Leaching of palladium from spent Pd/Al2O3 catalysts by coupled ultrasound-microwave technique

The recycling and reuse of spent Pd/Al2O3 catalysts have tremendous social, environmental, and economic benefits. Calcined-ClO2 and NH4Cl aqueous leaching method was investigated to recover Pd from spent Pd/Al2O3 catalysts, and ultrasound-microwave was introduced to intensify the leaching process, after which the surface morphology, valence, and content of major elements, and crystalline shape of carriers of the spent Pd/Al2O3 catalysts before and after the leaching were determined and analyzed. The results showed that ultrasonic-microwave-enhancement increased the leaching rate and shortened the leaching time of Pd in comparison with the conventional leaching experiments, and the leaching rate of Pd at 10 min of ultrasonic-microwave-enhanced leaching was 31.1 %, which was 13.6 % higher than that of the conventional conditions. The optimal process conditions: calcination temperature was 200 °C, calcination time was 90 min, the dosage of ClO2 was 15 % (V/Vall), the concentration of NH4Cl was 2 mol/L, the dosage of HCl (1 mol/L) was 15 % (V/Vall), the leaching power was 400 W, leaching temperature was 90 °C, leaching time was 60 min, and the liquid–solid ratio was 10:1. Eventually, the leaching rate of palladium was 80.6 %, and the average retention rate of carrier was about 94 %.