09. Clinker Burning Ordonez 2006

The clinker burning process and its influence on the refractory material Hugo Ordóñez

What is the goal of clinker burning ?

The production of

GOOD QUALITY cement... OF COURSE !

What is cement quality? Cement quality is defined in terms of characteristics & properties such as: •Chemical composition (Oxides content limits) •Physical properties (Strength, Workability, Setting behavior etc) in Standard Norms (DIN, ASTM, ISO) and is measured used standard methods.

Factors influencing cement quality Mechanical handling of clinker (grinding) Chemical & mineralogical composition of raw mix Chemical & mineralogical composition of clinker Burning process Chemical composition of fuels Circulation phenomena

QUALITY

Cement is produced by grinding clinker together with some admixtures: gypsum, slag, pozzolan, limestone, among others.

And consists mostly from mineral phases

C3S C2S C3A Free lime C4AF Gypsum

The most important mineral phases come from clinker

Some cement quality properties correlate with clinker mineralogical parameters: strength=F(stereological data) 90 days compressive strength N/mm2

Wt,% (Alit+Belit) DhAlit. Wt.% Alit 100

+ DhBelit. Wt.% Belit 100

Grindability of clinker in dependance of clinker mineralogical parameters : (Ak= belit corrected specific alit content)

Clinker Burning Air

Raw meal

Fuel Flue Gas

Clinker

Chemical Composition of cement raw materials and mix

Limestone

Marl

Clay

Sand

Raw mix

Weight % Ig. Loss

40-44

2-42

1-20

Up to 5

32-36

SiO2

0,5-3

3-50

37-78

80-99

12-16

Al2O3+TiO2

0,1-1

1-20

7-30

0,5-3

2-5

Fe2O3+Mn2O3

0,1-0,5

0,5-10

2-15

0,5-2

Up to 2

CaO

52-55

5-52

0,5-25

0,1-3

40-45

MgO

0,5-5

0,5-5

Up to 5

Up to 0,5

0,3-3

SO3

Up to 0,1

0,1-4

Up to 3

Up to 0,5

Up to 1,2

K2O

Up to 0,3

Up to 3,5

0,5-5

Up to 1

0,2-1,4

Na2O

Up to 0,1

Up to 0,2

0,1-0,3

Up to 0,5

Up to 0,3

Cl-

0,01-0,1

F-

0,02-0,07

Raw materials are mostly obtained from open Quarries

Calcite (CaCO3) Chrystals Rhombo

Cubic

Silica containing materials quartz

Silica sand

Clays Kaolinit

Caolinit

Raster Electron Microscope picture. Picture width 20 micrometers

35%

Clinker

100%

Mass, relative Raw meal

Chemical and Mineralogical processes during clinker burning

65% Clay minerals

Temperature°C

Summary of Reactions During Clinker Burning

• Below 200 °C: Drying: free water evaporation. No chemicalmineralogical changes

• Above 200 °C and below 450 °C : Dehydration: loss of chemically bonded water. chemical and mineralogical changes beginn.

Summary of Reactions During Clinker Burning..continued Between 400 °C and 700 °C: Decomposition of clay minerals. Formation of metakaolinite

Al4(OH)8Si4O10Æ2(Al2O3.2SiO2)+4H2O and highly reactive forms of silicic acid

Summary of Reactions During Clinker Burning..continued Between 600 °C and 900 °C: Decomposition of metakaolinite and other compounds. Formation of highly reactive oxide mixtures

Al2O3.2SiO2ÆAl2O3 + 2SiO2

Summary of Reactions During Clinker Burning..continued Between 600°C and 1000 °C Decomposition of limestone

CaCO3ÆCaO+CO2 Formation of calcium silicates and aluminates

3CaO+2SiO2+ Al2O3Æ2(CaO.SiO2) +CaO.Al2O3

Summary of Reactions During Clinker Burning..continued Between 800-1300 °C Uptake of lime by CS CA, CS + C Æ C2S 2C+S Æ C2S CA + 2C Æ C3A Formation of C4AF CA + 3C + F Æ C4AF

Summary of Reactions During Clinker Burning..continued Between 1250-1450 °C: Further uptake of lime by belite and formation of alite. C2S + C ÅÆ C3S

Thermo-Chemical profile of a long wet kiln

chains

Preheating zone 33 min

TIMEÆ

CALCINING ZONE 75 MIN

TRANS. ZONE 25 MIN

SINTER ZONE

COOLING ZONE 5 MIN

Thermo-Chemical profile of a 4 stage preheater kiln Length/diameter.......aprox. 14/1 DiameterXLength (2500 tpd) 4.8 X67 and 5x74 (mxm) Speed of rotation.......aprox. 2 rpm Secundary firing.....none Calcination extent in pre-calciner....aprox 40 %

Thermo-Chemical profile of a pre-calciner kiln

Length/diameter.......aprox. 14/1 DiameterXLength (2500 tpd) 4.0 X56 and 4.4x64 (mxm) Speed of rotation.......aprox. 3 rpm Secundary firing.....up to 65% with tertiary air Calcination extent in pre-heater....up to 95%

CALCINATION CaCO3ÅÆCaO+CO2

Decomposition rate of limestone is increased by CaCO3ÅÆCaO+CO2 • Increase in temperature of raw meal • Lowering CO2 partial pressure in combustion gases • Lowering dust load of combustion gases • Lowering particle size of raw meal • Decreasing crystal content of CaCO3 • Silicic acid formed by decomposition of clay minerals

Heating Rate and Dissotiation Speed of limestone

CaCO3ÅÆCaO+CO2 • Low HR (100 °K/min): DS depends on transport phenomena (gas-heat): heat flow to and gas transport from the inner of the limestone particles. • High particle size and HR (250 °K/min): DS hindered by low heat conductivity and high CO2 partial pressure. • High HR (450 °K/min): Increased reactivity of CaO with SiO2 (from 800 to 1000°C). No recrystalization and deffects in crystal lattice. • Kiln speed: lower rpm produce higher HR. • Alkalies (up to 2%) increase DS by lowering activation energy for limestone.

Sintering (Reactions with liquid phase) • Starts at 900-1000 °C (calcium silicates and aluminates. No C3S formation) • Free CaO starts decreasing at above 1250 °C (as melts appear and C3S formation accelerates). • Viscosity of melt affected by circulation phenomena (low melting point compounds) and AR. • Reversion from C3S to C2S can ocurr at isothermal conditions. C3S Æ C2S + C

Sintering Temperature • • • •

At 1250 °C: mostly C2S forms Below 1450 °C: C3S formation too slow. Reversion from C3S to C2S possible Above 1450 °C: quantity of melt increases, viscosity decreases. Sintering reaction rate increases. • Above 1500 °C: cost/benefit too high.

Heating rate between 900-1300 °C

Free lime

•High HR at 1450°C: high reactivity, no recrystalization •High HR at 1700 °C good strength due to high C3S with small crystal size. Too expensive. •Belite cement: possible through very fast cooling, 1000 °K/min (crystal lattice defects/disturbances) but technically unfeasible...yet!

Reaction Equilibria Belite + CaO ÅÆ Alite Shift in reaction equilibria by changes in: •

Temperature (+R;-L)

•

Quantity of melt ((+R; -L)

•

Melt viscosity (+L; -R)

•

Heating Rate between 1200-1450 °C (+R; -L)

•

Clinker cooling rate 1450-1200 °C (+eq. “Freezes” -L)

•

Time at isothermal conditions (above 1350°C) (+L; -R)

•

Fe++ content in Alite (reducing atmosphere) (+L; -R)

Fluxes Lower melting temperature and melt viscosity •Heavy metals (Ba, Sr, Ce, Cr, P,Ti, Zn) act as fluxes at up to 3% Wt. •They must bind with clinker to be active.

Effects on quality • (-) Sett. time • (+)Strenght (Cr) • (+)Grindability (V2O5,Cr2O3, BaO) at 0,5% wt.

Mineralizers •Accelerate C3S formation without melt appearance. •Ba, Cr, P, SO3 act as Min. At optimun conc. of 0,21% Wt. •Ce, Cr, Mn, Ti, Zn, Co, Pb at up to 4% act as Min. •F enables complete sintering at 1200-1300 °C (at 0,3-0,6% Wt.)

Effects on quality (+)Grindability (V2O5,Cr2O3, BaO) at 0,5% wt. Cr at up to 0,5% enhances C3S formation. At +0,5%, decomposes C3S Increase late strength (F) (-) grindability (F)

Kiln Atmosphere Oxidating atmosphere is required by economic and quality reasons. Clinker produced under Reducing conditions is brown Cement produced with clinker burned under reducing atmosphere: 1. Sett faster 2. Show lower mechanical stength at late age. (C3S incorporates Fe++ in gitter structure which makes it instable, lowering C3S content. 3. C3A content is higher as per calculations of Bogue.( Fe++ can not build C4AF). 4. Effects 2 and 3 are enhanced by slower cooling between 1450-1200°C!. 5. Excess O2 measured at kiln or preheater outlet is by no means “guaranty” that oxidating conditions prevail in all parts of the system. 6. AF and Petcoke use tend to cause “local” reducing atmospheres.

Chemical Composition LSF =

100CaO 2,8SiO2+1,18Al2O3+,65Fe2O3

1. Stoichiometric relationship. 2. Burnability of raw mix

Change in theorethical Sintering Temperature in dependence with LSF

°C

1510 1500 1490 1480 1470 1460 1450 1440

T=50 °C Lsf=6 88

90

92

lsf

94

96

98

Are 50 °C that important?

In reactions at ambient temperature, an increase of 10°K in temperature duplicates the reaction rate

Absolute Temperature °K

Influence of temperature on reaction time in accordance with Ahrrenius „Law“ Reaction Time at different temperatures 700 600 500 Time sec. 400 300 200 100 0 -1001440

1450

1460

1470 T °C

1480

1490

1500

1510

LSF Accumulated deviation from set point (211 hours, random period ) T2

8,00

T4

T3

7,00 6,00 5,00

T1

4,00 3,00 2,00 1,00

-1,00 -2,00

211

205

199

193

187

181

175

169

163

157

151

145

139

133

127

121

115

109

103

97

91

85

79

73

67

61

55

49

43

37

31

25

19

13

7

1

0,00

Chemical Composition of raw meal..continued SR =

SiO2 = Al2O3+Fe2O3

Solid Liquid

High SR values decrease burnability due to •Increased probability of having big SiO2 particles in raw meal •Decreased amount of clinker melt •A tendency for decreased homogeneity of raw meal (segregation)

Caution: increase in sinterization rates achieved by an increase in the quantity of liquid phase result in harder clinker as grindability of clinker varies inversely with porosity. Therefore, for economic reasons complete sintering with lowest posible temperature and quantity of melt shall be looked after!

Silica Ratio

Solid/liquid Ratio

Chemical Composition of raw meal..continued AR =

Al2O3 Fe2O3

=

viscous fluid

AR values are relevant for the viscosity of the clinker melt. •Low viscosity increase sinter rate •Alkalies and MgO can the viscosity of clinker melt lower. •At AR=1,38 clinker melt achieves optimun properties to promote sintering at lowest possible temperatures (1280-1340 °C).

Alumina Ratio

Typical particle size for selected materials

Raw Meal

Secund. Crusher Ground Coal

Micro Silica CEMENT

Primary Crusher Quarry

Fineness of raw meal

Appropiate surface area for chemical reaction shall be available Kaolin; picture width 200 micrometer

Scanning electron microscope photograph of a quartz grain 200 micrometer average diameter

Big quartz grains sourronded by belite ring do not let sintering to C3S to proceed due to lower reaction rate (low surface area).

1620

Alite crystal

Too big quartz grains (20x) hinder alite formation

Raw mix has to be homogeneous before burning! INHOMOGEN

HOMOGEN

Particle size criteria Tolerance for particles with average diameter + 100 micrometer (raw meal): Calcite, < 1% Quartz < 6% (by Weight) Optimun calcite average size for C3S formation 15 microns

Raw meal preparation

RM1

RM2

RM3

Homogeneization factor should be between 7-10:

RM4

feed

SDEVin/SDEVout=10 Separator

Coarse return Product

Blending Silo

Kiln feed

Mill Storage Silo

Kiln feed

Dust from kiln dust collector:

Problems?

•High alcali content •Partially calcinated (requieres less energy) •Concentration of fine particles

Fresh feed With Kiln feed

Separator

Coarse return Product

Blending Silo

Mill Chemical Composition: broad fluctuations

Storage Silo Kiln feed

Variability criteria Allowed variability of LSF, SR, AR in terms of variation coefficient: VC= Std. Dev./Average < 1% (clinker; hourly data)

Another example: Raw meal and AFR Feed

RM

AFR

LSF Clinker (24 hours S.C. DE CLINKER

102 Raw meal

100 98

AFR1

96 94 92 AFR3

90

AFR2

88 1% AFR 99% raw meal

86 84 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 time

Silica Ratio: Clinker+AFR´s 2,70 AFR1 2,60 2,50

AFR2

2,40 2,30 2,20

Raw Meal

AFR3

1% AFR, 99% raw meal

2,10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time

Alumina Ratio: Clinker+AFR 1,45

AFR1

1,40 1% AFR, 99% Raw meal 1,35 Raw Meal AFR2

AFR3

1,30

1,25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time

ASR in raw meal Normal value with brick wear rate of 9 mm/month. Life expec. 18 months

Introduction of AFR with high Na content Red spot in UTZ

AFRS May Contain Fluxing and Mineralizing Compounds in Dangerous Concentrations !!!!

Thank you very much for your attention!