Refractory aggregates and refractory powders in refractory castables are collectively referred to as main raw materials, and the rest are called secondary raw materials. Refractory aggregates are the +0.088mm or +0.1mm part of refractory castables. They are the main material in the organizational structure of refractory castables and play a skeleton role. Therefore, refractory aggregates are a component of the decisive factors of the physical and mechanical properties of the castable and the high-temperature performance. Usually, the raw materials used to prepare refractory aggregates should be high-quality raw materials with dense structure, low water absorption (generally less than 5%), high strength and low impurity content. Among the many types of refractory castables, corundum refractory castables are the most commonly used ones. In the preparation of corundum refractory castables, there are mainly 7 varieties of corundum raw materials used.
1.Sintered corundum
Sintered corundum, also known as sintered alumina or semi-molten alumina, is a refractory clinker made from calcined alumina or industrial alumina, ground into balls or blanks, and sintered at a high temperature of 1750~1900℃.
Sintered alumina containing more than 99% of aluminum oxide is mostly directly combined with uniform fine-grained corundum. The apparent gas fraction is less than 3.0%, the bulk density reaches 3.60%/cubic meter, the refractoriness is close to the melting point of corundum, it has good volume stability and chemical stability at high temperature, is not corroded by reducing atmosphere, molten glass and molten metal, and has good mechanical strength and wear resistance at room temperature and high temperature.
2.Fused Corundum
Fused corundum is an artificial corundum made by melting pure alumina powder in a high-temperature electric furnace. It has the characteristics of high melting point, high mechanical strength, good thermal shock resistance, strong corrosion resistance and small linear expansion coefficient. Fused corundum is the raw material for manufacturing high-grade special refractory materials. It mainly includes fused white corundum, fused brown corundum, sub-white corundum, etc.
3.Fused White Corundum
Fused white corundum is made of pure alumina powder and smelted at high temperature. It is white in color. The smelting process of white corundum is basically the process of melting and recrystallizing industrial alumina powder, without reduction process. The Al2O3 content is not less than 9%, and the impurity content is very small. The hardness is slightly lower than that of brown corundum, and the toughness is slightly lower. It is often used to make abrasives, special ceramics and advanced refractory materials.
4.Fused Brown Corundum
Fused brown corundum is made of high-alumina bauxite as the main raw material and coke (anthracite), and is smelted in a high-temperature electric furnace at above 2000℃. Fused brown corundum has a dense texture and high hardness, and is often used in ceramics, precision casting and advanced refractory materials.
5.Sub-white corundum
Sub-white corundum is produced by melting special or first-grade bauxite in a reducing atmosphere and under controlled conditions. Reducing agent (carbon), sedimentation agent (iron filings) and decarburizing agent (iron scale) are added during melting. Because its chemical composition and physical properties are close to those of white corundum, it is called sub-white corundum. Its bulk density is above 3.80g/cm3, and its apparent porosity is less than 4%. It is an ideal material for manufacturing advanced refractory and wear-resistant materials.
6.Chrome corundum
Add 22% chromium to the white corundum, and make it by arc furnace melting. The color is purple-red. The hardness is slightly higher than that of brown corundum, similar to that of white corundum, and the microhardness can be 2200-2300Kg/mm2. The toughness is higher than that of white corundum, and slightly lower than that of brown corundum.
7.Zirconia corundum
Zirconia corundum is a kind of artificial corundum made of aluminum oxide and zirconium oxide in an electric arc furnace through high-temperature smelting, crystallization cooling, crushing and screening. The main crystal phase of zirconium corundum is α-Al2O3, the secondary crystal phase is baddeleyite, and there is a small amount of glass phase. The crystal form and structure of zirconium corundum are important factors affecting its quality. Zirconium corundum has the characteristics of high hardness, good toughness, high strength, dense texture, strong grinding force, stable chemical properties, good thermal shock resistance, etc. It is widely used in abrasives and refractory industries. According to its zirconium oxide content, it can be divided into two product levels: ZA25 and ZA40.
