Cement manufacturing process
Raw materials
The essential raw material to manufacture cement is limestone or chalk,
which is used for producing clinker. Clinker, the intermediate product,
is the crux of cement and acts as raw material for manufacturing cement with
the additives such as bauxite, iron ore, and gypsum. Depending on the grade of
the limestone, the additives such as silica, alumina, and iron ore are added to
achieve the quality of the product. The required grade, constituting lime,
should be 40 - 41%, in the limestone and accordingly the additives are added.
Fuels like coal, pet coke, natural gas, or oil can be used. The industry
has even started considering the usage of alternate fuels like agro wastes,
waste oils, animal meal, rice husk, etc. owing to the shortage of fuels like
coal or rising fuel prices. The choice of fuel depends on the availability of
fuel, its cost, the efficiency level and the process used.
The general process of manufacturing cement from mining limestone from
quarry to the final product is as follows:
Limestone benching, drilling and blasting
It is necessary to have the required minimum constituents in limestone
to make cement. For that, the company takes the sample as the limestone are
found at various levels that is in the form of steps, which are called benches
and takes the quality assurance as quality varies and compares it with the
standard required. This is known as limestone benching. Limestone benching is
done so that they could add the additives accordingly for qualifying the basic
characteristics.
Once the limestone is benched, it is drilled with the drilling equipment
and broken into small pieces. This is known as blasting. Blasting is of two
types -Primary blasting and secondary blasting. Primary blasting is the first
time blasting where the limestone is broken into pieces and if these are not
small then there is a second blasting that takes place. Usually the pieces are
made by primary blasting.
After blasting, the excavation process takes place, which also known as
limestone is raising where the limestone is taken to the factory for taking it
to next stage of crushing.
Crushing
Limestone is crushed and reduced to a size suitable for storage and
blending. All the raw materials are then ground in a grinder. Size of the
crushed material required depends on the type of the grinding mill used.
Generally, crushing is done in two stages, in a primary crusher and a secondary
crusher. The primary crusher could be a fully mobile and self-propelled unit
operating near the quarry face, a semi-mobile unit moved at infrequent
intervals, or a static unit. The secondary crusher is a static unit and is used
if required.
Pre homogeneous stage
Crushed limestone is packed, as this is also important while
transporting the crushed limestone and taken to reclaimer stage. It is
pre-homogeneous stage where additives like silica, alumina, and iron ore are
added to spread it in such a way so as to make it in the uniform quality. This
helps in reducing the variations in the chemical characteristics of limestone.
Raw mill grinding
Reclaiming is followed by the raw milling stage. In the raw milling
stage, the raw-meal is ground into a fine powder to be burnt in the kiln. Raw
meal must be finely ground, so that it reacts fully on passing through the
kiln.
There are various types of milling systems with different equipments
namely Vertical roller mill (used for bigger capacity) and ball roller mill
(used for smaller capacity). The selection of a particular mill is influenced
by the type of raw material available, power consumed and the project outlay.
Modern milling systems use separators/classifiers, which separate the fine
product and return the coarse materials to the grinding unit.
Blending and storage
Raw milling is followed by blending and storage. Blending is done in
silos. Typically, two or more silos are used in a series or in parallel with a
total capacity of at least one day's raw-meal feed. The raw-meal is
continuously circulated and blended in the first silo; it then passes to the
second silo for further homogenisation (perfect blending and mixing of various
constituents). Alternatively, blending and storage may be combined in a single
large silo wherein the blending occurs on extraction through a series of orifices
in the base, with limited
fluidisation.
Pre heating stage and kiln
After the raw meal is blended, it is heated in a rotary kiln. In modern
cement plants, before the heat treatment in the kiln, the raw meal is heated in
a pre-heater or/and a precalcinator system, in order to ensure a higher degree
of burning and enhance the product quality. The vertical cyclone chambers are
used through which raw material passes through the kiln and the hot gases are
used to pre heat the material as they swirl through the cyclones which rises
more then 200 feet from the kiln.
The kiln is a refractory, lined with refractory bricks for insulation
throughout its high-heat zones. The kiln is cylindrical and marginally inclined
to a horizontal position (typically with a gradient of 3-4 degrees), and
rotates at 2-4 revolutions per minute. It is the important constituent of
cement making process.
The solid material passes down the kiln while it rotates. Solid material
flows in the opposite direction to the flame. Gas, oil, or pulverised coal is
used to ignite the flame at the lower or front end of the kiln. Various
processes occurring in the kiln include evaporation of water, thermal
decomposition of clay minerals (at 300-650 degree Celsius), calcite formation (at
800-950 degree Celsius), liquid formation (at around 1,250 degree Celsius), and
the formation of clinker (at over 1,400 degree Celsius).
Clinker from the kiln passes into a cooler, where convective airflow
cools the clinker for subsequent handling and grinding. The heat is reclaimed
and recycled to the kiln as secondary combustion air. Other gases reclaimed
from the suspension pre-heater (SP), precalcinator systems, and the cooler are
used as primary combustion air in the kiln. The excess air from the cooler is
cleaned and released into the atmosphere.
Process profile
There are four processes of heat treatment. These include, dry process,
wet process, semi-wet process, and semi-dry process. Cement industry had a
major share of the wet process of technology until the 70s, but use of
the dry process increased significantly since early 80s.
In 1950, there were only 33 kilns in India
However, in 2007, there were 164 kilns, out of which 135 were based on
dry process, 23 on wet process and six on semi dry process.
