Pyrochemical Separations - OECD Nuclear Energy Agency

The metal was either cast into the pre-melted fluoride salt (LiF(49.5 wt.%)-CaF2(38.5 wt.%)-Mo(12 wt.%) or
LiF(55.2 wt.%)-CaF2(42.8 wt.%)-Ru(2 wt.%)) or was put under the salt from the beginning. The experiment lasted
two hours at 790°C with a stirring rate of 40 rpm under argon or hydrogen. The final concentration of
Mo or Ru in the fluoride salt was determined by inductive coupled plasma spectrometry. The reactor
in which the experiments were carried out is shown in Figure 4.
Results and discussion
Results under argon
The first experiments were conducted under argon. Of all the liquid metals, Ru had the best
recovery rate; 95% of Ru was in the metal. Further experiments must be performed with smaller
particles. For Mo, Sn did not lead to a good recovery – 20 wt.% of Mo was extracted – whereas Sb and
Zn seemed to be good digesting metals – 75 wt.% Mo recovered. A complete recovery for Mo under
argon was never obtained. The samples obtained with Sn showed a 2 mm thick dark grey layer at the
salt-metal interface. With Sb and Zn, we had a dark grey mass on one point at the interface. An analysis
by SEM of these samples allows us to conclude that all the metallic particles not digested settled
through the salt and stayed in the dark grey accumulations. The presence of this dark grey layer and its
importance are a sign of how the digestion occurred.
We also carried out some experiments varying the time of contact between the molten salt and the
liquid metal. Those results are shown in Figure 5. It seems that time does not allow to eliminate the
problems that limit the digestion.
Moriyama, et al. [6-8] observed a similar dark grey layer at the interface in their study of
actinides/lanthanides separation. They had the same problem of digestion with the inter-metallic
compounds formed in the salt between the species reduced and the metal solvent. By studying the
influence of temperature, reducing agent concentration and salt composition, they found that the
extraction of the wanted elements was not controlled by the reduction but by the mass transfer of
the metallic species at the interface. The authors found that more than 15 h were necessary to get the
metallic compounds into the metal solvent.
Experiments with larger Mo particles (30 µm) gave good recovery (∼80% Mo recovered) for all
the digesting metals. This suggests the presence of an oxide film on the metal surface which prevents
the Mo particles from being in contact with the metal, especially in the case of Sn. Furthermore, an
AUGER spectrometry analysis of the Mo particles showed a superficial oxidation of Mo in MoO3.
These two phenomena entail a bad wettability of Mo by the metallic solvent and a good wettability of
Mo by the fluorides under argon, and thus a limited digestion. In the case where inter-metallic
compounds can be formed between Mo and the metal at the interface, wetting could begin on some
points of the particle and then allow the digestion under argon. Sb, Sn and Zn may form inter-metallic
compounds with Mo but the kinetics must be different.
Results under hydrogen
In order to see if the absence of the layer of oxide on the liquid metal and around the Mo particles
permits to digest these particles, we conducted the same experiments under hydrogen, which is
supposed to reduce all these oxides. The results are displayed in Figure 6.
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