricks, because the mullite crystals form a continuous staggered network structure, and the secondary crystal phase corundum is a granular structure that can be filled in the gaps of the mullite network. The interlacing and filling of the main and secondary crystal phases form a dense stacking and a high degree of bonding.

 

There are two types of synthetic mullite used in the process of manufacturing sintered mullite bricks: one is sintered synthetic mullite, which is industrial alumina (or aluminum hydroxide) and clay (or quartz stone), and some sillimanite raw materials are mixed and finely ground to make balls or billets, and calcined at a temperature above 1700℃ in a rotary dense or other high-temperature kiln; the other is fused mullite, which is made by adding the prepared raw materials into an electric arc furnace and melting them at 1900~2000℃. The content of both types of mullite can reach 95%. There is a mullite high-aluminum clinker sintered with selected natural bauxite, whose mullite content can reach 88%, which can be used with synthetic raw materials. Other raw materials should have few impurities and stable composition.

Mullite, bauxite clinker, corundum and other aggregates are crushed to particles below 3mm and graded reasonably. 2~3 kinds of mineral raw materials such as mullite, corundum, quartz sand, clay, sillimanite, etc. are selected and finely ground as the binding phase. In order to obtain low creep mullite bricks, 0.1~1mm high-purity quartz particles are often added to the ingredients. Quartz particles form an aluminosilicate layer at high temperature, and the expansion produced by the reaction can improve creep resistance. Add a binder to the prepared raw materials and mix them with a mixing mill; sometimes after one mixing, leave it for 24 hours and then mix it twice, and make brick blanks on a brick press. Large bricks need to be formed by vibration or wind hammer. After drying, the brick blanks are loaded into the kiln for firing. The firing temperature is determined according to the formula and product requirements, generally 1550~1600℃. The cooled products need to be inspected piece by piece, and the combined bricks need to be cut and ground and pre-laid.

Uses of sintered mullite bricks Commonly used in hot blast furnaces with high wind temperature above 1200C; parts of torpedo cars impacted by molten iron and slag lines; tops of electric arc furnaces for steelmaking; saggers and kiln tools for ceramic industrial kilns; material channels, regenerator arches, upper structures of glass melting furnaces, and pool walls of clarifiers; linings of cement and lime rotary kilns, etc.

Sintered mullite bricks should not be in contact with alkaline substances above 1450℃, otherwise the mullite will decompose. In a reducing atmosphere above 1370℃, mullite will also decompose, and part of SiO2 will become gaseous SiO and leave the brick body. At temperatures above 1650℃, mullite will decompose even if it is not a reducing atmosphere but at a lower oxygen partial pressure.

Due to the development of high-temperature technology, it is a development trend to replace traditional refractory materials made of natural raw materials with artificial synthetic materials.

Sintered Mullite Brick

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