Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 48,040
METHOD OF AQUIFER RESTORATION
BACKG~0UND OF THE IN~ENTION
Methods of solution mining uranium have been
perfected to the extent that the processes are now commer-
cial. In-situ leaching of uranium mineralized in uncon-
solidated sandstone layers underground is typically car-
ried out using an alkaline bicarbonate such as an ammonium
bicarbonate-ammonium carbonate solution along with an
oxidant such as a peroxide. The sandstone stata amenable
to in-situ leaching is generally contained between con-
fining shale or mudstone layers. The mineralization alsogenerally contain some amount of cla~ usually in the range
5 to 15%~ consisting predominantly of montmorillonite or
kaolinite with small amounts of illite and clinoptilolite
clays.
15The uranium that is leached from the in-situ ore
- body and is in the pregnant leach solu-tion is recovered by
hydrometallurgical methods, such as ion exchange. The
leach solution is recycled after restoring its chemical
strengths in the reagent and the oxidant. During the
in-situ leaching process due to the underground mineral-
ization, the ammonium ion in the leach solution may be
absorbed by the clay fraction. The counter ion released
from the clay may be calcium, magnesium, sodium, potas-
sium, etc., depending upon the state of the virgin clay.
One of the principa]. problems in the utilization of this
technology is the restoration of the leached mineraliza-
tion to a stable aquifer as mandated by state and federal
legislation. Until now, techniques for accomplishing the
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restoration of the aquifer were limited to pumping old
water out of -the aquifer and discarding it and permitting
ground wa~er to seep into the mine zone from the surround-
ing aquifer, a technique known as ground water sweep.
Anothe~ technique that was also used was called "clean
water recycle" which involved cleansing the solution
pumped from the aquifer through a reverse osmosis membrane
and pumping the cleansed water back underground until the
ammonium ions were removed ~rom the aquifer. Both these
techniques could require long periods of time to reduce
the ~mmcnium ion level to acceptable levels depending on
the characteristics of the clay.
SUMMARY 0~ THE INVENTION .
We have discovered a method of restoring an
aquifer which has been mined for uranium or other metals
using an ammonium ion containing solution mining tech-
niques. Basically, our invention involves the removal of
bicarbonate ion followed by the removal of the ammonium
ion and the removal o residual salts used in the process.
Our technique reduces ammonium ion concentration
much more rapidly than previous techniques and is capable
of reducing the concen-tration to levels which meet the
~ restoration criteria of various government agencies. Our
; process does not use expensive materials and is not unduly
: 25 elaborate.
DES~RIPTION OF THE INVENTION
The accompanying drawing is a block diagram
which illustrates a certain presently-preferred embodiment
o~ the process of this invention.
This invention is useful in restoring clay-con-
taining aquifers because it is in clay-containing aquifers
that ammonium ion retention is a problem. An aquifer
having about 5 to about 25% clay or more can be usefully
restored ~Ising the process of this invention and aquifers
having as low as about 2% clay can probabl~ also benefit
from the technology embodied in this invention. The
invention is applicable to aquifers which were m:ined using
lixiviants containing ammonium io~s. Usually, the solu-
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tions are used to mine uranium although they could also be
used to mine other metals.
Referring to the drawing, a solution in line l
is pumped out of the underground aqui-fer 2 from leached
ore 3 to lime softener ~. In the lime softener, the
bicarbonate ion i5 removed to prevent it from forming
precipitating calcite underground which could plug the
formation. The removal of ~he bicarbonate ion is accom-
plished by the precipitation of calcite, calcium carbon-
ate, CaC03. The calcium carbonate precipitation is accom-
plished by the addition of lime, Ca(OH)2, or calcium
oxide, CaO or mixtures thereof (line 5 in drawing). It is
preferable to use calcium o~ide as it is less expensive
than lime. The amount of calcium oxide or hydroxide added
should be at least stoichiometrically equi~alent to the
bicarbonate ion concentration in the solution and should
be sufficien~ to raise the pH to at least 9.5. Prefer-
ably, sufficient calcium oxide or hydro~ide is added to
raise the pH to about 10 to about 12. Higher pH's may be
used but require the addition of too much calcium oxide or
hydro~ide.
The precipitated calcium carbonate solids are
then separated from the remainder of the solution (line 6
in drawing). This can be accomplished by any standard
technique such as the use of a decantation or settling
device or a filter. ~ranium which is in solution also
tends to precipitate with the calcium carbonate and if its
concentration is high enough, recovery of it may be eco-
nomical.
From lime sotener 4, the remaining solution
passes through line 7 to an ammonium removal system 8
~here the a~nonium ion is remo~ed ~rom the solution. This
may be accomplished in se~eral ways. The preferred tech-
nique is ammonia air stripping, a well-k~own procedure.
In thi.s technique, air from line 9 is bubbled through the
solution which carries off ammonia gas according to the
equation
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NH4~ ~ OH ~ ~H3~ H20.
The ammonia gas can be recovered as dilute ammonium hy-
droxide by bubbling it through water or as an ammonium
salt such a~ (NH4)2S04, N~4Cl, or NH4~03 y
through a sol~tion of H2SO~ Cl, or HN03, respectively.
Alternatively, instead of using ammonia air stripping
(which is i-llustrated in the drawing), the ammonium ion
may be removed using clinoptillolite clay absorption in a
packed bed tower or in a slurry in a mix tank, which are
lo also well-known techniques. The ammonium ion is absorbed
onto the clay, and is la-ter removed from the clay by
dilution with a high pH solution such as sodium hydroxide
or calcium hydroxide. This technique is less preferred
than air stripping because -the pH of the solution must be
lowered to about 5 to 9 with an acid such as sulfuric acid
before it can be used, and it therefore requires more
steps and is more expensive.
An additional alternative for ammonia ion remov-
al is the use of standard biological denitrification
methods. In this technique, standard biological treatment
systems are used to convert the NH4+ to nitrogen gas, N2,
and thus, effect the ammonium ion removal.
The remaining solution is then recycled through
line 10 back underground to the aquifer. The solution is
recycled and the above steps are repeated until the bicar-
bonate ion concentration in the solution is low enough to
permit proceeding to the nex-t step without plugging the
formation by precipitating calcium carbonate underground.
If the formation is not very permeable, it is also neces-
sary during this recycling to add an acid such as hydro-
chloric acid from line 11 to reduce the pH to about the
range of 6 to 10 to prevent the underground precipitation
of calcite. Also, iE the aquifer begins to plug up at any
time due to the prec:ipitated calcite, the addition of
hydrochloric acid can be used to open it up again. If the
particular formation is a ~ery loose formation, one can
proceed to -the next step sooner and risk some underground
calcite precipitation.
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Once the bicarbonate ion concentration has been
reduced, high concentrations of calcium, magnesium, potas-
sium, or mix~ures thereof are introduced in line 12 to
displace the remaining ammonium ions from -the clay. The
introduction of -these ions can be accomplished by adding
any soluble salt of calcium, magnesium, potassium, or
mixtures thereof. Calcium, however, is preferred because
of its high af~inity for clay. The preferred anion is
hydroxide for calcium, sulfate for magnesium, and chloride
for potassium. If calcium hydroxide is used, the concen-
tration used is about 100 ppm to about saturation. lf
another soluble salt is used, the concentration is about
100 ppm up to about 10 grams per liter. The solution is
again recycled through lines ~, ~, and ~ as before except
~hat no hydrochloric acid is added from line 11. This
recycling step is continued until the ammonium ion concen-
tration has decreased to an acceptable level, usually
considered to be about 50 to about 100 ppm.
Finally, it is necessary to remove the residual
soluble salts of calcium, magnesium, or potassium and
other residual salts which may be present due to the
solution mining process which were previousIy added. This
can be accomplished using standard ground water sweeping
or clean water recycle techniques. Clean water is pumped
through the aquifer until the total dissolved solids in
the solution have been reduced to acceptable levels.
Alternatively, a clean water recycle technique can be
used. In this technique, the solution is recycled as
before except that after the ammonium strip tower 8, the
solution passes through line 13 to a reverse osmosis
membrane or ion exchange column 14 and back underground
through line 15 until the total dissolved solids have been
reduced.
It is also possible at this stage to process
solution 1 directly in the RO unit, 14, without going
through the lime so~tener, ~, or ammonia strip tower, 8,
as shown by line 16.
The following example further illustrates this
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invention.
EXAMPLE
Agitation leach tests were performed on the
~ollowing samples of ammoniated clay.
Ammonium Content
Clay Type meq/g
Montmorillonite No. 24, 1.0
California
Montmorillonite No. 27, 0.818
South Dakota
Kaolinite No. 9, 0.02
North Mexico
Illite Bearlng Shale, 0.159
Illinois
15 Clinoptilolite, 1.735
California
Five grams of the clay were~ agitated in 200
milliliters of water containing various concentrations of
calcium hydroxide. After 24 hours, the solutions were
analyzed to determine the amount of ammonia removed from
the clay. The following table gives the result.
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CALCIUM HYDROXIDE TESTS
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Volume of Solution Used = 200 ml
Amount of Clay Used ~ 5.0 g
Ca Con ~ . mg/ Ca Exchd . %NH3 Rej ect .
No. Clay_T ~ __ Feed Raff. meq/g (estimate) Fim~l Ph
Mont ., Calif. 895 168 1.454 100 11.4
2 Mon-t ., Calif. 452 12 0.880 88,0 10.4
3 Mont ., Calif. 225 12 0.426 42.6 10.5
4 Mont., S .Dak. 895 2501.290 100 11.7
10 5 Mont., S.Dak. 452 135 0.634 77.5 10.3
6 Mont ., S . Dak. 225 28 0.394 48.2 9.6
7 Kaolinite 895 622 0.546 100 11.8
8 Kaolinite 452 349 0.206 100 11.8
9 Kaolinite 225 155 0.140 100 11.3
1510 Illite 895 584 0.622 100 11.8
11 Illite 452 266 0.372 100 10.9
12 Illite 225 137 0.176 100 11.5
13 Clinoptilolite 895 95 1.600 92.2 11.1
14 Clinoptilolite 542 21 0.862 49.7 10.35
2015 Clinoptilolite 225 4 0.442 25.5 10.0
The above table shows that calcium hydroxide was
very effective in removing ammonia from -the clay.
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