INTERACTION OF SILICATE WITH GIBBSITE IN SODIUM ALUMINATE SOLUTION

One of the important steps in the production of gibbsite by the Bayer process is precipitation. Impurities retard the precipitation reaction. It is generally postulated that adsorption of impurities on seeded gibbsite crystals is the mechanism of the retardation. To test this postulate, sorption and desorption characteristics of silicate have been studied in sodium aluminate solution saturated with gibbsite under typical Bayer process conditions.; Sorption kinetics indicate silicate adsorbs and precipitates a sodium aluminum silicate phase by a three-step reaction scheme which is consistent with heterogeneous nucleation and crystal growth. The extent of sorption depends on silicate concentration, suspension concentration and temperature. Silicate increases the equilibrium solubility of gibbsite in sodium aluminate solution.; Sorption isotherm data corroborate the proposed reaction mechanism by revealing that lateral interaction among adsorbed silicate ions and cluster formation prevail at the onset of precipitation.; The extent of desorption depends on silicate concentration, temperature, the physical state of the reaction product and the number of successive extractions.