Clinker Burning
The kiln is a long cylinder lined with refractory materials and supported on rollers such that it can rotate on it's own axis and erected with a slight inclination from the horizontal. The raw meal now called kiln feed enters the kiln system at the top of the pre-heater. This is essentially a counter current heat exchanger with the hot gasses rising and kiln feed falling. The heat source is located at the lower end of the kiln. The kiln is essentially made up of four sections, drying , calcining , sintering and cooling. As the kiln feed moves towards the lower end of the kiln it undergoes successive reactions as shown below:
Temperature
Deg C |
Reactions |
Thermal Change |
Clinker Compounds Formed |
| 100 |
Evaporation of free water from raw meal |
Endothermic |
|
| 500+ |
Evolution of combined water from clay |
Endothermic |
|
| 800+ |
CaCO3 = CaO + CO2 |
Endothermic |
|
| 800-900 |
CaO + SiO2 = CaO.SiO2 |
Exothermic |
CS |
| 900-950 |
5CaO + 3Al2O3 = 5CaO.3Al2O3 |
Exothermic |
C5A3 |
| 950-1200 |
2CaO + SiO2 = 2CaO.SiO2
2CaO + Fe2O3 = 2CaO.Fe2O3 |
Exothermic
Exothermic |
C2S
C2F |
| 1200-1300 |
3CaO + Al2O3 = 3CaO.Al2O3 |
Exothermic |
C3A, C4AF |
| 1250-1280 |
Beginning of liquid formation |
Endothermic |
Molten |
| 1260-1450 |
3CaO + SiO2 = 3CaO.SiO2 |
Endothermic |
C3S |
| 1100-1300 |
Formation of clinker compounds |
"Cooling zone" |
C3S, C2S , C3A , C4AF |
The principal chemical compounds in the clinker are tricalcium silicate , C3S (40-60%) , dicalcium silicate, C2S (16-30%), tricalcium aluminate, C3A (7-15%) and tetracalcium aluminoferrite, C4AF (7-12%). The cooling zone in the kiln is actually quite short but it is in this zone that nodulisation of the melt occurs.
The red hot clinker is then discharged into the cooler , where it is quenched cooled to around 100 degrees centigrade. The heat dissipated by the clinker is used as secondary air for the combustion in the calciner. This hot gas is also used in the dryers at the raw materials preparation stage. Rapid cooling of the clinker is essential as this hampers the formation of crystals , causing part of the liquid phase to solidify as glass. The faster the clinker cooling the smaller the crystals will be when emerging from the liquid phase. Table 7 provides a summary of the principal compounds of Portland cement and their characteristics.
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