Triboelectrostatic fly ashTriboelectrostatic fly ash Beneficiation

Ing. Antonio Zucca IGAG – CNR – UOS CAGLIARIazucca@unica.it

Fly ash

Burning coal in electric power stations generates hugeBurning coal in electric power stations generates hugequantities of fly ash, giving rise to major technicalproblems and additional financial burden Therefore theproblems and additional financial burden. Therefore, there-use of this kind of material would offer significantadvantages dispensing with the need and cost of disposaladvantages, dispensing with the need and cost of disposalin suitable landfill systems and providing in additionsome return on the raw-material marketsome return on the raw material market.

A rough estimate puts at more than 300 billion tonnes peryear the amount of fly ash produced worldwide.

Fly ash is being used in building and constructionapplications (cement additive, lightweight aggregate),environmental rehabilitation (mine spoil ameliorate),waste management (toxic element immobilisation) andpolymers (functional filler).

Another important application is to use unburned carbonas filter media in place of activated carbon or as aas filter media, in place of activated carbon or as asubstitute for industrial carbon. The unburned coalparticles are structurally modified while passing throughparticles are structurally modified while passing throughthe combustor resulting in the formation of macro-poroussurfaces with presence of silica and alumina compounds,surfaces with presence of silica and alumina compounds,so that, after separation from the true ash, they mayconstitute a suitable precursor for the production of bulkconstitute a suitable precursor for the production of bulkadsorbents.

However the most common application is actually theHowever the most common application is actually theuse of fly-ash as a component of concrete.

Tests carried out by different organisations showed thatcarbon could be a deleterious substance in concretebecause it adversely affects air entraining admixturescommonly used to impart desirable properties toconcrete, including increased durability.

International standards (EN 450 fly ash for concrete)International standards (EN 450, fly ash for concrete)have established a 5% maximum content of unburnedcoal in fly ash usable for cement application althoughcoal in fly ash usable for cement application, althoughsome concrete producers set the maximum allowableLOI level at less than 4%LOI level at less than 4%.

The newer coal fired plants are equipped with low-NOxburners in the boilers which are operated at lower firingburners in the boilers which are operated at lower firingtemperatures.This negatively affects fly ash quality generating largerThis negatively affects fly ash quality, generating largerproportions of unburned carbon.

In order to reduce and prevent the landfilling prone toincreasing restrictions, as well as to increase the useand value of fly ash with respect to variousapplications, a beneficiation process becomesnecessary.

Regarding the choice of the technology, the mainrequirement is that it must be suitable for therequirement is that it must be suitable for thetreatment of very finely divided matter, taking intoaccount that fly ash recovered in electrostaticaccount that fly ash recovered in electrostaticprecipitators is usually below 150 micrometers, witha considerable proportion finer than 50 micrometersa considerable proportion finer than 50 micrometers.

A second and not less important requirement is thelow-cost of a treatment process, due to the fact thatfly ash has a low commercial value especially forthe most common applications.

Recently, application of a dry triboelectrostatic processhas been considered for the separation of unburnedhas been considered for the separation of unburnedcarbon from fly ash into economically valuableproductsproducts.

Apart from these methods, froth flotation and airclassification are also used for the separation ofunburned carbon from fly ash.

Some studies show that unburned carbon can berecovered with oil agglomeration processrecovered with oil agglomeration process.

Gravity and magnetic separation were also proposedfor removing not only unburned carbon, but alsomagnetic material as well.

Carbon Burnout.

Si l ifi ti t b i t ti hSize classification appears to be an interesting approachon account of the fact that unburned particles, often inl l b d t ll i iloosely bound aggregates, are generally coarser in sizethan non combustible matter.

Sieving can take advantage of the round particle shapeof non combustible matter that is favourable to a readyyflow through the sieve mesh without clogging.

Si l ifi ti f th ti f l bSize classification for the separation of a low-carbonfraction must be carried out very gently using

t t i l i t ti th lidapparatuses not involving strong actions on the solidparticles.

The suitability of electrostatic methods for theybeneficiation of fly ash has already been proven elsewhereeven at a commercial scale.

A separator has been developed by Separation TechnologyI b d l ti t ib h i f b d hInc., based on selective tribocharging of carbon and ashparticles [E.Tondu, W.G.Thompson, D.R.Whitlock,J D Bitt d A V ili k C i l ti fJ.D.Bittner and A.Vasiliauskas: Commercial separation ofunburned carbon from Fly ash; Mining Engineering, 6, 47-50 1996]50, 1996].

The machine, capable of treating 40 t/h of fly ash,, p g y ,consists of two parallel plate electrodes, between whichan open mesh polymer belt is moved producing a strongp p y p g gagitation and a cleaning action of the electrodes due tothe counter current flow in the upper and lower parts.pp pParticles migrate through the narrow gap between theelectrodes according to the respective charge and areg p gcarried towards the opposite ends where are collected.

Negative Electrode

Mark Shilling, P.E Carolina Power & Light Co. Carbon/Ash Separation.1999 International Ash Utilization Symposium, Center for Applied Energy Research, University of Kentucky, Paper #97.

About eighteen separators are used: the first one (1997)About eighteen separators are used: the first one (1997)North Carolina (Progress Energy – Roxboro Station)and the most recent applications are in Florida (Tampaand the most recent applications are in Florida (TampaElectric Co. Big Bend Station); England (EDF EnergyWest Burton Station Lafarge Cement UK); WalesWest Burton Station – Lafarge Cement UK); Wales(RWE Aberthau Station - Lafarge Cement UK); Poland(HATRA Cement Lafarge Cement Poland)(HATRA Cement - Lafarge Cement Poland).

An interesting article as been published in theProceedings of 2009 World of Coal Ash (WOCA)Conference. May 4-9, Lexington, KY, USA: “Fly AshCarbon Separation and Ammonia Removal at TampaElectric Big Bend” authors J.D. Bittner, S.A.Gasioroski and F.J.Hrach.

“Fly Ash Carbon Separation and Ammonia Removal at Tampa Electric Big Bend” authors J.D. Bittner, S.A. Gasioroski and F.J.Hrach.2009 World of Coal Ash (WOCA) Conference. May 4-9, Lexington, KY, USA

A i il b d ib h i i b iA similar concept based on tribocharging is beingfollowed again in the USA [J..M.Stencel, H.Bann, T.X.Li,J K N h d J L S h f D l iJ.K.Neathery and J.L.Schaefer : Dry electrostaticseparation of unburned carbon from coal ash ; inR Ci (Ed ) SWEMP’96 DIGITA C li i 1996R.Ciccu (Ed.): SWEMP’96, DIGITA, Cagliari 1996(pp.263-275)].

The charging device consists of a stainless steel duct,suitably shaped in order to enhance the probability ofy p p ycollisions of particles against each other and on the metalsurface itself. The separation of selectively chargedp y gparticles is achieved in a free fall chamber.

C A Lockert R Lister and J M Stencel “Commercialization Status of a PneumaticC. A. Lockert, R. Lister and J. M. Stencel Commercialization Status of a PneumaticTransport, Triboelectrostatic System for Carbon/Ash Separation.”2001 International Ash Utilization Symposium, Center for Applied Energy Research, University of Kentucky, Paper #46

In the last years, the IGAG (Environmental Geologyand Geoengineering Institute of CNR UOS ofand Geoengineering Institute of CNR, UOS ofCagliari, Italy) together with DIGITA (Dept. ofGeoengineering and Environmental technologiesGeoengineering and Environmental technologiesUniversity of Cagliari, Italy) has carried out aninvestigation aimed at verifying the possibility ofinvestigation aimed at verifying the possibility ofbeneficiating fly ash to obtain a product suitable forcement manufacturing or other advanced applicationscement manufacturing or other advanced applicationsand a low-heat fuel to be recycled back to the boileror to be used in immobilization techniquesor to be used in immobilization techniques.

Material characterization.A l f fl h f l t tiA sample of fly ash from a coal power stationlocated in the South of Italy has been tested. Fly

h hi fl h b f d t i t f i l t dash chiefly has been found to consist of isolatedparticles having prevailingly a spherical shape,

h b d b l t ithwhereas unburned carbon occurs as elements withirregular contour, often in clusters.Diff t t i l i l d th fDiffractometric analysis revealed the presence ofmullite and of a phase similar to graphite that mayb th d t f ki M tit i l tbe the product of coking. Magnetite is also presentas the final step of pyrite oxidation.Th l t f fl h i t f hThe larger part of fly ash consists of an amorphousphase with extremely variable chemical

iti i b i ti ti h dcomposition, as microprobe investigation showed.

Observation on polished briquettes revealed thatunburned coal particles show different degrees ofp gtransformation, either limited to a thin outer shell, inthe case of compact elements, which also appear top ppbe characterized by the presence of a disseminatedvitreous phase, or developed deep inside in the casep p pof porous particles.

It can be assumed that the first derive from maceralsIt can be assumed that the first derive from maceralslike inertinite and exinite and the second fromvitrinitevitrinite.

Table - Dry size analysis of fly ash from a power y y y pstation.

Size class[µm]

Weight[%]

L.O.I.[%]

L.O.I.Distr.[%]

+45 28.33 14.61 65.9

4 3 13 4 2 3-45+37 5.13 4.27 3.5

-37 66.54 2.89 30.6

Total 100.00 6.28 100.0

Before undertaking the separation tests the behaviourBefore undertaking the separation tests, the behaviourof selected samples of unburned particles and ofcarbon-free ash subjected to contact charging wascarbon free ash subjected to contact charging wasstudied by measuring the electric charge per unitmass with a Faraday well connected to anmass with a Faraday well connected to anelectrometer.

Ash matter is negatively charged by contact on thevibrating chute while unburned particles arevibrating chute, while unburned particles arepositively charged by conductance on a groundedmetal surface while passing through a static fieldmetal surface while passing through a static fieldgenerated by a negative voltage.

Two alternative flow-sheets have been tested:- direct treatment of fly ash by electric separationd ec ea e o y as by e ec c sepa a o- recovery of a low-carbon fraction by screeningfollowed by the electrostatic separation of the oversizey pfor the recovery of coarser ash and of a combustibleproduct.p

The proposal which includes a size classification fori h fi f i h l d irecovering the finest fraction that already contains

little combustible matter and eliminating unburnedb f h f i b f l icarbon from the coarser fraction by means of electric

separation is here reported.

Size classification tests have been carried outusing a Vibrowest circular screen with differentgmesh openings and variable setting of thecounterweights in order to control the kind ofgmovement of the fly ash mass flowing through themachine. Using a suitable set-up, capacity andg p p yresidence time, it was possible to obtain anundersize with a LOI content of 2,98 % and arecovery of 68 % [Ciccu R., Ghiani M., Muntoni A., Serci A., Peretti R.,Orsenigo L.G., Quattroni G., Zucca, A., 1999. The Italian Approach to the Problem ofFly Ash Proceedings of the International Ash Utilization Symposium LexingtonFly Ash. Proceedings of the International Ash Utilization Symposium. Lexington,Kentucky, USA, pp. 591-601. ISBN 0-9674971-0-8. ].

The amenability of the screen oversize to be treatedwith electrostatic separators has been tested usingdifferent electrostatic separators :

- - a classical drum separator [Ciccu et al., 1997]

- a “metal belt” prototype (Patent: RM 98 A 000156)

a “cone shaped” prototype;- a cone-shaped prototype;

Classical drum separator : Electrostatic separation of unburnt coal for fly-ash beneficiation.Proceedings of XX International Mineral Processing Congress[Ciccu R Ghiani M Peretti R Serci A Zucca A 1997 ][Ciccu R., Ghiani M., Peretti R., Serci A., Zucca A., 1997.]

A “metal belt” prototype (Patent: RM 98 A 000156,1998) Th t i t f f d d i f1998). The system consists of: a feed device forinitiating the charging process and discharging the

lid t t th b lt t l b lt ithsolid stream onto the belt; a metal belt, withadjustable inclination electrically grounded orconnected to a oltage so rce; a set of electrodes forconnected to a voltage source; a set of electrodes forcreating a static field, their arrangement and positionwith respect to the belt can be modified according towith respect to the belt can be modified according torequirements; a voltage source. [Ciccu R., Ghiani M., Peretti R.,Serci A., Zucca A, 2005. Development of a new electric belt separator for fine particles.Serci A., Zucca A, 2005. Development of a new electric belt separator for fine particles.Proceedings of the XI Balkan Mineral Processing Congress, Mineral processing insustainable development, Fetahu K. Ed., Albpaper Publisher, Tirane, Abania, pp. 259-266.]

Separation device. 1) metal belt; 2) vibrating feeder; 3) p ) ; ) g ; )electrodes; 4 and 5) trays

A “cone-shaped” prototype. The system consists of: a vibrating stainless steel feeding device; a stainless steelvibrating stainless steel feeding device; a stainless steel cone-shaped surface, connected to the ground or to a voltage source and revolving along its vertical axis; a setvoltage source and revolving along its vertical axis; a set of electrodes for the establishment of a static field, the arrangement of which and the position with respect to thearrangement of which and the position with respect to the carrier surface can be modified according to needs; a section for the separate collection of the products and forsection for the separate collection of the products and for cleaning of the surface; a voltage source.A New Electrostatic Separator For Fine Particles. Proceedings of the XXI International Mineral p g fProcessing Congress. [Ciccu R., Ghiani M., Peretti R., Serci A., Zucca A. ]

1 Feeder2 Electrodes3 "Unburned" discharge4 Revolving surface5 Shielding plates6 "Ash" discharge7 Brusches

Figure 1. Views of the separator prototype

Selective charging of mineral particles can takeSelective charging of mineral particles can takeplace in a suitable separate device like the“turbocharger” and/or in the same vibrating feederturbocharger and/or in the same vibrating feederto the separator as the result of friction betweenparticles themselves and against the metal surfaceparticles themselves and against the metal surface.

Charging intensity is controlled by the amplitudeand frequency of vibration and by the length andinclination of the chute.

Charging depends also on the conditions affectingthe feed material and the surrounding environmentthe feed material and the surrounding environment(temperature and humidity).

As the cone rotates, the layer is carried into thesection where the static field becomes active. Nonconductor particles charged with the same polarityof the electrode are pinned to the surface andremain attached to it all the way until they reachthe discharge zone where the residual layer ofadhering particles is brushed away and collectedinto the appropriate bin.

The separator was hosted in an encasement insidehi h d h idiwhich constant temperature and humidity

conditions were maintained.Th l h d 50°C b f h iThe sample was heated at 50°C before each test inorder to reduce the moisture.

Combination of dry screening and electrostatic separationy g p

Products Fly ash sample

Yield[%]

L.O.I.[%]

L.O.I.Distr.[%][ ] [ ] [ ]

Benef. Ash 86,56 3,12 41,01

Middlings 7,52 13,48 15,39

Comb 5 92 48 50 43 60Comb.fraction

5,92 48,50 43,60

Total 100,00 6,59 100,00Total 100,00 6,59 100,00

CARBOSULCIS

The coal Sulcis so named because of theThe coal Sulcis, so named because of thegeographical area extracted from the mineraldeposit Sulcis, is called “sub-bituminous coal longp , gflame”, according to the ATSM D 338 AmericanClassification.

The structure has a potential mining production of1,500,000 tons of coal per year, the estimatedreserves of coal deposit amount to over two billiontonnes.

H idit 10 5%

The characteristic parameters of coal sulcis

Humidity 10.5%

Volatile matter 39%

Ashes 16%

Fixed Carbon 52.5%

Sulfur 6.06%

Lower Calorific Value - Hi [kcal / kg] 5000 - 6000

SOTACARBOSO C O

The new research centre started in may 2008

The Centre is located near the Coal MuseumSerbariu Mine (Carbonia) and covered a surface ofmore than 12 000 square meters (about 1430 m2more than 12,000 square meters (about 1430 m2

laboratories and offices and 10000 m2 pilot plants).

- Member of “IEA Clean Coal Center”, set up asMember of IEA Clean Coal Center , set up as“Implementing Agreement” of the International EnergyAgency, aimed at developing Clean Coal Technologies(CCTs);

- Involved in the European “Hyways” Project aimed atp y y jdeveloping technologies for producing and usingHydrogen;

- involved in “Carbon Sequestration LeadershipForum” (CSLF) aimed at developing technologies forCO2 separation, sequestration, transport and storage;

- involved in “Coal Mine Methane Subcommittee”(CMS) aimed at developing the use and the develop oftechnologies for methane recovery from different

i iemissions.

DIGITA (Dept. of Geoengineering and environmentalG ( ept o Geoe g ee g a d e o e tatechnologies) and IGAG-CNR-UOS of Cagliari(Environmental Geology and Geoengineering Institute ofCNR).

- The influence of coal characteristics and operatingparameters on the rheological proprieties of coal-watermixes.

- Reduction of gas serra emissions in the atmosphere troughthe geological storage CO2.

- Coal flotation using Water Jet Technology.