Split-anion solvent extraction of light rare earths from concentrated chloride aqueous solutions to nitrate organic ionic liquids†
Despite its benefits, the extraction of rare earths (REEs) from chloride solutions with neutral or basic extractants is not efficient, so that separation is currently carried out by using acidic extractants. This work aims to improve this process by replacing the conventional molecular diluents in the organic phase by ionic liquids (ILs) which contain coordinating anions. The extraction of La(III), Ce(III) and Pr(III) from concentrated chloride solutions was tested with a quaternary ammonium and a phosphonium nitrate IL extractant. Dissolution of a trialkylphosphine oxide neutral extractant (Cyanex 923) in the nitrate ILs changed the preference of the organic phase from lighter to heavier REE and increased the overall extraction efficiency and the loading capacity of the organic phase. An increase of the CaCl2 concentration in the feed solution resulted in higher extraction efficiencies, due to a lower activity of water and hence to a poorer hydration of the REE ions. In that respect, chloride ions were not coordinating to the REE ion after extraction from concentrated chloride solutions. To achieve selectivity, one should fine-tune the loading by varying the CaCl2 and/or Cyanex 923 concentrations. Adjustment of the CaCl2 concentration in the feed and stripping solutions is essential for the separation of mixtures of REE. However, and unlike in the case of acidic extractants, no control of equilibrium pH is required. The split-anion extraction offers the possibility to separate mixtures of REEs in different groups without having to change the chloride feed solution. It leads to safer and environmentally friendlier extraction processes by (1) using solvents that are not volatile, not flammable and do no accumulate static electricity, (2) consuming no acids or alkali, (3) easy stripping with water and (4) avoidance to create nitrate-containing effluents.