Chemistry Reference
In-Depth Information
As in the case of the tetra-
n
-alkyl substituted BTP ligands, the tetra-
n
-alkyl
substituted BTBP compounds suffer from a weak chemical stability when they are
dissolved in HTP/
n
-octanol mixtures, because of the oxidation of their α-benzylic
hydrogens. However, 6,6′-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]tri-
azin-3-yl)[2,2′]bipyridine (CyMe
4
-BTBP, Figure 3.18), in which the labile α-benzylic
hydrogens have been replaced by methyl groups, exhibits a good hydrolytic stability
(comparable to that of CyMe
4
-BTP) and a better apparent radiolytic stability com-
pared with BTP compounds, probably due to its lower complex stoichiometry. In
fact, the mass-action law differs and is more favorable in the case of the BTBPs: the
D
M(III)
values, measured when investigating the effect of irradiation doses on BTBP
ligands, follow a two-fold order of the extractant concentration in the organic phase
(because M:L
2
complexes are formed), whereas in the case of the BTPs, the
D
M(III)
values follow a three-fold order of the extractant concentration (because M:L
3
com-
plexes are formed) (
207
).
CyMe
4
-BTBP was envisaged for the development of an An(III)/Ln(III) parti-
tioning process. The system formulation was optimized: [CyMe
4
-BTBP] = 0.01 M
and [DMDOHEMA] = 0.25 M in
n
-octanol, for the organic solvent, and [Glycolic
acid] = 0.5 M, neutralized to pH 4 by NaOH, for the stripping solution (
216
). A flow-
sheet was elaborated based on the extraction isotherms determined through a series
of test-tube experiments. Nevertheless, kinetic problems were identified when imple-
menting this system in centrifugal contactors at the FZJ and at the ITU (
217
). Due
to the small hold-up volume compared to the flow rate in centrifugal contactors, the
time for extraction was too short to reach the
D
M
values determined at equilibrium
in batch tests. Kinetics experiments performed to investigate the
D
M
dependence
of the flow rate showed that even with the lowest applicable flow rates, only 8% of
the equilibrium
D
Am
value was reached for Am(III) extraction in the single-stage
centrifuge spiked test and around 16% in the single-stage centrifuge hot test (the dif-
ference being due to the size of the centrifuges). Nevertheless, a countercurrent hot
test was attempted at the ITU (
218
). The An(III) + Ln(III) product issued from the
TODGA hot test carried out at the ITU in 2006 on a PUREX raffinate (see Section
3.3.1.1.6.1) was used as the feed, after increasing its acidity to 2 M. The concentration
of CyMe
4
-BTBP was increased to 15 mM in the mixture DMDOHEMA (0.25 M)/
n
-
octanol to compensate for the low apparent
D
M(III)
values measured in single-stage
centrifuge experiments. The extraction section comprised nine stages followed by a
three-stage scrub and a four-stage strip. All flow-rates were set to 10 mL/h due to the
slow extraction/back-extraction kinetics of the CyMe
4
-BTBP system (Figure 3.21).
The “once-through” countercurrent test was successful, as more than 99.9% of
Am(III) and Cm(III) ended up in the product with less than 0.1% of the most domi-
nant lanthanides, thus demonstrating the ability of CyMe
4
-BTBP system to separate
An(III) from Ln(III). A validation of the process when recycling the solvent is now
awaited.
3.3.1.2.1.2 Sulfur-donor Extractants
The CYANEX 301 Process.
Va r ious
dialkyl-(mono/di)-thio(phosphoric/phosphinic) acids, such as di(2-ethylhexyl)-di-
thiophosphoric acid (HDEHDTP), bis(2,4,4-trimethylpentyl)-thiophosphinic acid
(CYANEX 272), bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (CYANEX
