Calcination and sintering process of refractory raw material

Except for some raw materials, most of the raw materials used as aggregates of refractory materials need to be calcined at high temperature. A series of physical and chemical reactions occur when the raw materials are calcined, which plays a very important role in improving the composition, mineral composition and microstructure of the refractory materials, ensuring the volume stability of the refractory products and the accuracy of the appearance dimensions. The indicators that characterize the sintering of materials include apparent porosity, bulk density, and water absorption.
 

The ultimate goal of raw material calcination is to achieve sintering. Sintering is performed at a high temperature, and the temperature at which the material starts to sinter is often consistent with the temperature at which the particles begin to migrate.
 

Liquid phase often occurs during the sintering process. The dynamics of liquid phase sintering and solid phase sintering are both surface tension (or surface energy). The sintering process can be roughly divided into three stages:

① Particle rearrangement produces a small amount of liquid phase wrapped in the surface of the particle during sintering, and forms a neck where the two particles contact;
 

② The solid phase and the liquid phase have similar chemical properties in the raw materials of particle melting and precipitation. When the sintering progresses to a certain degree, dissolution and precipitation begin to occur:
 

③ particles grow, the cementation between particles, the liquid phase fills the pores, and the dissolution and precipitation between different curved surfaces continue. The purer the raw material, the more difficult it is to sinter. For example, high-purity natural dolomite requires high temperature above 1750 ^ C for real sintering: while purified high-purity magnesia requires more than 190 ~ 000 to sinter. This obviously brings a series of problems to the calcination equipment and fuel consumption of raw materials.