Portland Cement is made primarily from a combination of a calcareous
material, such as limestone or chalk, and of silica and alumina found as clay
or shale.The manufacturing process consists essentially of grinding the raw
materials into a very fine powder, mixing them intimately in a predetermined
proportion, and burning them in a large rotary kiln at a temperature of about
1400 ˚C (2550 ˚F). When the material sinters and partially fuses into clinker, the clinker is cooled and ground to fine powder with some gypsum added, and
the resulting product is the commercial Portland Cement .
First stage: Extraction of Raw Materials
The raw materials needed to produce cement (calcium
carbonate, silica, alumina and iron ore) are generally extracted from limestone
rock, chalk, clayey schist or clay. These raw materials are extracted from the quarry by blasting. They
are then crushed and transported to the plant where they are stored and
homogenized.
Second stage : Raw Grinding and Burning
Once the homogenization process is complete, the raw mix –
or “kiln feed” – is fed into a preheating tower where it is heated and starts
to react chemically by counter flow process; as raw meal moves downwards by
gravity, and hot gases resulting from the fuel combustion in the rotary kiln
move upwards. Kiln feed is heated from approximately 80°C up to 850°C, causing
a series of chemical reactions that include moisture evaporating, limestone
calcination and primary bonds between raw materials are created. During this
process, the emitted gases – which include CO2 as a result of the calcinations
process – are cooled down from 1,050°C to 350°C, to be used afterwards to dry
out the wet raw materials during the grinding process. Calcined raw materials
are fed to the rotary kiln where they are burnt at 1,450°C to produce the main
cement constituent “Clinker”.
Third stage: Cement Grinding and Shipping
While grinding cooling stage, gypsum is added to the clinker
and the mix is then finely ground to a form of grey powder called “cement” in a
special cement mill, which is similar to a raw material mill but relatively larger
in size. The ground cement is then mechanically packaged in paper bags and
loaded onto trucks to be transported to the marketplace.
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