Manufacture
of Portland cement:-
Raw
Materials –
1. Calcareous – limestone or chalk and Marlde
2. Argillaceous – shale or clay-slate, laterite
kaoline.
Process:-
1. Grinding
the raw materials, mixing them intimately in certain proportions depending upon
their purity and composition and burning them in a kiln at a temperature of
1300-1500˚C at which temperature the material sinters and partially fuses to
form a modular shaped clinker.
2. The
clinker is cooled and ground to a fine powder with addition or about 3-5% gypsum.
The product
formed using this procedure is called Portland cement.
There are two
process of manufacturing Portland cement-
A. Dry
process &
B. Wet
process
Wet Process:-
1. Limestones
brought from the quarries is first crushed to smaller fragments then it is
taken to a ball or tube mill where it is mixed with clay or shale and ground
with fine consistency with the addition of water.
The slurry is a liquid of creamy consistency with water
content of about 30-50%, wherein particles crushed to the fineness of Indian
standard sieve number 9 is held in suspension.
2. The
the slurry is pumped to slurry tanks or basins where it is kept in an agitated
condition by means of rotating arms with chains or blowing compressed air from
the bottom to prevent the setting of limestone and clay particles.
3. The
composition of the slurry is tested to give the required chemical composition
by blending slurry from different storage tanks and the slurry is kept in a
homogeneous condition.
4. The
corrected/checked periodically slurry is sprayed on to the upper end of a
rotary kiln against hot heavy hanging chains.
Read:- tender-what-is-tender.
The rotary kiln, an important
component of a cement factory, is a steel cylinder of dia. ranging from 3-8 m,
lined with refractory materials, mounted on roller bearings and capable of
rotating about its own axis at a specified speed. The length of the rotary kiln
may vary from 30 m to 200 m.
The slurry on
being sprayed against a hot surface of flexible chain loses moisture and
becomes flakes.
These flakes
peel off and fall on the floor. The rotation of the rotary kiln causes the
flakes to move from upper end towards the lower end of the kiln subjecting
itself to a higher and higher temperature . The kiln is fired from the lower
end.
5. The
fuel is either powdered cool, oil or natural gas.
6. By
the time the materials rolls down to the lower end of the rotary kiln. Where
the temperature is in the order of 1500˚C, about 20 to 30% of the material get
fused. This is where the oxides in raw materials will be combined to form
compounds in clinker. The clinker drops into a rotary cooler where it is cooled
under controlled condition and stored in silos or bins.
7. The
cooled clinker is then ground in a ball mill with the addition of 3-5% of
gypsum in order to prevent flash-setting of the cement.
A ball mill consist of several compartments charged
with progressively smaller hardened steel balls. The particles crushed to the
required fineness are separated by currents of air and taken to storage silos
from where the cement is bagged or filled into barrels for bulk supply to dams
or other large work sites.
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