Granulometry of Clinker

Presenter : Eng. Ali Nadeem Mohd. NadeemProduction Engineer

Qatar National cement company

Clinker burning is a very complex procedure. Although clinker is produced for more than 150 years; there is still no full clear picture about the multitude of physical & chemical reactions occurring in the rotary kiln.

Clinker liquid phase or clinker melt, isthe fraction of kiln feed that meltsbetween the transition and burningzones, which occurs at a temp.between about 1260 ºC and 1310 ºCwith further rise in temp. the proportionof liquid face increases to around 20 –30% (by weight) at 1450 ºC. The liquidphase practically corresponding to 23 -28% of clinker mass is mainlycomposed of calcium ferrites andaluminates, and in a smaller extensionof silicates.

When the kiln load / charge arrives to the upper transition zone, small part of it melts into liquid when kiln feed moves in sintering zone, it’s temperature increases & so does the amount of liquid phase. The amount of liquid at any given temp. can be estimated with the help of formulas. Too much liquid is harmful to the brick lining and to clinker grindability. Too little liquid severely impairs alite C3S formation and requires more fuel to reduce the free lime content in the clinker. For a given temperature, the amount of liquid increases with iron, alumina, magnesium, sodium, potassium and sulfur. The insufflation of calcium chloride in the burning zone has a similar effect. The liquid phase has an important role in clinker granulation and also in clinker mineral development and its properties.

The influence of the liquid phase upon clinker properties and refractory performance is so important that it deserves further consideration. The LP has an important role on clinker nodulization and also on clinker mineral development and properties. if the raw mix consisted only of C-S-A-F it would start melting at 1338 ºC , the so-called eutectic point for the system C-S-A-F . at the eutectic temperature, the liquid composition would be 55 % C, 6% S,23% A, and 16% Iron. Such composition is saturated in lime and un saturated in silica. Therefore it’s very aggressive to refractory products containing silica or silicates in their compositions. This explains why high alumina bricks exhibit Duck nesting.

The actual proportions being dependenton the chemical composition of thematerial. For example: the proportion ofliquid formed is less accordingly as thesilica modulus is higher. As shown in thefollowing graph:

Liqu

id p

hase

at 1

450o C

in %

by

wei

ght

Alumina modulus Al2O3=2.2 TM

Fe2O3

lime standard = 96 KSt35

30

25

20

151.50 2.00 2.50 3.00 3.50 4.00

Silica modulus SiO2

Al2O3 + Fe2O3

Dissolved in the liquid phase alsoappear chlorides and alkaline solutionsthat modify the rheological properties offused phase (viscosity and surfacetension), thus increasing the reactivityand aggressiveness of the liquid. Theeffect of such oxides on the rhelogicalproperties of liquid phase may beresumed like this:

K2O + Na2O

Surface tension

decreases.

Viscosity increases.

Clinker formation rate

reduces.

K2O + Na2O + MgO

Surface tension

increases insignificantly

Viscosity decreases

Clinker formation

rate increases

SO3

Surface tension and viscosity

decrease

Clinker formation rate

increases

It is also seen that the viscosity ofliquid phase increases withtemperature reduction or aluminaratio increase. The viscosity of liquidphase exercises great influence on:

Clinker nodulisation.

Ring formation.

Clinker formation rate.

Thickness and stability ofcoating.

Clinker liquid phasepenetration in the refractory.

Specific consumption incement grinding.

In order to improve the burn-ability andincrease the clinker formation rate , wecan:

Reduce the liquid phase viscosity.

Increase the surface tension.

Increase the liquid phase quantity.

Grind calcite and quartz more finely.

High alumina ratio (AL2O3 / Fe2O3) & lowalkalis Equivalent (% Na2O+0.659 % K2O)increase surface tension of the meltwhile a low burning zone temperaturewill result in increased liquid viscositywhich is lower with decreasing aluminaratio (Al2O3 / Fe2O3).

When Iron Ore (Fe2O3) increases, thesurface tension decreases, due to lessviscosity & more mutability then IronOre is a “surfactant”- which is asubstance that lowers surface tension-.Temperature has the most pronouncedeffect on liquid phase viscosity.Increasing the burning temperature by93 ºC reduces liquid viscosity by 70%,for Ordinary Portland Cement. Thissimple fact explains why hotter-than-normal temperature are so beneficial toclinkering & so harmful to the refractorylining. MgO, alkali sulfates, fluorides&chlorides also reduce liquid phaseviscosity. Extreme caution should beexerted when insufflating** calciumchloride into the burning zone to reducealkali in the clinker. The injection ofsodium carbonate in the burning zone isalso detrimental to the refractory lining.

Free alkali and phosphorus increaseliquid phase viscosity, but this effect iscompensated by MgO and SO3. Onlyclinkers with a sulfate/alkali ratiolower than 0.83 and which are low inMgO would experience the negativeeffects of high liquid viscosity.

Another important property of theliquid phase is its surface tension.The surface tension has a directimpact on clinker fineness, coatingadherence to the lining & clinkerquality.

High surface tension values favornodule formation (granulometry) andliquid penetration through thenodules.

The resulting clinker contains less dust(fraction below 32 mesh) and lower freelime content. A liquid phase with highsurface tension has less tendency to‘wet’ the brick surface, thereforereducing clinker coat-ability oradherence to the lining.

Alkali, MgO and SO3 reduce liquidsurface tension. So does temperature.Sulfur and potassium have thestrongest effects, followed by Na2O andMgO. Therefore, MgO, SO3 and K aregood coating promoters.

The mineralogical nature of the rawmaterial conditions the raw mealgranulometric behavior. For examplelimestone of sedimentary origin showsbetter grinding capacity thanmetamorphic limestone.

Thus by keeping the same conditionsof grinding limestone of metamorphicorigin will show a coarsergranulometry in the raw mix, ifcompared to sedimentary limestone.There are two qualities of clinkersbeing manufactured. One is based onmetamorphic limestone and the otheruses sedimentary fine-grainedlimestone. Metamorphic crystallinelimestone deposits are characterizedby high-energy requirements forcalcining, since the calcinationtemperature is 930-1050oC against850-900oC with sedimentary finegrained limestone. This increase of80-150oC in calciner equates to anincrease in fuel consumption by 40-75Kcal/Kg clinker.

The material temperature at the kiln inletgoes up from the desired 800 oC to 900oC.kilns are designed for a materialtemperature of 800oC at the kiln inlet. Therate of burning is reduced, which affectsclinker quality. Liquid formation is quitedifferent in both types of limestone as canbe seen below.

Calcination of different limestone Metamorphic Sedimentary

Start of appearance of liquid 1290oC 1270 oCCompletion of liquid formation 1390oC 1320oCClinkerization temperature. 1450oC 1450oCC2S.C3S formation less time More timeClinker quality Inferior Good

Effect of burning rate on CO2 emission Metamorphic Sedimentary

Material kiln inlet 900oC 800oC

Residence time 20 minutes 20 minutes

Burning rate / minute 27.5oC 32.5oC

C2S to C3S zone 1390-1450oC 1320-1450oC

C2S to C3S time 2.18 minute 4.0 minute

C3S + exothermic heat Low High

CO2 emission High Low

The use of fluxes to reduce the calcination temperature and increase the burning rate, would improve the kinetics of reaction and accelerate the clinkering process.

Once this is done thequality of clinkerproduced frommetamorphic limestonewould be improved andthis would in turnreduce fuelconsumption and CO2emission -CO2 is themain greenhouse gas(GHG) responsible forglobal warming. Use offluxes would also allowa higher percentage offly ash and slag additioninto blended cements,for improved businessmargins.

The mineralogical composition, forexample, affects the pelletizeabilityof the raw meal and also affects thewater content needed in raw slurry,while the burning behavior and thespecific heat requirement aremodified, inter alia, by the mineralcomponents of the raw meal. Themineral character of clays andcoarsely crystalline quartz, inparticular, is a major influencingfactor, but crystal lattice dislocations,crystal size and intergrowth,admixtures and impurities, naturalblending of the phases in the rawmaterial, and other factors, also playa part.

The rates at which reactions takeplace are generally dependent on theparticle size of the reactants, i.e., onthe reactive surface areas. Hence theraw meal should be of much finenessthat in the burning process even itscoarsest particles will react ascompletely as possible. As a rule ,this condition is satisfied by cementraw meal with a residue of not morethan 5 – 20 % (by weight) retained onthe 90 micro sieve, the actualmaximum acceptable percentagebeing dependent on the compositionof the meal and the type of kilnsystem.

The raw meal fineness constitutes one of the most important factors in the aptitude to clinkering. It is known the finer the raw meal, the easier to burn & the lower the clinkering temperature.

The lime combination Degree decreases as raw meal granulometry increases, allowing an increase in the content of free lime. As shown in the graph below :

%

Cao

free

60 1000 oC

50 1100 oC

40 1200 oC

30 1290 oC

20 1320 oC

10 1380oC

01450 oC

20 40 60 80 100 120µm

average particle diameter đp

Fig. : Effect of limestone particle size on freeCaO content at various burning temperatures đp=average particle size of a fraction : lime Std.KStl = 96 : silica modulus = 3.0 : aluminamodulus=2.2

The following shown figure represents araw meal with L.S.F.= 95 % & SR= 2.5,with three different fineness (5, 10 &15%) represented by the residue in thescreen No. 170. We can observe thebehavior of free lime with the variationof temperature and fineness.

Big grain size of calcium and quartzaffecting negatively the combination ofraw mix, causing bad granulation. Whensilica ratio (SiO2/Al2O5+Fe2O3)increases, raw meal burnabilitydecreases and aptitude of clinkergranulation decreases.

The result is fine clinker that generatesa dust cycle between kiln and cooler,that causes a deficiency in the coolerprocess thermal efficiency and wear inclinker handling equipments.

CaO (%)

4 LSF=95SR=2.5

3 15%10%

2

5%1

1450 1500 1550 1600 ( oC )

Content of the free lime as a function of raw mix fineness and burning temperature.

It is important to avoid variable orhard burning mixes as the harderburning and longer retention timeinvolved in controlling free lime, resultin large alite -C3S and worse largebelite C2S crystals which cause poorgrindability. This large crystals tend tocause dusty clinker (normal clinkershould contain less than 2% - 1mmparticle size) which in turn causes ahigh recirculation load between cooler& kiln, can exacerbate snow manformation in the cooler and overloadthe cooler dust collector.

The final major factor is the rate ofreaction in the kiln, after calcination iscomplete, transition to melt formationshould be as rapid as possible tominimize growth of belite and CaOcrystals.

This transition is delayed by a long lazyflame, which may be due to poor fuel -air mixing, or insufficient burner tipmomentum. Too much liquid phase orclinker melt is harmful to brick lining andgrind-ability.

Example of liquid phase control is thecase of production of sulphate resistingportland cement. The characteristicdifference between OPC & SRCcements is the limited value forTriCalcium Aluminate for which attributedthe increase in the formation of ettringitewhich when it develops and due to it’shigh crystallization pressure causes thehardened cement paste to disintegrate.

C3A< 3.5 Target & How todecrease C3A ? C3A = 2.65A-1.692F

Increase F to decrease C3A but consequences silica modulus is decreased accordingly.

SM = SiO2Al2O3 + Fe2O3

liquid phase portion increased withit’s probable negative impact onoperation stability, coating profile,plugging, infiltration of brick liningetc.

Appropriate action is the increaseof SiO2 by addition of Sand

ELEMENT OPC Meal SRC MealSiO2 13.35 13.32

Al2O3 02.95 02.42Fe2O3 01.90 02.76Cao 41.44 41.39

MgO 02.27 02.14SO3 00.30 00.22

Na2O 00.16 00.09K2O 00.58 00.42TiO2 00.15 00.12MnO 00.03 00.03P2O5 00.06 00.04SrO 00.10 00.05Cl 00.107 00.100

LSF 97.6 98.1SM 02.75 02.57AM 01.56 00.88C3A 07.31 02.79

C4AF 09.10 13.31AW 23.97 25.58

QNCC Typical OPCClinker Composition

Range OptimumSiO2 20.90Al2O3 4.60Fe2O3 3.05Cao 64.35MgO 3.9SO3 0.80Na2O 0.26K2O 0.68TiO2 0.23MnO 0.061P2O5 0.12SrO 0.22Sum 99.17F. Lime 0.85Cl 0.036LSF 0.964 90 to 98 96.00SM 2.73 2 to 3 2.70AM 1.51 1.3 to 2.5 1.55C3S 61.45 45 to 65 60.00C2S 13.56 10 to 30 15.00C3A 7.03 5 to 15 7.25C4AF 9.28 5 to 12 8.00AW 25.12 20 to 35 25.00L.Ph 23.99 23 to 28 25.00F. Temp. 1434 1450B.I 3.8 2.6 to 4.5 3.50B.F. 109.2 100 to 120 110.00Alk. (M) 0.99 0.6 to 1.4 0.90SO3 (M) 0.95 0.83 to 1.0 0.85

Kiln-cooling zone is playing animportant role for clinker nodulecrystalline structure. Clinker meltsstarts to solidify in the cooling zoneand size of clinker granule start toincrease due to their agglomerationduring cooling process. Normally nocoating is built in the Kiln – coolingzone. When clinker cooling is notsatisfactory, C3S crystals will explodedue to thermal and mechanicalstresses, resulting in fine dusty clinker,which is the form of bad clinkergranulation.

Qualitative clinker granulation isessential as fine & dusty clinkerresults in uneven air distribution inthe cooler & commonly a red river ofhot clinker extending well down thecooler.

To enhance clinker granulation andavoid the phenomena of fine clinkerthe following should be adjusted:-

Consistent kiln feed – quantity and qualitySilica ratio to be decreased relatively.Lime saturation factor to be kept stable in lower side - high LSF results in more fine clinker due to less liquid phase %.

Well homogenized kiln feed (raw meal)Medium flame and stable kiln thermal zones graduation.

Enough retention time for material in the kiln to confirm completion of all chemical reactions.Control of liquid phase percentage considering alkali ,sand ,MgO etc.

Satisfactory cooling – zone length ,fast cooling (quenching)…etc

Kiln stable operation.

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