Mechanisms of Gibbsite Crystallization from Partially Neutralized Aluminum Chloride Solutions

Abstract

Abstract--To interpret the erratic conditions, rates, and extent of gibbsite crystallization from partially neutralized A1C13 solution, the following hypothesis is proposed: The initial OH-AI polymers in the freshly prepared solutions were probably unstable and transformed into either gibbsite or stable OH-AI polymers via two different reaction paths. In the presence of nuclei, the OH-A1 polymers dissociated into monomeric ions, which then deposited onto nuclei to form gibbsite. In the absence of nuclei, the unstable polymers slowly converted to stable polymers. The erratic stability of OH-A1 solutions and gibbsite crystallization are therefore attributed to the relative magnitudes of these two reaction paths which, in turn, are attributed to two key factors: (1) the distribution of unstable vs. stable OH-A1 polymers; and (2) the presence or absence of nuclei. The duration of aging of the parent solution governed the distribution of unstable vs. stable polymers. The rate of neutralization resulted in varying localized high alkalinity in OH-AI solution preparation and thus varying development of nuclei.

Evidence and importance of intermediate nanostructures in the journey from molecular precursors to allophane and imogolite nanocrystals

“…At slow NaOH injection rate, the condensation of Al is slowly driven by injection of HO − in a continuous and progressive way. At large injection rate, concentration gradient effects have led initially to a local over-condensation of Al polycations [36,37]. As soon as NaOH injection stops, the large obtained polycations are restructured and their average size decreases.…”