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Transcript of Study Material Training
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CEMENT & ITS MANUFACTURING PROCESS
PREPARED BY: GUID ED BY:
DEEPAK KUMAR, SHRI R.S. SONI,
ENGINEER-I I (PROD.), SR. GM (PROD.),
JSCP, SIDHI. JSCP, SIDHI.
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State-of the-art equipment, instrumentation and control systems have been installed at each of the plants. The
successive units have incorporated improved measures for facilitating easy operation, maintenance, and higher
energy conservation and pollution control. Consistent and high quality cement production is achieved through
extensive instrumentation and fully automatic and computerized process control systems, custom- designed quality
software such as the QSO Expert and CADES in the mines, Cross belt analyzer using the Prompt Gamma Neutron
Activation Analysis (installed for the first time in India), X-ray fluorescence and X-ray diffraction analyzers and
optical microscope.
The Jaypee cement is a market leader among all blended/ composite cements in the markets of Uttar Pradesh, Bihar,
Madhya Pradesh, Punjab, Haryana, Delhi and Nepal. The companys unique design and blend, with high strength
clinker and superior quality fly ash has made its cement a popular choice for a large number of strong and durable
structures in these states. Keeping pace with the advancements in the IT industry, all the 260 cement dumps are
networked using TDM/TDMA VSATs along with a dedicated hub to provide 24/7 connectivity between the plants
and all the 120 points of cement distribution in order to ensure track the truck initiative and provide seamless
integration. This initiative is the first of its kind in the cement industry in India. In the near future, the group plans
to expand its cement capacities via acquisition and Greenfield additions to maximize economies of scale and build
on vision to focus on large size plants from inception.
JAYPEE SIDHI CEMENT PLANT
Jaypee Sidhi Cement Plant is one of the modern and energy efficient plant of the group situated in Sidhi district of
Madhya Pradesh, India. Its annual capacity is 1.4 MnTPA and is started in the January2009.
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HIGHLIGHTS OF PLANT (JSCP) MACHINERY
S. No. Name of Machinery Capacity Model/Size Suppliers1. LIMESTONE CRUSHER 800 TPH APPM 1822 M/S. - L & T
KHANSBAL2. RAW MILL ( VRM) 330 TPH RM 54 / 27 M/S. - POLYSIUS
AG
3. COAL MILL 38 TPH CM 25 / 12 M/S. - POLYSIUSAG
4. PREHEATER CYCLONE 4200 TPD 6 STAGE (DOUBLESTRING)
M/S. - L & TPOWAI MUMBAI
5. ROTARY KILN 4200 TPD 4.35m X 66m L M/S. - L & TPOWAI MUMBAI
6. CLINKER COOLER 4200 TPD ETA COOLER 877 M/S. - L & TPOWAI MUMBAI
7. CEMENT MILL 185 TPH 5m X 15.5m L M/S. - L & TPOWAI MUMBAI
Introduction - Cement and its Manufacture
The dictionary meaning of cement is to join. Cement may be defined as adhesive substances capable of uniting
fragments or masses of solid matter to a compact whole. Constructions are the backbone of civilization and
development of society. From the time immemorial, constructions have evolved around some type of building
blocks, joined together with some binder. Mud blocks, pieces of stone cut and dressed to size, or bricks are examples
of building blocks, which are joined with, may be mud mortar (specially in climates with little or no rain), lime or
cement mortar, or even bitumen, and laid one over the other to erect the structure. Seen in this perspective, cement is
the essential binder of the present day constructions, and concrete, again made with cement, is the leading building
material the world over.
The use of cementing materials is very old. The ancient Egyptians used calcined impure gypsum. The Greeks and
Romans used calcined limestone and later learned to make lime mortar by adding other substances like sand,
crushed stone or brick, and broken tiles, to lime and water. From the chemistry viewpoint, present-day cements bear
similarity with lime, which was known as the classical building material for many centuries. The process of lime
manufacture and use in lime mortar involves the following steps;
1. Burning of lime CaCO3 CaO + CO22. Slaking of lime CaO + H2O Ca(OH)2, and3. Carbonation, hardening Ca (OH) 2 + CO2 CaCO3 + H2O.
Burning of limestone to make quicklime means release of CO2 occupying 44 percent by weight. Such loss of mass
makes quicklime porous. In contact with water, it absorbs water quickly, with considerable evolution of heat (280
cal/gm). This is hydration of lime. The resultant volume is 20 percent greater than that of the reactants. It gives rise
to expansive forces. Depending upon the amount of water added slacked lime will have powdery appearance or
highly dispersed paste, also known as milk of lime. The particles of hydrated lime are extremely fine and develop
cohesive forces. Upon drying, it will harden and develop strength; which is not very high may be of the order of
30 40 kg/cm2. The actual hardening of lime mortar is completed on carbonation with atmospheric CO2.
Silica (SiO2), as obtained in natural rocks and stones, occurs mostly in crystalline form as in quartz. In crystalline
form, Silica is very stable and non-reactive. When heated to high temperature, even the ordinary quartz variety of
silicon becomes chemically reactive. In a silica-containing aqueous solution mixed with lime, a precipitate forms
rapidly, giving rise to a gel-like mass of colloidal dimensions having cohesive forces. Around 1756, it was
discovered that lime made from limestone containing a considerable proportion of clayey matter gave better results.
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This was the beginning of most important advances in the knowledge of cements. It was soon recognized that if a
mixture of lime and silica, in finely ground form and in required proportions, is heated to sufficiently high
temperatures, calcium silicates having hydraulic properties (i.e. capable of hydrating and hardening in contact with
water) are formed. This is the basis of cement manufacture.
BRIEF PROCESS DESCRIPTION
LIMESTONE FROM MINES
CRUSHING AND STACKING PROCESS(Crushed Limestone)
STOCK PILE
RECLAIMING PROCESS(Blended Limestone along with Laterite & Iron Ore)
RAW MEAL GRINDING PROCESS(Raw Meal)
RAW MEAL STORAGE IN CF SILO & HOMOGENISATION(Kiln Feed) (Fine Coal)
PYRO PROCESS(Hot Clinker)
CLINKER COOLER(Cold Clinker)
CLINKER STORAGE IN SILO & YARD(Clinker + Gypsum + Flyash)
CEMENT GRINDING PROCESS(Cement)
CEMENT STORAGE IN SILO
PACKING & DISPATCH
MININGThe cement manufacturing process starts from the mining of limestone, which is the main raw material for makingcement. Limestone is excavated from open cast mines after drilling and blasting and loaded on to dumpers whichtransport the material and unload into hoppers of the limestone crushers.
CRUSHING, STACKING & RECLAIMING OF LIMESTONEThe LS Crusher (Impact Crusher) crushes the limestone to minus 80-90 mm size and discharge the material onto a
belt conveyor which takes it to the Stacker via the Bulk Material Analyzer. The material is stacked in longitudinalstockpiles by Chevron method. Limestone is extracted transversely from the stockpiles by the Reclaimer (BridgeScrapper type) and conveyed to the Raw Mill hoppers for grinding of raw meal.
CRUSHING, STACKING & RECLAIMING OF COALThe process of making cement clinker requires heat. Coal is used as the fuel for providing heat. Raw Coal isdropped on a belt conveyor from a hopper and is taken to and crushed in a crusher. Crushed coal discharged fromthe Coal Crusher is stored in a longitudinal stockpile from where it is reclaimed by a reclaimer and taken to the coalmill hoppers for grinding of fine coal.
Coal crushing, stacking
& reclaiming.
Coal grinding process.
Fine Coal Storage.
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RAW MEAL DRYING/GRINDING & HOMOGENISATIONReclaimed limestone along with some laterite & Iron ore stored in their respective hoppers is fed to the Raw Mill forfine grinding. The hot gasses coming from the Clinkerisation section are used in the raw mill for drying andtransport of the ground raw meal to the Bag House, where it is collected and then stored and homogenized in theconcrete silo. Raw Meal extracted from the silo (now called Kiln feed) is fed to the top of the Preheater for Pyro-
processing.
CLINKERISATIONCement Clinker is made by Pyro-processing of Kiln feed in the Preheater and the rotary kiln. Fine coal is fired as
fuel to provide the necessary heat in the kiln and the Precalciner located at the bottom of the 6 stage Preheater. Hot
clinker discharged from the Kiln drops on the ETA - Cooler and gets cooled. The cooler discharges the clinker onto
the pan / bucket conveyor and it is transported to the clinker stockpiles / silos.
CEMENT GRINDING & STORAGEClinker and Gypsum (for OPC) and also Pozzolana (for PPC) are extracted from their respective hoppers and fed tothe Cement Mills. The Ball Mill ground the feed to a fine powder and the Mill discharge is fed to an elevator, which
takes the material to a separator, which separates fine product and the coarse. The latter is sent to the mill inlet forregrinding and the fine product is stored in concrete silos for packing & dispatch.
PACKING & DISPATCHCement extracted from silos is conveyed to the automatic electronic packers where it is packed in 50 Kg. Polythenebags and dispatched in trucks.
RAW MATERIALS AND MINING
The raw materials for cement manufacture which are subject of geological exploration are mainly limestone and
clay. Both occur in the nature as rocks (mostly sedimentary rocks). In addition, certain materials are incorporated in
the raw mix in smaller quantities, in order to correct its chemical composition in terms of moduli values discussed
later. These are called additives and comprise materials providing iron and alumina in the required proportions.
There are a number of parameters which determine the suitability of the raw materials for use in cement
manufacture. This is particularly important for limestone deposits, which, in India, comprise more than 90 percent of
the mass of the raw mix.
Parameters of Quality of Limestone Deposit
Suitability of limestone is mainly judged on the basis of its chemical composition. Limestones consist
predominantly of CaCO3, as Calcite. In addition, they often contain Mg, Al and Fe combined as carbonates and
silicates; as well as silica as quartz. The reactions in the cement burning process takes place between the individualphases present in the kiln feed. In few instances, it has been possible to have a limestone of composition with LSF =
1, and silica ratio and alumina ratio within desirable range for raw mix composition. Such a limestone is more
favorable, because the components are naturally present in a well blended form and can be used singly in cement
manufacture. However, most of the time, the composition deviates from such ideal. It should be noted that mixture
of pure limestone and pure clay will react less favourably. Limestone containing some admixture of clay
minerals is to be preferred. The following classification in terms of CaCO3 content is applicable;
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Pure limestone > 95% CaCO3Marly limestone 85 95% CaCO3
Lime marl 70 85% CaCO3
Marl 30 70% CaCO3
Clay Marl 15 30% CaCO3
Marly Clay 5 15% CaCO3
Clay < 5% CaCO3
For cement manufacture, Cement grade limestone should, conventionally have 46 47 % CaO, or > 82 % CaCO3.
If MgO is present (as MgCO3), the Total Carbonates (TC) should be about 90%, with the proviso that MgO
content does not exceed 6% as per IS requirements.
High Silica, low lime limestone with SiO2 > 14% and CaO < 44% is classified as low grade. For intermediate
compositions i.e. CaO > 44 % but < 46 %, it is called marginal grade limestone. On the other hand, limestone with
CaO >48 % can be called high grade and used as a sweetener with limestone of lower CaO content, to yield a
satisfactory raw mix.
The mode of origin of limestone e.g. whether sedimentary, igneous or metamorphic, influence the mineral form,
degree of crystallinity, grain size, cementing medium, intensity of compaction etc. Each of these parameters
influences the thermo-chemical reactivity. i.e. dissociation characteristics and chemical combinability. For cementmanufacture, fine-grained sedimentary limestone deposits are preferred.
Clay Component
The clay mineral component is generally a soft and loose-textured sedimentary rock deposit of clays, slate, shale and
crystalline slates. Depending upon the particle size, these are classified as
Clay ( 2mm).
Suitability is judged by chemical composition in terms of silica ratio (SR) and alumina ratio (AR). Mineralogical
form of silica is important. Large amount of free quartz is not desirable.
Assimilation of coal ash in the clinker provides a source of silica in raw mix composition. With high ash coal used
in India, often no clay is required to be added separately.
Corrective materials
Small amounts of materials other than limestone and clay are needed, containing high proportion of oxides which
may be deficient in the main raw materials. Laterite or quartz sand is used for increasing SiO 2; roasted pyrites or
iron ore for Fe2O3; and bauxite for Al2O3. Mineralisers are added with marginal or low grade raw materials or whenthe raw mix has low burnability.
Exploration and Reserve Estimation -
For the manufacture of 1 tonne of cement, on an average, 1.5 tonnes of limestone are required. For setting up a one
MT cement plant, limestone deposit of the order of at least 60 MT are required for a life span of 40 years, and much
more for longer life and/or larger plant size and future expansion. Hence, the deposit of limestone has to be properly
explored, assessed and evaluated. The main purposes of exploration are;
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1. Verifying the quality of raw materials;2. Establishing the range of variation in the quality of raw materials throughout the working life of the
deposit; and3. Estimating and verifying the workable reserves of raw materials.
Availability of limestone in any country is, prima facie, known through geological exploration carried out by thenodal governmental agencies (GSI and State Geological Surveys in India). With this information as the starting
point, more focused and dedicated investigation is required to establish availability of cement grade limestone. The
exploration is carried out in three stages;
Stage I Field inspection of a number of deposits, surface tests, large area mapping and a limited number of
exploratory borings. The main aim is to find out the quality of the deposit.
Stage II One or more deposits are selected for detailed investigation. Comprehensive drilling programme enables
the deposits to be broadly studied. Chemical characteristics, bedding conditions, ground water and possibilities of
working the deposit are established over extensive areas. Suitability of the site for quarrying or open-cast mining are
known.
Stage III Detailed exploration is carried out with the help of grid of closely spaced bore-holes to determine the
chemical composition of limestone and the variation over short distances.
The amount of exploration and the number of bore-holes depend upon the variation in the chemical characteristics;
more intense exploration being necessary when the variation is large and vice-versa. The variance of the
geochemical data of the deposit enables it to be classified as under;
Simple when the co-efficient of variation is 4 percent,Complex - 10 percent, andIntricate - > 10, 20 percent.
There is regional imbalance in the availability with deposits being located in the states in southern, western, northern
and eastern zones, in that order. States like Andhra Pradesh, Karnataka, Gujarat, Madhya Pradesh and Rajasthan
account for nearly 75 percent of cement grade limestone deposits and large plants are set up in these states in large
numbers. Next comes states like Himachal Pradesh, Meghalaya and J&K etc.; while states like West Bengal have no
deposits and no large plants. Similar is the situation in Bangladesh. Now a large cement plant is being set up in
Bangladesh for which limestone from Meghalaya will be transported by a belt conveyor.
Mining and Quarrying of Limestone
As indicated before, a million tonne cement plant producing 3000 T of cement per day (330 working days in a year)
will need more than 5000 T of limestone to be quarried every day. Proper planning and deployment of mining
equipment is necessary. The usual method is of large scale open-cast mining using benching technique, in which the
material in the deposit is quarried in several benches or steps, one above the other. The operations are seldom
confined to a single bench or single location at a time. In order to compensate for variation in chemical
characteristics of the rocks at different depths and different locations in the deposit, several benches at several
working points are worked upon simultaneously.
Computer-aided mine planning programme developed for such purposes helps in selecting locations and benches, as
mining progresses. The software developed by NCB enables reliable estimation of limestone reserves, its grade and
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quality variation and help in proper planning pit design and cost effective mining. The software is used by Indian
cement industry. Various quarry floor or base levels should be connected to each other and the general ground level
by ramps, forming sufficiently wide roads with gentle slope (1 : 10) for haulage and movement of vehicles. If
ground water is likely to be encountered, provision for pumping out should be made. The mining of limestone
involves the following steps;
Drilling, Blasting / ripping, Loading and haulage to the site of primary crusher.
Drilling Large-hole blasting with blast holes in single row parallel to the slope of quarry face is common. The
holes are more than 12 m in depth may be 20 to 30 m, and the holes are of 50 to 250 mm diameter. Another
alternative is surface blasting with holes distributed over an area instead of single row. Rotary drilling or
percussive rotary drills are used for forming the blast holes. Depending upon the characteristics of the rock and the
machine deployed, step bit, roller bit or cross bits are used. The drilling machines are with fully hydraulic drive
system and have capacities of 30 m/hour or so. The entire assemblage comprising the power pack, hydraulic units,
drilling mast, rod magazine and other accessories are mounted on a traction unit, which can be moved from one
bench and location to other, as required.
The purpose of blasting is to loosen and fragment the rock mass. The blast holes are charged with explosives and
fired with the help of delay-type electric detonators (blasting caps). The explosives can be powder type (ANC
ammonium nitratecarbon) or gelatinous (gelatins, gelignite). The specific explosive consumption can be of the
order of 200 to 400 gm per m3 of solid rock to be blasted. Careful planning of blast hole location grid, its spacing,
depth, diameter and the amount of explosive charge is required to break the mass and to minimize the ground
vibration, noise and other hazards. When the surface is inaccessible for blasting, tunneling method can be adopted,
but it is not very common. In this method, tunnels are driven into the face of the deposit and fairly large amount of
charge is fired.
Secondary blasting In normal course, the rock should be fragmented after blasting to a size, which can be taken
to the primary crusher for reducing to smaller size. In case oversize pieces or boulders are obtained, they have to bebroken up further, for ease of loading, haulage and crushing. This is called secondary blasting. For this purpose,
small diameter blast holes are drilled into the boulder and explosive charge @ 60 to 90 kg / m3 fired. Another
method is to apply gelatin type high explosive on the surface of the rock mass and fire; this is called mud capping.
It is rather noisy and not preferred.
Ripping In case of heavily fissured rocks having thin bedding and coarsely crystalline structure, ripping methods
can be used. Ripping teeth are mounted at the rear end of a heavy crawler tractor. As the tractor travels, the teeth
penetrate into the rock. The teeth can be straight or curved. Mechanical methods with pneumatic or hydraulic
breaking hammers are also possible in case of softer rocks and smaller deposits.
Loading and haulage
The blasted rock is loaded in cable-operated excavators, hydraulic operators or wheel loaders. Haulage to the
primary crushing plant can be on rail-mounted vehicles or rubber-tired heavy trucks or combination thereof. The
trucks have payload of 15 to 20 T. Belt conveyors can be used, when the primary crusher is located at the quarry
site, so that the limestone after primary crushing is transported for further size reduction. When the terrain condition
is difficult, aerial ropeways can be used. The range varies from 1 to 100 km and capacity is about 500 T/hour. The
speed of the ropeway is of the order of 4 m/sec.
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Grinding Mills
At JSCP, VRM is used for grinding Raw Materials & Raw Coal whereas Ball Mill is used for grinding of Clinker &
other additives for making cement.
RAW MILL (Vertical Roller Mill)
Feed Moisture Content Max. 8 %
Lump Size 0-120 mm
Year of Construction 2007
Mill Type RM/54/27/470
Finished Material Output 330 TPH
Finished Material Fineness 15 % R 90 m
Finished Material Moisture Content 1 %
Mill Power Requirement 3510 kW
I/P & O/P (rpm) 990 & 23.53
Description of Functioning: -
In roller mill three work operations are performed.
Grinding.
Drying.
Separating
The material to be ground is fed to the roller mill via a flow regulating device and a feed chute. The mill feed
material falls on to the centre of the rotating grinding table and is carried under the rollers by the centrifugal force
generated by the rotation of the grinding tables. Due to the centrifugal force, the crushed material spills over the
edge of the grinding table and is entrained in the stream of gas from the nozzle ring. All or part of the material
falling from the grinding table is dried in the hot gas stream and carried to the dynamic or static separator located
above the grinding chamber.
The separator classifies the material entrained in the hot gas stream into finished product & oversize. The oversize
material falls back onto the centre of the grinding table, while the finished product is carried by the gas stream to the
dedusting filter where it is collected. In the roller mills two roller pairs run on a rotating grinding table. The principle
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of the double roller pairs leads to a lower speed differential between the rollers and the grinding table. This cut down
the slippage of material between rollers and grinding table and thereby minimizes wear.
The grinding roller guide and simple grinding force application into the system allows the roller pairs freedom of
movement in all three planes. This means that they can yield vertically & pivot horizontally on their guide axis. If
the inner roller is raised by coarse feed, the outer roller pressures down with greater force on the material. Each
roller thus supports the action of the other. This interactive functioning of the roller results in a highly efficient
grinding process. The nozzle ring is the bottom neck in the roller mill. Here, the hot gas stream emerges at a high
velocity, takes up to the crushed material falling off the grinding table and carries it upwards to the separator. The
nozzle ring cross-section is adjustable from outside. The hot gas velocity in the nozzle ring is a decisive parameter
for the grinding process. If the gas velocity is reduced by enlarging the open cross section of the nozzle ring, the
amount of material falling through the nozzle ring increases. The recirculating bucket elevator is therefore supplied
with a large quantity of material. The recirculated material is returned to the dynamic separator or direct to the mill
feed unit.
Hydraulic System: - Force required for grinding the feed material is generated by a hydraulic system. Two
hydraulic cylinders per roller pairs ensure that the grinding force is evenly applied to the rollers while maintaining
the maximum freedom of roller movement. The hydraulic system permits infinite adjustment of pressure in order to
adapt the grinding force to actual operating conditions. When the mill is started up, the hydraulic pressure is lower inorder to reduce the force applied to the grinding rollers. This is turn reduces the starting torque. Each hydraulic
cylinder is connected to a piston accumulator which dampens the shocks generated by the rollers running over the
bed of materials.
BALL MILL
A cement mill/Ball mill is the equipment used to grind the hard, nodular clinker stored in the silo from the cement
kiln into the fine grey powder that is cement.
Materials ground
Portland clinker is the main constituent of cements. In Portland cement, a little calcium sulfate (typically 3-10%) is
added in order to retard the hydration of tricalcium aluminate. The calcium sulfate may consist of natural gypsum,
anhydrite, or synthetic wastes such as flue gas desulfurization gypsum. In addition, up to 5% calcium carbonate and
up to 1% of other minerals may be added. It is normal to add a certain amount of water, and small quantities of
organic grinding aids and performance enhancers. "Blended cements" and Masonry cements may include large
additions (up to 40%) of natural Pozzolana, fly ash, limestone, silica fume or metakaolin. Blast furnace slag cement
may include up to 70% ground granulated blast furnace slag. See cement. Gypsum and calcium carbonate are
relatively soft minerals, and rapidly grind to ultra-fine particles. Grinding aids are typically chemicals added at a rate
of 0.01-0.03% that coat the newly-formed surfaces of broken mineral particles and prevent re-agglomeration. They
include 1, 2-propanediol, acetic acid, triethanolamine and lignosulfonates.
Temperature control
Heat generated in the grinding process causes gypsum (CaSO4.2H2O) to lose water, forming bassanite (CaSO4.0.2-
0.7H2O) or -anhydrite (CaSO4. ~0.05H2O). The latter minerals are rapidly soluble, and about 2% of these in cementis needed to control tricalcium aluminate hydration. If more than this amount forms, crystallization of gypsum ontheir re-hydration causes "false set" - a sudden thickening of the cement mix a few minutes after mixing, which thins
out on re-mixing. High milling temperature causes this. On the other hand, if milling temperature is too low,
insufficient rapidly-soluble sulfate is available and this causes "flash set" - an irreversible stiffening of the mix.
Obtaining the optimum amount of rapidly-soluble sulfate requires milling with a mill exit temperature within a few
degrees of 115C. Where the milling system is too hot, some manufacturers use 2.5% gypsum and the remaining
calcium sulfate as natural -anhydrite (CaSO4). Complete dehydration of this mixture yields the optimum 2% -
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anhydrite. In the case of some efficient modern mills, insufficient heat is generated. This is corrected by
recirculating part of the hot exhaust air to the mill inlet.
Process Description
A ball mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis,
imparting a tumbling and cascading action to the balls. Material fed through the mill is crushed by impact and
ground by attrition between the balls. The grinding media are usually made of high-chromium steel. The smaller
grades are occasionally cylindrical ("pebs") rather than spherical. There exists a speed of rotation (the "critical
speed") at which the contents of the mill would simply ride over the roof of the mill due to centrifugal action. The
critical speed (rpm) is given by: nC = 42.29/d, where d is the internal diameter in meters. Ball mills are normallyoperated at around 75% of critical speed, so a mill with diameter 5 meters will turn at around 14 rpm.
The mill is usually divided into at least two chambers, allowing the use of different sizes of grinding media. Large
balls are used at the inlet, to crush clinker nodules (which can be over 25 mm in diameter). Ball diameter here is in
the range 60-80 mm. In a two-chamber mill, the media in the second chamber are typically in the range 15-40 mm,
although media down to 5 mm are sometimes encountered. As a general rule, the size of media has to match the size
of material being ground: large media can't produce the ultra-fine particles required in the finished cement, but small
media can't break large clinker particles. A current of air is passed through the mill. This helps keep the mill cool,
and sweeps out evaporated moisture which would otherwise cause hydration and disrupt material flow. The dusty
exhaust air is cleaned, usually with bag filters.
Liners & Diaphragm:
The High Carbon Steel, Cast Manganese Steel and Ni-hard liners are essential part of the mills and are designed to
give optimum production. Ensuring the optimum level of material filling ratio in each chamber of Ball Mills is the
key to high performance and maximum use of grinding energy of the Mill.
Clinker hardness
The hardness of clinker is important for the energy cost of the grinding process. It depends both on the clinker's
mineral composition and its thermal history. The easiest-ground clinker mineral is alite, so high-alite clinkers reduce
grinding costs, although they are more expensive to make in the kiln. The toughest mineral is Belite, because it is
harder, and is somewhat plastic, so that crystals tend to flatten rather than shatter when impacted in the mill. The
mode of burning of the clinker is also important. Clinker rapidly burned at the minimum temperature for
combination, then rapidly cooled, contains small, defective crystals that grind easily. These crystals are usually also
optimal for reactivity. On the other hand, long burning at excess temperature, and slow cooling, lead to large, well-
formed crystals that are hard to grind and un-reactive. The effect of such a clinker can be to double milling costs.
Capacity of cement mills
The cement mills on a cement plant are usually sized for clinker consumption considerably greater than the output of
the plant's kilns. This is for two reasons: The mills are sized to cope with peaks in market demand for cement. In
temperate countries, the summer demand for cement is usually much higher than that in winter. Excess clinker
produced in winter goes into storage in readiness for summer demand peaks. For this reason, plants with highly
seasonal demand usually have very large clinker stores. Cement milling is the largest user of electric power on acement plant, and because they can easily be started and stopped, it often pays to operate cement mills only during
"off-peak" periods when cheaper power is available. This is also favorable for electricity producers, who can
negotiate power prices with major users in order to balance their generating capacity over 24 hours. More
sophisticated arrangements such as "power shedding" are often employed. This consists of the cement manufacturer
shutting down the plant at short notice when the power supplier expects a critical demand peak, in return for
favorable prices. Clearly, plenty of excess cement milling capacity is needed in order to "catch up" after such
interruptions.
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PYROPROCESSING AND CLINKER FORMATION
In pyroprocessing, the raw mix is heated to produce Portland cement clinkers. Clinkers are hard, gray, sphericalnodules with diameters ranging from 0.32 - 5.0 cm (1/8 - 2") created from the chemical reactions between the rawmaterials. The pyroprocessing system involves three steps: drying or preheating, calcining (a heating process inwhich calcium oxide is formed), and burning (sintering). During the process of cement manufacture, the following
operations take place at the temperatures indicated;
1. Evaporation of free and chemically combined moisture 100 - 550OC,2. Preheat and evolution of CO2 from the carbonates (calcinations) 805 OC,3. Formation of phases and Clinkerisation 800 to 1400 OC,4. Clinker discharge and cooling.
These heat transfer functions in cement manufacture take place in the kiln system comprising the Preheater and
precalcinator, the kiln, the cooler and the refractory. The heat for pyroprocessing is supplied by the fuel, ignited by
hot air and gases from the system. In this section, the kiln, refractories, fuels and the flame characteristics will be
described. The heat transfer mechanisms and heat balance involved in cement manufacture will be discussed.
The dimensions of the kiln i.e. the length and the diameter depend upon the capacity of the kiln. It should be notedthat the capacity is not the volume of the kiln, as the loading in a kiln with kiln feed is only about 6 to 10 percent of
the cross section. The length to diameter (l/d) ratio in a wet or dry process kiln without precalcinator (called long dry
kiln) is of the order of 30 to 35; in case of a Preheater and precalcinator the l/d ratio could be from 12 to 18 (short
rotary kiln). It is supported on two sets of rollers when l/d = 12 to 14 and three supports when l/d = 16 to 18. The
kiln is set in an inclination of 3 to 4.5 %, with the feed end being higher and discharge end lower.
Clinker Coolers - These are fitted at the discharge end of the kiln. It performs two functions;
To cool the hot clinker discharged from the kiln, and To supply the kiln with necessary air for combustion of the fuel. Valuable heat from the clinker is
recuperated and enters the kiln as hot secondary air.
CHEMISTRY OF CEMENT
Chemical Composition and Phases in Portland cement
Portland cement, being made from calcareous and argillaceous raw materials, is, as to be expected, composed of
compounds of oxides of calcium (CaO), silicon (SiO2), aluminum (Al2O3) and iron (Fe2O3). It is customary to
express the chemical formulae of the compounds in cement as the sum of the oxides e.g. Ca3SiO5 is expressed as
3CaO.SiO2. It does not mean that the constituent oxides exist separately within the structure of the phases. One
exception is MgO, originating from impurities in the main raw materials. Among the main oxides, CaO occurs as
Calcite (CaCO3) in limestone. It decarbonates at about temperature of 680OC and above.
[CaCO3 CaO + CO2]
We had taken note of this reaction in connection with lime manufacture. It is an important reaction in cement
manufacture, both in terms of thermal energy consumed, reduction in solid volume and the amount of CO 2 emitted
to the environment. SiO2 occurs as aluminosilicates in clay or shale, rather than the oxides. Silica, SiO2 exists
naturally in the pure state as different crystalline polymorphs (e.g. quartz, Crystobalite and Tridymite). Poorlycrystalline or amorphous forms are also found as in Opal and Flint etc.
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Al2O3 and Fe2O3 are not essential for the hydraulic properties, but their presence is essential as flux in the hottest
zone of the kiln in cement manufacture, to keep the thermal energy required to manageable economical levels.
Minor components are those, whose presence even in small amounts (
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Loss on Ignition (LOI) 0.7 1.7
Insoluble residue (IR) 0.5 1.7
Two terms in the above composition need explanation.Loss on ignition is the loss of mass when heated to 1000OC,
which could be due to any moisture or CO2 attracted during storage, CO2 in the limestone filler when permitted to be
added, any water content in gypsum not driven out during manufacture, or any adulteration. Insoluble residue iswhat is insoluble in hydrochloric acid; indicate free quartz, impurities from rock gypsum, or insoluble in adulterants.
When industrial wastes like fly ash, which are mostly acid-insoluble, are permitted to be added in composite
cements, the values of IR are higher. In Indian cements, the values for CaO content could be somewhat lower,
depending upon the quality of limestone. Proportion of MgO in some cases can be higher, the upper limit being 5%.
The Phases in Portland cement
Before the phases present in cements are identified, let us consider how the oxides interact at different compositions
and different temperature, as binary, ternary and quaternary systems. These are best studied in terms of phase
diagrams. What is described here is a simple explanation.
The System CaO SiO2
The equilibrium phase diagram for the system CaO SiO2 is shown below. It contains four binary compounds CS,
C3S2, C2S and C3S. The melting point of pure CaO is about 2750OC, and that of SiO2 is about 1698
OC. However,
presence of each other lowers the liquidus temperature. Out of the four phases, CS and C3S2 are not hydraulic, nor
are they formed during cement manufacture.
C2S has a number of polymorphs. On cooling from higher temperatures, - C2S ultimately transforms to form at630OC. At still lower temperature transformation to - C2S, which is not hydraulic, takes place. Fortunately, C2Scan be stabilized to low temperatures by quenching, or by forming solid solutions with a large number of oxide
impurities like boric oxide (B2O3) or Phosphorous pentaoxide (P2O5) and MgO.
C3S is the most abundant phase. Although a lower limit of solubility exists for C3S at about 1250OC, its
decomposition to C2S and CaO does not occur on cooling; C3S exhibits a range of metastable modifications between
room temperature and 1100
O
C, which are;
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CaO Al2O3 System
The relevant phase diagram is reproduced below. Presence of Al2O3 (and Fe2O3) lowers the liquidus temperature of
CaO and helps in reducing the burning temperature for cement manufacture. This system contains stable phases
C3A, CA, C12A7, CA2 and CA6. Except CA6, all other phases are hydraulic.
For compositions common in ordinary Portland cement manufacture, C3A is formed, which is responsible for early
hydration and setting. Gypsum is used to control the rapid hydration of C 3A phase and initial set. CA and C12A7are predominant phases in aluminous cements. CA2 and CA6 phases may be present in those with high alumina
content.Ternary System CaO SiO2 Al2O3
Lime rich binary phases comprise nearly 90 % of cement composition. The equilibrium phase assemblage at the
burning temperature for the cement manufacture consists ofC3S, C2S and a liquid phase of composition Lc shown in
the figure below. This liquid phase is containing the aluminous components. The final phase assemblage obtained on
cooling is more complex. It will consist of C3S, C2S, C3A and a melt. The melt, on cooling would crystallize to give
C2S, C3A and C12A7. In practice, all the four phases - C3S, C2S, C3A and C12A7 are obtained.
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The System CaO Al2O3 Fe2O3
The most significant phase in this system has a composition Ca2 (Alx Fe1-x)2O5, where x is between 0.48 and 0.7.If
CaO is also present x is fixed at 0.48, giving a composition close to C4AF called the Ferrite phase. This phase
contains a number of other ions in solid solution. Fe3+ also dissolves in C3A, C12A7 and CA, up to about 4.5% at
1325oC.
The Quaternary System CaO SiO2 - Al2O3 Fe2O3
We can now take a look at the phases present in Portland cement, which is a quaternary system in the four oxides,
besides minor components. More than 95% of the composition of Portland (and aluminous) cements comprise
compounds containing CaO, SiO2, Al2O3 and iron oxides. There are no quaternary phases and at equilibrium, any
mix composition will crystallize the following four phases;
Alite It is tricalcium silicate - Ca3SiO5, which is written as 3CaO.SiO2 or expressed as C3S. Pure C3S consists of
73.7% CaO and 26.3% SiO2. In industrial clinkers, the composition is modified by incorporation of Mg2+, Al3+ and
Fe3+ ions in solid solutions, the total of impurity oxides is of the order of 3 to 4 percent. The crystal structure of
cement minerals have been studied in details, which is not discussed here. In Indian cements, C3S content typically
varies from 30 to 48 percent.
Belite It is dicalcium silicate Ca2SiO4, which is written as 2 CaO.SiO2 or expressed as C2S. Pure C2S consists of65.1% CaO and 34.9 % SiO2. In industrial clinkers, the composition of Belite is modified by incorporation of
foreign ions, usually Al2O3 and Fe2O3, the total impurity oxides being 4 to 6 percent. Limit of K 2O substitution is 1.2 percent. In Indian cements, C2S content typically varies from 27 to 45 percent.
It is to be noted that in Portland cements, the total of C3S + C2S content is between 70 to 80 percent, perhaps closer
to 75 percent.
Aluminate phase It is tricalcium aluminate Ca3Al2O6, which is written as 3 CaO.Al2O3 or expressed as C3A.
Pure C3A contains 62.3% CaO and 37.7% Al2O3. The composition is modified by incorporation of foreign ions like
Si4+, Fe3+, Na+ and K+. The substitution is substantial it could be 13 percent for cubic and 20 percent for
orthorhombic modification of the crystal structure. In Indian cements, C3A content typically varies from 4 to 10
percent.
Ferrite phase It is tetra calcium alumino ferrite Ca2AlFeO5, also written as 4 CaO.Al2O3.Fe2O3, or abbreviated
as C4AF. Ferrite phase can be prepared with any composition in the solid solution series Ca2 (Alx Fe1-x) 2O5, where x
is between 0 and 0.7. C4AF is a particular form for x = 0.5. Pure C4AF contains 46.1 percent CaO, 21 percent Al2O3and 32.9 percent Fe2O3.The composition is substantially modified by variation in A/F ratio and incorporation of
foreign ions like Mg2+, Si4+ and Ti4+. Mn3+ can replace all the Fe3+ ions or up to 60 percent of Al3+ ions. The ferrite
phase makes up about 5 to 15 percent of ordinary Portland cement clinkers.
Calculation of Amount of Different Phases
Chemical analysis of cement or clinker is obtained in terms of oxides, as shown in the example given above.
Estimation of the relative proportions of the phases is accomplished by the procedure known as Bogue calculation.
It is based on the following steps; Assume that the compositions of the four major phases are C3S, C2S, C3A and C4AF. Assume that Fe2O3 occurs in C4AF. Assume balance of Al2O3 occurs in C3A. Deduct from the CaO content, the amounts attributable to C3A and C4AF and solve two simultaneous
equations to calculate the amounts ofC3S and C2S.
The formulae for Clinker are;
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C3S = 4.0710 CaO 7.6024 SiO2 6.7187 Al2O3 1.4297 Fe2O3. C2S = - 3.0710 CaO + 8.6024 SiO2 + 5.0683 Al2O3 + 1.0785 Fe2O3.
= 2.8675 SiO2 0.7544 C3S.
C3A = 2.6504 Al2O3 1.6920 Fe2O3. C4AF = 3.0432 Fe2O3.
For Cement, put (CaO 0.7 SO3) in place of CaO.The above gives the potential phase composition and differs from the true phase composition because;
Equilibrium may not be maintained during cooling, and Composition of clinker phases differ from the pure compounds, as explained before.
The total of four phases calculated plus free lime will not add up to 100 percent, because minor oxide components
are ignored. It is implied that all the MgO occurs as periclase.
Summary of Reactions Occurring during Clinker Formation
Reaction type Examples and comment
Decomposition CaCO3 CaO + CO2; one or more volatile substances form in thecourse of reaction
Diffusion 2 CaO +SiO2 Ca2SiO4; solid-state reaction between componentsMelting Formation of C-A-F-S eutectic at 1338 C between C3S, C2S, C3A
and C4AF.
Liquid phase sintering 3 CaO + SiO2 Ca3SiO5; precipitation of Alite from melt, inwhich the reactants are dissolved.
Polymorphic transformation -C2S and/or - C2S; polymorphic changes occurring duringcooling.
Evaporation / condensation (Liquid)K2SO4 (vapor)-K2SO4; evaporation and condensationof alkali sulphates.
In summary, we may note;
Below 1300OC The reactions that take place are;
a. Decomposition of calcite at 680OC and above,b. Activation of silicates through removal of water and changes in the crystal structure at up
to 700OC,c. Decomposition of clay minerals 900OC,d. Initial combination of calcite or lime from it with activated quartz and Al 2O3 and Fe2O3 700 -
900OC,e. Belite forms at 900 1200 OC.
Between 1300 1450OC Clinkering.
A melt is formed mainly from aluminate and ferrite phases. By 1450OC, some 20 to 30 percent of the
mix is liquid. Much of Belite and nearly all the lime react in the presence of the melt to form Alite. The
material nodulises to form clinker.
During cooling The liquid crystallizes giving mainly aluminate and ferrite. Polymorphic
transformations of the Alite and Belite occur.
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Moduli Values
In the manufacture of Portland cement, it is desirable that C3S content is optimized and thefree (uncombined) lime
contentis minimized. For such a situation, the CaO content is given by;
[CaO = 2.8SiO2 + 1.18Al2O3 + 0.65Fe2O3.]
CaO
The ratio -------------------------------------------- is called lime saturation factor (LSF).
2.8SiO2 + 1.18Al2O3 + 0.65Fe2O3
LSF largely shows the proportion of Alite and Belite in the clinker. In Portland cements, it is typically in the range
of 0.92 to 0.98, although specifications allow a range of 0.66 to 1.02. For LSF> 1.0, free lime will be present. Please
note that the above formula is for clinkers; for cement, use (CaO 0.7SO3) in place of CaO.
Some other useful limits are;
Silica ratio (SR) = [SiO2/ (Al2O3 + Fe2O3)]. SR is typically in the range of 2.0 to 3.0. Increase in SR means
proportion of liquid is lower and the clinker becomes difficult to burn.
Alumina ratio (AR) = Al2O3/ Fe2O3. The value of AR can vary between 1.0 to 4.0. AR determines the quantity of
liquid formed at low temperature. At 1338oC, the quantity of liquid is theoretically maximum at AR of 1.38.
Role of Mineralisers and Flux
It has been discussed how the presence of Al2O3 and Fe2O3 in the raw mix, although not essential to the constitution
of Portland cement, constitute a cheap source of fluxes lowering the thermal energy required in cement manufacture.
Al2O3 and Fe2O3 form a liquid melt, in which much of the Belite (C2S) and nearly all the CaO react, to form Alite
(C3S). Other volatiles like MgO, Na2O and K2O also act as flux. When the ratio of A/F is > 1.38 and the amount of
MgO present is < 2 percent, the amount of flux present at different temperatures is given by the formulae;
Flux at 1338OC = 6.1 x% Fe2O3 + % MgO + % Na2O + % K2O. Flux at 1400OC = 2.95 x % Al2O3+ 2.2 x% Fe2O3 + % MgO + % Na2O + % K2O. Flux at 1450OC = 3.0 x % Al2O3+ 2.25 x% Fe2O3 + % MgO + % Na2O + % K2O.
Where the ratio A/F is < 1.38,
Flux at 1338OC = 8.5 x % Al2O3 5.22 x% Fe2O3 + % MgO + % Na2O + % K2O.If the level of flux is less than 19 percent, the coating formation is inadequate and refractories are likely to be
damaged. Excessive high flux levels at 1338OC will lead to ring formation at the point where the kiln feed enters the
burning zone. It is, therefore, necessary to ensure the optimum level of flux by careful design of the raw mix. The
desirable range could be about 19 to 26 percent. Somewhat higher level may be required in case of difficult raw
materials, but not exceeding 28 29 percent.
Mineralisers
These are minor components or additives in the raw mix in small amounts, which increase the activity of the clinker
phases, facilitate formation of clinker having adequate properties with lower grade limestone or at lower
temperatures. From material science, it is known that the activity of a single crystal may be enhanced by introducing
defects in the crystal structure. Extrinsic defects can be induced in the clinker phases by crystallochemical
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substitution forming solid solution, which contribute to the reactivity of the phases. Such levels of impurity-induced
defects can take place in the C3S and C2S phases in industrial clinkers
The role of mineralisers can be summarized under the following headings;
Fluxing action Temperature of first formation of liquid are reduced; the viscosity and surface tension of the liquid
are modified; and crystal morphology is modified.
Phase relations The relative thermodynamic stabilities of the clinker minerals are changed due to solid solution
effects;
Hydraulic activity The reactivity of clinker minerals (Alite and more particularly, Belite) are modified by solid
solution and/or crystal symmetry effects (defects).
In cement manufacture, liquid formation is assisted by traces of halides like Cl- and F-. Presence of chlorine (Cl-) in
the cement invites opposition from the users, because of risk of corrosion of reinforcing steel. Fluoride addition has
the unique combination of acting as a flux, lowering the temperature of melt formation, reducing the viscosity and
surface tension of the liquid and enhancing the thermodynamic stability of C 3S, enabling its formation attemperature < 1250OC. Formation of liquid may take place at lower temperatures also due to presence of sulphates
and carbonates, which have low melting point eutectics ( 800 - 900OC).
Refractory
In order to protect the steel shell of the kiln from the high temperature encountered during manufacture of cement, a
lining of refractory material is provided. The refractory materials are usually bricks of appropriate size and shape
so that they can be laid on the round periphery of the kiln. Alternatively, the material is cast in a plastic stage inside
the kiln and allowed to harden, much like laying concrete lining. These are called Castables.
The requirement of refractoriness varies from zone to zone in the kiln, depending upon the temperature. It is the
greatest in the burning zone. What helps the matter is the formation of a coating on to the refractory lining.Coating is a mass of clinker and the dust particles that adheres to the lining, having changed from a liquid or semi-
liquid to a solid state. A kiln feed with a high liquid content at clinkering temperature is more effective in coating
formation. This happens in easy-burning raw mixes with adequate amount of flux i.e. iron, alumina, magnesia and
alkalis. Ash in coal also helps in coating formation. The flame characteristics (to be discussed later) also have an
influence in coating formation.
Requirements of refractory From the above, it is clear that the refractory should possess the following
characteristics;
1. Resistance to temperature encountered in the zone,2. Spalling resistance ability to withstand high temperature changes inside the kiln, caused by start up, shut
down or upset operating conditions,
3.
Coatability ability to pick up a coating and maintain it,4. Abrasion resistance,5. Slag resistance, i.e. resistance against chemical attack from the materials inside the kiln.
Choice of refractory material depends upon test results and records of prior use. Tests are routinely conducted for
compressive and tensile strength, modulus of elasticity, chemical analysis, thermal conductivity, density, porosity
and gas permeability. Specific performance tests are for melting point, deformations under hot load, linear shrinkage
etc. One important test is called refractoriness under load (RUL) which gives value of deformation of the
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refractory brick when subjected to a specified temperature under a specified load for a specified time. The results of
hot load are expressed as the temperature at which a definite deformation takes place.
Types of Refractories In line with different temperature profiles, refractory materials of different chemical
composition and properties are used in different zones of the kiln system. The requirements of the burning zone are
most demanding. The following types are used in the burning zone;
Alumina silica group the main components being alumina and silica. Refractoriness increases withalumina content and high-alumina (Al2O3 > 70 percent) refractory is used in the burning zone. Withincreasing alumina content, strength, conductivity and spalling resistance also increase. The bricks aremanufactured by dry press or a stiff mud brick.
Basic group manufactured mainly from periclase (dense crystalline magnesia), dead burned magnesiteand chrome ore. For rotary kilns, majority of refractory used are of magnesia-chromium classification(Mag-Chrome) with periclase forming the major component (85 % +). In Chrome Mag bricks, chromeore forms the major component. In these formulations, periclase is responsible for high refractoriness andvolume stability and chrome supplies hot strength and spalling resistance. Basic refractories take to coatingformation in a better manner than high-alumina bricks as the coating fuses with the surface layer of therefractory and are thus preferred for the burning zone.
Dolomite bricks - composed mainly of CaO and MgO and have close affinity to the chemical compositionof the kiln feed. Once the kiln attains the operating temperature, they form a coating very rapidly.
Spinnel bonded bricks these are newer development, having 10 15 % alumina and 80 85 % MgO.They offer much longer service life than mag-chrome liners. However, these cost more than other bricks.
For other places, refractory materials of different compositions are used. Details are given in the Table below.
Service life and reasons of failure For a kiln operator, kiln shutdown due to refractory failure is the main concern.
This manifests as a red spot in the kiln shell, where shell temperature becomes excessive due to lack of protection
by the refractory lining. This can lead to warping of the kiln shell and replacement may be called for. The ideal
refractory life should coincide with the planned annual shutdown. Thus, service life of 11 or 23 months is ideal.
While in the cooler regions in the kiln system, refractory materials last for up to 5 years, in the burning zone, the life
Could be as short as a few days or extend up to two years. The reasons of unsatisfactory service life of refractory are
as follows;
Frequency of kiln shutdowns,
Too rapid heating of the refractory, Overheating the kiln, improper kiln operation, Quality of the refractory material, improper installations, Chemical composition and uniformity of the kiln feed, Mechanical condition of the tire and the kiln shell, and Poor location and poorly directed flame pattern, as discussed later.
Types of refractory material for different locations
Location Type and composition RUL,OC
Cooling zone High alumina (>70%) bricks or mag-
chrome bricks
1500
Burning zone Dolomite bricks (MgO >96%) 1600 - 1700
Transition zone Alumina or high alumina bricks; mag-
chrome bricks (MgO > 65%)
1600
Preheating zone Fireclay brick with Al2O3 content
decreasing towards feed end; lightweight
bricks
1350
Clinker cooler Acid fireclay brick 1300
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Fuels, Combustion and Flame Characteristics
For cement manufacture in India, solid fuels in the form of coal are traditionally used; elsewhere liquid (heavy oil)
or gaseous fuels (natural gas, LPG) are also used. In addition, the recent trend is to use waste materials having heat
value as alternative fuels in cement manufacture.
The origin of coal is from decomposition of vegetation with anaerobic bacteria and subjected to thermal andmechanical loads for centuries. The main constituents in coal as fuel are carbon, hydrogen, oxygen and sulphur,
besides moisture and ash. The combustible part of coal is composed of complex organic molecules. The volatile
matter, which is the matter released on heating to 800 OC, should be of the order of 18 to 22 percent, and ash content
low. A classification system for coal is on the basis of combustion behavior. It includes four main coal ranks,
starting from most difficult to ignite to the easiest; anthracites, bituminous, sub-bituminous and lignite in that
order. These are also according to their ages; anthracite being the oldest and lignite the youngest. With age and
residence time at high temperature and pressure, older coal formations have less volatiles and more fixed carbon,
thereby the difficulty in ignition. Some details of different types of coals and other fuels used in abroad are given
below;
Details of coal types and other Fuels
Group Moisture,
%
Volatile
matter, %
Fixed Carbon, % Ash, % Calorific value,
kcal/kg
Anthracite 2 3 12 88 67 7 18 7525 6800
Bituminous, 3 12 18 40 74 39 4 9 8000 - 6380
Sub-bituminous 16 - 25 28 34 39 8 12 5400 4800
Lignite 35 28 50 30.8 6 4000
Heavy fuel oil 0.1 0.2 86 # - 9500
Natural gas - - 58 # - 7500
Total carbon content
Indian coals are mostly sub-bituminous, while imported coals e.g. from Australia are bituminous. The ash content in
Indian coals is high and calorific value low. Although, 20 to 25 % ash content is considered satisfactory, often ash
content in excess of 30 35 percent and calorific value 3,500 kcal/kg are encountered. The coal ash getsincorporated in the clinker structure during sintering and should be taken into account in working out the raw mix
proportions. Chemical composition of ash is similar to that of fly ash discussed before, with SiO2 content in excess
of 55 percent. If the weight of coal used is about 20 percent of the weight of clinker, and the coal contains 25 percent
ash with 55% silica, about 2 - 3 percent SiO2 of clinker weight is contributed by coal alone.
Coal should be dried and pulverized to a fine powder before firing. The fineness of coal is expressed as residue in 90
sieve (R90). For coal with ash content up to 20 25 percent, it should be of the order of about 10 percent. Theresidue on 200 sieve should 1 to 2 percent and nil on 500 sieve. A rule of thumb for such coal (ash 20%) is R90= half of volatile matter. Lignite coals have high volatile matter (> 50%) and may pose explosion and fire hazard
during grinding and storage. Often, it may have to be flooded with inert gases.
Depending on its chemical composition (percent C, H, S, O and water, W), the net calorific value of coal can be
calculated as;
Hu = 33,900 * C + 1, 21,400 *(H 1/8 * O) + 10,500 * S 2,500 * W, in KJ / kg units,
Or
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Hu = 8,096 * C + 28,994 *(H 1/8 * O) + 2,508 * S 597 * W, in Kcal / kg units.
For proper combustion of fuel, supply of sufficient amount of oxygen (through air) and the required temperature to
ignite the fuel-oxygen mixture are necessary. As the fuel is injected in the system, it remains in suspension in the
kiln atmosphere. Sufficient time must be available to accomplish complete combustion while the fuel is in
suspension in the kiln. The requirement of oxygen for combustion is worked out as follows;
(i) C + O2 = CO2; 12 units of C combine with 32 units of O, i.e. C kg of carbon requires 8/3 * C kg ofoxygen.
(ii) 2 * H2 + O2 = 2 * H2O; 2 units of H combine with 16 units of O, i.e. H kg of hydrogen requires 8 * Hkg of oxygen.
(iii) S + O2 = SO2; 1 unit of S combines with 1 unit of O, i.e. S kg of hydrogen requires S kg of oxygen.Taking into account the amount of oxygen already present in the coal, combustion of 1 kg coal requires (8/3 * C + 8
*H O + S) kg of oxygen. I kg of air contains 0.23 kg of oxygen and 0.77 kg of nitrogen. Hence air requirement for
combustion of 1 kg coal is
(8/3 * C + 8 *H O + S) / 0.23 kg; i.e.
(11.6 * C + 34.78 * H 4.35 * O + 4.35 * S) kg of air.
In other countries having good quality fuel, on an average, 1 kg of fuel requires 10.5 kg of air. The coal required for
cement manufacture depends on the moisture content and heat value. It was mentioned that in India, the weight of
coal consumed in dry process precalcinator plants is about 20 % of the weight of clinker produced. This will give
some idea about the volume of air to be supplied for combustion of coal.
Incomplete combustion of coal due to inadequate supply of oxygen results in formation of carbon mono-oxide (CO)
rather than CO2, and only 31 percent of potential heat following complete conversion of C to CO2, is generated. For
satisfactory operations, it is essential that incomplete combustion must be avoided and there should be no trace of
CO in the kiln exit gases. If, in order to ensure complete combustion, the amount of air supplied is more than what is
required, presence of oxygen will be noticed in the exit gases. Such excess air means larger volume of air thanrequired has to be raised to the operating temperature in the kiln, less heat available for actual burning process,
lower flame temperature and greater heat loss.
Ideally, there should be neither any trace of CO nor of oxygen in the kiln exit gases. In practice, this is difficult to
achieve. The burning conditions in the kiln not being the ideal, often traces of CO and free oxygen are found
together in the exit gas. A practical solution is to work on a range of free oxygen in the exit gas being not less than
0.7 percent, or more than 3.5 percent under stable operating conditions, as shown in the figure below. The most
desirable range of oxygen in the kiln exit gas is between 1 to 1.5 percent.
Flame characteristics
To an experienced kiln operator, the flame is characterized by many parameters length, shape, point of ignition,
direction and colour. A typical flame is sketched below, which shows that on leaving the nozzle of the burner pipe,
the flame first has a plume, followed by the point of ignition and then, finally the flame. The length could be the
total flame length from the tip of the nozzle to the end or the ignited flame length from the point of ignition.
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The percentage of combustion air present and the velocity of fuel-air mixture at the tip of the burner are the main
factors influencing the length of the flame. It is always preferable to have as short a flame as possible, with the fuel
getting ignited soon after it leaves the burner. Ignition takes place in the burning zone and the flame extends to the
calcining zone. Lack of combustion air makes the flame long, as it searches for oxygen further back in the kiln.
A high tip velocity of about 70 80 m/s gives exceptionally good flame pattern, as it happens in indirect firing. If
the velocity is about 45 66 m/s, as it happens in case of direct firing, a lazy swirling flame results. In any case, the
tip velocity should be more than 35 m/s to keep the coal particles in suspension in the circular burner pipe and not
allow them to settle down. The quantity of total air primary air, secondary air and any parasite air due to leakages,
is controlled by the speed of ID fan; the greater the speed, more is the air. The primary air velocity should be twice
that of flame propagation speed, so as to prevent a flashback of the flame. Generally, under stable operating
condition, there is no change in the flame length, as long as the air-fuel ratio is maintained constant.
The initial ignition of the fuel is primarily dependant on sufficient heat to ignite the fuel and on sufficient air to
obtain combustion of the fuel. During the initial start of the kiln, when the kiln temperature is too low to ignite the
fuel, a pilot burner (auxiliary torch) is placed at the mouth of the burner pipe to obtain good ignition. The
temperature of the flame is given by the relation;Hv
T = -------------------,
1.11 * A *s
Where,
T = Flame temperature, OC,
Hv = Heat value of the fuel, kcal/kg,
A = amount of air required for combustion, kg/kg of fuel,
s = specific heat of combustion gas, adopt 0.29.
The values obtained by this calculation are of the order of 2000 to 2650 OC. In practice, lower values are obtained
and the colour of the flame changes. The following is a guide;Temperature of flame and colour
Flame Temperature,OC Colour of the
flame
White to dazzling white >1540
Light yellow to white 1320 1540
Yellow to light yellow 1090 1320
Orange to yellow 900 1090
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Bright cherry red to orange 825 900
Cherry red to bright cherry red 750 825
Dark red to cherry red 650 750
Lowest visible red to dark red 475 650
Lowest visible red 475 >
Heat Transfer
Total heat transfer in the entire kiln system involves all the three modes conduction, convection and radiation.
Some examples are;
Radiation: Flame ----------------------- Feed bed
Hot kiln wall ---------------- Feed bed
Conduction: Heat from kiln interior --- Kiln shell
Kiln chains ----------------- Feed bed
Convection: Hot kiln exit gases ------- steam in boiler tubes.
Heating the feed bed inside the kiln is primarily by radiation. In the burning zone, the kiln feed in a sticky
condition is in a constant state of agitation. Aided by the rough and uneven surface of the coating, it rises along the
upward-moving side of the kiln and then tumbles back. The hot particles coming in contact with colder parts,
transfers heat by conduction. Meanwhile, new particles are exposed to heat transfer by radiation. The coating
receives heat from hot gases by radiation. Part of the heat then radiates to the bed of the feed and part of the part is
transferred by conduction as the feed comes in contact with the wall tumbling back. As a result, the temperature of
the wall is lowest when it emerges from the bed and highest immediately before it comes into contact with the feed
bed (see figure below).
Heat exchangers in the form of chains were common in wet process plants and sometimes used in dry process plants
also. The chains get heated by the gases and transfer heat to the feed by conduction, as they come in contact. A
similar action takes place when heat is transferred from the kiln wall to the feed. The flame radiates heat to the
coating. Part of the heat is radiated to the feed bed and part is transferred by conduction, when the feed comes in
contact with the wall. The heat transfer in Preheater tower is more efficient, as the time taken is small. The
difference between the material and gas temperatures is small.
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CYCLONE SEPARATORS
Definition:
Cyclone separators are devices that utilize centrifugal forces and low pressure caused by spinning motion to separate
materials of differing density, size, and shape. Gas cyclones are widely used in industry for the separation of
particles from gas and air streams. Cyclones are popular because they are simple and inexpensive to manufacture,
require little maintenance, contain no moving parts, and have the ability to operate at high temperatures and
pressures.
Operating Principles:
The gas is injected at high velocities through the inlet pipe, which is positioned tangentially to the body of the
cyclone. The shape of the cone induces the stream to spin, creating a vortex. Larger or denser particles are forced
outward to the walls of the cyclone where the drag of the spinning air as well as the force of gravity causes them to
fall down the sides of the cone into an outlet. Meanwhile the lighter and/or less dense particles (gas) exit through the
top of the cyclone via the low pressure center. The separation process in cyclones requires a steady flow, free of
fluctuations or short term variations in the flow rate. Cyclone separators are customarily operated with the top andbottom open to the atmosphere so that there is no pressure difference between the two.
Sizing and Selection:
There are many sizes and types of cyclone separators available. The two main types of cyclones are axial and
tangential. They both operate on the same principles; however, in the axial flow cyclones the material enters from
the top of the cyclone and is forced to move tangentially by a grate at the top. In tangential cyclones the material
enters from an inlet on the side which is positioned tangentially to the body. Axial flow cyclones are the most
commonly used. The sections of cyclone separators are manufactured in varying proportions of the body diameter.
The efficiency of cyclone separators is dependent upon the cyclone diameter and the pressure drop between the inlet
and outlet of the cyclone.
Types of CementPortland cement
1. Ordinary Portland cement (OPC)2. Rapid hardening Portland cement3. Portland slag cement (PSC)4. Low heat Portland cement5. Portland Pozzolana cement (PPC)6. High strength Portland cement7. Super sulphate cement8. High alumina cement9. White Portland cement10. Colored Portland cement11. Hydrophobic cement
Special Cement
1. Quick Setting cement2. Calcium Chloride cement3. High alumina cement4. Slag cement
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5. Expansive cementTechnical Details of cement
1. Specific Gravity= 2.642. Weight per Cubic meter
Loosely filled =1120Kg Tamping and filled as per IS: 1911=1602 Kg 1 liter of cement=1.44kg
3. Co efficient of thermal Expansion= Varies from 11x10^-6 to 20x10^-6 /c4. Fineness of cement= should not exceed more than 10%5. Soundness of cement=Expansion should not exceed 10 MM6. Initial setting time= Not be less than 30 Min7. Final setting time=Not be more than 600 Min
Reduction in Cement Strength Due To Storage
S. No. Storage Period Reduction in Strength
1 Fresh Cement Nil
2 Three months old 20%
3 Six months old 30%
4 12 months old 40%
5 24 months old 50%
Lab Test
Fineness of Cement
Aim: To determine the fineness modulus of the Cement
Apparatus: IS 90 micron sieves, balance with weights 100gm of cement etc.Theory: The purpose of this test is to detect the defect in grinding process of cement; finely grained cement shouldbe preferred for superior works. The residue on 90 Micron sieve weights more than 10% the cement should berejected ,use finally grained cement provides larger surface area higher will be the rate of hydration and hence fasterthe development of strength.Procedure:
1. Weigh accurately 100 gm of cement and take it on a STD IS Sieve.2. Powder all the coarse lumps with finger and sieve the sample continuously for 15 Min3. Weigh the residue left on the sieve this should not be more than 10% of original weight.
Observation
Sl No Description Weight
1 Wt of cement taken W1 gms 100
2 Wt of residue left in the sieve W2 gms 7
3 % of residue by weight or fineness=((W2/W1)x100) 7%
4 Fineness modulus of cement=Fineness/100 0.07
Results:The fineness modulus of given cement is 7%
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Conclusion:The Fineness of cement should not Exceed 10% otherwise it will be rejected.
Normal Consistency test on cement
Aim: To determine the normal consistency of Cement using STD vicat apparatus
Apparatus: ARE sieves, balance gauge measuring jar vicat apparatus, plunger, mould etc.Theory: For finding out initial setting time, final setting time & soundness of cement a parameter known as stdconsistency has to be used. The consistency of cement paste is defined as that consistency which will permit a vicatplunger, having 10 mm diameter to penetrate to depth of 30mm - 35mm from top of the mould i.e. 5mm from thebottom of the mould.Procedure:
1. 400gms of cement passing through Is Sieve is taken &stirred with 25% of water in 35 mingauging time.
2. Vicat mould resting upon a non porous plate is filled with cement paste the top surface issmoothened
3. This complete assembly is placed under the rod attached to the std consistency plunger; theplunger is lowered and made to touch the top of the surface mould
4. The plunger is released & penetration with the top of the surface of the mould is recorded5. The procedure is repeated with different pastes with varying % of water until the plunger
penetrates to depth of 33.5mm from the top of the surface of mould6. The % of water that allows the plunger penetration 33.5mm from the top is recorded as normal
consistency.Observation
S. No Description Remarks
1 Wt of cement taken gms 400 gms
2 Normal consistency plunger length 50 mm
3 Normal consistency plunger diameter 10 mm
4 Cement mould diameter 75 mm
5 Cement mould depth 40 mm
Results:
Normal Consistency of cement =30.5%Conclusion:The normal consistency varies as changes from cement & depends upon humidity of the testing room.
Setting time of cement
Aim: To determine the initial & final setting time of cementApparatus: Balance gauge measuring jar vicat apparatus with initial and final setting time needle, plunger, mouldtraveler etc.Procedure:
1. Weigh about 400 gms of cement
S.NoWt of cement in
gms
% of water
added
Vol. of water
added in ml
Penetration from bottom in
mould in mm
Normal
consistency
1 400 25% 100 28
2 400 27% 108 34
3 400 30% 120 32 30.50%
4 400 31% 124 40
5 400 30.50% 122 45
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2. Measuring 0.85 times of the percentage of water as determined in consistency test3. Mix the paste well & fill it to the mould4. Attach a square needle of c/s area 1x1mm to the moving rod of vicat apparatus5. Quickly release the vicat rod and allow the needle to penetrate the cement paste in the beginning.
The needle is penetrate completely it is taken out and set a fresh plane. The procedure is repeatedat regular intervals until the penetration is 5mm from the bottom of mould
6.
The time taken to achieve should not be more than 30 min 7 is called initial setting time.7. The time after which the needle falls to make an impression on the surface is the final setting timeObservation
Sl No Description Remarks
1 Wt of cement taken gms 400 gms
2 Water to be added =Normal consistency x Wt of cement=(30.5/100) X 400) x .85 103.7 cc
3 Time when water is added 3.50 pm
4 Time when needle did not pierce 4.05 pm
5 Initial setting time 19 min
6 Time when the needle false fails to make an impression 4.09 pm
7 Final setting time 6 Hrs
Results:Initial setting time of cement=19 minFinal setting time of cement=6 hrs
Soundness of Cement
Aim: To determine the soundness of the Cement using Le-Chatelier's apparatus.Apparatus: Le-Chatelier's apparatus, two glass plates, temperature controlled water bath, Scale.Theory: Excess of free lime and magnesia present in cement slakes very slowly and cause appreciable change involume after setting. In consequence cracks, distortion and disintegration results, thereby giving passage to waterand atmospheric gases which may have injurious effect on concrete and reinforcement .the expansion is prevented
by limiting the quantity of free lime and magnesia in cement.Procedure:
1. Take 100gms of cement with .78 times the water required to make a paste of Standardconsistency.
2. Mix the paste thoroughly and fill the Le Chatelier's apparatus with it.3. Place this arrangement between two glass plates and immerse the entire assembly in water for 24
hrs.4. Remove it from water and measure initial the distance between two pointers.5. Transfer the arrange into temperature controlled water bath and keep it boiling for 3 hrs.6. Remove the mould from water and allow to cool and measure the final distance between two
pointers.7. The difference between initial and final distance gives the soundness value of cement.
Observation
Sl No Description Remarks1 Weight of cement taken 100 gms
2 Water to be added by volume V=.78x30.5 24 cc
3 initial distance between two needles after immersions for 24 hrs in water D1 14 mm
4 ) Final distance between two needles after bath D2 16 mm
5 The difference between initial and final distance =D2-D1 2 mm
Results:
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The Soundness of given cement is= 2 MMConclusion:
According to I.S specification the expansion should not exceed 10 MM
Compressive strength of cement
Aim: To determine the Compressive strength of cement.Apparatus: UTM, Balance