Raw Material Charact-Burnability

Kiln Process and Operation Raw Material Characteristics

Raw Material Characteristics (Including Burnability and Volatile Matter)

Training Course: Kiln Process and Operation

This subject is related to the Computer Based Training program which can be downloaded for free from this website.

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Kiln Process and Operation Raw Material Characteristics

TABLE OF CONTENTS RAW MATERIAL MIXTURES AND THEIR CHARACTERISTICS .......................... 4 4.1 INTRODUCTION ...................................................................................................... 4 4.2 BURNABILITY OF RAW MIX ................................................................................ 5 4.2.1 Coarse grains........................................................................................................ 5 4.2.1.1 Coarse grains - determination ....................................................................... 7 4.2.2 Effects of chemical composition.......................................................................... 7 4.2.3 Alternative burnability determination................................................................ 10 4.2.3.1 Burnability index. ....................................................................................... 10 4.2.3.2 The burnability factor ................................................................................. 10 4.3 CLINKER FORMATION ........................................................................................ 11 4.3.1 Effect of changes in chemistry........................................................................... 13 4.3.1.1 Reduction in the alumina/iron ratio ............................................................ 13 4.3.1.2 Lowering of the silica modulus................................................................... 14 4.3.1.3 Lowering the lime saturation factor............................................................ 14 4.4 THE BEHAVIOUR OF VOLATILE MATTER...................................................... 15 4.4.1 Mechanism of circulation of volatile matter...................................................... 15 4.4.1.1 Evaporation in the kiln................................................................................ 15 4.4.1.2 SO2 gas and volatile matter exiting the preheater ...................................... 16 4.4.1.3 Condensation in the preheater..................................................................... 16 4.4.2 Affinity between the volatile components ......................................................... 16 4.4.3 Volatility of the compounds of volatile components......................................... 17 4.4.3.1 Definition of volatility ................................................................................ 17 4.4.3.2 Average evaporation factors: ...................................................................... 18 4.4.3.3 Molecular ratio of sulphur and alkalis ........................................................ 19 4.4.3.4 Vapour pressure .......................................................................................... 20 4.4.4 A mathematical model ....................................................................................... 21 4.4.4.1 Definitions; evaporation factor and valves ................................................. 22 4.4.4.2 Rules of computations ................................................................................ 23 4.4.4.3 Average values for ε and valves ................................................................. 23

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4.4.5 Three aspects of volatile matter ......................................................................... 24 4.4.5.1 Content of volatile matter in clinker ........................................................... 25 4.4.5.2 Emission to the environment ...................................................................... 26 4.4.5.3 Operational aspects ..................................................................................... 26 4.4.6 Solving problems with sulphur .......................................................................... 30 4.4.7 Solving problems with chloride ......................................................................... 31 4.4.8 Solving problems with alkali ............................................................................. 32 4.4.9 Setting up a mass and volatile matter balance ................................................... 33 4.5 CONCLUSION......................................................................................................... 37

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Kiln Process and Operation Raw Material Characteristics

RAW MATERIAL MIXTURES AND THEIR CHARACTERISTICS

4.1 INTRODUCTION Designing a raw mix is a complex process involving many factors. Often it is possible to obtain desired chemical properties of the clinker using a different blend of the available raw materials. The behaviour of a new and untried raw mix in the kiln can to a large degree be predicted by the tools provided in the concept of Burnability. However, there are definitely cases where more complex investigations are required to explain kiln behaviour and the observed clinker properties. In this case microscopy examinations on the clinker is widely used to provide further information. While the goal is to produce clinker with uniform and predictable quality, it is necessary that this is done with a minimum amount of energy that the kiln operation is smooth and that expensive downtime of the kiln is avoided. To achieve this goal it is necessary to know •

which parameters in the raw mix influence the kiln operation

•

how and why they influence operation

•

what can be done about it

In this module, three central concepts in the relation between raw meal characteristics and kiln operation is treated, namely: •

the burnability of a raw mix,

•

the clinker formation treated as a physical agglomeration process and

•

the circulation phenomena of volatile matter in a kiln system.

Each of these concepts serve to explain how various characteristics of the raw mix influence the kiln operation and explain what appropriate action can be taken in kiln operation.

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Kiln Process and Operation Raw Material Characteristics

4.2 BURNABILITY OF RAW MIX Different types of raw mix are designed to give proper amounts of the clinker minerals: alite (C3S), belite (C2S), aluminate (C3A) and ferrite (C4AF). The readiness with which a raw mix is transformed into these minerals in the course of a high temperature treatment is determined by their chemical, mineralogical and granular composition or a combination of these elements. Such readiness of transformation into the clinker minerals is called the burnability. Burnability is the deciding factor for the temperature necessary in the kiln burning zone in order to obtain a satisfactory clinker product. The burnability is determined in the laboratory on a small representative sample of the kiln feed. One procedure used in laboratories consists essentially of an isothermal treatment of the calcined raw meal, one part at 1400 °C, another at 1450 °C, and a third at 1500 °C, all for 30 minutes. After cooling, the content of free lime in the clinker is determined, and the results, relative to those of a well-known standard sample similarly treated, are taken as a measure of the burnability. Different types of raw mix with the same chemical composition and equal fineness may have greatly differing burnabilities. The reason for the differences in burnability can be found in the differences in mineralogical composition of the raw mix types. A formalised description of the various minerals in the raw mix according to their chemical reaction rates during burning has been developed. The results gained from the theoretical work together with the experience gained from practical implementation of the description have revealed that poor burnability of a raw mix may be caused by coarse grains from two mineral groups: (1) Calcite and marl rich in CaCO3 (2) Quartz and flint rich in SiO2

4.2.1 Coarse grains The clinker consists of Alite, Belite, solidified Liquid phase, Free lime, and Pores. The effect of coarse grains in the raw mix can be illustrated by looking at the distribution of the clinker minerals in the resulting clinker. This is done by making a polished section of the clinker sample and examining the clinker by microscopy. The optimum distribution, as seen through microscopy, can be seen in Figure 4.1. However, if the chemical reaction has not gone to completion the result may be as seen on Figure 4.2. The reason for this is local inhomogeneity in the raw mix. This may stem from course grains of calcite or coarse grains of quartz. Figure 4.2 shows a clinker with a large belite cluster (small dark grains) surrounding what was previously a quartz grain. The cluster has not reacted with CaO to form the Page 5

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Kiln Process and Operation Raw Material Characteristics

desired C3S during the clinkerisation, and therefore the unused CaO is present somewhere in the clinker as free lime.

Figure 4.1 Homogenous Clinker

Figure 4.2 Inhomogeneous clinker with large belite cluster from coarse quartz.

If the original coarse grain had been calcite, a cluster of C3S would have been formed around a grain of free lime. In both cases the clinker will present a high free lime content. So both coarse quartz and coarse calcite may result in poor burnability, with corresponding high free lime content and too little C3S in the clinker.

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4.2.1.1 Coarse grains - determination The question is how to determine what constitutes a coarse grain? From calculations, experience, and many tests using standard sieves, the following particle sizes have been found to be critical for residual free lime after burning for 30 minutes: quartz and silicates:

+45 µm

calcite:

+125 µm

It has been found that after burning for 30 minutes at 1400oC, an increase in the amount of coarse particles results in the following increases in free lime: +1% quartz + 45 µm leads to

+0,93% free CaO

+1% calcite + 125 µm leads to

+0,56% free CaO

(From the above can be deducted that coarse grains of Quartz is more serious than coarse grains of Calcite). If improved burnability of the raw mix is desired, a reduction of the coarse particles can be achieved by finer grinding. However, this may only have a significant effect on the amount of coarse calcite since this is the softer material. If a high content of the hard quartz particles must be reduced, separate grinding may be necessary. The content of the coarse particles in the raw mix is determined in the polarising microscope. The coarse quartz is determined in the acid insoluble residue + 45 µm and coarse calcite is determined in the total sieving residue on 125µm.

4.2.2 Effects of chemical composition In addition to the coarse grains, both the lime saturation factor and the amount of liquid phase in the raw mix at the burning temperature influence the burnability. Knowing the effect on burnability caused by the coarse grains, the effect attributable to chemical composition can be determined. Statistical treatment of the data from a large number of raw meals of different types has shown that the chemical parameters of significance are the lime saturation factor LSF, and the silica modulus MS (MS is inversely related to the content of the liquid phase). Page 7

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The following formula may be used for estimating the free CaO after burning for 30 minutes at 1400°C:

CaO1400o C = 0,33 ⋅ (LSF − 95) +1,8 ⋅ (MS − 2)

+ 0,93 ⋅ SiO2 (+ 45µ ) + 0,56 ⋅ CaCO3 (+ 125µ )

As expected, an increase in LSF and MS will result in higher free lime. Figure 4.3 shows the correlation between free lime calculated according to this equation and the corresponding measured free lime after laboratory burnability tests. Each total free lime consists of three contributions. It is clear that the various raw mixes are very different from each other, with respect to which of the three factors (coarse quartz, coarse calcite, or chemistry) contributes to the bulk of the potential free lime.

Figure 4.3 Correlation between measured and estimated free lime

Using this formula makes it possible to optimise the burnability of a raw mix consisting of given raw materials, not only in laboratory experiments but also under plant conditions. Assuming that the contributions from the calcite, the quartz and the chemical composition are known, the free lime will be as shown in figure 4.4 on the following page (represented by the bar on the left).

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Figure 4.4 Effect of raw mix changes on resulting free lime

It is shown in figure 4.4 that grinding to a 12% residue on the 90 µm sieve instead of to 17%, improves the burnability from 7,8 to 6,5% free lime due to a reduction in the amount of calcite grains coarser than 125 µm. At the plant from which this example is taken, the clinker was burned to an average free lime of 2,5%. After the feed fineness was increased, this free lime level could be achieved at a lower burning temperature, or its equivalent - a shorter burning time. In this case, a 10% reduction in burning time was realised. This corresponds, for practical purposes, to a 10% increase of the kiln output. The burnability may also be improved by changing the chemistry. Changing the lime saturation factor from 98 to 94 improves the burnability from 6,5% to 5,2% free lime, as shown in Figure 4.4. Decreasing MS will also improve the burnability. An alternative way of reducing the temperature at which clinkerisation takes place is by the addition of mineralisers and fluxors. In the pure four component system of CaO, SiO2, Al2O3 and Fe2O3, C3S does not form below 1250°C. A mineraliser is a minor component which encourages the formation of C3S by lowering this lower limit of stability of C3S. An example of a mineraliser that is widely used is fluoride. A fluxor is a minor components which reduces the temperature at which liquid phase is first formed, and modifies the viscosity and the surface tension of the clinker liquid to promote the formation of the clinker minerals. Sulphate is often used as a fluxor and introduced as gypsum.

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4.2.3 Alternative burnability determination Quantitative microscopy is a rather time consuming technique not available at most cement laboratories. It is therefore encouraging that an adequate description of burnability may also be obtained by using the raw mix acid insoluble residue > 45 µm and the total residue > 125 µm, instead of the microscopically determined parameters. The burnability expression obtained with LSF, Ms, acid insoluble residue A45 > 45 µm, and total residue T125 > 125 µm as parameters is slightly different:

CaO1400o C = 0,35⋅ (LSF − 96) +1,58⋅ (MS − 1,6) + 0,55⋅ A45 + 0,12 ⋅ T125 For an evaluation of burnability, other formulas have been developed by earlier authors of which we will mention the burnability index and the burnability factor. Both formulas only consider the chemical influence on the burnability and disregards the influence of the mineralogy. Hence the accuracy of the formulas is limited.

4.2.3.1 Burnability index. An early formula for the estimation of raw meal burnability is Kuehl's burnability index defined as:

Burnabilit y index =

C3 S C 4 AF + C 3 A

The higher the content of C3S with corresponding lower contents in C4AF or C3A, the harder the clinker is to burn. The index is indirectly dependent on the LSF and the MS and roughly proportional to the earlier mentioned formula for CaO1400°C without regarding the contribution of coarse grains.

4.2.3.2 The burnability factor The factor is developed based on pure empirical notions and observations and hence might be suspect in its fundamental reasoning. The coefficients x, y and z are best determined by multiple regression analysis performed on laboratory tests.

Burnabilit y factor = x ⋅ LSF + y ⋅ MS + z ⋅ (MgO + alkalies ) In the originally published formula, the coefficients were stated as x = 100, y = 10 and z = 3. Page 10

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4.3 CLINKER FORMATION The distribution of the cement clinker size is very important with respect to kiln operation and cement mill operation and therefore directly affects the production cost of the cement. Dusty, finely grained clinker seriously endangers satisfactory maintenance free operation of grate coolers, and generally strongly reduces the brick life in the burning zone of the kiln. Dust returned from the clinker cooler often leads to the formation of a porous, unstable and pumice-like coating on the brick lining, instead of the desired dense, stable coating. Furthermore, the handling of very dusty clinker is problematic and the grindability is often inferior to that of a coarser, dust free clinker. Experience shows that energy consumption in the cement mills may increase 40% when the clinker granulometry changes from clinker nodules to the dusty or pumice like type. It is of interest to discuss whether, and if so, how this dusty type of clinker can be avoided. Reflections on clinker formation, especially regarding agglomeration, have led to a model in which two processes compete. 1) One is the physical agglomeration and nodulisation of the material in the burning zone due to the formation of the clinker liquid phase. 2) The other is the formation and growth of C3S particles, which counteract the nodulisation process.

Nodulisation Pelletisation or nodulisation in drums is well known in many industries. The particles are held together by the capillary forces of the liquid. Theory and experiments have shown that the rate of formation of nodules is a function of the amount of liquid, particle size, and speed of drum revolution. There is every reason to assume that the same physical principles determine the agglomeration processes in a rotary kiln.

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At the entrance to the burning zone a system of particulate material and a liquid is present. Figure 4.5 demonstrates the amount of liquid as a function of temperature. The liquid is formed in a narrow temperature interval at approximately 1330°C. After this, the amount of liquid increases very little with increasing temperature, and may be regarded as constant; at this time, as long as the particles are not too coarse, nodulisation may take place.

Figure 4.5 % Liquid versus temperature

The formulas for calculating the amount of liquid phase from the chemical composition of the clinker is presented in module 1, section 1.2.8.1. C3S is formed in the material in the rotary kiln during the burning process. Numerous investigations of clinker samples have shown that dusty and pumice like clinker - an un-nodulised product - is often made up of sintered C3S particles. These particles consist of several individual crystals, which have grown together. When two C3S crystals meet, they have a high probability of growing together to form coarse C3S particles; increasing the particle size in a nodulising system will slow down and eventually stop the nodulisation process.

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Figure 4.6 schematically shows the combination of nodulisation caused by the liquid phase and the counteraction due to the formation and sintering of C3S particles. The uppermost curve demonstrates the mean size of the nodules versus time, and the lower curves show the formation of C3S. The curves demonstrate the situations for different mean temperatures of the burning zone: The higher the temperature, the higher the formation rate of C3S.

Figure 4.6 Formation and size of nodules, and formation of c3s at various temperatures, both as a function of time

The more C3S is formed, the higher the probability of getting large particles consisting of several C3S crystals grown together. Above a certain quantity of C3S, the number of C3S particles is so large that further formation of nodules is impossible. It is inferred from the diagram that a high mean temperature of the burning zone will result in a small average nodule size.

4.3.1 Effect of changes in chemistry The most noticeable changes in clinker formation result from changes in the ratios of alumina/iron, lowering of silica modulus and LSF.

4.3.1.1 Reduction in the alumina/iron ratio This change causes formation of a greater amount of liquid phase at a lower temperature. The maximum effect is obtained by reducing the ratio from 2,0 to 1,6, which is the optimum value. As this will not influence the formation of C3S, it means that we have a Page 13

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longer time with enough liquid phase for nodulising at the entrance to the burning zone, and thereby improved nodulisation. An increase of the alumina/iron ratio will cause the opposite to happen.

4.3.1.2 Lowering of the silica modulus When the silica modulus decrease, the amount of liquid phase increases. If the clinker is dusty because of too little liquid, a decrease in the silica modulus will result in improved nodulisation. The clinker will be easier to burn and require lower burning temperature/shorter burning time.

4.3.1.3 Lowering the lime saturation factor It is often seen that a decrease of the LSF (i.e. from 98% to 95%) has given better clinker formation even if the change does not result in a corresponding change in the amount of liquid phase in the clinker. The decrease in LSF makes the clinker easier to burn and also decreases the total potential alite content; thus the extent of alite sintering is reduced. The necessary lower temperature also implies a shrinkage of the maximum temperature zone in relation to the liquid zone. This, together with the reduced amount of alite sintering, provides for improved nodulisation.

Figure 4.7 Dependence of kiln temperature profile on mix burnability Page 14

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Kiln Process and Operation Raw Material Characteristics

When the raw mix is difficult to burn, the temperatures in the burning zone may be very high and The Maximum Temperature Zone is too long, at the expense of The Heating and Liquid Zones, as shown in figure 4.7. Insufficient nodulisation takes place, and the kiln produces a dusty clinker. For certain kiln systems, the temperature profile and average burning zone temperature is critical with respect to whether "good" or "bad" nodulisation is obtained. The amount of fuel burned in the kiln will influence the average burning zone temperature and the material temperature profile. An elevated heat consumption may therefore have a similarly adverse effect on the nodulisation process as that of a raw mix with poor burnability.

4.4 THE BEHAVIOUR OF VOLATILE MATTER Minor components introduced in the kiln system with raw materials and fuel can give rise to difficulties in kiln operation when present in high concentrations within the system. The minor components normally considered are the alkalis potassium and sodium + sulphur and chlorine, but other components such as fluorine and heavy metals, although so far of less practical importance, follow a similar pattern. The concentration of these components is increased in the kiln system due to internal and external circulation as the components will evaporate at the burning zone temperature, condense when cooled in the kiln back end and be brought back to the burning zone with the material. In many older plants, such problems are unknown because the kiln system design allows the evaporated components to escape through the chimney. However, in installations built in the eighties and nineties with efficient pre-heaters and filters, it can become a problem.

4.4.1 Mechanism of circulation of volatile matter 4.4.1.1 Evaporation in the kiln Upon approaching the burning zone in the rotary kiln, a fraction of the volatile components will evaporate depending on the degree of volatility of the component and be transported with the smoke gas back to the colder zones in the kiln system. Here the components will condense on either the surrounding relatively colder surfaces or on the raw meal and re-enter the burning zone with the raw meal where a fraction reevaporates. This repeated evaporation and condensation results in an internal circulation where the concentration of some components can be increased in the kiln material up to fifty times the input concentration. When an equilibrium state is reached, the output of the volatile components is equal to the total input by the raw materials and the fuels. Especially in a kiln system equipped with a preheater tower, almost all of the volatile matter will finally leave the kiln with the clinker, as only a small fraction succeeds in passing through the cyclones and escape with the exit gas. The concentration of the Page 15

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component in the kiln system at this equilibrium state can be very high, depending on the degree of volatility of the component. If the concentration of volatile matter in the kiln system becomes too high, either because of a large input of volatile components or due to a high degree of volatility, the installation of a kiln gas bypass is necessary to extract part of the volatile components from the kiln system.

4.4.1.2 SO2 gas and volatile matter exiting the preheater An often insignificant part of the volatile matter is discharged with the smoke gas into the atmosphere. In the case of sulphur however, a part of the sulphur in the raw meal can be present as pyrite, FeS, or organic sulphur. These are burned to SO2 gas in the preheater upper cyclones in the temperature range of 400 – 600°C and a part of this gas formed will be expelled as gas from the preheater tower. The small part of the volatile matter that escapes the kiln preheater with the dust in the smoke gas is effectively precipitated in the filters and will normally be reintroduced with the kiln feed. This recirculation of volatile matter from the kiln system to the filter and back again to the kiln is termed the external circulation.

4.4.1.3 Condensation in the preheater The condensing of volatile matter vapour in the colder zones takes place on the raw meal particles and on the surrounding walls. Condensation products are in part complex compounds with low melting points and are therefore present as liquid in the preheater tower. This presence of moisture in the raw meal has several consequences: it reduces the flowability of raw meal and glues particles to the walls. This may cause build-ups on cyclone walls and riser ducts, which in turn restricts the passage areas and causes blockage of cyclones when pieces are loosened and sticks in the cyclone outlets. Strictly speaking, the SO2 gas that is liberated in the kiln and passes from the kiln up through the preheater does not condense. It combines with the calcined raw meal in the lower cyclone stages where CaO is readily available as follows: CaCO3 + heat →

CaO + CO2

calcination

CaO + SO2 + ½O2

→

sulphur reaction

CaSO4

4.4.2 Affinity between the volatile components The volatility of the different compounds differs greatly, and with it the volatility of the individual elements in the compounds. For instance, potassium combined with chloride as KCl will evaporate nearly 100% in the burning zone while potassium combined with sulphate as K2SO4, to a large extent, will leave the kiln with the clinker. Therefore it is of great interest to know what compounds are present in the kiln system.

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Below is given a schematic and simplified list indicating the order of which volatile components have been observed to combine with other components. •

Chloride reacts primarily with the alkalis, forming NaCl and KCl. Any chloride in excess of alkali will combine with calcium to form CaCl2.

•

A part of the alkalis in excess of chloride combine with sulphur to form Na2SO4, K2SO4 and double salts such as Ca2K2(SO4)2.

•

Alkalis not combined with chloride or sulphur will be present as Na2O and K2O embedded in the clinker minerals.

•

Sulphur in excess of alkali combines with CaO to form CaSO4.

4.4.3 Volatility of the compounds of volatile components 4.4.3.1 Definition of volatility By volatility or evaporation factor ε of a volatile element or compound is understood that fraction of the element that evaporates in the kiln burning zone instead of leaving the kiln with the clinker as illustrated in figure 4.8.

Figure 4.8 Definition of evaporation factor E, valves V, Page 17

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circulation factor K and residual component R

The evaporation factor is defined as;

Evaporatio n factor =1 −

% within clinker % at kiln inlet [LOI free basis ]

ε = 1 means all evaporates and nothing leaves with the clinker ε = 0 means nothing evaporates and all leaves with the clinker

4.4.3.2 Average evaporation factors: Average values for the evaporation factor of various compounds is given in Figure 4.9

Evaporation factor

KCl

Cl-free K2O

Na2O

Cl

Alkali SO3

Excess SO3

0,990 – 0,996

0,10 – 0,40

0,10 – 0,25

0,990 – 0,996

0,30 – 0,90

0,75

0,05

0,15

0,05 – 0,20

0,05

0,05 – 0,25

0,42

0 – 0,10

0 – 0,10

0 – 0,15

0 – 0,10

0 – 0,20

0,80

Preheater valve Filter valve

Figure 4.9 Evaporation factors and values

Chloride compounds KCl, NaCl and CaCl2 are seen to have an evaporation factor of 0,990 – 0,996 in the kiln. At approximately 800°C these compounds are melted (Figure 4.10 and 4.11) and at 1200 – 1300°C they are almost entirely evaporated. K Compound

Na

Melting point

Boiling point

Melting point

Boiling point

[°C]

[°C]

[°C]

[°C]

decomp.

350

sublim

1275

-Carbonate

894

decomp.

850

decomp.

-Sulphate

1074

1689

884

-

-Chloride

768

1411

801

1440

-Hydroxide

360

1320

328

1390

-Oxide

Figure 4.10 Melting points and boiling points, °C.

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Sulphate compounds with alkalis such as K2SO4 and Na2SO4 will in general be more stable than CaSO4, which is the form that sulphur in excess of alkalis take. Alkali sulphates have evaporation factors in the range of 0,30 – 0,90 and are normally in the lower part of the range, while excess sulphur has a value of 0,75. It is therefore desirable that all sulphur is combined with alkalis to the widest extent. This can be investigated by looking at the molecular ratio of sulphur to alkalis.

4.4.3.3 Molecular ratio of sulphur and alkalis The concentration of sulphur and alkali may increase to a point where it affects kiln operation by causing build-ups. Both overall quantity and relative proportions may be the cause. Individually they are more damaging to kiln operation than their sulphate compounds. K2SO4 and Na2SO4, as K2SO4 has such a high evaporation temperature, that it enters the clinker without evaporating. The same applies to approximately half the Na2SO4, despite the fact that this sulphate begins to split into Na2O and SO3 at temperatures as low as 900°C. Sulphur in excess of alkalis will form the more volatile CaSO4 which has a high evaporation factor in the kiln burning zone. A number of equations have been developed for the estimation of the optimum molecular ratio between sulphur and alkalis in the kiln system. Two such equations are mentioned below: 1)

SO3 80

 SO3  = ≈1,1   K O Na O Alk 2   optimum + 0,5 ⋅ 2 94 62 The sulphur and alkalis is the total input. If the ratio exceeds 1,1 it is held that an amount of sulphur is present in the kiln material which is not covered by alkalis, and as "excess" sulphur will form CaSO4. The amount of excess sulphur (E.S) is expressed in gram SO3 per 100 kg clinker and calculated according to the equation

E.S . =1000 ⋅ SO3 − 850 ⋅ K 2 O − 650 ⋅ Na2 O

[gr SO3 /100 kg clinker]

The limit on excess sulphur is given to be in the range of 250 – 600 g/100 kg clinker. For easy burning raw mix the high value 600 gram SO3/100 kg clinker should present no problems for the kiln operation, but for a hard burning raw mix the lower value is the limit. Above these limits, the sulphur will give rise to coating problems in the preheater tower. Page 19

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2)

SO3  SO3  80 =    Alk  optimum K 2 O + Na 2 O − Cl 94 62 71 The equation subtracts the chloride from the alkalis and states that the optimum range is approximately 0,7 – 1,2

4.4.3.4 Vapour pressure The equilibrium vapour pressure for compounds of volatile components is seen in Figure 4.11 as a function of the temperature. Recalling the principle of vapour pressure curves, the equilibrium vapour pressure of a component is the partial pressure exerted by the components gas phase when the liquid phase and gas phase are in equilibrium. For instance, on the curve the vapour pressure for NaCl is seen to be 340 mmHg at 1300°C, which means that if we start with vacuum above an infinite amount of pure NaCl(liquid) of 1300°C in a closed container, NaCl will evaporate until the NaCl(gas) has a pressure of 340 mmHg. If we were to remove the NaCl(gas) as it is formed, NaCl(liquid) will keep evaporating, striving to obtain the equilibrium vapour pressure. The driving force for the evaporation is then proportional to the difference between the equilibrium and the actual vapour pressure. Inside the kiln, the vapour is constantly removed and the vapour pressure is therefore practically zero. In this case, the volatility of the compound can be assumed to be proportional to the equilibrium vapour pressure shown in figure 4.11. A high equilibrium vapour pressure at a given temperature therefore indicates a high evaporation factor. For instance, a vapour pressure of 760 mmHg means that the boiling point is reached of the component. The nature of the compounds in which the volatile matter is present is seen to be important and it is inferred from the graph in figure 4.11 that the alkali chlorides will evaporate before the alkali sulphates. This is in accordance with the experience that chlorides evaporate nearly 100% in the burning zone and that chloride facilitates the evaporation of alkalis.

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Figure 4.11 Equilibrium vapour pressures of volatile compounds

Vapour pressures are seen to be exponential functions of the temperature, increasing sharply with higher temperatures to the point where the vapour pressure can almost double within 100°C. It follows that high temperatures in the kiln burning zone will promote high evaporation factors and that small changes in the temperature will have a big influence on the evaporation factor.

4.4.4 A mathematical model For evaluating the behaviour of the volatile matter in an existing system and for predicting, with reasonably accuracy, the behaviour of the matter in an installation where major alterations are planned, a mathematical model can be used. Rather pragmatically a simple model of the circulation circuit is established and then this model is applied to actual measurements in order to obtain actual values for the factors in the formula.

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4.4.4.1 Definitions; evaporation factor and valves The definitions of the factors are given in figure 4.8. The fraction of the total, which evaporates in the burning zone, is, as seen earlier, termed the "evaporation factor", ε. The fraction escaping through the back end is termed the "valve", V, and the circulation factor, K, is the factor by which the concentration in the burning zone of a component is increased by circulation.

Figure 4.12 Rules of computation

Figure 4.12 shows that the circulation factor K and the content in the clinker, R, per feed unit can be calculated when the evaporation factor ε and the valves (V) are known. Page 22

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Kiln Process and Operation Raw Material Characteristics

4.4.4.2 Rules of computations Rules of computations are shown in figure 4.12 where valves in series are seen to be the product of the individual valves. Hence the valve of a 4-stage preheater tower in theory is a product of the four valves of each cyclone stage. The valve of the stack is a product of a number of valves, namely one for each settling point, V-kiln, V-cyclones, V-raw mill etc.

4.4.4.3 Average values for ε and valves Average values for evaporation factors and valves are compiled in figures 4.13 and 4.9. These values are based on a number of complete mass balances for circulating components in various kiln systems. A valve of 1,0 means that nothing is retained, i.e. all components leave the system with the gas. Figure 4.13 shows that values for kiln valves can vary widely according to the kiln type. For instance, from 0,20 valves for a long dry kiln to 1,0 for a precalciner kiln. The preheater valves are small and of course very small for 4-stages of cyclones as the valves for each stage theoretically should be multiplied by each other to give the total valve. VOLATILE MATTER Typical Values for ε and V Symbol

K 2O

Na2O

Cl

SO3

ε

0,20 - 0,40

0,10 – 0,25

0,990 – 0,996

0,35 – 0,80

- wet module-op.kiln

Vo

0,50

0,70

0,70

0,60

- wet dust-op.kiln

Vo

0,40

0,60

0,60

0,40

- long dry kiln

Vo

0,20

0,50

0,60

0,40

- 1 stage kiln

Vo

0,55

0,80

0,60

0,35

- 2 stage kiln

Vo

0,70

0,85

0,80

0,60

- 4 stage kiln

Vo

-1

-1

-1

-1

-1

-1

-1

-1

Evaporation factor Kiln valve

- precalciner kiln Cyclone preheater valve

Vc

- 1 stage

Vc

0,5

0,50

0,35

0,45

- 2 stages

Vc

0,20

0,45

0,20

0,30

- 4 stages

Vm

0,15

0,40

0,05

0,15 – 0,50

Vkt

0,60

0,70

0,50

0,55

Vt

0,60

0,80

0,70

0,30

-1

-1

-1

-1

0,40

0,70

0,30

0,50 – 0,80

Dedusting cyclone valve Raw mill valve Cooling tower valve Elec.precipitator valve

Figure 4.13 Typical values for ε and V.

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Kiln Process and Operation Raw Material Characteristics

In practice it is found that the above characteristics vary considerably due to their close dependence on: 1. Calciner kilns tend to be in the lower end of the scale of evaporation factors 2. Burnability of raw mix (chem. comp., fineness, coarse silica, fluxes, etc.) 3. Nature of impurity-bearing compounds (sulphates-sulphides-organic S) 4. Sulphur/Alkali ratio 5. Presence of other non-volatile impurities (phosphates) 6. Operational Conditions (flame formation, excess air, etc.)

The valves and evaporation factor for sulphur are difficult to evaluate. As previously discussed, sulphur combined with alkali has a different behaviour than sulphur existing as excess sulphur. Therefore in figure 4.9 it is seen that alkali sulphur and excess sulphur are given different evaporation factors and valves. Excess sulphur is seen to evaporate more rapidly and furthermore to pass more willingly through the preheater tower than alkali sulphates.

4.4.5 Three aspects of volatile matter There are three aspects to consider when regarding volatile matter

Content of volatile matter in clinker The introduced alkali, sulphur and chloride will end up in the clinker if not removed elsewhere. A high alkali content in the clinker may often not be desired and limitation on the content of all such matter in the cement have to be taken into account.

Emission to the environment Evaporated sulphur discharged through the stack as SO2 gas and the amount of enriched dust from the EP that is either discarded or blown away, constitutes a troublesome emission source.

Operational aspects Operational problems often arise when the circulating components reach high concentrations within a kiln system. In the kiln there is the formation of rings in the inlet section and formation of dusty clinker. Problems in the preheater cyclones include the formation of build-ups, unsteady material flow and frequent blockages of cyclones.

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Kiln Process and Operation Raw Material Characteristics

4.4.5.1 Content of volatile matter in clinker To comply with the most common cement norms and for maintaining a high clinker quality, the following limits on the content of volatile matter in the clinker apply: Na2O + K2O :

1,5 %

SO3

:

1,6 %

Cl

:

0,1 %

Low Alkali clinker Limits for volatile matter in the clinker when producing low alkali clinker are often given in the standards as: Na2Oeq

:

0,6 %

SO3

:

1,6 %

Cl

:

0,1 %

where Na2Oeq =

Na2O + 1,5·K2O

When the objective is to obtain a low content in the clinker of a certain volatile compound, i.e. for the production of low alkali clinker, the ideal is to have raw materials with a low content of volatile matter. If this is not the case, the volatile component will have to be evaporated and removed in the kiln system. When producing low alkali clinker with a raw material containing more alkali than can be tolerated in the clinker, it is necessary to have a high evaporation factor ε and preferably a large preheater valve V.

Increasing the evaporation factor For the production of low alkali clinker, the evaporation factor can be increased by various measures such as by increasing the temperature of the burning zone. This is widely done by simply increasing the silica ratio of the raw meal and burning the clinker hard to low free lime values. Other means of increasing the volatility of alkalis is by reducing the sulphur input and by adding chloride to the system, such as CaCl2 or by the burning of chlorinated organic solvents in the kiln. When producing a clinker with a normal or low content of alkali from alkali rich raw materials that contains more alkalis than desired in the clinker, it becomes necessary to remove the alkalis with a kiln gas bypass. This is to avoid operational problems due to a large circulation of alkalis in the kiln system. Page 25

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4.4.5.2 Emission to the environment Emission of particulate matter When the particulate volatile matter is found as ultra fine particles it can escape through the precipitator, but normally there is no substantial emission of solid circulating matter.

Emission of gaseous matter The only gaseous volatile component in the preheater exit gas is SO2, of which a fraction will escape through the stack. The amount escaping depends mostly on the kiln system. SO2 gas is formed by the combustion process in the rotary kiln and in the calciner vessel where the sulphur in the fuel will be liberated as SO2 gas. SO2 is also formed in the burning zone by the thermal decomposition of the sulphates as described previously. In a SP and calciner kiln the SO2 gas formed in the kiln is swept with the smoke gas through the preheater tower where the SO2 is almost completely absorbed by the calcined raw meal in the lower stages to form CaSO4. This is due to the good contact between the gas and the material. Investigations show that the SO2 gas formed in the calciner by the combustion process, is instantaneously and completely absorbed by the CaO simultaneously formed in the calcining process. In the raw meal a small part of the sulphur can be present as pyrite FeS or organic sulphur which combusts to SO2 gas in the upper cyclones in the temperature range of 400 – 600 °C. Part of this will be expelled as SO2 gas from the preheater. However, a considerable part, 30 – 95 %, of the SO2 gas that exits the preheater is absorbed by the raw meal in the raw mill installation. Thus the emission from the stack under normal operating conditions is minute. For the wet and long dry kiln, the SO2 gas formed in the kiln will only to a very limited degree be reabsorbed in the kiln due to a poor contact between material and gases. Here, the amount of SO2 gas that escapes through the stack is normally in the range of 30 – 50 % of the total sulphur input to the system.

4.4.5.3 Operational aspects Operational problems due to circulating components The day to day kiln operation can become seriously hampered by formation of build-ups in the system when the concentration of circulating components increases in the kiln system. As a consequence of the build-ups in riser pipes and cyclones, the pressure drop in the system increases and maintaining the draft in the kiln becomes increasingly difficult. Page 26

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The first consequence of the reduced draft will often be that the maximum kiln production is reduced. It is very tempting to compensate for the reduced draft in the kiln by operating with less excess air, but this will only make things worse as the circulation of sulphur increases significantly at low oxygen levels. Another consequence of high levels of sulphur circulation is an increasing tendency to the formation of a very dusty clinker. The reason for this phenomenon is not very well known. The operational problems encountered with a dusty clinker are foremost linked with the moving of heat from the burning zone up towards the kiln inlet, as the hot dust is transported with the smoke gas. This creates an undesired heating of the zones before the burning zone and has a detrimental effect on the kiln coating, leading to the formation of an unstable coating at the burning zone inlet. The resulting kiln operation is poor with symptoms as: •

Frequent kiln stops due to cyclone cleaning and clogging problems

•

Higher heat consumption

•

Reduced kiln production rate

Limits on volatile components within the SP kiln system Experience shows that surpassing the normal limits of volatile matter, shown in the table in Figure 4.14, will lead to a growing tendency of cyclone blockages and the formation of build-ups in exposed areas of the kiln system. Frequent cleaning must be expected. Kiln operation frequently becomes impossible and frequent kiln stops are to be expected for cleaning away deposits and blocked cyclones if values are beyond the maximum limits shown in figure 4.14.

Limits on volatile matter in bottom cyclone stage in a SP kiln system on LOI free basis normal limit

maximum limit

(%)

(%)

K2Oeq = K2O + 1,5·Na2O

3,7

6

Chlorine as Cl-

0,8

2,0

Sulphur as SO3

2,5

5

Figure 4.14

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Kiln Process and Operation Raw Material Characteristics

Limits on input of volatile matter to the SP kiln without bypass The circulation process naturally sets an upper limit to the acceptable input of the different volatile components with the raw mix and the fuel for kiln system without bypass. These limits are given in Figure 4.15 with a normal and a high limit. The high limit only applies when the raw mix is easy burned and the alkali/sulphur ratio, as discussed in Section 4.3.1 is ideal.

Maximum allowable input of volatile components for SP kiln system without bypass on LOI free basis normal limit

maximum limit

(%)

(%)

1

1,5

Chlorine as Cl-

0,023

0,023

Sulphur as SO3

1

1,6

K2O + 0,65·Na2O

Figure 4.15

Note! In the expression for alkalis, it is not the K2Oeq that is calculated. Rather it is an empirical formula where the sodium is given less weight in accordance with the volatility of sodium being lower than the volatility of potassium. If these limits are surpassed and the total input of volatile matter is higher, the kiln system must be equipped with a bypass through which some of the kiln gas can be extracted from the system before reaching the preheater.

Limits on input of volatile matter to the Calciner kiln without bypass As a consequence of the many different designs of precalciner kilns, some aspects regarding their behaviour with volatile matter is different for each calciner system. Calciner kilns and SP kilns, both without bypass, can tolerate equal amounts of alkalis. However, the calciner kiln can tolerate less sulphur

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Kiln Process and Operation Raw Material Characteristics

The allowable limits on inputs are given in figure 4.16 with a normal and a high limit. The high limit only applies when the raw mix is easy burning and the alkali/sulphur ratio as discussed earlier is ideal.

Maximum allowable input of volatile components for calciner kiln system without bypass on LOI free basis normal limit

maximum limit

(%)

(%)

1

1,5

0,015

0,015

0,8

1,2

K2O + 0,65·Na2O Chlorine as Cl

-

Sulphur as SO3 Figure 4.16

If these limits are surpassed and the total input of volatile matter is higher, the calciner kiln system must be equipped with a bypass through which some of the kiln gas can be extracted from the system before reaching the preheater.

Why Calciner kilns are more sensitive to Volatile matter Calciner kiln systems will to a larger extent than SP kilns be sensitive to input of chloride and sulphur and will require kiln gas bypass installations for lower input levels of these components than SP kilns. The reason for this higher sensitivity is found in the lower kiln smoke gas to clinker ratio [Nm3/kg clinker] in the calciner kiln where only 320 kcal/kg clinker or 40% of the total combustion takes place. The concentration of the volatile matter in the smoke gas from the calciner rotary kiln expressed as gram/Nm3 will therefore reach critical values with less chloride and sulphur in the raw material.

Discarding filter dust To reduce the level of circulating matter in the kiln system, the input of circulating matter can in some cases be reduced by discarding the fine fraction of the filter dust, which is enriched with volatile matter. This is an interruption of the external circulation. The impact on the internal circulation within the kiln will normally be very limited, but can in every case be evaluated by establishing a mass and volatile balance as described under the mathematical model. More effective in combating high levels of circulating components, is the reduction of the volatility of the component. This can be done by reducing the burning zone temperature and by other measures described in the following. Page 29

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Kiln Process and Operation Raw Material Characteristics

Reducing the volatility of compounds When kiln operation is troubled by coating formations and high concentration of volatile matter in the system, the aim is to reduce the circulation factor K. Such problems are foremost found in 4-stage preheater kilns, which have small natural valves, combined with high temperature in the kiln. Reducing the circulation factor can be achieved by reducing the evaporation of the volatile components, for instance by lowering the burning zone temperature. Reducing the volatility is the most practical measure when dealing with problems caused by excessive amounts of alkali and sulphur.

4.4.6 Solving problems with sulphur Reducing burning zone temperature Volatility of the sulphur compounds and especially of CaSO4 is an increasing function of the temperature. CaSO4 will start to decompose slowly at temperatures above 1220°C and this thermal decomposition is best avoided by lowering the burning zone temperature. This may be achieved by making the raw meal easy to burn, which means finer grinding of the raw meal and if necessary changing the chemical composition, i.e. lowering the silica ratio, to make an easier burning raw mix. For kiln operation it also implies that unnecessary overheating of the material has to be avoided and a higher free lime content may have to be accepted.

Molecular ratio Observing the optimum molecular ratio of sulphur to alkali and ensuring that the excess sulphur is minimised are important steps for reducing the sulphur volatility.

Kiln atmosphere

Oxygen in kiln atmosphere The dissociation of sulphur compounds can be described as Alk-SO4 + heat

→

Alk-O + SO2 + ½ O2

The equilibrium of this balance is shifted to the left favouring the formation of Alk-SO4 with increasing O2 and SO2 partial pressure. It is therefore important that enough oxygen is present in the kiln atmosphere. It is known that for increasing oxygen con-tent up to approximately 2%, volatility of sulphur is progressively reduced while increasing the oxygen beyond 2% has a limited effect.

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Kiln Process and Operation Raw Material Characteristics

Reducing conditions in kiln atmosphere In a generally reducing atmosphere or with local reducing conditions in the charge due to incomplete combustion of fuel, the presence of free carbon in the charge will give rise to the following reactions: CaSO4

+

C

→

CaO

Alk2SO4

+

C

→

Alk2O + SO2 + CO

+ SO2 + CO

This significantly increases the sulphur circulation as it promotes the decomposition of the otherwise reasonably stable CaSO4, as well as the stable alkali sulphates. It is therefore important that oxidising conditions are maintained in the kiln with O2 > 1,5 – 2% and CO < 0,1% at the kiln inlet. The incomplete combustion of fuel and coal dropping out of the flame is strictly to be avoided.

Thermal load Reducing the thermal load in the kiln has shown to have a positive effect on the sulphur circulation. The use of a precalciner kiln system or implementing secondary firing in the kiln riser will thus have a positive effect. Furthermore the implementation of firing in the kiln riser pipe will increase the content of oxygen in the kiln atmosphere adding to the positive effect.

Discarding EP dust For dry process kilns with 4-stage preheaters, no perceptible reduction in sulphur circulation can be achieved by discarding precipitator dust as there is only a relatively minor amount of settlement due to the cleaning effect of the cyclones.

4.4.7 Solving problems with chloride Reduction of volatility of chloride Evaporation of chloride is always high and can hardly be significantly reduced due to their extremely high volatilisation degree in the burning zone. Measures against chlorides will therefore not be centred around the reduction of the evaporation. Other ways of reducing the chloride content in the kiln system is therefore necessary.

Input from raw materials and other sources As the evaporation factor of chloride is high, limiting the input is one of the few ways to control the chloride cycle. While it may not be possible to avoid the chloride in the main raw materials, avoiding a minor component with significant chloride content is often feasible. The natural in-homogeneity of the raw materials can sometimes lead to substantial peaks in the input of chlorine to the system, which must be avoided. Here the Page 31

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pre-homogenisation of the raw materials together with a good knowledge of the quarry deposit can help to avoid such peaks.

Discarding of dust In a preheater kiln, this measure is normally not very effective as the amount of chlorides escaping the preheater is small. If there is a sizeable external circulation where the filter dust is enriched with chloride, discarding this dust will have some effect (however, environmental aspects must be carefully looked into and discussed with the Authorities). A mass and volatile matter balance will help to clarify the effect on the internal circulation of this measure.

Installation of a kiln gas bypass If the total input of chloride to the SP kiln system exceeds 0,015% Cl on raw meal basis, 0,023 %Cl on clinker basis, the installation of a bypass is normally required and a kiln gas bypass is most frequently used for the removal of chloride. For SP kilns the rule of thumb applies that the percent bypass should be total input %Cl on raw meal basis times 100. %bypass = % Cl in raw materials (raw meal basis) · 100 Example:

In raw materials (raw meal basis)

= 0,10% Cl

required bypass of kiln gas

= 10 %

Reducing the sulphur evaporation As the clogging of cyclones and formation of build-ups in riser ducts can be viewed as a process where both sulphur and chloride is involved, the reduction of the sulphur evaporation will probably enable the system to function with a higher level of chlorides.

4.4.8 Solving problems with alkali Alkali volatility If not covered with sulphur, the volatility of alkali is very high. In such cases the addition of sulphur to the kiln system in the form of gypsum can be contemplated to reduce the volatility. As the alkalis are normally combined with either sulphur or chloride they have been dealt with under sections 4.4.6 and 4.4.7.

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Kiln Process and Operation Raw Material Characteristics

4.4.9 Setting up a mass and volatile matter balance An example is provided below to illustrate a simple balance of volatile matter: Inputs Raw meal: Raw meal basis:

LOI free basis:

Na2O :

0,15%

Na2O :

0,23%

K2O

:

0,45%

K2O

:

0,69%

SO3

:

0,375%

SO3

:

0,58%

Cl

:

0,005%

Cl

:

0,008 %

LOI

:

34,9%

Coal: Coal Analysis Coal basis:

Coal Ash basis:

Na2O :

0,10%

Na2O :

1,00%

K2O

:

0,05%

K2O

:

0,50%

SO3

:

2,50%

SO3

:

25,0%

Cl

:

0,013%

Cl

:

0,13%

Net Value

Hnet

:

5600 kcal/kg

Ash content

:

10%

Specific Heat Consumption :

800 kcal/kg clinker

Ash content in clinker

800/5600·10% = 1,4%

:

Outputs Clinker:

Na2O :

0,24%

LOI free basis

K2O

:

0,68%

SO3

:

0,90%

Cl

:

0,007%

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Kiln Process and Operation Raw Material Characteristics

Kiln inlet. Lowest cyclone Stage Raw Meal basis

LOI free basis

Na2O :

0,24%

Na2O :

0,29%

K2O

:

1,64%

K2O

:

2,00%

SO3

:

3,85%

SO3

:

4,70%

Cl

:

0,66%

Cl

:

0,80%

LOI

:

18%

Balance of inputs and outputs Na2O (%)

K2O (%)

SO3 (%)

Cl (%)

Raw meal

0,23

0,68

0,57

0,008

Coal

0,01

0,01

0,36

0,002

Total

0,24

0,69

0,93

0,010

Clinker

0,24

0,68

0,90

0,007

Total

0,24

0,68

0,90

0,007

Input LOI free basis

Output LOI free basis

The amount of K2O, SO3 and Cl leaving the kiln with the clinker is apparently smaller than the input with the raw materials and the coal. Small deviations are quite normal due to sampling not being representative. In the case of sulphur, part of the sulphur in the raw meal could be sulphides which burns off in the preheater tower and leaves as SO2 gas.

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Kiln Process and Operation Raw Material Characteristics

Checking the input of Volatile matter (LOI free basis)

Limit (%)

Actual (%)

K2O+0,65·Na2O

1,0

0,85

0,023

0,010

1,0

0,93

Cl SO3

All OK for operation without bypass and no problems expected.

Sulphur to Alkali ratio

SO3

Alk

=

0,93 80

0,69 0,24 + 0,5 ⋅ 94 62

=1,25

The ratio is higher than the limit of 1,1 and therefore there is excess sulphur in the system present as CaSO4. Excess Sulphur E.S. = 1000·0,93 – 850·0,69 – 650·0,24 = 187 gram SO3/100 kg clinker OK, not too much excess sulphur and no problems are expected. Content in lower stage (LOI free basis)

Limit (%)

Actual (%)

K2Oeq

3,5

2,0 + 1,5·0,29

OK

Cl

0,8

0,8

OK

SO3

2,5

4,70

High!

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Kiln Process and Operation Raw Material Characteristics

Content in clinker (LOI free basis)

Limit (%)

Actual (%)

Na2O+K2O

1,5

0,92

OK

Cl

0,1

0,007

OK

SO3

1,6

0,90

OK

Evaporation factors of volatile components

ε Na O = 1 − 2

εK

=1−

2O

0, 24 = 0,17 0, 29

0,68 = 0,66 2,00

The evaporation factor of K2O must for a more detailed study be calculated separately for the evaporation of KCl and Cl-free K2O. The typical evaporation factors for these compounds are seen in figure 3.9.

ε SO = 1 − 3

ε Cl = 1 −

0,90 = 0,81 4,70

0 , 007 = 0 ,991 0 ,80

The evaporation factor for Na2O is seen to be in line with factor shown earlier. The content of SO3 in the lower stage material is very high and is caused by a high evaporation factor of 0,81 in the kiln. Operation of the kiln is expected to be very problematic with many kiln stops for cleaning of the cyclones. The high evaporation factor of sulphur must be investigated further to determine the cause.

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Kiln Process and Operation Raw Material Characteristics

4.5 CONCLUSION This module has related the characteristics of the raw mix to burnability clinker formation and the circulation phenomena of volatile matter in a kiln system. It has been demonstrated that the average burning zone temperature is dependent on raw mix characteristics and kiln operation. It has also been demonstrated that a high burning zone temperature may adversely affect the clinker formation and produce problems in the preheater tower as regards circulating volatile matter. Further, it has been demonstrated that the specific choice of raw materials has a large impact on the behaviour of the kiln. It may cause cyclone blockages or increase the need for frequent cleaning in the riser ducts. It has likewise been shown that the coating formation in the kiln and the clinker size may be greatly affected by the substitution of one minor component by another, even if the chemical analysis of the two appears to be practically identical. It can be difficult to identify the factors responsible for the change in the kiln behaviour. Sometimes the use of a specific material has to be abandoned due to the problems caused in the kiln system. Some disturbances introduced in the kiln by changing the type of raw mix are difficult to counter in the daily operation of the kiln. It is therefore important to learn to identify the symptoms in kiln behaviour, which can be traced back to the raw mix so that corrections can be made. Other disturbances can to some extent be alleviated by taking appropriate measures in the kiln operation as has been discussed. Finally, the volatile content of the raw material has been discussed and a model presented for analysing and evaluating the behaviour of the volatile matter.

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