Objectives :

• Significance of pyroprocessing.

• Burnability of Rawmix

•Effect of Raw mix in burning process.

•Formation of clinker and Granulation

• Effect of S, Cl, Na, K in burning process

“Heating of the raw meal to the required temperature so as to produce the desired clinker compounds in an economic way at higher productivity in the preheater & kiln.”

Pyro processing

Production of Good quality clinker from the kiln depends on :

Good raw mix design which in turn depends on

1. Desired clinker minerals

2. Allowable free lime

3. Nature of the raw material

Liquid ratio & Residence time in the kiln

Fuel quality & Combustion

Fineness of the raw meal to the desired level

Reactions involved in Pyro processing

1250Formation of liquid phase5

1400Completion of reaction & Equilibrium6

1200Formation of C2S, C3A 4

805Evolution of CO2 from CaCO3 and MgCO3

3

550Evolution of chemically bonded water2

100Evaporation of free water from the feed1

Required minimum temperature, Deg C

Reactions taking placeSl no

BURNABILITY OF RAWMIX

• The readiness with which a rawmix is transformed into clinker minerals in the course of high temperature treatment

• Determined by their chemical, mineralogical and granular composition

• Deciding factor for the temperature necessary for a good clinker product

• Different rawmix with the same chemical composition and equal fineness may differ in their burnability due to their different mineralogical composition

• Poor burnability of rawmix may be caused by coarse grains of Calcite (rich in CaCO3) or Quartz (rich in SiO2)

COARSE GRAINS• Clinker consists of alite, belite, liquid phase, free

lime and pores • The effect of coarse grains is studied with optical

microscope• Coarse quartz and calcite results in poor burnability,

high free lime and too little C3S in the clinker• Critical size of particle for residual free lime after

30 minutes Quartz - 45 micronsCalcite - 125 microns

• 1% increase of Quartz +45mic ----> 0.93% Free CaOCalcite +125mic ----> 0.93% Free CaO

Free CaO1400 = 0.33 (LSF –95)

(30minutes) + 1.8 (SM-2)

+ 0.93 SiO2 +45 micron

+ 0.56 CaCO3 +125 micron

FREE LIME

Kuehl’s Burnability Index

BI = C3S / (C4AF+ C3A)

Example :

BI = 45 / (14.5 + 7.5 )

= 2.045

• For the desired free lime, the required fineness can be found out to improve the burnability.

• The burnability can also be improved by reducing the LSF and Silica Modulus

• A mineraliser like flouride or sulfate (gypsum) can also be added to reduce the temperature at which liquid phase is formed

• mineraliser modifies the viscosity and surface tension of the clinker liquid to promote the formation of clinker minerals.

Chemical Composition

Parameter Variation Burnability

LSF

AR

SR

Parameter Variation Burnability

C3S

C4AF + C3A

Flux & Mineraliser

Desired Clinker QualityMineralogy of Raw meal

Chemical CompositionFineness

% Liquid

Effect of Raw mix in burning

FLUX & MINERALISER

Flux : Reduce the temperature of liquid formation.

Examples : Fe2O3, Al2O3

Mineraliser : Reduce the temperature for the Alite formation

Examples :TiO2, P2O5

LIQUID CONTENT

• Raw meal melts at more than 1200 deg C depending on the amount of fluxing material

•Liquid formation is important for

Effective Granulation

Stable coating

Protecting the refractory

% Liquid = 2.92 Al2O3

+ 2.25 Fe2O3

+ Mgo

+ Alkali

CLINKER FORMATIONPoor granulometry and Dusty Clinker leads to.• More wear rate in cooler and more maintenance

• Formation of unstable, porous coating instead of dense, stable coating

• Poor grindability of clinker

• Problematic clinker handling and dust nuisance

• Unstable kiln operation

NODULISATION• Particles are held together by capillary forces of the

liquid• Nodulisation depends on the amount of liquid,

particle size and the speed of the kiln• In the kiln, the liquid is formed in a narrow temp

interval at aprox 1300 degC• Formed C3S crystals sinter together to form coarse

C3S particles and slow down the nodulisation process• Nodulisation is enhanced by liquid phase and

counteracted by large C3S particles.

• At higher BZ temp, the formation of C3S particles is faster and hence smaller will be the nodule size

• Reducing the Alumina / Iron ratio (to 1.6) will improve the nodulisation as the formation of liquid phase starts at lower temperature

• Lowering of the silica modulus increases the amount of liquid and thereby improves the nodulisation

• Reducing the LSF reduce the potential C3S and thus increase the nodulisation

CHANGES IN CHEMISTRY & CLINKER NODULISATION

Nodulisation

N

Amount of C3S

% Liquid

Length of burning

zone

Particle size

Temperature Residence time

Common Morphological features of Clinker Phases

SlNo

ClinkerPhases Colour Shape Microstructures

1 Alite – C3S(3CaO.SiO2)

Straw yellow,brown,yellowishbrown,brownish yellow

Hexagonal,Pseudo-hexagonal, Lath,Subhedral,Anhedral etc.

Fused grains,Stretched, Twinned,Granulated, Brokenout line grain

2 Belite – C2S(2CaO.SiO2)

Blue, Bluishyellow,yellowish blue,greenish yellow,yellowish green

Rounded, Sub-rounded,Elliptical,Subhedral,Anhedral etc.

Clusters of varioussize, Fused grains,Twinned grains,Corroded grainmargins, Striationson belite grainsurface asinclusions

3 Free lime–CaO

Multiple highorderinterferencecolours of pink,green, yellow,blue etc.

Rounded,Sub-rounded,Subhedral,Anhedrtal etc.

Clusters, Striationson the grain surfaceas inclusions.

Characteristic Optical Properties of Clinker Phases

Major Clinker mineralsBouge’s Formulae:

when A.R > 0.64

C3S = 4.071 C - 7.6 S - 6.718 A- 1.43 F - 2.852 SO3

C2S = 2.867 S - 0.754 C3S

C3A = 2.650 A - 1.692 F

C4AF = 3.04F

VOLATILE MATTER

Volatile Matters present

SO3

Cl

K2O

F

OH

Na2O

Circulation of volatile matter• A fraction of the volatile components evaporates in

the kiln burning zone and condense in the back end or rawmeal and re-enter the burning zone

• The repeated evaporation and condensation results in an Internal circulation where the concentration can go up to fifty times the input concentration

• At equilibrium state, the output of volatiles along with clinker is equal to the total input from rawmeal and fuel

• Higher degree of volatiles concentration exists either due to more input or due to a high degree of volatility

External circulation• Volatile matter in rawmeal like sulfur, is burnt to

SO2 gas in the preheater upper cyclones at around 400 - 600 degC and expelled out from preheater but effectively precipitated in ESP

Condensation in Preheater• Volatile matter with low melting point condenses in

preheater walls and rawmeal particles causing build ups on cyclone

• SO2 gas combines with calcined rawmeal and condenses as CaSO4

CaO + SO2 + 1/2 O2 --------> CaSO4

Raiser Duct &

4 CycloneKiln

S in Raw meal a3

S

Kiln firing

a2

SO3 (b)

Through Raw meal

SO3 thro’ Clinker (c)

INTERNAL CIRCULATION

Recycled SO3 (e)

S

PC firing

a1

PREHEATER FIRST STAGE ESP

S in Raw meal

SO3

LOWER STAGES

SO3

IN HOTGAS

SO3 thro’ stack

EXTERNAL CIRCULATION

Recycled Raw meal

kiln

1

23

4

ESP

Raw meal

PC

Kiln Coal

Types of Circulation

Internal Circulation

External Circulation

Melting Pt & Boiling Pt of Volatile Matter

CompoundK Na Ca

MPt BPt MPt BPt MPt BPt

Oxide dec 350 sub 1275 2850 -

Carbonate 894 dec 850 dec dec 825

Sulphate 1074 1689 884 - 1190 -Chloride 768 1411 801 1440 772 1600

Hydroxide 360 1320 328 1390 580 -

MPt : Melting Point, BPt : Boiling Pt,dec : Decompose, Sub:Sublimate

Affinity between the Volatile Components

• Chloride reacts first with alkalis to form NaCl and KCL• Excess chloride reacts with Calcium to form CaCl2

• Alkalis in excess of chloride combine with sulphur to form Na2SO4 and K2SO4

• Alkalis not combined with chloride or sulphur will be present as Na2O or K2O

• Sulfur in excess of alkali combines with CaO to form CaSO4

VOLATILITY• Volatility is expressed in terms of evaporation factor

% within clinkerEvap. Factor E = 1 - -------------------------

% at kiln inlet LOI free basis• Chloride compounds KCl, NaCl and CaCl2 are having

an E of 0.99 to 0.996. At 800 degC they are melted and at 1200 - 1300 degC they are almost entirely evaporated

• Sulphate compounds with alkalis K2SO4, Na2SO4 are more stable ( E = 0.30 - 0.90), which is very desirable

Molecular Ratio of Sulfur and Alkali• Sulfur and alkali overall concentration and their

proportion is very important• Individually they are more damaging than their

sulphate compounds(K2SO4, Na2SO4) as K2SO4 has such a very high evaporation temp and they come out along with clinker

• Only half of Na2SO4 comes out as it begins to split into Na2O and SO3 at around 900 degC

• Sulfur in excess of alkali will form a more volatile CaSO4, which has a high evaporation factor

• An optimum molecular ratio is a must to avoid the presence of the damaging individual components

Optimum (SO3 / Alkali) Ratio

SO3 SO3 / 80

------ = ----------------------- = 1.1

Alk K2O + 0.5 * Na2O------ -------- 94 62

Operational Aspects of VC• Formation of build-ups in preheater riser

pipes and cyclones reduces the air volume • Reduced kiln production and increased

circulation of sulfur compounds due to less availability of excess oxygen

• Higher heat consumption• Dusty clinker formation

Nor mal Limits

Max Limits

K 2O eq=K 2O+1 .5Na2O 3.70% 6%

Chlor ine as Cl - 0.80% 2.00%

Sulf ur as SO3 2.50% 5%

Limits On Volatile Components In Bottom Cyclone Stage in a SP kiln system on LOI free Basis

CONTROL LIMITS

Nor mal Limits

Max Limits

K 2O+0.65*Na2O 1 .00% 1 .5%

Chlor ine as Cl - 0.02% 0.02%

Sul f ur as SO3 1 .00% 1 .6%

Max Allowable Input of Volatile Components for a SP kiln system Without bypass on LOI free Basis

CONTROL LIMITS

Nor mal Limits

Max Limits

K 2O+0.65*Na2O 1 .00% 1 .5%

Chlor ine as Cl - 0.01 5% 0.01 5%

Sul f ur as SO3 0.80% 1 .2%

Max Allowable Input of VC for a CALCINER Kiln system Without bypass on LOI free Basis

CONTROL LIMITS

Raw mix control

SO3 / Alkali ratioKiln by pass

Excess airFlame adjustments

Reducing Evop factor

ControlMethods

Discard the filter dust

Kiln Inlet Coating PROBLEMS• CO formation in kiln inlet• reduced atmosphere • increased kiln inlet draught• reduction in kiln inlet Nox content• reduced kiln feed• blockage of meal chute and riser duct• premature refractory failure• dusty clinker formation and poor clinker quality• less intake of secondary air • kiln hood positive

Kiln Inlet Coating Remedial Measures• increased oxygen content in kiln inlet ( to be

maintained around 2 %)• higher fineness of fine coal• smooth surface of kiln inlet lining• maintaining the alkali sulfur ratio near unity• using air blasters in kiln inlet and riser duct• using coating repellant refractory castable at kiln

inlet like silicon carbide castable• periodical cleaning of kiln inlet using compressed

air lance during plant running condition

Solving the Problem with Sulfur• Reducing the volatility- especially of CaSO4, by

lowering the BZ temp• By making the rawmeal easy to burn by means of

higher fineness / lower silica ratio • Maintaining optimum molecular ratio of sulphur to

alkali to avoid excess sulfur circulation• Reducing thermal load in the kiln• Increasing the availability of oxygen in kiln to shift

the equilibrium towards Alkali Sulphate• In the Case of reducing atmosphere, free carbon in

fuel reacts with alkali sulphates to liberate alkali oxides which increases the VC circulation

Kiln burning zone SO2 Formation :

Fuel S + 02 SO2

Alk SO4 <=> Alk-O + SO2 + ½O2

SO2 absorption :

Na2O + SO2 + ½O2 <=> Na2S04

K2O + SO2 + ½O2 <=> K2SO4

CaO + SO2 + ½O2 <=> CaSO4

Ca SO4 + C => CaO + SO2 + CO

Alk2 SO4 + C => Alk2O + SO2 + O

During Reducing Atmosphere in Kiln

Calcining zone SO2 Formation :

Fuel S + 02 S02

CaSO4+ C <=> CaO + SO2 + CO

SO2 absorption :

CaO + SO2 <=> CaSo3

CaCO3 + SO2 <=> CaS03 + CO2

CaSO3 + ½O2 <=> CaSo4

PREHEATER SO2 Formation :

Sulphides(FeS) + O2 Oxides + SO2

Pyrite

Organic S + O2 SO2

SO2 absorption :

CaCO3 + SO2 <=> CaS03 + CO2

CaSO3 + ½O2 <=> CaSo4

Solving the Problems with Chloride• As the evaporation factor of chloride is very high,

it is very difficult to reduce volatility.• Avoiding the chlorine rich raw material• Avoiding the peaks of chlorine content in rawmeal

by pre homogenization of raw material with respect to chlorine

• Discarding the ESP dust• Installation of alkali by pass system

Solving the Problems with Alkali• Maintaining the optimum the alkali sulfur ratio

• Addition of sulfur in the system in the form of gypsum