The Two Key Indicators For Evaluating Mullite Quality Are Actually These!

Mullite materials are generally produced by direct synthesis using kaolinite, sillimanite-group minerals, aluminum hydroxide, or alumina and silica. Clay materials react with alumina, or sillimanite-group minerals react with industrial alumina during heating to form primary and secondary mullite. Primary mullite forms in the range of 1000–1200 °C; further increases in temperature only enlarge the crystals. The formation of secondary mullite is usually completed at 1650 °C. A two-step sintering method is commonly used to produce dense mullite products.

Mullite has two crystal morphologies: acicular and prismatic. Acicular mullite reinforces the glass phase. For materials with the same chemical composition, the refractoriness of acicular mullite materials is higher than that of prismatic mullite materials. Rapid heating of kaolinite to above 1400 °C results in the formation of acicular mullite. In contrast, slow heating to a lower temperature leads to the formation of prismatic mullite. Tubular and spherical mullite have also been reported. The former is presumed to form due to tension caused by the mismatch in the sizes of SiO₄ and AlO₄ tetrahedra, while the latter refers to the so-called nitrogen-containing mullite. The anisotropic thermal expansion of mullite gives it excellent thermal stability. When high-grade mullite materials are used as feeder components, they can be directly replaced in an operating feeder without preheating.

Mullite is a refractory raw material whose main crystalline phase is 3Al₂O₃·2SiO₂. It is classified into two categories: natural mullite and synthetic mullite. Natural mullite is rare, so artificial synthesis is generally adopted.

The chemical composition of mullite is 71.8% Al₂O₃ and 28.2% SiO₂. It has an orthorhombic crystal system, with crystals arranged in long columnar, acicular, or chain-like forms. Acicular mullite crystals interpenetrate each other in products to form a strong framework.

Mullite is divided into three types:

α-mullite: equivalent to pure 3Al₂O₃·2SiO₂, abbreviated as the 3:2 type;

β-mullite: a solid solution with excess alumina, with a slightly expanded lattice, abbreviated as the 2:1 type;

γ-mullite: a solid solution containing a small amount of titania and ferric oxide.

Mullite has stable chemical properties and is insoluble in hydrofluoric acid. Its density is 3.03 g/cm³, Mohs hardness is 6–7, melting point is 1870 °C, thermal conductivity (at 1000 °C) is 13.8 W/(m·K), linear expansion coefficient (20–1000 °C) is 5.3 × 10⁻⁶ /°C, and elastic modulus is 1.47 × 10¹⁰ Pa.

Due to its excellent high-temperature mechanical and thermal properties, synthetic mullite and its products feature high density and purity, high high-temperature structural strength, a low high-temperature creep rate, a low thermal expansion coefficient, strong chemical corrosion resistance, and good thermal shock resistance.

The key indicators for evaluating the quality of mullite are its phase composition and density.

Mullite synthesis methods can be divided into the sintering process and the fused (electrofused) process. The sintering process is further classified into the dry process and the wet process according to raw material preparation.

In the dry process, the batch materials are co-ground, pelletized or pressed into shapes, and then fired in a rotary kiln or tunnel kiln. In the wet process, the batch materials are milled with water into a slurry, dewatered by pressure filtration to form a filter cake, vacuum-extruded into billets or shapes, and then fired.

The fused (electrofused) process involves charging the batch into an electric arc furnace, melting it at the high temperature generated by electric arcs, and cooling it to crystallize. When natural raw materials (such as bauxite) are used, they can be directly crushed to particles smaller than 1.5 mm without grinding, and then uniformly mixed with other powdered raw materials in a mixer.

Sintered synthetic mullite is generally produced at 1650–1700 °C. The main technological factors influencing sintered mullite synthesis are raw material purity, raw material fineness, and firing temperature. Sintered mullite synthesis is mainly achieved via solid-state reactions between Al₂O₃ and SiO₂. Therefore, increasing the dispersion of the raw materials accelerates the solid-state reaction. Particles smaller than 8 μm are particularly effective for mullite formation and sintering. Thus, thorough mixing and fine grinding of raw materials are critical for complete solid-state reactions during mullite synthesis.

Mullite formation starts at approximately 1200 °C and finishes at 1650 °C, at which point it exists as microcrystals. The crystals become well developed when the temperature exceeds 1700 °C. Firing temperature directly affects mullite formation and crystal growth. Accordingly, heating to a sufficient firing temperature and holding for an appropriate time are necessary conditions for synthetic mullite production.

Strict purity is required for raw materials used in mullite synthesis, since even small amounts of impurities reduce the mullite content. Impurities inevitably introduced in industrial production mainly include Fe₂O₃, TiO₂, CaO, MgO, Na₂O and K₂O. Among them, Na₂O and K₂O are the most harmful: they suppress mullite formation, induce a large amount of silica-rich glass phase, and lower the mullite content. Fe₂O₃ retards the mullitization process and increases the glass phase content. A small amount of TiO₂ allows some Ti ions to enter the mullite lattice to form a solid solution, thereby promoting mullite formation and crystal growth. However, excessive TiO₂ acts as a flux.

Fused mullite is produced by melting the batch in an electric arc furnace and crystallizing mullite from the melt during cooling. Its crystallization process is similar to that shown in the Al₂O₃–SiO₂ phase diagram. When the Al₂O₃ content in the batch is higher than the theoretical 71.8% in mullite, a mullite solid solution with excess Al₂O₃ (i.e., β-mullite) forms. The corundum phase appears only when Al₂O₃ > 80%. The mineral phase composition of fused mullite generally consists of mullite crystals and a glass phase.

In China, the industry standard YB/T 104-2004 Fused Mullite classifies fused mullite into two grades, DM-1 and DM-2, based on Al₂O₃ content. The relevant technical specifications can be found in the corresponding national standards.

Compared with sintered mullite, fused mullite has well-developed crystals, a large grain size, and few defects-its crystal size is hundreds of times that of sintered mullite. Consequently, it exhibits better high-temperature mechanical properties and corrosion resistance.

According to the binary phase diagram of the Al₂O₃–SiO₂ system, the optimum composition for mullite is approximately 79% Al₂O₃ and 21% SiO₂. However, considering the presence of a certain amount of impurities in the raw materials, which react with Al₂O₃ and SiO₂ at high temperatures and thereby alter the Al₂O₃/SiO₂ ratio, the Al₂O₃/SiO₂ ratio used in actual batching is slightly lower.

Regarding impurities such as TiO₂, CaO, MgO, and R₂O (alkali oxides), only a small amount of titanium can enter the mullite solid solution, while the remainder remains in the glass phase and cannot be removed by reduction. Therefore, an increase in impurity content leads to a higher content of glassy phase and corundum and promotes the formation of a coarse-grained mullite structure. This results in higher apparent porosity of the products, poorer thermal shock resistance and corrosion resistance, and a greater tendency for cracking.

To further improve the chemical corrosion resistance and thermal shock resistance of mullite and reduce its expansion coefficient, ZrO₂ can be introduced into the Al₂O₃–SiO₂ system to modify the mullite structure. Mullite containing zirconia is known as zirconia mullite. Zirconia mullite is generally produced by the electrofusion process.

The introduction of ZrO₂ into mullite serves two functions:

1.It forms a solid solution, activates the crystal lattice, and creates vacancies, thereby promoting sintering.

2.Based on the phase transformation toughening mechanism of ZrO₂, the high-temperature mechanical properties are improved.

When the ZrO₂ mass fraction is between 15% and 30%, stress-induced phase transformation toughening is dominant.

When the ZrO₂ content exceeds 30%, microcrack toughening becomes the main mechanism.

Zirconia mullite is mainly used in new-type casting sliding plates, sizing nozzles, and long nozzles for continuous casting tundishes, as well as in key components of glass furnaces.