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In this study, the leaching behavior of palladium and silver from spent catalysts generated in petro-chemicals was investigated using nitric and hydrochloric acid as lixiviant solutions. The effects of nitric and hydrochloric acid concentrations, different leaching temperatures and times, and different pulp densities on the leaching efficiencies of palladium and silver were examined, and results were compared. Based on these parameters, optimal leaching conditions were established. When nitric acid (2.0 M HNO3, 90°C, 1 h) was applied, silver was completely leached out, whereas the leaching efficiency of palladium was found to be low at approximately 45%. By contrast, the use of hydrochloric acid as a leachate agent (4.0 M HCl) at 90°C for 1 h yielded nearly 100% efficiency for each of the two metals. The rate of palladium leaching was governed by a mixed (chemical and diffusion) control model with an activation energy of 6.4 kcal/mol in the temperature range of 25-90°C.

Leaching Behavior of Palladium and Silver from Spent Petrochemical Catalysts by Nitric and Hydrochloric Acid Solutions