Clinker Formation Concepts

7/27/2019 Clinker Formation Concepts

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Introduction:

Why study clinker burning?

To understand the influence of changes in kilnoperation conditions

Normal kiln operation

Influence of chemistry, fineness, mineralogychanges

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 dispersedmaterial)

-Clinker minerals are not stable at normal temperature!

-Quality of product is determined by:

Clinker chemistry

Clinker microstructure

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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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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
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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


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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 FeCO3calcite

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

H2Opyrites FeS2, anhydrite CaSO4, gypsum CaSO4

Fluorides

fluorspar CaF2


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Intermediate products encountered during clinker production

Type Mineral Name Formula

Simple Sulfates anhydrite CaSO4


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arcanite K2SO4

Ca SO4Compound Sulfates sulfate-spurrite 2(C2S)

calcium-langbeinite K2Ca2 (SO4)3

CaCO3Compound Carbonates spurrite 2(C2S)

Simple Chlorides sylvite KCl

Al2O3 7 Al2O3CaO Calcium Aluminates mayenite 12 CaO

Calcium Ferrites Fe2O3 2 CaO

SiO2 Al2O3 Calcium Alumino-Silicates gehlenite 2 CaO

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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


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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 temperature 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 oC = 6.1 Fe2O3 + MgO + Na2O + K2O if AR 1.38

8.2 Al2O3 5.22 Fe2O3 + MgO + Na2O + K2O if AR 1.38

1400 oC = 2.95 Al2O3 + 2.2 Fe2O3 + MgO + Na2O + K2O for MgO 2%

1450 oC = 3.0 Al2O3 + 2.25 Fe2O3 + MgO + Na2O + K2O for MgO 2 %

Quantitative change of liquid phase with temperature in several group

plants (influence of MgO, Na2O and K2O included)


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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 weightratio of 1.38


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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 quartzstructural change high quartz

CaO + CO2decomposition CaCO3

C2Scombination 2CaO + SiO2

2. according to the state of matter:

belitesolid solid quartz and free CaO

crystallisation of aluminate + ferritesolid liquid liquid phase

CaO + CO2solid gas CaCO3

liquid liquid -

liquid gas drying process, volatilisation of alkalis

CO2gas gas CO + 1/2 O2

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


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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 C3S 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 Fe2+ , H2O and K2SO4 /

CaSO4 melts


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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 reactants 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


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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
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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 contactsurface between raw mix components

(frequency factor A)

decreases with higher activation energy Eafor 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:


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development of suspension preheater

In practice, the most convenient method offollowing the reaction is by measuring the

rate of decrease of non-combined lime (i.e.

free lime).

This technique is illustrated in the followingfigures that show two raw mixes, I and II,

ofidentical chemistry (LS = 95, SR = 3.2

and AR = 2.2) and similar fineness (R200m

= 0.5 %, R90m = 7 % and R60m = 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.


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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 %


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Pyrite traces

Feldspars traces

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Assessment of Raw Meal Burnability

In practice, simple methods are mostly applied to asses the burnabilityof amix, 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 obtainedby the Mark burnability test

o The 1350, 1400 and 1450 free limevalues 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: limesaturation, silica ratio, alumina ratio,

K2O + Na2O, MgOo 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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Physiochemical Burnability Model

the amount of uncombined lime depends ono Specific reaction area (area of contact

between grains)

o Local oversaturation (grain size ofindividual minerals)

o Ambient conditions (pressure,temperature, burning time)

o Diffusion coefficient of CaO through theliquid phase (composition of the liquid

phase)

o Amount of liquid phase formed duringburning

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 isgoverned 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 beaccommodated within the liquid phase and

in which it can diffuse and thus react, is

inversely proportional to the silica ratio. Alinear 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 notbeing completely combined as also results

from grains whose lime saturation is over

100 %.


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For the Holderbank burnability testconditions the following grain diameters

were found to be critical limits:

o quartz 32 mcalcite 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 (heatliberated)

o Coal (C) + O2 CO2o Lime (CaO) + H2O Ca(OH)2o Cement + H2O Cement Hydrateso Liquid K2SO4 Solid K2SO4

Examples for endothermic reactions (heatabsorbed)

o H2O (liquid) H2O (steam)o CaCO3 CaO + CO2

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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


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170dehydration of clays

1990decarbonisation of calcite

105heat of melting

2050heating of raw materials 0 1450 C

4315Total endothermic

Exothermic processes kJ/kg clinker

-40crystallization of dehydrated clay

-420heat of formation of clinker minerals

-105crystallization of melt

-1400cooling of clinker

-500cooling of CO2 (ex calcite)

-85cooling of H2O (ex clays)

-2550Total 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 )


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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

Clinker Formation Concepts

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