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NOY 21 1967
HELP r.
O A K RIDGE N A T I O N A L LABORATORY operated by
UNION CARBIDE CORPORATION NUCLEAR DIVISION for the
U.S. ATOMIC ENERGY COMMISSION
ORNL - TM - 2036 COPY NO.
-
r; -I '/if
. .
Reduction of I r o n Dissolved i n Molten LiF-ThF4 C. J. Barton and H. H. Stone
ABSTMCT Additions of 59Fe t r a c e r t o LiF-ThF,: (73-27 mole 'r A&' I .
4)
permitted r a p i d and
s e n s i t i v e measurements of t h e i r o n content of f i l t e r e d samples of molten material.
More than 40 hours were required f o r nearly complete removal of
i r o n from t h e m e l t by hydrogen reduction a t about 6OO0C while reduction of i r o n by m e t a l l i c thorium a t t h e same temperature w a s v i r t u a l l y complete a f t e r t h r e e hours.
Disappearance of a r e l a t i v e l y l a r g e q u a n t i t y of s o l i d thorium
on long exposure t o the molten s a l t w i l l r e q u i r e f u r t h e r i n v e s t i g a t i o n .
Com-
parison of t h e data obtained by use of 59Fe t r a c e r counts w i t h t h e results of colorimetric i r o n determinations by two d i f f e r e n t l a b o r a t o r i e s seems t o i n d i c a t e t h a t t h e colorimetric i r o n method employed when t h e s e t e s t s were performed d i d not give r e l i a b l e i r o n results a t low i r o n concentrations. Colorimetric n i c k e l determinations by t h e two l a b o r a t o r i e s give divergent data f o r more than h a l f of t h e samples.
NOTICE This document contains information of a preliminary nature and was prepared primarily for internal use a t the Oak Ridge National Laboratory. It i s subiect to revision or correction and therefore does not represent a final report.
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-3-
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Reduction of I r o n Dissolved i n Molten LiF-ThF4 C. J. Barton and H. H. Stone
INTRODUCTION
Many samples of LiF-ThF4 (73-27 mole $) from protactinium recovery experiments ' j 2 j 3
have been analyzed f o r i r o n and some f o r nickel.
I n some cases it
has been possible t o c o r r e l a t e t h e i r o n and protactinium r e s u l t s b u t t h e low i r o n and n i c k e l concentrations expected i n f i l t e r e d s a l t samples reduced w i t h m e t a l l i c thorium have seldom been confirmed by t h e a n a l y t i c a l data.
We decided,
therefore, t o conduct an experiment i n which 59Fe t r a c e r would be used t o follow t h e reduction of i r o n dissolved i n molten LiF-ThF4 and t o compare t h e 59Fe r e s u l t s obtained with a n a l y t i c a l values obtained by two l a b o r a t o r i e s t h a t r o u t i n e l y perform i r o n and n i c k e l determinations using colorimetric methods.
It appears t h a t s i m i l a r s t u d i e s have been performed a t least twice before during t h e h i s t o r y of t h e molten salt project at O m .
The r e s u l t s were not
documented i n any d e t a i l on e i t h e r occasion, b u t t h e d a t a obtained a r e i n general agreement with t h e findings of t h e present i n v e s t i g a t i o n .
EXPERIMENTAL PROCEDUFB The LiF-ThF4 (73-27 mole
9) used
as t h e solvent material i n t h i s experi-
ment w a s supplied by J. H. Shaffer (Reactor Chemistry Division, ORNL) as p a r t of a l a r g e (3.5 kg) batch t h a t had received t h e usual p u r i f i c a t i o n treatment including hydrofluorination with a HE'-H2 mixture followed by prolonged hydrogen reduction t o remove iron, nickel, and other reducible impurities.
W e obtained approximately one m i l l i c u r i e of 59Fe t r a c e r by purchasing
I
I
10 mg of i r o n i n t h e form of Fe2O3 t h a t had been enriched i n 58Fe isotope and i r r a d i a t i n g it i n t h e LITR,
The i r r a d i a t e d m a t e r i a l was mixed w i t h enough
i n a c t i v e Fez03 t o give approximately 600 ppm of Fe when completely dissolved i n 320 g of salt.
The s a l t and i r o n oxide were placed i n a n i c k e l container, 0
heated t o approximately 600 C i n flowing helium, t r e a t e d with mixed helium
and anhydrous HF' followed by a b r i e f hydrogen treatment and then with mixed hydrogen and HF i n an e f f o r t t o remove oxygen and t o dissolve t h e added i r o n . We believe, on t h e b a s i s of thermodynamic data? t h a t t h i s treatment a l s o reSubsequent analyses of f i l t e r e d samples indicated t h a t
duced Fe3+ t o Fe2+.
p a r t of t h e added i r o n w a s not dissolved by t h i s treatment. w a s then s t a r t e d .
Hydrogen reduction
This gas w a s p u r i f i e d by passing it through a Deoxo u n i t t o
convert any oxygen present t o water and then through a column of D r i e r i t e and a liquid-nitrogen-cooled t r a p t o remove water.
Hydrogen treatment continued
u n t i l t h e 59Fe count on f i l t e r e d samples indicated t h a t only a s l i g h t t r a c e of i r o n remained i n t h e m e l t .
The m e l t temperature varied during t h i s period
0
from about 590 t o 625 C due t o d a i l y v a r i a t i o n s i n t h e l i n e voltage t h a t supplied t h e furnace. The melt was treated with mixed hydrogen-HF and with helim-HF t o redissolve t h e hydrogen-reduced iron.
When t h e 59Fe counts indicated t h a t no f u r t h e r i n -
crease i n t h e i r o n content of f i l t e r e d sarwples w a s occurring, w e gave t h e m e l t a b r i e f hydrogen treatment (1-1/2 hours) t o e f f e c t a t least p a r t i a l reduction
of dissolved N i 2 + and removal of dissolved HF.
Thorium rods approximately 1/4
inch i n diameter were exposed t o t h e m e l t for three, l - h r periods and one, 16-hr period, taking a f i l t e r e d sample each t i m e a f t e r t h e rod w a s removed. were cleaned by f i l i n g before they were reused.
The rods
This experiment was performed W
-5I
i n a hood located i n t h e High Alpha Molten S a l t Laboratory since no protactinium
was added t o t h e m e l t .
The samples were handled i n t h e hood as far as possible
because of t h e hazard of airborne thorium. Most samples were removed by f i l t e r i n g through s i n t e r e d copper f i l t e r s following t h e procedure previously described
5
.
The frozen s a l t samples were
removed from t h e f i t e r s t i c k s and crushed i n porcelain mortars.
One-gram
portions of t h e samples were placed i n s m a l l p l a s t i c v i a l s , sealed i n p l a s t i c bags, and given t h e following a n a l y t i c a l treatment,
The gamma a c t i v i t y of t h e
s o l i d samples was f i r s t measured by use of a multichannel analyzer.
The samples
were then dissolved i n t h e High Level Alpha Radiation Laboratory (Building 3508) and analyzed f o r i r o n and n i c k e l content by colorimetric methods.
Portions of
t h e solutions were t r a n s f e r r e d t o t h e General Hot Analysis Laboratory (Building 2026) f o r similar determinations and t h e t h e Radioisotopes Radiochemistry Laboratory (Building 3019) f o r 59Fe counting.
Selected samples were a l s o sub-
mitted f o r spectrographic a n a l y s i s as indicated i n Table I. ANAIYTICAL DATA, .-
Most of t h e a n a l y t i c a l d a t a obtained from t h e experiment are displayed i n Table 1. It w a s necessary t o c a l c u l a t e a f a c t o r f o r converting t h e 59Fe counts i n t o i r o n concentrations.
This was accomplished by choosing one o r two samples
f o r which t h e 2026 and 3508 colorimetric analyses agreed reasonably w e l l , and dividing t h e average of t h e i r results by t h e 59Fe count ( p e r minute).
The values
calculated from t h e 59Fe counts obtained with solutions were s l i g h t l y more c o n s i s t e n t than t h e values calculated from t h e 59Fe counts on s o l i d samples using l e s s refined counting techniques.
The i r o n concentration values shown
i n parentheses i n t h e 59Fe column of Table 1 a r e t h e values assumed t o be U
c o r r e c t f o r t h e c a l c u l a t i o n of t h e conversion f a c t o r .
The f a c t o r used f o r t h e
-6hydrogen reduction phase of t h e experiment w a s not applicable i n t h e l a s t p a r t of t h e experiment.
Although t h e 59Fe counts were approximately t h e same f o r
samples 14 and 15 as f o r samples 2-5, t h e colorimetric i r o n values were much higher.
W e believe t h a t some i r o n was introduced i n t o t h e s a l t by use of
s t a i n l e s s s t e e l samplers a t a time when no copper samplers were a v a i l a b l e o r t h a t t h e second hydrofluorination treatment w a s more e f f e c t i v e i n dissolving t h e added i r o n oxide than t h e i n i t i a l treatment.
Good agreement among t h e r e s u l t s of i r o n determination of t h e t h r e e laborat o r i e s i s noted f o r about 1/3 of t h e samples analyzed.
The l a r g e s t discrepancies
between colorimetric determinations and 59Fe count values were obtained with samples t h a t were almost c e r t a i n l y contaminated.
(Samples 12, 13, 19, and 21).
Exclusing these samples, reasonable agreement w a s obtained w i t h almost h a l f of t h e samples analyzed by t h e t h r e e l a b o r a t o r i e s .
I n general, agreement w a s
poorest where t h e i r o n concentration calculated from t r a c e r counts w a s less than
0.10 Fe/g.
An e f f o r t was made t o a s c e r t a i n whether t h e discrepancy observed a t low i r o n concentrations was due t o some deficiency i n t h e colorimetric i r o n method
or ‘m sample contamination.
Several samples, including t h e as received s a l t ,
were subinitted f o r spectrographic analysis.
The value reported f o r t h e as
received salt, 0.011 mg/g, w a s lower than any of t h e colorimetric values obtained. These ranged from 0.03 mg/g (General Analysis Laboratory) t o 0.13 mg/g (2026 l a b . ) . O f course, no t r a c e r result w a s obtained with t h i s sample.
The spectrographic
concentrations determined f o r t h e o t h e r t h r e e samples analyzed by t h i s method were a l l higher than t h e values calculated from 59Fe counts.
I n each case, t h e
spectrographic result w a s i n good agreement with a t least one colorimetric value but it w a s lower than most of t h e d a t a obtained by t h i s method.
If t h e spectro-
graphic d a t a are correct, then w e must assume t h a t t h e samples were s l i g h t l y
I b
-'I -
Y
contaminated with i r o n e i t h e r i n our laboratory or i n t h e a n a l y t i c a l laboratory. Any s o l i d i r o n o r n i c k e l or compounds of e i t h e r metal, t h a t w a s introduced i n t o a sample a f t e r it w a s removed from t h e m e l t would have been dissolved and thus would contaminate t h e sample solution.
E t appears, however, t h a t t h e c o l o r i -
metric i r o n method tends t o give high results with samples having a low i r o n concent ration. Much less a t t e n t i o n has been given t o t h e colorimetric n i c k e l data because w e had no t r a c e r f o r t h i s element.
The r e s u l t s obtained by t h e 2026 laboratory
were lower than those reported by t h e 3508 laboratory f o r a majority of t h e samples but t h e cause of t h e observed discrepancies has not been determined. Since N i 2 + i s thermodynamically incompatible w i t h Feo a t 6OO0C, high n i c k e l values i n f i l t e r e d , reduced samples of s a l t are unlikely t o be c o r r e c t unless t h e samples were contaminated or m e t a l l i c n i c k e l p a r t i c l e s were small enough t o pass through t h e sampler f i l t e r s .
IRON REDUCTION The p l o t of i r o n concentration as a function of time i s shown i n Fig. 1. The hydrogen reduction process i s obviously q u i t e slow and t h e reduction r a t e seems t o diminish with decreasing i r o n concentration.
It i s not c l e a r whether
any significance can be attached t o t h e apparently l i n e a r rates during t h e i n i t i a l and middle f r a c t i o n s of t h e reduction period as indicated i n Fig. 1 b u t t h e data i n d i c a t e t h a t t h e f i r s t 10% of t h e i r o n was reduced i n l e s s than t h r e e hours while approximately 1 2 hours were required t o remove t h e l a s t 10%. The thorium reduction process was q u i t e rapid i n comparison t o hydrogen
reduction and t h e r e was no i n d i c a t i o n of a change i n reduction r a t e during t h e i n i t i a l three-hour period when 97.5% .of t h e i r o n a c t i v i t y was removed from W
solution.
A puzzling aspect of t h i s experiment was t h a t during t h e 16-hr period
between samples 22 and 23, t h e thorium rod (estimated t o weigh about 12 g) used t o reduce t h e i r o n completely disappeared.
Samples 25-28 were obtained
during t h e post-mortem phase of t h e experiment when t h e n i c k e l pot was cut through and i t s contents were removed f o r examination and analysis.
Black
lumps of vaqying s i z e were removed from t h e frozen s a l t , ground t o pass a 40mesh sieve and submitted f o r analysis.
i s given i n Table 2.
The complete a n a l y s i s of samples 25-27
I n addition t o t h e chemical analysis, which i s not e n t i r e l y
s a t i s f a c t o r y because none of t h e t o t a l s came close t o loo%, sample 25 ( t h e l a r g e s t black lumps) w a s submitted f o r X-ray d i f f r a c t i o n examination.
The only
d e f i n i t e l y i d e n t i f i e d component of t h e material was Li3ThF.l b u t LiF and LiThFs were reported t o be possibly present and a number of u n i d e n t i f i e d l i n e s were a l s o found. I f we assume t h a t a l l t h e f l u o r i d e ions were combined e i t h e r w i t h l i t h i u m
or thorium, c a l c u l a t i o n s show t h a t 170 mg/g of thorium w a s present as metal i n sami12.e 25 and 290 mg/g i n sample 26.
Since m e t a l l i c n i c k e l and thorium were
not found i n sample 25 by X-ray d i f f r a c t i o n , it i s p o s s i b l e t h a t t h e s e m e t a l s
were present as an i n t e r m e t a l l i c compound of unknown composition. The f a c t t h a t t h e black material composing samples 25 and 26 could be ground t o small p a r t i c l e s seems t o i n d i c a t e t h a t t h e metals present were deposi t e d from t h e m e l t . The disappearance of a s i g n i f i c a n t quantity of s o l i d thorium on long exposure t o molten LiF-ThF4 served as a reminder of similar behavior i n an experiment, Run 2-22 (66), reported earlier.
5
There, a 6-hr exposure r e s u l t e d i n
removal of 28 g of thorium from a l a r g e r rod than t h a t used here.
In that
W b
instance, it w a s speculated t h a t t h e thorium rod came i n contact with t h e bottom of t h e n i c k e l pot causing a current t o flow t h a t eroded t h e thorium rod. I n both experiments t h e thorium rod was supported by a 1/8-in n i c k e l rod t h a t
was e l e c t r i c a l l y i n s u l a t e d from t h e container by a Teflon plug.
Black magnetic
m a t e r i a l removed from t h e n i c k e l pot a f t e r cooling t o room temperature i n t h e e a r l i e r experiment analyzed 45% n i c k e l and 30% thorium, while non-magnetic material contained 229 n i c k e l and 49.5% thorium.
The chunks of black material
found i n t h e pot were q u i t e b r i t t l e , as i n t h e present experiment, which w a s i n t e r p r e t e d t o mean t h a t they were aggregates of f i n e l y divided thorium and nickel particles. While t h e same explanation of thorium l o s s given e a r l i e r could be offered here, an a l t e r n a t i v e explanation can be given although it i s purely speculative
a t present.
This assumes t h a t t h e r e a c t i o n 3ThF4
+
Tho
-+
4ThF3
can occur i n t h e molten mixture and t h a t t h e ThF3, when it d i f f u s e s t o t h e n i c k e l w a l l , disproportionates because of formation of Th-Ni i n t e r m e t a l l i c compounds.
F a i l u r e t o f i n d X-ray evidence of such compounds i n sample 25
weakens t h e argument f o r t h i s explanation, b u t since t h e form of t h e n i c k e l present has not been determined, t h e question remains open.
Since ThF3 i s not
observed i n our frozen s a l t samples, and it has not been reported i n t h e l i t e r 0
ature, we must assume t h a t if t h e above reaction occurs a t 600
it must be
reversed on cooling. Since ThF3, i f it e x i s t s , may be strongly colored, w e plan t o expose molten LiF-ThF4 t o s o l i d thorium 3.n a furnace t h a t allows v i s u a l observation of t h e melted material. W
-10-
W
Conclusions 1. Use of 59Fe t r a c e r gives a s e n s i t i v e measure of t h e i r o n content of
f l u o r i d e s a l t samples. 2.
The colorimetric i r o n method p r e s e n t l y employed by t h e 2026 and 3508
l a b o r a t o r i e s does not appear t o give r e l i a b l e results a t low i r o n concentrations.
3.
There i s a l a r g e and p r e s e n t l y unexplained discrepancy i n t h e n i c k e l
analyses by t h e 2026 and 3509 l a b o r a t o r i e s f o r a l a r g e f r a c t i o n of t h e samples.
4.
Disappearance of a comparatively l a r g e amount of thorium metal on
long exposure t o molten LiF-ThF4 raises t h e p o s s i b i l i t y t h a t a lower-thannormal valence state of thorium may occur i n melts exposed t o s o l i d thorium. I n addition t o i t s s c i e n t i f i c i n t e r e s t , t h i s reaction could affect the use of s o l i d thorium as t h e reductant f o r protactinium and we are planning f u r t h e r examination of t h i s phenomenon.
b
Table 1. Analysis of Samples from Iron Reduction Experiment Sample No.
I r o n Concentration ( w / g ) 2026
0 1 2 3 4 5 6 7 5: 3
10 11 12 13
14 15 16
17 18 19 20 21 22 23 24
25 26 27 28 a
3508
Lab. 0.13
-
< 0.01
0.52 0.27
0.18 0.29 0.29 0.25 0.20 0.21 0.18 0.12 0.14 0.67a 1.85" 0.13 0.53 0.43 0.48 0.38 14.3 b
0.17 2 . 89b
0.14
< 0.01 < 0.01
-
Total
9Fe count
0.10 0.05 0.02 0.10
-
0.02 0.04 0.34 0.30 0.22 0.01 0.09 0.27 0.17 0.16
-< -
0.61a 2.10" 0.17
0.21 0.31 0.33 (0.315) (0.295) 0.22 0.13 0.07 0.01 0.030 0.10 0.002 0.n7 0.003 0.021 0.07 0.55C
0.58
(0.55)
0.47 0.54 0.47
0.50 0.52 0.43 0.11 0.13 0.07 0.01 0.04 1.05 1.32 1.36 0.37 25.7
14.9 0.20 2. 99b 0.05
< 0.01 0.27 1.03 1.84 0.53 34.5
Nickel Conc. (mg/g) Spec.
2026
Anal. Lab. 0.011 < 0.01
< < < < < < 0.15 0.04
0.03
0.06
0.01 0.01 0.01 0.01 0.01 0.06 0.01 0.07
< 0.01 0.09 0.21 0.29 < 0.01 < c.01 0.33 < 0.01 < 0.01
0.18
< 0.01 0.16 0.33 < 0.01 240
-
Sample Description
3 508 Lab. -
0.04 0.02 0.26 0.12
0.35 0.32 0.08 0.17 0.17 0.14 0.29 0.12 0.24 0.27 0.04 0.17 0.27 0.03 0.11 0.22 0.11 0.14 0.50 2.23 205 140-218 131-137 3.7-5.5
-
S a l t a s received F i l t e r e d s a l t - lL h r He-HF Filtered s a l t 2-f h r He-HF P h r H2 Filtered s a l t 12 Filtered salt 1 h r Ha-HF Filtered s a l t 2 h r H2 Filtered s a l t 7 s h r H2 F i l t e r e d s a l t - 18 h r H2 Filtered s a l t 26 h r I12 F i l t e r e d s a l t - 31l h r H2 Filtered s a l t 41-f h r H2 Filtered salt 4 9P~ h; r H2 Filtered salt 1 h r H2-HF Filtered s a l t 3 hr H2"F Filtered s a l t 5$ h r H2-HF Filtered s a l t 16 h r He-HF Filtered s a l t - 2 h r H2-HF Filtered salt 13 h r H2 Filtered s a l t 1 h r Th exp. F i l i n g s from Th rod Filtered salt 2 h r Th exp. F i l i n g s from 2nd Th rod F i l t e r e d s a l t - 3 h r Th exp. Filtered salt 1 9 h r Th exp. Crust from N i support rod Large black lumps from s a l t S m a l l black lumps from s a l t Ground u n f i l t e r e d s a l t Material leached from v e s s e l w a l l by acid
-
-
Coctaminated by s t a i n l e s s steel sampler.
bProbably c o n t d n a t e d by i r o n from f i l e used t o remove surface of Th rod. C The higher i r o n concentration i n t h i s and subsequent samples, a s compared t o e a r l i e r uncontaminated samples, i s possibly due t o use of stainless s t e e l smrplers f o r samples 1 2 and 13, or t o s o l u t i o n of some of t h e added i r o n t h a t did not dissolve i n t h e i n i t i a l hydrofluorination treatment.
-12-
Table 2.
Analysis of Material
Removed from Nickel Pot
Sample
Concentration (mg/g)
No.
Th
Li
F
Fe
Ni
Total
25
524
21.2
174
1.03
218
936
26
646
24.1
150
1.84
134
956
27
561
44.1
310
0.53
615
49.5
335
-
Theoretical (pure s a l t )
4.6
-
920 1000
-13-
ORNL-DWG 67-10339
100
90 c
80 u
2 0)
v)
70
0
.c ..&
60
Y-
O
-s?z 50 G
40
z W
30 V
z
g -
20 10
0
12
24
36
48
DURATION OF HYDROGEN OR THORIUM TREATMENT (hr)
Fig. 1.
Reduction of Fe2+ in LiF-ThF4 (73-2'7mole $I) as Indicated by 59Fe Counts on Filtered Samples.
References
1. C. J. Barton and H. H. Stone, "Protactinium Studies i n t h e High-Alpha Molten
S a l t Laboratory,"
MSR Program Semiann. Progr. Rept. Feb. 28, 1966, ORNL-3936,
p. 148.
2.
I b i d , ORNL-4119, p. 153.
3.
C. J. Barton, :'Recovery of Protactinium from Breeder Blanket Wxtures,"
MSR Program Semiann. Progr. Rept. August 31, 1965, ORNL-3872, p. 137.
4.
A. Glassner, "The Thermochemical P r o p e r t i e s of t h e Oxides, Fluorides, and
Chlorides t o 25OO0K, ANL-5750 (1957). 5.
C. J. Barton, H. H.
Stone, "Removal of Protactinium from Molten Fluoride
Breeder Blanket Mixtures,"
ORNL-TM-1543, June, 1966.
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