SODIUM GLUCONATE POISONING OF PRIMARY AND SECONDARY NUCLEATION OF GIBBSITE FROM SYNTHETIC BAYER LIQUORS

Nucleation of gibbsite from synthetic Bayer liquor has been studied in the presence and absence of sodium gluconate as a model precipitation poison. Both primary and secondary nucleation mechanisms were examined. The mechanisms of primary nucleation were inferred from induction period measurements obtained using multi-angle laser light scattering (MALLS). The advantage of MALLS is its ability to detect changes in clear, unseeded, supersaturated sodium aluminate solutions earlier than conventional induction period measurement techniques. The secondary nucleation experiments examined the relationship of seed surface area and sodium gluconate dose to the induction period and precipitation rate.

From the unseeded experiments and classical theory, the interfacial tension and critical nucleus size were determined, and shown to be independent of the presence of the poison. These experiments showed that both the homogeneous and heterogenous mechanisms operate within the supersaturation range studied. Addition of poison increased the supersaturation range within which homogenous nucleation was the dominant mechanism. Samples were removed from unseeded experiments and examined using in-lens field emission scanning electron microscopy (IFESEM).

The seeded experiments suggest that the growth rate suppression from the addition of sodium gluconate poison is a surface process, and that it results from a reduction in the available surface area and not from a change in overall surface activity. The mechanism of poisoning was inferred from micrographs collected by scanning electron microscopy (SEM), and by using measurements of the growth rate and induction period for both poisoned and unpoisoned gibbsite precipitations.

The results from both the seeded and unseeded experiments are used to demonstrate that the secondary nucleation of gibbsite is from the surface of the seed and not the adjacent solution.