ORNL-4191 - the Molten Salt Energy Technologies Web Site

One alternative to the distillation process for
decontaminating MSBR fuel salt involves reductive
extraction of the rare earths (and other fission
products) after the uranium has been recovered
from the salt by fluorination. ’-’ The reductant
currently being considered is ’Li dissolved in
molten metals such as bismuth or lead which are
immiscible with the salt. Studies in the Reactor
Chemistry Division’ n 2 j 4 have defined the equilibrium
distribution of several rare earths between
Li-Ri solutions and LiF-BeF, (66-34 mole X) at
600°C. The results of these studies indicate that
the rare earths can be effectively extracted in
such a system.
The apparent stoichiometry of the extraction
reaction is such that, under the experimental conditions
employed, the lithium concentration in the
metal phase should not change detectably, even
if the rare earth were quantitatively extracted.
However, in these, and similar, experiments,
usi-ially 50 to 75% of the lithium originally present
in the Li-Bi solution appainntly was consumed.
The mechanism by which lithium is “lost” in
these systems has not yet been elucidated.
In order to properly evaluate the practicality of
the reductive extraction process, the behavior of
lithium in these systems must be clearly defined.
Knowledge of its behavior is especially important
in designing a multistage extraction system, because
the extent to which the rare earths are ex-
‘R. B. B~iggs, MSR Program Semiann. Pro@. Rept.
AI@. 31, 1966, ORNL-4037.
’M. W. Rosenthal, MSR Program Semiann. Progr.
Rept. Feb. 28, 1967, ORNL-4119.
’D. E. Ferguson, Chem. Technol. Div. Ann. Progr.
Rept. Map 31, 1966, ORNL-3915.
‘W. R. Cirmles, Reartor Chern. Uiv. Ann. Progr.
Rept. Dec. 31, 1966, ORNL-4076.
L. M. Ferris C. E. Schilling J. F. Land
248
tracted (at equilibrium) is directly related to the
lithium concentration in the metal phase. 1 r 2 , 4
Consequently, more experiments have been con-
ducted, with the primary objective being the de-
termination of the cause of the apparent lithium
loss. As discussed below, these experiments did
not produce a definite answer to the question,
and more study will be required. In each experi-
ment a rare-earth fluoride, usually EuF,, was
added to the salt to provide a basis for compari-
son with the results of earlier studies and to allow
us to obtain preliminary information on the effect
of temperature on the distribution of europium be-
tween the two phases.
Experimentally, solutions of lithium in bismuth
(usually about 7 at. % Li) and of EuF, in LiF-
BeF, (66-34 mole %; mole fraction of EuF,,
about lo-‘) were prepared at 600°C in separate
mild steel or graphite crucibles, using pure argon
as a blanket gas. Then, the salt was transferred
to the crucible containing the Li-Bi alloy, and the
system was equilibrated at high temperature in an
argon atmosphere, Filtered samples (stainless
steel samplers) of both phases were removed
periodic a1 1 y for analysis.
No detectable “loss” of lithium occurred during
preparation of the Li-Bi alloys in either mild
steel or graphite crucibles. When the system was
not agitated (by sparging with argon), the time
required to achieve equilibrium was generally
about 24 hr. The low rate of dissolution of lithium
in bismuth when the system is quiescent has been
observed by others.’ The rate-controlling step
may be the dissolution of Li,Bi, a high-nelting,
insoluble conpund that probably forms rapidly
when mixtures of the two metals are heated.
After about 24 hr at 600°C, the lithium concentra-
tion in the solution, as determined by chemical