A new process for Al₂O₃ production from low-grade diasporic bauxite based on reactive silica dissolution and stabilization in NaOH-NaAl(OH)₄ media

Abstract

A new process of Al₂O₃ production from low-grade diasporic bauxite based on the reactive silica dissolution and stabilization in concentrated NaOH-NaAl(OH)₄ solutions is proposed and proved feasible. NaOH and Al₂O₃ concentrations and leaching temperature were found to be the main factors affecting the leaching process of reactive silica. The A/S (mass ratio of Al₂O₃/SiO₂) of diasporic bauxite was enhanced from 5.4 to 15 by reactive silica removal under the optimum operation conditions. Two obvious steps control the whole leaching process of reactive silica in NaOH-NaAl(OH)₄ media: reactive silica dissolution and desilication products (DSPs) precipitation. The kinetics data of two controlling steps fit a shrinking core model based on the calculation of OH⁻ activity with the aid of OLI platform and an empirical kinetic model well, respectively. Apparent activation energies of reactive silica leaching in the temperature range from 80 to 110 °C are 101.91 and 58.65 kJ mol⁻¹ for the two steps, respectively. The stabilization mechanism of reactive silica in concentrated NaOH-NaAl(OH)₄ solution was also elucidated based on the complexation of aluminum-bearing species and the calculation of supersaturation to DSP. It was found that the concentration of OH⁻ sharply decreases due to the formation of Al(OH) species with increasing aluminum concentration, suppressing greatly DSP precipitation. This proposed process paves the way for Al₂O₃ production from low-grade diasporic bauxite with high-reactive silica content. © 2011 American Institute of Chemical Engineers AIChE J, 2012