Manufacturing process of cement involves various raw materials and processes. It explained chemical reactions for manufacturing. Cement is a greenish grey colored powder made from calcined mixtures of clay and limestone. It mix with water becomes a tough.
The history of cement goes back to Roman Empire . The modern day cement, hydraulic cement was first produced by a British stone mason, Joseph Aspdin in 1824, who cooked cement in his kitchen. Limestone and clay powder in his kitchen and mixture into powder to prepare cement. The powder mix with water and provide Hard mass.
Manufacture Process of Cement
The manufacture procedures described below.
1. Mixing of raw material
4. Storage and packaging
1. Mixing of raw material
The major raw materials utilized in the manufacture of cement are Calcium, Silicon, Iron and Aluminum. These minerals are use in different form as per the supply of the minerals.
a) Dry Process
Calcareous and argillaceous raw materials are crushed within the crushers to 2-5 cm size pieces separately and crushed materials are again grinded to urge fine particles into ball or tube mill. Finely grinded material is stored in hopper after screening and these powdered minerals are mixed in required proportion to urge dry raw mix which is then stored in silos and kept able to be sent into rotary kiln. Raw materials are mixed in specific proportions in order that the typical composition of the ultimate product is maintained properly.
b) Wet Process
Raw materials are first crushed and made into powdered form and stored in silos then clay is then washed in washing mills to get of adhering organic matters found in clay. The powdered limestone and water washed clay are sent to flow within the channels and transfer to grinding mills where they’re completely mixed and therefore the paste is made so called slurry. Then the slurry is led into collecting basin where composition are often adjusted. The slurry contains 38-40% water which stored in storage tanks and kept ready for the rotary kiln.
2. Burning of Raw Materials
The burning process is monitored within the rotary kiln while the raw materials are rotated at 1-2 rpm at its longitudinal axis. The rotary kiln is formed from steel tubes having the diameter of two .5-3.0 meter and therefore 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 ball bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mixture of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the assistance of powdered coal or oil or hot gases from the lower end of the kiln in order 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 within the material is evaporated at 400oC temp, so this process is understood 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 + CO2
The 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 collect to make small and hard stones are referred to 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 make hard and little stones referred to 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 bottom of the rotary kiln. The cooled clinkers are collected in small trolleys.
3. Grinding of Clinkers
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 doesn’t settle quickly when comes in touch with water.
After the initial setting time of the cement, the cement becomes stiff and therefore 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
4. Storage and packaging
The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags.