Rare earth metal oxide process including extracting rare earth metal from acidic solution with an 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; recovering the rare earth metal from the non-aqueous phase; and processing the recovered rare earth metal into a rare earth metal oxide.

Claims

1. A method for preparing a rare earth metal oxide from a mixture of rare earth metals, said method comprising: contacting an acidic solution of the mixture of rare earth metals with a composition which comprises an ionic liquid to form an aqueous phase and a non-aqueous phase into which a rare earth metal has been selectively extracted; recovering the rare earth metal from the non-aqueous phase; and processing the recovered rare earth metal into a rare earth metal oxide, wherein the ionic liquid has the formula:
[Cat.sup.+][X.sup.−] in which: [Cat.sup.+] represents a cationic species having the structure: ##STR00037## 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; 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 rare earth metal is recovered from the non-aqueous phase by stripping with an acidic stripping solution to produce an acidic stripping solution comprising the recovered rare earth metal.

3. The method of claim 2, wherein processing the recovered rare earth metal into a rare earth metal oxide comprises: contacting the acidic stripping solution comprising the recovered rare earth metal with oxalic acid to give a rare earth metal oxalate; and converting, by calcination, the rare earth metal oxalate into the rare earth metal oxide.

4. The method of claim 3, wherein the rare earth metal oxalate is calcined at a temperature of at least 500° C.

5. The method of claim 1, wherein the method further comprises processing the rare earth metal oxide into a magnet.

6. 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, recovering the first rare earth metal from the non-aqueous phase, and processing the recovered first rare earth metal into a rare earth metal oxide.

7. The method of claim 6, 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.

8. The method of claim 6, wherein: the first rare earth metal is dysprosium, and the second rare earth metal is neodymium; or the first rare earth metal is europium, and the second rare earth metal is lanthanum.

9. The method of claim 1, wherein: the acidic solution from which the rare earth metal is extracted has a pH of from 2 to 4; the composition is added to the acidic solution in a volume ratio of from 0.5:1 to 2:1; 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; the acidic solution is contacted with the composition for from 1 to 40 minutes; and/or the method comprises contacting and physically mixing the acidic solution of the rare earth metal and the composition.

10. 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.

11. The method of claim 1, 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.+, where: 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; or a cyclic cation selected from: ##STR00038## where: 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; or a saturated heterocyclic cation having the formula: ##STR00039## where: 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.

12. The method of claim 11, wherein [Y.sup.+] represents a cyclic cation selected from: ##STR00040## 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.

13. The method of claim 1, wherein L.sub.1 represents: a linking group selected from C.sub.1-10 alkanediyl and C.sub.1-10 alkenediyl groups.

14. The method of claim 1, wherein each L.sub.2 represents: a linking group independently selected from C.sub.1-2 alkanediyl and C.sub.2 alkenediyl groups.

15. 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 or C.sub.1-6 alkyl.

16. The method of claim 1, wherein [Cat.sup.+] represents one or more ionic species having the structure: ##STR00041## 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.

17. 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, phosphinates, methides, borates, carboxylates, azolates, carbonates, carbamates, thiophosphates, thiocarboxylates, thiocarbamates, thiocarbonates, xanthates, thiosulfonates, thiosulfates, nitrate, nitrite, tetrafluoroborate, hexafluorophosphate, perchlorate, halometallates, amino acids, borates and polyfluoroalkoxyaluminates.

18. The method of claim 1, wherein the composition further comprises a lower viscosity ionic liquid and/or one or more organic solvents; and/or the anion of the ionic liquid is present in the composition in a concentration of at least 0.001 M.

19. The method of claim 1, wherein the method comprises preparing the acidic solution of rare earth metal by leaching the rare earth metal from its source with an acid.

20. The method of claim 19, wherein the method further comprising preparing the acidic solution of rare earth metal by a process which comprises: dissolving the rare earth metal source in a first mineral acid; adding a base to precipitate the rare earth metal as a salt; converting the salt into an oxide; converting the oxide into a halide salt; and dissolving the halide salt in a second mineral acid to form the acidic solution comprising rare earth metal.

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.

(4) FIG. 3 is a graph showing extraction of a selection of rare earth metals using [MAIL.sup.+][R.sub.2P(O)O.sup.−].

(5) FIG. 4 is a graph showing extraction of a selection of rare earth metals using [MAIL-6C.sup.+][NTf.sub.2.sup.−].

(6) FIG. 5 is a graph showing extraction of a selection of rare earth metals using [MAIL-Ph.sup.+][NTf.sub.2.sup.−].

EXAMPLES

Example 1: Synthesis of Ionic Liquid

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

(8) 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° 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

(9) [MAIL.sup.+][NTf.sub.2.sup.−]:

(10) ##STR00012##

(11) 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.

(12) 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.−]). [MAIL-6C.sup.+][NTf.sub.2.sup.−]:

(13) ##STR00013##

(14) ##STR00014##

(15) A mixture of potassium pthalimide (10.0 g, 54.0 mmol) and 1,6-dibromobutane (9.91 mL, 64.8 mmol) in dry DMF (100 mL) was stirred at room temperature for 12 days. The mixture was concentrated and extracted with chloroform (3×30 mL) and washed with deionised water (3×80 mL) and brine (100 mL). The organic layer was dried over magnesium sulfate and concentrated to give a white syrup. The syrup was triturated with hexanes, filtered and dried to give a white solid product (3) (14.3 g, 85%).

(16) ##STR00015##

(17) To NaH (0.645 g, 26.9 mmol) in THF was added at 0° C. under N.sub.2, imidazole (1.21 g, 17.7 mmol) in THF was added over 30 mins, and stirred for a further 30 mins at 0° C. 3 (5.00 g. 16.1 mmol) in THF was added at 0° C. and the mixture stirred for 1 hour at room temperature, then refluxed at 70° C. overnight. The mixture was filtered and the residual NaBr was washed with THF. The filtrate was concentrated to give a syrup which was dissolved in DCM to give a yellow solution which was then washed with water and dried over sodium sulfate and triturated with hexanes to precipitate a white solid which was filtered and washed with hexanes (4) (1.52 g, 32%).

(18) ##STR00016##

(19) 4 (0.750 g, 2.54 mmol) was dissolved in a EtOH:H.sub.2O mixture (160 mL, 3:1) and hydrazine hydrate (50-60%, 0.174 mL, 5.55 mmol) was added at room temperature and the mixture refluxed overnight. The solution was cooled to room temperature and concentrated HCl (2 mL) was added, the reaction mixture changed from colourless to yellow to red to light yellow during the addition. The mixture was stirred at reflux for 6 hours and filtered. The solution was concentrated and dissolved in distilled water to give a yellow solution. Sodium hydroxide was added until the mixture reached pH 11, it was then extracted with chloroform (4×40 mL), dried over magnesium sulfate and concentrated to give an orange oil (5) (0.329 g, 78%).

(20) ##STR00017##

(21) To a high pressure vessel was added 5 (0.257 g, 1.54 mmol), triethylamine (0.623 g, 6.16 mmol), N,N-diisobutyl-2-chloroacetamide (0.950 g, 4.62 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 140° C. on an oil bath for 16 hours. The reaction mixture was washed with pH 1 HCl (40 mL), Na.sub.2CO.sub.3 (2×40 mL) then water (4×40 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give a viscous dark brown liquid (6) (0.648 g, 59%).

(22) ##STR00018##

(23) To a round bottom flask was added 6 (0.6255 g, 0.88 mmol) followed by DCM (50 mL). LiNTf.sub.2 (0.7572 g, 2.64 mmol) was added followed by water (50 mL). The reaction mixture was stirred at room temperature for 24 hours. The aqueous layer was removed and the organic layer washed with deionised water (4×40 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a black viscous liquid, [MAIL-6C.sup.+][NTf.sub.2.sup.−], (0.7467 g, 89%).

(24) [MAIL-Ph.sup.+][NTf.sub.2.sup.−]:

(25) ##STR00019##

(26) ##STR00020##

(27) To a high pressure vessel was added 1-(3-aminopropyl)imidazole (0.200 g, 1.60 mmol), triethylamine (0.647 g, 6.39 mmol), 2-chloro-N,N-diphenylacetamide (1.18 g, 4.49 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 145° C. on an oil bath for 16 hours. The reaction mixture was washed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give an orange/brown solid (7) (0.883 g, 70%).

(28) ##STR00021##

(29) To a 50 mL round-bottom flask was added 7 (0.444 g, 0.560 mmol) followed by DCM (20 mL). LiNTf.sub.2 (0.484 g, 1.69 mmol) was added followed by deionised water (20 mL). The reaction mixture was stirred at room temperature for 24 hours. The aqueous layer was removed and the organic layer washed with deionised water (5×15 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a viscous brown liquid, [MAIL-Ph.sup.+][NTf.sub.2.sup.−], (0.351 g, 65%).

(30) The phosphinate ionic liquid [MAIL.sup.+][R.sub.2P(O)O.sup.−] (R=2,4,4-trimethylpentyl) was also synthesised by ion exchange.

(31) Synthesis of Phosphonium Ionic Liquids

(32) [MAIL-PPh.sub.3.sup.+][NTf.sub.2.sup.−]:

(33) ##STR00022##

(34) ##STR00023##

(35) To a 50 mL round-bottom flask equipped with a magnetic stir bar triphenylphosphine (0.836 g, 3.19 mmol), 3-bromopropylamine hydrobromide (1.00 g, 4.57 mmol), and acetonitrile (25 mL) were added. The suspension was then heated and stirred at reflux for 16 hours. The reaction was cooled to room temperature, and the solvent was removed under reduced pressure, and the resulting white solid was then dried in vacuo, and used in subsequent steps without further purification (1.01 g, 79%).

(36) ##STR00024##

(37) To a 50 mL round-bottom flask was added (3-Aminopropyl)(triphenyl)phosphonium bromide (1.01 g, 0.252 mmol) followed by DCM (20 mL). LiNTf.sub.2 (2.17 g, 7.55 mmol) was added followed by deionised water 20 mL). The reaction mixture was stirred at room temperature for 24 hours. The aqueous layer was removed and the organic layer washed with deionised water (5×15 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a white solid (1.26 g, 84%).

(38) ##STR00025##

(39) To a high pressure vessel was added (3-Aminopropyl)(triphenyl)phosphonium bistriflimide (0.200 g, 0.333 mmol), triethylamine (0.135 g, 1.33 mmol), N,N-diisobutyl-2-chloroacetamide (0.137 g, 0.666 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 145° C. on an oil bath for 48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give a viscous dark brown liquid, [MAIL-PPh.sub.3.sup.+][NTf.sub.2.sup.−], (0.282 g, 90%).

(40) [MAIL-PPh.sub.3.sup.+][R.sub.2P(O)O.sup.−]:

(41) ##STR00026##

(42) ##STR00027##

(43) To a high pressure vessel was added (3-Aminopropyl)(triphenyl)phosphonium bromide (1.01 g, 2.53 mmol), triethylamine (1.03 g, 10.1 mmol), N,N-diisobutyl-2-chloroacetamide (1.04 g, 5.07 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 145° C. on an oil bath for 48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give a viscous dark brown liquid (0.981 g, 56%).

(44) ##STR00028##

(45) To a 50 mL round-bottom flask was added the phosphonium diamide (0.898 g, 1.29 mmol) followed by DCM (20 mL). R.sub.2P(O)OH (R=2,4,4-trimethylpentyl) (0.356 g, 1.29 mmol) was added followed by a KOH solution (40%, 20 mL). The reaction mixture was stirred at 50° C. for 16 hours. The aqueous layer was removed and the organic layer washed with deionised water (5×15 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a white solid, [MAIL-PPh.sub.3.sup.+][R.sub.2P(O)O.sup.−], (0.943 g, 77%).

(46) [MAIL-P.sub.444.sup.+][NTf.sub.2.sup.−]:

(47) ##STR00029##

(48) ##STR00030##

(49) To a 50 mL round-bottom flask equipped with a magnetic stir bar tributylphosphine (0.823 g, 4.07 mmol), 3-bromopropylamine hydrobromide (0.890 g, 4.07 mmol), and acetonitrile (25 mL) were added. The suspension was then heated and stirred at reflux for 48 hours. The reaction was cooled to room temperature, and the solvent was removed under reduced pressure, and the resulting oil was then dried in vacuo, and used in subsequent steps without further purification (1.24 g, 89%).

(50) ##STR00031##

(51) To a 50 mL round-bottom flask was added (3-Aminopropyl)(tributyl)phosphonium bromide (0.559 g, 1.64 mmol) followed by DCM (20 mL). LiNTf.sub.2 (1.41 g, 4.93 mmol) was added followed by deionised water (20 mL). The reaction mixture was stirred at room temperature for 24 hours. The aqueous layer was removed and the organic layer washed with deionised water (5×15 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a colourless oil (0.304 g, 34%).

(52) ##STR00032##

(53) To a high pressure vessel was added (3-Aminopropyl)(tributyl)phosphonium bistriflimide (0.200 g, 0.370 mmol), triethylamine (0.150 g, 1.48 mmol), N,N-diisobutyl-2-chloroacetamide (0.152 g, 0.740 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 145° C. on an oil bath for 48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give a viscous dark brown liquid, [MAIL-P.sub.444.sup.+][NTf.sub.2.sup.−], (0.250 g, 77%).

(54) [MAIL-P.sub.888.sup.+][NTf.sub.2.sup.−]:

(55) ##STR00033##

(56) ##STR00034##

(57) To a 50 mL round-bottom flask equipped with a magnetic stir bar trioctylphosphine (0.872 g, 2.35 mmol), 3-bromopropylamine hydrobromide (0.500 g, 2.28 mmol), and acetonitrile (25 mL) were added. The suspension was then heated and stirred at reflux for 48 hours. The reaction was cooled to room temperature, and the solvent was removed under reduced pressure, and the resulting oil was then dried in vacuo, and used in subsequent steps without further purification (0.889 g, 85%).

(58) ##STR00035##

(59) To a 50 mL round-bottom flask was added (3-Aminopropyl)(trioctyl)phosphonium bromide (0.564 g, 1.11 mmol) followed by DCM (20 mL). LiNTf.sub.2 (0.954 g, 3.32 mmol) was added followed by deionised water (20 mL). The reaction mixture was stirred at room temperature for 24 hours. The aqueous layer was removed and the organic layer washed with deionised water (5×15 mL). The organic layer was dried over magnesium sulfate and concentrated. The product was dried overnight to give a colourless oil (0.542 g, 69%).

(60) ##STR00036##

(61) To a high pressure vessel was added (3-Aminopropyl)(trioctyl)phosphonium bistriflimide (0.200 g, 0.282 mmol), triethylamine (0.114 g, 1.13 mmol), N,N-diisobutyl-2-chloroacetamide (0.116 g, 0.564 mmol) and chloroform (5 mL). The vessel was stoppered and stirred at 145° C. on an oil bath for 48 hours. The reaction mixture was washed with pH 1 HCl (15 mL), then water (4×150 mL). The organic layer was dried over magnesium sulfate and concentrated in vacuo to give a viscous dark brown liquid, [MAIL-P.sub.888.sup.+][NTf.sub.2.sup.−]0.313 g, 99%).

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

(62) General Procedure for Extraction of Rare Earth Metals

(63) 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.

(64) 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).

(65) 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).

(66) 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.

(67) The separation of rare earth metals was also performed by the above method using 0.0075 M of the ionic liquids [MAIL.sup.+][R.sub.2P(O)O.sup.−], [MAIL-6C.sup.+][NTf.sub.2.sup.−] and [MAIL-Ph.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−]. These ionic liquids were also found to differentially extract rare earth metals at pH 1 to pH4 as shown in FIGS. 3, 4 and 5.

(68) Recycling of Ionic Liquid

(69) Dy was extracted from an aqueous solution of Dy (180 ppm) at pH4 using 0.025 M [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] (>95% extracted) and the ionic liquid stripped at pH 1 using HCl (1:1 ionic liquid to stripping solution ratio) in 4 contacts. The ionic liquid was washed with deionised water to raise the pH to 7, and was used in further extractions. The amount of Dy extracted dropped by around 20% compared to the first extraction, but remained at a constant level over four subsequent extractions.

(70) Separation of Dy and Nd

(71) 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.+][INTf.sub.2.sup.−] in [P.sub.666(14).sup.+][INTf.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.

(72) 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).

(73) The above separation was repeated using 0.0075M of an ionic liquid in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] at pH2. The extraction was performed using [MAIL.sup.+][NTf.sub.2.sup.−], [MAIL.sup.+][R.sub.2P(O)O.sup.−], [MAIL-6C.sup.+][NTf.sub.2.sup.−], [MAIL-P.sub.444.sup.+][NTf.sub.2.sup.−], [MAIL-P.sub.888.sup.+][NTf.sub.2.sup.−], [MAIL-PPh.sub.3.sup.+][NTf.sub.2.sup.−] and [MAIL-PPh.sub.3.sup.+][R.sub.2P(O)O.sup.−] and the results are shown in Table 2. As can be seen, ionic liquids described herein can be used to completely selectively extract Dy from Nd. Completely selective extraction of Dy from Nd using [MAIL.sup.+][NTf.sub.2.sup.−], [MAIL.sup.+][R.sub.2P(O)O.sup.−] and [MAIL-6C.sup.+][NTf.sub.2.sup.−] was also observed at pH 1.8, with extraction of more than 50% Dy.

(74) TABLE-US-00002 TABLE 2 Ionic liquid Dy % Extraction Nd % Extraction [MAIL.sup.+][NTf.sub.2.sup.−] 82 0 [MAIL.sup.+][R.sub.2P(O)O.sup.−] 86.5 0 [MAIL-6C.sup.+][NTf.sub.2.sup.−] 83 0 [MAIL-P.sub.444.sup.+][NTf.sub.2.sup.−] 89 0 [MAIL-P.sub.888.sup.+][NTf.sub.2.sup.−] 87 0 [MAIL-PPh.sub.3.sup.+][NTf.sub.2.sup.−] 90 0.6 [MAIL-PPh.sub.3.sup.+][R.sub.2P(O)O.sup.−] 90 0

(75) Separation of Eu and La

(76) 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.

(77) Separation of Tb and Ce

(78) 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.−.]

(79) Dy(III) (80 ppm) was stripped from an organic phase at pH 0.25 comprising [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] (0.0075 M) in 3 successive contacts. The organic phase was contacted with an equal volume of an aqueous HCl solution (0.55 M) and was equilibrated for 15-30 minutes on a wrist action shaker. 67 ppm of Dy(III) was stripped in the first contact, 10 ppm was stripped in the second contact, and 2 ppm was stripped in the third contact.

(80) 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.

(81) 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.

(82) 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.

(83) 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.

(84) 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.

Example 4: Preparation of Rare Earth Metal Oxides

(85) An aqueous HCl stripping solution composition comprising Dy is contacted with oxalic acid at room temperature and pressure. The mixture is stirred. Dysprosium oxalate (Dy.sub.2(C.sub.2O.sub.4).sub.2) precipitates from the solution. The precipitate is separated from the acidic stripping solution by filtration, and is washed with water. The dysprosium oxalate precipitate is calcined at 1000° C. to give dysprosium oxide (Dy.sub.2O.sub.3).

Example 5: Extraction of Rare Earth Metals from a Magnet Sample

(86) A magnet sample containing rare earth metals was obtained in powdered form and was converted to the chloride form as follows. The magnet feed was dissolved in 2 M H.sub.2SO.sub.4. The undissolved impurities were removed by filtration. The pH was raised to 1.5 using ammonium hydroxide at 60° C. At 60° C. the rare-earth sulphates crash out of solution leaving the iron sulphate impurity in solution. The separated rare-earth sulphate was converted to the oxalate (by contacting with oxalic acid to and washing the rare-earth oxalate with water) and calcined at 900° C. to form the rare-earth oxide. The rare-earth oxide is converted into the rare-earth chloride by leaching into a HCl solution and recrystallised.

(87) A feed solution of 0.2 g rare-earth chloride salt in 50 mL pH 2 solution (HCl) was prepared. The feed solution had an initial concentration of 20.93 ppm Dy and 1573.81 ppm Nd.

(88) Separate extractions were carried out as described in Example 2, using 0.0075 M [MAIL.sup.+][NTf.sub.2.sup.−] or [MAIL.sup.+][R.sub.2P(O)O.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] at pH 2. The ionic liquids were both found to extract more than 90% of the Dy in the solution after 4 contacts, whilst extracting less than 5% Nd.