Development of a solvent extraction system with 1,2-bis(2-methoxyethylthio)benzene for the selective separation of palladium(II) from secondary raw materials

The chelating dithioether 1,2-bis(2-methoxyethylthio)benzene, a novel solvent extractant for Pd(II), is aimed to be utilised in the selective recovery of palladium from spent automotive catalysts. For that, the extraction system has been further customised, including the choice of an appropriate diluent (1,2-dichlorobenzene) as well as an effective stripping agent (0.5 M thiourea in 0.1 M HCl), which both have been selected from a number of potential agents. It is shown in batch experiments that the selectivity for Pd(II) is maintained when the organic phase (10− 2 M 1,2-bis(2-methoxyethylthio)benzene in 1,2-dichlorobenzene) is used several times to extract an oxidising leach solution. According to the McCabe–Thiele plot two theoretical stages are needed to extract more than 98% of the Pd(II) contained in that solution. The calculation of the thermodynamic quantities ΔΗ°, ΔS° and ΔG° reveals that the reaction is entropy driven — the temperature has only a slight influence on the extraction yield. It is demonstrated that the mono-oxidised extractant has a catalytic effect on the extraction kinetics when the aqueous phase contains highly concentrated hydrochloric acid. HPLC measurements prove the presence of small quantities of 1-(2-methoxyethylsulfinyl)-2-(2-methoxyethylthio) benzene in the organic phase.

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► 1,2-Bis(2-methoxyethylthio)benzene is a highly selective extractant for Pd(II).
► Extraction system adapted to requirements in industry (diluent, stripping agent)
► Organic phase can be used several times without decline of efficiency or selectivity.
► Determination of ΔΗ°, ΔS° and ΔG° shows that the extraction is entropy driven.
► Small quantities of mono-oxidised extractant can enhance the extraction rate.