Usually there are two processes employed for the manufacture of Portland cement. These are:-

1. Dry process
2. Wet process

The selection of process for the manufacture of cement is normally dependent upon the nature of raw material. If the raw material is had then dry process is normally preferred. Whereas if the raw material is soft, then wet process is preferred. Other than nature of raw material, the factors like consumption of fuel, availability of raw material, the climate of the factory place are also considered for the selection of process for manufacture of cement. Normally in both processes, the basic processes involves following major operations:-

1. Selection of raw materials
2. Crushing and grinding
3. Storage of slurry
4. Burning the ground mix in a rotary kiln
5. Cooling of hot clinkers
6. Grinding of clinkers
7. Storage and packing

WET PROCESS MANUFACTURING OF PORTLAND CEMENT

In this process, the materials are finely ground and blended in the required proportion and stored in big storage tanks, known as silos. Argillaceous materials are thoroughly mixed with water for washing to remove organic matter, if any and then stored. Powdered calcareous and washed wet clay are allowed to flow in proportioning tank. From this, two raw materials led to grinding mills where they are mixed intimately to form paste called as slurry. This slurry is introduced into a rotary kiln. This process involves following major operations:-

• MIXING OF RAW MATERIALS

1. Limestone(Calcareous materials) are crushed in crusher and forwarded to tube mills to convert ground and fine powder, which is further stored in storage tank known as silos, fine clay or shale is mixed with water for washing to remove organic matter and stored in.
2. Powdered limestone and washed wet clay are allowed to flow in a channel in the right proportions and led to grinding mills where they are mixed homogenously to form paste called slurry.
3. Then slurry is led to connecting basin where the chemical composition is adjusted if necessary. This slurry is finally stored and fed to rotary kiln.

• BURNING OF RAW MATERIALS

1. Burning of raw materials is usually done in rotary kiln which is made up of steel tube, lined inside with refractory bricks and rotating at speed of 0.5 to 2 rotations per minute.
2. The kiln is slightly inclined so that the material fed in at the upper end travels slowly to the lower and firing and discharging end.
3. The kiln is supported by several tiers which run on rollers and the kiln is driven by an AC commulator motor.
4. The slurry of raw materials enters from the upper end of the rotary kiln while the burning fuel like pulverised coal, oil or natural gas and air are induced from the lower end of kiln.
5. The slurry gradually descends in the kiln into different zones of increasing temperatures. A typical rotary kiln is shown above.

CHEMICAL REACTIONS IN VARIOUS ZONES OF ROTARY KILN

With reference to the temperature, there are three zones in which various reactions take place. The zones and reactions are discussed as below.

• Drying Zone

The upper part of kiln is known as Drying zone, where the temperature is about 400 degree Celsius. In this zone, most of the water is driven out of slurry due to hot gases.

• Calcination zone

The central part of kiln is called calcination zone, where the temperature is about 1000 degree Celsius. In this zone, limestone of slurry undergo decomposition to form quick lime and carbon dioxide which is latter escaped out. The material forms small lumps called nodules. The following reaction takes place in this zone.

$CaCO_3 \rightarrow CaO + CO_2$

• Clinkering zone

This is the lower part of kiln, where the temperature is between 1500-1700 degree Celsius. Here lime and clay undergo chemical interaction that is fusion, yielding various products are shown in following reactions:-

$2CaO + SiO_2 \rightarrow Ca_2SiO_4 (C_2S)$

$3CaO + SiO_2 \rightarrow Ca_3SiO_5(C_3S)$

$3CaO + Al_2O_3 \rightarrow Ca_3Al_2O_6(C_3A)$

$4CaO + Al_2O_3 + Fe_2O_3 \rightarrow Ca_4Al_2Fe_2O_{10}(C_4AF)$

The aluminates and silicates of calcium then fuse together to form small, hard greyish stones called clinkers.

• GRINDING

The hot clinkers discharging from the kiln is cooled by various systems such as planetary coolers, rotary coolers. In the coolers, clinker is cooled with atmospheric air. The cooled clinker is then  tinely pulverised together with 2 to 6% gypsum in long tube mills. The finer the cement, greater is the strength.

• PACKING

The ground cement is stored in concrete storage silos. Moisture free compressed air is used  to agitate the cement and to keep it free from compaction by its own weight. Further it is fed to automatic packing machines. Usually, cement is packed in jute bags each holding 50kg of cement.