The seeded precipitation of gibbsite from caustic aluminate solutions is accomplished by a combination of three crystallisation processes, secondary nucleation, agglomeration and growth. They usually occur together during precipitation: but the contribution of one can be promoted over the others by careful selection of precipitation conditions, so that the effects of variables on each part of the precipitation process can be studied.
In this study, the effects on gibbsite agglomeration of substituting potassium and caesium for sodium in synthetic Bayer liquors were investigated using Hydral 710 seed.
Precipitation data obtained as a function of time from sodium aluminate solutions, supplemented by particle size data and the examination of scanning electron micrographs of gibbsite samples, showed that agglomeration was a rapid process, accompanied by significant gibbsite deposition. It was confirmed that for the selected conditions, agglomeration was the dominant precipitation process. Similar data obtained under identical conditions for samples from potassium and caesium aluminate solutions revealed unexpectedly high levels of secondary nucleation.
The results are discussed in relation to the role of the cation in the precipitation process, and their implications for our understanding of supersaturation and crystallisation driving force.