AU2021390705A2 - Method for selectively recovering rare earth metals and uranium(vi) from an aqueous solution of phosphoric acid - Google Patents

Abstract

The invention relates to a method for recovering rare earth metals and uranium(VI) present in an aqueous phosphoric acid solution by liquid-liquid extraction that enables not only highly efficient co-extraction of the rare earth metals and the uranium(VI) from this aqueous solution but also subsequent selective recovery of the rare earth metals and of the uranium(VI). Applications: treatment of aqueous solutions from the leaching of natural phosphates by sulfuric acid in order to utilize the rare earth metals and uranium(VI) present in these aqueous solutions.

Description

METHOD FOR SELECTIVELY RECOVERING RARE EARTH ELEMENTS AND URANIUM(VI) FROM AN AQUEOUS SOLUTION OF PHOSPHORIC ACID

Description

Technical field

The invention relates to the field of mining. More specifically, the invention relates to a method for recovering the rare-earth

elements and the uranium(VI) present in an aqueous solution of phosphoric acid by

liquid-liquid extraction which not only allows very efficiently co-extracting the rare-earth elements and the uranium(VI) from this aqueous solution, but also selectively recovering

afterwards the rare-earth elements on the one hand, and the uranium(VI) on the other hand.

The invention finds particular application in the treatment of aqueous solutions

resulting from the leaching of natural phosphates by sulphuric acid in order to recover the rare-earth elements and the uranium(VI) present in these aqueous solutions.

Prior art

The rare-earth elements group together metals that are characterised by similar properties, namely scandium (Sc), yttrium (Y) as well as all lanthanides, these

corresponding to the 15 chemical elements listed in the periodic table of the elements of Mendeleev that have an atomic number ranging from 57 for lanthanum (La) to 71 for

lutetium (Lu). The close properties of the rare-earth elements are related to their particular

electronic configuration and, in particular, to their unsaturated 4f electron subshell which

confers unique chemical, structural and physical properties thereon. These properties are used in varied and sophisticated industrial applications: metallurgy, catalysis, glass

industry, optics, ceramics, luminescence, magnetism, electronics, etc.

Consequently, rare-earth elements are part of the so-called "technological"

metals, the supply of which is strategic, but also threatened by the effect of the growth in world demand for these particular metals.

In contrast with what their name suggests, rare-earth elements are widely distributed in nature. Indeed, they are present in many natural ores and, in particular, in

natural phosphates (or phosphate ores) which are exploited for the manufacture of phosphoric acid and phosphate fertilisers up to a hundred to several thousand of ppm.

Besides containing recoverable rare-earth elements, natural phosphates contain uranium(VI) in the range of ten to a few hundred ppm. The recovery potential of the

uranium contained in these phosphate ores is 14,000 t/year, which represents a non

negligible source of uranium supply. The treatment of natural phosphates for the production of phosphoric acid and

phosphate fertilisers starts with an acid attack, or leaching, of these phosphates, crushed and ground beforehand, by an acid, which is most often sulphuric acid which transforms

tricalcium phosphate into phosphoric acid H 3 P0 4 , 30% phosphate anhydride P 2 05 , as well as insoluble calcium sulphate (gypsum).

This acid attack solubilises, partially or totally, the metals present in the natural phosphates, including the rare-earth elements and uranium, which are therefore found in

the aqueous solution of phosphoric acid. Similarly, a large number of other metals are

also solubilised, such as iron, vanadium, molybdenum, aluminium, etc. One of the means for recovering one or more metallic element(s) of interest

present in an acidic aqueous solution consists in subjecting this aqueous solution, after filtering and concentration, to a hydrometallurgical treatment based on the liquid-liquid

extraction technique. This technique consists in bringing the aqueous solution into contact with an organic solution, also called solvent, comprising one or more extractant(s)

in a diluent, to obtain a transfer of the element(s) of interest in the organic solution; such an extraction should preferably be simultaneously efficient and selective with respect to

the other metals present in the aqueous solution. Then, the element(s) of interest is/are

stripped afterwards from the organic solution by a reverse mechanism.

Given the strategic nature shown not only by the supply of rare-earth elements

but also that of uranium, it would be desirable to have an industrialisable process which allows recovering very efficiently both rare-earth elements and uranium present in an

aqueous solution of phosphoric acid resulting from the leaching of natural phosphates by sulphuric acid and that being so, with not only good selectivity with respect to the other

metals present in this type of solution but also a good rare-earth elements/uranium selectivity since rare-earth elements and uranium do not have the same industrial uses.

Recent studies concerning the valorisation of uranium derived from phosphoric acid production routes have allowed achieving major advances through the development

of bifunctional compounds, comprising both an amide function and a phosphonic acid or

phosphonate function. Amongst these compounds, which are described in PCT international application WO-A-2013/167516, hereinafter reference [1], mention may be

in particular of butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, or DEHCNPB, which shown excellent properties for the extraction of uranium(VI) from an aqueous solution of phosphoric acid with high distribution coefficient of uranium(VI) and

selectivity with respect to iron. Moreover, some bifunctional compounds described in reference [1] have been

studied in a mixture with a surfactant such as sodium bis(2-ethylhexyl)sulfosuccinate, or AOT, for the extraction of rare-earth elements from an aqueous solution of phosphoric

acid. It has been demonstrated in the PCT international application WO-A-2018/051026, hereinafter reference [2], that such a mixture allows extracting very effectively the rare earth elements, both heavy and light, from an aqueous solution of phosphoric acid

comprising rare-earth elements and iron - thanks to the existence of a synergistic effect between the bifunctional compounds and the surfactant - and that being so, with an

excellent selectivity with respect to iron. Yet, in the context of their works, the Inventors have noticed, on the one hand,

that the synergistic effect and the selectivity with respect to iron which are reported in reference [2] for the extraction of the rare-earth elements by the mixtures described in

this reference are totally preserved when the aqueous solution of phosphoric acid from

which these rare-earth elements are extracted also comprises uranium(VI), and, on the other hand, that the extraction of uranium(VI) from an aqueous solution of phosphoric acid by the mixtures described in reference [2] is quantitative while being selective with respect to iron.

Furthermore, they have noticed that it were possible, once the rare-earth elements and the uranium(VI) have been co-extracted from said aqueous solution of

phosphoric acid by said mixtures, to selectively strip them from this solution so that it is possible to separately recover the rare-earth elements on the one hand, and the

uranium(VI) on the other hand. And it is on these experimental findings that the present invention is based.

Disclosure of the invention

Hence, an object of the invention is a method for recovering the rare-earth

elements and the uranium(VI) present in an aqueous solution Al of phosphoric acid, which comprises:

a) a co-extraction of the rare-earth elements and the uranium(VI) from the

aqueous solution Al by at least one contacting of the aqueous solution Al with a water immiscible organic solution, which comprises an extractant in an organic diluent, followed

by a separation of the aqueous solution Al from the organic solution, the extractant of the organic solution comprising:

- a compound of formula (I) hereinafter:

0 0 R1 -,OR4 "N YN'R5

(I) wherein: R' and R2 , identical or different, represent a linear or branched alkyl group comprising from 6 to 12 carbon atoms;

R 3 represents a linear or branched alkyl group comprising from 1 to 12 carbon

atoms, or a saturatedorunsaturated, monocyclic hydrocarbon group comprising from 3 to 8 carbon atoms and possibly one or more heteroatom(s); R 4 and R 5, identical or different, represent a hydrogen atom or a linear or branched alkyl group comprising from 2 to 8 carbon atoms; and

- an anionic or zwitterionic surfactant; then b) a separation of the rare-earth elements from the uranium(VI) present in the

organic solution resulting from a), this separation comprising:

- either a stripping of the rare-earth elements from the organic phase resulting from a) followed by a stripping of uranium (VI) from the organic phase resulting

from the stripping of the rare-earth elements;

- or a stripping of uranium(VI) from the organic phase resulting from a) followed by a stripping of the rare-earth elements from the organic phase resulting from the stripping of uranium(VI).

In the foregoing and next, by "linear or branched alkyl group comprising from 6 to 12 carbon atoms", it should be understood any alkyl group which comprises a total of

6, 7, 8, 9, 10, 11 or 12 carbon atoms and which has a straight chain or one or more

branch(es). Analogously:

- by "linear or branched alkyl group comprising from 1 to 12 carbon atoms", it should be understood any alkyl group which comprises a total of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 carbon atom(s) and which has a straight chain or one or more branch(es); and

- by "linear or branched alkyl group comprising from 2 to 8 carbon atoms", it should be understood any alkyl group which comprises a total of 2, 3, 4, 5, 6, 7 or 8

carbon atoms and which has a linear chain or one or more branch(es). By "saturated or unsaturated, monocyclic hydrocarbon group comprising from 3

to 8 carbon atoms and possibly one or more heteroatom(s)", it should be understood any cyclic hydrocarbon group which comprises only one ring and whose cycle comprises 3, 4,

5, 6, 7 or 8 carbon atoms. This ring may be saturated or, on the contrary, contain one or more double or triple bond(s), and may comprise one or more heteroatom(s) or be

substituted by one or more heteroatom(s) or by one or more substituent(s) comprising a heteroatom, this or these heteroatom(s) typically being N, 0 or S. Thus, this group may in

particular be a cycloalkyl, cycloalkenyl or cycloalkynyl group (for example, a cyclopropyl, cyclopentyl, cyclohexyl, cyclopropenyl, cyclopentenyl or cyclohexenyl group), a saturated

heterocyclic group (for example, a tetrahydrofuryl, tetrahydrothiophenyl, pyrrolidinyl or piperidinyl group), an unsaturated but non-aromatic heterocyclic group (for example,

pyrrolinyl or pyridinyl), an aromatic group or a heteroaromatic group.

In this respect, it is specified that by "aromatic group", it should be understood any group whose ring meets HOckel's rule of aromaticity and therefore has a number of

delocalised electrons a equal to 4n + 2 (for example, a phenyl or tolyl group), whereas by "heteroaromatic group", it should be understood any aromatic group as it has just been

defined but whose ring comprises one or more heteroatom(s), this heteroatom or these heteroatoms typically being selected from among the atoms of nitrogen, oxygen and

sulphur (for example, a furyl, thiophenyl or pyrrolyl group). Moreover, the terms "aqueous solution" and "aqueous phase" are equivalent

and interchangeable, just like the terms "organic solution" and "organic phase" are equivalent and interchangeable.

In accordance with the invention, in the formula (1) hereinabove, it is preferred that R' and R2 , which are identical or different, are a linear or branched alkyl group

comprising from 8 to 10 carbon atoms.

What is more, it is preferred that R' and R 2 are identical to each other and are a branched alkyl group comprising from 8 to 10 carbon atoms, the 2-ethylhexyl group being

quite particularly preferred. Moreover, in the formula (1) hereinabove, R3 is advantageously a linear alkyl

group comprising from 1 to 12 carbon atoms, or a 6-membered monocyclic aromatic group, preferably a phenyl or ortho-, meta- or para-tolyl group.

What is more, R 3 is preferably a methyl, n-octyl or phenyl group.

Finally, in the formula (1) hereinabove, R 4 is preferably a linear or branched alkyl

group comprising from 2 to 8 carbon atoms and, still better, 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 quite particularly preferred, whereas R 5 is preferably a hydrogen atom.

Compounds of formula (1) hereinabove which feature these characteristics are in particular:

- ethyl 1-(N,N-diethylhexylcarbamoyl)ethylphosphonate, denoted DEHCEPE,

which corresponds to formula (1) hereinabove wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is a methyl group, R 4 is an ethyl group and R5 is a hydrogen atom;

- ethyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted DEHCNPE,

which corresponds to formula (1) hereinabove wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is an n-octyl group, R 4 is an ethyl group and R5 is a hydrogen atom;

- butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted DEHCNPB, which corresponds to formula (1) hereinabove wherein R1 and R 2 are a 2

ethylhexyl group, R 3 is an n-octyl group, R 4 is an n-butyl group and R5 is a hydrogen atom;

- butyl 1-(N,N-dioctylcarbamoyl)nonylphosphonate, denoted DOCNPB, which corresponds to formula (1) hereinabove wherein R1, R2 and R 3 are an n-octyl group,

R 4 is an n-butyl group and R 5 is a hydrogen atom; and

- ethyl 1-(N,N-diethylhexylcarbamoyl)benzylphosphonate, denoted DEHCPBE, which corresponds to formula (1) hereinabove wherein R1 and R 2 are a 2

ethylhexyl group, R 3 is a phenyl group, R 4 is an ethyl group and R5 is a hydrogen atom.

Amongst these compounds, DEHCNPB is quite particularly preferred. As indicated before, the surfactant may be selected from among cationic or

zwitterionic (also called amphoteric) surfactants provided that it allows forming a micro emulsion in an organic solution immiscible with water, i.e. a water-in-oil dispersion which

is isotropic and thermodynamically stable. For the selection of such a surfactant, the reader is invited to refer to the books

"Chemistry and Technology of Surfactants" (Richard J. Farn, 2006 Blackwell Publishing Ltd,

ISBN-13: 978-14051-2696-0) and "Self-Organised Surfactant Structures" (Tharwat F.

Tadros, 2011, John Wiley & Sons, ISBN: 978-3-527-63265-7), hereinafter references [3] and [4].

Preferably, the surfactant is an anionic surfactant, i.e. a surfactant which dissociates in aqueous solution into an amphiphilic cation and an anion, in which case it

could in particular be selected from among salts (sodium, potassium, magnesium, calcium, ammonium, ... ) of alkylsulfates, alkylarylethersulfates, alkylethersulfates,

alkylarylethersulfates, alkylethercarboxylates, alkylarylethercarboxylates, alkylsulfosuccinates, alkylarylsulfosuccinates, paraffin sulfonates, alkylisothionates,

alkylsarcosinates, the alkyl group(s) of these compounds preferably comprising from 6 to

24 carbon atoms and, still better, from 6 to 12 carbon atoms, and the aryl group(s) of these compounds preferably being phenyl group(s).

In accordance with the invention, the surfactant is preferably a salt of a di(C6

C 1 2)alkylsulfosuccinate such as a salt, for example of an alkali metal (Na, K, ... ), of di (n hexyl)sulfosuccinate, di(2-ethylhexyl)sulfosuccinate, di(n-octyl)sulfosuccinate (also called docusate), di(i-octyl)sulfosuccinate or di(n-decyl)sulfosuccinate, the salts of di(2 ethylhexyl)sulfosuccinate and, in particular, the sodium salt of di(2

ethylhexyl)sulfosuccinate, also called AOT and whose formula is recalled hereinafter,

being quite particularly preferred.

0 CH 3

H 3C O O CH3 O=s=O0O H 3C ONa Moreover, the extractant preferably has a molar ratio of the compound of formula (1) to the surfactant ranging from 20/80 to 80/20 and, still better, from 40/60 to

60/40, a molar ratio of 60/40 being quite particularly preferred in the case of an extractant comprising DEHCNPB as a compound of formula (1) and AOT as a surfactant since it leads to a maximum synergistic effect in the extraction of rare-earth elements and

uranium(VI) from an aqueous solution of phosphoric acid.

In accordance with the invention, the organic diluent that comprises the organic

solution used in step a) may be any aliphatic or aromatic hydrocarbon or any mixture of aliphatic or aromatic hydrocarbons, provided that the hydrocarbon or the mixture of

hydrocarbons is not water-miscible. Thus, it may in particular consist of n-octane, iso

octane, n-decane, n-dodecane, hydrogenated tetrapropylene, isoparaffins such as those

marketed under the trade name Isane (IsaneTM IP-175, IsaneTM IP-185, etc.), or toluene. Moreover, the organic solution used at step a) preferably comprises from 0.005

mol/L to 1 mol/L and, still better, from 0.1 mol/L to 0.5 mol/L of extractant. In accordance with the invention, the stripping of the rare-earth elements from

the organic solution resulting from step a) or from the organic solution resulting from the stripping of uranium(VI) preferably comprises at least one contacting of this organic

solution with an aqueous solution A2 followed by a separation of the organic solution from the aqueous solution, the aqueous solution A2 comprising at least one compound selected from among:

- oxalic acid and its salts such as sodium, potassium or ammonium oxalates;

- aminopolycarboxylic acids such as nitrilotriacetic acid (or NTA), ethylenediaminetetraacetic acid (or EDTA), diethylenetriaminepentaacetic acid (or DTPA)

and the salts thereof such as their alkali metal salts, for example, sodium or potassium;

- nitrilotrimethylphosphonic acid (or NTMP);

- hydroxamic acids such as acetohydroxamic acid;

- hydrophilic diglycolamides, i.e. diglycolamides of formula: R(R)N-C(O) CH 2-0-CH 2-C()-N(R1 2)R or R 10(R)N-C()-CH 2-0-CH 2-COOH wherein R 10 , R, R 1 2 and R 3, which may be identical or different, are typically alkyl groups and the total number of

carbon atoms is at most equal to 16; such hydrophilic diglycolamides are, for example, N,N,N',N'-tetramethyldiglycolamide (or TMDGA), N,N,N',N'-tetraethyldiglycolamide (or

TEDGA), N,N,N',N'-tetrapropyl-diglycolamide (or TPDGA) and N,N-dipropyldiglycolamic acid (or DPDGAc).

The aqueous solution A2 preferably comprises an oxalate, which is

advantageously a sodium, potassium or ammonium oxalate and, still better, ammonium oxalate which is preferably used at a concentration ranging from 0.05 mol/L to 2 mol/L and, still better, of 0.2 mol/L. Like for the stripping of uranium(VI) from the organic solution resulting from step a) or from the organic solution resulting from the stripping of the rare-earth elements, it preferably comprises at least one contacting of this organic solution with an aqueous solution A3 followed by a separation of the organic solution from the aqueous solution, the aqueous solution A3 comprising a carbonate or a mixture of carbonates.

In particular, the carbonate(s) may be selected from among alkali metal carbonates and, in particular, from among sodium and potassium carbonates, alkaline

earth metal carbonates and, in particular, from among calcium and magnesium

carbonates, as well as from among the carbonates of non-metallic elements such as ammonium carbonate.

The aqueous solution A3 preferably comprises ammonium carbonate which is preferably used at a concentration ranging from 0.05 mol/L to 2 mol/L and, still better, of 0.5 mol/L.

As indicated before, it is possible to firstly strip the rare-earth elements from the organic solution resulting from step a), then to secondarily strip uranium(VI) from the

organic solution resulting from the stripping of the rare-earth elements or, conversely, to firstly strip uranium(VI) from the organic solution resulting from step a), then to

secondarily strip the rare-earth elements from the organic solution resulting from the stripping of uranium(VI). Nonetheless, in the context of the invention, it is preferred to firstly strip the

rare-earth elements from the organic solution resulting from step a), then to subsequently strip uranium(VI) from the organic solution resulting from the stripping of

the rare-earth elements. In accordance with the invention, the aqueous solution Al is preferably a

solution resulting from the leaching of a natural phosphate by sulphuric acid. The concentration of phosphoric acid in this solution may vary within a wide

range and may, in particular, range from 1 mol/L to 8 mol/L of phosphoric acid, the range

from 5 mol/L to 8 mol/L being particularly preferred.

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

following complementary description. It goes without saying that this complementary description is given only as an

illustration of the object of the invention and in no way forms a limitation of this object.

Detailed disclosure of particular modes of implementation

The experimental results which are reported in the examples hereinafter have been obtained using acidic aqueous phases comprising two light rare-earth elements,

namely lanthanum (La) and neodymium (Nd), three heavy rare-earth elements, namely europium (Eu), dysprosium (Dy) and ytterbium (Yb), uranium (U) and iron (Fe).

The distribution coefficients, the separation factors and the stripping yields

which are reported in the following examples, have been determined in accordance with the conventions of the liquid-liquid extraction field, namely:

- the distribution coefficient of a metallic element M, denoted DM, between

two phases, respectively organic and aqueous, is determined by the following equation:

[M]org,f_ [M]aq,i - [M]aq,f

[M]aq,f [M]aq,f

wherein:

[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 separation factor of a metallic element M1 with respect to a metallic element M2, denoted FSM1/M2, is determined by the equation:

- DM1 FSM1/M2 DM2

wherein:

DMl is the distribution coefficient of M1, and DM 2 is the distribution coefficient of M2; whereas

- the stripping yield of a metallic element M, denoted RM, is determined by the following equation:

RM - [M]aq,desex,f [M]aq,desex,f__ [M]aq,desex,f

[M]org,desex,i [M]org,f [M]aq,i - [M]aq,f

wherein:

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

[M]org,dsex,i is the concentration of M in the organic phase before stripping, and

[M]org,f, [M]aq,i and [M]aq,f have the same meaning as before.

EXAMPLE 1: Co-extraction of uranium(VI) and the rare-earth elements from an aqueous

solution of phosphoric acid by a DEHCNPB/AOT mixture in iso-octane

As reported in reference [2], the DEHCNPB/AOT mixture has demonstrated an

optimum synergism in the extraction of rare-earth elements at the 60/40 molar ratio. The preservation of this optimum synergism of extraction of the rare-earth

elements in the presence of uranium(VI) as well as the possibility of quantitatively

extracting uranium(VI) by a DEHCNPB/AOT mixture have been demonstrated by extraction tests which have been performed using:

- as aqueous phases: solutions obtained by dissolving five salts of rare-earth elements in the oxidation state (Ill)in the respective forms La(NO3) 3, Nd(NO 3)3, Eu(NO3)3, Dy(NO 3)3 and Yb(NO 3)3, at a rate of 0.25 g/L of rare-earth elements, an iron salt in the

oxidation state (Ill) in form of Fe(NO 3)3, at a rate of 2.5 g/L of Fe and, with or without uranium(VI) resulting from a stock solution of uranyl nitrate U0 2 (NO 3 ) 2 at 10 g/L in 4%

nitric acid, at a rate of 0.25 g/L of U, in solutions comprising 5 mol/L or 8 mol/L of

phosphoric acid in ultra-pure water (i.e. Milli-Q water, resistivity > 18 Q/cm at 25C); and - as organic phases: solutions comprising 0.1 mol/L of an extractant in iso

octane, this extractant consisting either of DEHCNPB alone, or of AOT alone, or of a

mixture of DEHCNPB and AOT at the 60/40 molar ratio. The extraction tests have been carried out using an aqueous phase/organic phase

(A/O) volume ratio of 1. The aqueous and organic phases have been brought into contact for 1 hour at room temperature (22°C) with a vibrating stirrer (550 rpm), after which they have been centrifuged (7,000 rpm) for 5 minutes at 22°C then separated from each other.

The concentrations of the rare-earth elements, uranium and iron in the aqueous

phases have been measured by inductively-coupled plasma optical emission spectrometry (ICP-OES) before and after extraction.

The results of these tests are reported in tables 1, 2 and 3 hereinbelow, in which are indicated:

- Tables 1 and 2: the distribution coefficients of the rare-earth elements, uranium and iron, denoted DM, as a function of the molar fraction of AOT, denotedXAOT,,

of the extractant as obtained for a concentration of the phosphoric acid of 5 mol/L and 8 mol/L respectively; XAOT = 0 for the extractant consisting of DEHCNPB alone;XAOT = 1 for the extractant consisting of AOT alone whereas XAOT= 0.4 for the extractant consisting of

the mixture of DEHCNPB and AOT at the 60/40 molar ratio;

- Table 3: the separation factors of the rare-earth elements and uranium compared to iron, denoted FSM/Fe, as obtained at the two acidities for XAOT- 0,4. Table 1

H 3 PO 4 Dm 5 mol/L XAOT La Nd Eu Dy Yb U Fe

0 0.1 0.2 0.3 0.3 0.5 - 0.02 Rare-earth 0.4 5.7 4.6 2.5 1.2 0.8 - 0.04 elements 1 0.4 0.2 0.1 0.1 0.1 - 0.01

0 0.1 0.3 0.3 0.3 0.5 > 115 0.01 Uranium(VI) + rare-earth 0.4 6.1 4.7 2.5 1.3 0.7 > 115 0.04 elements 1 0.7 0.3 0.2 0.2 0.1 0.0 0.01

Table 2

H 3 PO 4 Dm 8mol/L XAOT La Nd Eu Dy Yb U Fe

0 0.0 0.1 0.1 0.1 0.1 - 0.01 Rare-earth 0.4 4.6 2.8 1.5 0.7 0.3 - 0.03 elements 1 0.6 0.3 0.2 0.2 0.1 - 0.04

0 0.0 0.1 0.1 0.1 0.1 > 115 0.01 Uranium(VI) + rare-earth 0.4 4.7 2.6 1.3 0.6 0.3 > 115 0.04 elements 1 0.5 0.3 0.2 0.2 0.1 0.0 0.06

Table 3

Uranium(VI) + FSM/Fe rare-earth XAOT elements La Nd Eu Dy Yb U

H 3 PO4 5 mol/L 0.4 175 135 73 36 21 > 2,500 H 3 PO4 8 mol/L 0.4 105 58 29 14 6 > 2,500

These results show that with or without uranium(VI) in aqueous solution and regardless of the concentration of phosphoric acid (5 mol/L or 8 mol/L) in this solution,

the distribution coefficients obtained for the rare-earth elements with an extractant consisting of a mixture of DEHCNPB and AOT are greater than the sum of the distribution

coefficients that are obtained by, on the one hand, an extractant consisting of DEHCNPB alone, and, on the other hand, an extractant consisting of AOT alone. Hence, a synergistic

effect of the mixture of DEHCNPB and AOT on the extraction of all of these rare-earth

elements is obtained regardless of the composition of the aqueous solution. In addition, they show that the extraction of uranium is quantitative (Du > 115)

and, herein again, regardless of the concentration of phosphoric acid in the aqueous solution.

They also show that the separation factors of the rare-earth elements compared to iron are high, despite the presence of uranium(VI) in the aqueous solution. The

selectivity of extraction of the uranium(VI) compared to iron is very high, despite the presence of the rare-earth elements (FSU/Fe > 2,500) and regardless of the concentration

of the phosphoric acid in the aqueous solution.

EXAMPLE 2: Co-extraction of the uranium(VI) and the rare-earth elements from an aqueous solution of phosphoric acid by a DEHCNPB/AOT mixture in toluene

Extraction tests have been carried out using:

- as aqueous phases: phases comprising rare-earth elements, uranium (VI), iron and phosphoric acid at a concentration of 5 mol/L and having been obtained as

described in Example 1 hereinbefore; and

- as organic phases: phases comprising 0.1 mol/L of a mixture of DEHCNPB

and AOT in the 60/40 molar ratio (XAOT= 0.4) diluted in toluene. The extractions are carried out under the same conditions as those described in Example 1 hereinbefore.

The results of these tests are reported in Table 4 hereinafter, in which are

indicated the distribution coefficients of the rare-earth elements, uranium and iron, denoted DM, as well as the separation factors with respect to the iron, denoted FSM/Fe.

Table 4

H 3 PO 4 Dm 5 mol/L La Nd Eu Dy Yb U Fe

10.2 7.0 3.8 1.8 0.6 > 115 0.04 Uranium(VI) FSM/Fe + rare-earth elements La Nd Eu Dy Yb U 239 164 90 42 15 > 2,500

This table shows that the use of toluene as a diluent also allows having a very

effective co-extraction of the rare-earth elements and the uranium(VI) with a quantitative

extraction of uranium (Du> 115). Just like the use of iso-octane as a diluent, the use of toluene allows obtaining a

better extraction of the light rare-earth elements compared to the heavy rare-earth elements. However, when the diluent is toluene, the extraction of the rare-earth

elements is 1.4 to 1.7 times better than is the case when it consists of iso-octane except for ytterbium where the extraction is equivalent.

The distribution coefficient of Fe being identical for the two diluents (DFe= 0.04), the separation factors FSM/Fe are therefore higher when the diluent is toluene.

EXAMPLE 3: Selective stripping of uranium(VI) and the rare-earth elements after co

extraction of these metals from an aqueous solution of phosphoric acid

This example follows on from Examples 1 and 2 hereinbefore in the cases where

the extractions have been carried out from a solution of phosphoric acid (5 mol/L or 8 mol/L) comprising both rare-earth elements and uranium(VI) and using as an extractant, a mixture of DEHCNPB and AOT with the 60/40 molar ratio. All organic phases, with iso-octane or toluene as a diluent, loaded with rare

earth elements and uranium(VI) have been subjected to two successive strippings in order to strip the rare-earth elements in a first step and uranium(VI) in a second step,

with:

- for the first stripping, an aqueous phase, hereinafter referred to as "aqueous

phase No. 1", comprising 0.2 mol/L of ammonium oxalate, and

- for the second stripping, an aqueous phase, hereinafter referred to as "aqueous phase No. 2", comprising 0.5 mol/L of ammonium carbonate.

All strippings have been carried out using an aqueous phase/organic phase (A/O)

volume ratio of 1. The aqueous and organic phases have been brought into contact for 30 minutes at room temperature (22°C) with a vibrating stirrer (550 rpm), after which they

have been centrifuged (7,000 rpm) for 5 minutes at 22°C, then separated from each other. The concentrations of the rare-earth elements and uranium in the aqueous

phases are measured by ICP-OES before and after stripping. The results of these tests are reported in table 5 hereinafter, in which are

indicated the distribution coefficients and the stripping yields of the rare-earth elements

and the uranium(VI), denoted respectively DM and RM, as a function of the concentration of phosphoric acid (5 mol/L or 8 mol/L) in the aqueous phase from which these metals

have been co-extracted. Table 5

pase o. IAqueous Aqueus XAOT D Aqueousphseo.t phase No. 2 0,4 La Nd Eu Dy Yb U

H 3 PO 4 Iso-octane Dm 0.01 0.01 0.01 0.03 0.13 0.15

5mol/L Rm 99% 99% 99% 97% 87% 87% Dm 0.01 0.01 0.01 0.01 0.02 0.004 Toluene RM 99% 99% 99% 99% 98% 100%

H 3 PO 4 Dm 0.02 0.01 0.02 0.05 0.14 0.52 8mol/L RM 98% 99% 98% 95% 87% 66%

This table shows that the stripping of the rare-earth elements is done selectively

during the first stripping with the aqueous phase No. 1 (ammonium oxalate at 0.2 mol/L)

and that the stripping of uranium(VI) is done selectively during the second stripping with

the aqueous phase No. 2 (ammonium carbonate at 0.5 mol/L). - With iso-octane used as a diluent:

Lanthanum, neodymium and europium are stripped quantitatively (RM > 98%) during the first stripping. The obtained RM values for dysprosium are slightly lower (RM= 97% and 95% respectively for H 3 P4 at 5 mol/L and H 3 P04 at 8 mol/L). Ytterbium is

stripped up to 87% for the two concentrations of H 3 P04 essentially because it is initially extracted in a smaller amount than the other rare-earth elements. In addition, the

concentration of the rare-earth elements in the aqueous phase No. 2 after stripping is 3,

2, 3, 5 and 10 mg/L respectively for La, Nd, Eu, Dy and Yb, which is negligible.

The uranium is stripped very well during the second stripping. It is better stripped when it has been extracted from an aqueous solution comprising 5 mol/L of

phosphoric acid (RM = 87%) than when it has been extracted from an aqueous solution comprising 8 mol/L of phosphoric acid phosphoric (RM= 66%). - With toluene used as a diluent:

All rare-earth elements are stripped quantitatively (RM 98%) during the first

stripping. In turn, the uranium(VI) is stripped quantitatively (RM = 100%) during the second

stripping.

Cited references

[1] WO-A-2013/167516

[2] WO-A-2018/051026

[3] "Chemistry and Technology of Surfactants", Richard J. Farn, 2006 Blackwell Publishing Ltd, ISBN-13:978-14051-2696-0

[4] "Self-Organized Surfactant Structures", Tharwat F. Tadros, 2011, John Wiley & Sons, ISBN:978-3-527-63265-7

Claims (17)

Claims

1. A method for recovering the rare-earth elements and the uranium(VI) present in an aqueous solution Al of phosphoric acid, which comprises:

a) a co-extraction of the rare-earth elements and uranium(VI) from the

aqueous solution Al by at least one contacting of the aqueous solution Al with a water immiscible organic solution, which comprises an extractant in an organic diluent, followed

by a separation of the aqueous solution Al from the organic solution, the extractant of the organic solution comprising:

- a compound of formula (1) hereinafter:

0 0 R N _OR4 OR3

wherein:

R' and R2 , identical or different, represent a linear or branched alkyl group

comprising from 6 to 12 carbon atoms; R 3 represents a linear or branched alkyl group comprising from 1 to 12 carbon

atoms, or a saturated or unsaturated, monocyclic hydrocarbon group comprising from 3 to 8 carbon atoms and possibly one or more heteroatom(s);

R 4 and R 5, identical or different, represent a hydrogen atom or a linear or branched alkyl group comprising from 2 to 8 carbon atoms; and

- an anionic or zwitterionic surfactant; then b) a separation of the rare-earth elements from the uranium(VI) present in

the organic solution resulting from a), this separation comprising:

- either a stripping of the rare-earth elements from the organic phase resulting from a) followed by a stripping of uranium (VI) from the organic phase resulting

from the stripping of the rare-earth elements;

- or a stripping of uranium(VI) from the organic phase resulting from

a) followed by a stripping of the rare-earth elements from the organic phase resulting

from the stripping of uranium(VI).

2. The method according to claim 1, wherein R 1 and R 2 are identical to each other and represent a branched alkyl group comprising from 8 to 10 carbon atoms.

3. The method according to claim 2, wherein RI and R2 represent a 2-ethylhexyl group.

4. The method according to any one of claims 1 to 3, wherein R3 represents a linear alkyl group, comprising from 1 to 12 carbon atoms, or a 6-membered monocyclic

aromatic group.

5. The method according to claim 4, wherein R 3 represents a methyl, n-octyl or phenyl group.

6. The method according to any one of claims 1 to 5, wherein:

- R4 represents an alkyl group, linear or branched, comprising from 2 to 8 carbon atoms, and/or

- R5 represents a hydrogen atom.

7. The method according to any one of claims 1 to 6, wherein the compound is

selected from among:

- ethyl 1-(N,N-diethylhexylcarbamoyl)ethy phosphonate which corresponds to formula (1) wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is a methyl

group, R 4 is an ethyl group and R5 is a hydrogen atom;

- ethyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate which corresponds to formula (1) wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is an n-octyl group, R 4 is an ethyl group and R5 is a hydrogen atom;

- butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate which corresponds to formula (1) wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is an n-octyl

group, R 4 is an n-butyl group and R5 is a hydrogen atom;

- butyl 1-(N,N-dioctylcarbamoyl)nonylphosphonate which corresponds to

formula (1) wherein R1, R2 and R 3 are an n-octyl group, R 4 is an n-butyl group and R 5 is a hydrogen atom; and

- ethyl 1-(N,N-diethylhexylcarbamoyl)benzylphosphonate which corresponds to formula (1) wherein R1 and R 2 are a 2-ethylhexyl group, R 3 is a phenyl group, R 4 is an ethyl group and R 5 is a hydrogen atom.

8. The method according to claim 7, wherein the compound is butyl 1-(N,N diethylhexylcarbamoyl)nonylphosphonate.

9. The method according to any one of claims 1 to 8, wherein the surfactant is

a salt of adi(C6-C12)alkylsulfosuccinate.

10. The method according to claim 9, wherein the surfactant is a salt of di(2 ethylhexyl)sulfosuccinate.

11. The method according to claim 10, wherein the surfactant is the sodium salt

of di(2-ethylhexyl)sulfosuccinate.

12. The method according to any one of claims 1 to 11, wherein the stripping of the rare-earth elements from the organic solution resulting from a) or from the organic

solution resulting from the stripping of uranium(VI) comprises at least one contacting of the organic solution with an aqueous solution A2 followed by a separation of the organic

solution from the aqueous solution, the aqueous solution A2 comprising at least one compound selected from among oxalic acid, oxalates, aminopoly-carboxylic acids, salts of

aminopolycarboxylic acids, nitrilotrimethyl-phosphonic acid, hydroxamic acids and

hydrophilicdiglycolamides.

13. The method according to claim 12, wherein the aqueous solution A2 comprises a sodium, potassium or ammonium oxalate.

14. The method according to any one of claims 1 to 13, wherein the stripping of

uranium(VI) from the organic solution resulting from a) or from the organic solution resulting from the stripping of the rare-earth elements, comprises at least one contacting

of the organic solution with an aqueous solution A3 followed by a separation of the organic solution from the aqueous solution, the aqueous solution A3 comprising a

carbonate or a mixture of carbonates.

15. The method according to claim 14, wherein the aqueous solution A3

comprises a sodium, potassium or ammonium carbonate.

16. The method according to any one of claims 1 to 15, wherein the separation of the rare-earth elements from the uranium(VI) present in the organic solution resulting

from a) comprises a stripping of the rare-earth elements from the organic phase resulting from a) followed by a stripping of uranium(VI) from the organic phase resulting from the

stripping of the rare-earth elements.

17. The method according to any one of claims 1 to 16, wherein the aqueous

solution Al is a solution resulting from the leaching of a natural phosphate by sulphuric acid.