Solvent Extraction Equilibria For Rare Earth Nitrate Tributyl Phosphate Systems PDF Download

In this paper the application of different classes of solvent-extraction reagents to the recovery and separation of the rare-earth metals is reviewed, and some aspects of the chemistry of these systems are discussed. Solvating extractants such as tri-n-butyl phosphate (TBP) are useful both in the recovery of mixed rare-earth products from nitrate liquors (complexes of the type Ln(NO3)3(TBP)3 being formed in the organic phase) and in the highly selective separation of cerium(IV) from the trivalent lanthanides (when complexes of stoichiometry Ce(NO3)4(TBP)2 or H2Ce(NO3)6(TBP)2 are extracted). Cation-exchange extractants, especially organophosphorus acids such as D2EHPA, are invaluable in the preliminary division of the rare earths into light, middle and heavy fractions, as well as for the subsequent separation of individual metals within these fractions. In some cases, as for the lanthanum-cerium pair, improved separations can be obtained with extractants of the phosphonic and phosphinic acid types. The characteristics of the extraction of the trivalent lanthanides and yttrium by different carboxylic acids can be related to the steric parameter of the substituent alkyl group. For sterically hindered acids such as Versatic 10, complexes of stoichiometry LnA3(HA)3 are extracted, and the pH(0,5) values decrease steadily from lanthanum to lutetium. For non-hindered acids such as 3-cyclohexylpropanoic acid, dimeric complexes such as (LnA3(HA)3)2 are formed, and the pH(0,5) values decrease to a minimum near the middle of the lanthanide series and thereafter increase again. Anion-exchange extractants, particularly quaternary ammonium salts, offer distinct advantages over other reagent types in certain instances, such as the separation of yttrium from the heavy rare-earth fraction in thiocyanate media, and the separation of neodymium from praseodymium in nitrate media. In these systems, complexes of the types (R4N)LnX4 and (R4N)2LnX5 are extracted, where X represents the thiocyanate or nitrate anion. The order of extraction of the individual rare earths is reversed in the nitrate system (La > Ce... > Lu) compared with the thiocyanate system (La