The influence of organic additives on the crystallization and agglomeration of gibbsite

During the crystallization of gibbsite Al(OH)₃, which leads to the synthesis of alumina in the Bayer process, crystals go through a step of agglomeration. In this work, we present a study concerning the influence of different organic compounds, polycarboxylic acids and polyalcohols on the crystallization and agglomeration of gibbsite crystals. It has been determined that they act as crystal habit modifiers and inhibitors of the agglomeration occurring during the formation of alumina crystallites. Simply by following the kinetics of the reaction, it has been observed that polyalcohols are stronger inhibitors than polycarboxylic acids, which can be linked to the structure of gibbsite crystals. The influence of the characteristics of the additive, such as the number of carboxylic groups and the distance between them for polyacids, the stereochemistry and the length of the carbon chain in the case of polyalcohols, is demonstrated to have a significant effect.     

Modelling of multiple-mechanism agglomeration in a crystallization process

A three-level agglomeration model coupled with crystal growth is developed. It accounts for Brownian, laminar, and turbulent agglomeration. The desupersaturation profiles, the particle size distributions, the average sizes, and variances (or standard deviations), as well as the instantaneous agglomeration degrees for each mechanism, can be calculated as functions of time. The model is applied to the crystallization of an amorphous solid into a crystalline polymorph in a batch crystallizer. A runaway phenomenon is detected for agglomeration when crystals are switching over from the Brownian regime to the laminar one: this switchover significantly affects the desupersaturation curve and the crystal shapes.