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Mild extraction of PdAg precious metal components and efficient recovery of Al2O3 supports from spent acetylene hydrogenation catalyst

Qianyuan Zhang,Qihui Wang,Chunli Li,Pinggui Tang,Yongjun Feng

Addressing the challenges of simultaneously extracting Pd and Ag components from spent acetylene hydrogenation catalysts, severe structural damage to the Al 2 O 3 supports, and high wastewater discharge, here, we propose a “hydrochloric acid–ammonium chloride–ammonia” (HCl–NH 4 Cl–NH 3 ) complexation system that enables efficient extraction of PdAg leaching alongside recovery of the Al 2 O 3 supports. The leaching rates of Pd and Ag exhibit an increasing trend with rising concentrations of HCl, NH 4 Cl, and NH 3 , the increase of reaction temperature, and the extension of reaction time. Nevertheless, excessively high HCl concentration, overly elevated reaction temperature, and unduly prolonged reaction time caused some damage to the structure of the alumina support and reduce its mechanical strength. Under optimized conditions (0.2 M HCl, 1.0 M NH 4 Cl, 120 °C, 6 h, 5 wt% ammonia), the PdAg components were efficiently leached while effectively preserving the crystal phase and pore structure of the Al 2 O 3 beads. The leaching rates of Pd and Ag reached 95.6 % and 94.4 %, respectively; and the recovered Al 2 O 3 beads have a pore diameter of 30.8 nm, a specific pore volume of 0.244 mL g −1 , a specific surface area of 29.2 m 2 /g, and a crushing strength of 53.2 N, corresponding to retention rates of 95.2 %, 96.0 %, 100.2 %, and 98.6 %, respectively. The HCl–NH 4 Cl–NH 3 complexation system, combined with cyclic washing, shows advantages of low cost, mild extraction conditions, and reduced wastewater discharge, which keeps to the principles of green development.