Slide Background
Slide Background


Grocott, S.C. and Rosenberg, S P.

A mechanism is proposed to explain the incorporation of sodium into aluminium hydroxide during the hydrate precipitation stage of the Bayer process. The proposed mechanism also explains the effect of variables such as temperature and aluminate supersaturation.

The mechanism for soda incorporation into the hydrate crystal structure involves the trapping of sodium ions in a growing layer of gibbsite. These sodium ions do not come directly from Bayer liquor. Rather, they result from the polymerisation of sodium aluminate that occurs during the hydrate precipitation reaction. If the sodium ions are not allowed sufficient time to diffuse away from the growing hydrate surface, they can be incorporated into the hydrate crystal structure, thereby giving rise to soda in alumina.

The proposed sodium incorporation mechanism also provides an understanding of how process liquor impurities can affect soda levels in hydrate. Compounds sharing the common characteristic of relatively acidic, adjacent hydroxyl groups were found to significantly increase soda concentrations in hydrate. These types of impurities are found in Bayer liquors.