Manufacture of Portland cement:
The raw materials required for the manufacture of cement are calcareous materials such as limestone or chalk and argillaceous materials such as shale or clay. Cement factories where established where these raw materials are more or in plenty. Cement factories have come up in many regions in India, eliminating the inconvenience of long distance transportation of raw and finished materials.
The process of manufacture of cement consists of grinding the raw materials , mixing the intimately in certain proportions depending upon their purity and composition and burring them in kiln at a temperature of about 1300 to 1500 Celsius at which temperature the materials sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and ground to fine powder was addition of about 3-5% of gypsum. The product formed by using this procedure is Portland cement.
There are two processes known as “wet” and “dry “ process depending upon whether the mixing and grinding of raw materials is done in wet or dry conditions. With a little change in the above process we have the semi dry process also where the raw materials are ground dry and the mixed with about 10-14 per cent of water and further burnt to clinkering temperature.
For many years the wet process remained popular because of the possibility of more accurate control in the mixing of raw materials. The technique of intimate mixing of raw materials in powder form was not available then. Later, the dry process gained the momentum with the modern development of the technique of dry mixing of powdered materials using compressed air. The dry process requires much fuel as the materials are already in a dry state, where as in the wet process the slurry contains about 35-40 per cent water. To dry the slurry we thus require more fuel. In INDIA most of the cement factories used the wet process. Recently a number of factories have been commissioned to employ the dry process method. Within next few years most of the cement factories will adopt dry process system.
1.In the wet process the limestone brought from the quarries are crushed to smaller fragments.
2.Then it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to fine consistency of slurry with the addition of water.
3.The slurry is a liquid of creamy consistency with water content of about 35-50 per cent, where in particles are crushed to fineness of Indian standard sieve of number 9, are held in suspension.
4.The slurry is pumped to slurry tanks or basins where it is kept in an agitated condition by means of rotating arms with chains or blowing compressed air from the bottom to prevent setting of limestone’s and clay particles.
5.The composition of the slurry is tested to give the required chemical composition and corrected periodically in the tube slurry is stored in the final storage tanks and kept in a homogenous condition by the agitation of slurry.
6.The corrected slurry is sprayed on to the upper end of rotary kiln against hot heavy hanging chains. The rotary kiln is an important component of a cement factory.
7.It is a thick steel cylinder of diameter of anything from 3 meters to 8 meters lined with refractory materials, mounted on a roller bearing and capable of rotating about its own axis at specified speed.
8.The length of the rotary surface of flexible chain loses moisture and becomes flakes.
9.These flakes peel off and fall on the floor.
10.The rotation to the rotary Kline causes the flakes to move from the upper end toward the lower end of the kiln subjecting itself to higher and higher temperatures. The kiln is fired from the lower end.
11.The fuel is either powered coal oil or natural gas.
12.By the time the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, Where the temperature is in the order of 1500 Celsius about 20-30 per cent of the materials get fused.
13.Lime, silica and alumina get recombined. The fused mass turns into nodular form of size 3mm to 20mm known as “clinker”.
14.The clinker drops into a rotary cooler where it is cooled under controlled conditions.
15.The clinker is stored in silos or bins.
16.The clinker weighs about 1100 to 1300 Gms per liter.
17.The liter weight of clinker indicates the quality of clinker.
18.The clinker is cooled is then grind in ball mill with the addition of 3-5 per cent of gypsum in order to prevent flash-setting of the cement.
19.A ball mill consists of several compartments charged with progressively smaller hardened steel balls.
20.The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement Is bagged or filtered in barrels for bulk supply to dams or other large work sites
Manufacture of Portland cement: