Process of manufacturing of cement

• Mining of limestone.
• Raw material preparation / raw mix preparation.
  • Crushing.
  • Homogenization.
  • Proportioning of raw materials.
  • Grinding.
  • Storing and Blending.
• Burning of raw mix.
• Clinker grinding along with gypsum – Cement.
  • Grinding.
  • Storing.
  • Packing.

The prime raw material limestone after blasting in mines is broken into big boulders. Then it is transported by dumpers, tipplers to limestone crusher where it is crushed to 15 to 20mm size.

After crushing, the crushed limestone is piled longitudinally by an equipment called stacker / reclaimer. The stacker deposits limestone longitudinally in the form of a pile. The pile is normally 250 to 300 m long and 8-10 m height. The reclaimer cuts the pile vertically, simultaneously from top to bottom to ensure homogenization of limestone.

The crushed limestone from pile is transported through belt conveyor to hopper. Similarly, other raw materials like clay, bauxite, iron ore etc. are also transported by belt conveyor from storage yard to respective hoppers. All raw materials are proportioned in requisite quantity through weigh feeders.

After grinding, the powdered raw mix, is stored in a raw meal-silo where blending takes place. Blending is done by injecting compressed air. Generally blending ratio is 1:10. This powder material (Raw meal) is fed to the kiln for burning.

The proportioned raw materials are transported by belt conveyor to Raw Mill for grinding into powder form.

• The powder raw mix is fed into 4 to 6 stage preheater from top by air pressure. The hot gases from kiln enters preheater from bottom. When powdered raw mix slides down through cyclones and comes in contact with hot air which travels from top to bottom .
• In preheater the temperature of raw meal rises to 900oC to 1000oC and nearly 90% Calcinations (removal of CO2 from CaCO3) takes place before entering the kiln.

Rotary kiln is generally 50-60 meters long, 3 to 3.5 meter diameter tube. This is placed horizontally with some inclination to facilitate the raw mix movement inside the kiln. Powdered raw mix enters the kiln at one end and the burner is situated at the opposite end. The rotary kiln rotates at the speed of 1 to 3 revolution per minute (RPM). The raw mix in the kiln melts first into liquid form and then transforms into nodules due to the effect of the rotation of the kiln. There are two zones inside the kiln, namely calcining zone and burning zone. The zone where raw mix enters into the kiln is called calcining zone. Where temperature would be 950 – 1000oC. Burning zone starts after this zone where temperature would be 1350 - 1450oC.

• The hot clinker from kiln discharge is cooled very quickly/ quenched in air with the help of efficient coolers. The temperature of clinker is brought to 50oC from 1400oC. Fast cooling is very essential to get good quality clinker . If cooling is not quick, the compound stability in clinker will be adversely affected resulting in lower strength of cement after grinding.
• All operations of feeding of raw meal, coal, burning, temperature control and cooling are automatic through fuzzy logic / computer control. These operations are controlled from Central Control Room (CCR) which is the nerve center for plant.

Clinker from clinker silo is transported to clinker hopper by belt conveyor. Similarly, gypsum, fly ash and other additives are transported to their respective hoppers by belt conveyors. The proportioning of clinker, gypsum and HRS is done by electronic weigh feeders. Clinker and gypsum are pre crushed in a Roller press and subsequently fed into Cement mill for fine grinding.

• Cement is packed by high efficiency rotary packers which are electronically controlled. Packers are highly precise with tolerance of +/- 0.5%. The main feature of electronic packers are that.
• It shows the display of weight of each bag.
• It ensures the bags to fall from nozzles only after filling of cement to accurate weight of 50.2 kg.
• The electronic weighing machine is installed just by the side of conveyor belt and bag weight can be checked randomly for cross checking. The records of calibration of electronic packers, weighing machine and bag weights are maintained in packing plants on daily basis. The packed bags are conveyed by conveyor belts and are loaded into trucks by truck loaders.