USE OF LIPOPHILIC DERIVATIVES OF AMINOPOLYCARBOXYLIC ACIDS FOR THE EXTRACTION OF RARE EARTHS FROM AN ACIDIC AQUEOUS SOLUTION

TECHNICAL FIELD

[0001]The invention relates to the field of extraction and recovery of rare earths present in acidic aqueous solutions, with a view to recycling these rare earths.

[0002]More specifically, the invention relates to the use of a lipophilic derivative of an aminopolycarboxylic acid as an extractant, to extract one or more rare earths from an acidic aqueous solution.

[0003]The invention finds applications in particular in the production of rare earths from concentrates derived from “urban ores”, that is to say “mines” consisting of industrial and domestic waste comprising rare earths and, in particular, in the recycling of rare earths present in waste electrical and electronic equipment (also called “WEEE” or “W3E”).

[0004]More particularly, the invention finds an application in the recycling of rare earths contained in used or discarded permanent magnets, and, in particular, in permanent magnets of the Neodymium-Iron-Boron (or NdFeB) type.

[0005]However, it can also be used to produce rare earths from concentrates derived from natural ores such as monazites, bastnaesites, apatites or xenotimes, or from concentrates derived from residues of natural ores such as, for example, tin slag.

PRIOR ART

[0006]The particular physical and chemical properties of rare earths (scandium, yttrium and lanthanides) currently make them essential chemical elements in many industrial fields: glass and ceramics industries, catalysis, metallurgy, manufacture of permanent magnets, optical devices, luminophores, etc.

[0007]Rare earths are therefore part of the metals called “technological” metals whose supply is strategic.

[0008]The global demand for rare earths continues to grow and is estimated to increase by 50% over the next ten years. However, as the number of rare earth producing countries remains limited—with China currently dominating the global production of rare earths—there is a significant risk of a shortage of rare earth supplies in the long term, hence the need to optimise all the avenues for producing them.

[0009]Recycling of rare earths present in used materials is increasingly favoured. Recycling allows to reconcile the reduction of supply risks and the environmental challenges related to mining activities.

[0010]One of the first markets in terms of volume and market value for rare earth recycling relates to permanent NdFeB magnets found in a certain number of WEEE waste (computer hard drives, audio or video equipment speakers, magnetic devices, etc.). This resource for recycling rare earths has the advantage of comprising interesting and recoverable proportions of rare earths, typically of the order of 30% by mass. The composition of permanent NdFeB magnets varies according to the applications of these magnets and the manufacturers, but they typically contain heavy rare earths (dysprosium and, to a lesser extent, gadolinium, terbium) which are very recoverable as well as light rare earths (neodymium and praseodymium in particular).

[0011]The hydrometallurgical route, based on the liquid-liquid extraction technique, is commonly considered one of the most commercially suitable routes to recover rare earths from the environment wherein they are found.

[0012]Hydrometallurgical methods, which are currently used industrially to recover rare earths from an acidic aqueous solution, preferentially employ organophosphorus extractants such as phosphoric acids, phosphonic acids, phosphinic acids, carboxylic acids and alkyl phosphates. They are, for example, di-2-ethylhexylphosphoric acid (or HDEHP), 2-ethylhexylphosphonic acid (or HEH[EHP]), bis(trimethyl-2,4,4-pentyl)phosphinic acid (or Cyanex™272), neodecanoic acid (or Versatic™ 10) and tri-n-butyl phosphate (or TBP).

[0013]The use of other types of extractants has been proposed in recent years, such as N,N-dibutylacetamide (see European patent application 3 323 899, hereinafter reference [1]), lipophilic symmetrical diglycolamides such as N,N,N′,N′-tetraoctyl-3-oxapentanediamide (or TODGA) (see PCT international application WO 2016/046179, hereinafter reference [2]) and amphiphilic dissymmetrical diglycolamides (see PCT international application WO 2019/197792, hereinafter reference [3]).

[0014]Moreover, the idea of developing lipophilic derivatives of aminopolycarboxylic acids emerged in the mid-1970s and then spread over the following years, mainly with the aim of providing compounds for medical imaging. Thus provision was made of contrast agents for medical imaging by magnetic resonance comprising a paramagnetic ion, for example Mn²⁺, complexed with a lipophilic derivative of ethylenediaminetetraacetic acid (or EDTA) in U.S. Pat. No. 5,762,910, hereinafter reference [4], and with a lipophilic derivative of trans-1,2-diaminocyclohexanetetraacetic acid (or CyDTA) in PCT international application WO 2016/135523, hereinafter reference [5].

[0015]The use of lipophilic EDTA derivatives, incorporated into a polymer membrane, has also been proposed for the extraction of alkaline-earth metals, in particular calcium and magnesium, by Erne et al., Helv. Chim. Acta 1980, 63(8), 2264-2270, hereinafter reference [6].

[0016]Finally, provision was made in U.S. Pat. No. 8,785,691, hereinafter reference [7], to use lipophilic derivatives of EDTA, in solution in 1-octanol, to extract by liquid-liquid extraction americium(III) and curium(III) selectively with respect to the lanthanides(III) from a raffinate resulting from the implementation of the PUREX spent fuel treatment method. Thus, according to this reference, the lanthanides which represent 15 of the 17 rare earths would not be extractable or only very poorly extractable by lipophilic derivatives of EDTA.

[0017]However, in the course of their work, the inventors found that, contrary to the teaching of reference [7], lipophilic derivatives of aminopolycarboxylic acids and, in particular, EDTA and CyDTA can very effectively extract rare earths and in particular neodymium, praseodymium and dysprosium from acidic aqueous solutions.

[0018]And it is on these experimental observations that the invention is based.

DESCRIPTION OF THE INVENTION

[0019]The invention therefore relates to the use of a lipophilic derivative of an aminopolycarboxylic acid as an extractant, to extract at least one rare earth from an acidic aqueous solution.

[0020]This derivative corresponds to the general formula (I) or (II) below:

[0035]Moreover, in the foregoing and the following, the expressions “from . . . to . . . ” and “comprised between . . . and . . . ” are equivalent and are intended to mean that the limits are included.

[0036]Likewise, the terms “solution” and “phase” are equivalent and perfectly interchangeable.

[0037]It goes without saying that, in the general formulae (I) and (II) above, the meanings of R¹ to R⁸, X¹ to X⁶ and R⁹ may be chosen according to the degree of lipophilicity that it is to be conferred on the derivative. Thus, in particular, for a high degree of lipophilicity, the presence in these formulae of one or more alkyl groups comprising from 12 to 40 carbon atoms, preferably from 12 to 36 carbon atoms and, even more, from 18 to 24 carbon atoms will be entirely conceivable.

[0038]In the general formula (I), it is preferred that m be equal to 0.

[0039]Moreover, in the general formula (I), it is preferred that R¹ and R², identical or different, represent a hydrogen atom, a linear or branched C₁ to C₄₀ alkyl group, a C₅ or C₆ cycloalkyl group, a monocyclic aryl group, or together form a cyclohexyl or phenyl group, optionally substituted one or more times by a hydrogen atom, a linear or branched C₁ to C₄₀ alkyl group, a C₅ or C₆ cycloalkyl group or by a monocyclic aryl group.

[0040]Therefore, the derivative preferably responds to the particular formula (Ia), (Ib) or (Ic) hereinafter:

[0044]In these particular formulas, it is preferred that X¹ and X² represent a group —NHR or —NRR′ with R and R′ representing a linear or branched C₈ to C₂₀ alkyl group, a C₅ or C₆ cycloalkyl group or a monocyclic aryl group, in which case it is X³ and X⁴ that represent a hydroxyl group.

[0045]Furthermore, it is preferred that X¹ and X² represent a group —NRR′ wherein R and R′ are identical and represent a linear or branched C₈ to C₂₀ and, more particularly, C₈ to C₁₂ alkyl group such as an n-octyl, 2-ethylhexyl, n-decyl or n-dodecyl group.

[0046]Among the derivatives of particular formula (Ia), (Ib) and (Ic), all preference is given to the derivatives of particular formula (Ia) or (Ib) as previously defined.

[0051]In the general formula (II), it is preferred that X⁵ and X⁶ represent a group —NRR′ with R and R′ representing a linear or branched C₆ to C₂₀ alkyl group, a C₅ or C₆ cycloalkyl group or a monocyclic aryl group.

[0052]Furthermore, it is preferred that X⁵ and X⁶ represent a group —NRR′ wherein R and R′ are identical and represent a linear or branched C₈ to C₂₀ and, more particularly, C₈ to C₁₂ alkyl group such as an n-octyl, 2-ethylhexyl, n-decyl or n-dodecyl group.

[0053]More particularly, it is preferred that X⁵ and X⁶ represent a group —NRR′ wherein R and R′ represent a 4-hexyldodecyl group.

[0054]As for R⁹, it preferentially represents a —CH₂COOH group, in which case R¹⁰ is advantageously a —COOH group.

[0055]In accordance with the invention, the rare earth is preferably extracted from the aqueous solution A1 by liquid-liquid extraction, in which case this extraction comprises at least contacting the aqueous solution A1 with an organic solution immiscible with water, comprising the derivative in an organic solvent, then separating the aqueous solution A1 from the organic solution.

[0056]However, it goes without saying that it is also possible to extract the rare earth from the aqueous solution A1 by solid-liquid extraction, in which case this extraction may in particular comprise contacting this aqueous solution with a solid material insoluble in water and preliminarily impregnated with an organic solution immiscible with water, comprising the derivative in an organic solvent, then separating the aqueous solution from the solid material.

[0057]The aqueous solution A1 preferably comprises from 0.1 mmol/L to 0.01 mol/L of an inorganic acid, which is advantageously nitric acid or hydrochloric acid. However, it goes without saying that other inorganic acids such as sulphuric acid or phosphoric acid can also be used.

[0058]As for the organic solution, it may comprise from 0.01 mol/L to 0.1 mol/L of the derivative, it being understood that the most appropriate concentration is likely to vary from one derivative to another and can be easily determined, for the derivative to be used, by first carrying out extraction tests with different concentrations of this derivative.

[0059]The solvent for the organic solution may be any non-polar solvent in which the derivative, at the concentration at which it is intended to be used, can be solubilised. Suitable organic solvents are in particular 1,3-diisopropylbenzene, chloroform, 10-undecen-1-ol, methyl isobutyl ketone (or MIBK), 3-heptanone, TBP as well as n-dodecane, alone or mixed with 1-octanol, for example in a volume ratio of 93/7.

[0060]In accordance with the invention, the extraction of the rare earth from the aqueous solution A1 is preferably followed by a back-extraction of this rare earth from the organic solution obtained at the end of its extraction, which back-extraction advantageously comprises at least contacting the organic solution obtained at the end of the extraction with an aqueous solution A2, then separating the organic solution from the aqueous solution A2.

[0061]This aqueous solution A2 may in particular be an acidic aqueous solution having a pH comprised between 0 and 3.

[0062]Moreover, to promote the back-extraction of the rare earth, the aqueous solution A2 may comprise a metal complexing agent such as an aminopolycarboxylic acid of the nitrilotriacetic acid (or NTA), EDTA, diethylenetriaminepentaacetic acid (or DTPA) type or a salt thereof such as a salt of an alkali metal (sodium or potassium in particular), for example at a concentration ranging from 0.005 mol/L to 0.05 mol/L and, better still, 0.01 mol/L.

[0063]In accordance with the invention, the use as just described is preferably implemented to extract neodymium, praseodymium and/or dysprosium from an acidic aqueous solution.

[0064]This acidic aqueous solution may in particular be a solution derived from the dissolution in an acidic medium of an urban ore concentrate and, in particular, of a W3E waste concentrate.

[0065]As such, it may in particular be a solution derived from the dissolution in an acid medium of a material in a divided form (powder, fragments, etc.) and resulting from a treatment (for example, demagnetisation+grinding as described in particular in PCT international application WO 2014/064587, hereinafter reference [8]) of used or discarded NdFeB permanent magnets.

[0066]Other features and advantages of the invention will emerge from the additional description which follows, which relates to experimental tests which have allowed to validate the use of an aminopolycarboxylic derivative as previously defined as a rare earth extractant.

[0067]It goes without saying that this additional description is given only as an illustration of the object of the invention and must in no case be interpreted as a limitation of this object.

BRIEF DESCRIPTION OF THE FIGURES

[0068]FIG. 1 illustrates the influence of the initial pH of aqueous hydrochloric solutions on the extraction coefficient of neodymium(II), denoted E %_Nd, as observed in extraction tests carried out using a lipophilic derivative of EDTA as an extractant, in solution in different solvents.

[0069]FIG. 2 illustrates the influence of the initial pH of aqueous hydrochloric solutions on the distribution coefficient of neodymium(II), denoted D_Nd, as observed in extraction tests carried out using a lipophilic derivative of EDTA as an extractant, in solution in different solvents.

[0070]FIG. 3 illustrates the influence of the initial pH of aqueous nitric solutions on the extraction coefficient of neodymium(II), denoted E %_Nd, as observed in extraction tests carried out using a lipophilic derivative of EDTA as an extractant, in solution in different solvents.

[0071]FIG. 4 illustrates the distribution isotherm of neodymium(III) as obtained following tests aimed at extracting this element from an aqueous solution comprising 0.01 mol/L of either hydrochloric or nitric acid and using a lipophilic derivative of EDTA as an extractant, in solution in 1,3-diisopropylbenzene.

[0072]FIG. 5 illustrates the influence of the initial pH of aqueous hydrochloric solutions on the extraction coefficient, denoted E %_M, of neodymium(III), praseodymium(III) and dysprosium(III) as observed in extraction tests carried out using a lipophilic derivative of EDTA as an extractant, in solution in 1,3-diisopropylbenzene.

[0073]FIG. 6 illustrates the influence of the initial pH of aqueous nitric solutions on the extraction coefficient, denoted E %_M, of neodymium(III), praseodymium(III) and dysprosium(III) as observed in extraction tests carried out using a lipophilic derivative of EDTA as extractant, in solution in 1,3-diisopropylbenzene.

[0074]FIG. 7 illustrates the influence of the concentration, noted [RP2] and expressed in mol/L, of a first lipophilic derivative of CyDTA on the extraction coefficient, noted E %_M, of neodymium(II), praseodymium(III) and dysprosium(III) as observed in tests aimed at extracting these three rare earths from an aqueous nitric solution using this derivative as an extractant, in solution in n-dodecane.

[0075]FIG. 8 illustrates the influence of the concentration, noted [RP4] and expressed in mol/L, of a second lipophilic derivative of CyDTA on the extraction coefficient, noted E %_M, of neodymium(II), praseodymium(III) and dysprosium(III) as observed in tests aimed at extracting these three rare earths from an aqueous nitric solution using this derivative as an extractant, in solution in n-dodecane.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Example 1: Use of a Lipophilic Derivative of EDTA

[0089]The liquid-liquid extraction tests reported below were carried out using as extractant a lipophilic derivative of EDTA, namely the derivative of particular formula (Ia) wherein R¹, R², R³ and R⁴ represent a hydrogen atom, X¹ and X² represent a —N(C₁₀H₂₁)₂ group while X³ and X⁴ represent an —OH group.

1—Synthesis of the Derivative:

[0090]The derivative was preliminarily synthesised by reacting EDTA dianhydride (commercially available) with an excess of didecylamine.

[0091]For this purpose, in a 100 mL flask, 1 g (3.9 mmol) of EDTA dianhydride and 2.55 g (8.58 mmol) of didecylamine were dissolved in 33 mL of anhydrous dimethylformamide (or DMF) under nitrogen and heated to 90° C. with vigorous stirring for 12 hours. After cooling, the mixture was poured into 330 mL of milli-Q™ water and the precipitate was collected by vacuum filtration. The solid was washed with 50 mL of milli-Q™ water then dissolved in 150 mL of methanol and evaporated under reduced pressure to a dry residue. The latter was recrystallised in 50 mL of ethyl acetate at 4° C. The crystals were collected by vacuum filtration, washed with cold ethyl acetate and dried under high vacuum. 2.6 g of the expected derivative were thus obtained in the form of a white solid (Yield: 78%).

2—Nd(III) Extraction Test:

[0095]Each test was performed by placing 2 mL of an aqueous solution and 2 mL of an organic solution (that is to say an O/A ratio of 1) in a tube and subjecting the tube to vigorous stirring (400 rpm) for 30 minutes at room temperature, followed by centrifugation at 11,000 g for 5 minutes.

[0096]Afterwards, the concentrations of Nd(III) remaining in the aqueous solutions were determined by inductively coupled plasma emission spectroscopy (or ICP-OES) on aliquots of these solutions after dilution in 1 M hydrochloric or nitric acid. The calibration range was established from ICP standards (PlasmaCAL™) at 1,004±5 μg/mL.

[0097]The concentrations of Nd(III) present in organic solutions were deduced from those obtained for aqueous solutions after a simple mass balance.

[0098]The extraction coefficients, E %_Nd, and distribution coefficients, D_Nd, of Nd(III) were calculated from the concentrations thus determined.

[0103]Moreover, FIG. 4 illustrates the distribution isotherm of neodymium(III) in the form of a curve which shows the equilibrium relationship existing between the concentration of Nd(III) in organic solution, noted [Nd]_org,eq and expressed in g/L, and the initial concentration of this same element in aqueous solution, noted [Nd]_aq,init and expressed in g/L, as obtained for the tests wherein neodymium(III) was extracted from an aqueous solution comprising 0.01 mol/L of hydrochloric acid or nitric acid (pH 2) using as organic solution, a solution comprising 0.015 mol/L in 1,3-diisopropyl benzene.

3—Nd(III) Back-Extraction Tests:

[0107]Each test was carried out following an operating protocol similar to that described in point 2 above.

[0108]The analysis of Nd(III) concentrations in aqueous and organic solutions after their separation was also carried out as described in point 2 above.

[0109]These tests showed that it is possible to extract almost all of the neodymium(II) from an organic solution wherein it has been previously extracted, using an aqueous solution containing DTPA at a level of 0.01 mol/L.

4—Nd (III), Pr(III) and Dy(III) Extraction Tests:

[0113]Each test was carried out following an operating protocol similar to that described in point 2 above.

[0114]The analysis of the concentrations of the three rare earths in the aqueous and organic solutions after their separation was also carried out as described in point 2 above.

[0115]Their extraction coefficients, E %_M, and distribution coefficients, D_M, were calculated from the concentrations thus determined, then the factors of separation of neodymium(II) from praseodymium(III) on the one hand, and dysprosium(II) on the other hand, FS_Nd/Pr and FS_Nd/Dy, were calculated from the D_M thus obtained.

[0120]These results show that the derivative allows the extraction of neodymium(III), praseodymium(III) and dysprosium(III) from an aqueous hydrochloric or nitric solution with a pH ranging from 2 to 4.

[0121]They also show that the derivative has more affinity for neodymium (111) than for the other two rare earths, this affinity being in the order: Nd>Pr>Dy for the extractions carried out with the derivative in solution in 1,3-diisopropylbenzene while it is in the order: Nd>Dy>Pr for the extractions carried out with the derivative in solution in the n-dodecane/1-octanol mixture (93/7, v/v).

Example 2: Use of Two Lipophilic Derivatives of CyDTA

1—Synthesis of Derivatives:

[0125]The derivatives were preliminarily synthesised by reacting the dianhydride of CyTDA with an excess of didodecylamine for the derivative RP2 and dioctylamine for the derivative RP4.

Synthesis of CyTDA Dianhydride:

[0126]12.64 g (36.7 mmol) of trans-1,2-diaminocyclohexane tetraacetic acid monohydrate (CyDTA·H₂O) and 11 mL of pyridine were introduced into a 250 mL single-necked flask. Then, 66 mL of acetic anhydride was added and the mixture was left stirring overnight. The solution obtained was poured dropwise into 350 mL of diethyl ether and the suspension formed was then filtered through a sintered glass of porosity 3. The precipitate was washed with diethyl ether (3×100 mL) then dried under vacuum. 7.27 g of the expected dianhydride were thus obtained in the form of a yellowish powder (Yield: 69%).

Synthesis of the Derivative RP2:

[0127]1 g (3.22 mmol) of the dianhydride of the previously obtained CyDTA and 2.51 g (2.2 eq., 7.08 mmol) of didodecylamine were introduced into a 100 mL two-necked flask equipped with a septum, topped with a condenser and placed under an argon atmosphere. Then, 40 mL of DMF was added, the mixture was brought to 60° C. and left stirring overnight. The DMF was then evaporated under vacuum and the residual crude was solubilised in 100 mL of dichloromethane (or DCM) and poured into a 250 mL separating funnel. The organic phase was washed with a 3 M hydrochloric acid solution (2×100 mL) then with deionized water (Milli-Q™—2×100 mL). The organic phase was then dried over sodium sulphate (Na₂SO₄) and filtered under reduced pressure. The hydrated salts were rinsed with DCM (3×40 mL) and the filtrate was evaporated under reduced pressure. The oily residue was purified by reverse-phase flash chromatography (C18 column) and elution by methanol/isopropanol gradient (from 100/0 to 80/20). 2.19 g of the derivative RP2 were thus obtained in the form of a white paste (Yield: 67%).

Synthesis of the Derivative RP4:

[0128]1.43 g of the derivative RP4 in the form of a yellowish oil were obtained by following a protocol similar to that described for the synthesis of the derivative RP2, except that 1 g (3.22 mmol) of the CyDTA dianhydride was reacted with 1.71 g (2.2 eq., 7.08 mmol) of dioctylamine and that the elution of the C18 column used for the purification was carried out by a methanol/water gradient from 95/5 to 100/0 (Yield: 56%).

2—Extraction Tests:

[0132]Each test was carried out following an operating protocol similar to that described in point 2 of Example I above.

[0133]The analysis of the concentrations of the three rare earths in the aqueous and organic solutions after their separation was also carried out as described in point 2 of Example I above.

[0134]Their extraction coefficients, E %_M, were calculated from the concentrations thus determined.

[0135]The results are illustrated in FIGS. 7 and 8 which present the extraction coefficients thus obtained as a function of the concentrations of RP2 (FIG. 7) and RP4 (FIG. 8) of the organic solutions.

[0136]These figures show that, unlike the lipophilic derivative of EDTA tested in Example I above, the two lipophilic derivatives of CyDTA, in solution in n-dodecane, have an identical or almost identical affinity for neodymium(II), praseodymium(III) and dysprosium(II).

[0137]These results are extremely interesting because they mean that the invention offers a panel of extractants capable of enabling both an extraction of neodymium(III) selective with respect to praseodymium(II) and dysprosium(III)—if such extraction is sought—and an extraction of all three rare earths.

REFERENCES MENTIONED