A process is provided for separating, by solvent extraction, metal compounds contained in aqueous solutions. The aqueous solution is contacted with an organic phase containing at least one substituted hydroquinoline. The process of the invention is particularly applicable to the separation of at least one element selected from among the group consisting of the rare earths and yttrium from an aqueous solution containing it.
Yttrium is selectively separated from other rare earth values by contacting a mixed aqueous solution thereof that contains nitrate ions with a water-immiscible organic phase which includes a selective extractant for such other rare earth values, whereby such other rare earth values are selectively liquid/liquid extracted into this organic phase, the selective extractant comprising: (a) an anionic compound having the general formula (I): ##STR1## in which M is nitrogen, phosphorus or arsenic, Z is an anion, and R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which may be identical or different, are each an aliphatic hydrocarbon radical having from 1 to 18 carbon atoms or an aromatic radical, and (b) a substituted hydroxyquinoline having the general formula (II): ##STR2## in which R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9 and R.sub.10, which may be identical or different, are each a hydrogen or halogen atom, or a substituted or unsubstituted alkyl, alkenyl, cycloaliphatic or aromatic hydrocarbon radical, with the proviso that at least one such radical is a hydrocarbon radical having from 8 to 20 carbon atoms.
