Recycling Oriented Characterization of PGMs from Spent Automotive Catalysts (ACCs)

Platinum Group Metals (PGMs) are key critical metals for Europe, that is dependent on export from the main producing countries, mainly South Africa and Russia, which together are responsible for 75 to 80% of the PGM world mine supply. On the demand side, the EU legislation on pollution emission standards has pushed the use of automotive catalytic converters (ACCs) in vehicles, making that the manufacturing of ACCs by far the largest PGMs (Pt, Pd & Rh) consuming application. The concentrations of PGMs in ACC urban mines is much higher (>2000 ppm) than conventional mines in South Africa and Russia (2-10 ppm) and therefore recycling of ACCs presents an important source of PGMs for sustainably supply and reduced environmental footprint.
Hydrometallurgical recovery methods are seen as sustainable alternatives to well established and industrially applicable pyrometallurgical techniques. However, research efforts have not yet led to industrialization. A better understanding of complex ACC material in terms of morphology, liberation, metallurgical associations, metal speciation and metal liberation should improve the process performances.
Recycling oriented characterization of different types of ACCs available on the market was performed using SEM-EDS. These included cordierite based diesel oxidation (DOC) and three-way catalysts (C-TWC), silicon carbide based diesel particle filter (DPF), and metallic based three way catalyst (M-TWC). Structural design differences were noticed between different catalyst types and all ACCs contained complex compositional complexity. Washcoat distribution in DPF was found to be different than other ACCs. Catalysts were found to contain secondary deposition layers of poisons with varying degree of contamination of Zn, Ca, P, K, S, Pb, etc. PGM particles were found to be uniformly distributed in the washcoat. They were also trapped in the poisoning layer, attached to or contained within the washcoat components such as ceria and alumina. PGM and washcoat components were heavily sintered and grown in size. Sintering had blocked the active surfaces/PGM liberation by loss of surface area which also results in reduction of porosity.
SEM-EDS proved to be a powerful tool to analyze several phenomena that have happened to ACCs over the operational lifetime. The information obtained at nanoscale provided for better understanding of ACCs and variance within ACC urban deposit useful for subsequent pretreatment or hydrometallurgical PGM recovery routes.