Introduction
Mullite is a high-quality refractory raw material, although it is a relatively rare mineral. It forms from aluminosilicates at high temperatures. Artificial heating of aluminum silicate results in the formation of mullite. Naturally occurring mullite crystals are typically slender needles arranged in radiating clusters.
Mullite is widely used in the production of high-temperature refractory materials and serves as a thermal barrier coating in carbon/carbon (C/C) composites. In the Al₂O₃–SiO₂ system, the only stable binary compound at normal pressure is 3Al₂O₃·2SiO₂.
Natural mullite is scarce and is usually synthesized through sintering or by the fusion method. Its general chemical formula is AlₓSi₂₋ₓO₅.5–0.5ₓ. It has a density of 3.16 g/cm³, a Mohs hardness of 6–7, and a refractoriness of up to 1800 °C. It remains stable up to 1810 °C, at which point it decomposes into corundum and a liquid phase.
Advantage
Mullite is a high-quality refractory material, primarily including high-purity electrofused mullite, regular electrofused mullite, natural bauxite concentrate, sintered mullite, and lightweight sintered mullite.
High temperature resistance:
Can withstand direct flame contact on the working lining, enabling high-temperature operation and improved energy efficiency.
Low thermal conductivity and good insulation:
Special lightweight mullite aggregates ensure volume stability, high strength, and a tightly sealed structure with excellent insulation performance.
Lightweight:
Reduces structural weight by 40%–60%, eliminating hidden risks such as collapsed roof-hanging bricks.
Quick drying and high porosity:
Allows smooth exhaust and faster kiln heating, reducing oven time and bringing significant economic benefits.
Type
Sintered Mullite
Sintered Mullite products are made from high-quality coal kaolin as the raw material and processed through high-temperature calcination. These products are characterized by a dense texture, high hardness, low thermal expansion coefficient, excellent abrasion resistance, and good thermal and chemical stability. They are mainly used in metallurgy, building materials, the chemical industry, refractories, and the military industry.
Electrofused Mullite
Electrofused mullite is produced from high-quality mullite ore that is melted at high temperature using an advanced industrial electric furnace. It features uniform thermal expansion, excellent thermal shock resistance, a high load softening point, low creep at high temperatures, high hardness, and strong resistance to chemical corrosion. It is an ideal material for use in metallurgy, ceramics, building materials, the chemical industry, electric power, and the casting industry.
Physical and chemical indicators
Sintered Mullite
|
Al2O3 |
SiO2 |
Fe2O3 |
R2O |
Water absorption (%) |
|
| M70 | 68-79 | 22-28 |
≤0.3-1.2 |
≤0.3 |
≤3 |
| M60 |
58-62 |
33-28 |
≤1.1 |
≤0.3 |
≤3 |
| M45 |
42-45 |
53-55 |
≤0.4 |
≤1.6 |
≤2 |
Electrofused Mullite
| Chemical Formula |
Al(1+x)Si(2-x)O(5.5-0.5x) |
|
Density |
3.16g/cm3 |
|
Mohs Hardness |
6~7 |
|
Melting Temperature |
About 1910°C |
|
Fire resistance |
Remains stable at 1800°C, decomposes to corundum and liquid phase at 1810°C |
Application
Refractory materials are widely used in industries such as ceramics, metallurgy, foundry, and electronics due to their high temperature resistance, high strength, low thermal conductivity, and significant energy-saving effects. They are suitable for applications in oil cracking furnaces, hot blast furnaces in metallurgy, ceramic roller kilns, tunnel kilns, electric porcelain drawer kilns, glass crucible kilns, and various types of electric furnace linings. These materials can be used in direct contact with flame and, after testing by relevant technical supervision departments and real-world application, have been shown to meet the technical standards of similar products manufactured abroad.
differentiation
| Performance Indicators | Sintered Mullite | Fused Mullite |
|---|---|---|
| Refractoriness | 1700-1800℃ | 1800-2000℃ (close to the theoretical melting point of mullite, 1910℃) |
| Bulk Density | 2.5-3.2 g/cm³ | 3.0-3.2 g/cm³ (close to the theoretical density of 3.16 g/cm³) |
| Flexural Strength (1400℃) | 30-80 MPa | 100-200 MPa (slow attenuation of high-temperature strength) |
| Thermal Shock Resistance | Good (300-500℃ temperature difference cycle) | Poor (dense structure with large thermal expansion coefficient, prone to cracking) |
| Chemical Stability | Moderate resistance to acidic slag erosion | Extremely strong resistance to acidic/alkaline slag erosion (especially resistant to glass melt erosion) |
Precise Differentiation of Refractory Applications
Sintered Mullite – For medium- to high-temperature scenarios, prioritized for its cost-performance balance
Metallurgical industry: Used in blast furnace bodies and ladle middle linings, leveraging its thermal shock resistance and cost advantages (30%–50% cheaper than electrofused mullite).
Ceramic kilns: Applied in saggers and kiln car deck tiles, capable of withstanding sintering temperatures of 1300–1600 °C, combining good refractory performance with thermal shock resistance.
Industrial kiln insulation: Used in combination with lightweight materials to reduce overall construction cost.
Electrofused Mullite – For high-end applications under extreme working conditions
Glass melting kilns (critical areas): Used in liquid-flow holes and top bricks that come into direct contact with molten glass at around 1500 °C, requiring excellent erosion resistance.
Metallurgical furnace slag lines: Applied in ladle slag line bricks and refining furnace linings, where high resistance to alkaline slag erosion (CaO–SiO₂–Al₂O₃ system) is essential.
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