Application of Mullite and Its Composite Refractory Materials in Different Industries
Application of Mullite in the Ceramic Industry
Kiln furniture (saggers, shelves, push plates, etc.) is a tool used to space, support, cushion and protect the baked blanks in the industrial kiln during the baking process. The development of high-performance kiln furniture is of great significance for the firing of high-quality products. Since fused mullite has good thermal shock resistance, high-purity fused mullite is the best raw material for preparing high-quality kiln furniture.
Mullite-corundum kiln furniture
Mullite-corundum refractory material is one of the mainstream materials of kiln furniture at present. It has good high temperature strength, thermal shock resistance and chemical stability, and is particularly suitable for supporting soft magnetic (ferrite) materials and electrical insulating ceramics. Using M75 fused mullite and fused corundum as aggregates, aluminum glue, α-Al₂O₃ micropowder and SiO₂ micropowder as bonding matrix, mullite-corundum high temperature push plate with good thermal shock resistance was prepared. After 2 thermal shocks (1100℃⇌water cooling), the flexural strength retention rate was 78%, and no fracture occurred after 23 thermal shocks. The thermal shock resistance of corundum-mullite kiln furniture can be further improved. The mechanism of improved thermal shock resistance is: zircon decomposes into ZrO₂ and SiO₂ during the firing process. On the one hand, SiO₂ migrates outward from the ZrO₂ aggregate, and closed pores are generated at the position of SiO₂. On the other hand, the annular microcracks caused by thermal mismatch between ZrO₂ aggregates and the surrounding mullite matrix can disperse the stress generated during thermal shock cycles.

Mullite-cordierite kiln furniture
Cordierite has a low thermal expansion coefficient (2.5×10⁻⁶℃⁻¹ from room temperature to 1000℃) and has excellent thermal shock resistance. The thermal expansion coefficient of cordierite is smaller than that of mullite. Due to the mismatch in the thermal expansion coefficients of the two, microcracks are easily formed at the interface between the two phases, which is beneficial to improve the thermal shock resistance of mullite-cordierite materials. The shelf is a special kiln furniture for supporting porcelain parts, and the material is mostly cordierite-mullite composite material. With M60 mullite and cordierite as the main raw materials and dextrin as a binder, a high-strength cordierite-mullite shelf is prepared, and the high-temperature flexural strength at 1200℃ can reach 17.7MPa. Because the skeleton of the material is composed of mullite and cordierite aggregates, the two are firmly connected by a “connecting bridge” composed of mullite, cordierite and low-aluminum high-silica glass phase, and this structure is conducive to improving the high-temperature mechanical properties of the material.
With the development of new energy vehicles, electronic mobile devices and energy storage fields, the demand for lithium batteries is increasing, and the kiln tools used for firing their positive electrode materials are also increasingly attracting attention.
Mullite-aluminum titanate kiln tools
Compared with cordierite, aluminum titanate has a lower thermal expansion coefficient (room temperature to 1000℃, 1.5×10⁻⁶℃⁻¹) and a higher melting point. It is currently the best material with high temperature resistance among low expansion materials. When mullite and aluminum titanate are used in combination, mullite-aluminum titanate kiln tools with good thermal shock resistance and high operating temperature can be obtained. Studies have shown that when mullite is added to the aluminum titanate matrix, the lattice stability of aluminum titanate can be improved, thereby preventing the decomposition of aluminum titanate. It was found that when mullite exists in aluminum titanate materials, the stability of aluminum titanate can reach 80%.
Mullite-silicon carbide kiln furniture
Silicon carbide has high strength, high thermal conductivity, wear resistance, chemical corrosion resistance and other properties. Therefore, silicon carbide kiln furniture has excellent thermal shock resistance, high wear resistance and strength at room temperature and high temperature. In order to improve the oxidation resistance of silicon carbide kiln furniture, Shi Jinxiong et al. prepared silicon carbide-mullite kiln furniture using silicon carbide, M70 sintered mullite and SiO₂ micropowder as raw materials.
Application of Mullite in Metallurgical Industry
The application of mullite in metallurgical industry is mainly reflected in steel smelting. Mullite bricks made of mullite as the main raw material have the characteristics of small thermal expansion coefficient, low creep rate, high high temperature strength, good thermal shock resistance and strong chemical corrosion resistance. It can be used for blast furnace, continuous casting, the dome of hot blast furnace and the middle and upper parts of the combustion chamber, etc., and can also be used as ceramic burner bricks. Amorphous refractory materials containing mullite also have good thermal shock resistance and mechanical properties, etc., and are used in blast furnaces, permanent linings of ladles, tundishes, ignition and insulation furnaces, hot metal desulfurization spray guns, etc.

Mullite-corundum refractory materials
Mullite-corundum bricks have high density, low porosity, high strength and good resistance to molten iron and slag erosion. They are ideal refractory materials for blast furnace ceramic linings, furnace walls, tuyere and furnace bottom. Mullite-corundum castables can be used as lining materials for blast furnace hot blast furnaces and heating furnaces.
The purity of mullite raw materials affects the performance of mullite-corundum refractory bricks. Corundum-mullite bricks prepared with high-purity sintered mullite as raw materials have excellent resistance to alkali vapor and CO erosion. Suitable for use in high-temperature reducing atmosphere conditions such as hot blast furnaces. Compared with mullite-corundum castables prepared with fused mullite as aggregate, mullite-corundum castables prepared with microcrystalline mullite as aggregate have higher room temperature compressive strength, room temperature flexural strength, high temperature flexural strength, and better thermal shock resistance.
Introducing other refractory raw materials on the basis of mullite and corundum can further improve the performance of mullite-corundum refractory materials. Adding an appropriate amount of boron carbide to the silica sol combined mullite-corundum castable can further improve the room temperature flexural strength, room temperature compressive strength, high temperature flexural strength and thermal shock resistance of the castable.
Mullite-cordierite refractory materials
In order to meet the requirements of good thermal shock resistance, scouring resistance, high temperature resistance, erosion resistance, high load softening temperature and other requirements of refractory materials for ceramic burners, M70 sintered mullite, M70 fused mullite and cordierite are used as the main raw materials, and the mullite-cordierite ceramic burner composite bricks are prepared through the casting-centrifugal molding process.
The lining bricks of the coke oven door need to have good wear resistance, volume stability, carbon resistance and thermal shock resistance. With M70 sintered mullite and cordierite as the main raw materials and aluminate cement as the binder, a mullite-cordierite precast brick with low linear expansion rate, low porosity and good thermal shock resistance was prepared. The precast brick was applied to the door of a large coke oven. After one year of use, the surface of the precast brick was smooth and crack-free.
Mullite-alumina refractory materials
Casting materials made with mullite and bauxite as the main raw materials use less water and exhibit high mechanical strength over a wide temperature range.
In order to further improve the wear resistance and thermal shock resistance of mullite-alumina-based castables, a mullite-alumina-silicon carbide castable with excellent mechanical properties and wear resistance was prepared with M60 mullite, bauxite and silicon carbide as the main raw materials and calcium aluminate cement as the binder.
Mullite-magnesium aluminum spinel refractory
Magnesium aluminum spinel has excellent thermal shock resistance, corrosion resistance and wear resistance. Studies have found that the introduction of nano-magnesium aluminum spinel into mullite-based castables generates tension ring stress around the spinel and forms microcracks in the matrix due to the mismatch in thermal expansion coefficients between mullite and magnesium aluminum spinel. This is conducive to strengthening the internal structure of the material, thereby limiting the expansion of cracks. Therefore, the prepared mullite-magnesium aluminum spinel castable has good thermal shock resistance.
Mullite-silicon carbide refractory
Silicon carbide has high strength, high thermal conductivity, wear resistance and other properties, and can be combined with mullite to prepare mullite-silicon carbide castables with excellent performance. Mullite-silicon carbide wet jet castable. The wet jetting castable has good rheological properties, showing a pseudoplastic fluid with low yield value and apparent viscosity, suitable for pumping pipeline transportation, and can be used in kilns such as ladles, electric furnaces, blast furnaces, and torpedo tanks. Adding 6% (w) zircon to the mullite-silicon carbide castable can further improve the wear resistance of the mullite-silicon carbide castable, making it suitable for lining parts with strict requirements for thermal shock resistance and wear resistance.
Mullite-bonded corundum-silicon carbide refractory
In order to solve the problem of reduced high temperature performance of corundum-silicon carbide castables bonded with ordinary binders (such as calcium aluminate cement), a mullite-bonded corundum-silicon carbide cement-free castable was prepared using corundum and silicon carbide as aggregates, M70 mullite powder, Al₂O₃ micropowder and SiO₂ micropowder as bonding matrix. Because the mullite powder introduced into the matrix can form a network structure of columnar mullite inside the castable after sintering, the room-temperature flexural strength, room-temperature compressive strength, high-temperature flexural strength and thermal shock resistance of the corundum-silicon carbide castable are improved.
Development of the Application of Mullite and Its Composite Refractory Materials
Mullite and its composite refractory materials have been widely used as linings for various kilns in traditional fields such as metallurgy, ceramics, cement and petrochemicals. The future development trend of mullite in refractory materials is mainly reflected in the following aspects:
- (1) Expand the application field of mullite refractory materials. In addition to the traditional high-temperature industrial field, mullite refractory materials have emerged in aerospace, military and other fields. However, the application of mullite refractory materials in these fields is quite limited at present, and its application in aerospace, military and other fields will continue to be promoted in the future.
- (2) Promote the development of lightweight mullite. With the advancement of energy conservation and emission reduction, the development of lightweight and heat-insulating refractory materials is of great significance to reducing kiln heat loss and improving work efficiency. Refractory materials prepared with lightweight mullite not only have good high-temperature performance, but also have good thermal insulation effect. The use of lightweight heat-insulating refractory materials with good high-temperature performance can thin or even eliminate the working layer. Refractory castables made of lightweight mullite can solve the problem that lightweight refractory castables made of perlite, vermiculite or ceramsite have low use temperature and cannot be used as working linings in direct contact with flames at high temperatures. Therefore, the development of lightweight mullite with large porosity, low bulk density, low thermal conductivity and high strength plays a positive role in the development of lightweight insulating refractory materials with good high-temperature performance that can achieve energy saving and consumption reduction.
- (3) Strengthen the application of nano-mullite in refractory materials. In recent years, nanotechnology has been successfully applied in refractory materials and has continuously achieved breakthroughs in material performance. The use of nano-mullite powder can further improve the performance of refractory materials. In the future, nano-mullite will play an increasingly important role in refractory materials.