Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for the Recovery of Uranium from Pregnant Liquor Solutions
The present invention is directed to a new more environmentally friendly
method for the
recovery of uranium from acid leach pregnant liquor solutions that comprise
high levels of
chloride by using an amino phosphonic functionalized resin.
Numerous minerals are present in subsurface earth formations in very small
quantities
which make their recovery extremely difficult. However, in most instances,
these minerals are
also extremely valuable, thereby justifying efforts to recover the same. An
example of one such
mineral is uranium. However, numerous other valuable minerals, such as copper,
nickel,
molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth
metals, etc., are also
present in small quantities in some subsurface formations, alone and quite
often associated with
uranium. Consequently, the recovery of such minerals is fraught with
essentially the same
problems as the recovery of uranium and, in general, the same techniques for
recovering uranium
can also be utilized to recover such other mineral values, whether associated
with uranium or
occurring alone. Therefore, a discussion of the recovery of uranium will be
appropriate for all
such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and
granitic
deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata
such as sandstones,
unconsolidated sands, limestones, etc. However, very few subterranean deposits
have a high
concentration of uranium. For example, most uranium-containing deposits
contain from about
0.01 to 1 weight percent uranium, expressed as U 3 0 8 as is conventional
practice in the art. Few
ores contain more than about 1 percent uranium and deposits containing below
about 0.1 percent
uranium are considered so poor as to be currently uneconomical to recover
unless other mineral
values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-
containing materials. One common technique is roasting of the ore, usually in
the presence of a
combustion supporting gas, such as air or oxygen, and recovering the uranium
from the resultant
ash. However, the present invention is directed to the extraction of uranium
values by the
utilization of aqueous leaching solutions. There are two common leaching
techniques (or
lixiviation techniques) for recovering uranium values, which depend primarily
upon the
accessibility and size of the subterranean deposit. To the extent that the
deposit containing the
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uranium is accessible by conventional mining means and is of sufficient size
to economically
justify conventional mining, the ore is mined, ground to increase the contact
area between the
uranium values in the ore and the leach solution, usually less than about 14
mesh but in some
cases, such as limestones, to nominally less than 325 mesh, and contacted with
an aqueous leach
solution for a time sufficient to obtain maximum extraction of the uranium
values. On the other
hand, where the uranium-containing deposit is inaccessible or is too small to
justify conventional
mining, the aqueous leach solution is injected into the subsurface formation
through at least one
injection well penetrating the deposit, maintained in contact with the uranium-
containing deposit
for a time sufficient to extract the uranium values and the leach solution
containing the uranium,
usually referred to as a "pregnant" leach solution (PLS), is produced through
at least one
production well penetrating the deposit. It is this latter in-situ leaching of
subsurface formations
to which the present invention is directed.
The most common aqueous leach solutions are either aqueous acidic solutions,
such as
sulfuric acid solutions, or aqueous alkaline solutions, such as sodium
carbonate and/or
bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of
uranium values.
However, aqueous acidic solutions generally cannot be utilized to extract
uranium values from
ore or in-situ from deposits containing high concentrations of acid-consuming
gangue, such as
limestone. While some uranium in its hexavalent state is present in ores and
subterranean
deposits, the vast majority of the uranium is present in its valence states
lower than the
hexavalent state. For example, uranium minerals are generally present in the
form of uraninite, a
natural oxide of uranium in a variety of forms such as UO 2, UO 3, UO.U 2 O 3
and mixed U 3 O 8
(UO 2 .2UO 3 ), the most prevalent variety of which is pitchblende containing
about 55 to 75
percent of uranium as UO 2 and up to about 30 percent uranium as UO 3 . Other
forms in which
uranium minerals are found include coffinite, carnotite, a hydrated vanadate
of uranium and
potassium having the formula K 2 (UO 2) 2 (VO 4) 2 .3H 2 0, and uranites which
are mineral
phosphates of uranium with copper or calcium, for example, uranite lime having
the general
formula CaO.2UO 3 .P 2 0 5 .8H 2 O. Consequently, in order to extract uranium
values from
subsurface formations with aqueous acidic leach solutions, it is necessary to
oxidize the lower
valence states of uranium to the soluble, hexavalent state.
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Combinations of acids and oxidants which have been suggested by the prior art
include
nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid,
in combination with air,
oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and
magnesium
perchlorate and dioxide, as oxidants. However, the present invention is
directed to the use of
sulfuric acid leach solutions containing appropriate oxidants and other
additives, such as
catalysts.
There are two commonly used methods for the recovery of uranium from pregnant
leach
solution (PLS). One technique, solvent extraction, employs the use of a non
aqueous solvent to
selectively extract uranium from the PLS.
The second method involves ion exchange technology. Strong and weak base anion
exchange resins are commonly used. This ion exchange method has become the
more preferred
method of uranium recovery in various regions of the world because of its
environmental
benefits as well as its safety benefits. Flammable toxic solvents need not be
used for the present
method as compared to the solvent extraction method where harmful chemicals
are employed.
Additionally it has been discovered that in environments where there is a
relatively high
concentration of sulfate, i.e. greater than 25g/L, based on the composition of
the PLS fouling of
the ion exchange resin occurs. This fouling results in a decreased loading
capacity of the resin.
US 4599221 uses an amino phosphonic functionalized resin to recover uranium
from phosphoric
acid; however a need exists for a method to recover uranium from acid leach in
high sulfate
environments. Recovery of uranium from phosphoric acid is a different process
from the acid
leach process because there are competing ions, such as sulfate, in an acid
leach solution that can
foul any recovery media. The phosphoric acid process does not have such.
Additionally, the
levels of uranium in a phosphoric acid process are relatively low, i.e. less
than 300ppm. In acid
leach, the loading capacity of uranium must be much greater as the levels of
uranium in acid
leach liquors can be present in up to 2000 mg/L (or ppm) It is known that for
the same
concentration of uranium in the PLS, the operating capacity is much greater in
acid leach liquor
than in phosphoric acid liquor. Therefore one of skill in the art would not
typically apply the
same techniques from the recovery of metals from phosphoric acid to acid
leach.
The present invention solves these problems of the art by proving an amino
phosphonic
functionalized resin type useful for the recovery of uranium that does not
foul in sulfate
environments of greater than 25g/L.
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The present invention provides a method for the recovery of uranium from a
pregnant
liquor solution comprising:
i) providing an amino phosphonic functionalized resin;
ii) providing a pregnant liquor solution comprising sulfate and uranium;
iii) passing the pregnant liquor solution over the amino phosphonic
functionalized resin
to separate the uranium from the pregnant liquor solution; and
iv) eluting the uranium
wherein the sulfate is present in an amount from 25 to 280 g/L.
As used herein the term amino phosphonic functionalized resin is meant to
include either
an amino phosphonic resin or an amino hydrophosphonic functionalized resins.
In the present invention the resin is a styrene polymer resin having active
amino
phosphonic functional groups linked to the polymer matrix. The term "styrene
polymer"
indicates a copolymer polymerized from a vinyl monomer or mixture of vinyl
monomers
containing styrene monomer and/or at least one crosslinker, wherein the
combined weight of
styrene and cross linkers is at least 50 weight percent of the total monomer
weight. The level of
cross linking ranges from 4 to 10%. All percentages herein are weight
percentages.
A crosslinker is a monomer containing at least two polymerizable carbon-carbon
double
bonds, including, e.g., divinylaromatic compounds, di- and tri-(meth)acrylate
compounds
anddivinyl ether compounds. Preferably, the crosslinker(s) is a
divinylaromatic crosslinker, e.g.,
divinylbenzene.
The structure of the polymer can be either gel or macroporous
(macroreticular). The term
"gel" or "gellular" resin applies to a resin which was synthesized from a very
low porosity (0 to
0.1 cm3/g), small average pore size (0 to 17 A) and low B.E.T. surface area (0
to 10 m2/g)
copolymer. The term "macroreticular" (or MR) resin is applied to a resin which
is synthesized
from a high mesoporous copolymer with higher surface area than the gel resins.
The total
porosity of the MR resins is between 0.1 and 0.7 cm3/g, average pore size
between 17and 500 A
and B.E.T. surface area between 10 and 200 m2/g. The resin is in appropriate
ionic form,
preferably acid or acidic form. The resin of the present invention may be in
sodium form.
The resin is used to treat an acid leach pregnant liquor solution (PLS). The
PLS of the
present invention comprises uranium and sulfate. Uranium is primarily present
in the form of
U308; although other commonly known forms and isotopes of uranium may be
present. As used
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herein, the term uranium refers to all forms and isotopes of uranium. Uranium
is present in the
PLS in an amount from 25 to 2000 mg/L, preferably from 50 to 1500mg/L, and
further
preferably from 60 to 1000 mg/L. Sulfate ion and sulfate complexes together as
"sulfate" is
present in the PLS in an amount from 25 to 280 g/L and preferably form 35 to
220g/L and
further preferably from 40 to 180 g/L. The PLS of the present invention may
optionally contain
a variety of other components. Such components include but are not limited to:
iron, sulfuric
acid, sodium, calcium, potassium, chloride, copper, phosphorus, and aluminum.
The pH of the
PLS is acidic and ranges from 0 to 4, preferably 0 to 3, more preferably 0 to
2, most preferably 0
to 1.8, . Furthermore, the PLS may be obtained from any method commonly known
to those of
skill in the art including but not limited to in situ leach, heap, leach,
resin in pulp, and in situ
recovery.
Uranium is separated from the PLS by passing the PLS over the amino phosphonic
functionalized resin. Techniques commonly used in the art to separate the
uranium from the PLS
may be applied. Such techniques include but are not limited to fixed bed, co-
current or
countercurrent fluidized bed, resin in pulp (RIP). The process may be batch or
continuous.
Typically the flow rate within the column or packed bed system is from 0.5 to
50BV/h.
The amino phosphonic functionalized resin retains the uranium from the PLS and
the uranium is
then recovered by elution. Methods of elution used by those of ordinary skill
in the art are used
herein. Preferably, the uranium loaded resin may be treated with a solution of
ammonia or
ammonia hydroxide. Afterwards, the resin is eluted with a solution of sodium
carbonate. The
uranium is then recovered from solution by known separations techniques, such
as for example
precipitation. It is beneficially found that the at least 10% of the uranium
found in the original
PLS may be recovered. Within the pH range of 0 to 4 of the PLS, uranium
recovery levels of up
to 25% may be achieved, preferably up to 10%. The uranium may be recovered at
levels ranging
from 5-25%, preferably 10-25%, and more preferably 15-25%.
In addition to ion exchange technology, solvent extraction technology may also
be
employed to recover uranium from PLS comprising high levels of sulphate.
Traditionally,
solvent extraction employs solvents with a tertiary amine functional group. In
the present
invention, affixing an amino phosphonic group or an amino hydrophosphonic
group to a solvent
molecule may be advantageously employed in lieu of tertiary amine functional
groups.
Conventional methods of solvent extraction may be utilized herein.
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Examples
Laboratory equipment used
Jacketed glass column (height 30cm, 0 2-3cm, fitted with sintered glass of
porosity 1). Peristaltic
pump with flexible tubings. 10, 100 graduated cylinder. 25 mL plastic flasks
for samples collections.
Stopwatch. Appropriate equipment for Uranium analysis (I.e: ICP). Standard
laboratory glassware
Resin used
AMBERSEPTM 940U, is a registered trademark of Rohm and Haas Company, a wholly
owned
subsidiary of The Dow Chemical Company. The resin is in sodium form having a
polystyrenic
matrix, crosslinked with divinyl benzene and containing aminophosphonic
functional groups.
Note
The resin was converted in its appropriate ionic form (i.e: acidic form)
before carrying out the
experiments.
Examples
Solution
Solution 1: A solution containing 500 mg/L of uranium (expressed as U), 25g/L
of sulfate, Og/L of
chloride 2g/L of iron (as Fe 3) was left in contact with a sample of
AMBERSEPTM 940U for 8 hours.
Solution 2: A solution containing 500 mg/L of uranium (expressed as U), 195g/L
of sulfate, 20g/L of
chloride 2g/L of iron (as Fe 3) was left in contact with a sample of
AMBERSEPTM 940U for 8 hours.
Solution 3: A solution containing 500 mg/L of uranium (expressed as U), 278g/L
of sulfate, Og/L of
chloride 2g/L of iron (as Fe3+) was left in contact with a sample of
AMBERSEPTM 940U at 2.5 BV/h.
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Experiments
All experiments were carried out at 25 C. 500 milliliters of solution was left
in contact with a 10
milliliters sample of AMBERSEPTM 940U. The ratio of 1 part resin to 50 parts
of solution was kept
constant in order to avoid any external perturbation. The pH of the solutions
was adjusted at different
values (i.e: 0, 1, 1.8, 2.5, 3) in order to determine the impact on the
loading capacity. After shaking for
8 hours, the analysis of the uranium residual in the supernatant was measured
and the resin loading
determined.
Results
pH
0 1 1.8 2.5 3
J
Solution 1 41.0 37.4 29.0 34.8 38.0
CL Solution 2 36.2 24.2 19.4 23.0 27.8
V
IM
Solution 3 24.8 21.9 16.7 16.2 21.1
0
The uranium loading increases when the pH decreases. The operating capacity
equates 36.2 g/LR
(expressed as U) when the pH is equal to 0 for a solution containing 195g/L.
The results prove that the resin AMBERSEPTM 940U exhibits very good
performance of uranium
recovery under very high concentration of sulfate.
It is remarkable that the lower the pH the better the operating capacity. Such
characteristic offers the
possibility to use sample of AMBERSEPTM 940U to recover uranium when the
concentration of
sulfate is very high. The resin performance (i.e: operating capacity) can be
improved by lowering the
pH.
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Elution
The loaded resin (obtained from experiment with the Solution 2 at pH 0) was
treated with 2 bed
volumes of a solution of ammonia hydroxide at a concentration of 1 mol/L (IN).
Afterwards, the
resin was eluted with a solution of sodium carbonate at a concentration of I
N.
The totality of uranium loaded was eluted within 7 bed volumes of sodium
carbonate solution.
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