Relation between silico-aluminous fly ash and its coal of origin

vailableonlineatwww.sciencedirect.comSciencedirectH PARTICUOLOGYELSEVIERParticuology 6(2008)85-92www.elsevier.com/locate/particRelation between silico-aluminous fly ash and its coal of origin"Jean-Charles Benezet Pierre Adamiec, Ali BenhassaineEcole des mines d'Ales6 Avenue de claviers, 303 19 ALES Cedex, franceReceived 26 June 2007: accepted 21 September 2007AbstractFly ashes are typical complex solids which incorporate at the same time intrinsic properties derived from the layers(various mineralogicaland dimensional spectra)and major transformations generated during prior processing. To use fly ashes in various applications, it is necessarycharacterise them completely. The first research to date carried out on silico-aluminous Ay ashes in order to characterise them physicallymorphologically, chemically and mineralogically, resulted in the recognition that they are relatively simple materials. In the present study, a silico-aluminous fly ash coming from the power station of Albi(France)was selected. Heat treatment at 450 and 1200 C together with coal simulatedthe treatment undergone by coal in the power station in order to mimic real coal residue. In conclusion, the diversity of the particles contained inAly ash could only be explained by the relation existing between the fly ash and its coal of origino 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B V.Keywords: Fly ash; Selection: Thermal treatment1. Introductionimportant parameter is the nature of the coal used in the thermalpower station producing the ash.Fly ash is a by-product of thermal power stations for electric- The physical, morphological, chemical and mineralogicaity production and is commonly used in the cement and concrete characterisation of silico-aous fly ashes up to the present(Erdogdu& Turker, 2000), and ceramic(Anderson& Jackson, date, suggests that they are relatively simple materials( Carles1983)industries. The long series of complex processes taking Gibergues, 1983; Clerc, 1993; Jarrige, 1971; Minoux, 1994,place in thermal power stations in forming fly ash is presented vienot Genta, 1993)Morphological simplicity: the ash particles are generallyermey ash is a typical complex solid that combines both prop- spherical in shape(Fig. 2). These ashes, such as used in thises intrinsic to mineral deposits(various mineralogical and study, are recovered in filters installed in the conduits of thermaldimensional spectra)and profound transformations caused by power stations.prior treatment processes. In order to properly use fly ash forChemical simplicity: silica and alumina are the majorvarious applications, its complete characterisation must be car- nents of the ash. Due to the relatively small content of Calried out first( Vassilev Menendez, 2005: Vassilev, Menendez, silico-aluminous population represented in the ternary diagramBorrego, Diaz-Somoano,& Martinez- Tarazona, 2004). Such a(Fig 3)could practically be reduced to a binary silico-aluminousharacterisation is especially crucial for the use of Aly ash in diagram(Wan, Wang, Ye, Guo, Gao, 2006)cement matrix For a given type of boiler, the nature of coal has Mineralogical simplicity(Fig 4): a single reactive amorphousan influence on the nature of the ash. For a given type of ash in a phase(silico-aluminous glass), which is often predominant, pluscement matrix, the anticipated properties would be unique. An inert crystallised phases(mullite, quartz and magnetite)Numerous authors( Carles-Gibergues, 1983; Jarrige, 1971;Venuat, 1981)have explained the physical, chemical, mineralog-a This article was adapted by the authors from their French original, titled ical and morphological properties of fly ash by a single process'Relation entre une cendre volante et son charbon published in poudres& of particle transformation in the boiler house through thermalgrains 15(3), 35-46(October 2005), in order touch more readers in the treatmfield中国煤化工 isation. They thus presentCorresponding author. Tel. +334 6678 5000: fax: +3346678 5365. fly ash-ticles of simple chemicalE-mail address: Jean-Charles Benezet @ema. fr .-C. Benezet)compCNMHGnds(silica, alumina and1674-2001/S-see inside back cover b 200 Chinese Society of Particuology and Institute of Process Engineering. Chinese Academy of Sciences. Published by Elsevier B V. All rights reserved.doi:0.l016 j- partic2007.09002C. Benezet et al. / Particuology 6(2008)85-92grindiFly ashFig. I. Coal treatment processes leading to the formation of fly ashferric oxide). They also identify three crystallised phases(mullite, quartz and magnetite)and a single(reactive? ) amorphoustypes of fly ash have resulted in a classification model for flyashshow that fly ashof several individual particle populations with particular speciginsmechanisms obsolete. The oldest model ( Watt Torme, 1965)was an associated model of fly ash and coal. the types of coallisted by these authors all consisted of the Sio2-Al2O3-K20system. Other ash formation mechanisms are based on a modelof fusion, expansion, spheroidisation and quenching(HemmingFig. 2. Morphology of fly ash.Berry, 1960; Raask, 1968). Hemmings model(HemmingBerry, 1960) attaches more importance to the calcium contentand fusion temperatures of the temary eutectics and the viscositof two populations of fly ash.In this study, the relation between silico-aluminous fly ashand its coal oftreatment does not completely erase the history of the material(size, nature and shape of the particles). The purpose of thisinvestigation was to characterise different fractions of a fly ashOthers fly ashesand a treated coal(Adamiec, 1998). The crossing of these resultsPortlandmust highlight the bond between coal and ashesAluminous2. Methods and materialsAlo32. MethodsAluminous ceThe coal was first subjected to fixed bed carbon calcinationFig 3. Keil-Rankin tat 450C for 10h, to eliminate the carbon fraction and recoveragram ( Minoux, 1994)the minerals present without significant modification. A second中国煤化工HCNMHGFig 4. Diffractogram of raw fly ash.J-C Benezet ef al. /Particuology 6(2008)85-92est density fraction (2.9 g/cm )and enrich it in alumina-richheterogeneous systemcompounds: mullite and corundum.Dimensional Separationse These fractioning operations produced the following 20 frac-ns(Fig. 6)N fractionsDimensional fractions: A, B, C and d(sieves: 80, 40 and25Densimetric SeparationHighest density fractionDensimetric fractions: 2, 3, 4 and 5 Table 1)Magnetic separationThe 1l most important populations in terms of mass werethen characterisedM main component fractionsFig. 5. Block diagram of ash treatment.3. 1. Characterisation offractionsthermal treatment after carbon calcination consisted in increas- The chemical composition is relatively constant in the variouing the temperature at 10 C/min up to 1200C, holding thetemperature for 8h, and finally cooling to ambient temperature fractions: silica and alumina are the main constituents,the minor(20C/min). This heat treatment is longer than that in a power elements being potassium, calcium and iron( Fig. 7)station because it was carried out in a fixed bed in the laboratorysed phases are identical in the ashes as a wholeParticles size of the samples were measured using a and in the sub-fractions: mullite, quartz and magnetite. The den-Beckman-Coulter laser granulometer (LS230. Each sample simetric spectrum is very broad, with density values varyingwas sieved in a sieve shaker using three sieves: 25, 40 and 80 umbetween I and 4.8 g/cm( Fig 8)Morphological investigations were carried out using a scana detailed typology of the Albi fly ash was obtained by morning electron microscope(SEM), JEOL (SM 35CP), equipped phological analysis of the particles coupled with local chemicalwith an energy dispersive x-ray spectrometry(EDS), Kevex analysis: solid spherical particles, hollow spheres, alveolar par-analyser. The mineralogical species of fly ash and coal were ticles, corundum particles, quartz particles, magnetic particlesstudied using X-ray fluorescence(XRF) and X-ray powder and unburnt particlesdiffraction(XRD)(Philips Pw1700)The compositions of the particles revealed by local chem-ical analysis were mostly the binary SiO2-Al2O3 system or2.2. Fractioning of the fly ashsimple termary systems. To properly represent the crystallisedhases(quartz, mullite, magnetite, hematite, free lime, anhy-drite, corundum), the whole set of results can be symbolised onIn order to verify the variation of fly ash, we attempted to link a diagram containing multiple temary systems. The amorphousfly ash diversity to the coal from which fly ash was produced. phases are strongly varied in composition. They belong eitherWe selected a silico-aluminous fly ash from Albi thermal power to the binary SiO2-Al2O3 system or simple temary systems andstation(France) for this stud, alogical procedures, a treat-In accordance with mineare shown in the hatched area of Fig. 9ment consisting of three-stage unit operations was adoptedThis diversity of chemical compositions is totally masked byhe predominance of silica and alumina, and also by the tradi(Figs. 5 and 6). The granulometric fractions were produced by tional representation of fly ash in the Keil-Rankin system(Fig. 3)a pneumatic selector(Alpine 50 ATP)The first stage( dimensional fractioning) yielded the fraction(Kutchko& Kim, 2006).denoted A, B, C and D. The second stage consisted in a densimetric separation of each of these fractions. The densities used 3.2. Srudy of the coalwere chosen a priori for separating the constituents ( Table 1).The third stage was a magnetic separation to collect the highMuch variability, often attributed to sampling methods or theperation of the power station, could result from the variety ofthe diluted mineral profile in the carbon phase. Much of thisTable 1Densimetric fractioningvariability could be inherited. The mineral profile derived fromtudied in threso as to simulate the treatmentNature of particleepsDensities(g/cm)and中国煤化工a. coal treated at40℃CNMHG8-2.9Silico-aluminous particles, quartz and glass3. 2.1. Study of non-thermally treated coal29Mullite, heavy iron oxide particles and corundumNearly all the morphological particle types already identifiedin the fly ash were also found in non-thermally treated coal, forFRACTIONINGLDsEDENSIMETRICSieve80μmFRACTIONINGDENSIMETRICFRACTIONINGd<1218d<29Sieve40μm12D>25μmDENSIMETRICSieve25μmFRACTIONINGD<25μmCsd<12DENSIMETRICFRACTIONINGC3d2.9d<12D31829MAGNETICFRACTIONING中国煤化工Fig. 6. Fractioning operations applied to fCNMHGJ-C. Benezet et aL. /Particuology 6(2008)85-92Fig. 7. Chemical composition of the fly ash fractions(CL; combustion loss).Fig 8. Densities of particle fractions(MP: magnetic particles).example the solid(Fig. 10)and hollow(Fig. lla and b) spherical Fe. These observations are not specific to coal from AlbiSimilar studies carried out on other coals have shown the exis-Semi quantitative analysis of these hollow spherical par- tence of these particles( Ferrand, 1998). Non-thermally treatedticles showed that their composition is similar to that of coal thus contains particles identical to those observed in flythe hollow spheres in the fyAL. K and a little ashQuartz中国煤化工CNMHGIAETUUnburntFig 9. Amorphous and crystallised phases of the Albi fly ashFig. 10. Solid spherical particlesJ.C. Benezet et al. Particuology 6(2008)85-92QuartzKOCaO10Coal - Unbumt Fes2+ Fe203Fig 13. Amorphous and crystallised phases after treatment of the coal at 450.C12). This profile largely belongs to the Sio2-Al2O3-K20nd the carbon-pyrite-calcite system, represented103. 3. Study of coal treated at 1200CFig. Il.(a and b) Hollow spherical partThe residue of this treatment has a different appearance fromthat of fly ash. The colours reddish, but microscopicobservation3.2.2. Study of coal treated at 450Cshows that it contains similar particlesKaolinite is the mineral most often cited as being presentin coal. It is also the mineral that is most easily altered by The quartz particles are fractured by the effect of heat; thetemperature, with its transformation starting at about 490Cclays dehydroxylate, expand and show crystallisations of mulThe shape of certain particles is sometimes altered, mostnotably the hollow spheres, where occluded gases can expand Round,or even spherical, magnetic particles were also foundso much that they disrupt the particles(Kolay singh, 2001).in this"ash'At 450C, the mineral profile consists of quartz, poorly Together with lamellar magnetite, hematite was found in allcrystallised muscovite(or illite); kaolinite and a vitreous domethe particlesQKaoMusMuw~MH出中国煤化工CNMHGFig. 12. Diffractogram of the Albi coal treated at.CJ.C. Benezet ef aL. /Particuology 6(2008)85-92M· MulliteMa-MagnetiteFig. 14, Diffractogram of the Albi coal treated at 1200C.K,O4. ConclusionsFirst, this study clearly shows that average macroscopic phys-ical and chemical values can mask the existence of particles ofhighly varied chemical composition and great morphologicalMullIn this study, the inherited aspects of the Albi fly ash wereAhoestablished. While the same chemical and mineralogical compo-sition of the fly ash was observed, it is no longer possible to useMullitethe term“ glass content” or even“ glass" as a single phase. Existing knowledge of the characteristics of coal helps to explain theformation of ashes. According to the characteristics of coal andthe parameters of the processes, we obtained ashes differingUnburntQuartz size and shape the nature of which depends on the parametersMagnetiteof the boiler house and the phenomena which occur inside theparticles: melting, coalescence and fragmenttationFig. 15. Amorphous and Crystallised phases after treatment of the coal at Given the breadth of the crystallographic domain, the vitre-1200°cous dome contains a wide spectrum of"disordered"phases andmakes the system difficult to understand.Hollow spheres of variable size(a few to 100 um)were The variability of fly ash system comes from the variety ofobserved in this“ashthe mineral profile and the respective quantities of each mineralThe existence of a common dome of minerals of varied comCharacterisation by X-ray diffraction of the coal treated at positions confirms that the concept of"silico-aluminous glass1200.C showed two crystallised phases: small quantities of linked to the idea of a single glassy phase, leads to a real riskquartz and large quantities of mullite( Fig. 14). A vitreous dome of misinterpretation in terms of reactivity. It appears preferablealso appeared.rather to think in terms of a mixture of disordered phases. Obvi-At 1200C, the mineralogical profile was transformed: ously, the first concern of a Ay ash user concerning reactivitymuscovite had disappeared and a new phase had crys- (Tangpagasit, Cheerarot, Jaturapitakkul, Kiattikomol, 2004)tallied. The phases disappeared had been replaced by a is to identify the origin of these disordered phasesdisordered phase: glass"which needs to be identified andThe thermal treatment of the Albi coal highlighted the linklocalised, then linked to the kaolinite and muscovybetween the ash and its coal sourceillite). The Ay ash seems to consist of the simple中国煤化工SiO2-Al2O3. The mineral profile of the Albi coal, essentially made up of the quartz-kaolinite-muscovite combination, QuaCNMHGcorrelated with the mineralogical constituents of a fly ash Kaolinite leads to the crystallisation of mullite and cristo-essentially made up of the quartz-mullite-glass combinationbalite(Fig. 15)Gibbsite leads to the crystallisation ofJ.-C. Benezet et al. 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