Industry Primer: Cement Sector
4/5/2012 9:00 PM Thursday
There are two general processes that are followed for producing cement in India – the dry process and the wet process. Almost 95 per cent of the cement is produced through the dry process, as it consumes less energy than the wet process.
1) The first step for making cement is to procure limestone from the mines, which are quite abundant in India. Approximately 1400 kg of limestone is required to make one tonne of cement, though the requirement may be reduced when blended cement is being manufactured. Limestone is big in size, and hence, needs to be crushed into manageable chunks of 20 mm.
2) Grinding - The processed limestone is then ground in the grinding machine. A vertical steel mill with huge rollers is used to grind the material. In this process, limestone (95 per cent) is mixed with other additives (secondary raw materials) like clay, bauxite, silicon aluminum and iron ore. These are crushed and milled into a raw meal. The secondary raw materials (materials in the raw mix other than limestone) used depend on the purity of the limestone.
3) Pre-Heating Process - The raw meal, which is ground, is then put into a six to seven stage pre-heater. Here, it meets
the hot gases rising from the kiln. The raw meal in the pre-heater is heated at temperatures between 600-800 degree
celsius. This process is to remove the Carbon Dioxide from the raw meal to form Calcium Oxide, also known as quick
lime. It also helps the company to save energy to be used in the clinkerisation (calcination) process.
4) Clinkerisation (Calcination)- Making Of Clinker - This is the most critical stage for any cement manufacturing process. Once the raw meal is pre-heated, the resultant Calcium Oxide, i.e. quicklime, is then fed into the kiln, which is a huge rotating furnace. This is further heated at 1450 degree celsius with the help of coal/pet coke or oil. 100-120 units of
electricity are required to produce one tonne of cement.
Power & fuel is one of the major cost components for a cement company, accounting for 15-20 per cent of the total cost of production. This is one of the reasons why many cement producers go for captive power plants when they set up cement plants, in a bid to keep their manufacturing cost under control.
5) Cement - The clinker is almost the final product. However, to bring more strength to ordinary portland cement, the clinker is mixed with other components like gypsum, fly ash etc. to finally get cement. Gypsum is used to keep the stickiness of the cement intact, and tends to set instantaneously when used with water. Gypsum is added to the extent of five per cent during the cement making stage. The mineral is naturally available in abundance in Rajasthan, Gujarat and Tamil Nadu. The other raw material that is used is the blast furnace slag (waste coming from iron smelting furnace) mostly used to produce portland blast furnace slag cement.
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