CA1102679A - In situ leach method for recovering uranium and related values - Google Patents

Abstract

IN SITU LEACH METHOD FOR
RECOVERING URANIUM AND RELATED VALUES
Abstract of the Disclosure An in situ leaching process for recovering uranium values from a subterranean formation containing calcium-based clays wherein the lixiviant used has a cationic composition essentially the same as that of the formation connate water with only minute quantities of innocuous leaching chemicals, i.e., CO2, and an oxidant, e.g., O2, present therein. The pH
of the lixiviant at the injection well is maintained at a value approximately equal to that of the formation connate water, i.e., 7.5 to 8Ø The pH control to this level is achieved by addition of limes without introducing additional cations to the leaching circuit. This lime addition also serves the purpose of calcium removal (water softening) from the lixiviant. Since there are no cations, e.g., NH4+, added to the lixiviant, there is no problem of contaminating the formation water and altering the nature of the formation, itself, thereby eliminating the need to restore the formation after the leach. Further, since the lixiviant is essentially of the same cationic composition as the formation water, swelling of clay in the formation is minimized.

Description

~343 Ba~s~ d o~ the Invention _ The present invention relates ~o an in si~u leaching me~hod ~or recovering uranium and xela~ed values~ e.g., molybdenum, rom a subterranean ~onmation. More particula~ly,~
the present invention rel~tes to a method wherein urani~m and rela~ed values are leached in si~u with a lixivian~ which does not contaminate either the formation or the recovered values. ~
In a typical i~ situ lea(h operation~ wells a~e completed into the uranium and related value bearing formation and a ~ixiviant is flowed between injection and produc~ion wells to dissolve the desired values. Since the uranium is a~most always in its tetravalen~ state9 an oxida~t is used to oxidize the ura~ium to its hexavalent state so .lat it is readily soluble in the lixiviant. The pregnant lixiviant i.B
produced to the surface where it is treated to recover the desired values from the lixiviant.
Unfortuna~ely, many presently known e~ective lixivi~nts not only leach the desîred values from the formation, but also react with certain formatîons to give up chemicals which r~main in the formation after the leach operation has ; been completed. Where the formation a~so con~ains ground wa~ers and~or a wa~er source whic~ would otherwise be fi~ or surface use, e'.g., animal consumption, these chemicals ret~ined by the fo~ation will b~eed from the formation into the connate water, thereby posing a ser~ous contamination problem. When this situation exists~ the fonmation will have to undergo an expensive treatme~t oyeration tG remo~e these ccr.taminants ~o ~2 7~

restore the purity of the water afte~ the leach has been com-pleted.
More specifically, in many of the known uranium bear ing formations, a substantial part of the formation matrix is S comprised of calcium bearing clays (e.g., smectite). In forma-tions of this type, one o~ the most e-ffective known lixiviants is an aqueous solution of ammonium carbonate and/or bicarbonate, see copending Canadian application No. 288,057 filed October 4, 1977. However! ammonium ions in the lixiviant exchange with ~;
calcium ions in the clays and are retained by the clay after the lixiviant passes. These ammonium ions bleed from the for-mation into the connate water present in the formation after the leaching operation and will contaminate this water as dis-cussed above. Also, exchanged calcium ions react with the car-bonate lixivian-t to form insoluble calcium carbonate (calcite~
and can present serious plugging throughout the leach circuit.
Other carbonate lixiviants have been proposed, e.g., sodium carbonate~ (see U.S. Patent 2~896,930) and while such lixiviants do not present as serious contamination problems as do the ammonium lixiviants, they may still present serious cal-cite problems. More importantly, sodium lixiviants are not normally considered as effective in the over-all leach process as ammonium lixiviants since the ammonia in the ammonium lixi-viants can be easily separated from the uranium values, i.e., yellowcake, during precipitation of the yellowca~e while sodium from the sodium lixiviants is known to precipitate as a con-taminant in yellowcake. This requires an undesired, additional processing step in the purificàtion oF the recovered product, thereby adding to the cost of the product~

'79 Summary o~ the ~Invention The present invention provides an in situ leaching method for recovering uranium and related values from a calcium bearing clay formation wherein a lixiviant is used which does not contaminate either the formation or the recovered values.
In the present process, a lixiviant is prepared by dissolving CO2 into water having essentially the same cationic composition as that of the formation connate water. The lixi~
viant is inejected along with an oxidant, e.g., 2' ihrough an injection well into the formation. The pH of the lixiviant at injection is maintained at substantially the same value, i.e., 7.5 to 8.0, as that of the formation connate water. As the lixiviant and oxidant flow through the formation, the uranium values are oxidized and are dissolved into the lixiviant. The lixiviant and dissolved values are produced from the formation through a production well.
Due to the reactions within the ~ormation, calcium ~rom the formation is carried from the formation along with the produced lixiviant. In oraer to control the pH of the lixiviant in the leach circuit within the desired range, substantial amounts of this calcium are chemically removed from the lixiviant by a water softening technique, preferably by addition of lime in a calcium precipitator. After the calcium is removed, the produced lixiviant is filtered to remove suspended solids and is passed through a uranium extraction means, e.g., an ion ex-change resin, to extract the desired values from the lixiviant. -~
The barren lixiviant is passed from the extraction means to a mix tank where carbon dioxide is added to make up fresh lixi-viant for recycle in the leach circuit. This fresh lixiviant is injected along with additional oxidant into the formation ~Z~79 and the process is continued until essentially all o~ the re-coverable desired values are recovered ~rom the ~ormation.
No cations, e.g., NH4+ or Na ~ are added to the lixi-viant so there will be substantially no contamination of either the formation or the recovered values. Also, since the cationic ~ ;
composition o~ the lixiviant is essentially the same as that of the formation connate water, there will be little, if any, swel- r ling of the clays in the formation, thereby preventing any sub- '~
stantial decrease in the permeability of the formation during the leach operation.
Brie~ Description of the Dra~ing The actual operation and the apparent advantages of the invention will be better understood by referring to the draw~
ing in which:
The figure is a schematical view of an in situ leach-ing circuit in accordance with the present invention. ~' Description of the Preferred ~mb~odiment Referring more particularly to the drawing, the figure discloses a simplified schematic of a typical in situ leach operation. At least one injection well 10 and at least one production well 11 are completed into a uranium bearing forma-tion 13. Although only one injection well and one production well are illustrated, it should be recognized that different known configurations of wells such as those used in various types of mineral recovery operations ~e.g., five~spot, seven spot, line-drive, and other patterns used for water-flooding for petroleum recovery~ may be used for carrying out the leach operation.
A lixiviant is prepared in chemical mix tank 14 and is injected into ~ormation 13 through line 15 in injection well 10. In accordance with the present invention as will be ~Z6~79 explained in more detail below, ur~nium bearing formation 13 is of the common type having large amounts of calcium compounds therein, e.g., calcium~based clays and/or limestones~ As is known in the art, uranium is present in such Eormations in its S reduced tetravalent state and must be oxidized to its hexava]ent state in order to make the uranium values soluble in the lixi-viant To accomplish this, an oxidant is normally injected into the formation prior to ox along with the lixiviant.
The lixiviant and oxidant ~low through formation 13 from injection well 10 to production weIl 11. The uranium values are oxidized and dissolve into the lixiviant. Calcium from the formation also reacts with the lixiviant and forms cal~
cite (calcium carbonate) which is carried by the pregnant lixi~
viant to production well 11. The pregnant lixiviant is produced through line 16 by means of submexsible pump 17 or the like to the surface for processing~
The pregnant lixiviant flows through line 16 through ;~
a calcium and/or calcite removal means 20, (e.g., a "Spiractor Precipitator" manufactured by Permutit Company, Paramus~ New Jersey) to remove substantial amounts of calclum and/or calcite from the lixiviant. The pregnant lixiviant then passes via line 21 through filter means 22 (e.g., a packed sand column) to re-move suspended solids from the lixiviant before the lixiviant passes via line 23 to uranium extraction means 24 (e.g., an ion exchange resin column). The barren lixiviant, now stripped of desired values, passes from extraction means 24 through line 25 to mix tank 14 for reuse in the leaching cycle.
The uranium and related values are removed from ex- ;
traction means 24 by passing an eluant from line 30, through .

*Trademark :` ~

means 24, and to recovery means 32 via line 31 where the uran-ium product ~e.g., "~ellowcake") and other values, e.g., molyb-denum, are separated from the eluant and passed to storage or the like via line 33. The above description describes a typical, in situ leach operation in general terms and reference is made to copending Canadian application Serial No. 288,057 filed October 4~ 1977, for more complete details of such an operation.
In a typical, in situ leach operation as described above (see Canadian application Serial No. 288,057) an alkaline 10 carbonate and/or bicarbonate ti.e., ammonium or sodium carbona-te) lixiviant has been found to be hi~hly efficient in leaching urani~
um and related values from formations having substantial amounts of calcium-based clays and limestones therein. In such a leach, the alkaline carbonate lixiviant and an oxidant ~e.g., hydrogen peroxide, sodium chlorate, ox~gen, air) are injected into the formation where the following reactions take place:

~Z6~7 9343 (1) Oxidation o~ uranium U2 ~ ~~ ~ H20---~rU02~2 ~ 20H 1

(2) Urani~un Complexation ~a) U02~2 ~ 3C03-2~ 1'02(C03)3;4~ or (b) U02+2 ~ 2C03-2~ 2(C03)2 2 D

The NH4~ or Na~ is presen~ in the lixivian~ for controllillg the pH thereo~ which ".n turn, controls the . solubillt.~t of C02 in the lixiviant. However, where t~e ~orma~ion contains substantial amounts o~ calcium bearing clays, the NH4~ or ~a~ will react with the clay~ and by ion-exchal~ge convert ~hem into their am~onium form or sodium form, respectively, during the c~urse of ~he le2ching operation, Where ammonium carbonate is used as ~he lixivian~;
the for~a.tion will normally have to be restored after a leach ;15 operation since the NH4~ which is ion~exchanged onto the clays will ble'ed rom the clay ln~o any ground waters present in the formation. This bleeding of NH4~ f~om the clay w;ll contaminate these waters~ thereby making them unsuitable ~or human and/or anlmal con~ump~ion.
Where sodium carbonate is used as a lixiviant, contamination of the formation is not a serious problem but sodium from ~he lixi~ian~ does ion-exchange with the calcium ~r~ the ormation and this ~nwanted calcium in the lixiv;ant adds to the potential plugging problems caused by calcite ~uild~up in the leach cixcuit~ Con~inuous addition o~ sodium 9343 carbonate introduces additional sodi~m ions which acc~ul~e in the leachi~g circuit7 thereby necessitating costly desalination and causing additional waste disposal problem-.
Further, sodium from the lixiviant has been found to precipita~e in the y~llowcake product, thereby cont2~ina~-irg the yellow-ake and requiring an othe~ise unnecessary processing step in purifying the r~covered uranium product.
In accordance with the present invention, an iIl situ leach operation is carried ou~ with a lixiviant whie~ is similar in catîonic composition to that of the water nonmally found in t~ie formation. No cations (e.g., NH~, Nat) are added to t~e lixiviant. By using ~uch a lixiviant, the pxoblem of ~onmation contamination is alleviate~ ~nd the fo~nation will not have to be rPstored after the leach operation has been comple~ed. Als~, since the lixiv}ant is basically the same in cationic composition as the natural formation water, the composition o~ the clays wi~l be relati~el~ undisturbed and there will be less likelihood o the clays swelling to decrease permeability of -he ~ormation~
~ More specifica~ly, in the present invention, the lixivian~ is comprised o~ CO~ dissolved in wa~er with no cations such as NH4~, Na~, etc., being added. The amo~t of C2 re~uired can be dissolved in water readily ~e.g., up to 2 grams per liter at 1 atmosphere)e I a greater amount o~
C02 is desired, the lixiviant can be saturated with C02 at elevated prPssures. Pre~erablyy the lixivîant ~s made by adding gaseous C02 to the barren lixiviant in mix tank 14.

~ 6~

9343 The compos:itions of a typical barren lixiviant and a fre~h .
lixiviant (a.~ter C0~ has been added) in accordance with the present in~ention are summarized in the following table:
~ Barren Fresh Solution Lixiviant- Lixiviant Total C03 , ppm~'S00 4~000 1,500-8,000 - S~4-9 ppm: 100-2,000 100-2~000 CJ. , ppm300-2,000 300-2,000 N~ , ppm 600-5,000 600-5,000 C~t ppm 1-20 1-20 ~0 pH 8.3 7.5 *~arts per million The pH of the barren solution is about ~.3 but upon making up with 1000 to 4000 parts per mill~~ (ppm) of C02, the pH gnes down to abou~ 7~5g close to the p~ of khe natural ground water in the formation. It is noted that because of the high ~oncentration of tot~l carbonate (C03=), the barren ~nd fresh lixivia~ts are highly bu~fered. In addition, since the for~ation has substantial amounts o~
l~mestone, the forma~ion, itself, possesses high buffer capacit~ for the lixiviant in the pH ra~ge 7-8.
Due to the follow~ng side reactions in the formations 9

(3) Side R2actions (a~ 2FeS2 ~ 15Co] + 7H2~ 2Fe(OH~3~ ~ 8H -~ 4S0 (D~ MoS2 ~ 9 Eo~ ~ 3~20 ~MoO4 2 + ~ ~ 2sO4-2 ~5 the over-all xeaction tends to make acid and ~ower the pH of the system. However, due to the high bu~fering c~pacity of the ~3~3 s~stem (for~ation plus lixiviant~,lthe pH o~ pregnant li~i~iant when it is pumped out of the ground will not be much di~fer~nt from the fresh lixivian~ at the injection well, i.e.~ pH o~
about 7.5 -~o 8Ø
S l~en the pH of the lixiviant is at the low end of this range (7.5)~ there is a tendeIIcy to produce C ~2 ions :;n the lixivi~nt, e.g., at a level of 100 ppm. I~ is believed ~hat these Ca~ ions arise from dissolution of CaC03 in the:
~ormation ~.,y tbe acid produc~O Exp`~r~mental evidence suggects ~hat it is more desirabIe to keep t~e pH o~ the lixlviant at the high end of t~e formatlon water pH range (~8), to minimize the Ca~2 ion level in the lixivianL (e.gO~ 20-30 ppm3 during the leach opera~ionO
The conve~tional method o~ pH control by adding cationic hy~roxides, e.g., NH~OH or;NaOH, is not desirable since i~ would Lntroduce undesired ~ations ~o the leaching circuit, thereby counteracting one of the desired ~ea~ures of the presen~ invention. The pH of t7.e lixiviant is controlled ~n the present invention by removing the Ca~ ions from the ~ixiviant in calcium removal means, precipita~or ~00 In precipita~ox 2Q alkali hydroxides~ alkali oxides, or alkali carbonates are added to the lixiviant to promote removal of - calcium rom the pregnant lixiviant and to maintain the pH
level o the entire leachlng circui~. The preferred additive is lime (CaO). It has been found that at higher pH leve~ of the leaching circuit, the leaching rate is higher and the calcium content in ~he pregnan~ lixiviant from the production wells is lower.

~ '7~

9~43 To remove calcium to a ~esired level~ e.g., 5 pp~t, the p~I o the lixiviant normally has to be increased to 8.0-8.5. Th;s increased pH will, n turn~ minimize the calcium level of the pregnant lixiliantO To do thîs, the l~me requirement may range from 0.~ to 0.4 grams per liter o~ lixi~icnt~ The calcium is remo~ed ~rom ~he lixiviant iD
precipitator 20 as calcite ~CaC03) 3 preferably in the form of beads whicl~ facilitates handling aIct disposal of the calciteO
To su~marize the present inven~ion, a lixlviant is prepared by adding C02 to the barren lixiviantO ~n ox;dant is either injected prior to or along with the lixiviant.
Although ~y known oxidant te.g., hydrogen peroxide, air, sodium chlorate) may be used9 preferably molecul2~ oxygen (~
is used because it is both eff~cti~re and relati~-~iy inexpensiveO
Also, oxygen does not ~orm any undesirable by-products in the 1 h ~ t eac clrcil O
The pH of the lixiviant is mainta;ned at from 7.5 ~o 8.0 at in~ection and due to the buffering capabilities of the lixiviant and o the formation, the pH o the l~xiviant wiil ~0 not change substantially as i~ flows through ~he fo~mation~
Since ~here are no cations, e.g., N~4~, presen~ ~n the lixiviant~ ~here is no serious problem of contaminating the formation and/or the water contained therein. This eliminates the need ~or a relatively ~xpensive restoration operation to remvve cationic contaminants from the formation a~ter the leach has been completed. Also, since the lixiviant ;s basically similar to the orma~ion water, there will be little 75~

if anyr swelling of the clays in the ~orm~tion which allows the permeability of the formation to remain virtually unchanged throughout the leach operation.
The pregnant lixiviant is produced to the surface where Ca ~ ions are removed by increasing the pF~, e.g., by adding lime, and passing the lixiviant through precipitator 20. Su5-pended solids are then filtered out and the lixiviant is flowed through an ion-exchange column to extract the uranium values from the lixiviant. The barren lixiviant is returned to the mix tank where it is mixed with CO2 to make fresh lixiviant for recycling.
In short, the present invention essentiall~ uses a lixiviant comprised of the formation connate water with the addition of onl~ minute quantities of innocuous leaching chemi-cals (1.0 grams per liter of CO2 and 0~4 grams per liter of 2)' the lixiviant having essentially the same p~I as the forma~
tion water.
The present invention may be used for an entire leach operation from beginning to end, or it may be implemented after an alkaline carbonate leach operation such as disclosed in Canadian application Serial No. 288~057 has been in effect for some period. For example, in a leach process where an ammonium carbonate and/or bicarbonate lixiviant is being used to leach a formation, a switch to the present lixiviant (water and CO2) at some stage before the leach is comple-te (eOg., at the halfway point) will not only recover the remaining, recoverable uranium values, but will also commence restoration of the ammonium contamination formation. The ammonium ions on the clays in the formation will bleed into water-CO2 lixiviant and will be carried from the formation, thereby eliminating the need ~ 367~

for extensive restoratiQn operati~ns a~ter the leach has been completed.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO-PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of recovering uranium values from a subterranean formation comprising:
injecting an oxidant into the formation to oxidize the uranium values to their hexavalent state;
injecting a lixiviant comprised of an aqueous solution of carbon dioxide into said formation and flowing said lixiviant through said formation to dissolve said uranium values into said lixiviant, and producing said lixiviant and said dissolved uranium values from said formation.

2. The method of claim 1 wherein said oxidant com-prises:
molecular oxygen.

3. The method of claim 1 wherein the pH of said aqueous solution of carbon dioxide is maintained in the range of 7.5 to 8.0 as it is injected into said formation.

4. The method of claim 1 wherein said oxidant is in-jected simultaneously with said lixiviant.

5. A method of recovering uranium values from a sub-terranean formation having substantial amounts of calcium-based clays and/or limestone therein, said method comprising:
injecting an oxidant into said formation to oxidize said uranium values to their hexavalent state;
flowing a lixiviant comprised of an aqueous solution of carbon dioxide through said formation to dissolve said uranium values into said lixi-viant;
producing said lixiviant and said dissolved uranium values from said formation;
removing calcium and/or calcite from the produced lixiviant; and removing the dissolved uranium values from the produced lixiviant.

6. The method of claim 5 wherein the pH of said aqueous solution of carbon dioxide is maintained in the range of 7.5 to 8.0 as it is injected into said formation.

7. The method of claim 6 including:
adding gaseous carbon dioxide to said produced lixiviant after said uranium values are removed to form fresh lixiviant for reinjection into said formation.

8. The method of claim 7 wherein said step of removing calcium and/or calcite comprises:
raising the pH of the produced lixiviant to a value of from 8.0 to 8.5; and flowing said produced lixiviant through a calcium precipitator.

9. The method of claim 8 wherein the step of raising the pH of the produced lixiviant comprises:
adding lime to the produced lixiviant prior to flowing said produced lexiviant through said calcium precipitator.

10. The method of claim 5 wherein said oxidant comprises:
molecular oxygen.

11. The method of claim 9 wherein said aqueous solution of carbon dioxide comprises:
approximately 2 grams of carbon dioxide per 1 liter of water at 1 atmosphere of pressure.