The dissolution kinetics of raw and mechanochemically treated kaolinites in industrial spent liquor - The effect of the physico-chemical properties of the solids

Using industrial spent Bayer-liquor solution and 95 °C temperature, the dissolution and desilication products formation kinetics of five raw kaolinite samples obtained from different sources were found to display profound differences. It was hypothesized that these differences were connected to the physico-chemical properties of the solids. The solids were characterized by powder X-ray diffractometry, N₂ adsorption-desorption isotherms, temperature-programmed NH₃ desorption, infrared and ultraviolet-visible diffuse reflectance spectroscopies, scanning electron microscopy connected with energy dispersive X-ray analysis, dynamic light scattering and thermal analysis. The detailed structural study of the as-received samples was supplemented by exploring the effect of mechanochemical treatment. It was found that beside specific surface area and pore volume, the amount of the Lewis acid sites proved to be the most important factors, and the picture was found to be modulated by the degree of aggregation of the primary particles. On mechanochemical treatment (60 min milling in a mixer mill), the number of the acid sites increased significantly (from 0.004-0.013 to 0.055-0.066 mmol/g), and largely as a result of this, the dissolution of all raw kaolinites became practically instantaneous and source-independent.