THE SOLVENT EXTRACTION OF RARE-EARTH METALS BY CARBOXYLIC ACIDS

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Abstract
The solvent extraction of the trivalent lanthanides and yttrium from nitrate media by solutions of carboxylic acids in xylene has been studied. Commercially available carboxylic acids such as Versatic 10 and naphthenic acids were used, as well as model compounds of known structure, such as 2-ethylhexanoic and 3-cyclohexylpropanoic acids. In a few cases, extraction of the metals from sulphate and chloride solutions was also investigated. The dependence of the extraction properties of the carboxylic acids on the atomic number of the lanthanide shows a definite relationship to the steric bulk of the carboxylic acid molecule quantified by means of the steric parameter, Es′ of the substituent alkyl; group. For sterically hindered acids (− Es′ > 2) the pH0.5 of extraction decreases more or less continuously from lanthanum through to lutetium, and the behaviour of yttrium most closely resembles that of the middle lanthanides, such as gadolinium or terbium. For straight-chain and other non-hindered acids (−Es′ < 1), a minimum pH0.5 value is reached near the middle of the lanthanide series after which the pH0.5 values begin to increase again. The behaviour of yttrium with these acids most closely resembles that of the lighter lanthanides, such as cerium and praseodymium. The stoichiometrics of the extracted complexes for representative light (La), middle (Gd) and heavy (Lu) rare-earth metals were investigated by the slope-analysis technique for a sterically hindered acid (Versatic 10 acid; − Es′ = 3.83) and an acid with low steric hindrance (3-cyclohexylpropanoic acid; − Es′ = 0.28). It was found that monomeric complexes of the type LnA3(HA)3 were extracted by Versatic 10 acid, whereas with 3-cyclohexylpropanoic acid, dimeric complexes of stoichiometry (LaA3(HA)3)2, (GdA3(HA)3)2 and (LuA3(HA)2)2 were formed. The difference in the extraction behaviour across the lanthanide series for these two acids can be rationalized in terms of the structures of the extracted complexes.