SYNERGISTIC EFFECTS IN SOLVENT-EXTRACTION SYSTEMS BASED ON ALKYLSALICYLIC ACIDS. IV. EXTRACTION OF THE TRIVALENT RARE-EARTH METALS IN THE PRESENCE OF BIFUNCTIONAL C=0, P=0 AND S=0 COMPOUNDS

Abstract

ABSTRACT Several bifunctional compounds of different types (diamides, diphosphonates and disulphoxides) were synthesized, and their effect on the extraction of the trivalent rare-earth metals from chloride media by solutions of some alkylsalicylic acids (HA) in xylene was investigated. Appreciable synergistic shifts in the individual pH50 values were observed in most cases, the shifts generally increasing in the order S=0 ≤ C=0 < P=0 for comparable bifunctional compounds containing these donor groups. For compounds with C=0 or P=0 groups, the synergism is greater when the two groups are separated by a one-carbon bridge (-CH2-) than by a two-carbon bridge (-CH2-CH2-), whereas the opposite is true for compounds containing S=0 groups. For a given bifunctional compound, the synergistic shifts were found to increase with increasing steric bulk of the alkylsalicylic acid The synergistic shifts produced by the addition of a given bifunctional compound generally decrease across the lanthanide series (La to Lu), which suggests that 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 (and lanthanum) 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 measurements of the solubility of the neodymium-alkylsalicylic acid complex in xylene solutions of the bifunctional ligands (L) are consistent with the formation of complexes of stoichiometry NdA3L.