Sustainable treatment of bimetallic (Ag-Pd/α-Al2O3) catalyst waste from naptha cracking process: An innovative waste-to-value recycling of precious metals.

Bimetallic (Ag-Pd/α-Al2O3) catalysts are essentially applied to naptha-cracking process with a controlled CO2 emission. After losing the catalytic properties in long run, the landfilling disposal of spent catalysts poses severe stress to the environment and deprivation of precious metals. Therefore, an innovative solvo-chemical recycling approach that involving the solid-liquid and liquid-liquid mass transfer phenomena was studied. The parametric variations for dissolving precious metals yielded >98% efficiency at a lixiviant concentration, 2.0 mol L-1 HCl; pulp density, 20% (wt./vol.); agitation speed, 300 rpm, temperature, 90 °C, and duration, 60 min. The activation energy of silver (6.9 kJ mol-1) and palladium (11.9 kJ mol-1) leaching indicated that the process was governed by a diffusion-controlled mechanism. Subsequently, silver and palladium were separated using 0.15 mol L-1 LIX 84-I at different acid concentration that yielding the maximum separation factor (β(Ag/Pd) = 12,501) at 2.0 mol L-1 HCl. Stripping of separately (Ag/Pd)-loaded organic solutions with different solutions of HNO3, (NH4)2SO4, and CH4N2S showed higher affinity for thiourea, yielding 56%, 38%, and 87% efficiency, respectively. Thus the counter-current extraction at an organic-to-aqueous (O:A) ratio of 1:2.5 and stripping with 0.5 mol L-1 CH4N2S at an O:A ratio of 2:1 yielded a five-fold enrich solutions of precious metals (75.2 mg L-1 Ag and 188.5 mg L-1 Pd) with a purity of >99.9%. The process essentially aims to Goal 12 under the United Nations' Sustainable Development Goals for sustainable recycling of industrial wastes consequently conserving the natural mineral reserves.