Modeling of platinum leaching kinetics in aqua-regia solutions using the Elovich equation

Platinum extraction from different secondary sources has been experimentally tested by many researchers. In this article, dissolution of platinum in aqua-regia solution from two types of spent catalysts (reforming and nitric acid production) was kinetically studied. A variety of kinetic models for heterogeneous solid-liquid reactions, including homogeneous diffusion model, shrinking core model, and the power law model were examined. However, these were not able to robustly predict the experimental data. Another model, the Elovich equation, fitted the kinetics data fairly well for both catalysts. The relation between the Elovich equation parameters (a and ?) and the process conditions, i.e., temperature and liquid/solid ratio was obtained. It was realized that different catalyst matrices only modestly affect the ? parameter, which is representative of the initial rates (only a function of initial concentrations and temperature). In contrast, the other term, a, which is a function of the catalyst structure and the liquid/solid ratio, was found to be rather different for the two catalysts.