Organic-inorganic hybrid material of use for extracting uranium (VI) from aqueous media containing phosphoric acid, processes for preparing same and uses thereof

This application is a U.S. national phase under the provisions of 35 U.S.C. § 371 of International Patent Application No. PCT/EP13/73429 filed Nov. 8, 2013, which in turn claims priority of French Patent Application No. 1351639 filed Feb. 25, 2013. The disclosures of such international patent application and French priority patent application are hereby incorporated herein by reference in their respective entireties, for all purposes.

The invention relates to the field of extraction of uranium(VI) from aqueous media containing phosphoric acid.

More specifically, the invention relates to an organic-inorganic hybrid material which allows extraction of uranium(VI) present in an aqueous medium comprising phosphoric acid and this, both very efficiently and with high selectivity towards the other metal cations which may be present in this medium and in particular, towards iron(III).

It also relates to methods which allow preparation of this organic-inorganic hybrid material.

It also relates to the uses of this organic-inorganic hybrid material for extracting uranium(VI) from an aqueous medium comprising, in addition to this uranium, phosphoric acid such as a solution from the attack of a natural phosphate with sulfuric acid and this, whether the purpose is to subsequently add value to this uranium or, on the contrary, treat it as a waste.

The invention notably finds application in the treatment of natural phosphates.

Natural phosphates (or phosphate ores) which are used for making phosphoric acid and fertilizers, contain uranium at contents which may vary from a few tens of ppm to several thousands of ppm as well as variable amounts of other metals.

The uranium present in natural phosphates is almost entirely found again in the aqueous solutions of phosphoric acid which stem from the sulfuric attack of these phosphates.

The potential for recovering the uranium contained in these phosphate ores is of 14,000 metric tons/year, i.e. about 25% of the present yearly production of uranium, which represents a non-negligible uranium supply source.

Also, many research teams have been interested in the problem of recovering uranium from an aqueous solution of phosphoric acid.

Three main techniques have been proposed to this day, i.e.:

1°) the liquid-liquid extraction, which consists of extracting uranium from the aqueous solution of phosphoric acid by putting this aqueous solution in contact with an organic solution which comprises one or several specific extracting agents such as, for example, a mixture of di-2-(ethylhexyl)phosphoric acid (or HDEHP) and of trioctylphosphine oxide (or TOPO), in solution in an organic diluent such, as for example, n-dodecane. This technique is efficient and relatively simple to apply, which results in the fact that to this day, it is the only one which has been applied industrially, known under the name of the URPHOS method (cf., for example, Hurst et al., Industrial & Engineering Chemistry Process Design and Development 1972, 11(1), 122-128, reference [1]). However, it requires the use of significant volumes of organic solutions which, after having stripped the uranium for recovering it, should be washed with different aqueous solutions in order to be able to reuse them, which makes it a technique not very adapted to dry climate conditions which prevail in a certain number of regions with mines of natural phosphates. Further, the problem is posed of a possible contamination of the uranium with chemical species stemming from the organic solutions.

2°) co-precipitation, which consists of forming in situ, i.e. in the aqueous solution of phosphoric acid, solid particles which comprise the uranium which one seeks to recover, by adding a chemical species in solution and then after decantation and filtration, by collecting these solid particles. This technique is efficient but it is relatively expensive because of the many treatments to which the co-precipitates then have to be subject in order to recover the uranium alone.

3°) solid-liquid extraction, which consists of extracting uranium from the aqueous solution of phosphoric acid by putting this aqueous solution in contact with a resin formed by an organic polymer insoluble in water which includes chemical groups capable of retaining the uranium either by ion exchange or by complexation. Thus resins bearing aminophosphonic, hydroxyphosphonic, phosphonic or polyfunctional sulfonic/phosphonic acid groups (cf, for example, Gonzalez-Luque and Streat, Hydrometallurgy 1983, 11(2), 207-225, reference [2]; Ketzinel, U.S. Pat. No. 4,599,221, reference [3]; Kabay et al., Industrial & Engineering Chemistry Research 1998, 37(5), 1983-1990, reference [4]) have been proposed.

The solid-liquid extraction has the advantage over the liquid-liquid extraction of not using organic solutions and therefore being without the different drawbacks related to the use of this type of solutions. However, it is found that the applications of resins which have been proposed to this day for extracting uranium from an aqueous solution of phosphoric acid are very limited because:

(1) most of these resins require that the uranium, which is naturally present in the state of oxidation VI in aqueous solutions of phosphoric acid stemming from the sulfuric attack of natural phosphates, be reduced beforehand to the state of oxidation IV;

(2) their capability of selectively retaining the uranium is generally low; and

(3) because of their organic nature, they tend to swell and float in the aqueous medium, which is an obstacle to their use in equipment which is typically used for carrying out continuous solid-liquid extractions such as fluidized beds.

Moreover materials with an organic solid support are known, which are more stable chemically than the materials with an organic solid support and which have consequently given rise recently to a certain number of studies as to the possibility of using them for extracting uranium from aqueous acid solutions, typically nitric solutions.

Thus the following were notably proposed:

materials with an inorganic support functionalized by molecules with amine groups (Donia et al., International Journal of Mineral Processing 2011, 101(1-4), 81-88, reference [5]; Sadeghi et al., Microchemica Acta 2012, 178(1-2), 89-97, reference [6]), or impregnated with trioctylamine (Ahmed et al., Hydrometallurgy 2013, 134-135(0), 150-157, reference [7]); however these materials prove to be not selective for uranium towards the other metal cations; materials with an inorganic support functionalized with molecules with phosphorus-containing groups: for example, Lebed et al. ( Chemistry of Materials 2012, 24(21), 4166-4176, reference [8]) have proposed a mesoporous silica functionalized with ethyltriethoxysilane diethylphosphonate groups at the surface of the pores of this silica, while Yuan et al. ( Dalton Transactions 2011, 40(28), 7446-7453, reference [9]) have proposed a mesoporous silica functionalized with diethylphosphato-ethyltriethoxysilane groups; however, in addition to the fact that the extraction tests described in both of these references are conducted without competing ions, their results show a low performance of the materials at pH 2; and materials with an inorganic support functionalized with molecules of the glycinylurea, salicylamide, acetamide phosphonate type (Fryxell et al., Environmental Science & Technology 2005, 39(5), 1324-1331, reference [10]) or further of the dihydroimidazole type (Yuan et al., Journal of Materials Chemistry 2012, 22(33), 17019-17026, reference [11]); however, the extraction tests described in these references are not poorly conclusive, either because the extraction of the uranium is very low or even zero at pHs of the order of 2, or because the selectivity of the extraction of uranium is not very selective, notably towards iron (reference [10]).

The inventors therefore set the goal of providing novel materials which allow extraction by the solid-liquid extraction technique, of uranium(VI) present in an aqueous medium comprising phosphoric acid and which are generally without the drawbacks which the solid materials proposed up to now have for applying this technique.

More specifically, the inventors set the goal that these novel materials allow extraction of uranium(VI) from an aqueous medium comprising phosphoric acid, very efficiently and with high selectivity towards the other metal cations which may be present in this solution and, in particular towards iron(III) and this, without it being necessary to subject beforehand this uranium to any reduction treatment.

They also set the goal that these novel materials be stable in an aqueous medium and that their preparation be relatively simple to apply, notably because it only requires reactions which are conventionally applied in the field of synthesis chemistry.

These objects and further ones are achieved by the invention which firstly proposes an organic-inorganic hybrid material, which comprises an inorganic solid support on which is covalently grafted a plurality of organic molecules fitting the general formula (I) hereafter:

wherein:

x, y and z are equal to 0 or 1, with the proviso that at least one of x, y and z is equal to 1; m is an integer ranging from 1 to 6; v and w are equal to 0 or 1, with the proviso that v is equal to 1 when w is equal to 0 and that v is equal to 0 when w is equal to 1; if x equals 0, R1 represents a hydrogen atom or a saturated or unsaturated, linear or branched hydrocarbon group comprising from 1 to 12 carbon atoms, while, if x is equal to 1, R1 represents a group bound to the inorganic solid support through at least one covalent bond (represented by the dotted line); if y is equal to 0, R2 represents a hydrogen atom or a saturated or unsaturated, linear or branched hydrocarbon group comprising from 1 to 12 carbon atoms, while, if y is equal to 1, R2 represents a group bound to the inorganic solid support through at least one covalent bond (represented by the dotted line); if z is equal to 0, R3 represents a hydrogen atom or a saturated or unsaturated, linear or branched hydrocarbon group comprising from 1 to 12 carbon atoms, while, if z is equal to 1, R3 represents a group bound to the inorganic solid support through at least one covalent bond (represented by the dotted line); R 4 and R 5 represent independently of each other, a hydrogen atom, a saturated or unsaturated, linear or branched hydrocarbon group comprising from 2 to 8 carbon atoms, or a monocyclic aromatic group.

Thus, the organic-inorganic hybrid material according to the invention comprises a solid support of an inorganic nature (which is therefore chemically more stable than are typically organic supports), on which are attached in a covalent way molecules which, themselves, are of an organic nature and comprise a diamidophosphonate unit which is capable of complexing uranium(VI) when the latter is present in an aqueous medium comprising phosphoric acid and of retaining it by this complexation mechanism.

Within the scope of the present invention, is considered as “inorganic”, any element (compound, material, etc.) which is able to be broken down at a temperature above 800° C., while is considered as “organic” any element which is able to break down at a temperature below or equal to 800° C.

By “saturated or unsaturated, linear or branched hydrocarbon group comprising from 1 to 12 carbon atoms” is meant any alkyl, alkenyl or alkynyl group with a linear or branched chain, which comprises at least 1 carbon atom but which does not comprise more than 12 carbon atoms. Such a group may therefore comprise 1 carbon atom, 2 carbon atoms, 3 carbon atoms, 4 carbon atoms, etc., up to 12 carbon atoms inclusive.

In a similar way, by “saturated or unsaturated, linear or branched hydrocarbon group comprising from 2 to 8 carbon atoms” is meant any alkyl, alkenyl or alkynyl group with a linear or branched chain, which comprises at least 2 carbon atoms but which does not comprise more than 8 carbon atoms. Such a group may therefore comprise 2 carbon atoms, 3 carbon atoms, 4 carbon atoms, etc., up to 8 carbon atoms inclusive.

By “monocyclic aromatic group” is meant any group with a single ring and for which the ring meets the Hückel aromaticity rule and therefore has a number of delocalized π electrons equal to 4n+2, for example a phenyl or benzyl group.

According to the invention, the inorganic solid support may be formed by any inorganic solid material on which it is possible to covalently bind organic molecules with one or several chemical reactions.

Thus, the inorganic solid support may notably be based (1) on a metal oxide and, in particular, on a transition metal oxide like a titanium oxide or a zirconium oxide (or zirconia), a post-transition metal oxide such as an aluminium oxide (or alumina), a metalloid oxide such as a silicon oxide (or silica), a silica glass or a germanium oxide, (2) on a metal mixed oxide such an aluminosilicate, an aluminosilicate glass, a zirconium silicate, a tin silicate or a cerium silicate, (3) a mixture of metal oxides such as a borosilicate or a borosilicate glass, or (4) on carbon (graphite, fullerenes including nanotubes, mesoporous carbon, . . . ), and may be in a great diversity of shapes (particles, granules, beads, membranes, fibers, felts, . . . ), of (nano-, micro- or macroscopic) sizes and structures (either porous or not, with interconnected pores or not, either ordered or not, . . . ).

However, in order to optimize the extraction performances of the organic-inorganic hybrid material according to the invention, it is preferred that the inorganic solid support has (after grafting of the organic molecules) a specific surface area greater than or equal to 100 m 2 /g (as determined by adsorption-desorption of gas with the BET method), which is made possible by the use of a porous material.

This porous material may be a microporous material, i.e. a material for which the diameter of the pores is less than 2 nm (according to the definition of the International Union of Pure and Applied Chemistry), a mesoporous material, i.e. a material for which the diameter of the pores is comprised between 2 and 50 nm (also according to the definition of the IUPAC), a macroporous material, i.e. a material for which the diameter of the pores is greater than 50 nm (always according to the definition of the IUPAC), or further a material with double porosity, for example both mesoporous and macroporous, or even with triple porosity. It may moreover be ordered or disordered.

As non-limiting examples of materials which may be suitable, mention may notably be made of ordered mesoporous silicas such as silicas of the MCM and SBA type, disordered porous silicas such as the porous silica glasses of the VYCOR™ type (notably available from Corning), mesoporous titanium oxides, mesoporous zirconias, ordered porous carbons such as mesoporous carbons of the CMK type and carbon nanotubes, and disordered porous carbons such as active coals.

Preferably, the inorganic solid support consists of a mesoporous or macroporous material and is, in particular, selected from mesoporous silicas, mesoporous titanium oxides, mesoporous zirconias and mesoporous carbons.

From among these materials, mesoporous silicas and mesoporous carbons are most particularly preferred, in particular ordered mesoporous silicas of the SBA type and ordered mesoporous carbons of the CMK type.

According to the invention, the organic molecules may be grafted on the inorganic solid support via R 3 , in which case R 3 preferably represents a group of formula —(CH 2 ) q —X 1 — wherein q is an integer ranging from 0 to 12, while X 1 represents a group selected from the groups:

more simply written as —SiO 3 — hereafter;

more simply written as —CH 2 —C— hereafter; and

—CH═CH—;

depending on the nature of the inorganic solid support.

It should be well understood that, when R 3 represents a group of formula —(CH 2 ) q —X 1 —, the covalent bond(s) between R 3 and the inorganic solid support is (are) ensured by the group —X 1 — and not by the group —(CH 2 ) q —.

Alternatively or additionally, the organic molecules may also be grafted to the inorganic solid support via at least one of R 1 and R 2 , in which case R 1 and/or R 2 represent(s), preferably, a group of formula (a), (b), (c), (d), (e), (f) or (g) hereafter:

—(CH 2 ) p —C(O)—NH—(CH 2 ) q —X 2 —  (a)

—(CH 2 ) p —NH—C(O)—(CH 2 ) q —X 2 —  (b)

—(CH 2 ) p —C(O)—O—(CH 2 ) q —X 2 —  (c)

—(CH 2 ) p —O—C(O)—(CH 2 ) q —X 2 —  (d)

—(CH 2 ) p —O—(CH 2 ) q —X 2 —  (e)

—(CH 2 ) p -triazole-(CH 2 ) q —X 2 —  (f)

—(CH 2 ) q —X 2 —  (g)

wherein p is an integer ranging from 1 to 6, q is an integer ranging from 0 to 12, while X 2 represents a group selected from the groups:

There also, it should be well understood that, when R 1 and/or R 2 represent(s) a group of formula (a), (b), (c), (d), (e), (f) or (g) hereinbefore, the covalent bond(s) existing between R 1 and/or R 2 on the one hand and the inorganic solid support on the other hand, is (are) ensured by the group —X 2 — and not by the group —(CH 2 ) p —.

In the case when the organic molecules are grafted on the inorganic solid support both via R 3 and via at least one of R 1 and R 2 , then X 1 (which belongs to R 3 ) is preferably identical with X 2 (which belongs to R 1 and/or R 2 ). Thus, for example, if R 3 represents a group —(CH 2 ) q —SiO 3 —, then R 1 and/or R 2 may fit any of the formulae (a) to (g) hereinbefore but wherein X 2 preferentially represents a group —SiO 3 —. Similarly, if R 3 represents a group —(CH 2 ) q —CH 2 —C—, then R 1 and/or R 2 may fit any of the formulae (a) to (g) hereinbefore but wherein X 2 preferentially represents a group —CH 2 —C—.

According to the invention, the organic molecules preferably fit the general formula (I) hereinbefore wherein v is equal to 1, w is equal to 0, in which case these organic molecules fit the particular formula (Ia) hereafter:

wherein x, y, z, m, R 1 , R 2 , R 3 , R 4 and R 5 have the same meaning as earlier.

Still further, molecules of the particular formula (Ia) hereinbefore wherein x and y have the value 0, R 1 and R 2 represent, independently of each other, a linear or branched alkyl group comprising from 1 to 12 carbon atoms; z is 1 and R 3 represents a group bound to the inorganic solid support through at least one covalent bond, while R 4 and R 5 represent, independently of each other, a hydrogen atom or a linear or branched alkyl group comprising from 2 to 8 carbon atoms, are preferred.

Advantageously, R 1 and R 2 are identical with each other and represent a branched alkyl group comprising from 6 to 12 carbon atoms, the group 2-ethylhexyl being most particularly preferred.

As for R 4 and R 5 , they preferentially represent, independently of each other, a hydrogen atom or a linear or branched alkyl group comprising from 2 to 4 carbon atoms such as an ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl group, the ethyl and n-butyl groups being most particularly preferred among these alkyl groups.

According to a particularly preferred arrangement, the inorganic solid support is based on a metal oxide, on a metal mixed oxide or on a mixture of metal oxides, in which case R 3 represents a group of formula —(CH 2 ) q —SiO 3 — wherein q is 1 to 5.

Alternatively, the inorganic solid support is based on carbon, in which case R 3 represents a group of formula —(CH 2 ) q —CH 2 —C— wherein q is 0 to 5.

According to the invention, the covalent grafting of the organic molecules on the inorganic solid support may be obtained by a method in a single step, which consists of reacting one or several reactive functions F 1 belonging to the inorganic solid support with one or several reactive functions F 2 of an organic compound which comprises, in addition to these reactive functions F 2 , the diamidophosphonate complexing unit, so as to obtain the covalent binding of this organic compound on the inorganic solid support by reaction of the reactive functions F 1 and F 2 with each other. In this case, the organic molecules of general formula (I) hereinbefore corresponds to what is left of the organic compound after the reactive functions F 1 and F 2 have reacted together.

Thus, for example, in the case when the inorganic solid support is based on silica, the covalent grafting of the organic molecules on the inorganic solid support may be obtained by reacting silanol functions (—SiOH) of the silica with a trialkoxy function (for example, trimethoxy-, triethoxy- or tripropoxy-silane function) of an organic compound which also comprises the diamidophosphonate complexing unit.

Alternatively, the grafting of the organic molecules on the inorganic solid support may be obtained by a method in two steps, which consists of:

firstly reacting one or several reactive functions F 1 belonging to the inorganic solid support with one or several reactive functions F 2 of a first organic compound, which comprises two different types of reactive functions, F 2 and F 3 respectively, so as to obtain the covalent binding of the first organic compound on the inorganic solid support by reaction of said reactive functions F 1 and F 2 with each other and functionalization of the inorganic solid support with the reactive function(s) F 3 ; and then subsequently reacting this or these reactive functions F 3 with one or several reactive functions F 4 belonging to a second organic compound which comprises, in addition to these reactive functions F 4 , the diamidophosphonate complexing unit, so as to obtain the covalent binding of the second organic compound on the first by reaction of said reactive functions F 3 and F 4 with each other. In this case, the organic molecules of general formula (I) hereinbefore correspond to the molecular assembly formed by what is left of the first and second organic compounds after the reactive functions F 1 and F 2 , and then F 3 and F 4 ave reacted together.

This second way of proceeding is well adapted to grafting organic molecules on inorganic solid supports based on one or several metal oxides or on carbon and will therefore be preferred for these type of supports.

Thus, for example, in the case when the inorganic solid support is based on silica, the covalent grafting of the organic molecules on the inorganic solid support may be obtained by firstly reacting the silanol functions (—SiOH) of the silica with a trialkoxysilane function of a first organic compound which also comprises an amine function, and then reacting this amine function with the carboxylic acid function of a second organic compound which also comprises the diamidophosphonate complexing unit.

The organic-inorganic hybrid material according to the invention has particularly high affinity and selectivity for uranium(VI) when it is present in an aqueous medium comprising phosphoric acid.

Also, the invention further has the object of using an organic-inorganic hybrid material as defined earlier for extracting uranium(VI) from an aqueous medium comprising phosphoric acid and wherein this uranium is present.

According to the invention, this aqueous medium may comprise phosphoric acid in a very large range of concentrations and in particular from 0.01 to 9 mol/L of phosphoric acid.

Such an aqueous medium may notably be an aqueous solution resulting from the attack of a natural phosphate with sulfuric acid.

The extraction of uranium(VI) from an aqueous medium by means of an organic-inorganic hybrid material is extremely simple to apply since it is sufficient to put this material in contact with the aqueous medium, for example in a reactor with stirring or in a column, for a sufficient time in order to allow uranium(VI) to be complexed by the material, and then for separating the latter from the aqueous medium. Typically, 0.01 to 1 L of aqueous solution will be used for 0.05 to 5 kg of material.

According to the goal in which uranium(VI) is extracted from the aqueous medium (recovery of the uranium with view to increasing its value or purification of the aqueous medium as regards uranium), it is then possible either to strip uranium(VI) of the organic-inorganic hybrid material, for example by means of a basic aqueous solution of the potash aqueous solution type for recovering it with view to adding value to it, or treating the organic-inorganic hybrid material as a waste.

Also, the object of the invention is further a method for recovering uranium(VI) present in an aqueous medium comprising phosphoric acid, which method comprises:

a) extracting uranium(VI) from the aqueous medium, by putting this aqueous medium in contact with an organic-inorganic hybrid material as defined earlier, and then separating of the aqueous medium and the organic-inorganic hybrid material; and

b) stripping of uranium(VI) from the organic-inorganic hybrid material obtained at the end of step a) by putting the organic-inorganic hybrid material in contact with a basic aqueous solution, for example a potash aqueous solution, and then separating the organic-inorganic hybrid material and the basic aqueous solution.

In this method, the aqueous medium comprising phosphoric acid which is used in step a) is advantageously an aqueous solution resulting from the attack of natural phosphate with sulfuric acid.

Other features and advantages of the invention will become better apparent upon reading the description addition which follows, which relates to examples for preparing organic-inorganic hybrid materials according to the invention as well as to examples for demonstrating the properties of these materials.

Of course, these examples are only given as illustrations of the object of the invention and by no means are a limitation of this object.

FIG. 1 schematically illustrates the preparation of a first organic-inorganic hybrid material according to the invention, in which the inorganic solid support is a mesoporous silica and in which the organic molecules fit the general formula (I) hereinbefore in which R 1 and R 2 both represent a 2-ethylhexyl group, R 3 represents a group —(CH 2 ) 3 —SiO 3 —, R 4 represents an ethyl group, while R 5 represents a hydrogen atom.

FIG. 2 schematically illustrates the preparation of a second organic-inorganic hybrid material according to the invention in which the inorganic solid support is a mesoporous carbon and in which the organic molecules fit the general formula (I) hereinbefore wherein R 1 and R 2 both represent a 2-ethylhexyl group, R 3 represents a group —CH 2 —C—, R 4 represents an ethyl group, while R 5 represents a hydrogen atom.

FIG. 3 schematically illustrates the reaction schemes of the synthesis of organic compounds useful for preparing the organic-inorganic hybrid materials shown in FIGS. 1 and 2 .

[1] Hurst et al., Industrial & Engineering Chemistry Process Design and Development 1972, 11(1), 122-128

[2] Gonzalez-Luque and Streat, Hydrometallurgy 1983, 11(2), 207-225

[3] Ketzinel, patent U.S. Pat. No. 4,599,221

[4] Kabay et al., Industrial & Engineering Chemistry Research 1998, 37(5), 1983-1990

[5] Donia et al., International Journal of Mineral Processing 2011, 101(1-4), 81-88

[6] Sadeghi et al., Microchemica Acta 2012, 178(1-2), 89-97

[7] Ahmed et al., Hydrometallurgy 2013, 134-135(0), 150-157

[8] Lebed et al., Chemistry of Materials 2012, 24(21), 4166-4176

[9] Yuan et al., Dalton Transactions 2011, 40(28), 7446-7453

[10] Fryxell et al., Environmental Science & Technology 2005, 39(5), 1324-1331

[11] Yuan et al., Journal of Materials Chemistry 2012, 22(33), 17019-17026

[12] Zhao et al., Science 1998, 279, 548-552

[13] Jun et al., Journal of the American Chemical Society 2000, 122, 10712-10713