CA1117862A - Uranium extraction from underground deposits - Google Patents

Abstract

URANIUM EXTRACTION FROM UNDERGROUND DEPOSITS

ABSTRACT OF THE DISCLOSURE
Uranium is extracted from underground deposits by passing an aqueous oxidizing solution of carbon dioxide over the uranium in the presence of calcium ions. Complex uran-ium carbonate or bicarbonate ions are formed which enter the solution. The solution is forced to the surface and the uranium removed from it.

Description

- - BACKGROUND OF rl'HE I ~
Underground deposlts of uranium have been ex~racted with solutions Or ammonium bicarbonate, usually also contain-lng hydrogen peroxide to oxidize any uranium in the ~4 state to the ~6 state necessary for the formation of soluble complex uranium carbonate and bicarbonate lons.~ Whlle ammonium bicarbonate solutions were successful in extracting - uranium, they were dlfflcult to work wlth because ion ex-changes occurred between the ammonlum ions and calcium ions in calcite deposits, leavlng ammonia underground which polluted water sources and added calcium to the solution.
As the solution was brought to the surface the decrease in hydrostatic pressure resulted in the release of carbon dioxide which raised the pH of the solution (C02 ~+2H20 7 H30 +HC03 ) precipitating calcium carbonate (CaC03 ~ +H30 +HC3 ~ Ca +2HC03 +H20) which clogged the pipes. Also, the use of hydrogen peroxide and ammonium bicarbonate added to the costs of the extraction process, both in the purchase of the chemicals and in metering them into the solution and monitoring their concentrations. Alkall metal carbonates and bicarbonates have also been used, but they also present .'. ~ .
, problems by polluting underground aquafers.
PRIOR ART
U.S. Patent 2,992,887 uses low pressure CO2 tomaintain the in-tegrlty of Na2CO3 in a solution used to leach underground depo~its of uranium.
U.S. Patent 3,130,960 uses a solution of CO2 to leach uranium.
SIJMMARY OF THE INVENTION
I have found that the use of ammonium and alkali metal carbonates and bicarbonates to extract uranium can be .
entirely eliminated. Instead, I can use only carbon dioxide and air. Not only are carbon dioxide and air very inexpen-sive to use, but they present virtually no pollution problems.
I have found that solutions of carbon dioxide and air extract almost as much uranium as do solutions of ammon-ium carbonate. Also, since ammonium ions are not present to exchange with calcium ions, precipitation of calcium carbon-ate is less of a problem. I have also found that hydrogen peroxide may precipitate uranium and decrease the amount recovered. By using the oxygen in air, I am able to elimin-ate the use of hydrogen peroxide except for deposits con-taining predominantly uranium in the ~4 state and for recovery of traces of uranium after extraction without hydrogen peroxide is complete.
DESCRIPTION O~ THE IN~ENTION
An oxidizing aqueous solution of carbon dioxide is prepared free of ammonium and alkali metal carbonates and bicarbonates. The solution must have a bicarbonate ion concentration of less than 5 grams/l as greater concentra-tion can result in the precipitation of CaCO3 whlch can clog the pipes. If no CaC03 is leached into the solution thesolution will have a bicarbonate concentration of less than 1 gram/l. The solution must contain oxygen or its equiva-lent (e.g. J hydrogen peroxide) at a concentration equivalent to at least one atmosphere of air, and preferably equivalent to at least one atmosphere of oxygen. These high oxygen concentrations are necessary because uranium in the +4 state is not solubilized by bicarbonate ions and will remain underground if it is not oxidized to the ~6 state. The solution is preferably prepared by injecting air and carbon dioxide, as a gas or as dry ice, into water as it is forced underground. Saturation with a blanket of carbon dioxide at atmospheric pressure can also be used but oxygen must then be sprayed in because the carbon dioxide drives out dis~
solved oxygen. The addition of chemicals to the solution which release carbon dioxide, such as calcium carbonate and hydrochloric acid, is not as desirable since the acid anion can create other problems. The use of oxygen gas may be desirable if the uranium in the underground deposit is largely in the +4 state, but usually the more-costly use of oxygen gas will not be necessary. Hydrogen peroxide can also be used but is even less desirable as it is expensive, it promotes the precipitation of calcium carbonate, and it may be conveniently objectionable due to the dissolution of other oxidizable species. However, it may be useful in areas of predominately ~4 state uranium or to recover uranium traces after remaining after extraction without hydrogen peroxide.
Since the concentration of bicarbonate ion is very small (i.e., less than 1 gm/l) unless calcium ions are ~3 ~

present, it is necessary to add calcium ions to the solution if the deposit does not contain enough calcium, which is often present in the ground as calcite3 CaCO3. If the deposit is deficient in calcium, it can be added to the solution by pumping it over a bed of limestone (CaCO3), dolomite (CaCO3 MgCO3), or lime (CaO), until the solution has picked up enough calcium ion or has become saturated.
When calcium is present, at unit activity, the bicarbonate concentration is about 0.014 molar at 1 atmosphere of CO2, which is ab.out the same bicarbonate concentration that would result from using 1 gm/l of NH4HCO3.
The solution, which has a pH of about 3 to about 7 before entering the ground and about 5.5 to about 7.5 immedi-ately after returning, preferably does not contain any additive, in addition to those described. It is pumped into the uranium deposit through one or more in~ection wells.
Due to the weight of the water and the pump pressure, the C2 pressure at the bottom is considered to be at least 2 atmospheres which further increases the bicarbonate ion concentration and therefore the amount of uranium extracted.
Some time later, the solution is pumped up again through one or more recovery wells. Upon reaching the surface, the uranium is removed from the solution by any suitable method.
A particularly useful method is to pass the solution through ion exchange columns which extract the uranium onto the column. The uranium is then washed off the column and is precipitated, for example, as ammoriium duranate (ADU~. The ADU can then be calcined to produce U3O8. The following examples further illustrate this invention.
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EXAMPLES
Initially, the following compositions were prepared # 1 # 2_ 50 g Lamprecht Uranium Ore (L.O.) 50 g LØ
100 ml ~l O 100 ml 0.02 M -A Air (1 a~m) CaC12 0.8 g C02 Air (1 atm) o.8 g C02 Same as above except Same as above solution contained except solution B 1 g/l H202 and contained 1 g/l 0.1% NH4HCo3 H22 and 0.1%

Each of the above formulations was to be contained in polypropylene liners inside a Parr bomb with a volume of - approximately 325 ml. Assuming a free volume of 200 ml after all components were placed in the bomb, the partial pressure of C02 was approximately 2 atm. and the partial pressure of 2 was approximately 0.2 atm. Each of the solutions from the above experiments were filtered and analyzed for U and ~a. The duration of contact between the solutions and the ore was noted carefully and no agitation was provided during the experiment.
Several hundred grams of Lamprecht uranium ore were obtained which reportedly contained approximately 0.1%
uranium. When it was discovered that a cold dilute H2S04 leach of the ore recovered approximately three times the amount of uranium reported to be present in the ore, a more accurate analysis of the ore was initiated. Three samples of Lamprecht ore from different bottles, obtained at different times were analyzed by the differential pulse polarographic (DPP~ technique. All of these samples passed a 20 mesh screen but were analyzed without further homogenization.
These ores contained 0.32, 0.34, and 0.36% uranium by the ~ ' 47,475 DPP method. Spectrophotometric analyses showed some serious disagreement but x-ray fluorescence analysis of the ore confirmed the DPP results. The analysis for calcium was performed by an atomic absorption procedure involving use of synthetic standards and appropriate dilutions of the samples.
The results of the various leaching experiments are contained in the table which follows. While examining this data it is important to note both the uranium content and the calcium content as a function of leach composition, The calcium analyses were performed several days after the leaching experiments were conducted. The importance of this point will become obvious in the discussion of the fate of uranium leached by solutions containing peroxide.
The pH of the solutions measured within a half-hour after filtering from the ore was consistently around 6.5. After several hours the pH's of the various solutions rose to values between 7.2 to 7.8. This behavior is similar to data reported from field tests of mining solutions and can be attributed to loss of C02 from the solutions.
The leach solutions containing Ca 2 were prepared by adding 2.222 g of CaC12 to one liter of water. This should have given a solution 0.02 M in CaCl2 or about 800 ppm Ca 2. The actual analysis of the solution indicated that it contained 775 ppm Ca 2. The leach solution utilized in Experiment #6 was prepared by adding 0.5 ml of 10% (NH)4C03 to 500 ml of H20 containing 775 ppm Ca 2 and 0.1% H202. A
precipitate of CaC03 formed immediately, thus removing some of the Ca 2 from solution~ The resulting Ca 2 concentration was determined to be 445 ppm Ca 2.
The data in the table which follows indicates relationships, or lack thereof, between calcite solubility and the leaching of uranium.
Experiments #1 and #2 were conducted simultaneously and were intended to show the effect of high Ca 2 content on the leaching of uranium. Unfortunately, the reaction vessel was not sealed properly in #2 and CO2 gradually leaked out during the experiment. Nonetheless, it is interesting to note that the solutlon containing 775 ppm Ca 2 leached approximately 67% as much uranium as did the solution which retained C02 pressure and contained no Ca 2 originally.
Furthermore, it is very significant that the Ca 2 content in Experiment #2 did not change even though 247 ppm of uranium were taken into solution. Since such high levels of uranium were recovered in these experiments in 16 hours, many sub-sequent experiments were conducted for only 70 minutes in order to get more data in a shorter period of time.
Experiments #3 and #4 were identical to #1 and #2 except that contact time was reduced from 16 hours to 70 minutes. Again, the solution containing no Ca 2 originally leached more uranium than the one containing 775 ppm Ca 2 originally. This time~ however, the solution containing Ca 2 leached more than 85% as much uranium as did the solu-tion containing no Ca 2. Also, note that the Ca 2 concen-tration actually declined slightly to 750 ppm in Experiment #4.
It is also interesting to compare Experiment #3 with #1. The 16 hour experiment leached 216 ppm more of uranium than did the 70 minute experiment but only leached 18 ppm more of calcium.
In order to determine why the presence of 775 ppm 3~

of Ca 2 apparently retards, albeit only slightly, the leach-ing of uranium if the extraction of uranium does not depend upon the dissolution of calcite another experiment (#13) was devised to show the effect of the presence of a different salt upon the leaching of uranium. The experiment was devised so that the ionic strength given by a 0.02 M solution of CaC12 would be matched by a NaCl solution. In order to have a NaCl solution whose ionic strength is equivalent to that of a 0.02 M CaC12 solution, the NaCl solution must be 0.06 M. The results of Experiment #13 show an even smaller amount of uranium leached (113 ppm versus 129) by a 0. o6 M
NaCl solution than was leached by the 0.02 M CaC12 solution.
In addition, the NaCl solution caused the dissolution of 175 ppm Ca 2 versus 118 ppm leached in Experiment #3 where no salt was added. The latter result is not surprising since it is well known that calcite is many times more soluble in sea water than in fresh water. This bodes ill for the operations in the solution mining program which introduce chloride into the leach solution.
Having conducted Experiments #1, #2, #3, and #4, it was desired to determine the effectiveness of the reagents currently being used in the field and compare these results to Experiments #3 and #4. The leach solutions in Experiments #5 and #6 are identical to those in Experiments #3 and #4 except that they contain 0.1% (NH4)2 C03 and 0.1% H202.
The results of Experiments #5 and #6 were, for a while, quite perplexing. The uranium content of these solutions when first measured were less than 70 ppm in Experiment #5 and less than 40 ppm in Experiment #6. These analyses were quoted as "less than" because the excess H202 gave a huge polarographic reduction wave which interfered with the measurement of the uranium (VI) peak. The uranium was obviously ~resent but its concentration declined after several hours and declined further af'ter several days until only a few ppm o~ uranium were left in solution. Concur-rently, the appearance of a yellow-green sludge was noted in the bottom on the sarnple container. Clear liquid decanted from this sludge would begin to precipitate more of the material after standing for a few hours. After filtering these solutions the residue remaining on a 0.45 Millipore filter had a bright yellow appearance. The partial char-acterization of this deposit is described in some detail below, but suffice it to say that the residue was found to contain major amounts of uranium and calcium. Note that the solution which had no calcium added was very low in calcium - and that the solution which originally contained 445 ppm Ca 2 increased in calcium content by only 40 ppm.
As soon as it was discovered that the uranium had been precipitating from the solutions from Experiments #5 and #6 another experiment (#9) was begun using the same leach solution but with a contact time of four hours. The uranium content of thls solution proved to be less than the amount found from Experiment #5. At ~irst glance one is tempted to dismiss this information as entirely predictable since the four hour contact time gave sufficient time for more uranium to be removed from solution by precipitation.
This observation is true but it also means that the precipi-tation took place in the simulated underground condition and was not a result of bringing the solution to atmospheric pressure with a concurrent loss of CO2 and rise in pH. This _g_ ÇJ~

implies that uranium, although oxidized underground, may also be precipitated as an insoluble calcium salt under-ground. The calcium level in this solution from Experiment #9 was very low, as it should be if it is being precipitated along with the uranium. The appearance of the yellow pre-cipitate in the bottom o~ the sample container was obvious.
Up to this point, no precipitate had been observed from any experiment except those containing (NH4)2C03 and H202. Experiments ~'7 and #8 had already been planned which would help pin down the conditions necessary to form the precipitate. If the C02 partial pressure or HC03 concen-tration were factors in producing the precipitate then we should be able to produce the precipitate with (NH4)2 C03 (Experiment #8) or by raising the partial pressure of CO2 (Experiment #7) which would have the effect of raising the HC03 concentration and would also enhance calcite solubility.
As shown in the table which follows, the uranium content of the solution from Experiment #7 is within 10% of that obtained from Experiment #3, the calcium content is about a factor of 2 higher, and no precipitate was observed even after several days. The solution from Experiment #8 contained essentially the same level of uranium as did Experiment #3 and the calcium content was 25 ppm lower than Experiment #3. Again, no precipitate was observed even after several days.
Upon completion of these experiments it was obvious that the only time precipitation of uranium occurred was when H202 was present in solution.
Two more experiments seemed desirable in order to answer two persistent questions. "Will the uranium content of the solutions continue to rise with longer contact times?"
and "Will water and air alone effectively leach uranium from the ore?"
Experiments #lO and #ll were designed to more closely approximate contact times in the field althou~h the 66 hours may still be cGnsiderably less than the field condition. Experiment #ll which was conducted with 0.1%
(NH4)2 ~03 extracted 580 ppm which is approximately 10% more uranium and about 60 ppm less calcium than did the solution in Experiment #lO which contained just C02, air, and H20.
In order to answer the second question asked above, Experiment #12 was conducted utilizing only water and air. Both uranium and calcium concentrations in this exper-iment were very low, suggesting that carbon dioxide is indeed needed for effective leaching of uranium.

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After careful examination of the data in the abo~Je Table, the following conclusions can be made:
(l) Uranium recoveries from leach solutions con-taining H202 are poor due to the precipitation of an lnsoluble calcium-uranium salt.

(2) Carbon dioxide, air, and water will effectively leach uranium from the ore, but water and air will not.

(3) (NHL~)2 C03 solutions may provide some slight advantage over C02, air, and water but this has not been conclusively proven, and furthermore calcite precipitation is introduced in the injection wells when (NH4)2 C03 is used.

(4) The amount of uranium in the various leach solutions does not appear to bear any relationship to the amount of calcite that has been solubilized and is affected only slightly by the amount of calcium in the leach solution, though some calcium must be present.
Characterization of the Calcium-Uranium Precipitate When the precipitate was first noted,-a small piece of filter paper containing a thin layer of the residue was dissolved in HN03 and H2S0~l and the solution was analyzed for uranium. The equivalent of 3.5 mg of uranium was found.
Although thls was a qualitative check for uranium, it was obvious that uranium was a major constituent of the residue.
An x-ray diffraction analysis of the residue was performed and showed strong patterns suggesting CaC03 and other compounds which obviously were due to some uranium compound, but it could not be positively identified.
An emission spectrographic analysis of the residue ~13-~.3 ~

was performed and showed that the ma~or constituents ~"e-re Ca and uranium with lesser amounts of sllicon, aluminum, mag-nesium, and titanium.
The Use of MHLIHCO3 The equllibrium concentration value for HCO3 in equilibrium with CaCO3 under a C02 atmosphere is [HCO3 ] =
1.41 x 10 2M. This conclusion is reached by utilizing the first acid dissociation constant for carbonic acid, assuming a partial pressure of CO2 = 1 atm, a pH of 6, and substi-tuting these values into the appropriate equilibrium expressionwhich is [H ] ~HCO3 ]
[C ~ = Ka = 4.4 x 10 7 The molar solubility of CO2 under 1 atm of CO2 is 3.4 x 10 Moles/l. Substituting all of these values into the above expression:

[HCO3 ] ~ 1.41 x 10 2 M.

~ ~:r, If 1 g/l of NH~HCO3 is added to the leach solution the bicarbonate concentration would be approximately 1.3 x M bicarbonate. This is essentially the same value that would be obtained by allowing water to come to equilibrium with calcite under 1 atm of CO2. If the partial pressure of C2 is greater than 1 atm then the bicarbonate ion concen-tration would be even higher than 1.41 x 10 M.
The point to be made here is that the particular ore tested is essentially an infinite source of bicarbonate and that it is not necessary to add to it. In addition, the NH4 may exchange with Mg 2 and Ca 2 in certain "ion exchange"
clays in the ore, ~hus raising their levels in solution and creating a more serious calcite precipitation problem.
The Use of H202 The experiments show that uranium is precipitatedvery slowly from leaching solutions containing exeess per-oxide. Although this precipitate has not been quantitatively characterized it is known to contain high amounts of uranium.
The use of hydrogen peroxide may very well be causing uranium oxidation in the ore body, but followed by uranium preeipi--tation in the ore body. Some of the above-ground preeipitates that occur in the surge tanks and silting in the ion-exehange columns may also be uranium-bearing compounds. It is popular to claim that H202 will decompose upon contaet with the ore and therefore, underground precipitation of uranium is not a faetor. However, the experiments showed that after four hours of eontact with the ore that the peroxide level was still very high. l'he quantitative precipitation of uranium can also occur in solutions of pH <3.5 in the presenee of H202, so incomplete precipitation of' uranium at pH 5.5 - 6.0 underground would not be surprising.
Calcite Precipitation The reason that there is a calcite precipitation problem on the in~ection side of the process is that the pH
of the leach solution has been raised by the additlon of NH3 or NH4HC03 to such a level that caleite has only limited solubility. The raising of the pH is an unnecessary and futile exercise since the solution eventually equilibrates with the ore to produce a solution whose pH is governed largely by the C02-H20-calcite equilibria.
If the pH of the leach solution is not raised but is instead lowered, calcite precipitation will not be a problem. This implies, of course, that the leach solution would be recirculated without removing the calcium and begs the question, "Can the uranium be effectively recovered without removing the calcium?" It is believed that the answer to this question is "Yes, it can," although the experimentation to date does not conclusively prove that point. Consider what the results of these experiments with regard to calcium concentration and uranium recovery have shown. The experiment with 800 ppm Ca 2 showed that the recovery of uranium was approximately 85% of the amount - recovered with no calcium present. This is not a drastic reduction and may be a higher absolute recovery than is presently obtained with the current process.
The calcite removal system is costly and causes increasing levels of Cl in the leach solutions which leads to an increased solubility of calcite in the ore, creating a "vicious circle" which can be avoided by eliminating the calcite removal system, (i.e., the use of NH4HCO3).
If CO2, air, and H2O is added to the solution as it comes off the uranium recovery ion exchange column and is returned to the field, the pH of the solution will be lowered so that no calcium will precipitate. The calcium levels in the mining solutions will remain fairly constant with a slight upward drift due to some gradual increase in ionic strength caused in part by the increase in Cl levels as a result of exchange of Cl for uranium species on the ion exchange columns. If it were not for the increase in ionic strength, the calcium content would be expected to rise to 30 an equilibrium level somewhere between 100 ppm and 1000 ppm ~16-at the recovery well and remain constant. It would remain constant because the leach solution would already have been in equilibrium with solid calcite in the ore body.
Some calcite precipitation as the solution is brought above ground due to the rise in pH and escape of C02 can be expected. However, less precipitation should occur under this invention since calcium levels should be less than they are using the NH4HC03 process which induces greater calcite solubility underground, and therefore, more precipi-tated calcite above ground.

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Claims (15)

1. What I claim is:
   l. A method of extracting uranium from under-ground deposits comprising:
   (1) forming a solution of carbon dioxide which has a ? bicarbonate concentration of less than 5 gms/1, which is free of alkali metal and ammonium carbonates and bicarbonates, and which contains oxygen or its equivalent at a concentration equivalent to at least one atmosphere of air;
   (2) contacting said underground deposits with said solution in the presence of calcium ions; and (3) forcing said solution to the surface.
2. 2. A method according to Claim 1 wherein said solution is saturated with carbon dioxide.
3. 3. A method according to Claim 1 wherein air is added to said solution to render it oxidizing.
4. 4. A method according to Claim 1 wherein said calcium ions result from the dissolution of underground deposits of calcite into said solution.
5. 5. A method according to Claim 1 wherein calcium ions are introduced into said solution by passing said solution over a calcium compound before said solution.
6. 6. A method according to Claim 5 wherein said calcium compound is CaCO3.
7. 7. A method according to Claim 5 wherein said calcium compound is CaO.
8. 8. A method according to Claim 1 wherein hydrogen peroxide is added to said solution.
9. 9. A method according to Claim 8 wherein the amount of hydrogen peroxide is about 0.5 to about l gm. per liter of solution.
10. 10. A method according to Claim 1 wherein after said method is applied to an underground uranium deposit, said method is repeated using a solution to which hydrogen peroxide has been added.
11. 11. A method according to Claim 1 including the additional last steps of passing said solution through an ion exchange column whereby the uranium in said solution is collected on said column.
12. 12. A method according to Claim 11 including the additional last steps of washing uranium off said column and precipitating it from the wash solution.
13. 13. A method according to Claim 12 wherein said uranium is precipitated and calcined to produce U3O8.
14. 14. A method according to Claim 1 wherein said solution contains oxygen or its equivalent at a concentration equivalent to at least one atmosphere of oxygen.
15. 15. A method according to Claim 1 wherein said solution is formed by carbon dioxide and air are injected into water.