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Alumina The Sintering

Hangzhou JWG Technology Co.Ltd | Updated: Sep 20, 2017

Alumina acid resistance, alkali slag erosion ability is strong, high temperature and high strength, widely used in steel, non-ferrous industry, but its poor nasty cold and hot resistance, high temperature and volume stability is bad, cause deformation of the material and peeling, reduce the service life of materials. At present, artificial synthetic mullite is used in China to overcome its shortcomings. However, the price of artificial synthetic mullite is high, which restricts its use. Alumina has a wide use in refractory and aluminum industries. The most basic use of alumina is in the metallurgical division of aluminium, which accounts for 85 per cent of bauxite consumption. The remaining bauxite consumption can be divided into special alumina and other two categories. Alumina is used in the production of aluminum nitride, alumina and activated alumina, which account for 10% of the total consumption of alumina. "Other" category refers to bauxite, which is used directly for mining such as refractory materials, cement and corrosive industries. In terms of market size and value, refractory bauxite occupies an important position in the ultimate use of non-metallurgical market.

Alumina sintering

The high temperature mechanical properties of sintered alumina materials depend on the glass effect and the crystallization effect. The closer to 70% of A12O3, the better the high temperature strength and the smaller the creep. The introduction of appropriate amount of MgO and zirconium quartz is beneficial to high temperature mechanical properties. But K2O, CaO, TiO2 and Fe2O3 are all harmful impurities and should be minimized. The heating changes of bauxite are divided into three stages: the decomposition stage, the secondary moat phase and the recrystallization phase.

Decomposition stage: at this stage, bauxite diaspore and kaolinite in the dehydration, the start of the 400 degrees Celsius to 450 violent reactions to six baidu, 700 ~ 800 ℃. When the water alumina dehydrates, it forms a false impression of corundum. It still retains the shape of the original alumina, but the edges are blurred and gradually transformed into a corundum at high temperature. High diaspore partial kaolinite is formed after dehydration, above 950 ℃ are kaolinite into mullite and amorphous SiO2. The latter converted to fang shi ying at high temperature.

Secondary mullite phase: in 1200 ℃ above, from the formation of diaspore dehydration corundum and kaolinite decomposition of free silica continue to react to form mullite. Secondary mullite petrochemical stone in about 10% of the volume expansion, in 1300 ~ 1400 ℃ at the same time of the following is bauxite impurities such as iron, titanium and other impurities and form liquid aluminum and silicon, iron and titanium in mullite crystal lattice to form solid solution. The formation of liquid phase contributes to the preparation of the second mo and the preparation of recrystallization sintering.

Recrystallization stage: in the secondary mullite phase due to the formation of the liquid phase, has begun a degree of sintering, but progress is slow, have to finish, only with the secondary mullite petrochemical recrystallization sintering began to rapidly. Above 1400 ℃ ~ 1500 ℃, due to the effect of liquid phase, corundum and mullite crystal grew up, at 1500 ℃ is about 1500 ~ 300 microns, 60 to 1700 ℃ is respectively and 90 microns sintering alumina materials of high temperature mechanical properties depending on the effect of glass and crystal effect. Since natural raw materials generally contain more impurities, especially the increase of alkali metal oxide will not only increase the content of glass, but also cause the decomposition of mullite in high temperature. Therefore, in order to reduce the adverse effects of impurities in the synthesis of mullite, the content of the synthetic materials should be high in molite, and the high purity raw materials should be selected as high as possible to reduce the quality. According to the characteristics of China's high bauxite resources, more than 70% of them are medium-low grade ore, with high content of impurities, uneven distribution of mineral distribution and hard sintering. The closer to 70% of A12O3, the better the high temperature strength and the smaller the creep. The introduction of appropriate amount of MgO and zirconium quartz is beneficial to high temperature mechanical properties. But K2O, CaO, TiO2 and Fe2O3 are all harmful impurities and should be minimized


Hangzhou JWG Technology Co.Ltd