Chemistry Reference
In-Depth Information
•
The desired separation and purification specifications are easily obtained
by successive extraction-scrubbing-stripping steps. Flowsheet design is
very flexible.
The separation processes involving all-liquid operations can be imple-
•
mented continuously, which, is very advantageous industrially, as it ensures
high throughputs.
The criticality risks can be almost completely cancelled by designing
•
devices that fulfill specific safety geometric requirements.
The energy input for running hydrometallurgical processes is usually small
•
(room temperature, normal pressure).
However, it is also fair to say that concerns about proliferation have been a prob-
lem, in that solvent-extraction methods have created the possibility to separate plu-
tonium. Furthermore, nuclear researchers have to address several problems when
developing partitioning processes, such as the use of flammable liquids, the disposal
of spent solvents, as well as the fate of the chemicals introduced in the various parts
of the processes to improve their performances (e.g., complexants and buffers added
to the feeds or stripping solutions).
3.2.2 t
H e r m o d y n a m i C S
of f
S
of l v e n t
e
X t r a C t i o in
P
e r t a i n i n g
t o
f
-e
l e m e n t S
3.2.2.1 Properties of trivalent Actinides and Lanthanides
Today, it is accepted that lanthanides (4
f
elements) and transplutonium actinides (5
f
elements) possess relatively similar physical and chemical properties (
28-31, 63
)
including:
•
A stable trivalent oxidation state in acidic aqueous solutions
•
Decreasing cation radii along the series resulting from the inability of the
relatively small spatial extension of the 4
f
and 5
f
electrons to compensate
for the steadily increasing nuclear charge
Similar cation radii for certain An(III) and Ln(III)
•
•
First-coordination-sphere hydration numbers ranging from 9 (at the begin-
ning of the series) to 8 (at the end of the series)
Hard electron-acceptor properties according to Pearson's theory of hard
•
and soft acids and bases (
64
), which favor electrostatic interactions with
oxygen-bearing ligands (hard donors), such as ligands bearing -C=O or
-P=O groups
From the early 1980s, it has also been assumed that the slightly greater spatial
extension of the 5
f
orbitals was responsible for the existence of some degree of cova-
lence (or polarizability) in the bonding interactions between An(III) and soft bases,
such as sulfur- or nitrogen-bearing ligands (
65- 69
). Recent thermodynamic studies
exploiting accurate analytical techniques, such as microcalorimetry, nuclear mag-
netic resonance, extended X-ray absorption fine structure, X-ray crystallography or
mass spectrometry, providing a better comprehension of the complexation/extraction
