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NaCl-CaO

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NaCl-CaO

    NaCl-CaO

    InternationalJournalofMinerals,MetallurgyandMaterials

    Volume17,Number6,December2010,Page791

    D01:10.10O7,s12613-010-0391-8

    CathodicbehaviorofmoltenCaCI2CaOandCaCI2-NaCI-CaO

    ShulanWang,WeiWang,Shi-chaoLi,andShanhuiCao

    DepartmentofChemistry,NortheasternUniversity,Sh~yang110004,China (Received:8January2010;m~sed:1February2010;accepted:19February2010) Abstract:ThecathodicbehaviorofmoltenCaCl,,CaCI2-Ca0andequimolarCaC12-NaC1.CaOwasstudiedbycyclicvoltammogramsand

    constantpotentialpolarizationattemperaturesof1123to1173Konmolybdenumandtitaniumelectrodes.ThediffusioncoecientofC

    (CaO)inmoltenCaC12.Ca0wascalculatedfromthelinearrelationshipbctweenthesquarerootofscanrateandthepeakcurrentdensity.

    Thedepositionpotentialsandthepotentialtemperaturecoe

    cientofCaOinmoltenCaCl,.0.5mol%CaOandCaCl,-NaCl-0.5mo1%CaO

    werealsoobtainedfromtheircyclicvoltammograms.TheresultshowsthatCaOismoreeasilyreducedthanCaC1,.TheadditionofNaCliIl

    moltenCaC12-CaOinducestheunderpotentialelectrodepositionofCaO. Keywords:moltensalts;calciumoxide;cyclicvoltammetry;polarization Th/sworkwasfinanciallysupportedbytheNationalNaturalScienceFoundationofChina(No.50674027).]

    1.Introduction

    MoltenCaCl,.CaOandCaC12.NaC1.Ca0ageimportant

    saltsandhavebeenintensivelyusedintheresearchoftita-

    niumelectrorefinementf1-18].Threenewelectrofining

    processesoftitaniumhavebeenproposedrecently,andtwo

ofthemtookCaC12-CaOastheelectrolyte2_41.ChenPf,.

    21studiedtheelectrodeoxidationoftitaniumdioxidein moltenCaC12-CaObyapplyingaDCvoltageOf_3.OVbe

    tweenthetitaniumdioxidecathodeandthegraphiteanode. ThisiscalledtheFFCprocess.Onoeta1.31electrodepos.

    itedcalciumfrommoltenCaCl,.CaOat3.OVandfurther

    reducedthecathodeoftitaniumdioxidebycalcium.Thisis calledtheOSprocess.Theelectrodeoxidationofchromium oxide,titaniumoxide,terbiumoxide,andmixedoxideofti. taniumandironwasalsostudiedinCaCI2.basedmolten salts[5-9].Thereactionmechanismofextractingtitanium bytheFFCprocesswasstudiedintensivelyfromelectro. chemicalandstructuralaspectsnl13].Thecathodicbe.

    haviorofmoltenCaCl,inthetitaniumdioxidescaledfita- niumcathodewasalsostudiedbythecyclicvoltammogragn technique[15].Mohamedieta1.[16]investigatedtheanodic processofNaC1-Na20,CaC12CaO,CaC12-NaC1CaO,

    NaCISrC12SrO,andNaC1-BaCI2BaOandconfirmedthe

    followingtwostepoxidationprocessofthemoltensaltson thegraphiteanode

    C+202一一4eCO2

    CO2+0CO;

    (1)

    (2)

    Moeta1.[17]studiedthecyclicvoltammogr'amsofmol- tenLiC1-KC1-NaCI-Na2CO3ondifferentanodesandfound thattheoxidationreactionofc0;onthegraphiteanode

    isirreversible.CO2dissolvedinmoltenCaCI2isreducedto carbonbythereversereactionofEq.(1),withadecreasein theelectricalefficiencyofthetitaniumelectroextracting

    process[18].Inthepresentwork,thecathodicprocessof CaCI2-CaOandequimolarCaC12-NaC1?-CaOwasstudiedby cyclicvoltammogramsandconstantpotentialpolarizationto getthefundamentalaspectofcathodicbehaviorofthemol

    tensalts

    Correspondingauthor:ShulanWangE-mail:slwang@mail.1ieu.edu.ci1 @UniversityofScienceandTechnologyBeijingandSpringer?VerlagBerlinHeidelberg201

    0Springer

    792

    2.Experimental

    caOwascalcinedat1200.Cfor4h.CaC12andNaC1 weredriedat473Kfor60h.CaC12andequimolar

    CaC12-NaC1wereputinagraphitecrucible(0minx80 nun),separately,withtheadditionofasmallamountofCaO. Thecruciblewasputinasealedfurnacewithargonflowing andkeptat473Kovernightbeforeheatingtoahighertern

    perature.Ire.electrolysiswasperformedat-0.5Vf0r20h betweentwographiterods(10millindiameter).Inthe measurementofcyclicvoltammograms,molybdenum(1.5 mmindiameter),nickel(2toniindiameter),andgraphite wereusedastheworkingelectrode.pseudoreferenceelec. trodes,andcounterelectrode,respectively.Inthestudyof polarizationataconstantpotential,titaniumf8n-lrnX 2mmx40n1wasusedastheworkingelectrodeandthe othertwoelectrodesweregraphite.Theelectrochemica1 measurementswereperformedbythePrincetonApplied ResearchModel273A.

    3.Resultsanddiscussion

    Fig.1showsthecyclicvoltammogramsofmoltenCaC12 andCaC12-0.5mo1%CaOatthemolybdenumelectrodeat

    1173K.InthecyclicvolmmmogramsofmoltenCaC12,the coupleofoxidationandreductioncurrentpeaksofCaC12are observed,andthecurrentchangesrapidlyasthepotential changes.Inthecyclicvoltammogramsofmolten CaCI2.0.5mo1%CaotheoxidationcurrentpeakofCaO appearsatamorepositivepotentialthanthatofCaC12. TheinitialreductionpotentialofCaOatl173Kis-1.34V vs.nicke1.andthepeakpotentialis-1.52Vvs.nicke1. BymeansofthispropertyofCaC12-CaO,Freydinaeta1. [14preparedPbCaalloysfrommoltenCaC12CaOatthe

    liquidleadcathode.ThereductioncurrentofCaOisgreater thanitscorrespondingoxidationcurrent,indicatingthatde

    positedcalciumisnotoxidizedcompletelyduringthere

    versescan.CalciumcandissolveinthemoltenCaCl,and thesolubilityofcalciuminthemoltensaltisashighas 5mo1%at1048K191.Calciumcatlalsoreactwithdis

    solvedoxygenandcarboninthemoltensalt,formingCaO [3,14]andCaC2[6].

    Fig.2showsthecyclicvoltammogramsofmolten CaCI2-NaCIandCaCl2-NaCICaOatmolybdenumelectrode

    at1173K.Inthecyclicvoltammogramsofmolten CaC1,-NaCI0.5mol%Ca0theoxidationandreduction peaksofCaOareobserved.Thereductionpeakpotential ofCaOinmoltenCaC1,.NaC10.5mo1%Ca0is-1.143V

    vs.nickelandisgreaterthanthatofCaOinmolten

    

    

    

    Int.Miner.Metall-Mater.,vot17,No.6,Dec2010 Efvs.N

    Fig.1.Cyclicvoltammogramsatmolybdenumelectrodein moltenCaCI2andCaCI2CaOat1173K(scanrate:0.2Ws).

    

    

    

    E}vs.N

    Fig.2.Cyclicvoltammogramsatmolybdenumelectrodein moltenCaCI2-NaCI-CaOat1173K(scanrate:0.1V/s). CaC12-0.5too1%CaO.Thissuggeststhattheattractionbe. tweenCaOandNaC1isweakerthanthatbetweenCaOand CaC1,.andCaOcanbeeasilydepositedbecauseofweakat- tractioninthemoltensalt.Thatis.NaClinducedtheunder- potentialelectrodepositionofCaOinmoltenCaCl2-NaC1. CaO.AstheconcentrationofCaOinmoltenCaCl2-NaCl decreasedfrom0.5mol%to0.3mo1%,thereductionpeak potentialofCaOdecreasesfrom-1.143to-1.268V.This processcanbeconfirmedbytheNernstequation. E=E+_R

    ZF

    Tl

    nac

    a

    2+(3)

    whereEisereductionpotentia1.Eethestandardreduc. tionpotential,Zthenumberofelectronstransferred,Fthe Faradayconstant,Rthegasconstant,Tthetemperature(K), andaC

a

    "theactivityofCaO.Boththeactivityandthe reductionpotentialofCaOincreasewiththeincreasein CaOconcentration.

    Fig.3showsthepeakcurrentdensityofthereductionof S.L.Wanget.CathodicbehaviorofmoltenCaCI2CaOandCaCI2-NaCI-CaO CaOinmoltenCaCI2.CaOasafunctionofthesquareroot ofscanrate.Thecurrentdensityincreaseswiththeincrease inscalarateandtheconcentrationofCaO.Thisrelationship Callbeexpressedasfollowing[20:

    jep=0.61((4)

    whereDistheiondiffusioncoefficient,c0theCaOconcen- tration(mol/L),andvthescanrate.Theaveragediffusion coefficientofCa2(CaO)at1173Kinthemoltensaltsis calculatedaS9.45x10-5cm2/sfromtheslopeofthestraight lineinFig.3.

    

    

    

    

    v(,,-s)

    Fig.3.Cathodicpeakcurrentdensitythesquarerootof

    scanrateinmoltenCaCI2-CaOat1173K.

    Fig.4showsthecyclicvoltammogramsofequimolar moltenCaCI2-NaC10.5mo1%Ca0at1123andll73K.re

    spectively.ereductionpeakpotentialofCaOmoved negativelyfrom-1.117t01.518Vwiththetemperature

    increasingetemperaturecoefficientofthereduction DeakpotentialofCaOiscalculatedas7.O×10-'Vfrom

    thereductionpeakpotentialsat1123and1173K.

    Thetemperaturecoemcientofthereductionpotentia1of CaOinmestandardstatecallbecalculatedbythermody- namics.ForthedecompositionreactionofCaO,assuming thestandardstatesofCaOandCaaretheirpuresubstances, thestandardfreeenergy,AG.,andthestandardcellpoten- tialCallbeexpressedbyF_qs.(5)and(6):

    CaO]=[Ca]+l/202,AG=593950+104.5T(5)

    AG=-ZFE.(6)

    BydifferentiatingEq.(6),thestandardtemperaturecoef- ficientisobtainedasshowninEq.(7).

    (=(7)

    793

    ForthedecompositionreactionofCaO.thestandard temperaturecoefficientis5.4×l0-V/Kandissmallerthan

    thattheabovemeasuredresuh.InmoltenCaCl2-NaCl

    O.5mo1%CaO.theconcentrationofCaOislowandCaOis mostlysurroundedbvCaC12andNaC1.Theelectrochemical behaviorofCaOinthemoltensaltdependsonthestructure andpropertiesofthemoltensaltandisdifferentfrompure CaO.Therefore,thetwotemperatureCOCfficientsaleofa significantdifference.

    

    言名

    lU

    E{vs.Ni

    F4.Cycficvoltammogramsatthemolybdenumelectrode inequimolarCaCI2-NaCI-0.5mo1%CaO(scanrate:0.1Ws). Fig.5showsthecathodicpolarizationc1.u'veofmolten CaCl2,CaC12.0.5mol%Ca0,andCaCl2NaCl-0.5mo1%Ca0

    atthetitaniumelectrodeatll73K.Thecathodicpolariza.

    tionofmoltenCaCl2startsat-2.42Vvs.graphite.andthe polarizationcurrentincreasesrapidlywhiletheapplied voltageisabove-2-42Vvs.graphite.Thecathodicpolali- zationofmoltenCaCI2.0.5mo1%CaOstartsat-1.18V. graphite,anddifferentfromthebehaviorofmoltenCaCI2, thepolarizationcurrentofmoltenCaC12-CaOincreases

    

    

    l0

    -

    E/V.graphi~

    Fig.5.PolarizationcurvesofmoltenCaCI2,CaCI2- O.5mo1%CaO.andCaCI2-NaCI-0.5mo1%CaOatthetitanium electrodeat1173K.

    gradually.ThepolarizationvoltageofmoltenCaCl2-NaC1. 0.5mo1%CaOis-0.93Vvs.graphiteandishigherthanthat ofmoltenCaCl'.Ca0.111eadditionofNaClinmolten CaC120.5moI%CaOincreasesthepolarizationvoltageof moltenCaCl,.Ca0.Thisconfirmstheconclusionobtained fromthecyclicvoltammogramsofmoltenCaC12-NaC1Cao

    thattheinteractionbetweenNaClandCaOisweakerthan atbetweenCaCl,andCaOinthemoltensalt.

    4.Conclusion

    Thecyclicvoltammogramsandthepolarizationcurvesof moltenCaCI2.CaC1,.CaOandCaCl,.NaClCaowerestud-

    ied.ThediffusioncoefficientofCfromCaOinmolten CaCl'_1.2mo1%CaOiscalculatedas9.45xl0-5cm%at l173K.1f11etemperaturecoefficientofthereductionpeak

potentialofCaOinequimolalmoltenCaCI2NaClis

    -

    7.0×lV/K.ThedecompositionvoltagesofCaC1,in moltenCaC1,,CaOinmoltenCaC12-0.5mo1%Caoand CaC1,.NaC10.5mo1%CaOaleobtainedfrompolarization curvesat-2.42,1.18,and0.93Vvs.graphite,respectively. ThecathodicbehaviorofCaOisdifferentfromCaC12.and CaOismoreeasilyreducedthanCaC12.CaOhasaweaker interactionwithNaClmanwithCaCl,andtheadditionof NaC1inmoltenCaCl,.CaOinducedtheunderpotentialelec

    trodepositionofCaO.

    References

    T.H.Okabe,M.Nakamura,T.Oishi,andK.Ono,Electro- chemicaldeoxidationoftitanium,MemlLTram.B,24(1993), p.449.

    C.Z.Chen,D.J.Fray,andT.W.Farthing,Directelectro

    chemicalreductionoftitaniumdioxidetotitaniuminmolten calciumchloride,Nature,407(2ooo),p.361. K.OnoandR.O.Suzuki,AnewconceptforproducingTi sponge:calciothermicreduction,JOM,54(2002),P.59. S.Q.JiaoandH.M.Zhu,Novelmetallurgicalprocessfortita- niumproduction,Mater.Res.,21(2006),P.2172. G.Z.Chen,E.Gordo,andD.J.Fray,Directelectrolyticprepa- rationofchromiumpowder,Me~H.Mater.Trans.B, 35(2004),p.223.

    M.F.Liu,Z.C.Guo,andW.C.Lu,Aninvestigationintoelec

    trochemicalreductionofTiO,pellet,Tram.1nst..Me~H. Sect.C,ll4(2005),p.87.

    /nLMiner.MetalLMater.,VoL17,No.6,Dec2010

[7]D.H.Wang,G.H.Qiu,x.B.Jin,eta1.,Electrochemicalmetal

    lizationofsolidterbiumoxide,Angew.Chem.1nt., 45(2006),p.2384.

    [8J.H.Du,Z.P.Xi,Q.Y.Li,eta1.,EffectofTiO2cathodeper- formanceonpreparationofTibyelectro-deoxidation,Tram. NonferrousMet.Soc.China,17(2007),P.s514.

    [9]X.L.Guo,Z.C.Guo,andZ,Wang,Directpreparation0fTiFe alloybyelectrolyticreductionfromTi02andFe203,Univ. Sci.Techno1.Beijing(inChinese),30(2008),No.6,p.620. [10C.Schwandt,D.T.L.Alexander,andD.J.Fray,Theelec- tro-deoxidationofporoustitaniumdioxideprecursorsin moltencalciumchlorideundercathodicpotentialcontrol, Electrochem.Acta,54(2009),P.3819

    [IlS.L.WangandY.J.Li,Reactionmechanismofdirectelec- trochemicalreductionoftitaniumdioxideinmoltentalcium chloride,ElecWoanaLChem.,571(2004),p.37.

    12c.SchwandtandD.Fray,Determinationofthekinetic

    pathwayintheelectrochemicalreductionoftitaniumdioxide inmoltencalciumchloride,Electrochem.Acta,51(2005), p.66.

    [13D.T.L.Alexander,C.SchwandLandD.J.Fray,Mierostruc? ruralkineticsofphasetransformationsduringelectrochemical reductionoftitaniumdioxideinmoltencalciumchloride, ActaMater.,54(2006),P.2933.

    [14E.B.FmydinaandD.J.Fray,SynthesisofPb-Caalloysby electrolysisofCaOinmoltensalts,Tram.1mt.Min,Metal1. Sect.G1l1(2002),P.C79.

    [15G.Z.ChenandD.J.Fray,Voltalnmetricstudiesoftheoxy- gen-titaniumbinarysysteminmoltencalciumchloride, E~cWochem.Soc.,249(2002),P.E455.

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