A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

SOLVENT EXTRACTIONSome text books still imply that the lanthanides and yttrium are all produced by ion exchangetechnology. Not so. Only a few heavy lanthanides are purified commercially on a small scale by ionexchange. For large-scale production the chosen path is solvent extraction, usually in nitratesolutions[1]. Furthermore, once the starting precursor has been prepared, this separationtechnology is independent of starting mineral and different feedstocks can ultimately be processedby the same separation routines and equipment.The liquid-liquid counter-current two-phase extraction procedure, commonly referred to assolvent extraction (SX), rely on the differential partitioning of metal soluble complexes betweenimmiscible aqueous and organic phases. The organic phase acts as a diluent for the extractant thatmay be selective towards cationic, neutral or anionic species. The reagents used are :• as extractants, or complexing agent, organic phosphates, carboxylic compounds, etc.,• as the aqueous phase, acidic solutions, e.g. nitric,• as the organic phase, commercial aliphatic or aromatic solvents.A component, say Ln A is distributed between the two phases and will have a distributioncoefficient, measured at equilibrium :D A = (Concentration in organic phase) / (Concentration in aqueous phase)For two components, Ln A and Ln B , both distributed between the organic and aqueous phases, aseparation factor can be defined:β A B = D A /D BThe closer this factor approaches '1' the more difficult it will be to separate those twoelements. The degree of separation is maximized by optimization of operating conditions. Toachieve any relatively complete separation, i.e for two lanthanides adjacent in the Periodic Table inorder to produce material of 99.99% or higher purity for the one element without contamination bythe other, many SX cells must be linked[1] Solvent Extraction Used in Industrial Separation of Rare Earths, C.G.Brown and L.G.Sherrington,J.Chem.Tech.Biotechnol., 1979, 29, 193 : Discovery and Commercial Separations, J.Kaczmarek, in "IndustrialApplications of Rare Earth Elements", ed. K.A.Gschneidner, ACS Symp. 164, publ. Am.Chem.Soc., 1981,p.13541