Ligand Development in the Coordination Chemistry and Selective Extraction of Platinum Group Metals

Abstract

Platinum group metals (PGMs) are widely used in industry, especially as automotive catalysts. Palladium, platinum and rhodium in particular are employed in automobile catalytic converters for converting exhaust emissions into nonpolluting gases. To meet the future market demand, it is necessary to develop economic and environmental-friendly processes to recover PGMs from secondary sources. Solvent extraction is one of the most effective techniques for PGMs recovery, and the extractant is essentially important. However, no perfect palladium extractants have been reported. The development of new extractants and new solvent extraction systems for PGMs recovery from secondary sources in a faster and cost-effective way is still the main problem to be solved. This thesis describes the development of a new solvent extraction system for PGMs recovery, especially Pd, from secondary sources such as spent catalytic converters in an effective and selective way, which is promising for implementation in industry. To achieve this goal, first, the dithiobiuret-, thiobiuret- and biuret-based ligands were successfully synthesized as potential extractants. The coordination chemistry of the synthesized ligands with Pd, Pt and Rh were separately investigated, and the coordination mode of the metal-ligand chelation was determined. Next, the dithiobiuret-based ligands were applied as extractants for palladium recovery in solvent extraction systems. The optimized extraction conditions for effectively extracting palladium have been developed resulting in an extraction efficiency of Pd that was greater than 99% with 2:1 ligand/Pd molar ratio and 98% with 1:1 ligand/Pd molar ratio under the conditions of 0.1 M HCl, 2 minutes extraction time and aqueous to organic phase ratio of 10:1, which indicated the sustainability of this system by using a minimum amount of organic solvent while still maintaining high Pd extraction efficiency. Finally, the dithiobiuret-based ligands were applied into the recovery of palladium from spent catalytic converters. Utilizing a ligand/Pd molar ratio of 1:1, the dithiobiuret extractant demonstrated an outstanding extraction efficiency and selectivity for Pd over Pt, Rh, Fe, Ni and Zn. The Pd recovery efficiency was higher than 97%, making it a highly efficient method for Pd recovery.