Densificationofflyashunderalkalinehydrothermalhotp

14thInternationalConferenceonthePropertiesofWaterandSteaminKyoto360DensificationofflyashunderalkalinehydrothermalhotpressingconditionsZ.Matamoros-Veloza1,*,J.C.Rendón-Angeles2,K.Yanagisawa3andM.M.Cisneros-Guerrero11DepartmentofMetal-Mechanical,TechnologicalInstituteofSaltillo,Coahuila25280,México*E-mail:[e-mail]2ResearchCenterforAdvancedStudiesoftheNPI,CampusSaltillo,Saltillo,Coahuila25900,México3ResearchLaboratoryofHydrothermalChemistry,KochiUniversity,Kochi780-8520,JapanLargevolumesofflyashareannuallyproducedatpowerplantCo.oftheFederalElectricityCouncil(Coahuila,MEXICO),andthissolidwastehasbecomeanenvironmentalproblembecauseofshortageofstoragerestrictionsandenvironmentalpollution.Theflyashconsistsofsphericalparticles(particlesizeof20Pm)andcontainsanamorphoussilicatephaseandtwocrystallinephases(MulliteandQuartz)asmajorconstituents.Flyashcompactswerepreparedwiththeadditionof15wt%ofanalkalinesolvent(NaOHsolution5and10M)byhydrothermalhotpressingattemperaturesintherangeof150-250ºC,fordifferentreactionintervals(30-120min)withaloadingpressureof17MPa.TensilestrengthofthecompactswasmeasuredbytheBraziliantest.ThestructuralchangesofthecrystallinephasesduringthetreatmentweredeterminedbyX-raydiffractionanalyses,andmicrostructuralaspectsofthespecimenswereobservedbyscanningelectronmicroscopy.AlthoughX-raydiffractionresultsshowedthatthecrystallineMulliteandQuartzphasesdidnotreacteveninconcentratedNaOHsolutionsathightemperature,thedensificationwasachievedbythedissolutionoftheamorphousphase.Alowstrength(3MPa)wasobtainedonthecompactpreparedwith5MNaOHsolutionat250°C.Incontrast,thestrengthoftheflyashcompactswasfurtherincreasedupto30MPabyincreasingtheNaOHconcentrationto10M.1.IntroductionNowadays,theannualproductionofflyashishigheachyear.Anamountbetween10-20wt%offlyashiscommonlyproducedbythecombustionofbituminouscoalinpowerplants.Thecombustionofbituminouscoalinpowerplantscommonlyproducesflyash,10-20wt%fromthecoal.Theflyashwastesolidiscollectedbycyclones,electricprecipitators,and/orbagfiltersfromthegasflowingattheburnertop,whileclinkerashisrecoveredfromthebottomasmassoffusedrock.Morethan150millionoftons/yearisgeneratedintheworld,andatleast,one-thirdisusedasrawmaterialinthefabricationofcement.In2002,1.68millionoftonswereproducedattheelectricplant“JoseLopezPortillo”Coahuila-Mexico,andonly4.5%wasrecycled.Thehalfoftheflyashfindsnoapplicationandmostofthiswasteisdisposedasasolidwasteinlandfills.Thisrouteforsolidwasterecyclinghasbeenexpensiveincreasinglyandbringsenormousenvironmentalcontaminationproblems.Fortunately,manyresearchworksarenowevaluatingnewroutesforflyashprocessing.However,attemptsforrecyclingoftheflyashhavebeenlimitedduetothechemicalcompositionofthisspecificwaste.ItiswellknownthatflyashmajorinorganicconstituentsarethecompoundssuchasSiO2andAl2O3.Thereareotherminorundesirable’selements,suchas:As,B,Ba,Cd,Cr,Pb,Hg,Mo,Se,S,whichareconsideringhighlycontaminants.Nevertheless,thepotentialuseforpowerplantflyasheswasreportedbymanyresearchers[1-5],aswellassomeapplications,inordertoincreasetheamountoftherecycledflyashsolidwastes[1-5].Hitherto,theflyashhadbeenusedasanadditiveforcementsandconcrete,becauseitslightweightcharacteristicandmorphologyofagglomeratedspheresorcenospheres(1-100µmindiameter).Theseaggregatesmainlyconsistofcrystallineinorganicphasesofquartz(Į-SiO),mullite(3Al2O3xSiO2),hematite(Fe2O3),magnetite(FeOxFe2O3)andasignificantamountofavitreousamorphousphase[6-7].Agreatnumberofexperimentshavebeencarriedoutunderhydrothermalalkalineconditions,inordertotransformflyashintoinorganiczeolite14thInternationalConferenceonthePropertiesofWaterandSteaminKyoto361materials.Undertheseconditions,differenttypesofcrystallinezeolitephaseswereobtainedatlowconcentrationofalkalinesolutions[2.0M]withinthetemperaturerangeof100-200ºC.Themainzeolitesobtainedaretheanalcime,Na-AandNa-P1typezeolite,andhydroxyl-sodalite.TheformerresultssuggestedthatthetypeofzeolitestronglydependedonthenominalSi/Alratiooftherawflyash[8-10].Inthepresentresearchwork,wehaveaimedtostudytheinfluenceofthealkalinesolutiononthepreparationofthecompactsunderhydrothermalhot-pressingconditions.Tensilestrengthandapparentdensityofthepreparedcompactsweremeasured.2.ExperimentalProcedureFlyashproducedatCFEpowerplantCo.(Coahuila,MEXICO)wasemployedasarawmaterial.Theflyashsphericalparticleswithanaverageparticlesizeof20Pm,consistedofamorphoussilicatephaseandtwocrystallinephases(mullite,3Al2O3·2SiO2andquartz,SiO2).Thechemicalcompositiondeterminedbywetanalyses,showedthatthemajorconstituentsareSiO2(62.37%),Al2O3(26.14%),andminorNa2O(0.34%),CaO(1.99%),MgO(0.85%)andTiO2(0.71%).Figure1showsthetypicalXRDpatternoftherawflyash,whichdepictsthatthismaterialisformedbycrystallinequartz,SiO2();andmullite,Al6Si2O13(~).Theflyash(5g)waswellkneadedwith15wt%oftheNaOHsolutionusedasasolventwithtwodifferentconcentrations,5and10M.Themixturewasthenplacedinthechamberofthehydrothermalhotpressingautoclave,andtheexperimentswereconductedatvarioustemperaturesbetween150-250ºCforseveralreactionintervals(30-120m