Fast microwave leaching of platinum, rhodium and cerium from spent non-milled autocatalyst monolith

This work presents a new microwave leaching approach to solubilize platinum group metals (PGMs) and cerium from spent autocatalyst monoliths. In the proposed process, the catalyst does not require to be milled/pulverized prior to the leaching step in 6 M HCl for 3 min in the absence of an oxidation agent and internal mixing. Therefore, processing costs associated to catalyst comminution could be avoided. The highest extraction yields were achieved at 220°C at a liquid to solid ratio of 10 (Pt: 99±4%, Rh: 95±1%, Ce: 104±3%). At this temperature, the composition of the in situ generated headspace gas, including Cl2 gas that acts as oxidant, and the enhanced porosity of cordierite, improved PGM and Ce leaching. Furthermore, the blocks preserved their honeycomb structure after leaching, simplifying downstream solid-liquid separation, e.g. using gravity filtration. SEM/EDX analyses of residues' cross-sections allowed for identifying the dissolution mechanism of the washcoating (150-220°C). Whereas, under the same conditions ball-milled catalyst material gave significantly lower PGM leaching (Pt:47±4%, Rh: 48.6±0.3%) and a greener leaching in 1 M HCl + 3.8 M NaCl, achieved higher selectivity towards PGMs, although overall yields decreased (Pt: 68±2%, Rh: 67±5%, Ce: 39±1%). Statement of Novelty and Significance: This work presents a novel approach to solubilize Pt, Rh and Ce from spent ceramic autocatalyst via a fast microwave leaching process (3 min) in a 6 M HCl solution at 220°C, without the need of (i) a milling pretreatment, (ii) an oxidizing/reducing agent, nor (iii) internal mixing. Therefore, the energy consumption, cost of chemicals and complexity of the process are reduced in comparison to previously described microwave assisted routes, which might benefit its upscaling. Additionally, the metal leachability was high (99±4% Pt, 95±1% Rh, 104±3% Ce), and the downstream solid-liquid separation was facilitated.