CA1088726A - Carbonate leaching of uranium - Google Patents

Abstract

ABSTRACT: In the hydrometallurgical mining of uranium with ammonium carbonate leaching solution containing hydrogen peroxide as the oxidant for solubilizing the quadrivalent uranium compounds, decomposition of the peroxide due to contact with iron bearing minerals is suppressed by incorporating 1-hydroxyethylidene-1,1-diphosphonic acid and anhydrous tetrasodium pyrophosphate in the leaching solution. The weight ratio of the diphosphonic acid to pyrophosphate is from about 1 to 10 to about 10 to 1, but is preferably about 1 to 6.

Description

i~8~7Z~ F~C 1679 This invention relates to the extraction of uranium values from subterranean ores, particularly by the carbo-nate leaching process.
The recovery of minerals by the in place leaching of underground ore deposits is known as hydrometallurgy~ It consists of introducing a leach solution, usually aqueous acid or alkali, into an ore zone through an injection well to effect dissolution of mineral values and removing the resultant pregnant leach solution through one or more recovery wells. The production liquors are processed to isolate dissolved minerals while the spent leaching solu-tion is reconstituted for recycling or discarded.
Hydrometallurgical mining has been applied to the recovery of uranium from subterranean ore beds using both acid and alkaline solutions. However, where the uranium ore has a high carbonate content, alkaline leaching with a carbonate solution is preferred to acid leaching because of the large amounts of acid needed to neutralize the carbonate rock.

' --.. ' ~

87;~f~
In the carbonate leaching process for ~ininq uranium, an aqueous solution of an alkaline carbonate and an oxi-dizing agent is in~ected into the uranium formation to dissolve out the uranium values. The purpose of the oxi-diæing aqent is to oxidize the insoluble quadrivalent uranium minerals, commonly in excess of 90% of the total uranium, to the soluble hexavalent form. ~fter the treat-ment aforesaid, the leach liguor contains the extracted uranium in the form of the stable, soluble, uranyl tri-carbonate complex anion.
Carbonate leachinq is employed where the calcite content is high and where the uranium is involved in carbonate mineralization. Ammonium carbonate leaching, with which the present invention is concerned, is pre-ferred in the case of sandstones where the uranium is present in the binder, usually calcite or gypsum, and it is not desirable to attac~ the gangue material (silica or clays). Sodium carbonate/bicarbonate leaching is required, however, when it is necessary to attack min-eralized uranium, especially in the case of carnotite ores (uranyl vanadates). Bicarbonate is required in the sodium carbonate leaching to reduce attack on silicas and aluminas and prevent decomposition of the uranyl carbonate complex and precipitation above pH 11 because the reaction produces hydroxyl ions as follows;
U03 + 3C03 + H20 = [U02(C03)3] 4 + 20H
and above pH 11,

2[Uo2(Co3)3] + 60H + 2Na = Na2U207 + 6C03 + 3H20 7Z~

The latter reaction is used to separate uranium, using caustic, and the liquor is regenerated with C02. Ammonium carbonate has the advantage of being easily decomposed with steam to NH3 and C02 and at the same time decomposing the uranyl carbonate complex to U03 (hydrated) + C02 with the possibility of using the NH3 and C02 to regenerate the leach liquor. The absence of sodium ions means higher U30 purity and less pollution of ground water.
There are many oxidizers capable of elevating lower valence uranium compounds to the soluble hexavalent state.
However, only a few of these entities are considered prac-tical candidates for use in conjunction with the leaching of uranium minerals. For instance, the oxidizer must not contaminate or in any way damage ground water which may be the source of community water supplies. Another important requirement is that the oxidizer exert minimal effect on other minerals such as pyrite, copper sulfides, molybdenite, often found in association with uranium bearing formations And of course the oxidizer must be stable in the presence of the leaching solution.
When economic factors are imposed on the requirements aforesaid, the practical choices of an oxidizer for use in the ammonium carbonate leaching process for mining uranium bearing ores narrows down to oxygen (as air or oxygen enriched air) and hydrogen peroxide. Of the two, hydrogen peroxide is the more effective but i5 prone to gassing in the presence of iron minerals commonly occurring with uranium ore. This behavior reduces the oxidative efficacy ,: ~ ' lOl~7~;

of peroxide while the evolved gasses interfere with the mining hydraulics. Gassing can be minimized by resorting to low levels of hydrogen peroxide; of the order of less than 0.25 g but at such dilute concentrations the peroxide has low extractive capacity.
It is, of course, well known to suppress the decom-position of hydrogen peroxide by means of stabilizers and numerous compounds have been advocated for this purpose over the years. For instance, sodium stannate and 8-hydroxyquinoline are effective in preserving acidic 35%-90% commercial hydrogen peroxide. However, in many of its industrial applications hydrogen peroxide is employed in dilute alkaline solutions and is much less stable than in acidic media. Several compounds are known capable of enhancing the stability of hydrogen peroxide in alkaline solution and in this connection magnesium salts are recog-nized as effective, particularly in combination with a soluble silicate or phosphate. The use of magnesium salts for the purpose aforesaid is disclosed in the following U.S. Patents: Eugen De Haen 1,482,477; Schmidt 1,155,102;
Schaidhauf 1,181,409, 1,181,410 and 1,278,389; Reichert et al, 2,160,391; Lind et al 2,254,434; Kauffman et al, 2,383,141; Campbell et al, 2,333,916; McEwen 2,527,563;
Sprout 2,838,459; and Dithmar 3,003,910.
Other references which disclose the use of phosphorus compounds to stabilize alkaline hydrogen peroxide solu-tions include: Belgian Patent 818,616 - alkylene and aminoalkylenediphosphonic acids in combination with ' -' , -- ' .

1()~2~

a phenol for use in molybdenite mining; U.S. Patent

3,860,391 - aminoalkylidenepolyphosphonates and/or hydro-xyalkylidenephosphonates with hydroxy compounds for use in textile bleaching baths; U.S. Patent 3,591,341 - hydro-gen peroxide with a stannate, pyrophosphate and orthophos-phoric acid to give pH of 9 to 11; U.S. Patents 3,811,833, 3,795,625 and 3,740,187 disclose the use of l-hydroxyethyli-dene-l,l-diphosphonic acid as an adjunct in alkaline hydrogen peroxide solutions for bleaching textiles. In U.S. Patent 3,687,627 is described stabilized acidic aqueous hydrogen peroxide containing a stannate, a soluble magnesium salt, an alkylidenediphosphonic acid and preferably a soluble pyrophosphate such as tetrasodium pyrophosphate. The mixture can be diluted to give stable alkaline hydrogen peroxide solutions.
So far as can be ascertained, however, the art has not ove~come the problem of hydrogen peroxide decomposi-tion as it occurs in aqueous alkaline ammonium carbonate leach solutions used as the fluid in tne hydrometallurgical mining of uranium from subterranean formations.
In accordance with the present invention the carbo-nate leaching process for mining subterranean uranium containing formations in which injection and production wells are drilled and completed within a uranium forma-tioni alkaline ammonium carbonate uranium leaching solu-tion and hydrogen peroxide are injected through the injec-tion wells into the formation whereby pregnant leaching solu-tion containing uranium values is taken from the production ., - - - .:: . . :.: ~ .
.
" .'' ' ' . ,' ' ~ `

: , ,.

1()~87Z~;

wells, is improved by providing in the leaching solution a mixture of l-hydroxyethylidene-l,l-diphosphonic acid and an alkali metal pyrophosphate in a weight ratio of from about 1 to 10 to about 10 to 1, the amount of said mixture being sufficient to inhibit decomposition of the hydrogen peroxide in the presence of iron containing minerals. The hydrogen peroxide is desirably added to the leaching solu-tion as an acidic stabilized premix consisting essentially of by weight from about 35% to about 70~ hydrogen peroxide;
l-hydroxyethylidene-l,l-diphosphonic acid; tetrasodium pyrophosphate and sufficient polyphosphoric acid to maintain the pH below about 4, the weight ratio of l-hydroxyethyli-dene-l,l-diphosphonic acid to total pyrophosphate being from about 1 to 10 to about 10 to 1, the total amount of diphosphonic acid and pyrophosphate being sufficient to stabilize the hydrogen peroxide after the premix is added to the ammonium carbonate leaching solution. As understood herein, tetrasodium pyrophosphate is the anhydrous form of the formula Na4P2 07.
In accordance with the present invention, the decom-position of hydrogen peroxide in aqueous ammonium carbonate leaching solutions of the type used in extracting uranium values from uranium-bearing ore zones is suppressed by maintaining such leaching solutions in contact with l-hydroxyethylidene-l,l-diphosphonic acid and a tetrasodium pyrophosphate. The weight ratio of diphosphonic acid to total pyrophosphate can vary from about 1 to 10 to about 10 to 1, but is preferably about 1 to ~. As to the overall ~08~Z6 amount of stahilizer, this will depend on the guantity of hydrogen peroxide in the ammonium carbonate leachinq solution. ~esirably, uranium values in the production liquor, using the ammonium carbonate leaching process in a typical hydrometallurgical installation, should assay about 500 mg of U308 per liter. Accordingly, hydrogen peroxide is added to the leach solution in sufficient amounts to oxidize the insoluble quadrivalent uranium minerals to the soluble hexavalent form. The production liquor is monitored on emerging from the recovery well and stabilized hydrogen peroxide of the invention intro-duced at the injection well as needed to sustain the output of U308 at the requisite level. The actual amounts o~ hydroqen peroxide required will be based on the assay of the uraniu~ deposits and how much of the uranium values are in the insoluble ~uadrivalent state, Such data are provided by an analysis of core samples taken from the mining site.
Generally speaking, the concentration of the ammonium carbonate leach solution is from about 0.05 molar to about 1.0 molar and contains from about 0.01 moles per liter to about O.l moles Per liter of hydroqen peroxide. On passage through the ore formation, such leach solution dissolves the uranium values as the complex [uo2(co3)3]-4. This solution is purified by solvent extraction or ion-exchange treatment after which the uranium is subsequently recovered as yellow cake.

,,,, ~. .. . .. . .. .. . . . .

10~7'~

So far as can be ascertained, the decomposition of hydrogen peroxide in ammonium carbonate solution during the hydrometallurgical mining of uranium deposits therewith is caused primarily by contact with iron bearing minerals.
Such decomposition is manifested by severe gassing due to release of oxygen by the peroxide and is promoted both by soluble iron (ferric ions) and by contact with crystalline iron minerals such as marcasite, pyrite pyrrhotite. In fact, the effectiveness of the l-hydroxyethylidene-l,l-diphosphonic acid/tetrasodium pyrophosphate system herein can be determined by the addition thereof to an aqueous solution of ammonium carbonate and hydrogen peroxide of the type used in the hydrometallurgical mining of uranium and mixing such solution with finely divided pyrite crystals such as about 100 mesh (U.S. standard) (A.S.T.M. - E-11-61).
Without the phosphonic acid and tetrasodium pyrophosphate, vigorous gassing occurs at the pyrite crystal surfaces due to decomposition of the hydrogen peroxide. For instance, when a 1% slurry of 100 mesh (U.S. standard) pyrite crys-tals was stirred with 0.5 M hydrogen peroxide in 1.0 M
ammonium carbonate, the half-life of the peroxide was less than an hour; a similar result was obtained using 10 parts per million (ppm) dissolved ferric iron in place of the pyrite. Addition of 0.6 to 1.2 g/l of tetrasodium pyrophos-phate plus 0.65 g/l of l-hydroxyethylidene-l,l-diphos-phonic acid increased the half-life of hydrogen peroxide by 30%;
a level of 6 g/l of pyrophosphate with 0.65 g/l of diphos-phonic acid extended the peroxide half-life to over 6 hours.

.

.

10~872~

Accordingly, the stabilizer syste~ of the invention is added to the ammonium carbonate leaching solution in an amount sufficient to suppress the deleterious effect of the iron minerals on the hydrogen peroxide stability and thereby maintain it at the concentration required to dissolve the uranium values and provide the requisite U38 assay in the producer liquor. Generally speaking, hydrogen peroxide decomposition from contact with iron minerals will be that decrease in hydrogen peroxide not utilized in oxidizing the quadrivalent uranium to the soluble hexavalent state. These losses in peroxide strength are determined by analyzing the producer liquor and such losses suppressed by introducing the stabilizer system of the invention at the injection well. Makeup hydrogen peroxide is blended with the leach solution to replace oxidant required to dissolve the uranium minerals and sustain the U308 assay at the requisite level.
As pointed out above, the stabilizer system herein and hydrogen peroxide may be simultaneously added to the ammonium carbonate leaching solution in the form of an acidic hydrogen peroxide premix solution. For reasons not fully understood, the stabilizer mixture is more effective when dissolved in the hydrogen peroxide than when blended separately with a leaching solution that may be contaminated with metal ions.
Although both l-hydroxyethylidene~ diphosphonic acid and tetrasodium pyrophosphate inhibit hydrogen per-oxide decomposition in an ammonium carbonate leaching '.

- ' .

7~

solution, the collective stabilizing power of the mixture, on a general weight basis, exceeds that of the individual components. ~pparently, a synergistic response results when the two compounds are employed together. Moreover, tetrasodium pyrophosphate proved to be uniquely effective among phosphorus acid salts in forming a synergistic com-bination with l-hydroxyethylidene~ diphosphonic acid.
Reference is now made to the following examples.
Peroxide and solution percentages in the examples and the specification are by weight unless otherwise stated.
Phosphoric acid concentration refers to the theoretical P2O5 content of the acid.

Example 1 Solutions were made up of l-hydroxyethylidene-l,l-diphosphonic acid (Dequest 2010) in combination with tetrasodium pyrophosphate (TSPP) in 35% H2O2. The use of "105% phosphoric acid" (nominal 42% pyrophosphate content) with TSPP is required to obtain a pH in the concentrated peroxide of 4 or less to maintain optimum stability. The systems made up were as follows (% based on total solution of 35% H2O2):
Sl: 2% Dequest 2010, 5% 105% phosphoric acid.
S2: 2% Dequest 2010, 3% 105% phosphoric acid, 2% TSPP.10 H2O
S3: 2% Dequest 2010, 2% 105% phosphoric acid, 3% TSPP-10 H2O
S4: 1% Dequest 2010, 3% 105% phQsphoric acid, 5% TSPP-10 H2O
These systems were then tested in~simulated leach solutions of 0.5 M (~H4)2CO3 containing 0.1 M and 0.3 M H2O2 stirred 7;~;

at 25C at a ratio of 20 g of uranium ore to 100 cc of solu-tion. Stability of the H2O2 was measured as percent remaining at a given time; the ore was a sandstone assaying 0.110%
U38 and 1% pyrite, with minor amounts of marcasite and pyrrhotite.

% H22 Remaining After System M H2O2 Hrs. 0.1 1/2 1 2 4 6 Control 0.3 100 47.9 4.4 ~1 Sl 0.3 100 96.7 90.8 77.7 42.5 19.5 S2 0.3 100 98.0 94.2 85.3 64.2 38.0 S3 0.3 100 96.4 90.8 7~.5 44.4 22.2 S4 0.3 100 95.6 91.5 78.? 46.3 22.3 Sl 0.1 100 96.3 ~7.4 68.9 24.9 8.7 S2 0.1 100 96.1 9~.3 85.2 52.5 22.0 S3 0.1 100 95.6 90.0 80.0 43.6 19.2 S4 0.1 100 93.7 86.6 68.9 37.8 15.7 The improvement in stability of the H2O2 is especially noteworthy in the case of S4 where the final diphosphonic acid concentration at 0.1 M H2O2 is only 100 ppm. System S4 constitutes a low cost, practical technique of stabilizing the hydrogen peroxide product in the ammonium carbonate solution mining of uranium minerals.
Example 2 The ore of Example 1 was leached to extract the uranium by treating 100 g samples with 100 cc of 0.5 M
(NH4)2CO3 leaching solutions containing various levels of both regular peroxide and S4 peroxide of Example 1.
Leaching was done under a static condition for 70 to 75 hours. The leach liquors were separated after the 70 to 75 hour period by filtering through filter paper (No. 42 Whatman ashless paper). The ore was washed 3 times with : .. . .
- - :- ,. :
~ ~" . - . ' ' ~ --.: . . ~ . ~ . . - - - . ... - . -.~: -: . .

10~'7'~

approximately 20 cc each of 0.1 M (N1~4)2CO3 and dried at ambient room conditions for a week. The leached ore samples were then analyzed for U3Oa content. A control sample was also leached with no peroxide added but only ambient air contact. The original U3O8 content of the ore was 0.110%
and the residual U3O8 in the extracted samples is as follows:

Ratio of H2 2 to OreGain in U3 8 lbs/ton (S) % U303 % lbs/ton(S) H202 (kg/ton (M)) LeftExtracted (kg/ton (M)) Commercial 35% 10/1 (49.84/1) .0108 90.2 0.281 (0.12) S4 3.3/1 (16.43/1)~.001 ~99.0 0.475(0.22) S4 1/1 (4.98/1) .0184 83.3 0.130(0.06) S4* 1/1 (4.g8/1) .0197 82.1 0.104(0.05) None (air only) -- .0249 77.4 0 0 (S) Short ton (M) i~etric ton *Added 15 ppm of wetting agent, cetyl betaine.
The new stabilized peroxide formulation can be seen to be more efficient than regular commercial 35~ H2O2. Even at a level of one lb/ton of ore, the gain in U3O8 is signifi-cant and economically advantageous.
Example 3 Synergizinq Effect of Phosphorous Acid Salts With l-Hydroxyethylidene-l,l-Diphosphonic Acid Solutions of 0.5 M (NH4)2CO3 were made 0.3 M in H2O2 from a solution of 2% of the diphosphonic acid (Dequest 2010) in 35% H2O2, and to these were added various levels of sodium salts of phosphoric acids. These solutions were then stirred with the ore of Example 1 at 25C at a ratio of 20 g ore/100 cc solution. The stability of the peroxide as percent remaining was measured with time as before. The final levei of phosphonic acid in all cases is 650 ppm based on solution.

.

- - - ' ' : ' .: .' :. ' ,:- . , -. :

. , . :

1()~b~7'~i % H202 Remainin~ After SaltConc.,g/lHrs. 0.1 1/2 1 2 4 6 None -- 100 95.4 87.2 68.9 21.6 8.3 NaPO3 10 100 96.4 90.8 73.5 32.7 13.1 NasP3Olu 10 100 96.1 94.3 85.2 52.5 22.0 NaH2PO4 5 100 95.1 90.6 74.4 36.9 11.8 NaH2PO4 10 100 96.1 92.3 80.7 45.1 14.8 NaH2PO4 50 100 96.3 93.3 84.2 69.5 37.3 Na4P2O7~l0H2o 1 100 96.7 92.5 78.7 40.0 17.5 Na4P2O7 lQH2O 2 100 97.0 91.7 75.8 38.7 19.2 Na4P2O7-10H20 5 100 97.4 91.8 81.7 57.2 34.8 Na4p2o7-loH2o 10 100 96.7 93.0 86.3 68.9 51.3 Na4p2o7-loH2o 50* 100 97.1 92.1 87.6 77.0 62.7 * No Phosphonic acid All of the sodium salts of phosphoric acids tested with the phosphonic acid made some improvement in H2O2 stability but the singularly effective one was the tetrasodium pyro-phosphate (TSPP). A level of only 1 to 2 g/l of the deca-hydrate (or 0.6 to 1.2 as anhydrous) appeared to be most efficient. Fifty (50) g/l of the TSPP decahydrate alone (30 g/l as anhydrous) is comparable in stabilizing power to 2 g/l of the diphosphonic acid.

' . , - ''. ' '-:, ', ' ',, ., , ,' .
.
: -

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the carbonate leaching process for the solution mining of subterranean uranium containing formations in which an injection well is drilled and completed within the uranium formation; alkaline carbonate uranium leaching solution and sufficient hydrogen peroxide are injected through the injection wells into the formation whereby uranium values are produced from production wells, the im-provement which comprises providing in the leaching solution a mixture of 1-hydroxyethylidene-1,1-diphosphonic acid and an alkali metal pyrophosphate in a weight ratio of from 1 to 10 to 10 to 1, the amount of said mixture being sufficient to inhibit decomposition of the hydrogen peroxide in said leaching solution.

2. The process according to claim 1 wherein the alkali metal pyrophosphate is tetrasodium pyrophosphate.

3. The process according to claim 1 wherein the weight ratio of 1-hydroxyethylidene-1,1-diphos-phonic acid to tetrasodium pyrophosphate is 1 to 6.

4. A composition of matter comprising by weight 35%
to 70% hydrogen peroxide; 1-hydroxyethylidene-1,1-diphos-phonic acid; tetrasodium pyrophosphate and sufficient polyphosphoric acid to maintain the pH of the composition at 4 or less, the weight ratio of 1-hydroxyethylidene-1,1-diphosphonic acid to total pyrophosphate being from 1 to 10 to 10 to 1, the total amount of diphosphonic acid and pyrophosphate being sufficient to stabilize the hydrogen peroxide after the composition is added as the oxidant to an alkaline carbonate uranium leaching solution.

5. The composition according to claim 4 wherein the concentration of the hydrogen peroxide is 35% by weight.