Phillipsitesynthesisfromflyashpreparedbyhydrotherm

AdvancedPowderTechnol.,Vol.17,No.4,pp.369–382(2006)©VSPandSocietyofPowderTechnology,Japan2006.Alsoavailableonline-∗,KAZUHIROARAI,KEIJIKANAYAMAandHIDETOYOSHIDADepartmentofChemicalEngineering,GraduateSchoolofEngineering,HiroshimaUniversity,Higashi-Hiroshima739-8527,JapanReceived29August2005;accepted3November2005Abstract—CoalflyashwastreatedhydrothermallywithNaOHaqueoussolutionat373Kusingmicrowaveheatingandconventionalheating.X-raydiffractionandenergydispersiveX-rayfluo-rescencemeasurementsindicatedthatthecrystalstructureandchemicalcompositionoftheproductpowderdonotdependontheheatingmethod.However,microwaveheatingreducestheparticlesizeofsynthesizedphillipsite.Moreover,microwaveheatingmakesthewaitingtimeforthebeginningofcrystallizationmuchshorterthanconventionalheating,althoughitreducesthecrystalgrowthrateslightly.Itwasalsofoundthatmicrowaveheatingpromotestherateofincreaseoftheadsorptioncapacityoftheproductpowder.ChangesintheionconcentrationsinNaOHaqueoussolutionandthepeakintensityandadsorptioncapacityoftheproductpowdersuggestthatmicrowaveheatingmainlyhaseffectsonnucleationandcrystalgrowthofphillipsite.Weproposedanewheatingprocedure,‘partialmicrowaveheating’,whichhastheadvantagesofbothmicrowaveheatingandconventionalheating.Thisheatingprocedurecandevelopthecrystallinityandadsorptioncapacityoftheproductwithoutreducingthecrystalgrowthrate.Keywords:Flyash;microwave;phillipsite;hydrothermaltreatment.1.INTRODUCTIONInJapan,over7.5millionstonsofflyasharegeneratedperyearbythermalpowerplants,etc.,asindustrialwastes.Theamountofdischargedflyashisexpectedtoincreasemonotonouslyintheyearsahead[1]and40%offlyashisreclaimedfromthesea[2].Therefore,effectivewaystoreuseflyashshouldbequicklydeveloped.Onenewwayproposedtoreuseflyashisbysynthesizingphillipsite,whichcanbeusedasadsorbents,catalysts,etc.,usingthehydrothermaltreatmentmethod[3–8].Previouslywehavereportedtheeffectsofparticlesizeandcompositionofflyash∗Towhomcorrespondenceshouldbeaddressed.E-mail:[e-mail]Haqueoussolutiononthegrowthrateandcrystalstructureofgeneratedzeolite[9–12].Furthermore,wehaveinvestigatedthemethodtosynthesizephillipsiteselectivelyfromamixtureoflowsilicacontentflyashandricehuskash,andproposedamethodtoimprovethepurityandyieldsofsynthesizedphillipsite[13,14].Ontheotherhand,microwavescanheatanobjectuprapidly,directlyandselec-tively,generatingheatbyrotatingthedipoleofanobject[15].Forthesereasons,microwaveheatingisnowwidelyusedtosynthesizefinepowdermaterials[16].However,theeffectsofmicrowaveirradiationonphillipsitesynthesisfromflyashhasnotbeendiscussedsufficiently.Itistheaimofthisworktoclarifywhichprocessesofphillipsitesynthesisfromflyashareintensivelyaffectedbymicrowaveirradiationandtoproposeaneweffectiveirradiationmethod.2.EXPERIMENTALCoalflyashsuppliedfromShin-Onodathermalpowerplant(ChugokuElectricPower)wasusedasrawmaterial.ThepropertiesofthetestedflyasharelistedinTable1.Thisflyashhasrelativelyahighsilicacontentandthecrystallinephaseofquartz.TheschematicdiagramofthemicrowaveheatingequipmentisshowninFig.1a.Microwaves(2.45GHz)generatedbythemagnetronweredirectedtothevesselbywayofawaveguide.Theslurry,whichcontains50.0mlofNaOHaqueoussolutionand2.0gofflyash,wasirradiatedandheatedupbythemicrowaves.ThetemperatureoftheslurrywascontrolledbyvaryingthemicrowaveoutputwithaPIDcontroller.Usingthisequipment,theflyashwastreatedhydrothermallyat373K.Here,thetreatmentpressurewas1.0×105Paandtherotationalspeedofthestirrerwassetat250r.p.m.Afterthehydrothermaltreatment,theslurrywascooleddowntoroomtemper-ature.TheproductpowderwasseparatedfromthehydrothermalsolutionbyaBuchnerfunnel,washedwithdistilledwaterthoroughlyanddriedat393Kfor48h.Therefore,theproductpowderconsistsofunreactedflyashandgeneratedzeolite.Table1.PropertiesofthetestedflyashFlyashAComponentSi(wt%)61.0Al(wt%)25.5Others(wt%)13.5Mediandiameter(μm)2.0CrystalstructureSiO2Phillipsitesynthesisfromflyash371(a)(b)Figure1.Schematicdiagramoftheexperimentalset-up.(a)Microwaveheatingequipment.(b)Conventionalheatingequipment.372K.Fukuietal.Asacontrolexperiment,theflyashwastreatedhydrothermallyunderthesameconditionswiththeconventionalelectricheatershowninFig.1b.Figure2showsthechangeintemperatureofNaOHaqueoussolutionwithtreatmenttimeformicrowaveheatingandconventionalheating.Inthecaseofmicrowaveheating,thetemperaturerapidlyreaches373Kin3min.However,thetemperatureofconventionalheatingrisesslowlyandittakesover15mintoreach373K.Inordertoreducethedifferenceoftheinitialtemperaturechange,inthecaseoftheconventionalheating,theflyashwasfedintotheNaOHaqueoussolutionassoonasthetemperaturereachedto373K.Thepropertiesoftheobtainedproductpowderwereevaluatedbythecrystalstruc-ture,ammoniumionadsorptioncapacity,chemicalcomponentsandmorphology.ThecrystalstructurewasidentifiedbyX-raydiffraction(XRD;RigakuRINT-2000).Theconcentrationsofammoniumionandchemicalcomponentsweremeasuredbyanionmeter(HoribaF23)andenergyd