Preparation of Fused Silica Brick
Fused silica brick has excellent thermal shock resistance, and no cracks are generated after rapid heating; the chemical composition and high-temperature performance are similar to ordinary coke oven silica bricks; high strength and other characteristics. Therefore, in terms of raw materials, only fused silica can be used as the basis for producing such bricks.
Characteristics of Fused Silica
Fused silica has a very small thermal expansion coefficient (0.5×10-6) and a small thermal conductivity coefficient (0.8w/m·k). It has good thermal shock resistance, chemical resistance to acid and chlorine, high viscosity at high temperature, and high strength.
The raw material for generating fused silica is ultra-high purity fused silica sand. In an electric arc furnace, fused silica is formed after heat treatment at 1700°C. It is solid in the X-ray amorphous state, and the SiO2 content of ultra-high purity fused silica sand is usually greater than 99.5%. It has extremely low thermal expansion and is extremely resistant to extreme cold and extreme heat. However, at around 1000°C, fused silica will form cristobalite due to crystallization and phase transformation, while losing its original low thermal expansion.
Damage Analysis of the Fused Silica Brick
In order to ensure good thermal shock resistance of fused silica brick, in the production process of fused silica brick, it is necessary to ensure that fused silica has as little crystal as possible. As for the damage of fused silica brick during service, it is mainly due to the fact that the structure of fused silica bricks is not dense enough, and there are many pores that lead to the invasion of the pores. And in the process of its use, it will accelerate the destruction of fused silica bricks. Therefore, in order to increase the life of fused silica bricks, it is necessary to increase the density of the product while reducing the amount of crystallization in the production process.
Density Control of Fused Silica Brick
The density of fused silica brick products can be controlled within a certain range through the following operations.
- Through reasonable particle grading and molding pressure, the green density can be improved.
- The reasonable choice of binder and admixture can control the impurity content in the molten silicon brick. To a certain extent, the conversion of the crystalline form of molten silicon bricks during firing and use is reduced.
- The firing system of molten silicon brick has a great influence on its density. A good firing effect will make the bonding between the fused silica brick particles denser. Thereby increasing the density of the product. In order to obtain denser molten silicon bricks, the firing temperature should not exceed 1200°C and the holding time should not be less than 3 hours.
Since fused silica does not undergo crystallization and the corresponding crystal form transformation below 1000°C, it can be fired at a rapid temperature rise below 1000°C and only briefly held at 1000°C to accelerate the solid-phase sintering of fused silica bricks.
Since the conversion of α-cristobalite to β-cristobalite (volume change -2.8%) usually occurs during its cooling process, the cooling stage of the fused silica brick firing process should have a strict temperature control system. Moreover, the furnace door cannot be opened until the temperature of the molten silicon brick drops below 150°C. Otherwise, the volume change caused by the crystal transformation will cause the cracking of the molten silicon brick.
The Role and Characteristics of Silica Powder
In addition to fused silica as the main raw material for the production of fused silica bricks, the role of silica fine powder in the production process of fused silica brick cannot be ignored. It is an amorphous SiO2 composed of submicron spherical particles.
- Its particle size is small, with an average of about 0.1～0.5µm;
- The activity is strong, the surface of the particles can form a silica gel film, and its high reaction activity can promote the low-temperature reaction and sintering of the molten silicon brick.
- When the silica fine powder is heated above 500°C, amorphous SiO2 is transformed into crystalline SiO2, namely cristobalite and scale quartz. It will not introduce too many impurities into the molten silicon brick and the crystal phase transformation will occur.
- Due to the characteristics of easy agglomeration and moisture absorption and agglomeration of SiO2 micro powder, it needs to be sieved before it is put into use in order to achieve better dispersion performance.
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