Alumina (Al2O3) refractories are the part of alumina- silica group of refractories. They differs from fireclay refractories in term of Al2O3 content and normally have Al2O3 content of more than 45 %. The raw material base for these refractories are different than the fire clay bricks.
As the Al2O3 content is increased, the melting point of the refractory increases to a maximum of 2054℃ which is the melting point of pure corundum. Refractoriness increases with the increase in the Al2O3 content. The only stable compound in the system is mullite, which has a defective space lattice and decomposes into corundum and liquid phase at around 1840℃.
According to the Al2O3 content, alumina refractories can be divided into:
Al2O3 less than 50%
Fireclay belongs to this group. Its main crystal phases are mullite and glass. It can contain free SiO2. Normally it is made from 100% fireclay. High quality fireclay is used at about 1600℃, Usually it contain 38 % to 42 % Al2O3 and are based on fireclay minerals.
Al2O3 50% or 60%
The main crystal phases of these refractories are mullite as major phase and glass as minor phase. They can contain free SiO2. These refractories cannot be made from 100 % clay since Al2O3 content of clay are lower. They are made from minerals containing 60% Al2O3 and contains some fireclay. They can be made with bauxite and clay and can be used for temperatures greater than 1700℃.
Al2O3 70%
The major phase of these refractories is mullite. They contain corundum, mullite and glass. They are made either from bauxitic clay or calcined bauxite and clay. They can be used for temperatures greater than 1750℃.
Al2O3 80% and 85%
These refractories are made from calcined bauxite. The main phases of these refractories are corundum as the major phase along with minor quantity of mullite and glass.
Al2O3 90%
These refractories consist of corundum as the major phase with a minor amount of mullite and glass. Fused cast alumina block belongs to this group. It is used in contact with liquid steel or glass due to its enhanced abrasion resistance and corrosion resistance. When in contact with molten glass, it barely produces any blistering or stones. so it is ideal for the high quality glass furnace and special glass furnace.
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