Cement Chapter 2

2 . 1 I N T R O D U C T I O N

2 . 2 R AW M A T E R I A L S

2 . 2 . 1 I N T R O D U C T I O N

2 . 2 . 2 P L A N N I N G

2 . 2 . 3 G E O L O G Y

2 . 2 . 4 E X T R A C T I O N

2 . 2 . 5 R E S T O R A T I O N

2 . 3 R AW M A T E R I A L P R E P A R A T I O N

2 . 3 . 1 W E T P R O C E S S

2 . 3 . 2 S L U R RY M O I S T U R E C O N T E N T

2 . 3 . 3 S E M I - W E T P R O C E S S

2 . 3 . 4 D RY P R O C E S S

2 . 3 . 5 S E M I - D RY P R O C E S S

2 . 4 K I L N F E E D

2 . 4 . 1 I N T R O D U C T I O N

2 . 4 . 2 C O A L G R I N D I N G

2 . 4 . 3 F I R I N G

2 . 4 . 4 F L A M E S

2 . 5 K I L N P R O C E S S

2 . 5 . 1 I N T R O D U C T I O N

2 . 5 . 2 C O M B I N A B I L I T Y

2 . 5 . 3 W E T P R O C E S S K I L N S

2 . 5 . 4 S E M I - W E T P R O C E S S K I L N S

2 . 5 . 5 D RY P R O C E S S K I L N S

2 . 5 . 6 S E M I - D RY P R O C E S S K I L N S

2 . 5 . 7 L O N G - D RY P R O C E S S K I L N S

2 . 5 . 8 P R E C A L C I N E R P R O C E S S K I L N S

2 . 6 C L I N K E R C O O L E R S

2 . 7 V O L A T I L E C O M P O N E N T

2 . 8 K I L N C O N T R O L

2 . 9 Q U A L I T Y C O N T R O L

2.10 M A T E R I A L S H A N D L I N G

2. Cement ManufacturingProcess

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 4 17

contents chapter 2 chapter 3


2.1 INTRODUCTIONA simplified outline of the overall process is shown in figure 18,which shows the process in the following stages:

1. Raw material extraction (or purchase)2. Proportioning (for LSF, Silica and Alumina

Ratio, etc)3. Grinding4. Blending5. Drying6. Preheating7. Calcining8. Sintering (+ fuel preparation)9. Cooling10. Cement Grinding (+ gypsum)

Figure 18. Simplified Cement Process.

The above is applicable to all process types, although there aresome important differences. The process types can generally beplaced in the following categories:-

- Wet- Semi-Wet- Semi-Dry- Dry or Pre-heater- Pre-Calciner

In the wet process, raw materials have high moisture content,say around 20%, and the raw material preparation andblending is carried out as a slurry (ie 30-40% moisture). Thekiln serves to carry out the drying through to sintering (5-8above). Overall fuel efficiency is low, as a result of the highenergy requirement associated with removal of the 30-40%moisture.

The semi-wet process represents a modification to the wetprocess in which the raw material extraction and preparation isthe same but that the water is partially removed by amechanical means prior to the kiln. This typically involves afilter press where moisture levels are reduced to around 20% orjust below.

The semi-dry process essentially refers to the Lepol processwhere the raw materials are nodulised (usually in a nodulisingdish) and then passed over a grate heat exchanger. Nodulescontain around 5-10% moisture. The heat exchanger consists ofa moving grate in which hot gases are passed through the bed ofnodules. The grate in effect carries out the drying and preheating stages outside of the kiln, with calcination and sinteringremaining inside the rotary kiln.

The dry-process can consist of long dry kilns, but more usuallyconsists of a suspension pre-heater, where the ground rawmaterial (raw meal) is passed through a series of cyclone stages(usually 4) for drying and pre heating prior to the kiln.

In the pre-calciner process some of the energy is applied to thekiln back-end(1) to a calciner vessel to achieve calcination of thefeed outside of the kiln. Thus the rotary kiln then essentiallycarries out the sintering stage only. The heat exchange takesplace in a series of cyclone stages (up to 6).

In the pre-calciner process the degree of calcination prior to thekiln can be around 90-95%. This compares to perhaps 30-40%in the conventional suspension preheater kiln. However manydry process kilns have been modified with secondary orauxilliary firing, in which some fuel (say 15% of the total) isfired at the back end (usually in a riser pipe). In the pre-calcinerprocess fuel can be split 50/50 or even 60/40 between the back-end and front end of the kiln.

The following sections provide further details for the principalprocess stages.

Note (1): Kiln front and back ends usually refer to the gasflow rather than the material flow.

2. CEMENT MANUFACTURING PROCESS



2.2 RAW MATERIALS2.2.1 INTRODUCTIONLimestone represents the most important raw material, and asalready discussed, is typically some 80% of the mix. This meansthat for 1 tonne of clinker it is necessary to have approximately1.25 tonnes of limestone (i.e. allowing for CO2 loss). For thisreason, the location of cement plants is dictated by theavailability of limestone.

However, before building a cement plant adjacent to a largesource of limestone there are a number of factors to beconsidered.

2.2.2 PLANNINGPlanning regulations, largely as a result of public environmentalpressure, are becoming increasingly complex and involved, and have resulted in more refusals or stricter conditions beingimposed on permissions.

Planning applications need to address:-- location of minerals- reason for extraction- need for extraction- method of extraction- time-scale for operation- likely noise levels- likely dust levels- lines of sight- vibration- night lighting- water table- agriculture- archaeology

2.2.3 GEOLOGYIn general terms raw materials for the manufacture of cementshould be:-- low cost- simple to prepare- easy to crush- easy to grind- of suitable chemistry

and, above all in a viable, suitable location.

The role of the geologist is to establish the following propertiesof the raw material reserve:-- overburden thickness- reserve thickness- chemical composition- hardness- water levels- variability- joint planes- faults- size of reserve

2.2.4 EXTRACTIONIn most cases raw materials are extracted in a quarryingoperation rather than a mine. Factors involved in a successfulquarry operation include appropriate planning of:-- overburden removal and tipping- mobile and fixed plant- method of operation- drilling/blasting requirements- geotechnical conditions- minimum cost extraction- appropriate regulations- manning requirements- final landform for restoration

Raw material extraction efficiency can be assessed in manyways, for example, in terms of:-- output per man hour- output per dumper tonne capacity- output per unit of capital employed- blasting rates

2.2.5 RESTORATIONThe end of the land and minerals cycle is restoration. It is nowno longer acceptable to leave quarries once extraction has beencompleted. Indeed, restoration often proceeds simultaneouslywith extraction. Restoration work can include:-- landforming- seeding- planting- field patterns- final use of land (e.g. farming, fishing, recreation, naturereserve, public open space, landfill or development potential)




2.3 RAW MATERIAL PREPARATION2.3.1 WET PROCESSIn the wet process raw materials are prepared as a slurry with amoisture content of usually 30-40%, but sometimes as low as25% and sometimes as high as 45%. The objective ofpreparation is to produce pumpable slurry which is fine enoughfor chemical combination in the kiln and which has the requireduniform chemistry.

Below is a brief description of the principal elements of a wetmilling plant. (See Figure 19).

Figure 19. Simplified Wet Process

Washmills:Many materials used in the wet process are soft and sticky withhigh moisture and do not require the high energy input of a ballmill. The necessary tearing/attrition can be achieved in awashmill. This essentially involves agitation with harrowshanging into a cylindrical tank from a centrally pivoted rotatingarm (10-15 rpm). The material is thrown against screens ofaround 5-6mm mesh. The slurry passes through whilst coarsermaterial is retained. A typical size is 10-11 metre diameter, 3-4metres deep with a 250-300kw drive.

However, coarse residues can be a problem dictating asecondary preparation stage in either a further washmill (higherspeed, smaller mesh, smaller size) or a ball mill.

Washdrums:A washdrum is used where there are hard inclusions in the rawmaterial (e.g. flint). Effectively it is a lined steel drum like a ballmill, but without media. The hard inclusions themselves buildup and act as grinding media. Slurry is discharged through theshell, whilst the hard pieces pass to a second chamber or arerejected.

The slurry can sometimes be screened, with the coarser sizespassing to a secondary grinding stage.

Wet Ball Mills:Ball mills can be used as primary grinders or secondary millsand can have single or 2 or 3 chambers depending on theirapplication and feed material size. Linings and diaphragms canbe rubber or steel. Media sizes are typically 80-100mmmaximum and 25mm minimum. Smaller sizes tend to "float" inlow moisture slurries.

Wet Classification:Classification to reject oversize particles can be achieved in anumber of ways, e.g.

- vibrating screen- hydrocyclone- sieve bend

The rejects can be returned to the primary mill, passed to asecondary stage or totally rejected.

Screens are generally unattractive to the cement works, sincepoor classification is achieved in low moisture slurries and wearrate is high.

The hydrocyclone consists of a cylindrical head, concentricoutlet nozzle and a conical body. Slurry is introducedtangentially where a vortex is formed. Particles in the slurry aretherefore subject to centrifugal force which pushes material to

the outer wall of the cone, and a viscous drag force opposingthis, which drags material towards the axis of the cone. (SeeFigure 20).

In the sieve bend the slurry is introduced inside a 270 arc ofscreen mesh. Material reaches the screen at right angles and thusthe cut size is somewhat smaller than the mesh size. This helpsovercome blinding of the screen. (See Figure 21).

Figure 20. Hydrocyclone


Tangential inlet

Outlet Nozzle

Vortex Finder

Rejects Nozzle

Chalk (Limestone)

Oversize/Rejects

Ball Mill

Fines

ClayWashmill

Kiln

Storage

&

Blending

Hydrocyclone



Figure 21. Sieve Bend

Blending:

Even though the feed materials are carefully proportioned in thepreparation plant, the resultant slurry will usually have somevariability as a result of changes in clay moisture, feed rates andchemistry. A blending and storage system thus aims to provide auniform kiln feed slurry.

Blending and mixing can either be a batch or continuousprocess.

2.3.2 SLURRY MOISTURE CONTENTNaturally, the higher the moisture content of the slurry thehigher will be the energy requirement in the kiln. Slurrymoisture will depend on a number of factors, such as:

- raw material properties- mix design- materials handling system- dust return

There are generally two ways to reduce the moisture content:- use of chemical slurry thinners- mechanical de-watering

Figure 22 represents a typical relationship between the kiln feedmoisture and the specific heat consumption of the kiln.

Figure 22. Effect of Kiln Feed Moisture on Energy Requirement

In essence, the slurry is a mixture of water and fine dispersedparticles. The mixture has a specific viscosity and thus specifichandling behaviour. Slurry thinners are used to reduce the watercontent for a given viscosity.

In general, each percent of water reduction equates to anincrease in kiln capacity of 1.5% and a 1% reduction in thekcals/kg e.g., as shown in Figure 22, reducing by 5% from 38%to 36.1% would yield approximately a 5% reduction inkcals/kg.

The slurry thinners are usually added to the grinding stage andthus can also act as grinding aids. Their effect depends on thephysical and chemical properties of the slurry and so the choiceof additive is usually based on experimental trials.

The ions and molecules of the chemical added are absorbedonto the particles of the raw mix thereby preventingagglomeration and reducing internal friction. Hence flowabilityof the slurry is improved.

There are two main groups of slurry thinners:-

- alkaline electrolytes- surface active organic substances

A list of substances used by the cement industry is shown inFigure 23. Combinations of these materials are often used. Thealkaline electrolytes can be limited due to their contribution tothe input of alkalis.

Mechanical de-watering represents the semi-wet process.

Figure 23. Examples of Slurry Thinners


Akaline Electrolytes Sodium SilicateSodium HhydroxideSodium CarbonateSodium Trypolyphosphate (STPP)

Surface Active Organics Lignin DerivativesHumic AcidsSulphite LiquorCalcium LignosulphonateCarbonaceous AdditivesMolasses

The curved screenis made of wedgeshaped wires

Slot width

Feed

Coarse Particlesrejecte

Slurry

Stream of slurrypasses across screen

at high velocity

Rejects Fines



2.3.3 SEMI-WET PROCESSIn the semi-wet process raw material extraction and preparationis the same as that used in the wet process. However, once theslurry has been produced the water content is mechanicallyreduced before introducing the raw feed to the kiln.

The slurry de-watering can be achieved in drum or disc filters incontinuous operation or in filter presses operated in batchmode. The latter results in a filter cake of 18-20% moisture.(See Figure 24).

Figure 24. Simplified Semi-Wet Process

In the filter press the slurry is pressed between metal platescovered by a filter cloth. The resultant cake is often "chopped"into briquettes and collected into a store before being fed to thekiln.

Another alternative for modifying the wet process concerns theuse of a spray dryer, which can achieve good heat transfer bythe close contact of gas and slurry presented as droplets. Thekiln then becomes shorter, like the dry process.

2.3.4 DRY PROCESSLike the wet process, the objective of preparation is to achieve adesired fineness and chemistry, but as the process name suggests,also to achieve drying.

The materials for the dry process are usually relatively low inmoisture and thus, in comparison to the wet process, are driedto produce a dry kiln feed powder rather than additional waterto produce a slurry.

The principal stages involved are described below:

Crushing:The objective of crushing is to provide raw materials of a sizesuitable for fine grinding. Sometimes only 1 stage of crushing isneeded, but more often there are 2 or 3 stages necessary for therequired size reduction.

The selection of the crushing circuit will depend on:-- feed size- required produce size- moisture content- stickiness- hardness- abrasivity

Crusher types in use include (See Figure 25)- gyratory- impact- roll- hammer- cone

Grinding:In grinding the main objectives are to:-

- produce the fineness required for the kiln process- remove the remaining moisture

This then achieves a product, usually referred to as raw meal,which is fine, dry and free flowing.

The most effective way of removing the moisture is during thegrinding process by passing a stream of hot gas through themill. This is normally waste heat from the kiln system.

There are three principal types of mill used in the cementindustry:-

- Ball mill- Aerofall mill

- Roller mill or Vertical Spindle mill

Figure 25. Examples of Crusher Types


Gyratory Crusher Cone Crusher



The Aerofall mill is a special case of a very short and largediameter rotating mill. There are only a relatively few largegrinding balls, the majority of grinding achieved by the materialitself.

Grinding circuits can appear quite complex and varied in rawmaterial grinding (when compared to those of cement grinding),and so it is not the intention to cover these in detail here.Below are some features of raw material grinding circuits:-- Circuits can often contain- drying- grinding stage(s)- transport- classification stage(s)- Drying can be in a separate stage, like a drum dryer, or in the

grinding stage (e.g. in a pre-grinding stage or in the mill) or in the classification stage

- Mills can be ball mills, Aerofall, double rotator (central discharge), roller mill or roll press. Pre-grinding is often achieved in hammer mills or impact crushers

- Classification can be achieved in static, mechanical, cyclone or high efficiency separators or a combination of them

- Material transport can be achieved in bucket elevators or by air sweeping or by a combination of them

Examples of such raw material grinding systems are shown inFigure 26.

Figure 26. Examples of Raw grinding Circuits

Figure 26. Examples of Raw grinding Circuits (continued).


Pre-Drying Chamber Pre-Drying Duct

Pre-Drying Impact MillPre-Drying Impact Mill,Central Dicharge Mill,and Air Separator

Pre-Drying, Pre-crushing Impact Mill

Drying in Air Separator Pre-Drying in Duct Air-Swept Mill

Impact Crusher Roll Crushers

Hammer Crushers

Single Roll

Double Roll

Single Rotor

Double Rotor



In general the product moisture should be below 0.5%. Feedmoistures of up to 8% can usually be dried by using pre-heaterexhaust gas, whilst above this it maybe necessary to useauxiliary heating.

In comparison the grinding circuit for a roller mill appearsrelatively simple since the mill has an internal classifier,generally internal material transport, and simultaneous grindingand drying.

Roller mills are becoming increasingly used for raw materialgrinding, although the roll press (see section 5) is also findingapplications in raw grinding circuits.

The roller mill is a development of the flour stone mills, whichwere driven by wind or water. Material is fed onto a rotatingtable and is crushed/ground by the action of 2, 3 or 4 rollers,which press onto the table. As material leaves the edge of thetable, airflow transports it to the classification stage. Rejects arereturned down the centre of the mill body to the grinding table.(See Figure 27).

Figure 27. Vertical Roller Mill.

Material is retained on the table by a retaining (or dam) ring,which allows the formation of a material bed on the table. Airflows through a "nozzle ring" at the edge of the table.

Some of the important operating features include:-- Feed properties (e.g. ability to form bed of material)- Bed depth

- Dam ring height- Nozzle ring velocity (can be 40-70 m/sec)- Gas flow (around 2kg gas/kg solids)- Use of water sprays (to modify material bed

properties and control outlet temperature)- Operating pressure (arising from static and

hydraulic forces)- Material feed size and feed rate- Differential pressure (proportioned to feed rate)- Separator rotational speed- Wear rate- Vibration (function of bed properties)

Blending:As for the wet process, the product from the grinding circuitwill usually require homogenisation to provide a uniform andconsistent kiln feed material.

Inhomogeneous kiln feed can adversely influence:-- cement quality- kiln output- fuel consumption- refractory life

Blending has already begun in the quarry, stockpiles and theraw milling circuit. However, the final stage is the blending silo.As with the wet process this can be batch or continuous inoperation.

Figure 28. Raw Meal Quadrant Blending System.

The blending silos achieve homogenisation through the use offluidising air to achieve a liquid like powder (i.e. fluidisedpowder).

Parts of the silo are fluidised, usually via porous tiles or canvas,in turn thereby creating movement of material from one part toanother. There are many variations of systems to achieve thisusing various designs of segments and high/low pressure airflow.Some of these are illustrated in Figure 28.

The blended raw meal is then stored in one of several kiln feedstorage silos. Some opportunity for final adjustments to the kilnfeed can then be achieved, if necessary, by simultaneouslyextracting from more than one silo.

2.3.5 SEMI-DRY PROCESSIn the semi-dry process ((Lepol) the raw feed is processed withsome 10% moisture in pre-wetting screws. The material is thenfed to a nodulisation process, such as an inclined dish, in whicha tumbling action processes the material which becomes"rolled" into nodules. These nodules are then pre-heated in agrate before passing to the kiln (See section 2.5).




2.4 KILN FUEL2.4.1 INTRODUCTIONAs already discussed in section 1, coal remains the most widelyused primary fuel.

Whilst coal has abundant supplies at relatively low cost, itpresents some handling problems, contains moisture and oftensignificant ash levels, requires preparation and can be variablein calorific value.

In comparison, oil has no moisture, low ash and is easy to handle,but can be expensive and tends to have a higher sulphur content.

2.4.2 COAL GRINDINGCoal grinding methods can generally be characterised into:-

- high speed impactor type- medium speed vertical spindle type- low speed tumbling type

High speed mills include the Attritor where grinding is achievedby impact by rotating hammers and attrition between particlecollision. Mill speeds can be 1700-1800 rpm with peripheralspeeds of around 80m/sec.

Vertical roller mills are like those used for raw material grindingalthough grinding can be achieved on the table under balls (in aring) or rollers.

Ball mills are also used for coal grinding.

Like the raw material grinding described in section 2.3, coalgrinding also involves simultaneous drying.

2.4.3 FIRINGThe air used in grinding is also used to convey the pulverisedcoal or fuel (usually referred to as pf) to the kiln.

Where fuel is conveyed directly from the mill to the kiln thesystem is referred to as a direct firing system.

Where the fuel and air are separated and the pf is stored in ahopper from which it is later fed into an air stream into thekiln, this is referred to as an indirect firing system.

A comparison of these two approaches is shown in Figure 29.

Figure 29. Comparison of Direct and Indirect Firing Systems.

Air for drying and conveying is usually taken from the kilnhood and is normally kept below 350C to avoid ignition byusing a cold air bleed.

Combustion requires about 10 kg air/kg coal

Fuel is introduced into the kiln via a firing pipe with 5-40% ofthe total combustion air. The firing nozzle is sized to give anozzle velocity of 30-100m/sec. The remainder of the airrequired for combustion is supplied, preheated, from the clinkercooler.

Naturally there are a certain number of safety requirementswhen handling powdered coal. Powdered coal suspended in airwill explode if in a concentration of 0.04-3kg/m3, depending onvolatiles and particle size. An ignition source, e.g. a spark is

required to cause an explosion, although damp coal in air canignite spontaneously if left for a period of time (can be onlydays).

In indirect systems where pf is stored, spontaneous fires canoccur.

2.4.4 FLAMESFlame characteristics are largely influenced by:-

- coal preparation- burner momentum- axial location- back end oxygen

Inappropriate characteristics can adversely affect economy,output, clinker quality and kiln lining. In particular poor burnerdesign can cause:-

- chemical reducing conditions- ash heterogeneity (i.e. poor combination of ash)- variable burning- under/over burning- damage to refractories

Thus the kiln flame can influence a large number of parameterssuch as:-

- 28 day strength- Workability- Setting- Fuel consumption- Volatile recirculation- Early/late strength relationship- Decreased refractory life- Increased cement grinding kWh/t- Increased pollutant emission


Direct Firing

Advantages: Relatively simple systemStorage of fine coal not necessaryLower capital cost

Disadvantages: Primary air (from mill system) dictated by the mill requirementsCoal feed rate linked to mill outputMill failure will require kiln shutdown

Indirect Firing

Advantages: PF storage permits kiln operation without millCoal fee rate readily adjustablePrimary air independent of mill rate

Disadvantages: Fine coal needs to be stored (safety hazard)Additional equipment needed for caol/air separation and re-entrainment from storage



2.5 KILN PROCESS2.5.1 INTRODUCTIONReferring to the stages shown in Figure 18, the kiln can achievethe 5 stages from raw meal production to clinker, i.e.:-

- drying- preheating- calcination- sintering (burning)- cooling

However these should not be seen as independent processstages, since considerable overlapping occurs.

Drying Zone:In the wet and semi-wet processes, slurry water is evaporatedoff by heat in a chain section in the kiln and the water vapourleaves the kiln with the back-end gasses. Temperatures canexceed 100C.

In the semi-dry and dry process the drying takes place outsidethe rotary kiln.

Pre-Heating Zone:In this stage the raw meal is gradually heated to remove waterof crystallisation and the temperature rises to 700-850C. Inthe semi-dry and dry process this again takes place outside ofthe kiln.

Calcination (Decarbonation) Zone:Calcium carbonate dissociates at about 1000C producingreactive lime and carbon dioxide gas.

In the wet and semi-wet process this takes place in the rotarykiln just before the burning zone. In the semi-dry and dryprocess around 30-40% of the decarbonation has taken placeoutside the kiln. In the precalciner process the majority (>80%)of decarbonation is achieved outside the kiln.

Burning Zone:Here the main reactions between lime and silica, alumina andferric oxide take place to produce the clinker minerals.

In these reactions heat is actually generated, adding to the heatfrom the flame, and thus the temperature rises rapidly to around1450C. Some 25% of the material (flux) become molten at thisstage.

Cooling Zone:Cooling begins immediately after the maximum temperature hasbeen reached. The resultant clinker is rapidly cooled by theincoming secondary air. The flux solidifies and the clinkerminerals become fixed before the clinker passes the nose ring.

2.5.2 COMBINABILITYIn general terms the main objective in the kiln process is toproduce as much C3S from the lime and silica as possible,minimising the amount of uncombined lime (free lime). Thusthe aim is to maximise the amount of oxides reacting with lime.In practice the kiln is operated by controlling the level of freelime. The target is often 1-1.5% free lime in the clinker.

Besides the percentage of Alite (C3S), the crystal sizes should bekept relatively small for best reactivity in the clinker (and hencethe cement). This is assisted by having a raw mix that can beburned to the target free lime as easily as possible.

The ease to which this can be achieved (referred to as the"combinability" or "burnability") is primarily a function of:-

- raw meal chemistry- raw meal fineness (and distribution of components)- raw material mineralogy

The combinability properties of a given raw meal can beevaluated in laboratory tests where the temperature for a givenfree lime is determined. (See Figure 30).

Figure 30. Typical Combinability Curves.

The combinability has a significant influence on:-- kiln fuel consumption- clinker microstructure (hence cement quality)- kiln operation (e.g. volatile recirculation and

build-ups)

Combination in the kiln is influenced by:-

Burning Temperature:From Figure 30 it is evident that the amount of uncombinedlime remaining in the clinker decreases as the burningtemperature increases. The actual relationship varies from oneraw mix to another and this will influence the relative ease atwhich a given free lime can be controlled.

Raw Meal Fineness:Finer grinding facilitates combination (See Figure 31), since finermaterials are more intimately mixed and have a higher surfacearea for reaction.




Figure 31. Combinability Effect of Raw Feed Fineness.

Lime Saturation Factor:As LSF increases combination becomes more difficult (SeeFigure 32a) and near to 100% LSF the combinabilitytemperature rises sharply.

Silica Ratio:As for LSF, an increase in silica ratio results in an increase incombinability temperature (See Figure 32b). This arises since assilica ratio increases the amount of flux available decreasesthereby reducing mobility and ease of reaction.

Figure 32a. Combinability Effect of Clinker LSF.

Figure 32b. Combinability Effect of Clinker Silica Ratio.

Alumina Ratio:Unlike LSF and silica ratio, there is an optimum alumina ratiofor minimum combinability temperature. (See Figure 33). Thisarises since at 1.4 - 1.6 alumina ratio the amount and viscosityof the flux are at an optimum for the oxides to move and react.In addition the maximum amount of liquid phase occurs at thestart of melt formation, thereby facilitating early combination.

Figure 33. Combinability Effect of Clinker Alumina Ratio.

Nature of Raw Materials:The achievement of close proximity between lime and the otheroxides can be influenced by fineness and the amount and natureof the flux, as discussed above. However it can also be stronglyinfluenced by the nature of the raw material minerals, such asthe heterogeneity of silica and calcareous residues. Thus thecomposition of the coarse fractions (e.g. at 90 microns) can beas important as the magnitude of the residue. These cansometimes be rich in siliceous or calcareous materials.

2.5.3 WET PROCESS KILNSAs already discussed the feed material typically contains 30-40% moisture. The kiln is a refractory lined steel cylindersupported on tyres and rollers. It is inclined by about 3 fromthe feed end (back-end) and rotates typically at 1-2 rpm. Therotational speed is designed to give circumferential speeds of 40-70 cm/sec. Material passes through the kiln as a result of theincline and rotation.

Large wet kilns, such as those at Lafarge UK (formerly BlueCircle), Northfleet, can be as large as 5.6m diameter by 200mlong, producing 80-90 tonnes per hour of clinker.

In the first part of the kiln there is a system of chains, whichincrease the overall thermal efficiency. Here drying is achieved


% 90 Micron Residue

Silica RatioAluminium Ratio

Aluminium Ratio

Lime Saturation Factor (%)



and material reaches around 150C. This can typicallyrepresent 30-34% of the kiln length.

In the next part material temperature gradually rises to around900C where de-carbonation occurs. This can represent afurther 40-50% of the kiln length.

It is in the latter part of the kiln, near the flame, where thesintering and clinkering reactions occur at 1300 - 1500C. Thiscan represent only some 15-20% of the kiln length.

Cooling commences in the last 2-3% of the kiln before materialpasses the nose ring into the cooler.

Approximately 2.45 tonnes of slurry (35% moisture) is requiredto produce 1 tonne of clinker together with around 0.2 tonnesof standard coal. Fuel consumption can be as high as 1400 -1700 kcal/kg clinker.

During this process a certain amount of dust is entrained withthe gases and leaves the kiln. This requires collection, usually inelectrostatic precipitators. Under adverse conditions the grossdust loss could be uneconomically high (as much as 30-40%),hence at least a proportion is usually returned to the kiln, e.g. by:-

- blowing into the kiln (via the burner or a separate pipe)

- returning to the slurry- dust scoops (scoops and seals located at the end of

the drying zone which feed material back through the kiln shell)

However, it is often necessary to discard some dust from thekiln system. This is referred to as the net dust loss and cantypically be 5-10%.

2.5.4 SEMI-WET PROCESS KILNSThese can be the same as wet process kilns although the chainsystem needs modification to accommodate the lower feedmoisture.

Alternatively the filter cake can be briquetted and pre-heatedoutside the kiln, for example on a moving grate similar to theLepol process.

In general around 2 tonnes of filter cake and 0.18 tonnes of coalare required for each tonne of clinker. Fuel consumption can bearound 1100 - 1300 kcals/kg.

2.5.5 DRY PROCESS KILNSIn the dry process, drying and preheating is achieved outside thekiln in a suspension preheater.

The suspension preheater (See Figure 34) consists of a series ofcyclones (usually 4) acting as heat exchangers. The raw mealpasses through these cyclones counter-current to the hot gases.

Figure 34. Suspension Pre-heater.

Referring to Figure 34, the raw meal is fed into the gas inlet ofstage I. The meal then drops from the base of stage I and passesinto the gas inlet of stage II, and so on through the stages. Thehot kiln gases first pass from the kiln to stage IV and then stageIII and so on.

It is essential that the cyclone design achieves good mixing andthus good heat transfer.

Material entering the kiln can be at around 1000C and some30% decarbonated. The preheater exit gases will be at around350C.

The rotary kiln is shorter than that used in the wet process sinceonly calcination and sintering is achieved in the kiln.

In general around 1.6 tonnes of dry raw meal and 0.12 tonnesof standard coal are required for each tonne of clinker. Sincethe overall heat transfer is relatively efficient, fuel consumptionis significantly lower at around 800 kcals/kg.

The dust leaving the preheater stack is usually returned to thesystem via the raw milling or blending silos.

In recent years the suspension preheater system has beenmodified with secondary or auxiliary firing. Here some 10-20% of the total fuel requirement is burned in the riser pipe tostage IV, thereby increasing the degree of decarbonation. Theenergy input is balanced to the decarbonation and thus there isno change in the temperature.

2.5.6 SEMI-DRY PROCESS KILNS (LEPOL)This is similar to the dry process but:-

- the raw feed contains some 10% moisture and is in the form of nodules

- preheating is achieved in a moving grate rather than cyclones

The Lepol grate is effectively an endless grate in a refractorylined chamber. A bed of nodules is moved by the grate and hotgases are passed through this. Heat exchange is relatively




efficient and fuel consumption as low as 800-820 kcals/kg canbe achieved.

Because the kiln feed is pre-formed into nodules, theassimilation of ash into the clinker is difficult. For this reason itis usual for the Lepol process to require low ash fuels.

2.5.7 LONG DRY PROCESS KILNSAs the name suggests these are like wet process kilns but wherethere is a dry raw meal. The kiln serves to complete dryingthrough to clinkering. Sometimes there maybe a single stagepreheater.

2.5.8 PRECALCINER PROCESS KILNSThe precalciner is development of the suspension preheater,where in addition to the cyclones, there is a chamber (orprecalciner) where some of the fuel is burnt. As much as 50-60% of the total fuel requirement can be burned in the calcinerand 90% decarbonation of the raw meal can be achieved. Asimplified flowsheet is shown in Figure 35. However there aremany designs of precalciner system.

The main advantage of this process is the large increase inclinker production that can be obtained from the kiln.Conversion of preheater operation to precalciner operation candouble kiln capacity.

Kilns of 6m diameter and 10,000 tonnes per day are inoperation.

Fuel consumption is generally similar to that of the preheateralthough fuel consumption of less than 800 kcals/kg are beingachieved.

Figure 35. Simplified Kiln with Precalciner.

A summary of the main features of a modern kiln system areshown in Figure 36.

Figure 36. Modern Kiln System Features.




2.6 CLINKER COOLERSAlthough an important part of cooling commences in the kiln,clinker leaves the kiln at around 1200-1300C and thus needscooling to around 100-150C for handling and heat recovery tominimise fuel consumption. An efficient cooler can recover asmuch as 70% (say 210 kcals/kg) of the heat leaving the kiln.

Cooling of clinker is achieved in one of three types of cooler:-- Rotary- Satellite or planetary- Grate

These are schematically shown in Figure 37.

Figure 37. Principal Types of Clinker Cooler.

Rotary Coolers:This is a simple rotary tube inclined like the kiln. The clinkertumbles as it passes along the length of the cooler and air isdrawn through the cooler and into the kiln.

Clinker can be cooled to around 150C, whilst the air ispreheated to around 700C.

Satellite or Planetary Coolers:Unlike the rotary cooler, which is a unit independent of the kiln,the satellite cooler consists of a number (often 10) of tubesattached to the kiln. Clinker passes directly to these tubes wherecooling takes place in a manner similar to that of the rotarycooler.

Grate Coolers:The most important and most common type of cooler consistsof a moving grate where a bed of clinker on the grate is cooledby passing air through it. Some of the heated air is used in thekiln, although a certain amount is exhausted through aprecipitator.




2.7 VOLATILE COMPONENTSThe behaviour of alkalis, sulphur and chloride in cementmanufacture can have a very significant influence on bothprocess operation and ultimate cement quality.

The actual effects are influenced by:-- input levels- alkali/sulphate ratio- process type- process operation

Only part of the alkalis and sulphur are fixed in the clinker,whilst part are volatilised in the burning zone and carried to theback-end of the kiln system. The latter condense onto solids inthe lower temperature regions of the back-end or preheater.

In the wet process the volatiles largely condense on the dustleaving the kiln and so the overall retention of alkalis can berelatively low, depending on the level of net dust loss. Typicallysome 60% of K2O and Na2O is retained in the clinker, whilstonly 25-35% of SO3 is retained. In general these volatiles donot build-up a very large circulation unless a high level of dustreturn is employed.The wet process is relatively tolerable to input of chloride,although the majority ends up in the dust.

In the Lepol process retention is a little higher since there isusually a lower net dust loss. Typically retention of alkalis willbe 70-80% and retention of SO3 will be around 30-50%.

In the suspension preheater there is little net dust loss from thesystem and hence overall retention is high (around 90%). Thealkalis, sulphur and chloride are volatilised in the burning zoneand generally condense in the last stage of the preheater. Thevolatilisation of SO3 can be significantly increased in thepresence of kiln reducing conditions.

Some condensation can occur on the cyclone walls causingproblematical build-ups or deposits. The majority condense onthe incoming feed, which returns the volatiles back to theburning zone for re-volatilisation. Hence large circulating loadscan build up in this way.

The nature of the deposits depends on the total input levels ofvolatiles, but also on the ratio of the individuial materials.

Where there is excess sulphur there is a greater risk of hard-based calcium sulphate deposits. For an excess of alkalis, thereis a risk of hard-based alkali carbonate. For a balanced alkaliand sulphate input alkali sulphate deposits form which seem tobe less problem causing with a tendency for self-removal.

Compared to alkalis and sulphur, chloride has a very highvolatilisation rate (around 97-99%) and thus a very largecirculating load forms. As a result the concentration of chloridein the lower preheater stages can often reach some 30-50 timesthe input level.

For this reason the dry process cannot tolerate a high level ofchloride input and chloride usually has to be limited to below0.03% on clinker.

In extremes, where the volatile input causes significantproblems, a gas bleed (or bypass) can be used. Here part of thekiln gases containing the volatiles is withdrawn from the kilnand the volatiles removed before they condense causingproblems. However there is a significant adverse influence onthe overall economy.

In most cases the threat of severe deposits or build-ups is takencare of by mechanical and pneumatic devices fitted at strategicplaces. These "shock" or "blast" any deposits therebypreventing their build-up.




2.8 KILN CONTROLIn common with the rest of the cement making process there isa high degree of process control in the kiln system. In recentyears there have been many developments in sophisticatedcomputer control.

The objective of computer systems to operate and control thekiln was to overcome the natural manual tendency of erring onthe side of caution. Potential benefits include:-

- lower burning temperature- fuel savings- increased capacity- reduced refractory wear- easier grindability clinker

The first stages of good kiln control involve:-- raw meal feed rate- coal feed rate- kiln speedkiln airflow- dust return

To assist in their adjustment the following parameters need tobe monitored:-

- kiln exit NOx- kiln exit O2- kiln exit CO- back end temperature- kiln amps- kiln speed- feed rate- fuel rate- damper position or fan speed

A list of potential reasons for kilns not remaining in optimumburning condition is shown in Figure 38.

Modern automated kiln control systems include the use of:-- expert systems- fuzzy logic- rule based control

Figure 38. Examples of Reasons for Kiln Instability.


Kiln Feed Compostional VariabilityKiln Feed Physical Changes (e.g. Residue)Kiln Feed Moisture Content Variation (especially WetProcess)Kiln Feed Rate VariabilityCoal/Fuel Chemical Compostional VariabilityCoal/Fuel Ash Content VariationCoal/Fuel Moisture Content VariabilityCoal/Fuel Physical Changes (e.g. Residue)Coal/Fuel Feed Rate VariabilityHeat Loss from Kiln VariabilityInleaking Air VariabilityChange in Kiln SpeedKiln Lining Coating BreakawayRefactory Brick LossKiln Dust Loading VariabilityKiln Airflow VariabilitySecondary Air Temperature VariationChanges in Cooler Conditions



2.9 QUALITY CONTROL2.9.1 INTRODUCTIONA quality assurance (QA) programme sets the policies,standards, methods and specifications for quality controlprocedures. It covers all the activities and functions concernedwith the achievement of quality. Quality is best described as"fitness for purpose".

Quality control concerns the day to day, hour to hourmonitoring and control of conformance to the Q.A.requirements.

In the cement industry, quality is often taken to refer to strengthand workability. However, consistency is probably the mostimportant parameter.

Some of the key parameters involved in good quality controlinclude:-

- kiln feed LSF, silica ratio and alumina ratio- clinker free lime- cement SSA- cement SO3

However, other parameters, such as alkalis (particularly thewater-soluble alkalis), cement 45-micron residue and loss onignition have a very marked influence in final cementperformance.

The following sections provide a brief outline of the input tooverall quality control throughout the cement making process.

2.9.2 QUARRYINGThere is an opportunity here to reduce variability as much aspossible so that control in later stages becomes easier. Thereforeit is important to have a good knowledge of the materialdeposits. This then allows the possibility of an optimumplanned extraction of materials.

2.9.3 CRUSHING AND STORAGEBefore the fine grinding stage the ex-quarry material is crushedand stored. Modern practice of stacking and reclaiming raw

materials can be an important stage in reducing variability ofchemistry, usually in terms of the LSF and silica ratio.

2.9.4 RAW MILLINGIn raw milling the various mix components are brought togetherusing controlled weighers. Continuous monitoring of LSF, silicaand alumina ratios allow adjustments to feeders. Fineness,usually at 90 and 300 microns, is also controlled to desiredlevels.

2.9.5 BLENDINGFinal kiln feed adjustments are made in the blending system.Variability can often be reduced by a factor of 10. Typicalvariations that are desired at this stage are:-

- LSF 1%- S/(A+F) 0.1- A/F 0.1- 90 micron 1%

Note: Between day standard deviations

2.9.6 KILN FUELSome of the key parameters for the fuel were discussed insection 1, and many of these are designed in the purchasespecification e.g. calorific value, volatile matter, sulphur,chloride, hardness, abrasivity and ash.

As discussed, ash in the fuel has to be treated as a raw material.In general a 1% change in ash will influence the LSF by about4%. Cement plants can operate with a wide range of ashcontents in the main fuel, but only if at constant levels.

Coal fineness is mainly a function of the volatile matter and istypically controlled by the 90-micron residue.

2.9.7 CLINKERKiln control was briefly discussed in section 2.8, where the mainobjectives of control are to produce a uniform clinker withrespect to chemistry, microstructure and free lime.

Part of the control involves gas temperatures, CO, O2, SO2,NOx. Generally free lime is targeted between 0.5 and 2.0%.Levels below 0.5% are often indicative of overburning, whilstabove 2.0% are indicative of unstable kiln operation or poorraw feed chemistry control.

High free lime clinker requires separate storage for controlledlow addition during cement grinding.

Naturally it is difficult to produce a good low variability clinkerfrom a poorly controlled raw feed.It is usual to make full chemical analyses (XRF) of clinker on aroutine basis.

2.9.8 CEMENT MILLINGA more detailed assessment of the relationship between cementquality and cement grinding is made in section 8. However thefollowing are the principal parameters to be controlled:-

- clinker feed rate- non-clinker component(s) feed rate- stored clinker feed rate- gypsum feed rate- SO3 content- blaine fineness- 45-micron residue- milling temperature

and also the grinding additive.

2.9.9 CEMENT PERFORMANCECement performance can be measured in many ways and this isdiscussed in Section 7. However for quality control purposes themain parameters are:-

- strength, EN196 mortar at 2, 7 and 28 days- SO3 level- alkali level- fineness- slump or equivalent

A summary of these parameters and typical between-daystandard deviations are shown in Figure 39.




Figure 39. Typical Between-Day Standard Deviation Targets.


Kiln Feed: LSF 1.0Silica Ratio 0.07Alumina Ratio 0.0590-micron Residue 1.0

Clinker: LSF 1.0Silica Ratio 0.07Alumina Ratio 0.05Free Lime 0.2 - 0.3SO3 0.1C3S 2.5

Cement: Specific Surface Area (M2/kg) 1045-micron Residue 1.0 - 1.5EN 196 Mortar Strength2-day 2.57-day 2.528-day 2.5SO3 2.5Alkalis, Eq. Na2O 0.03



2.10 MATERIALS HANDLING2.10.1 INTRODUCTIONThe production of cement involves a significant amount ofmovement of bulk materials between locations and between unitoperations. In fact to an outsider the operation could bedescribed as:-

1. Movement of large sized rocks and store2. Production of fine powder3. Move again and store again4. Heat and make large sized again5. Move again and store again6. Make small sized again7. Move once more and store once more

In considering the materials handling element it is necessary toexamine:-

- the type of bulk materials involved- the storage and extraction- the methods of conveying

2.10.2 MATERIAL TYPESIn materials handling terms, the full range of materials areencountered, i.e.:-

- coarse to very fine- wet to damp to dry- sticky to dusty- non-flowing to free flowing

These include raw materials, coal and pulverised fuel, clinkerand cement, and filter and precipitator dust.

2.10.3 STORAGE AND EXTRACTIONMaterials are stored in a wide range of equipment, such as:-

- stockpiles- sheds- silos- hoppers and bins

Once stored, the materials have to be extracted and this willoften involve a wide range of feeders such as:-

- rotary value- belt- apron- screw- vibratory- drag

The correct selection of an appropriate feeder will need toconsider many objectives, such as:-

- flow rates required- material properties- consistent flow rate- range of flow rates required- space available- temperatures involved

Even with good design of materials handling many unforeseenfactors can render a system ineffective. This can them sometimesbe overcome by the appropriate use of discharge assistance.These can include such devices as:-

- aeration pads- air cannons- air cushions- vibratory dischargers

As already discussed, one application of such devices is toovercome the build-ups that can often occur in the lower stagesof the preheater.

2.10.4 CONVEYING EQUIPMENTSimilarly the methods for transporting material depend on thenature of the materials and the volumes to be handled. Thefollowing equipment is common in the cement plant:-

- belt conveyer- bucket elevator- bucket conveyor- screw conveyor- drag chain conveyor (e.g. Redler)- vibratory- airslide- pneumatic (dense phase, screw type or blow tank)- air system (lean phase)

Cement flowability is discussed in section 7.



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Cement Chapter 2