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Explain manufacturing of Portland cement with labeled diagram of Rotary kiln.
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MANUFACTURING OF PORTLAND CEMENT :

Raw materials of Portland cement is,

(i) Calcareous Materials: It is lime stone.

(ii) Argillaceous Materials: It is clay, state,shale,ashes,cement &rocks

(iii) Gypsum (CaSO4) It regulates the setting of cement.

(iv) Pulverized Coal: It is used as an fuel in manufacturing process.

COMPOSITION OF RAW MATERIAL :

Sr. No. Component Formulae Percentage
1. Lime CaO 60-67
2. Silica SiO2 17-25
3. Alumina Al2O3 3-8
4. Iron Oxide Fe2O3 2-4
5. Magnesium Oxide MgO 1-5
6. Alkali Oxide Na2O 0.3-1.5
7. Sulphur Trioxide SO3 1-3

FUNCTIONS OF INGREDIENTS:

(I) LIME: It is main constituent of cement. Calculated amount of lime is added in the manufacturing of Portland cement, If it is excess in amount, it reduces strength of cement, because it makes the cement to expand and disintegrate.

(II) SILICA: Lime undergoes reaction with silica to form dicalcium silicate (C2S). These play important role in development of strength to cement.

(III) ALUMINA: Tricalcium aluminate is required for setting of cement. If it is an excess quantity, then cement undergoes setting very fastly, because with the reaction of water, there is evolution of large amount of heat.

(IV) IRON OXIDE: It is responsible for development of colour, strength and hardness to the cement.

(V) SULPHUR TRIOXIDE: It imparts soundness to cement.

(VI) ALKALIES: It causes the cement efflorescent.

MANUFACTURING PROCESSES:

There are two manufacturing processes

(i) Dry Process

(ii) Wet Process

Dry Process:

If raw material is hard and dry then this process is used for preparation.

Process :

Raw material is crushed into small pieces -----> Small pieces grind in fine ball mills and stored separately -------> Mixed in proper proportions ------->pulverized in tube mills and homogenized in a mixing mill.

This is called as dry raw mix, stored in stored in storage bins also called as silos and kept ready to be fed to be in to a rotary kiln.

WET PROCESS:

In this process, 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.

The 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.

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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 + 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 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 product 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.

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The clinkers 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.

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 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

Storage and packaging

The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags.

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