High temperature performance of spalling resistant high alumina bricks

The main mineral components of anti-stripping high alumina bricks are mullite, corundum and glass phase. With the increase of alumina content in high alumina bricks, the number of mullite and corundum phases also increases, the glass phase decreases accordingly, and the refractoriness and high temperature performance of the product increase. The increase in the number of glass phases and the decrease in viscosity will destroy the structure of high alumina bricks, especially the presence of KO and NaO not only reduces the temperature of the liquid phase, but also reduces the viscosity of the liquid phase, resulting in a rapid decline in the high temperature strength of the product. For high-alumina products with alumina content below 72%, the only high-temperature stable phase is mullite, which increases with the increase of alumina content; for high-alumina products with alumina content above 72%, the high-temperature stable crystal The phase is mullite and corundum. With the increase of alumina content, the amount of main steel is increased, and the amount of mullite is decreased, thereby increasing the high temperature strength of the product accordingly.

Therefore, high alumina bricks are divided into three grades according to the content of alumina. The first-grade high-alumina brick is more than 75% alumina; the second-grade high-alumina brick is 60%-75% alumina; the third-grade high-alumina brick is divided into three grades. Bricks with an aluminum content of 48%-60% and bricks with an alumina content of less than 48% are collectively referred to as clay bricks.

The performance of the third-grade high-alumina brick is similar to that of clay brick, and its main crystal phase is mullite and glass. Since its high temperature performance is superior to that of clay bricks, tertiary high alumina products can be used where clay bricks can be used. The main crystal phase of the secondary high alumina brick is mullite. The high-temperature performance of this type of product is obviously better than that of clay bricks: the main crystal phases of the first-grade high-alumina bricks are mullite and corundum. Since the chemical stability and fire resistance of corundum are higher than that of mullite, the higher the content of corundum in the product, the higher the high temperature resistance and corrosion resistance of the product. However, the coefficient of thermal expansion of corundum is much larger than that of mullite, so the higher the content of corundum, the lower its thermal shock resistance.

The important working properties of high alumina bricks are load temperature and high temperature creep. The load softening temperature increases with the alumina content of the product, as shown in the figure below. For high-alumina bricks with alumina content below 70%, the softening temperature under load depends on the quantitative ratio of mullite crystal phase to liquid phase, and mullite-corundum with aluminum content between 70% and 90% . For the product, the softening temperature under load does not increase significantly with the increase of alumina. This is because the Fe2O3 and TiO2 components in the raw material slightly increase with the increase of alumina, thereby changing the quantity and nature of the high-temperature liquid phase. At high temperature, the mullite crystal phase partially softens, and although the amount of corundum increases, it cannot form a skeleton, so the softening temperature under load will not increase significantly. Only when the alumina content in the product is more than 90%, or even more than 95%, the main crystal phase in the product is corundum, the direct bonding rate between the crystal grains is significantly improved, and the liquid phase only exists between the crystal grains in the void. , the softening temperature under load increases significantly.

The high temperature creep performance of high alumina bricks is expressed by creep rate. The torsional creep rate of the first-grade and second-grade high-alumina bricks is similar, and at 1200 °C, the creep rate is 0.25~0.29×10-5R.h, while the creep rate of the third-grade high-alumina brick is 3.5 at the same temperature ×10 R.h. -5r h, which is 10 times that of the first and second grade high alumina bricks. Phase analysis shows that the glass phase content of the first and second grade high alumina bricks is 7%~9%, and the glass phase content of the third and second grade high alumina bricks is 20%. The creep rate is not only related to the amount of glassy phase, but also to the composition of the glassy phase and its composition. high temperature viscosity. At 1200°C, the liquid phase viscosity of the third-grade high-alumina brick is only half of that of the first-grade high-alumina brick, and 26% of that of the second-grade high-alumina brick. Therefore, in the creep behavior of the three-stage high alumina brick, the glass phase plays a leading role, and besides the primary and secondary glass effects, the grain boundary creep also plays an important role. The higher the rate of direct bonding between crystal phases, the higher the grain boundary creep. more obvious. Obviously, improving the purity of raw materials in production, changing the chemical and mineral composition of the matrix, reducing the number of glass phases and adjusting the composition of glass phases are the keys to improving high temperature creep. At the same time, it can also improve high temperature volume stability and slag resistance.

The thermal shock stability of high alumina bricks is poor, which is closely related to the phase composition of the product. In production, measures such as adjusting the particle composition of the mud and improving the particle structure characteristics of the product are usually adopted to properly improve its thermal shock stability. Adding an appropriate amount of synthetic cordierite and zircon micropowder in the ingredients produces a product with high thermal shock stability and high alumina, which has achieved certain results.

High alumina bricks are widely used as lining materials for thermal equipment used in industrial production fields such as metallurgy, machinery manufacturing, petrochemical industry, electric power and light industry.