Production Technology of Mullite Products
Mullite refractory is a refractory product with mullite as main crystal phase, which is made of synthetic mullite as raw material. When the Al2O3 content of the product is lower than that of mullite theory, it also contains a small amount of square quartz, and when the Al2O3 content is higher than that of mullite theory, it contains a small amount of corundum. The high temperature vermicularity of mullite products is better than that of high aluminum bricks produced by natural bauxite raw materials. The high temperature properties of mullite products are comparable to those of siliceous and magnesia products, and the corrosion resistance of mullite products to acidic and low alkaline slag is better than that of magnesia products. There are two main types of mullite products: sintered mullite products and molten mullite products.
(1)At present, the main formulas used in the synthesis of mullite by sintering are as follows:
1)Natural high bauxite kaolin;
2)Industrial alumina clay (kaolin or waxite);
3)Industrial alumina silica;
4)Industrial alumina high bauxite (or sillimanite raw material concentrate);
5)Corundum (α-Al2O3) silica.
At present, the ingredients of synthetic mullite in China are mainly one and two.
When the industrial alumina and the natural raw material are used, the content of the Al2O3 in the mixture should be up to 68% or slightly higher, in which case the mullite content is the highest, and the corundum phase is not normally present. If the content of Al2O3 in the mixture exceeds 70%, the amount of corundum increases with the content of Al2O3, and the amount of mullite decreases. And the synthetic material is prepared according to the principle that the Al2O3 content is slightly larger than the highest forming amount of the mullite, and a small amount of corundum phase is present in the synthesis material and the glass phase is reduced. If the high-purity industrial alumina and silica are used, the Al2O3 content should be 1-3% higher than the theoretical value of mullite.
1)The production process of mullite by sintering method is the purity of raw material, the fineness of raw material and calcination temperature.
Dispersion fineness: the synthesis of mullite by sintering mainly depends on the solid state reaction between Al2O3 and SiO2. Increasing the subdivision divergence of raw materials will accelerate the process of solid state reaction. In particular, particles less than 8 μ m have great effect on the formation and sintering of mullite. According to some research data, the sintering temperature of synthetic mullite depends on the proportion of less than 4 μ m, and when the particle size of the material is below 4 μ m, the density of the material is the highest.
2)Calcination temperature: synthetic mullite usually begins to form at 1200 ℃ and terminates at 1650 ℃. At this time, it is microcrystalline, and the crystallization develops well when the temperature exceeds 1700 ℃. Therefore, the sintering temperature of synthetic mullite should be more than 1700 ℃. When natural raw materials are used, the calcination temperature is slightly lower because a small amount of impurities are brought into the raw materials. When the Al2O3 content in the proportion is slightly lower than the theoretical composition of mullite, the sintered mullite clinker with more than 90% mullite content can be obtained when the calcination temperature is 1630 ℃ and the temperature is 6 × 8 h.
3)Purity of raw materials: the purity of raw materials is very strict, a small amount of impurities will reduce mullite content. The most harmful is R _ 2O, which leads to the decomposition of mullite at high temperature and produces more silicon-rich glass phase. Fe2O3 can delay the process of mullite. When a small amount of TiO2 exists, some Ti4 ions enter the mullite lattice to form a finite solid solution, which promotes the formation and crystal development of mullite. When the content of TiO2 is too much, it acts as flux. It can be seen that it is the most basic technological condition to improve the purity of raw materials and reduce the impurity content as much as possible (generally less than 1.5%).
(2)Mullite was synthesized by electromelting. Electrofusion synthesis is a method to produce mullite clinker by mixing the batching into electric arc furnace. Raw materials are industrial alumina, sintered high quality bauxite, high purity silica and so on. The ingredients are reasonably selected according to Al2O3:SiO2=1.4~2.0 and mixed evenly in the mixer. Electric melting is carried out in a block three-phase arc furnace or a tilting three-phase arc furnace. Low voltage and high current system is used for smelting. After melting, the molten mullite molten block is cooled. After crushing and purifying, the electromelted mullite raw material with different particle size is made for user use. There are two types of production at present: high purity electricity The main performance indexes of molten mullite clinker and natural electromelted mullite clinker are shown in Table 11-45.
Main points of production process for firing mullite products.
The production process of the fired mullite product is similar to the production process of the high-alumina brick, the synthetic mullite is used as the particle material, the fine powder is synthetic mullite, or white corundum, quartz powder and pure clay are used to prepare the mixed fine powder which is equivalent to the mullite composition. The granular material and the fine powder are matched in proportion, and the common mixing ratio is 45-55% of the granular material and the fine powder (less than 0.088 mm = 55-45%). And performing high-pressure molding after the mixing is uniform. The sintering temperature is 1550-1600 DEG C. When the fused mullite clinker is used as the particle material, the sintering temperature of the product is more than 1700 DEG C, and the physical and chemical performance indexes of the product are shown in Table 11- 46
