CEMENT MANUFACTURING PROCESS

Definition:

• Defined as a product material obtained by calcination ofcalcareous (a material containing lime) and argillaceous(a material which contain silica) materials.• According to (specification stand as) ASTM (Americanstandard testing material)

“Cement is defined as a product obtained bypulverizing (crushing followed by grinding) clinkers towhich no addition has been made except H2O.”

Composition

• Chemically cement is a mixture of Silicates, Aluminateslike aluminoferrites with some amount of MgO andgypsum.

• Various complexes which are contained in cement are• (2CaO.SiO2) Di Calcium Silicate (C2S) 22-24%• (3CaO.SiO2) Tri Calcium Silicate (C3S) 50-58%• (3CaO.Al2O3) Tri Calcium Aluminate (C3A) 10-11%• (4CaO.Al2O3.Fe2O3) Tetra Calcium Aluminoferrite (C4AF) 8-9%• (6CaO.Al2O3.Fe2O3) Hexa Calcium Aluminoferrite (C6AF) traces• MgO <5%• Gypsum (CaSO4.2H2O) 4-5%

Various oxides

• Cement contains various oxides as follows

1) CaO 60-66%

2) Total silica 20-15%

a) Combined silica/active silica

b) Uncombined or inert silica

3) Alumina (Al2O3) 3-8%

4) Fe2O3 1-5%

5) MgO <5%

6)Alkalies <5%

Color

• The gray color in cement is due to presence of complexesof Ferric ions in O.P.C (ordinary Portland cement).

• White cement is produced from those raw materials whichcontain extremely low conc. of Iron & even like low conc.of Iron is converted from Fe+3 to Fe+2 (which can be doneby reducing or maintaining atmosphere in the kiln) &eventually the product appears to be white.

Raw materials used in the production ofO.P.C:

1) Lime stone

2) Clay

3) Gypsum

Lime stone:

• Lime stone on calcinations gives CaO which is an essentialcomponent of all the complexes (silicates, aluminates andaluminoferrites) present in cement. Lime stone is availablein two lime forms.

A) High lime:• High lime is that lime stone which on calcination gives

CaO of greater than 90% purity. The high lime containMagnesia (MgO) less than 5% and rest of the impuritiesmay be alumina, Fe2O3,SiO2.High lime is available inPakistan in the area of Banu District Kohat ,D.G khan ,D.I khan ,Salt area of Mianwali.

Cont..

B) Low lime• Low lime means the limestone which on decomposition

yields CaO of less than 90% purity. Alumina & MgO ispresent to an extent of greater than10%.

• Dolomite rich in MgCO3 should not be used in themanufacture of Portland cement.

Clay

• The complexes such as silicates, aluminates & aluminoferritesall contain SiO2, & Al2O3.These are added into O.P.C by clay.

• Ordinary clay found in the area close to the mines of lime stonehave high quality clay.

• The clay need for all production of O.P.C should meet thefollowing requirements.

1) It must be high in silica contents

2) Comparatively low in the amounts of Al2O3

3) Low in the amount of alkalis

• In case if clay doesn’t conform to the required specifications ofoxides then the clay has to be beneficiated before use.

Gypsum

• When H2O is added into the cement & it is not thoroughlymixed then tricalciumaluminate picks up H2O and rapidlyhardens. This phenomenon is called as flash setting.

• The flash setting in cement is retarded by the addition ofGypsum which forms the following complex(3CaOAl2O3.3CaSO4.31H2O)

• The time of hardening of cement is increased. Thus theGypsum containing cement gets hardened in comparativelylonger period of lime.

Fuel

• Prior to 1971 the cement industry used furnace oil as fuel incement industry. As an higher energy consuming industry inwhich energy is required for crushing, grinding, mixing &running of various units.

• Prior to 1971 most of the industries in Pakistan usedfurnace oil as a fuel. However after 1971 the majority of thecement industries was switched over to natural gas.

• In the past few years the importance of natural gas wasrealized for the production of petrochemicals and the cementplants were converted into furnace oil fired units now mostof the cement industry is being switched over to using coalas a fuel.Coal is available in Pakistan in Baluchistan, area ofMakarwal (Mianwali district)

• The indegeneous coal contains high % of organic as wellas inorganic sulphur which in some cases is as high as6.8%.

• High sulphur contents make the Pakistan coal inferior asthe S compound lower the calorific value of coal.

• If coal contains high contents of “S” is burnt, proportionalhigher quantity of SO2 or SO3 will be added into theexhaust of flue gasses.

Water

• If cement is produced by wet process large quantity of H2Ois needed. The H2O should not be saline in nature.

Various steps involved in theProduction of cement:

• Cement production involves following steps

1) Crushing & grinding of clay and lime stone.

2) Mixing of ground raw material to obtain an almosthomogeneous mass.

3) Transportation of material ground raw materials to rotarykilns & burning of raw materials to clinkers.

4) Grinding of clinkers in the presence of CaSO4.2H2O

• Step no 3 & 4 are as like on all the cement plants howeverstep no 1 & 2 may be carried out in two ways.

a) The crushed materials is ground & mixed in the presenceof H2O to obtain slurry containing 40%H2O &60% solids.This method is called as wet method.

b) If the crushed material is ground & mixed in absence ofH2O & dry homogenized mass is fed into the kiln forburning. The process is called as dry process.

Comparison between wet and dry processes:

Wet process1. Soft raw material is used

2. The crushed material is ground inthe presence of H2O to form aslurry paste .The slurry is thanfed in to the rotary kiln for themanufacture of cement

3. Grinding is done in rotary ballmills in the presence of H2O toobtain slurry of ground lime stone& clay.

Dry process1. Hard or soft any type of raw

material is used for theproduction of cement.

2. The crushed material is groundin dry &the feed given to the kilnis in the form of dry powder.

3. The grinding is conducted in theroller mill which combinesgrinding drying as well asclassification. In that roller millsparticles of dried size arecollected through aclassification.

4. Fuel consumption iscomparatively more as the slurryadded in the rotary kiln is 30-40%H2O and 60-70% solids.Substantial amount of heat is usedin evaporation of H2O

5. Exposure to the effect of heat isfor a longer period and hence theoperating life of the lining of thekiln is less.

6. The length of the rotary kiln ismore which may be between 300-400 ft usually and it may besometime even as long as 500 ft.

4. Since water has not to beevaporated so less fuelconsumption.

5. Exposure to the effect of heat isless & hence the operating life ofthe lining of the kiln is more.

6. The length of kiln is usuallyabout 150 ft which may in somecases be 250 ft.

7. The cost of insulation is high.

8. Thermal efficiency is higher asthe flue gases leave the kiln atabout 1000C.

9. The loss of raw material in theform of dust contained in the fluegas coming out of the kiln is less.It amounts to about 1.5% of theclinker produced.

7. The cost of insulation of kiln isless.

8. Thermal efficiency in the kiln isless as the gases leaving therotary kiln are about 300oC.

9. The loss of the raw material in theform of dust is much higher andabout 15% raw material is lostwith the flue gas coming out ofthe kiln.

10. The dust particles leaving withthe flue gas are recovered byinstalling scrubbers, cycloneseparators and by installinghigh density electricalprecipitators.

11. The recovery of heat from theflue gases is less.

10. Dust contained in the flue gas isrecovered in the same way as inwet process.

11. The heat recovery in dry processis 20% more in this process.

Crushing & grinding of clay and limestone

• The limestone and the clay needed for the production ofcement as excavated from the quarries are crushed toapproximately a particle size of 100mm using crusherswhich are called as the primary crushers.

• The secondary crusher installed in the same area crushesthe limestone and the clay to obtain a particle size of about10mm.

• In the wet process, the crushed lime and clay is fed into arotary grinding ball mill in the presence of water. Thegrinding is carried out using the technique of close circuit.

Mixing of ground raw material toobtain an almost homogeneous mass

• The lime and clay obtained from the classifier is received ina correcting tank where the composition w.r.t. limestoneand clay is adjusted. The slurry should contain 65-70%limestone and 30-35% clay in it. In case any of the two isless in quantity, the difficulty is made up by adding requiredquantity of deficient material.

• Then slurry containing corrected limestone and clay ispumped to a dark thickener where excess of water of any isremoved which in turn is reused for grinding in the ball mill.The slurry obtained from the dark thickener should contain30-40% water and 60-70% of the solids. This slurry is thenintroduced into the kiln for pyro processing (heat treatment).

Transportation of ground raw materials torotary kilns & burning of raw materials toclinkers

Construction and working of the rotary kiln:• The pyro-processing of slurry obtained in the wet method

and the dry ground raw materials in the dry process iscarried out in a rotary kiln. It is made from sheets of plaincarbon steel riveted together to obtain a shell.

• In the old versions the rotary kiln tapered towards the top.Now a days large no. of cement industry is using rotarykiln of uniform internal diameter.

• In the old versions the rotary kiln was placed in a way thatit made an angle of 15 degrees with the horizontal whichfacilitates the movement of material from the feed end tothe discharge end.

Construction and working of the rotarykiln:

• Now a days the inclination of shell with the horizontal hasbeen reduced to about 5-6 degrees. To facilitate themovement of the material from feed end to the dischargeend, chains are installed inside the rotary kiln.

• The kiln rotates in the clockwise direction by standing atthe discharge end. The revolutions of the kiln about its axisranges between half to one rpm in some cases, while in theothers it may be 1-3 rpm.

• This depends upon the size of the kiln which may be 300-400 ft in case of wet process and up to 150 ft in dryprocess. From the feed end to the discharge end the totaltime taken for the movement of material is between 2-3hours.

Cont..

• The temperature maintained inside the rotary kiln rangesbetween 300-1400 degree Celsius in dry process and

100-1400 degree Celsius in wet process.

• At elevated temperature, calcium oxide behaves as a verystrong base and it can attach the material of construction ofthe shell of rotary kiln. It is therefore necessary to takeappropriate measure to increase the working life of rotarykiln. For this purpose, lining is provided on the interior ofthe shell of rotary kiln.

Merits of Lining in rotary kiln

• A lining provided in the rotary kiln has the followingmerits:

1. The chemical attack of calcium oxide leading to corrosionof material of construction of shell be eliminated.

2. The lining used is of a refractory material which processesvery low thermal conductivity hence heat losses fromrotary kiln to atmosphere would be reduced.

Cont..

• The ordinary fire bricks which contain a very highpercentage of silica cannot be used for the purpose oflining as silica contained in it may react with CaO to formCaSiO2 which may solidify inside the kiln forming ringswhich may obstruct the movement of material inside thekiln from the feed end to discharge end.

• Usually high aluminium bricks produced from bauxite areused for the purpose of lining. In the old version of therotary kiln the lining of 9 inches were provided from first100 ft staring from discharge end where as the next 110ftwas lined with 7 inches thick brick of bauxite. Now a daysthe kiln usually processes a uniform lining.

Temperature Zones of Rotary kiln

Temperature Range

1. 100-300 0C

2. 300-600 0C

3. 400-800/900 0C

Working

1. Evaporation of water. This zoneshall not be existed in dryprocess.

2. (a)Evaporation of combined watermeans chemically bound water.

(b)Decomposition of carbonatestarts.

3. Burning of organic matter.

Cont..

4. 600-900oC

5. 900-12000C

4. Volatilization of alkalis.Volatilization of chlorides.Evolution of CO2 from CaCO3

and the existence of free CaOstart.

5. In this zone, main reactionbetween lime and clay takes placewhich is further subdivided intothe following:

a) 900-11000C (Formation oftricalcium aluminate andtetracalcium aluminoferrite.)

b) 1100-12000C (Max. formation oftricalcium aluminate anddicalcium silicate.)

Cont..

6. 1250-12800C

7. 1280-14000C

6. Commencement of liquidformation.

7. Further formation of moltenmass and the formation of mainconstituent of cement i.e.tricalcium silicate. In this rangemost of CaO from thedecomposition of lime stone getsassociated to form variouscomplexes.

Clinker Formation

• The product obtained due to high melting temperature getsconverted into the form of circular balls called as clinkerswhich vary in this size between 1/8 to 3/8 inches, theclinkers are collected in a cooler where they are cooledusing a draft of cold air. Wet air gets heated to 700-8000Cand used in the burning of fuel in the rotary kiln.

• The flue gases leaving at the feed end pass through anintegrated system of scrubbers, cyclone separator andelectrical precipitators to recover the dust particles of theraw material.

CLINKER GYPSUM

Grinding of the clinkers:

• The clinkers are ground in the presence of 4-5% gypsum toobtain fine powder which may confirm to the followingspecification:

• ASTM

The 78% of the ground material should pass through 200mesh sieve.

THE CEMENT MANUFACTURING PROCESS

1. BLASTING : The raw materials that are used to manufacture cement (mainly limestone and clay) are blastedfrom the quarry.

Quarry face

1. BLASTING 2. TRANSPORT

3. CRUSHING AND TRANSPORTATION : The raw materials, after crushing, aretransported to the plant by conveyor. The plant stores the materials before they arehomogenized.

quarry

3. CRUSHING & TRANSPORTATION

2. TRANSPORT : The raw materials are loaded into a dumper.

crushing

conveyor

Next

dumper

storage atthe plant

loader

THE CEMENT MANUFACTURING PROCESS

1. RAW GRINDING : The raw materials are very finely ground in order to produce the raw mix.

1. RAW GRINDING

Raw grinding and burning

2. BURNING

2. BURNING : The raw mix is preheated before it goes into the kiln, which is heated by a flame that canbe as hot as 2000 °C. The raw mix burns at 1500 °C producing clinker which, when it leaves the kiln, israpidly cooled with air fans. So, the raw mix is burnt to produce clinker : the basic material needed tomake cement.

conveyor

Next

Raw mix

kiln

cooling

Back

preheating

clinker

storage atthe plant

Raw mill

THE CEMENT MANUFACTURING PROCESS

1.GRINDING : The clinker and the gypsum are very finely ground giving a “pure cement”. Other secondaryadditives and cementitious materials can also be added to make a blended cement.

1. GRINDING

Grinding, storage, packing, dispatch

2. STORAGE, PACKING, DISPATCH

2. STORAGE, PACKING, DISPATCH :The cement is stored in silos before being dispatched either inbulk or in bags to its final destination.

Back

clinkerstorage

Gypsum and the secondary additives are addedto the clinker.

silos

Home page

dispatch

bags

Finish grinding