Clinker formation-concepts

Clinker formation-concepts

1. Introduction:


Why study clinker burning?


“To understand the influence of changes in kiln
operation conditions”

Normal kiln operation

Influence of chemistry, fineness, mineralogy
changes

Influence of new mix components (pozzolan,
AFR, sand, etc.)

Abnormal kiln operation

know causes of badly burnt clinker

understand why rings and deposits form

be able to suggest counter measures




Synthetic Hydraulic Minerals
Analogy to transformation of igneous and sedimentary rocks

into metamorphic rocks

Difference in T, p, t

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Two principal steps during transformation into clinker

Disintegration of original structure

Mechanical crushing and grinding

Thermal decomposition

Structural rearrangement on heating (e.g. polymorphism


Formation of new structures

Occurrence of intermediate products

Genesis and growth of final clinker minerals

Crystallization of liquid phase

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Features characterising the clinker formation process

-Complex system (series of diverse mechanisms)!

-Requires mechanical, thermal and electrical energy

-Reaction rate is slow (necessity of high temperatures, finely dispersed
material)

-Clinker minerals are not stable at normal temperature!

-Quality of product is determined by:

Clinker chemistry

Clinker microstructure

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3. Control of burning process

Material technological aspects

-Raw meal burning behaviour

Burnability

Dust formation

Coating behaviour

Granulation of clinker

etc.

-Quantity and properties of liquid phase

Process technological aspects

-Temperature profile

-Kiln atmosphere

-Fuel type

-Flame characteristics

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4. Reaction pathway
indicates the intermediate products occurring between reactants and products
Reaction mechanism
type(s) and reaction(s) taking place
Reaction kinetics
indicates rate at which the final products are produced
Reaction thermodynamics dictates whether reaction will be at all possible, and what the heat and
temperature requirements will be
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Reaction Pathways Encountered During Clinker Formation

Basic Sequence of Reactions

Mineralogical and Chemical Characteristics of Raw Mixes

Intermediate Products

Liquid Phase

5. The Overall Reaction Sequence

To fully describe the pathway of clinkering, it is necessary to consider
the following aspects:

-the chemical and mineralogical content of the raw mix

-the overall sequence of reactions

-the chemical and mineralogical nature of the intermediate products



Heating (°C)

20 – 100 Evaporation of H2O

100 – 300 Loss of physically adsorbed water

400 – 900 Removal of structural H2O (H2O and OH groups) from clay
minerals

>500 Structural changes in silicate minerals

600 – 900 Dissociation of carbonates

>800 Formation of belite, intermediate products, aluminate and ferrite

>1250 Formation of liquid phase (aluminate and ferrite melt)

~1450 Completion of reaction and re-crystallization of alite and belite

6. Cooling (°C)

1300 – 1240Crystallization of liquid phase into mainly aluminate and ferrite

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Mineralogical characteristics of raw mixes

Carbonates

MgCO3, ankerite CaCO3 (Mg,Fe)CO3 magnesite MgCO3, siderite FeCO3 calcite
CaCO3, dolomite CaCO3

Simple Oxides

quartz SiO2, hematite Fe2O3, magnetite Fe3O4

Feldspars

potassium feldspars (Na,K)Si3O8 and

plagioclase series (Na,Ca)(Si,Al)Al2Si2O8

Sheet silicates

minerals of the mica and chlorite groups
(e.g. biotite, muscovite, chlorite),

clay minerals (e.g. kaolinite, montomorillonite, illite, palygorskite)

Hydroxides

Al-hydroxides (e.g. boehmite),
Fe-hydroxides (e.g. goethite, limonite)

Sulfides / sulfates

H2Opyrites FeS2, anhydrite CaSO4, gypsum CaSO4

Fluorides

fluorspar CaF2

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I ntermediate products encountered during clinker production

T ype M ineral Name F ormula

S imple S ulfates anhydrite C aSO4

8. arc anite K2 SO4

C a SO 4Compound S ulfates ”s ulfate”-spurrite 2 (C2S)

”c alcium”-langbeinite K2Ca2 (SO 4)3

C aCO3Compound Carbonates s purrite 2 (C2S)

S imple Chlorides s ylvite KC l

A l2O3 7 A l2O3CaO Calcium A luminates mayenite 1 2 CaO

Calcium Ferrites F e2O3 2 C aO

S iO 2 A l2O3 Calcium A lumino-Silicates gehlenite 2 C aO

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Reasons for the formation of intermediate products

-Intermediate products are preferentially formed by kinetically faster reaction
rates

-Intermediate products are the reaction products of localised zones in the meal
charge,

i.e. local equilibrium but not overall equilibrium was reached (e.g. gehlenite
formation)

-Intermediate products are really the equilibrium products at the given
temperature and gas atmosphere,

but not at the final clinkering temperature (e.g. spurrite formation)


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Liquid Phase

basically created by early melting compounds such as Fe2O3 and Al2O3 and
some minor compounds such as MgO and Alkalis

9. The composition of the raw mix determines

-temperature at which liquid will first be formed

-amount of liquid formed at any given temperature

-the physical properties of the liquid at any particular temperature, especially
its viscosity

-Although most raw mixes show about the same minimum tempe rature of
liquid formation (eutectic point),

the quantity of liquid formed at this and progressively higher temperatures
varies according to the raw mix chemistry.

-In the Portland cement relevant parts of the system C – S – A – F, in which
melting begins at 1338 °C, the composition of the liquid is:

CaO - 55 %

SiO2 - 6 % Alumina ratio

Al2O3 - 23 % (AR) = 1.38

Fe2O3 - 16 %

Quantity calculation formulae acc. to LEA, considering different
temperature:

1338
o
C = 6.1 Fe2O3 + MgO + Na 2O + K2O if AR  1.38

8.2 Al2O3 – 5.22 Fe2O3 + MgO + Na 2O + K2O if AR 1.38

1400
o
C = 2.95 Al2O3 + 2.2 Fe2O3 + MgO + Na 2O + K2O for MgO  2
%

1450
o
C = 3.0 Al2O3 + 2.25 Fe2O3 + MgO + Na 2O + K2O for MgO  2 %

Quantitative change of liquid phase with temperature in several group
plants (influence of MgO, Na2O and K2O included)

10. Influence of Al2O3 and Fe2O3 alone on the quantity of liquid formed at
1338 °C.

The most effective use of Al2O3 and Fe2O3 – with respect to liquid
formation at 1338 °C – occurs when the two are used in the weight
ratio of 1.38

11. Viscosity of liquid phase

-The viscosity of the liquid phase diminishes exponentially with increasing
temperature and at 1400 °C is reduced by addition of fluxing components in
the following order:

Na2O < CaO < MgO < Fe2O3 < MnO

-With increasing SiO2 content of the melt and to a lesser extent with increasing
Al2O3, appreciable increases in viscosity occur.

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The overall reaction sequence, displayed based on qualitative change of
minerals from samples taken from an operational kiln

Minerals identified at different locations (long wet kiln)



Sequence of compound formation according to chemical composition

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Reaction Mechanisms

Definitions

State of matter

solid: definitive volume and definite shape

liquid: definitive volume, assumes shape of container

gaseous: neither definitive volume nor definite shape

Classification of reactions

1. according to their type:

low quartzstructural change high quartz

CaO + CO2decomposition CaCO3

C2Scombination 2CaO + SiO2

2. according to the state of matter:

belitesolid – solid quartz and free CaO

crystallisation of aluminate + ferritesolid – liquid liquid phase

CaO + CO2solid – gas CaCO3

liquid – liquid -

liquid – gas drying process, volatilisation of alkalis

CO2gas – gas CO + 1/2 O2

3. according to rate controlling step (kinetics of reaction)

14. diffusion formation of alite

belite (initial reaction)phase boundary quartz + free CaO

nucleation liquid phase crystallisation of aluminate + ferrite; alite formation


Examples

Structural changes: Arrangement of the atoms in low and high quartz



Structural changes: Calcite – Aragonite transition



Decomposition reactions (during clinker production)

-solid / gas type

15. De-hydroxylation of the clay minerals (kaolinite, etc.)

De-carbonation of the carbonate minerals (magnesite, dolomite, calcite,
spurrite)

-solid / solid type

decomposition of alite

Characteristic of this reaction type is that the single reactant is transformed
into two products.

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Decomposition reaction: Equilibrium dissociation pressure of calcite and
spurrite with temperature



Decomposition reaction: Decomposition of C 3S at 1175 °C

In the case of impure C3S, i.e. clinker alite, the rate of decomposition is
appreciably accelerated by:
 the presence of lime and C2S nuclei
 the presence of Fe
2+
, H2O and K2SO4 /
CaSO4 melts

16. Combination reaction: Formation of Belite

Belite formation is the result of a combination between the calcite and silica
components of the raw mix.



The rate limiting mechanism by which belite is formed (after an initial phase
boundary controlled reaction) depends on the diffusion of ions through the
solid state.

The rate of this reaction is thus dependent on:

the path distance that the diffusing species
have to travel

defects in the reactant’s crystal lattices.


Combination reaction: Formation of Alite

Formation of alite only at T > 1250 °C (lower stability limit). At that
temperature, the liquid phase is also starting to form: The formation of alite is
a liquid – solid reaction

17. The formation of alite and its stabilisation depends on the presence of
the liquid phase.

The rate of reaction is dependent on:

the path distance that the diffusing species
have to travel
 quantity and viscosity of liquid phase

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4. Kinetics of Clinker Burning

18. Theoretical consequences:

Rate increases with higher temperature (but also costs!)

Rate decreases with higher activation energy (different raw mix mineralogy)

Rate increases with higher frequency factor (larger contact surface, i.e. finer
mix)

The rate of reaction
 increases with temperature and contact
surface between raw mix components
(frequency factor A)
 decreases with higher activation energy Ea
for raw mix components.
To compensate for the slow reactivity of the less reactive minerals, a higher
burning temperature and / or longer burning period (longer clinkering zone) is
required.



Practical considerations:

19. development of suspension preheater


 In practice, the most convenient method of
following the reaction is by measuring the
rate of decrease of non-combined lime (i.e.
free lime).
 This technique is illustrated in the following
figures that show two raw mixes, I and II,
of identical chemistry (LS = 95, SR = 3.2
and AR = 2.2) and similar fineness (R200m
= 0.5 %, R90m = 7 % and R60m = 15 %).
 It is evident that the difference in
mineralogy and actual particle size of
the individual crystals influence both the
mechanism and rate of reaction, especially
at start of the clinker formation.

20. Limestone

Calcite 97 %

Dolomite ~ 2 %

Quartz traces

Chlorite -

Illite and Micas -

Pyrite traces

Feldspars -

Shale A

Calcite ~ 40 %

Dolomite -

Quartz ~ 25 %

Chlorite ~ 20 %

Illite and Micas ~ 10 %

Pyrite ~ 2 %

Feldspars ~ 2 %


Shale S

Calcite ~ 10 %

Dolomite -

Quartz ~ 55 %

Chlorite ~ 10 %

Illite and Micas ~ 20 %

21. Pyrite traces

Feldspars traces






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Assessment of Raw Meal Burnability

In practice, simple methods are mostly applied to asses the “burnability” of a
mix, i.e. the ease of formation of the clinker minerals. Three distinct methods
are practiced at HGRS:

Statistical burning model – in which ten material parameters influence the rate
of clinker formation. The non-combined CaO value, of any raw mix, relative to
that of a standard raw mix is calculated.

Physicochemical burning model – requires no standard raw mix. Only 4
parameters need to be considered.

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Statistical Burnability Model
 Quantitative evaluation of the data obtained
by the Mark burnability test
o The 1350, 1400 and 1450 free lime
values of other raw mixes from the
same raw material components can be
determined based on one single
burnability test of one mix
o Chemical Parameters: lime
saturation, silica ratio, alumina ratio,
K2O + Na2O, MgO
o Physical Parameters: residue on 200
m and 90 m sieves, quantity of
mica, quartz and iron minerals
NOTE : The burnability model can be used as an instrument for optimization of
raw mixes

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23. Physiochemical Burnability Model
 the amount of uncombined lime depends on
o Specific reaction area (area of contact
between grains)
o Local oversaturation (grain size of
individual minerals)
o Ambient conditions (pressure,
temperature, burning time)
o Diffusion coefficient of CaO through the
liquid phase (composition of the liquid
phase)
o Amount of liquid phase formed during
burning
o Supply and demand of CaO
 all these influencing factors may be
incorporated in four parameters: SR, LS,
amount of oversized quartz grains,
amount of oversized calcite grains .
(Pressure, temperature and burning time
are considered to be constant.)
 Silica ratio (SR) and lime saturation
(LS)
 The formation of C3S from C2S and CaO is
governed by the diffusion of CaO through
the melt. The silica modules and lime
saturation are sufficient to describe this
chemical reaction quantitatively.
 The amount of CaO which can be
accommodated within the liquid phase and
in which it can diffuse and thus react, is
inversely proportional to the silica ratio. A
linear relationship exists between max. lime
saturation and silica ratio values at which no
free lime can be observed.
 Quartz and calcite grains
 Whether a grain of material reacts fully
under given burning conditions depends on
its diameter, structure and chemical
composition.
 Too large calcite grains result in CaO not
being completely combined as also results
from grains whose lime saturation is over
100 %.

24.  For the “Holderbank” burnability test
conditions the following grain diameters
were found to be critical limits:
o quartz  32 m
calcite  90 m





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Thermodynamics of Clinker Formation

During clinker production, heat is both absorbed (endothermic heat changes)
and produced (exothermic heat changes)

Temp. (°C) Type of Reaction Heat Change

20 – 100 Evaporation of free H2 O Endothermic

100 – 300 Loss of physically adsorbed H2O Endothermic

400 – 900 Removal of structural H2O (H2O, OH groups from
clay minerals) Endothermic

600 – 900 Dissociation of CO2 from carbonate Endothermic

> 800 Formation of intermediate products, belite, aluminate and
ferrite Exothermic

> 1250 Formation of liquid phase (aluminate and ferrite
melt) EndothermicFormation of alite Exothermic

1300 – 1240 Crystallization of liquid phase into mainly (cooling
cycle) aluminate and ferrite Exothermic
 Examples for exothermic reactions (heat
liberated)
o Coal (C) + O2  CO2
o Lime (CaO) + H2O  Ca(OH)2
o Cement + H 2O  Cement Hydrates
o Liquid K2SO4  Solid K2SO4
 Examples for endothermic reactions (heat
absorbed)
o H2O (liquid)  H2O (steam)
o CaCO3  CaO + CO2
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26. DTA curves of typical cement raw meals

The greatest heat requirement occurs between 850 – 900 °C, i.e. for the
decomposition of the carbonate minerals.The total heat requirements for
dehydration, decarbonisation and melting exceed the heat liberated by the
formation of belite and the intermediate and final products.






Endothermic processes kJ/kg clinker

27. 170dehydration of clays

1990decarbonisation of calcite

105heat of melting

2050heating of raw materials 0 – 1450 °C

4315Total endothermic

Exothermic processes kJ/kg clinker

-40crystallization of dehydrated clay

-420heat of formation of clinker minerals

-105crystallization of melt

-1400cooling of clinker

-500cooling of CO2 (ex calcite)

-85cooling of H2O (ex clays)

-2550Total exothermic

Net theoretical heat of clinker formation  + 1765

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Heat balance of wet and dry kiln, kJ/kg clinker

( HFW Taylor: Cement Chemistry, 1998 )

28. Dry kiln Wet kiln

Evaporation of H2O 13 (0.4%) 2,364 (41.5%)


Heat of reaction 1,807 (54.6%) 1,741 (30.5%)

Heat losses through 711 (21.5%) 812 (12.3%)
gas, clinker, dust, etc.

Heat lost in air from cooler 427 (13.0%) 100 (1.7%)

Heat losses by radiation 348 (10.5%) 682 (12.0%)
and convection

3,306 kJ/kg 5,699 kJ/kg