USE OF A SYNERGISTIC MIXTURE OF EXTRACTS TO EXTRACT RARE EARTHS FROM AN AQUEOUS MEDIUM CONTAINING PHOSPHORIC ACID

Description

TECHNICAL FIELD

[0001] The invention relates to the field of rare earth extraction and recovery.

[0002] More specifically, the invention relates to the use of a mixture of synergistic extractants for the extraction of at least one rare earth present in an aqueous medium comprising phosphoric acid such as an aqueous solution of phosphoric acid obtained from the attack of a phosphate ore by sulfuric acid.

[0003] The invention finds particular application in the processing of phosphate ores in order to valorize the rare earths present in these ores. PREVIOUS STATE OF THE ART

[0004] Rare earths (hereinafter "RE") include metals characterized by similar properties, namely scandium (Sc), yttrium (Y) and all the lanthanides, the latter corresponding to the 15 chemical elements listed in Mendeleev's periodic table of elements ranging from atomic number 57 for lanthanum (La) to atomic number 71 for lutetium (Lu).

[0005] Within this group, we distinguish between "light" TRs, that is, with an atomic number of 61 or less (scandium, yttrium, lanthanum, cerium, praseodymium and neodymium), and "heavy" TRs, that is, with an atomic number of 62 or less (samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium).

[0006] The unique electronic configuration of TRs, and in particular their unsaturated 4f electron subshell, gives them unique chemical, structural, and physical properties. These properties are exploited in a wide variety of sophisticated industrial applications: glass and ceramics industries, polishing, catalysis (especially in the oil and automotive sectors), manufacturing of high-tech alloys, permanent magnets, optical devices (including cameras), phosphors, rechargeable batteries for electric or hybrid vehicles, alternators for wind turbines, etc.

[0007] Technological metals are, therefore, among the so-called "technology" metals whose supply is strategic, but also threatened by the growth in global demand for these particular metals.

[0008] Recycled minerals (RMs) are currently produced from conventional resources such as bastnaesite hard rock deposits and alluvial monazite and xenotime deposits. However, other unconventional resources exist, such as phosphate ores (also called natural phosphates), which are mined for the production of phosphoric acid and phosphate fertilizers. While the concentration of RMs in these ores is lower, they can still lead to profitable RM production.

[0009] The processing of phosphate ores for the production of phosphoric acid and phosphate fertilizers begins with the leaching of these ores, previously crushed and ground, using a concentrated acid, typically 98% sulfuric acid, which transforms tricalcium phosphate into phosphoric acid (H₃PO₄) and insoluble calcium sulfate (or phosphogypsum). This leaching results in aqueous phosphoric acid solutions with concentrations exceeding 4 mol / L, containing varying concentrations of tricalcium phosphate (TR) depending on the TR content of the starting ores and the processing method applied to them.

[0010] One way to recover TRs from an aqueous solution of phosphoric acid obtained from the leaching of a phosphate ore by sulfuric acid is to subject this aqueous solution, after filtration and concentration, to a liquid-liquid extraction, or solvent extraction, which consists of putting the aqueous solution in contact with an organic solution comprising one or more extractants in an organic diluent having an affinity for TRs so as to obtain a transfer of TRs into the organic solution.

[0011] Such extraction must be simultaneously efficient and selective with respect to the many other metals (hereinafter "metallic impurities") which are also present in aqueous solutions of phosphoric acid from the leaching of phosphate ores by sulfuric acid and, in particular, with respect to iron present in the form of Fe 3+< ions and whose concentration is generally greater than 1 g / L in this type of solution.

[0012] A review of the scientific literature shows that a limited number of extractants have been tested to extract TR from a phosphoric acid medium (see S. Wu et al., Chemical Engineering Journal 2018, 335, 774-800, reference below). [1]).

[0013] Two main classes of extractants were studied, namely: cation exchange extractants, also known as acid extractants, which are mainly organophosphorus compounds, such as organophosphoric acids, organophosphonic acids or organophosphinic acids; these include, for example, di-2-ethylhexylphosphoric acid (or D2EHPA or HDEHP), di(n-octylphenyl)phosphoric acid (or DOPPA), 2-ethylhexyl-2-ethylhexylphosphonic acid (or HEH[EHP] or PC88A) and bis(trimethyl-2,4,4-pentyl)phosphinic acid (marketed under the reference Cyanex™< 272); and solvating extractants, also known as neutral extractants, such as phosphates, phosphine oxides or diglycolamides; These include, for example, tri-n-butylphosphate (or TBP), trioctylphosphine oxide (or TOPO), and N,N,N',N'- tetraoctyldiglycolamide (or TODGA).

[0014] Regarding organophosphoric acids, it appears that the extraction of triglycerides (TRs) by these acids is highly dependent on the acidity of the medium in which the TRs are found. Thus, D2EHPA and its analogs (DOPPA, for example) allow for the proper extraction of heavy TRs at an acidity greater than 4 mol / L of phosphoric acid, but not at such a level. Indeed, quantitative extraction of light TRs can only be achieved at an acidity below 0.5 mol / L of phosphoric acid, and therefore, at least eight times lower than that of aqueous phosphoric acid solutions obtained from the leaching of phosphate ores with sulfuric acid.

[0015] Organophosphoric acids also have the disadvantage of exhibiting slow extraction kinetics and an affinity for transition metals, particularly iron. Thus, the competitive extraction of Fe³⁺ ions by D₂EHPA significantly reduces TR extraction by this extractant, whereas the presence of other metallic impurities such as Al³⁺, Ca²⁺, or Mg²⁺ appears to have little impact on this extraction (see L. Wang et al., Hydrometallurgy 2010, 101(1-2), 41-47, hereinafter referred to as reference). [2] ).

[0016] To overcome this poor selectivity, one possibility would be to reduce Fe³⁺ ions to Fe²⁺ ions (which are poorly extracted by organophosphoric acids) using a reducing agent before extracting the triglycerides (TRs). However, the costs of such an operation could be very high compared to the economic gain from the direct and selective recovery of the TRs. A second possibility would be to add operations aimed at removing iron from aqueous phosphoric acid solutions before extracting the TRs, for example, by selective iron precipitation followed by removal of the precipitate by filtration. However, this would lead, on the one hand, to a process that is cumbersome to implement and therefore not very industrially viable, and on the other hand, to a risk of altering the final quality of the phosphoric acid produced.

[0017] It is known that the use of extractant mixtures including, for example, a cation exchanger and a solvent exchanger, can in some cases significantly improve the performance of a liquid-liquid extraction compared to that obtained with the use of extractants alone.

[0018] However, studies show that in the presence of a medium containing phosphoric acid, mixtures comprising an organophosphoric acid such as D2EHPA and a solvating extractant such as TBP or a phosphine oxide (Cyanex™ < 923) have an antagonistic effect on TR extraction, in that the extraction performance (quantified by the TR distribution coefficients) is lower than that obtained with organophosphoric acid alone (see reference). [2] cited above; DK Singh et al., Desalination and Water Treatment 2012, 38(1-3), 292-300, hereinafter referred to [3]).

[0019] DGAs, for their part, represent a family of extractants which was developed by a Japanese team as part of studies on the treatment of spent nuclear fuels with the aim of co-extracting trivalent actinides and lanthanides from a raffinate of the PUREX process but which has also been studied for the recycling of TRs from NdFeB permanent magnet manufacturing waste.

[0020] Thus, it was shown in the international application PCT WO 2016 / 046179, hereinafter referred to as [4], that symmetric lipophilic DGAs with 24 or more carbon atoms, such as TODGA, allow the recovery of dysprosium, praseodymium and neodymium from an aqueous nitric acid solution from the treatment of NdFeB permanent magnets, not only quantitatively but also selectively with respect to other metallic elements present in this phase, in particular with respect to iron and boron.

[0021] It is indicated in the reference [4] While the aqueous solution from which TRs are extracted is preferentially a nitric acid solution, it could also be a sulfuric or phosphoric acid solution. However, no experimental results concerning TODGA extraction of TRs from a phosphoric acid solution—in which the phosphate ions present are known to be much more strongly complexing than the nitrate ions present in a nitric acid solution—are reported in this reference.

[0022] However, it was shown in the international application PCT WO 2016 / 177695, hereinafter referred to as [5],that the extraction of lanthanum, neodymium, gadolinium, dysprosium and ytterbium from an aqueous solution comprising 0.5 mol / L to 5 mol / L of phosphoric acid by an organic phase comprising TODGA leads to extraction percentages which are all less than 2%, and this for all concentrations of phosphoric acid tested.

[0023] The reference [5] This confirms that the extraction performance obtained when TODGA is used to extract TR from an aqueous solution of nitric acid is not applicable to the extraction of TR from an aqueous solution of phosphoric acid.

[0024] Finally, although they concern the processing of spent nuclear fuel and not the extraction of TR from aqueous phosphoric acid solutions obtained from the leaching of phosphate ores with sulfuric acid, it is worth mentioning the work of PK Nayak et al. as reported, firstly, in J. Environ. Chem. Eng. 2013, 1(3), 559-565, hereinafter referred to as [6], and, on the other hand, In Sep. Sci. Technol. 2014, 49(8), 1186-1191, hereinafter reference [7].

[0025] Indeed, this work shows that a mixture of extractants comprising an organophosphoric acid, in this case D2EHPA, and a lipophilic symmetric DGA, in this case the N , N , N ', NThe addition of β-tetra(2-ethylhexyl)diglycolamide (or TEHDGA) to n-dodecane leads to a highly significant extraction of Fe³⁺ ions present in an aqueous solution of high-activity nitric acid. For example, for a mixture containing 0.25 mol / L of D₂EHPA and 0.1 mol / L of TEHDGA in n-dodecane, the iron distribution coefficient is approximately 1.2 for a batch test, and more than 80% of the iron present is extracted in the organic phase for a mixer-settler test (see reference). [5]).

[0026] They further show that, when the concentration of nitric acid in an aqueous solution containing americium and europium is greater than 1 mol / L, the ability of a mixture of extractants including D2EHPA and TODGA to extract europium from this solution is the same as that obtained with TODGA alone (see reference). [6]).

[0027] These works therefore allow us to conclude that the use of a mixture of extractants including an organophosphoric acid and a DGA is of no interest compared to the use of a DGA alone when it comes to extracting TR from an aqueous solution of nitric acid including iron and / or a nitric acid concentration greater than 1 mol / L.

[0028] In view of the above, there is a real need to provide an extractant or mixture of extractants which allows the extraction of all TR, light and heavy, from an aqueous solution of phosphoric acid having an acidity of the type of aqueous solutions from the leaching of phosphate ores by sulfuric acid, and this both efficiently and selectively with respect to other metals likely to be present in this solution and, in particular, with respect to iron.

[0029] However, in the course of their work, the Inventors found that, unexpectedly, a mixture of extractants comprising an organophosphoric acid such as H2EHPA, and a lipophilic symmetric diglycolamide such as TODGA, makes it possible to extract efficiently and selectively with respect to iron all the TRs present in an aqueous solution comprising phosphoric acid, even at an acid concentration greater than 4 mol / L.

[0030] They also observed that such a mixture of extractants has a synergistic effect on the extraction of TR from such an aqueous solution of phosphoric acid because: On the one hand, as known from the prior art and verified by the Inventors (see example 2 below), organophosphoric acids such as D2EHPA are unsuitable for extracting light TRs from an aqueous medium containing more than 4 mol / L of phosphoric acid, and on the other hand, as known from the reference [5]and corroborated by the Inventors (see example 1 below), lipophilic symmetric DGAs such as TOGDA do not allow, when used alone, the extraction of rare earths from an aqueous medium containing more than 0.5 mol / L of phosphoric acid.

[0031] And it is on these experimental findings that the invention is based. DESCRIPTION OF THE INVENTION

[0032] The invention therefore relates to the use of a mixture comprising: a first extractant which is an organophosphoric acid and which corresponds to the formula (I) below: in which R1 and R2, identical or different, represent a linear or branched hydrocarbon group, saturated or unsaturated, comprising from 6 to 12 carbon atoms, or a phenyl group optionally substituted by a linear or branched hydrocarbon group, saturated or unsaturated, comprising from 1 to 10 carbon atoms; and a second extractant which is a lipophilic symmetric DGA and which corresponds to the formula (II) below: in which R 3< represents a linear or branched alkyl group, comprising 6 to 12 carbon atoms; to extract at least one TR from an aqueous medium comprising phosphoric acid.

[0033] In the preceding and following text, the term " linear or branched hydrocarbon group, saturated or unsaturated, comprising 6 to 12 carbon atoms ", any alkyl, alkenyl or alkynyl group, with a linear chain or with one or more branches, and comprising a total of 6, 7, 8, 9, 10, 11 or 12 carbon atoms.

[0034] Similarly, we mean by " linear or branched hydrocarbon group, saturated or unsaturated, comprising from 1 to 10 carbon atoms", any alkyl, alkenyl or alkynyl group, with a linear chain or with one or more branches, and comprising a total of 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.

[0035] Furthermore, we mean by " linear or branched alkyl group, comprising 6 to 12 carbon atoms ", any alkyl group, with a linear chain or with one or more branches and comprising a total of 6, 7, 8, 9, 10, 11 or 12 carbon atoms.

[0036] In the above and below, the terms "aqueous medium", "aqueous solution" and "aqueous phase" are equivalent and interchangeable, just as the terms "organic solution" and "organic phase" are equivalent and interchangeable.

[0037] The expression "from ...... to ......" is meant to signify, when applied to a range of concentrations, that the limits of that range are included.

[0038] According to the invention, in the formula (I) above, R 1< and R 2< , identical or different, preferably represent an alkyl group, linear or branched, comprising from 6 to 12 carbon atoms, or a phenyl group substituted by a linear or branched alkyl group, comprising from 1 to 10 carbon atoms.

[0039] Furthermore, it is preferred that R1< and R2<, whether identical or different, represent: a linear or branched alkyl group, comprising 8 to 10 carbon atoms such as an n-octyl, isooctyl, n-nonyl, isononyl, n-decyl, isodecyl, 2-ethylhexyl, 2-butylhexyl, 2-methylheptyl, 2-methyloctyl, 1,5-dimethylhexyl, 2,4,4-trimethylpentyl, 1,2-dimethylheptyl, 2,6-dimethylheptyl, 3,5,5-trimethylhexyl, 3,7-dimethyloctyl, 2,4,6-trimethylheptyl, etc.; or a phenyl group substituted by a linear or branched alkyl group, comprising 6 to 10 carbon atoms such as a group n -hexyl, isohexyl, n-heptyl, isoheptyl, n -octyl, isooctyl, n -nonyl, isononyl, n -decyl, isodecyl, 1-ethylpentyl, 2-ethylhexyl, 2-butylhexyl, 2-methylheptyl, 2-ethylheptyl, 2-methyloctyl, 2-methylnonyl, 1,5-dimethylhexyl, 2,4,4-trimethylpentyl, 1,2-dimethylheptyl, 2,6-dimethylheptyl, 3,5,5-trimethylhexyl, 3,7-dimethyloctyl, 2,4,6-trimethylheptyl, etc.

[0040] Furthermore, R1< and R2< are preferably identical to each other.

[0041] Among the extractants of formula (I) above, preference is given to those in which R 1< and R 2<, identical to each other, represent a branched alkyl group, comprising 8 to 10 carbon atoms.

[0042] One such extractant is, for example, D2EHPA which corresponds to the formula (I) above in which R 1< and R 2< represent a 2-ethylhexyl group.

[0043] According to the invention, in the formula (II) above, R3 represents, preferably, a linear or branched alkyl group, comprising 8 to 10 carbon atoms such that a group n -octyl, isooctyl, n -nonyl, isononyl, n -decyl, isodecyl, 2-ethylhexyl, 2-butylhexyl, 2-methylheptyl, 2-methyloctyl, 1,5-dimethylhexyl, 2,4,4-trimethylpentyl, 1,2-dimethylheptyl, 2,6-dimethylheptyl, 3,5,5-trimethylhexyl, 3,7-dimethyloctyl, 2,4,6-trimethylheptyl, etc.

[0044] Among the extractants of formula (II) above, preference is given to those in which R 3< represents a linear alkyl group comprising 8 to 10 carbon atoms.

[0045] One such extractant is, for example, TODGA which corresponds to formula (II) above in which R 3< represents an n-octyl group.

[0046] According to the invention, the mixture of extractants is preferably a mixture of D2EHPA and TODGA.

[0047] Furthermore, the extractant mixture is preferably used in solution in an organic diluent, which can be any nonpolar organic diluent whose use has been proposed for dissolving lipophilic extractants such as a hydrocarbon or a mixture of hydrocarbons, aliphatic and / or aromatic. Examples of such a diluent include... n -dodecane, hydrogenated tetrapropylene (TPH), kerosene and diluents which are marketed under the references Isane ™< IP-185 (Total), Isane ™< IP-175 (Total), Shellsol ™< D90 (Shell Chemicals) and Escaid ™< 110 Fluid (Exxon Mobil), preference being given to Isane ™< IP-185.

[0048] Furthermore, the extractant mixture is preferably used to extract the TR or TRs from the aqueous medium in which it / they are found by liquid-liquid extraction, in which case the use of this mixture includes at least one contacting of the aqueous medium with an organic solution immiscible with water, comprising the extractant mixture in an organic diluent, and then a separation of the aqueous medium from the organic solution, thereby obtaining an organic solution comprising the TR or TRs.

[0049] The organic solution, which is brought into contact with the aqueous medium, typically comprises 0.2 mol / L to 2 mol / L of the first extractant and 0.05 mol / L to 2 mol / L of the second extractant.

[0050] It goes without saying that the choice of a concentration for each of the first and second extractants in these ranges will depend on the extractants used as well as possibly on the TR or TRs whose extraction we wish to prioritize.

[0051] Thus, for example, for a mixture comprising D2EHPA as the first extractant and TODGA as the second extractant, the organic solution will preferably comprise 0.2 mol / L to 1.5 mol / L of D2EHPA and 0.1 mol / L to 0.5 mol / L of TODGA.

[0052] According to the invention, the extraction of the TR or TRs from the aqueous medium by liquid-liquid extraction is preferably followed by a de-extraction of this TR or these TRs from the organic solution in which it / they was / were extracted, in which case this de-extraction includes at least a contacting of the organic solution with an acidic or basic aqueous solution, then a separation of the organic solution from the aqueous solution, thereby obtaining an aqueous solution comprising the TR or TRs.

[0053] The aqueous medium from which the TR(s) is / are extracted advantageously comprises from 0.5 mol / L to 10 mol / L, preferably from 2 mol / L to 6 mol / L and, even better, from 4 mol / L to 5 mol / L of phosphoric acid.

[0054] Such an aqueous environment can notably be an aqueous solution of phosphoric acid resulting from the leaching of a phosphate ore by sulfuric acid.

[0055] Such an aqueous solution may contain TR at a total concentration ranging from 30 mg / L to 1,200 mg / L, more precisely from 100 mg / L to 1,000 mg / L, as well as a number of metallic impurities including iron but also magnesium, aluminum, calcium, zinc, chromium, vanadium, etc.

[0056] In any case, the TR or TRs is (are), preferably, chosen from yttrium, lanthanum, neodymium, dysprosium, ytterbium and mixtures thereof.

[0057] Other features and advantages of the invention will become apparent from the supplementary description that follows and refers to the attached figures.

[0058] It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no way be interpreted as a limitation of this object. BRIEF DESCRIPTION OF THE FIGURES

[0059] THE Figures 1A and 1B illustrate the results of extraction tests carried out on aqueous phases of phosphoric acid comprising five TRs - namely yttrium, lanthanum, neodymium, dysprosium and ytterbium - as well as iron, using organic phases comprising 0.5 mol / L of D2EHPA and a variable molar concentration of TODGA; the Figure 1Ashows the evolution of the distribution coefficients of TR and iron, denoted DM and reported on a logarithmic scale, as a function of the molar concentration of TODGA, denoted [TODGA], in the organic phases, while the figure 1B This shows the evolution of the separation factors between TR and iron, denoted FS TR / Fe and also reported on a logarithmic scale, as a function of the TODGA concentration; for comparison, the DM and FS TR / Fe values ​​obtained under the same operating conditions with an organic phase containing only D2EHPA as the extractant are also shown in these figures. figure 2This illustrates the results of extraction tests carried out on aqueous phases of phosphoric acid comprising the five aforementioned TRs and iron, using organic phases comprising a variable molar concentration of D2EHPA and 0.5 mol / L of TODGA; more specifically, this figure shows the evolution of the distribution coefficients of TRs and iron, denoted DM and reported on an arithmetic scale, as a function of the molar concentration of D2EHPA, denoted [D2EHPA], in the organic phases. figure 3 illustrates the deextraction yields, denoted RM and expressed as %, as obtained during deextraction tests carried out on an organic phase comprising the five aforementioned TRs, iron, 0.5 mol / L of D2EHPA and 0.25 mol / L of TODGA, using different acidic aqueous phases. Figures 4A and 4Billustrate, by way of comparison, the results of extraction tests carried out on aqueous phases of phosphoric acid comprising the five aforementioned TRs and iron, using organic phases comprising only D2EHPA as the extractant, at varying molar concentrations; the figure 4A shows the evolution of the distribution coefficients of TR and iron, denoted DM and reported on a logarithmic scale, as a function of the molar concentration of D2EHPA, denoted [D2EHPA], in the organic phases, while the figure 4B shows the separation factors between TR and iron, denoted FS TR / Fe, obtained for an organic phase comprising 1 mol / L of D2EHPA. figure 5This illustrates, for comparative purposes, the results of extraction tests carried out on aqueous phases of phosphoric acid comprising the five aforementioned TRs and iron, using organic phases comprising 0.5 mol / L of D2EHPA and a variable molar concentration of TBP; more specifically, this figure shows the evolution of the distribution coefficients of TRs and iron, denoted DM and plotted on an arithmetic scale as a function of the molar concentration of TBP, denoted [TBP], in the organic phases; also shown in these figures are the DMs obtained under the same operating conditions with an organic phase comprising only D2EHPA as the extractant. figure 6illustrates, for comparison, the results of extraction tests carried out on aqueous phases of phosphoric acid comprising the five aforementioned TRs and iron, using organic phases comprising 0.5 mol / L of D2EHPA and a variable molar concentration of TOPO; more specifically, this figure shows the evolution of the distribution coefficients of TRs and iron, noted DM and reported on an arithmetic scale as a function of the molar concentration of TOPO, noted [TOPO], in the organic phases; also indicated on this figure are the DMs obtained under the same operating conditions with an organic phase comprising only D2EHPA as extractant.

[0060] On the figures 3 And 4B The error bars correspond to a relative uncertainty of 10% which encompasses the various analytical and experimental uncertainties. DETAILED DESCRIPTION OF SPECIFIC IMPLEMENTATION METHODS

[0061] The extraction tests reported in the following examples were all carried out using as aqueous phases aliquots of a synthetic aqueous solution, representative of the aqueous solutions of phosphoric acid actually obtained during the production of H3PO4 by leaching of natural phosphates with H2SO4.

[0062] This synthetic aqueous solution comprises, in addition to phosphoric acid, three light tricyclic elements (TCEs), namely yttrium, lanthanum, and neodymium, two heavy TCEs, namely dysprosium and ytterbium, and one major and detrimental impurity, namely iron. It was prepared by dissolving the oxides of the metals corresponding to the +3 oxidation state in a concentrated H₃PO₄ solution and then adjusting the H₃PO₄ concentration of this solution to 4.6 mol / L.

[0063] Its mass composition in metallic elements is presented in Table I below. Tableau I Elements Y There Nd Dy Yb Fe [C] and mg / L 216 195 173 204 267 1525

[0064] The organic phases were prepared using Isane™< IP185 as an organic diluent and pre-equilibrated by contact with an aqueous solution comprising 4.6 mol / L of H3PO4.

[0065] Furthermore, the extraction and deextraction tests reported in the following examples were all carried out in microtubes with volumes less than 1.5 mL, at a temperature of 45°C, using a volume ratio between organic and aqueous phases (O / A) of 1 and subjecting these phases to a single 20-minute contact under agitation using a Vibrax™ shaker. After centrifugation, the organic and aqueous phases were separated by decantation.

[0066] The distribution coefficients, separation factors, and deextraction yields were determined in accordance with the conventions of the field of liquid-liquid extractions, namely that: The distribution coefficient of a metallic element M, denoted DM, between two phases, respectively organic and aqueous, is equal to: D M = M org , f M aq , f = M aq , i − M aq , f M aq , f with: [M]org,f = concentration of M in the organic phase after extraction (or de-extraction), [M]aq,f = concentration of M in the aqueous phase after extraction (or de-extraction), and [M]aq,i = concentration of M in the aqueous phase before extraction (or de-extraction); the separation factor between two metallic elements M1 and M2, denoted FS M1 / M2, is equal to: FS M 1 / M 2 = D M 1 D M 2 with: DM1 = distribution coefficient of the metallic element M1, and DM2 = distribution coefficient of the metallic element M2; the extraction yield of a metallic element M, denoted RM, from an organic phase is equal to: R M = M aq , f M org , i × 100 with: [M] aq,f = concentration of M in the aqueous phase after deextraction, and [M] org,i = concentration of M in the organic phase before deextraction.

[0067] Multi-element analyses of aqueous or organic phases including TR (initial synthetic solution, aqueous and organic phases after extraction, aqueous phases after de-extraction, etc.) were carried out by atomic emission spectrometry whose source is an argon plasma generated by inductive coupling, after dilution to bring the metallic elements to measurable concentrations. EXAMPLE 1: Extraction of TRs by mixtures of D2EHPA / TODGA extractants according to the invention 1.1- Extraction tests : ∗< First series of tests :

[0068] Extraction tests were carried out using as organic phases, solutions comprising 0.5 mol / L of D2EHPA and TODGA at a concentration of 0.1 mol / L, 0.25 mol / L, 0.5 mol / L or 1 mol / L and, for comparison, a solution comprising 0.5 mol / L of D2EHPA but free of TODGA.

[0069] There Figure 1A illustrates the evolution of the distribution coefficients of TR and iron, DM, obtained at the end of these tests as a function of the concentration of TODGA in the organic phases.

[0070] As this figure shows, the addition of TODGA to D2EHPA results in a strong increase in the distribution coefficients of TR and, therefore, in their extraction from an aqueous phase of phosphoric acid.

[0071] D2EHPA / TODGA mixtures exhibit a particularly high affinity for dysprosium, ytterbium and yttrium.

[0072] Their affinity is significantly lower for lanthanum and neodymium but remains nevertheless high (D La max< = 2.3) when compared to that of D2EHPA alone (D La < 0.01).

[0073] Furthermore, the addition of TODGA to D2EHPA results in a threefold decrease in the iron distribution coefficient compared to that obtained with D2EHPA alone. This distribution coefficient is very low (D Fe = 0.005) for TODGA concentrations up to 0.25 mol / L, clearly demonstrating the excellent iron selectivity of D2EHPA / TODGA mixtures for transfusions.

[0074] The evolution of the separation factors between TR and iron, FS TR / Fe, as a function of TODGA concentration in the organic phases is illustrated in the figure 1B .

[0075] This figure shows that the maximum values ​​of FS TR / Fe are very significant for heavy TRs such as ytterbium (FS Yb / Fe > 12,500) and remain very satisfactory for light TRs such as lanthanum (FS La / Fe = 460).

[0076] For comparison, the best separation factor between TR and iron obtained for D2EHPA alone, at a concentration of 0.5 mol / L in organic phase, is the separation factor between ytterbium and iron, which is at least 20 times lower (FS Yb / Fe ≈ 600) than that obtained with D2EHPA / TODGA mixtures. ∗< Second series of tests :

[0077] Tests were carried out using as organic phases, solutions comprising 0.5 mol / L of TODGA and D2EHPA at a concentration of 0.1 mol / L, 0.5 mol / L, 1 mol / L or 2 mol / L.

[0078] There figure 2illustrates the evolution of the distribution coefficients of TR and iron, DM, obtained at the end of these tests as a function of the concentration of D2EHPA in the organic phases.

[0079] As this figure shows, the addition of D2EHPA to TODGA results in a strong increase in the distribution coefficients of TR and, therefore, in their extraction from an aqueous phase of phosphoric acid.

[0080] A D2EHPA concentration greater than 0.1 mol / L in the mixture is necessary to obtain good extraction performance for all the TRs. For example, the lanthanum distribution coefficient is very low when the D2EHPA concentration in the mixture is 0.1 mol / L (DLa = 0.06) but increases very significantly when the D2EHPA concentration in the mixture is 0.5 mol / L (DLa = 2.2). By comparison, the affinity for lanthanum of D2EHPA alone at a concentration of 0.5 mol / L is very low (DLa = 0.01).

[0081] These results corroborate those presented in reference [5], namely that TODGA alone does not allow the extraction of TR from an aqueous phase of phosphoric acid.

[0082] On the other hand, they show that using a concentration of D2EHPA at least equal to that of TODGA and preferably twice that of TODGA allows for good extraction of all TRs. 1.2 - Extraction tests :

[0083] Extraction tests were carried out using: as organic phases: aliquots of the organic phase from the extraction test carried out in point 1.1 above with a mixture comprising 0.5 mol / L of D2EHPA and 0.25 mol / L of TODGA; and as aqueous phases: aqueous solutions comprising: * either 0.5 mol / L, 1 mol / L or 6 mol / L of H2SO4, * or 1 mol / L of H2SO4 and 0.125 mol / L of Na2SO4, * or 5 mol / L or 10 mol / L of H3PO4.

[0084] There figure 3 illustrates the deextraction yields, RM, obtained at the end of these tests.

[0085] As this figure shows, highly concentrated aqueous solutions of H3PO4 (5 mol / L or 10 mol / L) also allow the extraction of lanthanum and neodymium as well as iron, but they do not allow the extraction of yttrium, dysprosium and ytterbium which remain in the organic phase.

[0086] Dilute aqueous solutions of H₂SO₄ (0.5 mol / L and 1 mol / L) allow for the near-quantitative deextraction of lanthanum and neodymium, and the partial deextraction of yttrium, dysprosium, and ytterbium. A 1 mol / L H₂SO₄ solution provides better selectivity for iron deextraction of tritium-14 (TR) than a 0.5 mol / L H₂SO₄ solution. The addition of 0.125 mol / L Na₂SO₄ further improves this selectivity, but at the cost of reduced deextraction yields for yttrium, neodymium, and dysprosium.

[0087] There figure 3also shows that iron is partially but selectively removed by an aqueous solution comprising 6 mol / L of H2SO4.

[0088] We can therefore consider implementing a scheme in which the organic phase from the extraction of TR would be subjected to a washing step with an aqueous solution comprising, for example, 6 mol / L of H2SO4 to selectively remove the iron present in this organic phase, before being subjected to a de-extraction step of TR, for example by means of a dilute H2SO4 solution, possibly with added Na2SO4.

[0089] The results obtained for the aqueous solution comprising both sulfuric acid and sodium sulfate also allow us to consider a scheme in which the TRs found in aqueous solution after deextraction would be recovered by precipitation, for example in the form of double sulfates of TR and sodium, carbonates, oxalates, etc., this type of precipitation being known from the literature. EXAMPLE 2 : Extraction of TR using D2EHPA alone (comparative example)

[0090] For comparison, extraction tests were carried out using as organic phases, solutions comprising 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L of D2EHPA.

[0091] There figure 4A illustrates the evolution of the distribution coefficients of TR and iron, DM, obtained at the end of these tests as a function of the concentration of D2EHPA in the organic phases.

[0092] This figure clearly shows that the extraction of TRs by D2EHPA decreases with increasing ionic radius of the TRs. Thus, D2EHPA has a good affinity for TRs with low ionic radius such as yttrium, dysprosium and ytterbium but does not allow or hardly allows the extraction of TRs with higher ionic radius such as lanthanum and neodymium (MD < 0.1 regardless of the concentration of D2EHPA in the organic phase).

[0093] Furthermore, the separation factors between TR and iron, FS TR / Fe, obtained for a D2EHPA concentration of 1 mol / L in the organic phase are reported on the figure 4B These separation factors are satisfactory for ytterbium (FS Yb / Fe = 500). However, they are not at all satisfactory for the other RTs.

[0094] It should be noted that no significant variation in these separation factors is observed as a function of the concentration of D2EHPA in the organic phase. EXAMPLE 3: Extraction of TRs using D2EHPA / TBP and D2EHPA / TOPO mixtures (comparative example)

[0095] Since TODGA is a solvent extractant, extraction tests were carried out to verify whether mixtures including D2EHPA and a solvent extractant other than TODGA would be likely to exhibit the same synergistic effect as that observed when D2EHPA is used in a mixture with TODGA.

[0096] These extraction tests were carried out using, as organic phases, solutions comprising 0.5 mol / L of D2EHPA and: either tri-n-butyl phosphate (or TBP) at a concentration of 0.1 mol / L, 0.25 mol / L or 0.5 mol / L; or trioctylphosphine oxide (or TOPO) at a concentration of 0.1 mol / L, 0.25 mol / L or 0.5 mol / L.

[0097] The results obtained from these tests are illustrated in terms of distribution coefficients, DM, on the figure 5 for D2EHPA / TBP mixtures and on the figure 6 for D2EHPA / TOPO mixtures.

[0098] As shown by figure 5 The addition of TBP to D2EHPA results in a notable decrease in the distribution coefficients of TR and, therefore, in their extraction from an aqueous phase of phosphoric acid, except in the case of lanthanum and neodymium since these are not already extracted by D2EHPA alone.

[0099] This decrease in DM, which is all the more important as the concentration of TBP in the organic phase increases, thus highlights the existence of an antagonistic effect of D2EHPA / TBP mixtures on the extraction of TR from an aqueous solution of phosphoric acid.

[0100] Similarly, the figure 6 shows a constant decrease in the distribution coefficients of TR as a function of the concentration of TOPO in the organic phase, also demonstrating the existence of an antagonistic effect of D2EHPA / TOPO mixtures on the extraction of TR from an aqueous solution of phosphoric acid.

[0101] These results corroborate those presented in the aforementioned references [2] and [3] for D2EHPA / TBP and D2EHPA / TOPO mixtures, and confirm that the use of a mixture comprising an organophosphoric acid such as D2EHPA and a solvating extractant has a priori of no interest if one wants to extract TRs from an aqueous solution containing phosphoric acid. REFERENCES CITED

[0102] [1] S. Wu et al., Chemical Engineering Journal 2018, 335, 774-800 [2] L. Wang et al., Hydrometallurgy 2010, 101(1-2), 41-47 [3] DK Singh et al., Desalination and Water Treatment 2012, 38(1-3), 292-300 [4] International Application PCT WO 2016 / 046179 [5] International Application PCT WO 2016 / 177695 [6] PK Nayak et al., J. Environ. Chem. Eng. 2013, 1(3), 559-565 [7] PK Nayak et al., Sep. Sci. Technol. 2014, 49(8), 1186-1191

Claims

1. Use of a mixture comprising: - a first extractant of formula (I): wherein R1 and R2, the same or different, represent a saturated or unsaturated, linear or branched hydrocarbon group, comprising from 6 to 12 carbon atoms, or a phenyl group, optionally substituted by a saturated or unsaturated, linear or branched hydrocarbon group, comprising from 1 to 10 carbon atoms; and - a second extractant or formula (II): wherein R3 represents a linear or branched alkyl group, comprising from 6 to 12 carbon atoms; for extracting at least one rare earth from an aqueous medium comprising phosphoric acid.

2. Use according to claim 1, wherein R1 and R2 represent a linear or branched alkyl group, comprising from 6 to 12 carbon atoms, or a phenyl group substituted by a linear or branched alkyl group, comprising from 1 to 10 carbon atoms.

3. Use according to claim 1 or claim 2, wherein R1 and R2 represent a linear or branched alkyl group, comprising from 8 to 10 carbon atoms, or a phenyl group substituted by a linear or branched alkyl group, comprising from 6 to 10 carbon atoms.

4. Use according to any one of claims 1 to 3, wherein R1 and R2 are identical to each other.

5. Use according to any one of claims 1 to 4, wherein the first extractant is di(2-ethylhexyl)phosphoric acid.

6. Use according to any one of claims 1 to 5, wherein R3 represents a linear or branched alkyl group, comprising from 8 to 10 carbon atoms.

7. Use according to claim 6, wherein the second extractant is N,N,N',N'-tetraoctyldiglycolamide.

8. Use according to any one of claims 1 to 7, wherein the mixture of extractants comprises di(2-ethylhexyl)phosphoric acid and N,N,N',N'-tetraoctyldiglycolamide.

9. Use according to any one of claims 1 to 8, wherein the mixture of extractants is used in solution in an organic diluent.

10. Use according to any one of claims 1 to 9, which comprises at least one contact of the aqueous medium with an organic solution that is not miscible with water, comprising the mixture of extractants in an organic diluent, and then a separation of the aqueous medium from the organic solution, whereby an organic solution comprising at least the rare earth is obtained.

11. Use according to claim 10, wherein the organic solution comprises from 0.2 mol / L to 2 mol / L of the first extractant and from 0.05 mol / L to 2 mol / L of the second extractant.

12. Use according to claim 10 or claim 11, which further comprises at least one contact of the organic solution comprising at least the rare earth with an acid or basic aqueous solution, and then a separation of the organic solution from the aqueous solution, whereby an aqueous solution comprising at least the rare earth is obtained.

13. Use according to any one of claims 1 to 12, wherein the aqueous medium comprises from 0.5 mol / L to 10 mol / L of phosphoric acid.

14. Use according to any one of claims 1 to 13, wherein the aqueous medium is an aqueous solution of phosphoric acid resulting from the lixiviation of a phosphate ore by sulfuric acid.

15. Use according to any one of claims 1 to 14, wherein the rare earth is chosen from yttrium, lanthanum, neodymium, dysprosium, ytterbium and mixtures thereof.