THE INFLUENCE OF SOLID PHASE OXALATE (SPO) ON GIBBSITE SECONDARY NUCLEATION IN SYNTHETIC CAUSTIC-ALUMINATE SOLUTION

Gibbsite secondary nucleation represents a dominant mechanism for generating new crystals in the commercial Bayer precipitation process. The presence of sodium oxalate crystals in plant gibbsite precipitation circuits can lead to increased gibbsite secondary nucleation rates resulting in the generation of an excessive amount of fine crystals that adversely affect product quality and process stability, and prolong classification settling times.

This work reports on experimental findings from a range of precipitation experiments carried out in synthetic caustic-aluminate solutions to determine sodium oxalate solubility, critical oxalate concentration, effect of solid phase oxalate (SPO) on gibbsite precipitation, and critical conditions under which SPO induced gibbsite secondary nucleation occurs.

Sodium oxalate solubility in synthetic liquor rose as temperature rose and fell as caustic and carbonate concentrations rose. Neither gibbsite seed concentration nor alumina solution concentration showed a significant effect on oxalate solubility. Critical oxalate concentration in synthetic liquor was strongly affected by the presence of gibbsite seed, which effectively narrows the oxalate precipitation meta-stability zone.

Both stirred tank and bottle roller gibbsite precipitation experiments showed that needle-like SPO seed, with high specific surface area, strongly affects gibbsite precipitation, leading to a significantly higher total number of gibbsite particles and smaller particle mean size.

The measured critical alumina supersaturation level, required for SPO to induce gibbsite precipitation in synthetic liquors, decreased with increasing temperature and increasing gibbsite seed loading. This suggests that in Bayer precipitation circuits SPO induced gibbsite precipitation is most likely to occur at the front end of the circuit, in the high temperature precipitators.