Refining of the platinum-group metals

Liquid‐ and Solid‐based Separations Employing Ionic Liquids for the Recovery of Platinum Group Metals Typically Encountered in Catalytic Converters: A Review

“…The thermodynamic and kinetic differences between the metal ion species are the fundamental parameter on which their effective separation is based. [5,75] According to thermodynamic data, Pd has the highest solubility in its chloride form, followed by Pt and Rh. [76] Rh, particularly, is quite inert and not soluble or only slightly soluble in acids.…”

“…The solubility and extractability of PGMs is also largely affected by the size and the charge of the complex present in solution, since those directly affect the selectivity of the extracting agent. Taking into account the decisive effect that these two parameters impart on the extraction, it comes as no surprise that Rh is only poorly extracted compared to Pt and Pd [75] . Pt is commonly encountered as PtCl 4 2− and PtCl 6 2− with oxidation numbers of +2 and +4, respectively.…”

“…The variety and strength of complexes that PGMs can form is one of their unparalleled features [75,76] . They tend to form coordinated complexes with various nucleophiles, which are stronger than the ones corresponding to base transition metals.…”

“…[55] extracted compared to Pt and Pd. [75] Pt is commonly encountered as PtCl 4 2À and PtCl 6 2À with oxidation numbers of + 2 and + 4, respectively. Similarly, the most common Pd species are PdCl 4 2À and PdCl 6 2À with oxidation numbers of + 2 and + 4, respectively.…”

“…This can be attributed to the better overlap of 4d and 5d orbitals, as opposed to transition metals that only have the spatially smaller 3d orbitals [5] . PGMs are B‐type metals and tend to form stable complexes with soft (π‐bonding) ligands, such as chloride (Cl − ) [75] . PGM‐chlorocomplexes are particularly important in their separation and refining processes, since in this form PGMs are soluble in acidic and oxidizing environments.…”