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Kinetics of Carbothermic Reduction of MnO2 and Catalytic Effect of La2O3

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Kinetics of Carbothermic Reduction of MnO2 and Catalytic Effect of La2O3

    Kinetics of Carbothermic Reduction of

    MnO2 and Catalytic Effect of La2O3

JournalofUniversityofScienceandTechnologyBeijing

    ;Vo1.9(2002),No.j,P.40Materials

    ;KineticsofCarbothermicReductionofMnO2andCatalyticEffectofLa2O3 ;HuiqingTangI)1ZhanchengGuoXingminGuoShengbiZhang~’

    ;I)InstituteofProcessEngineering,ChineseAcademyofSciences,Beijing,100080 ;2)Metallurgyschool,UniversityofSciencesandTechnologyBeijing,Beijing100083,China ;(Received200I-03-25)

    ;Abstract:Kineticsofcarbomermicreductionofmanganeseoxideandthecatalytice

    ctofLa2O3onthereductionhavebeenstudied

    ;bythemeas~ementofmasslOSSinN2atmosphereatdifferenttemperaturesandfollowedbySEManalysis.Itisconcludedthatthekin-

    ;eticsofc~bothermicreductionofmanganeseoxideisdividedintothreestages:gasdiffusioncontrollings

    tage.carbongasificationcon-

    ;trollingstageandsolidstatediffusioncontrollingstage.La2O3hascatalyticeriectonthereduction.Thec

    atalyticeriectofLa2O3increases

    ;withtheaddedamountofLa2O3.SEManalysisshowsthatthecatalyticmechanismisthatLa2O3promotesthetransferofoxygenionsso

    ;matcarbongasifyingiscatalyzedandthusc~bothermicreductionofMnO2iscatalyzed. ;Keywords:lanthanumoxide;carbothermicreduction;catalyticeffect ;1lntroduction

    ;Prereductionofmanganeseorewithcarboncouldbe

    ;apromisingroutetoincreasecost.efiectivenessoffer.

    ;romanganeseproduction.Theenergyconsumptionfor

    ;producingFeMnalloyhasdroppedfrom3000kW?h/t

    ;toabout1600kW?h/twhenprereducedpelletswere

    ;usedinsteadoforeintheelectricfumace11.

    ;Extensivestudieshavebeenconductedonthemech.

    ;anismandkineticsofcarbothermicreductionofman.

    ;ganeseoxide[15],whichsuggeststhatthecarbother-

    ;micreductionofthemanganeseoxideisacomplex

    ;processandisinfluencedbythephysicochemicalna.

    ;tureoftheoxide,reductantsandcompositionofthe

    ;charge.Inthepresenceofcarbon,manganeseoxide

    ;willdecomposeunderlowtemperature21:

    ;MnOMn0,Mn,0广_?Mnof1)

    ;It’sdifficulttoreduceMnOtomanganeseormanga-

;nesecarbideevenathightemperature.Rankineta1.3]

    ;considerthemaincarbideasMn5C2andtheoxidere. ;ductionstepasfollows:

    ;7C+7CO=14CO

    ;+)5MnO+9C0=Mn5C2+7CO2

    ;5MnO+7C=Mn5C2+5C0

    ;(2)

    ;(3)

    ;(4)

    ;Ithasbeenobservedthatsomesaltshaveapositive ;catalyticaleffectonthecarbothermicreductionofiron ;oxide,nickeloxide,leadsulfideandsoon[6--11]. ;Correspondingauthor:HuiqingTang

    ;E?mail:hqtang@home.ipe.ac.cn

    ;However,thesesaltsarelimitedinsaltsofalkalianda1. ;kaliearthmetals.whichhavethedisadvantageoflow ;meltingpointandso,arenotdesirableasadditiveto ;catalyzethecarbothermicreductionofmanganeseox. ;ide.Here.therareearthoxideisconsideredascataly. ;ticaladditiveduetotherichresourceofrareearthin ;Chinaanditswideuseascatalysisinmanychemicalfi. ;elds[12],i.e.La203hasadistinguishedcatalyticalef. ;fectonCO.CO2interchange121.

    ;Inthepresentstudy.effortshavebeenmadeviaseri. ;esofreductionexperimentstodisclosethereduction ;mechanismofthecarbothermicreductionofmanga. ;neseoxide.Inaddition,theinfluenceofLa2O3asaddi. ;tivetomanganeseoxideandcarbonmixturewasinves. ;tigatedexperimentallyandthecatalyticmechanism ;wasinvestigated.

    ;2Experimental

    ;2.1Materials

    ;Themajorityoftheexperimentswereperformed ;usingsubstanceswithhighpurity.Thematerialsand ;theirspecificationswerelistedintable1. ;Table

    ;tion)

    ;1Chemicalcompositionofreagentsused(massfrac. ;%

    ;2.2Samplepreparation

    ;Allthechemicalagentsweresizedtolessthan0.097 ;

    ;H.Q.TangZ.C.Guo.X.M.Guo,etal

    ;inlT1.Anapproximately5%excessofgraphiterelativ-

    ;elytoastoichiometricamountofcarbonrequiredfor ;100%ofmanganesereductiontocarbide(Mn5C2)was ;usedinallexperiments.Manganeseoxide.calciumox- ;ide.andlanthanutnoxidewithdifferentamountssho- ;wnintable2weremixedtogetherinaglasscontainer ;andthenapastewasmadebyadding2.5%organicbin- ;derand2mLdistilledwater.Sanaples,shapedasacol- ;41

    ;umnofl6rainindiameter,about4rainhiIghandabout ;1.5gofmass,werepreparedfromthepaste.These ;sampleswereairdriedfor5hfollowedby2hofdrying ;inanelectricovenmaintainedat4l3K.Thefinished ;sampleswerestoredinadesiccator.Totally,fivekinds ;ofsampleweremade.Manganesecontent()ineach ;kindofsamplewasanalyzedandtheresultswerealso ;shownintable2.

    ;Table2Chemicalcomponentsofsamples

    ;Note:w——.massfraction.

    ;2.3Equipment

    ;Aschematicdiagramoftheexperimentalequipment ;isshowninfigure1.Themainelementsoftheset-up ;areaverticaltubefumaceandagaspurificationtrain. ;Figure1Schematicdiagramofexperimentdevice,l-N2;2

    ;deoxygeningfurnace;3--desiccationbottle;4_nowmeter;5

    ;

    ;hightemperatureres~tancefurnace;6--programmable ;temperaturecontroller;7--sample;&--sampleholder;9-

    ;watercooledpipe.

    ;Thefurnacehadarefractoryworkingtube,which ;was25millinintemaldiameter.Attheupendofthe ;tubeacoolzonewasprovidedbyattachingawater. ;cooledcoppercoil.Thetemperatureofthesamplewas ;monitoredbyaPt/Ptl3Rhthermocouplewhichwas ;placedsome10nllTlbeneaththesampleholder. ;Thegas,N2,beforeenteringthereactiontube,was ;deoxidizedbypassingitthroughapackedbedofmag. ;nesiumturningsheldat673Kandthenwasdriedby ;passingitthroughacolumncontainingdehydrator. ;2.4Experimentalprocedure

    ;Thereductionkineticsweredeterminedbythemass. ;1OSSmeasurementsunderisotherlTlalconditions.Both ;catalyzedanduncatalyzedtestswereperformed. ;After

    ;takingtheinitialmassofadrysample.thesamplewas

    ;placedinthesampleholder(madeofmolybdenum) ;suspendedwiththehelpofamolybdenumwire.The ;samplewasfh’stpre.heatedfor10minatthetopofthe

    ;workingtubeandthenplungedintotheconstanthea. ;tingzoneoftheworkingtube.Aninertgasatmosphere ;wasmadebypassingN2ataflowrateof200mL/min. ;Thesamplewasreducedforarequiredtime.Thesam. ;pieholderwasthenquicklydrawnouttothewater. ;coolingzoneoftheworkingtubeandthesamplewas ;cooledherefor10minemassofthereducedsample

    ;wasmeasuredonaneleconicbalanceandthemass

    ;lOSSwasrecorded.Similartestwasrepeatedunderre. ;quiredexperimentalconditions.

    ;Sampleswerethenpreparedforchemicalanalysisof ;manganesecontenttoverifymass-lossmeasurements ;andSEMtosomeofthereducedsamples.

    ;3ResultandDiscussian

    ;Thedegreeofreductionwasstudiedbymeasuring ;thepseudokineticparameter()withrespecttotime,

    ;whichrepresentsthereductionfraction.Ifmoistureand ;volatilematterinsamplecouldbethoroughlyremoved ;inthepre-heatstage,thispseudokineticparameteris ;givenasfollows:

    ;f--(O2removedfromMnO2+carbon

    ;removedfromsample)/(total02inMnO2+

    ;totalcarb.ninsample)ml

    ;pIc

    ;oss-

    ;+

    ;m~ara

    ;.

    ;pleWv(5)

    ;whereml..isthemassremovedfromsample;msaraplcis ;themassofsample;wo,

    ;wc,wvarethemassfractionof

    ;theremovableoxygen,carbonandthevolatilematterin ;sample,respectively.

    ;Duringthepreparationofsamples,

    ;alittlewaterand

    ;organicbinderwasadded.Thesewaterandvolatile ;mattermaynotbecompletelyremovedafterdrying. ;Takingsample2asanexample,oxygencontent,car. ;

    ;42

    ;boncontentandvolatilecontentinthefinishedsample ;couldbeestimatedbythefollowingmeans. ;ManganesecontentinthesamplewM=38.5%.That ;ismanganeseoxidemassfractionis60.92%.Andso ;oxygencontentWo22.42%.

    ;Massbalanceofmanganeseoxideinthesample ;gives1OxO.865=(8.65+3+1+0.4+)x0.6092, ;whereisthemassofwaterinsample,hence1.1g. ;Iforganicbinderandwatercouldberemovedatthe ;pre.heatingstage,Wvis

    ;w=1.1+0.4

    ;×100%=10.60%,

    ;and

    ;Wc-100%=21?20%?

    ;3.1EffectofCaOonsamplereduction

    ;Thereductionexperimentwascarriedoutwithsam

    ;pielatl473K,theexperimentdataasfplotisshown ;infigure2.Examinationoftheftplotshowsthatfis ;about20%after50min,samplelcontainsnocarbon ;andthemasslossisduetowaterremovalandmanga

    ;nesedecompositionunderhightemperature.should

    ;bemorethan50%asreposedbyJiang[2].Tllisdiffer

    ;enceisduetothehighlycompactedsampleandnot ;longenoughreductiontime.Buthoweverthereduced ;samplemaintainsgoodshapeandhasarelativelyhi ;strength.Inthepresentstudy,theeffectofCaOinthe ;samplewastokeeDtheshapeandstrengthofthesalT1. ;pieathightemperature.

    ;t|rain

    ;Figure2Variationoffwithtimeforsample1at1473Kun

    ;derN2atmosphere.

    ;3.2MechanismofMnO2carbothermicreduction ;Theresultofisothermalreductionofsample2atdif- ;ferenttemperaturesisshownisfigure3.Inorderto ;identifytherate.controllingstepofthecarbothermic ;reductionofMnO2,activationenergyvalues(are ;calculated.Sincethereductionofsampleinvolvesa ;changeinmechanismandsincethechangeisrelatedto ;temperatureand|,=theEvaluesarecalculatedbvthein. ;O

    ;O

    ;O

    ;O

    ;O

    ;Univ.Sci.Techno1.Beijing,9(2002),No.j ;Figure3Effectoftemperatureonreductionofsample2. ;tegrationmethod[13]

    ;Thismethoddoesn’trefertoanyparticularkinetic

    ;modelandgiveEvaluesatdifferentleveloff.Itisnot. ;edthatthereactionrateisrelatedtotemperatureand ;thedegreeofreductionbythedifferentialequation ;df

    ;df=(6)

    ;whereFOr)isafunctionoff,and=Aexp(E/R7)

    ;Therefore,equation(6)canberearrangedandwrit

    ;tenasfollows:

    ;=

    ;sF

    ;Foragivenvalueoff,onegets:

    ;,=

    ;Bexp(E/RT)

    ;(7)

    ;(8)

    ;wheretiisthetimerequiredtoattainafixedvalueoff, ;andA,Bareconstant.

    ;Hence

    ;

    ;El

    ;nt/+b(9)

    ;Asequation(9)shows,anArrheniusplotoflnag

    ;ainstreciprocaltemperatureshouldyieldastraightline ;andtheslopewouldgivethevalueE.Theplotoflnbvs. ;reciprocaloftemperatureisshowninfigure4. ;Itisfoundthattheactivationenergyvariesalotwim ;,=Tllisindicatesachangeinthemechanismofreduc. ;tion.Fromfigure4,thewholeprocesscouldbeclearly ;dividedintothreestagesdistinctly.When,?0.3.the

    ;activationenergyissome6lkJ/mo1.thisshowsinthe ;initialstageofreduction,thesampleraisestemperature ;andreleasesthevolatilematter,Mn02decomposesto ;produceO2,andMnO2reactswithCtoproduceCOand ;CO2.Inthisstage,themasslOSSiSmainlyduetothere. ;1easeofvolatile,CO2andO2.Therate.controllingstep ;ofthisstageisgasdiffusion.

    ;Duringthesecondstageofreduction,0.3<f<0.6,the ;activationenergyisbetweenl52kJ/molandl92kJ/ ;molwhichisnearlyequaltothatofBoudouardreaction ;

;HQTnng.Z.C.Guo.X.M.Guo,etal

    ;6

    ;6

    ;5

    ;5

    ;5

    ;7.5

    ;7.0

    ;6.O

    ;5.O

    ;r/×1OK

    ;6.36.46.56.66.76.8

    ;r/×1OK

    ;8

    ;r/×1OK

    ;r/×10K

    ;Figure4AnArrheniusplotforMnO2carbothermicreductionatdifferentreductionfractionleve1.

    ;141.Therefore,thegasificationreactionbetweencar- ;bonphaseandgas?-carbonissuggestedtobetherate?- ;controllingstep.

    ;Duringthethird,sloweststageofreduction,f>0.6. ;Activationenergyofthisstageis322kJ/mo1.There- ;mainingoxideisreducedbycarbondissolvedinthe ;carbidephaseattemperatureabove1473K.

    ;MnO(s)+C(carbide)=Mn(carbide)+CO(g)(1o) ;Thisstagewasconsideredtobecontrolledbythedif- ;fusionofcarbontothecarbide/oxideinterface. ;Inthepresentstudy,thegeneralschemeofmanga- ;neseoxidereductionhasthreestages:gasdiffusion ;controllingstage,carbongasificationcontrollingstage ;andsolidstatecarbondiffusioncotrollingstage. ;3.3EffectofLa203asadditive

    ;(1)Catalyzedreduction.

    ;Basedontheabovediscussionandexperimenton ;themechanismofcarbothermicreductionofmanga. ;neseoxide,thecatalyzedexperimentswerecarriedout ;at1473Kbecauseatthistemperaturethereductionof ;sampleismainlycontrolledbycarbongasificationas ;showninfigure3.Sample3.4and5wereusedinthe ;experimentsandtheresultsareshowninfigure5.Itis ;foundthatthecatalyticeffectoflathnuamoxideincre. ;aseswithanincreaseoflathnuamoxidecontentinthe ;sample.

    ;(2)Microstructure

    ;Thereducedsamplesofsample5weresubjectedto ;SEManalysis.Thestucturesofthereducedsamplesare ;f/min

    ;Figure5EffectofLa203contentonreductionofsamples. ;showninfigure6.Becauselathnuamismuchheavier ;thanotherelementsinthesample.it’seasytoidentify

    ;lathnuamoxideparticlesinSEMphotographfthebrig- ;htpointsinthephotographarewherelathnuamoxide ;particleslocate).SEMphotographesshowthatthearea ;aroundthelanthnaumoxidepariticleismuchmorepor. ;OUSthanotherareainthesampleandthistendencyin. ;creasesasthereductiontimelasts.Electronicmicro. ;probeanalysistothezonearoundlathnuamparticles ;showsthatnonewphasehasformedduringreduction. ;(3)Catalyticmechanism

    ;Basedonthecatalyzedexperiment,SEManalysis ;andthecatalyticeffectoflathnuamoxideonCO.CO ;interchange,thecatalyticmechanismisconsideredthat ;lathnuamoxide,asadditiveinthesample.elevates ;oxygenionstransferinthesecondstageofreductionso ;thatthecarbongasificationispromoted.

    ;Thefollowing

    ;mechanismappearsmostlikely:

    ;La2O3+CO2La2O3?O+CO

    ;76

    ;lI一昌一lI

    ;

    ;44

    ;La203-O+C=La20,+c[o

    ;c[o]---co

    ;Inequation(11),La203.Oisatransitionalproduct ;whichmeansthatoxygenionisadsorbedbyLa203.In ;thecatalyzedreduction,thecarbonparticlesaround ;La203particlewerecatalyzedtobegasifiedquicklyto ;formveryporousareaaroundLa203particles,thisef- ;fectwilllastuntilLa2O3?Odoesn’tcontactwithcarbon.

    ;Theelevatedcarbongasificationmeansthecarbother

    ;micreductionofmanganeseoxideiscatalyzed. ;4Conclusious

    ;(1)Thecarbothermicreductionofmanganeseoxide ;includesthreestages:gasdiffusioncontrollingstage, ;carbongasificationcontrollingstageandsolidstatedif- ;fusioncotrollingstage.

    ;(2,Lanthanumoxidehasadistinctcatalyticeffecton

    ;thecarbothermicreductionofmanganeseoxideand ;thiseffectincreaseswithanincreaseoflanthanumox- ;idecontentinthesample.

    ;f3)Themechanismofthecatalyticeffectoflantha

    ;nHITIoxideoncarbothermicreductionistoelevateoxy

    ;gentransferinthesecondstageofreductionsothatthe ;J.Univ.Sci.Teehno1Beijing,9(2002).No.1

    ;Figure6SEMmicrophotographofthe

    ;samplecontaining3%La2O3subjectedto

    ;reduceat1473KunderN2gasatmos-

    ;phere,(a)subjectedtoreducefor10min;

    ;(b)subjectedtoreducefor30min;(c)

    ;subjecttoreducefor50min.

    ;carbongasificationispromoted.

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