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

•Ultrasound-microwave coupled field technology improves leaching.•A method for short process and efficient recycling.•Weak acidic environments are less destructive to the carrier Al2O3, which can be utilized twice. 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 %.