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