Addai-Mensah, J., Ametov, I., Ralston J. and Prestidge, C.A.

AFM colloid probe and rheological studies were performed under conditions which simulate seeded Bayer Process crystallisation. The role of liquor cation, gibbsite crystal face and hydrodynamic conditions on the time-dependent surface forces and particle interactions were determined. Repulsive forces are observed between gibbsite particle surfaces in fresh liquors; these are considered to be both steric and hydrodynamic in origin. The latter is particle approach rate dependent, and may prevail under processing related conditions. Upon aging, the steric repulsion attenuates and a structured gibbsite interface develops with an associated adhesive force. Adhesive forces were greater at gibbsite non-(001) than (001) crystal faces; this reflects face-specific surface chemistry and different propensities for agglomeration. In seeded, synthetic Bayer liquors the repulsive-to-adhesive transition in particle interaction leads to flocculation and formation of a particle network, with the development of associated non-Newtonian flow, yield stresses and viscoelasticity. The onset of adhesive forces, yield stresses and elastic moduli occurs faster in sodium than potassium based liquors, which concurs with faster agglomeration kinetics in sodium based liquors. These findings together show that gibbsite particle interactions are controlled by the structure of the interfacial layer and influenced by whether sodium or potassium ions are present. Flocculation is a precursor to cementation and may be rate determining in gibbsite agglomeration. A more complete understanding of the gibbsite crystal growth and agglomeration mechanisms has resulted.