Influence of oxalic acid on the kinetics of Al(OH) 3 growth from caustic soda solutions

Abstract

Crystallization of Al(OH) 3 from caustic soda solutions is an important part of the Bayer process for alumina production. Nucleation, agglomeration and crystal growth processes occur simultaneously during crystallization of Al(OH) 3 and have different effects on the final product. In this study, we have investigated the influence of oxalic acid on the kinetics of Al(OH) 3 crystal growth. The results have shown that the overall growth rate of Al(OH) 3 crystals increased with increase of temperature of crystallization and decreased with caustic soda concentration, both in the presence and absence of oxalic acid. However, the presence of oxalic acid reduces the overall growth rate of Al(OH) 3 compared to the overall crystal growth rate from the pure caustic soda solutions, and the reduction is greater as the concentration of oxalic acid increased. In other words, the oxalic acid has the inhibitory influence on Al(OH) 3 crystal growth. The effectiveness of the oxalic acid on the Al(OH) 3 growth can be represented through a modified Langmuir adsorption isotherm, and adsorption of admixture on the surface of growing Al(OH) 3 crystals is favoured with a decrease in temperature and with an increase in oxalic acid concentration. Relatively high crystal growth rates (between 8 and 22 μm/h) result from an agglomeration process that could not be prevented because of the presence of small seed Al(OH) 3 particles and the newly formed Al(OH) 3 particles through the nucleation process. Investigation of agglomeration and nucleation processes has shown that agglomeration increased with increasing temperature and decreasing caustic soda concentration, while the nucleation process can be increased with decreasing temperature and caustic soda concentration.