Leaching of rare earth elements from coal ash using low molecular weight organocarboxylic acids: Complexation overview and kinetic evaluation

The study of thermodynamics and kinetics of leaching rare earth elements (REEs) is a fundamental aspect in understanding the mechanism behind the leaching process. Leaching of REEs from coal ash with aqueous solution of organocarboxylic acid is a heterogeneous fluid-particle system. In the present study, the leaching mechanisms of these three potential organocarboxylic acids, tartaric acid, lactic acid, and citric acid were examined over a range of temperature (30–90°C) at various leaching durations. The kinetic data thus obtained were found to follow deviation from the conventional shrinking core model (SCM). A mixed mechanism model was deduced to be the optimum fit to the data with high precision (R² > 0.95) and desired graphical linearity with closer interception to the origin. Aluminosilicate matrix remained unaltered after acid treatment which is the unchanged core concluded as from the kinetic mechanism. Morphological analysis using Scanning Electron Microscope (SEM) and particle size determinations were suggestive of significant reduction in grain size post leaching with organocarboxylic acids, tartaric acid being the most effective of all.