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Cement Manufacturing Process
Raw Materials
The hydraulic property of cement originates from its cement minerals ( calcium
silicates). The major components in terms of metal oxides are CaO, SiO2,
Al22O3,
and Fe2O3. In order to provide a suitable chemical composition
for sintering, raw materials are either quarried or collected from other industrial
processes
for the cement manufacturing process. Typically, Calcium (Ca) is provided from
limestone, Silicon (Si) from Sand or Flyash, Aluminum (Al) from Flyash or Clay,
and Iron (Fe) from iron ore or slag.
Proportioning
Typically, Limestone 80% , Silica 9% , Flyash 9% , Iron 2% are dosed precisely
to a predefined chemical composition, and are then fed into the process.
Raw Mill
The dosed raw materials are dried and finely ground in the Raw Mill to form an
intermediate product, called “raw meal”. The grinding provides
an increased surface area to enhance the heat exchange in the downstream heating
process.
Homo Silos
The “raw meal” is then stored in a homogenizing silo in which the
chemical variation is reduced. This homogenizing process is important to stabilize
the downstream sintering process as well as to provide a uniform quality product.
The “raw meal” is then transferred to the Preheater Tower.
Preheater
In the Preheater, the raw meal undergoes a series of concurrent heat exchanges
with the hot exhaust gas from the kiln system. The gas and material stream
are separated by cyclones after each heat exchange process. The raw meal temperature
increases from 80oC to 1000oC within 40 seconds. The
first chemical reaction also takes place in the Precalciner of the Preheater,
where limestone CaCO3
is decomposed into lime (CaO).
Kiln
The calcinated material entering the kiln, then undergoes a long heating process.
The material temperature rises from 1000oC to 1450oC.
Mineral matrixes of raw material are totally destroyed and cement minerals
are formed at the sintering
temperatures. A semi-product called “clinker” is formed. Coal and
other alternative fuels are used as energy sources for the process. The ash
from fuels is absorbed into the clinker matrix. The residual heat from the
clinker leaving the kiln is recovered by a grate cooler to reduce the energy
requirement.
Conditioning Tower
The flue gas exiting the preheater is directed to the raw mill for drying. Before
it enters the electrostatic precipitator ("EP"
) for its final dust removal process,
its temperature and humidity is regulated in a conditioning tower. This process
is essential as it affects the dust collecting efficiency of the electrostatic
precipitator ("EP").
Electrostatic Precipitator
Electrostatic precipitator is commonly used as the final dust removal device
for flue gases. It consists of chambers each of which contains a series of
collection plates and an overhead framework of suspended rigid high-voltage
electrodes.
Particles in the gas stream are charged by a high-voltage, direct current field
which is generated from the discharge electrodes, suspended between the collector
plates. Current applied directly to the discharge electrodes manifests a highly
active and visible glow in the electrode known as the "corona". In
the strong electrical field region near the electrode-emitting surface, large
numbers of both positive and negative ions are formed. As the discharge electrodes
have a negative polarity, the positive ions are attracted to them. Both negative
and positive ions are formed in equal amounts directly in the corona region
near the discharge electrodes and over 99 percent of the gas space between
the discharge electrodes and the collector plates contain only negative ions.
As the particles entrained in the gas stream pass through the corona field,
they are bombarded by negative ions and become charged in a fraction of a second.
They are then attracted to the grounded collector plates where they are collected.
Particulate matter on the collecting plates and high voltage electrodes is
removed by the impact of "rapper" mechanisms. Dislodged particles from the
high voltage electrodes and collector plates fall into a hopper directly below
each precipitator chamber.
ID Fan
Induced draft fans (ID fan) are installed in the process to drive the gas stream
movements.
Grate Cooler
The residual heat from the clinker leaving the kiln, is recovered by a grate
cooler (consisting of rows of grates). Cooling air is injected from the bottom
of the grate, and is forced into the clinker which is traveling
slowly on the grate. The heated air is then recycled as secondary air for combustion
in the kiln, or in the Precalciner..
Finish Mill
The final process of cement making is called finish grinding. Clinker dosed with
controlled amount of gypsum is fed into a finish mill. Typically, a finish
mill is a
horizontal steel tube filled with steel balls. As the tube rotates, the steel
balls are lifted, tumble and crush the clinker into a super-fine powder. The
particle size is controlled by a high efficiency air separator. Other additives
may be added during the finish grinding process to produce specially formulated
cement.
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