Methods for Manufacturing of Cement



MANUFACTURE OF CEMENT

There are two methods for the manufacturing of cement.

  1. Wet process.

  2. Dry Process Both the processes involve the following steps for the manufacture of cement:

  • Crushing and grinding of the raw material.

  • Mixing of the ground raw material to form a homogeneous mass.

  • Burning of the homogeneous mass to form clinkers.

  • Grinding of clinkers to powdered form with the addition of gypsum.

CHOICE OF THE PROCESS

The choice between the wet and dry processes depends on certain factors:

i-       Physical condition of the raw material.

ii-    The price of the fuel.

iii-  The local climate conditions of the factory.

WET PROCESS

The raw material used in wet process is soft in nature. The climate is fairly damp and fuel is cheap. They are mixed in presence of water. The raw material after being crushed is introduced into the grinding mill where slurry is formed. This slurry is passed  on to the classifier to make it homogeneous. It is then sent to the correcting tank to adjust its composition. The slurry is then filtered and the desired material is sent to the rotating kiln. An outlineof the apparatus used in wet process is shown below:

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1. GRINDING MILL

The raw material after being  washed with water as introduced into the grinding mill. The raw material is washed with water to remove the flint stones. The function of grinding mill is to reduce the size of the raw material in presence of Water.

  1. CLASSIFIER

The ground raw material is passed on to the classifier where  over sized particles are  retained and sent back to the grinding mill. The uniform and small sized material can easily pass through the classifier.

  1. CORRECTING TANK

The uniform material is sent to the  correcting tanks where slurry containing 60% Clay ‘and 40% water is also added. The mixture is stirred to make it homogeneous.

  1. PADDLE MIXER

The homogeneous mixture is passed on to the paddle mixer where it is finally homogenized. It is then subjected to filtration to remove excess of water. The filtered product  is then sent to rotatory kiln.

  1. ROTARY KILN

The rotary kiln is a rectangular chamber having a length of about 250 feet and a diameter of about 10 feet, The kiln is made of  Steel and internally lined with fire bricks. It is provided with screw conveyer arrangement for the feeding of slurry and has gears for its rotation. Mostly it rotates at the rate of half a revolution per minute. The fuel along with hot air is introduced from the lower end of the kiln and burned. The kiln may be divided into three zones.

6. DEHYDRATION ZONE

In this zone, temperature varies from 100 to 500 °C. Up to 100 °C evaporation of  the water take place. Up to 500 °C evaporation of  combined water from clay takes place.

7. CALCINATION  &  BURNING ZONE

In this zone, temperature varies from 500 °C to 800°c .Crystallization of amorphous dehydration products  of clay has been done in the range 500 °C to 800 °C.  At 800°C  decomposition of carbonate takes place with the evolution of CO2 & In burning zone, temperature varies from 900° to 1400 °C. It is honest zone and hence, the main reactions between lime and clay take place.

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The product coming out of the kiln is in the form of granular mass and is called “Clinker”.

8 .AIR CHAMBER 

From the rotary kiln, the clinkers are passed on to the air chamber where they are  cooled with the help of liquid  air.  The resulting hot air is used in the rotary kiln for burning the fuel.

  1. CLINKER STORAGE

From the air chamber, the clinkers are passed on to the storage tank win they are cooled to room temperature (25°C).

  1. BALL MILL

The clinkers are passed to the ball mill where they are finely ground in t presence of 2 to 4% gypsum.. The function of gypsum is to regulate the setting cement.

9. SIEVING AND PACKING

The ground material is then sieved and stored in multilayer paper bags.

DRY PROCESS 

The dry process is preferred in places where hard and crystalline raw materials are available. This process has the advantage over the wet process,  the fuel consumption is low. The raw material is first crushed in the dry state in multi chamber tube mill. This crushed raw material is then ground in the grin& mill. The dry powdered mixture is finely homogenized by means of compress air. This finely homogenized powdered material is then passed on to the rota kiln.

SETTING OF CEMENT

When cement is mixed with water, the resulting mass becomes hard and resistant to pressure with the passage of time. This process is called “setting cement”. The setting of cement is mainly due to hydration followed by hydrolysis and formation of new compounds. Setting of cement can be explained by mea of the following theories:

1. According to first theory, hydrolysis of tricalcium silicate and dicalcium aluminate causes the formation of a “gel” consisting of calcium hydroxi and aluminium hydroxide.

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This ‘`gel” on drying causes the setting of cement. .

These colloidal gels separate in the form of very small crystals, which • interlock with one another and form a hard mass.

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