Complexation-Distribution Separated Solvent Extraction Process Designed for Rapid and Efficient Recovery of Inert Platinum Group Metals

Previously, we have demonstrated that thermal-assisted techniques can accelerate the extraction of inert platinum group metals (PGMs), while they still have several concerns about difficulty of temperature control in actual extraction contactors and safety risks arising from heating organic solvents. In this study, we report a complexation-distribution separated extraction process for the accelerated extraction of inert PGMs. This extraction method includes two steps: (1) complexation of PGMs with extractants in aqueous solution and (2) distribution of the formed complex from the aqueous phase to organic one. We separately investigated the complexation and distribution processes for typical inert PGMs such as Ru(III) and Rh(III) in the presence of water-soluble N,N,N′,N′-tetra-alkylpyridinediamide ligands (PDA) and bis(trifluoromethylsulfonyl)amide (Tf2N-) counteranions. As a result, the water-soluble complexes of Ru(III) and Rh(III) with PDA can be formed in 0.5 M HNO3(aq) within 3 h under heating at 356 K. The formed complexes were extracted to the 1-octanol layer containing Tf2N- within 5 min at room temperature, where this hydrophobic anion plays an important role to promote extraction of PGMs as an anionic phase-transfer catalyst (PTC). Consequently, we successfully established and demonstrated the complexation-distribution separated extraction process for the accelerated extraction of inert PGMs using a water-soluble ligand and anionic PTC.