Dissolution kinetics of nickel from spent catalyst in nitric acid medium

In this study, the kinetics of nitric acid leaching of a spent NiO–Al2O3 catalyst was investigated. Influence of leaching parameters such as temperature (60–90 °C), acid concentration (1.5–5 M) and average particle size (75–601 μm) was studied. For average particle size ≤ 75 μm, dissolution data followed surface reaction controlled shrinking core model at all temperatures and acid concentration. However, for higher particle sizes product layer diffusion controlled the overall kinetics. The estimated activation energy for the surface reaction controlled mechanism was 83.44 kJ/mol and the empirical order of reaction with respect to [HNO3] was nearly 2. In general, the reaction mechanism shifted from surface reaction controlled to diffusion controlled when the extraction reached ∼90%. The shift in reaction mechanism was explained by calculating the theoretical reaction rates.

► The reaction mechanism of acid leaching of nickel spent catalyst was established through the shrinking core kinetic models. ► The reaction mechanism shifted from reaction controlled model to diffusion controlled model with particle size >75 μm. ► For lower particle sizes the reaction mechanism shifted to diffusion controlled at higher conversions (>90%). ► The estimated activation energy was 83.4 kJ/mol and the order of reaction with respect to [HNO3] was ∼2. ► The shift in rate controlling step has been explained through theoretical calculations.