What are the synthetic methods of mullite?

The synthesis method of mullite can be divided into sintering method and electric melting method. The sintering method is divided into dry method and wet method according to the method of preparing raw materials. The dry method is to grind the ingredients together, and then press the ball or compact Rotary kiln or tunnel kiln for firing: The wet process is to grind the batch materials with water and grind them into slurry, then press filter to dehydrate them to form mud cakes, and vacuum squeeze the mud into mud sections or mud blanks and then fire them.
 

The electric melting method is to add the batch to an electric arc furnace, melt it at the high temperature formed by the arc, and cool and crystallize. When using natural raw material ingredients (such as bauxite, etc.), the bulk material can be directly crushed without grinding. Particles less than 1.5mm are sufficient, and then mixed with other powdery raw materials in a mixer to be uniform.
 

Sintering synthetic mullite is generally performed at 1650 ~ 1700C. The main factors affecting the synthesis of mullite by sintering method are the purity of the raw materials, the fineness of the raw materials and the calcination temperature. The sintering synthesis of mullite mainly depends on the 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 smaller than 8 μm have a great effect on the formation and sintering of synthetic mullite. It can be seen that the raw materials are thoroughly mixed and finely milled, which is an important process condition for the solid phase reaction of synthetic mullite to proceed fully. Mullite generally begins to form at 1200'C and ends at 1650'C. At this time, it was microcrystalline, and the crystal developed well when the temperature exceeded 1700 ° C.

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 prolonging a certain holding time are necessary conditions for synthesizing mullite. The purity of the raw materials used to synthesize mullite is very strict. A small amount of impurities will reduce the content of mullite. In industrial production, it is inevitable to bring in various impurities, mainly Fe2O3, TiO2, CaO, MgO, NazO and K2O. Among them, the most harmful are Na20 and K2O. They inhibit the formation of mullite and cause a large amount of The generation of silica-rich glass phase reduces the mullite content. Fe2O3 will delay the mullite process and increase the number of glassy phases. When a small amount of TiO2 is present, some T1 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 functions as a flux.
 

The fused mullite is prepared by contacting the batch in an electric arc furnace. The mullite is cooled and crystallized from the melt. The crystallization process is similar to that of the Al2O3-SiO2 system phase diagram. When the amount of Al2O in the batch is higher than the theoretical composition of 71.8% in mullite, a mullite solid solution in which excess Al2O3 is dissolved, namely β-mullite, is formed, and the corundum phase only appears when Al2O3> 80%. The ore phase of fused mullite is composed of mullite crystal and glass phase. China's fused mullite industry standard YB / T104-2004 "Emulsion mullite" divides fused mullite products into two grades DM-1 and DM-2 according to the Al2Os content. For relevant technical conditions, see the corresponding National standard. Compared with sintered mullite, fused mullite has well-developed crystals with large grains and few defects, and the crystal size is hundreds of times that of sintered mullite. Therefore, high temperature mechanical properties and corrosion resistance are relatively good.