Low temperature rehydration of thermally dehydroxylated Bayer–gibbsite, evolution and transformation of phases

Abstract

Rehydration processes in water vapor medium of thermally decomposed Bayer precipitated aluminum hydroxides were investigated. Three different thermal treatment methods were applied with variant temperatures and retention times resulting different composition (amount of remaining gibbsite, evolved boehmite, amount and character of amorphous phase), and surface properties. Rehydrated products were analyzed by X-ray diffractometry, thermal analysis, N2 sorption analysis and scanning electron microscopy. Sample underwent the highest mass loss during thermal treatment has shown the highest water uptake. Significant transformation was shown by oxide-like amorphous phases, yielding hydrated amorphous phase, from which crystalline hydroxide phases emerged, under the given circumstances bayerite and nordstrandite. Partial amorphisation of remaining gibbsite was experienced in early stage of rehydration, which suggests the dissolution of gibbsite by adsorbed water and subsequent recrystallization of it. Initial boehmite content of samples did not transform into other phases.