Manufacture of cement
Cement is a composite mixture of calcium silicates & calcium aluminates which is a popular material used for construction.
1) Raw materials : Raw materials required for manufacture of cement are
i ) Lime stone which provides calcium
ii) Clay which provides aluminium and silica
2) Methods: Cement is manufactured by two methods
i ) Wet process
ii) Dry process
i) Wet method
1) In the wet process first the clay is purified by washing in a wash mill.
2) The lime stone is crushed into small particles and mixed with purified clay in proper proportion to get raw slurry.
3) The slurry contains 40% of water and is made homogenous uniform by using a mixer machine.
ii) Dry process
1) In the dry process the raw materials are mixed in proper proportion.
2) The mixture is dried, pulverized (crushed into fine pieces) and made uniform.
3) The resulting powder is called raw meal.
4) The raw slurry or raw meal obtained by one of the above processes called charge is introduced into a rotary kiln.
5) The rotary kiln consists of a steel cylinder about 150 m long and 4 m diameter and rotates 30 - 60 turns per hour.
6) At one end of the cylinder a screw conveyer is arranged which slowly allows the charge into the cylinder.
7) At the other end of the cylinder a burner is arranged.
8) Coal burning oil or gas is burnt at this end.
9) The charge entering the cylinder slowly moves towards the hot end.
10) During passage it loses water in the beginning and then CO2 .
11) At the burning end of the kiln, the temperature is around 17000 - 19000C.
12) At this end some chemical reactions take place betweenCaO and aluminium silicates.
13) A mixture of calcium silicates and calcium aluminates is formed.
14) The resultant product consists of gray hard balls called clinker cement.
15) The clinker is cooled, ground to fine powder and mixed with 2 - 3 % of gypsum.
16) It is transported in air tight bags.