Simultaneous leaching and separating of palladium from spent palladium catalyst by tribromide ionic liquids as non-volatile oxidizing solvents

The recovery of palladium (Pd) from secondary resources has attracted much attention because of its unique environmental effects and economic value. A novel technique for the oxidation, dissolution, and separation of precious metals is presented in this paper. Tribromide ionic liquids (ILs) with low viscosity, high stability, and strong oxidation were synthesized and applied for the one-step leaching and separation of Pd from spent Pd-Al2O3 catalysts. Through the preliminary performance test, [C1C12PYR]Br3 was selected as the optimal leaching agent from three kinds of tribromide ILs, and the reasons for the obvious difference in leaching ability were analyzed from the perspective of quantum chemistry.The optimum leaching conditions were determined as follows: leaching time was 12 h, leaching temperature was 45 °C, and the dosage of [C1C12PYR]Br3 was 0.8 g (the mass of fixed Pd was 20 mg). Under these conditions, the leaching efficiency (η%) can reach 94.82%. By optimizing the response surface methodology (RSM) model, the leaching temperature in the actual system was determined to be 50.28 °C, and the dosage of [C1C12PYR]Br3 was 0.74 g, which achieved the best economic value of the system. Using XPS, HPLC, and EDS, the leaching mechanism of Pd was demonstrated to be a process that combined redox with coordination and complexation. The kinetic study shows that the rate control step of the leaching process was a chemical reaction process. The 20% hydrazine hydrate can achieve efficient (99%) stripping of Pd in the IL phase. As a result of the leaching agent's good selectivity and reusability when used to recover spent Pd-Al2O3 catalyst, [C1C12PYR]Br3 is regarded as a promising precious metal leaching agent.