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1Y PROGRESS REPORT
y equilibration of Fe" with NaZrF,
in similar equipment. However, equilibration of
Cr" with the alkali fluoride eutectic produces
about 500 ppm of Cr in solution, and, based on
the findings reported in the preceding section,
this is, presumably, a mixture of Cr3' and Cr".
This high concentration of soluble chromium in
the NaF-KF-LiF system is surprising, since about
100 ppm of Crtt is obtained in a similar equilibration'
with NaZrF,. The reaction responsible
st
value is not yet known.
for the reaction of UF, with
olvent indicate that at 300°C about
s present in the equilibrium
filtrate; this is nearly the same concentration
. At 6OO0C, however, about
covered in the filtrate. It
that the complex reaction in
h more temperature sensitive
e reaction in NaZrF,.
PRODUCTION OF PURIFIED MOLTEN
FLUORIDES
F. F. Blankenship G. J. Nessle
L. G. Overholser
Materials Chemistry Division
Electrolytic Purification of Zirconium-Base
Fluorides
C. M. Blood H. A. Friedman
F. W. Miles F. P. Boody
Materials Chemistry Division
Interest in the extremely short process time and
high efficiency obtained in the electrolytic purifi-
cation of molten NaF-ZrF, mixtures led to an ex-
ploration of the limitations of electrolysis as a
means of purification. At cathode current densities
up to 0.3 amp/cm2, the presence of UF, in a salt
mixture was found to greatly reduce the efficiency
of electrolysis because the reduction of UF, to
at the cathode was counteracted by the
UF,
oxidation of UF, to UF at the anode. A steady
state was approached wten about 5% of the UF,
had been converted to UF,. Apparently the rapid
NaF-ZrF, mixtures cone,
in large part, to the
of zirconium metal on
removal of fluorine as HF
the stripping gas or as
was used. The reduced
e precipitated in the
cathode deposit. Any structural metal ions which
were not deposited as a primary electrode process
were precipitated by reaction with the zirconium
metal.
Electrolysis was most effective in purification
when two advantages were achieved: first, a rapid
rate of over-all reduction of the melt, or a good
current efficiency, as measured by a high rate
of removal of HF or fluorocarbons, and second,
the deposition of impurities as an adherent cathode
coat which could be removed from the cell. In
NaF-Li F-KF mixtures the electrolytically reduced
potassium was slightly soluble in the melt, and
therefore oxidation of dissolved potassium at the
anode nullified reduction at the cathode and the
current efficiency for over-all reduction was low.
Also, an adherent cathode deposit was not
regularly found. Presumably, both these ad-
vantages could be regained, not only for the
NaF-LiF-KF system but for UF4-containing melts
as well, by alterations in cell design, electrode
construction, and current densities.
Some of the incidental points noted were that
0-- and SO,-- could be removed by electrolysis
in NaF-ZrF, melts, and hence HF treatment is
not necessarily required in the purification of salt
mixtures. Platinum was unsuitable as an anode
material. The NaF-LiF-KF mixtures were par-
ticularly prone to show an "anode effect," or gas
polarization of the graphite anode, when there
was a slight contamination by air. The UF,-UF,
oxidation-reduction system showed no polarization
at graphite anodes or nickel cathodes.
The electrolyses were carried out in apparatus
of the type previously described.22 The graphite
anodes were 1.12 cm in diameter and were im-
mersed to a depth of about 10 cm to give an area
of about 25 cm2. The cathodes were constructed
of a ?-in.-dia cylinder of nickel gauze having a
8
calculated immersed area of about 70 cm2. The
wet areas of the pot and of the probes which were
occasionally used as electrodes were 500 and
10 cm2, respectively. Ordinarily, 10 amp was
passed during any continuous electrolysis step.
Preliminary results with SO,-- removal were
sufficiently encouraging that an electrolysis trial
with helium as the sweep gas was carried out on
a 3-kg batch of KaF-ZrF, (53-47 mole %) which
had received no previous purification other than
me
LL C. M. Blood et al., ANP Quar. Prog. Rep. Sept. 10.
1954, ORNL-1771, p 73.
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