Neodymium electrowinning into copper-neodymium alloys by mixed oxide reduction in molten fluoride media

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Ciumag, Mihaela Raluca

and Gibilaro, Mathieu

and Massot, Laurent

and

Laucournet, Richard and Chamelot, Pierre

Neodymium electrowinning into

copper-neodymium alloys by mixed oxide reduction in molten fluoride media. (2016) Journal

of Fluorine Chemistry, 184. 1-7. ISSN 0022-1139

Official URL: https://doi.org/10.1016/j.jfluchem.2016.02.001

Open Archive Toulouse Archive Ouverte

Neodymium

electrowinning

into

copper-neodymium

alloys

by

mixed

oxide

reduction

in

molten

_fluoride

media

M.

Ciumag

a,b

_,

_M.

_Gibilaro

a,

_*

_,

_L.

_Massot

a

_,

_R.

_Laucournet

c

_,

_P.

_Chamelot

a

a_{LaboratoiredeGénieChimiqueUMRCNRS5503,UniversitéPaulSabatier,118routedeNarbonne,31069ToulouseCedex9,France} b_{CEA-TechMidi-Pyrénées,135avenuedeRangueil,31400Toulouse,France}

c_{CEAGrenobleLITEN,17ruedesMartyrs,38054GrenobleCedex9,France}

ABSTRACT

Thepossibility ofneodymiumelectrowinningfromneodymiumoxideusing areducingagent(RA) producedin-situbysolventreductionwasinvestigatedinmoltenLiF–CaF2–Li2Obetween900!Cand

1040!_{C.Nd2O3galvanostaticelectrolyseswereperformedandreactionproductswereanalyzedbySEM,}

XRDandelectron-probemicroscopy.Metallicneodymiumwasnotdirectlyobtained,duetoseveral limitationsforNd2O3reduction:lowelectricalconductivity,unfavourablePilling–Bedworthcoefficient

andformationofadenseinsulatingCaOlayeronthesamplesurface.

Consequently,thereductionofapelletmadeofaneodymiumoxide-metallicoxidemixturewascarried outasanalternativepathway.Nd2O3-CuOreductionledtometallicneodymiumproductionintheform

ofliquidCu-Ndalloys.Areactionmechanismwasproposedbasedontheseexperimentalresults:

1.Introduction

Needs of metallic neodymium and its alloys are lately increasing,particularly in the fields of magnetism, energy and hightechnology,asinpermanentmagnets,lampphosphorsand rechargeable NiMH batteries [1]. Neodymium is industrially produced by both calciothermic reduction of NdF3, requiring

severalpurificationsteps,and bymoltensaltelectrolysis,which enablescontinuousoperationandismoresuitableformassmetal production.

Metallicneodymiumelectrodepositioncantakeplaceinfused chloridesalts[2–13],butthisprocesshasseveraldrawbacks,such astheevolutionofchlorinegasattheanodeandlowfaradicyield. Thus,neodymiummetaliscurrentlyproducedbyelectrowinning fromneodymiumoxideinamoltenfluoridemediacontaininghigh amountsofNdF3(upto87mass%)[14].

Severalauthorssuggesteddifferentreductionmechanismsof Nd2O3inNdF3-basedmoltensalt[16–19],butnoagreementhas

been found yet. Stefanidaki et al. [15,16] detailed neodymium

production in LiF–Nd2O3 and LiF–NdF3–Nd2O3 systems. In LiF–

Nd2O3nocharacteristicsignalforneodymiumoxidereductioninto

metalwasobservedandmetallicneodymiumdepositiondidnot occurin this molten mixture.In LiF–NdF3–Nd2O3, fluoride and

oxyfluoride complexes such as [NdF6]3" and [NdOF5]4" were

proposedtobeformed,andmetallicneodymiumwasobserved. Thefollowingelectrontransfermechanismwassuggested: Cathode:[NdF6]3"+3e"!Nd(s)+6F"

Anode:3[NdOF5]4"!3/2O2(g)+3Nd3++15F"+6e"

The authors concludedthat electrowinning of metallic neo-dymiumcanbeperformedbycontrolledcell–voltageelectrolysis inLiF–NdF3–Nd2O3.

Thudumetal.[17]proposedadifferentreductionpathwayfor Nd2O3inLiF-CaF2-NdF3,suggestingboth[NdFO5]4"and[NdF6]3"

complexeswerereduced tometallic neodymium,dependingon theOF/Fmolarratio:forlowratios,[NdF6]3"wasreduced,whereas

aboveacriticalvalue,[NdFO5]4"wasreduced.

DysingerandMurphy[18]testedNd2O3reductioninLiF–CaF2–

1060!_{C, andhighlightedseveral side reactionsoccurringatthe}

anode:

Theauthorsconcludedmetallicneodymiumelectrowinningfrom LiF–CaF2–NdF3–Nd2O3wasonlypossibleathighcurrentdensity

(1Acm"2_{)andhightemperature(T>1021}!_C=Ndfusion

tempera-ture).

Fe–NdalloyswereobtainedbyMorriceet al.[19],byNd2O3

electrolysisonreactiveironcathodeat990!_CinLiF–NdF

3(89mass

%).Later,Nd2O3electrolysisinfusedfluoridesforFe–Ndfabrication

was patented by Pechiney [20]. Alloys were obtained by long duration (>25h) Nd2O3 electrolyzes on reactive iron cathode,

performedinLiF–NdF3–BaF2–B2O3,between750!and1000!C,and

atconstantcellvoltage.

Nowadays, neodymium is industrially produced by Nd2O3

reduction inNdF3-basedmixtures(NdF3>70mass%[21])using

Mo, W or Fe as cathodic materials [22]. Nevertheless, several drawbackscanbepointedoutforthisprocess:

– ThesolubilityofNd2O3isonly5mass%(at1100!C)[23],which

makestheoxidefeedingintotheelectrolytedifficulttocontrol andcanleadtoformationofsludgeatthecellbottom[24]. – Theoverallcostoftheindustrialprocessishighduetotheprice

ofNdF3($1000euros/kg).

Theaimofthisworkistoinvestigatealowercostproduction process for neodymium electrowinning from Nd2O3 in molten

fluoridemedia,inabsenceofNdF3(LiF–CaF2eutecticscontaining

Li2O2mass%),byanewapproach:theneodymiumoxideisusedas

solidpelletsatthecathode,similarlytotheFFC—Cambridge[25]

andOSprocesses[26],andnomoreasthefeedmaterial(current industrialprocess).Thetwomainadvantagesofthisapproachare: firstly, Nd2O3solubilitylimitation,encountered for the

conven-tionalelectrolyticsystem,iseliminated.Secondly,theoverallprice oftheprocessisreduced,duetothelowercostofLiF–CaF2–Li2O:

1kgofLiF–CaF2–Li2Omixturecosts200euros($60mass%LiFat

100euros/kg,$40mass%CaF2at60euros/kgand2mass%Li2Oat

4800euros/kg);while1kgofLiF–NdF3mixturecosts$750euros

($30 mass % LiF at 100euros/kg, and 70 mass % NdF3, at

1000euros/kg).

Withthisnewapproach,Nd2O3electroreductionwasstudiedby

linear voltammetry and galvanostatic electrolyses at different operating parameters:temperaturesfrom900!_Cto1040!_{C and}

imposed currents from 0.15 to 0.8A. Elemental analyses of compositionsandmicrostructures(SEM,XRDandelectron-probe microanalyser)wereperformed,andthelimitingfactorsforNd2O3

reduction intoNdinmolten_fluorides, inabsenceofNdF3,were

elucidatedforthefirsttime.

Nd2O3electroreductioninpresenceofanelectricalconducting

metallic oxide was also investigated.Indeed, this strategy was already shown to be suitable for preparing Nd–Co alloys by electrochemical reduction of Nd2O3–Co3O4 mixtures in molten

chloridemedia[27].Otherrareearthsalloyswithheavymetals, suchasLa–Ni[28],Ce–Co[29],Ce–Ni[30],Tb–Fe[31]andTb–Ni

[32],werealsoobtainedbyelectrochemicalreductionofrareearth oxidesandmetallicoxidesmixtures.

Inthisstudy,theelectrochemicalbehaviourofNd2O3–CuOwas

investigatedatdifferenttemperaturesbylinearvoltammetryand

constantcurrentelectrolyses.Analysisofmicrostructuresshowed formation of Cu–Nd alloys and allowed proposing a reduction pathway.ForhighpuritymetallicNdrecovery,afurtherseparation of Cu and Nd can be easily realised with electrorefining, by exploitingtheelectrochemicalpotentialsofthetwoconstituents. 2. Resultsanddiscussion

2.1.Preliminarydiscussionandsolventselection

Rareearth oxides can beelectrochemically reduced by two differentpathways,dependingontheirelectricalconductivity: 1.Highlyconductiveoxidesaresubjecttodirectelectrochemical

reductionbyFFC—Cambridgeprocess[25],wheretheoxideis reducedintometalonthecathodesurface.

2. Oxides presenting low electrical conductivity are subject to indirectreductionbyOSprocess[26].Inthiscase,areducing agent(producedbysolventreduction)reactswiththerareearth oxidetoformametallicphase.

Nd2O3 has a very low electrical conductivity (

s

650!_,0,2barO2= 1,45% 10"6

_V

"1_cm"1_{[33])thereforeitisexpectedtobereduced}

followingtheindirectOSprocess,asillustratedintheequation: Nd2O3+3xRA!2Nd+3RAxO (1)

whereRAisthereducingagentandRAxOistheoxidizedformof

thereducingagent.

The reducing agent (RA) couldbe an alkaline (K,Na, Li)or alkaline-earth metal (Ca) obtained by cathodic reduction of fluoride solvents (KF, NaF, LiF, CaF2). The Gibbs energy of the

reaction(

D

rG!)betweenneodymiumoxideandthereducingagent

indicateswhethertheneodymiumoxideisreducedintometallic neodymium or not: if

D

rG! is negative, the reduction of

neodymiumoxidebythealkalineoralkaline-earthis thermody-namicallyachievable.AspresentedinTable1,neodymiumoxide canbereducedintometallicneodymiumbyCaonly.

Nevertheless,becauseofCaF2toohighfusiontemperature(1

418!_{C), a lower working temperature was chosen: LiF–CaF}

2

eutectics(Tfusion=767!C).BecausethereductionpotentialsofLi+

andCa2+_areclose(ELi+_/Li=

"5.31VvsF2/F"andECa2+/Ca="5.33

VvsF2_/F"_{),theirsimultaneousreductionatthecathodesurface}

wasalreadyobserved[34].

Table1

StandardfreeenergydataforNd2O3reductionwithdifferentalkaliand alkaline-earthmetals,followingequation(1)at900!_C.

RA K Na Li Ca

2.2.ElectrochemicalcharacterizationinLiF–CaF2bylinearsweep

voltammetry

ToprovideO2"_{ionsinthemolten}

fluoridemediaandensurethe anodicreaction,Li2Owasused.Gibilaroetal.[35]showedinLiF–

CaF2–Li2Omixture,Li2Ohadtobemaintainedataconcentration

higherthan1mass%topreventtheAuanodeoxidationand to ensuretheoxidationofO2"_intogaseousO

2[36].

Inthiswork,Nd2O3reductionmechanismwasstudiedinLiF–

CaF2–Li2O(2mass%).Thelinearvoltammogramobtainedat10mV.

s"1_and900!_{C,ispresentedinFig.1,comparedwiththesolvent.}

Thetwocurvesdisplaynosignificantdifference,meaningthat nospecificNd2O3 electrochemicalreactionoccurs.Therefore,its

reduction isindirectand theproposedreaction pathwayisthe following:

Cathode:Ca2+_+2e"_{!Ca (2)}

Spontaneousredoxreaction:Nd2O3+3Ca!2Nd+3CaO (3)

Anode:2O2"

!O2+4e" (4)

The Ca reducing agent [34] produced on the Mo cathode reducestheneodymiumoxideintometallicneodymium,andthe resultedO2"_{ionsareoxidizedintogaseousO}

2onthegoldanode.

2.3.Galvanostaticelectrolysesofneodymiumoxide 2.3.1.Neodymiumoxidereduction

ToinvestigateNd2O3reduction,experimentswerecarriedout

inconstantcurrentmode.Becausestablereferenceelectrodesin

fluoridesaltsarenotavailableyet(noaccuratecontrolofcathodic potential), the constant potential mode was not used during electrolyzes.

Several reduction tests were performed in LiF–CaF2–Li2O

(2mass%) at900!_{C. ThetheoreticalchargeQ}

thneededfor the

reductionofoneneodymiumoxidepelletiscalculatedasfollows: Q_th¼mNd2O3

MNd2O3

nF ð5Þ

whereQthisthetheoreticalchargecalculated(C),mNd2O3isthe

oxidepelletweight(g),MNd2O3istheoxidemolecular weight

(336.48g.mol"1_{),nisthenumberofelectronsexchangedpermole}

ofNd2O3,FistheFaradayconstant(96,480Cmol"1).

The pellet cross-section after electrolysis at I="0.4A and t=1320s(500%Qth),observedbySEM,showsthreedifferentzones

(Fig.2).Quantitativeelectron-probemicroanalysisallowed iden-tifyingthecompositionofthesezones,asfollows:

– Theinternalzone(1)ismainlyNdOF,formedbyspontaneous decomplexationofNd2O3:

Nd2O3+2F"!2NdOF+O2" (6)

ThisphenomenonwasalsoobservedbyNourry[37]whenadding O2"_ionsin

LiF–CaF2–NdF3.

– Zone2isformedbyrecrystallized_fluoridesalts(LiFandCaF2).

– AdenselayermainlycomposedoflowelectricalconductingCaO, formedbyO2"_andCa2+_{co-precipitation,isobservedoverallthe}

surfaceoftheneodymiumoxidepellet(zone3).TheCaOlayer actsasabarrier, limitingthediffusionofO2"_{fromthepellet}

towardsthemoltensalt,preventingtheNd2O3reductiontotake

place.ThisphenomenonexplainstheabsenceofmetallicNdat theendofelectrolysis.

2.3.2.Temperatureinfluence

ToincreaseCaOsolubilityinLiF-CaF2(from0.5mol%at730!C

to2.9mol%at1000!_{C,[38])andtoattempttoremovethelayeron}

the sample surface, experiments were performed at higher temperature,whereliquidmetallicneodymiumcouldbeformed.

Fig. 3 illustrates the cross-section of Nd2O3 sample after

electrolysisat1040!_{Cand"0.4A(500%Q}

th).

Asexpected,noCaOlayerwasformedonthesamplesurface, but X-ray diffractommetry (Fig. 4) didnot highlight either the presenceofmetallicNd.

WhencomparingFig.2(900!_{C)andFig.3(1040}!_C),itcanbe

pointedout thatthepelletstructureand compositionchanged: Nd2O3chemicaldecomplexationat900!CledtoNdOF,whileat1

040!_{C Nd}

2O3 clusters appeared, enhancing significantly the

Fig. 1.LinearsweepvoltammogramsonMogridinLiF–CaF2–Li2O(2mass%Li2O)at 10mVs"1_and900!_{C.Continuousline:solvent,dottedline:Nd2O3pellet.}

porosity. This phenomena was also confirmed by the X-ray diffractogram (Fig. 4) where Nd2O3, LiF and CaF2 are present,

butnoNdOFwasdetected.

To investigate the temperature effect, a Nd2O3 pellet was

exposedat1040!_{Cinthemoltensaltbathwithoutpolarisation,}

andSEManalysisshowedsimilarstructurestothosepresentedin

Fig.3.Batsanovetal.[39]hadalreadyobservedasimilarbehaviour of neodymiumoxide powder (grain size 1–10

m

m) under high pressure and temperature, where 100–1000

m

m crystalline aggregateswerecreated.

Theinfluenceoftheappliedcathodiccurrent(from 0.15Ato 0.8A)andneodymiumoxidepelletsfabrication(pressureapplied for sinterizing the oxide powder from 2 to 4tcm"2_{) was also}

studied,butmetallicneodymiumwasstillnotobtained.

To resume, Nd2O3 electrochemical reductiondidnot lead to

metallicneodymium,whatevertheoperatingparameters. More-over,severallimitingphenomenawerehighlighted:

1RelatedtoNd2O3physicochemicalcharacteristics:

- low electrical conductivity (

s

650!C,0,2barO2=1,45% 10"6

V

"1

cm"1_)[33];

- unfavourable Pilling-Bedworth coefficient (VNd/VNd2O3=0.89).

Pilling–Bedworthruleconsiderations[40],asillustratedbyLi etal. [41]and Gibilaroetal. [42],indicatethat duringoxide reduction,ifthemolarvolumeoftheformedmetalVmissmaller thanthemolarvolumeoftheoxideVo,themetalobtainedis porousenoughtoallowthemoltensaltelectrolyteaccessingthe underlying oxide. For neodymium, themetal tooxide molar volume ratio is VNd/V Nd2O3=0.89, meaning that volume

constrictionisnotenoughduringNd2O3conversionintometal:

themetallicneodymiumformedwouldthusactasadiffusion barrier.

2Relatedtosolvent:

- neodymiumoxidecanonlybereducedbyCametal,limitingthe selectiontoCaF2-basedsolvents;

- inCaF2-based solvent, at900!C, theprecipitationof a dense

insoluble CaO layer on the sample surface is observed, preventingNd2O3reduction.

2.4.Reductionofneodymiumoxideinpresenceofcopperoxide ToobtainmetallicNd,theadditionofanelectricalconducting oxideintheneodymiumoxidepelletwastested.CuOwaschosen foritsgoodelectricalconductivity(

s

127!_C=10"1

V

"1cm"1[43]) and its favourable Pilling–Bedworth coefficient VCu/VCuO=0.56

Fig.4. X-raydiffractogramofNd2O3pelletafterelectrolysis(I="0.4A,500%Qth)at1040!CinLiF–CaF2–Li2O(2mass%). Fig.3.MicrographofNd2O3pelletcross-sectionafterelectrolysis(I="0.4A,500%

[36], enhancing pellet porosity. CuO is therefore expected to undergodirectelectrochemicalreductionintometallicCu,andto confirmit,CuOreductionin LiF–CaF2–Li2O(2mass%) wasfirst

studied. The linear voltammogram obtained at 10mVs"1

(vol-tammogramcinFig.5,I)displaysareductionsignalat"1.3VvsPt, assigned to the direct electrochemical reduction of CuO into metallicCu,followingtheequation:

CuO+2e"_!Cu+O2" ₍₇₎

Moreover,theelectrolysisof aCuOpelletat"0.15Aand 900!_C

confirmedthis result: 100

m

mto1000

m

mmetallic Cu clusters (micrographinFig.5,II)wereobtained.

The current efficiency was also calculated, following the equation:

h

¼mexperimental mtheoretical

wheremexperimentalisthemassofthemetallicdepositobtainedby

electrolysis(g)andmtheoreticalisthetheoreticalmassofmetallic

depositexpectedtobeobtainedbyelectrolysis(g),calculatedas following:

m_theoretical¼I)t!

n)F)Mmetal

whereIistheintensityoftheimposedcurrent(A),tiselectrolysis duration(s),nisthenumberofexchangedelectronspermoleof metallic oxide (n=2 for CuO!Cu reduction), F is the Farady constant(96480Cmol"1_{), and M}

metalis theatomicmassofthe

metal(forCu,M=63,54gmol"1_).

The calculated current efficiency for CuO reduction into metallicCuwasverycloseto100%.

Nd2O3reductioninpresenceofCuOwasfurtherinvestigated.

Cu–Ndphasediagram[44]presentsseveralliquidandsolidalloys, andcompositionofNd2O3–CuOpelletswaschosentobeinthe

zonewhereliquidalloysareformed:65atomic%Ndand35atomic %Cu.ThelinearvoltammogramofNd2O3–CuOat10mVs"1inLiF–

CaF2–Li2O(2mass%)(voltammogrambinFig.5,I)showsalsoa

reduction signal starting from "1.4V vs Pt, due to the direct electrochemical reduction of CuO. Additional cathodic signals, between"1.0and"1.3VvsPt,wereassignedtoimpuritiespresent inthesolvent(comparedtovoltammogramainFig.5,I)andinthe 99%purityCuO(comparisonwithvoltammogramcin Fig.5,I). Afterelectrolysis at"0.15A (500% Qth)and 900!C, thesample

presentedseveralzones(Fig.5,III):

– Zone 1, composed of NdCu4, NdCu5 and NdCu6 droplets

(size< 50

m

m);

– Zone2,composedofrecrystallizedLiFandCaF2;

– Zone3,composed ofNdOFandCaO,but nocompactlayeris observedonsamplesurface.

TheseresultsconfirmNd2O3reductionispossiblewhenCuOis

simultaneouslypresent,leadingtoformationofCu–Ndalloys,and amulti-stepreductionmechanismwasproposed,asfollows:

1ststep:ChemicaldecomplexationofNd2O3intoNdOF:

Nd2O3+2F"!2NdOF+O2" (6)

2nd step: CuO direct reduction at the cathode, leading to metallicCu,behavingasreactivecathode:

Mocathode:CuO+2e"

!Cu+O2"_{+porosity (7)}

3rdstep:NdOFreductiononthesurfaceofmetallicCu,leading toCu-Ndalloys:

Metallic Cu reactive cathode: Cu+NdOF+3e"_!Cu–Nd+O2"₊

F" ₍₈₎

Thethirdstepofthismechanismwasconfirmedbyelectrolysesof Nd2O3-Cu metallic powder mixture, which also led to the

formation ofCu–Nd alloys.Nourryetal. presentedalso similar results, showing that Nd3+_{reduction oncopper cathodeled to}

formationofCu–Ndalloys[37,45,46].

Fig.5. ILinearsweepvoltammogramsinLiF–CaF2–Li2O(2mass%)at10mVs"1and900!C.Dottedline:solvent,continuousline:Nd2O3–CuO,interruptedline:CuO.II,III Micrographsofpelletscross-sectionsafterelectrolyses(I="0.15A,500%Qth)at900!CinLiF–CaF2–Li2O(2mass%).II.CuO;III.Nd2O3–CuO:65atomic%Nd"35atomic%Cu.

Several authorsprovedthereductionof rareearth oxidesin presenceofmetallicoxidestobesuitableforobtainingrareearth alloysinmoltenchlorides.Moreover,theyalsoproposedreduction mechanismswherethemetallicoxide(Co3O4,NiO,Co3O4,Fe2O3)

wasfirstreducedonthecathode,followedbythereductionofrare earthoxideandformationofalloys[27–32].

ThecurrentefficienciesforNd2O3reductioninpresenceofCuO

ormetallicCupowdercouldnotbeestimated,duetothelackof homogeneityof thesample andthe smallsizeof Cu–Ndalloys droplets (<50

m

mdiameter,Fig.5,III),whichcouldnotbefully recoveredtobeweighted.

Nevertheless,whenNd2O3–CuOelectrolyseswereperformedat

higher temperature (1040!_{C), coalescing of Cu–Nd droplets}

occurred (size$500

m

m to 2mm), facilitating theirrecovery at theendofelectrolysis.Inthiscase,theglobalcurrentefficiency couldbeestimatedusingtheequation(8)anditsvaluewas$70%. The remaining30% current fractioncould berelatedtosolvent reduction(Ca+andLi+)anduncompleterecoveryandseparationof Cu–Ndalloysfromthesalt.

Thereductionofneodymiumoxideinpresenceofcopperoxide presentsseveraladvantages:

- CuO directreductionledtocreationofporosityin thepellet, enablingthepenetrationofmoltensaltinthebulkofthesample and thusenhancingtheinteractionbetweenthesolvent, the metallicCuobtainedbyreductionandtheneodymiumoxide; - Underpolarization,themetalliccopperobtainedbyCuOdirect

reduction behaves as reactive cathodic material, leading to formationofCu–Ndalloys.

TorecoverhighpuritymetallicNd,CuandNdseparationcanbe easilyaccomplishedbytheelectrorefiningprocess,duetothelarge differencesoftheirelectrochemicalpotentials:inLiF–CaF2at840!

C,ECu2+/Cu=2.83V/Li+/LiandENd3+/Nd=0.35V/Li+/Li[37].Thus,Nd

fromCu-NdalloyscanbeselectivelyoxidizedintoNd(III),which willbefurtherdepositedashighpuritymetallicNdonthecathode. Meanwhile, pure metallic Cu remaining at the anode can be recycledandreusedforNd2O3reduction.

3. Conclusions

Inmoltenfluoridemedia,thereductionofNd2O3wasshownto

beindirect,similartotheOSprocess.Nevertheless,reductionof neodymiumoxideintometallicneodymiumdidnotoccurwhen usinganinertcathodicmaterial(Mo),regardlessoftheoperating parameters(temperature,current,fabricationoftheoxidepellet). Thelimitingfactorswererelatedtothe:

1.PhysicalcharacteristicsofNd2O3:lowconductivityand

unfav-ourablePilling-Bedworthcoefficient.

2. Solvent: Indirect reduction of Nd2O3 in molten salts is

thermodynamicallyachievable(

D

rG!< 0)forCa-basedsolvents only,whenmetallicCaisobtained.However,adenseinsoluble CaO layer was formed overall the sample surface, acting as diffusionbarrierforO2"_{,preventingthereductionoftheoxide}

pellet.

Moreover,Nd2O3behaviorwasinfluencedbytemperature:at

900!_{C, the oxide was decomplexed into insoluble chemically}

stable NdOF, while at 1040!_{C it ledto formation ofcrystalline}

aggregates. Thus, electrowinning of metallic neodymium from neodymiumoxidecannotbeperformedinmoltenfluoridemedia inabsenceofNdF3.

Addition of a conductive metallic oxide to the neodymium oxidewasfurtherinvestigated.WhenNd2O3–CuOandNd2O3–Cu

pelletswereelectrolyzed,metallicneodymiumwasrecoveredas

alloyedCu–Nddroplets,andathighertemperature(T>1040!_C)a

bettercoalescencewasobserved.Thefollowingreduction mecha-nismwasproposed:

TheCu,initiallypresentinmixturewithNd2O3orproducedby

CuOdirectelectroreduction,actsasadepolarizingreactivecathode forNd3+_{(NdOF)reduction,leadingto}

Cu–Ndalloys.Thisstrategyis theonlyoneviable forobtainingmetallicNdfromneodymium oxideinmolten_fluorideswithoutNdF3.Furthermore,highpurity

metallicNdcanberecoveredbyasimpleelectrorefiningprocess, while the metallic Cu can be recycled and reused for Nd2O3

reduction. 4. Experimental

Thecelldesignconsistedinavitreouscarboncrucibleplacedin acylindricalvesselmadeofrefractorysteel.Theinnerpartofthe cellwallwasprotectedagainst_fluoridevapoursbyagraphiteliner. Experimentswereperformedunderinertargonatmosphere.The moltensalt(200g)wascomposedofLiF-CaF2eutectic,dehydrated

byheatingundervacuumfromroomtemperatureuptomelting point(767!_C).Li

2Opowder(Cerac99.5%)wasusedtoprovideO2"

ionsinthebath,toensuretheanodicreaction(2O2"

!O2+4e").

Nd2O3(Aldrich99.9%),CuO(Merck99%)andCumetallicpowder

(Goodfellow99.8%)wereusedintheformofpelletssinteredby applyingapressureof3.2tcm"2_{toseveralmilligrams(from100to}

300mg) of powder, at 25!_{C. The pellets, attached with a}

molybdenumgridconnectedtothecurrentleadbyamolybdenum wire,wereusedasworkingelectrodes.Theauxiliaryelectrodewas agoldwireorplate,withalargesurfacearea(S=3cm2_),andall

potentialswerereferredtoa platinumwire (0.5mmdiameter), actingasaquasi-referenceelectrodePt/PtOx/O2".

The electrochemical experiments were performed with an AutolabPGSTAT 30potentiostat-galvanostat.Afterresin embed-ding and polishing, the cathode bulk was examined with a scanningelectronmicroscopeSEM(LEO435VP)equippedwith and EDS probe (Oxford INCA200). XRDcharacterisations were performed with an Equinox 1000 diffractometer. Quantitative analysis wereperformed witha Cameca SXFive electronprobe microanalyser.Nevertheless,neitherSEM-EDSnorelectron-probe microanalysisdonotprovidedataonLi.Meanwhile,SEM-EDSdo notprovidedataanalysisonlightelementssuchasfluorideand oxygen.

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