In dry and semi dry processes for manufacturing of cement, the raw materials are crushed and fed in the correct proportions into a grinding mill. The raw materials are dried and reduced in size to a fine powder in to grinding mill. The dry powder is called the raw meal. The raw meal is pumped into a blending silo. Adjustment in the proportion of the materials required for the manufacture of cement is done in silo.
The raw meal blended by passing compressed air to obtain a uniform and intimate mixture. Compressed air induces upward movement of powder and decreases apparent density. The air is pumped over one quadrant of silo at a time and this permits the apparently heavier material from the non-aerated quadrants. Therefore the aerated powder tends to behave like a liquid and by aerating all quadrants in turn for a total period of about one hour, a uniform mixture is obtained. In some cement manufacturing plants continuous blending is used. The blended meal is further passed into sieve and then fed into a rotating disc called granulator. A quantity of water about 12 % by weight is added to make the blended meal into pellets.
In semi-dry process, the blended meal is now sieved and fed into a rotating dish called a granulator, water weighing about 12 percent of the meal being added at the same time. In this manner hard pellet about 15 mm in diameter are formed. This is necessary, as cold powder fed direct into kiln would not permit the air flow and exchange of heat necessary for the chemical reactions of formation of cement clinker.
The pellets are baked hard in a pre-heating grate by means of hot gases from the kiln. The pellets then enter the kiln where temperature is about 1450°C. The total consumption of coal in this method is only about 100 kg when compared to the requirement of about 220 kg for producing a ton of cement in the wet process.
The dry material undergoes a series of chemical reactions in the hottest part of the kiln and some 20 to 30 percent of the material becomes liquid, and lime, silica and alumina recombine. The fused mass turns fuses into balls of diameter 3 to 25 mm known as clinker. The clinker drops into coolers where it is cooled under control condition. Cooled clinker and 3 to 5 percent of gypsum are ground in ball mill to required fineness and then taken it to storage silos from where the cement is bagged.
The equipments used in the dry process kiln are comparatively smaller. The process is quite economical. The methods are commonly employed for direct control of quality of clinker.
