Mechanism and kinetics of aluminum and iron leaching from coal fly ash by sulfuric acid

The mechanism and kinetics of leaching of aluminum and iron from Coal Fly Ash (CFA), by sulfuric acid, involves a rate controlling step of mass transfer. It is shown that, in the leaching process, particles follow the shrinking core model with respect to formation of unreacted core that is encapsulated by a leached, porous, layer. Formation of diffusion resistant calcium sulfate precipitates, within the pores and on the surface, induces a self-inhibition effect to mass transfer at the leaching sites. The removal of calcium from the ash, by a preleaching conditioning step, diminishes the self-inhibition effect, and enhances the kinetics of subsequent leaching with sulfuric acid. However, no selective removal of calcium is possible as all metals are dissolved from the same active compounds. The major self-inhibition effect is shown to result from in situ precipitation of calcium sulfate, where dissolved calcium reacts with sulfate ions at the active leaching sites. A self-inhibition model for the description of the leaching kinetics is developed. The model incorporates the buildup of resistance to mass transfer, by diffusion, due to increased pore length, and formation of calcium sulfate barriers as precipitates. The model verifies that the rate of leaching is an exponentially decreasing function of concentration of dissolved aluminum, and consequently the latter follows a logarithmic relation with time. This, experimentally established, logarithmic relation, is characterized by two kinetic parameters. The proportional increase of these parameters with the CFA content was established both experimentally and theoretically. Finally, the model facilitates prediction of the leaching kinetics, in the range of CFA content studied, once it is calibrated with appropriate sample data. (C) 1998 Elsevier Science Ltd. All rights reserved.