ORNL-1816 - the Molten Salt Energy Technologies Web Site

the drying which occurred on heating to 600OC.
When the current was applied at 6OO0C, 30 v was
required to obtai amp. The anode was removed
for examination, found to be in good condition,
and reinstal led. pparently, this was an "anode
effect," and it probably due to the presence
of oxygen, which is frequently encountered in the
electrolysis of fluoride melts with carbon anodes.
By using a helium sweep rate of 200 cm3/min,
CF, was found to be the major constituent (0.33%)
in the effluent gas. Also, CO was present to an
extent of 0.07%. On raising the melt temperature,
the anode effect diminished and then suddenly
disappeared at 710OC. The response to temper-
ature was reversible; the anode effect was pro-
nounced at 7OOOC and absent at 725OC. The
electrolysis was continued at 725'C.
At low current densities (1 amp), where easily
reducible impurities were presumed to be carrying
the current, the predominant constituent recog-
nizable by mass spectrometry of the effluent
helium was CO, which was produced at an ef-
ficiency of about 0.2 equivalent per faraday.
At slightly higher current densities (1.3 amp),
where a plateau in the E-1 curve occurred because
the current was controlled by rate of diffusion
of easily reducible ions and where very little
increase in current resulted from an increase in
applied voltage, CF, began to appear in the
effluent gas.
At still higher current densities, the decom-
position potential of ZrF, was exceeded, and CF,
became more prominent, along with small amounts
of higher fluorocarbons. For 6.0 v and 3.6 amp
PERIOD ENDING DECEMBER 10,7954
the efficiency of production of CO was 14%, and
that of CF, was 16%.
As shown in Table 5.5, impurities (94G ppm of
Fett and smaller amounts of Ni" and Cr")
theoretically equivalent to 0.15 faraday were
initially present. Additional electrolysis beyond
five times the theoretical amount apparently had
little effect on the Fe analysis. The experiment
demonstrated that NaF-ZrF, mixtures may be
effectively purified by electrolysis under helium
without any preliminary HF treatment. No notice-
able corrosion of the electrolysis apparatus OC-
curred.
When H, was used as the sweep gas while
electrolyzing NaF-ZrF, (53-47 mole %) at 8OO0C,
HF could be removed at a rate proportional to
the current used (0.32 equivalent of HF per
faraday). These results were obtained with a
purified melt from which zirconium meial was
being deposited.
In view of a poor efficiency found for the elec-
trolyses of 250-ib batches containing UF,, an
experiment on 3 kg of NaF-ZrF,-UF, (53..5-40-6.5
mole %) was carried out by using H, arid elec-
trolysis simultaneously. A filtered sample showed,
prior to electrolysis, 740 ppm Fe, 90 ppm Ni,
40 pprn Cr, and 46 ppm S. The HF level when
the filtered sample was withdrawn was 1 x
mole/liter. Simultaneous electrolysis and H,
stripping were started. Periodically the elec-
trolysis was stopped and the HF concentration
in the effluent H, was measured to ohain an
index of the degree of reduction achieved. The
HF concentrations prevailing during electrolysis