AU2021393802A1 - Mixtures of quaternary ammonium salts for extracting uranium(vi) from aqueous solutions of sulfuric acid - Google Patents

Abstract

The invention relates to the use of a mixture of quaternary ammonium salts as an extractant for extracting uranium(VI) from an aqueous sulfuric acid solution. It also relates to mixtures of quaternary ammonium salts and to a method enabling recovery of the uranium(VI) present in an aqueous sulfuric acid solution from the leaching of a uranium-bearing ore using sulfuric acid and utilizing a mixture of quaternary ammonium salts as an extractant. Applications: treatment of uranium-bearing ores (uraninite, pitchblende, coffinite, brannerite, carnotite, etc.) for the purpose of utilizing the uranium(VI) present in these ores.

Description

MIXTURES OF QUATERNARY AMMONIUM SALTS FOR EXTRACTING URANIUM(VI) FROM AQUEOUS SOLUTIONS OF SULPHURIC ACID DESCRIPTION

Technical field

The invention relates to the field of extracting uranium(VI) from aqueous solutions of sulphuric acid.

More specifically, it relates to the use of a mixture comprising at least two different quaternary ammonium salts as extractant, for extracting uranium(VI) from an

aqueous solution of sulphuric acid in which it is present, such as a solution resulting from

the leaching of a uranium ore with sulphuric acid. It also relates to mixtures of quaternary ammonium salts. It further relates to a method for recovering the uranium(VI) present in an

aqueous solution of sulphuric acid resulting from leaching a uranium ore with sulphuric

acid and in which a mixture of quaternary ammonium salts is used as extractant. The invention can be applied notably to the processing of uranium ores (uraninite,

pitchblende, coffinite, brannerite, carnotite, ... ) for the purpose of recovering the uranium(VI) present in these ores.

Prior art

Uranium ores are extracted from mines, crushed and ground to the consistency of

a fine sand, then subjected to an attack, also known as leaching, with sulphuric acid (unless their gangue is naturally alkaline, in which case this leaching would require a prohibitive

consumption of sulphuric acid). Sulphuric acid has been selected for two reasons: on the one hand, it is the

cheapest strong acid, this acid can be produced at the site of uranium ore processing plants from sulphur by a process known as "double catalysis", and, on the other hand, its use

produces effluents which are relatively easy to process as the sulphate ions can be largely eliminated by precipitation with lime.

During the leaching, in addition to uranium, many other elements such as

aluminium, iron and silica are solubilised, which generally form the gangue elements, as well as elements which vary from one ore to another, both in their nature and their

quantity, such as molybdenum, titanium, zirconium, copper, nickel and arsenic. After filtration for removing insolubles, the aqueous solution derived from

leaching with sulphuric acid, which generally contains 0.1 g/L to 10 g/L uranium(VI), is sent to a purification unit where it is not only purified but also concentrated either by passing it

through ion exchange resins or by liquid-liquid extraction (i.e. by means of a solvent phase or organic phase). Then its pH is adjusted and it undergoes precipitation, which results in

an uranium concentrate of yellow colour, commonly referred to as "yellow cake".

This uranium concentrate is filtered, washed, dried and optionally calcined (to obtain uranium sesquioxide U 3 08 ), before being put into drums and sent to a refining/converting plant, where the uranium is converted to UF6 of nuclear purity.

Two processes are currently used for purifying the aqueous solution from the

leaching with sulphuric acid by liquid-liquid extraction: the AMEX process ("AMine EXtraction") on the one hand, and the DAPEX process ("DiAlkylPhosphoric acid EXtraction")

on the other hand. The AMEX process (which is described by Coleman et al. inIndustrial& Engineering

Chemistry 1958, 50, 1756-1762, hereinafter reference [1]), uses as extractant a commercial

mixture of trialkylate tertiary amines with C8 to C10 alkyl chains, for example Adogent TM 364 orAlamineTM 336, insolutioninakerosene-type hydrocarbon, optionally with the addition

of a heavy alcohol (C 1 0 to C 13) which has the role of phase modifier, whereas the DAPEX process (which is described by Blake et al., Oak Ridge National Laboratory Report, 18

December 1956, hereinafter reference [2]) uses as an extractant a synergistic mixture of di(2-ethylhexyl)phosphoric acid (HDEHP) and tri-n-butylphosphate (TBP), in a solution of

kerosene-type hydrocarbon. As the AMEX and DAPEX processes are not completely satisfactory, it has recently

been proposed to extract uranium(VI) from an aqueous solution of sulphuric acid by using

as extractants:

- bifunctional compounds comprising both a phosphoric acid or phosphonate function and an amide function linked to one another by a methylene bridge (cf. WO-A

2014/139869, hereinafter reference [3]); and

- bifunctional compounds comprising both a phosphine oxide function and an

amine function linked to one another by a methylene bridge (cf. WO-A-2016/156591, hereinafter reference [4].

These two types of compounds make it possible to extract uranium(VI) very effectively from an aqueous solution of sulphuric acid while being, on the one hand, more

selective with respect to uranium than the mixture of tertiary amines used in the AMEX process and, on the other hand, less sensitive to acids than these tertiary amines. In

addition, they make is possible to obtain rapid kinetics for the uranium(VI) extraction, contrary to the synergistic mixture of HDEHP and TBP used in the DAPEX process.

However, since it would be desirable to further expand the range of extractants

that can be used industrially to extract uranium(VI) from aqueous solutions of sulphuric acid, the inventors have set themselves the goal of providing new extractants which can

advantageously replace the extractant mixtures used in the AMEX and DAPEX processes. Since the work of Rogers et al. (in: K.C. Liddell, D.J. Chaiko (Eds.), MetalSeparation

Technologies Beyond 2000, The Minerals, Metals & Materials Society, Warrendale, PA, 1999, 139-147, hereinafter reference [5]) and Dai et al. (Journal of the Chemical Society,

Dalton Transactions 1999, 8, 1201-1202, hereinafter reference [6]) which pioneered the use of ionic liquids in the liquid-liquid extraction of metals from aqueous solutions, many

authors have been interested in these ionic liquids.

Ionic liquids, which are salts which have a melting temperature of below 100°C, or even below ambient temperature, in normal pressure conditions, do have many

advantages, such as their almost zero vapour pressure, their low flammability and their high thermal stability. In addition, some of their physical characteristics, such as their

immiscibility in water and their viscosity, can be modulated as a function of the cation and anion which constitute them.

With regard to the extraction of uranium(VI) from aqueous acid solutions, studies which have been reported to date relate to:

- the use of ionic liquids to replace hydrocarbons of the kerosene or n dodecane type typically used as organic diluents in liquid-liquid extraction; thus, for

example, Zhang et al. proposed (Separation Science and Technology 2014, 49, 1895-1902, hereinafter reference [7]) extracting uranium(VI) from aqueous solutions of nitric acid by

using organic phases comprising NNN',N'-tetraoctyl-3-oxapentanediamide (TODGA) as the extractant, and 1-alkyl-3-methylimidazolium hexafluorophosphate (or [Comim]+[PF]

with n = 6 or 8) as the diluent;

- the use of ionic liquids with specific tasks, commonly referred to as "task specific ionic liquids" or "TSILs", which are ionic liquids comprising a cation onto which a group known for complexing uranium(VI) has been grafted; thus, for example, it was

proposed by Xie et al. (Scientific Reports 2017, 7, 15735, hereinafter reference [8]) to extract uranium(VI) from aqueous solutions of nitric acid using TSILs consisting of a

bis(trifluoromethylsulfonyl)imide of an imidazolium cation functionalised by a complexing group -P(O)(Obutyl) 2 as extractants, diluted in 1-butyl-3-methylimidazolium bis(trifluoro

methylsulfonyl)imide ([C 4mim]+[N(CF 3 SO 2 ) 2 ]-);

- the use of ionic liquids which are referred to as "simple" as opposed to TSILs; thus, for example, Scheiflinger et al. (Journalof Radioanalytical and Nuclear Chemistry

2019, 322, 1841-1848, hereinafter reference [9]) published the results of a study on testing

the ability of 10 ionic liquids based on 3 different cations (methyltri-n-octylammonium,

methyltri-n-octylphosphonium and tetra-n-decyltri-n-hexylphosphonium) and 5 different anions (anthranilate, 2-hydroxy-5-nitrobenzoate, 4-aminosalicylate, thiosalicylate and 2

(hexylthio)acetate) to extract uranium(VI) from aqueous solutions in which it is present in nitrate or acetate form; and

- the use of a commercial mixture (AliquatTM 336) of methyltri-n octylammonium and methyltri-n-decylammonium chlorides for extracting uranium(VI)

from a leaching liquor of a uranium ore with sulphuric acid (cf. El Sayed, Hydrometallurgy 2003, 68, 51-56, hereinafter reference [10]; Amaral et al., Minerals Engineering 2010, 23, 498-503, hereinafter reference [11]).

However, to the knowledge of the inventors, no study has been published to date on the possible use of a mixture of two ionic liquids comprising different anions for extracting uranium(VI) from an aqueous solution of sulphuric acid, such as a solution resulting from the leaching of a uranium ore with sulphuric acid. In the course of their work, the inventors have found that a mixture of two quaternary ammonium salts with different anions can very advantageously replace the mixture of trialkylated tertiary amines used in the AMEX process, with, in particular, a better affinity for uranium(VI), a significantly higher uranium(VI) loading capacity and therefore the possibility of dispensing with the use of a phase modifier.

The present invention is based on these experimental findings.

Disclosure of the invention

Therefore, the invention primarily relates to the use of a mixture of quaternary

ammonium salts comprising:

- at least one salt of formula (1): ([NR1R 2 R 3R 4]+)mAim -, and

- at least one salt of formula (2): [NR 5 6R R 7 Rs]+A2-,

where:

Ri and R 5, identical or different, represent a hydrogen atom or a linear or branched alkyl group comprising 1 to 12 carbon atoms;

R 2 , R3 , R4 ,R ,6R 7 and R ,8identical or different, represent a linear or branched alkyl group

comprising 6 to 12 carbon atoms;

Aim- represents an anion: * halide, of formula: Cl-, Br- and I-, * perchlorate, of formula: [C10 4 ]-, * sulphate, of formula: [SO 4 ] 2 * hydrogen sulphate, of formula: [HSO 4]-, * nitrate, of formula: [NO 3]-, * thiocyanate, of formula: [SCN]-, * dicyanamide, of formula: [N(CN) 2]-,

* acetate, of formula: [CH 3 CO 2]-,

* phosphate, of formula: [P0 4] 3 * hydrogen phosphate, of formula: [HPO 4] 2 -

* dihydrogen phosphate, of formula: [H 2 PO 4]-, * methyl hydrogen phosphate, of formula: [MeHPO 4 ]-,

* methyl hydrogen phosphonate, of formula: [MeH 2 PO 3 ]-,

* diethylphosphate, of formula: [P(OCH 2CH 3) 20 2 ]- ; m corresponds to the degree of oxidation of the anion Aim-; whereas

A2- represents an anion: * tris(pentafluoroethyl)trifluorophosphate, of formula: [P(F 3)(CF 2CF 3 )3]-,

* bis(trifluoromethylsulfonyl)imide, of formula: [N(CF 3 SO 2 ) 2 ]-, * hexafluorophosphate, of formula: [PF6 ]-,

* bis(fluorosulfonyl)imide, of formula: [N(SO 2 F) 2 ]-, * fatty acid carboxylate, of formula: [CH3(CH 2)xCO 2]- where x is an integer ranging from 6 to 18,

* tetrafluoroborate of formula: [BF 4 ]-, or trifluoromethylsulfonate, (also known as triflate), of formula: [CF 3SO 3]-; as an extractant, for extracting uranium(VI) from an aqueous solution of sulphuric acid.

Thus, the mixture of quaternary ammonium salts used according to the invention comprises at least two salts having:

- as the cation: an ammonium cation substituted by three C 6 to C 1 2 alkyl chains

or by four alkyl chains, three of which are C6 to C 12 and one of which is Ci to C1 2, this cation being able to be identical or different in the two salts, and

- as the anion: an anion which, in the case of the salt of formula (1), is hydrophilic and strongly complexes uranium(VI) and, in the case of the salt of formula (2),

is hydrophobic and weakly complexes uranium(VI), the salt of formula (2) having the function of reducing the viscosity of the mixture and allowing this mixture to be liquid at

ambient temperature (20°C-25°C). In the above and in the following,"linear or branched alkyl group, comprising 1 to

12 carbon atoms", is defined as any alkyl group which comprises a total of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 carbon atoms and, in the case where it comprises more than 2 carbon

atoms, has a linear chain or one or more branches.

Similarly, "linear or branched alkyl group, comprising 6 to 12 carbon atoms", is

defined as any alkyl group which has a total of 6, 7, 8, 9, 10, 11 or 12 carbon atoms and has a linear chain or one or more branches.

Furthermore, the terms "aqueous medium", "aqueous solution" and "aqueous phase" are equivalent and interchangeable similarly the terms "organic solution" and "organic phase" are equivalent and interchangeable.

Also, the terms "from ... to ...... " and "between ...... and ...... ", applied to a range

(either a range of numbers of atoms, concentrations or other), imply that the boundaries of this range are included.

According to the invention, it is preferred that R et R , identical or different, represent a hydrogen atom or a linear or branched alkyl group comprising 1 to 4 carbon

atoms (methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl), in which case the alkyl group is advantageously a linear group and even more advantageously a

methyl or ethyl group.

Furthermore, it is preferred that R 2 , R 3, R 4, R6 , R 7 and R8 , identical or different, represent a linear or branched alkyl group, comprising 8 to 10 carbon atoms (n-octyl,

isooctyl, n-nonyl, isononyl, n-decyl, isodecyl, 2-ethylhexyl, 2-butylhexyl, 2-methylheptyl, 2 methyloctyl, 1,5-dimethylhexyl, 2,4,4-trimethyl-pentyl, 1,2-dimethylheptyl, 2,6

dimethylheptyl, 3,5,5-trimethylhexyl, 3,7-dimethyloctyl, 2,4,6-trimethylheptyl, etc.), preference being given to a linear C8 to C10 alkyl group and, more preferably, to an n-octyl or n-decyl group.

Thus, salts of formula (1) and formula (2) which are particularly preferred are salts whose cations, identical or different, are selected from the cations:

* tri-n-octylammonium, of formula: [HN(n-CH 17 3 ], * methyltri-n-octylammonium, of formula: [N(CH 3)(n-CH 17 3 ], * ethyltri-n-octylammonium, of formula: [N(C 2 H)(n-CH 1 7 3 ], * tri-n-decylammonium, of formula: [HN(n-C1 oH 21) 3]*,

* methyltri-n-decylammonium, of formula: [N(CH 3)(n-C1oH 21)3]*,

* ethyltri-n-decylammonium, of formula: [N(C 2 H)(n-C1oH 2 1 ) 3]*, * n-octyldi-n-decylammonium, of formula: [HN(n-CH 17)(n-C1oH 21)2]*,

* methyl-n-octyldi-n-decylammonium, of formula: [N(CH 3)(n-C 8 H 17)(n

C10H21)2]*,

* ethyl-n-octyldi-n-decylammonium, of formula: [N(C2H)(n-CH 17)(n-C1oH 21)2]*,

* di-n-octyl-n-decylammonium, of formula: [HN(n-CsH1 7) 2(n-CoH 2 1)]*, * methyldi-n-octyl-n-decylammonium, of formula: [N(CH 3 )(n-C 8 H 17)2 (n

C1oH 21)]*, and * ethyldi-n-octyl-n-decylammonium, of formula: [N(C2H)(n-CsH1 7)2(n-C1oH 21)]*. From the above, preference is given to salts of formula (1) and of formula (2)

whose cations, identical or different, are selected from tri-n-octylammonium, methyltri-n

octylammonium, tri-n-decylammonium, methyltri-n-decylammonium, methyl-n-octyldi-n decylammonium and methyldi-n-octyl-n-decylammonium cations.

Furthermore, it is preferred that the Ai m- anion is a sulphate ion [S0 4 ]2- and that the A 2 - ion is selected from bis(trifluoromethylsulfonyl)imide [N(CF 3SO 2)2]-, bis(fluoro

sulfonyl)imide [N(SO2 F) 2 ]- and hexafluorophosphate [PF 6]- ions.

Even more preferably, the A 2- anion is a bis(trifluoromethylsulfonyl)imide ion

[N(CF 3 SO 2 ) 2 ]-, more simply denoted [NTf2 ]- in the following. Thus, most preferably,

- the salt of formula (1) is selected from:

* tri-n-octylammonium sulphate, of formula: ([HN(n-CsH 1 7)3]*)2 [SO 4 ] 2 -, more simply denoted (TOAH*) 2 SO 4 2- in the following, * methyltri-n-octylammonium sulphate, of formula: ([N(CH 3)(n C8H 17)3 ]) 2 [SO 4 ] 2-, more simply denoted (MTOA*) 2SO 42- in the following, * tri-n-decylammonium sulphate, of formula: ([HN(n-C 1 oH 21) 3 ]*)2 [S0 4 ] 2 -, more simply denoted (TDAH*) 2 SO 4 2- in the following,

* methyltri-n-decylammonium sulphate, of formula: ([N(CH 3)(n

C 1oH 2 1 ) 3 ])2 [SO 4 ] 2 -, more simply denoted (MTDA*) 2 SO 4 2- in the following, * methyl-n-octyldi-n-decylammonium sulphate, of formula: ([N(CH 3)(n

4 ]2-, more simply denoted (MODDA*) 2SO 42- in the following, and CsH 17)(n-C 1 oH 2 1 ) 2 ])2 [SO

* methyldi-n-octyl-n-decylammonium sulphate, of formula: ([N(CH 3)(n

CsH 1 7)2 (n-C1oH 2 1 )]+) 2 [SO 4 ]2-, more simply denoted (MDODA+) 2SO 42- in the following; whereas

- the salt of formula (2) is selected from:

* tri-n-octylammonium bis(trifluoromethylsulfonyl)imide, of formula:

[HN(n-CsH 17)3 ][N(CF 3SO 2)2 ]-, more simply denoted TOAH+NTf 2- in the following,

* methyltri-n-octylammonium bis(trifluoromethylsulfonyl)imide, of

formula: [N(CH 3)(n-CsH 17)3]+[N(CF 3SO 2 )2 ]-, more simply denoted MTOA+NTf 2- in the

following, * tri-n-decylammonium bis(trifluoromethylsulfonyl)imide, of formula:

[HN(n-C 1 H 21) 3 ][N(CF 3SO 2) 2 ]-,more simply denoted TDAH+NTf 2- in the following, * methyltri-n-decylammonium bis(trifluoromethylsulfonyl)imide, of formula: [N(CH 3 )(n-C 1 oH 21)3 ]+[N(CF 3SO 2)2 ]-, more simply denoted MTDA+NTf 2- in the following,

* methyl-n-octyldi-n-decylammonium bis(trifluoromethylsulfonyl)imide, of formula: [N(CH 3)(n-CsH 17)(n-C 1oH 21) 2 ][N(CF 3 SO 2) 2 ]-, more simply denoted MODDA*NTf 2- in

the following, and * methyldi-n-octyl-n-decylammonium bis(trifluoromethylsulfonyl)imide, of

formula: [N(CH 3)(n-CsH 17)2(n-C 1 oH 21 )][N(CF 3 SO 2) 2 ]-, more simply denoted MDODA*NTf 2- in the following.

According to the invention, the mixture of quaternary ammonium salts can comprise several salts of formula (1) that are different from one another and/or several

salts of formula (2) that are different from one another. In which case, it is preferred that the salts of formula (1) differ from one another

in their cation but have the same anion Aim- just as it is preferred that the salts of formula

(2) differ from one another in their cation but have the same anion A 2 -. Thus, for example, the mixture of quaternary ammonium salts can comprise in

particulartwo or more salts of formula (2) selected from the salts TOAH+NTf 2-, MTOA*NTf 2-, TDAH+NTf 2-, MTDA+NTf 2-, MODDA+NTf 2- and MDODA+NTf 2- such as, for example

MTOA+NTf 2- together with MTDA+NTf 2-, or MTOA+NTf 2- together with MTDA+NTf 2-, MODDA+NTf2- and MDODA+NTf2-.

In any case, the molar fraction of the anion of the salt of formula (1) or of all the anions of the salts of formula (1), if there are several of them, is advantageously between

0.25 and 0.75 and, preferably, between 0.4 and 0.6 in the mixture of salts. This molar

fraction corresponds to the ratio between the number of moles of the anion of the salt of formula (1) or of all of the anions of the salts of formula (1) which the mixture of salts comprises and the total number of moles of the anions that this mixture comprises.

In the context of the invention, it is preferred that the uranium(VI) is extracted

from the aqueous solution of sulphuric acid by liquid-liquid extraction, in which case the use of the salt mixture comprises at least contacting the aqueous solution with an organic

solution comprising this mixture, followed by a separation of the aqueous solution from the organic solution.

In this case, the organic solution is, preferably, formed by the salt mixture, i.e. the organic solution does not comprise anything other than this mixture, in which case the

mixture of salts represents 100% by mass of the mass of the organic solution. However, it is of course entirely possible to provide that the organic solution

additionally comprises an organic diluent of the acyclic hydrocarbon or acyclic hydrocarbon mixture type, for example n-dodecane, hydrogenated tetrapropylene (TPH), kerosene, a

isoparaffinic solvent of the IsaneTM IP-185T or IsaneTM IP-175T type. In which case, the mixture of salts represents, preferably, at least 20% and, more preferably, at least 50% by

weight of the weight of the organic solution. In any case, the volume ratio between the aqueous solution of sulphuric acid and

the organic solution is preferably between 1 and 3. According to the invention, the aqueous solution of sulphuric acid is, preferably, a

solution which is derived from leaching a uranium ore with sulphuric acid, in which case this aqueous solution comprises typically 0.01 mol/L to 0.5 mol/L sulphuric acid, 0.1 g/L to

10 g/L uranium and optionally 0.1 mol/Lto 2 mol/L sulphate ions, these sulphate ions being

provided by the addition of an inorganic sulphate to said solution. The inorganic sulphate is for example ammonium sulphate, a sulphate of an alkali metal such as sodium sulphate or potassium sulphate, or a sulphate of an alkaline earth metal such as calcium sulphate or magnesium sulphate. From the mixturesof quaternary ammonium salts used accordingtothe invention, some, to the knowledge of the inventors, have never been described in the prior art. Also, the object of the invention is also the following salt mixtures:

- mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2-,

- mixture of (MTOA+) 2 SO 4 2- and MTOA+NTf 2-,

- mixture of (TOAH+) 2 SO 4 2- and MTOA+NTf 2-,

- mixture of (MTOA*) 2 SO 4 2- and TOAH+NTf 2-,

- mixture of (TOAH+) 2 SO 4 2 , MTOA*NTf2 and MTDA*NTf2[, and

- mixture of (TOAH+) 2 SO 4 2-, MTOA+NTf 2-, MTDA+NTf 2-, MODDA+NTf 2- and

MDODA+NTf 2-. A further object of the invention is a method for recovering uranium(VI) present

in an aqueous solution of sulphuric acid, resulting from leaching a uranium ore with

sulphuric acid, which comprises the steps of: a) extracting uranium(VI) from the aqueous solution by at least one contact of

the aqueous solution with an organic solution comprising a mixture of quaternary ammonium salts as defined above, followed by a separation of the aqueous solution from

the organic solution; and

b) stripping uranium(VI) from the organic solution obtained at the end of step a) by at least one contact of the organic solution with an aqueous solution comprising at least

one carbonate, for example, sodium or ammonium carbonate, followed by separating the organic solution from the aqueous solution.

As before, it is preferred that the mixture of salts represents 100% by weight of the weight of organic solution used in step a).

Furthermore, it is preferred that the aqueous solution comprises sodium carbonate in a concentration of 0.1 mol/L to 2 mol/L. Also as before, the aqueous solution of sulphuric acid comprises typically

0.01 mol/L to 0.5 mol/L sulphuric acid, 0.1 g/L to 10 g/L uranium and, optionally, 0.1 mol/L

to 2 mol/L sulphates ions.

Other features and advantages of the invention will appear upon reading the

following description which makes reference to the appended figures. Of course, this further description is given only to illustrate the object of the

invention and is in no way intended to limit this object.

Brief description of the figures

Figure 1 illustrates the distribution coefficients of uranium(VI), denoted Du, obtained after extraction tests which were carried out on aqueous phases of sulphuric acid

comprising uranium(VI) and iron(Ill) by using organic phases consisting of mixtures of quaternary ammonium salts having respectively [S04 ] 2- and [NTf2 ]- as anions, as a function

of the molar fraction of the anion [S0 4 ] 2 -, denoted xSo2-, present in these mixtures; in this

figure, the curve denoted 1 corresponds to mixtures of (TOAH+) 2 SO 4 2- and M Aliquat T 336NTf 2-; the curve denoted 2 corresponds to mixtures of (TOAH+) 2 SO 4 2- and

MTOA+NTf 2-; the curve denoted 3 corresponds to mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf2-whereas the curve denoted 3bis corresponds to mixtures of (TOAH+) 2 SO 4 2- and

TOAH+NTf 2- having been preferably contacted with an aqueous phase comprising 0.1 mol/L sulphuric acid; the Du are also illustrated having been obtained in the same conditions but

by using, on the one hand, organic phases consisting of (TOAH+) 2 SO 4 2 (Xs- =1),

Aliquat T M 336+NTf 2-, MTOA*NTf 2- or TOAH+NTf 2- (xs0 2- = 0) and, on the other hand, an

organic phase comprising 0.1 mol/L tri-n-octylamine in n-dodecane (dotted line denoted

R). Figure 2 shows the distribution coefficients of iron(ll), denoted DFe, as obtained

from extraction tests carried out on aqueous phases of sulphuric acid comprising uranium(VI) and iron(Ill) using organic phases consisting of mixtures of quaternary

ammonium salts having [S0 4 ] 2 - and [NTf 2]- respectively as anions, as a function of molar

fraction of the [S0 4 ] 2 -, denoted xSo2-, present in these mixtures; in this figure, the curve

denoted 1 corresponds to mixtures of (TOAH+) 2 SO 4 2- and Aliquat TM 336+NTf 2- ; the curve

denoted 2 corresponds to mixtures of (TOAH+) 2 SO 4 2- and MTOA*NTf 2-; the curve denoted 3 corresponds to mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2-whereas the curve denoted 3bis corresponds to mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- having been previously contacted with an aqueous phase comprising 0.1 mol/L sulphuric acid; the DFe are also illustrated having been obtained in the same conditions but by using, on the one hand, organic phases consisting of (TOAH+) 2 SO 4 2 (xs0 2- = 1), Aliquat TM 336+NTf 2-, MTOA*NTf 2 orTOAH+NTf 2- (Xs 0 2- = 0) and, on the other hand, an organic phase comprising 0.1 mol/L tri-n-octylamine in n-dodecane (dotted line denoted R).

Figure 3 shows the separation factors between uranium(VI) and iron(ll), denoted

FS/Fe, as obtained from extraction tests carried out on aqueous phases ofsulphuric acid comprising uranium(VI) and iron(Ill) by using organic phases consisting of mixtures of

quaternary ammonium salts having [SO 4 ] 2 - and [NTf2 ]- respectively as anions, as a function

of the molar fraction of anion [S 4 ] 2 -, denotedXSo2-, present in these mixtures; in this

figure, the curve denoted 1 corresponds to mixtures of (TOAH+) 2 SO 4 2- and Aliquat T M 336+NTf 2- ; the curved denoted 2 corresponds to mixtures of (TOAH+) 2 SO 4 2- and

MTOA+NTf 2-; the curve denoted 3 corresponds to mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf2- whereas the curve denoted 3bis corresponds to mixtures of (TOAH+) 2 SO 4 2- and

TOAH+NTf 2- having been previously contacted with an aqueous phase comprising 0.1 mol/L

sulphuric acid; the FSU/Fe are also illustrated having been obtained in the same conditions

but using, on the one hand, organic phases consisting of (TOAH+) 2 SO 4 2 (xs 0 2- = 1),

Aliquat T M 336+NTf2-, MTOA+NTf2- or TOAH+NTf2- (Xs 0 2- = 0) and, on the other hand, an

organic phase comprising 0.1 mol/L tri-n-octyl-amine in n-dodecane (dotted line denoted

R). Figure 4 illustrates the distribution coefficients of uranium(VI), denoted Du, as

obtained from extraction tests that were carried out on aqueous phases of sulphuric acid comprising uranium(VI) by using organic phases consisting of mixtures of (TOAH+) 2 SO 4 2

and TOAH+NTf 2- in A/O volume ratios of 1, 1.4, 1.6, 2 and 3, as a function of the molar

fraction of the [SO 4 ] 2 anion, denotedXSo2-, present in these mixtures; the Du are also

illustrated having been obtained in the same conditions but by using organic phases

consisting of (TOAH+) 2 SO 4 2 (xs0 2- = 1) or TOAH+NTf 2- (Xso2- = 0).

Figure 5 illustrates the concentrations of uranium(VI) in the organic phase, denoted [U]org and expressed in mg/L, as obtained from extraction tests that were carried out on aqueous phases of sulphuric acid comprising uranium(VI) by using organic phases consisting of mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf2- in A/Ovolume ratios of 1, 1.4, 1.6,

2 and 3, as a functionofthemolarfraction of the [S 2-anion, 4] denoted xSo2-, present in

these mixtures; concentrations of uranium(VI) in an organic phase are also illustrated

having been obtained in the same conditions but by using organic phases consisting of

(TOAH+) 2 SO 4 2 (xso2- = 1) orTOAH+NTf 2- (Xs 0 2- = 0).

Figure 6 illustrates the total concentrations of uranium(VI) in organic phases,

denoted [U]org,tot and expressed in g/L, consisting of mixtures of (TOAH+) 2 SO 4 2- and

TOAH+NTf 2- having a molar fraction xSo2- of 0.5 or of 0.75 after tests consisting in bringing

these organic phases into contact several times with an aqueous phase of sulphuric acid

comprising uranium(VI) so as a load them with the maximum amount of uranium(VI); on the abscissa of this figure, the cumulative concentrations of uranium(VI), present in the

aqueous phase, denoted [U]aq,tot and expressed in g/L, with which the organic phases were brought into contact; the [U]org,tot are also illustrated having been obtained in the same

conditions but by using organic phases consisting of (TOAH+) 2 SO 4 2 (xs0 2- = 1).

Figure 7 illustrates the distribution coefficients of uranium(VI), denoted Du, as obtained from extraction tests having been carried out on aqueous phases of sulphuric acid

comprising uranium(VI) and iron(Ill) using organic phases consisting of mixtures of

(TOAH+) 2 SO 4 2- and TOAH+NTf 2- having a molar fraction xSo2- of 0.5 or 0.75, as a function

of time denoted t and expressed in hours, during which the aqueous phases were contacted with the organic phases; Du are also shown having been obtained in the same conditions

but by using organic phases formed by (TOAH+) 2 SO 4 2 (xs0 2- = 1).

Figure 8 illustrates the percentages of uranium(VI) stripping, denoted E%, as obtained from stripping tests having been carried out on organic phases consisting of

mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- previously charged with uranium(VI) by using aqueous phases comprising 1 mol/L sodium carbonate or ammonium sulphate, as a

function of the molar fraction xSo2- presented by the organic phases; the E% are also

illustrated that have been obtained in the same conditions but using organic phases

consisting of (TOAH+) 2SO 42 (xs0 2- = 1).

Description of the embodiments

EXAMPLE 1: Synthesis of quaternary ammonium salts:

1.1- Synthesis by acid-base neutralisation: The quaternary ammonium salts with [S0 4 ] 2- as anion can be obtained by simple

reaction of the corresponding tertiary amine with sulphuric acid. Thus, for example, (TOAH+) 2 SO 4 2- is synthesised by adding dropwise 1.19 mL (21.8 mmol) sulphuric acid (98 % mass/volume or 18 mol/L) to 20 mL of a 98% pure solution of

tri-n-octylamine (i.e. 16 g or 44.33 mmol), with stirring and in an ice bath to remove excess heat produced by the neutralisation reaction. The solution obtained in this way is then

solubilised in 50 mL absolute ethanol, then it is all evaporated under reduced pressure (rotary evaporator). After cooling, the resulting product is dried at 60°C in a vacuum for 2

hours to obtain a viscous solution.

1.2 - Synthesis by anionic metathesis:

The quaternary ammonium salts with [NTf 2 ]- as anion can be obtained by anionic

metathesis. This metathesis can be performed:

- by mixing a quaternary ammonium salt comprising the cation ammonium that is intended to be associated with the anion [NTf 2]- with an aqueous solution of lithium

bis(trifluoromethylsulfonyl)imide (LiNTf2, available from ABCR for example), at ambient temperature and with mechanical stirring,

- subjecting the mixture obtained in this way to centrifugation at 8000 rpm for 20 min at 20°C for separating the ionic liquid phase from the aqueous phase,

- washing the ionic liquid phase thus separated 10 times with deionised water,

- solubilising the ionic liquid phase thus washed in 50 mL absolute ethanol,

- evaporating the thus obtained solution under reduced pressure (rotary evaporator), then

- after cooling, drying the resulting product at 60°C in a vacuum for 2 hours. Thus with a quantitative yield the following were synthesised:

- TOAH+NTf 2- by using 10 g (0.012 mol) (TOAH+) 2 SO 4 2- obtained at point 1.1 above and 25 mL of an aqueous solution of LiNTf2 at 1 mol/L;

- a mixture of MTOA*NTf 2- and MTDA*NTf 2-, referred to simply as Aliquat T M 336+NTf 2- in the following, by using 10 g (0.024 mol) Aliquat TM 336 (mixture of

methyltri-n-octylammonium and methyltri-n-decylammonium chlorides available from Alfa Aesar) and 25 mL of an aqueous solution of a 1 mol/L LiNTf 2

. EXAMPLE 2: Densities, water contents and viscosities of useful mixtures according to the invention:

Three types of salt mixtures are prepared, respectively referred to as type 1, type 2 and type 3 mixtures in the following, comprising:

- mixtures of type 1: (TOAH+) 2 SO 4 2- + Aliquat TM 336+NTf 2-;

- mixtures of type 2: (TOAH+) 2 SO 4 2- + MTOA*NTf 2- (this latter salt being available from Sigma-Aldrich);

- mixtures of type 3: (TOAH+) 2 SO 4 2- + TOAH+NTf2-; and in which the molar fraction of the [S0 4 ] 2 - anion varies from 0.15 to 0.75. This molar fraction corresponds to the ratio between the number of moles of the

[S04 ]2 - anion which comprises a mixture of salts and the total number of molesof [S 4 ]2

and [NTf 2 ]- anions that this mixture comprises. It is denoted xSo2- in the following.

All of the salts were dried before being mixed. Their water content is less than 1000 mg/L.

2.1 - Densities: The densities of the type 1, 2 and 3 mixtures are measured before contacting and

after contacting with an aqueous phase comprising 0.1 mol/L sulphuric acid, 0.1 mol/L ammonium sulphate, 250 mg/L uranium(VI) and 250 mg/L iron(Ill), in an aqueous

phase/organic phase (A/O) volume ratio of 2, for 1 hour, at ambient temperature (25°C° 1C) and with stirring.

These measures are carried out using a digital density meter (Anton-Paar DSA

500), at a temperature of 20°C and at atmospheric pressure.

For comparison, the densities of (TOAH+) 2 SO 4 2-, AliquatMT 336+NTf 2-, MTOA+NTf 2 and TOAH+NTf2- were also measured separately in the same conditions.

The results are presented in Tables 1A, 1B, 2A and 2B in the following, Tables 1A and 1B relate to the densities obtained before contacting with the aqueous phase and

Tables 2A and 2B relate to the densities obtained after contacting with the aqueous phase.

Table 1A

Densities (g/cm3) Xso 2- Mixtures of type 1 Mixtures of type 2 Mixtures of type 3 (TOAH+) 2SO 42- (TOAH+) 2SO 42 (TOAH+) 2SO 42 AliquatT M 336+NTf2- MTOA+NTf2- + TOAH+NTf2 0.15 1.0578 1.085 1.0785

0.25 1.0468 1.0617 1.0626

0.5 0.9982 1.043 1.0168

0.75 0.9679 0.9793 0.9746

Table 1B

Densities Salts (g/cm 3 )

(TOAH+) 2SO 42- 0.9214 AliquatTM 336+NTf2 1.0584 MTOA+NTf2 1.0967 TOAH+NTf2 1.0998

Table 2A

Densities

Xso 2- F7 Mixtures of type 1 (g/cm3) Mixtures of type 2 Mixtures of type 3 (TOAH+) 2SO4 - 2 (TOAH+) 2SO4 - 2 (TOAH+) 2SO4 2 TM Aliquat 336NTf 2- MTOA+NTf 2- + TOAH+NTf 2 0.15 1.045 1.075 1.069

0.25 1.038 1.052 1.048

0.5 1.007 1.010 1.006

0.75 0.975 0.965 0.963

Table 2B

Density Salts (g/cm 3 (TOAH+) 2SO 42- ) 0.956 AliquatTM 336+NTf2 1.055 MTOA+NTf2 1.088 TOAH+NTf2 1.091

These tables show that the densities of the three types of mixtures, as measured

before and after contacting with the aqueous phase, fall between the values of the

densities presented by the quaternary ammonium salts entering into the constitution of these mixtures.

They also show that the densities of the three types of mixtures, as measured before and after contacting with the aqueous phase, decrease linearly with the increase of

the molar fraction xSo2- presented by these mixtures.

They also show that the densities that were greater than 1 before contacting with the aqueous phase decrease after contacting with the aqueous phase while the densities

which were less than 1 before contacting with the aqueous phase increase after contacting with the aqueous phase, and this for all three types of mixtures as well as for the salts

entering into the constitution of these mixtures.

2.2 - Water content:

The type 1, 2 and 3 mixtures are contacted with a first aqueous phase consisting

solely of water, in an A/O volume ratio of 2, for 1 hour, at ambient temperature (25°C

1C)and with stirring, then separated from this first aqueous phase. Then, they are contacted with a second aqueous phase comprising 250 mg/L (or

1.005 mmol/L) uranium(VI) and 0.1 mol/L sulphuric acid, in an A/O volume ratio of 2, for 1 hour, at ambient temperature (25°C ±1C) and with stirring, then separated from this

second aqueous phase. The water contents of the three types of mixtures are measured bythe Karl Fischer

(titrator: Metrohm Titrando 809) after each contacting.

For comparison, the same test is conducted in the same conditions with (TOAH+) 2 SO 4 2-, Aliquat T M336+NTf 2-, MTOA+NTf 2- and TOAH+NTf 2- taken separately.

The results are presented in Tables 3A and 3B in the following.

In Table 3A, xSo2- corresponds, as before, to the molar fraction xSo2- presented by type 1, 2 and 3 mixtures.

-j

co M r- l m

m (0 f_ 6 6 6 0) 1-)1-"

4, 0

4- .4-Z

o ? r-.

w 0 r = M .4-

S4

.4-o

m3 m m~ 00 ~ 4-i C.4- 0 m. WCCN c0 j 4 *Lr-4

) o-~ U 0 0

.

coOJ Lfl 00 M .4-' Lfl r- 00 C CT .4-'

.4

(1 3 L 00 00 r,4 00 4 a- (1 < n r 0- C 1 Z "

4- .4-M

~ .4-'

.4- o

0 1 CJ (N M r o 61 C 6

Table 3B

Water content

Salts (mol/L) After contact with After contact with 1staqueous phase 2nd aqueous phase 2- (TOAH+) 2 SO 4 5.57 5.01 AliquatTM 336+NTf2 0.84 0.85 MTOA+NTf2 0.84 0.85 TOAH+NTf2 0.84 0.85

These tables show that after contacting with the second aqueous phase, which

contains uranium, the watercontents of the three types of mixtures are slightly higherthan after contacting with the first aqueous phase which consists solely of water.

They also show that since (TOAH+) 2 SO 4 2- is the salt which charges most water, the

water contents of the three types of mixture increase with the molar xSo2- presented by

these mixtures, and this regardless of the aqueous phase with which the mixtures have

been contacted.

2.3 - Viscosities: The viscosities of type 3 mixtures are measured.

These measurements are carried out by means of a glass capillary kinematic viscometer (Anton-Paar AMVn), at a temperature of 20°C and at atmospheric pressure.

For comparison, the viscosity of (TOAH+) 2 SO 4 2- was also measured in the same

conditions. The results show that the viscosity of (TOAH+) 2 SO 4 2- is 4.9 Pa.s and is therefore

much too high for this salt to be used as the sole extractant in an industrial uranium(VI) extraction process.

On the other hand, adding TOAH+NTf 2- to (TOAH+) 2 SO 4 2- leads to mixtures which have a much lower viscosity, this viscosity being about 1 Pa.s for a mixture having a molar

fraction xSo2- of 0.5 and less than 1 Pa.s for mixtures with a molar fractionXSo2- of less

than 0.5.

EXAMPLE 3: Synergistic effect of useful mixtures according to the invention in the

extraction of uranium(VI) from an aqueous phase of sulphuric acid: The capacity of useful mixtures according to the invention for extracting

uranium(VI) from an aqueous solution of sulphuric acid and the influence of the molar ratio

[S04 ] 2 -/NTf 2 - presented by these mixtures on their extracting properties are assessed by

extraction tests which are performed by using:

- as aqueous phases: aqueous solutions comprising 250 mg/L (or 1.005 mmol/L) U(VI), 250 mg/L Fe(Ill), 0.1 mol/L sulphuric acid and 0.1 mol/L ammonium sulphate; and

- as organic phases: phases which consist of: * either one of the mixtures of type 1, type 2 and type 3 prepared in the

example 2 above; * or one of the mixtures of type 3 prepared in example 2 above but after pre-contacting these mixtures with an aqueous phase comprising 0.1 mol/L sulphuric acid;

* or (TOAH+) 2 SO 4 2-, Aliquat TM 336+NTf2-, MTOA+NTf2- or TOAH+NTf2-.

The extraction tests are performed by using an A/O volume ratio of 2. The aqueous

and organic phases are contacted in 15 mL tubes with mechanical stirring, at ambient temperature (25°C ±1C)for 1 hour. They are then separated from one another by

centrifugation (8000 rpm) for 20 minutes at 20°C. The concentrations of uranium and iron in the aqueous phases are measured by

inductively coupled plasma optical emission spectrometry (ICP-OES) before and after extraction, after dilution with water to bring these concentrations to measurable values

(between 0 and 15 mg/L).

For reference, an extraction test is also carried out in the same conditions but by using as the organic phase, a phase comprising 0.1 mol/L tri-n-octylamine in n-dodecane.

The results of these tests are presented in figures 1, 2 and 3 which illustrate respectively:

- the distribution coefficients of uranium, denoted Du, obtained as a function of the molar fraction xSo2- presented by the organic phases (figure 1);

- the distribution coefficients of iron, denoted DFe, obtained as a function of the molar fraction xSo2- presented by the organic phases (figure 2); and

- the separation factors between uranium and iron, denoted SU/Fe, as a function

of the molar fraction xSo2- presented by the organic phases (figure 3).

According to the conventions in the field of liquid-liquid extractions:

- the distribution coefficient of a metal M (here, U(VI) or Fe(Ill)) is determined by the formula:

Vaq _ _org,f Vaq M]aq,i - [Mlaq,f Dm= X = X Vorg [Moaq,f org [M]aq,f

where:

Vaq is the volume of the aqueous phase,

Vorg is the volume of the organic phase,

[M]org,f is the concentration of M in the organic phase after extraction,

[M]aq,f is the concentration of M in the aqueous phase after extraction, and

[M]aq,i is the concentration of M in the aqueous phase before extraction; - the coefficient of selectivity of a metal M1 (here, U(VI)) relative to a metal M2 (here, Fe(Ill)), denoted SM1/M2, is determined by the formula:

SM1/M2 - DM1

DM2

where:

DM is the distribution coefficient of M1, and DM 2 is the distribution coefficient of M2.

Figure 1also shows that the Du obtained with the useful mixtures according to the invention, in the absence of any pre-contacting of the organic phases with an acidic

aqueous solution, are greater than the sum of the Du obtained with each of the salts entering into the constitution of these mixtures (cf. curves 1, 2 and 3). This is characteristic

of a synergistic effect of the useful mixtures according to the invention in the extraction of uranium(VI).

Figure 1also shows that for the mixtures of types 1, 2 and 3, the highest synergistic

effect is observed for a molar fraction xSo2- of 0.5, obtaining, on the one hand, Du of in

the order of 300 for mixtures of types 1 and 2 and, on the other hand, a Du greater than

600for the mixture of type 3. By comparison, the Du obtained with the reference system is

55 (cf. dotted line R). Figure 1also shows that a pre-contacting (or pre-equilibration) of mixtures of type

3 with an acidic aqueous phase leads, for molar fractions xSo2- greater than 0.5 to Du equal

to or greater than 1000, i.e. much higher than those obtained for the non-pre-equilibrated

mixtures of type 3 (cf. curve 3bis).

Furthermore, figures 2 and 3 show that the useful mixtures according to the invention have a capacity to selectively extract uranium(VI) from iron(III).

EXAMPLE 4: Influence of the A/O volume ratio on the efficacy of useful mixtures according to the invention in extracting uranium(VI) from an aqueous solution of

sulphuric acid: The influence of the A/O volume ratio on the efficacy of the useful mixtures

according to the invention in extracting uranium(VI) from an aqueous solution of sulphuric acid is assessed by extraction tests which are carried out by using:

- as aqueous phases: aqueous solutions comprising 2500 mg/L (or 10.05 mmol/L) U(VI), 0.1mol/L sulphuric acid and 0.1 mol/L ammonium sulphate; and

- as organic phases: phases which are formed either by one of the mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2 - prepared in example 2 above (mixtures of type 3), or

(TOAH+) 2 SO 4 2- or TOAH+NTf 2-.

These extraction tests are carried out in the same operating conditions as those

described in example 3 above, except that volume ratios A/ of 1, 1.4, 1.6, 2 and 3 are used.

The results of these tests are presented in figures 4 and 5 which illustrate, for each of the tested volume ratios A/O:

- the distribution coefficients of uranium(VI), denoted Du, obtained as a

function of the molar fraction xSo2- presented by the organic phases (figure 4), and

- uranium(VI) concentrations measured in the organic phases at the end of

tests, denoted [U]org and expressed in mg/L, as a function of the molar fraction xSo2

presented by these organic phases (figure 5).

Figure 4 shows that the synergistic effect of mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf2- in the extraction of uranium(VI) is observed for the volume ratios A/O greater

than or equal to 1.6, with a synergy peak which is obtained for each of these ratios for extractions carried out with the organic phases consisting of the mixture of (TOAH+) 2 SO 4 2

et de TOAH+NTf 2- having a molar fraction xso,- of 0.75.

Figure 5 shows that, for volume ratios A/O less than or equal to 1.6, the

uranium(VI) concentrations found in the organic phases after extraction reach a plateau for a molar fraction xSo2- of 0.25 whereas, for the volume ratios A/O 2 and 3, this plateau is

only reached for a molar fraction xSo2- of 0.5.

EXAMPLE 5: Loading capacity with uranium(IV) of useful mixtures according to the

invention: The uranium(IV) loading capacity of useful mixtures according to the invention is

assessed by tests consisting of contacting multiple times organic phases formed either by

a mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- having a molar fraction xSo2- of 0.5 or 0,75,

or (TOAH+) 2 SO 4 2 (Xs 0 2- = 1) with an aqueous phase comprising 2500 mg/L (or 10.05

mmol/L) U(VI), 0.1 mol/L sulphuric acid and 1 mol/L ammonium sulphate, in order to

achieve a maximum loading rate of uranium(IV).

The results of these tests are presented in figure 6 which illustrates, for each of the organic phases used, the total concentrations of uranium(VI) as found in these organic

phases, denoted [U]org,tot and expressed in g/L, as a function of the cumulative concentration of uranium(VI), present in aqueous phase, with which the organic phases

have brought into contact, denoted [U]aq,tot and expressed in g/L. This figure shows that the loading capacity of uranium(VI) is respectively 16.6 g/L

for the mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- having a molar fraction xSo2- of 0.5, of

56.3 g/L for the mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- having a molar fraction xSo2- of

0.75 and 63.8 g/L for (TOAH+) 2 SO 4 2 These loading capacities are all much higher than that of an organic phase

comprising 0.1 mol/L tri-n-octylamine in n-dodecane and which is about 7.5 g/L.

Furthermore, figure 6 shows that the loading capacity with uranium(VI) does not

evolve linearly as a function of the molar fraction xSo2-.

EXAMPLE 6: Study of the extraction kinetics of uranium(VI) by useful mixtures according

to the invention: The influence of the aqueous phase/organic phase contacting time (i.e. the time

during which the aqueous phase from which the uranium(VI) is to be extracted is contacted

with the organic phase which is used for stripping this uranium) on the efficacy of useful mixtures according to the invention in extracting uranium(VI) from an aqueous solution of

sulphuric acid is assessed by extraction tests which are carried out by using:

- as aqueous phases: aqueous solutions comprising 250 mg/L (1.005 mmol/L) U(VI), 250 mg/L Fe(Ill), 0.1 mol/L sulphuric acid and 0.1 mol/L ammonium sulphate; and

- as organic phases: phases consisting of either a mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- having a molar fraction xSo2- of 0.5 or 0.75, or (TOAH+) 2 SO 4 2 (Xs 0 2- = 1).

These tests are carried out in the same conditions as those described in example 2 except that the contacting time between the aqueous phases and the organic phases is

varied from 30 min to 4 hours. The results are presented in figure 7 which illustrates, for each of the tested

organic phases, the distribution coefficients of uranium(VI), denoted Du, obtained as a

function of the contacting time between the contact aqueous phase/organic phase. This figure shows that the Du obtained for an organic phase vary very little as a

function of time during which this organic phase has been contacted with the aqueous phase and a contacting time of 30 min is therefore sufficient to achieve extraction

equilibrium.

EXAMPLE 7: Stripping tests: Stripping tests are performed by using:

- as organic phases: phases which consist either of one of the mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- prepared in example 2 above (mixtures of type 3), or (TOAH+) 2 SO 4 2- or TOAH+NTf2-, these organic phases having been previously charged with uranium(VI) by contact with aqueous solutions comprising 250 mg/L (1.005 mmol/L) U(VI),

250 mg/L Fe(III), 0.1 mol/L sulphuric acid and 0.1 mol/L ammonium sulphate; and

- as aqueous phases: aqueous solutions comprising 1 mol/L sodium carbonate (Na 2 CO 3 ) or ammonium sulphate ((NH 4 )SO 4 ).

Stripping tests are carried out by using an A/O volume ratio of 2. The organic and aqueous phases are brought into contact in 15 mL tubes with mechanical stirring, at

ambient temperature (25°C ±1C)for 1 hour. These phases are then separated from one

another by decantation and the concentrations of uranium(VI) in the aqueous phases are measured by ICP-OES, after dilution with water to bring these concentrations to

measurable values (between 0 and 15 mg/L). The results are shown in figure 8 which shows, for each type of aqueous phase

used (Na 2 CO 3 or (NH 4 ) 2 SO 4 ), the percentages of uranium stripping, denoted E%, obtained as a function of the molar fraction xSo2- presented by the organic phases.

The percentage of uranium stripping is determined by the formula:

Vaq [Ulaq,f E(%) = 100 x V x[a org [U]org,ini

wherein:

Vaq is the volume of the aqueous phase,

Vorg is the volume of the organic phase,

[M]aq,f is the concentration of the uranium in the aqueous phase after stripping,

[M]org,ini is the concentration of the uranium in the organic phase after stripping.

Figure 8 shows that, when uranium is stripped by means of an aqueous solution

of sodium carbonate, E% is greater than 92% and increases slightly until the molar fraction

XSo2- 4 presented by the mixtures of (TOAH+) 2 SO 4 2- and TOAH+NTf 2- are 0.75. For this molar

fraction, more than 96% uranium is stripped from the organic phase in a single contact with the aqueous solution of sodium carbonate.

It also shows that when uranium is stripped by means of an aqueous solution of

ammonium sulphate, E%, which is 75% for a molar fraction xSo2- equal to 0.15, decreases as the molar fraction xSo2- increases. E% is less than 10% for the molar fractionsXSo2 above 0.5. The use of an aqueous solution of a carbonate such as sodium carbonate therefore appears to be preferable for effectively stripping uranium(VI) from an organic phase consisting of a mixture of (TOAH+) 2 SO 4 2- and TOAH+NTf 2-.

References

[1] Coleman et al., Industrial & Engineering Chemistry 1958, 50, 1756-1762

[2] Blake et al., Oak Ridge National Laboratory Report, 18 December 1956

[3] WO-A-2014/139869

[4] WO-A-2016/156591

[5] Rogers et al., in: K.C. Liddell, D.J. Chaiko (Eds.), MetalSeparation Technologies Beyond

2000, The Minerals, Metals & Materials Society, Warrendale, PA, 1999, 139-147

[6] Dai et al., Journal of the Chemical Society, Dalton Transactions 1999, 8, 1201-1202

[7] Zhang et al., Separation Science and Technology 2014, 49, 1895-1902

[8] Xie et al., Scientific Reports 2017, 7, 15735

[9] Scheiflinger et al., Journal of Radioanalytical and Nuclear Chemistry 2019, 322, 1841 1848

[10] El Sayed, Hydrometallurgy 2003, 68, 51-56

[11] Amaral et al., Minerals Engineering 2010, 23, 498-503

Claims (19)

1. Use of a mixture of quaternary ammonium salts comprising:

- at least one salt of formula (1): ([NR1R 2R 3 R 4]+)mAj m -, and

- at least one salt of formula (2): [NR 5 6R R 7 R8 ]*A 2 -,

wherein: R 1 and R 5, identical or different, represent a hydrogen atom or a linear or branched

alkyl group comprising 1 to 12 carbon atoms; R 2 , R3 , R4 , R ,6 R 7 and R ,8 identical or different, represent a linear or branched alkyl

group comprising 6 to 12 carbon atoms;

Aim - represents a Cl-, Br-, I-, [C104]-, [SO 4 ] 2 -, [HSO 4]-, [NO 3]-, [SCN]-, [N(CN) 2]-,

[CH 3CO2 ]-, [P04 ]3-, [HPO4 ] 2-, [H 2 PO 4 ]-, [MeHPO 4]-, [MeH 2 PO 3]-, [P(OCH 2CH 3) 20 2 ]- anion;

m corresponds to the oxidation state of the anion Aim-; and

A 2- represents a [P(F 3)(CF 2CF 3)3 ]-, [N(CF 3SO 2) 2]-, [PF]-, [N(SO ]-, [CH 3(CH 2)xCO 2] 2 F)2

where x is an integer ranging from 6 to 18, [BF 4]- or [CF 3SO 3]- anion;

as an extractant, for extracting uranium(VI) from an aqueous solution of sulphuric acid.

2. Use according to claim 1, wherein R1 and R5 , identical or different, represent

a hydrogen atom or a linear or branched alkyl group comprising 1 to 4 carbon atoms, the

alkyl group being preferably linear and, still better, a methyl or ethyl group.

3. Use according to claim 1 or claim 2, wherein R 2, R 3, R4, R6 , R 7 and R8 , identical or different, represent a linear or branched alkyl group comprising 8 to 10 carbon atoms,

the alkyl group being preferably linear and, still better, an n-octyl or n-decyl group.

4. Use according to any of claims 1 to 3, wherein [NR1R2R3R 4]' and [NR 5 6R R 7Rs]*,

identical or different, represent a tri-n-octyl-ammonium, methyltri-n-octylammonium,

ethyltri-n-octylammonium, tri-n-decyl-ammonium, methyltri-n-decylammonium, ethyltri

n-decylammonium, n-octyldi-n-decyl-ammonium, methyl-n-octyldi-n-decylammonium, ethyl-n-octyldi-n-decylammonium, di-n-octyl-n-decylammonium, methyldi-n-octyl-n decylammonium or ethyldi-n-octyl-n-decylammonium cation.

5. Use according to claim 4, wherein [NR 1 R 2 R 3R 4]' and [NR5 R6 R 7 Rs]*, identical or

different, represent a tri-n-octylammonium, methyltri-n-octylammonium, tri-n

decylammonium, methyltri-n-decylammonium, methyl-n-octyl-di-n-decylammonium or methyldi-n-octyl-n-decylammoniumcation.

6. Use according to any of claims 1 to 5, wherein A- is [S 4 ] 2- and A 2- is selected from [N(CF 3SO 2) 2 ]-, [N(SO 2F)2 ]-, [PF 6 ]-, A 2- being preferably [N(CF 3SO 2) 2]-.

7. Use according to any of claims 1 to 6, wherein the salt of formula (1) is tri-n octylammonium sulphate, methyltri-n-octylammonium sulphate, tri-n-decylammonium

sulphate, methyltri-n-decyl-ammonium sulphate, methyl-n-octyldi-n-decylammonium sulphate or methyldi-n-octyl-n-decylammonium sulphate.

8. Use according to any of claims 1 to 7, wherein the salt of formula (2) is tri-n

octylammonium bis(trifluoromethylsulfonyl)imide, methyltri-n-octylammonium bis(trifluoromethylsulfonyl)imide, tri-n-decylammonium bis(trifluoro

methylsulfonyl)imide, methyltri-n-decylammonium bis(trifluoromethylsulfonyl)imide, methyl-n-octyl-di-n-decylammonium bis(trifluoromethylsulfonyl)imide or methyldi-n octyl-n-decylammonium bis(trifluoromethylsulfonyl)imide.

9. Use according to any of claims 1 to 8, wherein the mixture of salts comprises

several salts of formula (1) that are different from one another and/or several salts of formula (2) that are different from one another.

10. Use according to any of claims 1 to 9, wherein the molar fraction of the anion

of the salt of formula (1) or of the totality of the anions of the salts of formula (1) is between

0.25 and 0.75, preferably between 0.4 and 0.6, in the mixture of salts.

11. Use according to any of claims 1 to 10, which comprises at least one contact of the aqueous solution with an organic solution comprising the mixture of salts, followed

by a separation of the aqueous solution from the organic solution.

12. Use according to claim 11, wherein the mixture of salts represents 100% in weight of the weight of the organic solution.

13. Use according to any of claims 1 to 12, wherein the solution of sulphuric acid

further comprises an inorganic sulphate.

14. Use according to any of claims 1 to 13, wherein the aqueous solution of

sulphuric acid is issued from the leaching of a uranium ore with sulphuric acid.

15. Use according to claim 14, wherein the aqueous solution of sulphuric acid

comprises 0.01 mol/L to 0.5 mol/L sulphuric acid, 0.1 g/L to 10 g/L uranium(VI) and, optionally, 0.1 mol/L to 2 mol/L sulphate ions.

16. Mixture of quaternary ammonium salts, characterised in that it is:

- a mixture of tri-n-octylammonium sulphate and tri-n-octylammonium bis(trifluoromethylsulfonyl)imide,

- a mixture of methyltri-n-octylammonium sulphate and methyltri-n octylammonium bis(trifluoromethylsulfonyl)imide,

- a mixture of tri-n-octylammonium sulphate and methyltri-n-octylammonium bis(trifluoromethylsulfonyl)imide,

- a mixture of methyltri-n-octylammonium sulphate and tri-n-octylammonium

bis(trifluoromethylsulfonyl)imide,

- a mixture of tri-n-octylammonium sulphate, methyltri-n-octylammonium bis(trifluoromethylsulfonyl)imide and methyltri-n-decylammonium

bis(trifluoromethylsulfonyl)imide, or

- a mixture of tri-n-octylammonium sulphate, methyltri-n-octylammonium bis(trifluoromethyl-sulfonyl)imide, methyltri-n-decylammonium

bis(trifluoromethylsulfonyl)imide, methyl-n-octyldi-n-decylammonium bis(trifluoromethylsulfonyl)imide and methyldi-n-octyl-n-decylammonium

bis(trifluoromethylsulfonyl)imide.

17. Method for recovering the uranium(VI) present in an aqueous solution of

sulphuric acid issued from the leaching of a uranium ore with sulphuric acid, which comprises the steps of:

a) extracting uranium(VI) from the aqueous solution by at least one contact of the aqueous solution with an organic solution comprising a mixture of quaternary

ammonium salts as defined in any of claims 1to 10, followed by a separation of the aqueous solution from the organic solution; and

b) stripping uranium(VI) from the organic solution obtained at the end of step a) by at least one contact of the organic solution with an aqueous solution comprising a

carbonateora mixture of carbonates, followed bya separation of the organic solution from

the aqueous solution.

18. Method according to claim 17, wherein the mixture of salts represents 100% in weight of the weight of the organic solution.

19. Method according to claim 17 or claim 18, wherein the aqueous solution of

sulphuric acid comprises 0.01 mol/L to 0.5 mol/L sulphuric acid, 0.1 g/L to 10 g/L uranium(VI) and, optionally, 0.1 mol/L to 2 mol/L sulphate ions.