Issues about Mullite

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What is Mullite?

Mullite is a refractory raw material primarily composed of the crystalline phase 3Al₂O₃·2SiO₂. Mullite can be categorized into two types: natural mullite and synthetic mullite. Natural mullite is rare, so synthetic mullite is commonly used.

The chemical composition of mullite is 71.8% Al₂O₃ and 28.2% SiO₂. It has a monoclinic crystal structure, and its crystals are arranged in long columnar, needle-like, and chain-like formations. The needle-shaped mullite interlaces with each other to form a sturdy framework in the product. Mullite can be classified into three types: α-mullite, which is equivalent to pure 3Al₂O₃·2SiO₂ and is abbreviated as 3:2 type; β-mullite, which contains excess alumina and slightly expanded lattice, abbreviated as 2:1 type; γ-mullite, which contains small amounts of titanium oxide and ferric oxide. Mullite has stable chemical properties and is insoluble in hydrofluoric acid.

Due to its excellent high-temperature mechanical and thermophysical properties, synthetic mullite and its products have high density and purity, high temperature structural strength, low creep rate at high temperatures, low thermal expansion, strong resistance to chemical corrosion, and thermal shock resistance.

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

mullite

Synthesis of Mullite

The synthesis of mullite can be achieved through sintering and fused cast methods. The sintering method can be further divided into dry and wet methods based on the preparation of raw materials. In the dry process, the ingredients are co-ground, and then after pelletizing or forming, they are fired in a rotary kiln or tunnel kiln. The wet process involves grinding the raw materials with water to form a slurry, followed by pressure filtration and dewatering to obtain mud cakes. The mud cakes are then extruded under vacuum to form clay segments or clay billets, which are subsequently fired.

The fused cast method involves adding the raw materials to an electric arc furnace, where they melt at high temperatures formed by the arc. After cooling and crystallization, mullite is obtained. When using natural raw materials (such as alumina clay), grinding is not necessary, and the block-shaped raw materials can be directly crushed to particles smaller than 1.5mm and then mixed uniformly with other powdered raw materials in a mixer.

Sintered Mullite

The sintering synthesis of mullite is typically carried out at temperatures ranging from 1650 to 1700℃. The main process factors affecting the synthesis of mullite by sintering method are the purity and fineness of raw materials, as well as the calcination temperature. The synthesis of mullite through the sintering method mainly relies on the solid-phase reaction between Al₂O₃ and SiO₂. Therefore, improving the dispersion of raw materials accelerates the progress of the solid-phase reaction, especially for particles smaller than 8μm, which have a significant impact on the formation and sintering of mullite. Therefore, thorough mixing and fine grinding of raw materials are important process conditions for promoting the complete solid-phase reaction of mullite. Mullite typically starts to form at 1200℃ and completes around 1650℃. At temperatures above 1700℃, crystal development is enhanced. Thus, heating to a certain calcination temperature and extending a specific holding time are necessary for mullite synthesis. The purity requirements for the raw materials used in the synthesis of mullite are stringent, as even small amounts of impurities can lower the mullite content. In industrial production, various impurities are inevitably introduced, including Fe₂O₃, TiO₂, CaO, MgO, Na₂O·K₂O. Among them, Na₂O and K₂O have the greatest harmful effects as they inhibit the formation of mullite and result in the production of a large amount of silica-rich glass phase, reducing the mullite content. Fe₂O₃ delays the mullitization process and increases the quantity of glass phase. When a small amount of TiO₂ is present, some Ti ions enter the mullite lattice to form a solid solution, promoting the formation and growth of mullite crystals. However, when the TiO₂ content is too high, it still acts as a flux.

Fused Cast Mullite

Fused cast mullite is produced by melting the raw materials in an electric arc furnace, and mullite is obtained by cooling and crystallization from the melt. The crystallization process is similar to the crystallization process in the Al₂O₃-SiO₂ phase diagram. When the Al₂O₃ content in the raw materials exceeds the theoretical composition of mullite (71.8%), a mullite solid solution with excess Al₂O₃, known as β-mullite, is formed. The corundum phase only appears when Al₂O₃ > 80%. The mineral phase composition of fused cast mullite generally consists of mullite crystals and a glass phase. In China, the industry standard YB/T104-2004 “fused cast Mullite” classifies fused cast mullite products into two grades, DM-1 and DM-2, based on their Al₂O₃ content. The relevant technical conditions can be found in the corresponding national standard. Compared to sintered mullite, fused cast mullite has well-developed crystals with larger grain size and fewer defects. The crystal size is hundreds of times larger than that of sintered mullite, resulting in better high-temperature mechanical properties and resistance to erosion.

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