Note: Descriptions are shown in the official language in which they were submitted.
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CONTINUOUS ION EXCHANGE PROCESS INTEGRATED
WITH MEMBRANE SEPARATION FOR RECOVERING URANIUM
FIELD
The present invention is directed toward a continuous ion exchange process for
recovering
uranium from pregnant liquor solutions.
INTRODUCTION
Continuous ion exchange (CIX) processes have been used since the 1970's to
recover
uranium from pregnant liquor solutions (PLS). A brief overview of the process
is described by:
Anton R. Hendriksz and Ronald R. McGregor, "The extraction of uranium from in-
situ leach
solutions using NIMCIX ion exchange contactor," Annual Uranium Seminar
(proceedings) 1980,
4th, pages 121-124. In general, the CIX process involves the use a uranium
recovery circuit
including of a plurality of ion exchange beds, commonly arranged in carousal,
which repetitively
cycle through individual process zones including uranium loading and elution.
Various anions (e.g.
chloride, sulfate, carbonate, bicarbonate) present in the PLS can also absorb
on resin exchange sites
during the resin loading phase of the process. The extent to which these
anions ultimately compete
with uranium anions is influenced by their relative concentration and affinity
for the resin along with
the pH and temperature of the leach solution. The recycling of barrens or
residual eluant exacerbates
this problem by effectively concentrating these competing anions to the point
where they result in a
loss of separation efficiency, e.g. lower resin capacity, more frequent resin
elution, eluant
replacement, dilution of PLS, and the like.
SUMMARY
The present invention includes a continuous ion exchange system and method for
recovering uranium from a pregnant liquor solution that integrates the use of
one or more membrane
separations to reduce the concentration of competing anions. In one
embodiment, the method
includes recovering uranium from an alkaline pregnant liquor solution
including uranium, carbonate
and chloride. The pregnant liquor solution is passed through a plurality of
ion exchange beds (12,
14) resin that cycle through process zones as part of a repeating uranium
recovery circuit. The
method includes the steps of: (a) treating the pregnant liquor solution (16)
with a membrane (28) to
produce: i) a leach permeate solution (30) at least partially depleted of
uranium and carbonate and ii)
a leach concentrate solution (30') having a relatively higher concentration of
uranium and carbonate
and which is at least partially depleted of chloride; (b) passing the leach
concentrate stream (30')
through an ion exchange bed to load uranium onto a strong base anion exchange
resin and produce
an untreated barren (18) solution depleted of uranium, (c) passing an eluant
solution (20) comprising
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bicarbonate through the loaded ion exchange bed to strip uranium from the
strong base anion
exchange resin and produce an eluate (22) comprising uranium and bicarbonate,
(d) precipitating
uranium (24) from the eluate (22) to produce a residual eluant solution (26)
depleted of uranium, and
(e) repeating steps (a)-(d).
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of an embodiment of the present continuous
exchange system.
DETAILED DESCRIPTION
The invention includes a system and method for recovering uranium from a
pregnant liquor
solution, ("PLS"). The source of the PLS is not particularly limited but is
typically produced by
heap leaching, in-situ leaching, vat leaching or pressure leaching of
carbonate-containing uranium
ores. In one embodiment, the leach ores reside in a lixiviation tank from
which the PLS is drawn.
The PLS comprises an alkaline solution preferably having a pH of at least 9
and more preferably at
least 10; and further includes uranium, bicarbonate, carbonate, sulfate and
chloride anions along
with their counter cations and corresponding salts. Even though the
concentration of these anions is
dynamic, they are preferably maintained within the following ranges:
carbonate: 10-60 g/L;
bicarbonate: 1-20 g/L; chloride: 0 to 10g/L; sulfate: 0-25 g/L. While the
tetravalent uranyl
tricarbonate complex anion UO2(CO3)3 4-- predominates, a divalent ion
UO2(CO3)22- = 2H20 may
exist at low carbonate concentration. During the loading phase of the process,
the mobile exchange
ion ("X," e.g. chloride, hydroxyl, etc.) initially adsorbed on the exchange
resin (R) and the uranium
anions in solution will proceed as follows:
4RX + UO2(CO3)34 ¨> R4UO2(CO3)3 +4K
When an anion exchange resin is provided in the carbonate form, the loading
reactions proceeds as
follows:
2(R+)2C032 + UO2(CO3)34 ¨> (R+)4. UO2(CO3)34 + 2 C032
During the elution phase, an eluant solution (e.g. from 50g/L to saturated
aqueous bicarbonate
solution) is passed through the uranium loaded ion exchange bed and exchanges
eluant anions for
uranium anions.
As part of the present method, the PLS is subject to continuous ion exchange
(CIX)
including the step of passing PLS through a plurality of ion exchange beds
containing strong base
anion exchange resin. The beds pass through individual process zones as part
of a repeating uranium
recovery circuit schematically illustrated in Figure 1. More specifically, a
CIX unit is generally
shown at 10 including a plurality of ion exchange beds (12, 14) containing a
strong base anion
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exchange resin that sequentially pass through individual process zones (e.g.
A, B) as part of a
uranium recovery circuit. Each zone preferably includes at least one ion
exchange bed or column,
and in practice may include a plurality of individual beds. The method
includes the following
sequential steps:
(a) passing the PLS (16) through an ion exchange bed (zone A) to load
uranium onto
the ion exchange resin and produce an untreated barren solution (18) which is
depleted of uranium,
and (b) passing an eluant solution (20) through the uranium loaded
ion exchange bed(s)
(zone B) to strip uranium from the ion exchange resin and produce an eluate
(22). The eluate (22)
may be then treated to precipitate out uranium (24) leaving a residual eluate
solution (26) that may
be optionally reused. The method may include additional process zones as is
well known in the art,
e.g. rinsing, washing, scrubbing, etc. Processed uranium ore may be stored in
a lixiviation tank (27)
from which PLS is drawn. PLS and eluant may be maintained in tanks (16'),
(20'), respectively.
The tanks are in selective fluid communication with the ion exchange beds (12,
14). Fluid flow may
be controlled by a plurality of values and a control panel (not shown) as the
beds (12, 14) cycle
through the individual process zones (A and B). CIX equipment for performing
the subject method
is available from PuriTech (e.g. IONEXTm), Ionex Separations and Calgon Carbon
(e.g. ISEPTM)
and is also described in US 7594951. Suitable ion exchange resins include
AMBERSEPTm 400
strong base anion exchange resin available from The Dow Chemical Company. This
resin includes
a styrene-divinylbenzene copolymer (gel) matrix with functional quaternary
ammonium groups.
The resin may be initially provided in various ionic forms, e.g. sulfate,
carbonate, hydroxyl and
chloride.
In order to reduce the concentration of competing anions (e.g. chloride)
present in the PLS
(16), at least a portion of the PLS is be treated with a membrane (28) to
produce: i) a leach permeate
solution (30) at least partially depleted of uranium and carbonate and ii) a
leach concentrate solution
(30') having a relatively higher concentration of uranium and carbonate and
that is at least partially
depleted in monovalent anions (e.g. chloride) as compared with the untreated
PLS (16). The leach
permeate solution (30) may be disposed or reused. For example, the leach
permeate solution (30)
may be subject to further membrane treatment (not shown), e.g. with a reverse
osmosis membrane
(e.g. FILMTECTm XLE-440). The concentrate solution resulting from such a
reverse osmosis
treatment includes most of the remaining ionic species (e.g. chloride,
sulfate) and can be disposed;
whereas the permeate solution can be recycled and used in the lixiviation tank
(27) to replace
evaporative loss, used to make fresh bicarbonate solution added to the
lixiviation tank (27), or used
to dilute the PLS (16) or leach concentrate solution (30').
The leach concentrate solution (30') (and optional blended PLS (16)) is passed
through an
ion exchange bed (12) to load uranium onto the strong base anion resin and
produce an untreated
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barren solution (18) depleted of uranium. The untreated barren solution (18)
may be disposed of,
recycled back to the lixiviation tank (27), or in a preferred embodiment,
subject to further treatment
with a membrane (31). For example, all or a portion of the untreated barren
solution (18) may be
treated with a membrane (31) to produce: i) a barren permeate solution (32) at
least partially
depleted of carbonate (and other anions optionally including sulphate and
chloride) and ii) a barren
concentrate solution (32') having a relatively higher concentration of
carbonate. The barren
permeate solution (32) may be optionally recycled to (i.e. combined with) the
PLS (16) or leach
concentrate solution (30') for use in the loading phase of the process. The
barren concentrate
solution (32') may be optionally disposed (34) or recycled, e.g. all or a
portion may be recycled to
the lixiviation tank (27). In a preferred embodiment, the barren concentrate
solution (32') is subject
to further membrane treatment (not shown), e.g. with a reverse osmosis
membrane with the resulting
permeate being used in the lixiviation tank (27) to replace evaporative loss,
used to make fresh
bicarbonate solution added to the lixiviation tank (27), or used to dilute the
PLS (16) or leach
concentrate solution (30').
The eluant solution (20) passes through the uranium loaded ion exchange bed(s)
(zone B) to
strip uranium from the ion exchange resin and produce an eluate (22). The
eluate (22) may be then
treated to precipitate out uranium (24) leaving a residual eluate solution
(26). By way of example,
the eluate may be neutralized with sulfuric acid and uranium can be
precipitated with hydrogen
peroxide. In this example, the resulting residual eluate solution (26)
includes sodium sulfate along
with carbonate/bicarbonate. This residual eluate solution (26) may then be
disposed of, recycled to
the lixiviation tank (27) or preferably subject to further membrane treatment.
For example, at least a
portion of the residual eluate solution (26) may be treated with a membrane
(38) to produce: i) a
residual eluate permeate solution (40) at least partially depleted of
bicarbonate (and uranium) and ii)
a residual eluate concentrate solution (42) having a relatively higher
concentration of bicarbonate
(and uranium) than the residual eluate solution (26). The residual eluate
concentrate solution (42)
can be recycled directly to the lixiviation tank (27) the PLS (16) or the
leach concentrate solution
(30'). The residual eluate permeate solution (40) may be disposed, or further
treated with
membranes (not shown). For example, the residual eluate permeate solution (40)
may be further
treated with a reverse osmosis membrane (e.g. FILMTECTm XLE-440 or FILMTECIm
BW30
XFR-400/34i or FILMTECTm XFRLE-400/34i available from The Dow Chemical
Company. This
treatment creates a second permeate solution that is depleted of almost all
ions (e.g. over 98%
rejection of chloride) and a second concentrate solution including most of the
ions and salts that
were present in the residual eluate permeate solution (40). This second
permeate solution can be
recycled to the lixiviation tank (27), used to prepare fresh bicarbonate
solution for addition to the
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lixiviation tank (27) or for diluting the PLS (16) or leach concentrate
solution (30'). The second
concentrate solution can be disposed.
Different membranes may be used depending upon the degree of ion separation
desired.
Applicable membranes (28, 31 and 38) include nanofiltration and reverse
osmosis elements such as
FILMTECTm NF90 and NF 270, FILMTECIm XLE-440 or FILMTECTm BW30 XFR-400/34i or
FILMTECTm XFRLE-400/34i available from The Dow Chemical Company.
Many embodiments of the invention have been described and in some instances
certain
embodiments, selections, ranges, constituents, or other features have been
characterized as being
"preferred." Characterizations of "preferred" features should in no way be
interpreted as deeming
such features as being required, essential or critical to the invention.
Stated ranges include end
points. The entire subject matter of each of the aforementioned patent
documents is incorporated
herein by reference.
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