Solvent extraction of platinum-group metals from hydrochloric acid solutions by dialkyl sulphoxides

The solvent extraction of palladium(II), platinum(II), platinum (IV), rhodium(III), iridium(III) and iridium(IV) from hydrochloric acid solutions by some dialkyl sulphoxides of the types R2SO, RR'SO and R-2'SO, where R=alkyl and R'=cycloalkyl, was investigated. The order of extraction of these metals by 0.50 M di-n-hexyl sulphoxide in xylene from solutions containing 1 to 6 M hydrochloric acid was found to be Pd(II) > Pt(II) > Ir(IV) > Pt(IV) > Rh(III) > Ir(III). Metal-distribution studies suggest that iridium(IV) and platinum(IV) are extracted in the form of ion-pairs such as (L-3.H3O+)(2)IrCl62- and (L-2.H3O+) (L-3 . H3O+)PtCl62-, where L=dialkyl sulphoxide. In the case of palladium(II), and probably platinum(II) also, the sulphoxide apparently displaces one or two of the chloride ligands from the chlorometallate anion (MCl42-), and the distribution data are consistent with the extraction of PdCl2L2 at low hydrochloric acid concentrations (1-2 M) and (L-2 . H3O+)PdCl3L- at higher concentrations (4-6 M). The slow rates of extraction of rhodium (III) and iridium(III) from solutions containing the MCl63- species suggests a ligand substitution reaction for these metals also, with the probable formation of species such as (L-2 . H3O+ RhCl4L2- and (L . H3O+)(2)IrCl5L2- in the organic phase. Comparison of the uv-visible spectra of the metal-loaded organic phases with those obtained using tri-n-octylammonium chloride as extractant confirms that simple chlorometallate anions are present in the dialkyl sulphoxide extracts only for platinum(IV) and iridium(IV).