Separation of precious metals by split-anion extraction using water-saturated ionic liquids

A split-anion solvent extraction process was developed for the separation of precious metal ions Au( iii ), Pt( iv ), Pd( ii ) and Rh( iii ) from aqueous chloride media using water-saturated ionic liquids. The metal extraction and stripping behavior of the chloride form [A336][Cl], bromide form [A336][Br] and the iodide form [A336][I] of the quaternary ammonium ionic liquid Aliquat 336 were compared. The three ionic liquids extracted Au( iii ), Pd( ii ) and Pt( iv ) quantitatively in most cases, whereas the co-extraction of Rh( iii ) was strongly dependent on the acidity and the chloride concentration. Among the studied ionic liquids, [A336][I] achieved the highest separation factors between Pd( ii )/Rh( iii ), Pt( iv )/Rh( iii ), and Au( iii )/Rh( iii ) at 6 mol L −1 Cl − . Additionally, the selective stripping of the individual metal ions Pd( ii ), Au( iii ), and Pt( iv ) was only possible from loaded [A336][I] using ammonia solution (NH 4 OH), sodium thiosulfate (Na 2 S 2 O 3 ), and thiourea ((NH 2 ) 2 CS), respectively. A closed-loop flow sheet was designed for the recovery of the precious metals from chloride media using split-anion extraction with [A336][I]. The integrated process was demonstrated to be suitable for the purification of Rh( iii ), Pt( iv ) and Pd( ii ) from a complex metal feed such as the leachate of spent automotive catalysts. The ionic liquid-based split-anion extraction process is simple, selective and effective for the sustainable separation of the precious metals, using only one green extractant [A336][I], which can be regenerated for consecutive extraction-stripping cycles. Split-anion extraction with water-saturated ionic liquids is a benign, efficient and selective process for recovery of precious metals from chloride media.