Synthesis 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 burn it by rotary kiln or tunnel kiln after pressing ball or compact. The wet process is to grind the compound material into slurry with water, then press and filter to dehydrate into mud cake, and vacuum squeeze mud into mud section or mud blank and then burn.
The electric melting method is to add the compound material to the arc furnace, melt in the high temperature formed by the arc, cool crystallization, the use of natural raw material ingredients (such as bauxite, etc.) can be crushed directly to <1.5mm particles without grinding, and then mixed with other powdered raw materials in the mixer.
Synthesis of mullite by sintering is generally carried out at 1650~1700℃. The main factors affecting the synthesis of mullite by sintering are purity of raw material, fineness of raw material and calcination temperature. The synthesis of mullite by sintering method mainly depends on Al? O? The solid phase reaction between SiO2 and sio2 is completed, so improving the dispersion of raw materials will accelerate the process of solid phase reaction. In particular, the 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 raw materials is an important process condition to promote the solid phase of the synthesis of mullite. Mullite generally begins to form at 1200 ° C and ends at 1650 ° C. At this time, it is microcrystalline, and the crystallization develops well when the temperature exceeds 1700℃. It can be seen that the combustion temperature directly affects the formation and crystal development of mullite. Therefore, heating to a certain firing temperature and extending a certain holding time are necessary conditions for the synthesis of mullite. The purity of the raw materials used to synthesize mullite is very strict, and a small amount of impurities will reduce the content of mullite. In industrial production, it is inevitable to bring a variety of impurities, mainly Fe? O? TiO? , CaO, MgO, Na? O·K? O, the most harmful of which is Na? O, K? O, they inhibit the formation of mullite and lead to the production of a large number of silicon-rich glass phases, reducing mullite content. Fe? O? It will slow down the mullite process and increase the amount of glass phase. Be TiO? When a small amount of Ti ions exist, part of Ti ions enter the mullite lattice to form a solid solution, promoting the formation of mullite and crystal development and growth, when the TiO2 content is too high, it still acts as a flux.
Electromelting mullite is prepared by melting the compound in an electric arc furnace, and the mullite is cooled out of the melt. The crystallization process is similar to that of Al? O? -SiO? The crystallization process of the system phase diagram is similar. When the Al? O? When the theoretical composition of mullite is higher than 71.8%, dissolved excess Al is formed. O? The solid solution of mullite, beta-mullite, is only Al? O? The corundum phase only appears at 80%. The mineral phase composition of fused mullite is generally mullite crystal and glass phase. China's electric melt mullite industry standard YB/T104-2004 "Electric melt mullite" the electric melt mullite product according to Al? O? The content is divided into DM-1 and DM-2 grades, and the relevant technical conditions can be seen in the corresponding national standards. Compared with sintered mullite, fused mullite crystals are well developed with large grains and fewer defects, and the crystal size is hundreds of times that of sintered mullite, so the high temperature mechanical properties and erosion resistance are relatively good.