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SEPARATION OF RARE EARTH METALS

20200080169 ยท 2020-03-12

    Inventors

    Cpc classification

    International classification

    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 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, 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 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.

    2. The method of claim 1, wherein the method comprises recovering the rare earth metal from the non-aqueous phase.

    3. The method of claim 2, wherein the rare earth metal is recovered from the non-aqueous phase by stripping with an acidic stripping solution.

    4. The method of claim 3, wherein the acidic stripping solution comprises an aqueous hydrochloric acid or nitric acid solution.

    5. The method of claim 3 or claim 4, wherein the acidic stripping solution has a pH of 1 or lower.

    6. The method of any one of claims 3 to 5, wherein the acidic stripping solution has a pH of 0 or higher.

    7. The method of any of claims 1 to 6, wherein the method comprises extracting a rare earth metal from a mixture of two or more rare earth metals.

    8. The method of any of claims 1 to 7, wherein the acidic solution comprises a first and a second rare earth metal, and the method comprises: (a) preferentially partitioning the first rare earth metal into the non-aqueous phase.

    9. The method of claim 8, wherein the method further comprises, in step (a), 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 optionally recovering the second rare earth metal therefrom.

    10. The method of claim 9, wherein the first rare earth metal is recovered from the non-aqueous phase in step (a), and said non-aqueous phase is recycled and used as the composition in step (b).

    11. The method of any one of claims 8 to 10, wherein the first rare earth metal is dysprosium, and the second rare earth metal is neodymium.

    12. The method of any one of claims 8 to 10, wherein the first rare earth metal is lanthanum, and the second rare earth metal is europium.

    13. The method of any of claims 8 to 12, wherein 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).

    14. The method of any of claims 1 to 13, wherein the acidic solution from which the rare earth metal is extracted has a pH of from 2 to 4.

    15. The method of any of claims 1 to 14, wherein the composition is added to the acidic solution in a volume ratio of from 0.5:1 to 2:1, preferably 0.7:1 to 1.5:1, more preferably 0.8:1 to 1.2:1, for example 1:1.

    16. The method of any of claims 1 to 15, wherein 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.

    17. The method of any of claims 1 to 16, wherein the method comprises contacting the acidic solution of the rare earth metal and the composition for from 10 to 40 minutes, preferably from 15 to 30 minutes.

    18. The method of any of claims 1 to 17, wherein the method comprises contacting and physically mixing the acidic solution of the rare earth metal and the composition.

    19. The method of any of claims 1 to 18, 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, preferably a 5 membered ring.

    20. The method of any of claims 1 to 19, 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.

    21. The method of any of claims 1 to 19, wherein [Y.sup.+] represents 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.

    22. The method of claims 1 to 19, wherein [Y.sup.+] represents a saturated heterocyclic cation selected from cyclic ammonium, 1,4-diazabicyclo[2.2.2]octanium, morpholinium, cyclic phosphonium, piperazinium, piperidinium, quinuclidinium, and cyclic sulfonium.

    23. The method of claim 22, 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.sup.e and R.sup.f, are as defined in claim 21.

    24. The method of any of claims 20 to 23, wherein at least one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.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.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.

    25. The method of claim 24, wherein at least one of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.e and R.sup.f is 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.

    26. The method of claim 25, 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, preferably C.sub.4 alkyl, for example i-Bu.

    27. The method of claim 26, 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: ##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, preferably C.sub.4 alkyl, for example i-Bu.

    28. The method of any of claims 20 to 27, 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.

    29. The method of claim 28, wherein the remainder of R.sup.a, R.sup.b, R.sup.c, R.sup.d, R.sup.e and R.sup.f are H.

    30. The method of any of claims 21 or 24 to 29, wherein [Y.sup.+] represents a cyclic cation selected from: ##STR00018##

    31. The method of claim 30, wherein [Y.sup.+] represents the cyclic cation: ##STR00019##

    32. The method of claim 30 or claim 31, 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.

    33. The method of any of claims 1 to 32, wherein L.sub.1 represents a linking group selected from C.sub.1-10 alkanediyl and C.sub.1-10 alkenediyl groups.

    34. The method of claim 33, wherein L.sub.1 represents a linking group selected from C.sub.1-5 alkanediyl and C.sub.2-5 alkenediyl groups.

    35. The method of claim 34, wherein L.sub.1 represents a linking group selected from C.sub.1-5 alkanediyl groups.

    36. The method of claim 35, wherein L.sub.1 represents a linking group selected from CH.sub.2, C.sub.2H.sub.4 and C.sub.3H.sub.6.

    37. The method of any of claims 1 to 36, wherein each L.sub.2 represents a linking group independently selected from C.sub.1-2 alkanediyl and C.sub.2 alkenediyl groups.

    38. The method of claim 37, wherein each L.sub.2 represents a linking group independently selected from C.sub.1-2 alkanediyl groups.

    39. The method of claim 38, wherein each L.sub.2 represents a linking group independently selected from CH.sub.2 and C.sub.2H.sub.4.

    40. The method of any of claims 1 to 39, 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 or C.sub.1-6 alkyl.

    41. The method of claim 40, wherein each EDG represents an electron donating group independently selected from 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.

    42. The method of claim 41, wherein each -L.sub.2-EDG represents an electron donating group independently selected from: ##STR00020## 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, preferably C.sub.4 alkyl, for example i-Bu.

    43. The method of claim 42, wherein each -L.sub.2-EDG represents an electron donating group independently selected from: ##STR00021## wherein R.sup.y=R.sup.z, and wherein R.sup.y and R.sup.z are selected from C.sub.3-6 alkyl, preferably C.sub.4 alkyl, for example i-Bu.

    44. The method of any of claims 1 to 43, 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.

    45. The method of claim 44, 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.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 (e.g. [R.sup.2OCS.sub.2].sup., thiocarbamates (e.g. [R.sup.2.sub.2NCS.sub.2].sup.), thiocarboxylates (e.g. [R.sup.1CS.sub.2].sup.), thiophosphates (e.g. [(R.sup.2O).sub.2PS.sub.2].sup.), thiosulfonates (e.g. [RS(O).sub.2S].sup.), 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, SS(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)O R.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.

    46. The method of claim 45, wherein [X.sup.] represents 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; and preferably from bistriflimide and triflate anions.

    47. The method of any of claims 1 to 46, wherein [Cat.sup.+] represents one or more ionic species having the structure: ##STR00022## where: [Z+] 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.

    48. The method of any of claims 1 to 47, wherein the composition further comprises a lower viscosity ionic liquid.

    49. The method of claim 48, 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.

    50. The method of claim 49, wherein the cation of the lower viscosity ionic liquid is selected from phosphonium, imidazolium and ammonium groups.

    51. The method of claim 50, wherein the cation of the lower viscosity ionic liquid is selected from:
    [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.

    52. The method of claim 51, wherein the cation of the lower viscosity ionic liquid is [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, preferably C.sub.2-6 alkyl, and R.sup.6 is selected from C.sub.4-20 alkyl, preferably C.sub.8-14 alkyl.

    53. The method of claim 51, wherein the cation of the lower viscosity ionic liquid is [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, preferably C.sub.6-10 alkyl, and R.sup.6 is selected from C.sub.1-4 alkyl, preferably C.sub.1-2 alkyl.

    54. The method of claim 50, wherein the cation of the lower viscosity ionic liquid is selected from imidazolium cations substituted with one or more C.sub.1-20 alkyl, C.sub.3-8 cycloalkyl and C.sub.6-10 aryl groups, preferably substituted with two C.sub.1-10 alkyl groups.

    55. The method of any of claims 48 to 54, wherein the anion of the lower viscosity ionic liquid is as defined in any of claims 44 to 46.

    56. The method of any of claims 1 to 55, wherein the composition comprises less than 25% halide or pseudohalide anions as a proportion of the total anions.

    57. The method of any of claims 1 to 56, wherein the composition further comprises one or more organic solvents.

    58. The method of any of claims 48 to 57, wherein the ionic liquid is present in the composition in a concentration of at least 0.001 M, preferably from 0.005 M to 0.01 M, for example 0.0075 M.

    59. The method of any of claims 1 to 58, wherein the acidic solution is obtainable by leaching the rare earth metal from its source using an acid.

    60. The method of claim 59, wherein the source of the rare earth metal is a mineral or a waste material.

    61. An ionic liquid as defined in any of claims 1 and 19-47.

    62. A composition as defined in any of claims 1 and 19-58.

    63. The composition of claim 62, wherein the composition further comprises a rare earth metal.

    64. A method for preparing an ionic liquid as defined in claim 61, said method comprising reacting: ##STR00023## where: LG represents a leaving group.

    65. Use of an ionic liquid as defined in claim 61, or a composition as defined in claim 62 or claim 63, for extracting rare earth metals.

    66. The use of claim 65, wherein the ionic liquid or the composition is used to preferentially extract a first rare earth metal from a solution which comprises a first and a second rare earth metal.

    67. Use of a composition as defined in claim 63, for electrodeposition of a rare earth metal.

    68. Use of a composition as defined in claim 63, for precipitation of a rare earth metal.

    Description

    [0147] The present invention will now be illustrated by way of the following examples and with reference to the following figures in which:

    [0148] 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

    [0149] 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

    [0150] 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.

    Synthesis of an Imidazolium Ionic Liquid

    [0151] ##STR00012##

    [0152] 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.

    [0153] 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

    [0154] 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.

    [0155] 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).

    [0156] 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).

    [0157] 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

    [0158] 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.DyNd of 1085.

    [0159] 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

    [0160] 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.EuLa of 211.

    Separation of Tb and Ce

    [0161] 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.TbCe of 162.

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

    [0162] 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.

    [0163] 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.

    [0164] 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: NdDy 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.

    [0165] 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.

    [0166] 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.

    [0167] 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 , non-bonding) than in the case of Lu (2.6 , 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.