Thermodynamics and kinetics of alumina extraction from fly ash using an ammonium hydrogen sulfate roasting method

A novel method was developed for extracting alumina (Al2O3) from fly ash using an ammonium hydrogen sulfate (NH4HSO4) roasting process, and the thermodynamics and kinetics of this method were investigated. The thermodynamic results were verified experimentally. Thermodynamic calculations show that mullite present in the fly ash can react with NH4HSO4 in the 298-723 K range. Process optimization reveals that the extraction rate can reach up to 90.95% when the fly ash reacts with NH4HSO4 at a 1:8 mole ratio of Al2O3/NH4HSO4 at 673 K for 60 min. Kinetic analysis indicates that the NH4HSO4 roasting process follows the shrinking unreacted core model, and inner diffusion through the product layer is the rate-controlling step. The activation energy is calculated to be 16.627 kJ/mol; and the kinetic equation can be expressed as 1 - (2/3)alpha - (1 - alpha)(2/3) = 0.0374t exp[-16627/(RT)], where alpha is the extraction rate and t is the roasting temperature.