The Physical and Chemical Changes In the Firing Process of Silica Brick

During the firing process, silica brick will have many crystalline changes and volume changes. The physical and chemical changes in the firing process of silica brick can be summarized as follows:

1) <150℃  remove the residual moisture in the bricks;

2) 450-550℃  Ca (OH) begins to decompose; at 550℃, the decomposition is complete; the bonding between the silica brick particles and CaO is destroyed; the strength of the brick is decreased and the brick becomes brittle;

3) 550-650℃  β- quartz is converted into α-quartz and the strength is improved; 

4) 600-700℃  solid phase action occurs between CaO and SiO2;

5) 800-1100℃  liquid phase appears in the brick and the strength of brick is sharply increased; at 1100℃, the transforming speed of the quartz is increased and Low specific gravity quartz variants have a great volume expansion;

6) 1300-1350℃  due to the increased amount of tridymite and cristobalite, the true specific gravity of bricks decreases and volume expansion and cracking phenomenon occur;

7) 1350-1470℃  the conversion and expansion of quartz are greatly enhanced; the liquid phase is more and more viscous, which makes the transforming speed faster and faster and improves the possibility of cracking. 

 

In an oxidizing atmosphere, α- quartz is mainly converted into α- cristobalite. In particular, when the mineralization agent is insufficient, white spots appear in the brick and the brick begins to expand and crack. 

 

According to the above changes, the operating points of silica brick are as followings:

1) In order to maximize the amount of tridymite in the firing process, control the temperature rising rate in different temperature ranges. Below 600℃, the rising of temperature should be slow; at 600-1000℃, the rising of temperature can be accelerated; at 1100-1300℃, the rising of temperature should be slow; At 1300 to the firing temperature, the rising of temperature should be the slowest. The fired silica bricks should be cooled at 600℃ and at 300℃, the cooling should be slow.  

2) In order to make sure the temperature of the kiln is rising slowly and evenly to promote the generation of tridymite and the mineralization, at high temperature of the firing process, the kiln should be in a reducing atmosphere.