Refining of Palladium, Platinum, and Rhodium from a Pregnant Leach Solution of Spent Automotive Catalysts by Continuous Counter-Current Solvent Extraction with Ionic Liquids in Mixer-Settlers

Assessment of solvent extraction in continuous mode is crucial to determine the feasibility of upscaling the processes. At the moment, limited information is publicly available on how to operate counter-current solvent-extraction equipment and how to tackle problems that may emerge during the use of these contactors with real feeds. In this work, the performance of a solvent-extraction process in mixer-settlers is evaluated. The extractant Aliquat 336 iodide was synthesized in mixer-settlers by contacting the commercially available Aliquat 336 diluted in p-cymene with an aqueous solution of potassium iodide in a counter-current mode. Next, palladium­(II) and platinum­(IV) were extracted by Aliquat 336 iodide from a highly impure pregnant leach solution of spent automotive catalysts. Selective stripping was achieved with solutions of aqueous ammonia and thiourea. In total, 12 L of pregnant leach solution from spent automotive catalysts was processed in mixer-settlers using two stages for the extraction of palladium­(II) and platinum­(IV) and four stages for each stripping step. The chosen organic to aqueous (O/A) ratios allowed to enrich the solutions of palladium and platinum up to 6 and 2.5 times, respectively, whereas rhodium remained in the raffinate. The output solutions obtained during solvent extraction were further treated downstream to produce highly pure (>99%) platinum group metal (PGM) nitrate solutions. The main challenges of running a solvent-extraction process using ionic liquids at a minipilot scale are critically discussed.