Note: Descriptions are shown in the official language in which they were submitted.
~4~
Descriptlon
Process For Recovering Uranium From
Wet-Process Phosphoric Acid Using
Alkyl Pyrophosphoric Acid Extractants
Technical Field
This invention relates to the recovery of
uranium from "wet process" phosphoric acid which is a
intermediate in the conversion of phosphate rock to
certain fertilizers.
Background Art
Mineable phosphate is found in a number of
places throughout the world, and in many of these
deposits, small quantities of uranium are found
complexed with the phosphate values. The large
phosphate deposit in Central Florida, for example,
contains from 0.01 to 0.02 weight percent uranium.
This uranium is taken into solution when the phos-
phate is acidulated with mineral acid to produce what
is known as l'wet process" phosphoric acid. It is
estimated that over six million pounds of uranium,
expressed as U3O8, is now dissolved each year in the
United States in the production of wet process phos-
phoric acid. Uranium is a valuable energy resource
and is used as fuel in nuclear power reactors.
Interest in recovering uranium from wet
process phosphoric acid arose about three decades ago
when the then U,S. Atomic Energy Commission was
searching for a domestic supply of uranium for nuclear
weapons use. This early work is described in a
publication of the U.S. Atomic Energy Commission
known as DOW-81 and entitled "The Recovery of Uranium
from Industrial Phosphoric Acids by Solvent E~trac-
tion". Alkyl pyrophosphoric acids (APPA) were found
to be very efficient in selectively extracting uranium
, ~
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from phosphoric acid and several flow sheets utiliz-
ing this type extractant are shown in U.S. Patent No.
~,866,680, to Ray S. Long. Although this type extrac-
tant has a very high distribution coefficient for
uranium, in favor of the extractant, upon contacting
with wet process phosphoric acid when the uranium and
iron in the acid are in their lower valence states,
the coefficient is still substantial even when the
uranium and iron in the phosphoric acid are in their
oxidiæed states. It even continues to be well above
one ~Ea = 1) when contacted with concentrated phos-
phoric acid, a potential stripping agent from which,
after dilution, the uranium could be re-extracted for
further purification. Therefore, prior processes
using a pyrophosphoric acid extractant, such as
disclosed i~ the prior ong patent, were forced to
rely upon precipitation as the means of separating
the uranium from the extractant. The resulting
product was impure, requiring redissolution and
further purification before it could be put to use.
Another disadvantage of the APPA extractant as used
in prior methods is that it is not stable and, thus,
requires continual regeneration with phosphoric
oxide.
In view of these disadvantages, work contin-
ued in an effort to ~ind a more stable extractant and
one that produced a more pure uranium precipitate.
This work resulted in the discovery of a synergistic
mixture of di-2-ethylhexyl phosphoric acid (D2EHPA)
and trioctyl phosphine oxide (TOPO), which was speci~ic
for uranium in the hexavalent state and which was
quite stable. A process utilizing the D2EHPA-TOPO
extractant was developed at Oak Ridge National Labora-
tory and is described in a U.S. Atomic Energy Commis-
sion report ORNL-TM-2522, entitled "Solvent Extraction
of Wet-Process Phosphoric Acid". More recently,
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octylphenyl acid phosphates (OPAP) were found to have
good extraction characteristics for uranium in the
tetravalent state in wet process phosphoric acid.
This work was originally performed in India by T.K.S.
Murthy et al and is described in IAEA-SM-135/11,
"Study of Some Phenyl Phosphoric Acids for Extraction
of Uranium from Phosphoric Acids", an International
Atomic Energy Agency report of papers presented in
Brazil in 1970. This extractant was tested at the
Oak Ridge National Laboratory and is the subject of
U.S. Patent No. 3,835,214 to Hurst et al. The dis-
tribution coefficient (Ea) with octylphenyl acid
phosphates in reduced acid is about twice that of the
D2EHPA TOPO extractant of equal concentration in
oxidized acid, and it is relatively stable, but its
distribution coefficient is only one-twentieth of the
distribution coefficient of APPA for uranium under
the same conditions. However, wet process phosphoric
acid from Central Florida has the general composition
shown in the Table I, appearing on the following
page, and certain components of commercial OPAP are
found to form a complex with the iron in the acid,
and the extractant is thus lost as a precipitate.
A serious drawback found in both of the
more stable extractants D2EHPA-TOPO and OPAP arises
from the fact that their distribution coefficients
may decrease by as much as 5~% or more after extended
use in a uranium recovery process. Possible causes
of this loss are from flotation additives used in
rock beneficiation, from foam depressants used in
acid manufacture, and from additives used to settle
out solids when the acid is clarified. Minor metallic
impurities in the starti~g acid, such as vanadium,
may also be taken up by the extractant and lessen its
affinity for uranium. The number of possible inhibit-
ing additives are many and, although they may be
present as only a few parts per million, they have
:
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TYPICAL COMPOSITION
~T PROCESS PHOSPHORIC ACID FROM CENTRAL FLORIDA ROCK
.
COMPONENT NOMINAL WEIGHT PERCENTAGE
CONTAINED
S ~3PO4 40
Al 0.45
Ca 0.30
Cl 0.00
Fe 0.75
F 1.6
K 0.05
Mg 0.18
Mo 0.001
Na 0.1
Si 0.4
so4 2.5
U 0.017
Inorganic Solids 2.0
Organic Solids (humates) 1.0
EMF -300 mv
been found to increase in concentration upon repeated
cycling of the extractant with a corresponding in-
crease in effect upon the extractant's long term
performance until the point is reached where it must
be replaced. Further, we have also found that losses
of these extractants by entrainment in the acid and
from solids separation can approach as much as two
percent of the extractant during each cycle. This
amounts to a substantial cost, since these two more
stable extractants are relatively expensive. An
important advantage of the process of this invention
is that only a small quantity of APPA is needed in
the recovery process in view of its high extraction
~4417~
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coefficient, and the quantity of APPA necessary is
about equal to ordinary losses of the D2EHPA-TOPO and
OP~P extractants used in the other known processes.
Disclosure of I~vention
Our invention comprises as its basic feature
a method for stripping uranium from an extractant of
the alkyl pyrophosphoric acid type ~APPA) into an
aqueous phosphoric acid solution from which the
uranium is readily re-extracted for further purifica-
tion and precipitation as high purity yellow cake,
thus permitting the use of this relatively inexpensive
extractant in a practical process for recovering
uranium from wet process phosphoric acid. Hence, by
our method, an APPA extractant, which is highly
efficient even with the more concentrated wet process
phosphoric acid produced by the more advanced proces-
ses, can be used without intermediate precipitation
and redissolution steps in a process for recovering a
uranium product of commercial purity.
More specifically, we have discovered that,
after loading an APPA extractant with uranium by
extraction from wet process phosphoric acid, if the
loaded extractant is heated at a temperature of about
70C to about 140C for a period of from about one-
half hour to about six hours the distribution co-
efficient of the APPA extractant for uranium in
phosphoric acid is lowered by a factor of from 4 to
10, or possibly more, where it there levels off.
Thus, without expensive or lengthy treatment, it
becomes possible to achieve a practical stripping of
the uranium from the extractant using a phosphoric
acid stripping solution by reason of the change in
the extractant's distribution coefficient. Further,
since APPA type extractants are not particularly
sensitive to phosphoric acid concentrations, our
invention can be used to extract uranium values from
wet process phosphoric acid having concentrations of
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as low as 25~ H3PO4 by weight to as high as 55% H3PO4
by weight. Suitable phosphoric acid strip solutions
can have acid concentrations in the range of 25% to
85% by weight H3P04, but a concentration in the range
of 35% to 55% by weight H3PO4 is more desirable and
about 40% by weight ~3PO4 is preferred.
The various APPA extractants that can be
used in our invention are those disclosed in U.S.
Patent No. 2,866,680 and include those in which the
substituent alcohol has a chain length of from 7
carbon atoms to 17 carbon atoms. We prefer alcohols
containing 8 to 10 carbon atoms.
In view of the high distribution coefficient
(Ea) of the APPA extractants for uranium in phosphoric
acid, only a small guantity is necessary in accordance
with our invention. An organic diluent containing
0.5% to 5.0% by weight APPA can be used with 1.0 to
2.5% more desirable and about 2% preferred. Further,
when treating more concentrated phosphoric acid
starting solutions (near 55% H3PO4 by weight~, and
depending upon cost considerations of reagent mate-
rials, it may be desirable to use higher than 5% APPA
solutions, even up to 10%. Kerosene and Stoddard
solvents have been found to be excellent solvents for
use as a diluent with these alkyl pyrophosphoric acid
extractants. However, many other materials are
satisfactory including petroleum materials such as
diesel oil, aromatic oils, distillates, various
commercial organic solvents, and petroleum ethers.
Benzene, chlorobenzene, toluene, hexane, chlorinated
aliphatic hydrocarbons, and ethers are also suitable
with the selection of any particular solvent being
made generally on the basis of economic considera-
tions. ~efined ~erosene is presently considered the
preferred.
Further, we have discovered that if a small
amount of ferric ion, in the range of about 0.1% to
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1.5% by weight, preferably 0.25 to 1.0%, is present
in the phosphoric acid stripping reagent when con-
tacting the pregnant APPA extractant, stripping of
the uranium is enhanced by a factor of 10 or more.
Thus, we have found in accordance with our invention
that by heat treating the pregnant APPA extractant
and stripping with a phosphoric acid strip solution
containing 0.1 to 1.5% by weight ferric ion, it is
possible to produce an overall distribution coeffi-
cient for stripping of uranium from the loaded APPAextractant in the neighborhood of 0.20.3 ~Ea = 0.2 to
0.3) or less. Accordingly, nearly complete stripping
of the uranium from the APPA extractant can be ac-
cQmplished and the uranium transferred directly into
a phosphoric acid solution from which it can then be
re-extracted without dilution for recovery. For
example, the uranium can be re-extracted from the
phosphoric acid strip solution into an extractant
such as a D2EHPA-TOPO synergistic mixture, from which
a uranium product of high purity can be precipitated.
Such direct re-extraction has the advantage of avoid-
ing the cost of reconcentrating the strip acid before
it can be reused.
Furthermore, we have found it advantageous
to add a small amount of o~idizing agent, such as
hydro~en peroxide, sodium chlorate, potassium perman-
ganate, or the like, to the loaded APPA extractant.
By the addition of such oxidant, the uranium strip-
ping from the APPA by the phosphoric acid strip
solution is further improved by a factor of 2 to 3.
The quantity of oxidizing agent added should be
sufficient to oxidize all of the uranium and iron
ions contained in the extractant to their +6 and +3
states, resp~ctively.
114~.74
g
Preparation of the Alkyl Pyrophosphoric
~.cid Extractant
_
While the preparation procedure described
in Long U.S. Patent No. 2,866,860 for preparation of
the APPA extractants is suitable for the present
invention, we have discovered surprisingly that
preparation of the alkyl pyrophosphoric acid solution
at a high reaction temperature, about 80C to 140C,
in the absence of excessive moisture, results in a
clear extractant solution that remains stable far
longer than similar extractants described by Long.
More specifically, we have found that if the alcohol
(ROH), such as octyl or decyl alcohol, is reacted
with the phosphoric oxide (P4Olo) which has been
slurried in an organic diluent, such as refined
kerosene, at a high reaction temperature of about
80C to 140C, preferably 85 to 100C, in the absence
of excessive moisture, an alkyl pyrophosphoric acid
solution results which is clear and remains stable
far longer than indicated by Long in U.S. Patent No.
2,866,680. The stability was determined by the
di~tribution coefficient of the extractants remaining
essentially constant for at least three weeks when
stored in an ordinary, opaque, storage tank at ambient
temperatures.
In preparing APPA extractants in accordance
with our invention, we prefer a mole ratio of about
four, alcohol to phosphoric oxide (ROH:P401o = 4);
however, the mole ratio may deviate slightly from the
preferred. Suitable organic diluents in which the
phosphoric oxide is slurried are kerosene, benzene
and ether and the oxide concentration is preferably
0.1 to 0.2 grams P4Olo per ml. of diluent. Since the
APPA extractants used to recover uranium in accor-
dance with this invention are dissolved in a sub-
stantial quantity of organic diluent, it is prefer-
1~4~i`74
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able to slurry the phosphoric oxide for preparationof the APPA extractant in the same organic diluent
that is to be used to dilute the APPA extractant for
the uranium recovery. Refined kerosene has been
found most suitable. The reaction time is quite
short; the reaction going to completion usually is
less than about 30 minutes.
APPA extractants produced in accordance
with the above description appear to have consider-
ably improved storage characteristics compared tothose previously produced and described by Long in
U.S. Patent No. 2,866,680, thus permitting the pre-
paration of larger and more economic batches. This
extractant can therefore be fed continuously into the
process without fear of loss of extraction capability
while in the feed tank or loss of usefulness from
long standing during plant outages. Further, we have
determined that when an APPA extractant prepared in
accordance with the above procedures is introduced
directly into the extraction contactor circuit at
normal wet process phosphoric acid temperatures of
about 50C to 60C, it is sufficiently stable to
perform effectively through at least four contact
stages, thus permitting recoveries well in excess of
90% of the uranium originally contained in the wet
process acid. Hence, it is possible with the APPA
extractants to extract the uranium from the wet
process phosphoric acid without the expense of
artificially cooling the acid before extraction as
previously practiced, or reheating the acid which
must be done for it to proceed on its way to becoming
a product of commerce.
Brief Description of the Drawinqs
Figure l shows a typical flow sheet for the
recovery of uranium in accordance with the present
invention where like numerals are referred to herein-
after.
1~44~7~
13est Mode for Carrying Out the Invention
Wet process phosphoric acid having an
analysis as set forth in Table I is first treated to
xemove the organic solids, such as by the process
described in U.S. Patent No. 4,087,512. The cleaned
acid is then contacted with a reducing agent, as at
lOl, such as scrap iron or ferrosilicon or other
known reductant which, as it dissolves in the acid,
reduces any U 6 to U 4 and reduces Fe 3 to Fe 2. As
is apparent from the references previously cited, it
is not necessary that all of the Fe+3 be reduced to
the lower valence state in order to effectively
extract the uranium from wet process acid. In fact,
an advantage of the instant process is that extractant
concentration may be varied to offset consumption of
reductant by operating with more or less Fe+3 present
as their relative costs dictate and without loss of
extraction efficiency.
The cleaned and reduced wet process acid
in an concentration range of 25% to 55% H3PO4 by
weight is introduced into a counter~urrent liquid-
liquid solvent extraction system 103 where it is
contacted with an organic diluent, such as refined
kerosene, to which about 2% APPA by weight is added
as it enters the system at 106. At this concentra-
tion of APPA, the distribution coefficient ranges
from 20 to 30 IEa = 20 to 30) when contacted with wet
process acid at its normal temperature of about 55C
and in which 80% to 90% of the iron content has been
reduced to the ferrous state, the coefficient being
highest in the first contactor and the lowest after
final contact with the acid. A distribution coef-
ficient of this magnitude has a number of advantages,
some of which include: (l) a small volume of organic
may be used compared to the volume of acid contacted,
thus smaller, less costly equipment is required for
downstream processing of the pregnant organic; (2) a
.;
~44~74
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]arge concentrating effect on the uranium is
obtained; and (3) fewer extraction stages are
required to obtain good recovery. While four stages
of extraction are shown and preferred, as few as two
or as many as ten might be used depending upon a
number o~ factors, such as the concentration of the
APPA in the organic diluent.
The process in Figure 1 also contemplates
an agueous to organic ratio of ten (A:0 = 10).
However, ratios in the range of about 1:1 to 20:1 can
be employed. It will be appreciated that concentra-
tions of the extractant in the organic phase and the
phase ratio are interrelated and that the particular
choice of values for these ~ariables will depend on a
variety of factors including solubilities of the
relevant materials. A uranium concentration in the
organic of about 1.5 grams per liter compared to
about 0.15 grams per liter in the acid when using
four contact stages will achieve a recovery of 96% of
the uranium in the feed acid.
Following extra~tion of the uranium, the
wet process acid is returned to the acid plant for
further processing as shown by line 105. The organic
is then heated at a temperature of about 80C for
about one hour at 107. After such heat treatment,
the distribution coefficient drops and levels out in
the range of 3 to 5 (Ea= 3 to 5~ for acid concentra-
tions of about 40% H3PO4 by weight. A small quantity
of oxidizing agent, such as hydrogen peroxide, chlorate
ion (sodium chlorate), permanganate ion (potassium
permanganate), or other known oxidizing agent, is
then added to oxidize the uranium and iron ion content
of the extractant to hexavalent uranium and ferric
iron. The amount of oxidizing agent added should be
an excess of the stoichiometric amount required to
achieve the desired oxidation. A 50~ by weight
solution of H O is suitable and an addition of about
2 2
~14~174
0.1% by volume of extractant should accomplish the
requisite oxidation in the process shown in Figure 1.
The oxidation further depresses the distribution
coefficient to about 1 to 2 (Ea = 1 to 2). While the
oxidizing agent as shown in Figure 1 is to be added
after heating, it is contemplated for our invention
that the oxidizing agent may be added during heating,
or even before. It is only necessary that it be
added before commencing the stripping.
The organic is now ready to be stripped of
the uranium. This is accomplished in a countercur-
rent liquid-liquid system 109, using say 10 con-
tactors which are operated in the range of 60C to
70C to achieve good phase disengagement and more
favorable stripping efficiency. The stripping
solution comprises 40~ H3P04 by weight and contains
about 0.5% by weight ferric ion dissolved therein.
The ferric ion concentration in the strip solution
can be obtained by adding iron metal to the acid in a
quantity sufficient to establish the desired ferric
concentration. Ferric ion can be added directly by
addition of a ferric salt, such as ferric sulfate.
Further, since ferric ions are extracted by the APPA
extractant from the wet process phosphoric acid, and
therefore necessarily will carry over to the strip
solution, iron or ferric ion addition is not
essential since ferric ion will necessarily build up
in the strip solution after a few stripping cycles.
During the stripping, the ferric ion acts
to salt the uranium out of the extractant. The
distribution coefficient in this system is less than
0.1 (Ea = 0.1). All but a few milligrams per liter
of the uranium can be removed from the extractant in
ten contact stages of this relative small equipment.
Again, however, the number of stripping stages can
vary from as low as 4 to as many as 12 or more de-
pending upon concentrations, phase ratios, etc.
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Also, the organic to aqueous ratio in the system
shown is about eight (O:A = ~), but again can vary.
For this system shown in Figure 1, a uranium con-
centration of about 12 grams per liter is obtained in
the strip solution. The barren extractant 111 is
recycled to extraction. It now consists principally
of diluent and decomposition products of APPA,
believed to be mostly alkyl orthophosphoric acids
which themselves have an appreciable distribution
coefficient for the U+4 in the wet process acid.
While it is not understood precisely how
the heat treatment chemically alters the loaded APPA
extractant to reduce the distribution coefficient in
a phosphoric acid system, it is believed that heating
the APPA extractant to about 70 to 140C for 1/2 to 6
hours serves to destroy some of the pyroesters re-
sponsible for generating the initial high distribu-
tion coefficients of the alkyl pyrophosphoric acids
for uranium in phosphoric acid solutions. Heat
treatment at temperatures in the lower end of the
temperature range requires longer heating times
toward the upper end of the time range and, converse-
ly, a higher heating temperature requires less time
to achieve the desired reduction of the APPA distribu-
tion coefficient. Thus, heat treatment at about 70Crequires about 4 to 6 hours whereas heating to 130 to
140C requires only about 1/2 hour or less heat
treatment. It is believed that heating to about 80F
and for about 1 hour is the most practical for altering
the distribution coefficient of the APPA extractant
in accordance with the instant invention.
Experience has shown that extractant losses
in the commercial separation of uranium from wet
process acid due to entxainment, solubility in the
acid, losses to interface emulsions, etc. is approxi-
mately two percent. In this process, the quantity o
new APPA added to the extractant solution can be
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varied by adjusting other parameters somewhat to
match losses without lowering uranium yield. A bleed
als at 113 is also provided to maintain extractant
equilibrium. Spent APPA can be removed from the
system by contacting the extractant solution with a
phosphoric acid solution, having, for example, a 40%
by weight H3PO4 concentration, which contains an
excess of about 1.5% by weight ferric ion. Such an
excess ferric ion concentration precipitates an
organic-iron complex. This can then be removed from
the extractant solution before recycle (not shown),
such as by filtration. By relying upon APPA for only
one use and then only for a short period which occurs
immediately upon its introduction into the extraction
system, the long-term effects that lower the distribu-
tion coefficient of the stable extractants are avoided.
The strip solution containing about 12
grams of uranium per liter in the example of Figure 1
is then contacted as at 115 with a synergistic mixture
of dialkyl phosphoric acid and trialkyl phosphine
oxide in an organic diluent such as refined kerosene
as described in ORNL-TM-2522. Six re-extraction
stages are sufficient to remove 99% of the uranium
into the extractant which is maintained as the
continuous phase in the re-extraction system. The
high concentration of uranium in the strip solution
feed to re-extraction provides sufficient uranium to
nearly saturate the extractant with uranium. As it
approaches saturation, uranium replaces other metallic
species in the extractant, leaving fewer impurities
to precipitate with the uranium. The uranium content
of the loaded extractant is about 15 grams per liter.
Entrained phosphoric acid is next scrubbed out of the
extractant, as at 117, to minimize phosphate contamina-
tion of the product, and the washed extractant iscontinuously fed to the precipitation system 119.
Here, the extractant is contacted with either ammonium
i,
~i4~7~a
carbonate to precipitate ammonium uranyl tricarbonate
(AUT) or ammonia may be used to precipitate ammonium
diuranate (ADU). AUT produces a more filterable
precipitate under these conditions than does ADU.
The precipitate is filtered off, dried and calcined.
The yellow cake decomposes to commercially pure U3o8
and ammonia or ammonia and carbon dioxide gases,
which gases are redissolved and reused. The ammoniated
extractant is reacidulated, as at 121, with mineral
acid and recycled to re-extraction.
As is apparent from the description, our
discoveries have permitted us to devise a process
with many cost saving advantages over the processes
previously used to recover uranium from wet process
phosphoric acid and to produce a commercially pure
uranium oxide product. Other advantages and other
systems which may be readily apparent from this
disclosure to one skilled in the art are considered
to be within the scope of this invention.
