Method for extracting tantalum and niobium from acidic aqueous solutions

Abstract

The present invention relates to a method and apparatus in which tantalum and optionally niobium are selectively extracted by cyclic extraction between low temperature single phase and high temperature two phase regions in a ternary phase prism away from the critical point, using a ternary system comprising an acidic aqueous solution, a water-immiscible organic solvent and a heat-sensitive co-solvent.

Description

Technical Field

[0001] The present invention relates to the field of extraction and purification of tantalum, and in particular to a process for extracting and purifying tantalum and optionally niobium from an acidic aqueous solution. Background Art

[0002] Strategic metals are found in many everyday products, such as laptop hard drives, mobile phones, cars, batteries and printed circuit boards. Similar to salt and pepper in food, these high-tech materials are usually used in small quantities, but they are essential ingredients in these devices. In order to meet the challenges of dwindling resources and growing demand for them, mining, recycling or alternative solutions appear to be important means to extract metals from low-grade ores, recover material streams from production waste or treat more complex waste products.

[0003] Due to the dramatic growth of numerous innovations in the aforementioned technological fields, the demand and prices of rare earth elements (REEs) are rising, and a shortage in the supply line can be observed.

[0004] Selective extraction of different metals with the same physical and chemical properties is difficult. Tantalum (Ta) and niobium (Nb) are important transition metals and are widely used in various high-tech industries. In particular, tantalum is an important chemically inert high melting point element used in alloys and electronic components such as wires, surgical instruments and capacitors, while niobium is used in steel and high temperature alloys, superconducting magnets, medical devices and jewelry.

[0005] The main sources of Ta and Nb are tantalite (Fe, Mn)(Ta, Nb)2O6, columbite (Fe, Mn)(Ta, Nb)2O6 and aplite (Ca, Na)2(Ta, Nb)2(O, OH, F)7. Therefore, Ta and Nb are always combined geologically and are difficult to separate due to their very similar physical and chemical properties.

[0006] Liquid-liquid extraction is currently the most effective method for separating tantalum and niobium. Methods known in the art are mainly based on the use of binary mixtures of aqueous solutions and organic solvents, which contain 1-octanol or 2-octanol alone or mixed with other organic compounds such as octane or methyl isobutyl ketone (MIBK) (Zhourou Lu et al. "Solvent extraction technology for the separation and purification of indium and tantalum: areview", Hydrometallurgy 107, 2011, pp. 1-12).

[0007] However, these methods are not entirely satisfactory because in practice the amount of electrolyte usually available in the "feed" leads to a large increase in the viscosity of the solvent phase, and the flash point of the most effective known extractants (MIBK) is low, and solvents such as cyclohexanone have poor miscibility with the extractants.

[0008] Therefore, there remains a need for methods that can selectively and efficiently extract tantalum and optionally niobium from aqueous solutions.

[0009] The above needs are met by the method described hereinafter and constituting the object of the present invention. Summary of the invention

[0010] The present invention relates to a process wherein at least tantalum in the form of anionic salts is selectively extracted from an acidic aqueous solution using a thermal and ternary system.

[0011] The invention is particularly applicable to the treatment of natural and / or industrial wastes coming from concentrates of natural minerals rich in tantalum and optionally niobium, such as monazite, bastnasite, xenotime, pyrochlore or colombo-tantalite, or from concentrates obtained from the treatment of tantalum-rich minerals, such as concentrates from "urban mines" consisting of industrial and household post-consumer waste containing tantalum, in particular electronic equipment waste or tantalum products.

[0012] The present invention firstly relates to a method for extracting tantalum in the form of a cationic salt from an acidic aqueous solution, said method comprising at least the following steps:

[0013] i) providing a ternary mixture at a first temperature T1, the ternary mixture comprising: a) an acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted, b) an organic solvent immiscible with water, c) an amount Q1 of a thermosensitive co-solvent, the thermosensitive co-solvent comprising a hydrophilic portion and a hydrophobic portion, T1 being between 15°C and 35°C and below the critical temperature of the ternary mixture,

[0014] ii) heating the ternary mixture at a second temperature T2 at which the mixture exhibits a two-phase structure consisting of an aqueous phase and an organic phase rich in at least tantalum, wherein T2 is between 40°C and 75°C, is above the critical temperature of the ternary mixture and differs from T1 by 15°C to 50°C,

[0015] iii) separating the organic phase and the aqueous phase from the two-phase structure obtained in step ii),

[0016] iv) extracting tantalum from the organic phase,

[0017] in:

[0018] - the ternary mixture of step i) is in the phase boundary pre-Ouzo region, the pre-Ouzo region being characterized by an ultra-soft surfactant-free microemulsion comprising aggregates of about 20 to about 300 molecules linked together in the form of dynamic aggregates,

[0019] - the hydrophilic part of the cosolvent comprises at least one ethoxy group,

[0020] - The molar concentration of the acidic aqueous solution is at least 2M.

[0021] The invention also relates to an apparatus for implementing the method according to the invention to extract at least tantalum in the form of anionic salts from an acidic aqueous solution. More specifically, said apparatus comprises at least a mixing chamber and suitable cooling means and heating means for said extraction. Said apparatus also comprises the necessary means to ensure recycling and separation means, such as Figure 2 shown. DETAILED DESCRIPTION

[0022] In a first aspect, the present invention relates to a method for extracting tantalum in the form of a cationic salt from an acidic aqueous solution, the method comprising at least the following steps:

[0023] (i) providing a ternary mixture at a first temperature T1, the mixture comprising: a) an acidic aqueous solution containing a mixture of metal elements, the metal elements including at least tantalum to be extracted, b) an organic solvent immiscible with water, c) an amount Q1 of a thermosensitive co-solvent, the thermosensitive co-solvent comprising a hydrophilic portion and a hydrophobic portion, T1 being between 15°C and 35°C and below the critical temperature of the ternary mixture,

[0024] (ii) heating the ternary mixture at a second temperature T2 at which the mixture exhibits a two-phase structure consisting of an aqueous phase and an organic phase at least partially loaded with at least tantalum, wherein T2 is 40° C. to 75° C., above the critical temperature of the ternary mixture, and differs from T1 by 15° C. to 50° C.,

[0025] (iii) separating the organic phase and the aqueous phase from the two-phase structure obtained in step ii),

[0026] (iv) extracting at least tantalum from said organic phase,

[0027] in:

[0028] - the ternary mixture in step i) is in the phase boundary pre-Ouzo region, the pre-Ouzo region being characterized by an ultra-soft surfactant-free microemulsion comprising dynamic aggregates of about 20 to about 300 molecules bound together,

[0029] - said hydrophilic part of said cosolvent comprises at least one ethoxy group,

[0030] - The molar concentration of the acidic aqueous solution is at least 2M.

[0031] Ternary mixtures in the “pre-Ouzo” region, also known as “surfactant-free microemulsions” or “ultra-soft microemulsions”, are characterized by a combination of the following properties:

[0032] - Aggregates comprising more than 30 molecules, which are partially covered by a partial monolayer of a cosolvent, as defined, for example, in Th. Zemb et al., Proceedings of the National academy of science of the USA, 2016, 113(16), 4260- 4262.

[0033] - Static light scattering gives an apparent average aggregate mass that is more than 30 times the molecular mass of the solvent used,

[0034] The ternary mixture is far from its critical point, wherein for each component, in the case of tantalum and niobium, the difference between said critical point and its mass percentage is at least 3%, preferably at least 5%, more preferably at least 8%.

[0035] According to the present invention, "cosolvent" is a compound soluble in both aqueous and organic phases. More specifically, a cosolvent is a compound that dissolves a hydrophobic compound in an aqueous solution. Cosolvents are generally composed of a hydrophilic portion and a hydrophobic portion (e.g., a surfactant), but the hydrophobic portion is generally too short to cause spontaneous self-aggregation and micelle formation (J.Mehringer, Werner Kunz, 2021, Advances in Colloid and Interface Science 294,102476). There are acidic electrolytes, alkaline electrolytes or salt electrolyte cosolvents and non-electrolyte cosolvents. The molecular volume of the cosolvent is greater than 0.090nm 3 And less than 0.5nm 3 According to the invention, the "cosolvent" is miscible with water and an organic solvent that is immiscible with water and, when present in an amount Q1 (also referred to as the "minimum cosolvent concentration" (MHC)), allows at least tantalum to be significantly dissolved in the form of anionic salts. The cosolvent reduces the surface tension of water, but the cosolvent itself does not form micelles. The existence of a stable phase between surface pressure and cosolvent concentration proves the existence of such micellization.

[0036] According to the present invention, the term "thermosensitive" cosolvent refers to a cosolvent capable of introducing thermosensitivity into the layered composition of the ternary mixture of said cosolvent, organic solvent and hydrophobic agent.

[0037] According to the present invention, the term "dynamic aggregates" refers to aggregates in aqueous solution that exhibit a critical aggregate concentration (determined by any standard technique such as surface tension) equal to or greater than 0.1 M at room temperature (S. Schöttl and D. Horinek, Current opinion in Colloid & Interface Science, vol. 22 (2016) pp. 8-13).

[0038] The process according to the invention has the advantage of using a simple ternary system, wherein the properties of the organic solvent and the co-solvent are combined in a single single-phase ternary mixture. In the process according to the invention, steps (i) to (iii) can be repeated in a cyclic process comprising n reaction cycles by combining the fluxes of the aqueous phase and the organic phase in a countercurrent arrangement.

[0039] In the method according to the invention, the diameter of the aggregates in the ternary mixture in step (i) can be determined by any suitable method known to a person skilled in the art, preferably by X-ray light scattering with a correlation length greater than 2 nm.

[0040] According to the present invention, by "surfactant-free single-phase microemulsion structure" is meant a single-phase ternary mixture with a microemulsion-like structure in the absence of any surfactant in the mixture. The absence of surfactant or detergent provides the advantage that the method according to the present invention is easier to implement and avoids the necessity of eliminating the detergent.

[0041] The acidic aqueous solution containing the mixture of metal elements including at least tantalum to be extracted according to the method of the present invention is, for example, an acidic concentrate of ore or waste. The concentration of tantalum in the acidic aqueous solution is preferably 0.1 g / L to 10 g / L.

[0042] According to the present invention, the acidic solution may be any solution comprising a strong acid and having a molar concentration of 2 M to about 12 M. According to a preferred embodiment, the strong acid is sulfuric acid or nitric acid, sulfuric acid being particularly preferred.

[0043] According to a particular embodiment of the invention, the molar concentration of the acidic aqueous solution is at least 5 M. In this case, the method according to the invention is capable of extracting not only tantalum but also niobium. Therefore, according to this particular embodiment, the molar concentration of the acidic aqueous solution is at least 5 M and the method according to the invention is used for the selective extraction of tantalum and niobium from the acidic aqueous solution.

[0044] The amount of the aqueous solution in the ternary mixture of step (i) may be about 10 wt. % to about 90 wt. %, and preferably about 15 wt. % to about 80 wt. %, relative to the total weight of the ternary mixture.

[0045] According to the present invention, the organic solvent "immiscible with water" refers to an organic solvent that cannot mix with water and form a multiphase mixture. More specifically, the definition includes solvents whose miscibility in water is less than 0.001 mole fraction, and wherein the miscibility of water in the solvent is greater than 0.1 mole fraction.

[0046] According to a particular embodiment of the invention, the organic solvent is chosen from fatty alcohols having a carbon chain comprising at least 8 carbon atoms, Guerbet alcohols, ethyl acetate, aldehydes, ketones, phosphorus-based nonionic liquids, amides, carbollides, carboranes and phosphonates.

[0047] In a specific and preferred embodiment of the present invention, the organic solvent is selected from the group consisting of: 1-octanol, 2-octanol, methyl isobutyl ketone (MIBK), cyclohexanone and tributyl phosphate (TBP). Among these organic solvents, 1-octanol and 2-octanol are particularly preferred.

[0048] The amount of the organic solvent in the ternary mixture of step (i) may be about 5 wt % to about 70 wt % (wt. %), and preferably about 5 wt % to about 50 wt %, relative to the total weight of the ternary mixture.

[0049] According to the present invention, the amount of the co-solvent Q1 is generally greater than 10% by weight of the ternary mixture, preferably 10% to 55% by weight of the total weight of the ternary mixture, and more preferably 20% to 50% by weight of the ternary mixture.

[0050] Examples of the co-solvent are propylene glycol alkyl ethers, the alkyl group being selected from the group consisting of methyl, ethyl, propyl, isopropyl, butyl, isobutyl and pentyl, such as 1-propoxy-2-propanol (PnP) and dipropylene glycol n-propyl ether (dPnP).

[0051] Among these cosolvents, 1-propoxy-2-propanol is particularly preferred.

[0052] In a particular embodiment of the process according to the invention, step (i) first comprises mixing the organic solvent and the co-solvent and then comprises adding the acidic aqueous solution to prepare the ternary mixture of step (i).

[0053] According to the invention, both T1 and T2 are different from the critical temperature of the ternary mixture, wherein the difference between T1 or T2 each and the critical temperature of the ternary mixture is at least 3°C, preferably at least 5°C, more preferably at least 10°C.

[0054] The critical temperature of a given ternary mixture is readily determined by established methods, such as that described in: Alberto Arce, Alberto Arce, Jr. and Oscar Rodriguez, J. Chem. Eng. Data 2022, 67, 286-296.

[0055] According to a preferred embodiment of the present invention, T1 is from about 15°C to about 35°C, more preferably from about 20°C to about 30°C.

[0056] According to a preferred embodiment of the present invention, T2 is from about 45°C to about 70°C, more preferably from about 50°C to about 65°C.

[0057] In step (ii), the ternary mixture provided in step (i) can be heated from temperature T1 to temperature T2 by any means or apparatus known to those skilled in the art to suitably heat the mixture. As an example, a microwave oven or a water bath (Bain Marie) can be used.

[0058] In step (iii), the organic phase and the aqueous phase may be separated by any appropriate means, for example, by draining, pipetting, centrifugation, and the like.

[0059] In step (ii), the temperature T2 is maintained for a period of time sufficient to allow phase separation to occur and to extract at least tantalum from the ternary mixture into the organic phase by solvent, the extraction typically lasting from 1 minute to 20 minutes. If intensification is required, the process can be accelerated a hundredfold using a centrifugal extractor at low acceleration (<3000g).

[0060] According to the present invention, both T1 and T2 are different from the critical temperature of the ternary mixture, wherein the difference between T1 or T2 and the critical temperature of the ternary mixture is at least 3°C, preferably at least 5°C, more preferably at least 10°C. In a particular embodiment of the present invention, in step ii), the ternary mixture may be centrifuged. According to the method of the present invention, the centrifugation performed in step ii) is intended to promote and / or accelerate the extraction of at least tantalum.

[0061] According to a specific and preferred embodiment of the present invention, before performing step (iv) and after returning the ternary mixture to temperature T1, steps (i) to (iii) are repeated at least once, more preferably 2 to 20 times, even more preferably 3 to 6 times.

[0062] In this case, according to a first mode of realization of this particular embodiment, after carrying out step (iii) and before each repetition of steps (i) to (iii), the method comprises at least the following sub-steps:

[0063] (1a) cooling the aqueous phase recovered in step (iii) to a temperature T1,

[0064] (2a) mixing the aqueous phase with the organic solvent and the co-solvent to obtain the ternary mixture, and

[0065] (3a) adjusting the amount of the co-solvent in the ternary mixture to the amount Q1.

[0066] This first mode of realization is preferably performed when a single iteration of steps (i) to (iii) is insufficient to extract all the tantalum initially present in the initial acidic aqueous solution or to further extract niobium from said initial acidic aqueous solution. In the case of further extraction of niobium, the molarity of the acidic aqueous solution is adjusted to at least 5M to extract niobium.

[0067] According to a second implementation mode of this particular embodiment, after performing step (iii) and before each repetition of steps (i) to (iii), the method comprises at least the following sub-steps:

[0068] (1b) cooling the organic phase recovered in step (iii) to a temperature T1,

[0069] (2b) mixing the organic phase with a new acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted and the co-solvent to obtain the ternary mixture, and

[0070] (3b) adjusting the amount of the co-solvent in the ternary mixture to the amount Q1.

[0071] Preferably, this second mode of realisation is carried out so as to obtain an organic phase highly concentrated in at least tantalum.

[0072] In both the first and second modes of realisation of this particular embodiment, it is necessary to adjust the amount of said co-solvent in said ternary mixture to said amount Q1 in step (3a) or step (3b) in order to maintain effective properties of the distribution coefficient in step (ii).

[0073] In both the first and second modes of realization of this particular embodiment, cooling step (1a) and step (1b) of cooling the temperature from temperature T2 to temperature T1 can be performed by any cooling means such as an ice bath or a refrigeration device.

[0074] In step (iv), tantalum may be extracted from the organic phase by washing the organic phase with an acidic aqueous solution. As an example, when the organic solvent of the organic phase is octanol, the organic phase may be washed with a 0.1M to 0.2M aqueous sulfuric acid solution to transfer tantalum from the organic phase to the aqueous sulfuric acid solution. As another example, when the organic phase contains both tantalum and niobium, the organic phase is first washed with a 3.5M to 4.0M first aqueous sulfuric acid solution to transfer niobium from the organic phase to the first aqueous sulfuric acid solution, and then the organic phase is washed again with a 0.1M to 0.2M second aqueous sulfuric acid solution to transfer tantalum from the organic phase to the second aqueous sulfuric acid solution.

[0075] In a second aspect, the present invention relates to an apparatus for extracting at least tantalum in the form of anionic salts according to the method defined in the first aspect of the invention.

[0076] More specifically, the present invention relates to an apparatus for implementing the method of the present invention, wherein the apparatus comprises at least:

[0077] a mixing chamber for preparing a ternary mixture comprising: a) an acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted, b) an organic solvent immiscible with water, c) an amount Q1 of a cosolvent comprising a hydrophilic part and a hydrophobic part, said hydrophilic part comprising at least one ethoxy group,

[0078] - a heating device for heating the ternary mixture,

[0079] - cooling device,

[0080] - means for conveying said acidic aqueous solution, said organic solvent, said thermosensitive co-solvent and said mixture, and

[0081] - means for controlling at least the flow rates and the temperatures of said solutions and mixtures.

[0082] Other features and advantages of the present invention will become apparent from the following detailed description of embodiments and the accompanying drawings, in which:

[0083] Figure 1 A shows the phase diagram of the ternary mixture of water phase, PnP and 2-octanol according to Example 1 at 25°C and 60°C. Figure 1 B shows the Ta / Nb separation factor depending on the single-phase starting composition.

[0084] Figure 2Flowchart examples of several reaction cycles are shown that combine the flux of aqueous and organic phases in a countercurrent arrangement. In the first stage, in the first mixing chamber 2a, at a temperature T1, the initial acidic aqueous solution 1 is mixed with the organic phase 5b from the second mixing chamber 2b and a thermosensitive co-solvent to form a uniform nanostructured ternary mixture 3a, the initial acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted, wherein the amount of the co-solvent is equal to Q1. The ternary mixture 3a is then separated into two phases by means of a heating device (not shown) that brings the temperature from T1 to T2 and their density differences. This produces an aqueous raffinate 4a containing mainly one or more than one undesirable metal and an organic phase 5a at least partially loaded with at least tantalum. In the second stage, the aqueous raffinate 4a is fed into the second mixing chamber 2b and mixed with the organic phase 5c from the third mixing chamber 2c and a sufficient amount of a thermosensitive co-solvent from the container 6b to form a uniform nanostructure ternary mixture 3b, wherein the amount of the co-solvent is readjusted to the amount Q1, and the co-solvent is the same as the co-solvent used in the first step of the method. Then, the ternary mixture 3b is separated into two phases by means of a heating device (not shown) that makes the temperature reach T2 from T1 and their density difference. This step produces an aqueous raffinate 4b containing mainly one or more than one undesirable metal and an organic phase 5b at least partially loaded with at least tantalum. In the third stage, the aqueous raffinate 4b is fed into the third mixing chamber 2c and mixed with a new organic solvent 7 and a sufficient amount of a thermosensitive co-solvent from the container 6c to form a uniform nanostructure ternary mixture 3c, wherein the amount of the co-solvent is readjusted to the amount Q1, and the co-solvent is the same as the co-solvent used in the first and second steps of the method. When the process is finished, each of the tantalum-loaded organic phases 5a, 5b, and 5c can be recovered, and the aqueous phase 8 from the third mixing chamber 2c and containing the undesired metal element can be removed.

[0085] Figure 3 A scheme for extraction in cascade form according to the invention is shown, which allows the recycling of the co-solvent. In the first step, at a temperature T1, [A] an initial acidic aqueous phase containing water, sulfuric acid, PnP (1-propoxy-2-propanol) and a metal salt is mixed with [B] an initial organic phase containing PnP and 2-octanol to form [C] a homogeneous nanostructured ternary mixture in a mixing chamber. The mixture is then separated into two phases by means of [D] a heating device (spiral) that brings the temperature from T1 to T2 and [E] their density difference. This step produces [F] an aqueous raffinate containing mainly one or more than one undesirable metal and [G] an organic phase partially loaded with tantalum. By adding [A] a new aqueous phase, this step can be [T] repeated until [H] an organic phase completely loaded with tantalum is obtained. [F] The aqueous raffinate can be recovered by loading the desired metal again.

[0086] Example: Separation of Tantalum and Niobium

[0087] This example describes the selective extraction of tantalum and niobium by the process of the present invention.

[0088] A ternary mixture is prepared comprising:

[0089] • A 4M aqueous sulfuric acid phase containing niobium and tantalum

[0090] • 1-Propoxy-2-propanol (PnP) was used as a thermosensitive cosolvent to ensure the miscibility of the aqueous phase and the organic solvent.

[0091] •2-Octanol acts as an organic solvent which is immiscible with water.

[0092] The phase diagram and separation factor of the system at 25°C and 60°C are shown in Figure 1 A and Figure 1 As shown in B.

[0093] As shown, the distribution coefficient (kD) as well as the Ta / Nb separation factor (InS) show two maxima around the compositions reported in Table 1 below:

[0094] Table 1

[0095]

[0096] In the first step, the aqueous phase, PnP and 2-octanol were mixed in the following manner: PnP and 2-octanol were mixed in a ratio of 0.82 and 0.18 with a tolerance of + / - 0.02, and then the aqueous solution was added so that the mass fractions of PnP / 2-octanol / aqueous solution were 0.4 / 0.1 / 0.5 with a possible variation of 0.03 for each component (the point is located between the two lines drawn at 25°C and 60°C).

[0097] Mixing the three components at a low temperature T1 of 25° C. allows the formation of a homogeneous nanostructured microemulsion that exhibits certain advantages, such as a high contact surface between the aqueous and organic phases, low viscosity and rapid attainment of thermodynamic equilibrium. At this temperature, the ions are distributed between the existing aqueous phase and the organic pseudophase according to the distribution coefficient.

[0098] In the second step, the temperature was raised to 60°C (T2). Due to the temperature sensitivity of the cosolvent, the phase boundary of the ternary system was Figure 1 The result of this change is a phase separation into a water-rich phase and an organic-rich phase. Figure 1As shown, the distribution coefficient is above 50, favoring the solvent phase, allowing the separation and purification of the associated elements from the iron by cascade. This step can be accelerated using a centrifuge. The volume ratio between the two phases is determined by the connecting line of the phase diagram at 60°C. Tantalum is mainly transferred to the organic-rich phase, while niobium remains mainly in the aqueous phase. The separation coefficient is high and depends on the initial composition of the phase diagram (see Figure 1 B).

[0099] like Figure 2 As shown, by combining the flux of aqueous phase and organic phase in a countercurrent arrangement, the number of circulation steps can be increased, thereby promoting the separation between the two metals. In each step, aqueous phase and organic phase are mixed at room temperature in an appropriate volume ratio to obtain a uniform phase. Thereafter, the temperature is raised again to separate the two resulting phases, and the two resulting phases are transferred to the next circulation chamber. 3 to 20 cycles including step ii) and step iii) can be carried out, preferably 6 to 10 cycles are carried out.

Claims

1. A method for extracting tantalum in the form of a cationic salt from an acidic aqueous solution, the method comprising at least the following steps: (i) providing a ternary mixture at a first temperature T1, the ternary mixture comprising: a) an acidic aqueous solution of a mixture containing metal elements, the metal elements including at least tantalum to be extracted, b) an organic solvent immiscible with water, c) an amount Q1 of a thermosensitive co-solvent, the thermosensitive co-solvent comprising a hydrophilic portion and a hydrophobic portion, T1 being between 15°C and 35°C and below the critical temperature of the ternary mixture, (ii) heating the ternary mixture at a second temperature T2 at which the mixture exhibits a two-phase structure consisting of an aqueous phase and an organic phase at least partially loaded with at least tantalum, wherein T2 is 40°C to 75°C, which is higher than the critical temperature of the ternary mixture and differs from T1 by 15°C to 50°C, iii) separating the organic phase and the aqueous phase from the two-phase structure obtained in step ii), (iv) extracting at least tantalum from said organic phase, in: - the ternary mixture in step i) is in the phase boundary pre-Ouzo region, the pre-Ouzo region being characterized by an ultra-flexible surfactant-free microemulsion comprising dynamic aggregates of about 20 to about 300 molecules bound together, - said hydrophilic part of said cosolvent comprises at least one ethoxy group, - The molar concentration of the acidic aqueous solution is at least 2M.

2. The method according to claim 1, wherein: The concentration of tantalum in the acidic aqueous solution is 0.1 g / L to 10 g / L.

3. The method according to claim 1 or 2, wherein: The molar concentration of the acidic aqueous solution is 2M to 12M.

4. The method according to any one of claims 1 to 3, wherein: The acidic aqueous solution comprises a strong acid selected from sulfuric acid and nitric acid.

5. The method according to any one of claims 1 to 4, wherein: The molar concentration of the acidic aqueous solution is at least 5 M, and the method is used to selectively extract tantalum and niobium from the acidic aqueous solution.

6. The method according to any one of claims 1 to 5, wherein: The amount of the acidic aqueous solution in the ternary mixture of step (i) is 10 wt % to 90 wt % relative to the total weight of the ternary mixture.

7. The method according to any one of claims 1 to 6, wherein: The organic solvent is selected from 1-octanol, 2-octanol, methyl isobutyl ketone, cyclohexanone and tributyl phosphate.

8. The method according to any one of claims 1 to 7, wherein: The amount of the organic solvent in the ternary mixture of step (i) is 5 wt % to 70 wt % relative to the total weight of the ternary mixture.

9. The method according to any one of claims 1 to 8, wherein: The cosolvent is 1-propoxy-2-propanol or dipropylene glycol n-propyl ether.

10. The method according to any one of claims 1 to 9, wherein: The amount of the co-solvent in the ternary mixture of step (i) is in the range of 10 wt % to 55 wt % relative to the total weight of the ternary mixture.

11. The method according to any one of claims 1 to 10, wherein: T1 is 20°C to 30°C.

12. The method according to any one of claims 1 to 11, wherein: T2 is from about 45°C to about 70°C.

13. The method according to any one of claims 1 to 12, wherein: Before performing step (iv) and after returning the ternary mixture to temperature T1, steps (i) to (iii) are repeated at least once.

14. The method according to claim 13, wherein: After performing step (iii) and before each repetition of steps (i) to (iii), the method comprises at least the following sub-steps: (1a) cooling the aqueous phase recovered in step (iii) to a temperature T1, (2a) mixing the aqueous phase with the organic solvent and the co-solvent to obtain the ternary mixture, and (3a) adjusting the amount of the co-solvent in the ternary mixture to the amount Q1.

15. The method according to claim 13, wherein: After performing step (iii) and before each repetition of steps (i) to (iii), the method comprises at least the following sub-steps: (1b) cooling the organic phase recovered in step (iii) to a temperature T1, (2b) mixing the organic phase with a new acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted and the co-solvent to obtain the ternary mixture, and (3b) adjusting the amount of the co-solvent in the ternary mixture to the amount Q1.

16. An apparatus for carrying out the method as defined in any one of claims 1 to 15, wherein: The device at least comprises: a mixing chamber for preparing a ternary mixture comprising: a) an acidic aqueous solution containing a mixture of metal elements including at least tantalum to be extracted, b) an organic solvent immiscible with water, c) an amount Q1 of a cosolvent comprising a hydrophilic part and a hydrophobic part, the hydrophilic part comprising at least one ethoxy group, - a heating device for heating the ternary mixture, - cooling device, - Tools for delivering acidic aqueous solutions, organic solvents, heat-sensitive co-solvents and mixtures, and - means for controlling at least the flow rates and the temperatures of said solutions and mixtures.

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