Describe manufacturing of Portland cement. OR Describe dry and wet process of manufacturing of cement with merits and demerits. OR Describe various stages of manufacturing of cement with chemical reactions and diagram.
Chart for understanding only:
Manufacturing process can be divided in 4 stages as below:
(1) Mixing of raw materials:
This can be done by (a) dry process or (b) wet process.
(a) Dry process:
-The raw materials (lime stone and clay) are crushed into small pieces.
-The crushed raw materials are powdered in ball mills.
They are stored separately.
-Both type of raw materials are mixed in required ratio and stored in ‘silos’.
(b)Wet process:
-lime containing raw materials are crushed, powdered and stored in ‘silos’.
-Clay materials are mixed with water in wash mills and washed to remove organic materials.
-Powdered limestone and washed clay are mixed in required ratio in channel.
-This mixture is sent to grinding mills for making a slurry.
-The slurry contains 38-40% water and it is stored in storage tanks.
[note: scroll dawn for merits and demarits of dry and wet process].
(2) Burning:
-Burning is done in rotary kiln made of steel.
-Rotary kiln is 2.5-3 meters broad and 90-120 meter long.
-Refractory bricks are lined inside the wall.
-It is rotated at the speed of 1 r.p.m.
-It is inclined at small angle.
Process:
-The raw-mix or slurry are injected from upper end.
-Burning fuel (coal/oil) and air are injected from lower end. This produces long
flame which heats the kiln upto 17500
C temp.
(i) Upper part is known as ‘drying zone’. Temp. is about 4000
C and most of the
water gets evaporated.
(ii) Central part is known as ‘calcination zone’. Temp. is about 10000
C. Limestone
gets decomposed to quick lime and CO2. Small lumps are formed.
CaCO3 CaO+CO2
(iii) Lower part is known as ‘clinkering zone’. Temp. is about 17000
C. Lime and
clay gets converted to calcium aluminates and silicates. They fuse together to
from small grey stones called ‘clinkers’.
2CaO + SiO2 Ca2SiO4
OR 2CaO.SiO2
(C2S)
3CaO + SiO2 Ca3SiO5 OR 3CaO.SiO2
(C3S)
3CaO + Al2O3 Ca3Al2O6 OR 3CaO.Al2O3
(C3A)
4CaO + Al2O3+ Fe2O3 Ca4Al2Fe2O10 OR 4CaO.Al2O3
.Fe2O3
(C4AF)
Clinkers are very hot (10000
C). They are cooled using cool air and collected.
(3)Grinding:
Cooled clinkers are powdered in ball mills or tube mills and 2-3% gypsum is added
to delay quick setting of cement.
Gypsum delays quick setting by forming insoluble tricalcium sulphoaluminate
(3CaO.Al2O3.xCaSO4.7H2O). This prevents dissolution of C3A and early reactions of
setting and hardening.
(4) Packing:
Cement after grinding is stored in ‘silos’ and then packed using automatic
machines.
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✪ AUTHOR: SAHIL SIR
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