05-02-2013, 01:05 PM
CEMENT INDUSTRIES
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PROCESS DESCRIPTION
Cement industries typically produce portland cement, although they also produce masonry
cement (which is also manufactured at portland cement plants). Portland cement is a fine,
typically gray powder comprised of dicalcium silicate, tricalcium silicate, tricalcium aluminate,
and tetracalcium aluminoferrite, with the addition of forms of calcium sulfate. Different types of
portland cements are created based on the use and chemical and physical properties desired.
Portland cement types I - V are the most common. Portland cement plants can operate continuously
for long time periods (i.e., 6 months) with minimal shut down time for maintenance.
The air pollution problems related to the production, handling, and transportation of portland
cement are caused by the very fine particles in the product.
Exhibit 1 illustrates the stages of cement production at a portland cement plant:
1. Procurement of raw materials
2. Raw Milling - preparation of raw materials for the pyroprocessing system
3. Pyroprocessing - pyroprocessing raw materials to form portland cement clinker
4. Cooling of portland cement clinker
5. Storage of portland cement clinker
6. Finish Milling
7. Packing and loading
1. Raw Material Acquisition
Most of the raw materials used are extracted from the earth through mining and quarrying and
can be divided into the following groups: lime (calcareous), silica (siliceous), alumina (argillaceous),
and iron (ferriferous). Since a form of calcium carbonate, usually limestone, is the
predominant raw material, most plants are situated near a limestone quarry or receive this
material from a source via inexpensive transportation. The plant must minimize the transportation
cost since one third of the limestone is converted to CO2 during the pyroprocessing and is
subsequently lost. Quarry operations consist of drilling, blasting, excavating, handling, loading,
hauling, crushing, screening, stockpiling, and storing.
2. Raw Milling
Raw milling involves mixing the extracted raw materials to obtain the correct chemical configuration,
and grinding them to achieve the proper particle-size to ensure optimal fuel efficiency in
the cement kiln and strength in the final concrete product. Three types of processes may be
used: the dry process, the wet process, or the semidry process. If the dry process is used,
the raw materials are dried using impact dryers, drum dryers, paddle-equipped
rapid dryers, air separators, or autogenous mills, before grinding, or in the grinding process
itself. In the wet process, water is added during grinding. In the semidry process the materials
are formed into pellets with the addition of water in a pelletizing device.
3. Pyroprocessing
In pyroprocessing, the raw mix is heated to produce portland cement clinkers. Clinkers are
hard, gray, spherical nodules with diameters ranging from 0.32 - 5.0 cm (1/8 - 2") created from
the chemical reactions between the raw materials. The pyroprocessing system involves three
steps: drying or preheating, calcining (a heating process in which calcium oxide is formed), and
burning (sintering). The pyroprocessing takes place in the burning/kiln department. The raw
mix is supplied to the system as a slurry (wet process), a powder (dry process), or as moist
pellets (semidry process). All systems use a rotary kiln and contain the burning stage and all
or part of the calcining stage. For the wet and dry processes, all pyroprocessing operations
take place in the rotary kiln, while drying and preheating and some of the calcination are performed
outside the kiln on moving grates supplied with hot kiln gases.
4. Clinker Cooling
The clinker cooling operation recovers up to 30% of kiln system heat, preserves the ideal
product qualities, and enables the cooled clinker to be maneuvered by conveyors. The most
common types of clinker coolers are reciprocating grate, planetary, and rotary. Air sent through
the clinker to cool it is directed to the rotary kiln where it nourishes fuel combustion. The fairly
coarse dust collected from clinker coolers is comprised of cement minerals and is restored to
the operation. Based on the cooling efficiency and desired cooled temperature, the amount of
air used in this cooling process is approximately 1-2 kg/kg of clinker. The amount of gas to be
cleaned following the cooling process is decreased when a portion of the gas is used for other
processes such as coal drying.
5. Clinker Storage
Although clinker storage capacity is based on the state of the market, a plant can normally store
5 - 25% of its annual clinker production capacity. Equipment such as conveyors and bucket
elevators is used to transfer the clinkers from coolers to storage areas and to the finish mill.
Gravity drops and transfer points typically are vented to dust collectors.
6. Finish Milling
During the final stage of portland cement production known as finish milling, the clinker is
ground with other materials (which impart special characteristics to the finished product) into a
fine powder. Up to 5% gypsum and/or natural anhydrite is added to regulate the setting time of
the cement. Other chemicals, such as those which regulate flowability or air entrainment, may
also be added. Many plants use a roll crusher to achieve a preliminary size reduction of the
clinker and gypsum. These materials are then sent through ball or tube mills (rotating, horizontal
steel cylinders containing steel alloy balls) which perform the remaining grinding. The
grinding process occurs in a closed system with an air separator that divides the cement
particles according to size. Material that has not been completely ground is sent through the
system again.
7. Packing and Loading
Once the production of portland cement is complete, the finished product is transferred using
bucket elevators and conveyors to large, storage silos in the shipping department. Most of the
portland cement is transported in bulk by railway, truck, or barge, or in 43 kg (94 pound)
multiwalled paper bags. Bags are used primarily to package masonry cement. Once the
cement leaves the plant, distribution terminals are sometimes used as an intermediary holding
location prior to customer distribution. The same types of conveyor systems used at the plant
are used to load cement at distribution terminals.
B. SOURCES OF POLLUTION
Although portland cement plants generate the same final product using similar processes,
plant layouts vary according to fuels and raw materials used, location, climate, site topography,
and the manufacturer of the equipment. The flow diagram in Exhibit 1 depicts the manufacturing
process at a portland cement plant and indicates emission points throughout the process.
C. POLLUTANTS AND THEIR CONTROL
This section briefly discusses the nature of the pollutants generated from, and controls used at,
several sources in the cement manufacturing process. Air pollutants are typically of greater
concern than solid or liquid wastes.
1. Air Pollutants
Air pollutants generated during the cement manufacturing process consist primarily of particulates
from the raw and finished materials, and fuel combustion by-products. Exhibit 2 indicates
sources of air pollution, and differentiates between particulates and other air pollutants.
Controlling particulate emissions from sources other than the kiln usually entails capturing the
dust using a hood or other partial enclosure and transporting it through a series of ducts to the
collectors. The type of dust collector used is based on factors such as particle size, dust
loading, flow rate, moisture content, and gas temperature. The best disposal method for collected
dust is to send it through the kiln creating the clinker. However, if the alkali content