| Abstract |
- Partitioning of trivalent actinides (in particular, americium and curium) from the
fission produced light lanthanides is a major concern of used nuclear fuel reprocessing for
the purposes of waste disposal. Several solvent extraction processes have been developed
to address these chemically difficult separations. The historically employed TALSPEAK
Process utilizes di-2-ethylhexyl phosphoric acid (HDEHP), a cation exchange extractant
as an organic phase extractant, while the Advanced TALSPEAK Process utilizes 2-
ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]), its phosphonic acid
analog as an organic extractant to alleviate processing problems. The newly developed
mixed extractant ALSEP Process utilizes either HDEHP or HEH[EHP] in conjunction
with either tetraoctyldiglycolamide (TODGA) or tetra-2-ethylhexyldiglycolamide
(T2EHDGA), collectively DGA, to perform two separation steps in a single process. The
nature of organic phase chemical interactions in these systems must be clarified in order
to effectively model and operate them on an engineering scale.
Neodymium speciation in HDEHP organic phases, the organic extractant used in
the TALSPEAK Process, have been subject to multiple interpretations necessitating
further investigations. UV-Visible spectroscopy coupled with Karl-Fischer titrations reveal
that water in the inner neodymium coordination sphere results in an increase in the metal
coordination number. Furthermore, the extraction of water was found to be independent
of metal concentration, but was dependent on the HDEHP concentration. Investigating
across the lanthanides revealed that light f-elements (americium, praseodymium, and
neodymium) can contain inner-coordination sphere water, while heavier lanthanides
(samarium, holmium, and erbium) cannot, presumably due to their smaller ionic radii. For
HEH[EHP] organic phases, the Advanced TALSPEAK extractant, water extraction was
found to be similarly independent of metal concentration and dependent on HEH[EHP]
concentration, although a weaker water extractant. Compared the HDEHP, only the
lightest lanthanide measured (praseodymium) revealed a change in UV-Vis spectra
resembling a speciation change, suggesting that only the lightest of lanthanides can occupy
water in the inner-metal coordination sphere.
Characterization of the organic phase of the ALSEP Process is necessary to advise
scaling efforts and to identify any potential difficulties which may exist due to inter-ligand
interactions. The ALSEP ligand combinations tested were HDEHP - T2EHDGA,
HEH[EHP] - TODGA, and HEH[EHP] - T2EHDGA. IR spectroscopy shows that the
ability of the ALSEP ligand combinations to form intermolecular adducts in metal-free
organic phases are relatively similar, although approximately an order of magnitude lower
for the HDEHP - CMPO combination proposed for the previously developed
TRUSPEAK system. Metal loaded IR spectroscopy shows that, when trivalent lanthanides
are extracted from an aqueous phase which should not allow for the formation of neutral
DGA solvates, the nature of the DGA carbonyl bond is shifted for the HDEHP -
T2EHDGA system and the HEH[EHP] - TODGA system, but not for the HEH[EHP] -
T2EHDGA system. Job’s Method further reveals that, for the ligand combinations which
are measured to form ternary species, a 1:6:1 ratio of the cation exchange extractant to the
DGA forms with metals in the organic phase. UV-Vis titrations further reveal that
HDEHP - T2EHDGA forms ternary species the strongest with extracted metals, followed
by HEHEHP] - TODGA and HEH[EHP] - T2EHDGA. Furthermore, americium forms
ternary species the strongest with ALSEP ligand combinations, followed by neodymium
and holmium.
Finally, the dependence of organic phase nitrate on the complexation of metals in
the ALSEP process was investigated for advising process operations. IR spectroscopy
revealed that titration of an organic phase containing a DGA loaded with nitric acid shifts
the metal coordination from the cation exchange ligand to the DGA similar to extraction
of metal from a high concentration nitric acid aqueous phase. UV-Vis titration addition of
nitrate to a metal loaded organic phase reveals that, in general, TODGA is more able to
accommodate metal-nitrate complexes than T2EHDGA and the neodymium more
strongly complexes with DGAs in the presence of nitrate than holmium. Also seen is that
holmium exists in a complicated coordination environment at higher organic phase nitrate
concentration, potentially indicating some process issues.
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| Additional Information |
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Previous issue date: 2017-01-06
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