Two key indicators for evaluating the quality of mullite

Two key indicators for evaluating the quality of mullite

Two Common Forms of Mullite in Mullite Materials

Mullite materials are generally produced by directly synthesizing kaolinite, sillimanite group minerals, aluminum hydroxide or alumina, and silica. Clay substances react with alumina or sillimanite group minerals with industrial alumina under heating conditions to produce primary and secondary mullite. Primary mullite forms in the range of 1000-1200℃; further increasing the temperature only increases the crystal size. The formation of secondary mullite usually completes at 1650℃. A two-step sintering method is commonly used to produce dense mullite products.

Mullite has two crystalline forms: acicular and prismatic. Acicular mullite reinforces the glassy phase; for the same chemical composition, acicular mullite materials have a higher refractoriness than prismatic mullite materials. Rapid heating of kaolinite to above 1400℃ forms acicular mullite. Otherwise, slow heating to lower temperatures forms prismatic mullite. There are also reports of tubular and spherical mullite. The former is speculated to be caused by tension resulting from the size mismatch between silicon-oxygen and aluminum-oxygen tetrahedra, while the latter is the so-called nitrogen-containing mullite. The anisotropic thermal expansion of mullite gives it good thermal stability. When high-grade mullite is used as a feeder accessory, it can be directly replaced onto an operating feeder without preheating.

What is Mullite?

Mulite is a refractory material with 3Al₂O₃·2SiO₂ crystalline phase as its main component. Mullite is divided into two categories: natural mullite and synthetic mullite. Natural mullite is rare; it is generally synthesized artificially.

The chemical composition of mullite is 71.8% Al₂O₃ and 28.2% SiO₂. Its mineral structure is orthorhombic, with crystals arranged in long columnar, acicular, or chain-like formations. Acicular mullite interweaves to form a strong framework in finished products. Mullite is classified into three types: α-mullite, equivalent to pure 3Al₂O₃·2SiO₂, abbreviated as 3:2 type; β-mullite, with excess alumina in solid solution, resulting in a slightly expanded crystal lattice, abbreviated as 2:1 type; and γ-mullite, with small amounts of titanium oxide and ferric oxide in solid solution. Mullite is chemically stable and insoluble in hydrofluoric acid. Its density is 3.03 g/cm³, Mohs hardness is 6-7, melting point is 1870℃, thermal conductivity (1000℃) is 13.8 W/(m·K), coefficient of linear expansion (20-1000℃) is 5.3*10⁻⁶/℃, and elastic modulus is 1.47*10¹ºPa.

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

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

Mullite Synthesis

The synthesis of mullite can be divided into sintering and electrofusion methods. Sintering methods, based on the raw material preparation, are further divided into dry and wet methods. The dry method involves grinding the ingredients together, pressing them into balls or blanks, and then firing them in a rotary kiln or tunnel kiln. The wet method involves adding water to the ingredients, grinding them into a slurry, then pressing and dewatering them to form a mud cake, vacuum extruding the mud into mud segments or blanks, and then firing them.

The electrofusion method involves adding the ingredients to an electric arc furnace, melting them in the high temperature generated by the electric arc, and then cooling and crystallizing them. When using natural raw materials (such as bauxite), the lumpy raw materials can be directly crushed to particles <1.5mm without grinding, and then mixed evenly with other powdered raw materials in a mixer.

Sintering for mullite synthesis is generally carried out at 1650~1700℃. The main process factors affecting the sintering synthesis of mullite are the purity of the raw materials, the fineness of the raw materials, and the calcination temperature. The sintering method for synthesizing mullite primarily relies on the solid-state reaction between Al₂O₃ and SiO₂. Therefore, increasing the dispersion of the raw materials will accelerate the solid-state reaction process. Particularly, particles <8μm play a significant role in the formation and sintering of synthesized mullite. It is evident that thorough mixing and fine grinding of the raw materials are crucial process conditions for ensuring the full progress of the solid-state reaction in mullite synthesis. Mullite generally begins to form at 1200℃ and terminates at 1650℃, at which point it exists as microcrystalline particles. Crystallization develops well at temperatures exceeding 1700℃. Thus, the combustion temperature directly affects the formation and crystal development of mullite. Therefore, heating to a certain firing temperature and extending the holding time are necessary conditions for mullite synthesis. The purity requirements for the raw materials used in mullite synthesis are very strict; even a small amount of impurities will reduce the mullite content. In industrial production, various impurities are inevitably introduced, mainly Fe₂O₃, TiO₂, CaO, MgO, and Na₂O·K₂O. Among these, Na₂O and K₂O are the most harmful, inhibiting mullite formation and leading to the formation of a large amount of silica-rich glassy phase, thus reducing the mullite content. Fe₂O₃ slows down the mullification process and increases the amount of glassy phase. When a small amount of TiO₂ is present, some Ti ions enter the mullite lattice to form a solid solution, promoting mullite formation and crystal growth. When the TiO₂ content is too high, it still acts as a flux.

Electrofused mullite is produced by melting the batch in an electric arc furnace, and then cooling and crystallizing the mullite from the melt. Its crystallization process is similar to that of the Al₂O₃-SiO₂ system phase diagram. When the Al₂O₃ content in the batch exceeds the theoretical composition of mullite (71.8%), a mullite solid solution containing excess Al₂O₃, i.e., β-mullite, is formed. Only when Al₂O₃ > 80% will the corundum phase appear. The mineral phase composition of fused mullite is generally mullite crystals and a glassy phase. Compared to sintered mullite, fused mullite has well-developed crystals, larger grains, and fewer defects; its crystal size is hundreds of times larger than that of sintered mullite, resulting in better high-temperature mechanical properties and corrosion resistance.

What are the roles and effects of the aluminum-silicon ratio and impurity composition in the production of fused mullite?

According to the Al₂O₃-SiO₂ binary phase diagram, a mullite composition of approximately 79% Al₂O₃ and 21% SiO₂ is preferable. However, considering the presence of certain impurities in the raw materials, these impurities will react with Al₂O₃ and SiO₂ at high temperatures, correspondingly altering the Al₂O₃/SiO₂ ratio. Therefore, in actual batching, the Al₂O₃/SiO₂ ratio will be slightly lower.

As for impurities such as TiO₂, CaO, MgO, and R₂O, except for a small amount of titanium which can enter the mullite solid solution and exist in the glassy phase, the rest cannot be removed by reduction. Therefore, an increase in impurities will increase the glassy and corundum content, causing mullite to form a coarse-grained structure. This leads to increased porosity during crystal formation, reduced thermal shock resistance and corrosion resistance, and a greater susceptibility to cracking.

Zircon mullite

To further improve the chemical resistance, thermal shock resistance, and reduce the coefficient of thermal expansion of mullite, ZrO₂ can be introduced into the Al₂O₃-SiO₂ system to improve the mullite structure. Mullite containing zirconium dioxide is called zircon mullite. Zircon mullite is generally produced by electrofusion.

The introduction of ZrO₂ into mullite has two functions: 1. It forms a solid solution, activates the crystal lattice, and creates vacancies; it can promote sintering; 2. Utilizing the phase transformation toughening mechanism of ZrO₂, it improves high-temperature mechanical properties. When the mass fraction of ZrO₂ is between 15% and 30%, stress-induced phase transformation toughening is the main process; when ZrO₂ is greater than 30%, microcrack toughening is the main process.

Zircon mullite is mainly used in new types of steel casting slides, sizing nozzles and long nozzles of continuous casting ladles, and key parts of glass furnaces, etc.