KINETICS OF SILICATE REACTION WITH GIBBSITE

The reaction of silicate with gibbsite in sodium aluminate solution was studied. Sorption kinetics indicated that sorption of silicate was surface-induced. Silicate is believed to have adsorbed and precipitated as sodium aluminum silicate (SAS) on gibbsite by a three-step reaction; a first fast step, a lag period step, and a slow step. The fast and slow steps conformed to first-order rate laws with respect to silicate concentration. The extent of sorption depended on silicate concentration, suspension concentration and temperature. Further, silicate increased the equilibrium solubility of gibbsite in sodium aluminate solution. For a silicate concentration of 500 μgSiO2/cm3, adsorption/precipitation accounted for 86% of the reduction in gibbsite crystallization, while the remaining 14% was due to increased solubility of gibbsite. Thus, our adsorption-precipitation observations confirm the postulate that the reduction in gibbsite crystallization in the Bayer process is due mainly to the adsorption of silicate on gibbsite seed crystals. © 1991 Academic Press, Inc. All rights reserved.