Concentration variation of aluminate ions during the seeded precipitation process of gibbsite from sodium aluminate solution

The changes of aluminate ions in the Bayer seeded precipitation process were analyzed by the semi-quantitative FTIR analytical method and the ion structure of attached-liquid on the seed surface was determined using ATR-FTIR in order to investigate the gibbsite precipitation mechanism from sodium aluminate solution. The results show that the concentration of tetrahedral aluminate ions corresponding to the bands at 720 cm−1 and 880 cm−1 declines remarkably as the precipitation process proceeds, while the concentration of Al2O(OH)62− corresponding to the band at about 550 cm−1 increases slightly, suggesting that the gibbsite precipitation from sodium aluminate solution was mainly caused by the tetrahedral aluminate ions. The concentration of tetrahedral aluminate ions in the spent liquor declines with time, resulting in a reduction of the driving force of cluster formation of Al(OH)4−, which in turn leads to the reduction of the cluster size. Thus the clusters formed by Al(OH)4− could not get to the critical size to transform into Al(OH)3 and consequently dissolve in the solution. So the rate of nucleation and growth is very slow in the late period of seeded precipitation and the content of Al(OH)4− in the attached-liquid on the aluminum trihydroxide surface remains constant.

Research Highlights
► Al–OH tetrahedral aluminate ions are beneficial for gibbsite precipitation.
► The dimeric aluminate species are disadvantageous to gibbsite precipitation.
► The content of Al(OH)4− in the attached-liquid on the Al(OH)3 surface remains constant.