57-paper-Desulfurization and Deoxidation of Molten

DesulfurizationandDeoxidationofMoltenIronwithInSituProducedMagnesiumVaporJianYANG,MamoruKUWABARA,ZhongzhuLIU,KeijiOKUMURAandMasamichiSANO(GraduateSchoolofEngineering,NagoyaUniversity,Nagoya,Aichi-ken464-8603,Japan)Abstract:Novelhotmetaldesulfurization,andmoltensteeldesulfurizaitonanddeoxidationprocesseshavebeendevelopedwithmagnesiumvaporproducedin-situbyaluminothermicreductionofmagnesiumoxide.Thecheapermagnesiumoxide,insteadofmagnesiummetal,isusedasthesecondarysourceofmagnesiumsothatthecostofmagnesiumdesulfurizaitonanddeoxidationisgreatlyreduced.MgO-Alpelletsarechargedintoatube,whichisimmersedintothemolteniron.Heatedbyheatconductionfromthemolteniron,magnesiumoxideinthepelletsisreducedbyaluminiumtoproducemagnesiumvapor,whichisinjecteddirectlyintothemoltenirontogetherwithArcarriergastoreactwithsulfuroroxygendissolvedinit.Thedesulfurizationanddeoxidationprocessesarecharacterizedbyeasyandsafeoperationandlowcost,andbothofthesulfurandoxygenconcentrationsinthemoltenironcanbequicklydecreasedtoaverylowlevel.TherefiningefficiencyofmagnesiumcanbegreatlyimprovedbyadjustingthemagnesiumpartialpressureintheinjectedbubbletotheoptimumvaluethroughchangingtheArcarriergasflowrate.TheMgOinclusionsformedinthedeoxidationprocessareusuallyverysmall.Moreover,theycanberapidlyremovedbyadheringtothesurfaceoftherisingbubbles.Keywords:Desulfurization;Deoxidation;Magnesiumvapor;MgOinclusions;Molteniron;Aluminothermicreduction1IntroductionItiswellknownthatmagnesiumisastrongdesulfruizationanddeoxidationreagentinironandsteelmakingprocesses.1,2)Asadesulfurizer,magnesiumhastheadvantagesthatitcanpromptlyreducethesulfurcontentinhotmetaltoanultra-lowlevel,andthenecessaryamountofslagismarkedlycutdown,whichnotonlyisfavorabletotheenvironment,butalsoreducesthetemperaturedropduringthetreatment.3-9)Withmagnesiumasadeoxidationreagent,theMgOinclusionsformedinthedeoxidationprocessareusuallysosmallthattheyarenotharmfultothemechanicalpropertiesofsteelproduct.10-13)However,sincethenormalboilingpointofmagnesiumof1380Kismuchlowerthanthetemperatureofironandsteelmaking,refiningwithmagnesiumhasthedrawbacksoflowrefiningefficiencyandsafetyproblemduetoviolentmagnesiumvaporization.Furthermore,metalmagnesiumishighinprice.Inthepresentstudy,anewmethodofdesulfurizationanddeoxidationofmoltenironwithmagnesiumvaporproducedinsitubyaluminothermicreductionofmagnesiumoxideisproposed.Thecheapermagnesiumoxide,insteadofmagnesiummetal,isusedasthesecondarysourceofmagnesiumsothatthecostofdesulfurizaitonanddeoxidationwithmagnesiumisgreatlyreduced.Pelletsmadeofmixtureofmagnesiumoxideandaluminumpowdersarechargedintoanimmersiontube.Theproducedmagnesiumvaporisinjecteddirectlyintothemelt.Theeffectsofexperimentaltemperatureandcarriergasflowrateontherefiningratearestudied.Akineticmodeldescribingtherateofdesulfurizationanddeoxidaionwithmagnesiumisproposedtoexplaintheexperimentalresults.Anovelmethodofimprovingtherefiningefficiencyofmagnesiumisproposedbyadjustingthemagnesiummolarratiointhebubbletotheoptimumvaluethroughchangingcarriergasflowrate.ThecharacteristicsofinclusionsformedintheMgdeoxidationprocessarealsoinvestigated.2ExperimentalApparatusandProcedureFigure1schematicallyshowstheexperimentalapparatus.Ahighfrequencyinductionfurnace(15kW,100kHz)wasusedtomeltelectrolyticironof350ginacrucibleof40mmi.d.and100mminheight.Inthecaseofdesulfurizationofmolteniron,agraphitecruciblewasused;inthecaseofdeoxidationofmoltensteel,amagnesiacruciblewasused.Thegraphiteormagnesiaimmersiontubeof11mmi.d.and15mmo.d.had5holesof1.0mmdiameteratitslowerpart,throughwhichthemagnesiumvaportogetherwithargoncarriergaswasinjectedintothemelt.Thepelletschargedintothetubewerethemixturecomposedofmagnesiapowder(0.4μminaveragediameterandpurityhigherthan99.99pct)andaluminumpowder(75-150μminsizeandpurityhigherthan99.5pct).Withmixingthemagnesiumoxideandaluminumpowdersatamolarratioof3:2or4:2,thepelletswereformedbyuseofacoldisostaticpressunderapressureof150MPaforonehour.Aftermeltingtheelectrolyticiron,theinitialsulfurconcentrationwasadjustedbyaddingFeStothemolteniron,andtheinitialoxygenconcentrationwasadjustedbyblowingthemixturegasofoxygenandargonontothemeltsurface.Thenthetubechargedwiththepelletswasimmersedintotheironmelt.Themagnesiumvaporproducedinsitubyaluminothermicreductionofmagnesiumoxidewasinjecteddirectlyintothemoltenirontogetherwiththeargoncarriergas,toreactwithsulfuroroxygendissolvedinit.Duringalltheexperiments,desulfurizationanddeoxidationprocesseswereexaminedbytakingsamplesfromthemeltatappropriatetimeintervalstoanalyzesulfurandoxygencontents.3PrincipleofReductionofMagnesiumOxideThereductionofmagnesiumoxidebyaluminumtakesplaceintwostages.14)Atthefirststage,thefollowingreactionsoccurtoproducemagnesiumvapor,aluminaandMgO･Al2O3spinel:3MgO(s)+2Al(l)=3Mg(g)+Al2O3(s)(1)4MgO(s)+2Al(l)=3Mg(g)+MgO・Al2O3(s)(2)Atthesecondstage,thespinelisfurtherreducedbyaluminumaccordingtoGraphitetubeCarriergasArgasinletQuartztubeInductioncoilMoltenironProtectioncrucibleGraphitecrucibleDensealuminatubeSamplingholeGasoutletPelletFigure1ExperimentalApp