Grinding effect on kaolinite-pyrophyllite-illite natural mixtures and its influence on mullite formation

Abstract

The effect of dry grinding (mechanochemical activation) on a natural mixture of aluminium silicates composed of kaolinite, pyrophyllite and potassium mica (illite) was studied. The evolution of the ground material was followed by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, scanning electron microscopy and nitrogen adsorption methods. Grinding produces a strong structural alteration of the silicates that constitute the mixture, increasing the surface area and decreasing particle size, with progressive amorphization and formation of agglomerates as detected by X-ray diffraction and scanning electron microscopy. Marked changes were observed in the thermogravimetric analysis and differential thermal analysis curves as influenced by grinding, with weight losses at lower temperatures and the appearance of an exothermic differential thermal analysis effect at 985‡C, which increases in intensity and sharpness as grinding time proceeds. After grinding using the pure silicates, kaolinite, pyrophyllite and illite, it was found that the increase in intensity and sharpness of this exothermic effect was only associated with the presence of pyrophyllite in the natural mixture irrespective of the presence of kaolinite and/or illite, at least up to 325 min grinding. This effect is produced in the ground mixture by mullite formation mainly from pyrophyllite by combined mechanical (by grinding) and thermal treatments.