Extracting rare earth metal from acidic solution by contacting with ionic liquid composition

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. A method for extracting a rare earth metal from a mixture comprising 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, wherein the 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 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 or uronium; each EDG represents an electron donating group; L.sub.1 is selected from C.sub.1-10 alkanediyl, C.sub.2-10 alkenediyl, C.sub.1-10 dialkanylether or C.sub.1-10 dialkanylketone groups; each L.sub.2 is independently selected from C.sub.1-2 alkanediyl, C.sub.2 alkenediyl, C.sub.1-2 dialkanylether or C.sub.1-2 dialkanylketone; and [X.sup.−] represents an anionic species.

2. The method of claim 1, wherein the method comprises recovering the rare earth metal from the non-aqueous phase by stripping with an acidic stripping solution comprising at least one of: an aqueous hydrochloric acid, nitric acid solution or an acidic stripping solution having a pH of 0 or higher.

3. The method of claim 1, wherein the method comprises extracting a rare earth metal from a mixture of two or more rare earth metals.

4. The method of claim 1, wherein the acidic solution comprises a first and a second rare earth metal, and the method comprises: (a) partitioning the first rare earth metal into the non-aqueous phase and separating the non-aqueous phase from the acidic solution; and (b) contacting the acidic solution depleted of the first rare earth metal with the composition which comprises an ionic liquid, and recovering the second rare earth metal therefrom.

5. The method of claim 4, wherein at least one of: the first rare earth metal is dysprosium and the second rare earth metal is neodymium; the first rare earth metal is lanthanum and the second rare earth metal is europium; or the acidic solution has a pH of less than 3.5 in step (a), and the acidic solution has a pH of greater than 3.5 in step (b).

6. The method of claim 1, wherein at least one of: the acidic solution from which the rare earth metal is extracted has a pH of from 2 to 4; or prior to contacting the composition with the acidic solution of the rare earth metal the composition is equilibrated with an acidic solution having the same pH as the acidic solution of the rare earth metal.

7. The method of claim 1, wherein the composition is added to the acidic solution in a volume ratio of from 0.5:1 to 2:1.

8. The method of claim 1, wherein the method comprises contacting the acidic solution of the rare earth metal and the composition for from 10 to 40 minutes, and/or wherein the method comprises contacting and physically mixing the acidic solution of the rare earth metal and the composition.

9. The method of claim 1, 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.

10. The method of claim 1, wherein [Y.sup.+] represents an acyclic cation selected from at least one of:
[—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; a cyclic cation selected from: ##STR00014## wherein: R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.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.sup.e attached to adjacent carbon atoms form an optionally substituted methylene chain —(CH.sub.2).sub.q— where q is from 3 to 6; a saturated heterocyclic cation selected from cyclic ammonium, 1,4-diazabicyclo[2.2.2]octanium, morpholinium, cyclic phosphonium, piperazinium, piperidinium, quinuclidinium, and cyclic sulfonium; a saturated heterocyclic cation having the formula: ##STR00015## wherein: R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.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.sup.e attached to adjacent carbon atoms form an optionally substituted methylene chain —(CH.sub.2).sub.q— where q is from 3 to 6.

11. The method of claim 10, wherein one or more of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.e and R.sup.f is selected from at least one of: 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.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, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from hydrogen or C.sub.1-6 alkyl; a C.sub.1-3 alkyl group substituted with —CO.sub.2R.sup.x, —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; 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; or a group selected from: ##STR00017## wherein R.sup.y═R.sup.z, and wherein R.sup.y and R.sup.z are selected from C.sub.3-6 alkyl.

12. The method of claim 10, wherein one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.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.sup.e and R.sup.f are independently selected from H and unsubstituted C.sub.1-5 alkyl groups.

13. The method of claim 1, wherein [Y.sup.+] represents a cyclic cation selected from: ##STR00018## optionally wherein 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.sup.e and R.sup.f are independently selected from H and unsubstituted C.sub.1-5 alkyl groups.

14. The method of claim 1, wherein L.sub.1 represents a linking group selected from C.sub.1-10 alkanediyl groups, C.sub.1-10 alkenediyl groups, C.sub.1-5 alkanediyl groups, C.sub.2-5 alkenediyl groups, —CH.sub.2—, —C.sub.2H.sub.4— or —C.sub.3H.sub.6—.

15. The method of claim 1, wherein each L.sub.2 represents a linking group independently selected from C.sub.1-2 alkanediyl groups, C.sub.2 alkenediyl groups, —CH.sub.2— or —C.sub.2H.sub.4—.

16. The method of claim 1, wherein each EDG represents an electron donating group independently selected from —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.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, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from H, C.sub.1-6 alkyl or C.sub.3-6 alkyl.

17. The method of claim 16, wherein each -L.sub.2-EDG represents an electron donating group independently selected from: ##STR00019## 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.

18. The method of claim 1, wherein [X.sup.−] represents one or more anionic species selected from: hydroxides, halides, perhalides, sulphates, sulphites, sulfonates, sulfonimides, phosphates, phosphites, phosphonates, methides, carboxylates, azolates, carbonates, carbamates, thiophosphates, thiocarboxylates, thiocarbamates, thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate, nitrite, tetrafluoroborate, hexafluorophosphate, halometallates, amino acids, borates, polyfluoroalkoxyaluminates; or 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) an 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.2OSO.sub.2O]; 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.1SO.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-dicyano-imidazolate], [4-nitroimidazolate], [tetrazolate]; o) a sulfur-containing anion selected from: thiocarbonates, thiocarbamates, thiocarboxylates, thiophosphates, thiosulfonates, thiosulfates; p) a nitrate or nitrite anion; q) a tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate or perchlorate anion; r) a carbonate anion selected from [CO.sub.3].sup.2−, [HCO.sub.3].sup.−, [R.sup.2CO.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-C.sub.6 alkoxy, C.sub.2-C.sub.12 alkoxyalkoxy, C.sub.3-C.sub.8 cycloalkyl, C.sub.6-C.sub.10 aryl, C.sub.7-C.sub.10 alkaryl, C.sub.7-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-C.sub.6 alkyl, —S(O)O(C.sub.1-C.sub.6)alkyl, —OS(O)(C.sub.1-C.sub.6)alkyl, —S(C.sub.1-C.sub.6)alkyl, —S—S(C.sub.1-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)N R.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-C.sub.6 alkyl, or wherein R.sup.1 is fluorine, chlorine, bromine or iodine; or one or more anionic species selected from bistriflimide, triflate, tosylate, perchlorate, [Al(OC(CF.sub.3).sub.3).sub.4.sup.−], tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, tetrakis(pentafluorophenyl)borate, tetrafluoroborate, hexfluoroantimonate and hexafluorophosphate anions.

19. The method of claim 1, wherein [Cat.sup.+] represents one or more ionic species having the structure: ##STR00020## where: [Z.sup.+] is 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, or uronium.

20. The method of claim 1, wherein the composition further comprises a lower viscosity ionic liquid, optionally wherein the cation of the lower viscosity ionic liquid is 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; or [N(R.sup.3)(R.sup.4)(R.sup.5)(R.sup.6)].sup.+ and [P(R.sup.3)(R.sup.4)(R.sup.5)(R.sup.6)].sup.+ wherein R.sup.3, R.sup.4, R.sup.5 and R.sup.6 are each independently selected from optionally substituted C.sub.1-20 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups; [P(R.sup.3)(R.sup.4)(R.sup.5)(R.sup.6)].sup.+, wherein R.sup.3, R.sup.4, R.sup.5 are selected from C.sub.1-10 alkyl or C.sub.2-6 alkyl, and R.sup.6 is selected from C.sub.4-20 alkyl or C.sub.8-14 alkyl; [N(R.sup.3)(R.sup.4)(R.sup.5)(R.sup.6)].sup.+, wherein R.sup.3, R.sup.4, R.sup.5 are selected from C.sub.4-14 alkyl or C.sub.6-10 alkyl, and R.sup.6 is selected from C.sub.1-4 alkyl or C.sub.1-2 alkyl; or imidazolium cations substituted with one or more C.sub.1-20 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups, or substituted with two C.sub.1-10 alkyl groups.

21. The method of claim 20, wherein the anion of the lower viscosity ionic liquid comprises one or more anionic species selected from: hydroxides, halides, perhalides, sulphates, sulphites, sulfonates, sulfonimides, phosphates, phosphites, phosphonates, methides, carboxylates, azolates, carbonates, carbamates, thiophosphates, thiocarboxylates, thiocarbamates, thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate, nitrite, tetrafluoroborate, hexafluorophosphate, perchlorate, halometallates, amino acids, borates, polyfluoroalkoxyaluminates; or 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) an 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.2OSO.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.1SO.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-dicyano-imidazolate], [4-nitroimidazolate], [tetrazolate]; o) a sulfur-containing anion selected from: thiocarbonates, thiocarbamates, thiocarboxylates, thiophosphates, thiosulfonates, thiosulfates; p) a nitrate or nitrite anion; q) a tetrafluoroborate, hexafluorophosphate, hexfluoroantimonate or perchlorate 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-C.sub.6 alkoxy, C.sub.2-C.sub.12 alkoxyalkoxy, C.sub.3-C.sub.8 cycloalkyl, C.sub.6-C.sub.10 aryl, C.sub.7-C.sub.10 alkaryl, C.sub.7-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-C.sub.6)alkyl, —S(O)O(C.sub.1-C.sub.6)alkyl, —OS(O)(C.sub.1-C.sub.6)alkyl, —S(C.sub.1-C.sub.6)alkyl, —S—S(C.sub.1-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-C.sub.6 alkyl, or wherein R.sup.1 is fluorine, chlorine, bromine or iodine; or one or more anionic species selected from bistriflimide, triflate, tosylate, perchlorate, [Al(OC(CF.sub.3).sub.3).sub.4.sup.−], tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, tetrakis(pentafluorophenyl)borate, tetrafluoroborate, hexfluoroantimonate and hexafluorophosphate anions.

22. The method of claim 20, wherein the ionic liquid is present in the composition in a concentration of at least 0.001 M, or from 0.005 M to 0.01 M.

23. The method of claim 1, wherein the acidic solution is obtainable by leaching the rare earth metal from its source with an acid.

Description

(1) The present invention will now be illustrated by way of the following examples and with reference to the following figures in which:

(2) 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

(3) 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

(4) 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. CHC.sub.3, CH.sub.2Cl.sub.2, etc.) or an aromatic solvent (e.g. toluene, xylene, etc.) at 60 to 70° 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.

(5) Synthesis of an Imidazolium Ionic Liquid

(6) ##STR00012##

(7) 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.

(8) 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.−.]

(9) General Procedure for Extraction of Rare Earth Metals

(10) 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.

(11) 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).

(12) 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).

(13) 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.

(14) Separation of Dy and Nd

(15) 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.

(16) 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).

(17) Separation of Eu and La

(18) 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 (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—La of 211.

(19) Separation of Tb and Ce

(20) 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 [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.−.]

(21) 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.

(22) 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.

(23) 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.

(24) 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.

(25) 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.

(26) 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.