SOLVENT EXTRACTION OF TRIVALENT LANTHANIDES AND YTTRIUM FROM NITRATE MEDIA BY DIALKYL SULPHOXIDES

Abstract

The solvent extraction of the trivalent rare-earth metals (the lanthanides and yttrium, Ln3+) from nitrate media by xylene solutions of some dialkyl sulphoxides (R2SO, where R=alkyl) was studied. The salting-out effect of added metal nitrate (5.00 M) increases in the order: NH4NO3>NaNO3>LiNO3. Extraction is independent of pH in the range from 2 up to at least 4, but decreases gradually below pH 2 due to the competing extraction of nitric acid. Vapour-pressure osmometry data indicate that the dialkyl sulphoxides are not appreciably self-associated under the conditions studied (0.10 to 0.60M in xylene or toluene solutions at 30°C), which permits the use of organic-phase concentration terms (in place of activities) in the slope-analysis studies. The distribution ratios for the extraction of yttrium, gadolinium and lutetium show a third-order dependence on the concentration of the sulphoxide in the organic phase, with a slightly higher dependence (3.3) for lanthanum. In addition, the mole ratio of nitrate to metal in the loaded organic phase is close to 3, which suggests that the stoichiometry of the extracted complexes is Ln(NO3)3(R2SO)3. Increasing the steric bulk of the substituent alkyl groups (R) in the sulphoxides causes a marked decrease in metal extraction, and a reasonable correlation exists between the extraction of a given metal and the sum of the steric parameters of the alkyl groups in each of the sulphoxides used. In traversing the lanthanide series, the extraction increases in the interval La to Sm, and decreases thereafter to the end of the series. The behaviour of yttrium most closely resembles that of thulium or ytterbium.