Why must clinker be rapidly cooled when it leaves rotary kiln? First, let me look at the purpose of rapid cooling of clinker:
a. It can prevent or reduce the decomposition of C3S.
b. It can prevent β-C2S from converting into γ-C2S at 500°C, thereby preventing clinker from pulverizing and losing its hydraulic properties.
c. It can prevent C3A from crystallizing coarsely to prevent cement from setting quickly.
d. It can prevent or reduce MgO from generating periclase, thereby reducing the destructive effect of MgO on the stability of cement paste.
e. It can increase the internal stress of hot materials, which is beneficial to improving grindability.
In the rotary kiln system, clinker must be rapidly cooled when it leaves the kiln, and the faster the cooling rate, the better. The reasons are analyzed as follows.
a. From the perspective of ensuring the quality of clinker, the following physical and chemical changes may occur after the clinker leaves the kiln: at about 1250 degrees Celsius, C3S→C2S+CaO; at 525~600 degrees Celsius, a crystal transformation occurs, β-C2S→γ-C2S. After the crystal transformation, the volume expands by 10%, causing the clinker to pulverize. Both reactions occur when the clinker is slowly cooled. The secondary free calcium oxide and γ-C2S minerals formed seriously reduce the quality of the clinker. During rapid cooling, the clinker liquid phase does not have time to crystallize and most of it turns into glass. Even if it crystallizes, the crystal grains are smaller than those during ordinary cooling. Rapid cooling maintains the quality of the clinker. At the same time, rapid cooling fixes most of MgO and C3A in the glass, which is beneficial to the improvement of the stability of the clinker and its resistance to chemical corrosion.
b. From the perspective of saving energy and reducing the heat consumption of clinker, only a fast cooling speed can ensure that a large amount of heat is absorbed by the cooling air in the high temperature section of the grate cooler. In this way, the temperature of the secondary and tertiary air of the rotary kiln is increased, providing a large amount of heat source for the kiln.
c. From the perspective of reducing “snowman” failures, slow cooling can easily cause the clinker to pulverize after the crystal transformation, and the fine powder produced will circulate and gather repeatedly at the kiln mouth, thus creating conditions for the appearance of “snowman”.
d. From the perspective of improving cement grinding efficiency, the clinker particles after rapid cooling will inevitably produce thermal stress and cracks, thereby improving the grindability of the clinker, which creates very favorable conditions for increasing production and saving energy in cement grinding.
In summary, Fodamon engineers said that in order to ensure the quality of clinker, reduce heat consumption, save energy, and improve cement grinding efficiency, it is necessary to ensure rapid cooling of clinker when it leaves the kiln.