Recycling of Palladium from Spent Catalysts Using Solvent Extraction—Some Critical Points

Ana Paiva
2017 Metals  
Electrical and electronics equipment and automotive and industrial catalysts are some examples of top technological devices whose functioning rely on the use of platinum-group metals (PGMs). The PGMs' high economic value and difficult to replace technological properties, together with their scarcity in the Earth's crust, justify concerns about their critical condition and reinforce the importance of developing recycling practices for PGM end-of-life materials. This article presents and
more » ... recent advances regarding the use of hydrometallurgical solvent extraction to recover one PGM, palladium, from spent catalysts. Two different tendencies are implicit in the literature concerning Pd(II) extraction: a few groups focus on the adjustment and optimization of current commercial extractants, while others prefer to design new extracting compounds. Actually, the leach solutions obtained from the treatment of anthropogenic materials generally exhibit different compositions when compared to those coming from the primary resources. The pros and cons of both approaches are critically discussed, and the assumptions backing some of the reported achievements are also appraised. Metals 2017, 7, 505 2 of 16 Hydrometallurgical SX basically includes an extraction and a stripping stage. In extraction, the feed aqueous phase contacts with an efficient and selective extractant (the component of the organic solvent able to chemically interact with the metal species to be recovered through the formation of ion-pairs, by complexation or by solvation). The loaded solvent is then equilibrated with a stripping aqueous medium, causing the transfer of the metal to the new aqueous solution- Figure 1 . Prior to the stripping step, and if some undesired contaminants accumulate in the solvent, contact with a scrubbing solution may additionally be included [5] . Successful SX schemes effectively separate the metal of interest from the impurities, left in the raffinate, and also allow the reutilization of the solvent in successive extraction-stripping cycles, meaning that the solvent did not suffer degradation. Metals 2017, 7, 505 2 of 15 Hydrometallurgical SX basically includes an extraction and a stripping stage. In extraction, the feed aqueous phase contacts with an efficient and selective extractant (the component of the organic solvent able to chemically interact with the metal species to be recovered through the formation of ion-pairs, by complexation or by solvation). The loaded solvent is then equilibrated with a stripping aqueous medium, causing the transfer of the metal to the new aqueous solution- Figure 1 . Prior to the stripping step, and if some undesired contaminants accumulate in the solvent, contact with a scrubbing solution may additionally be included [5] . Successful SX schemes effectively separate the metal of interest from the impurities, left in the raffinate, and also allow the reutilization of the solvent in successive extraction-stripping cycles, meaning that the solvent did not suffer degradation. Figure 1. The solvent extraction scheme applied for metals recovery. The distribution ratio (D) is one of the main parameters used to evaluate the efficiency of a liquid-liquid extraction process and is defined by the International Union of Pure and Applied Chemistry (IUPAC) as the "total analytical concentration of a substance in the organic phase to its total analytical concentration in the aqueous phase, measured at equilibrium". To assess the selectivity of a SX system for the metal A over the metal B, for instance, the separation factor (SF or β) of A towards B is defined as DA/DB [5] . A final recovery stage is then necessary to obtain the purified end-product, which can be the metal itself or a salt containing the metal. There is more interest in the recycling of platinum, palladium, and rhodium from the urban mining, since these three PGMs have a larger worldwide utilization (particularly the former two [3]). Chloride media is traditionally used to solubilize the PGMs, as these solutions show a better performance than others (e.g., nitric or sulfuric acids) [5] . The predominating chlorocomplex species found in acidic and concentrated chloride media for Pt, Pd, and Rh are typically [PtCl6] 2− , [PdCl4] 2− , and [RhCl6] 3− , respectively [5] [6] [7] . Rh(III) SX is very difficult [8] , and the separation of Pt(IV) from Pd(II) is also challenging, as both metals are often co-extracted [5] [6] [7] . Pt(IV) is almost exclusively recovered through ion-pairs ([PtCl6] 2− is inert, being extracted as such), and Pd(II) can be removed either by the formation of ion-pairs and/or by complexation [6, 7] . Palladium displays the highest supply and demand values within the above mentioned PGMs [3]. This review comments on the most recent research findings focusing on the SX of Pd(II) from chloride solutions, particularly those envisaging the application of SX systems to the leaches coming from the hydrometallurgical treatment of catalysts, either automotive or industrial, and critically discusses some dubious claims and assumptions found in the literature.
doi:10.3390/met7110505 fatcat:bngyhx3jw5bfph36i33towy2di