Recovery of palladium from a spent industrial catalyst through leaching and solvent extraction

A hydrometallurgical approach to recover palladium from a spent industrial hydrogenating catalyst is described. The work aimed to enhance palladium and minimize aluminum recoveries. The efficiency of chloride leaching has been evaluated by varying the most determinant parameters affecting the leaching process. HCl solutions and mixtures of HCl and two chloride salts (MgCl2 and NH4Cl) were assessed. The use of H2O2 as oxidant was found essential to improve the Pd leaching to over 90%, together with low levels of chloride ion concentration (2.0M) and temperature (25°C). The use of the chloride salts enhanced the selectivity of Pd leaching over Al, but nevertheless Al is always present in the leachates. Aqueous phases composed by HCl+H2O2 and the respective salts, with optimized concentrations, were selected to be applied in liquid-liquid extraction using N-methyl-N-cyclohexyloctanthioamide (MCHTA) and N,N′-dimethyl-N,N′-dicyclohexylthiodiglycolamide (DMDCHTDGA). The results obtained from equilibrium Pd(II) extraction isotherms and reutilization experiments, using toluene as diluent and 0.1M thiourea in 1.0M HCl as stripping phase, show that MCHTA and DMDCHTDGA have similar loading capacities, with molar [extractant]:[Pd(II)] ratios of 3.5, but DMDCHTDGA exhibits a more reproducible extraction performance in the reutilization experiments. The liquid-liquid extraction efficiency of DMDCHTDGA for Pd(II) recovery from HCl+MgCl2+H2O2 and HCl+NH4Cl+H2O2 media was also evaluated. Molar [extractant]:[Pd(II)] ratios of 3.8 for the former and 3.5 for the latter medium were achieved; accordingly, the overall results obtained for the three leaching phases are similarly encouraging. The reutilization experiments depict the recyclability robustness of the solvent to recover Pd(II). The whole data reported are promising regarding Pd recovery, but simultaneously evidence the deleterious effect Al has on the solvents, since it progressively accumulates in the organic phases.