With the implementation of the dual carbon strategy, high-temperature industries are facing increasingly stringent requirements for energy conservation and environmental protection, leading to an increase in the research and use of lightweight refractory materials. Mullite castables offer excellent properties such as a high refractoriness under load, good thermal shock resistance, and strong erosion resistance. They are widely used in industrial furnace linings, ladle covers, and tundish linings. However, the dense aggregate and matrix result in high thermal conductivity in castables, leading to significant heat loss and high energy consumption in high-temperature thermal equipment. Therefore, efforts to lightweight castables have led to the development of lightweight, high-strength mullite castables that offer high-temperature resistance, low thermal conductivity, high strength, and good thermal shock resistance.
Lightweighting castables can be achieved through lightweight aggregates or lightweight matrices. Aggregates are the primary contributor to castable strength, and only high-performance lightweight aggregates can produce castables with high strength and low thermal conductivity. Using porous mullite microspheres, α-Al₂O₃, and SiO₂ fine powders as the primary raw materials, silica sol as a binder, and AlF₃·H₂O and V₂O₅ as additives, a large number of whiskers were generated in the sample, resulting in a high-strength, low-thermal-conductivity mullite castable. Using spherical lightweight mullite aggregate, mullite fine powder, and kyanite as the primary raw materials, a lightweight mullite castable with good flowability, a bulk density of 1.73 g·cm⁻³, a thermal conductivity of 0.580 W·(m·K)⁻¹, and excellent mechanical properties was achieved.
The matrix can also be lightweighted by adding pore-forming agents. When polystyrene foam spheres are added to a corundum-mullite castable at 50% of the total slurry volume, the castable has a porosity of 61% and a thermal conductivity as low as 0.220 W·(m·K)⁻¹. Alumina hollow spheres are often used as aggregate in the preparation of porous ceramics due to their high refractoriness and low thermal conductivity. They are also a new type of inorganic pore-forming agent. When 30% (w/w) of alumina hollow spheres are added to a mullite castable, the bulk density of the castable is reduced to 1.96 g·cm⁻³ and the thermal conductivity is 0.890 W·(m·K)⁻¹.
By simultaneously lightweighting the matrix and aggregate, a castable with even lower thermal conductivity can be produced while maintaining good mechanical properties, thereby reducing energy consumption in high-temperature thermal equipment. Therefore, a lightweight and high-strength mullite castable was prepared, with microporous mullite, mullite fine powder, alumina powder, and silica powder as main raw materials, alumina hollow spheres as pore-forming agent, and cement as binder.
Effect of Alumina Hollow Sphere Addition on the Properties of Mullite Castables
Lightweight mullite castables were prepared using microporous mullite, mullite fine powder, alumina fine powder, alumina hollow spheres, and silica fine powder as raw materials, with cement as a binder. The experimental raw materials included: microporous mullite aggregates of 5-3, 3-1, and 1-0.086 mm; mullite fine powder ≤0.045 mm; silica fine powder, alumina fine powder; alumina hollow spheres of 2-1 mm as a pore former; CA71 cement binder; and sodium tripolyphosphate.
By optimizing the closest packing principle, the castable component ratios were optimized to address the difficulty in combining alumina hollow spheres with lightweight mullite aggregate. The effects of alumina hollow sphere addition (mass fractions of 0, 3%, 6%, and 9%) on the properties of the mullite castables were investigated. The results show that:
Alumina hollow spheres increase the porosity of the sample and promote the formation of mullite phases between alumina and silica powders. This resolves the issue of significantly reduced mechanical properties due to increased apparent porosity.
With increasing the amount of alumina hollow spheres added, the apparent porosity of the sample significantly increases, the bulk density decreases, the thermal conductivity significantly decreases, the thermal shock resistance improves, and the strength decreases minimally. When the alumina hollow sphere addition reaches 9% (w), the sample exhibits higher apparent porosity, lower bulk density and thermal conductivity after heat treatment at 1400°C, but still exhibits high room-temperature strength.
Mullite Castable in RS Monolithic Refractories Factory
What is the Most Suitable Temperature for Mullite Refractory Castables?
Mullite castables are used in industrial furnace linings and are most suitable for temperatures up to 1400°C.
Mullite castables offer excellent flexibility and stable high-temperature performance. At medium and high temperatures, aluminum and silicon react to form mullite, which acts as a binder. Secondary mullitization then improves and enhances the castable’s flexibility. Mullite castables also exhibit excellent thermal shock resistance, allowing them to withstand the thermal stresses of high temperatures. They also have high slag resistance, resisting erosion and infiltration from high-temperature slag.
Mullite refractory castables have excellent flowability and plasticity, making them easy to apply and form. This shortens the construction cycle and improves production efficiency. While offering high thermal stability, they also provide excellent thermal insulation properties, improving the thermal efficiency of the furnace lining and reducing fuel consumption.
Mullite castables are suitable for a variety of industrial furnace linings, particularly at the burner end. Due to their excellent flexibility and plasticity, they offer significantly superior performance compared to castables made from other materials. The effect is particularly pronounced in temperatures below 1400°C. Mullite has an orthorhombic crystal structure, with crystals arranged in long columns, needles, and chains. The interspersed needle-shaped mullite within the product creates a strong framework for the castable.
Mullite castables are considered high-grade refractory castables. They can also be combined with corundum and silicon carbide to create composite refractory castables, maximizing the mullite high-temperature bonding phase, resulting in excellent microstructure and thermal shock resistance. Mullite castables can also be used to create prefabricated burner bricks, effectively protecting the burner from erosion and erosion caused by wind and fire.
