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

HAL Id: hal-01890092

https://hal.archives-ouvertes.fr/hal-01890092

Submitted on 8 Oct 2018

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Neodymium electrowinning into copper-neodymium

alloys by mixed oxide reduction in molten fluoride media

Mihaela Raluca Ciumag, Mathieu Gibilaro, Laurent Massot, Richard

Laucournet, Pierre Chamelot

To cite this version:

Mihaela Raluca Ciumag, Mathieu Gibilaro, Laurent Massot, Richard Laucournet, Pierre Chamelot.

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

media. Journal of Fluorine Chemistry, Elsevier, 2016, 184, pp.1-7. �10.1016/j.jfluchem.2016.02.001�.

�hal-01890092�

OATAO is an open access repository that collects the work of Toulouse

researchers and makes it freely available over the web where possible

Any correspondence concerning this service should be sent

to the repository administrator:

tech-oatao@listes-diff.inp-toulouse.fr

This is an author’s version published in: http://oatao.univ-toulouse.fr/20547

To cite this version:

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

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– NdF3 on tungsten cathode, for temperatures from 1030!C to

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

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.

References

[1]K.Binnemans,P.T.Jones,B.Blanpain,T.VanGerven,Y.Yang,A.Walton,M. Buchert,J.CleanerProd.51(2013)1–22.

[2]E.Morrice,E.S.Shed,T.A.Henrie,U.S.Bur.MinesRep.Invest.7146(1968). [3]E.Morrice,M.M.Wong,Miner.Sci.Eng.11(1979)125–136.

[4]Y.Qiqin,L.Guanqun,F.Zhongan,S.Lichang,RareMet.8(1989)9. [5]M.F.Chambers,J.E.Murphy,U.S.Bur.MinesRep.Invest.9391(1991). [6]A.Kaneko,Y.Yamamoto,C.Okada,J.AlloysCompd.193(1993)44–46. [7]C.Shiguan,Y.Xiaoyong,Yu.Zhongwing,Lu.Qingtao,RareMet.13(1994)46. [8]J.E.Murphy,D.K.Dysinger,M.F.Chambers,LightMet.(1995)1313–1320. [9] D.W.Dees,J.P.Ackerman,UniversityofChicago,USPatent0045835A1,(2004). [10]Y.H.Kang,S.C.Hwang,H.S.Lee,E.H.Kim,J.Met.Process.Technol.209(2009)

5008–5013.

[11] R.Fujita,H.Nakamura,K.Mizuguchi,S.Kanamura,T.Omori,K.Utsunomyia,S. Nomura,KabushikiKaishaToshiba,USPatent0314260A1,(2010). [12] B.H.Park,I.C.Choi,J.-M.Hur,J.Chem.Eng.Japan45(2012)888–892. [13]E.-Y.Choi,J.W.Lee,J.J.Park,J.-M.Hur,J.-K.Kim,Chem.Eng.J.514(2012)207–

208.

[14]S.Seetharaman(Ed.),TreatiseonProcessMetallurgy,vol.3,Elsevier,2014,pp. 995–1069.

[15]E.Stefanidaki,C.Hasiotis,C.Kontoyannis,Electrochim.Acta46(2001)2665– 2670.

[16]E.Stefanidaki,G.M.Photiadis,C.Kontoyannis,A.Vik,T.Østvold,J.Chem.Soc. (2002)2302–2307.

[17]R.Thudum,A.Srivastava,S.Nandi,A.Nagaraj,R.Shekhar,Min.Process.Extr. Metall.(Trans.Inst.Min.Metall.C)119(2010)88–92.

[18]D.K.Dysinger,J.E.Murphy,USBur.MinesRep.Invest.9504(1994). [19]E.Morrice,E.Shedd,T.Henrie,USBur.MinesRep.Invest.7146(1968). [20] Y.Bertaud,AluminiumPechiney,EuropeanPatentEP0289434(A1),1988. [21]D.Chen,SolutionofNeodymiumandformationofslimeduringthe

neodymiumelectrolysis,J.RareMet.32(2008)482–483Translatedfrom ChinesebyJ.Wang,inChinese.

[22]W.Li,Utilizationrateofneodymiumoxideinproducingmetallicneodymium, Non-ferrousSmelting4(50)(2001)35–36TranslatedfromChinesebyJ.Wang. [23]E.Morrice,R.G.Reddy,Solubilityofrareearthoxidesinfluoridemelts,

SymposiumonHighTemperatureandMaterialsChemistry,Berkeley, California,1989.

[24]X.Guo,J.Sietsma,Y.Yang,1stEuropeanRareEarthResourcesConference2014, MilosIsland,Greece,4–7September,2014,pp.149.

[25]G.Z.Chen,D.J.Fray,T.W.Farthing,Nature407(2000)361–364. [26]K.Ono,R.O.Suzuki,J.Min.MetalsMat.Soc.54(2002)59–61. [27]A.M.Abdelkader,D.J.Hyslop,A.Cox,D.J.Fray,J.Mater.Chem.20(2010)

6039–6049.

[28]B.J.Zhao,L.Wang,L.Dai,G.G.Cui,H.Z.Zhou,J.AlloysCompd.468(2009) 379–385.

[29]L.Dai,S.Wang,Y.-H.Li,L.Wang,Trans.NonferrousMet.Soc.China22(2012) 2007–2013.

[30]Y.Zhang,H.Yin,S.Zhang,D.Tang,Z.Yuan,T.Yan,W.Zheng,J.RareEarths30 (2012)923–927.

[31]G.Qiu,D.Wang,M.Ma,X.Jin,G.Z.Chen,J.Electroanal.Chem.589(2006) 139–147.

[33]K.A.GschneiderJr.,L.Eyring,HandbookonthePhysicsandChemistryofRare EarthsCh.27,North-HollandPublishingComp.,1979,pp.386.

[34]M.Gibilaro,S.Bolmont,L.Massot,P.Chamelot,J.Electroanal.Chem.726(2014) 84–90.

[35]M.Gibilaro,L.Cassayre,O.Lemoine,L.Massot,P.Chamelot,J.Nucl.Mater.414 (2011)169–173.

[36]L.Massot,L.Cassayre,P.Chamelot,P.Taxil,J.Electroanal.Chem.606(2007)17– 23.

[37]C.Nourry,Thèsededoctoratdel’UniversitéPaulSabatier,Toulouse(2007)51. [38]D.-G.Kim,M.-A.VanEnde,C.Liebske,C.vanHoek,S.vanderLaan,P.Hudon, I.-H. Jung,9thInternationalConferenceonMoltenSlags,FluxesandSalts,Beijing, China,27–30May,2012,pp.110.

[39]S.S.Batsanov,A.A.Deribas,FizikaGoreniyaIVzryva1(1965)103–108English version:Combustion,ExplosionandShockWaves1(1965)77–80(10.1007/ 2FB00757157)..

[40]N.B.Pilling,R.E.Bedworth,J.Inst.Met.29(1923)529–591. [41]W.Li,X.Jin,F.Huang,Angew.Chem.Int.Ed.49(2010)3203–3206. [42]M.Gibilaro,J.Pivato,L.Cassayre,L.Massot,P.Chamelot,Electrochim.Acta56

(2011)5410–5415.

[43] A.A.Samokhvalov,N.A.Viglin,B.A.Gizhevskîi,N.N.Loshkareva,V.V.Osipov,N. I. Solin,Y.P.Sukhorukov,Zh.Eksp.Teor.Fiz.103(1993)951–961English version:J.Exp.Theor.Phys.76(1993)462–468(http://www.jetp.ac.ru/cgi-bin/ dn/e_076_03_0463.pdf)..

[44] BinaryAlloyPhaseDiagrams,S.E.,ASMInternational,1996.

[45]C.Nourry,L.Massot,P.Chamelot,P.Taxil,J.Appl.Electrochem.39(2009) 927–933.

[46]C.Nourry,L.Massot,P.Chamelot,P.Taxil,J.Appl.Electrochem.39(2009) 2359–2367.