EVALUATION OF THE SELECTIVITY ON THE LA(III)-CE(III) SEPARATION WITH CHELATING EXTRACTANTS

Rare earths elements are usually recovered by the processing of ore concentrates (25-30% La2O3; 44-50% CeO2) by acid digestion followed by solvent extraction. The separation of lanthanides by liquid-liquid extraction is widely achieved by TBP and quaternary ammonium compounds (from nitrate solutions); versatic acids and alkylphosphoric acids (mainly D2EHPA) from chloride solutions. Separation factors 1.5-2.5 for neighboring lanthanide, but most stages are required for the separation section. The selective separation of lanthanides Ln could be improved by a proper application of more selective chelating reagents HR. The investigation was concerned with the utilisation of these extractants and in particular nitrogen-bearing reagents. The experimental work was directed towards a quantification of the different extractability of lanthanum(III) and cerium(III) in aqueous chloride solutions as a function of pH, extractant concentration, ligand and metal concentrations. Two commercial reagents have been evaluated: SME529 (an alkyl-hydroxyoxime) and KELEX100 (an alkyl-hydroxy-quinoline). Extraction equilibrium depends mainly on the pH for a particular extractant. The most promising selective reagent has been KELEX100 which has achieved separation factors greater than 200. The behaviour of lanthanides for a particular liquid-liquid system has been explained according to the molecular structure of the metalchelate. The ease of proton release and the donor properties of the nitrogen group of the chelating reagent molecule have been outlined and discussed.