Cement manufacture with flow diagram

Cement is a fine grey powder made by burning a mixture of calcareous (limestone) and argillaceous (clay) materials into clinker and grinding it with gypsum. 53 grade OPC (IS 12269) is made by the dry process in modern plants, because this process gives a closely controlled, fuel-efficient kiln feed.

Raw materials

• Calcareous: limestone, chalk, marl (supplies CaO, about 60-65% of the cement).
• Argillaceous: clay, shale (supplies SiO₂, Al₂O₃, Fe₂O₃).
• Correctives: bauxite, iron ore, silica sand (to adjust the composition).
• Gypsum (added after burning) to control setting.

Flow diagram

 Limestone + Clay (+ correctives)
            |
      Crushing & Pre-blending
            |
   Raw mill (dry grinding to fine powder)
            |
     Blending / Homogenising silo
            |
   Preheater (cyclones, 800-900 °C)
            |
   Rotary kiln (1400-1500 °C) -> CLINKER
            |
        Clinker cooler
            |
 Clinker + Gypsum (3-5%) -> Cement mill
            |
         Cement silo
            |
   Packing (50 kg bags) / Bulk dispatch

Steps

1. Quarrying and crushing: limestone and clay are quarried and crushed to about 25 mm.
2. Proportioning and dry grinding: the materials are mixed in the right ratio (about 75-80% limestone, 20-25% clay) and ground in the raw mill to a fine, dry powder called raw meal (moisture below 1%).
3. Blending: the raw meal is homogenised with compressed air in silos so that its chemical composition is uniform (the lime saturation factor and the silica and alumina moduli are checked).
4. Preheating: hot kiln gases heat the meal in cyclone preheaters, and calcination begins (CaCO₃ → CaO + CO₂).
5. Burning: in the rotary kiln (inclined at 3-4%, rotating at 1-3 rpm) the meal passes through the drying, calcination and clinkering zones. At about 1450 °C, partial fusion forms nodules of clinker (C₃S, C₂S, C₃A, C₄AF).
6. Cooling: the clinker is cooled rapidly in a grate cooler.
7. Grinding: the clinker with 3-5% gypsum is ground in a ball mill to a fineness of at least 225 m²/kg (Blaine).
8. Storage and packing: the cement is stored in silos and packed in 50 kg bags.

53 grade cement has more C₃S and is ground finer, giving a 28-day compressive strength of at least 53 MPa.