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Keywords:

  • reactive silica;
  • kinetic modeling;
  • Bayer process;
  • stabilization;
  • dissolution;
  • precipitation

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)math image 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