Analytical applications
RDCH 702: Lecture 9 Separations Part 3Separation methodsSolvent
extractionPUREXIon exchangeVolatilityElectrochemistrySpecific
actinide separations
Basic concept of separationsOxidation stateIonic
radiusDevelopment of advanced separationsTrivalent
actinidesNecessary for fuel cycle due to formation of mixtures due
to fissionActinides TransuranicsFission productsSe (Z=34) to Dy
(Z=66)
Tributyl phosphate (TBP)
9-#PyroprocessesElectrorefining/ElectrochemistryUses molten salt
as solventKCl-NaClAvoids problems associated with aqueous
chemistryHydrolysis and chemical instabilityReduction of metal ions
to metallic stateDifferences in free energy between metal ions and
saltThermodynamic data at hand or easy to obtainSequential
oxidation/reductionCations transported through salt and deposited
on cathodeDeposition of ions depends upon redox potential
9-#Electrochemical SeparationsSelection of redox potential
allows separationsCan use variety of electrodes for separationFree
energiesnoble metalsiron to zirconiumactinides and rare earthsGroup
1 and 2Solubility of chlorides in cadmiumExplored at part of
Integral Fast Reactor (IFR) program
9-#
Molten SaltVolatility of chlorides as separation basisGaps
between groupsHigh vapor pressures achievable at low
temperature
9-#Molten Salt ExampleOxide fuel dispersed in CaCl2 /CaF2 at 800
C Inclusion of Ca metal reduces U salt to a metalReduced metals
dissolved in receiver alloyExample Cu - 40% Mg - Ca Uranium exceeds
solubility limits receiver alloy and precipitatesPu, other
actinides, rare-earths, and noble metal fission products accumulate
in receiver alloySeparated by distillationAlkali metals (Rb and
Cs), alkali-earths (Sr and Ba),and remaining iodine and bromine
accumulate in the CaCl2/CaF2 salt.Salt contains CaO from the
reduction processCaO is electrolytically reduced to metal for
reuse
9-#Molten Salt LiCl-KCl-UCl3 at 450-500 oCChopped metallic fuel
added to anode basketsU oxidized at anode to U(III)Reduced back to
metal at cathodeTransuranics and fission products
oxidizedCo-deposition of actinides with cadmium cathode
9-#Pu ProcessingDOR reduces plutonium dioxide to metal PuO2 and
Ca/CaCl2Formation of Pu metal and CaOCaO treated with Cl2
Pu metal purified by electrorefiningPu metal melted in NaCl/KCl
at 800CPu oxidized to PuCl3, dissolves in molten saltPuCl3 migrates
to cathode and reducedPu metal drips from cathode and collects in
annular region outside cupring of pure Pu produced
9-#Ionic liquidsRoom temperature ionic liquid (ILs) composed of
ions that are liquid below 373 K Composed of a bulky, unsymmetrical
organic cation and an organic or inorganic anionRange of possible
pairs, can be task specificLow vapor pressureAbility to dissolve
organic and inorganic compoundsConductive Wide electrochemical
windowIonic liquids are tunable to obtain properties needed for
particular applicationSolubilityReaction RateElectrochemical
windowSpecific metal ion interaction
9-#8Introduction: Ionic liquidsEthylammonium nitrate
electrochemistry in 1914
Initial efforts focused on chloroaluminate anion
(AlCl4-)Electroplate aluminum J. Electrochem. Soc. 98, 203
(1951)Detailed studies, identification of limitations with
chloroaluminate Moisture sensitivity Requires non-ambient
conditionsInorg. Chem. 17, 2728 (1978)Newer ionic liquids have
non-hydrolyzing or stable anionsMoisture stable ionic liquids
developedJACS, 965 (1992)
9-#9Examples of IL cations and anions1018 possible
combinationsRoute for task specific propertiesGroup actinide
separation
Can functionalize ILs
9-#10Ionic liquids in separationsIonic liquids can replace
traditional solvents in the PUREX Studies showed favorable
extraction when ILs used in conjunction with extractantsChem.
Commun. 1765 (1998)Possible recovery of metals through
electrodeposition direct from the organic phase following solvent
extraction
From J. Nucl. Radiochem. Sci., 10(1), 1-6 (2009)
9-#11f-element reduction in ionic liquidsHaloaluminates not
stable to reduction of An(III) or Ln(III) to metal stateDevelopment
of moisture-stable ILs good cathodic stability large ~6V
electrochemical windowsApplication based upon the molten salt
system (450 - 800C)Ionic liquids eliminateSpecialized corrosion
resistant cell Operation at elevated temperaturesProduction of
caustic side reactionsElectrochemistry of Sm3+, La3+, Eu3+ and Th4+
in [Me3NBu][TFSI]Reported reduction of Sm, La, and Eu to the
metallic stateTh was reported to reduce to Th0 but subsequently was
converted to ThO2 by moisture in ionic liquidDalton Trans, 24,
4532-4534 (2002)
Role of water central, useful in dissolution by problematic in
full reduction
9-#12Choice of Ionic LiquidA number of ionic liquids initially
investigatedbis(trifluoromethanesulfonyl)imide (TFSI) anion
selected for study at UNLVFunctionalized TFSI evaluatedpropyl and
imidazoliumlarge potential window with Au, Pt, and Glassy carbon
electrodesSuitable for reduction of lanthanides and actinides Ionic
liquidN-trimethyl-N-butylammonium
bis(trifluoromethanesulfonyl)imideUsed with actinidesWritten as
[Me3NBu][TFSI]
9-#13Electrochemistry in ionic liquidsILs have wider potential
window Actinide electrochemistry possibleMust limit water in the
systemTFSI is a poor ligandMoves deposition to favorable
potentialElectrode can influence windowGlassy Carbon (GC) widest
potential window
9-#U TFSI compound synthesisAll reactions were performed in an
argon filled glove boxK(TFSI)(s) synthesized by adding H(TFSI) and
KH THF solutionK(TFSI) separated by decanting excess THF90%
yieldK(TFSI) and UI3(THF)4 dissolved in THFExcess of K(TFSI) added
to ensure reaction completionClear pale yellow solution was stirred
for 24 hoursFinal orange red solutionContained off white KI solid
precipitateTHF solution contained product U(TFSI)3Solid was
isolated by evaporating excess THF74% yield
9-#Formation of Uranium DepositsConstant potential of -1.5 V
versus NHESEM, EDS, and XRD analysis performedSEM of U deposited on
Au foilXRD evaluation of sampleAlpha U metalSEM EDSNo oxygen at 5.5
keV
Au electrodeU-metal deposit
9-#16Direct dissolution of U3O8 into ionic liquidOriginal
dissolution conditions: 45 mg U3O8 with 3.27 M HTFSI in Me3NBuTFSI
Stirred for weeks with no change
RESULT: dissolution of materialoxidizing gas usedO3, NO2Addition
of HTFSIPromote formation of uranium-TFSI complex
Ozone (From compressed air)t = 24 hrs
9-#17Direct dissolution of U3O8 into RTILProposed dissolution
mechanism:
U3O8 + 6 HTFSI + O3 3 UO22+ + 6 TFSI- + 3 H2O + O2
Supporting EXAFS data of the original dissolution solution
confirmed presence of UO22+ coordinated with five to six equatorial
oxygens.
2 O @ 1.75 6 1 O @ 2.38 U --- O multi scattering @ 3.56
9-#18RDCH 702: Lecture 9 Separations Part 3Separation
methodsSolvent extractionPUREXIon
exchangeVolatilityElectrochemistrySpecific actinide separations
Basic concept of separationsOxidation stateIonic
radiusDevelopment of advanced separationsTrivalent
actinidesNecessary for fuel cycle due to formation of mixtures due
to fissionActinides TransuranicsFission productsSe (Z=34) to Dy
(Z=66)
Tributyl phosphate (TBP)
9-#