Mullite is the only stable binary compound in the Al₂O₃-SiO₂ system, with the chemical formula 3Al₂O₃·2SiO₂. The theoretical composition is w(Al₂O₃)=71.8% and w(SiO₂)=28.2%. It has excellent properties such as stable chemical properties, good corrosion resistance, good thermal shock resistance, high melting point (1870℃), and high hardness (Mohs hardness 6-7). Mullite is mostly synthesized artificially by sintering and electrofusion methods. According to the content of Al₂O₃, sintered mullite can be divided into four grades: M75, M70, M60, and M45, while electrofused mullite can be divided into two grades: M75 and M70. Refractory materials prepared from sintered mullite and electrofused mullite have their own characteristics: the grains of sintered mullite are finer, so mullite bricks prepared from it have higher strength, and the high-temperature flexural strength at 1400℃ is more than twice that of mullite bricks produced from electrofused mullite; Electrofused mullite has better high-temperature volume stability, so refractory materials prepared from it are not easy to deform after long-term use at high temperatures and show good thermal shock resistance.
Pure mullite and its composite refractory materials prepared from mullite as raw materials are widely used in metallurgy, ceramics, cement, glass, petrochemicals, electric power and other fields as linings of various kilns and high-temperature kiln furniture. In addition to the above traditional application fields, in recent years, materials prepared with mullite as the main raw material have been gradually applied to aerospace and military industries. Therefore, the application progress of mullite in metallurgy, ceramics, aerospace, military and other fields is reviewed, and the development trend of mullite in refractory applications is prospected.
The Application of Mullite in the Ceramic Industry
Kiln furniture (such as saggar, shelf plate, pusher plate, etc.) are utensils that play the roles of spacing, supporting, cushioning and protecting the fired blanks during the roasting process in industrial kilns. The kiln furniture used for firing traditional ceramic products (building ceramics, sanitary ceramics, daily-use ceramics, advanced ceramics) needs to have good mechanical properties and thermal shock resistance because it undergoes multiple cycles from room temperature to firing temperature under load conditions. The kiln furniture used for firing lithium battery cathode materials in the new energy field should not only have excellent mechanical properties and thermal shock resistance, but also have excellent corrosion resistance. The development of high-performance kiln furniture is of great significance for firing high-quality products. Due to the good thermal shock resistance of electrofused mullite, high-purity electrofused mullite is the best raw material for preparing high-quality kiln furniture.
1.1 Mullite-corundum kiln furniture
Mullite-corundum refractory materials are one of the mainstream materials for kiln furniture, featuring good high-temperature strength, thermal shock resistance, and chemical stability. They are particularly suitable for supporting and firing soft magnetic (ferrite) materials and electrical insulating ceramics.
Chen Guihua et al. used M75 electrofused mullite and electrofused corundum as aggregates, combined with aluminum sol, α-Al₂O₃ micro-powder, and SiO₂ micro-powder as the bonding matrix, to prepare mullite-corundum high-temperature pusher plates with excellent thermal shock resistance. After two thermal shock cycles (1100°C ⇌ water cooling), the flexural strength retention rate was 78%, and no fracture occurred after 23 thermal shock cycles.
Hou Xiaojing et al. prepared corundum-mullite kiln furniture with excellent thermal shock resistance using tabular corundum particles, fine powder, and electrofused mullite particles as the main raw materials. After three thermal shock cycles (1100°C ⇌ air cooling), the room-temperature flexural strength retention rate reached 100%. Meanwhile, this material exhibited high high-temperature flexural strength (22.8 MPa at 1400°C).
Shou Kedi et al. found that introducing zircon into corundum-mullite refractory materials prepared with tabular corundum and electrofused mullite as the main raw materials can further improve the thermal shock resistance of corundum-mullite kiln furniture. The mechanism for this improvement is as follows: during the firing process, zircon decomposes into ZrO₂ and SiO₂. On one hand, SiO₂ migrates outward from the ZrO₂ aggregates, generating closed pores in the position of SiO₂. On the other hand, the circular micro-cracks formed between ZrO₂ aggregates and the surrounding mullite matrix due to thermal mismatch can disperse the stress generated during thermal shock cycles.
In terms of mullite raw materials, Zhang Yongchen et al. compared the properties of mullite-corundum pusher plates prepared with the same addition amount of microcrystalline mullite and ordinary electrofused mullite as raw materials. It was found that the compressive strengths of the two pusher plates were similar, but in terms of thermal shock resistance, the number of thermal shocks of the pusher plate prepared with microcrystalline mullite as raw material was 20% higher than that of the pusher plate prepared with ordinary electrofused mullite. This is because the internal microstructure of electrofused microcrystalline mullite is conducive to absorbing thermal stress, thereby improving thermal shock resistance.
1.2 Mullite-Cordierite Kiln Furniture
Cordierite has a low thermal expansion coefficient (2.5×10⁻⁶℃⁻¹ from room temperature to 1000℃) and excellent thermal shock resistance. Since its thermal expansion coefficient is lower than that of mullite, the mismatch between the two creates fine cracks at the phase interface, which enhances the thermal shock resistance of mullite-cordierite materials. Shelf plates, as specialized kiln furniture for supporting ceramic parts, are mostly made of cordierite-mullite composites.
Fang Binxiang et al. prepared high-strength cordierite-mullite shelf plates using M60 mullite and cordierite as main raw materials and dextrin as a binder. The high-temperature flexural strength at 1200℃ reached 17.7 MPa, as the material's skeleton consists of mullite and cordierite aggregates firmly connected by a "bridge" of mullite, cordierite, and low-alumina high-silica glass phase-this structure improves high-temperature mechanical properties.
Although mullite-cordierite refractories exhibit good performance and low pollution to ceramic parts, cordierite has a low melting point, limiting the maximum service temperature of its products to <1350℃. With the development of new energy vehicles, electronic mobile devices, and energy storage fields, the demand for lithium batteries has increased, drawing attention to kiln furniture for firing cathode materials. Common lithium battery cathode materials (e.g., LiCO₃, LiOH, LiCoO₂) are alkaline and corrosive to saggars, and the thermal cycling process imposes higher requirements on chemical stability and thermal shock resistance compared to ordinary ceramic kiln furniture.
Mullite resists corrosion by Li and Co ions, making it suitable for lithium battery cathode firing kiln furniture. Zhai et al. studied the erosion mechanism of battery cathode materials on mullite saggars and found that the reaction products LiAlSiO₄ and LiAlSi₂O₆ formed by Li₂O and mullite cause micro-cracks due to thermal expansion mismatch, leading to surface spalling. This indicates that single-phase mullite saggars have insufficient corrosion resistance.
To improve the performance of single-phase mullite saggars for lithium battery cathodes, Shan Zhilin et al. used M70 mullite and cordierite as aggregates to prepare mullite-cordierite saggars, which formed a protective reaction layer resistant to cathode material erosion. Xie Huajing et al. studied the erosion mechanism of LiCoO₂ (synthesized from cobalt basic carbonate and lithium carbonate) on mullite-cordierite saggars, noting that LiO₂ from Li₂CO₃ decomposition reacts with mullite and cordierite to form LiAlO₂, β-LiAlSiO₄, and Li₄SiO₄. Volume changes and thermal expansion mismatch generate horizontal micro-cracks and surface spalling, but the reaction gradually fuses mullite and cordierite particles with the matrix, making the penetration layer denser and hindering further LiO₂ diffusion, thus enhancing corrosion resistance.
Chen Yang et al. prepared cordierite-mullite saggars using electrofused mullite and cordierite as main raw materials, which showed good resistance to Li₂(Ni₀.₈Co₀.₁Mn₀.₁)Oχ at 800℃ and retained a flexural strength of 3.0–4.5 MPa after 5 thermal shocks (1100℃, water cooling). Chen Ning et al. developed mullite-cordierite kiln furniture for lithium battery cathode firing, with room-temperature flexural/ compressive strengths of 9.5/58.8 MPa and high-temperature flexural strength of 11.4 MPa at 1100℃. Zhao Mengxi et al. used low-cost M60 mullite and cordierite with wax binder to prepare saggars via hot-press injection molding.
Introducing magnesium aluminate spinel or aluminum titanate into mullite-cordierite systems further enhances performance. Zhai et al. indicated that magnesium aluminate spinel has minimal reaction with battery materials, offering better lithium corrosion resistance. Liu Pengcheng et al. prepared mullite-cordierite-magnesium aluminate spinel saggars with good mechanical properties using M70 mullite, cordierite, and electrofused spinel. Xu Haisen synthesized in-situ cordierite and spinel with M70 mullite as aggregate to create saggars resistant to LiCoO₂ and thermal shock. Zhao Yingna et al. found that adding aluminum titanate to mullite-cordierite refractories improves thermal shock resistance, with room-temperature flexural strength reaching 9.4 MPa after 5 thermal shocks (1100℃, water cooling).
1.3 Mullite-Aluminum Titanate Kiln Furniture
Compared to cordierite, aluminum titanate has a lower thermal expansion coefficient (1.5×10⁻⁶℃⁻¹ from room temperature to 1000℃) and a higher melting point, making it the best high-temperature resistant material among low-expansion materials. Combining mullite with aluminum titanate yields mullite-aluminum titanate kiln furniture with excellent thermal shock resistance and high service temperature.
Huang et al. showed that adding mullite to an aluminum titanate matrix improves lattice stability and prevents decomposition. Kim et al. found that the stability of aluminum titanate can reach 80% in the presence of mullite. To address the low strength of aluminum titanate as a high-temperature kiln material, Yin Hongfeng et al. in-situ synthesized mullite-aluminum titanate kiln furniture using M60 mullite, industrial alumina, titanium oxide, and Suzhou clay, which exhibited good thermal shock resistance and high-temperature flexural strength (11.4 MPa at 1400℃), suitable for firing hard magnetic materials.
Ren Yun et al. in-situ synthesized mullite-aluminum titanate kiln furniture with a service temperature <1400℃ and load ≤8 kg using M60 mullite, titanium dioxide, industrial alumina, and Suzhou clay. The thermal expansion mismatch between mullite and aluminum titanate creates micro-cracks that relieve stress concentration during thermal shock, reducing elastic strain energy and improving thermal shock resistance. This kiln furniture can partially replace expensive mullite-corundum kiln furniture for soft magnetic materials and substitute mullite-cordierite kiln furniture for daily-use, building, sanitary, and artistic ceramics at low temperatures (≤1250℃).
1.4 Mullite-Silicon Carbide Kiln Furniture
Silicon carbide features high strength, thermal conductivity, wear resistance, and chemical corrosion resistance, endowing silicon carbide kiln furniture with excellent thermal shock resistance, high abrasion resistance, and room/ high-temperature strength. However, silicon carbide oxidizes significantly above 1300℃, limiting its application temperature.
To improve oxidation resistance, Shi Jinxiong et al. prepared silicon carbide-mullite kiln furniture using silicon carbide, M70 sintered mullite, and SiO₂ micro-powder. As mullite content increased from 5% to 25% (w), oxidation and thermal shock resistance improved after calcination at 1300℃ in air.
The Application of Mullite in the Metallurgical Industry
Mullite's application in the metallurgical industry is primarily reflected in steelmaking. Mullite bricks prepared with mullite as the main raw material feature a low thermal expansion coefficient, low creep rate, high high-temperature strength, good thermal shock resistance, and strong chemical corrosion resistance. They can be used in blast furnaces, continuous casting, the arch roofs and middle-upper parts of hot blast stove combustion chambers, as well as bricks for ceramic burners. Unshaped refractory materials containing mullite also exhibit good thermal shock resistance and mechanical properties, and are used in blast furnaces, ladle permanent linings, tundish, ignition insulation furnaces, hot metal desulfurization guns, etc.
2.1 Mullite-Corundum Refractory Materials
Mullite-corundum bricks have properties such as high density, low porosity, high strength, and good resistance to hot metal and slag erosion, making them ideal refractories for blast furnace ceramic linings, furnace walls, tuyeres, and furnace bottoms. Mullite-corundum castables can be used as lining materials for blast furnace hot blast stoves and reheating furnaces. The quality of mullite raw materials significantly influences the performance of mullite-corundum refractories.
He Xian et al. found that the purity of mullite raw materials affects the properties of mullite-corundum refractory bricks. Corundum-mullite bricks prepared from high-purity sintered mullite have excellent resistance to alkali vapor and CO erosion, suitable for high-temperature reducing atmospheres in hot blast stoves.
Zhang Yongchen et al. indicated that compared with mullite-corundum castables using electrofused mullite as aggregate, those using microcrystalline mullite as aggregate show higher room-temperature compressive strength, room-temperature flexural strength, high-temperature flexural strength, and better thermal shock resistance. The internal mullite network structure in the castable enhances thermal shock resistance, while micro-cracks generated by the large thermal expansion coefficient difference between mullite and corundum absorb elastic strain energy, reduce the energy driving main crack propagation, and disperse/deflect main cracks.
Li Zhigang et al. found that introducing 25% (w) M75 electrofused mullite into silica sol-bonded corundum castables reduces their elastic modulus. The added mullite connects via mass transfer of high-temperature liquid phase to form a mullite network, enhancing the castable's strength. Cai Wei et al. developed a corundum-mullite castable for converter anhydrous sub-guns with excellent thermal shock resistance using tabular corundum and M70 sintered mullite as main raw materials and CA80 cement as binder, with a flexural strength retention rate of 48% after 3 thermal shocks (1100°C ⇌ water cooling).
To address the long curing (2–3 days) and oven-drying (7 days) times of cement-bonded mullite-corundum castables, as well as cracking during oven-drying, silica sol is used as a binder. Xiong Jiquan et al. developed silica sol-bonded mullite-corundum castables using M60 mullite and electrofused tabular corundum, which can be demolded within half a day and oven-dried in 1 day, applied to blast furnace tuyeres and ignition furnace roofs/walls in WISCO.
To avoid performance degradation caused by low-melting-point phases from cement binders, ρ-Al₂O₃ binders are used. Wang Delun et al. prepared ρ-Al₂O₃-bonded mullite-corundum castables for tundish covers, which showed fewer cracks and spalling and a 50% longer service life than calcium aluminate cement-bonded ones.
2.2 Mullite-Cordierite Refractory Materials
To meet the requirements of ceramic burners for good thermal shock resistance, erosion resistance, and high load softening temperature, Zhu Xinjun et al. prepared mullite-cordierite combined bricks for ceramic burners via casting-centrifugal forming, using M70 sintered mullite, M70 electrofused mullite, and cordierite. Applied in hot blast stove ceramic burners of Guangzhou Iron and Steel Group, they meet the needs of large and medium-sized hot blast stoves.
2.3 Mullite-Bauxite Refractory Materials
Castables prepared from mullite and bauxite have low water consumption and high mechanical strength across a wide temperature range. Xia Changyong et al. developed bauxite-mullite castables for sintering ignition furnaces using bauxite aggregate and M75 sintered mullite fines, applied to ignition furnace roofs, partition walls, and side walls in Magang, Bengang, and WISCO, with good service performance and a service life exceeding 4 years in some furnaces.
2.4 Mullite-Magnesium Aluminate Spinel Refractory Materials
Magnesium aluminate spinel has excellent thermal shock resistance, corrosion resistance, and wear resistance. Khalil et al. found that introducing nano-magnesium aluminate spinel into mullite-based castables creates tensile ring stress around spinel and micro-cracks in the matrix due to thermal expansion mismatch, enhancing internal structure and limiting crack propagation, thus endowing the castables with good thermal shock resistance.
2.5 Mullite-Silicon Carbide Refractory Materials
Silicon carbide (SiC) features high strength, thermal conductivity, and wear resistance, and combined with mullite, yields high-performance mullite-SiC castables. Wang Yajuan et al. prepared mullite-SiC wet-spray castables with a high-temperature flexural strength of 10 MPa at 1400°C, suitable for ladles, electric furnaces, blast furnaces, and torpedo ladles due to their pseudoplastic fluid properties (low yield value and apparent viscosity) for pipeline pumping.
2.6 Mullite-Bonded Corundum-Silicon Carbide Refractory Materials
To address the high-temperature performance degradation of ordinary binder (e.g., calcium aluminate cement)-bonded corundum-SiC castables, Zhang Zhanying et al. prepared mullite-bonded corundum-SiC cementless castables using corundum and SiC as aggregates and M70 mullite powder, Al₂O₃ micro-powder, and SiO₂ micro-powder as the bonding matrix. The sintered mullite powder forms a columnar mullite network, improving room-temperature/high-temperature flexural/compressive strengths and thermal shock resistance.
The Application of Mullite in the Aerospace Industry
The rocket launch platform is one of the key ground equipment. During the rocket launch and take-off process, the gas flow from the engine gradually acts on the side walls of the diversion holes of the launch platform and the upper surface of the launch platform from the diversion channel, so the launch platform is eroded and impacted by the gas flow. In order to ensure that the main structure of the launch platform is not eroded and damaged during the rocket launch process, the metal structure of the launch platform must be effectively protected by thermal protection materials. Aiming at the launch conditions of the Long March 5 launch vehicle, the largest in-service one in China, the author and others developed a refractory material suitable for the thermal protection of the Long March 5 rocket launch platform using mullite, cordierite, etc. as the main raw materials and aluminate cement as the binder, which has good ablation resistance and thermal insulation performance.
After coating a 30mm-thick refractory material on the metal steel plate, the average linear ablation rate is 0.86mm・s⁻¹ and the temperature rise of the back of the metal steel plate is about 30℃ after the simulated gas flow ablation by the scaled-down test engine. The developed refractory material is applied to the Long March 5 rocket launch platform of the Wenchang Satellite Launch Center in Hainan. The use after the rocket launch shows that the material can withstand the ablation and scouring of the gas flow when 10 engines, including 8 120t LOX-kerosene engines in the 4 boosters of the Long March 5 launch vehicle and 2 hydrogen-oxygen engines in the core stage rocket, are ignited simultaneously. The temperature rise of the back of the metal structure of the launch platform is about 70℃, and the ablation area of the refractory material is less than 10% of the total area.
The Application of Mullite in Other Industries
In addition to the above application fields, high-performance refractory materials prepared from mullite are also used in high-temperature equipment in industries such as glass, cement, petrochemicals, and power. Electrofused mullite can be used to prepare glass kiln lining bricks and alkali-free glass sedimentation tank mullite bricks; mullite-tin groove bricks prepared from mullite are key refractory materials for furnace construction in the forming stage of float glass kilns; mullite-corundum castables prepared from mullite are also applied to glass kilns.
Low-aluminum alkali-resistant mullite bricks prepared with low-aluminum mullite (M45) as the main raw material have good alkali corrosion resistance, thermal shock resistance, high-temperature resistance, high load softening temperature, low bulk density, and thermal conductivity. They are suitable not only for medium, low, and high-temperature areas of cement rotary kilns such as the preheating zone, safety belt, tertiary air duct, burning zone, and cooling zone but also for rotary kilns used in active lime calcination, chromium salt roasting, and alumina production by the alkali-lime sintering method. These bricks enable kiln lightening, reduce cylinder heat loss, and play a role in energy conservation and consumption reduction. Mullite-based castables prepared from mullite as the main raw material are used in parts such as the kiln mouth of cement rotary furnaces, cement kiln covers, and the head coolers of tertiary air ducts. Mullite-based refractory castables can also be applied to petrochemical industries such as petrochemical liquid catalytic converters.
| Field | Application Parts | Examples |
|---|---|---|
| Ceramics | Kiln furniture, kiln linings | Mullite - corundum kiln furniture, mullite - cordierite kiln furniture, mullite - aluminum titanate kiln furniture, mullite - silicon carbide kiln furniture, mullite bricks |
| Metallurgy | Blast furnaces | Mullite - corundum bricks, mullite - corundum castables, mullite - silicon carbide castables |
| Hot blast stoves | Mullite - based low - creep bricks, mullite - andalusite bricks, mullite - cordierite bricks for ceramic burners, mullite - corundum castables | |
| Reheating furnaces, regenerative reheating furnaces | Mullite - corundum castables | |
| Ignition furnaces | Mullite - corundum castables, bauxite - mullite castables | |
| Continuous casting (sliding nozzles, slide plates, permeable plugs) | Mullite composites around the casting holes of upper and lower sliding plates, mullite - corundum injection blowing pipes for argon blowing | |
| Electric furnaces, flame furnaces and intermittent kilns | Mullite - silicon carbide castables, mullite - corundum bricks | |
| Coke ovens | Mullite - cordierite precast bricks | |
| Cement | Cement kilns | Mullite - based castables |
| Petrochemicals | Reactors, catalyst converters, pipelines | Mullite - based castables |
| Glass | Glass tank furnaces, forehearths, end walls of charging chambers | Mullite bricks, mullite - corundum castables |
| Aerospace | Rocket launch platforms, rocket launch vehicles | Mullite - cordierite coatings |
| Military industry | Warship decks | Mullite - based unshaped refractory materials |
Mullite and its composite refractory materials, as linings for various kilns, have been widely used in traditional fields such as metallurgy, ceramics, cement, and petrochemicals. Since these devices operate in high - temperature and high - pressure environments, mullite refractory materials possess good corrosion resistance and high - temperature stability, which can effectively extend the service life of the devices and improve production efficiency. The future development trends of the application of mullite in refractory materials are mainly reflected in the following aspects:
(1) Expand the application fields of mullite refractory materials. Besides the traditional high - temperature industrial fields, mullite refractory materials have already shown promise in fields like aerospace and military industry. However, currently, the application of mullite refractory materials in these fields is rather limited. In the future, their application in fields such as aerospace and military industry will continue to be promoted.
(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 for reducing the heat loss of kilns and improving work efficiency. Refractory materials prepared from lightweight mullite not only have good high - temperature performance but also have a relatively 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. The refractory castables prepared from lightweight mullite as raw materials can solve the problem that lightweight refractory castables prepared from perlite, vermiculite, or ceramsite as raw materials have a low service temperature and cannot be directly used as working linings in contact with flames at high temperatures. Therefore, the development of lightweight mullite with high porosity, low bulk density, low thermal conductivity, and high strength plays a positive role in promoting the development of lightweight heat - insulating refractory materials with good high - temperature performance that can achieve energy conservation 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.