Kinetics of kaolinite dissolution and hydrosodalite precipitation during alkali leaching of diasporic bauxite

Abstract

Alkali leaching is an effective desilication method for improving the alumina-silica mass ratio (A/S) of bauxite. The paper aims to study the kinetics of kaolinite dissolution and hydrosodalite precipitation during alkali leaching of kaolinite-rich diasporic bauxite. Alkali leaching of bauxite in NaOH solutions was studied at Na₂O concentrations of 200–260 g/L, temperatures of 90–105 °C, and times of 30–150 min. The leached bauxites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques. The rate equations describing kaolinite dissolution and hydrosodalite precipitation were derived by fitting the Avrami model. The results show that kaolinite with finer particle sizes was preferentially dissolved during alkali leaching, providing a material basis for the precipitation of hydrosodalite. Hydrosodalite was heterogeneously nucleated on the dissolved edges of some kaolinite and grew in both one- and two-dimensions, presenting acicular and lamellar morphology. The dissolution rate of kaolinite and the crystallization mechanism of hydrosodalite were primarily influenced by Na₂O concentration and temperature. Both kaolinite dissolution and hydrosodalite precipitation were controlled by chemical reactions with activation energies of 70.152 (± 1.429) kJ/mol and 289.089 (± 2.063) kJ/mol, respectively, and the orders of reaction with respect to Na₂O of 0.733 (± 0.070) and 7.165 (± 0.047), respectively. The kinetic equations describing kaolinite dissolution, hydrosodalite precipitation and even the leaching of SiO₂ were eventually modeled with Na₂O concentration, temperature and time as variables.