Enhanced separation of rare earth metals

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+][X−]
• in which:
• [Cat+] represents a cationic species having the structure:
• where:
• [Y+] 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
• L1 represents a linking group selected from C1-10 alkanediyl, C2-10 alkenediyl, dialkanylether and C1-10 dialkanylketone groups;
• each L2 represents a linking group independently selected from C1-2 alkanediyl, C2 alkenediyl, C1-2 dialkanylether and C1-2 dialkanylketone groups;
• and
• [X−] represents a phosphinate anion.