Manufacturing process of Portland Cement
There are two processes of manufacture cement,
i) Wet Process
iI) Dry process
The raw materials required for manufacture are (a) calcareous such as lime stone or chalk (b) argillaceous materials such as slate or clay, The process of manufacture consists essentially of grinding the raw materials into a very fine powder mixing them intimately in predetermined proportions and burning them in a temperature of about 1400 degree C, when the material sinters and partially fuses into clinker. The clinker is cooled and ground to a powder with some about 3 to 5 % gypsum added and the resulting product is the commercial portland cement.
The mixing and grinding of raw materials can be done either in water or in a dry condition and hence the name, becomes wet and dry process. The mixture is fed into a rotary kiln (in wet process) as large as 7 M in diameter and 230 M long.at the kiln is slightly inclined. The mixture is fed into the upper end while pulverized coal is blown in by an air blast at the lower end of the kiln where the temperature may reach about 1500 degree C.
As the mixture of raw material move down the kiln ,it encounters a progressively higher temperature so that various chemical changes take place along the kiln. First any water is driven off and CO2 is liberated from the calcium carbonate. As the materials moves further down, the dry materials undergoes a series of chemical reaction until, finally in the hottest part in the kiln,some 20 to 30 % of the material becomes liquid and lime, silica and alumina recombine. The mass then fuses into balls ,3 to 25 mm dia ,known as clinker.
Afterwards, the clinker drop into cooler which provide means for exchange of heat with the air subs equally used for the combustion of the pulverized coal. The cool clinker, which is very hard,is interground with gypsum in order to prevent flash setting of the cement.
In the dry and semi dry process the raw materials are crushed dry and fed dry in correct proportions into a grinding mill where they are further dried and reduced to fine powder. The dry powder called the raw meal is then further blended and corrected for its right composition and mixed by means of compressed air. The areated powder tends to behave almost like liquid and in about one hour of aeration a uniform mixture is obtained. The equipment used in the dry process kiln is comparatively smaller and economical.
The strength properties of cement are considerably influenced by the rate of cooling of clinker. It is found that moderate rate of cooling results in higher strength that is 27 and 33 MPa (28 days compressive strength) for Normal cement and high Early strength respectively. Moderate cooling implies that the clinker is cooled from about 1200 degree C to 500 degree C in about 15 minutes time and 500 degree C to normal atmospheric temperature in 10 minutes time.