Manufacturing of cement involves various raw materials and processes. Each process is explained chemical reactions for manufacture of Portland Cement.
Cement is a greenish grey colored powder, made of calcined mixtures of clay and limestone. When mixed with water becomes a hard and strong building material.
Manufacture Process of CementThe manufacture procedures of Portland cement is described below.
- Mixing of raw material
- Storage and packaging
1. Mixing of raw materialThe major raw materials used in the manufacture of cement are Calcium, Silicon, Iron and Aluminum. These minerals are used in different form as per the availability of the minerals. Table shows the raw materials for Portland cement manufacture
- Dry process
- Wet process
a) Dry ProcessThe both calcareous and argillaceous raw materials are firstly crushed in the gyratory crushers to get 2-5cm size pieces separately. The crushed materials are again grinded to get fine particles into ball or tube mill. Each finely grinded material is stored in hopper after screening. Now these powdered minerals are mixed in required proportion to get dry raw mix which is then stored in silos and kept ready to be sent into rotary kiln. Now the raw materials are mixed in specific proportions so that the average composition of the final product is maintained properly.
Fig: Manufacture of Cement by Dry Process
b) Wet ProcessThe raw materials are firstly crushed and made into powdered form and stored in silos. The clay is then washed in washing mills to remove adhering organic matters found in clay. The powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills where they are completely mixed and the paste is formed, i.e., known as slurry. The grinding process can be done in ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln.
Fig: Manufacture of Cement by Wet Process
Comparison of dry process and wet process of Cement Manufacture
|Criteria||Dry process||Wet process|
|Hardness of raw material||Quite hard||Any type of raw material|
|Time of process||Lesser||Higher|
|Quality||Inferior quality||Superior quality|
|Cost of production||High||Low|
|Physical state||Raw mix (solid)||Slurry (liquid)|
2. Burning of Raw MaterialsThe burning process is carried out in the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal axis. The rotary kiln is made up of steel tubes having the diameter of 2.5-3.0 meter and the length differs from 90-120meter. The inner side of the kiln is lined with refractory bricks. The kiln is supported on the columns of masonry or concrete and rested on roller bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gases from the lower end of the kiln so that the long hot flames is produced. As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at the speed of 15m/hr. In the upper part, water or moisture in the material is evaporated at 400oC temp, so this process is known as Drying Zone. The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place. The remaining material is in the form of small lumps known as nodules after the CO2 is released.
CaCO3 = CaO + CO2The lower part (clinkering zone) have temperature in between 1500-17000C where lime and clay are reacts to yielding calcium aluminates and calcium silicates. This aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10mm. The lower part i.e. clinkering zone has the temperature around 1500-1700C. In the region lime and clay reacts to yield calcium aluminates and calcium silicates. This products of aluminates and silicates of calcium fuses together to form hard and small stones known as clinkers. The size of the small and hard clinkers varies from 5 to 10mm.
2CaO + SiO2 = Ca2SiO4 (declaim silicate (C2S))
3CaO + SiO2 = Ca3SiO5 (tricalcium silicate (C3S))
3CaO + Al2O3 = Ca3Al2O6 (dicalcium aluminate (C2A))
4CaO + Al2O3 + Fe2O3 = Ca4Al2Fe2O10 (tetracalcium aluminoferrite(C4AF))The clinker coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys.
The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in ball mill or tube mill. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement that does not settle quickly when comes in contact with water. After the initial setting time of the cement, the cement becomes stiff and the gypsum retards the dissolution of tri-calcium aluminates by forming tricalcium sulfoaluminate which is insoluble and prevents too early further reactions of setting and hardening.
3CaO.Al2O3 + xCaSO4.7H2O = 3CaO.Al2O3.xCaSO4.7H2O