Clinker cooling
Descriptions of the formation of cement clinker generally concentrate on those reactions that take place as the kiln feed heats up and passes through the process to the burning zone. The part of the operation after the burning zone and up until the clinker’s eventual delivery to the cement mill can also have a significant effect on the quality of cement produced.
Figure 1: zoned C3S crystal with C2S inclusions in a finely divided groundmass. (Field of view is 90µm)
In a precalciner kiln, cement kiln feed passes through a temperature range from about 1000°C at the kiln inlet to about 1450°C in the burning zone then exits the kiln at around 1300°C depending on a number of factors, including the secondary air temperature and the closeness of the burning zone to the outlet. As the temperature increases through the kiln, liquid forms in the mid-1300°C range from alumina and iron oxide, combined with lime. Further free lime and silica are also dissolved in the melt to give something like 25-27 per cent liquid in the clinker through the burning zone. This may be supplemented by a separate liquid phase formed from sodium and potassium oxides, combined with sulphate and further lime which condenses from volatiles or melts directly into the kiln feed before the kiln is reached.
