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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
ORAL-Q-2359
Conf 66080
INTERFACE AND TRANSFERRING SPECIES IN AMINE
ORNL - AEC - OFFICIAL
9961 ” 228
ORNL - AEC - OFFICIAL
EXTRACTION OF URANIUM
SEP 2 2 1966
W. J. McDowe11
c. F. Coleman
CESTI PRICES
Oak Ridge National Laboratory
Oak Ridge, Tennessee
H.C. $ 1.00; MN_$?
For presentation at the
INTERNATIONAL CONFERENCE ON SOLVENT EXTRACTION CHEMISTRY
August 27 - September 1, 1966, Göteborg, Sweden
and publication in the Proceedings

RELEASED FOR ANNOUNCEMENT
IN NUCLEAR SCIENCE ABSTRACTS
LEGAL NOTICE
This report was prepared as an account of Government sponsored work. Neither the United
Slates, nor the Commission, nor any person acung 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 infurmation contained in this report, or that the use
of any information, apparatus, method, or process disclosed in thiu report may not infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damages resulung from the
use of any information, apparatus, method, or process disclosed in this report.
As used in the above, "person acting on behalf of the Commission" includes any em-
ployee 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 to any information pursuant to his employment or contract
with the Commission, or his employment with such contractor.
Research sponsored by the U.S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
MUL ONE- OFFICIAL
ORNI - AFC - OFFICIAL
INTERFACE AND TRANSFERRING SPECIES IN AMINE
EXTRACTION OF URANIUM
W. J. McDowe11
C. F. Coleman
ORNL - AEC - OFFICIAL
Oak Ridge National Laboratory
Oak Ridge, Tennessee, USA
*Research sponsored by the u.s. Atomic Energy Commission under contract with
the Union Carbide Corporation.
ABSTRACT
In the extraction of uranyl sulfate by amine sulfates a question of in-
terest is whether neutral U02504 or an anion such as Uo2(804)22- actually
crosses the interface; this cannot be assessed by equilibrium measurements.
We have investigated this question and the general structure of the interface
by measuring interfacial tensions and the kinetics of 35504 transfer between
organic (di-n-decylamine sulfate in benzene) and aqueous (acid sodium sulfate)
phases.
Interfacial tension measurements indicated the organic-aqueous interface
to be saturated with a close-packed layer of the amine sulfate, sulfates
oriented toward the aqueous phase. The amine sulfate-uranium complex was
much less interface active. The kinetic experiments with 35504 allowed de-
tection of anionic species transfer aqueous to organic. Evidence was obtained
for anion exchange when anionic species predominate in the aqueous phase and
for neutral species transfer when neutral or cationic aqueous phase species
predominate.
Salts of the long-chain amine sulfates have been useful extractants [1]
for a number of years and are often called liquid anion exchangers in analogy
with resinous ion exchangers, although it is usually not certain in either case
t_ST
that actual exchange of anions is involved.
To obtain information regarding
ORNL - AEC - OFFICIAL
-
transferring species, kinetic studies are required and these are complicated,
in solvent extraction systems, because of the two phases involved; reactions
ORNI - AEC - OFFICIAL
in the aqueous phase, diffusion to the interface, transfer across the inter-
face, diffusion away from the interface, and perhaps further reaction in the
organic phase must all be considered.
In this investigation, the extraction
of uranium from acidic sulfate solutions by benzene solutions of di-n-decyl-
amine sulfate has been examined by interfacial tension measurements and by
the transfer of 355-tagged sulfate between the two phases. A model is pro-
posed, derived in part from the Interfacial tension studies, and then it is
tested by following the kinetics of sulfate transfer.
1. THE MODEL
Figure 1 will show the proposed structure of the system in question.
F1
The two phases are known to contain the species indicated [2]. The amine
sulfate exists in loosely-bonded micelles in the bulk organic phase [3].
Its distribution to the aqueous phase is very low, resulting in aqueous
phase concentrations of < 10-8 M. Interfacial tension measurements show
that the amine sulfat:: is interface active and that the interfacial tension
is little affected by the ionic strength of the aqueous phase (see table 1).
TI
Other anion forms show considerably different interfacial tensions, suggest-
ing that the anion is not completely dissociated at the interface, but
rather tightly bound to the ammonium ion.
The uranyl complex is much less
interface active than the amine sulfate.
The extraction of uranium by such
a system would proceed as follows: Some uranyl species would approach the
interface by convection and/or diffusion and exchange with or become attached
to the sulfates associated with the alkyl ammonium ion. The uranium would
then be dragged through the interface into the organic phase during or
after which it would associate with additional amine sulfates to form the
final complex. Two reactions expressing such a transfer are as follows:
ORNL -- AEC - OFFICIAL
TYRO
3R2NH2)2SO4 + UO2SO4 =
VO2(504)4(R2NH2)
-274. INYO
ORNL - AEC - OFFICIAL
3(R2NH2)2SO4 + U02(S04)22 7
102 (S04)4(R2NH2). + 5042- .
The dotted underlines indicate organtc phase species. Although equation (1)
Ladicates the transfer of a neutral uranyl sulfate and equation (2) the trans-
fer of a uranyl disulfate through the interface, equilibrium-wise these equa-
tions are identical, and thus can be distinguished only by kinetic studies.
On the aqueous-phase side the uranium may move to the interface as any
of the species in which it exists in the aqueous phase. Allen has shown
that the uranyl ion is the most effective in moving uranium to the interface
and explains this effectiveness on the basis of its higher ionic mobility as
compared with uranyl sulfate and uranyl disulfate [4). The question we wish
to examine hert is what species actually remain intact and cross the aqueous-
organic interface. That is, does the uranium cross as U02504 or does Uo2(504)22-
cross the interface intact in actual exchange for sulfate lons? This problem
will be examined by a quite unique method in the following section.
2. SPECIES TRANSFERRING ACROSS THE INTERFACE
There is one difference between reaction (1) and reaction (2) which we
may exploit. If an anion is exchanged as in reaction (2), a sulfate ion from
the organic phase is transferred to the aqueous phase.
This does not occur
in neutral uranyl sulfate transfer.
The use of radioactively-tagged sulfate
(355) in the organic-phase amine sulfate will allow us to differentiate be-
tween organic-phase and aqueous.phase sulfates and to distinguish between
reactions (1) and (2) by following the transfer of sulfate from the organic
to aqueous phase under various conditions and comparing rates of transfer
during uranium extraction with those for sulfate self-diffusion with no
uranium present or present but at equilibrium.
If reaction (1) predominates
ORNL - AEC - OFFICIAL
and there are no unsuspected side effects there should be no difference in
vivisiu-gi- inndu
the rate at which 35s transfers from aqueous to organic phase whether uranium
18 absent, present at equilibrium, or is being extracted. If, however, an
anion exchange mechanism such as reaction (2) predominates, the extraction
of uranium should prod’ıce an increased rate of transfer of 358 from organic
to aqueous phase. These effects could be seen, of course, only during non-
ORNI ~ AEC - OFFICIAL
equilibrium conditions, both in the isotopic equilibrium between the two
phases and in the uranium distribution between the two phases.
In these experiments the equilibrations were made in an apparatus
(ref. [3]) in which both phases were stirred in opposite directions but not
rapidly enough to break the interface.
Thus, in all cases, the transfers
were observed through a limited quiescent interface of constant geometry.
The temperature was held constant at 25 t .05°C. In each experiment the
acidity of the system was carefully adjusted to that the organic phase con-
tained no bisulfate. This was important, since the extraction of uranium
into an amine solution that is partly in the bisulfate form returns sulfuric
acid to the aqueous phase in a way which would be impossible to duplicate
with a uranium-free system. With experimental conditions thus controlled
the mixing and diffusion effects remain the same and we should be able to
observe the effect of uranium being extracted on the 35s transfer. Samples
of both phases were removed at various time intervals and analyzed for
uranium and/or 355.
Figure 2 shows a set of 35s transfer curves for 0.050 M aqueous sul-
fate. These are typical of the type of data obtained at various sulfate
F 2
levels. Table 2 lists the aqueous sulfate concentrations examined along
witii the fraction of uranium in the various species in each sulfate con-
centration and the relative rate of 35s transfer. In the low aqueous sul-
fate system where most of the aqueous-phase uranium exists as vo22, the
ORNL - AEC - OFFICIAL
35s transfer from organic to aquecus 18 slowed by uranium extraction.
This
result was not predicted by our starting hypothesis. Among possible ex-
ORNL - AEC - OFFICIAL
ORNI - ASC-OFFICIA'
planations for this, we consider that a temporary blocking of the interface
by an amine sulfate uraniun complex is most likely. Since most of the aqueous
phase uranium exists as 10227, it is necessary that this charged species find
and attach a 8042- before it can be transferred to the organic phase. How-
ever, there appears to be no reason why this charged species cannot adsorb
on or coordinace to the layer of sulfates associated with the interface. The
opportunity for this to occur would be high because of the high mobility of
the uranyl ion.
The attached uranyl ion would probably remain at the inter-
face until it could catch a sulfate ion.
Such a process would result in a temporary biocking of the interface.
This proposed explanation is supported by the fact that the slowing effect
is proportional to the amount of uranyl ion remaining in the aqueous phase.
That is, the data can be made to fit an adsorption isotherm equation of the
form
kı'/ky « [1022+jn
where ki is the rate constant for
s transfer with uranium at equilibrium
and kı' with uranium being extracted. These considerations suggest strongly
that at low aqueous sulfate concentrations uranium is transferred as a neutral
species rather than by anion exchange.
, Referring to table 2, we see that experiments at higher (8042- Jaq give
different relative positions of the 35s curves. In 0.025, 0.050, 0.50, and
1.0 M aqueous sulfate the 35s approaches isotopic equilibrium more rapidly
during uranium extraction than with no uranium present, as was expected on
the basis of the original hypothesis" for anion exchange.
This difference
increases as the (S042 Jaq increases. Table 2 shows the parallel increase
ORNL - AEC - OFFICIAL
in anionic aqueous uranium species.
Thus, in these systems we have a
positive indication of the transfer intact of anionic species from aqueous
ORNI - ASC-065!CIA!
ORNL - AEC - OFFICIAL
to organic phase.
Attempts to reach a more detailed evaluation of the relative contri-
butions of anion exchange and neutral transfer encounter difficulties, in
UT
that the observed change of 35s transfer rate is a net balance between
speeding up by exchange and slowing by interface blocking such as was dis-
cussed above. Une can only conclude that if there is a preference for either
mechanism it is for anion exchange.
Aside from the main question, two other striking trends exist in the
data.
The first is a decrease in the rate at which isotopic equilibrium
is approached as (S042 Jaq increases. We attribute this to the large increase
in the absolute amount of 355-tagged sulfate which must be transferred to
bring about isotopic equilibrium when the aqueous sulfate is high. The
second is in the relative position of the curves for 35s approach to equilib-
rium when uranium is present but already at equilibrium. (In all cases more
than 99% of the uranium was in the organic phase.) As the [SO42- Jaq increases,
the uranium-at-equilibrium curves do not fall as much as do the no-uranium
curves; in fact, they remain nearly identical from < 0.01 to 0.05 M aqueous
sulfate. At 0.5 M the uranium-ac-equilibrium curve has fallen considerably,
but is about twice as high as the no-uranium curve. At 1 M it is about
three times as high as the no-uranium curve, and higher than the during-
uranium-extraction curve.
A possible explanation for this is an effect due to the chemical form
in which the tracer was added to the system, which in all cases was as the
amine sulface. In a test designed to investigate the effect of the chemical
form of the tracer, the aqueous phase was 1 M, the uranium was already at
equilibrium, and the tracer was added to the organic phase at time zero, as
ORNL - AEC - OFFICIAL
(1) (DCA)23580., (2) (DDA)8102(35804)4 and (3) an equilibrium mixture of
these two species. Situation (1) transferred 35s to the aqueous phase most
rapidly, (2) significantly more slowly, and (3) at an intermediate rate.
A primary conclusion from this must be that 35s exchange between the two
ORNI - AEC - OFFICIAL
ANIAC-OFFICIAL
organic phase species is slower than the 1sotopic equilibrium across the
interface, otherwise there would be no difference in rate due to the three
methods of addir.g the tracer. Such slow exchange of free and bound ligands
is well known in aqueous complex species and is generally accepted as evidence
for "inner coordination" type complexes. Here we have no other indication of
bond type in the organic complex, but the presence of strong coordinate bonds
seems a reasonable explanation for the slow exchange.
That the tracer in the amine sulfate form should transfer more rapidly
than that in the amine-sulfate-uranium complex form with uranium present at
equilibrium is consistent with the interfacial tension data indicating that
the amine sulfate saturates the interface. The 35s present as amine sulfate
would go to the interface and become immediately available for exchange with
aqueous sulfates, and thus to the extent that the uranium exists in the aqueous
phase as 102(SO4)2-, its transfer from aqueous to organic (under dynamic
equilibrium) should force increased sulfate exchange between (DDA)2S04 and
aqueous sulfate. This effect should increase with increasing (S042- Jaq
since the proportion of vo2(SO4)22- increases, tending to compensate for the
concomitant slowing effect due to the absolute increase in 35s transfer re-
quired.
The transfer of sulfate from aqueous to organic phase has also been
observed by similar techniques. A11 of the aqueous sulfate levels listed
in table 2 were examined except the very low one. At each sulfate level
Voulddio - vsi - ringo
the equilibration of 35s is more rapid during uranium extraction than with
ORNL - AEC - OFFICIAL
.
.
uranium at equilibrium or no uranium present. The number of sulfates trans-
ORNI - AEC - OFFICIAL
ferred per uranium was calculated from the kinetic data and the known 35s/5504
ratio. The difference between 35s transferred during uranium extraction and
ORNI - AEC - OFFICIAL
that transferred with no uranium present was called net 35s transferred, and
1t8 ratio to the number of moles of uranium transferred, uc 35s/mole uranium,
was plotted vs time. Extrapolation of this plot to t = O gave a value of uc
35s/mole uranium Independent of back-transfer effects or any change from
initial concentration of sulfate or uranium and thus representative of the
average number of sulfates crossing the interface as a part of the uranium
species.
The values obtained for the various aqueous sulfate concentrations
were:
(S04Elaq
0.025 M
0.05 M
0.5 M
1.0 M
50,-
trans.
1.64
2.7
2.0
Although the value 2.7 moles S04/mole uranium at 0.5 M ES04 18 somewhat high,
the otheir values are close to the expected trend, which should begin at 1 for
very low aqueous sulfate concentrations and rise to 2 at higher values. With-
in the iairly large (~ + 25%) deviation in these numbers they are in good
accord with our original hypothesis.
--
3. CONCLUSIONS
-
-
-
-
.--.
-
Uranyl sulfate extraction by amine sulfate proceeds by both anion ex-
change and neutral transfer, the proportion shifting according to the pro-
T
.
:
-*-
.-
portion of anionic uranium present, with probably some bias toward anion
exchange.
vividjin -usi-
ORNL - AEC - OFFICIAL
REFERENCES
YOY IYO.
1. Van Ipenburg, K., Recueil 80, 269 (1958).
Coleman, C. F., Atomic Energy Review, Vol. 2, No. 2, p. 3 (1964).
ORNI - AFC-NEFCIni
Moore, F. L., Liquid-Liquid Extraction with High-Molecular-weight
Amines, AEC ORNL CF-61-1-77 (January 1961).
2. McDowell, W. J., and Baes, C. F., Jr., J. Phys. Chem. 62, 777 (1958).
3. Allen, K. A., J. Phys. Chem. 62, 1119-1123 (1958).
4. Allen, K. A., J. Phys. Chem. 64, 667-670 (1960).
ORNL - AEC - OFFICIAL
FIGURE CAPTIONS
ORNI - AEC - OFFICIAL
ORNL-DRAWING 78077A
Fig. 1.
Proposed Structure of Extraction System.
ORNL-DRAWING 64-2752
Fig. 2.
35s Approach to Equilibrium Distribution. Transfer from
organic, 0.05 M DDAS to aqueous, 0.05 M NaS04, uranium present,
0.012 M where indicated.
-
- -
- .
-
-
.
ORNI - AEC - OFFICIAL

ORNL-LR-DWG® 78077:
12(R,NH226(50q14002
BASE (R2NH2)2504
F
aso (RQNH? -
ORGANIC PHASE
di-n=DECYLAMINE SULFATE
IN BENZENE
V (R₂Ntg/2504
PP2NH2
(R2 NH2)
On (R₂NH2)
(R₂NH₂)
(R₂NH₂)
(R2 NH2)
AQUEOUS PHASE
URANYL SULFATE, SCDIUM
SULFATE AND SULFUR. C ACID
so
vozi
VO2(SO4)2
Na - -
Ht som
- AEC - OFFICIAL
ORNL - AEC - OFFICIAL

VIJIJIO-
V - INYO
TVIJIJO- 33V - INDO
UNCLASSIFIED
ORNL-DWG 64-2752
FRACTION APPROACH TO
ISOTOPIC EQUILIBRIUM
LO DURING URANIUM EXTRACTION
A URANIUM AT EQUILIBRIUM
O NO URANIUM PRESENT
- URANIUM CAME TO
EQUILIBRIUM HERE
0
50
100
150
200
250
300
350
400
TIME (min)
Os Approach to Equilibrium Distribution. Transfer
from Organic, 0.05 M DDAS to Aqueous, 0.05 M
Na2SO4, Uranium Present, 0.012 M Where Indicated.

..
13
Table 1.
Interfacial Tensions
ORNI - AEC - OFFICIAL
Interfacial Tensions
Dyne/cm
System
Benzene - H20
31.12
27.9
26.0
22.1
- 0.01 N H2S04
DDA - H20
DDANO3 - 0.01 N HNO3
(DDA)2S04 - 0.01 N H2S04
(DDA)2S04 - 0.995 M Na2S04 - 0.005 M H2S04
(DDA)2804 Uranium Complex - 0.001 M H2S04
(DDA)2SO4 + 22% as Uranium Complex - 0.001 M H2S04 ·
(DDA)2SO4 + 1% as Uranium Complex - 0.001 M H2SO4
Each number is the average of at least three determinations up and three
down through the interface; average deviation ~ 0.5 per set.
UDA = di-n-decylamine, TOA = tri-n-octylamine; the amines were 0.10 N in
benzene.
1
-
-
An
-
i
A
Fig Le - AEC - OFFICIAL
nga...ver-ncercia
mga crin.
mw,
Table 2. Uranium Species Distribution and 355 Transfer Rates
at Various Aqueous Sulfate Levels
.-355 Transfer Organic to Aqueous Phase-
.
Relative Race of
Fraction of Aqueous Uranium
-
0 0227 002504 002(504)22-
(S042- Jaq
M
a 8
35s Transfer
U at Equilib.
No U
U Extracting
.
.
.
Fastest Slightly slower
Slowest
.
.
.
.
-
re-
>0.5
0.35
0.19
Slowest Intermediate
Fastest
I
<0.01
0.025
0.050
0.50
1.0
<0.5
0.60
0.68
0.25
.
Slowest
Intermediate
Fastest
-'
0.05
0.13
0.75
0.87
*...-
Slowest Intermediate
Fastest
is-
* :-
*
0.13
1,5
Slowest Fastest
Intermediate
1-
---
.
.
. . .
*= -:
-
. ---...
-
-, ... - E
ns
END
DATE FILMED
10/24/66







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