ORNL-TM-2036.txt

NOV 27 1967

 

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
NUCLEAR DIVISION
for the
U.S. ATOMIC ENERGY COMMISSION

ORNL- TM- 2036

 

£ pay
COPY NO. -~ & #

DATE - 11/2/6’7-

g@&\\i

Reduction of Iron Dissolved in Molten LiF-ThF,

C. J. Barton and H. H. Stone
ABSTRACT

Additions of 2%Fe tracer to LiF-ThF, (73-27 mole %) permitted rapid and
sensitive measurements of the iron content of filtered samples of molten

material. More than 40 hours were required for nearly complete removal of

P
@ L

iron from the melt by hydrogen reduction at about 600°C while reduction of
ilron by metallic thorium at the same temperature was virtually complete after
three hours. Disappearance of a relatively large quantity of solid thorium
on long exposure to the molten salt will require further investigation. Com-
parison of the data obtained by use of 59Fe tracer counts with the results

of colorimetric iron determinations by two different laboratories seems to
indicate that the colorimetric iron method employed when these tests were
performed did not give reliable iron results at low iron concentrations.
Colorimetric nickel determinations by the two laboratories give divergent

data for more than half of the samples.

N\

‘._.'v NOTICE This document contains information of a preliminary nature

' and was prepared primarily for internal use ot the Oak Ridge National
Laboratory. It is subject to revision or correction and therefore does
not represent a final report.
 

 

 

LEGAL NOTICE

This report was prepared as an account of Government sponsored wark, Neither the United States,

nor the Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty or representation, expressed or implied, with respect to the accuracy,
completeness, or usefulness of the information contained in this report, or that the use of
any information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; or

B. Assumes ony liabilities with respect to the use of, or for damages resulting from the use of
any informotion, apporatus, method, or process disclosed in this report.

As used in the above, "‘person acting on behalf of the Commission' includes any employee or

contractor of the Commission, or employee of such contractor, to the extent that such employee

or contractor of the Commission, or employee of such contractor prepares, disseminates, or
provides access te, any information pursuant to his employment or contract with the Commission,

or his employment with such contractor,

 

 

‘4
LEGAL NOTICE

This report was prepared 2s an account of Government sponsored work, Neither the United
States, nor the Commigsion, nor any person acting on behalf of the Commission:

A. Makes any warranty or representation, expressed or implied, with respect to the accu-

racy, completeness, or usefulness of the information contained in this report, or that the uge
....3._ of any information, apparatus, method, or process disclosed in this report may not infringe
privately owned rights; or

B. Assumes any liabilities with respect to the use of, or for damages resulting from the
use of any information, apparatus, methed, or precess disclosed in this report,

As used in the above, “person acting on behalf of the Commission®’ includes any em-
ployee or contractor of the Commissicn, or emplovee of such contractor, to the extent that
such employee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides access to, any information pursuant to his empioyment or contract
with the Commission, or his empioyment with such contractor.

Reduction of Iron Dissolved in Molten LiF-ThF,

C. J. Barton and H., H. Stone

INTRODUCTION

 

Many samples of LiF-ThF, (73-27 mole %) from protactinium recovery experi-
mentsl’2’3 have been analyzed for iron and some for nickel. In some cases it
has been possible to correlate the iron and protactinium results but the low
iron and nickel concentrations expected in filtered salt samples reduced with
metallic thorium have seldom been confirmed by the analytical data. We decided,
therefore, to conduct an experiment in which °9Fe tracer would be used to follow
the reduction of iron dissolved in molten LiF-ThF,; and to compare the >%Fe
results obtained with analytical values obtained by two laboratories that
routinely perform iron and nickel determinations using colorimetric methods.

Tt appears that similar studies have been performed at least twice before
during the history of the molten salt project at ORNL. The results were not
documented in any detall on either occasion, but the data obtained are in

general agfeement with the findings of the present investigation.

The LiF~-ThF, (73-27 mole %) used as the solvent material in this experi-
ment was supplied by J. H. Shaffer (Reactor Chemistry Division, ORNL) as part
of a large (3.5 kg) batch that had received the usual purification treatment
including hydrofluorination with a HF-Hp, mixture followed by prolonged hydrogen

reduction to remove iron, nickel, and other reducible impurities.
Ll

We obtalned approximately one millicurie of 5°Fe tracer by purchasing
10 mg of iron in the form of Fe,03 that had been enriched in 58pe isotope and
irradiating it in the LITR, The irradiated material was mixed with enough
inactive Fe,03 to give approximately 600 ppm of Fe when completely dissolved
in 320 g of salt. The salt and iron oxide were placed in a nickel container,
heated to approximately 600°C in flowing helium, treated with mixed helium
and anhydrous HF followed by a brief hydrogen treatment and then with mixed
hydrogen and HF in an effort to remove oxygen and to dissolve the added iron.
We believe, on the basis of thermodynamic dataf that this treatment also re-
duced Fe3T to Fe2+. Subsequent analyses of filtered samples indicated that
part of the added iron was not dissolved by this treatment. Hydrogen reduction
was then started, This gas was purified by passing it through a Deoxo unit to
convert any oxygen present to water and then through a column of Drierite and
a liquid-nitrogen-cooled trap to remove water. Hydrogen treatment continued
until the °?Fe count on filtered samples indicated that only a slight trace
of iron remained in the melt. The melt temperature varied during this period
from about 590 to 625°C due to daily variations in the line voltage that
supplied the furnace.

The melt was treated with mixed hydrogen-HF and with helium-HF to redissolve
the hydrogen-reduced iron. When the °°Fe counts indicated that no further in-
creagse in the iron content of filtered samples was occurring, we gave the melt
a brief hydrogen treatment (1—1/2 hours) to effect at least partial reduction
of dissolved Ni2¥ and removal of dissolved HF. Thorium rods approximately 1/4
inch in diameter were exposed to the melt for three, 1-hr periods and one, 1l6-hr
period, taking a filtered sample each time after the rod was removed, The rods

were cleaned by filing before they were reused. This experiment was performed
~Be

in a hood located in the High Alpha Molten Salt Laboratory since no protactinium

was added to the melt., The samples were handled in the hood as far as possible
because of the hazard of airborne thorium.

Most samples were removed by filtering through sintered copper filters
following the procedure previously described5. The frozen salt samples were
rembved from the fiter sticks and crushed in porcelain mortars. One-gram
portions of the samples were placed in small plastic vials, sealed in plastic
bags, and given the following analytical treatment. The gamma activity of the
solid samples was first measured by use of a multichannel analyzer. The samples
were then dissolved in the High Level Alpha Radiation Laboratory (Building 3508)
and analyzed for iron and nickel content by colorimetric methods, Portions of
the solutions were transferred to the General Hot Analysis Laeboratory (Building
2026) for similar determinations and the the Radioisotopes Radiochemistry
Laboratory (Building 3019) for 59Fe sounting. Selected samples were also sub-

mitted for spectrographic analysis as indicated in Table I.
ANATYTICAL DATA

Most of the analytical data obtained from the experiment are displayed in
Teble 1. It was necessary to calculate a factor for converting the 5%Fe counts
into iron concentrations., This was accomplished by choosing one or two samples
for which the 2026 and 3508 colorimetric analyses agreed reasonably well, and
dividing the average of their results by the 59Fe count (per minute). The values
calculated from the °%Fe counts obtained with solutions were slightly more
consistent than the values calculated from the °?Fe counts on solid samples
using less refined counting techniques, The iron concentration values shown
in parentheses in the 5%9Fe column of Table 1 are the values assumed to be

correct for the calculation of the conversion factor. The factor used for the
-6

hydrogen reduction phase of the experiment was not applicable in the last part
of the experiment., Although the 59Fe counts were approximately the same for
samples 14 and 15 as for samples 2-5, the colorimetric iron values were much
higher, We believe that some iron was introduced into the salt by use of
stainless steel samplers at a time when no copper samplers were available or
that the second hydrofluorination treatment was more effective in dissolving
the added iron oxide than the initial treatment.

Good agreement among the results of iron determination of the three labora-
tories is noted for about 1/3 of the samples analyzed. The largest discrepancies
between colorimetric determinations and °°Fe count values were obtained with
samples that were almost certainly contaminated. (Samples 12, 13, 19, and 21).
Exclusing these samples, reasonable agreement was obtained with almost half of
the samples analyzed by the three laboratories., In general, agreement was

poorest where the iron concentration calculated from tracer counts was less than
0.10 Fe/g.

An effort was made to ascertain whether the discrepancy observed at low
iron concentrations was due to some deficiency in the colorimetric iron method
or to sample contamination, Several samples, including the as received salt,
were submitted for spectrographic analysis. The value reported for the as
received salt, 0,011 mg/g, was lower than any of the colorimetric values obtained.
These ranged from 0.03 mg/g (General Analysis Laboratory) to 0.13 mg/g (2026 lab.).
Of course, no tracer result was obtained with this sample. The spectrographic
concentrations determined for the other three samples analyzed by this method
were all higher than the values calculated from °°Fe counts. In each case, the
spectrographic result was in good agreement with at least one colorimetric value
but it was lower than most of the data obtained by this method. If the spectro-

graphic data are correct, then we must assume that the samples were slightly
-T-

contaminated with iron either in our laboratory or in the analytical laboratory.
Any solid iron or nickel or compounds of either metal, that was introduced into
a sample after it was removed from the melt would have been dissolved and thus
would contaminate the sample solution. It appears, however, that the colori-
metric iron method tends to give high results with samples having a low iromn
concentration.

Much less attention has been given to the colorimetric nickel data because
we had no tracer for this element. The results obtained by the 2026 laboratory
were lower than those reported by the 3508 laboratory for a majority of the
samples but the cause of the observed discrepancies has not been determined.
since Ni2¥ is thermodynamically incompatible with Fel at 6OOOC, high nickel
values in filtered, reduced samples of salt are unlikely to be correct unless
the samples were contaminated or metallic nickel particles were small enough to

pass through the sampler filters.
IRON .REDUCTION

The plot of iron concentration as a function of time is shown in Fig. 1.
The hydrogen reduction process is obviously quite slow and the reduction rate
seems to diminish with decreasing iron concentration. It is not clear whether
any significance can be attached to the apparently linear rates during the
initial and middle fractions of the reduction period as indicated in Fig. 1
but the data indicate that the first 10% of the iron was reduced in less than
three hours while approximstely 12 hours were required to remove the last 10%.

The thorium reduction process was quite rapid in comparison to hydrogen
reduction and there was no indication of a change in reduction rate during
the initial three-hour period when 97.5% of the iron activity was removed from

solution,
8-
TnORTUM LOSS

A puzzling aspect of this experiment was that during the 16-hr period
between samples 22 and 23, the thorium rod (estimated to weigh sbout 12 g)
used to reduce the iron completely disappeared. Samples 25-28 were obtained
during the post-mortem phase of the experiment when the nickel pot was cut
through and its contents were removed for examination and analysis. Black
lumps of varying size were removed from the frozen salt, ground to pass a 40-
mesh sieve and submitted for analysis. The complete analysis of samples 25-27
is given in Table 2. In addition to the chemical analysis, which is not entirely
satisfactory because none of the totals came close to 100%, sample 25 (the
largest black lumps) was submitted for X-ray diffraction examination. The only
definitely identified component of the material was LisThFy but LiF and LiThFs
were reported to be possibly present and a number of unidentified lines were
also found.

If we assume that all the fluoride ions were combined either with lithium
or thorium, calculations show that 170 mg/g of thorium was present as metal in
sample 25 and 290 mg/g in sample 26. Since metallic nickel and thorium were
not found in sample 25 by X-ray diffraction, it is possible that these metals
were present as an intermetallic compound of unknown composition.

The fact that the black material composing samples 25 and 26 could be
ground to small particles seems to indicate that the metals present were depos-
ited from the melt.

The disappearance of a significant quantity of so0lid thorium on long ex-
posure to molten LiF-ThF, served as a reminder of similar behavior in an experi-
ment, Run 2-22 (66), reported earlier.5 There, a 6-hr exposure resulted in

removal of 28 g of thorium from a larger rod than that used here. In that
9=

instance, it was speculated that the thorium rod came in contact with the
bottom of the nickel pot causing a current to flow that eroded the thorium rod.
In both experiments the thorium rod was supported by a l/8-in nickel rod that
was electrically insulated from the container by a Teflon plug. Black magnetic
material removed from the nickel pot after cooling to room temperature in the
earlier experiment analyzed 45% nickel and 30% thorium, while non-magnetic
material contained 22% nickel and 49.5% thorium. The chunks of black material
found in the pot were quite brittle, as in the present experiment, which was
interpreted to mean that they were aggregates of finely divided thorium and
nickel particles.

While the same explanation of thorium loss given earlier could be offered
here, an alternative explanation can be given although i1t is purely speculative

at present. This assumes that the reaction

3thF, + Th® -  A4ThF,

can occur in the molten mixture and that the ThF;, when it diffuses to the
nickel wall, disproportionates because of formation of Th-Ni intermetallic
compounds. Failure to find X-ray evidence of such compounds in sample 25
weakens the argument for this explanation, but since the form of the nickel
present has not been determined, the question remains open. Since ThF; is not
observed in our frozen salt samples, and it has not been reported in the liter-
ature, we must assume that if the above reaction occurs at 600° it must be
reversed on cooling.

Since ThFs, if it exists, may be strongly colored, we plan to expose
molten LiF-ThF, to solid thorium in a furmace that allows visual observation

of the melted material.
=10~

Conclusions
1. Use of °°Fe tracer gives a sensitive measure of the iron content of
fluoride salt samples,
2. The colorimetric iron method presently employed by the 2026 and 3508
laboratories does not appear to give reliable results at low iron concentrations.
3. There is a large and presently unexplained discrepancy in the nickel
analyses by the 2026 and 3509 lsboratories for a large fraction of the samples.
4, Disappearance of a comparatively large amount of thorium metal on
long exposure to molten LiF-ThF, raises the possibility that a lower-than-
normal valence state of thorium may occur in melts exposed to solid thorium.
In addition to its scientific interest, this reaction could affect the use of
solid thorium as the reductant for protactinium and we:are planning further

examination of this phenomenon.
Table 1. Analysis of Samples from Iron Reduction Experiment

 

 

 

 

 

Sample
No. Iron Concentration (mg/g) Nickel Conc. (mg/g) Sample Description
2026 3508 *9Fe Spec, 2026 3508
Lab. Lab. Count Anal, Lab. Lab.
0 0,13 0,07 - 0,011 < 0,01 0.04 Salt as received
1 0.52 < 0,01 0.21 0.06 0.02 Filtered salt - 1% hr He-HF
2 0.27 0,02 0.31 < 0,01 0.26 Filtered salt - ¥ hr He-HF
3 0.18 0,04 0.33 < 0.01 0.12 Filtered salt - 13 hr Hp
4 0.29 0.34 (0.315) < 0,01 0.35 Filtered salt - 1 hr Hy-HF
5 0.29 0.30 (0.,295) < 0.01 0.32 Filtered salt - 2 hr Hp
6 0.25 0.22 0,22 < 0.01 0.08 Filtered salt - 73 hr Hp
7 0.20 0.01 0.13 0,06 0.17 Filtered salt - 18 hr H,
8 0.21 0.09 0.07 < 0,0L 0.17 Filtered salt - 26 hr Hz
9 0,18 0.10 - 0,27 - < 0,01 0.030 0.15 0,07 0.14 Filtered salt - 31% hr H
10 0.12 0,05 - 0,17 - 0,10 0.002 0.04 < 0,01 0.29 Filtered salt ~ 41? hr Hp
11 0,14 0,02 - 0,16 - 0,07 0,003 0.03 0.09 0.12 Filtered salt - 49 hr H
12 0.67% 0,10 - 0,612 0,021 0.21 0.24 Filtered salt - 1 hr Hp-HF
13 1,858 2.10% 0.07 0.29 0,27 Filtered salt - 3 hr Hp-HF :
14 0.13 0,17 0,55¢ < 0,01 0,04 Filtered salt - 5% hr Hp-HF -
15 .53 0.58 (0.55) < C.01 0.17 Filtered salt - 16 hr He-HF &
16 0.43 0,47 G.50 0.33 0.27 Filtered salt - 2 hr Hp-HF
17 0.48 0.54 0.52 < 0,01 0,03 Filtered salt - 13 hr Hp
18 0,38 0.47 0.43 < 0.01 0.11 Filtered salt - 1 hr Th exp.
19 14.3 b 114.9 P 0.11 0.18 0,22 Filings from Th rod
20 0.17 0.20 0,13 < 0,0L 0.11 Filtered salt - 2 hr Th exp.
21 2,89P 2,99P 0.07 0.16 0.14 Filings from 2nd Th rod
22 0.14 0,05 0.01 0.33 0.50 Filtered salt - 3 hr Th exp.
23 < 0,01 < 0,01 0.04 < 0.01 2.23 Filtered salt - 19 hr Th exp.
24 < 0,01 0,27 1.05 240 205 Crust from Ni support rod
2 - 1,03 1.32 - 140-218 Large black lumps from salt
26 - 1,84 1.36 - 131-137 Small black lumps from salt
27 - 0.53 0.37 - 3.7-5.5 Ground unfiltered salt
28 Total 34.5 25.7 - - Material leached from vessel

wall by acid
Ycontaminated by stainless steel sampler,
bProba.bly contaminated by iron from file used to remove surface of Th rod.

“The higher iron concentration in this and subsequent samples, as compared to earlier uncontaminated samples, is
possibly due to use of stainless steel samplers for samples 12 and 13, or to solution of some of the added iron
that did not dissolve in the initial hydrofluorination treatment,
Table 2.

Removed from Nickel Pot

~12-

Analysis of Material

 

 

 

Sample Concentration (mg/g)
No. Th Li F T'e Ni Total
25 524 2L.2 174 1,03 218 936
26 646 24.1 150 1.84 134 956
27 561 béi 1 310 0.53 4.6 920

Theoretical

(pure salt) 615 49,5 335 - - 1000

 
-13-

ORNL-DWG 67-10339

 

 

 

 

 

 

 

 

 

 

 

100 @

\
0 T®

\ ® COUNT ON SOLUTIONS
80 + \ A COUNT ON SOLID SAMPLES
70—\
o

60 \
50 \ HYDROGEN REDUCTION

 

 

 

40 \JK‘
30

X

 

 

IRON CONCENTRATION (% of initial °°Fe count)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

®
20 \‘\
10 —THORIUM REDUCTION \.‘
\ .
o2 é J
0 {2 24 36 48

DURATION OF HYDROGEN OR THORIUM TREATMENT (hr)
Fig. 1. Reduction of Fe2t in LiF-ThF, (73-27 mole %) as Indicated by 5%Fe

Counts on Filtered Samples.
References

C. J. Barton and H. H. Stone, "Protactinium Studies in the High-Alpha Molten
Salt Laboratory,” MSR Program Semisnn. Progr, Rept. Feb. 28, 1966, ORNL-3936,
p. 148,

Tbid, ORNL-4119, p. 153.

C. J. Barton, 'Recovery of Protactinium from Breeder Blanket Mixtures,"

MSR Program Semiann., Progr. Rept. August 31, 1965, ORNL-3872, p. 137.

A, Glassner, "The Thermochemical Properties of the Oxides, Fluorides, and
Chlorides to 25OOOK, ANTL,-5750 (1957).

C. J. Barton, H, H. Stone, "Removal of Protactinium from Molten Fluoride

Breeder Blanket Mixtures,"” ORNL-TM-1543, June, 1966.
~15-

INTERNAL DISTRIBUTION

1. R. F. Apple

2. C. F. Baes

3-12. C. J. Barton
13. E. S. Bettis

14, F. F. Blankenship
15. E. G. Bohlmann
16. G. E. Boyd

17. J. Braunstein
18, M. A, Bredig
19, R. B. Briggs

20, H. R. Bronstein
2l. S. Cantor

22. L. T, Corbin

23. S. J, Ditto

24. D. E. Ferguson
25, L. M, PFerris

26. W. R. Grimes

27. A. G. Grindell
28, P, N. Haubenreich
29. P. R. Kasten
30, M. T. Kelly
31l. C. E. Lamb
32. R. E. MacPherson
33. H. E. McCoy

34, H, F. McDuffie
35. R. L. Moore
36. E. L. Nicholson
37, L. C. Oakes
38. A, M., Perry
39-40, M. W. Rosenthal

41, Dunlap Scott
42, J. H. Shaffer
43-44, M. J. Skinner

45, H. H. Stone
46, R. E. Thoma
47. J. R. Weir
48. M. E. Whatley
49, J. C., White

50-51, Central Research Library
52-53. Document Reference Section
54-55. TILaboratory Records

56. Laboratory Records, ORNL R.C.

EXTERNAL: DISTRIBUTION

57-71. Division of Technical Information Extension (DTIE)
72. Laboratory and University Division, ORO

ORNL-TM-2036