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.