The extraction of platinum group metals from catalytic converters: non conventional solvents and pressure effects

Griffiths, Clive Vinee O’niell

doi:https://doi.org/10.25381/cput.30576374

The extraction of platinum group metals from catalytic converters: non conventional solvents and pressure effects

Author(s)

Griffiths, Clive Vinee O’niell

Date Issued

2025

Type

Thesis

Publisher

Cape Peninsula University of Technology

DOI

https://doi.org/10.25381/cput.30576374

Abstract

Secondary sources of precious metals, such as catalytic converters, contain up to 200 times higher concentrations of platinum group metals (PGMs) compared to natural ores, making them increasingly important for sustainable metal recovery. Catalytic converters contain platinum, palladium, and rhodium in approximate ratios of 4:4:1, with market values in the ratio of 1:1:4, respectively. Current recovery methods using conventional organic solvents achieve high recoveries for platinum and palladium but significantly lower yields for rhodium, while also posing environmental, health, and safety concerns. Deep eutectic solvents (DES) have emerged as environmentally benign alternatives to conventional organic solvents for metal extraction. However, their high viscosity limits mass transfer efficiency, reducing extraction kinetics and overall recovery yields. This study investigates, for the first time, the combination of DES with supercritical CO₂ (sCO₂) to overcome viscosity limitations and enhance PGM recovery from spent catalytic converters. Conductor-like Screening Model for Real Solvents (COSMO-RS) identified choline chloride and oxalic acid as the optimal hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) pair for DES formulation. Extraction experiments were conducted using both water bath and pressure-assisted processes with compressed CO₂. The effect of water addition on DES viscosity and the influence of compressed CO₂ on extraction efficiency were systematically investigated. The combined DES-sCO₂ approach achieved unprecedented recovery yields from the solid residue: 86.5% for platinum, 84.7% for palladium, and above 77% for rhodium. This represents the first time such high rhodium recovery has been achieved using compressed CO₂-assisted extraction. However, a significant challenge was identified in the poor absorption of metals into the DES phase, with only 17.8%, 17.3%, and 20.5% absorption for platinum, palladium, and rhodium, respectively. This work demonstrates that while DES-sCO₂ systems can effectively leach PGMs from catalytic converter matrices, future research must focus on optimizing metal-DES complex formation to improve absorption efficiency. The findings provide a foundation for developing more sustainable PGM recovery processes and highlight the potential of pressure-assisted extraction using environmentally benign solvents.

Additional information

Thesis (MEng (Chemical Engineering))--Cape Peninsula University of Technology, 2025

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