Cement Manufacturing Process


Ordinary Portland cement is produced by grinding cement clinker in association with gypsum (3-5%) to specified fineness depending on the requirements of the cement consumers. Limestone is the major raw material used for manufacture of cement and about 35% of raw materials are lost in the atmosphere in the form of gaseous compounds of which carbon dioxide is the major one. Therefore cement units are necessarily located near the cement grade limestone deposit. The major steps or unit operations involved in cement manufacturing process include: Mining, Crushing, Pre-homogenisation, Grinding and Final Blending of raw materials for preparation of kiln feed. Pyroprocessing of kiln feed in presence of combustion gas/ flame generated from combustion of pulverized coal, mineral oil or natural gas. Grinding of cement clinker along with Gypsum for production of OPC Gypsum and other additive / blending components for production of cement other than OPC. Packing and despatch of cement. The various unit operations from mining of raw materials to cement despatch at Reliance Cements are discussed as follows
MINING OF LIMESTONE
As mentioned above the major quantity of limestone is obtained from the captive limestone mines of the plant. However, depending upon the proportions of different cement clinker phase forming components, the additive materials including high grade / low grade limestone are purchased from outside parties in required quantities.
MINING METHODS
Limestone mining operation traditionally is carried out generally as Opencast mining. However eco-friendly Surface Miners are used in limestone mines today.
DRILLING / BLASTING METHODS OF MINING
Drilling and blasting is one of the most commonly used methods to extract limestone from mines. Equipment used in this method of open cast mining includes HM-25 dumpers, Poclain & Demag shovels, Hindustan and Beml loaders, Beml dozers etc. Usage of sequential timer blasting machines in operations reduces the problem of noise, ground vibrations and improves fragmentation.
MINING WITH SURFACE MINER
Surface mining involves removing the soil and rock overlying the mineral deposit. It is considered a notch above the drilling and blasting techniques because it is eco-friendly and allows selective mining. The capacity of each machine is 200 tonnes per hour limestone. ;
CRUSHING OF LIMESTONE
The big boulders extracted are then transported to be crushed into the required feed size for grinding. This process may take place in a single stage or in double stages. Jaw crushers as well as impact crushers / hammer crushers of different capacities are employed in this process.
PREHOMOGENISATION
The crushed limestone is then transferred to stacker reclaimer site through belt conveyor / rope ways and stacked in layers. The reclaiming machine further cuts the limestone and mixes the layers to reduce the variation in quality. The limestone is now ready for reclaiming and feeding to raw mill hoppers.
GRINDING OF RAW MATERIALS
The raw mill hoppers use continuous weighing machines to create a proportioned limestone mix to maintain quality of the cement. This process may also involve combining purchased sweetner or additive materials with captive mines limestone. Vertical Roller Mill and Tube Mill Grinding machines are often used for production of pulverized raw mix.
HOMOGENISATION
The raw meal ground in the raw mill is thoroughly blended in vertically tall blending silos of capacity up to 10,000-15,000 tones or more. The characteristics of blending raw meal should satisfy the requirement of standard deviation variation in the range of (+/-) 0.2% CaO of raw meal. The moisture content of raw meal powder is less than 1%. The properly blended raw meal is now ready for burning to produce cement clinker in cement rotary kiln.
PYROPROCESSINGS
The modern pyroprocessing system comprises of preheating and precalcining, clinkerisation and cooling. Precalcining of raw meal is carried out in separate vessel vertically held and placed in-between preheating and clinkerisation section. The clinkerisation reaction is carried out in a rotary kiln. The kiln is inclined at an angle of about 3-5o from horizontal from preheating to the cooling end. The dry and properly blended raw meal is air lifted or lifted by a bucket elevator from the bottom of raw meal blending / storage silo to the top of the preheater, and fed at the top stage of cyclone inlet duct with the help of screw conveyor and rotary air lock. Raw meal weigh-feeders are installed for continuous weighing of raw meal for feeding the same to preheater at a constant rate.
COAL GRINDING SYSTEM
Pulverized coal is pushed into the burning zone of rotary kiln to generate high temperature combustion gas. This gas transfers its heat to the raw meal causing it to attain a precalcination of about 35-40%. The heating process progressing until there is 100% calcination. The final clinkerisation of raw meal is achieved between the temperature range of 1250-1450oC. The clinker is then cooled down to a temperature of 100-150oC. The cold clinker is crushed continuously and then transported for storage.
CEMENT GRINDING
After storing the clinker for few days it is ground in cement grinding mills along with appropriate quantity of gypsum and other additive materials for production of finely pulverized cement. Efforts are being made to reduce the level of energy consumption in this process.
CEMENT STORAGE PACKING & DESPATCH
The pulverized different types of cements are stored in different silos installed with different capacities. Depending upon the market requirements, the cement is loaded in bulk or packed in specially designed Reliance 50 kg bags with the help of conventional rotary packs or electronic packs, loaded on to trucks and finally dispatched to their destinations.

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Cement Manufacturing Process