Method for recovering uranium from seawater by complexation of uranium and membrane filtration

Abstract

The invention relates to a method for recovering uranium from seawater, which uses complexation of uranium and filtration on a membrane made of an inorganic material and which comprises at least the steps of: a) complexing the uranium present in the seawater with at least one water-soluble organic polymer, comprising one or more groups complexing the uranium at the pH of the seawater, 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 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.

Description

[0001] PROCESS FOR RECOVERING URANIUM FROM MARINE WATER BY URANIUM COMPLEXATION AND MEMBRANE FILTRATION

[0002] DESCRIPTION

[0003] Technical field

[0004] The invention relates to the field of recovering uranium from marine water, i.e. water from a sea or ocean, more commonly called seawater.

[0005] More specifically, the invention relates to a process which, by implementing selective complexation of uranium in combination with membrane filtration, makes it possible to very effectively separate the uranium present in marine water from other metal 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.

[0006] State of the prior art

[0007] The Earth's surface is covered by 71% oceans and seas. This area represents a total volume of seawater of 1.37.10 9 km 3 whose main constituents are chlorine, sodium, sulfates, magnesium, calcium, potassium, strontium and hydrogen carbonates at concentrations ranging from around a hundred ppm to several g / L.

[0008] Uranium is present in seawater at an average level 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 tonnes of exploitable uranium, a resource approximately 500 times higher than terrestrial uranium.

[0009] Although the global nuclear fleet and, consequently, the demand for uranium continue to increase, the costs and economic impacts of uranium recovery are evolving. Uranium reserves must therefore be estimated in relation to the costs of recovering this element.

[0010] There are two main obstacles to exploiting seawater as a uranium resource:

[0011] - uranium is present in seawater in a highly complexed anionic form, namely tricarbonatouranate(VI), of formula (UChfCOsh 4- , which, by association with the calcium and magnesium cations also present in macroconcentrations or micro-concentrations in this water, forms neutral complexes Ca2UO2(CO3)3 and Mg2UO2(CO3)3;

[0012] - the recovery of metals from large volumes of seawater is extremely complicated and its implementation is very expensive; to be economically viable, this recovery can be carried out by solid-liquid extraction using an adsorbent which, in addition to being efficient and selective for the pH and ionic strength of seawater or freshwater, must be practically insoluble in aqueous media and resistant to biological corrosivity and fouling (or biofouling).

[0013] The first work on recovering uranium from seawater dates back to the early 1960s and many research teams have been interested in it, whether in Japan, the United States, India, China or Europe.

[0014] 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 against pathogenic or non-pathogenic microorganisms such as bacteria, or for desalinating it.

[0015] However, very few studies appear to have been conducted on the recovery of uranium from seawater or freshwater by membrane filtration. These studies appear to be four in number, three of which were conducted 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 conducted using an inorganic filtration membrane (Y. Chung et al., Water Sci. Technol. Water Supply 2019, 19, 789-795).However, in the course of their work, the inventors found that by subjecting seawater to membrane filtration in which the uranium in this water is complexed by an organic polymer capable of being retained by the filtration membrane both in the decomplexed and complexed state, it is, unexpectedly, possible to separate this uranium very effectively from the other metal cations present in this water and to recover it in the form of a concentrated solution.

[0016] And it is on these experimental observations that the invention is based.

[0017] Statement of the invention

[0018] The invention therefore relates to a process for recovering uranium from marine water, which implements a complexation of the uranium and a filtration on a membrane made of an inorganic material (such as metal oxides, like alumina, titanium dioxide, zirconium, etc.), which comprises at least the steps of: a) complexation of the uranium present in the marine water by a water-soluble organic polymer, comprising one or more groups complexing the uranium at the pH of the marine water, that is to say at a pH typically between 6 and 9, the organic polymer being retained by the membrane in the uncomplexed state and in the complexed state; b) passage of the water obtained at the end of step a) through the membrane, whereby a uranium-depleted permeate and a uranium-enriched retentate in the form of complexes are obtained on either side of the membrane; then c) collection of the retentate and dissociation of the uranium complexes present in the retentate.

[0019] In the foregoing and the following, the term "polymer" means both a homopolymer which results from the polymerization of a single monomer and which is therefore formed from a single repeating unit, and a copolymer which results from the polymerization of several (i.e. two or more than two) different monomers and which is therefore formed from several different repeating units.

[0020] As mentioned above, the organic polymer used in the process of the invention must be soluble in water, that is to say that it has a solubility at least equal to 1% by mass per unit volume of water at 20°C. This organic polymer is chosen so that, on the one hand, it is capable of complexing uranium in seawater, which obviously implies that it is solubilizable in an aqueous medium, and, on the other hand, it can be retained by the membrane when it is in the decomplexed state, that is to say when it does not form complexes with uranium. To do this, one of the criteria for choosing this polymer may be that its molecular mass in the uncomplexed state is greater than the cut-off threshold of the filtration membrane used (steric exclusion).

[0021] The organic polymer is also chosen so that it forms complexes with the 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 cut-off threshold of the membrane (steric exclusion), or to their charge which can be of the same sign as that of the polarized membrane following the passage of sea water, in which case the complexes are retained on the surface of the membrane by electrostatic interaction (so-called Donnan exclusion), or even to a combination of the two mechanisms, steric exclusion and Donnan exclusion.

[0022] Advantageously, the organic polymer is a polymer which comprises, as group(s) complexing or chelating uranium (complexing and chelating being considered synonymous in the context of the invention), one or more groups chosen from the groups -COOH, -C(O)NHOH, -SO3H, -P(O)(OH)2, -NH-(CH2)3-C(OH)(P(O)(OH)2)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:

[0023] R = linear or branched alkyl group, comprising from 1 to 8 carbon atoms,

[0024] R' = hydrogen atom or linear or branched alkyl group, comprising from 1 to 8 carbon atoms,

[0025] Ar(OH)2= cathecol group, and

[0026] R" = hydrogen atom or -COOH or -SO3H group.

[0027] Thus, for example, organic polymers that may be suitable are: - polyethylene glycols, or PEGs, the polymer chain of which comprises at least one group of the aforementioned type, such as α-methoxy-β-cathecol poly(ethylene glycol), α-hydroxy-β-phosphonic acid poly(ethylene glycol), α-methoxy-β-carboxylic acid poly(ethylene glycol), α,β-bis(carboxylic acid) poly(ethylene glycol), α-methoxy-β-phosphonic acid poly(ethylene glycol), α,β-bis(phosphonic acid) poly(ethylene glycol), α,β-bis(succinic acid) poly(ethylene glycol) and α,β-bis(acetic acid) poly(ethylene glycol), all of these poly(ethylene glycol) being commercially available;

[0028] - polyacrylamides, such as poly( / V-isopropylacrylamide), or polyNIPAM, the polymer chain of which comprises at least one group of the aforementioned type;

[0029] - polyvinyl alcohols, or PVA, whose polymer chain comprises at least one group of the aforementioned type;

[0030] - polyvinylpyrrolidones, or PVP, whose polymer chain comprises at least one group of the aforementioned type; and

[0031] - polymers comprising a repeating acryloyl unit and the polymer chain of which comprises at least one group of the aforementioned type, this repeating unit being able to be associated or not with another repeating unit, acryloyl or not, such as an acryloylmorpholine unit.

[0032] It is specified that the expression "whose polymer chain comprises at least one group of the aforementioned type" means that said at least one group can be carried by any unit of the polymer chain or, in other words, that it can be carried by one of the two units forming the ends of this chain as well as by any other unit of the chain.

[0033] 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, i.e. a group which comprises at least one phosphorus atom linked to three oxygen atoms, the phosphorylated group preferably being, in both cases, a phosphonate group (-P(O)(OH)(OR) or -P(O)(OR)2), phosphonic acid (-P(O)(OH)2) or alendronic acid (-NH-(CH2)3-C(OH)(P(O)(OH)2)2). Among these, all preference is given to a poly( / V-acryloylmorpholine)- poly( / V-acryloylalendronate).

[0034] In accordance with the invention, step a) preferably comprises dissolving, with stirring, the organic polymer in the marine 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 greater than the number of moles of uranium present in the marine water to be treated.

[0035] As mentioned previously, the filtration membrane used in step b) is an inorganic membrane, this type of membrane having, in fact, many advantages compared to organic membranes such as better mechanical resistance allowing the use of high pressures during filtration, better tolerance to pH and temperature variations, less tendency to fouling and lower operating costs.

[0036] Thus, the filtration membrane may 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.

[0037] Among these membranes, preference is given to a ceramic membrane and, even more so, to a ceramic membrane with two or more metal oxides, with all preference being given to a TiCh / ZrCh membrane.

[0038] According to the invention, the membrane filtration is preferably nanofiltration or ultrafiltration.

[0039] Therefore, the filtration membrane preferably has a cut-off threshold between 500 daltons and 50,000 daltons and, even better, between 1,000 daltons and 5,000 daltons.

[0040] 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.

[0041] 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 capable of effectively complexing the uranium.

[0042] Preferably, this modification of the pH comprises an addition to the retentate of 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.

[0043] Other characteristics and advantages of the method of the invention will emerge from the additional description which follows and which relates to tests which have enabled this method to be validated.

[0044] It goes without saying, however, that this additional description is given only as an illustration of the method of the invention and must in no case be interpreted as a limitation thereof.

[0045] Detailed presentation of specific implementation methods

[0046] The tests whose results are reported below are carried out using:

[0047] - a tangential nano / ultrafiltration pilot (SIVA) equipped with a single-channel tubular membrane in TiCh / ZrCh with a cut-off threshold of 1 kDa or 5 kDa (TAMI Industries); and

[0048] - as seawater to be treated, samples of Mediterranean sea doped with uranium so as to give these seawater samples a uranium concentration easily detectable by inductively coupled plasma optical emission spectroscopy (or ICP-OES) or by inductively coupled plasma mass spectrometry (or ICP-MS); and

[0049] - as organic polymers complexing uranium, the following three polymers:

[0050] * oc-methoxy-cD-cathecol polyethylene glycol, available from Specific Polymers, noted 1 below,

[0051] * a,o-bis(phosphonic acid) polyethylene glycol, available from Specific Polymers, noted 2 below, and * a poly( / V-acryloylmorpholine)-poly( / V-acryloylalendronate), or (poly(NAM-co-ALNAm)), noted 3 below, synthesized from 4-acryloylmorpholine and succinimidyl / V-acrylate modified to introduce the alendronate group, according to a synthesis scheme of the type described by S. Ida et al. in Gels 2019, 6(1):2, 1-11. The chemical formulas of these polymers and their characteristics in terms of number-average molar mass, noted Mn and expressed in kDa or g / mol, number-average degree of polymerization, noted n or m, mass of available complexing sites per gram of polymer, noted m sc and expressed in g / g, are presented in Table 1 below. Table 1

[0052] * Uranium doping of Mediterranean seawater:

[0053] For the doping of Mediterranean seawater, 10 mL of a uranium stock solution containing 2 g / L of uranium and 200 mg of sodium carbonate, NaHCCh, are added to 1 liter of this water to prevent uranium from precipitating at the basic pH of seawater. The uranium stock solution was prepared by dissolving uranyl nitrate pentahydrate, UChfNChh.SFbO, in a sodium carbonate medium (3.36 g / L), diluting it in seawater and adjusting the pH to that of seawater (i.e. 8.4).

[0054] * Contacting uranium-doped seawater samples with one of polymers 1, 2 and 3:

[0055] Before being introduced 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, with stirring until the polymer is completely dissolved.

[0056] Example 1: Kinetic study

[0057] In this study, which aims to estimate the time required for the filtration of seawater samples doped with uranium (20 mg / L) and added with one of the polymers to reach equilibrium, these samples are subjected to filtration for more than 3 hours 30 minutes by taking samples of the retentates and permeates every 30 minutes.

[0058] 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.

[0059] The concentrations of U, Ca, Mg, Na and K are measured by ICP-OES and ICP-MS in the retentate and permeate samples and the rejection rates of these elements are determined from these concentrations.

[0060] In this regard, it is specified that the rejection rate or retention rate of a solute, noted TR, is defined by the following relation:

[0061] / C P\

[0062] TR = 1 - 7- X 100 \ C R / in which CP and CR are the concentrations of the solute in the permeate and in the retentate respectively.

[0063] 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 according to the polymer and the cut-off threshold of the membrane used.

[0064] Table 2 Table 3 below details the rejection rates obtained for U, Ca, Mg, Na and

[0065] K as a function of time and polymer used.

[0066] Table 3

[0067] These tables show that, of the 3 polymers used, polymer 3 has the greatest affinity for uranium and the best capacity to capture it and separate it from other elements, with a uranium rejection rate of 96% and rejection rates for other elements of less than 5% after a 3.5-hour filtration.

[0068] 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 below 6% after a 3.5-hour filtration.

[0069] With these two polymers, maximum uranium separation is achieved at 3.5 hours of filtration.

[0070] The maximum uranium rejection rate obtained with polymer 1 is lower than those obtained with polymers 2 and 3, being 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 lower than 2%, which makes polymer 1 potentially interesting.

[0071] 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 rejection of uranium with these polymers remaining, in fact, constant since the beginning of filtration. The low rejections of Ca, Mg, Na and K guarantee an effective concentration of uranium and its separation from the elements mainly present in seawater.

[0072] Example 2: Study of the rejection capacity of polymers

[0073] In this study, which aims to assess the rejection capacity of polymers as a function of the 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 2 g / L uranium stock solution so as to increase the uranium concentration from 20 mg / L to 75 mg / L.

[0074] Here too, samples of retentates and permeates are taken every 30 minutes for analysis and calculation of rejection rates.

[0075] The rejection rates thus obtained are presented in Table 4 below according to the uranium concentration and the polymer used. Table 4

[0076] This table shows that the uranium rejection 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.

[0077] However, it also shows that the systems implemented make it possible to separate high levels of uranium from other metallic elements present in seawater.

[0078] Example 3: Study of the decomplexation of uranium in an acid medium

[0079] In this study, which aims to assess the possibility of recovering uranium by dissociation of 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 nitric acid or sulfuric acid solution is added (to avoid a significant variation in volume) to the feed tank of the nano / ultrafiltration pilot.

[0080] After acidification to a level of 0.1 mol / L of nitric acid, uranium rejection rates for polymers 1, 2 and 3 are obtained which are respectively of the order of 6%, 15% and 66%, after one hour of filtration.

[0081] These rejection rates indicate that the uranium is no longer in complexed form but in free form and is therefore no longer retained by the membrane.

[0082] Uranium decomplexation is less important for polymer 3 since it has a greater affinity for uranium than the other two polymers. However, it is possible to overcome this limitation by increasing the acid concentration while remaining at an acidity that does not degrade the constituent elements of the filtration module of the nano / ultrafiltration pilot. Thus, the uranium rejection rate for polymer 3, 66% for a nitric acid content of 0.1 mol / L, drops to less than 10% for a nitric acid content of 0.86 mol / L. Similar results are obtained with sulfuric acid.

[0083] References cited

[0084] A. Favre-Reguillon et al., Ind. Eng. Chem. Res. 2003, 42, 23, 5900-5904

[0085] A. Favre-Réguillon et al., Water Res. 2008, 42, 4-5, 1160-1166

[0086] O. Raff et R.D. Wilken, Desalination 1999, 122, 147-150 Y. Chung et al., Water Sci. Technol. Water Supply 2019, 19, 789-795

[0087] S. Ida et al., Gels 2019, 6(1):2, 1-11

Claims

CLAIMS 1. A method for recovering uranium from marine water, implementing complexation of the uranium and filtration through a membrane made of an inorganic material, and comprising at least the steps of: a) complexation of the uranium present in the marine water with at least one water-soluble organic polymer, comprising one or more groups complexing uranium at the pH of the marine 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;wherein the organic polymer is a polyethylene glycol, a polyacrylamide, a polyvinyl alcohol, a polyvinylpyrrolidone or a polymer comprising an acryloyl unit, the polymer chain of which comprises at least one group -COOH, -C(O)NHOH, -SO3H, -P(O)(OH)2, -NH-(CH2)3-C(OH)(P(O)(OH)2)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.

2. Method according to claim 1, in which the organic polymer is a polyethylene glycol whose polymer chain comprises at least one phosphorylated group.

3. The method of claim 1, wherein the organic polymer comprises an acryloyl repeating unit functionalized with a phosphorylated group.

4. A method according to claim 2 or claim 3, wherein the phosphorylated group is a phosphonate group, a phosphonic acid group or an alendronic acid group.

5. The method of claim 3, wherein the organic polymer is a poly( / V-acryloylmorpholine) (NAM) - poly( / V-acryloylalendronate).

6. Method according to any one of claims 1 to 5, in which step a) comprises dissolving with stirring the organic polymer in sea water.

7. Method according to any one of claims 1 to 6, in which the filtration membrane is a ceramic membrane, preferably with two or more metal oxides and, even more so, a TiCh / ZrCh membrane.

8. A method according to any one of claims 1 to 7, wherein the membrane filtration is nanofiltration or ultrafiltration.

9. Method according to any one of claims 1 to 8, in which 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.

10. A method according to any one of claims 1 to 9, wherein the dissociation of the uranium complexes present in the retentate comprises a modification of the pH of the retentate.

11. The method of claim 10, wherein modifying the pH of the retentate comprises adding to the retentate a solution of a strong acid.

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