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

Abstract

• Liquid-liquid separation of PGMs’ from HCl medium using Cyphos IL 101 was studied. • Increasing acid and total Cl – ion concentration inhibited the extraction efficiency. • Poorly extracted Rh(III) efficiently scrubbed from organic phase in high acid solution. • Outer sphere complexation of [PtCl 6 ] 2− and [PdCl 4 ] 2− with ionic liquid was observed. • Quantitative stripping of Pt(IV) with NaSCN and Pd(II) with CH 4 N 2 S was achieved. 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 ~ 22% Rh(III). Depending upon the HCl and total Cl − ions concentrations, species like [PtCl 6 ] 2− and [PdCl 4 ] 2− exhibited remarkable ability for ion-pairing with Cyphos IL 101 unlike the higher charge density of [RhCl 6 ] 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 × 10 −1 mol.L −1 NaSCN in two stages, which exhibiting ~ 99% efficacy of Pt(II) stripping. Subsequently, Pt(IV)-depleted organic phase was stripped with 1 × 10 −2 mol.L −1 CH 4 N 2 S 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.