PRODUCTION OF HIGH VALUE

COAL FLY ASH

Giulio Belz, Pompilio Caramuscio

ENEL Produzione Ricerca - Italy

Abstract

Fly ash represents the particulate matter captured from exhausted gases of coal burning

thermoelectric power plants by electrostatic precipitators. Collected at a rate of about 20 t/h for

a 660 MW unit, fly ash production reaches 1 Mt/y in Italy and it is close to 40 Mt/y in Europe.

Reduced storage capacities characteristic of power plants, generally corresponding to no more

than one week of fly ash production, and unacceptable economical consequences of fly ash

landfill oblige power plant management to a complete fly ash reutilization.

According to the legislation about not-hazardous wastes reutilization and with modalities

specified into dedicated technical standards, fly ash is nowadays commonly used as secondary

raw material in the production of blended cements and concrete mixtures. In these construction

products fly ash work as pozzolanic addition and micronic filler, improving their mechanical

properties and their durability to environmental agents aggression.

Nevertheless, to guarantee a full and constant fly ash reutilization regime, obtaining at the same

time an economic valorisation of its role with respect to substituted compounds in

cementicious mixtures, it is necessary to fulfil specific quality control requirements that make

fly ash management procedures more and more similar to those typical of an industrial

“product”.

In this contest, ENEL Production Research has started a study to detect the influence of the

principal variables of the thermoelectric process on fly ash quality and the applicability of fly

ash beneficiation post-production treatments. Particular attention was addressed to the

possibility to reduce the unburned carbon content, which certainly represents the most critical

quality control parameter, and to increase fly ash fineness, which, on the other hand, is related

to its rheological and pozzolanic contribution to fresh and hardened hydraulic mixtures.

With this scope, extended fly ash characterisation campaigns were conducted, monitoring its

quality in correspondence of several different running plant configurations, and a fly ash

sieving pilot plant was also assembled for beneficiation tests. It was also verified the technical

and economical possibility to produce ultrafine fly ashes to be adopted in special, high value

applications. Prototype samples were obtained utilising wet and dry milling treatments and by

means of fly ash air classification. Properties of treated ash particles, all less than 10 microns in

diameter, were experimentally tested in the production of high strength and self-

compacting/self-levelling concretes (HSC and SCC). Main results of all the above

investigations are reported in the present paper.

1. Formation and composition of coal fly ash

Fly ash is the principal product of transformation of mineral impurities present in coal after its

combustion in pulverised fuel furnaces. The coal inert fraction, mainly formed by quartz,

mullite, pheldspates, pyrite, and carbonate, typically ranges between 6 and 12 % of the fuel

weight. During the thermal process it melts at the higher furnace temperatures (1400-1500°C)

and a small part of it falls in the bottom of the boiler producing the so-called bottom ash. The

main part is instead dragged by the exhausted gas stream, cooling quickly, and solidify in the

form of small vitreous spherical particles, which form the fly ash. Fly ash is separated by the

fumes by means of electrostatic precipitators and collected in the hoppers beneath. Evacuated

pneumatically and stocked dried in silos, fly ash is then transported by trucks or shipped to its

different destinations of reuse.

With particle diameters mainly between 1 and 100 µm, fly ash appears like a fine grey powder,

which fineness compares to that of cements. Its chemical composition is characterised by the

presence of high contents of silica, alumina and ferrous oxides in an amorphous phase, and can

be considered equivalent to that of a volcanic pozzolana.

2. Production and destination of coal fly ash in Europe

Referring to statistical data elaborated by the ECOBA (European Coal Combustion Products

Association), the production of coal combustion residues reaches in Europe 60 Mton/year,

mainly represented by fly ash (66% , equal to 39 Mton), bottom ash (10%) and FGD gypsum

(18%). Principal producers are Germany, Greece, Spain and UK, while the production in Italy

not overcomes 1 Mt/y.

In accordance to national and communitarian legislation, fly ash fields of destination in Europe

are summarised in Figure 1. Environmental restoration (open cast mines, quarries and pits) and

construction industry applications, with also underground mining, are certainly the most

important. Only 5 % of the total fly ash production is disposed of. As raw material for the

construction industry, fly ash is utilised for the production of concrete, cement clinker, blended

cements and sub-base component for road construction.

With respect to the prices of the raw materials substituted by fly ash in these applications,

concrete and blended cement productions are certainly the most valuable destinations, where

economical profits from fly ash may become significant.

Figure 1 – Fields of destination of coal fly ash in Europe in the year 2000 (by ECOBA)

In Italy, national legislation about reutilisation of not-hazardous wastes indicates for coal fly

ash the production of concretes, cements, clay tiles and aggregates, excluding other potential

destinations, like environmental restoration and road construction. Consequently, cement and

concrete industries absorb in Italy more than 90% of the total fly ash production, making

fundamental for power-plants management to satisfy technical requirements for these sectors,

not only for an economical profit but also to guarantee normal running conditions for

thermoelectric production.

3. Fly ash quality requirements for concrete and cement production

Chemical and physical properties for fly ash to be utilised in blended cement and concrete are

specified by two communitarian standards: EN 197-1 “Cement – composition, specifications

and conformity criteria for common cement” and EN 450 “Fly ash for concrete – definitions,

requirements and quality control”. In Table 1 requirements of standard EN 450 are

summarised, indicating chemical-physical properties and statistical control criteria for fly ash

to be utilised in the production of structural concretes conforming to the standard EN 206.

According to EN 206, fly ash can be introduced as a pozzolanic addition in concrete mix-

design with an equivalent factor k = 0.2 or 0.4, depending on the cement class, with respect to

Temporarystockpile(6%)

Environmentalrestoration(43%)

Constructionindustry(46%)

Disposal(5%)

Cementraw material(24%)

Roadconstruction(22%)

Others(4%)

Concrete(39%)

Blendedcement(11%)

the minimum Portland cement content required by the standard for each class of environmental

exposition of the final product.

4. Role and benefits of coal fly ash in cementitious mixtures

Fly ash is essentially adopted in cementitious mixture in partial replacement of Portland

cement. This can be made both producing blended cements or directly in the concrete

manufacturing.

Property Acceptance limit Testprocedure Frequency

Loss on ignition (%) ≤ 5.0 (≤ 7.0 (1)) EN 196-2 1/dayChloride (Cl-) (%) ≤ 0.10 EN 196-21 1/monthSulphate (SO3) (%) ≤ 3.0 EN 196-2 1/monthFree CaO (%) ≤ 1.0 (≤ 2.5) EN 451-1 1/weekExpansion (mm) (if CaOfree=1÷2.5%) ≤ 10.0 EN 196-3 1/weekFineness (%)(wet residue at 45 µm) ≤ 40.0 1/dayFineness uniformity (%) Mean value ± 5.0 EN 451-2 1/dayPozzolanic activity index (%) ≥ 75.0 at 28 d

≥ 85.0 at 90 dEN 450 &EN 196-1 1/ 2 weeks

Density (kg/m3) Mean value ± 150 EN 196-6 1/month(1) limit valid on national basis

Table 1 – Fly ash quality control requirements according to standard EN 450

During hydration, fly ash acts in cementitiuous mixtures as an artificial pozzolana. It is able to

chemically react at environmental temperature with the excess of calcium hydroxide, Ca(OH)2,

released by Portland cement hydration, forming the same hydrated calcium silico-alluminate

produced by the cement binding reaction. Besides contributing to the final strength of the

cementitious matrix, the pozzolanic reaction also increase the chemical resistance (durability)

of it by reducing its permeability and the content of free Ca(OH)2, which is water soluble and

vulnerable to the attack of carbon dioxide and sulphate solutions. The slower velocity of

exothermic hydraulic reaction observed in mixtures with fly ash also reduces the temperature

raising in the material, avoiding the risk of producing cracks due to differential expansion,

especially in massive casts.

Fly ash fineness and spherical shape make even of them an affective filler in improving the

rheologycal behaviour of fresh cementitious mixture, increasing its workability (slump) and

pumpability, while reducing the risk of bleeding and component segregation during moulding.

In accordance with these contributions, the above fly ash quality requirements have the scope

to control the material pozzolanic behaviour and fineness, while verifying the absence of

deleterious substances. Among these, the limit for LOI (loss on ignition) indirectly indicates

the residual content of unburned carbon (UBC) in fly ash, obtained measuring the loss of

weight of a dried ash sample heated at 950�25°C for 1 hour. Carbon particles are undesirable

because they partially inhibit the corrective action of several concrete admixtures (aerating,

setting and fluidisating agents), which are selectively captured by the porous carbon particles

due to their common organic nature.

5. Management of fly ash at the power plant

Produced at a rate of about 20-25 t/h from each unit of 660 MW and typically collected in a

2000 m3 silo per each unit, power plant fly ash stocking capacity not overcomes 4-5 days of

full production. This pushes ash producers to look for the creation a stable market of

destination, formed by several utilisers able to withdraw with continuity its by-product, making

fly ash a reference raw material for those industrial processes. This result can be reached

fulfilling quality requirements and creating operational procedures and infrastructures able to

guarantee fly ash quality control and delivery. Up to 20 trucks per day are necessary to

transport fly ashes produced by each 660 MW unit.

Fly ash quality is persecuted inside the power plant by detecting and monitoring the influence

of the principal variables of the thermoelectric process. Time curves of thermal load, O2

excess, CO content in fumes, boiler temperatures distribution, start up and shut down transitory

phases are compared to the quality of the ash sampled by the final silos. Fly ash properties, and

particularly its content of residual carbon, are also controlled by dedicated on-line equipments,

such as the MITER®, specifically developed by ENEL Production Research for sampling and

analysis fly ash in continuous (1 measure every 7 minutes), directly from the exhausted gas

stream at the exit of the boiler. An example of correlation between UBC in fly ash measured by

the MITER and CO concentration in fumes is reported in Figure 2 (the interval amplitude was

voluntary increased for test purposes).

Figure 2 – Correlation between on-line measurements of UBCin fly ash and CO content in fumes

Obtained correlations help power plant personnel to address the process toward the production

of better quality ashes, compatibly with other relevant running conditions.

Different coals are also frequently blended with the scope to control the resultant coal ash

content, its grindability, the content of volatile matter and any other parameter able to

influence, among others, fly ash composition and properties.

6. Reduction of fly ash unburned carbon content

While all other fly ash quality control requirements generally result satisfied in standard coal

power plant running conditions, UBC frequently represents a critical parameter for the modern,

high productivity generation of boilers, characterised by smaller dimensions and consequent

reduced fuel residence time. The introduction of low-NOx burner also produced an UBC

increase, while continuous variations in the quality of the coal provisioned, depending on the

economics of the international market, force power plant conductors to set mills, burners, and

electrostatic precipitators on averaged operational configurations among those optimal for each

coal type. A careful power plant operation allows to reduce LOI variations, but this is often not

enough to satisfy standard limits. In this cases, further solutions are necessary.

Internationally, several processes have been proposed for the beneficiation of fly ash; some of

them have also been applied on an industrial scale. Examples are the Separation Technology

Inc. triboelectrostatic treatment, the Carbon Burn Out fluidised bed technology and the DUOS

tumbler screening system. Nevertheless, their diffusion is still reduced to few applications as

their investment and running costs are generally excessive with respect to actual economical

revenues from fly ash marketing. Besides, as these technologies usually represent fly ash post-

treatments, which operate independently by the power-plant process and frequently are

managed by external companies off-site, they normally treat the whole ash production, with the

consequence to need great plants and infrastructures.

An alternative, more promising way may be represented by the utilization of the same power-

plant fly ash capturation and extraction system as the basic tool for fly ash quality selection.

After that, simple treatments, such as furnace reburning or dimensional ash sieving, may be

eventually applied only to small streams of selected ashes. In this way, due to the smaller

treatment capacity required, beneficiation systems might also be better integrated to the power

plant, whit a sensible reduction in the resultant costs for fly ash quality improvement.

6.1. Fly ash selection and partial boiler recirculation

The electrostatic precipitator (ESP) for fly ash capturation is composed of a series of modular

units, dimensioned and assembled in a number sufficient to guarantee a resultant separation

efficiency greater than 99%. Typically, four to seven parallel lines of six to eight sided cells,

each of them containing a positively charged plate for fly ash deposition, form the ESP of a

660 MW thermoelectric unit. A correspondent scheme of underlying hoppers collect the ash

that, charged and separated from the exhausted gas stream by the electrostatic field action,

precipitates beneath the positive electrode after loosing their charge.

Ash is then evacuated pneumatically by the periodical connection of each hopper of the ESP to

a vacuum pipeline that operates its extraction and transportation to the stocking silo.

In Figure 3 a typical ESP scheme and evacuation system lay out is presented. Ashes from each

line of hoppers are evacuated in sequence and blended in the final silo.

Figure 3 – ESP scheme and evacuation system lay out

Quantities of ash collected from each ESP line can be estimate by the time occurred for its

evacuation. The record of some evacuation cycles is reported in Figure 4, with also the

ESP side SX

ESP side DX

FUMES

I II III IV V VILines

ESP

silos

I II III IV V VI

05

1015202530

35

I II III IV V VI

ESP line

Fine

ness

(d50

v m

icro

ns)

05

1015202530

35

I II III IV V VI

ESP line

Fine

ness

(d50

v m

icro

ns)

obtained distribution of the total ash capacity. The on-line measure of the vacuum level in the

pipeline for ash transportation automatically governs each evacuation step. Its fall corresponds

to the hopper emptying, since the aspiration system comes in connection with the inside room

of the ESP.

Figure 4 – Plotted record of some ESP evacuation cycles and obtained fly ash distribution

The progressive action of the ESP on the gas stream also produces a significant variation in the

quality of the ash fallen along it. Fineness and UBC greatly change. Results of mean values,

per ESP line, of LOI% and dv50 (50 percentile particle diameter) measured on ash samples

from each hopper are summarised in Figure 5. First two-three ESP lines, where it is collected

from 50 to 80% of the total ash production, are characterised by ashes with UBC sensibly

lower, that increases in the last ones up to double values.

46

9

1312

15

02468

10121416

I II III IV V VI

ESP line

LOI %

Figure 5 – LOI and fineness fly ash values vs ESP lines

With the availability of few intermediate silos, the evacuation system can then be easily

modified to operate the differential destination of these ashes. In this way, low LOI ash can be

selected for a better marketing, but high LOI ash destination would represent a problem.

A possible solution of it could to be the ash recirculation in the boiler. This would consent its

recovery, as in the new mass equilibrium an equivalent increase of ash quantity will be

registered in the stream of selected ash. Moreover, because of the combustion of the UBC in

the ash recirculated, resultant combustion efficiency of the boiler would be improved. Selected

0

20

40

60

80

100

>150 150-75 75-45 <45Granulometric fraction (microns)

% in

wei

ght

05

101520253035

raw >150 150-75 75-45 <45Granulometric fraction (microns)

LOI %

ash quality would also be bettered, as a consequence of the equivalent raise in the coal ash

content and the diluting effect of it on the final UBC in fly ash.

Results of simulation models point out that the beneficiation effect of a 20-25% ash

recirculation can be estimated in a 12-15% reduction of selected fly ash LOI. With this

practice, best profit is anyway represented, as mentioned above, by the possibility to improve

an ash selection without generating a refused stream.

The increase in the inert mass going through the boiler doesn’t seem to be critical for its

efficiency, and no raise in fouling/slugging phenomena should be expected. In fact, if we

consider that excessive UBC in fly ash is especially observed with coals characterised by a low

ash percentage, typically less than 8%, a rate of 25% of ash recirculation would correspond to

an equivalent coal ash content up to 10%, still largely acceptable for power plant design and

widely experimented in the normal plant running configurations.

Ash recirculation, whose quantities correspond to an addition of 1-2% on the coal mass, can be

operated in different ways: by dosing the ash on the coal belt direct to the mill; adding it to the

coal in the mill classificator or in the pulverised coal tubes direct to the burners, or, finally,

pumping the ash directly inside the boiler under depression.

6.2. Integrated fly ash beneficiation treatments

As seen above, fly ash with typical LOI values up to 8 % can be beneficiated to less than 5%

by means of its selection and partial recirculation. If higher carbon content could be expected,

it will be necessary to adopt further solutions.

A competitive alternative is represented by fly ash sieving. It is in fact well known that

unburned coal particles are characterised by greater diameter and lower density with respect to

silico-aluminous ash particles. Figure 6 presents a typical distribution of mass and UBC in

different granulometric intervals of fly ash sieved samples, while in Figure 7 the SEM image of

the fraction rejected at the 75 µm sieve is showed, with an apparent almost exclusive carbon

particles separation.

Particle Size Distribution

0.1 1 10 100 1000 Particle Size (µm)

0 0.2 0.4 0.6 0.8

1 1.2 1.4 1.6 1.8

2 2.2 2.4 2.6 2.8

3 3.2 3.4 3.6 3.8

4 4.2 4.4 4.6 4.8

5 5.2

Vol

ume

(%)

N°17383, giovedì 20 febbraio 2003 10.19.52 N°17385, giovedì 20 febbraio 2003 10.31.30 N°17387, giovedì 20 febbraio 2003 11.08.17N°17389, giovedì 20 febbraio 2003 11.23.28 N°17391, giovedì 20 febbraio 2003 11.33.31 N°17393, giovedì 20 febbraio 2003 11.44.20

Particle size (µm)

Vol

ume

(%)

I

II

IIIIV

VVI

Figure 6 – Typical weight distribution and LOI values for fly ash granulometric fractions

Figure 7 – SEM images of UBC particles rejected at 75 microns sieve

Actually, due to the fact that UBC particles enclose in their porous structure a significant

quantity of smaller ash particles, beneficiation efficiencies greater the 25-30% have never been

reported for sieving treatments applied on raw fly ash. This means that no more than 2-2,5% of

LOI reduction can be obtained on beneficiated ashes. Furthermore a 20% of the treated mass

remains rejected onto the sieve. For these reasons, the sieving treatment alone can’t be

considered a sufficient solution for LOI reduction.

On the contrary, granulometric distribution analysis on ashes coming from different ESP lines

suggest that sieving efficiency could be greatly increased when applied on selected ashes. An

example is reported in Figure 8 where laser granulometric distributions of ash samples from

lines I to VI of an ESP are compared. For ashes from lines III to VI, it is clearly evident the

two-phase nature of the material, characterised by two separate peaks in the particles

distribution. Peaks on the left always belong to the silico-aluminous ash fraction decreasing

with the order of the ESP line. On the other side, right peaks correspond to carbon particles

enriched fractions.

200 µm 20 µm

Figure 8 – Granulometric laser distribution of ash samplescollected from different ESP lines

The two-phase nature is particularly evident for ash from line III, where a distinctive minimum

value at 40-50 µm separates the carbon enriched fraction. It can be efficiently removed by

sieving at 75 µm, as shown by laboratory tests were productivity higher than 90% and carbon

separation efficiency near to 40% were measured. In this case, ashes with LOI values up to 9 %

can be beneficiated up to the standard limit of 5%.

Looking at the previous ash management scheme, in which ashes from first ESP lines were

selected for reuse and ashes from lasts were reburned, it can be easily implemented to the new

scheme in Figure 9. In it the possibility to treat the ash from an intermediate ESP line as been

introduced. In this case, it will be sufficient to dimension the sieve to treat not more than 25%

of the total ash mass to guarantee a 100% ash quality. With an ash productivity of about 20-25

t/h, it corresponds to 5-7 t/h of treatment capacity, that can be easily guarantee by a single

industrial screening machine.

Figure 9 – Improved fly ash management scheme

7. Implementation of fly ash fineness

According to considerations in §4, fly ash pozzolanic and rheologycal contribution in

cementitous mixture increases with its fineness.

ESPI II III IV V VI

To the boiler

I & II III IV, V & VI

To the user

To an extent, more and more fine fly ash would tend to a behaviour similar to that of silica

fume, which is characterised by a mean particles diameter of 0,1 µm and an almost totally

amorphous phase. Silica fume acts in cementitious mixtures as a strong addition able to

accelerate setting time and increase early and final strength development.

Looking at silica fume market price, up to five times the cement one, treatments for fly ash

micronisation seem to offer the opportunity to create new high value niche sectors for fly ash

utilisation. Among these, of particular interest appear the production of high strength concrete

(HSC), with compressive strength Rck > 75 MPa, and, even more, the preparation of self-

levelling, self-compacting concretes (SCC), characterised by slump capability greater than

600%. In these applications, micronised fly ash could represent an alternative material,

optimised with respect to both raw fly ash and silica fume, able to gather their advantages

without their respective lacks. Raw fly ash, in fact, reduces setting time and strength

development but improves fresh concrete pumpability, while silica fume significantly increases

the former with a contemporary dramatic worsening of the latter.

7.1. Techniques for fly ash micronisation

Fly ash micronisation, for the obtainment of a material entirely smaller than 10 microns, can be

operated by grinding it or by its aerodynamic classification. On the contrary, the sieving

separation is no more possible at these diameters on an industrial scale. Grinding can be made

adopting dry or wet milling technologies.

An assessment of more than 10 applicable technologies has been conducted by sending ash

samples to as many industrial machine producers in Europe. A selection of the most promising

tests is presented in Table 3, with also the main results obtained for optimised ash fineness,

specific consumes of energy and maximum treatment capacities.

Micronization technology Quantitytested

d90v(µm)

Esp(KWh/kg)

Cmax(1)

(t/h)dry ball mill 500 kg 8 0,18 1,500

wet micro-sphere mill 400 kg 8 0,25 3,000

air-classification 1000 kg 15 0,1(p(2)=35%) 18,000

(1) - Maximum capacity of industrial available machines(2) - Productivity of air-classification with respect to fly ash input

Table 3 – Technologies tested for fly ash micronisation and main results obtained

7.2. Micronised fly ash utilisation tests for high performance concretes application

Preliminary tests were conducted to verify the influence of fly ash fineness on its pozzolanic

and rheologycal contribution in cementitious mixtures.

Ash samples obtained in laboratory and coming from best industrial trials were selected to

compose an homogeneous series of fineness values, with d90v ranging from 103 µm (for raw

fly ash) to 6 µm (for wet mill micronised fly ash).

Ashes were introduced in standard mortar mixtures, prepared adopting a CEM I 52,5 cement

and water/cement ratios of 0.4 and 0.5, operating a 25% in wt. cement partial replacement.

Binders water requirement, mortars initial/final setting times and fluidity were compared.

Figures 11 presents obtained correlations between mixtures fluidity and setting times versus fly

ash fineness. Results are also compared with those of mixtures without cement substitution.

Micronised ash introduction in partial cement replacement delayed setting time and improve

workability (as raw fly ash does) up to a d90v value of about 8 microns. Under it there seems

to be an inversion, and micronised ash behaviour start to migrate towards that observed for

silica fume.

Figure 11 – Correlations between setting times and fluidity vs fly ash finenessfor cement pastes and mortars with 25% in wt. of fly ash replacement

By adjusting mixtures fluidity to equivalent values, mortar samples were also moulded for

compressive strength tests. Results at 28-days of curing are presented in Figure 12, in

comparison with those of mixtures containing the cement only. As expected, the pozzolanic

contribution of ashes increases with their fineness and the equivalence to the 100% cement

performance seems to be reached for d90v values between 20 and 30 microns.

0.00

1.12

2.24

3.36

4.48

6.00

1101001000Fineness (d90v in microns)

Setti

ng ti

me

(h.m

in)

ti

tf

ti 100% cement mixt.

tf 100% cement mixt.

50

55

60

65

70

75

80

85

020406080100120

Fly ash fineness (d90v in microns)

Mor

tar s

lum

p (%

)

reference mortar (CEM I 52,5)

50

60

70

80

90

100

110

120

1101001000Fly ash fineness (d90v microns)

Rck

(MPa

)

FA mixtures w/c=0,5

FA mixtures w/c=0,4

CEM I 52,5 w/c=0,5

CEM I 52,5 w/c=0,4

Figure 12 – Compressive strength at 28-days of mortars with 25% in wt. cement replacementversus fly ash fineness and comparison with reference 100% cement mixtures

8. Conclusions

Environmental legislation and technical standards in Europe consider coal fly ash as a usable

and profitable raw material for the obtainment of several products, with particular relevance in

the concrete and cement production. Fly ash role and benefits have been recognised, but ash

producers are everyday more and more pushed to pursue their quality if they want to create a

stable market of destination and guarantee normal running conditions in coal thermoelectric

power plants. Unburned carbon content often results as the most critical parameter, while fly

ash fineness is its most valuable characteristic.

Fly ash post-treatment beneficiation processes for carbon removal have the disadvantage to

treat the entire ash production, with consequent high costs of investment and exercise.

Fly ash selection inside the power plant and its partial recirculation into the boiler could

represent simpler alternatives for fly ash quality improvement. Selection could be based on

natural differences in fly ash quality when collected at electrostatic precipitator hoppers, while

reburning of high carbon fractions will avoid the definition of a refused ash stream.

Eventually, further demand of carbon separation could be persecuted by sieving selected ash

streams, with the advantage of reducing the treatment capacity and obtaining a better efficiency

and productivity with respect to those typical of the raw material treatment.

For niche destinations, an improvement in fly ash marketing value can be even persecuted

producing ultra fine ashes, particularly suitable for special applications in the construction

industry, such as the production of high strength or self levelling concrete mixtures.

Micronisation of fly ash by mills or air-classification cyclones results, in fact, in a great

enhancement of its pozzolanic and filler capacity in cementitious mixtures.