MULLITE

Mullite [Al₆Si₂O₁₃ = 3Al₂O₃·2SiO₂], with 71.8 wt.% Al₂O₃ ,  is an important silicate mineral that occurs in high-silica alumina refractories. Mullite exhibits a high melting point of 1810°C combined with low thermal expansion coefficients (i.e., 4.5 × 10^–6 K^–1 parallel to the a-axis and 5.7 × 10^–6 K^–1 parallel to the c-axis), a good mechanical strength with a tensile strength of 62 MPa, and resilience at elevated temperatures that make mullite a highly suitable mineral for highly refractory materials. In nature, mullite is an extremely scarce mineral that occurs only in melted argillaceous inclusions entrapped in lavas from the Cenozoic Era on the Island of Mull, Scotland, but no deposit was found to be economically minable.

colorless; rhomb, a=0.7557nm, b=0.76876 nm, c=0.28842 nm; hardness: hot pressed 13.6GPa, sintered 12.7GPa; indentation microfracture 2.02MPa ·m1/2; submicrometer powder(s) can be prepared by hydrolysis of mixed alkoxides, followed by drying and calcining up to 1600°C; sinter, microstructure [SOM91]; phases equilibria in mullite [PAS88]; other data in [HIR89] [ROB67] [MIZ89]

Mullite is present in the form of needles in porcelain, it is used in high-mullite based products is in hot blast stove checker bricks, melting and firing processes, and is used as a refractory material, manufacture of a type of crucible, steel industry, electronic substrates and protective coatings.

Artificial mullite, or synthetic mullite, a ceramic material made by a prolonged fusing in the electric furnace of a mixture of silica sand or diasphoric clay and bauxite, has the composition 3Al₂O₃·2SiO₂, has a melting point of 1810 C, and softens at 1650 C.
Mullite occurs in nearly all ceramic products containing alumina and silica but, with the exception of refractories, is seldom introduced as such except as calcined kyanite.
The bricks are resistant to flame and to molten ash, and have a low, uniform coefficient of thermal expansion and a heat conductivity only slightly above that of fireclays. Normally, mullite has very fine crystals that change form and become enlarged after prolonged heating, making the product porous and permeable. For stable high-temperature refractories the mullite is prefused to produce larger crystals. At very high temperatures mullite tends to decompose to form corundum and alkali-silicate minerals of lower heat resistance. Mullite is also used for making spark plugs, chemical crucibles, and extruding dies, and a foamed mullite is used as a uniformly latticed honeycomb structure for lightweight, heat-resistant structural parts.

Synthetic mullite is formed in high-silica alumina refractories during the firing process at high temperature, the major raw materials being kaolin, alumina, and clays and to a lesser extent kyanite, when available. Actually, when a fire clay is fired, its major phyllosilicate mineral, the kaolinite Al₄(Si₄O₁₀)(OH)₈, first gives off its water, and above 800°C an important chemical change takes place with the formation of one of the three aluminosilicate polymorphs (Al₂SiO₅), i.e., andalusite, kyanite, or sillimanite, and free silica according to the following chemical reaction:
Al₄(Si₄O₁₀)(OH)₈ —> 2Al₂SiO₅ + 2SiO₂ + 4H₂O (at T > 800°C)
If firing is carried out above 1595°C, the highly refractory mineral mullite then forms with an additional liberation of free silica that melts according to the following chemical reaction:
3Al₂SiO₅ —> Al₆Si₂O₁₃ + SiO₂ (at T >1600°C)
For that reason, high-silica alumina refractories containing less than 71.8 wt.% Al₂O₃ are limited in their use to temperatures below 1595°C. Above 71.8 wt.% Al₂O₃, mullite alone or mullite plus corundum (α-Al₂O₃) coexists with a liquidus at 1840°C. Therefore, the use of high-alumina refractories is suited for iron- and steelmaking for firebrick and ladles and furnace linings.

Two grades of synthetic mullite are available for refractories: sintered mullite is obtained by calcination of bauxitic kaolin or a blend of bauxite, aluminas, and kaolin or, to a lesser extent, kyanite; electrofused mullite is made by the electrothermal melting at 2200°C of a mixture of silica sand and bauxite or diasporic clay in an electric-arc furnace. Mullites are formulated to produce dense shapes, some in a glass matrix to yield maximum thermal shock resistance and good mechanical strength. Dense electrofused mullite in a glassy matrix formulated to offer a high-quality economical insulating tubing for thermocouple applications is an extremely versatile and economically viable material. Its workability allows for an extensive range and flexibility in fabrication.
It is well suited for the casting of special shapes. Its typical applications are insulators in oxidizing conditions for noble-metal thermocouples used in conditions up to 1450°C, spark plugs, protection tubes, target and sight tubes, furnace muffles, diffusion liners, combustion tubes, radiant furnace tubes, and kiln rollers. Major producers of sintered mullite are C-E Minerals, Andersonville, GA in the USA, followed by several Chinese producers, while Washington Mills Electro Minerals Corp. in Niagara Falls, NY leads the production of electrofused mullite.