Mechanism and kinetics of gibbsite-seeded sodium aluminosilicate crystallisation from synthetic spent Bayer liquor

• Aluminosilicate scaling is a serious issue for Bayer refineries.
• Si solubility increases with increased liquor [Al].
• Scaling decreases on gibbsite seed dissolution due to decreased Si supersaturation.
• Desilication occurs predominately by heterogeneous nucleation on gibbsite seeding.
• Increasing spent liquor [Al] in single steam plants may help reduce scaling.

Despite a significant number of studies having been carried aimed at developing strategies for scale inhibition during the Bayer process, particularly in heat exchanger systems, there is still limited understanding of the underlying mechanisms and kinetics of scaling, in particular for single stream plants. The aim of this study was to further this understanding for sodium aluminosilicate crystallisation in spent Bayer liquor seeded with gibbsite. Gibbsite is present in pre-desilicated bauxite which is mixed with spent liquor and then passes through the heat exchanger system in single stream Bayer plant circuits.The solution desilication rate, at 140 °C, was found to increase significantly with increasing seed loading (or surface area) from 45 g L− 1 to 90 g L− 1 even though partial, and in some cases complete, dissolution of the gibbsite seed resulted in increased liquor Si solubility. SEM, particle size distribution and X-ray diffraction analysis of the solid residues demonstrated that heterogeneous nucleation and growth of sodalite on the gibbsite seed results in reduced scaling on steel coupon surfaces as compared to without seeding. These findings suggest that the gibbsite, inherent in pre-desilicated bauxite, may be effective in reducing scale on heat exchanger surfaces in a single stream process environment.

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