Recovery of Precious Metals from Secondary Sources
The project aim is to develop recyclable systems based on ionic liquids (ILs) or deep eutectic solvents (DESs), in combination with ligands and/or oxidants, for the recovery of Pd and Pt from catalytic converters and of Au from electronics using non-toxic reagents. The synthesis of dione and dithione ligands (1,4-dimethyl-2,3-piperazinedione, 1,4-dimethyl- 2,3-perhydrodiazepinedione, 1,4-dimethyl- 2,3-piperazinedithione, and 1,4-dimethyl-2,3- perhydrodiazepinedithione) and their iodine adducts (1,4-dimethyl-2,3-piperazinedithione/ triiodide, 1,4-dimethyl-2,3-perhydrodiazepinedithione/ bisdiiodine), for the dissolution of Pd and Pt was investigated. 1,4-dimethyl-2,3-piperazinedione, 1,4-dimethyl-2,3-perhydrodiazepinedione, and 1,4-dimethyl-2,3-piperazinedithione were successfully synthesised using conventional methods. 1,4-dimethyl-2,3-piperazinedione and 1,4-dimethyl-2,3- perhydrodiazepinedione were also synthesised mechanochemically while 1,4-dimethyl-2,3- piperazinedithione can be synthesised sonochemically. The adduct 1,4-dimethyl-2,3- piperazinedithione/triiodide was also successfully synthesised. The mild conditions which would lead to the dissolution of Pd, Pt, and Au were examined. The best results were achieved in DESs, where metal solutions up to 200 mM (Reline 200) and 300 mM (Ethaline 200) were obtained, using a Pd:iodine ratio of 1:4. The highest Au concentration (57.1 mM) was obtained in [BMIM]OTf using a triphenylphosphine:iodine ratio of 1:1. Mixed chloride/iodide square-planar Pd(II) complexes are believed to form in the DESs. Solutions of Pd and iodine in either DES produce CVs that are similar to each other. Dissolved Pd was recovered by electrodeposition from Reline 200 and the mixture could be recycled at least twice. Pd was also successfully recovered from a used catalytic converter as proven by cyclic voltammetry, SEM, and EDX spectroscopy. A triphenylphosphine:iodine mixture in [BMIM]OTf was able to dissolve Au at room temperature without the need of anhydrous/anaerobic conditions. The Au complex formed has not been confirmed but appears to be a unique, stable product. Approximately 53% of Au was successfully recovered via electrodeposition and the remaining solution successfully re-used for further Au dissolution.
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