PROCESS FOR RECOVERING URANIUM FROM NATURAL WATER BY COMPLEXING URANIUM AND MEMBRANE FILTRATION

Abstract

The invention relates to a process for recovering uranium from natural water, which involves complexing uranium and filtering it through a membrane in an inorganic material and which comprises at least the steps of: a) complexing the uranium present in the natural water with at least one water-soluble organic polymer, comprising one or more groups that complex uranium at the pH of the natural water, the organic polymer being retained by the membrane in the uncomplexed and complexed states; b) passing the water obtained from a) through the membrane, thereby obtaining, on either side of the membrane, a uranium-depleted permeate and a uranium-enriched retentate in the form of complexes; and c) collecting the retentate and dissociating the uranium complexes present in the retentate.

Description

Title of the invention: METHOD FOR RECOVERING URANIUM FROM NATURAL WATER BY COM URANIUM PLEXATION AND MEMBRANE FILTRATION Technical field

[0001] The invention relates to the field of uranium recovery from natural water such as seawater, groundwater (water from a groundwater aquifer or underground river), surface freshwater (water from a natural or artificial lake, water from a stream such as a torrent, river or stream) or brackish water (lagoon water for example).

[0002] More specifically, the invention relates to a process which, by implementing a selective complexation of uranium in combination with membrane filtration, makes it possible to separate very efficiently the uranium present in natural water from other metallic cations likely to be present in this water at concentrations much higher than that of uranium, such as Na, K, Mg, Ca and Sr, and to recover this uranium in the form of a concentrated solution. Prior art

[0003] The Earth's surface is 71% covered by oceans and seas. This area represents a total volume of seawater of 1.37109 km3, the main constituents of which are chlorine, sodium, sulfates, magnesium, calcium, potassium, strontium and hydrogen carbonates at concentrations ranging from a hundred ppm to several g / L.

[0004] Uranium, for its part, is present in seawater at an average concentration of 3.3 pg / L. This concentration, which can be considered very low, given the concentrations of alkali and alkaline earth metals in seawater, nevertheless represents the largest uranium resource on Earth with approximately 4.5 billion tons of exploitable uranium, a resource about 500 times greater than terrestrial uranium.

[0005] Notwithstanding the fact that the global nuclear fleet and, consequently, the demand for uranium, continues to increase, the costs and economic impacts of uranium recovery are evolving. Uranium reserves must therefore be estimated in light of the costs of recovering this element.

[0006] This is why the exploitation of natural waters such as seawater or freshwater as uranium resources presents two main obstacles: - Uranium is present in seawater and freshwater in a strongly complexed anionic form, namely tricarbonatouranate(VI), with the formula (UO2(CO3)34, which, by association with calcium and magnesium cations also present in macro-concentrations or micro-concentrations in this water, forms neutral complexes Ca2UO2(CO3)3 and Mg2UO2(CO3)3; - the recovery of metals from large volumes of seawater or freshwater is extremely complicated and its implementation is very costly; to be economically viable, this recovery can be carried out by solid-liquid extraction using an adsorbent which, in addition to being effective and selective to the pH and ionic strength of seawater or freshwater, must be practically insoluble in aqueous medium and resistant to corrosivity and biological fouling (or biofouling).

[0007] The first work on the recovery of uranium from seawater dates back to the early 1960s and many research teams have been interested in it in Japan, the United States, India, China and Europe.

[0008] Membrane filtration is a technique that offers the advantage of accelerated recovery kinetics without mass transfer. It has proven its effectiveness in water treatment, whether for depolluting, making it potable, purifying it from pathogenic or non-pathogenic microorganisms such as bacteria, or for desalinating it.

[0009] However, very few studies appear to have been carried out on the recovery of uranium from seawater or freshwater by membrane filtration. These studies appear to number four, three of which were carried out using organic filtration membranes (A. Favre-Reguillon et al., Ind. Eng. Chem. Res. 2003, 42, 23, 5900-5904; A. Favre-Réguillon et al., Water Res. 2008, 42, 4-5, 1160-1166 and O. Raff, RD Wilken, Desalination 1999, 122, 147-150) and one was carried out using an inorganic filtration membrane (Y. Chung et al., Water Sci. Technol. Water Supply 2019, 19,789-795).

[0010] However, in the course of their work, the inventors found that by subjecting seawater or fresh water in which the uranium in this water is complexed by a polymer capable of being retained by the filtration membrane in both the uncomplexed and complexed states to membrane filtration, it is unexpectedly possible to separate this uranium very efficiently from the other metallic cations present in this water and to recover it in the form of a concentrated solution.

[0011] And it is on these experimental findings that the invention is based. Description of the invention

[0012] The invention therefore relates to a process for recovering uranium from natural water, which involves complexing the uranium and filtering it through a membrane into an inorganic material (such as metal oxides, such as alumina, titanium dioxide, zirconium dioxide...), and which includes at least the following steps: a) complexation of uranium present in natural water by a water-soluble organic polymer comprising one or more groups that complex uranium at the pH of natural water, i.e., a pH typically between 6 and 9, the organic polymer being retained by the membrane in both its uncomplexed and complexed states; b) passage of the water obtained at the end of step a) through the membrane, thereby obtaining, on either side of the membrane, a uranium-depleted permeate and a uranium-enriched retentate in the form of complexes; then c) collection of the retentate and dissociation of the uranium complexes present in the retentate.

[0013] In the preceding and following, "polymer" means both a homopolymer which is obtained from the polymerization of a single monomer and is therefore formed of a single repeating motif, and a copolymer which is obtained from the polymerization of several (i.e. two or more) different monomers and is therefore formed of several different repeating motifs.

[0014] As mentioned previously, the organic polymer used in the process of the invention must be soluble in water, that is to say, it must have a solubility of at least 1% by mass per unit volume of water at 20°C.

[0015] This organic polymer is chosen so that, on the one hand, it is capable of complexing uranium in natural water, which obviously implies that it is solubilizable in an aqueous environment, and, on the other hand, it can be retained by the membrane when it is in its uncomplexed state, that is to say, when it does not form complexes with uranium. To this end, one of the criteria for choosing this polymer may be that its molecular mass in the uncomplexed state is greater than the cutoff threshold of the filtration membrane used (size exclusion).

[0016] The organic polymer is also chosen so that it forms complexes with uranium comprising uranium and at least one polymer molecule, these complexes also being retained by the filtration membrane. This retention of the complexes formed can be linked either to their molecular mass, in particular if the organic polymer itself has a molecular mass greater than the membrane's cutoff threshold (steric exclusion), or to their charge, which can be of the same sign as that of the polarized membrane following the passage of natural water, in which case the complexes are retained on the membrane surface by electrostatic interaction (so-called Donnan exclusion), or even to a combination of the two mechanisms, steric exclusion and Donnan exclusion.

[0017] Advantageously, the organic polymer is a polymer which comprises, as group(s) complexing or chelating uranium (complexing and chelating being considered synonymous within the scope of the invention), one or more groups chosen from the groups -COOH, -C(O)NHOH, -SO3H, -P(O)(OH)2, -P(O)(OR)2, -P(O)(OH)(OR), -P(O)(OH)R, -P(O)(OH)-CH(R')-P(O)(OH)2, -P(O)(OH)-CH(R')-P(O)(OH)(OR), -P(O)(OH)-C(OH)(R')-P(O)(OH)2, -P(O)(OH)-C(OH)(R')-P(O)(OH)(OR) and -Ar(OH)2, with: R = linear or branched alkyl group, comprising 1 to 8 carbon atoms, R' = hydrogen atom or linear or branched alkyl group, comprising 1 to 8 carbon atoms, Ar(OH)2 = catechol group, and R” = hydrogen atom or -COOH or -SO3H group.

[0018] Thus, for example, suitable organic polymers include: - polyethylene glycols, or PEGs, whose polymer chain includes at least one group of the aforementioned type, such as α-methoxy-co-cathecol poly(ethylene glycol), α-hydroxy-co-phosphonic acid poly(ethylene glycol), α-methoxy-co-carboxylic acid poly(ethylene glycol), α,co-bis(carboxylic acid) poly(ethylene glycol), α-methoxy-co-phosphonic acid poly(ethylene glycol), α,co-bis(phosphonic acid) poly(ethylene glycol), α,co-bis(succinic acid) poly(ethylene glycol) and α,co-bis(acetic acid) poly(ethylene glycol), all of these poly(ethylene glycol) being commercially available; - polyacrylamides, such as poly(A-isopropylacrylamide), or polyNIPAM, whose polymer chain includes at least one group of the aforementioned type; - polyvinyl alcohols, or PVA, whose polymer chain includes at least one group of the aforementioned type; - polyvinylpyrrolidones, or PVP, whose polymer chain includes at least one group of the aforementioned type; and - polymers comprising a repeating acryloyl motif functionalized by a phosphorylated group, typically a phosphonate group, phosphonic acid or alen-dronic acid, this repeating motif being associated or not with another repeating motif, acryloyl or not, such as an acryloylmorpholine motif.

[0019] Among these polymers, preference is given to polyethylene glycols whose polymer chain comprises at least one phosphorylated group and to polymers comprising an acryloyl repeating unit functionalized by a phosphorylated group, the phosphorylated group being preferably, in both cases, a phosphonate, phosphonic acid, or alendronic acid group. Among these, preference is given to a poly(A-acryloylmorpholine)-poly(A-acryloylalendronate).

[0020] According to the invention, step a) preferably comprises dissolving the organic polymer under stirring in the natural water to be treated (i.e., from which the uranium is to be recovered), the number of moles of organic polymer introduced into this water being advantageously at least equal to and preferably significantly higher than the number of moles of uranium present in the natural water to be treated.

[0021] As mentioned previously, the filtration membrane used in step b) is an inorganic membrane, this type of membrane having, in fact, many advantages over organic membranes such as better mechanical resistance allowing the use of high pressures during filtration, better tolerance to variations in pH and temperature, less tendency to fouling and lower operating costs.

[0022] Thus, the filtration membrane can in particular be a carbon membrane, a ceramic membrane comprising one or more metal oxides such as an alumina, zirconia, titanium dioxide, zirconia / alumina, titanium dioxide / alumina or titanium dioxide / zirconia membrane.

[0023] Among these membranes, preference is given to a ceramic membrane and, even more so, to a ceramic membrane with two or more of two metal oxides, with every preference being given to a TiO2 / ZrO2 membrane.

[0024] According to the invention, membrane filtration is preferably nanofiltration or ultrafiltration.

[0025] As a result, the filtration membrane preferentially has a cut-off threshold between 500 daltons and 50,000 daltons and, even better, between 1,000 daltons and 5,000 daltons.

[0026] In this regard, it is specified that the cut-off threshold of a filtration membrane can be defined as the molar mass of the smallest solute which is retained at 90% by this membrane, the unit of this cut-off threshold typically being the dalton.

[0027] Advantageously, in step c), the dissociation of the uranium complexes present in the retentate is obtained by modifying the pH of this retentate so as to bring this pH to a value such that the organic polymer is no longer able to effectively complex uranium.

[0028] Preferably, this pH modification includes adding to the retentate a solution of a strong acid such as hydrochloric acid, nitric acid or sulfuric acid, the acid concentration of this solution being advantageously chosen so as to obtain a dissociation of the uranium complexes without corroding the constituent elements of the filtration module.

[0029] According to the invention, the natural water to be treated can be marine water (i.e., water from a sea or ocean), groundwater such as groundwater or underground river water, surface freshwater such as water from a natural or artificial lake, a stream such as a torrent, river or stream, or brackish water such as water from a lagoon, preference being given to marine water.

[0030] Other features and advantages of the process of the invention will become apparent from the following supplementary description, which relates to tests which have validated this process.

[0031] It goes without saying, however, that this additional description is given only as an illustration of the process of the invention and should in no case be interpreted as a limitation thereof.

[0032] Detailed description of particular implementation methods

[0033] The tests whose results are reported below were carried out using: - a tangential flow nano / ultrafiltration (SIVA) pilot plant equipped with a single-channel tubular TiO2 / ZrO2 membrane having a cutoff threshold of 1 kDa or 5 kDa (TAMI Industries); and - as natural water to be treated, samples of Mediterranean seawater doped with uranium so as to give these seawater samples a uranium concentration easily detectable by inductively coupled plasma optical emission spectroscopy (ICP-OES) or by inductively coupled plasma mass spectrometry (ICP-MS); and - as organic polymers that complex with uranium, the following three polymers: * α-Methoxy-co-cathecol polyethylene glycol, available from Specify Polymers, noted as 1 below, * α,co-bis(phosphonic acid) polyethylene glycol, available from Specify Polymers, noted as 2 below, and * a poly(A-acryloylmorpholine)-poly(A-acryloylalendronate), or (poly(NAM-co-ALNAm)), denoted 3 below, synthesized from 4-acryloylmorpholine and succinimidyl A-acrylate modified to introduce the alendronate group, according to a synthetic scheme of the type described by S. Ida et al. in Gels 2019, 6(1):2, 1-11.

[0034] The chemical formulas of these polymers and their characteristics in terms of number-average molar mass, denoted Mn and expressed in kDa or g / mol, number-average degree of polymerization, denoted n or m, mass of complexing sites available per gram of polymer, denoted msc and expressed in g / g, are presented in Table 1 below.

[0035] [Tables 1] Table 1 Polymer Ma (g / mL) nm (g / g) 1 .A. .-A-, .3. . • ?" -- '".O. W 2000-2100 44 - 45 - 0.51 2 OK î ...... ...... ho- ; - ■ '-a3 -■ i. ï / WQ 2200-2285 44 - 46 - 0.36 3 S:»' 'A. <■>' y'- tS A 36300150500.36

[0036] * Uranium doping of Mediterranean seawater:

[0037] For doping Mediterranean seawater, 10 mL of a uranium stock solution containing 2 g / L of uranium is added to 1 liter of this water, along with 200 mg of sodium carbonate, NaHCO3, to prevent the uranium from precipitating at the basic pH of seawater. The uranium stock solution was prepared by dissolving uranyl nitrate pentahydrate, UO2(NO3)2.5H2O, in sodium carbonate (3.36 g / L), diluting it in seawater, and adjusting the pH to that of seawater (i.e., 8.4).

[0038] * Contacting the uranium-doped seawater samples with one of the Polymers 1, 2 and 3:

[0039] Before their introduction into the feed tank of the nano / ultrafiltration pilot, the uranium-doped seawater samples are supplemented with one of the polymers 1, 2 and 3 at a rate of 1 g or 2 g of polymer per liter of uranium-doped seawater, under agitation until complete dissolution of the polymer. Example 1: Kinetic study

[0040] In this study, which aims to estimate the time required for the filtration of samples of seawater spiked with uranium (20 mg / L) and supplemented with one of the polymers reach equilibrium, these samples are subjected to filtration for more than 3 hours 30 minutes, taking samples of the retentates and permeates every 30 minutes.

[0041] The temperature within the nano / ultrafiltration pilot is maintained at 22 °C. The transmembrane pressure is 3 bars for a retentate flow rate of 100 L / h and a permeate flow rate of 0.1 L / h.

[0042] The concentrations of U, Ca, Mg, Na and K are measured by ICP-OES and ICP-MS in samples of the retentates and permeates and the rejection rates of these elements are determined from these concentrations.

[0043] In this regard, it is specified that the rejection rate or retention rate of a solute, denoted TR, is defined by the following relationship:

[0044] [Math.l] TR = (1-^ ) x 100 in which CP and CR are respectively the concentrations of the solute in the permeate and in the retentate.

[0045] The rejection rates of U, Ca, Mg, Na and K corresponding to a maximum separation between U and the 4 other elements as well as the time after which this maximum separation is obtained are presented in Table 2 below as a function of the polymer and the membrane cut-off threshold used.

[0046] [Tables2] Table 2 Membrane cutoff threshold (kDai) Maximum separation time (h) Rejection rate U Ca Mg Na K 1 Ug / U 1 2 77.6 0.3 1.6 0.7 1.2 2 U g / Q 1 92.4 3.5 1.0 5.8 1.4 3 (1 g / L) 5 31 95.7 4.8 xs 0.3 3.2

[0047] Table 3 below details the rejection rates obtained for U, Ca, Mg, Na and K depending on the time and the polymer used.

[0048] [Tables3] Rejection Rate Time U Ca Mg Mi K 1 2 3 1 2 3 1 2 3 1 2 3 I 2 3 05 96.4 93.8 96.6 79.5 67.0 65.1 28.7 34.9 63.5 95.9 93.6 S6.1 34.8 91.8 64.6 1 88.2 80.5 96.6 24.1 35.3 210 12.3 17.1 21.5 27.4 23.7 20.4 29.0 23.8 21.3 13 89.1 88.7 97.0 28.2 14.3 24.6 1.1 42.6 26.5 10.5 17.5 24.6 5.3 19.3 26.0 2 77.6 33.2 86.4 0.3 5.6 18.7 1.6 10J 144 0.7 4.6 13.1 ' J 3.9 13;3 3 74f6 92.4 95.7 1.2 3 sy 15 1.0 1.8 1.3 5.8 0.3 y 3.2

[0049] These tables show that, of the 3 polymers used, polymer 3 has the greatest affinity for uranium and the best ability to capture and separate it from other elements, with a uranium rejection rate of 96% and rejection rates of other elements of less than 5% after a filtration of 3.5 hours.

[0050] Polymer 2 also shows very good performance in capturing uranium and separating it from other elements, with a uranium rejection rate of 93% and rejection rates of other elements of less than 6% after a filtration of 3.5 hours.

[0051] With these two polymers, the maximum separation of uranium is achieved at 3.5 hours of filtration.

[0052] The maximum uranium rejection rate obtained with polymer 1 is lower than those obtained with polymers 2 and 3, at 78%. However, this rejection rate is obtained after a shorter filtration time (2 hours versus 3.5 hours) and with rejection rates of other elements below 2%, which makes polymer 1 potentially interesting.

[0053] These tables also show that 2 g of polymer 2 and 1 g of polymer 3 can capture 20 mg of uranium without reaching their maximum complexation capacities, the uranium rejection with these polymers remaining, in fact, constant from the start of filtration. The low rejections of Ca, Mg, Na, and K ensure efficient concentration of uranium and its separation from the predominantly uranium elements. present in seawater.

[0054] Example 2: Study of the rejection capacity of polymers

[0055] In this study, which aims to assess the polymer rejection capacity as a function of uranium concentration, an operating protocol identical to that described in Example 1 above is followed, except that uranium is added every hour to the feed tank of the nano / ultrafiltration pilot via the uranium stock solution at 2 g / L so as to increase the uranium concentration from 20 mg / L to 75 mg / L.

[0056] Here too, samples of retentates and permeates are taken every 30 minutes for analysis and calculation of rejection rates.

[0057] The rejection rates thus obtained are presented in Table 4 below as a function of the uranium concentration and the polymer used.

[0058] [Tables4] Table 4 Uranium concentration WD Uranium release rate R 1 2 3 20 673 87.7 883 30 56.8 893 87.1 50 68.2 79.7 87.5 75 66.5 70.6 86.1

[0059] This table shows that the uranium release rates, for a uranium concentration of 75 mg / mL, are significantly lower than those obtained for lower uranium concentrations, which indicates saturation of the polymers with uranium.

[0060] However, it also shows that the systems implemented make it possible to separate high levels of uranium from other metallic elements present in seawater.

[0061] Example 3: Study of the decomplexation of uranium in an acidic medium

[0062] In this study, which aims to assess the possibility of recovering uranium by dissociating the complexes formed by polymers 1, 2 and 3, an operating protocol identical to that described in Example 1 above is followed, except that a concentrated solution of nitric acid or sulfuric acid (to avoid a significant volume variation) is added to the feed tank of the nano / ultrafiltration.

[0063] After acidification with 0.1 mol / L of nitric acid, uranium rejection rates for polymers 1, 2 and 3 are obtained which are respectively around 6%, 15% and 66%, after one hour of filtration.

[0064] These rejection rates indicate that the uranium is no longer in complexed form but in free form and therefore is no longer retained by the membrane.

[0065] Uranium decomplexation is less significant for polymer 3, as it exhibits a greater affinity for uranium than the other two polymers. However, this limitation can be overcome by increasing the acid concentration while maintaining an acidity level that does not degrade the components of the nano / ultrafiltration pilot's filtration module. Thus, the uranium rejection rate for polymer 3, which is 66% at a nitric acid concentration of 0.1 mol / L, drops to less than 10% at a nitric acid concentration of 0.86 mol / L.

[0066] Similar results are obtained with sulfuric acid. References cited

[0067] A. Favre-Reguillon et al., Ind. Eng. Chem. Res. 2003, 42, 23, 5900-5904 A. Favre-Réguillon et al., WaterRes. 2008, 42, 4-5, 1160-1166; O. Raff and RD Wilken, Desalination 1999, 122, 147-150 Y. Chung et al., Water Sci. Technol. Water Supply 2019, 19, 789-795 S. Ida et al., Gels 2019, 6(1):2, 1-11

Claims

Claims

1. A method for recovering uranium from natural water, implementing a complexation of the uranium and filtration on a membrane made of an inorganic material, and comprising at least the steps of: a) complexation of the uranium present in the natural water by at least one water-soluble organic polymer, comprising one or more groups complexing uranium at the pH of the natural water, the organic polymer being retained by the membrane in the uncomplexed state and in the complexed state; b) passing the water obtained at the end of step a) through the membrane, whereby, on either side of the membrane, a uranium-depleted permeate and a uranium-enriched retentate in the form of complexes are obtained; then c) collecting the retentate and dissociating the uranium complexes present in the retentate.

2. A method according to claim 1, wherein the uranium-complexing group(s) are selected from the groups -COOH, -C(O)NHOH, -SO3H, -P(O)(OH)2, -P(O)(OR)2, -P(O)(OH)(OR), -P(O)(OH)R, -P(O)(OH)-CH(R')-P(O)(OH)2, -P(O)(OH)-CH(R')-P(O)(OH)(OR), -P(O)(OH)-C(OH)(R')-P(O)(OH)2, -P(O)(OH)-C(OH)(R')-P(O)(OH)(OR) and -Ar(OH)2, with: R = linear or branched alkyl group, comprising from 1 to 8 carbon atoms, R' = hydrogen atom or linear or branched alkyl group, comprising from 1 to 8 carbon atoms, Ar(OH)2 = cathecol group, and R” = hydrogen atom or -COOH or -SO3H group.

3. The method of claim 1, wherein the organic polymer is a polyethylene glycol, a polyacrylamide, a polyvinyl alcohol or a polyvinylpyrrolidone whose polymer chain comprises at least one group -COOH, -C(O)NHOH, -SO3H, -P(O)(OH)2, -P(O)(OR)2, -P(O)(OH)(OR), -P(O)(OH)R, -P(O)(OH)-CH(R')-P(O)(OH)2, -P(O)(OH)-CH(R')-P(O)(OH)(OR), -P(O)(OH)-C(OH)(R')-P(O)(OH)2, -P(O)(OH)-C(OH)(R')-P(O)(OH)(OR) and -Ar(OH)2, with: R = linear or branched alkyl group, comprising from 1 to 8 carbon atoms, R' = hydrogen atom or linear or branched alkyl group, comprising from 1 to 8 carbon atoms, Ar(OH)2 = cathecol group, and R” = hydrogen atom or -COOH or -SO3H group.

4. A method according to claim 3, wherein the organic polymer is a polyethylene glycol whose polymer chain comprises at least one phosphorylated group, preferably phosphonate, phosphonic acid or alendronic acid.

5. The method of claim 1, wherein the organic polymer comprises an acryloyl repeating unit functionalized with a phosphoryl group, preferably a phosphonate, phosphonic acid or alendronic acid group.

6. The method of claim 5, wherein the organic polymer is poly(A-acryloylmorpholine) (NAM) - poly(A-acryloylalendronate).

7. A method according to any one of claims 1 to 6, wherein step a) comprises dissolving with stirring the organic polymer in natural water.

8. A method according to any one of claims 1 to 7, wherein the filtration membrane is a ceramic membrane, preferably with two or more metal oxides and, moreover, a TiO2 / ZrO2 membrane.

9. A method according to any one of claims 1 to 8, wherein the membrane filtration is nanofiltration or ultrafiltration.

10. A method according to any one of claims 1 to 9, wherein the filtration membrane has a cut-off threshold of between 500 daltons and 50,000 daltons, preferably between 1,000 daltons and 5,000 daltons.

11. A method according to any one of claims 1 to 10, wherein the dissociation of the uranium complexes present in the retentate comprises a modification of the pH of the retentate.

12. The method of claim 11, wherein modifying the pH of the retentate comprises adding to the retentate a solution of a strong acid.

13. A method according to any one of claims 1 to 12, wherein the natural water is marine water, fresh groundwater, fresh surface water or brackish water, preferably marine water.