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Recovery of Uranium by Electro deposition in Molten Chlorides Media Pierre Taxil(1), Pierre Chamelot (1),
Karine Serrano(1) and Catherine Bessada(2) (1)
Laboratoire de Génie Chimique, UMR CNRS 5503, Université Paul-Sabatier,
118, route de Narbonne, 31062 Toulouse cedex 4, France. (2) CNRS, Centre de Recherche
sur les Matériaux à Haute Température, 1D, Avenue de la Recherche Scientifique,
45071 Orléans cedex 2, France The separation of
minor actinides and rare earth compounds
is a problem relevant to the nuclear waste processing. The
electrochemical way for this separation is based on the difference of the potential of electro reduction
between these two categories of elements. According to data on the free
energies of chlorides, uranium is less reactive than most of lanthanides, so
that it can be removed electrochemically of a mixture of wastes containing both
uranium and rare earth chlorides by a selective electro deposition process. In this paper, we
have examined the conditions of obtention of an uranium deposit in a chloride
bath, on various cathodic substrates. The electrolyte consists of an eutectic
mixture
NaCl-KCl as solvent (fusion
point 650°C) and a tetravalent Uranium compound, UCl4, as solutes.
The results presented stepwise in the paper are focused on the following
points: -
The
electrochemical behaviour of Uranium ions, investigated by cyclic voltammetry
and chronopotentiometry: This first part allows to conclude that uranium metal on a cathode is only
possible if Uranium IV is pre-reduced in Uranium III. Uranium is obtained in a
single step of electro reduction of trivalent uranium ions. -
Chronoamperometric
measurements showed, after modelling, that the formation of nuclei proceeds
instantaneously at the first time of electrolysis. The final coating results on
the further growth of these initial crystals. The observation by Scanning Electron Microscopy (SEM) of
samples evidences that the major part of the area is covered with small grain
of metals growing slowly, while on a few sites of the cathode, needles of
uranium are growing rapidly. Consequently, the final coatings are dendritic,
poorly adherent on the substrate and porous. -
Series of
electro deposition runs was carried out, in a chloride bath containing Uranium III, in the frame of an
optimal design. The results put into evidence faradic efficiencies close to one
in any conditions even at high current densities, low compacities of the
coating and a change of the morphology of the crystals at temperature above
710°C, attributed to the crystallographic transition α→β, occurring in this
range of temperature. -
SEM
observation of cross section of uranium coatings on the cathode evidenced the
influence of the cathodic substrate on the adherence of the coating: on copper
and stainless steel, alloy layers are observed under the dendritic coating,
while, in the case of steel substrate, a quite compact layer of pure uranium is
formed on 20-30 μm prior to the dendritic layer. This work showed the feasibility of the process for removing uranium
from chlorides solutions containing trivalent ions. Further investigations on
the physical phenomena promoting dentrites should allow to understand their
mode of formation, and further to propose a strategy to prepare compact
coatings . In situ Xray diffraction experiments will be carried
out to investigate the structural evolution of the metallic deposits with
temperature. High temperature NMR and EXAFS experiments will be used to
characterize the molten electrolyte and to propose a structural description of
the different species present in the bath.
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