Mullite brick is a kind of fire brick that uses synthesized mullite as raw material and the principal crystalline phase is mullite. When Al2O3 content is lower than the mullite theoretical ratio, mullite brick contains a small amount of periclase. When Al2O3 content is higher than the theoretical proportion of mullite, there is a small amount of corundum. Mullite products have a better high-temperature creep property than high alumina bricks made from natural bauxite. Mullite bricks’ high-temperature property is equivalent to silica-based products and magnesia-based products. Mullite fire bricks also have better acid slag resistance and weak basic slag corrosion resistance than magnesia products. There are two kinds of mullite refractory bricks: sintered mullite bricks and fused cast mullite bricks.
Good thermal shock resistance
Low high temperature creep rate
Good acid and weak alkaline slag corrosion resistance.
High refractoriness and mechanical strength.
Sintered mullite bricks adopt synthesized mullite aggregates, and fines, or white corundum, quartz, and pure clay powder mixture with the same chemical composition of mullite. The raw material is evenly mixed, then molded under high pressure, and finally sintered at the temperature of 1550~1600℃.
| Item | Hot blast furnace mullite brick | Stopper head mullite brick | Blast furnace wall brick | Fused synthesized mullite brick |
| Al2O3 % | 80.0 | 68.3 | 72.0 | 75.1 |
| SiO2 % | 18.2 | — | — | 20.1 |
| Fe2O3 % ≤ | 0.76 | 0.84 | 1.7 | 3.35 |
| Apparent porosity % | 15.6-16.5 | ≤16.7 | ≤22 | 12-16 |
| Cold compression strength/Mpa | 197-267 | ≥77 | ≥49 | 210-250 |
| Rupture under load/℃ | ≥1700 | ≥1600 | ≥1650 | ≥1700 |
| Bulk density g/cm3 | 2.77-2.79 | 2.53 | 2.68 | 2.56-2.66 |
Fused cast mullite bricks’ main raw materials are high alumina bauxite or industrial alumina, clay, or silica with proper ratio. The mixed material is melted in electric arc furnaces, cast into shape, and then annealed to make the final mullite bricks. The main mineral ingredient of fused cast mullite bricks is mullite, which’s chemical composition is 3Al2O3·2SiO2.
| Item | Hot blast furnace mullite brick |
| Al2O3 % | 70.1-75.47 |
| SiO2 % | 19.32-21.6 |
| Fe2O3 % | 0.92-1.90 |
| Apparent porosity % ≤ | 10 |
| Cold crushing strength/Mpa | 250-500 |
| Rupture under load/℃ | 1700 |
| Bulk density g/cm3 | 2.9-3.3 |
| Refractoriness ℃ | 1730-1800 |
| Thermal expansion rate %(20-1000 ℃) | 0.60-0.65 |
Mullite bricks are mainly used in the inner lining of various parts of thermal kilns and furnaces for different industries, such as the steel and iron industry, chemical industry, glass industry, ceramic industry, and so on. For example, mullite bricks are widely used in hot blast furnaces for iron making, used in glass kiln main body feeding channel, regenerative chamber, clarification tank wall. Mullite bricks are also used in rotary cement kiln and rotary lime kiln inner lining. The physiochemical index of mullite bricks is different for different applications.
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