53-paper-Reaction Mechanism between Solid CaO and

ReactionMechanismbetweenSolidCaOandSlagintheHotMetalDephosphorizationProcessTasukuHAMANO,ShinyaFUKAGAIandFumitakaTSUKIHASHIDepartmentofAdvancedMaterialsScience,GraduateSchoolofFrontierSciences,TheUniversityofTokyo,Kashiwa,Chiba277-8561Japan.Abstract:SolidCaOandFeOx-CaO-SiO2-P2O5slagwerereactedfor2to2400sat1573and1673K.TheinterfacebetweensolidCaOandmoltenslagwereobservedandanalyzedbySEM/EDS.TheCaO-FeOlayerwasobservedadjacenttosolidCaO.ThethicknessoftheCaO-FeOlayerincreasedwithreactiontime.NexttotheCaO-FeOlayer,2CaO·SiO2phasewasformedandFeO-CaO-SiO2phasewithhighFeOcontentexistedadjacentto2CaO·SiO2.TheestimatedactivityofFeOfor2CaO·SiO2saturatedphaseislargerthanthatfortheCaO-FeOlayerandthebulkslag.Therefore,Fe2+diffusesfrom2CaO·SiO2saturatedphasetosolidCaOandbulkslagandthentheCaO-FeOlayerisformedbesidesolidCaO.TheactivityofCaOintheCaO-FeOlayerismuchlargerthanthatinthe2CaO·SiO2saturatedFeO-CaO-SiO2phaseandbulkslag.Therefore,theCa2+iondiffusesfromsolidCaOtobulkslagthroughCaO-FeOlayerduetotheactivitygradientofCaO.Thereactionmechanismisdescribedas(1)SolidCaOdissolvesintotheslagandthecontentofCaOinthemeltincreases.(2)TheCaOandSiO2areconsumedtoform2CaO·SiO2inthemelt,andthecontentofFeOintheslagincreases.(3)AccordingtotheactivitygradientofFeO,Fe2+diffusestobothsolidCaOandbulkslag.(4)CaO-FeOlayerisformedbesidesolidCaO.Keywords:CaOdissolution;Hotmetaldephosphorization;Multiphaseflux1.IntroductionThedemandforproductionoflowphosphorussteelhasbeenincreasing.Thehotmetaldephosphorizationprocesshasbeendevelopedtomeetincreasingrequirementsforproductionofhighqualitysteel.Thebasicandoxidativeslagiseffectivefordephosphorization.ThereforetheFeOx-CaO-SiO2systemisthetypicalslagforsteelmaking.BecauseofhighmeltingpointofCaO,solidCaOexistsintheslaginthehotmetaldephosphorizationprocess.Althoughmanyresearchesonhot-metaldephosphorizationreactionhavebeenconducted,theslagistreatedashomogeneousliquidslagandthecontributionofsolidphasesuchasundisolvedCaOand2CaO·SiO2todephosphoriazationisnottakenintoaccount.Recently,itisrequiredtoreducetheamountofsteelmakingslagproduction.TheeffectivemethodforreducingsteelmakingslagvolumeistheincreaseofutilizationefficiencyofsolidCaO.ThusitisimportanttorevealthereactionmechanismofCaOdissolutionintoslag.Sincecalciumsilicateandcalciumphosphateformstablesolidsolutionorcompoundathotmetaltemperatures,ifphosphateintheslagwerecondensedinthesolidcalciumsilicatephase,itwouldbepossibletousethemultiphasefluxincludingsolidphasetothehotmetaldephosphorizationprocess.SeveralinvestigationshavebeenconductedtoclarifythereactionmechanismofsolidCaOandslag.1-13)Inthepresentstudy,solidCaOandFeOx-CaO-SiO2-P2O5slagwerereactedat1573and1673K.TheinterfaceofCaOandslagwasobservedandanalyzedbySEM/EDS.ThemicroscopicbehaviorofphosphorusinthemultiphasefluxwasinvestigatedandthereactionmechanismbetweensolidCaOandFeOx-CaO-SiO2-P2O5slagwasdiscussed.2.ExperimentalTheslagsamplewaspreparedbymixingthesynthesizedwüstite(FetO),CaO,reagentgradeSiO2andCa3(PO4)2.AdenselysinteredCaOpiecewasusedforsolidCaOspecimen.TheoxygenpartialpressurewascontrolledbyCO-CO2mixture.ThemixtureratioofCO/CO2was100andtheoxygenpartialpressurewascalculatedtobe1.8×10-9Paat1573Kand2.3×10-8Paat1673K,respectively.Tengramsof25.3mass%FeOx-30.8%CaO-33.1%SiO2-10.8%P2O5slagsamplechargedinanaluminacrucibleweresetinaelectricfurnaceandCO-CO2gaswasflownfromtopofthereactiontube.After1800s,solidCaOpiece(3g)wasputinthecrucible.Afterthereactiontime,theCaOpiecewasquicklytakenoutfromthefurnaceandquenchedinflushingargon.TheinterfaceofCaOandslagwasobservedbySEMandthelayersofphaseswereanalyzedbyEDS.3.ResultsanddiscussionFigures1(a)to(d)showtheSEMimagesofCaO-slaginterfaceat1573Kreactedfor2to30s.ThesamplenamesareCS-2,CS-10,CS-20andCS-30,wherethenumbersmeanthereactiontimeinseconds.TheresultsofEDSanalysisforpositionsshownintheSEMimagesaretabulatedinTable1.TheCaO-FeOlayerwasobservedadjacenttosolidCaO,andnexttotheCaO-FeOlayertheCaO-SiO2orCaO-SiO2-P2O5phasewiththeFeO-CaO-SiO2meltwereobservedineachSEMimage.Figure1SEMimagesforsolidCaOandFeOx-CaO-SiO2-P2O5slaginterfaceat1573K.(a)CS-25μm234567891011121314151CaOCaO-FeOCaO-SiO2(-P2O5)FeO-CaO-SiO25μm5μm234567891011121314151CaOCaO-FeOCaO-SiO2(-P2O5)FeO-CaO-SiO2(a)CS-25μm234567891011121314151CaOCaO-FeOCaO-SiO2(-P2O5)FeO-CaO-SiO25μm5μm234567891011121314151CaOCaO-FeOCaO-SiO2(-P2O5)FeO-CaO-SiO25μmCaOCaO-SiO23195671081112131415174162CaO-FeOFeO-CaO-SiO2CaO-SiO2-P2O55μm5μmCaOCaO-SiO23195671081112131415174162CaO-FeOFeO-CaO-SiO2CaO-SiO2-P2O5(b)CS-105μmCaOCaO-SiO23195671081112131415174162CaO-FeOFeO-CaO-SiO2CaO-SiO2-P2O55μm5μmCaOCaO-SiO23195671081112131415174162CaO-FeOFeO-CaO-SiO2CaO-SiO2-P2O5(b)CS-105μmCaO234567891CaO-FeOCaO-SiO2FeO-CaO-SiO25μm5μmCaO234567891CaO-FeOCaO-SiO2FeO-CaO-SiO2(c)CS-205μmCaO234567891CaO-FeOCaO-SiO2FeO-CaO-SiO25μm5μmCaO234567891CaO-FeOCaO-SiO2FeO-CaO-SiO2(c)CS-205μmCaOCaO-FeOCaO-SiO223456789101112131FeO-CaO-SiO25μm5μmCaOCaO-FeOCaO-SiO223456789101112131FeO-CaO-SiO2(d)CS-305μmCaOCaO-FeOCaO-SiO223456789101112131FeO-CaO-SiO25μm5μmCaOCaO-FeOCaO-SiO2234567891