Physical and chemical properties of sillimanite bricks and andalusite bricks for glass kilns
The so-called sillimanite products are made by adding a certain proportion of sillimanite, andalusite or kyanite and other trace elements to the ingredients of high-alumina bricks, thereby improving the thermal shock resistance of the products, especially the High temperature creep rates are reduced. Some people also call this kind of brick low creep high alumina brick.
The heating phase change behavior of sillimanite, andalusite and kyanite is the same. They all decompose to form mullite and Si02 phases. The latter can form cristobalite or glass phase. The difference is that the volume expansion of sillimanite and andalusite into mullite is small: while kyanite is extremely large, and its reaction proceeds as follows, 3(AL0,·2Si0,)---3AL0,·2Si0 ,+Si0
Since sillimanite and andalusite have only a small volume effect during the thermal mulliteization process, they can be directly used to make bricks. It can be burned or not burned. The AI203 content of sillimanite bricks is generally 50% to 60%. Its significant advantages are that its high temperature volume stability, load softening point and thermal shock resistance are better than those of high alumina bricks. In modern large-scale furnaces, sillimanite bricks must be used to ensure the stability of the furnace structure;
Sillimanite bricks are generally fired below the temperature at which sillimanite decomposes. During use, if the use temperature is slightly lower than the decomposition temperature of sillimanite, the volume stability of the brick is very good and there is no obvious volume effect; if it is higher than the decomposition temperature, sillimanite decomposes into mullite and liquid phases. A small amount of volume expansion can offset the shrinkage caused by the generation of liquid phase and maintain a relatively stable volume. By using high-temperature firing, that is, firing at a temperature higher than the decomposition temperature of sillimanite and andalusite, mullite chemical products can be obtained. By controlling the volume expansion of bricks during firing, products with superior performance can be obtained. Research results show that the microstructure of bricks produced by sillimanite and andalusite without pre-burning clinker is better. This can be explained as: the mullite crystal network formed by pre-burning clinker will be destroyed when crushed, and it is impossible to form a mullite bonding network during further firing, which will reduce the density and high temperature of the product. performance.
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