SYNERGISTIC EFFECTS IN SOLVENT-EXTRACTION SYSTEMS BASED ON ALKYLSALICYLIC ACIDS - IV - EXTRACTION OF THE TRIVALENT RARE-EARTH-METALS IN THEPRESENCE OF BIFUNCTIONAL C=O, P=O AND S=O COMPOUNDS

Several bifunctional compounds of different types (diamides, diphospho nates and disulphoxides) were synthesized, and their effect on the ext raction of the trivalent rare-earth metals from chloride media by solu tions of some alkylsalicylic acids (HA) in xylene was investigated. Ap preciable synergistic shifts in the individual pH(50) values were obse rved in most cases, the shifts generally increasing in the order S=O l ess than or equal to C=O < P=O for comparable bifunctional compounds c ontaining these donor groups. For compounds with C=O or P=O groups, th e synergism is greater when the two groups are separated by a one-carb on bridge (-CH2-) than by a two-carbon bridge (-CH2-CH2-), whereas the opposite is true for compounds containing S=O groups. For a given bif unctional compound, the synergistic shifts were found to increase with increasing steric bulk of the alkylsalicylic acid. The synergistic sh ifts produced by the addition of a given bifunctional compound general ly decrease across the lanthanide series (La to Lu), which suggests th at coordination of the neutral ligand is somewhat dependent on steric factors. For the diamides and diphosphonates containing a -CH2- bridge , the synergistic effects in the extraction of neodymium land lanthanu m) are substantially greater than for their monofunctional analogues, which suggests that these bifunctional compounds may be coordinated to these metals in a bidentate mode. Slope analysis studies and measurem ents of the solubility of the neodymium-alkylsalicylic acid complex in xylene solutions of the bifunctional ligands CL) are consistent with the formation of complexes of stoichiometry NdA(3)L.