From quarrying to bagging, cement manufacturing takes place in several steps.
Step 1: Extracting raw materials
Limestone and clay are extracted from quarries located near cement plants.
Blown off with rocks and boulders, limestone and clay are carried by large trucks to crushers in order to reduce their size and get stones with a diameter less than 10 cm. These raw materials are subject to rigorous selection. Their composition should include lime, silica, alumina and iron oxide in well-defined proportions.
Step 2: Preparation of the raw materials
Once crushed, the materials undergo a pre-mixing process in the pre-homogenization rooms, which make it possible to obtain a greater regularity of composition.
The materials are then dried and crushed in a grinder (ball or roller mill) in order to reduce their size to a few tens of microns. The crossing of the oven gases into the grinder allows the materials to dry. The fine powder then obtained is called raw meal, hence the expression of raw mill.
Throughout the process, features like sampling, transport and chemical analysis continuously check that the proportions of the material materials correspond to the desired chemical composition.
Step 3: Cooking
- The oven
The meal introduced in a powder like form in a preheating tower where it is preliminarily heated by the gases produced in the rotating oven (pre-calcination phase).
Under the influence of the gravity, the flour thus goes down against the flow of the hot oven gases. The tower has several cyclone stages, which makes it possible to increase the temperature of the flour up to 850 Celsius degrees (dry-way process). The flour will then be fed into an inclined rotating oven that is packed with refractory bricks. The oven is supplied with heat thanks to a large capacity burner. The flame temperature reaches over 2000 Celsius degrees.
During this cooking process, there are many complex chemical reactions undergoing. Firstly, the limestone decarbonizes under a heat temperature close to 950 Celsius degrees and then transforms into lime, while the clays decompose into silicates and aluminates. The whole set is then recombined in silicates and aluminates of lime at a temperature of about 1450 Celsius degrees.
This chemical process leads to the production of a semi-finished product called clinker, which has the hydraulic binder properties.
The clinker is then conveyed through the cooler where it restores a good part of its thermal energy and is cooled by 100 Celsius degrees blowing air.
The chemical composition and homogeneity of the material at the oven entrance, as well as its regularity over time, are fundamental elements in the conduct of the production process. The oven has bag filters that filter dust that ensure limiting discharges to the environment.
Step 4: Obtaining Cement
The clinker is stored in halls (or silos) and then, in a new grinding stage, passes through ball mills. Its rotation causes, by shocks and crushing, the cut into very fine grains.
The obtained powder is cement.
Gypsum is added to regularize the setting time of the cement. Further additions can be made: limestone filler or flying ashes (issued from coal combustion in the thermal power plants) in order to improve workability and consistency, blast furnace slags (sub-products of steel mills), or even natural or artificial pozzolans to improve hydraulic properties.
The obtained cement is stored in silos of several thousand tons that are sometimes divided into compartments allowing the preservation of several cement qualities.
It is then delivered in bulk or bag.