An in situ synchrotron X-ray diffraction investigation of lepidocrocite and ferrihydrite-seeded Al(OH)3 crystallisation from supersaturated sodium aluminate liquor

Lepidocrocite and ferrihydrite-seeded Al(OH)3 crystallisation from supersaturated sodium aluminate liquor at 70 °C was investigated using in situ synchrotron X-ray diffraction. The presence of iron oxides and oxyhydroxides in the Bayer process has implications for the nucleation and growth of scale on process equipment, and a greater understanding of the effect they have on Al(OH)3 crystallisation may allow for development of methods for Al(OH)3 scale prevention. The early stages of both crystallisation reactions were characterised by nucleation of gibbsite on the seed material. This was followed by a rapid increase in gibbsite concentration, which coincided with the appearance of the bayerite and nordstrandite polymorphs of Al(OH)3. The lepidocrocite-seeded reaction then proceeded via a mechanism similar to that which has been observed previously for goethite, hematite and magnetite-seeded Al(OH)3 crystallisation. Different behaviour was observed in the ferrihydrite-seeded experiment, with nucleation as well as growth occurring during the period of rapid increase in gibbsite concentration, followed by a period of diffusion controlled growth.

► Seeded Al(OH)3 crystallisation investigated using in situ X-ray diffraction.
► Three Al(OH)3 polymorphs crystallised from the liquor.
► Lepidocrocite-seeded crystallisation proceeded via a constant mechanism.
► Three-stage mechanism for ferrihydrite-seeded crystallisation proposed.