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

Publication . Paiva, A. P.; Piedras, Francisco Vega; Rodrigues, Pedro G.; Nogueira, Carlos

ABSTRACT: 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.& nbsp;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 (R) 471X and Cyphos (R) IL 101. Cyanex (R) 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 (R) IL 101 were very encouraging, as only Fe(III) and Zn(II) were appreciably co-extracted, however, scrubbing of the contaminating metals, and Pd(II) and Pt(IV) stripping, did not work. Hence, investigation to find proper scrubbing/stripping agents for Cyphos (R) IL 101 SX system, to recover PGMs from recycled SACs, is further needed.

2022-04Journal article

Open access

Ionic Liquids in the Extraction and Recycling of Critical Metals from Urban Mines

Publication . Paiva, A. P.; Nogueira, Carlos

ABSTRACT: Ionic liquids (ILs), salts with a melting temperature below the boiling point of water, are one of the most recent fashion trends in modern chemistry. Nowadays, and taking into account the extensive research found in literature, it seems hard to imagine a sustainable world in the near future without the involvement of ILs, since they have low vapor pressure, are non-flammable, and display excellent chemical/electrochemical/thermal stabilities. Accordingly, ILs are considered to be advantageous replacers of the traditional organic solvents, therefore, much more environmentally-friendly. ILs can be easily produced to tune their physicochemical properties to specific applications, and that is the case currently occurring for several separation processes. This review aims to highlight and discuss some of the most relevant key-achievements, developed at laboratory scale, focusing on the use of ILs for the hydrometallurgical extraction of critical metals from urban mines, particularly the platinum-group metals (PGMs): ruthenium, rhodium, palladium, osmium, iridium, and platinum. A few decades of investigation brought a well-recognized scientific knowledge, still with a wide space to go, but work has yet to be conducted on testing the most promising ILs for the recycling of metals from real urban mines, and at a scaled-up level. Nevertheless, based on the most significant case-studies, the upcoming of ILs to recover critical metals from end-of-life devices for further valorization is assessed.

2021-04Journal article

Open access