CrystallizationofKLandKWzeolitesfromflyash

Resources,ConservationandRecycling24(1998)167–181CrystallizationofK–LandK–Wzeolitesfromfly-ashG.Belardia,S.Massimillab,L.Pigaa,*aMineralProcessingInstitute,CNR,ViaBolognola,7,Rome,ItalybGeologist,Sapri,Salerno,ItalyAccepted14April1998AbstractTheexperimentalconditionsforthesynthesisofK–LandK–Wzeolitesbyhydrothermalalkalinetreatmentofcoal-firedpowerplantsfly-ash(FA)havebeenestablished.Thankstotheirhighstabilityinacidenvironments,thesezeolites—intheirsodicform—canbeutilizedtoremovetheammoniumionNH4frommunicipalandindustrialwastewaters.TheadditionofSiO2toFAortothealkalinesolutionincreasestheSi:Alratiooftheslurry.Thisbooststheselectivityofthesynthesis,ensureslargeramountsofK–LandK–Wzeolites,andpreventstheformationofphasessuchaskaliophilitethatareunsuitableforabsorption.Thezeolitesthussynthesizedexhibithighthermalstabilityupto600°C.IndustrialuseofsuchzeolitescouldgenerateadditionalincomeforpowerproducersandreducetheexpenditureincurredinstoringFAorwastingittolandfill.©1998ElsevierScienceB.V.Allrightsreserved.Keywords:Fly-ash;Zeolites;K–L;K–W;Alkalineactivation;Coal1.IntroductionAccordingtoEurostat[1]about25%oftheEuropeanUnion’s(EU)energydemandismetfromsourcessuchascoalandlignite,resultinginthegenerationofsome60Mtonne:yearoffly-ash(FA)asaby-product[2].FAiscurrentlyutilized*Correspondingauthor.Tel:3968804361;fax:3968804361;[e-mail]0921-3449:98:$-seefrontmatter©1998ElsevierScienceB.V.Allrightsreserved.PIIS0921-3449(98)00016-0G.Belardietal.:Resources,Conser6ationandRecycling24(1998)167–181168fortheproductionofcementandconcrete,asbackfillinmining,andinagriculture;aswellasinavarietyofcivilengineeringapplications[3–5].Therateofre-useofFAvariesfromtheexceptional100%inHolland,tolessthan20%inotherEU-countriesasfoundoutbyKema.UnrecycledFAisgenerallyburiedincontrolledlandfillstoavoidpollutionofthesurroundingareasowingtoitsfineness.However,accordingtoKemathecostofmaintenanceofsuchlandfillshasrisenfrom$9:tonnein1985to$65:tonneinsomecountries,thusincreasingthecostofthecoal-burningprocessasawhole.Moreover,eventhoughlandfillingreducestheriskofcontamination,thefactremainsthattheextensiveareasoflandneededforsuchwastedumpsarenolongeravailableforproductivepurposes.There-useofFAinproductsofimprovedenvironmentalqualitylikecheapabsorbentssuchaszeolitesforenvironmentalprotection,wouldlengthenthelifeoflandfillsites,sinceonlyapartoftheFAwouldneedtobedumped.TheremovalofNH3:NH4frommunicipalandindustrialwastewatersisachievedbybiologicalandchemicaltreat-ments,andalsobyionexchangeutilizingnaturalzeoliteslikeclinoptiloliteandphillipsite[6–9].TheNaionreleasedbysuchzeolitesduringexchangewiththeNH4ion,favoursthecoagulationoforganiccolloidalparticlesinthewastewaters,thusimprovingclarificationandreducingthebiologicaloxygendemand(BOD)[10]thatisanindicatorofthewaterpollution.Synthesisofthemostadvancedzeolitesresultsinahighqualityproductwithtailor-madeaperturesforspecializedapplica-tions.However,whenproducedfrompurechemicalreagentssuchzeolitesareexpensive,pricesrangingfrom$560:tonneforbulkproductsupto$33:kgforthosedestinedforspecialapplications.Naturalzeolites,ofwhichtherearehugereserves,aregenerallymixturesofafewspecies.Theseareimpure,andhaveonlylimitedporesizeandvolume,sotheirretentioncapacityisalsolimited[11].Naturalzeolitescanbeupgradedbutthisisanexpensiveprocessandthequalityisnotbeasgoodasthatofartificialzeolites.Furthermore,theextractionofnaturalzeolitesbyopencastmininginevitablycauseslandscapedespoliationandthereisalsotheneedtotransportthemtothesitewheretheywillbeused.Whenvolcanicashinteractswithhydrothermalsolutionsorgroundwater,itisfrequentlytransformedintozeolites.FAissimilartovolcanicashintermsofmajorelementcompositionandabundanceofvitreousphase.ThepossibilityofproducingzeolitesforbulkapplicationsderivesfromthepresenceintheFAofthislargevitreousphase(amorphousSiO2)—whichishighlyreactiveinhotalkalinesolu-tions—togetherwithquartz(SiO2cryst.)andmullite(3Al2O3*2SiO2)—SiandAl-bearingrespectively—whicharethetwobasicelementsofzeolites.Theadvan-tagesofthisconversionlieinthepossibilityofre-cyclingawastetoproducezeoliteswhichhavebetterabsorbingpropertiesthannaturalzeolitesandcanbeproducedintheareawheretheywillbeused.Severalkindsofzeoliteshavebeensynthesized.Some20patentsdealingwithzeolitizationofFAhavebeenfiledsince1976andover40articleshavebeenpublished.Zeolitesreportedtohavebeenproducedareanalcime[12–14],phillipsite[15–19],faujasite[20–24],hydroxysodalite[25–28]andzeoliteA[29–35],F[14,36],NaP[37–41],andY[42].NospecificattempthasbeenmadeofproducingK–L(K9AL9SI27O72nH2O)zeolitefromFA,whileafewpapershavebeenpublishedonK–W(K9AL9Si23O64nH2O)zeolite[14,43].TheseG.Belardietal.:Resources,Conser6ationandRecycling24(1998)167–181169twolastzeolitesarerareinnatureastheyderivefrommonocationicpotassicsolutions.K–LzeolitehasahexagonalstructureandaccordingtoBarrer’sclassifi-cationbelongstoChabazitegroup.TheSi:Almolarratiorangesbetween2.6and3.5,hencetheK–Listhesilicarichestpotassiczeolite.AsaconsequenceK–Lzeoliteshowsalowcationicexchangecapacity(c.e.c.)butagreatst