CEMENT INDUSTRY

Manufacturing Process

The history of Portland Cement may be said to date back to the time when it was found that by burning limestones containing clay and silica, a cementing agent was produced which hardened under water and after hardening was not soluble in water. As this end product somewhat resembled Portland stone in colour and character, it was named Portland Cement. Earlier cements were incompletely burnt as the material was not heated to a temperature sufficiently high for sintering to occur. It was soon found that higher strengths could be obtained by burning the material more completely i.e. beyond decarbonising and upto sintering which is a stage immediately preceding the melting of the mix. Basically, the ground raw mix containing suitable mixture of calcium oxides, silicon oxides, aluminium oxides and iron oxides respectively occurring as limestone, sand, clay, bauxite, laterite etc. is after fine grinding and blending subjected to burning process inside the kiln. As the temperature rises, carbon dioxide is first evolved at temperatures between 700 deg and 900 deg C transforming the calcium carbonate into lime. Lime being strongly basic reacts with other materials in the raw mix when the temperature further rises and calcium carbonate into lime. Lime being strongly basic reacts with other materials in the raw mix when the temperature further rises and in this way silicates of calcium, aluminium and iron, which are basic constituents of Portland Cement, are formed. At a temperature of 1350 deg C the process of sintering begins inside the kiln and is normally completed between 1400 deg and 1450 deg C. At this stage the material which by now has acquired a greenish black colour is converted into what is known as clinker. This clinker after cooling is ground in finish mills along about 5% gypsum to give the finished product known as Ordinary Portland Cement (OPC).

Raw materials which already possess correct composition in their natural state are found in very few places. Hence in the vast majority of cases, cement is made from an artificially proportioned mixture of raw materials.
The manufacturing process over the years of development may be classified under the following categories :

- wet process

- semi-wet / semi-dry process

- dry process

a) Wet Process

In the wet process raw mix is fed into the kiln in the form of slurry which may have water content of 30 to 40%. The slurry which is easy to blend and homogenise is directly fed into the kiln which in the case of wet process is a relatively long tube. The wet process becomes indispensable in those cases where the naturally occurring raw materials have high moisture content of more than 12% like chalk and marl. This is also essential where relatively poor grade limestone have to be enriched through the process of beneficiation requiring use of water as a process media. In fact, in the earlier times i.e. before 1950 most of the kilns were wet process kilns due to the fact that in the form of slurry it is easy to blend and homogenise the various components of the raw mix. In this process the fuel consumption is the highest (in the region of 1300 to 1600 K.cal/Kg of clinker) the power consumption is lower at 110-115 Kwh/tonne of cement.

b) Semi-wet/Semi-dry Process

This process was evolved to counter the main drawback of the wet process which is high fuel consumption. In this process powdered raw meal is either converted into nodules by adding controlled quantity of water in a nodulising pan or by dewatering slurry in a filter press to form filter cake of the raw material. These nodules or the cake thus formed are fed on to a moving grate where the raw meal gets partially calcined. This partially calcined raw mix in the form of nodules/cake is further charged into a rotary kiln for complete calcining and sintering in the form of clinker. However, this process poses a number of operational problems and capacity problems. The fuel consumption however, improves reasonably to about 900-1100 K.cal/Kg of clinker but the power consumption increases to 115-120 Kwh/tonne of cement.

c) Dry Process

In the dry process, the raw materials are dried in a combined drying and grinding plant to reduce the moisture content below 1%. The drying of materials is achieved by using kiln exhaust gases which may be supplemented by auxiliary hot furnaces during rainy season. The ground raw mix is homogenised in large silos. In fact, development of suitable homogenising and blending systems are mainly responsible for making the dry process popular and practicable. The blended and homogenised raw is fed into either a long dry kiln or a short kiln with air suspension preheater in which partial calcination of the raw mix takes place. In fact, long dry kilns have now practically gone out of use and the dry process is mainly confined to the use of air suspension preheater. This process gives the maximum benefit as far as the heat consumption figures are concerned. As a further refinement and development of the dry process, the air suspension preheaters are now being fitted with Precalcinators which ensure complete calcining of the raw mix before it enters the kiln. Fuel consumption is lowest in this process and is in the range of 750-950 Kcal/Kg of clinker. The power consumption is in the range of 120-125 Kwh/tonne of cement. A flow process sheet of all the cement production processes including Pre-calcinator system is indicated in Fig.

Reference Book :
Precalcinator technology in indian cement industry
Department of Science and Industrial Research
Ministry of Science and Technology
Government of India 

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