Solvent‐Polymeric Membranes for Separation of Li+ From Other Alkali Metal and Alkaline Earth Ions

Lithium may be recovered from the Dead Sea brines by a process which combines membrane separation with ion‐exchange. Solvent‐polymeric membranes based on alkyl‐arylphosphates cause selective permeation of lithium ions with Br−3 as counter ions. Addition of the derivatives of neutral “crown” ethers did not improve their performance and an adverse effect, due to the decrease in the fluidity of the membrane system, was observed. Incorporation of ionizable “crown” ethers compatible with the system may, however, be advantageous; pH gradients could act as a driving force for transport of lithium in such systems. Membranes prepared with (2‐ethylhexyl)‐diphenyl phosphate (Santicizer 141) gave the best results from the point of view of selectivity of Li+ transport vs. Mg2+ and Ca2+. Maintenance of ca. 10−3 M concentrations of Br2 in the end‐brine solutions gives optimal membrane performance. No significant change in membrane permeability and selectivity occurred during six months of their operation. Lithium ions in the product solution of the membrane separation process may be further separated from the residual Mg2+ and Ca2+ ions and concentrated up to 1 M by ion exchange processes. Lithium may be precipitated from such solutions, free from alkaline earth ions, as Li2CO3.