PROCESS FOR LIQUID-LIQUID EXTRACTION OF RARE EARTH ELEMENTS OR ACTINIDES VIA THE COMBINATION OF A HYDROTROPIC CO-SOLVENT AGENT WITH CHELATING OR ANIONIC EXTRACTANTS

Abstract

The invention relates to a process for the liquid-liquid extraction of at least one salt of a metal chosen from the elements of the rare earth group and the actinide group from an acidic aqueous phase containing them, said process comprising in particular a step of mixing an acidic aqueous phase comprising said salt of said metal and at least one non-ionic hydrotropic agent with an organic phase comprising at least one non-ionic hydrotropic agent and at least one extracting agent. Figure for abstract: none

Description

Title of the invention: PROCESS FOR LIQUID-LIQUID EXTRACTION OF RARE EARTH ELEMENTS OR ACTINIDES VIA THE COMBINATION OF A HYDROTROPIC CO-SOLVENT AGENT WITH CHELATING OR ANIONIC EXTRACTANTS Technical field

[0001] The present invention relates to the field of extraction and purification of elements of the rare earth or actinide group from acidic aqueous solutions in which they are found. The process according to the invention is particularly suitable for the extraction of uranium (U), lanthanum (La), neodymium (Nd), europium (Eu), dysprosium (Dy), erbium (Er), ytterbium (Yb) or a mixture thereof. State of the art

[0002] Modern metallurgy mainly covers hydrometallurgical, pyrometallurgical, electrometallurgical and nuclear approaches. The hydrometallurgical approach is the dominant reference technology for the extraction and purification of elements of the rare earth group and the actinide group.

[0003] With the exception of natural deposits of gold, silver, copper and platinum, metals occurring in nature mainly appear in the form of salt and minerals due to natural processes of reactions with the surrounding chemical elements. Generally, the desired metal is extracted from minerals or ores. The most well-known ore containing uranium is uraninite or formerly pitchblende. Rare earth elements are found in various minerals such as bastnaesite, monazite, xenotime, loparite or apatite. The mining process is followed by steps of mechanical and chemical separation, enrichment and purification.The resulting raw materials, such as rare earth oxides of sufficient purity or uranium concentrate (also known as "orange concentrate" or "yellowcake") for nuclear fuels, are then used to create products. Rare earth elements are essential compounds for modern consumer electronics and are also used as magnetic compounds in wind turbine generators or motors for electric mobility. These metals are therefore essential for a wide range of modern inventions, indispensable for the development of efficient, carbon-free and environmentally friendly technologies. They are an important ingredient for modern batteries. hydride, hybrid and electric cars, smartphones, HD displays, laser technology, crude oil refining, catalysis as well as for permanent magnets as magnetic compounds in generators.

[0004] The actinides are a family in the periodic table comprising the 15 chemical elements from actinium (number 89) to lawrencium (number 103). These heavy metals are named after actinium, the first in the family, because of their related chemical properties. They are all f-block elements, except for lawrencium, which is in the d-block. All actinides are radioactive, releasing energy through radioactive decay. They all fission into fast neutrons, and a few into thermal neutrons. Uranium, thorium, and plutonium are the most abundant actinides on Earth. Unlike the lanthanides, which occur in nature in appreciable quantities (with the exception of promethium), most actinides are very rare elements. The most abundant naturally occurring elements are thorium and uranium; and the easiest to synthesize is plutonium; the others are only found in trace amounts.

[0005] Therefore, the accessibility of rare earths and actinides is of paramount importance for the development and proliferation of carbon-free technologies. Nuclear power plants make it possible to produce energy without emitting carbon dioxide. The objective of the Energy Transition Act adopted by the French government in 2015 is to achieve a more diversified and low-carbon electricity production scheme. By 2030, a total share of nuclear energy of around 50% is planned. Nuclear energy and the nuclear fuel cycle will therefore remain important for French energy policy.

[0006] Furthermore, the lifespan of any product is limited. For electronic devices containing rare earth elements, such as smartphones, the lifespan is generally estimated at two or three years, while magnets containing rare earth elements can be used for approximately 15 years. The residence time of a nuclear fuel rod depends on the type of reactor and the fuel used, but the fuel generally remains in the reactor for four to six years. When the end-of-life state of a metal-containing product is reached, recycling strategies are important to reduce the amount of waste created and increase the amount of reusable materials. For nuclear fuels and rare earth elements, there are various recycling and waste management strategies.

[0007] Any recycling strategy must be efficient, inexpensive and applicable on an industrial scale. In addition, ecological, sustainable and green chemistry aspects must be taken into account throughout the life cycle of metals.

[0008] One of the oldest and now considered "mature" separation procedures is liquid-liquid extraction, also called liquid-liquid extraction. solvent. This procedure is generally applied between the extraction and production of raw materials, and after the end of life of the products to recover raw materials and sort waste. In the field of nuclear fuel cycle management, liquid-liquid extraction allows the separation and isolation of radioactive fission products (notably the PUREX and DIAMEX processes in France). For example, the PUREX chemical process (acronym for "Plutonium, Uranium, Reduction, Extraction") is a method for treating spent nuclear fuel, used since 1947 to separate plutonium and uranium independently of each other from minor actinides and fission products by a liquid-liquid extraction method and during which uranium and plutonium are extracted by an organic solvent composed of 30% tributylphosphate (TBP) in dodecane.The fission products are then recovered in a nitric acid phase and then the plutonium is extracted from the uranium / plutonium solution by reduction of the plutonium.

[0009] These processes allow the separation of uranium and plutonium from fission products (e.g. cesium) and transuranic elements (americium, curium, etc.). Although previously considered waste, these transuranic elements can be reused in a closed fuel cycle for 4th generation generators. In addition, liquid-liquid extraction allows the selective separation of rare earth elements from raw ores and electrical and electronic waste (WEEE).

[0010] Liquid-liquid hydrometallurgical extraction is defined by IUPAC (International Union of Pure and Applied Chemistry) as the process of transferring a dissolved substance from one liquid phase to another (immiscible or partially miscible) in contact with it. The distribution of solute species between the two phases (distribution coefficient) allows the efficiency of a given extraction method to be estimated. If more than one compound is dissolved and can be extracted, the selectivity of the chosen approach is important to achieve the desired results.

[0011] The ultimate goal of a liquid-liquid extraction process is to selectively transfer the (desired) ions from phase A to phase B, while the unwanted ions remain in phase A. Back-extraction is the reverse step. Successive extraction and back-extraction cycles, with controlled temperature and pH, form the basis of any separation of rare earths or actinides and other metals present such as iron for example.

[0012] The term "hydrometallurgy" describes the recovery of metals from ores, concentrates and recycled or residual materials. In both the mining and recycling sectors, the main steps are leaching, concentration and recovery. In the context of hydrometallurgy, phase A represents in most cases an acidic aqueous phase loaded with desired and unwanted ions. This This phase is also called "feed solution" and results from the treatment of ores and minerals or - in the case of recycling - valuable waste, e.g. electronic and metal scrap or spent nuclear fuel rods, with acid. This wet chemical process is called "leaching" and is the first step in the extraction cycle.

[0013] In a second step, called the concentration step, the feed solution is brought into contact with a formulated organic phase. It is designed to extract the desired ions in the organic phase, where we enter the field of liquid-liquid extraction which uses ternary or quaternary formulations. In all currently known and used processes, the organic phase contains one or more complexing extractants pure or diluted in a diluent and often a phase modifier.

[0014] After loading the organic phase with the desired metals, the third step is "back-extraction" during which said metals are re-extracted from the organic phase - "the extract" - into a fresh aqueous phase. The organic solvent is recovered and can be reused for another extraction cycle.

[0015] The success of any liquid-liquid extraction process depends on the distribution of a compound between the two phases. This distribution, following the IUP AC convention for aqueous / organic systems, is described by the distribution ratio (DA) on a molar concentration scale. It is defined as "the total analytical concentration of an element in the organic phase relative to its total analytical concentration in the aqueous phase, generally measured at equilibrium". This distribution ratio Da corresponds to the ratio between the molar concentration of a solute species A in the organic phase ([A]org) and the molar concentration of the same solute species A in the aqueous phase ([A]aq).

[0016] The higher Da, the more solute A is present in the organic phase, and the more efficient the extraction. In the case where more than one solute (species A and B) is extracted, the selectivity of the solvent extraction system is then described by the separation factor (S) according to the following equation (1):

[0017] [Equation 1] „ 1'1 ■[«] / (1) V _ ^A / _ wg aq / ^A / B~ / DB~ / [A] aq or g in which: Da and Db are the distribution ratios for the two species A and B, respectively. The greater the difference between the two distribution ratios DA and DB, the more selective the liquid-liquid extraction process is towards the species with the higher D.

[0018] One of the main obstacles to recycling rare earths is the fact that the amount of rare earths used in most products ranges from milligrams to several kilograms. This, combined with the complexity of their applications, the difficulty of separating the different elements from each other to obtain pure and unique elements due to their similar properties, means that less than 1% of rare earths are currently recycled. Liquid-liquid extraction is a possible way to go further, as some of the processes for extracting lanthanoid cations have already been applied on an industrial scale.

[0019] The main liquid-liquid extraction processes used in industry differ from each other in particular by the type of solvents used or by the back-extraction and washing methods used. The solvents used in these processes make it possible to extract various rare earths with a purity generally greater than 99.9%. Commercially, the most widely used solvents are organophosphorus extractants such as phosphoric acids, phosphonic acids and phosphinic acids; carboxylic acids and alkyl phosphates.Examples include di-2-ethylhexylphosphoric acid (HDEHP), 2-ethylhexylphosphonic acid (HEHEHP), bis(trimethyl-2,2,4-pentyl)phosphinic acid (such as the product sold under the trade name Cyanex ® 272), mixtures of branched carboxylic acids such as the products sold under the trade names Versatic™ Acid 10 and Versatic™ Acid 911, tri-n-butylphosphate (TBP) or Aliquat 336.

[0020] Tributyl phosphate (TBP) has long been the most widely used solvent by large industrial groups in extraction processes. Quaternary ammoniums and tertiary carboxylic acids (versatic acids) are also used commercially.

[0021] Despite the use of these formulations, often diluted in petroleum fractions (Isopar®, Isane®, Kerosene, etc.), in tried and tested and efficient processes for the production and purification of rare earths, certain problems linked to their use concern scientists and technologists.

[0022] The separation of trivalent actinoid cations (An3+) in an aqueous solution of nitrate salts also containing dissolved trivalent lanthanoid species (Ln3+) is challenging. The difficulties in separating trivalent actinoid and lanthanoid cations arise from the similarity of ionic radii and hydration indices. Therefore, in a first step, a Ln3+ / An3+ co-extraction is carried out with PUREX raffinate at a very high concentration of nitric acid. This process, called DIAMEX (DIAMide Extraction), uses malonamide-based extractant molecules, such as A,A'-dimethyl,A,A'-dioctylhexylethoxymalonamide (DMDOHEMA). In a second step, the lanthanoid and actinoid cations are separated by the SANEX (Separation of Actinoids by Extraction) extraction process under less acidic conditions. The minor actinoid cations Am3+ and Cm3+ are finally separated by another process similar to DIAMEX.

[0023] N,N-Dialkylamides (or equivalently, monoamide extractants) have proven to be a promising group of extractants for actinide extraction. They exhibit good affinities for hexavalent and tetravalent ions of actinides and low affinities for the main fission products. This group of extractants is poorly soluble in aqueous solution and stable against chemical degradation and radiolysis. Synthesis and purification are quite simple and the physicochemical properties and selectivities can be easily adjusted by varying the three hydrocarbon chains. The main advantage of N,N-dialkylamides is the possibility of simultaneous extraction of uranium and plutonium without the need for a redox step and additional reducing agents as is the case for TBP. Separation of uranium and plutonium can be achieved by adjusting the pH of the aqueous solution.

[0024] On the other hand, it has been observed and modeled that increasing the uranium concentration in the organic phase, composed of monoamide diluted in an aliphatic diluent, leads to an increase in viscosity that can be prohibitive for industrial implementation in conventional extractors (M. Pleines, “Viscosity-control and prediction of microemulsions. Thesis of the University of Montpellier,” PhD Thesis, 2018). Thus, if the solvation of these compounds is poor, the organic phase in equilibrium with the aqueous phase separates into two distinct phases. This is the so-called “3rd phase” phenomenon.

[0025] Whatever the nature of the formulations studied, the formation of the “3rd phase” is the major obstacle in the use of known solvents, because in all industrial devices, the 3rd phase is a stable, viscous emulsion which stops the liquid-liquid extraction, requiring complete emptying-cleaning of the containers, extremely costly in the case of rare earths and in practice impossible in the case of extraction containing radioactive elements. The risk of the appearance of a third phase is a sizing element limiting the intensification of processes, and therefore the cost. The rare theoretical models of the appearance of a third phase published (C. Erlinger et al., “Attractive Interactions between Reverse Aggregates and Phase Separation in Concentrated Malonamide Extradant Solutions”, Langmuir, vol. 15, no. 7, pp. 2290-2300, Mar. 1999, doi: 10.1021 / la980313w) are not yet predictive enough to serve as a guide, and the third phase is avoided in practice by holding systematic trial-and-error test campaigns in pilots or pre-pilots. This leads operators to proceed in conditions where the concentration of acid and metal are lower than the LOC (Limiting Organic Concentration) which limits in . practical the loading capacity of conventional commercial solvents to values ​​below 100 gL *. To achieve this maximum loading capacity, several methods are used to increase the LOC such as increasing the temperature and polarity of the diluent or the use of a phase modifier in the solvent such as fatty alcohols, monoamides like DHOA in the TODGA-dodecane system or phosphates like TBP used in the CMPO-dodecane system of the TRUEX actinide(III) / lanthanide(III) co-extraction process, with in all cases unfavorable consequences for the extraction in terms of efficiency as well as selectivity.

[0026] There is therefore a need for a process for selectively extracting one or more elements belonging to the rare earth group or the actinide group in an efficient, inexpensive and environmentally friendly manner, while avoiding this 3rd phase phenomenon.

[0027] It is therefore in order to meet this need that the inventors have developed the extraction process which is the subject of the present invention.

[0028] Very surprisingly and according to reasoning which goes completely against the teachings of the state of the art, the inventors discovered that by replacing all or part of the branched alkane type diluents usually used in liquid-liquid extraction with a hydrotropic agent, the 3rd phase phenomenon could be completely avoided.

[0029] More specifically, the invention relates to an extraction process designed for the recovery of one or more elements belonging to the rare earth group or to the actinide group, in which process a hydrotropic agent (co-solvent) combined with a solvating or ionic (cationic or anionic) extractant is used as a new extraction system. Said new extraction system replaces the organic phase conventionally used in liquid-liquid extraction processes and which comprises an extractant molecule, an often aliphatic diluent and a phase modifier.

[0030] The present invention therefore relates to a process for the liquid-liquid extraction of at least one salt of a metal chosen from the elements of the rare earth group and the actinide group from an acidic aqueous phase containing them, said process being characterized in that it comprises at least the following steps: (i) preparing a balanced acidic aqueous phase (Phaq e) comprising said at least one salt of said metal and at least one non-ionic hydrotropic agent in an initial quantity Ql, (ii) preparing an organic phase (Phorg) comprising at least one non-ionic hydrotropic agent in an initial quantity Q2 and at least one extracting agent,(iii) bringing said organic phase prepared above in step (ii) into contact with said balanced acidic aqueous phase prepared above in step (i), to obtain a M mix, (iv) stirring said mixture M, then (v) separating said organic phase from the balanced acidic aqueous phase.

[0031] Thanks to the liquid-liquid extraction method according to the present invention, it is possible to extract metals from the rare earth group and from the actinide group from an acidic aqueous phase efficiently and selectively, while reducing the increase in viscosity when said metals pass into the organic phase and reducing or eliminating the so-called "3rd phase" effect usually observed when carrying out extraction methods using conventional solvents such as hydrocarbons. In particular, the extraction method according to the present invention makes it possible to extract from an acid solution at least one rare earth and / or one actinide likely to be present in the acid solution, with high extraction yields and high selectivity with respect to metallic impurities likely to also be present in said acid solution and, in particular, with respect to iron.One of the advantages of the process according to the invention is also that it can be implemented in an extraction device comprising several stages (typically from 2 to 10 stages) operating countercurrently according to techniques well known to those skilled in the art. The liquid-liquid extraction process according to the invention can also be carried out in a closed circuit, which makes it possible to minimize effluents and drastically reduce costs. Finally, the process according to the invention is easily adaptable to any liquid-liquid extraction process and to any extraction installation existing in the industry.

[0032] The invention finds a particular application in the purification of uranium before manufacturing fuel elements, in the treatment of irradiated nuclear fuels or in the extraction of uranium from phosphates originating from mining concentrates.

[0033] The invention also finds an application in the production of rare earths whether from concentrates of natural ores rich in rare earths such as monazites, bastnaesites or xenotimes, from concentrates resulting from the processing of natural ores other than ores rich in rare earths such as concentrates from “urban mines”, that is to say from “mines” consisting of post-consumer industrial and domestic waste comprising rare earths and, in particular, waste from electrical and electronic equipment (also called “WEEE” or “D3E”), or from concentrates of scrap from the manufacture of products comprising rare earths as well as in the treatment of aqueous solutions resulting from leaching in order to recover the rare earths present in these aqueous solutions.

[0034] The rare earth group includes scandium (Sc), yttrium (Y) and the 15 lanthanide elements, namely cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), terbium (Tb), thulium (Tm) and ytterbium (Yb).

[0035] The actinide group includes the 15 chemical elements ranging from actinium (no. 89 - Ac) to lawrencium (no. 103 - Lr), and in particular uranium (no. 92 - U).

[0036] The process according to the present invention is particularly suitable for the extraction of metals chosen from so-called "light" rare earths, such as lanthanum (La) and neodymium (Nd), so-called "heavy" rare earths such as europium (Eu), dysprosium (Dy), erbium (Er) and ytterbium (Yb), or even uranium, and mixtures thereof.

[0037] According to the invention, a “hydrotropic agent” is a molecule which makes it possible to increase the solubility limit of any hydrophobic solute in water, without itself forming micelles in the water. There are acidic, basic or salt-forming electrolyte hydrotropes and non-electrolytes. Their molecular volume is greater than 0.090 nm3 and less than 0.5 nm3.

[0038] The non-ionic hydrotropic agent may in particular be chosen from primary, secondary or tertiary alcohols with a short or medium chain, as well as from alkyl ethers of alkylene glycols. For the purposes of the invention, a “short chain” is a carbon chain having from 1 to 4 carbon atoms and a “medium chain” is a carbon chain having 5 or 6 carbon atoms.

[0039] According to a particular embodiment of the invention, the non-ionic hydrotropic agents used during steps (i) and (ii) are chosen from: - alkylene glycol alkyl ethers, the alkyl being chosen from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl and pentyl and the alkylene being chosen from ethylene and propylene and dipropylene; and - primary alcohols containing a C2-C3 alkyloxy group.

[0040] Among such hydrotropic agents, mention may be made in particular of l-propoxy-2-propanol (PnP), dipropylene glycol n-propyl ether (DPnP) and ethylene glycol monopentyl ether (C5Ei). Among these, l-propoxy-2-propanol is particularly preferred.

[0041] According to a very particularly preferred embodiment of the invention, the non-ionic hydrotropic agent present in the balanced acidic aqueous phase is identical to the non-ionic hydrotropic agent present in the organic phase.

[0042] By “acidic aqueous phase” is meant a solution of an organic or inorganic acid, said acid being described as “strong” or “weak”.

[0043] The acidic aqueous phase may in particular be an acidic solution of a concentrate of a natural or urban ore comprising the salts of said metals.

[0044] The acid present in the acidic aqueous phase used according to the different stages of the The process according to the invention may in particular be chosen from strong acids such as nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid and mixtures thereof, weak acids such as acetic acid, formic acid, citric acid and tartaric acid and mixtures thereof, as well as from mixtures of at least one strong acid and at least one weak acid.

[0045] According to a particular embodiment of the invention, the balanced acidic aqueous phase comprises at least one strong acid in a concentration ranging from 1.10 4 to 6 mol / L, and preferably from approximately 0.01 to 4 mol / L.

[0046] According to a particular embodiment, and when the acid present in the acidic aqueous phase is a strong acid, and such as for example nitric acid, then the acid concentration preferably varies from 0.01 to 3 mol / L.

[0047] According to another particular embodiment of the invention, the acidic aqueous phase comprises at least one weak acid, such as for example acetic acid, in a concentration ranging from 1 to 6 mol / L, and preferably from 1 to 3 mol / L approximately.

[0048] The concentration of metal salt in the acidic aqueous phase is preferably between approximately 0.01 mol / L and 1.0 mol / L inclusive.

[0049] According to the extraction process according to the present invention, step (i) is a pre-equilibration step consisting of adding to an acidic aqueous solution containing at least one salt of a metal chosen from the elements of the rare earth group and the actinide group, a pre-equilibration phase comprising at least one hydrotropic agent in an initial quantity Ql, to obtain a balanced acidic aqueous phase (Phaq.e).

[0050] The pre-equilibration step allows the saturation of said acidic aqueous phase by said pre-equilibration phase and therefore the stability of the aqueous and organic phases during the extraction (steps (iii) and (iv)).

[0051] According to a particularly preferred embodiment of the invention, the pre-equilibration phase is pure l-propoxy-2-propanol (PnP) or pure dipropylene glycol n-propyl ether (DPnP) or pure ethylene glycol monopentyl ether (C5Ei). In this case, the initial quantity Ql of hydrotropic agent corresponds to the volume of pure hydrotropic agent which is used to saturate the acidic aqueous phase.

[0052] Preferably, step (i) is carried out while respecting a pre-equilibration phase / acid aqueous phase volume ratio of approximately 1:1 to 1:10 and even more preferably of approximately 1:3.

[0053] The duration of step (i) generally varies from 5 to 60 min, preferably from 5 to 20 minutes, and even more preferably this duration is approximately 10 minutes. A duration of 10 minutes makes it possible to obtain saturation of the aqueous phase with said pre-equilibration phase.

[0054] Step (i) is generally carried out at a temperature ranging from approximately 20 to 50°C. Preferably, step (i) is carried out at room temperature, i.e. at a temperature of approximately 20°C to 25°C.

[0055] Step (i) is preferably carried out at a pressure at least equal to atmospheric pressure, and even more preferably at atmospheric pressure.

[0056] According to a particular embodiment, the method according to the invention may further comprise, before step (i), at least one preliminary leaching step. This preliminary leaching step may be carried out in a conventional manner, for example by bringing a solid material comprising the rare earths and / or the actinides to be extracted into contact with a solution comprising high concentrations of a strong acid such as sulfuric acid, hydrochloric acid or nitric acid most often, or even a weak acid such as acetic acid. This leaching step makes it possible to obtain a solution of elements chosen from the elements of the rare earth group and the actinide group.

[0057] According to the invention, the organic phase comprises at least one hydrotropic agent in an initial quantity Q2 and at least one extracting agent. This organic phase generally has a density different by at least 0.1 mg / L compared to the density of the acidic aqueous phase, which makes it possible to easily separate the aqueous and organic phases during step (v).

[0058] The extracting agent present in the organic phase prepared in step (ii) may be chosen from charged or neutral molecules which are specific for at least one of the elements to be extracted from the balanced acidic aqueous phase and mixtures of these molecules. The extracting agent may in particular be chosen from non-ionic extractants based on phosphorus, amides, carboxylates and certain bifunctional molecules. According to a preferred embodiment, the extracting agent is chosen from bis(2-ethylhexyl) phosphoric acid (HDEHP), N,N'-dimethyl-N,N'-dioctylhexyl-ethoxy-malonamide (DMDOHEMA), N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA), trioctylamine (TOA) and mixtures thereof.

[0059] According to a particular embodiment of the process according to the invention, the metal to be extracted from the balanced acidic aqueous phase is a lanthanide and the extracting agent is chosen from HDEHP, DMDOHEMA and their mixtures.

[0060] According to a particularly advantageous embodiment of the process according to the invention, and when the metal to be extracted from the balanced acidic aqueous phase is a lanthanide, then the extracting agent is a mixture of HDEHP and DMDOHEMA. Indeed, as demonstrated in the examples below, the extraction yields of the lanthanides which are obtained with an extracting agent consisting of such a mixture are higher than the sum of the extraction yields which are obtained by an extracting agent consisting solely of HDEHP or consisting solely of DMDOHEMA, which indicates a synergistic effect of the mixture of HDEHP and DMDOHEMA on the extraction yield of rare earths according to the process according to the invention.

[0061] When the extracting agent is a mixture of HDEHP and DMDOHEMA, the molar fraction of DMDOHEMA in the DMDOHEMA / HDEHP mixture preferably varies from 0.3 to 0.8. Most preferably, the molar fraction of DMDOHEMA in the DMDOHEMA / HDEHP mixture is 0.5, which corresponds to the highest synergistic effect.

[0062] According to a particular embodiment of the method according to the invention, the metal to be extracted from the balanced acidic aqueous phase is an actinide, and in particular uranium, and the extracting agent is chosen from DEHiBA, TOA and their mixtures.

[0063] According to a preferred embodiment of the process according to the invention, the organic phase consists solely of at least one extracting agent and one non-ionic hydrotropic agent, that is to say that it does not comprise anything else, in particular that it does not comprise any organic diluent. It is in fact simpler to manage organic effluents with two constituents on an industrial scale than organic effluents with several constituents.

[0064] According to a preferred embodiment of the invention, the initial quantity Q2 of hydrotropic agent present in the organic phase varies from approximately 0.1 to 10 mol / L, and even more preferably from approximately 1.5 to 8 mol / L.

[0065] Although this does not represent a preferred embodiment of the process according to the invention, the organic phase may nevertheless comprise, in addition to the extracting agent and the non-ionic hydrotropic agent, an organic diluent of aliphatic type. In such a case, the organic phase preferably comprises at least 0.5 mol / L of non-ionic hydrotropic agent, preferably at least 4 mol / L and even better 5 mol / L of non-ionic hydrotropic agent.

[0066] Step (iii) of contacting can be carried out by simple mixing of the balanced acidic aqueous phase and the organic phase.

[0067] According to a preferred embodiment, the aqueous and organic phases are mixed in a volume proportion ranging from 1 / 1 to 1 / 4, and even more preferably in a volume proportion equal to 1 / 1.

[0068] The stirring step (iv) corresponds to the extraction step itself during which the salts of the metals initially present in the balanced acidic aqueous phase pass in whole or in part into the organic phase.

[0069] Step (iv) is generally carried out at a temperature of 20 to 50°C, preferably at a temperature of 20 to 30°C, and even more preferably at a temperature of 20 to 25°C, i.e. at room temperature.

[0070] The separation of the organic phase from the balanced acidic aqueous phase during step (iv) can be carried out for example by centrifugation.

[0071] According to a particular and preferred embodiment of the invention, the method is implemented in an extraction device comprising several stages, preferably operating countercurrently, each of the stages making it possible to implement steps (iii) to (v). In this case, the method then further comprises, after each step (v) and before each step (iii), at least one intermediate step of readjusting the quantities of non-ionic hydrotropic agents present respectively in the acidic aqueous phase and in the organic phase to values ​​identical to the initial values ​​Q1 and Q2.

[0072] The salts of the metals extracted from the balanced acidic aqueous phase and present in the organic phase after separation can then be recovered.

[0073] Thus, according to a preferred embodiment, the method further comprises at least: - a step (vi) of back-extraction of the salts of the metals present in the organic phase at the end of step (v), said step (vi) comprising at least one contacting of said organic phase with an aqueous phase called “back-extraction solution”, said solution containing at least one non-ionic hydrotropic agent in an initial quantity Q3, then - a step (vii) of separating the organic phase from said extraction phase to recover a extraction phase comprising the salts of said metals.

[0074] The de-extraction solution may consist of water, in particular distilled water, that is to say that it only comprises water and said non-ionic hydrotropic agent or else a mixture of water and at least one acid and said non-ionic hydrotropic agent.

[0075] The initial concentration Q3 of ionic hydrotropic agent in the extraction solution preferably varies from approximately 0.1 to 5 mol / L, and even more preferably from approximately 0.5 to 4 mol / L.

[0076] According to a particularly advantageous and preferred embodiment of the invention, the non-ionic hydrotropic agent present in the stripping solution is identical to the non-ionic hydrotropic agent of the organic phase, itself preferably being identical to the non-ionic hydrotropic agent present in the balanced acidic aqueous phase.

[0077] When the stripping solution contains an acid, then said acid is preferably the same as that present in the balanced acidic aqueous phase.

[0078] According to a preferred embodiment of the invention, steps (vi) and (vii) are implemented in a multi-stage back-extraction battery, preferably operating countercurrently in each of the stages enabling steps (vi) and (vii) to be implemented. In this case, the method according to the invention then further comprises, after each step (vii) and before each step (vi), at least one intermediate step of readjusting the quantity of non-ionic hydrotropic agent present in the extraction solution to a value identical to the initial value Q3.

[0079] A second object of the invention is the use of an organic phase as defined above, that is to say comprising at least one extracting agent and one non-ionic hydrotropic agent for the extraction of a metal chosen from the elements of the rare earth group and the actinide group from an acidic aqueous phase containing said metals and a non-ionic hydrotropic agent.

[0080] According to this use, the organic phase preferably comprises only one or more extracting agents and a non-ionic hydrotropic agent. In this case, said organic phase does not comprise anything else, in particular it does not comprise any organic diluent.

[0081] According to a preferred embodiment of this use, the non-ionic hydrotropic agent of the organic phase is identical to the non-ionic hydrotropic agent of the acidic aqueous phase.

[0082] Other features and advantages of the invention will emerge from the detailed description of the examples which follow, as well as from the appended figures in which:

[0083] [Fig-1]: [Fig.l] illustrates the influence of the nature of the acid in the aqueous phase on the extraction of europium (Eu) from this acidic aqueous phase at two different acid concentrations: [Fig.l] (a): 1 M and [Fig.l] (b): 0.03 M;

[0084] [Fig.2]: [Fig.2] shows the evolution of the selectivity coefficients of rare earths (La, Nd, Eu, Dy, Er, Yb) with respect to iron, noted SLn / Fe depending on the nature of the acid. [Fig.2] (a): Nitric acid, [Fig.2] (b): phosphoric acid, [Fig.2] (c): sulfuric acid and [Fig.2] (d): hydrochloric acid;

[0085] [Fig.3]: [Fig.3] shows the variation of the europium extraction yield as a function of time;

[0086] [Fig.4]: [Fig.4] illustrates the influence of PnP on the relative viscosity of the phase organic after extraction;

[0087] [Fig.5]: [Fig.5] shows the load capacity of the organic phase containing the hydrotrope compared to conventional extraction systems;

[0088] [Fig.6]: [Fig.6] illustrates the influence of the concentration of the acid in the aqueous phase and the concentration of the extractant in the organic phase on the extraction of europium;

[0089] [Fig.7]: [Fig.7] illustrates the influence of the nature of the acid in the aqueous phase on the extraction of europium (Eu) from this acidic aqueous phase at two different acid concentrations: [Fig.l] (a): 0.3 M and [Fig.l] (b): 1 M;

[0090] [Fig.8]: [Fig.8] shows the load capacity of the organic phase containing the hydrotrope compared to an organic phase containing n-dodecane;

[0091] [Fig.9]: [Fig.9] shows the variation in the extraction yield of europium in function of time

[0092] [Fig.10]: [Fig.10] shows the influence of the use of PnP on synergy;

[0093] [Fig. 11]: [Fig. 11] shows the evolution of the distribution coefficients noted DLneq, as a function of the molar fraction of the DMDOHEMA extractant, noted xDMDOHEma;

[0094] [Fig. 12]: [Fig. 12] shows the evolution of the distribution coefficients of Europium depending on the nature of the hydrotrope used;

[0095] [Fig. 13]: [Fig. 13] shows the distribution coefficients of europium in function of the acidity of the aqueous phase with two different extractants. [Fig. 13] (a): HDEHP and [Fig. 13] (b): DMDOHEMA;

[0096] [Fig. 14]: [Fig. 14] shows the evolution of the land distribution coefficients rare as a function of the acidity of the aqueous phase for two different extractants. [Fig. 14] (a): HDEHP and [Fig. 14] (b): DMDOHEMA;

[0097] [Fig. 15]: [Fig. 15] shows the evolution of the uranium distribution coefficient depending on the concentration of the DEHiBA extractant;

[0098] [Fig. 16]: [Fig. 16] shows the evolution of the uranium distribution coefficient depending on the concentration of the TOA extractant; and

[0099] [Fig. 17]: [Fig. 17] schematically represents an example of implementation of the recovery process according to the invention designed for the extraction, on an industrial scale, of rare earths and uranium, from an acidic aqueous phase.

[0100] EXAMPLES

[0101] The experimental results reported in the examples below were obtained using acidic aqueous phases comprising light rare earths, namely lanthanum (La) and / or neodymium (Nd), and heavy rare earths, namely europium (Eu), dysprosium (Dy), erbium (Er) and / or ytterbium (Yb) or an actinide, namely uranium. In order to evaluate the selectivity of the extraction systems studied, iron was also added to the aqueous phase.

[0102] As extracting agents, bis(2-ethylhexyl) phosphoric acid (HDEHP) and N,N'-dimethyl,N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) were used to extract rare earths while uranium was extracted using di-ethylhexyl-isobutyramide (DEHiBA) or trioctylamine (TOA). The extraction of these elements from an aqueous solution of nitric, sulfuric, phosphoric or hydrochloric acid was carried out by bringing into contact a pre-equilibrated aqueous solution containing the element(s) to be extracted, with an organic phase comprising the extracting agent at a concentration of 0.1 to 2 mol / L diluted in one of the following hydrotropic agents (diluents): l-propoxy-2-propanol (PnP), dipropylene glycol n-propyl ether (DPnP) or ethylene glycol monopentyl ether (C5E1).The results obtained were then compared to those obtained by dissolving the same extracting agents in “classic” aliphatic or aromatic diluents such as n-dodecane, tetra-. hydrogenated propylene (TPH), toluene, isooctane or isoparaffinic solvents such as Tisane ® (IP-175).

[0103] The distribution coefficients, extraction efficiencies and selectivity coefficients, which are reported in the following examples, were determined in accordance with the conventions of the field of liquid-liquid extractions, namely that: - the distribution coefficient of a metallic element M, noted DM, between two phases, respectively organic and aqueous, is determined by the following equation (2): [Equation 2]: [M] (2) Fj - or8 [M] aq

[0104] in which: [M] is the concentration of the metallic element M in the organic phase after extraction, [M] is the concentration of the metallic element M in the aqueous phase after extraction.

[0105] - the extraction yield of a metallic element M, noted EM, from a phase aqueous is determined by the following equation (3): [Equation 3]: [ M ] „ (3) F __________O1'g _ [M] . .-Dm+1 aq, initiai in which: [M] , and DM have the same meaning as before; whereas org [ M ] is the concentration of the metallic element M in the aqueous phase aq, initial before extraction; while the selectivity coefficient of a metallic element Ml with respect to a metallic element M2, noted SMi / m2, is determined by the following equation (4): [Equation 4]: _ DM1 (4) dm2 in which: is the distribution coefficient of the metallic element Ml, and DM2 is the distribution coefficient of the metallic element M2.

[0106] EXAMPLE 1: Separation of lanthanides (La, Nd, Eu, Dy, Er and Yb) according to the process of the invention using an anionic extractant (HDEHP)

[0107] 1.1 Influence of the nature of the acid:

[0108] Tests aimed at extracting rare earths from an acidic aqueous phase were first carried out in tubes using: - As organic phases: phases consisting of the extracting agent HDEHP at a concentration fixed at 0.6 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel. For comparison, the isoparaffinic solvent Isane® IP 175 was taken as a reference; and - As aqueous phases: aqueous solutions previously pre-equilibrated with PnP alone comprising 10 mM hydrated europium nitrate, and 0.03 M or 1 M nitric acid, phosphoric acid, sulfuric acid or hydrochloric acid in a PnP / aqueous phase volume ratio of 3:1.

[0109] The organic and aqueous phases were brought into contact with each other in an organic phase (Org) / aqueous phase (Aq) ratio (Org / Aq) of 1 (v / v), for 1 h, at room temperature and with rotary stirring. The organic and aqueous phases were then separated by centrifugation at 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered by inductively coupled plasma optical emission spectrometry (ICP-AES, Spectro Arcos device marketed by AMETEK) and by energy dispersive X-ray fluorescence spectrometry (EDXRF - SPECTRO brand device, XEPOS model, marketed by AMETEK).

[0110] The results of these tests are reported in the attached [Fig. 1] which represents the distribution coefficients of Europium, noted DEu, as a function of the nature of the acid used. In these figures, the results given by the white bars correspond to the comparative extraction process not in accordance with the invention and carried out with Isane IP 175 as solvent and the hatched bars correspond to the extraction process in accordance with the present invention and carried out using PnP as hydrotropic agent. [Fig.l] (a) shows the case where the concentration of the different acids used was fixed at 1 M while [Fig.l] (b) shows the case where the concentration was fixed at 0.03 M.

[0111] As shown in [Fig.l] (a), at a concentration of 1 M of acid, no measurable extraction of europium is observed in the case where the extracting agent HDEHP is diluted in Isane IP 175. On the other hand, these results demonstrate that the extraction of europium by the extracting agent HDEHP is largely favored by the replacement of Isane IP 175 by PnP in the organic phase since an increase in the distribution coefficient of at least a factor of 10 is observed.

[0112] [Fig.l] (b) shows that by carrying out the extraction of europium using Isane IP 175 as solvent and at a concentration of 0.03 M of acid, the distribution coefficients have considerably increased compared to [Fig.l] (a) and an ame A net improvement in extraction is observed in the case of PnP with at least a factor of 2. This is consistent with the literature which, to the inventors' knowledge, does not report any study showing the possibility of extracting rare earths from an aqueous phase with an acidity greater than or equal to 1 M with conventional diluents.

[0113] 1.2. Study of selectivity towards iron:

[0114] The selectivity of the extraction of rare earths with respect to iron, which constitutes the main impurity in all the processes, was assessed by tests which were carried out in tubes using:

[0115] - As organic phases: phases comprising, as extracting agent, HDEHP at a concentration set at 0.6 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel. For comparison purposes, the isoparaffinic solvent Isane® IP 175 was taken as a reference; and - As aqueous phase: aqueous solutions comprising lanthanum (La), neodymium (Nd), europium (Eu), dysprosium (Dy), erbium (Er), ytterbium (Yb) and iron (Fe) at rare earth concentrations of 10 mM each and 30 mM for iron. These aqueous solutions were diluted in an acid solution comprising 1 M nitric, phosphoric, sulfuric or hydrochloric acid.

[0116] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1 h, at room temperature and with rotary stirring. The organic and aqueous phases were then separated by centrifugation at 5000 rpm for 20 minutes. The concentrations of rare earths as well as iron were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated above in point 1.1)

[0117] The results of these tests are reported in the attached [Fig.2] which represents the selectivity coefficients of rare earths with respect to iron, noted SFLn / Fe, for the different acids used: (a) nitric, (b) phosphoric, (c) sulfuric and (d) hydrochloric. In these figures, the results given by the white bars correspond to the comparative extraction process not in accordance with the invention and carried out with Tisane IP 175 as solvent and the hatched bars correspond to the extraction process in accordance with the present invention and carried out using PnP as hydrotropic agent.

[0118] As shown in these figures, the selectivity of the process with respect to the extraction of all rare earths compared to iron is considerably improved with the replacement of Tisane IP 175 by PnP in accordance with the process according to the present invention.

[0119] 1.3 Kinetics of extraction

[0120] The influence of time on the efficiency of extraction was assessed by tests which were carried out in tubes using: - As organic phases: phases comprising, as extracting agent, HDEHP at a concentration fixed at 0.6 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel. For comparison, the isoparaffinic solvent Isane® IP 175 was taken as a reference; and - As aqueous phase: aqueous solutions comprising 10 mM europium (Eu) in an acid solution comprising 0.03 Mol / L nitric acid in ultrapure water (i.e. Milli-Q water, with a resistivity greater than 18 MQ / cm at 25°C).

[0121] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1, 3, 10, 30, 60, 120 and 180 min, at room temperature and with rotary stirring. The organic and aqueous phases were then separated by centrifugation at 5000 rpm for 20 minutes. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated above in point 1.1).

[0122] The results of these tests are reported in the attached [Fig.3] which represents the variation of the europium extraction yield (E in %) as a function of time (in min.). In this figure, the curve with the solid circles corresponds to the experiments carried out using Isane® IP 175 as solvent according to a process not in accordance with the invention and the curve with the solid squares corresponds to the experiments carried out using PnP as a hydrotropic agent according to the process in accordance with the invention.

[0123] As shown by the results presented in [Fig.3], the kinetics of europium extraction is faster and reaches a yield plateau of 80% after 10 min and 99% after 30 min using PnP according to the process according to the invention, whereas it takes twice as long, i.e. 60 min, to obtain this same yield of 99% with the process not according to the invention. 1.4 Viscosity of organic phases after extraction

[0124] Viscosity in the organic phase is a crucial parameter on an industrial scale that must be taken into account. To do this, the influence of the concentration of salts as well as the extracting agent on the viscosity of the organic phases after extraction was assessed by tests that were carried out in tubes using: - as organic phases: phases comprising the HDEHP extractant at concentrations ranging from 0.6 mol / L to 2 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel. For comparison, the isoparaffinic solvent Isane® IP 175 was taken as a reference; and - As aqueous phase: aqueous solutions comprising 10 mM of dysprosium (Dy) in an acid solution comprising 0.03 Mol / L of nitric acid in ultrapure water (i.e. Milli-Q water, with a resistivity greater than 18 MQ / cm at 25°C).

[0125] The organic and aqueous phases were brought into contact with each other in an Org / Aa ratio of 1 (v / v), for 60 min, at room temperature and with rotary stirring. The organic and aqueous phases were then separated by centrifugation at 5000 rpm for 20 minutes. The viscosities of the organic phases were measured at 25°C, before and after extraction using an automated rolling ball viscometer, reference AMVn (Anton Paar Company, Graz, Austria).

[0126] The results of these tests are reported in [Fig.4] attached in which the relative viscosity (in mPa.s) is expressed as a function of the initial dysprosium concentration (initial C in mol.L-1) (figure 4a) or as a function of the initial concentration of extracting agent (initial CHDEHP in mol.L-1) (figure 4b). In these figures, the results given by the white bars correspond to the comparative extraction process not in accordance with the invention and carried out with Tisane IP 175 as solvent and the hatched bars correspond to the extraction process in accordance with the present invention and carried out using PnP as hydrotropic agent.

[0127] As shown in [Fig. 4], when the extraction is carried out according to the process according to the present invention, the relative viscosity of the aqueous phase hardly varies with the concentration of dysprosium in the aqueous phase and varies very little depending on the concentration of the extractant and does not exceed 1.5 times the viscosity of the organic phase before extraction. On the other hand, when the extraction is carried out using Tisane IP 175 according to the comparative process not according to the invention, it is observed that the viscosity of the organic phase after extraction can increase up to 5 times compared to the viscosity before extraction.

[0128] 1.5. Load capacity

[0129] In order to evaluate the efficiency of the extraction process according to the present invention, load capacity tests were carried out and compared to the load capacities of extraction systems using “conventional” diluents.

[0130] Load capacity tests were assessed and were carried out in tubes using: - As organic phases: phases comprising, as extracting agent, HDEHP at a concentration fixed at 0.6 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel, namely with n-heptane, dodecane, isooctane and toluene. - As aqueous phase: aqueous solutions comprising europium (Eu) at concentrations ranging from 10 mmol / L to 400 mmol / L in an acid solution comprising 0.03 Mol / L of nitric acid in ultrapure water (i.e. Milli-Q water, with a resistivity greater than 18 MQ / cm at 25°C).

[0131] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 60 min, at room temperature and with rotary stirring. The organic and aqueous phases were then separated by centrifugation at a speed of 5000 rpm for 20 minutes. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated above in point 1.1).

[0132] The results of these tests are reported in the attached [Fig.5] which represents the variation of the concentration of europium in the organic phase at equilibrium, noted CEu,org and expressed in mmol.L *, as a function of the concentration of europium in the initial aqueous phases (CEu, initial / mmol.L *). In this figure, the curve with the solid lines corresponds to the extraction in the presence of PnP according to the process in accordance with the invention. The other curves correspond to the extractions carried out with conventional solvents according to processes not in accordance with the invention: curve with the solid squares n-heptane, curve with the solid triangles point up: dodecane, curve with the solid triangles point down: dodecane + 5% of PnP, curve with the solid diamonds: isooctane and curve with the empty circles: toluene.

[0133] [Fig.5] shows that a start of saturation of the europium extraction is observed in the case of heptane, dodecane, isooctane and toluene with a formation of a 3rd phase for the first three diluents. On the other hand, no saturation or 3rd phase was observed for PnP according to the process of the invention, and a loading capacity greater than 250 mmol / L can be expected for europium concentrations in the aqueous phase greater than 400 mmol / L. Concerning the mixture n-dodecane + 5% PnP, the loading capacity is less important than in the case where n-dodecane is completely replaced with PnP. Nevertheless, the 3rd phase is also avoided in the concentration range tested, even with a quantity of PnP of 5%.

[0134] In any case, these results show the possibility of loading the organic phase with rare earths using PnP beyond 400 mmol / L, which usually represents the loading limit of the extraction systems conventionally used in the field of rare earth extraction.

[0135] 1.6 Influence of the acidity of the aqueous phase and the concentration of the ex- agent tractant in the organic phase

[0136] The influence of the acidity of the nitric aqueous phase as well as the concentration of the extracting agent in the organic phase on the capacity of this system to extract europium was assessed by tests which were carried out in tubes using: - As organic phases: phases comprising, as extracting agent, HDEHP at concentrations ranging from 0.01 mol / L to 3 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using a conventional solvent were carried out in parallel with dodecane. - As aqueous phase: aqueous solutions comprising 10 mmol / L of europium (Eu) in a nitric acid solution at concentrations ranging from 0.1 Mol / L to 3 mol / L of nitric acid in ultrapure water (i.e. Milli-Q water, with a resistivity greater than 18 MQ / cm at 25°C).

[0137] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 60 min, at room temperature and with rotary stirring, then separated by centrifugation at 5000 rpm for 20 minutes. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated above in point 1.1)

[0138] The results of these tests are reported in the attached [Fig.6]. [Fig.6] (a) shows the variation of the europium distribution coefficient (DEueq) as a function of the initial concentration of HDEHP (CHdehp, initiai) in mol.L 1 in the organic phase, whereas [Fig.6] (b) shows the variation of the europium distribution coefficient (DEu,eq) as a function of the initial concentration of nitric acid in the aqueous phase (Chncb, initiai) in mol.L *. In this figure, the curves with the solid circles correspond to the extraction carried out using PnP according to the method of the invention while the curves with the solid squares correspond to the extraction carried out using dodecane according to a method not forming part of the invention.

[0139] [Fig.6] (a) shows that increasing the concentration of extracting agent improves the efficiency of the extraction without the appearance of a 3rd for PnP according to the process according to the invention, contrary to what is observed using a conventional solvent, dodecane, according to a process not forming part of the invention. [Fig.6] (b) shows that increasing the acidity of the aqueous phase results in a progressive decrease in the extraction of europium by PnP. Beyond 1 mol / L of nitric acid, the values ​​of the distribution coefficient tend towards zero for the extraction process carried out with dodecane while the decrease in the distribution coefficient remains negligible when the extraction is carried out in the presence of PnP. This widens the range of acid concentrations to be used according to the process in accordance with the invention.

[0140] EXAMPLE 2: Process for separating lanthanides: La, Nd, Eu, Dy, Er and Yb according to the process in accordance with the invention using DMDOHEMA as extracting agent

[0141] 2.1 Influence of the nature of the acid:

[0142] Tests aimed at extracting rare earths from an acidic aqueous phase were first carried out in tubes using: - As organic phases: phases consisting of the extractant DMDOHEMA at a concentration fixed at 0.6 mol / L diluted in PnP. In order to be able to compare the results of the extraction experiments according to the method according to the invention with a comparative extraction method not forming part of the invention, extraction experiments using conventional solvents were carried out in parallel. For comparison, the isoparaffinic solvent Isane® IP 175 was taken as a reference; and - As aqueous phases: aqueous solutions previously pre-equilibrated with PnP alone comprising 10 mM hydrated europium nitrate, 1 M or 3 M nitric, phosphoric, sulfuric or hydrochloric acid.

[0143] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1 h, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5,000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0144] The results of these tests are reported in the attached [Fig.7] which represents the Europium distribution coefficients, noted DEueq, as a function of the nature of the acid used. [Fig.7] (a) shows the case where the concentration of the different acids used is fixed at 1 M whereas in [Fig.7] (b), the concentration is fixed at 3 M. In these figures, the white bars correspond to the results obtained by implementing an extraction process using Tisane® IP175 not in accordance with the invention whereas the hatched bars correspond to the results obtained by implementing an extraction process in accordance with the invention and using PnP.

[0145] As shown in [Fig.7] (a), at a concentration of 1 M of acid, no measurable extraction of europium is observed in the case where the extracting agent is diluted in Tisane® IP 175. On the other hand, the extraction of europium is favored by the replacement of Isane® IP 175 by PnP in the organic phase and an increase in the distribution coefficient of at least a factor of 20 is observed.

[0146] [Fig.7] (b) shows that with Isane® IP 175 as diluent and at 3 M extractant concentration, the distribution coefficients have considerably increased compared to [Fig.7] (a) and a clear improvement in extraction is observed in the case of PnP with at least a factor of 2.

[0147] 2.2 Load capacity

[0148] In order to evaluate the efficiency of the extraction process according to the invention, load capacity tests were carried out and compared to the load capacities of extraction systems using “conventional” diluents.

[0149] Load capacity tests were assessed and were carried out in tubes using: - As organic phases: phases comprising the extracting agent DMDOHEMA at a concentration fixed at 0.6 mol / L diluted in PnP. To be able to compare the results, extraction experiments with conventional diluents were carried out in parallel with dodecane. - As aqueous phase: aqueous solutions comprising concentrations ranging from 10 mmol / L to 400 mmol / L of europium (Eu) in an acid solution comprising 3 Mol / L of nitric acid in ultrapure water (i.e. Milli-Q water, with a resistivity greater than 18 MQ / cm at 25°C);

[0150] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 60 min, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0151] The results of these tests are reported in the attached [Fig.8] which represents the variation of the concentration of europium in the organic phase at equilibrium, noted CEueq and expressed in mmol / L, as a function of the concentration of europium in the initial aqueous phases (CEUjinitiai in mmol.L *). In this figure, the curve with the solid circles corresponds to the results obtained by implementing the extraction process in accordance with the invention and using PnP while the curve with the solid squares corresponds to the results obtained by implementing the extraction process not in accordance with the invention and using dodecane.

[0152] [Fig.8] shows that the beginning of saturation of the europium extraction is observed in dodecane with the formation of a 3rd phase. On the other hand, no saturation or 3rd phase was observed for PnP, a loading capacity greater than 250 mmol / L can be expected for europium concentrations in the aqueous phase greater than 400 mmol / L.

[0153] In any case, these results show the possibility of loading the organic phase with rare earths using PnP beyond 400 mmol / L, which represents the loading limit of extraction systems conventionally used in the field of the extraction of rare earths.

[0154] 2.3 Kinetics of extraction

[0155] The influence of time on the extraction efficiency was assessed by tests carried out in tubes using: - As organic phases: phases comprising, as extracting agent, DMDOHEMA at a concentration fixed at 0.6 mol / L diluted in PnP. To be able to compare the results, extraction experiments with conventional diluents were carried out in parallel with dodecane; and - As aqueous phase: aqueous solutions comprising 10 mM europium (Eu) in an acid solution comprising 3 Mol / L nitric acid in ultrapure water (i.e. Milli-Q water, with resistivity greater than 18 MQ / cm at 25°C).

[0156] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1.3, 10, 30, 60, 120 or 180 min, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0157] The results obtained are reported in the attached [Fig.9] in which the variation in the europium extraction yield (E%) is expressed as a function of time (in min.). In this figure, the curve with the solid squares corresponds to the results obtained by implementing the process according to the invention using PnP, while the curve with the solid circles corresponds to the results obtained by implementing a process not according to the invention and using dodecane as the solvent for the extracting agent.

[0158] As shown in [Fig.9], the extraction kinetics of europium is faster and reaches an efficiency value of 80% after 15 min and a yield plateau of 99% after 60 min using PnP according to the method of the invention.

[0159] EXAMPLE 3: Extraction process according to the invention using two extractants in anionic and solvating synergy (HDEHP and DMDOHEMA) for the separation of lanthanides: Lanthanum, Neodymium, Europium, Dysprosium, Erbium and Ytterbium

[0160] The synergistic effect of a mixture of HDEHP and DMDOHEMA on the extraction of rare earths from an aqueous nitric acid solution as well as the influence of the DMDOHEMA / HDEHP molar ratio of this mixture on these extracting properties were assessed by extraction tests carried out using the following phases: - As aqueous phase: phases obtained by dissolving five rare earth salts in oxidation state (III) in the respective forms: La(NO3)3, Nd(NO3)3, Eu(NO3)3, Dy(NO3)3 and Yb(NO3)3, at a rate of 10 mmol / L of each of these salts, in solutions comprising 1 mol / L of nitric acid in ultrapure water (i.e. water Milli-Q, with resistivity greater than 18 MQ / cm at 25°C); and - As organic phases: phases comprising 0.6 mol / L of an extracting agent in PnP, this extracting agent being composed either solely of DMDOHEMA, or solely of HDEHP, or of a mixture of DMDOHEMA and HDEHP in which the molar ratio of DMDOHEMA to HDEHP is varied (from 0 to 1). To be able to compare the results, similar extraction experiments but with a conventional diluent were carried out in parallel with Tisane® IP175.

[0161] Since iron is naturally present as a major impurity in ores, and in particular natural ores, the aqueous phases also included 30 mmol / L of iron(III) nitrate.

[0162] The extraction tests were carried out using an aqueous phase / organic phase (Aq / Org) volume ratio of 1 (v / v). The aqueous and organic phases were brought into contact for 1 hour at a constant temperature (25°C), after which they were separated from each other by centrifugation (5000 rpm) for 20 minutes at 25°C. The concentrations of rare earths were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0163] The results of these tests are presented in the attached figures 10 and 11, in which the rare earth distribution coefficients are indicated, noted DLn>eq, as a function of the molar fraction of DMDOHEMA relative to HDEHP, noted x dmdohema- In these figures, the curves with the solid squares correspond to the results obtained by implementing the process in accordance with the invention and using PnP, while the curves with the solid circles correspond to the results obtained by implementing the process not in accordance with the invention and using Tisane® IP175.

[0164] [Fig. 10] compares the distribution coefficients of lanthanum ([Fig. 10] (a)) and europium ([Fig. 10] (b)) in the case of PnP (solid squares) and Isane® IP 175 (solid circles), as a function of the molar fraction X of DMDOHEMA noted XDMDOHEma-

[0165] [Fig. 11] shows the evolution of the distribution coefficients of the different lan-thanides noted DLn, eq, as a function of the molar fraction X of the extracting agent DMDOHEMA, noted XDMDOHEma- In this figure, La = curve with solid circles, Nd = curve with solid squares, Eu = curve with solid triangles pointing up, Dy = curve with solid triangles pointing down, and Yb = curve with solid diamonds.

[0166] In Figures 10 and 11, the results given for xDMDOhema= 0 correspond to the results obtained when the extracting agent consists solely of HDEHP while the results given for xDMDOhema=1 correspond to the results obtained when the extracting agent consists solely of DMDOHEMA.

[0167] These results show that, for all rare earths, the extraction yields which are obtained with an extracting agent consisting of a mixture of DMDOHEMA and HDEHP are higher than the sum of the extraction yields which are obtained, on the one hand, by an extracting agent consisting solely of DMDOHEMA, and, on the other hand, by an extracting agent consisting solely of HDEHP, which indicates a synergistic effect of the mixture of DMDOHEMA and HDEHP on the extraction of all these rare earths.

[0168] They also show that the highest synergistic effect of the mixture of DMDOHEMA and HDEHP is observed for a DMDOHEMA molar fraction of 0.5 and higher extraction yields in the case of PnP compared to dodecane.

[0169] EXAMPLE 4: Extraction of earth according to the process in accordance with the invention and using PnP, DPnP or C5Ei as hydrotropic agent:

[0170] Attempts to replace the PnP hydrotrope with other cosolvent hydrotropic agents to extract rare earths from an acidic aqueous phase were first carried out in tubes using: - As organic phases: phases comprising, as extracting agent, HDEHP at a concentration fixed at 0.6 mol / L diluted in PnP, DPnP or C5Ei; and - As aqueous phases: aqueous solutions previously pre-equilibrated with the hydrotropic agent to be used alone and comprising 10 mM of hydrated europium nitrate and 0.03 M of nitric acid.

[0171] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1 h, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0172] The results of these tests are reported in the attached [Fig. 12] which represents the europium distribution coefficients, noted DEu, according to the nature of the hydrotropic agent used. In this figure the europium distribution coefficients (DEUeq) are given for each of the hydrotropic agents used: the white bar corresponds to PnP, the bar with the diagonal hatching corresponds to DPnP and the bar with the vertical hatching corresponds to C5Ei. As shown in [Fig. 12], the three hydrotropic agents are effective for the extraction of rare earths.

[0173] EXAMPLE 5: Extraction process according to the invention comprising a preliminary leaching step and using weak organic acids for leaching, extraction and back-extraction

[0174] In this example the extraction of europium was carried out under “mild” or “green” conditions, i.e. without using strong acid.

[0175] Tests aimed at extracting rare earths from an aqueous phase comprising a weak acid such as acetic acid were first carried out in tubes using: - As organic phases: phases comprising, as extracting agent, HDEHP or DMDOHEMA at a concentration fixed at 0.6 mol / L diluted in PnP. In order to be able to compare the results, extraction experiments with a conventional diluent were carried out in parallel with dodecane; and - As aqueous phases: aqueous solutions previously pre-equilibrated with PnP alone and comprising 10 mM of hydrated europium nitrate and 1 M to 6 M of acetic acid.

[0176] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1 h, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0177] The results of these tests are reported in the attached figures 13 and 14. [Fig. 13] represents the distribution coefficients of Europium, noted DEu>eq, as a function of the concentration of acetic acid. [Fig. 13] (a) shows the case where the process was implemented with HDEHP as the extracting agent, whereas in [Fig.13] (b), the extracting agent used is DMDOHEMA.

[0178] [Fig. 14] shows the evolution of the rare earth distribution coefficients (DL>>eq) as a function of the initial acidity of the aqueous phase (Cacid>initial in mol.L *) for two different extracting agents. [Fig. 14] (a): HDEHP and [Fig. 14] (b): DMDOHEMA. In these figures 13 and 14, the curves correspond to: solid circles: La; solid squares: Nd; Solid triangles pointing up: Eu; Solid triangles pointing down: Dy; and solid diamonds Yb.

[0179] As shown in [Fig. 13] (a), the extraction of europium with HDEHP is effective and considerably improved by replacing dodecane with PnP by a factor of at least 2.

[0180] In [Fig. 13] (b), no measurable extraction of europium is observed in the case where the extracting agent is diluted in dodecane while the extraction becomes effective (distribution coefficient increases from 0.6 to 2.8 at 6 M acetic acid). Thus, according to the extraction process according to the present invention, the use of a hydrotropic agent makes it possible to dispense with the presence of a strong acid. The use of a hydrotropic agent acting as a co-solvent makes it possible to implement known extraction processes with strong but not sufficiently effective acids without having to synthesize new complicated organic molecules. In this example of implementation of the process according to the invention, a synergy of the actions is observed of hydrated weak organic acid and of all hydrotropic agents.

[0181] In [Fig.14] (a), the extraction efficiency decreases drastically with increasing acidity for light rare earths and very little for heavy rare earths. Acidity plays an important role in the extraction efficiency and induces selectivity between light and heavy rare earths. [Fig. 14] (b) shows that with DMDOHEMA the effect is opposite. Extraction is more efficient at high acetic acid concentrations.

[0182] EXAMPLE 6: Extraction of uranium according to the process in accordance with the invention

[0183] Tests aimed at extracting uranium from an acidic aqueous phase were first carried out in tubes using: - As organic phases: phases comprising, as extracting agent, DEHiBA or TOA at a variable concentration of 0.01 to 2 mol / L diluted in PnP. To be able to compare the results, extraction experiments with conventional diluents were carried out in parallel, in particular with dodecane, toluene, TPH and isooctane. - As aqueous phases: aqueous solutions previously pre-equilibrated with PnP alone comprising 250 ppm of uranium nitrate or sulfate diluted in 3 M of nitric or sulfuric acid.

[0184] The organic and aqueous phases were brought into contact with each other in an Org / Aq ratio of 1 (v / v), for 1 h, at room temperature and with rotary stirring, then separated by centrifugation at a speed of 5000 rpm for 20 min. The concentrations of europium were measured in the aqueous and organic phases thus recovered (ICP-AES and EDXRF as indicated in Example 1 above in point 1.1).

[0185] The results of these tests are illustrated in [Fig. 15] which represents the distribution coefficients of uranium, noted Du (in mol.L-1) as a function of the concentration of DEHiBA diluted in different solvents (solid circles: TPH, solid squares: dodecane, solid triangles pointing up: toluene and solid triangles pointing down: PnP).

[0186] [Fig. 15] shows that uranium extraction is significantly better with PnP without the appearance of a 3rd phase or an increase in viscosity of the organic phase than with conventional solvents.

[0187] [Fig. 16] represents the uranium distribution coefficients (Du>eq) as a function of the concentration of the TOA extracting agent diluted in different solvents (solid circles: dodecane, solid squares: isooctane, solid triangles pointing up: toluene and plain triangles pointing down: PnP).

[0188] [Fig. 16] shows that uranium extraction is significantly better with PnP without the appearance of a 3rd phase or an increase in viscosity of the organic phase, whereas with conventional solvents such as dodecane for example, the 3rd phase appeared from a concentration of 0.1 mol / L. This allows a more compact implementation with lower quantities of reagents. The extraction process according to the present invention therefore makes it possible to ensure that a third phase accident cannot occur for fundamental reasons, thus avoiding the use of solvent modifiers, but also to increase the safety margins against criticality accidents compared to current processes.

[0189] EXAMPLE 7: Example of an industrial installation for implementing the extraction process in accordance with the invention

[0190] The extraction method according to the present invention and illustrated in examples 1 to 5 above can for example be implemented on an industrial extraction installation 1 as shown in the attached [Fig. 17].

[0191] The installation 1 comprises a tank 2 containing an acidic aqueous phase 21 loaded with metals, a tank 3 containing a non-ionic hydrotropic agent 31, the tanks 2 and 3 respectively supplying via the pipes 211 and 311 a container 4 for mixing the acidic aqueous phase 21 loaded with metals with the hydrotropic agent 31 in order to obtain a balanced acidic aqueous phase 4L. A pipe 412 makes it possible to return the non-ionic hydrotropic agent to the tank 3. The installation 1 also comprises a tank 5 containing an organic phase 51 comprising at least one extracting agent and a non-ionic hydrotropic agent.A battery of extraction stages 6 comprising several stages (not shown) is supplied in a middle stage with balanced acidic aqueous phase 41 via a pipe 411, with organic phase 51 via a pipe 511 at the first stage of the extraction battery and with an aqueous washing solution comprising a non-ionic hydrotropic agent via a pipe 1711 leaving a reservoir 17 at the last stage of the extraction battery. In this way the aqueous and organic phases circulate counter-currently in the extraction battery according to the rules of the art. Each stage of the battery comprises a device for mixing the phases and a device for separating these same phases after contact (not shown).The reservoir 17 is supplied on the one hand with non-ionic hydrotropic agent by means of a pipe 1811 coming from a non-ionic hydrotropic agent reservoir 18 and on the other hand with aqueous washing solution 191 by means of a pipe 1911 coming from an aqueous washing solution reservoir 19. A pipe 1712 at the outlet of the reservoir 17 makes it possible to return non-ionic hydrotropic agent to the reservoir 18. After mixing the acidic aqueous phase 21 and the organic phase 51 in the extraction battery 6, at least a portion of the elements belonging to the rare earth group or to the actinide group has been transferred into the organic phase 51. The organic phase 51 thus loaded with metal is then transferred to the first stage of a stripping battery 7 comprising several stages. (not shown) via a pipe 611. The balanced acidic aqueous phase freed at least in part from the elements belonging to the rare earth group or to the actinide group is sent to a stripping unit 23 via a pipe 612. The stripping unit 23 makes it possible to recover the non-ionic hydrotropic agent contained in said acid phase, said non-ionic hydrotropic agent in a mixture with a solvent 121, preferably composed of a mixture of aliphatic compounds such as Isane. The solvent 121 is sent to the reservoir 13 via a pipe 2311. At the outlet of the stripping unit 23, the acidic aqueous phase freed from the non-ionic hydrotropic agent (raffinate) is transferred via a pipe 2312 to a container 24 which can then be emptied.

[0192] The installation 1 also comprises a tank 8 containing a back-extraction solution 81 comprising a non-ionic hydrotropic agent. The tank 8 is supplied with an aqueous solution 91 from a tank 9 via a pipe 911, as well as with said non-ionic hydrotropic agent 101 from a tank 10 via a pipe 1011. A pipe 812 makes it possible to return the hydrotropic agent to the tank 10 and a pipe 811 makes it possible to supply the back-extraction battery 7 with back-extraction solution 81 at the level of the last stage of the battery. At the outlet of the back-extraction battery 7, the back-extraction solution, which comprises at least a portion of the elements belonging to the rare earth group or to the actinide group 71, is transferred to a back-extraction unit 11 via a pipe 711.A reservoir 12 containing a solvent 121 of the Isane type is connected to the reservoir 11 via a pipe 1211. The introduction of the solvent 121 into the de-extraction unit 11 makes it possible to recover the non-ionic hydrotropic agent contained in the de-extraction solution 81. The de-extraction solution freed from the non-ionic hydrotropic agent but loaded with elements belonging to the rare earth group or to the actinide group is sent to a container 20 via a pipe 1112. The mixture of the solvent 121 and the non-ionic hydrotropic agent is transferred via a pipe 1111 to a reservoir 13 itself connected to a distillation unit 14 via a pipe 1311. The distillation unit 14 makes it possible to separate the non-ionic hydrotropic agent from the solvent 121.The non-ionic hydrotropic agent leaving the distillation unit 14 is recovered in a reservoir 15 via a pipe 1411 and then returned to a reservoir 16 via a pipe 1511. The solvent 121 leaving the distillation unit 14 is transferred via a pipe 1412 to the reservoir 12. This reservoir 12 also makes it possible to supply the de-extraction unit 23 with solvent is supplied with solvent 121 via a pipe 1212. Furthermore, at the outlet of the de-extraction unit . traction 7, the organic phase freed at least in part from the elements belonging to the rare earth group or to the actinide group is transferred to the reservoir 16 via a pipe 712. The reservoir 16 makes it possible to readjust the quantity of non-ionic hydrotropic agent of the organic phase before its transfer to the reservoir 5 via a pipe 1611. The elements belonging to the rare earth group or to the actinide group contained in the de-extraction solution present in the container 20 can then be recovered by conventional techniques such as for example by precipitation of these elements in said solution.

Claims

Claims

1. A process for the liquid-liquid extraction of at least one salt of a metal selected from the elements of the rare earth group and the actinide group from an acidic aqueous phase containing them, said process being characterized in that it comprises at least the following steps: (i) preparing a balanced acidic aqueous phase (Phaq e) comprising said at least one salt of said metal and at least one non-ionic hydrotropic agent in an initial amount Ql, (ii) preparing an organic phase (Phorg) comprising at least one non-ionic hydrotropic agent in an initial amount Q2 and at least one extracting agent, (iii) bringing said organic phase prepared above in step (ii) into contact with said balanced acidic aqueous phase prepared above in step (i), to obtain a mixture M, (iv) stirring said mixture M, then (v) separating said organic phase from the balanced acidic aqueous phase.

2. Method according to claim 1, characterized in that said metal is chosen from lanthanum, neodymium, europium, dysprosium, erbium, ytterbium, uranium and their mixtures.

3. Process according to claim 1 or 2, characterized in that the non-ionic hydrotropic agents used during steps (i) and (ii) are chosen from: - alkylene glycol alkyl ethers, the alkyl being chosen from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl and pentyl and the alkylene being chosen from ethylene and propylene and dipropylene; and - primary alcohols comprising a C2-C3 alkyloxy group.

4. Method according to any one of the preceding claims, characterized in that the non-ionic hydrotropic agent is chosen from l-propoxy-2-propanol, dipropylene glycol n-propyl ether and ethylene glycol mono-pentyl ether.

5. A method according to any one of the preceding claims, characterized in that the non-ionic hydrotropic agent present in the balanced acidic aqueous phase is identical to the non-ionic hydrotropic agent present in the organic phase.

6. A method according to any preceding claim, ca- characterized in that the balanced acidic aqueous phase comprises at least one strong acid in a concentration ranging from 1.10 4 to 6 mol / L, and preferably from approximately 0.01 to 4 mol / L.

7. Method according to any one of the preceding claims, characterized in that step (i) is a pre-balancing step consisting of adding to an acidic aqueous solution containing at least one salt of a metal chosen from the elements of the rare earth group and the actinide group, a pre-balancing phase comprising at least one hydrotropic agent in an initial quantity Ql, to obtain a balanced acidic aqueous phase.

8. A method according to claim 7, characterized in that the pre-equilibration phase is pure l-propoxy-2-propanol or pure dipropylene glycol n-propyl ether or pure ethylene glycol monopentyl ether.

9. Process according to any one of claims 7 or 8, characterized in that step (i) is carried out while respecting a pre-equilibration phase / acid aqueous phase volume ratio of 1:1 to 1:

10.

10. Method according to any one of the preceding claims, characterized in that it further comprises, before step (i), at least one preliminary leaching step.

11. Process according to any one of the preceding claims, characterized in that the extracting agent present in the organic phase prepared in step (ii) is chosen from bis(2-ethylhexyl)phosphoric acid, N,N'-dimethyl-N,N'-dioctylhexyl-ethoxy-malonamide, N,N-di-(2-ethylhexyl)isobutyramide, trioctylamine and mixtures thereof.

12. A method according to any one of the preceding claims, characterized in that the metal to be extracted from the balanced acidic aqueous phase is a lanthanide and the extracting agent is chosen from bis(2-ethylhexyl) phosphoric acid, N,N'-dimethyl-N,N'-dioctylhexyl-ethoxy-malonamide and mixtures thereof.

13. A method according to claim 12, characterized in that the extracting agent is a mixture of bis(2-ethylhexyl) phosphoric acid and N,N'-dimethyl-N,N'-dioctylhexyl-ethoxy-malonamide.

14. Process according to any one of claims 1 to 11, characterized in that the metal to be extracted from the balanced acidic aqueous phase is an actinide and the extracting agent is chosen from N,N-di-(2-ethylhexyl)isobutyramide, trioctylamine and mixtures thereof.

15. A method according to any preceding claim, ca- characterized in that the aqueous and organic phases are mixed in a volume proportion equal to 1 / 1.

16. Method according to any one of the preceding claims, characterized in that it is implemented in an extraction device comprising several stages, each of the stages making it possible to implement steps (iii) to (v), and in that said method then further comprises, after each step (v) and before each step (iii), at least one intermediate step of readjusting the quantities of non-ionic hydrotropic agents present respectively in the acidic aqueous phase and in the organic phase to values ​​identical to the initial values ​​Q1 and Q2.

17. Method according to any one of the preceding claims, characterized in that it further comprises at least: - a step (vi) of back-extraction of the salts of the metals present in the organic phase at the end of step (v), said step (vi) comprising at least one contacting of said organic phase with an aqueous phase called "back-extraction solution", said solution containing at least one non-ionic hydrotropic agent in an initial quantity Q3, then - a step (vii) of separation of the organic phase from said back-extraction phase to recover a back-extraction phase comprising the salts of said metals.

18. Method according to claim 17, characterized in that steps (vi) and (vii) are implemented in a multi-stage back-extraction battery, each of the stages making it possible to implement steps (vi) and (vii), and in that said method then further comprises, after each step (vii) and before each step (vi), at least one intermediate step of readjusting the quantity of non-ionic hydrotropic agent present in the back-extraction solution to a value identical to the initial value Q3.

19. Use of an organic phase comprising at least one extracting agent and a non-ionic hydrotropic agent for the liquid-liquid extraction of a metal chosen from elements of the rare earth group and the actinide group from an acidic aqueous phase containing said metals and a non-ionic hydrotropic agent.

20. Use according to claim 19, characterized in that the non-ionic hydrotropic agent of the organic phase is identical to the non-ionic hydrotropic agent of the acidic aqueous phase.

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