Clinker Manufacture – Formation of Clinker

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Clinker Manufacture – Formation of Clinker

 

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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 measure

Analogy to transformation of igneous and sedimentary rocks
into metamorphic rocks

Difference in T, p, t

 

Two principal steps during transformation into clinker

Disintegration of original structure

-Mechanical crushing and grinding
-Thermal decomposition
-Structural rearrangement on heating (e.g. polymorphism)
-melt

Formation of new structures

-Occurrence of intermediate products
-Genesis and growth of final clinker minerals
-Crystallisation of liquid phase

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

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
etc.

Aspect

-Reaction pathway

-Reaction mechanism

-Reaction kinetics

-Reaction thermodynamics

Characteristic feature

-indicates the intermediate products occurring between reactants and products
-type(s) and reaction(s) taking place

-indicates rate at which the final products are produced
-dictates whether reaction will be at all possible, and what the heat and temperature requirements will be

 

Overview

1. Introduction

2. Reaction Pathways Encountered During Clinker Formation

-Basic Sequence of Reactions
-Mineralogical and Chemical Characteristics of Raw Mixes
-intermediate Products
-Liquid Phase
-The Overall Reaction Sequence

3. Reaction Mechanisms

4. Kinetics of Clinker Burning

-Practical Considerations
-Assessment of Raw Meal Burnability

5. Thermodynamics of Clinker Formation

 

2. Reaction Pathways

 

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

 

Sequence of reactions occurring in a rotary kiln

 

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

Cooling (°C)

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

Typical chemical characteristics of raw mixes

 

Parameter               X mean                X min                  X max

L.o.I.                                             35.5                                     33.8                                   37.3
SiO2                                            14.4                                     12.8                                    16.0
Al2O3                                          3.2                                       2.4                                       4.6
Fe2O3                                        1.8                                         1.0                                       3.8
CaO                                           42.4                                      39.0                                     44.2
SO3                                           0.37                                      0.08                                      1.1
Na2O                                        0.17                                      0.04                                      0.58
LS                                             94.0                                       85.4                                      103.4
SR                                             2.9                                         1.8                                        3.9
AR                                            1.9                                          0.7                                         3.2

 

Intermediate products encountered during clinker production

 

Type                                 Mineral Name                                 Formula

Simple Sulfates                          anhydrite                                                     CaSO4
arcanite                                                        K2SO4

Compound Sulfates                 “sulfate”-spurrite                                        2(C2S) . Ca SO4

“calcium”-langbeinite                                      K2Ca2 (SO4)3

Compound Carbonates               spurrite                                                  2(C2S) . CaCO3

Simple Chlorides                           sylvite                                                        KCl

Calcium Aluminates                      mayenite                                        12 CaO . 7 Al2O3
CaO .Al2O3

Calcium Ferrites                                                                                      2 CaO.Fe2O

Calcium Alumino-Silicates           gehlenite                                          2 CaO.Al2O3. SiO2

 

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)

 

Liquid Phase

 

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

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

 

Temperature of Liquid Formation

-The following are the lowest melting temperatures of various mixtures pertinent to Portland cement production:

-Due to the inclusion of additional oxides (e.g. K2O, TiO2, Mn2O3, etc.) the lowest formation temperature of the liquid phase in clinker is in reality around 1250°C.

 

Liquid Phase Quantity of Liquid

-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 %

 

 

 

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

 

 

displays graphically the influence of Al2O3and Fe2O3 alone on the quantity of liquid formed at 1338 °C, the solid contours representing the percentage of liquid formed. It can be seen that the effect of adding Fe2O3or Al2O3on the proportion of liquid formed at 1338 °C is influenced strongly by the alumina ratio. in a clinker composition containing 4.5 % Al2O3 and 2 % Fe2O3, for example, the Fe2O3 content is held constant and the Al2O3 content is increased, the percentage of liquid remains unchanged at 14 %, because the iso-Fe2O3 contour lies parallel to the iso-liquid contour in this region.

If, however, the iron content is increased and the Al2O3content remains constant, the proportion of liquid at first increases
rapidly until at 3.3 % Fe2O3 it attains a maximum of
21 %. With further increase in Fe2O3the quantity of liquid decreases.
Therefore, the most effective use of Al2O3and Fe2O3- with respect to liquid formation at 1338 °C – occurs when the two are used in the weight ratio of 1.38.

 

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
AR = Alumina ratio OB, GM, UN, VN = Different Plants At 1400 °C and above increasing Al2O3 and / or Fe2O3always result in a larger quantity of liquid phase, Al2O3 being the one with the larger influence.

 

 

Overview

1. Introduction
2. Reaction Pathways Encountered During Clinker Formation

-Basic Sequence of Reactions
-Mineralogical and Chemical Characteristics of Raw Mixes
-Intermediate Products
-Liquid Phase
-The Overall Reaction Sequence

3. Reaction Mechanisms

4. Kinetics of Clinker Burning

-Practical Considerations
-Assessment of Raw Meal Burnability

5. Thermodynamics of Clinker Formation

 

3. Reaction Mechanisms – Definitions

 

Classification of reactions

 

2. according to the state of matter:

 

-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:

 

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

3. Reaction Mechanisms – 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

-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.

 

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

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.

 

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

 

Overview

 

1. Introduction

2. Reaction Pathways Encountered During Clinker Formation
-basic Sequence of Reactions

-Mineralogical and Chemical Characteristics of Raw Mixes
-intermediate Products
-Liquid Phase
-The Overall Reaction Sequence

3. Reaction Mechanisms
4. Kinetics of Clinker Burning
-Practical Considerations
-Assessment of Raw Meal Burnability
5. Thermodynamics of Clinker Formation

 

4. Kinetics of Clinker Burning

 

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

-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

-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

 

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:

-Holcim burnability test – involves the heating of a raw mix under well defined laboratory conditions, and subsequent laboratory determination of the non-combined lime.
-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

 

 burnability test

 

-The test is based on the isothermal burning at 1400 °C of raw meal nodules
-The test allows the determination of the relative influence of the various material parameters to be ascertained, free from the influence of process technological disturbances.

Evaluation                           Free Lime %

very good                          0 – 2
goo                                   d 2 – 4
moderate                         4 – 6
poor                                6 – 8
very poor >                        8

 

Statistical Burnability Model

 

-Quantitative evaluation of the data obtained by the Holcim burnability test

-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

-Chemical Parameters: lime saturation, silica ratio, alumina ratio, K2O + Na2O, MgO
-Physical Parameters: residue on 200 m and 90 m sieves, quantity of mica, quartz and iron minerals

The burnability model can be used as an
instrument for optimization of raw mixes

Physiochemical Burnability Model

the amount of uncombined lime depends on

Specific reaction area (area of contact between grains)
Local oversaturation (grain size of individual minerals)
Ambient conditions (pressure, temperature, burning time)
Diffusion coefficient of CaO through the liquid phase (composition of the liquid phase)
Amount of liquid phase formed during burning
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 %.
For the “Holderbank” burnability test conditions the following grain diameters were found to be critical limits:
quartz > 32 um
calcite > 90 um

Overview

1. Introduction
2. Reaction Pathways Encountered During Clinker Formation

Basic Sequence of Reactions
Mineralogical and Chemical Characteristics of Raw Mixes
Intermediate Products
Liquid Phase
The Overall Reaction Sequence

3. Reaction Mechanisms
4. Kinetics of Clinker Burning

Practical Considerations
Assessment of Raw Meal Burnability

 

5. 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)                              Endothermic
Formation of alite Exothermic
1300 – 1240            Crystallization of liquid phase into mainly (cooling cycle) aluminate and ferrite      Exothermic

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.

 

 

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