Hydrometallurgical recycling of palladium and platinum from exhausted diesel oxidation catalysts

Abstract

An efficient reclamation of the precious metals remained in exhausted diesel oxidation catalyst (DOC) is desirable due to their high economic values and depletion of primary minerals. Aiming to the resource recovery from exhausted DOC, herein, HCl leaching followed by the selective precipitation of platinum and palladium has been investigated. The parameters like acid concentration (2.0-10.0 mol/L), H2O2 addition dosage (0.5-5.0 vol %), temperature (25-70 degrees C), time (10-180 min), and pulp density (5-20 wt./vol.%) were varied to determine the optimum condition of leaching. Experimental results revealed that the efficient formation of metal-chloro complexes was achieved by the addition of 3 vol% H2O2 with 8.0 mol/L HCl solution at 55 degrees C and 180 min of leach time that yielded approximately 94% palladium and 90% platinum into the leach liquor. Under the studied condition, the leaching process was found to be governed by a diffusion-controlled mechanism with the apparent activation energy of palladium and platinum was calculated to be 14.4 kJ/mol and 10.8 kJ/mol, respectively by following the logarithmic model. Subsequently, the leach liquor was processed to selectively recover platinum salt by adding a 300 g/L solution of NH4Cl, precipitating (NH4)(2)PtCl6. Palladium from platinum-depleted leach liquor was recovered by additional introducing NaClO3 crystals under heating, yielding red precipitates of (NH4)(2)PdCl6. The present process can potentially contribute to the circular economy of precious metals through the hydrometallurgical recycling of exhausted DOC.