Hydrometallurgical recovery of platinum-group metals from spent auto-catalysts – Focus on leaching and solvent extraction

• Published in: Separation and purification technology 2022-04, Vol.286, p.120474, Article 120474
• Main Authors: Paiva, Ana Paula, Piedras, Francisco Vega, Rodrigues, Pedro G., Nogueira, Carlos A.
• Format: Article
• Language: English
• Subjects: Hydrometallurgical recycling; Hydrometallurgy; Leaching; Platinum-group metals; Recycling; Solvent extraction; Spent auto-catalysts
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2022.120474

•Recovery of platinum and palladium from spent auto-catalysts by hydrometallurgy.•Best conditions for leaching: 11.6 mol L−1 HCl, 1%vol H2O2, 60 °C, L/S = 2 L kg−1, 3 h.•Best solvent extraction systems for Pd and Pt: Cyanex® 471X and Cyphos® IL 101.•Efficient stripping agents for Pt and Pd from Cyphos® IL 101 need further research. To ensure the supply of raw materials for products of extreme importance in strategic sectors, the recovery of critical metals from secondary sources becomes increasingly urgent. Platinum group metals (PGMs), being rare and very valuable, fall into this demand, and the catalytic converters that contain them are recognized as one of the main sources. Hydrometallurgical processes have been proposed as an alternative to pyrometallurgical ones, with leaching and separation by solvent extraction being core operations in this type of processing. This article investigates these two operations, seeking to optimize conditions and propose new arrangements to improve them. Two different catalyst samples were considered in the experimental work. Leaching was carried out involving concentrated HCl solutions (with H2O2 as oxidant) and low liquid/solid ratios, seeking to maximize PGMs recovery, guaranteeing their high concentration in leachates and minimizing aluminum co-dissolution. Cerium leaching was also followed since this is a rare-earth metal with potential interest. Temperature, HCl concentration, liquid/solid ratio (L/S), time and particle size factors were evaluated, and the optimized conditions found were 11.6 mol L−1 HCl, 1%vol H2O2, 60 °C, L/S = 2 L kg−1 and 3 h, leading to PGM yields of 90–98% Pt, 99% Pd and 70–96% Rh, and leachate compositions of 0.41–0.78 g L−1 Pt, 1.6 g L−1 Pd, 0.062–0.066 g L−1 Rh, depending on the catalyst sample. For solvent extraction (SX), several commercial extractants dissolved/diluted in toluene were checked, firstly with a model solution, and then applied to the real spent auto-catalyst (SAC) leachates produced in-situ. The overall results showed that the most promising SX systems among those tested were Cyanex® 471X and Cyphos® IL 101. Cyanex® 471X allowed the quantitative extraction of Pd(II) and Fe(III), but the latter was conveniently scrubbed by water prior to Pd(II) stripping by an acidic thiourea solution. Pt(IV) and Pd(II) extraction values by Cyphos® IL 101 were very encouraging, as only Fe(III) and Zn(II) were appreciably co-extracted, however, scrubbing of the contaminating metals, and Pd(I