Pyrolysis characteristics and stage division of red mud waste from the alumina refining process for cyclic utilization

Abstract

• Three stages are established of Weiqiao red mud pyrolysis. • Divided stages are verified by FTIR and previous studies. • Corresponding reactions at the three stages are proposed. • Reaction mechanisms are explored by model-fitting and model-free method. Red mud is a kind of strongly alkaline industrial waste from the alumina refining process. A large quantity of red mud not only occupies land resources, but also pollutes the groundwater and land. In order to dispose this waste by thermal conversion process, it is crucial to understand its pyrolysis characteristics and kinetics. Based on thermogravimetric and Fourier transform infrared experiments, three main stages of red mud pyrolysis were put forward: (1) the dehydroxylation of gibbsite ( Al(OH) 3 ) into boehmite ( AlOOH ), (2) the decomposition of boehmite and (3) the decomposition of calcite ( CaCO 3 ). Furthermore, the reaction mechanisms corresponding to the three stages were estimated by coupling the model fitting method and model-free method: g(α) = ((1−α) −1/3 −1) 2 , g(α) = (−ln(1-α)) 3 and g(α) = α + (1−α) ln(1−α) , respectively. Finally, the current kinetic parameters in the three stages were validated by the comparison of previous studies. The obtained individual pyrolysis characteristics of red mud would be of great reference value for co-pyrolysis or co-combustion for red mud-based mixture.