Selective Extraction of Palladium and Platinum from Hydrochloric Acid Solutions by Trioctylammonium-Based Mixed Ionic Liquids

Mixtures of the protic ionic liquids trioctylammonium bis(trifluoromethanesulfonyl)amide ([TOAH][NTf2]) and trioctylammonium nitrate ([TOAH][NO3]) were investigated as extractants for platinum-group elements. The mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] has low viscosity (274.2 mPa s) and low aqueous solubility (2.6 × 10–5 mol dm–3 as the cation). With the ionic liquid mixture, Pd(II) and Pt(IV) were extracted almost quantitatively from 0.10 mol dm–3 hydrochloric acid. Under the same conditions, Na(I), Mg(II), K(I), Ca(II), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ru(III), Rh(III), and Cd(II) were only slightly extracted. It was found that the extraction of Pd and Pt increases with increasing content of [TOAH][NO3] in the mixture. The metals could be back-extracted from the ionic liquid mixture with nitric acid solution, and selective stripping of Pt was possible by controlling the concentration of nitric acid. The metal extractability of the ionic liquid mixture after the back-extraction was equivalent to that of the fresh mixture, indicating that the species composition of the mixture returned to the original state through the back-extraction process. This extraction system provides an efficient separation method for Pd and Pt in acidic chloride media using ionic liquid mixtures, which are recyclable, easy to handle, safe, and environmentally friendly.