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Silicon recovery from silicon-iron alloys by
electrorefining in molten fluorides
Laurent Massot, Anne-Laure Bieber, Mathieu Gibilaro, Laurent Cassayre,
Pierre Taxil, Pierre Chamelot
To cite this version:
Laurent Massot, Anne-Laure Bieber, Mathieu Gibilaro, Laurent Cassayre, Pierre Taxil, et al.. Silicon
recovery from silicon-iron alloys by electrorefining in molten fluorides. Electrochimica Acta, Elsevier,
2013, vol. 96, pp. 97-102. �10.1016/j.electacta.2013.02.065�. �hal-00805734�
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Eprints ID : 8620
To link to this article : DOI: 10. 1016/j.electacta.2013.02.065
URL : http://dx.doi.org/10.1016/j.electacta.2013.02.065
To cite this version : Massot, Laurent and Bieber, Anne-laure and
Gibilaro, Mathieu and Cassayre, Laurent and Taxil, Pierre and
Chamelot, Pierre Silicon recovery from silicon-iron alloys by
electrorefining in molten fluorides. (2013) Electrochimica Acta,
vol. 96 . pp. 97-102. ISSN 0013-4686
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㩷
Silicon
recovery
from
silicon–iron
alloys
by
electrorefining
in
molten
fluorides
L.
Massot
∗, A.L.
Bieber, M.
Gibilaro, L.
Cassayre, P.
Taxil, P.
Chamelot
UniversitédeToulouse;UPS,INP,CNRS;LaboratoiredeGénieChimique;DépartementProcédésElectrochimiques;118RoutedeNarbonne,31062Toulouse,France
Keywords: Moltenfluorides Silicon Electrorefining Si–Fealloys Voltammetry
a
b
s
t
r
a
c
t
Electrorefiningofasilicon-ironmaterial(Si–4.7at%Fe)inmoltenNaF-KFat850◦Cwasinvestigatedin
viewofrecoveringpureSi,usingelectrochemicaltechniques,ScanningElectronMicroscopycoupled withEnergyDispersiveSpectroscopy(SEM-EDS)andInductivelyCoupledPlasmawithatomicemission spectroscopy(ICP-AES)analyses.TheselectiveelectrochemicaldissolutionofSiwasevidenced. Elec-trorefiningrunsledtoamaximumrecoveryof80%ofSiinitiallycontainedinthematerial,intheform ofadensedepositatthecathode,withveryhighcurrentefficiencies.TheSipuritywasexaminedandno FewasdetectedbyICP-AESanalysis:therecoveredSipuritywasassumedtobehigherthan99.99%.
1. Introduction
Photovoltaictechnologyisaveryimportantrenewablesource
ofelectricalenergyproductionwiththehighestannualgrowthrate
[1,2]andthemostcommonbasematerialforphotovoltaiccellis
SolarGradeSilicon(SoG-Si),witharequiredpurityof99.9999%.Due
toagrowingdemandforSoG-Si,thedevelopmentofnewprocesses
allowingtheproductionofcheaperSoG-Siisamainissueforthe
solarenergyindustry[3].Inthe1980’s,siliconelectrodepositionin
moltensaltshasbeenconsideredasanattractiveoptionforSoG-Si
production[4].However,theelectrolyticSiproductionprocesshas
notfoundanycommercialapplicationuptonow.Intheframeof
morerecentconcernsaboutcarbon-freeenergyproduction,such
kindofprocesshasneverthelessbeenreassessed.Thus,basic
inves-tigationsonSi(IV)reductionmechanismhavebeencarriedoutin
moltenfluorides[5–14]and more recently,thesaltsproperties
andsilicondepositionstrategieswereexaminedinourlaboratory
[15,16].TheseresultsconcludedthatSi(IV)ionsarereducedinto
Siinaone-stepprocessexchangingfourelectronscontrolledby
diffusioninthesolution.
Thefinalobjectiveofthestudyistoevaluatethefeasibilityof
usingelectrorefining processin moltenfluoridesas preliminary
stepof purificationofmetallurgicalgradeSi(MG-Si)withgood
quality.
Table1comparesthecompositionofMG-Siandthemaximum
admissibleimpuritycontentinSoG-Si.Thesedatashowfirstthat
∗ Correspondingauthor.Tel.:+33561558194;fax:+33561556139. E-mailaddress:[email protected](L.Massot).
oxygenandironarethemostimportantpollutantsofMG-Siand
then,that thehighesttolerance inSoG-Siis forcarbon, oxygen
andiron[17–19].Inthiswork,onlymetallicimpurities(Mi)were
takenintoaccount.Morespecifically,thepresentworkisfocused
ontheseparationofsiliconandironbyelectrolysisatlaboratory
scale,inamoltenfluoridemixture,throughthedissolutionofa
Si–Feanode,withtheoperatingconditionsdefinedinreferences
[15,16].Thestudywasdividedintothreeparts:(i)determinationof
theSi–Fedissolutionpathwaybyelectrochemicaltechniques
cou-pledwithSEMobservationsandEDSanalysis,(ii)silicondeposits
purityexaminationwithEDSandICP-AESanalysesand(iii)anodic
andcathodiccurrentefficienciesevaluationbyweightingthe
elec-trodes.
2. Experimental
2.1. Cell
Thecellwasavitreouscarboncrucibleplacedinacylindrical
vesselmadeofrefractorysteelandclosedbyastainlesssteellid
cooledbycirculatingwater.Thedescriptionofthiscellhasbeen
detailedinpreviouswork[20].Theexperimentswereperformed
underaninertargonatmosphere(Linde).Thecellwasheatedusing
aprogrammablefurnaceandthetemperaturewasmeasuredusing
a chromel-alumelthermocouple. Theelectrolyticbathconsisted
oftheNaF-KF(40–60mol%)eutecticmixture(CarloErba99.99%).
Themixturewasinitiallydehydratedbyheatingunder vacuum
(6×10−2mbar)fromambienttemperatureuptoitsmeltingpoint
Table1
ComparisonbetweenimpuritiescontentinMG-Siandmaximumadmissiblecontent (ppm)inSoG-Si.
Impurities MG-Si SoG-Si(Maximumcontent)
B 20–50 <1 P 25–50 <1 O 3000 <10 C 10–250 <10 Fe 1000–3000 <10 Al,Ca 100–1500 <2
Othermetals(Ti,Cr, Mn,Ni,Cu,Zr...)
5–150 <1
theformofsodiumhexafluorosilicateNa2SiF6(AlfaAesar99.99%)
powder.
2.2. Electrodes
Iron, copper, nickel, titanium, silicon plate
(4mm×8mm×11mm) (Goodfellow 99.99%), molybdenum
(Goodfellow 99.95%), chromium, manganese and zirconium
(Goodfellow99.5%)wires(1mmdiameter)and95.3at%Si–4.7at%
Fealloy(8mmdiameter)wereusedaselectrodes.Thesurfacearea
oftheworkingelectrodewasdeterminedaftereach experiment
bymeasuringtheimmersiondepthinthebath.Theauxiliary
elec-trodes were silicon plate(4mm×8mm×11mm)and graphite
carbonrods(3mmdiameter–Mersenspectroscopicquality)with
a largesurfacearea(3cm2).Aplatinum wireimmersed inthe
moltenelectrolytewasusedasaquasi-referenceelectrode.
2.3. Techniques
Theelectrochemicaltechniqueusedfortheanodicbehaviour
investigationofeachmaterialwaslinearvoltammetry,atascan
rateof10mVs−1,whileelectrorefiningrunswerecarriedoutby
impositionofaconstantcathodiccurrent.Alltheelectrochemical
polarizationswereperformedwithanAutolabPGSTAT30
poten-tiostat/galvanostatcontrolledbya computerusingtheresearch
softwareGPES4.9.
The electrodessurface and cross-section werecharacterized
byScanningElectronMicroscope(SEM)coupledwithEnergy
Dis-persiveSpectroscopy (EDS)probe,afterresidualsalt removalin
HCl–AlCl3–H2Omixtureat50◦Candultrasonicwaves.
SiandFecontentofbothsaltanddepositsampleswere
mea-sured,afterdissolution,usingInductivelyCoupledPlasma-Atomic
EmissionSpectroscopy(ICP-AES,HORIBAUltima2).
3. Resultsanddiscussion
3.1. Operatingconditionsforsiliconelectrorefining
Electrorefiningisamethodappliedforpurifyingmetalsusingan
electrolysiscell.Itconsistsinananodicdissolutionofthematerial
tobepurifiedandadepositionofthetargetmetalwithout
impu-ritiesonthecathode. Threemainissueshavetobeconsidered:
theanodicbehaviourofthematerial,thestabilityofthedissolved
speciesinthemoltensaltandtheelectrodepositionconditionsat
thecathode.In thespecificcaseof Sielectrorefining, twosteps
(Si(IV)stabilityin thesalt andSideposition) havealreadybeen
investigatedinourlaboratory[15,16],asdetailedbelow,whilethe
Siselectivedissolutionremainstobedemonstrated.
ConcerningtheSi(IV)stability,ithasbeenshownthatinmolten
fluoridesalts,Si(IV)ionsaresolvatedasSiF4+xx−inequilibriumwith
gaseousSiF4,accordingtotheequilibrium(1).
SiF4+xx−= SiF4(g)+xF− (1) -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 -1.5 -1.0 -0.5 0.0 0.5 1.0 j/ A cm -2 E vs.Pt/V Fe MnMn FeCr Ni Mo Zr Si Ti Cu
Fig.1.Comparisonoflinearvoltammograms(anodicpart)onvariousmetalsat 10mVs−1inNaF-KFat850◦
C;Aux.El.:vitreouscarbon;RefEl.:Pt.
Bieberetal.showsthatSiF4+xx−stabilityishighlyinfluenced
by thefluoroacidity (free F− ions content) and that SiF
4+xx− is
more stable in basic molten baths: themost suitable one was
proventobetheeutectic NaF-KF(40–60mol%)mixture[15].In
ordertominimizeSilossesin thegaseous phase,thissalt
mix-ture(Tmelting=718◦C)hasthusbeenselectedfortheelectrorefining
runs.
IthasbeenstatedthattheelectrochemicalreductionofSi(IV)
ionsis aone-stepprocessexchanging4electrons,whateverthe
fluoridesaltcomposition,asindicatedinreaction(2).
Si(IV)+4e−
= Si (2)
Adetailednucleationandelectrodepositionstudyhasbeen
per-formed[16]andledtothefollowingconclusions:
-anoperatingtemperatureof850◦C,about130◦Cabovethe
sol-ventmeltingpoint,allowstoenhanceSielectricalconductivity,
-graphiteorSoG-Sicathodicsubstratesensurethebestadherence
toSideposits,
-aSi(IV)concentrationhigherthan 0.6molkg−1 inhibits
potas-siuminsertionintothedepositatacathodiccurrentdensityof
−50mAcm−2.
TheseexperimentalconditionshavethusbeenappliedtotheSi
electrorefininginmoltenfluorides.
3.2. Impactofimpuritiesontheelectrorefiningselectivity
3.2.1. Preliminarydiscussion
Theselectivityoftheelectrorefiningprocessdependsonthe
dis-solutionpotentials(Ediss)ofthemetalstobetreated.Intheframe
ofelectrorefiningofSibasematerialscontaininganimpurityMi,
threecaseshavetobeconsidered:
-case1:EMi
diss>EdissSi .Theimpurityisnotdissolvedduringanodic
dissolutionofSi:theselectivityoftheprocessisthusgoverned
bytheanodepotential.
-case2:EMi
diss<EdissSi .TheimpurityisdissolvedtogetherwithSi
butisnotreducedatthecathodeduetoalowerreduction
poten-tial:theselectivityoftheprocessisensuredbythecontrolofthe
cathodepotential.
-case3:EMi
diss∼E Si
diss.Theimpurity is dissolvedtogetherwithSi:
completeelectrochemicalreductionbehaviourhastobe
stud-iedinordertoevaluatethepossibilityofitsco-depositionwith
Fig.2.SEMmicrographsofthecrosssectionoftheSi–4.7at%Feanode:(a)bulk;(b)externalsurface(lightgrey:FeSi2;darkgrey:Si).
Fig.3.Fe–Siphasediagram[9].
Theelectrochemicalbehaviourofseveralmetalsconsideredas
commonpollutant in SoG-Sihasbeen studiedby linearsweep
voltammetryinNaF-KFat850◦C.Linearvoltammogramsrecorded
at10Mvs−1areplottedinFig.1.
Thethreecasespreviouslymentionedarehighlightedinthis
figure:Cu,Mo,Ni,CrandFecorrespondtocase1,Zrtocase2,Ti
andMntocase3.
Iron(case1)wasselectedasa“modelimpurity”duetoits
rel-ativelycloseoxidationpotentialcomparedtoSi(1E∼450mV).In
ordertodemonstratethefeasibilityofSi–Feseparationandto
eval-uatecurrentefficiencies,acommercialSi–4.7at%Fealloywitha
ratherhighFecontentwasused.
3.2.2. CharacterizationoftheSi–4.7at%Feanode
Themicrostructureoftheanodematerial,asillustratedinFig.2
bySEMmicrographsofitscross-section,iscomposedoftwophases:
asiliconmatrixandanintermetalliccompound,FeSi2,in
agree-mentwiththeSi–Fephasediagram(Fig.3)[21].Basedonavailable
densitydata((Si)=2.3gmL−1and(FeSi
2)=4.7gmL−1[22]),the
volumeoftheintermetallicphasewithinthematrixisabout11%.
-0.05 0 0.05 0.1 0.15 0.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 I/ A E vs. Pt/V Si Si-4.7at%Fe Fe
Fig.4. ComparisonoflinearvoltammogramsofSi,FeandSi–4.7at%Feat10mVs−1
inNaF-KFat850◦
Fig.5. SEMmicrographsoftheSi–4.7at%Feanodesurface:(a)beforeelectrolysisand(b)afterelectrolysisinNaF-KF-Na2SiF6(C=0.73molkg−1)during25hat50mAcm−2
at850◦
C(Q=8659C).
Fig.4exhibitstheanodicbehaviourofSi,FeandSi–4.7at%Fe
recordedbylinearsweepvoltammetryintheNaF-KFsystemat
850◦Cand10mVs−1.ThisfigureverifiesthatthedissolutionofFe
occursatamorepositivepotentialthanSi.ConcerningtheSi–Fe
alloy,theshapeofthelinearvoltammogramcouldbeexplainedby
thetwo-phasestructureoftheanode:SiandFeSi2.Asobservedin
Fig.2,thesampleiscomposedofpureSimatrixwithFeSi2phase.As
theSioxidationpotentialislowerthanFeandaccordingtoNernst
law,onlythepureSiphase isfirstoxidized,explainingwhythe
equilibriumpotentialisthesamethanSisample.Then,aftershort
polarizationduration,SiisoxidizedfrombothpureSiandFeSi2
phases;andaccordingtoNernstlawforSiactivitylowerthan1,
theSioxidationrateislowered.
3.3. Sielectrorefiningprocess
TheoptimizedexperimentalconditionsforSi electrorefining,
remindedinSection3.1,havebeenappliedtotheSi–4.7at%Fealloy.
3.3.1. Si–Feanodedissolutionpathway
Theselectiveanodicdissolutionwasdemonstratedfirst.
Elec-trolysisrunswereperformedinNaF-KF-Na2SiF6 (0.73molkg−1)
system,at850◦Candacurrentdensityof50mAcm−2during25h,
andFig.5showsSEMmicrographsoftheSi–Feanodesurfacebefore
(a)andafter(b)electrolysis.Beforeelectrolysis,bothSiandFeSi2
phasesareobservedwhereas,afterelectrolysis,theresidual
struc-turehasa“sponge-like”shapeandiscomposedofaround94.2at%
ofFe.
Magnificationsof the anode cross sections are presented in
Fig.6,forshort(5h)andlonger(25h)electrolysisdurations.They
indicate the selective dissolution of silicon: for short duration
(Fig.6a),thepuresiliconphaseisremovedwhiletheFeSi2phaseis
depletedinSiattheanode-bathinterface,formingFeSiandaphase
containing27.5at%ofSi.Forlongerelectrolysis(Fig.6b),the
sili-conmatrixismoredeeplydissolved(∼600mm),whileSi–Fephases
remaininthisarea.
Thecompositionprofileoftheanodeafteranelectrorefining
runof 25hwasinvestigated (seeFig.7).This profileevidences
thedecreaseoftheSicontentinintermetallicphasesfrom66.5
to27.5at%,fromthecoretotheelectrodesurface,exhibitingthe
followingdissolutionbehaviour:
-thepureSiphasewasdissolvedupto600mm.
- SiintheFeSi2phasewasselectivelyremovedtotheresidualvalue
of5.8at%,throughtheformationofFe-richcompounds
previ-ouslymentioned:FeSiandFe–27.5at%Si,inagreementwiththe
dissolutionofasolidsolution[23].
3.3.2. Sidepositspurity
Fig. 8 shows silicon coatings obtained after electrorefining
experimentsontwocathodes: SoG-Si(a)andgraphite(b),after
residual salt removal. TheSi deposits were adherentand their
roughnesswasrelativelyhigh.
EDSanalysisindicatesthatthepurityofsilicondepositishigher
than99.9%,correspondingtothedetectionlimitoftheapparatus
forelementstitration(seeEDSspectrumofFig.9).Formore
accu-ratequantificationofFe,theSicoatingwasdissolvedintoKOH
15molL−1 and Fewasanalysedby ICP-AESinthis solution:no
Fewassignificantlydetected,meaningthattheFeconcentration
islessthanthedetectionlimitwhichisaround10mgL−1.TheSi
purityisthenbetterthan99.99%.Ironconcentrationinthesaltwas
Fig.6. SEMmicrographsoftheSi–4.7at%FeanodecrosssectionafterelectrolysisinNaF-KF-Na2SiF6(C=0.73molkg−1)at50mAcm−2and850◦C:(a)duration:5h(Q=2448C)
Fig.7.ConcentrationprofileofSiintheSi–FeintermetallicphaseafterelectrolysisinNaF-KF-Na2SiF6(C=0.73molkg−1)at50mAcm−2and850◦C;duration25h;inset:SEM
observationoftheanodeafterelectrolysisshowingthelocationofSianalysis.
Fig. 8.Photographs of cathodes after electrolysis runs in NaF-KF-Na2SiF6
(C=0.73molkg−1)at50mAcm−2 at 850◦
C:(a)SoG-Si cathodeafter 39h;(b) graphitecathodeafter25h.
alsotitratedaftertheelectrolysisandwasnotdetected,proving
thattheSidissolutionisselective.
3.3.3. Currentefficiencies
Anodicandcathodiccurrentefficiencies,anodicdissolutionand
SirecoveryprogresseswereestimatedbycomparingthefinalSi
Fig. 9. EDS analysis of cathodes after electrolysis runs in NaF-KF-Na2SiF6
(C=0.73molkg−1)at50mAcm−2 at 850◦
C:(a)SoG-Si cathodeafter 39h;(b) graphitecathodeafter25h.
massofanodeandcathodetotheinitialmassofSiinSi–4.7at%Fe
anodes.Thesevalueswerecalculatedaccordingto:
Anodicdissolutionprogress= 1mdissolved
mSi into Si–Feanode
(3)
1mdissolved (g)isthemassofdissolvedSievaluatedfromthe
massbalanceoftheanode,andmSiintoSi–Feanode (g)isthemass
ofsiliconinitiallypresentintheanode.Thismasswascalculated
bymeasuringtheinitialanodemassandcorrecting itbytheSi
composition(90wt%Si).
Sirecoveryprogress= mSi deposited
mSi intoSi–Feanode
(4) mSidepositedisthesiliconmassdeposited.
Table2gathersthevaluesobtainedinseveralelectrorefining
runs,leadingtothefollowingconclusions:
Table2
AnodicSidissolutionandcathodicSirecoveryprogressesandanodicandcathodicfaradicefficienciesofelectrorefiningrunsperformedinNaF-KF-Na2SiF6(C=0.73molkg−1)
at50mAcm−2and850◦ C. Q(C) Anode:Siinitial mass(g) Anodedissolved mass(g) Dissolution progress Anodicfaradic efficiency Cathode:Si depositedmass(g) Sirecovery progress Cathodicfaradic efficiency 1 2448 1.851 0.178 10% 89% 0.205 11% 100% 2 5480 1.028 0.360 35% 98% 0.399 39% 92% 3 8659 1.095 0.548 50% 90% 0.648 59% 90% 4 10,391 1.076 0.646 60% 90% 0.716 67% 94% 5 13,622 1.111 0.878 79% 94% 0.989 89% 99% 6 22,248 1.755 1.457 83% – 1.619 92% 100% 7 23,425 1.818 1.636 90% – 1.809 99% 100%
-the faradic efficiencies are close to 100% and the difference
tounityislikely attributedtomateriallossduringfrozensalt
removalfromelectrodes,
-forexperiments6and7,theanodefeltintothebathbeforethe
endoftherun.Thisindicatesthatthelimitofdissolutionprogress
isaround80%,duetotheanodestructure(11vol%Fe).Athigher
dissolutionrate,theanodesbecometoobrittletoensurecohesive
structure,
-thecathodicefficiency(Sirecovery)reaches99%.
4. Conclusion
Thisworkdemonstratedthatelectrorefininginabasicmolten
fluoridesolventisanoptiontobeconsideredforpuresilicon
recov-eryfromsiliconmaterialscontainingFe.
Previous studiesstated onthe bestoperating conditions for
electrodeposition of Si: solvent (eutectic NaF-KF), temperature
(850◦C),Si(IV)concentration(>0.6molkg−1)andcurrentdensity
(50mAcm−2).
ElectrorefiningrunsevidencedthatelectrodissolutionofSifrom
aSi–Feanodeinvolveda selectiverelease ofSiwithinthe
elec-trolyte,aremainingFerich“skeleton”structureanda complete
recoveryofpuresiliconatthecathode.Itislikelythat,inthecase
ofmaterialswithlowerinitialFecontents,theremainingFerich
phasewillnotformasolidstructurebutratherfalloffandsediment
inthebottomofthecell.
Moreover,theanodicandcathodiccurrentefficienciesareclose
to100%andtheSidepositspurityisassumedtobehigherthan
99.99%,asnoFewasdetectedbyICP-AESanalysisofthedeposit.
Theelectrorefiningmethod,demonstratedinthisworkinthe
specificcase ofFe, is mostprobably suited for thepurification
ofSimaterialscontainingTi,Mn,Cr,Ni,MoandCusinceitwas
shownthat intheNaF-KF mixturetheirdissolution potentialis
moreanodicthanFe.
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