Common Forms and Synthesis Methods of Mullite

Mullite Introduction Mullite is a refractory raw material with 3Al2O3·2SiO2 crystalline phase as the main component. Mullite is divided into two categories: natural mullite and artificial mullite. Natural mullite is rare and is generally synthesized artificially. The chemical composition of mullite is Al2O3 71.8%, SiO2 28.2%. The mineral structure is orthorhombic, and the crystals are arranged in long columns, needles, and chains. Needle-shaped mullite is interspersed in the product to form a solid skeleton.

Rognsheng Fused Mullite

Mullite is divided into 3 types: α-mullite, which is equivalent to pure 3Al2O3·2SiO2, referred to as 3:2 type. β-mullite, which has excess Al2O3 in solid solution and slightly expanded character, referred to as 2:1 type. γ-mullite, which has a small amount of TiO2 and Fe2O3 in solid solution. Mullite has stable chemical properties and is insoluble in HF. Its density is 3.03g/cm3, Mohs hardness is 6-7, and melting point is 1870℃. Thermal conductivity (1000℃) is 13.8W/(m·K). Linear expansion coefficient (20-1000℃) is 5.3×10-6℃. Elastic modulus is 1.47×1010Pa. Mullite has good high-temperature mechanical and thermal properties. Therefore, synthetic mullite and its products have the advantages of high density and purity, high-temperature structural strength, low high-temperature creep rate, low thermal expansion rate, strong chemical erosion resistance, and good thermal shock resistance.

Common Forms of Mullite

Mullite materials can generally be produced by direct synthesis using kaolinite, sillimanite minerals, aluminum hydroxide or alumina and silicon dioxide. Clay materials react with alumina or sillimanite minerals with industrial alumina under heating conditions F to form primary and secondary mullite. Primary mullite is formed in the range of 1000-1200°C. Further increasing the temperature will easily increase the crystallization. The formation of secondary mullite is usually completed at 1650°C. In order to produce dense mullite products, a two-step sintering method is often used.

RS Sintered Mullite

Mullite has two crystal forms: needle-shaped and prismatic. Needle-shaped mullite strengthens the glass phase and the chemical composition phase of the material. At the same time, the refractoriness of needle-shaped mullite materials is higher than that of prismatic mullite materials. Kaolinite is quickly heated to above 1400°C to form needle-shaped mullite. Otherwise, slow heating to a lower temperature will form prismatic mullite. Tubular and spherical mullite has also been reported. The former is speculated to be caused by the tubular shape due to the tension caused by the incoordination of the sizes of silicon oxide and aluminum oxide tetrahedrons, and the latter is the so-called nitrogen-containing mullite. The anisotropic thermal expansion of mullite gives it good thermal stability. When high-grade mullite material is used as a feeder accessory, it can be directly replaced on the running feeder without preheating.

Synthesis Method of Mullite

The synthesis method of mullite can be divided into sintering method and electric melting method. Sintering method can be divided into dry method and wet method according to the way of raw material preparation. The dry process is to grind the ingredients together, and then use a rotary kiln or tunnel kiln to burn them after pressing balls or pressing blanks. The wet process is to grind the ingredients into slurry with water, then filter and dehydrate them into mud cakes, vacuum squeeze the mud into mud segments or mud blanks, and then burn them.

The electric melting method is to add the ingredients to the electric arc furnace, melt them in the high temperature formed by the electric arc, and cool them to crystallize. When using natural raw materials (such as bauxite, etc.), the block raw materials can be directly crushed into particles <1.5mm without grinding. Then mix them evenly with other powdered raw materials in a mixer.

The sintering synthesis of mullite is generally carried out at 1650-1700℃. The main process factors affecting the synthesis of mullite by sintering are the purity of the raw materials, the fineness of the raw materials and the calcination temperature. The sintering method of synthesizing mullite mainly relies on the solid phase reaction between Al2O3 and SiO2. Therefore, improving the dispersion of the raw materials will accelerate the process of the same phase reaction. In particular, particles <8μm have a great effect on the formation and sintering of synthetic mullite. It can be seen that the full mixing and fine grinding of the raw materials is an important process condition for promoting the full solid phase reaction of synthetic mullite.

Mullite generally starts to form at 1200℃ and ends at 1650℃. At this time, it is in the shape of micro-grains. When the temperature exceeds 1700℃, the crystallization is well developed. It can be seen that the combustion temperature directly affects the formation of mullite and the development of crystals. Therefore, heating to a certain firing temperature and extending a certain insulation time are necessary conditions for synthesizing mullite. The purity requirements of the raw materials used to synthesize mullite are very strict, and a small amount of impurities will reduce the content of mullite.

In industrial production, various impurities are inevitably introduced, mainly Fe2O3, TiO2, CaO, MgO, Na2O, and K2O, among which Na2O and K2O are the most harmful. They inhibit the formation of mullite and lead to the generation of a large amount of silicon-rich glass phase, reducing the content of mullite. Fe2O3 will delay the process of mullitization and increase the number of glass phases. When TiO2 exists in small amounts, some Ti ions enter the mullite lattice to form a solid solution, promoting the formation of mullite and the growth of crystals. When the TiO2 content is too high, it still acts as a flux. Fused mullite is made by melting the batch in an electric arc furnace and cooling the mullite from the melt to crystallize. Its crystallization process is similar to the crystallization process of the Al2O3-SiO2 system phase diagram. When the Al2O3 of the batch is higher than the theoretical group 71.8% in mullite, a mullite solid solution with excess Al2O3 is formed, namely β-mullite. The corundum phase will only appear when Al2O3>80%. The mineral phase composition of fused mullite is generally mullite crystals and glass phases. Compared with sintered mullite, fused mullite crystals are well developed, with large grains and fewer defects. The crystal size is hundreds of times that of sintered mullite. Therefore, the high-temperature mechanical properties and corrosion resistance are relatively good.