ORNL-1771 - Oak Ridge National Laboratory

ANP QUARTERLY PROGRESS REPORT
ammeters, and a recording ampere-hour meter.
In the first experiments the nickel container was
used as the cathode, and helium gas was continu-
ously bubbled through the melt in order to stir and
to remove gaseous electrode products, The melt
was first thoroughly purified by a conventional HF
treatment followed by reduction with hydrogen.
Then current-voltage curves were taken before and
after the addition of 1980 ppm Fet+ as FeF,, The
effect of the added irnpurity was readily noticeable.
The purified melt gave a curve like plot D of
Fig. 5.9, while the FeF,-contaminated melt yielded
a curve similar to plot A. Extrapolation of the
curve obtained from the FeF,-bearing melt gave a
decomposition potential of 0.9 v.
An electrolyzing current of 5 amp was employed,
but numerous current-voltage curves were also
obtained so that at the end of 5 hr, 20.9 amp-hi had
passed, This was approximately twice the theo-
retical ampere-hours required to reduce the Fe";
a sample removed at this time analyzed 50 ppm Fe
and demonstrated that a fairly complete reduction
had been achieved with at least 50% current ebfi-
ciency. No evolution of fluorine was noted, nor
was there an appreciable attack on the nickel con-
tainer. The wall which had been in contact with
the vapor appeared to be covered with a fine dust,
presumably NiF,.
For the next series of experiments a new batch
of freshly purified melt was used, hydrogen vms
substituted for helium, and a current of 10 amp was
passed. Three purifications were carried out in
succession with addition of FeF, and NiF, to
simulate impurities, as shown in the first part of
Table 5.12. Apparent current efficiencies based on
.......
74
data from Table 5.12 are deceptive because some
ofthe reduction is accomplished by hydrogen, which
gives high values, and also because the current
efficiency for purification becomes very low when
the amount of impurity to be reduced is very small.
When the containerwas used as a cathode, 6 v was
the maximum applied potential required to obtain a
current of 10 amp. An HF concentration in the
effluent gas of about lom3 mole of HF per liter of
hydrogen was noted at 5 amp.
In another series of experiments a removable
cathode, insulated from the pot, was used. The
cathode was a Y8 x 4 in. cylinder of nickel gauze
welded to a '/8-ins nickel rod; the gauze was made
of 15-mil wire, 25 wires to the inch, The dimen-
sions were chosen to be in the correct proportion
for a small-scale model of a 250-lb capacity pro-
duction reactor. Toobtoin a relatively high level of
initial impurity, approximately 20 g of FeF, and
209 of NiF, were mixed with the starting materials.
After the usual HF treatment before electrolysis,
a sample was removed to determine the iron con-
centration. The results of the trial are summarized
in the second part of Table 5.12.
At the end of 12.5 amp-hr the cathode was re-
moved; it had gained 10.3 g of metallic iron (as
computed from chemical analysis), representing a
97% yield. A new cathode was then used until a
total of 37 amp-hr had passed; this cathode gained
0.2 g of iron, and thus the percentage recovery
had been increased to 99%. A current of 10 amp
gave current densities of 11.7 amp/cm2 of anode
surface. By doubling the nominal area of gauze
used as the cathode to obtain the total surface
area both inside and out, the current density at the
TABLE 5.12, ELECTROLYTIC PURlFlCATiON OF NaF-ZrF4 MELTS WITH HYDROGEN STIRRING
Impurities Before Impurities After
Cathode Electrolysis (ppm) Ampere-Hours Electrolysis (ppm)
Arrangement
____...
Passed _____--
Fe
Ni
Fe
Ni
- ...... ___ ........ .... ___..... ...... __
Nickel container 1980 20 20 15.2 95 55
as cathode 1890 2000 23.6 Not sampled
1980 None 11.8 100 40
* 150
Removable nickel 4660 1 2.5
gauze cathode
~ .........
** 37.0 80
......... .... ~
*Unfortunately, a reliable analysis of the nickel concentration was not obtained.
**Continued without additional impurity.