There are two methods for the manufacturing of cement.
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.
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 outline–of the apparatus used in wet process is shown below:
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.
The ground raw material is passed on to the classifierwhere over sized particles are retained and sent back to the grinding mill. The uniform and small sized material can easily pass through the classifier.
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.
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.
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 dehydrationproducts 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.
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.
From the air chamber, the clinkers are passed on to the storage tank win they are cooled to room temperature (25°C).
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.
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.
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.