The drying and dehydration of refractory castables are very important, which are related to the quality and service life of the castables refractory.
In the lower temperature range of refractory castable, the bonding agent will decompose and lose strength, and the steam released by the castable will cause the castable to burst. Therefore, we must do a good work of drying and dehydrating castables.
The dehydration of refractory castables is divided into three stages. The first stage is at a temperature of 100°C. The mechanism is to remove free water, and the dehydration speed starts at a temperature between 50 and 60°C.
Second stage. A large amount of water vapor is generated in this stage. The water vapor pressure becomes the main driving force of the conduction process and accelerates the dehydration speed of the castable. The refractory castable shrinks greatly from the inside, slowing the steam exhaust, resulting in the maximum dehydration rate in the temperature range of 100-170 ℃. At this stage, some hydrates have already started to dehydrate.
The third stage of drying and dehydration occurs in the temperature range of 200-400℃. This stage is mainly dehydration of hydrates. The heating rate has a great relationship with the dehydration speed. The relationship between the dehydration temperature, dehydration speed and furnace temperature of the hydrate of calcium alumina cement is more complicated. When it is close to 100℃, it has been partially dehydrated and it has been affected by many factors such as the heating rate.
The dehydration temperature of the bauxite raw material and its gel in the refractory castable is 210-300℃. But the dehydration temperature of crystal water is about 530-550C. Therefore, the dehydration of calcium aluminate cement hydration products will not end until about 550℃.
The dehydration process of refractory castables causes the structure to relax and the strength to decrease. At the same time, a large amount of water vapor is discharged, causing a lot of pressure in the refractory castable body. Therefore, the refractory castable is prone to burst during the baking process.
In order to improve the burst resistance of refractory castables, some explosion-proof agents need to be added. Making the refractory castable form micropores at a temperature lower than dehydration is conducive to the discharge of water vapor during dehydration. Adding organic fiber as casting material is a safer and more effective method. Fibers include natural fibers such as paper fibers, straw fibers, hemp or cotton fibers, and polyolefins and polypropylenes. The principle is that the fibers overlap each other in the casting material. During the heating and baking process, they will melt or burn off to form a connecting air hole, which is conducive to the discharge of water vapor in the casting material.
