Calcined Resin Microsphere Pelletization (CRMP) a novel process for sintered metallic oxide pellets synthesis.

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synthesis.

E. Remy, S. Picart, S. Grandjean, T. Delahaye, N. Herlet, P. Allegri, O.

Dugne, Renaud Podor, Nicolas Clavier, P. Blanchart, et al.

To cite this version:

E. Remy, S. Picart, S. Grandjean, T. Delahaye, N. Herlet, et al.. Calcined Resin Microsphere Pelleti-

zation (CRMP) a novel process for sintered metallic oxide pellets synthesis.. Journal of the European

Ceramic Society, Elsevier, 2012, 32 (12), pp.3199-3209. �10.1016/j.jeurceramsoc.2012.04.011�. �cea-

02349240�

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JournaloftheEuropeanCeramicSocietyxxx(2012)xxx–xxx

Calcined resin microsphere pelletization (CRMP): A novel process for sintered metallic oxide pellets

E. Remy

^a,∗

, S. Picart

^a

, S. Grandjean

^a

, T. Delahaye

^b

, N. Herlet

^b

, P. Allegri

^c

, O. Dugne

^c

, R. Podor

^d

, N. Clavier

^d

, P. Blanchart

^e

, A. Ayral

^f

aRadioChemistryandProcessesDepartment,SCPS,LC2A,CEA,NuclearEnergyDivision,F-30207Bagnols-sur-Cèze,France

bFuelCycleTechnologyDepartment,SDTC,LEMA,CEA,NuclearEnergyDivision,F-30207Bagnols-sur-Cèze,France

cFuelCycleTechnologyDepartment,SGCS,LMAC,CEA,NuclearEnergyDivision,F-30207Bagnols-sur-Cèze,France

dInstitutdeChimieSéparativedeMarcoule,UMR5257CEA-CNRS-UM2-ENSCM,F-30207Bagnols-sur-Cèze,France

eHeterogeneousMaterialsResearchGroup,CentreEuropéendelaCéramique,F-87068Limoges,France

fInstitutEuropéendesMembranes,UMR5635CNRS-ENSCM-UM2,CC047,UniversitéMontpellier2,F-34095MontpellierCedex5,France Received8February2012;receivedinrevisedform5April2012;accepted9April2012

Abstract

Thisstudydealswiththepreliminarydevelopmentofapowder-freeprocesscalledcalcinedresinmicrospherepelletization(CRMP)usedfor thefabricationofmetallic oxidepellets.Thisdustlessprocesscouldbeusedforthefabricationof mixedU1−yAmyO2±x pelletsdedicatedto thetransmutationofAminfastneutronreactors.Inthisstudy,porousCeO2 microspheres,usedasasurrogateofAmO2,wereobtainedafter mineralizationofceriumloadedionexchangeresinbeads.Thesemillimetricoxidemicrospheresweredie-pressedintopelletswhichwerethen sinteredunderairtoformceramicpellets.Theirdensitiesapproached95%oftheoreticaldensityofCeO2andahomogeneousmicrostructurewas obtainedbyusingoptimizedmicrospheres.Theinfluenceofcalcinationparametersonthecharacteristicsofmicrospheresandonthepropertiesof sinteredpelletsisdiscussed.

Keywords:Microsphere;Calcination;Pressing;Sintering;Precursors-organic

1. Introduction

Oneof the most promising optionsunder study toreduce theradiotoxicityandtheheatloadofnuclearwastestorageis thetransmutationofthelong-livedradionuclidessuchasminor actinides (MA),americium (Am) for instance,in afast neu- tronreactor.¹Inthisrecyclingprocess,MAaretransformedby nuclearreactionsintoshort-lived orstablenuclides.Different modesof transmutationare investigated: homogeneoustrans- mutation,consistingof dilutingMA inthewholecoreof the

Abbreviations: MA, minor actinides; MABB, minor actinide bearing blanket;SGMP,sol–gelmicrospherepelletization;CRMP,calcinedresinmicrospherepelletization;FT,finaltemperature;HR,heatingrate;TD,theoretical density;PHWR,pressurizedheavywaterreactor;WAR,weakacidresin;IER, ionexchangeresin;LMFBR,liquid-metalcooledfast-breederreactors;Qweight, scientificweightcapacity;Qvol,technicalvolumecapacity;meq,milliequivalent.

∗Correspondingauthor.Tel.:+33466791827;fax:+33466796567.

E-mailaddress:elodie.remy@cea.fr(E.Remy).

reactor andheterogeneoustransmutation,whereMAarecon- centratedinuraniumdioxidefuelsintroducedintheperiphery ofthecore.²Thesespecificfuelsareoxidematerialscalledminor actinide bearing blanket (MABB). Their fabrication requires special attention due tothe highradiotoxicity of MA.³ Cur- rently, the fabricationof these mixed oxide pellets follows a powder metallurgyroutewhichconsists innumerousstepsof grindingandmillingofUO2andAmO2powders,pressingand sintering.^4,5Preparationofgranularmaterialbeforecompaction andespeciallygranulation,whichisrequiredtoproducefree- flowing feed material for automatic pellet presses, generates fineandhighlycontaminantparticles.Dustlessprocesseswhich avoidthepresenceofhighlycontaminatingpowderinthefab- ricationline,usingoxideintheformoffreeflowingspherules withmillimetricsize,arerecommended.⁶

The firstprocess developedinthe 70s, basedonthe compaction ofmillimetric spherestoform pellets,wascalled the sphere-calprocess.^7–9AuthorsstudiedthecompactionofUO2

andThO2beads,obtainedbysol–gelprocess,intopelletsand

http://dx.doi.org/10.1016/j.jeurceramsoc.2012.04.011

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thesubsequentsinteringtoformsimpleormixedoxidefuelpel- letsforadvanced pressurizedheavywaterreactors (PHWRs).

Their work focused especially on the influence of the gela- tionandcalcinationprocessesonbeadcompactionintopellets andonthesubsequentsintering.Exhaustiveinvestigationswere carried out tooptimize heattreatment parameters inorderto obtainUO2microspheressuitableforpelletization.Morerecent studieshavebeen published on thisprocessrenamed sol–gel microsphere pelletization (SGMP) to produce ThO2–UO2

pellets.^10–12 Pelletizationof soft porous microsphereswith a low crushing strengthleadsto thesynthesis of homogeneous and dense sintered pellets (≥94% TD). The SGMP process was then studied for the elaboration of UO2 pellets^13,14 and mixeduranium,cerium orplutonium oxidepelletsfor liquid- metalcooledfast-breederreactors(LMFBR).Inthesestudies, cerium was used as a surrogate of plutonium.^15,16 Pellets with high density (≥95% TD) anda tailored microstructure containing uniformlydistributed closed pores were produced by using porous microspheres, obtained by the addition of carbon black as a pore former or by adjusting gelation parameters.^14,17 Theviabilityof the SGMProuteforfabrica- tion of (U,Pu)C and (U,Pu)N has also been established.^18,19 Afterwards, uranium–plutonium nitrides as low-density pellets have been prepared and characterized for transmutation application.²⁰

Aswiththesphere-calortheSGMPprogramme,ourstudy dealswiththedevelopmentofaspheruleroutewhichiscalled the calcinedresinmicrospherepelletization(CRMP) process.

It consists of the synthesis of mixed oxide precursors of sphericalformandmillimetric sizeobtainedbyanadaptation of the weak acid resin (WAR) process^21–24 and their com- pactionintopelletsbeforesintering.Themicrospheresynthesis is based on the fixation of cerium cations into beads of ion exchangeresin(IER)andtheirmineralizationinairtoformthe oxide.

Thispresentstudyconstitutesafirststepintheelaborationof mixedoxidepelletsbytheCRMPprocesswiththeelaborationof cerium(asasurrogateofamericium)oxidepellets.Itdescribes theconversionmethod,theoptimizationofthecalcinationstep andtheresultsofcompactingandsinteringbyexaminationof thepellets’microstructureanddensity.

2. Materialsandmethods

2.1. Startingmaterials

TheacrylicresinwasobtainedfromDowChemicalsCom- pany (RohmandHaas, Chauny, France)and consistedof an IMACHP335geltypeacrylicexchangerintheformofbeads of3differentsizedistributions(batches1,2and3).Theirchar- acteristicsarelistedinTable1.

Concentratedammoniasolution(25%,Merck,ProAnalysis) and concentrated nitric acid solution (70%, Fisher Chem- ical, Certified ACS Plus) were used to prepare washing solutions. Hexahydrate cerium(III) nitrate (Ce(NO3)3·6H2O, 99.99%pure,Prolabo)wasusedtoprepare0.25MCe(III)stock solution.

Table1

Characteristicsofthe3batchesofIMACHP335ionicexchangeresin(nc,not communicated).

IMACHP335resin Commercial form:batch1

Finebeads:

batch2

Coarsebeads:

batch3

Structure Gel

Functionalgroup COOH

Physicalform Opaquebeads

Matrix Polyacrylic

Exchangecapacity(meq/mL RH)

≥3.85 4.19 4.02 Moistureholdingcapacity 52–58% 58.6 54.6

Density(H⁺form) 1.14–1.18 nc nc

Harmonicmeansize(␮m) 500–700 <300 >1000

2.2. Solutionanalysis

Theceriumcontentofthedifferentsampleswasdetermined by atomicemissionspectrometry (ICP-AES) usingan Activa HORIBALABequipment.Acalibrationwasperformedusing diluted Ce certifiedstandard (Merck, Certipur, 1000mg/L in 2–3%HNO3)in0.5M HNO3.Scandiumwasalsoused asan internalstandard(Merck,Certipur,1000mg/Lin2–3%HNO3) atafixedconcentrationof25mg/L.Twowavelengthswerecho- senforceriumanalyses(394.275and413.380nm)andonefor scandiummeasurement(358.963nm).

The ammoniumconcentration wasmeasured byacid–base titration carried out with aTitrando 809 Metrohmautomatic titratorinthepresenceofpotassiumoxalateusedtoshiftcerium hydrolysis. The evolution of pH was followed continuously usingaConsortC861analyserandaglasselectrode(Blueline 14pH,SchottInstruments).

2.3. Synthesisofthesolidoxidemicrosphereprecursor 2.3.1. Preparationofresinbeads

ThewetresinwasfirstmechanicallyscreenedwithanAS200 BasicRetschapparatusunderdeionizedwaterthroughsucces- sivelyfinerstandardProlabosieves(100␮m,250␮m,400␮m, 630␮m,800␮m,1000␮mand1250␮m).Thenamanualsec- ondscreeningwasperformedoneachsizerangeinordertolimit cloggingofthesieves.

The selected resin waswashed inacolumn intwo cycles consisting of percolating successively a 1M aqueous nitric acidsolution,deionizedwater,1Maqueousammoniasolution and deionized water. Two cycles were necessary to remove contaminants such as sodium and synthesis impurities. The resin inits finalammonium formwas drainedand recovered semi-wet.

2.3.2. Resinmetalloading

Abatchmethodwasusedtoperformthecationexchangeon thewashedresininitsammoniumform.Thoseresinbeadswere fullyrehydratedonacolumn2hbeforeexchangeindeionized waterandtheirpackedbedvolumewasmeasured(15–45mL).

Different size distributions of resin were contacted with the 0.25M cerium nitratestocksolution (100–300mL)for 4hin

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avesselatroomtemperature(22±2^◦C)undergentleagitation withamagneticstirringbar.Theresinwasthentransferredtoa columnandwashedwithdeionizedwater.Afterdrainingunder vacuum,theloadedresinwasdriedat110^◦Covernight.During exchange,thebulksolutionwassampledtofollowceriumand ammoniumconcentrations.

2.3.3. Calcination

Calcinationof theloadedresinwasperformedinatubular furnace(Nabertherm,HTRH100-300;aluminatube1200cmin lengthand10cmininternaldiameter)undersyntheticairflow (AirLiquide,20%O2–80%N2).Temperaturesintherangeof 500–1400^◦Cweretested.Themicrosphereswereplacedas a singlelayerinanaluminacrucibleinordertoobtainahomoge- neouscalcinationasmentionedbyFerreiraetal.²⁵

2.3.4. Pressingandsintering

Ceriumoxidemicrospheresobtainedaftercalcinationwere coldcompactedintogreenpelletsina10TpneumaticEnerpac pressat500MPa.Tungstencarbidedie-punchsetsof5.165mm diameterwereusedandwerelubricatedbyzincstearatepriorto eachcompaction.Thegreenpelletsweresinteredat1400^◦Cfor 6hinthesamefurnaceandsameairflowaspreviouslydescribed.

Theheatingandcoolingrateswerefixedat10^◦C/min.

2.4. Solidcharacterization 2.4.1. Loadedresin

An elemental analysis was performed on asample of the 630–800␮m loaded resin. A mass of 150mg of this sample was placed in a column and contacted by percolating 25mLof1MHNO3during100min.Thepercolatewasrecov- eredin a50mL volumetric flask and completed by washing waterfromtheresin.ICP-AESanalyseswerethenconducted on this solution to determine the concentration of cerium released during dissolving corresponding to the resin metal content.

Themetal content wasalso estimated bythermogravimet- ricanalysis(TGA)anddifferentialscanningcalorimetry(DSC) performedwithaSTA449CNetzschequipment.Microspheres wereplacedinanaluminacruciblefittedwithacoverandwere calcinedupto1100^◦Cinairflow(3NL/h)ataheatingrateof 5^◦C/min withan isothermaltreatmentof 1hat1100^◦C.The residuewaspureceriumoxidewhichwasconfirmedbyX-ray diffraction(XRD)andcarbonanalysis.

2.4.2. Oxidemicrospheres

The effective density²⁶ of oxide microspheres was measured by helium pycnometry (Accupyc 1330 Micromeretics pycnometer). The pycnometer was equipped with a 10cm³ volume cell and a sample module of 1cm³. It worked with helium (Alphagaz; Air Liquide, 99.995% pure). The 1cm³ volumemodulewascalibratedusinga0.718502cm³standard ball.Eachsamplewasdegassedbeforemeasurementovernight at140^◦C under vacuumusing a VacPrep061 Micromeretics apparatus.

Apparentdensity²⁶ofmicrosphereswasdeterminedbyvol- ume and mass measurements of asample of a few hundred beads.Numberofspheresandthesamplevolumewereestimated from image analysis of a layer of microspheres (considered as perfect spheres). Images were taken by an optical video microscope(FortSVM01)andtreatedbyEllixpatternrecog- nitionsoftware(Microvision).Unitaryweightequivalenttothe weightofoneoxidemicrospherewasalsodeterminedbythis method.

SpecificsurfaceareawasobtainedfromN2adsorptionand desorption at77Kusing the Brunauer–Emmett–Teller(BET) method.ATristar II3020 Micromereticsapparatus wasused formeasurements.Sampleswerepretreatedinavacuumoven toremovevolatiles.

Theresidualcarboncontentinoxidemicrosphereswasmea- suredwithaCS230LECOcarbon–sulphuranalysercalibrated withcarbonstandardsobtainedfromLECO(501-024,501-676 and502-809).The crystallographicstructure of cerium oxide microsphereswascharacterizedbypowderXRD(D8advanced BRUCKERdiffractometer,CuK␣radiation).Gold(NBSStan- dard)wasaddedtoallsamplesasaninternalstandardtocalibrate theangularpositionsoftheobservedXRDlines.XRDdiagrams weretreatedbyDiffracplusEvasoftware.Thelatticeparameters oftheoxidewererefinedbypatternmatchingusingFULLPROF Suite.²⁷

2.4.3. Greenandsinteredpellets

Effectivedensityofsinteredpelletswasmeasuredbyhelium pycnometry. Bulkdensity of pellets was obtained byvolume measurementandgravimetricanalysis.Adilatometrictestwas carried outonagreenpellet withaSetsys evolutionSetaram dilatometerwhichallowedcontinuousmonitoringoftheshrink- agekineticsupto1400^◦Cwithaheatingrateof10^◦C/minunder air flow (syntheticair, Alphagaz, Air Liquide, 20% O2–80%

N2).

2.4.4. Microstructureanalysis

Scanningelectron microscopy (SEM)micrographsof pel- letsandmicrosphereswerecarriedoutwithaSupra55ZEISS instrument.Microsphereswerecoldmountedwithepoxyresin withinaringmouldandthengrindedwithSiCsandpaper(1200, 2400and4000gritsizes)toaccessinternalmicrostructure.A diamondsuspension(10␮m)wasusedtopolishsamplesanda finalpolishingwithcolloidalSiO2suspensionwasperformed.

Internal microstructure of microsphereswas observed on the polishedsurface.

Sintered pelletswere cut andsections were coldmounted andpolishedfollowingthesameprocess.Insituhightempera- tureenvironmentalscanningelectronmicroscopy(HT-ESEM) experimentswereperformed usingafieldemissiongun envi- ronmentalscanningelectronmicroscope(modelFEIQUANTA 200 ESEM FEG) equippedwitha 1500^◦C hot stage.²⁸ Dur- ingthiswork,observationswereperformedunderwatervapour atanoperatingpressureof233Paandaspecificdetectorwas usedforinsitugaseoussecondaryelectronimagingathightem- perature.Anindividualbeadofceriumloadedresinwasfixed

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Table2

SizedistributionsoftheIMACHP335resin.

Batch 1 2 3

Sieveaperture(␮m) Noncumulativetotal(betweensieves)(%)

100–250 – 100 –

250–400 5.4 – –

400–630 47.2 – –

630–800 40.9 – –

800–1000 6.4 – 19.7

1000–1250 – – 79.0

1250–1400 – – 1.3

onrefractorycementandwasheatedto1000^◦C.Micrographs wererecordedatregularintervalsduringtheheattreatmentof themicrosphere.

3. Resultsanddiscussion

3.1. Exchangecapacity

Theresinwassortedfirsttoworkonhomogeneousbatches forexchangekinetics.Thesizedistributionsofthethreebatches ofIMACHP335resinwereobtainedbysieving(seeTable2).

Foursizerangeswerequalifiedinthisstudy:100–250,400–630, 630–800and1000–1250␮m.

Exchangecapacityoftheresinwasdeterminedbymeasuring theamountofammoniumandprotonexchangedduringthelast washingcycle.Averagevaluesofscientificweightcapacityof twosize distributions(Qweight)inmilliequivalentpergramof driedresininprotonform(meq/gdryH⁺ form)andtechnical volumecapacity²⁹ofeachsizedistribution(Qvol)inmilliequiv- alent per millilitre of packed bed (meq/mL packed bed) are shown inTable 3. The swelling factorwhich corresponds to volumeexpansionofthepackedbedafterH⁺/NH4+conversion isalsomentioned.Swellingisimportantbecausetheactionof ammoniaccausescompleteiondissociationintheresinbutalso introducesalargesolvatedammoniumcounterion.^29,30

The scientific weight capacity does not vary significantly withsizedistributionandisequalto12.6meq/gdryH⁺ form, which is not far from the theoretical value of 13.3meq/g dry H⁺ form (knowing the weight of 1mol of carboxylic moiety to be 75g, which represents 1 equivalent, the theoretical weight capacity corresponds to 1/75=13.3meq/g dry H⁺).

3.2. Kineticsoffixation

EvolutionsofpH,ammoniumandceriumconcentrationsdur- ing conversion step of the 630–800␮m size distribution are plottedinFig.1.

The amount of cerium in solution rapidly decreased during the first hour of conversioncorresponding to the loading ofceriumintheresinandthesubsequentreleaseofammonium.

Accordingly,thequantityofNH4+insolutionincreased.After 1h,thepHstabilizedatavaluebetween5and6corresponding toachievementofequilibrium.

Fig.1.EvolutionofCe³⁺andNH4+concentrationsinsolutionduringthecon- versionstep.

3.3. Ceriumcontentintheloadedresin 3.3.1. Balancefromsolutionanalyses

Experimentalconditions andresults of conversionof each sizedistribution,suchasfinalmassoftheloadedresin,quantity ofceriumfixedintheresin,quantityofammoniumreleasedin solution, effectiveexchange volumecapacity andweightper- centageofceriuminthedriedloadedresinarelistedinTable4.

Thequantityofceriumfixedintheresiniscalculatedonthe basis of the differencebetween the initialquantity of cerium andtheamountofCefoundinthesolutionatequilibrium,in samplingandwashing.TheweightpercentagesofCe(wt%Ce) for everysizedistributionsarequitehomogeneousvaluesand theirmeanvalueequals35±3%.

Thequantityofammoniumreleasediscalculatedinthesame waybyaddingtheamountofammoniuminsolutionatequilib- rium,samplingandwashing.

The stoichiometry of the exchange reaction (ratio of the quantity of ammonium releasedin solutionover the quantity of ceriumfixedintheresin)iscloseto3whichconfirms that the lanthanidecationtakesup3 exchangingsitesintheresin and thereforereleases 3 ammoniumcations insolution.This ratioof3hasalreadybeenreportedintheliterature³¹forLa³⁺, Ce³⁺,Pr³⁺inthecaseofaNa-formresinatpH6–7.Thecation exchangereactioncanbewrittenasEq.(1):

3RNH4+Ce³⁺→ R3Ce +3NH4+ (1) whereRrepresentsanexchangingsiteoftheresin.

3.3.2. Balancefromsolidcharacterization

Elementalanalysisalsoindicatesthattheweightpercentage of cerium equals 33±2% for the630–800␮mloadedbeads.

Metal contentinthe fully loadedresin canalsobe measured after calcinationunderairat800^◦Cfor eachsizedistribution consideringthattheresidueispureceriumoxidefreeofcarbon (confirmedbycarbonanalysespresentedlaterinTable6),the residualmassisrepresentativeofthemetaloxidematerialsleft aftercalcinationandallowsassessmentoftheinitialmetalcon- tentintheresin.TheweightpercentageofCeissimilarforeach sizedistributionandthemeanvalueis34.4±0.2%.

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Table3

TechnicalvolumeandscientificweightcapacitiesofdifferentsizedistributionsofIMACHP335.

Sizedistributions(␮m) 100–250 250–400 400–630 630–800 800–1000 1000–1250 Meanvalues

QvolH⁺form(±0.2)(meq/mL) 4.1 4.1 3.8 3.7 3.6 3.9 3.9±0.1

QvolNH4+form(±0.1)(meq/mL) 2.66 2.56 2.52 2.67 2.63 2.53 2.60±0.04

QweightH⁺form(±0.1)(meq/g) 12.5 – 12.6 – – – 12.6±0.1

Swellingfactor(±0.1) 1.4 1.5 1.4 1.4 1.4 1.4 1.4

Table4

ResultsofCeconversionfordifferentsizedistributions.

Sizedistribution(␮m) 100–250 400–630 630–800 1000–1250

VresinNH4+(mL) 15.0±0.3 15.0±0.3 42.0±0.6 15.0±0.3

VCe(NO3)3(mL) 100.0±0.6 100.0±0.6 300.0±0.6 100.0±0.6

FinalpH 4.38±0.05 4.68±0.05 5.28±0.05 5.22±0.05

n(Ce³⁺)loaded(mmol) 14.0±0.8 12.6±0.8 38.9±2.0 13.0±0.8

n(NH4+)released(mmol) 43±2 40±2 121±1 39±2

Effect.Capa.(meq/mL) 2.8±0.1 2.5±0.1 2.8±0.2 2.6±0.1

EquivalentNH4+/Ce³⁺ 3.1±0.3 3.2±0.3 3.1±0.4 3.0±0.3

mresinCe(g) 5.88±0.01 5.43±0.01 14.25±0.01 4.92±0.01

wt%Celoadedresin 33±2 33±2 38±5 37±2

Thesizedistributionhasnoinfluenceontheweightpercent- ageofCeinthedriedresinwhichisabout34%accordingtoall characterizations.Thoseresults areinagreement withresults concerning the fixation of lanthanide(III) cations studied by Mokhtari.³² Thetheoreticalvalueof weightpercentageof Ce inthedriedresin corresponding toafullexchange is38.7%, calculatedfromthefollowingequation:

wt% Ceth= MW(Ce) MW(Ce)+MW(R)

= 140.12

140.12+3×74 =38.7 (2)

with MW being the molecular weight and R the carboxylic inorganicgroupinitsdeprotonatedform.

Theresinexchangecapacityisnotfullyused owingtothe establishmentofequilibriumgeneratedbytheaccumulationof NH4+ insolution duringthe batch exchange.³³ The effective capacityreaches90%oftheoreticalcapacity.

3.4. Solidanalyses 3.4.1. TGA

Thethermalconversionofasampleof630–800␮mloaded resinwasinvestigatedusingTGAataheatingrateof5^◦C/min fromroomtemperatureto1100^◦Cinair(Fig.2).Thethermal behaviourofoneindividualmicrospherewasalsocharacterized byHT-ESEMwhichenablestofollowtheevolutionofthemicro- spherediameterwithtemperature(seeFig.2andAppendixB).

SeriesofimagesisgiveninFig.3.Thefirstsignificantweight loss(5%)betweenroomtemperatureand290^◦Ccanbeassigned todehydrationandlossofweaklyadsorbedwatercoupledwith reduction of3% inmicrospherediameter. Theelimination of carbonmatterintheloadedresinbeadsoccurredinthetemper- aturerangeof290–690^◦Cwithalossof52%oforiginalmass.

Thisprocesscanbedividedintothreeexothermicmechanisms, withafirstdegradationstepbetween290^◦Cand420^◦Ccaus- ingadiameterdropof12%followedbyasecondstepbetween 420^◦Cand490^◦Cconcomitantwitha3%diameterdecrease andafinaldegradationuntil690^◦Ccorrespondingtoadiame- terreductionof4%.Themicrospherecontinuestoshrinkabove 690^◦C.Thisphenomenonisattributedtothebeginningofsin- tering.Upto800^◦C,adecreaseof24%inmicrospherediameter isobserved.

3.4.2. Microspherecharacteristics

Thediametralandvolumetricshrinkagesoftheloadedresin (R3Ce)forcalcinationupto800^◦Cweredeterminedbyimage analysisforeachsizedistribution.Meanvaluesare22±3%and 53±8%respectively.Theformervalueofdiametralshrinkage isinagreementwiththedataof24%obtainedfromHT-ESEM images(Fig.2).

Fig.2.TGAandDSCcurvesof630–800␮mloadedresinbeadsandevolution byHT-ESEMofthediameterofoneloadedresinbeadduringheattreatment.

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Fig.3.EvolutionbyHT-ESEMofoneloadedresinbeadduringcalcination.

Fig.4.LatticeparametersatdifferenttemperaturesrefinedbyFULLPROFand X-raydiffractionpatternoftheCeO2 microspherescalcinedat800^◦C(Au standard).

3.4.3. Crystallineevolution

X-ray diffraction patterns were obtained on oxide microspheres calcined at 600^◦C, 800^◦C and 1000^◦C. For all calcinationtemperatures,thediffractionpatternsarecharacter- isticoftheface-centredcubicCeO2.Thepatterncorresponding

Table5

LatticeparametersatdifferenttemperaturesrefinedbyFULLPROF.

Temperatureof calcination(^◦C)

Latticeparameter (±0.001)( ˚A)

Averagecrystallite size(±3)(nm)

600 5.407 19

800 5.407 32

1000 5.408 48

tothetemperatureof800^◦CisreportedinFig.4.Therefined latticeparameter(Table5),a=5.408 ˚Aisnotsignificantlydiffer- entfromthevalueof5.411 ˚Apublishedintheliterature.³⁴The oxideisalreadyformedat600^◦C.Thevariationofthecrystal- lizationstatereportedinFig.4isestimatedthroughtheaverage fullwidthathalfmaximum(fwhm)fromthemoreintenseXRD lines. The crystallite size increases withtemperature and the evolutionisingoodagreementwithpublisheddata.³⁵

3.4.4. Microstructuralevolution

ESEMmicrographsrealizedonthecalcinationofaunique loaded resin microsphere enabled identification in situ the microstructuralchangesofthemicrosphereduringtheheating process(Fig.5).

Afterdehydrationandatthebeginningofmassloss,cracks appearatthesurfaceofthebeadscorrespondingtothebeginning

Fig.5.MicrostructuralevolutionofaceriumloadedresinmicrosphereduringthermaltreatmentobtainedbyHT-ESEM.

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oftheremovalofcarbonintheformofcarbondioxide.Open porosityiscreatedatthismomentandseemstodisappearwhen temperatureincreasesandthediameterofthespheredecreases.

SEM micrographs performed on as-polished sections of CeO2beadsrevealthehomogeneityoftheirinternalmicrostruc- tureandmicrometre-sizedgrains(Fig.6).Theporosityevolution versustemperaturewasinvestigatedandresultswillbepresented hereafter.

3.5. Calcinationoptimization

Differentthermalcycleswereappliedto630–800␮mbeads tocomparetheinfluenceofeachparameterofthecyclesuchas:

heatingrate(HR)andfinaltemperature(FT).Adwelltimeof4h wasappliedatfinaltemperaturetoallbatchesofmicrospheres exceptformicrospherescalcinedat500^◦C.

3.5.1. Heatingrate(HR)

Differentheatingrates wereappliedduringthe calcination oftheloadedresinbeads:0.1,0.2,0.5,1,2.5,5^◦C/min(witha FT=800^◦C).

Heatingratehasanimportantinfluenceonthemicrosphere fractureasshowninFig.7.

An excessive heating rate of 5^◦C/min causes violentout- gassingofCO2,leadingtoimportantmechanicalstressonthe beadsandtheirdisintegration.Thepercentageofbrokenbeads calcinedat5^◦C/minisabout20%.Moreover,anincreaseofheat- ingrateresultsingreaterresidualcarboncontentintotheoxide bead(2000ppmat5^◦C/mincomparedto570ppmat1^◦C/min).

Removalofcarbonisnotcompleteforexcessiverate.

Aslowheatingrate(≤1^◦C/min)wasthenselectedduringthe mineralizationtoformhomogeneousoxidemicrosphereswith enoughmechanicalstrengthtoavoidemissionofsmallparticles duringthehandlingstep.

3.5.2. Finaltemperature(FT)

Finaltemperaturesof500^◦C(withnodwelltime)and600^◦C, 800^◦C,1400^◦Cweretestedforoptimizingthecalcinationof loadedresin.Characteristicsofcalcinedbeadsaresummarized inTable6.

Afirstobservationisthattheeffectivedensityofoxidebeads increases with temperature which is linked mostly with the decreaseofclosedporosityfrom4%at500^◦Ctoavalueinfe- riorat1%at1400^◦C.Openporosityalsodecreasesfrom800^◦C butstillremainselevatedforbeadscalcinedathightemperature (68% at1400^◦C) as we can observe inFigs. 8 and 9 which explainsthatbulkdensityofbeadsislimitedto31%ofTDin thoseconditions.

Thoseresultsareinagreementwiththoseof TGAandthe evolutionofmeandiameterduringmineralization(Fig.2).They revealthedensificationofmicrosphereabove690^◦Cinrelation withagraingrowthobservableinFig.9andadecreaseofthe specificsurfaceofoxidebeadswithtemperature(from45m²/g at500^◦Cto0.7m²/gat1400^◦C).

A second observation concerns the evolution of carbon contentintotheoxidewhichisabout0.15%below600^◦Cinde-

pendentlyofdwelltimeandwhichisnotsignificantabovethis Table6 Characteristicsofoxidemicrospheresobtainedunderdifferenttemperaturesofcalcination. ThermalcycleofmicrosphereContentofresidualBulkdensityBulkdensityEffectivedensityOpenporosityClosedporositySpecificsurfaceMeandiameterUnitaryweight 3aab2calcination(FT;HR)carbon(ppm)(±0.15)(g/cm)(±1%)(%TD)(±1%)(%TD)(±1%)(%V)(±1%)(%V)(±0.1)(m/g)(±20)(␮m)(±6)(␮g/u)bulkbulk (500,1)1350±1501.27188278444.948979 (600,1)1360±1001.341993801–44664 (800,1)570±751.18169683≤18.249175 (1400,1)–2.24319868≤10.739272 a343TD,theoreticaldensityofCeO(7.216g/cm).2 b%V:percentageofthebulkvolume.bulk

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Fig.6.SEMmicrographsperformedonas-polishedsectionofCeO2beadsheattreatedat1400^◦C.

Fig.7.CeO2microspheresobtainedbycalcinationinairwithdifferentTR:(a)0.2^◦C/min;(b)1^◦C/minand(c)5^◦C/min.

Fig.8.Evolutionofeffectivedensity,openandclosedporosityofCeO2beads calcinedatdifferenttemperatures.

temperature inconformity withtheendof masslossinTGA curve.

Unitary weight corresponding to the weight of one oxide microsphere can be also calculated and is about 75␮g/microsphereequivalenttoapproximately61␮gofCeper microsphere.Thosedata haveagreatinterest inthedetermi- nationofcapacityrequirementsasmentionedbyHaasetal.³⁶ Forinstance,thevolumeofinitialresininitsammoniumform, necessary for the productionof 1g of CeO2 (mCeO2) which representsapproximately1/75×10⁻⁶=13,300beadsofoxide (n_␮spheres),isabout10mLofresinbed.Detailedcalculationsare giveninAppendixA.

Finaltemperatureisacriticalparameterandenablessynthesis ofmicrosphereswithdifferentfinalamountsofcarbon,specific surface,densityandporosity.Theseparameterswillhaveimpor- tantconsequencesforthemicrostructureofsinteredpellets,as willbediscussedinthefollowingsection.

Fig.9.SEMmicrographsofthesurfaceofCeO2beadscalcinedat(a)(600,1)and(b)(1400,1).

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Fig.10.Dilatometricanalysisperformedonagreenpelletobtainedbycom- pactionof630–800␮mbeadscalcinedat800^◦Cat1^◦C/minfor4h.

3.6. Compactingandsintering

Microspherescalcinedabove800^◦Cwereuniaxiallypressed at500MPaina5.165mmmatrix.Themicrospheresobtainedat lowertemperatureswerediscardedbecausetheircarboncontent wastoohigh.Thegreenpelletsweresubsequentlysinteredat 1400^◦C for 6h in air. A dilatometric analysis performed on agreenpelletfollowingthisthermalcycleisshowninFig.10.

Sinteringbeginsat800^◦Cwithamaximumlinearshrinkagerate at1090^◦C.Thesinteringisendedat1400^◦C.Thefinallinear shrinkageafter coolingisabout 14%.The shrinkagecurveis classicalandinaccordancewiththesinteringbehaviourofgreen

pelletsobtainedfromthecompactionofceriumoxidepowder preparedfromoxalateprecursorsevenifthefinalshrinkageis slightlyinferior: ameanvalue of18% isobtained withthese pellets.³⁷

Thegreenpelletdimensionsweremeasuredbeforesintering todeterminebulkdensity.Effectiveandbulkdensitiesofceramic pelletsareshowninTable7.

The bulk density of green pellets is found to be between 62 and65% of TD which is greater than the value of 52%

obtainedforCeO2pelletsfabricatedbypowdercompaction.³⁷ Aslightinfluenceofcalcinationparametersonthegreendensity isobserved,whichincreaseswithheatingrate.

Greenpelletsobtainedfromthecompactionofmicrospheres calcinedat1400^◦Cweretoobrittleandnotsuitableforhandling.

Thiscanbeexplainedbythehardnessofbeadswhichistoohigh andnotappropriateforpressing.Thispropertyhasbeenwidely describedintheliteratureasaconsequenceoftheuseof“non- porous”andhardmicrosphereswhichdonotdisintegrateduring pelletization.^9,25

Sinteringofgreenpelletsat1400^◦Cfor6hresultedineffec- tivedensitybetween85and94%ofTDdependingontheheating rate. In particular, aheating rate of 5^◦C/min duringcalcina- tionleadstosinteredpelletswithanimportantclosedporosity approaching15%probablyduetoaheterogeneouspopulationof beadsbeforecompactionasmentionedpreviously.SEMmicro- graphsofas-polisheddiametersectionsof sinteredpelletsare giveninFig.11.

Pellets obtained from compaction of beads calcined at a low rate of 1^◦C/min up to 800^◦C are dense and present a

Fig.11.SEMmicrographsofas-polishedsectionsofsinteredpelletsobtainedfrommicrospherescalcinedat(a)800^◦Cat1^◦C/minfor4hand(b)800^◦Cat5^◦C/min for4h.

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Table7

Densitiesofgreenandsinteredpellets.

Thermalcycleofmicrospheres calcination(FT;HR)

Bulkdensityofgreen pellets(±0.2)(g/cm³)

Sinteredpellets Bulkdensity (±0.2)(g/cm³)

Effectivedensity (±0.02)(g/cm³)

Effectivedensity (±0.2)(%TD)

Open porosity(%)

Closedporosity (±0.1)(%)

(800,0.2) 4.5 6.8 6.66 92.3 ∼0 7.7

(800,0.5) 4.5 6.8 6.69 92.7 ∼0 7.3

(800,1) 4.5 6.8 6.71 93.0 ∼0 7.0

(800,2.5) 4.6 6.8 6.78 94.0 ∼0 6.0

(800,5) 4.7 6.3 6.15 85.2 ∼0 14.8

(1400,1) Brokenaftercompaction

homogeneous microstructurewith6% of closedporosity and noopen porosity.Onthe contrary,ahighheatingrate causes aheterogeneousandporousmicrostructure.Afinalcalcination temperatureof800^◦Cproducesbeadswithspecificsurfaceof 8.2±0.1m²/gandbulkdensityof1.18±0.15g/cm³andseems tobeanoptimizedtreatmenttoobtaindenseandhomogeneous pellets.Actually,thosetwocharacteristicshavebeendescribed intheliterature^8,25asfundamentalpropertiesinthecaseofthe optimizationofthepelletizationprocessofmicrosphereprecur- sorsobtainedfromsol–geltechnology.Theauthors’conclusion istouselowdensifiedkernel(∼2g/cm³)withlowspecificsur- face(∼5m²/g)obtainedat850^◦C.

4. Conclusion

Thisworkconstitutesafirstapproachconcerning theelab- oration of dense cerium oxide pellets by the calcined resin microspherespelletization(CRMP)processbasedonthecom- paction of oxide microspheres into green pellets and their subsequent sintering to obtain dense ceramic pellets. Those homogeneousandporousoxidebeadsof150–750␮mindiam- eterwereobtainedfrommineralizationofresinbeadsloadedin ceriumfollowingdifferentthermaltreatments.Differentcalci- nationtemperaturesandheatingrateshavebeeninvestigatedto produceoxidebeadssuitable forpressing.Inparticular, com- pactionofporousoxidebeadscalcinedatslowmineralization rate(≤1^◦C/min)andatemperatureof800^◦Cresultsindense andhomogeneousmetaloxidepellets(95%ofTD).

In contrast, an overly high temperature of calcination (1400^◦C)resultedinhardbeadsnotsuitableforcompaction.

Themechanicalbehaviourundercompressionofthoseoxide beadsproducedindifferentcalcinationconditionsisunderstudy andshouldimprovethecontroloftheceramicmicrostructure.

EventuallytheoptimizedCRMPprocessshouldbeappliedtothe elaborationofmixed(U,Ce)O2pelletsand(U,Am)O2ceramics.

Acknowledgements

The authors would like to thank the MATINEX French ResearchGroupforitsfinancialsupportandD.Lacoumefrom RohmandHaas,France(subsidiariesof DowChemicals)for the supplyof the IMAC HP335 ionicexchange resin.Agnès Grandjean andCyrielleReyare thankfully acknowledgedfor

thecarbonanalyses.TheauthorsaregratefultoBénédicteArab- ChapeletforherhelpwithXRD.

AppendixA. Determinationofthevolumeofinitial resininitsammoniumform,necessarytotheproduction of1gofCeO2(mCeO2)

mRCe= mCeO2

1−WL = 1

1−0.57 =2.32 (A.1)

wheremRCerepresentsthemassoftheceriumloadedresinand WL isthe weightloss inpercentageduringcalcinationtaken fromTGAequalto57%.

mCe=wt% Ce×mRCe= 0.344×2.32=0.8 (A.2) withwt%Cerepresentingtheweightpercentageof Ceinthe loadedresinwhichisequalto34.4%.

So

VRH(mL)= mCe

MWCe×3×1000 QvolH⁺

= 0.8

140.12×3×1000

2.6 =6.6 (A.3)

whereMWCeistheCemolecularweightandQvolH⁺isthetech- nicalvolumecapacityoftheprotonresinexpressedinmeq/mL (Table3).

The estimation of the resin bed volumein its ammonium form (VRNH₄)isobtainedby takingintoaccount theswelling factor described in Table 3: in this case, VRNH₄ is equal to 6.6×1.4–10mL.

AppendixB. Supplementarydata

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/

j.jeurceramsoc.2012.04.011.

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