cement chapter 10 -1

10. Cement AdditiveTechnology

C E M E N T T E C H N O L O G Y N O T E S 2 0 0 5 117

1 0 . 1 I N T R O D U C T I O N

1 0 . 2 G R I N D I N G A I D M E C H A N I S M S

1 0 . 2 . 1 I N T R O D U C T I O N

1 0 . 2 . 2 R E H B I N D E R E F F E C T

1 0 . 2 . 3 A G G L O M E R A T I O N A N D C O A T I N G

1 0 . 2 . 4 M I L L R E S I D E N C E T I M E

1 0 . 2 . 5 S E P A R A T O R P E R F O R M A N C E

1 0 . 2 . 6 C E M E N T P E R F O R M A N C E

1 0 . 3 I N F L U E N C E O N M I L L P E R F O R M A N C E

1 0 . 3 . 1 I N T R O D U C T I O N

1 0 . 3 . 2 M I L L C O A T I N G

1 0 . 3 . 3 M I L L H O L D - U P

1 0 . 3 . 4 S P E C I F I C C O N S T R A I N T S

1 0 . 4 C U S T O M E R O B J E C T I V E S

1 0 . 5 P O T E N T I A L B E N E F I T S

1 0 . 5 . 1 I N T R O D U C T I O N

1 0 . 5 . 2 R E D U C E D K W H / T O N N E

1 0 . 5 . 3 I N C R E A S E D P R O D U C T I O N

1 0 . 5 . 4 R E D U C E D R U N H O U R S

1 0 . 5 . 5 I M P R O V E D M A T E R I A L S H A N D L I N G

1 0 . 5 . 6 I M P R O V E D C E M E N T P E R F O R M A N C E

1 0 . 5 . 7 E C O N O M I C S

1 0 . 6 E N V I R O N M E N T A L I S S U E S F O R A D D I T I V E S

contents chapter 11


10.1 INTRODUCTIONCement grinding additives have been used since the early1930's. One of the first references to the use of grinding aidswas given in a British patent in 1930, where the addition ofsmall amounts of resins was defended.

The development of "TDA" started in the early 1930's andachieved U.S. patent coverage in 1935/36.

HEA2 was patented in 1965 and the use of formulated qualityimprovers began in the early 1970's.

The objective of today's modern grinding additives is to assist inminimising manufactured cost, while maximising cementquality. There has been continued research and developmentactivity over the last 60 years or so together with analysis oftheir influence on the cement grinding process and resultantquality characteristics. However, reported results, even forapparently "similar" products in "similar" circumstances, havebeen quite wide ranging. As the understanding of their usecontinues to improve, so the probability of successfulapplications also improves. This requires a continualimprovement in our knowledge of the mechanisms involved aswell as increased awareness of our customers' grinding plant,cement performance characteristics and their market criteria.

Therefore, to ensure success in the application of cementgrinding additives, we need to assess:-

- the mechanisms of additives- the influence on mill performance- the economics involved- the quality characteristics and customer objectives- any environmental concerns

These are discussed in the following sections in more detail.

10. CEMENT ADDITIVE TECHNOLOGY

contents chapter 10 chapter 11


10.2 GRINDING AID MECHANISMS10.2.1 INTRODUCTIONThe mechanisms by which additives influence grindingperformance have been discussed in the literature for manyyears and reference is usually made to the role of:-

- decrease in the resistance to comminution (so-called Rehbinder Effect)

- the prevention of agglomeration and mills internal coating

- a decrease in mill residence time- an improved separator efficiency

In addition, there is reference to various influences on thecement properties (See Section 10.3).

The Rehbinder effect has increasingly become seen as lessimportant with the majority of emphasis placed on the de-agglomeration and coating. The influence on powder flowabilityand the reduction in residence time is often discussed butwithout sufficient assessment of the quantified effects ongrinding efficiency.

10.2.2 REHBINDER EFFECTAccording to Rehbinder's hypothesis, grinding additives areabsorbed on to the surface of particles, including intomicrocracks, thereby making crack propagation easier byreducing their valency forces, i.e. prevention of rejoining ofcracks as they form.

Theoretically a reduction in the surface energy and a consequentreduction in the critical stress of crack propagation would beexpected in the presence of grinding aids. This would then beexpected to produce an increase in the impact breakage.

However this assumes that the velocity of additive absorptiontakes place at a similar rate to that of crack propagation.Increasingly, it is recognised that the velocities of crackpropagation are very much higher than the velocities of grinding

additive diffusion. Hence it seems unlikely that the absorptionof additives can positively influence the rate of impact breakage.Indeed, a number of researchers have found this to be the case.However some workers (e.g. Moothedath and Ahluwalia -Powder Technology, 71, (1992), 229-237) found that the surfacehardness of materials, and their subsequent resistance toattrition, changes when grinding additives are present. Theyconcluded that attrition was increased when additives were usedin low dosages but reduced at higher concentration (probablybecause of lubrication).

10.2.3 AGGLOMERATION AND COATINGWe have already seen that, according to Rittinger, the area ofnew surface produced by grinding is directly proportioned tothe useful energy input. Hence the grinding efficiency in termsof cm2/j (See Section 6) is constant for any level of fineness (SeeFigure 111).

However in reality the energy input increases by an amount inexcess of this as a result of the negative influences ofagglomeration and coating. As grinding progresses and grainsbecome smaller the attractive thermodynamic, mechanical andchemical forces result in strong adhesions of particles. Thiscauses agglomeration, which limits the increase in specificsurface area, and coating of the mill internals, which results in areduction of the rate of breakage. (See section 6.3).

The grindability curve was discussed in Section 5 (See Figure 56).

If we assume that Eg is the energy for grinding and that Ec isthe energy lost because of coating and agglomeration, then wecan consider 3 regions of the curve (See Figure 110).

Figure 110. Grindability Curves – Rittinger.

In region 1, Ec = O (or negligible), and thus the relationshipbetween Eg and SSA is linear (as Rittinger).

In region 2, Ec becomes increasingly high but remains below Eg.Hence there is an increasing deviation from the Rittinger linearrelationship. Region 2 typically starts at around a SSA of 200-250 m2/kg.

At the interface of region 2 and region 3 grinding in effectceases, Ec = Eg. In other words, the energy lost due toagglomeration and coating equals that applied. This can happenat around 500-700 m2/kg and can be referred to as the “GrindEnd-Point”.

In region 3, Ec > Eg and no further grinding (increase in SSA) isseen. And agglomeration is occurring.

All grinding additives contain chemicals, which neutralise thesurface charges on cement grains and shield against the inter-particle attractive forces. This reduces the tendency foragglomeration and adhesion to media and linings and thus the




efficiency of grinding is enhanced. Thus, in effect, Ec is reduced.This means that region 1 is prolonged, the deviation in region 2is reduced and region 3 is delayed or virtually eliminated.However additives are not capable of achieving the linearrelationship of Rittinger (See Figure 111).

Figure 111. Mill Efficiency versus Mill Exit Fineness

From Figure 111, which aims to show a typical influence on thecm2/j versus SSA of an additive, we would expect the followingincreases in efficiency, and hence output:-

SSA Increase in Efficiency(m2/kg) %

300 5%350 10%400 12%450 20%500 30%

Thus, the effectiveness of grinding aids would be expected toincrease with increasing fineness and thus result in increasingmill efficiency.

10.2.4 MILL RESIDENCE TIMEIn Sections 4 and 6 we examined the parameters of millresidence time, hold-up and void filling. These can besummarised as (for steady-state):-

- the residence time refers to the mean time that material remains in the mill

- the hold-up refers to the tonnes of material (not media) in the mill

- the void filling refers to proportion (fractional or percentage) of the voidage in the media filled by material

The simplest way to compare material levels in the mill is to usethe void filling, since the others require some qualification byother parameters (such as tonnes/hour, circulating load, millsize, media tonnages).

In Section 6 we saw that there is an optimum filling of 85% ofthe media voids occupied by material. At this filling the grindingefficiency is maximised and both higher and lower levels willresult in a reduction in the grinding efficiency (See Figure 92).

The chemicals contained in grinding additives reduce thetendency for agglomeration and coating (See Section 10.2.3) andas a result also reduce the powder cohesion and thereforeincrease the flowability.

Many factors influence the material void filling (See 10.3), butan increase in material flowability will reduce the head ofmaterial required in the mill to discharge from the outlet.Hence, for other parameters constant, the void filling is reducedby the addition of a grinding aid. Hence the residence time andhold-up are also reduced.This reduction in residence time was considered in detail by

Frank Mardulier with the use of sodium fluorescein tracer toassess residence time (See Section 4 and 6). Residence timedistribution curves for a mill, with and without the use of agrinding aid is shown in Figure 112.

Figure 112. Residence Time Distribution

In this case the mean residence time was reduced by 30% (Peakof 3.5 minutes compared to 5 minutes). This would thusrepresent a 30% reduction in the hold-up and a 30% reductionin the void filling (for constant total mill throughput andmaterial density).

Since many mills operate with a high filling level (i.e. above theoptimum) the application of an additive therefore moves thefilling level to the optimum (or closer to). Hence there is anincrease in the overall grinding efficiency. (See 10.3).

10.2.4 SEPARATOR EFFICIENCYAs we have already considered, separators operate by exerting aforce balance on individual particles (of which there are many)(See Section 6). Separator efficiency is reduced for increasedloading of the separator as a result of poor dispersion, particleagglomeration and fines entrainment. Thus it can be expectedthat the improved dispersion resulting from the presence ofgrinding additives should assist in the separation efficiency.

Because the Tromp Curve is significantly influenced by thecirculating load, comparisons of separation with and without




additive should only be made at constant or similar conditions.Such a comparison is shown in Figure 113.

Figure 113. Separator Performance.

The often lower by-pass and less pronounced fish hook (SeeSection 6) means that less fines are returned to the mill and

hence their negative influence on the overall fineness balancearound the mill is reduced. Thus overall grinding efficiency canbe improved.

10.2.5 CEMENT PERFORMANCEIt is not the intention to review in detail the mechanisms ofadditives on cement properties. This remains complex and onlypartially understood.

We have seen that cement additives are formulated to providebenefits to the grinding process, but many are also formulatedto additionally provide a benefit to the cement performancecharacteristics, with an influence on cement hydration mechanisms.

Cement additives contain various combinations of organic andinorganic salts. Some components are able to behave as catalysisfor the hydration reactions of C3S and water to produce earlierinitial set and strength. Others have retarding properties andinteract with the C3A, leading to the formation of stablecomplexes in solution that later precipitate coating the C3A phase.Such an ability to form a complex is correlated with improvedflow and set properties of cement. The inorganic and organic saltsas Na/Ca Chlorides and Na Acetate/Formate are known to bestrong accelerators for C3S, increasing early strengths of cement.

Other polymers are adsorbed on the surface of the cementparticles, and impart repelling charges to the particles, releasingthe water normally entrapped by the surrounding cementparticles, which can then contribute to the fluidity of the mix.The properties of CBA additives has been researched in detailand the role of C4AF in limiting overall silicate hydration,together with the proposed mechanism of facilitated transportfor iron, at least partially explains the enhancement in strengthdevelopment. (See Figure 113a).

A comprehensive understanding of the fundamental mechanismsinvolved would help in the success of applying and developingnew additives. However this assumes that there is equally acomprehensive understanding of the influence of the manynatural cement characteristics on cement performance.Therefore it is likely that any improved knowledge will alsohave to include careful statistical analysis of data from the useof additives in laboratory and plant studies.

The principal parameters that are known to influence cementperformance were discussed in section 1, and it is likely thatmany, if not most or all, of these will also influence thebehaviour of cement additives.

If this is considered in combination with the way in whichcement performance can be judged (See Section 7), then there isa rather complex picture.

However we continue to understand the role of additives, by,for example, differentiating between certain clinker types andcertain additive types, e.g.

Figure 113a. The “Facilitated Iron Transport” mechanism ofstrength enhancement by CB100.

Cement types:-- Pure or composite- High or low fineness- High or low alkali- High or low clinker SO3 (alkali solubilisation)- High or low D.SO3 (availability of soluble CaSO4)- High or low Free Lime- Hard or soft burned- Fresh or surface conditioned- High or low C3A- High or low C4AF

Additive types:-- according to active components- according to dosage




10.3 INFLUENCE ON MILL PERFORMANCE10.3.1 INTRODUCTIONIf we conclude that grinding efficiency is principally influencedby cement grinding additives by their influence on:-

- de-agglomeration- reduction in void filling

then, to assess the likely influence on the performance of anymill system, we need to assess the characteristics of the millsystem in terms of the above parameters.

In simple terms, we can expect good results of using a cementadditive where milling efficiency is significantly below optimumas a result of agglomeration and coating, poor separatorefficiency and high void filling.

To ensure the appropriate application of a cement grindingadditive we therefore need to consider:-

- the propensity for agglomeration and coating (including separator efficiency)

- the mill filling level (void filling)- any specific constraints of the mill system- current cement performance- customer objectives- economics involved- environmental concerns

10.3.2 MILL COATINGThe higher the degree of agglomeration and coating present, thegreater the deviation will be from Rittinger’s straight linerelationship for SSA versus kWh/tonne (and the poorer theseparator efficiency will be).

Agglomeration and coating of mill internals is stronglyinfluenced by temperature, fineness and pre-hydration.

As temperature in the mill rises the degree of agglomeration andcoating increases. Up to around 110°C thus may not be asignificant factor, but at higher temperatures the negativeinfluence on mill output increases.

Milling temperature (See Section 4) can be expected to rise for:-- hotter clinker- higher kWh/tonne (either through inefficiency or

higher fineness)- poor ventilation- inappropriate, or absence of, water injection

Particle agglomeration and coating can also be expected tobecome more severe where surface hydration occurs, e.g.:-

- high moisture input in materials- excessive water injection- inadequate ventilation- nature of stored clinker (weathered)

As we have already discussed, the deviation from Rittinger’s lawincreases with increasing fineness (at the mill exit). This will beinfluenced by:-

- Product SSA target- Mill circuit type and efficiency- Circulating load- Separator efficiency

Non-clinker components, such as slag (if not with excessivemoisture), sand, pfa can reduce coating whilst others, such aslimestone and some pozzolans can exacerbate the coating.

If anything, larger ball sizes appear to coat more severely.However grinding efficiency probably reduces more rapidlywhere small media become coated.

Coating often seems to be more severe where the void filling ishigh (it maybe also that the filling level becomes higher withmore coating and hence less flowability). Thus, for whateverreasons, the more severe the agglomeration and coating, themore likely there will be substantial benefits of a grindingadditive.

It can be possible to make a direct assessment of coating duringa mill inspection (See Section 11).

The influence of these factors on agglomeration and coating willalso influence the performance of the separator in the millsystem circuit.

10.3.3 MILL HOLD-UPAs discussed previously there is an optimum void filling andgrinding additives reduce the mill hold-up and residence time,perhaps by around as much as 20-40%. Thus the increase inefficiency will very much depend on the existing filling levelbefore an additive is used. For a high void filling level of say120%, a 30% reduction will give a new void filling of 84% (i.e.close to the optimum level). As a result the grinding efficiencywould increase (From Figure 92) by around 7-8%.

On the other hand for a lower void filling level of say 90%, a30% reduction would produce a new level of only 63%, whichcould reduce the grinding efficiency by around 5%.

The overall influence of the additive on mill efficiency would ofcourse also depend on the other parameters, such as the positiveinfluence on the agglomeration and separation. The void fillingin the mill is principally influenced by:-

- the total mill throughput (See Figure 93)- the media grading (See Figure 97)- the mill ventilation rate- diaphragm design and condition- number of diaphragms/chambers- separator efficiency- volume loading- mill speed- mill length- material flowability

The combination of these parameters will result in acharacteristic void filling for any given mill system. It maybepossible to assess the void filling by careful consideration of theabove parameters or by discussion with plant personnel or by amill inspection (See Section 11).




Therefore, by considering just the two areas, i.e. coating andvoid filling, we can begin to see why even with similar mills,systems and additives we can find variations in the influence ofadditives. Thus the largest increase in output would be expectedfor mills with evidence of high levels of coating and excessivevoid filling. Conversely, for mills where there is no evidence ofagglomeration or coating and where the void filling is low,increases in output maybe small or even negative (See Figure 114).

10.3.4 SPECIFIC CONSTRAINTSIn any given mill system it is often found that there are specificcharacteristics related to that system. Some of these may havean indirect influence on the application of an additive, forexample:-

- low void filling (as discussed in section 10.3.3)- poor first chamber performance (e.g. near to

limit such that it is not possible to increase clinker tonnage)

- restricted circulating load (e.g. bucket elevator)- restricted product conveying system- restricted feed system- mill control system- outlet diaphragm blockages- high temperatures- restricted cement quality- existence of national standards- poor economics (see later)

Figure 114. Effectiveness of Cement Grinding Additives

Key: ***** Excellent Effectiveness**** Very Good Effectiveness*** Good Effectiveness** Moderate Effectiveness* Minimal Effectiveness

Note: Effectiveness of grinding additives refers to grinding efficiency only, i.e. mill output and kWh/t, not to effectiveness of enhancing cement performance


“Coating”

Void FillingHeavy Moderate Light/None

>1.100.90 - 1.10

<0.90

***** **** ******* *** ***** ** *



10.4 CUSTOMER OBJECTIVESThe application of an additive needs to be matched to theobjectives of the customer. These will not always be fullyappreciated, and it is therefore often necessary to have aniterative process, i.e. mutual discussion of the potential benefits(See 10.5) and all of the desired cement characteristics.

In general it will be very important to achieve economic use ofan additive. Often it will only be possible to evaluate additivecost versus reduced kWh/tonne and cement composition. Theinfluence on cement volume, for example by increasedtonnes/hour or from increased cement tonnes from a givenclinker tonnes, may also be relevant.

Any potential changes in cement performance should reflect:-- current cement performancecement competitors' capability- market requirements- standards

Often these will concentrate on:-- flowability- water demand- setting- strength development

However it maybe advantageous to consider other non-standardproperties (See Section 7), such as bleeding, very early strengths(e.g. at 8 or 16 hours).




10.5 POTENTIAL BENEFITS10.5.1 INTRODUCTIONThe application of additives mainly concerns the advantageousreduction in kWh/tonne and the potential to increase non-clinker components, where the additive is a quality improver.However there are other potential benefits in connection with:-

- the increase in throughput- the reduction in run time- the improved materials handling- the change in cement performance

10.5.2 REDUCED KWH/TONNEThe reduction in kWh/tonne is brought about by the enhancedgrinding efficiency and the possibility to reduce the targetfineness where properties are improved as a result of a narrowerparticle size distribution and/or improved hydrationcharacteristics.

In general, the actual kW of the plant can be considered to beindependent of throughput - hence a higher throughput will beproportional to the decrease in kWh/tonne.

The total mill system kW should be used in calculating thekWh/tonne costs. This will normally involve the plant from theclinker store through to cement storage. Ancillaries to the mill canrepresent some 15-30% of the mill kW. Hence for a cement millkWh/tonne of 35, the overall system kWh/tonne could be 40-46.

10.5.3 INCREASED PRODUCTIONNaturally the reduction in kWh/tonne is achieved by theincrease in production, i.e. higher tonnes/hour. However thehigher production can also have additional advantages.

- increased sales- ability to reach peak demand- postponement/cancellation of capital spend- maximise utilisation of efficient mill systems- de-commissioning of older, lower efficiency systems

In a declining market, the ability to provide increased sales arenot likely to be an advantage. However in high demandmarkets, particularly for grinding-only plants, increased

tonnes/hour can mean the ability to achieve increased sales. Thisprovides a very strong economic case for using an additive.

Cement sales are usually very seasonal and thus in some monthsthe demand can be significantly higher than the average (annualsales divided by 12). Thus an increase in production can reducethe impact of these peak periods.

If a plant is near to its capacity, costly capital investment willoften be required in order to increase capacity. This cansometimes be postponed or even cancelled by increasingproduction of existing equipment. So the additive cost can beoffset against capital investment.

Where there is more than one milling system it maybe possibleto maximise production on the more efficient system. Thussavings in kWh/tonne will often be calculated for the leastefficient mill.

In the extreme it maybe possible to cease using older lessefficient systems, with associated savings in high kWh/tonne andrepair and maintenance costs (See 10.5.4).

10.5.4 REDUCED RUN HOURSFor a fixed annual production the successful use of a grindingaid can reduce the operating hours required. This can thenresult in:-

- avoiding peak price electricity- reduced repair and maintenance costs- improved maintenance schedules- Increased tonnes/manhour

As briefly discussed in Section 3, electricity pricing can be rathercomplex involving various kWh prices as well as fixed chargesand rebates. Clearly a reduction in run time increases theflexibility of avoiding higher cost electricity periods. Thussavings will often be made for the highest unit energy cost.

Repair and maintenance (R+M) costs can be quite a largeproportion of the total grind department costs. Generally thecost/tonne is calculated using only the annual cost divided by

the tonnes produced. However a cost/tonne does not reflect thecause of this cost since, often the cost is fixed to the tonnesground and hence increased production involves increased costwith no cost/tonne saving.

In practice, the repair and maintenance costs can be treated as afunction of the plant operating hours. Hence where tonnes/houris increased the cost/tonne is decreased.

For example, considering the following:-

Tonnes Produced = 300,000Tonnes/hour = 50Run hour = 6000R+M Cost/tonne = 0.64

If grinding aid can achieve 55 tonnes/hour, then we have:-

Tonnes Produced = 300,000Tonnes/hour = 55Run hour = 5454

If the initial case the R+M cost can be equated to:-

R+M cost/tonne = 0.64Total cost = 0.64 x 300,000 = 192,000Cost/hour = 192,000 ÷ 6000 = 32

In the case with additive, we now have:-

Cost/hour = 32 (unchanged)Run hours = 5454Total cost = 174,528Cost/tonne = 0.58

Less run hours will result in greater flexibility for scheduling ofrepair and maintenance.

In the extreme, a reduction in run hours could directly influencelabour costs.




10.5.5 IMPROVED MATERIALS HANDLINGThe reduction in cohesive properties of cement when an additiveis used can lead to:-

- improved flowability- reduced packset- improved loading and unloading times- reduced pumping costs- improved utilisation of silo capacity

Cement flowability was discussed in Section 7. Improvedflowability and reduced packset will be most advantageouswhere cement is transported over long distances and/or storedover long periods. This is particularly so for bulk transport inships where difficult, and therefore long, unloading can entailsignificant financial penalties.

Silo capacity can be improved as a result of less "dead" cementand greater bulk density.

10.5.6 IMPROVED CEMENT PERFORMANCECement performance is influenced in two ways:-

- as a result of the increased grinding efficiency- by appropriate use of formulated additives

As already discussed, as grinding performance increases theparticle size distribution becomes narrower. This results in amore efficient use of the clinker since the resultant lower residuesproduce a more complete hydration (See Sections 3 and 7).

Quality improvers can, of course, be designed to provide a widerange of modifications to cement properties, such as waterdemand, setting and strength development, and thus satisfy thedemands of all cement types (See Figure 115).

The improvement to cement performance may permit a higherselling price, a new higher quality cement type, advantage todownstream concrete operations or, more commonly, areduction in cement production costs by allowing higher clinkerreplacement levels for the same quality or by allowingutilisation of lower quality raw materials.

Figure 115. Cement Additive Technology.


Service + Products + Optimization =70 years ofexperience

ComprehensiveTechnical Services

Specialist TechnicalSupport

IndependentOrganisation withGlobal Activities

Continued R & DCommitment

PerformanceOptimization

MillOptimization

GraceQuality

Improver

GraceMasonryCement

Additives

GraceGrinding

Aids

Enhances CementPerformance

Characteristics

Reduces CementCompostional Costs

ImprovesFlowability, Lowers

Handling Costs

Reduces GrindingCosts

Increases Production,Reduces Run Time

Increased Profitability



10.5.7 ECONOMICSThe success to using Cement Additives to reduce costs clearlyrequires a detailed analysis of the constraints and flexibility ofany given cement manufacturing business together withknowledge of the capabilities of cement additives.

In some cases this could involve a relatively straightforwardanalysis for the impact of increased tonnes/hour and reducedkWh/tonne on production costs against the used cost of thecement additive. However, where clinker substitution isconsidered, a more detailed analysis will be required, forexample in understanding the influence of the non-clinkermaterial and of the cement additive on process and cementperformance characteristics. This in turn will involve acombination of prior experience, known relationships,laboratory testing and plant evaluations.

The overall economics of using a cement additive will thereforemainly depend on:-

- effective kWh cost- reduction in kWh/tonne achieved- raw material costs- clinker replacement achieved- repair and maintenance costs

The raw material costs refer to the mill feed components. Careshould be taken when assessing clinker costs. Often it will onlybe realistic to use the variable cost elements. These can besignificantly lower than the total clinker cost.

As already discussed, electricity prices continue to increase,particularly in comparison to other cement manufacturing costs.In most case they are likely to (or already have) exceed theclinker fuel costs. As prices rise, and a combination of lowerprice primary fuels and energy efficiency improvements reduceclinker production costs, the kWh/tonne consumed is rapidlybecoming the highest cost energy requirement of the cementindustry.

In recent years the ratio of additive cost to kWh cost has movedin a direction where additives have become more attractive.

As an example, in the UK, grinding aid and kWh costs havechanged as below:-

1971 kWh = £0.005Additive = £0.18/kg

2001 kWh = £.03 - £0.04Additive = £0.55/kg

i.e. whilst electricity has increased by a factor of 6-8, theadditive has only increased by a factor of 3.



10.6 ENVIRONMENTAL ISSUES FOR ADDITIVESAwareness of the health and safety issues concerned with thehandling of the additive itself is not considered here. Howeverthere has been increasing awareness of what happens to theadditives once they are in the milling system.

As the cement industry moves forward with sustainabilityinitiatives the role of additives, for example to provide areduction in CO2 per tonne of cement, will become moreimportant. However the contribution that additives can make tothe VOC (volatile organic component) emission in a cementplant will also need addressing. Emission limits are beingdiscussed and are often considered as the carbon equivalent.

A VOC has been defined as:-

"Any organic substance or mixture which can release vapour tothe atmosphere and with the potential to cause environmentaleffects at low atmospheric levels."

A study of emission was undertaken by the VDZ in 1985 (ZKG10/86 577-580). In this Rechenberg reported emissions ofbetween 10 and 20 mg/m3, which represented some 2-13% ofthe total additive input. The data further showed that emissionwas higher for lower SSA and higher airflow. The averageemission amounted to some 4-5% of the additive input.

Figure 116, considers the mass balance for an additive. In thisassumptions were made for:-

- the retention of additive- the mill airflow- the filter airflow

Figure 116. Potential VOC Emission. For a mill of 2100 kW, at 60 tonnes/hour, with a filter airflowof 25,000 m3/hour, 25% of in-leak, a 300g/tonne dosageindicates the following potential emission:-

% Retention Emission (mg/m3)(organics) carbon equivalent

99 4 297 13 695 22 1190 43 22

The emission as carbon equivalent was determined assuming theadditive solids were 60% and 50% as carbon.

Thus, with an additive retention in excess of 95%, emissionswill be low and within likely stringent limits.

In addition to meeting environmental emission limits, the impactof any loss of additive will also need to be balanced against theenvironmental benefits achieved, for example by lowerkWh/tonne and as a result of clinker substitution and less CO2

per tonne of cement.

A first approximation is to consider the VOC emission in termsof its equivalent Greenhouse Gas potential. This can beestimated by considering the VOC components on a carbonbasis in terms of methane (CH4). This is then multiplied using afactor of 21, to estimate the equivalent CO2 GHG effect.

In the example in Figure 116, an additive retention of 97% (3%loss) would result in the following:

Additive loss (per tonne of cement) = 5.4gEquivalent CH4 (per tonne of cement) = 3.6gEquivalent CO2 (per tonne of cement) = 75g

In contrast, if the additive had been able to provide 3% higherlevel of clinker substitution, then the reduction in CO2 per tonneof cement would be around 25,000g (assumes 0.8 tonnes ofCO2 per tonne of clinker).


Mill Power (kW)Mill Tonnes/hourAdditive Dosage(g/tonne)Additive TotalOrganic Content(%)Additive CarbonEquivalent (% oforganics)Filter Airflow(Nm3/hour)Filter Inleak (%)

2100 210060.0 60300 300

60 60

50 50

25000 25000

25 25

210060

300

60

50

25000

25

2100 210060 60

300 300

60 60

50 50

25000 25000

25 25

AdditiveRetention (%)

99.9 99 97 95 90

ProductkWh/tonneMill Airflow(kg/kg cement)

UnaccountedOrganic (g/hour)UnaccountedOrganic (g/tonnecement)

35.0 35.0

0.40 0.40

11 108

0.18 1.8

35.0

0.40

324

5.4

35.0 35.0

0.40 0.40

540 1080

9 18

PotentionalEmission(mg/Nm3)PotentionalEmission, C(mg/Nm3)

0.4 4

0.2 2

13

6

22 43

11 22


cement chapter 10 -1

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