Aluminum Hydroxide Impurities Occlusions and Contamination Sources

This paper is reporting the data concerning impurities occlusion in the dried, milled and classified aluminum hydroxide, the sources of contamination and the ways to control the purity of classified aluminum hydroxide as raw material for special aluminas. Mainly, all the micronic size particles, floating in the super-saturated Bayer liquors, are potential sources of occluded impurities in the aluminum hydroxide particles. There are several mechanisms for embedding the impurities in crystalline substances. Of these, most probable ones in the Bayer alumina process are: a) occlusion of the spent liquor drops containing impurities inside the polycrystalline aluminum hydroxide congregates; b) hetero-nucleation of aluminum hydroxide on the surface of particles or colloids containing one or more impurities (the foreign particles are seized inside a crystals or inside of a crystalline multi-particulate association); c) incorporation of available ions or molecule reactive fragments in the poor crystalline structures of aluminum hydroxide after nucleation, during different growth stages of all already aggregated particles, under certain super-saturations. d) building up bridges between the scanty aggregated particles or filling the inside hollows of these aggregates with new quickly crystallized material, including the particulate impurities, mainly, during large fluctuations of the super-saturation. Using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy EDS (Apollo SSD detector, EDAX), the contributions of each of these mechanisms can be investigated simply and assumed from the collected data. It was shown that well crystallized phases originating directly from bauxite (like the aluminum substituted goethite and substituted hematite, rutile or quartz), as well as the well as the crystallized new born phases during specific Bayer reactions (like cancrinite, are not promoting directly the impurities occlusion. Poor crystalline phases (like sodalite and katoite or other secondary phases and their micronic size fragments are really sustaining impurities occlusion through all the acknowledged mechanisms.