Alumina refractory bricks with A1203 content greater than 48% are collectively referred to as high alumina refractory bricks. According to the content of A1203, it is divided into three grades: Class I (A1203>75%); Class II (60%-75% for A1203); Class III (48%-60% for A1203). According to the mineral composition, it can be divided into: low mullite (including sillimanite) and mullite (A1203 is 48% to 71.8%), mullite-corundum and corundum-mullite (A1203 71.8% to 95%), corundum (A1203 is 95% to 100%) and other refractory bricks.
The firing temperature of high alumina refractory bricks depends on the sinterability of the bauxite raw materials. When using special grade and I grade bauxite clinker (bulk density ≥ 2.80g/cm3), the structure of the raw material is uniform, and the impurity content is high, which makes the green body easy to sinter, but the sintering temperature range is narrow, which is easy to cause overburning. or underburned. When using grade II bauxite clinker cake (bulk density ≥ 2.55g/cm3), the green body is not easy to sinter due to the expansion and loosening effect caused by secondary mulliteization, so the sintering temperature is slightly higher. When using grade III bauxite clinker (bulk density ≥ 2.45g/cm3), the structure is dense, the content of A1203 is low, and the firing temperature is low, generally slightly higher than the firing temperature of clinker clay refractory bricks 30 ~ 50 ℃. High alumina refractory bricks are fired in an oxidizing flame.
Because the softening temperature of high alumina refractory bricks under load is an important property. The experimental results show that it varies with the content of A1203 in the high-alumina refractory brick: when the content of A1203 is lower than the theoretical composition of mullite, the equilibrium phase in the high-alumina refractory brick is mullite-glass phase. The content of mullite increases with the increase of the content of A1203, and the softening temperature under load also increases accordingly.
The thermal shock resistance of high alumina refractory bricks is worse than that of clay refractory bricks, and the 850 ℃ water cooling cycle is 3 to 5 times. The main reason is that the thermal expansion of corundum is higher than that of mullite, and there is no crystal transformation. Moreover, the thermal shock resistance of high-alumina refractory bricks such as I and II is worse than that of high-alumina refractory bricks such as III.
In production, the method of adjusting the composition of mud particles is usually adopted to improve the particle structure characteristics of high-alumina refractory bricks, thereby improving their thermal shock resistance. In recent years, a certain amount of synthetic cordierite has been added to the ingredients of high-alumina refractory bricks to manufacture high-alumina refractory bricks with high thermal shock resistance, and obvious results have been achieved.
The slag resistance of high alumina refractory bricks also increases with the increase of A1203 content. Reducing the impurity content is beneficial to improve the corrosion resistance.
The difference between high-alumina refractory bricks and clay refractory bricks is that high-alumina refractory bricks have good performance, so they have a longer service life than clay refractory bricks, and they have become one of the most widely used refractory bricks in the building materials industry.
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