The effect of mechanical and thermal treatments on the dissolution kinetics of kaolinite in alkaline sodium aluminate solution under conditions typical to Bayer desilication

Abstract

The kinetics of dissolution of kaolinite in industrial spent liquor followed by the precipitation of the desilication product was studied by using calcined and dimethyl sulfoxide intercalated Eckalite1 as starting material. These studies were aimed at understanding the influence of the combination of different pre-treatments on the variations in the maximum silica concentrations and shift (or in some cases, the disappearance) of the trigger point in the Bayer desilication process. In addition to the quantitative characterization of the dissolution kinetics, a variety of physico-chemical parameters of the kaolinites, like the specific surface area, Lewis acidity, hydrodynamic diameter, zeta potential, optical band gap and thermogravimetric behaviour was also determined. From the experimental data, it can be concluded that the thermal/mechanochemical dehydroxylation and the formation of metakaolinite/mechanochemically amorphized kaolinite are responsible for the increase in the concentration of the dissolved silica in the supersaturated sodium aluminate solution. The variation in the position of the trigger point could be connected to the deformation, fragmentation/delamination, and amorphization of the kaolinite structure evolved during the mechanochemical pre-treatment. The joined application of pre-calcination and pre-milling verified the conclusions drawn. In addition, the combined treatment can be optimized to end up with a dissolution process that is carried out in a time and energy saving manner. The extent of the desilication products obtained at the end of the dissolution tests showed a considerable dependence on the applied pre-treatments: the formation of the metakaolinite phases was found to facilitate the evolution of the ball of yarn-type lepispherical structures with sizes ranging from 600 to 3000 nm.