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unfavorable with respect to TBP. In view of the fact that physicochemical properties
of amides can be tuned by the judicious choice of the alkyl groups, this group of
extractants has received attention as an alternative to TBP. An increase in the chain
length, particularly adjacent to the carbonyl group, improves the extraction ability
and LOC of metal ions. However, it adversely influences the phase disengagement
time, hydraulic behavior, and the aqueous solubility of degradation products.
Musikas and co-workers studied extensively the extraction behavior of inorganic
acids and U/Pu extraction chemistry with
N,N
-dialkyl amides (
202-205
). Based on
the extraction data, they proposed certain dialkyl amides suitable for the reprocess-
ing of irradiated nuclear fuels in nitric acid media. Most of the work reported earlier
on amides referred to either aromatic or substituted aliphatic hydrocarbons employed
as diluents. However, these diluents are not suitable for commercial-scale reprocess-
ing due to their poor radiation and chemical stability in the presence of nitric acid, as
well as their tendency to form a three-phase system.
Recently, a systematic attempt has been made in the authors' laboratory to investi-
gate (a) linear dialkyl amides as an alternative to TBP (as in the PUREX process) for
their recovery and purification of Pu, and (b) branched dialkyl amides as alternatives to
TBP (as in the THOREX process) for the recovery and purification of uranium (
66-72,
74 -78
). Several dialkyl amides were synthesized and evaluated for their extraction
behavior toward U(VI), Pu(IV), Am(III), and fission products from nitric acid medium
employing
n
-dodecane as the diluent.
N,N
-dihexyl derivatives of hexanamide (DHHA),
octanamide (DHOA), and decanamide (DHDA) were found to be promising among a
large number of extractants studied. These ligands readily dissolved in
n
-dodecane and
did not form third phases with nitric acid (up to 7 M). The nature of extracted species
formed in the organic phase, the corresponding two-phase extraction constants, the
influence of U loading on third-phase formation and on distribution data, and the effect
of gamma irradiation as well as of temperature have been investigated (
74 -78, 205
).
Laboratory batch studies as well as mixer-settler studies were performed with DHOA
and compared with those of TBP. Figure 2.5a shows the differences in the behavior of
(a)
(b)
10
3
6
10
3
5
U
org.
- DHOA
U
org.
- TBP
10
2
5
4
10
2
[HNO
3
]
org.
- TBP
[HNO
3
]
org.
- DHOA
[U]
aq
.
- DHOA
[U]
aq
.
- TBP
10
1
4
3
[HNO
3
]
aq
.
- TBP
[HNO
3
]
aq
.
- DHOA
10
1
10
0
3
2
10
-1
2
10
0
1
10
-2
1
10
-1
10
-3
0
0
0123456789
0123456789
Stage number
Stage number
FIGURe 2.5
(a) Stage-analysis data of DHOA and TBP in uranium extraction cycle. (b)
Stage-analysis data of DHOA and TBP in uranium stripping cycle. (From Manchanda, V.K.;
Pathak, P.N.,
Sep. Pur. Technol
., 35, 85-103, 2004. With permission.)
