Aluminum-silicon Refractory Products: Silicon Brick

Aluminum-silicon refractory products are widely used in iron and steel, building materials, colored, glass and other high temperature industries, and occupy a very important position in refractory industry. Aluminum-silicon refractory products are based on Al2O3 and SiO2, which are mainly siliceous, aluminosilicate and corundum refractories. Aluminum silicate refractory products can be divided into three categories:

Semi-siliceous products: Al2O3 content is less than 30%;

Clay products: Al2O3 content is 30 ≤ 48%;

The content of Al2O3 in high aluminum products is more than 48%.
Silicon brick refers to refractory products with SiO2 content above 93%. Silicon brick is made from silica with SiO2 content not less than 96%as raw material, adding mineralization agent(such as iron scale, lime milk)and binding agent(such as nitrite pulp waste liquid), and is prepared through mixing, molding, drying and burning. The higher the content of SiO2 in silica raw material, the higher the fire resistance of the product. The impurity components K2O, Na2O, etc. are harmful, which will greatly reduce the fire resistance of refractory products. Fe2O3, CaO, MgO and other impurities act as flux. The raw materials of high-grade silicon bricks shall be specially treated to remove impurities. See Table11-1  for chemical composition of silicon brick.  

Its mineral composition is scale quartz, cristobalite, a small amount of residual quartz and glass formed at high temperature. According to the production process of silicon brick and the properties of raw materials used, the mineral composition fluctuates greatly, as shown in Table 11-2.

Silicon brick is an acid refractory, which has a strong ability to resist the erosion of acid slag or acid melt, but the resistance to alkaline material erosion is poor, which is easy to be destroyed by Al2O3,K2O,Na2O and other oxides, and has good resistance to CaO,FeO,Fe2O3 and other oxides.

The high load-softened temperature is the excellent characteristic of silicon brick, which is generally 1640 ≤ 1680 ℃, which is close to the melting point of scale quartz (1670 ℃) and cristobalite melting point (1713 ℃).

The volume of silicon brick is stable from above 300 ℃ to close to the melting point (scale quartz or cristobalite). When heated to 1450 ℃, the volume expansion of silicon brick is about 1.5% and 2.5%, which is beneficial to ensure the structural strength and air tightness of masonry.

The biggest disadvantage of silicon brick is that the thermal shock resistance of silicon brick is poor (the number of water-cooled cycles is 1 ≤ 4 times), followed by the low refractory degree (generally 1690 ≤ 1730 ℃), and its application range is very limited.

The properties and process of silicon brick are closely related to the crystal transformation of SiO2. The true density of silicon brick is one of the important indicators to judge the degree of crystal transformation. Therefore, true density is an important quality index of silicon brick. The general requirement is below 2.38g/cm3, and the high quality silicon brick should be below 2.35g/cm3. The true density is small, the number of quartz and cristobalite in the reaction brick is large, the amount of residual quartz is less, so the residual expansion is small, and the strength of the reaction brick decreases little.

Silicon brick is mainly used in carbonation chamber, combustion chamber and partition wall of coke oven (about 73% of refractory used in a coke oven is silicon brick), glass pool kiln and kiln top, pool wall, high temperature load bearing part of hot blast furnace, carbon calciner and other thermal kilns. In order to improve the production capacity of modern large coke oven, it is necessary to reduce the carbonation chamber, combustion chamber and partition wall of coke oven, so it is necessary to use high density high thermal conductivity silicon brick.

There are many kinds of silicon bricks, which can be divided into coke oven silicon brick, hot blast furnace silicon brick, electric furnace silicon brick, glass kiln silicon brick and so on. According to the complexity of brick, it can be divided into standard brick, general brick, special brick and so on.

When using silicon brick, the following two points should be paid attention to: (1) due to the change of crystal form of residual quartz at low temperature, the volume changes greatly, so it is not suitable to temperature up below 600 ℃ too fast, and to cool slowly to 600 ℃ to avoid cracks. (2) it is not in contact with alkaline slag as much as possible.

With the large size of coke oven (carbonation chamber increases from 4m to 7m), the quality and shape size of refractories used are further improved. In the future, the production technology of silicon brick should be developed in the direction of increasing the content of SiO2 and reducing the content of impurities, especially the content of Al2O3, and in the aspect of properties, the density and compressive strength of the products should be improved and the porosity should be reduced.

In the production process of silicon brick, it is suitable to use many kinds of ingredients and multi-level ingredients to ease the harm caused by volume expansion when the product is fired, to ensure the reasonable accumulation density of mud and to further increase the volume density of brick.

In order to make the silicon brick have high compressive strength, the mechanical forming method should be used as far as possible except for the brick with special shape and less number of production blocks.