SEPARATION OF RARE EARTH METALS

Abstract

A method for extracting a rare earth metal from a mixture of one or more rare earth metals, said method comprising contacting an acidic solution of the rare earth metal with a composition which comprises an ionic liquid to form an aqueous phase and a non-aqueous phase into which the rare earth metal has been selectively extracted.

Claims

1-68. (canceled)

69. An ionic liquid has the formula:
[Cat.sup.+][X.sup.−] in which: [Cat.sup.+] represents a cationic species having the structure: ##STR00013## where: [Y.sup.+] comprises a group selected from ammonium, benzimidazolium, benzofuranium, benzothiophenium, benzotriazolium, borolium, cinnolinium, diazabicyclodecenium, diazabicyclononenium, 1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium, furanium, guanidinium, imidazolium, indazolium, indolinium, indolium, morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium, iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium, pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium, quinazolinium, quinolinium, iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium, triazinium, triazolium, iso-triazolium and uronium groups; each EDG represents an electron donating group; and L.sub.1 represents a linking group selected from C.sub.1-10 alkanediyl, C.sub.2-10 alkenediyl, C.sub.1-10 dialkanylether and C.sub.1-10 dialkanylketone groups; each L.sub.2 represents a linking group independently selected from C.sub.1-2 alkanediyl, C.sub.2 alkenediyl, C.sub.1-2 dialkanylether and C.sub.1-2 dialkanylketone groups; and [X.sup.−] represents an anionic species.

70. The ionic liquid of claim 69, wherein when the nitrogen linking L.sub.1 to each L.sub.2 and one of the EDG both coordinate to a metal, the ring formed by the nitrogen, L.sub.2, the EDG and the metal is a 5 or 6 membered ring.

71. The ionic liquid of claim 69, wherein [Y.sup.+] represents an acyclic cation selected from:
[—N(R.sup.a)(R.sup.b)(R.sup.c)].sup.+, [—P(R.sup.a)(R.sup.b)(R.sup.c)].sup.+ and [—S(R.sup.a)(R.sup.b)].sup.+, wherein: R.sup.a, R.sup.b and R.sup.c are each independently selected from optionally substituted C.sub.1-30 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups.

72. The ionic liquid of claim 69, wherein [Y.sup.+] represents a cyclic cation selected from: ##STR00014## wherein: R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sub.e and R.sup.f are each independently selected from: hydrogen and optionally substituted C.sub.1-30 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups, or any two of R.sup.a, R.sup.b, R.sup.c, R.sup.dand R.sub.e attached to adjacent carbon atoms form an optionally substituted methylene chain —(CH.sub.2).sub.q— where q is from 3 to 6.

73. The ionic liquid of claim 69, wherein [Y.sup.+] represents a saturated heterocyclic cation having the formula: ##STR00015## wherein: R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sub.e and R.sup.f are each independently selected from: hydrogen and optionally substituted C.sub.1-30 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups, or any two of R.sup.a, R.sup.b, R.sup.c, R.sup.d and R.sub.e attached to adjacent carbon atoms form an optionally substituted methylene chain —(CH.sub.2).sub.q— where q is from 3 to 6.

74. The ionic liquid of claim 72 or claim 73, wherein at least one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sub.e and R.sup.f is a C.sub.1-5 alkyl group substituted with —CO.sub.2R.sup.x, —OC(O)R.sup.x, —CS.sub.2R.sup.x, —SC(S)R.sup.x, —S(O)OR.sup.x, —OS(O)R.sup.x, —NR.sup.x(O)NR.sup.yR.sup.z, —NR.sup.xC(O)OR.sup.y, —OC(O)NR.sup.yR.sup.z, —NR.sup.xC(S)OR.sup.y, —OC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)SR.sup.y, —SC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)NR.sup.yR.sup.z, —C(O)NR.sup.yR.sup.z, —C(S)NR.sup.yR.sup.z, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from hydrogen or C.sub.1-6 alkyl.

75. The ionic liquid of claim 74, wherein at least one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.e and R.sup.f represents a group selected from: ##STR00016## wherein R.sup.y=R.sup.z, and wherein R.sup.x, R.sup.y and R.sup.z are each selected from C.sub.3-6 alkyl.

76. The ionic liquid of claim 72 or claim 73, wherein one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sub.e and R.sup.f is a substituted C.sub.1-5 alkyl group, and the remainder of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sub.e and R.sup.f are independently selected from H and unsubstituted C.sub.1-5 alkyl groups.

77. The ionic liquid of claim 69, wherein L.sub.1 represents a linking group selected from C.sub.1-10 alkanediyl and C.sub.1-10 alkenediyl groups.

78. The ionic liquid of claim 69, wherein each L.sub.2 represents a linking group independently selected from C.sub.1-2 alkanediyl and C.sub.2 alkenediyl groups.

79. The ionic liquid of claim 69, wherein each EDG represents an electron donating group independently selected from 13 CO.sub.2R.sup.x, —OC(O)R.sup.x, —CS.sub.2R.sup.x, —SC(S)R.sup.x, —S(O)OR.sup.x, —OS(O)R.sup.x, —NR.sup.x(O)NR.sup.yR.sup.z, —NR.sup.xC(O)OR.sup.y, —OC(O)NR.sup.yR.sup.z, —NR.sup.xC(S)OR.sup.y, —OC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)SR.sup.y, —SC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)NR.sup.yR.sup.z, —C(O)NR.sup.yR.sup.z, —C(S)NR.sup.yR.sup.z, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from hydrogen or C.sub.1-6 alkyl, such as wherein each EDG represents an electron donating group independently selected —CO.sub.2R.sup.x and —C(O)NR.sup.yR.sup.z, wherein R.sup.x, R.sup.y and R.sup.z are each independently selected from C.sub.3-6 alkyl.

80. The ionic liquid of claim 69, wherein each -L.sub.2-EDG represents an electron donating group independently selected from: ##STR00017## wherein R.sup.y=R.sup.z, and wherein R.sup.x, R.sup.y and R.sup.z are each selected from C.sub.3-6 alkyl.

81. The ionic liquid of claim 69, wherein [X.sup.−] represents one or more anionic species selected from: hydroxides, halides, perhalides, pseudohalides, sulphates, sulphites, sulfonates, sulfonimides, phosphates, phosphites, phosphonates, methides, borates, carboxylates, azolates, carbonates, carbamates, thiophosphates, thiocarboxylates, thiocarbamates, thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate, nitrite, tetrafluoroborate, hexafluorophosphate and perchlorate, halometallates, amino acids, borates, polyfluoroalkoxyaluminates.

82. The ionic liquid of claim 81, wherein [X.sup.−] represents one or more anionic species selected from: a) a halide anion selected from: F.sup.−, Cl.sup.−, Br.sup.−, I.sup.−; b) a perhalide anion selected from: [I.sub.3].sup.−, [I.sub.2Br].sup.−, [IBr.sub.2].sup.−, [Br.sub.3].sup.−, [Br.sub.2C].sup.−, [BrCl.sub.2].sup.−, [ICl.sub.2].sup.−, [I.sub.2Cl].sup.−, [Cl.sub.3].sup.−; c) a pseudohalide anion selected from: [N.sub.3].sup.−, [NCS].sup.−, [NCSe].sup.−, [NCO].sup.−, [CN].sup.−; d) a sulphate anion selected from: [HSO.sub.4].sup.−, [SO.sub.4].sup.2−, [R.sup.2OSC.sub.2O].sup.−; e) a sulphite anion selected from: [HSO.sub.3].sup.−, [SO.sub.3].sup.2−, [R.sup.2OSO.sub.2].sup.−; f) a sulfonate anion selected from: [R.sup.1SC.sub.2O].sup.−; g) a sulfonimide anion selected from: [(R.sup.1SO.sub.2).sub.2N].sup.−; h) a phosphate anion selected from: [H.sub.2PO.sub.4].sup.−, [HPO.sub.4].sup.2−, [PO.sub.4].sup.3−, [R.sup.2OPO.sub.3].sup.2−, [(R.sup.2O).sub.2PO.sub.2].sup.−; i) a phosphite anion selected from: [H.sub.2PO.sub.3].sup.−, [HPO.sub.3].sup.2−, [R.sup.2OPO.sub.2].sup.2−, [(R.sup.2O).sub.2PO].sup.−; j) a phosphonate anion selected from: [R.sup.1PO.sub.3].sup.2−, [R.sup.1P(O)(OR.sup.2)O].sup.−; k) a methide anion selected from: [(R.sup.1SO.sub.2).sub.3C].sup.−; l) a borate anion selected from: [bisoxalatoborate], [bismalonatoborate] tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, tetrakis(pentafluorophenyl)borate; m) a carboxylate anion selected from: [R.sup.2CO.sub.2].sup.−; n) an azolate anion selected from: [3,5-dinitro-1,2,4-triazolate], [4-nitro-1,2,3-triazolate], [2,4-dinitroimidazolate], [4,5-dinitroimidazolate], [4,5-dicyanoimidazolate], [4-nitroimidazolate], [tetrazolate]; o) a sulfur-containing anion selected from: thiocarbonates (e.g. [R.sup.2OCS.sub.2], thiocarbamates (e.g. [R.sup.2.sub.2NCS.sub.2].sup.−), thiocarboxylates (e.g. [R.sup.1CS.sub.2]), thiophosphates (e.g. [(R.sup.2O).sub.2PS.sub.2].sup.−), thiosulfonates (e.g. [RS(O).sub.2S]) , thiosulfates (e.g. [ROS(O).sub.2S]).sup.−); p) a nitrate ([NO.sub.3].sup.−) or nitrite ([NO.sub.2].sup.−) anion; q) a tetrafluoroborate ([BF.sub.4.sup.−]), hexafluorophosphate ([PF.sub.6.sup.−]), hexfluoroantimonate ([SbF.sub.6.sup.−]) or perchlorate ([ClO.sub.4.sup.−]) anion; r) a carbonate anion selected from [CO.sub.3].sup.2−, [HCO.sub.3].sup.−, [R.sup.2CO.sub.3].sup.−; preferably [MeCO.sub.3].sup.−; s) polyfluoroalkoxyaluminate anions selected from [Al(OR.sup.F).sub.4.sup.−], wherein R.sup.F is selected from C.sub.1-6 alkyl substituted by one or more fluoro groups; where: R.sup.1 and R.sup.2 are independently selected from the group consisting of C.sub.1-C.sub.10 alkyl, C.sub.6 aryl, C.sub.1-C.sub.10 alkyl(C.sub.6)aryl and C.sub.6 aryl(C.sub.1-C.sub.10)alkyl each of which may be substituted by one or more groups selected from: fluoro, chloro, bromo, iodo, C.sub.1 to C.sub.6 alkoxy, C.sub.2 to C.sub.12 alkoxyalkoxy, C.sub.3 to C.sub.8 cycloalkyl, C.sub.6 to C.sub.10 aryl, C.sub.7 to C.sub.10 alkaryl, C.sub.7 to C.sub.10 aralkyl, —CN, —OH, —SH, —NO.sub.2, —CO.sub.2R.sup.x, —OC(O)R.sup.x, —C(O)R.sup.x, —C(S)R.sup.x, —CS.sub.—2R.sup.x, —SC(S)R.sup.x, —S(O)(C.sub.1 to C.sub.6)alkyl, —S(O)O(C.sub.1 to C.sub.6)alkyl, —OS(O)(C.sub.1 to C.sub.6)alkyl, —S(C.sub.1 to C.sub.6)alkyl, —S—S(C.sub.1 to C.sub.6 alkyl), —NR.sup.xC(O)NR.sup.yR.sup.z, —NR.sup.xC(O)OR.sup.y, —OC(O)NR.sup.yR.sup.z, —NR.sup.xC(S)OR.sup.y, —OC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)SR.sup.y, —SC(S)NR.sup.yR.sup.z, —NR.sup.xC(S)NR.sup.yR.sup.z, —C(O)NR.sup.yR.sup.z, —C(S)NR.sup.yR.sup.z, —NR.sup.yR.sup.z, or a heterocyclic group, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from hydrogen or C.sub.1 to C.sub.6 alkyl, wherein R.sup.1 may also be fluorine, chlorine, bromine or iodine.

83. The ionic liquid of claim 69, wherein [Cat.sup.+] represents one or more ionic species having the structure: ##STR00018## where: [Z.sup.+] represents a group selected from ammonium, benzimidazolium, benzofuranium, benzothiophenium, benzotriazolium, borolium, cinnolinium, diazabicyclodecenium, diazabicyclononenium, 1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium, furanium, guanidinium, imidazolium, indazolium, indolinium, indolium, morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium, iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium, pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium, quinazolinium, quinolinium, iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium, triazinium, triazolium, iso-triazolium and uronium groups.

84. The ionic liquid of claim 69 further comprising a lower viscosity ionic liquid, selected from ammonium, benzimidazolium, benzofuranium, benzothiophenium, benzotriazolium, borolium, cinnolinium, diazabicyclodecenium, diazabicyclononenium, 1,4-diazabicyclo [2.2.2]octanium, diazabicyclo-undecenium, dithiazolium, furanium, guanidinium, imidazolium, indazolium, indolinium, indolium, morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium, iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium, pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium, quinazolinium, quinolinium, iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium, triazinium, triazolium, iso-triazolium and uronium groups.

85. The ionic liquid of claim 69 comprising than 25% halide or pseudohalide anions as a proportion of the total anions.

86. The ionic liquid of claim 69 further comprising one or more organic solvents.

87. The ionic liquid of claim 86, further comprising a rare earth metal.

88. A method for preparing an ionic liquid as defined in claim 69, said method comprising reacting: ##STR00019## where: LG represents a leaving group.

Description

[0146] The present invention will now be illustrated by way of the following examples and with reference to the following figures in which:

[0147] FIG. 1 is a graph showing the distribution factors for the extraction of a selection of rare earth metals according to an embodiment of the present invention; and

[0148] FIG. 2 shows the crystal structure of the [MAIL].sup.+cation coordinating to Nd after extraction from an acidic (HCl) solution containing NdCl.sub.3.6H.sub.2O.

EXAMPLES

Example 1: Synthesis of ionic liquid

General Procedure for the Synthesis of an Ionic Liquid According to Embodiments of the Invention

[0149] A reaction mixture comprising 3 moles of an N,N-dialkyl-2-chloroacetamide and a substrate having the structure H.sub.2N-L.sub.1-[Z] were stirred in a halogenated solvent (e.g. CHCl.sub.3, CH.sub.2Cl.sub.2, etc.) or an aromatic solvent (e.g. toluene, xylene, etc.) at 60 to 70°0 C. for 7 to 15 days. After cooling, the solid was filtered off and the organic phase was repeatedly washed with 0.1 to 0.2 M HCl until the aqueous phase showed milder acidity (pH≥2). The organic phase was then washed with 0.1 M Na.sub.2CO.sub.3 (2-3 washes) and finally was washed with deionized water until the aqueous phase showed a neutral pH. The solvent was removed under high vacuum to give the ionic liquid product (with a chloride anion) as a highly viscous liquid. This ionic liquid could be used as it was or the chloride anion could be exchanged with different anions (e.g. bistriflimide, triflate, hexafluorophosphate etc.) using conventional metathesis routes, for example, by reacting with an alkali metal salt of the desired anion with the ionic liquid in an organic solvent.

Synthesis of an Imidazolium Ionic Liquid

[0150] ##STR00012##

[0151] 1-(3-Aminopropyl)-imidazole (0.05 mol) was added to of N,N-diisobutyl-2-chloroacetamide (0.15 mol) in a 500 ml three necked round bottom flask. Triethylamine (0.11 moles) was then added along with chloroform (200 ml). The reaction was stirred for 6 hours at room temperature and then stirred at 60 to 70° C. for 7 days. The reaction mixture was then cooled and after filtration it was successively washed with 0.1 M HCl, 0.1 M Na.sub.2CO.sub.3 and deionized water (as described in general procedure). The solvent was removed from the neutralised organic phase at 8 mbar (6 mm Hg) and finally at 60° C. and 0.067 mbar (0.05 mmHg). The ionic liquid [MAIL.sup.+]Cl.sup.− was recovered as a highly viscous yellow liquid.

[0152] Ionic liquid [MAIL.sup.+]Cl.sup.− (0.025 mol) was dissolved in chloroform and lithium bis-(trifluoromethane) sulfonamide (LiNTf.sub.2) (0.03 mol) was added. The reaction mixture was stirred for 1 hour and then the organic phase was repeatedly washed with deionized water. Finally the solvent was removed from the organic phase under vacuum (0.13 mbar, 0.1 mm Hg) at 65° C. to yield the bistriflimide anion form of the ionic liquid ([MAIL.sup.+][NTf.sub.2.sup.−]).

Example 2: Liquid-Liquid Extraction of Rare Earth Metals Using [MAIL.SUP.+.][NTf.SUB.2..SUP.−.]

General Procedure for Extraction of Rare Earth Metals

[0153] Equal volumes (2 to 5 ml) of the ionic liquid extractant ([MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−]) and an acidic aqueous feed solution containing rare earth metals in HCl were equilibrated for 15-30 minutes on a wrist action shaker. The phases were centrifuged and the aqueous phase was analysed for rare earth metal content using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), though it will be appreciated that any suitable analysis technique may be used. The proportion of the rare earth metals extracted into the ionic liquid (organic) phase was determined through mass balance using the ICP-OES measurement.

[0154] The distribution ratio of an individual rare earth metal was determined as the ratio of its concentration in the ionic liquid phase to that of it in the aqueous phase (raffinate). D.sub.M=[M].sub.IL/[M].sub.Aq, where IL represents ionic liquid phase and Aq represents the aqueous phase (raffinate).

[0155] The separation factor (SF) with respect to an individual rare earth metal pair is expressed as the ratio of the distribution ratio of a first rare earth metal with the distribution ratio of a second rare earth metal. For example, the separation factor of dysprosium with respect to neodymium=D.sub.Dy/D.sub.Nd. It will be appreciated that separation factors estimated from independently obtained distribution ratios will be lower than the actual separation factors, obtained during the separation of mixtures of rare earth metals during a competitive separation (as exemplified below).

[0156] Distribution ratios for individual rare earth metals were obtained in separate extractions according to the general procedure above, using 0.0075 M ([MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] and a 200 mg/l (ppm) HCl solution of the relevant rare earth metal chloride (where 200 ppm refers to the concentration of the elemental metal in the solution). FIG. 1 shows a plot of the distribution ratios for each rare earth metal as a function of pH, showing that the ionic liquid according to the present invention may be used to extract rare earth metals across a range of pH values.

Separation of Dy and Nd

[0157] An aqueous HCl solution containing DyCl.sub.3.6H.sub.2O (60 mg/l (ppm) Dy) and NdCl.sub.3.6H.sub.2O (1400 mg/l (ppm) Nd) at pH 3 was extracted with the ionic liquid extractant (0.005 M [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−]) according to the general procedure above. A single contact (extraction) gave D.sub.Dy=13.45, D.sub.Nd=0.0124, giving a SF.sub.Dy—Nd of 1085.

[0158] This separation factor (1085) is considerably higher than the separation factors obtained for Dy/Nd separation by the systems in the prior art shown in Table 1 (maximum 239).

Separation of Eu and La

[0159] An aqueous HCl solution containing EuCl.sub.3.6H.sub.2O (65 mg/l (ppm) Eu) and LaCl.sub.3.7H.sub.2O (470 mg/l (ppm) La) at pH 3 was extracted with the ionic liquid extractant extractant (0.005 M [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−]) according to the general procedure above. A single contact (extraction) gave D.sub.Eu=9.3, D.sub.La=0.044, giving a SF.sub.Eu—Le of 211.

Separation of Tb and Ce

[0160] An aqueous HCl solution containing TbCl.sub.3.6H.sub.2O (530 mg/l (ppm) Tb) and CeCl.sub.3.6H.sub.2O (950 mg/l (ppm) Ce) at pH 3 was extracted with the ionic liquid extractant (0.0075 M (0.005 M [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−]) according to the general procedure above. A single contact (extraction) gave D.sub.Tb=11.2, D.sub.Ce=0.068, giving a SF.sub.Tb—Ce of 162.

Example 3: Stripping of Rare Earth Metals from [MAIL.SUP.+.][NTf.SUB.2..SUP.−.]

[0161] Dy(III) (200 ppm) was stripped from an organic phase at pH 3 comprising [MAIL.sup.+][NTf.sub.2.sup.−]in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] (0.005 M) in 2 successive contacts. The organic phase was contacted with an equal volume of an aqueous HCl solution (0.2 M) and was equilibrated for 15-30 minutes on a wrist action shaker. 140 ppm of Dy(III) was stripped in the first contact and 55 ppm was stripped in the second contact.

[0162] Similarly, from observation of the distribution ratios in FIG. 1, it is clear that heavy rare earth metals such as Tm, Yb and Lu have significantly reduced distribution factors with increasing acidity. Thus, it is also expected that heavy rare earth metals may be stripped from the ionic liquid of the present invention at relatively high pH values.

[0163] The above examples show that a large increase in the separation factors between key rare earth metal pairs may be obtained by use of an ionic liquid according to the present invention (e.g. Nd/Dy: Nd—Dy magnet, Eu/La: white lamp phosphor, Tb/Ce: green lamp phosphor). The rare earth metals may also be advantageously stripped from the ionic liquid at relatively high pH compared to prior art systems.

[0164] Without wishing to be bound by any particular theory, it is believed that a more pronounced increase in distribution ratios is observed for heavier rare earth metals than lighter rare earth metals as a result of increased formation of the more hydrophobic doubly coordinated rare earth metal species M.([MAIL.sup.+][NTf.sub.2.sup.−]).sub.2 over the singly coordinated species M.([MAIL.sup.+][NTf.sub.2.sup.−]). It is believed that the more hydrophobic species will be more easily extracted into the organic phase during separation, leading to increased distribution ratios.

[0165] Nuclear magnetic resonance, infra-red and mass spectrometry studies have shown that the doubly coordinated species is more abundant in solutions of Lu and the ionic liquid compared to solutions of La and the ionic liquid, highlighting the differentiation between the heavy and light rare earth metals achieved by the ionic liquid of the present invention.

[0166] Furthermore, optimised geometries of the complexes LaCl.sub.3.([MAIL.sup.+][Cl.sup.−]).sub.2 and LuCl.sub.3.([MAIL.sup.+][Cl.sup.−]).sub.2 show that the distance between the tertiary central nitrogen of the ionic liquid cation and the metal is much longer in the case of La (˜2.9 Å, non-bonding) than in the case of Lu (˜2.6 Å, bonding), which also supports the weaker bonding of the ionic liquid to lighter rare earth metals. At the same time, the electron donating groups, in this case amides, linked to the nitrogen atom bond to the metal in a very similar way in both cases. This result shows that the central motif of the ionic liquid cation having a tertiary nitrogen donor is important for the differentiation obtained between the heavier and lighter rare earth metals and the improved selectivity that results therefrom.