Sintered fused alumina, also known as sintered alumina, is a refractory clinker made of calcined alumina or industrial alumina powder, which is finely ground into balls or blanks and sintered at a high temperature of 1750-1900℃. Sintered fused alumina with W (Al2O3) of more than 99% is mostly directly combined with uniform fine-grained corundum, with an apparent porosity of less than 3.0% and a bulk density of 3.60g/cm3. Its refractoriness is close to the melting point of corundum. It has good volume stability and chemical stability at high temperatures, is not easily corroded by reducing atmosphere, molten glass and molten metal, and has good mechanical strength and wear resistance at room temperature and high temperature.
The bulk density and crystal size of sintered fused alumina are closely related to factors such as the type of raw materials used in the firing process, the method of forming balls or blanks, the initial density of the blank, the type and amount of additives, and the calcination temperature. Adding less than 1% (mass fraction) of TiO2 can reduce the sintering temperature and increase the density of sintered alumina; adding more than 2% to 3% (mass fraction) of TiO2 will reduce its density; another effect of adding TiO2 is to produce large-particle clinker, and large-particle crystallized sintered alumina has good slag erosion resistance. 1% to 2% (mass fraction) of H3BO3 can also be added to promote the growth of corundum grains. Another purpose of adding H3BO3 is to react with Na2O in the raw material to generate Na3BO3 volatilization and reduce the Na2O content in the raw material. Sometimes a small amount of MgO can be added to inhibit the abnormal growth of corundum crystals. The calcination of sintered fused alumina is mostly carried out in a downdraft kiln or a rotary kiln.
Sintered fused alumina is mainly used as a raw material for the production of sintered fused alumina bricks and amorphous refractory materials, and its usable temperature is quite high.
However, this raw material causes crystal growth when the secondary heating time is long, which may result in a decrease in the solid solution limit and the formation of precipitates at the grain boundary, which affects its high-temperature performance.
Tabular alumina, also known as tabular corundum, is a pure sintered fused alumina that is completely sintered without any additives.
Tabular alumina is made of alumina powder. After grinding, forming and drying, it is sintered in an ultra-high temperature kiln at a temperature slightly lower than the melting point of corundum, under the conditions of rapid heating and cooling, to form many tightly bound plate-columnar crystals with large grains and a median diameter of 40 to 200 microns. The two-dimensional morphology of the crystals is flat and interlaced with each other. There are many 5 to 15 micron round closed pores inside the crystals, and there are fewer open pores, generally 2% to 3%.
Because tabular alumina does not add any additives, has high purity, high melting point, high grain hardness, high heat load strength, low heat shrinkage, high density and low permeability, good wear resistance, especially because there are many closed small pores in the grains, and good thermal shock stability. Except for hydrofluoric acid and phosphoric acid, most alkalis and inorganic acids have no effect on tabular alumina and are resistant to chemical erosion.
Due to a series of characteristics of tabular alumina, it has become an ideal refractory raw material. It can be used as aggregate in refractory materials such as A12O3-C, A12O3-SiO2-C and Al2O3-Cr2O3, and can also be added to other high-alumina amorphous refractories. Tabular alumina amorphous refractories bonded with phosphate and calcium aluminate cement are widely used. All parts where fused corundum or white corundum are used can be replaced by tabular alumina, so that the performance of refractory materials is improved.
The calcination of tabular alumina is mostly carried out in high-temperature vertical kilns.
The application of tabular alumina in the steel industry almost includes all parts where alumina refractories are used, from blast furnaces, hot blast furnaces, mixed iron furnaces, torpedo tanks to converters, secondary refining furnaces, mold casting and continuous casting ladles, tundishes, soaking furnaces and heating furnaces.
In the ceramic industry, tabular alumina is used in kiln furniture to significantly improve its performance. For example, it can be used in ceramic rollers to improve their thermal shock stability and deformation resistance; it can be used in the manufacture of saggers and slabs for firing high-temperature ceramics such as spark plugs and alumina balls, and the service life is significantly improved. Plate-shaped alumina can also be used as insulators and as raw materials for the manufacture of catalyst carriers for high-temperature reactions that require low porosity, high strength and high purity.
Sintered mullite and sintered spinel are also refractory clinkers, but they are synthetic raw materials.
