WELCOMETOLECTURE ON PROCESSFOR NONPROCESSENGINEERSK.P.Pradeep kumar
Cement is a substance (often a ceramic) that by a chemicalreaction binds particulates aggregates into a cohesive structure.( hydraulic binder). The quality of raw material is the main pointin maintaining of quality of cement. The mineral compoundscontaining the main components of cement: lime, silica, aluminaand iron oxide are used in cement manufacturing process.Therefore it is usually necessary to select a measured mixture of ahigh lime component with a component which is lower in lime,containing however more silica, alumina and iron oxide(clay component). The purpose of calculating the compositionof the raw mix is to determine the quantitative proportions of theraw components, in order to give the clinker the desired chemicaland mineralogical compositionWhat is cement ?
In 1824, Joseph Aspdin, a British stone mason, obtained a patent for acement he produced in his kitchen. The inventor heated a mixture of finelyground limestone and clay in his kitchen stove and ground the mixture into apowder create a hydraulic cement-one that hardens with the addition ofwater.Aspdin named the product portland cement because it resembled astone quarried on the Isle of Portland off the British Coast. With this invention,Aspdin laid the foundation for todays portland cement industryHistory of CementManufacture of cement has a history, which traces back to millennia. TheRomans who were prolific builders used burnt calcareous (calcium bearing)rocks along with pozzolanic materials in an era Before Christ. The structuresbuilt by them, like the Pantheon, are still there for us to see proving thegoodness of cementitious materials as input material for construction. TheRoman called it as Opus cementum and pozzalana as Pozzolui.Post industrialization and as infrastructure development started globally,demands for cement have been growing steadily both quantitatively &qualitatively.
BackgroundAlthough the use of cements (both hydraulicand non-hydraulic) goes back many thousandsof years (to ancient Egyptiantimes at least), the first occurrence of portlandcement" came about in the 19th century. In1824, Joseph Aspdin, a Leeds mason took out apatent on a hydraulic cement that he coined"Portland" cement (1824) He named thecement because it produced a concrete thatresembled the color of the natural limestoneQuarried on the Isle of Portland, a peninsula inthe English Channel Since then, the name"portland cement" has stuck and is written in alllower case because it is now recognized as atrade name for a type of material and not aspecific reference to Portland, England.
few years later, in 1845, Isaac Johnson made the first modernPortland Cement by firing a mixture of chalk and clay at much highertemperatures, similar to those used today. At these temperatures(1400C-1500C), clinkering occurs and minerals form which are veryreactive and more strongly cementitious.While Johnson used the same materials to make Portland cement aswe use now, three important developments in the manufacturingprocess lead to modern Portland cement:- Development of rotary kilns- Addition of gypsum to control setting- Use of ball mills to grind clinker and raw materialsRotary kilns gradually replaced the original vertical shaft kilns used formaking lime from the 1890s. Rotary kilns heat the clinker mainly byradiative heat transfer and this is more efficient at highertemperatures, enabling higher burning temperatures to be achieved.Also, because the clinker is constantly moving within the kiln, a fairlyuniform clinkering temperature is achieved in the hottest part of thekiln, the burning zone.
Raw materials required to make cement• Lime stone ( calcareous material , Calcium carbonate)• Shale , low grade lime stone , clay ( argillaceous materials, Silica)• Aluminous mateial ( clay, bauxite or Laterite)• Ferrous material, haematite ( iron ore , ferric oxide )
Cement quality – type of cementClinker qualityFuel chemistryRaw mix designOPC, PPC, WC, OWC, SRC,SCOrdinary portland cement,Pozalona portland cementWhite cement,Oil well cement,Sulfate resistant cement,Slag cementOther cements for special applicationGpsum&fly ash orOther additive quality
qualityFactors influencing the cement quality1. Mechanical handling of clinker2. Chemical and mineralogicalcomposition of raw mix3. Chemical and mineralogical compositionof clinker4. Burning process & cooling process5. Chemical composition of fuels (ash)6. Circulation phenomena (volatiles)
Mining• Core drilling (bore) holes to explore the mines• Drill holes for blast• Blasting• Excavation and haulage• Transportation to crusher• Size reduction for process requirements
MiningQuarry planning• Ensure - that the required quality & quantity ondaily / monthly/ yearly is available to the plant• Minimise – total operating and capital costs• Optimise - raw feed quality• Fulfil – all the safety and legislative requirement• Maximise – return on capital employed• Achieve - peak quarring / plant efficiency
overburdenGood quality lime stoneModerate qualityPoor qualityoverburdenGood quality lime stoneModerate qualityPoor qualityselfish mining – short term benefitsEfficient mining –for well blended – long term benefitsWell developed mineLong term benefitsBench = 10 M
Picture of a well developed mineAll benches are used effectivelyto improve the mine blend andincrease the reserve for long termbusiness
Mining on hill & under ground miningis challenge to the mining engineer
The released energy of the explosiveis converted into various other formsof energy• heat• seismic energy ( stress waves)• new surface energy ( rock fragmentation)• concussion and noise ( airblast)Explosion• kinetic energy of spoil ( throw) rockdisplacement
CrushingCrushing is a process which does size reductionCrushers are chosen depending upon the material characteristicssuch as hardness ,abrasiveness, feed input size,moisture content etcThe commonly used crushers are hammer crushers,Impact crushers, roll crushers, gyratory crushers, jaw crushers.Size reduction depends upon the grinding system to be adoptedie., ball mill or vertical millSize reduction ratioMax feed size ( linear edge dimension)Maximum feed size of crushed product=---------------------------------------------------( linear edge dimension)
Advantage and disadvantages of circular and linear pilesCircular pileAdvantages• space saving and hence low capital cost• end cone problem is avoided• un interrupted operationDisadvantages• pile correction is not possible and it depends on mines operationwith less variationLinear pileAdvantages• it occupies more space• while shunting the operation has interruption• end cone problemDisadvantages• Pile correction is possible if quality varies
Chevcon method ( at Ariyalur)Chevcon - was developed for a circular stockpile arrangement.the stacker boom slews back and forth over the curved stockpileridge maintaining a constant pile length. With each individualmovement, the end of one movement or the start of the next movementis advanced by the dimension ∆L. In that way many layers - similar tothe Chevron mode - are superimposed and the stockpile growscontinuously in one direction.Chevcon configuration refers on to circular stock piles and relates to Chevronwhen it is applied to a circle. In this cofiguration the chevcon layers areinclined as in the side of a cone , each layer runs from the full height of thestock pile to the ground
Well blended slice without end coneEnd coneproblemsLinear stock pileBlending ratio = S in / S outMore variation, high stdLess variation, low std
X (t)Quantity(t)X (t)ReclaimingSlices transverselyStacking inEqual layersMaterial quantityPer layer = tMaterial quantityPer slice = qVariations in the raw material composition homogenisedin the blending bed∆τ ∆τ∆Q ∆Q∆Q ∆Q∆τ∆τ∆τ
Assessment of blending methodS inS outBlending ratio =S inS outBlending efficiency n n = number of layersn = V*(S*3600) / dd = volume discharged cum/hrS = cross sectional area, sq mV = travelling speed of the stackerη
Homogenising systems3.1 Variabilitv and standard deviationThe normally accepted method of measuring variability is in the form of aterm called standard deviation. The standard deviation of a property canbe calculated by taking a number of measurements on the property (suchas LSF, SR etc.), and applying the following formula:-Where X is the measured variable (e.g. LSF)X is the variable mean (or average)N is the number of measurements or observationsTable 1 illustrates a worked example using actual kiln feed LSF data:-Blending ratio = Std in/ Std out , = 1 for an ideal blending system.σ =Σ ( X - X ) 2N - 1
Main parameters for raw mix designLime saturation factor = CaO / (2.8 SiO2+1.65Al2O3 + 0.65 Fe2O3)( LSF)Silica modulus = SiO2 / ( Al2O3+Fe2O3)Alumina modulus = Al2O3 / Fe2O3AlMHere we have apply the formula (as per British Standard)CaO-0.7SO3(2.8*SiO2 + 1.2* Al2O3 + 0.65*Fe2O3)(SIM)LSF =
Lime saturation factor on clinker basisIf MgO is below 2 %LSF = 100( CaO – free CaO+0.75 MgO)(2.85 SiO2) + ( 1.18 Al2O3) +(0.65 Fe2O3)If MgO is above 2 %LSF = 100( CaO – free CaO+1.5 MgO)(2.85 SiO2) + ( 1.18 Al2O3) +(0.65 Fe2O3)95 –harder to burn, tendency to high free lime & C3S clinker, high early strength high fuel consumption< 95 , easy to burn , excess coating , excess liquid phase ,possible brick infiltration reduced cement strength , low free limeacceptable standard deviation = 1.2
Advantages of vertical mills• Energy consumption is less compared to ball mills• Flexiblity in operation as all forces can be controlled• Drying capcity is better than ball mills• Noise level (noise pollution) is much less than ball mills• Particle size distribution better than ball mills
Roller presscompressed & Caked materialCompression zone
In homogeneous homogeneousKiln feed uniformity index (KFUI)KFUI= n ( C3S actual - C3S target )2ni - nC3S actual = the calculated C3S of one instantaneous daily sample of kiln raw mix feedC3S Target = the C3S target established for the mill productn = number of samples ( calculation of average C3S is done monthly)Target for KFUI is < 10( an instantaneous sample is one made up of 5 consecutive increments taken at short intervals)
PreblendingVariation is a devil in any process
Blending siloThe efficient blending silo does efficient blending withminimum energyThe variation in chemistry at the silo outlet is to be at theminimum possible ,Standard deviation of LSF < 1Standard deviation of CaO < 0.2Standard deviation of Silica ratio < 0.1Standard deviation of A/F < 0.01
Flow properties of powders :• Importance of measuring flow properties• Various problems in powder handling and storageArching Channeling Segregation
Controlled flowinverted coneblending SiloCapacity = 18000 t18 M40 MAdvantages• low inventory• low capital costDisdvantages• can not be operated on low stock as raw mill operation directly affectsilo effciency and hence the quality and production.• as the buffer stock is only for 1 day the incoming raw meal std mustbe < 1 for LSF and Silical modulus < 0.1
Pyro process• Wet process• Semi dry process• Semi wet process• Dry process( wet milling and slurry is fed into the kiln )(dry milling , water sprinkled to makenodulation, nodules are fed into the kiln)(wet milling , dried in vacuum drier, cakeddried , powedered and fed into kiln(dry milling , dry meal is fed into kiln)• VSK processVertical shaft kiln( First process invented in cement process )
Wet process Semi wet processSemi wet process Semi dry process
Long dryprocess kilnDry kiln , suspension preheater kilnDry kiln , suspension preheater kilnWith pre calciner
Kiln = 4500 tpd4.35 M * 67 M% filling = 9 – 11 %Material retention time =18 mtscalciner
Clinker manufacture• Calcite – CaCO3• Dolomite –CaMg(CO3)2• Quartz – SiO2• Clay minerals• Micas• Feldspars• Aluminum oxide• Pyrite• Iron oxide• Gypsum / anhydrite• Alite,C3S• Belite,C2S• Aluminate,C3A• Ferrite,C4AF• Free lime(un wanted)• Periclase(un wanted)• Alkalisulfates(unwanted)Mineral phases in raw meal Mineral phases in clinkerTemperaturePressureTime
AliteCaOBeliteLiquidCaCO3BetaquartzGammaquartz C3ACalcining zone Transition zone Burning zone coolingzone1400120010008006004002001450 OCDeg CPre heatingzoneC12A7 C2(A, F)C4AFClinkering process
RefractoriesThe function of the refractories are• to protect the shell from the heat• to insulate to reduce heat losses• to withstand thermal stresses• to with stand thermo-chemical stresses• to withstand thermo-mechanical stresses
Kiln refractory liningRefractories are lined inside the kiln shell and preheater cyclones to the metalfrom heat as well as to insulate to conserve heat.The bricks used are lowalumina , high alumina bricks, magchrome bricks and spinel bricks. Mag chromebricks are banned due to health hazard.Chromium is poisenous.For severeconditions special bricks like zirconia based , are used.1400-1500 deg C1200 -1250 deg C1000-1100 deg C1100-1200 degc Gas temperatureRefractory brick
Always to be rememberedIf coal is mixed it is burntIf flame is wrong everything goes wrongwhatever you may do with chemistry orhigher heat input through calciner or kiln.The burning zone needs heat and it can beonly obtained from well shaped radiantflame.i.e., short, snappy and convergentflame .
7 8 9Burner positioningWe do positioning of theburner for centering theflame.The positions1,2,3, 4 and 7are closeto the refractory andthey are away from thecharge.Positions9 and 8are close to charge .Only 5 is close to chargeand refractory and this isbest as the flame in thisgives the best thermaldistribution to doeffective burning.Position 8 & 9 is veryclose to charge if coal istrapped it has seriousnegativeimpact.Position 1,4 & 7is very close to refractoryand it can burn therefractory.4 5 61 2 3
Heat exchange in kiln is• mainly radiation of heat from flame to refractory walland to charge• conduction of heat from refractory and to charge• convection of heat within the charge ( particle to particlecontact)radiationconductionconvectionHeat flows from hotter body to colder bodyGases flow from high pressure area to low pressure area
Lower rpm , high % filling , less activeLayer , high free lime, high radiationlosseshigh rpm , low % filling , more activeLayer , low free lime and low radiationlossesInfluence of revolutions / minute on kiln operationOptimum % filling = 9 – 11 with raw meal retention time of 20 -25 minutesunfavorable favorablePassivelayeractivelayer
Different flamesNormal flameFlame with lowSecondary air tempDistorted nozzleFlame –poorhood geometryOr distorted nozzleFlame at the centerFlame downwardFlame upward
Flame lengthLong flame, unstable coating,High back end tempLow shell temperatureShort intense divergent flameGood for burningLow back end temperaturePoor refractory life, highShell temperatureConvergent flameGood for burningGood for refractoryStable coatingLow shell temperature
The Ideal Flamehot !short !stable !T"long" flame"short" flameComplete combustion:- CO = 0- SO2, NOX ↓Homogeneous:- no temperature peaks- no local CO on the clinker bedLonger flame increase the back end temperature resulting inHeat loss at kiln exit and hot meal clogging
Heat transferby radiationand convectionHeat movesto clinker edgeby conductionAir flows overclinker coolingsurfaceHow cooling is accomplished800 O C100 O C
• Convection - Surface to Air• Conduction - Inside to Surface• Heat transfer is driven by temperaturedifference• Takes place at the clinker surface• To maximize it:– Increase the air/material contact timewith:• Deeper bed ( ⇒ more power)• Slower air flow (⇒ larger cooler)Heat transfer in clinker
Old conventional grate platescreate sand blasting effect or fluidizationThis creates poor heat exchangeModern cooler plates flow resistancebranch off the air , createsless fluidization , better heat exchangeCross flowCounter currentMechanical flow regulator
TemperatureBedthicknessclinkerairFixed bedFluidized bedAir inAir outClinkerAir inAir outClinkerTemperatureBedthicknessclinkerairMore efficient recovery with fixedbedAir flow requirementHas reduced from4 kg air/ kg.cl to2.2 kgair / kg clHeat exchange between clinker and air
1. The hotter the inlet temperature the hotter the clinkeroutlet temperature.2. The hotter the cooling air temperature the hotter theclinker outlet temperature.3. The longer the air/material contact time the cooler theclinker outlet temperature.General truths ( all coolers)4. Quicker the clinker cooling ( quenching) the smaller thecrystals, results in micro cracks of the minerals, improvesthe soundness of the clinker ( when MgO % exceeds 1.5 %)
C4AFC3SC2SMgoCaOC3APictoral representation of clinker micrograph
• MicroscopicA mixture of different mineral phasesParticle size ≈ 0 – 100 µm• MacroscopicA gray, granulated, rocky materialGrain size ≈ 0 – 50 mmWhat is portland cement clinker
Uniform nodule SizesRather uniform-sized nodules areingeneral an advantage regardingburning efforts and uniform degree ofburning.
Quickly cooled clinkers are favourable for the early strength potential; noalite is lost. The fine crystalline liquid phase prevents aluminate from an earlyhydration. The influence of aluminate on the setting time is limited in quicklycooled clinker.
Influence of cooling on clinker phasesFast coolingWell distributedsmall crystalsSlow coolingLarger crystals
C3SClinker when it is quenched in cooler it creates micro cracks whichneeds less energy for comminution during grinding.C3SClinker coolingC2S
Fuels used in cement industry• Solid fuels ( coal , pet coke, lignite, anthracite )• Liquid fuels ( furnace oil)• Gas fuels ( natural gas)• Alternate fuels ( shredded tyres,waste woodchemical waste, animal meal)
Solid fuel preparationFuel lumps are crushed to suitable size depending on the grinding systemand Hard groove index of fuel. The residue depends on the volatile matter
Roller press•Pressure applied to materialvaries from 3,000 a 4,000kg/cm2. They are overdimensioned in order to operateat lower pressures (2500).•Requires a subsequent de-lump, in order to separate theresulting paste, except in thecase the roller press feeds a ballmill.•Pressure application angleshould be around 6°.•Press consumes 20 to 25kWh/ton of cement.•Circulation factors range from 6to 10.•Requires great maintenance.•Wear out elements expectedlifetime: 10,000 hours
Horizontal roller mill•Rotates at hypercriticalspeed (1.2 times criticalspeed), having no feed.•Pressure on materialranging from 700 to 1,000kg/cm2.•Pressure applicationangle: 15 to 20°.•Circulation factors: 3 a 8.•Requires greatmaintenance.•Consumes 25 to 30kWh/ton of cement.•Expected lifetime: 10,000hrs.Roller and ball mills hybrid.Being the most recent one,its utilization is notwidespread.
Tubular mill (ball mill)•Rotates at 0.7-0.8 of critical speed.•Lacks pressure system.•Lacks application angles.•Consume 35 to 40 kWh/ton.•Circulation factors: 1 to 3.•Requires little maintenance.•Expected lifetime measured inyears.It is the most widely used forcement milling. Its drying capacity isproportional to D2, so in cement theL/D proportion is 3.In raw meal milling L/D is 1.5, ifhumidity is not greater than 3%, asingle chamber mill isrecommended. In case the materialhas humidity greater than 7%, it isnecessary to incorporate a flashdryer or change to a vertical mill.The % of material in suspension willdetermine which type of mill shouldbe used.
Cement mill coolingThe setting properties depend the water molecules ofGypsum CaSO4.2H2OIf water is dehydrated ( at 125 deg C) it results in false setIf it is partially dehydrated, CaSO4.1/2 H2O, calledHemihydate it contributes to initial strength. Hencecement temperature is to be maitained > 100 deg c and< 125 deg C
Vertical mill (roller mill)•Pressure on material:300 to 500kg/cm2.•Application angle: 12°. The widthof the material layer is proportionalto this angle and to the rollersdiameter.•Consumes 25 to 35 kWh/ton ofcement.•Circulation factors: 3 to 5.•Requires great maintenance.•Wear out elements expectedlifetime: 15,000 hrs.•Recommended in cases wherehumidity is greater than 7%, takinginto consideration that abrasivecontent must remain low. This iswhy it is commonly used in rawmeal milling. It works better than aball mill on plastic materials (clay).
Migration velocity and collection efficiencyηω =q Ep( 4 π µ r)= 1 – exp ( - W.A / Q)ω = migration velocityEp = strength of field in which particles are collected , volts/ meterµ = Viscosity of gas Pa-sr = radius of the particle- µ mη = fractional collectional efficiencyA = collection surface of the particlesQ = gas volumetric flow rateW = drift velocity
PulseCleanGas/dust distributionClean gas outletRaw gas with dust inletDust drop outCollected dustDust up flowbetween bagsCake formation
No dampers:Only possible to do on-line cleaning.Maintenance:On-line not possible.Example AOne dirty gas chamber.One clean gas chamber.With inlet and outlet dampers:Possible to do on and of-line cleaning.Maintenance:On-line possible when separate hoppersMultiple dirty gas chambers.Multiple clean gas chambers.Example BFabric FilterOptional Arrangements
Quantitative rules of thumbC3S : 1 – 28 d : + 0.5 Mpa / %Ks ; 1 d : + 4 Mpa / %: 3 d : + 4 Mpa / %: 7 d : - 2 Mpa / %: 28 d : - 10 Mpa / %(SO3) tot : 1 - 28 d : - 5 Mpa / % fromoptimumBlaine : 1 d : + 0.04 Mpa / (m2/ kg)3- 28 d : + 0.08 Mpa / (m2/ kg)
One day strength is contributed mainly byC3A , Soluble alkalies, and C3S3 day is contributed mainly by C3S7 days strength is contributed by mainlyC3S28 days strength is mainly contributed byC2SApart from the above cement strength is enhanced byhigher fineness of cementLess C3S crystal size achieved by rapid burningand quenching the clinker in coolerHigher fineness of rawmeal also reduces thecrystal size of clinker minerals , ie ., C3S & C2S whichenhances the hydraulic reactivityCement strength – influencing parameters
Wk , prehydration of clinkerPrehydration of clinker minerals can occur1. As a result of incorrect internal water cooling in cement mill2. when storing too hot cement in a silo3. When clinker and especially cement is exposed to humidityPlease note:If clinker has more soluble alkalis and sulfates it is highlyhygroscopic especially when pet coke is fired.In cement silos they form Syngenite , K2SO4.2 CaSO4. H2O whichforms lumps and block the cement silos. Hence venting is mustto evacuate moisture and silo cleaning.cements having soluble alkalis and sulfates preferably packed inpaper bags to avoid depletion of strength.
Thumb rule formulae for prediction of strengthFLS predicted the formula for cement ground to 300 kgs/ m2With 4 % gypsumstrength,d28 = 52 - 10.( Ks) + 0.15.(C3S)The content of soluble alkalis Ks is dependent on the total alkalicontent and SO3 content in clinker.As per Knofel it isF 28 = (3*C3S)+ (2*C2S) + C3A – C4AF N / mm2Strength predictionfor 3 d = 97 + 35.8 Ma + 38.1K2SO4 + 28.7 Ms – 1.3 C3S Kg/ cm27 d = 300 + 13.4 Ms + 2.8 C2S + 56.1 Ma – 15.4 K2SO4 + 15.5 Na2O28 d = 490 – 55.3 K2SO4 + 1.3 C3S (or)= 490 – 86 K2O + 2 C3S – 26 Na2O
Influence of fineness on cement strengthFor cements with the same specific surface the increase of the uniformityfactor results in increase of strength of all ages.1. The specific surface , the percentage of fractions 3- 32 mm and theuniformity factor n really influences the development of cement strength.The influence of 3 - 32 mm fraction and the uniformity factor is higher incement with higher in specific surface ( > 3400 cm2/ g)2. The fractions with particle size less than 3 mm contributes only to earlystrength while the fraction with particle size more than 24 mm influencesstrength development significantly.3. While the fractions 3 – 16 mm and 16 – 32 mm seems to be moresignificant factor for specific surface 3500 – 4000 cm2/ g) . This isrelevant only if the granulometric distribution is continuous and steep.4. The optimistic granulometric distribution of a cement is a continuousand steep ( with high uniformity factor) distribution with a high (65 %)content in 3 – 32 mm fraction and specifically in 16 - 24 mm fraction andlow content of fine particles ( < 3 mm , 10 % ) and specific surface of2500 – 3000 cm2/ g according to Blaine.( high efficiency separator andgrinding media distribution plays significantly here)
Properties of cement mineralsCharacteristics C3S C2S C3A C4AFSetting quick slow rapid nilHydration rapid slow rapid nil3 days heat 1.1 cal / g 0.4 cal / g 2 cal / g nilliberationEarly strength high upto low upto not much nilContribution 14 days 14 days beyond one dayLate strength less later high later nil nilcontributionResistance to moderate high poor highChemical attackDrying shrinkage nil low nil nil
Problems and solutions1. Grinding problemsa) False set lower cement mill temperatureadd less gypsumadd part anhydriteb) reduced strength high mill temperatureless water coolingcorrect water cooling2. Silo storagea) False set short storage timecooling of cement < 70 deg cb) reduced strength increase gypsum dehydration inmillc) lump formation and add less gypsum, use partlysilo blockage (syngenite anhydrate , decrease K2O contentformation, K2SO4.2CaSO4.H2O to avoid the formation of SyngeniteProblems solutions
3 ) Bag storagea. reduced strength short storage timeb. lump formation add TEA during grinding(tri ethanal amine)add hydrophobic agentsc. crust formation plastic coated bagsd. abnormal setting plastic covering pallets
Customer is the king. He is a better business man than you.Tomorrow’s market is competitive with quality/ price ratio.Customer creates the customers.Bon’t brand the quality alone , brand your service too.