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.