Separation of platinum group metals from model chloride solution using phosphonium-based ionic liquid

The present study dealt with liquid-liquid separation of PGMs (Pt, Pd, and Rh) from a model chloride-based leach solution using an undiluted phosphonium-based ionic liquid, Cyphos IL 101. Based on the parametric optimization studies, 30 min equilibration of both phases at an organic-to-aqueous volume ratio of 1 showed the highest PGMs' extraction of > 99% Pt(IV) and Pd(II), and similar to 22% Rh(III). Depending upon the HCl and total Cl- ions concentrations, species like [PtCl6](2) and [PdCl4](2-) exhibited remarkable ability for ion-pairing with Cyphos IL 101 unlike the higher charge density of [RhCl6](3-) which significantly lowered down to be < 5% at an aqueous feed of 0.1 mol.L-1 HCl. Thermodynamic properties with negative changes in enthalpy and entropy indicated that the PGMs' extraction process exhibit exothermic behaviour with the formation of outer-sphere complexes in the ionic liquid phase. With the supports of experimental results and spectral analysis, the possible structure of Pt (IV)-Pd(II)-loaded ionic liquid could also be revealed. The poorly extracted Rh(III) was efficiently scrubbed by contacting with 5.0 mol.L-1 HCl solution, leaving the organic phase free from Rh(III). The selectivity in back-extraction of Pt(IV) and Pd(II) was achieved by first contacting the loaded ionic liquid with 1 x 10(-1) mol.L-1 NaSCN in two stages, which exhibiting similar to 99% efficacy of Pt(II) stripping. Subsequently, Pt(IV)-depleted organic phase was stripped with 1 x 10(-1) mol.L-1 CH4N2S that yielded > 99% Pd(II) recovery in a single contact. Results of extraction-stripping cycles that were showing similar efficiency of PGMs extractability clearly indicated the stability and reusability of ionic liquid.

Automated summary

The study successfully separated Pt, Pd, and Rh through liquid-liquid extraction using an undiluted phosphonium-based ionic liquid, Cyphos IL 101, with high extraction efficiency for Pt and Pd. Rh, however, showed lower extraction efficiency but could be efficiently scrubbed by contacting with HCl solution. Selective back-extraction successfully recovered Pt and Pd, demonstrating the stability and reusability of the ionic liquid.