CA1113725A

CA1113725A - Process for extracting uranium from its ores by using alkaline earth carbonates and bicarbonates solutions in the presence of carbon dioxide

Info

Publication number: CA1113725A
Application number: CA293,658A
Authority: CA
Inventors: Antoine G.L. Floreancig; Robert E. Schuffenecker
Original assignee: Wyoming Mineral Corp; Minatome Corp; Uranerz USA Inc
Current assignee: Wyoming Mineral Corp; Minatome Corp; Uranerz USA Inc
Priority date: 1976-12-28
Filing date: 1977-12-21
Publication date: 1981-12-08
Also published as: FR2376215B1; ZA777280B; FR2376215A1

Abstract

PROCESS FOR EXTRACTING URANIUM FROM ITS ORES
WITH ALKALINE EARTH CARBONATE AND BICARBONATE
SOLUTIONS IN THE PRESENCE OF CARBON DIOXIDE

ABSTRACT OF THE DISCLOSURE
Uranium is extracted from its ores, whether the ores are still in underground deposits or removed therefrom by oxidizing the uranium to a valency of 6 by means of oxygenated water or oxygen in the presence of carbon diox-ide, dissolving the uranium by means of alkaline earth carbonate and bicarbonate lixiviant solutions in the pre-sence of carbon dioxide under a pressure below 60 bars and at a temperature of from O tc 100°C, then separating the insoluble substances from the lixiviant after expansion to 4 bars or less to produce a calcium uranyl carbonate liquor.
Uranium is recovered from this Iiquor after expansion under vacuum by either: (a) a limewash treatment at a pH of 8 to 9 which precipitates calcium carbonate and separating a liquid fraction which is treated in turn with a limewash at a pH of 10 to 12 and allows the calcium uranate to be deposited, or (b) separation of the calcium carbonate resulting from the ex?ansion and subjecting the liquor to a limewash treatment at a pH of 10 to 12 to form a mixture of calcium uranate and calcium carbonate. After atmospheric expansion and separa-tion of the calcium carbonate resulting from this expansion, the liquor may be treated with hydrogen peroxide to precipi-tate uranium peroxide or it may be passed over a bed of anionic resins to fix the uranyl carbonate ions.

Description

BACKGROUND OF THE INYENTION
The known processes for extracting uranium usually comprise three main phases- a phase of oxidizing the ore to ' ~, . .. . .
.

,: .
.'" ' , .
.

`` ~1137~5 47,499 form a soluble uranium, a lixiviation phase to dissolve the uranium and allow the insoluble gangue to be separated, and finally a recovery phase which may be a treatment such as passing over ion exchange resins or through a selective membrane or the preferential extraction of a solvent, or precipitation with an alkaline hydroxide or magnesium, for example. The first two phases may be carried out simultan- -eously.
With more particular regard to the second so-called lixiviation phase, two general processes are avail-able at present. The first of these uses a sulphuric acid solution as the lixiviant and the other uses either a sodium carbonate and bicarbonate solution or an ammonium carbonate and bicarbonate solution. The choice between these acid and alkaline methods is based on the physical properties of the ore, on its degree of reactivity in relation to the reactive agents and on the composition of the host minerals which contain the uranium. Thus the acid method could be pro-hibited because of the considerable acid consumption in the -~e 5t~ ~
case of ores which are rich in-~me ~ .~ Therefore, in the case of such ores the use of an alkaline carbonate is preferred. The carbonate also has the advantage of only being slightly corrosive and of leading to a more selective extraction process. However, whatever the particular ad-vantages offered by each of the two methods, they both have -the disadvantage of entailing the use of polluting reactants.

Now, there is a growing tendency to try and ex-tract the elements from the ores by means which are similar to the natural means, so as to conserve the original form of the rock containing the element. The re-absorption of

-2-. .
.

~137ZS
47,499 solutions which are rich in salt could cause considerable environmental problems. These methods, which generally lead to slow lixiviations by means of processes carried out at ambient temperature, may be adapted to mined ores crushed more or less finely or merely hammered as well as to ores which are left "in situ" if the rocks are sufficiently permeable and within reach of percolation phenomena. It is obviously in the second or "in situ" case that the use of a non-polluting method is particularly important.
PRIOR ART
U.S. 3,130,960 discloses the lixiviation of miner-al values, including uranium, by means of carbonated water.
U.S. 2,992,887 discloses the use of C02 to re-plenish sodium carbonate lixiviants.
SUMMARY OF THE INVENTION
The present invention relates to a process for extracting uranium from its ores, whether these ores are still in their deposit or drawn out of the soil, in which the uranium is oxidized to the hexavalent state and is lixiviated by means of carbonate and bicarbonate solutions of an alkaline earth metal, in the presence of carbon di-oxide, in order to solubilize it in the form of a uranyl carbonate of these metals. The uranium is then recovered by means of a suitable treatment as an enriched product which may be used immediately for the production of pure uranium or of its derivatives.
- BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1, 2, 3 and 4 are each schematic or flow diagrams illustrating examples of the process.

~13'7Z5 47,499 DESCRIPTION OF THE INVENTION
Strongly motivated by this ecological aspect of the problem of treating ores, we have found that it was not necessary in the case of uranium to use acids or concen-trated solutions of alkaline carbonates and bicarbonates which leave noxious residual elements but that solutions based on alkaline earth carbonates and bicarbonates, in the presence of carbon dioxide, were suitable to be used. In fact, it is known that the carbonate and alkaline earth ions Ca2+ and Mg2+, in the presence of uranium, lead to the formation of salts of the uranyl carbonate type having the possible formula [U02(C03)3] Ca2 or [U02(C03)3] g2 the presence of C02 increases their solubility allowing values of the order of 10 g of U/l at pH 6.7 to be reached under normal pressure of C02 with the calcium salts and of ~-60 g of U/1 with magnesium salts, this taking place at ambient temperature. On the other hand, the present concern with maintaining the natural environment is complied with since the main concept is to treat the ores with solutions ~ --which are similar to the so-called hard waters, which nor-mally circulate in the calcarceous sub-soil. If the alka-line earth elements have been withdrawn from the soil, it is sufficient to increase the pH in order to replace them in their deposit.
We have developed a process for extracting uran-ium, based on this data, in which we employ an original, novel, lixiviation phase with the oxidation and recovery phases. More precisely, the process according to the in-- vention, which relates to the extraction of uranium from its ores either directly in the deposit itself or after the ores ~1~37Z5 47,499 have been removed from the soil, comprises an oxidation/
lixiviation phase followed by a recovery phase and is char-acterized in that carbonate and bicarbonate solutions of an alkaline earth metal are used for lixiviation in the pre-sence of carbon dioxide under a pressure of between 1 and 60 bars at a temperature of 0 to 100C.
The essential characteristic of the process is therefore to use carbonate and bicarbonate solutions of an alkaline earth metal. In practice, calcium and magnesium metals are used as these are the only ones which are economi-cally feasible. These solutions are usually obtained from specific elements external to the ores, such as carbonic anhydride, natural or precipitated carbonates, possibly calcinated in the form of oxides and rehydrated, which are placed in suspension in the water and treated with carboxy-lic gas to dissolve the alkaline earth element in the form of bicarbonate. These solutions are also formed by recycl-ing solutions which are by-products of the process. In the case of ores which are rich in sufficiently reactivated calcium or magnesium carbonates, these solutions are pre-pared with the ore itself by merely injecting carbonic anhydride (carbon dioxide) into the aqueous suspension of the ore or into the water circulating in the ore. The presence of natural so-called hard waters in the vicinity of certain deposits also provides a valuable contribution for forming lixiviation solutions. Whatever their origin, these solutions are used in the presence of carbon dioxide, the pressure of which is either the equilibriu~ pressure cor-responding to the quantity of dissolved bicarbonate or a pressure above this value. Since carbon dioxide dissolves 47,499 the ore better at a high pressure this pressure may be anything between O and 60 bars in this process; however, for technological and economical reasons, lower pressure ranges of the order of 0.5 to 4 bars are preferred, particularly in the case of lixiviation "in situ" when the C02 pressure must not exceed the hydrostatic pressure to prevent bubbles from forming. On the other hand, if a rise in temperature pro-duces an improvement in the speed of dissolution, it con- -tributes to the reduction of the solubility of the carbon dioxide, and although this process may be carried out at widely varying temperatures, ambient conditions are per-fectly suitable. In this way, the temperature of the lixi-viation solutions in the soil may be adjusted to that of the medium, thus removing any "thermal pollution".
Since the properties of the reactants according to the invention: a solution of carbonates and bicarbonates and carbon dioxide have been defined, lixiviation, that is ;~
placing these reactants in contact with the ore to cause the uranium to pass from the solid phase to the liquid phase, 20 may be validly carried out by any methods which are cur- - -rently used in this sphere. Thus, in the case of extraction in permeable soils, the reactants according to the invention may readily be used for injecting in one position and pump-ing in another position in conditions which cause a prefer-ential current which is naturally confined between the positions under consideration. In fact, with solutions of -this type which reproduce the natural phenomena of the soil in a controlled and increasing manner, the erratic migrations which are not recovered do not cause any problem, the dis-solved uranium may rapidly reprecipitate upon contact with 1$137Z5 47,499 the reducing zones or by means of a natural increase of the pH, despite a higher content than normal, by leaving a solution which is almost identical to the solution which circulates in the soil. Furthermore, it is possible to restore the aquifer after exhausting the ore by replacing the lixiviation solution with a solution the pH of which has been increased to between 8 and 9.5 by adding alkaline earth hydroxide, so that it allows the salts which had been di-splaced during lixiviation to be redeposited in the bed in solid form.
With regard to the ores which are extracted from their deposit at a degree of mere pounding or hammering, percolation over a column or in heaps on sealed beds and washing in mine galleries by means of alkaline earth solu-tions also produce good results. Similarly, after being ~ -extracted from the soil and by crushing the ores more finely, these reactants may be used in any conventional method for making contact with a solidJliquid pulp such as by means of agitated, Pachuea and autoclave tanks. These, which promote an increase of the carbon dioxide content in the reaction medium, lead to rapid dissolution of the uranium, allowing the process to be compared to a conven-tional alkaline attack, except for the polluting nature of the reactants.
In each case, this lixiviation by alkaline earth carbonates and bicarbonates also has the advantage of a selective dissolution effect, owing to the pH being near to 7. In particular, molybdenum and vanadium which are fre-quently present in uranium ores are not usually attacked or dissolved in our lixiyiant. This enables a later separating _7_ ~113725 47,499 treatment over resins which is generally difficult with other conventional lixiviation solutions which would dis-solve these other metals.
Once the uranium has been exchanged between the liquid and solid phases, the lixiviation phase follows with a separation of the solid products contained in the lixivia-tion product so as to obtain a clear lixiviation liquor which is able to be subjected to the recovery phase direct-ly. The quantity of solid separated depends upon the type of lixiviation used. Thus, in the case of lixiviation in situ, most of the insoluble matter in the ore remains in the soil and it is sufficient to remove the products in suspen-sion. On the other hand, if the ore is lixiviated in appa-ratus, not only the substances in suspension but also all of the gangue are separated. Before carrying out this separa-tion, the pressure of the carbon dioxide used during the ~ dissolution of the uranium should be taken into considera-; tion. If its pressure is less than or equal to 4 bars, as is usually the case with lixiviation in situ, the lixivia-20 tion product is subjected to the separation operation none- -theless. If the pressure is above 4 bars, this last opera-tion is preceded by an expansion of the gaseous atmosphere above the lixiviation product so as to bring it down to 1 to 4 bars. In this case, a calcium carbonate precipitate appears in the lixiviation product as a result of a displace-ment in the equilibrium of the reaction:

C2 + H20 + CaC03 <~~~ Ca(HC03)2 This precipitate is then eliminated from the circuit at the same time as the gangue. The carbon dioxide which is re-leased during expansion is recovered for drying by a con-1~13~Z5 47,499 ventional process and is then compressed before partici-pating in the preparation of the reactants again with additional C02 and the carbonates which are recycled into the process. The separation described above may take place in any conventional type of apparatus such as a decanter, filter, centrifuge, etc. The lixiviation liquor obtained is then subjected to a uranium recovery phase.
However, the extraction process according to the invention comprises an oxidation phase since the dissolution of the uranium by carbonated solutions takes place by means of a uranium of valency 6. In order to oxidize the uranium at this valency, which is often present in the ore at valency 4, any active oxidizer at the pH of these solutions may be used. These therefore, include potassium permanga-nate, oxygen in the form of aeration of the liquor~ or suspension of the ores, and oxygenated water which i5 usual-ly unstable in an alkaline medium but which is nonetheless well suited to the carbonated media because of the lower pH.
Furthermore, these last two oxidizers do not disturb the - 20 natural medium. This oxidation phase may be carried out simultaneously with the lixiviation but may also precede it, thus forming a pre-oxidation phase which is particularly valid in the case of in situ extraction where oxygenated water is circulated, for example in the ore, under CO2 pressure thus also benefiting from the stabilizing action of this gas on the oxidizing agent.
The process according to the invention also com-prises a phase of recovering the uranium contained in the lixiviation liquor. Owing to the nature and to the compo-sition of these liqu~rs, it i9 possible to carry out this _9_ , 47,499 phase by different methods.
A particularly interesting method consists infirstly relaxing the pressure exerted by the carbon dioxide above the liquor (partial pressure) to 1 bar absolute and then in reducing this pressure to a value of about 0.1 bar.
During this degassing, the C02-calcium bicarbonate equi-librium is broken and part of this salt in solution precip-itates to the carbonate state. The suspension formed by this reaction is then treated with limewash so that it has a pH of 8 to 9. In these conditions, a new quantity of calcium carbonate precipitates owing to the combination of the lime with the calcium bicarbonate which remained in the solution.
All of the calcium carbonate contained in the suspension is ;
then separated and a very pure calcium uranyl carbonate liquor is obtained and again subjected to the action of a ~- -limewash but increasing the pH to between 10 and 12. With this type of OH ion concentration, the uranium precipitates in the form of a lime uranate, CaU207, accompanied by cal-cium carbonate which is separated from the mother liquors.
During these operations, the recovered calcium carbonate as well as the mother liquors are conveyed to a stage of the proces~ known as preparation of the reactants where, under the action of the C02, they reconstitute a solution which may be used for lixiviation by dissolving the carbonate into bicarbonate. The C02 used in this preparation is partly derived from gas which is introduced into the circuit, but most of it is a by-product of the atmospheric and vacuum expansion operations. After these operations, the accom-panying water has been removed from the gas and the gas then recompressed to a valu,e which is suitable for manufacturing 111~7Z5 47,499 reactants and for carrying out subsequent oxidation or lixiviation. This method has the advantage of using a precipitant which is homogeneous with the lixiviation re-actant and which leads to uranium-rich concentrates such as: >30% U30g.
Another method which also makes use of lime con-sists in separating the calcium carbon~te which has been precipitated from the lixiviation liquor after the atmos-pheric and vacuum expansion operations similar to those described above, and in immediately treating the liquid thus obtained with a limewash at a pH of between 10 and 12. This results in the precipitation of a calcium uranate/calcium carbonate mixture, which is not as rich as the first in uranium but which only requires a single precipitation thus avoiding recycling small quantities of uranium which may accompany the calcium carbonate if it is formed at a pH of 8 to 9.
A third method relates to the use of oxygenated water (hydrogen peroxide) as a precipitant. In this case, some of the C02 is removed from the lixiviation liquor by expansion to atmospheric pressure. This is accompanied by precipitation of the calcium carbonate which is separated from the liquor and recycled in the same way as the expanded C02. A solution of (peroxide) oxygenated water is added to the liquor, the combination of which with the uranium leads to the formation of a uranium peroxide precipitate which is recovered while recycling the mother liquors. This method has the advantage of being very selective.

A fourth method consists in the direct treatment of the lixiviation liquor without expansion, and therefore 47,499 without intermediary separation of calcium carbonate, in ananionic resin column. In this operation, the uranyl car-bonate ion contained in the liquor is fixed on the resin whereas the Ca2+ ions are not retained. The resins having a Cl radical may be used but it is preferable to be limited to the R-HC03 resins which have the advantage of leading to a Ca(HC03)2 effluent which is homogeneous with the rest of the process. In this case, however, a slight over-presqure f C2 of between 0.1 and 1 bar must be maintained at the inlet of the column so as to prevent the CaC03 from pre-cipitating within the resin which might happen if the HC03 ions emitted by the resin during fixation of the uranium reach a concentration above that allowed by the equilibrium with the C02 which is present above the solution. This method is particularly simple and does not lead to the precipitation of carbonates, thus avoiding the operations of suspension transfer and of re-dissolution at the manufactur-ing state of the reactants.
These different methods illustrate possible ways of recovering uranium from liquors which are by-products of the claimed proceqs, but it is obvious that any other method which is applicable to solutions of this type would still be :
in the framework of the subject of this invention.
The present invention will be understood better with reference to the four attached figures, each of which shows the succession of the operations forming the proceqs in diagrammatic form by way of non-limitative example and according to a predetermined variation. The four examples have been selected from the case where there is pre-oxidation 30 of the ore or where lixiviation is carried out at a pressure ;

~113~ZS
47,499 of above 4 bars and therefore requires an additional expan-sion operation before separating the insoluble substances.
All four of the figures comprise a common section which relates to the oxidation, lixiviation and C02 recycl-ing phases but they are distinguished from each other by the recovery phase. Thus Figure 1 relates to the lime treatment in two stages, Figure 2 relates to the same reactant, but it i~ only applied once, Figure 3 relates to precipitation by means of oxygenated water (peroxide solution), and Figure 4 relates to the case of ion exchanging resins.
The following successive operations are therefore found in Figures 1, 2, 3, and 4 for the common section:
oxidation 1 in which the ore is treated with an oxidizer in the presence of C02; lixiviation 2 where the oxidized ore is placed in contact with the alkaline earth bicarbonate solution under C02 pressure L7 derived from the operation of preparing the reactants 7. A product is formed from this lixiviation under pressure L2 which in the present case undergoes an expansion to 4 bars in the operation 3 by the outlet of C02, the flow or flux G3 of which is subjected to drying 5 to give a flux G5 before being recompressed in 6 in the form G6 and reused in 7 where it partially ensures the dissolution of the recycled calcium carbonate and of the lime in the bicarbonate state and maintains the pressure required for lixiviation with the assistance of a fresh gas supply if necessary: the product which is expanded to 4 bars is then separated in 4 into a solid fraction S4, which is transferred, and into a liquor L4 to which the selected ~-recovery phase is then applied.
According to Figure 1, this phase comprises an - - , . -47,499 :

atmospheric expansion 8 followed by a vacuum expansion 9 leading to the formation of gaseous fluxes G8 and Gg which undergo drying 5 like G3 and recompression in 6 before participating in the manufacture of the reactant reagents in 7. The degassed liquor or Lg is then treated in 10 with a -limewash L14 which is manufactured from quick lime and water in 14. At a pH of 8 to 9, the Ca++ ions which are still in solution precipitate in the form of carbonate and are added to the carbonate which has already been formed during ex-pansions 8 and 9. The carbonate i9 separated in 11 in the form of a solid (Sll which is recycled to the preparation of the reactants 7 whereas the liquid fraction Lll i~ conveyed towards 12 where it undergoes a new treatment with a lime-wash but at a pH of between 10 and 12. The resultant ~ r Q~t~d suspension is s~earatcd in 13 into a liquid L13, which is recycled in 7, and a solid S13, which is composed of calcium uranate and calcium carbonate.
According to Figure 2, the recovery phase also comprises the same atmospheric 8 and vacuum 9 expansion operations and the recycling of G8 and Gg as in Figure 1.
The solid S15 which has been formed during the expansions is then separated in 15 and recycled as calcium carbonate to the preparation of the reactants 7. A limewash L14 derived from 14 is then added to the liquor L15 resulting from the :
operation 15 in a sufficient quantity to obtain a pH of between 10 and 12 in 16. The resulting precipitate of mixed calcium uranate/calcium carbonate S17 is then separated in 17 from the mother liquors L17 which are recycled for preparing the reactants 7.
According to Figure 3, the C02 atmosphere of the 47,499 liquor L4 derived from the lixiviation phase is brought to ambient pressure in 8 and the gas G8 is recycled as above.
The calcium carbonate formed during this expansion is then separated in 18 and subsequently recycled as S18 to prepar-ation of reagents 7. The liquid L18 resulting from this separation is subjected to the action of the oxygenated water in 19 to produce uranium peroxide S20 which is iso-lated in 20 from its mother liquors L20 which are recycled for lixiviation via 7.
According to Figure 4, the liquor L4 which is a by-product of lixiviation passes directly through an anionic resin column 21 maintained under an overpressure of C02 (G21) where it gives off its uranium which is recovered in the form of a uranyl carbonate solution L21 and produces a liquid L'21 which is returned to 7.
The following examples illustrate the practical implementation of the invention:

250 cm3 of a saturated solution of lime bicarbo-nate under a C02 pressure of 1 bar and containing 1 g/l ofH202 are added to 50 g of an ore containing 0.13% of uran-ium. This mixture is placed in a 500 cm3 flask under the C2 atmosphere of 1 bar with stirring for 24 hours at room temperature. The final pH is 6.4. After filtering in a vacuum and washing over a Buchner funnel, 300 cm3 of a solution titrating 0.152 g/l of uranium is obtained. The solid residue is analyzed after being oven-dried and is found to contain 390 ppm of uranium. The recovery yield of the uranium, solid basis, is therefore 70%.

1~137Z5 47,499 3000 cm3 of a saturated solution of lime bicar-bonate are added to 500 g of ore containing 0.872~ of uranium under 1 bar of C02. This ~olution also contains 0.5 g/l of H202. The mixture is placed in a flask in which there is a C02 pressure of 1 bar. A magnetic stirrer main-tains the solid in suspension. After carrying out this treatment for 172-1/2 hours at ambient temperature, the mixture is filtered. The residue is washed and the wash waters are placed aside.
The solid obtained has a titre of 0.07% of uranium, the extraction yield therefore being 92%.
The liquid obtained during filtration, that is a volume of 2502 cm3, and which has not been collected with the wash waters is placed in contact with C02 under 1.1 bar for half an hour. This solution is then passed through a .~ - JM ~-~n;onl~ ion ~x~h~
~1~ column provided with Dowex 21 K~resin in its bicarbonated form. A solution which only has a titre of 3.2 mg/l of uranium is collected at the outlet of the column. The resin ~ -is drained and then eluted. A concentration of 6.68 g/1 ofuranium is found in the eluate.

1250 g of an ore containing 0.0832% of uranium are treated with 1000 cm3 of a saturated solution of lime bi-carbonate in equilibrium with a C02 pressure of 3 bars.
This mixture is placed in an autoclave with 3 bars of C02 and 11 bars of 2 in the gaseous phase, the total pressure being 14 bars. After stirring for 72 hours at ambient temperature the mixture is flltered; the cake i8 wa8hed three time~ with a total of 500 cm3 of di tllled water.
; -16-.

47,499 These wash waters are added to the first filtrates giving the total volume of 1312 cm3.
The residue from the attack contains 216 ppm of uranium that is a recovery yield of 74%.
In the resultant of 1312 cm3 of solution, the uranium is at a concentration of 0.586 g/litre. 5 cm3 of a solution of oxygenated water (hydrogen peroxide) at 303 g/litre is added to this solution. A uranium peroxide is thus precipitated and after 4 hours the solution contains 72 mg/l of uranium and 0.945 g/l of H202. The recovery yield of the uranium in the solution is thus 87.7%.

125 g of an ore containing 0.0832% of uranium are stirred for 12 hours with a solution of 0.3 g/l of H202 which is saturated with C02, the pH of which is about 4.3.
After filtering and washing, this pre-oxidized ore is placed in contact with 1250 cm3 of a natural water having the following composition:
Calcium 0.359 g/l Magnesium 0.036 g/l Sodium 0.022 g/l Potassium 0.005 g/l S04-- 0.322 g/l HC03- 0.864 g/l -The pH of the medium is found to be 6.1. After 24 hours of reaction at ambient temperature, the mixture is filtered through a Buchner funnel under a vacuum of 360 mm of mercury. The cake is washed with 300 cm3 of distilled water and the wash waters are added to the first filtrate;
30 1303 g of this soluti~n are collected. The residue has a ~ -` 1~13~Z5 47,499 -titre of 0.0556~ of uranium, the extraction yield thus being 33-2%.
After 20 hours, the solution has deposited 80 mg of calcium carbonate in the form of calcite. This precipi-tate is filtered and washed in a Buchner funnel. 1362 g of solution with the following compositions are collected:
Calcium 0.280 g/l Magnesium 0.057 g/l Sodium 0.022 g/l 10 Potassium 0.015 g/l so4 - 0.362 g/l HC03 0.765 g/l U 0.025 g/l Pure lime is added to this solution within 8 hours in small quantities so as to obtain a final pH of 11.5. A
mixed compound containing 1.7% of uranium is precipitated.

75 g of an ore containing 0.872% of uranium are -treated with 250 cm3 of a saturated lime bicarbonate ~olu-20 tion under 9 bars of C02 and containing 0.25 g/l of hydrogen peroxide.
After 6 hours of reaction in an autoclave at 42C, atmospheric pressure is re-assumed, with filtration and washing with 50 cm3 of distilled water in a Buchner funnel under a vacuum of 275 mm of mercury.
The solid collected contained 0.2% of uranium which provided a recovery yeild of uranium of 77%. ~ -The filtrate, having a volume of 278 cm3, depo-sited 251 mg of calcite in 12 hours; it was filtered and washed with 20 cm3 of distilled water.

.

~37~ 47,499 The pH of the filtered solution was increased to8.7 by adding lime in the form of limewash, another 512 mg of a product containing 1.92% of uranium, that is 9.8 mg of uranium, and 40.5% of calcium precipitated.
Finally, after filtering under vacuum and washing, then adding limewash until a pH of 11.3 is obtained, 1350 mg of precipitate containing 36.6% of urahium and 22.2% of calcium are obtained.
Only about 1 mg/l of uranium remains in solution.

50 g of an ore containing 0.525% of uranium are treated with 250 cm3 of magnesium bicarbonate solution containing 20.5 g of this product. This solution is then saturated under an atmosphere of C02 by bubbling C02 through. -- The suspension is filtered after 24 hours. The presence of 339 mg/l of uranium is noted in the filtrate.
32.3% of uranium contained in the ore has thus been recovered.
70~ oxygenated water is then added to the solution so as to obtain 2.01 g/l of H202. After 6-1/2 hours, uran- ~-ium oxide is precipitated and the uranium content of the solution is 55 mg/1.

50 g of the same ore containing 0.525% of uranium are treated in the same conditions with a solution contain-ing 20.5 g of magnesium bicarbonate, then saturated with C2 in 1 atmosphere, by bubbling this gas through; the solution also contains 0.785 g/l of H202.
The suspension is filtered after 24 hours and the -.

47,499 presence of 0.824 mg/l of uranium is noted in the filtrate.
78.5% of the uranium content has thus been re-covered.

50 g of the same ore containing 0.525% of uranium is placed in an autoclave with 250 cm3 of a solution con-taining 0.326 g/l of H202 and 42 g of magnesium carbonate.
The autoclave is then placed under 30 atmospheres of C02.
After stirring for 63 hours, the suspension is filtered. The cake is washed with distilled water and after oven-drying at llO~C for 1 night, it is analyzed. Its uranium content turns out to be 0.06%. The recovery yield of this element is therefore 88.5%.
` EXAMPLE 9 -~ `
50 g of the same ore containing 0.525% of uranium are placed in a flask with 250 cm3 of a saturated lime bicarbonate solution under 1 atmosphere of C02 and contain-ing 0.08 g/l of potassium permanganate.
The mixture is placed at 60C and the suspension is stirred for 23 hours. At the end of thi~ period, the mixture is filtered. The solid residue is washed with distilled water. After drying, it is noted that only 0.282%
of uranium remains in this residue, that is an extraction rate of uranium of 46.2%.

Claims

What we claim is:

1. A process for the extraction of uranium from its ores, either directly in such an ore deposit itself or after such ores have been removed from the soil, comprising an oxidation/lixiviation phase followed by a recovery phase, characterized in that carbonate and bicarbonate solutions of an alkaline earth metal are used for lixiviation under a pressure of carbon dioxide of below 60 bars and at a temper-ature of 0 to 100°C.

2. A process for extraction according to claim 1, further characterized in that the oxidation phase is carried out prior to lixiviation by means of an oxygenated water solution in the presence of carbon dioxide.

3. A process for extraction according to claim 1, further characterized in that the oxidation phase is carried out prior to lixiviation by means of a solution containing a mixture of oxygen and carbon dioxide under pressure.

4. A process for extraction according to claim 1 wherein calcium is the alkaline earth metal.

5. A process for extraction according to claim 1 wherein magnesium is the alkaline earth metal.

6. A process for extraction according to claim 1, further characterized in that a solution formed of hard natural water is used for lixiviation.

7. A process for extraction according to claim 1, further characterized in that a carbonate and bicarbonate solution which is formed from the alkaline earth compounds present in the ore are used for lixiviation.

8. A process for extraction according to claim 1, further characterized in that after passing the uranium into solution, the C02 pressure above this solution is reduced back to 4 bars by expansion if this pressure is above this value, while at the same time recycling the C02 released by the expansion and in maintaining the C02 pressure as it is if it is lower than or equal to 4 bars, then in separating the insoluble substances from the aqueous fraction known as lixiviation liquor in these conditions.

9. A process for extraction according to claim 8, further characterized in that the recovery phase comprises first expanding the gases accompanying the lixiviation liquor to atmospheric pressure followed by expansion under vacuum, the released C02 being recycled during these oper-ations, then in treating the degassed liquor with a limewash at a pH of 8 to 9, this leading to the formation of a lime carbonate precipitate which is separated from the liquid portion and recycled, finally in treating this liquid por-tion with a limewash at a pH of 10 to 12, whereby a uranate and calcium carbonate precipitate is formed, recovering the precipitate by separation from its mother liquors, said liquors being recycled.

10. A process for extraction according to claim 8, further characterized in that the recovery phase consists firstly in expanding the gases accompanying the lixiviation liquor to atmospheric pressure, followed by expansion under vacuum, the released C02 therefrom being recycled, then in separating and recycling the calcium carbonate which has been precipitated from the liquor during these two expan-sions and in treating the latter liquor liquid with a lime-wash at a pH of 10 to 12, whereby a precipitate mixture of calcium uranate and calcium carbonate is formed, separating said precipitate from its mother liquor and recycling said mother liquor.

11. A process for extraction according to claim 8, further characterized in that the recovery phase com-prises expanding the gases accompanying the lixiviation liquor to atmospheric pressure, the C02 released during this operation being recycled, separating and recycling this calcium carbonate which has precipitated from the liquor during the expansion, and treating the latter liquor with a solution of hydrogen peroxide to form uranium peroxide separating the uranium peroxide from its mother liquor and recycling the mother liquor.

12. A process for extraction according to claim 8, further characterized in that the recovery phase consists in passing the lixiviating liquor through a bed of ion exchange resins under pressure.

13. A process for extraction according to claim 12, further characterized in that the recovery phase in-cludes increasing the carbon dioxide pressure of the gases accompanying the lixiviation liquor by from 0 to 1 bar and in passing the liquor through a bed of anionic resins of the formula R-HC03 under pressure.

14. A process for extraction according to claim l, further characterized in that when it is applied in situ to recover the uranium from the minaral deposit itself, the aquifier is restored by increasing the pH of the lixiviation solution to a value of between 8 and 9 by means of an alka-line earth hydroxide solution.