CA1144856A - Method of aquifer restoration - Google Patents

Abstract

48,040 ABSTRACT OF THE DISCLOSURE
A method is disclosed for restoring a clay rich aquifer which has been solution mined with ammonium ions.
Calcium carbonate is precipitated from a solution pumped through the aquifer by the addition of calcium oxide, cal-cium hydroxide, or a mixture thereof in quantities suffi-cient to raise the pH to at least about 9.5. The precipi-tated calcium carbonate is separated from the solution and the ammonium ion is then removed from the solution by standard techniques. The solution is recycled through the aquifer and these steps are repeated until the bicarbonate ion concentration has been reduced to such an extent that the addition of at least one soluble salt of calcium, magnesium, or potassium to the solution does not result in precipitation of calcite underground, plugging the aqui-fer. Recycling with the addition of the soluble salt is continued until the ammonium ion concentration is reduced to a desired level. Finally, residual amounts of the salt are removed from the aquifer using standard techniques.

Description

8~ 6 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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2 48,040 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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3 48,040 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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~ 8,040 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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48,040 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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' 6 48,040 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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7 ~8,040 CALCIUM HYDROXIDE TESTS
__ 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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Claims (14)

8 48,040 CLAIMS:

1. A method of restoring a clay-containing aquifer which has been solution mined with a solution containing ammonium ions comprising:
(1) precipitating calcium carbonate from a solution pumped through said aquifer by the addition of calcium oxide, calcium hydroxide, or a mixture thereof in quantities sufficient to raise the pH to at least about 9.5;
(2) separating said precipitated calcium carb-onate from said solution;
(3) removing ammonium ion from said solution;
(4) recycling said solution through said aqui-fer and repeating steps (1), (2), and (3) until the bicar-bonate ion concentration in said solution has been reduced to an extent such that step (5) can be performed without plugging said aquifer;
(5) adding at least one soluble salt of cal-cium, magnesium, potassium, sodium, or mixtures thereof to said solution;
(6) recycling said solution through said aqui-fer and repeating steps (1), (2), (3), and (5) until the ammonium ion level is reduced to a desired level; and (7) removing residual amounts of said salts introduced in step (5) from said aquifer.

2. A method according to Claim 1 wherein said calcium carbonate is precipitated using calcium oxide.

3. A method according to Claim 1 wherein the amount of calcium oxide, calcium hydroxide, or mixture 9 48,040 thereof added in step (1) is at least stoichiometrically equivalent to the bicarbonate ion concentration in said solution.

4. A method according to Claim 1 wherein said pH is raised to about 10 to about 12 in step (1).

5. A method according to Claim 1 wherein said ammonium ion is removed by ammonia air stripping.

6. A method according to Claim 1 wherein said ammonium ion is removed by clay absorption.

7. A method according to Claim 1 wherein said ammonium ion is removed by biological denitrification methods.

8. A method according to Claim 1 including the additional step during the initial recycling between steps (3) and (4) of adding acid to said solution to reduce the pH of said solution to about 6 to about 10.

9. A method according to Claim 1 wherein said soluble salt is at least one calcium salt.

10. A method according to Claim 9 wherein said calcium salt is calcium hydroxide.

11. A method according to Claim 1 wherein step (6) is continued until the ammonium ion concentration is reduced to about 50 to about 100 ppm.

12. A method according to Claim 1 wherein step (7) is accomplished by standard groundwater sweeping or clean water recycle techniques.

13. A method according to claim 6 wherein said clay used for ammonium absorption is clinoptillolite clay.

14. A method according to claim 8 wherein said acid is hydrochloric acid.