COUNTERCURRENT RARE EARTH SEPARATION PROCESS

Abstract

A method for extracting a rare earth metal from a mixture of one or more rare earth metals, said method comprising countercurrently 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 countercurrently 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: ##STR00032## 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; 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; and 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 countercurrently contacting the acidic solution and composition in a series of extraction stages, e.g. comprising from 2 to 10, preferably from 3 to 8, and more preferably from 4 to 6 extraction stages.

3. The method of claim 2, wherein the acidic solution and composition are contacted and separated into an aqueous phase and a non-aqueous phase in each extraction stage, e.g. using a centrifugal extractor or a combined mixer-settler.

4. The method of any of claims 1 to 3, wherein the composition is contacted with 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.

5. The method of any of claims 1 to 4, wherein the flow rate of the acidic solution is from 100 to 10000 L/hr, preferably from 200 to 7500 L/hr, and more preferably from 250 to 5000 L/hr.

6. The method of any of claims 1 to 5, wherein the acidic solution is contacted with the composition at a temperature of from 35 to 100° C., preferably from 40 to 70° C., and more preferably from 50 to 60° C.

7. The method of any of claims 1 to 6, wherein the acidic solution from which the rare earth metal is extracted has a pH of from 2 to 4.

8. The method of any of claims 1 to 7, 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.

9. The method of any of claims 1 to 8, wherein the method comprises contacting the acidic solution of the rare earth metal and the composition for from 1 to 40 minutes, preferably from 5 to 30 minutes.

10. The method of any of claims 1 to 9, wherein the method comprises recovering the rare earth metal from the non-aqueous phase, for instance by stripping with an acidic stripping solution, e.g. an aqueous hydrochloric acid or nitric acid solution, the acidic stripping solution preferably having a pH of 1 or lower and preferably a pH of 0 or higher.

11. The method of claim 10, wherein the ionic liquid is countercurrently contacted with the acidic stripping solution, for instance in a series of stripping stages.

12. The method of claim 10 or claim 11, wherein the method further comprises recycling the stripped non-aqueous phase for use as the composition comprising ionic liquid in the countercurrent extraction.

13. The method of any of claims 1 to 12, 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.

14. The method of claim 13, 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; and preferably 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); and/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).

15. The method of claim 13 or claim 14, 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.

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

17. The method of any of claims 1 to 16, 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.1-10 aryl groups; or a cyclic cation selected from: ##STR00033## 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: ##STR00034## 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.

18. The method of claim 17, wherein [Y.sup.+] represents a cyclic cation selected from: ##STR00035## and preferably represents the cyclic cation: ##STR00036## wherein preferably 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.

19. The method of any of claims 1 to 18, wherein L.sub.1 represents: a linking group selected from C.sub.1-10 alkanediyl and C.sub.1-10 alkenediyl groups; preferably a linking group selected from C.sub.1-5 alkanediyl and C.sub.2-5 alkenediyl groups; more preferably a linking group selected from C.sub.1-5 alkanediyl groups; and still more preferably a linking group selected from —CH.sub.2—, —C.sub.2H.sub.4— and —C.sub.3H.sub.6.sup.−.

20. The method of any of claims 1 to 19, wherein each L.sub.2 represents: a linking group independently selected from C.sub.1-2 alkanediyl and C.sub.2 alkenediyl groups; preferably a linking group independently selected from C.sub.1-2 alkanediyl groups; and more preferably a linking group independently selected from —CH.sub.2— and —C.sub.2H.sub.4.sup.−.

21. The method of any of claims 1 to 20, 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; and preferably 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.

22. The method of claim 21, wherein each -L.sub.2-EDG represents an electron donating group independently selected from: ##STR00037## and preferably from: ##STR00038## 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.

23. The method of any of claims 1 to 22, wherein [X.sup.−] represents one or more anionic species selected from: hydroxides, halides, perhalides, pseudohalides, 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 and perchlorate, halometallates, amino acids, borates, polyfluoroalkoxyaluminates; preferably selected from: bistriflimide, triflate, bis(alkyl)phosphinates such as bis(2,4,4-trimethylpentyl)phosphinate, tosylate, perchlorate, [Al(OC(CF.sub.3).sub.3).sub.4], tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, tetrakis(pentafluorophenyl)-borate, tetrafluoroborate, hexfluoroantimonate and hexafluorophosphate anions; and more preferably selected from: bistriflimide, triflate and bis(2,4,4-trimethylpentyl)phosphinate anions.

24. The method of any of claims 1 to 23, wherein [Cat.sup.+] represents one or more ionic species having the structure: ##STR00039## where: [Z.sup.+] represents a group selected from ammonium, benzimidazolium, benzofuranium, benzothiophenium, benzotriazolium, borolium, cinnolinium, diazabicyclodecenium, diazabicyclononenium, 1,4-diazabicyclo[2.2.2]octanium, diazabicyclo-undecenium, dithiazolium, furanium, guanidinium, imidazolium, indazolium, indolinium, indolium, morpholinium, oxaborolium, oxaphospholium, oxazinium, oxazolium, iso-oxazolium, oxothiazolium, phospholium, phosphonium, phthalazinium, piperazinium, piperidinium, pyranium, pyrazinium, pyrazolium, pyridazinium, pyridinium, pyrimidinium, pyrrolidinium, pyrrolium, quinazolinium, quinolinium, iso-quinolinium, quinoxalinium, quinuclidinium, selenazolium, sulfonium, tetrazolium, thiadiazolium, iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, thiophenium, thiuronium, triazinium, triazolium, iso-triazolium and uronium groups.

25. The method of any of claims 1 to 24, wherein the composition further comprises a lower viscosity ionic liquid and/or one or more organic solvents.

Description

[0169] The present invention will now be illustrated by way of the following examples and with reference to the following figures in which:

[0170] 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.

[0171] 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.

[0172] FIG. 3 is a diagram showing an apparatus (10) suitable for carrying out an extraction process of the present invention. An acidic solution (12) comprising a first and second rare earth metal is passed to a series of extraction stages (E) where it is countercurrently contacted with a composition (14) which comprises an ionic liquid. The first metal is selectively extracted from the acidic solution (12) into the non-aqueous phase. The non-aqueous phase is then countercurrently contacted with an acidic solution in a series of back-extraction (B) phases, in which second rare earth metal that has undesirably been partitioned into the non-aqueous phase may be partitioned back into an aqueous phase. The second metal may be recovered (16) from the aqueous phase. The composition comprising the first metal is passed to a series of stripping stages (S) where it is countercurrently contacted with an acidic stripping solution (18). The first metal is extracted into the acidic stripping solution, from where it may be recovered (20).

[0173] FIG. 4 is a diagram showing another apparatus (10) suitable for carrying out an extraction method of the present invention. In the apparatus shown in FIG. 4, countercurrent extraction is carried out in a series of 6 extraction stages (E1 to E6) and countercurrent stripping is carried out in a series of 4 stripping stages (S1 to S4). In each stage, the aqueous and non-aqueous phases are mixed and separated, with the aqueous phase passed to the next stage (e.g. from E2 to E3) and the non-aqueous phase passed in the opposite direction to the previous stage (e.g. from E2 to E1). Thus, the streams are countercurrently contacted and the rare earth metal concentration gradient is maintained between the aqueous and non-aqueous phase across the series of extraction/stripping stages. The stripped non-aqueous phase is recycled and used as the composition (14) comprising the ionic liquid in the countercurrent extraction. During recycling, the pH of the non-aqueous phase is equilibrated with an acidic solution (22) having the same pH as the acidic solution (12) of the rare earth metal in a pH adjustment stage (A1), with the spent aqueous effluent (24) withdrawn from the adjustment stage (A1) after equilibration.

[0174] FIG. 5 is a diagram showing an extraction train having three countercurrent extraction stages (E1-E3) and two back-extraction stages (B1-B2). The apparatus in each extraction and back extraction stage comprises a mixing chamber (28) and a separating chamber (30). A composition (14) comprising an ionic liquid is introduced into the extraction train in the third countercurrent extraction stage (E3). An acidic back-extraction solution (26) is introduced into the extraction train in the second back-extraction stage (B2). An acidic solution (12) comprising first and second rare earth metals is introduced into the first extraction stage (E1), where it is mixed with the non-aqueous ionic liquid phase in a mixing chamber (28). The aqueous and non-aqueous phases are then separated in a separating chamber (30). The non-aqueous ionic liquid phase is passed to the first back-extraction stage (B1)—where any second rare earth metal that has been taken-up by the ionic liquid may be back-extracted into the acidic back-extraction solution—while the aqueous phase is passed to the second countercurrent extraction stage (E2). The ionic liquid composition (14′) that leaves the extraction train may be processed further, e.g. it may be stripped of first rare earth metal using an acidic stripping solution. The acidic solution (26′) depleted of first rare earth metal may also be processed further, e.g. in a second rare earth metal recovery stage.

[0175] FIG. 6 is a graph showing extraction of a selection of rare earth metals using [MAIL.sup.+][R.sub.2P(O)O.sup.−].

[0176] FIG. 7 is a graph showing extraction of a selection of rare earth metals using [MAIL-6C.sup.+][NTf.sub.2.sup.−].

[0177] FIG. 8 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

[0178] General Procedure for the Synthesis of an Ionic Liquid According to Embodiments of the Invention

[0179] 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.

[0180] Synthesis of an Imidazolium Ionic Liquid

##STR00013##

[0181] 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.

[0182] 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.−]).

##STR00014##

[0183] A mixture of potassium pthalimide (10.0 g, 54.0 mmol) and 1,6-dibromobutane (9.97 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%).

##STR00015##

[0184] 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%).

##STR00016##

[0185] 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%).

##STR00017##

[0186] To a high pressure vessel was added b 02/g, 1.4 mmol) triethylamine (63 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%).

##STR00018##

[0187] 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%).

##STR00019##

[0188] 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%).

##STR00020##

[0189] 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%).

[0190] The phosphinate ionic liquid [MAIL.sup.+][R.sub.2P(O)O.sup.−] (R=2,4,4-trimethylpentyl) was also synthesised by ion exchange.

[0191] Synthesis of Phosphonium Ionic Liquids

##STR00021##

[0192] 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%).

##STR00022##

[0193] 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%).

##STR00023##

[0194] 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%).

##STR00024##

[0195] 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%).

##STR00025##

[0196] 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%).

##STR00026##

[0197] 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%/L)

##STR00027##

[0198] 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%).

##STR00028##

[0199] 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%).

##STR00029##

[0200] 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%).

##STR00030##

[0201] 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%).

##STR00031##

[0202] 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.−.]

[0203] General Procedure for Extraction of Rare Earth Metals

[0204] 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.

[0205] 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).

[0206] 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).

[0207] 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.

[0208] 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. 6, 7 and 8.

[0209] Recycling of Ionic Liquid

[0210] 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.

[0211] Separation of Dy and Nd

[0212] 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.

[0213] 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).

[0214] 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.

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

[0215] Separation of Eu and La

[0216] 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.

[0217] Separation of Tb and Ce

[0218] 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][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.˜]

[0219] 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.

[0220] In a further experiment, Dy(III) (160 ppm) was stripped from an organic phase at pH 1 comprising [MAIL.sup.+][NTf.sub.2.sup.−] in [P.sub.666(14).sup.+][NTf.sub.2.sup.−] (0.0225 M) in 5 successive contacts. The organic phase was contacted with an equal volume of an aqueous HCl solution (0.1 M) and was equilibrated for 15-30 minutes on a wrist action shaker. 98 ppm of Dy(III) was stripped in the first contact, 38 ppm was stripped in the second contact, 16 ppm was stripped in the third contact, and 8 ppm was stripped in the fourth contact.

[0221] 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.

[0222] 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.

[0223] 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.

[0224] 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.

[0225] 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: Comparison of Extraction Stage Equipment

[0226] Extraction of rare earth metal from an acidic solution was carried out using a Rousselet Robatel™ mixer-settler, a CINC™ V02 centrifugal separator and a Rousselet Robatel™ centrifugal separator.

[0227] Each of the devices gave good levels of rare earth metal extraction. However, the centrifugal devices were preferred, since they gave better phase separation with no more than 10% contamination of the aqueous phase with the non-aqueous phase. The CINC™ V02 centrifugal separator gave particularly good results, with less than 1% contamination of the aqueous phase.

Example 5: Continuous Laboratory-Scale Countercurrent Extraction

[0228] 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) 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.−]) using a series of three CINC™ V02 centrifugal separators. The ionic liquid extract and aqueous solution were used in a phase ratio of 1:1, with each phase flowing through the equipment at a rate of 5 L/hr. The experiment was repeated at three different pH levels. The results are shown in the following table:

TABLE-US-00003 % Extraction pH Dy Nd 2 25-35 0.04 3 55-65 8.00 4 65-75 8.00

[0229] It can be seen that high levels of selectivity are achieved by countercurrently contacting the acidic solution and ionic liquid composition in a series of extraction stages.

Example 6: Simulation of Larger-Scale Countercurrent Extraction

[0230] A simulation of a Dy-Nd countercurrent extraction process was carried out. The simulation showed that over 99.99% purity can be obtained in both the Dy and Nd product streams under the following conditions:

TABLE-US-00004 Volumetric flow rate pH Extraction and back-extraction stages Acidic solution comprising Dy and Nd 1.74 m.sup.3/h 2.5 Acidic back-extraction solution 1.74 m.sup.3/h 2.5 Composition comprising ionic liquid 1.74 m.sup.3/h HCl free Stripping stages Acidic stripping solution 1.74 m.sup.3/h 1
and with the following number of stages:

TABLE-US-00005 Extraction Back-extraction Stripping Number of stages 6 4 4

Example 7: Extraction of Rare Earth Metals from a Magnet Sample

[0231] 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.

[0232] 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.

[0233] 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.