Selective separation of La(III) and Ce(III) using hollow fiber membranes: influence of pH and extractant systems

Abstract

The selective separation of adjacent rare earth elements (REEs), such as La(III) and Ce(III), is a critical challenge in hydrometallurgy due to their similar chemical properties. This work evaluates the performance of non-dispersive solvent extraction (NDSX) using hollow fiber (HF) membranes for this purpose. Initial solvent extraction (SX) equilibrium experiments with Cyanex® 272 in kerosene determined that the aqueous phase’s optimal pH for selectivity is 5.6, achieving a selectivity of αCe/La =12.7. NDSX experiments demonstrated enhanced selectivity αCe/La = 34 after 120 min, benefiting from the additional mass transfer resistance provided by the HF membrane. Maintaining a constant pH of 5.0 with NaOH improved extraction rates but slightly reduced selectivity to αCe/La = 26. Experiments using 1,1,1-trifluoro-2,4-pentanedione (HTFAC) in the ionic liquid (IL) [Omim][Tf2n] as the receiving phase showed lower extraction rates but achieved comparable selectivity values (αCe/La = 22) in just 20 min, thanks to the IL’s viscosity limiting La(III) extraction. The impact of HF membrane design was also assessed; increasing the membrane’s surface area significantly improved extraction rates but reduced selectivity due to reduced mass transfer resistance. These results demonstrate the potential of NDSX systems for selective REE separation, particularly by leveraging controlled mass transfer and operating conditions. However, further work is needed to optimize system design. The findings highlight the advantages of NDSX over traditional SX, offering a promising pathway for sustainable and efficient REE processing.