The synthesis method of mullite can be divided into a sintering method and an electrofusion method. The sintering method is divided into a dry method and a wet method according to the method of preparing the raw material, and the dry method is to grind the ingredients together, after pressing the ball or pressing the blank. It is fired in a rotary kiln or a tunnel kiln; the wet process is to grind the batch with water to form a slurry, and then dewatering by pressure filtration to form a mud cake, and the vacuum is squeezed into a mud section or a mud body and then fired.
The electrofusion method is to add the batch material into the electric arc furnace, melt in the high temperature formed by the arc, and cool and crystallize. When the natural raw material is used (such as bauxite), the bulk material can be directly crushed without grinding. <1. 5mm particles can be mixed with other powdery materials in a mixer.
Sintered Mullite
Sintered synthetic 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 synthesis of mullite by sintering is mainly carried out by solid phase reaction between Al2O3 and SiO2. Therefore, increasing the dispersion of the raw materials will accelerate the progress of the solid phase reaction. In particular, particles of <8 μm have a great effect on the formation and sintering of synthetic mullite. It can be seen that the raw material is thoroughly mixed and finely ground, which is an important process condition for promoting the solid phase reaction of the synthetic mullite. Mullite is usually produced at 1200 and ends at 1650. At this time, it is microcrystalline, and when the temperature exceeds 1,700, the crystal develops well. 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 holding time is a necessary condition for synthesizing mullite. The purity of the raw materials used to synthesize mullite is very strict, and a small amount of impurities will lower the content of mullite. In industrial production, it is inevitable to bring in various impurities, mainly Fe2O3, TiO2, CaO, MgO, Na2O.K2O, among which the most harmful are Na2O, K2O, which inhibit the formation of mullite and lead to a large number of The production of a silicon-rich glass phase reduces the mullite content. Fe2O3 will delay the progress of the mullite and increase the amount of glass phase. When TiO2 is present in a small amount, part of the Ti ions enter the mullite lattice to form a solid solution, which promotes the formation of mullite and crystal growth. When the TiO2 content is too high, it still acts as a flux.
Fused Mullite
The fused mullite is obtained by melting the batch material in an electric arc furnace and cooling and crystallization of mullite from the melt. The crystallization process is similar to the crystallization process of the phase diagram of the Al2O3-SiO2 system. When the Al2O3 content of the batch is higher than the theoretical composition of 71.8% in mullite, the mullite solid solution in which excess Al2O3 is dissolved is formed, that is, β-mullite, and the corundum phase appears only when Al2O3>80%. The mineral phase composition of fused mullite is generally mullite crystals and a glass phase. China's electric melting mullite industry standard YB/T 104- 2004 "Electrofused mullite" will be divided into two grades of DM-1 and DM-2 according to the A2O3 content. The relevant technical conditions can be seen. National standard. Compared with sintered mullite, fused mullite crystals are well developed with large crystal grains and few defects. The crystal size is hundreds of times that of sintered mullite, so the high temperature mechanical properties and corrosion resistance are relatively good.
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