Cation effects during aggregation and agglomeration of gibbsite particles under synthetic Bayer crystallisation conditions

Rheological methods have been used to study the influence of the liquor cat ion (sodium versus potassium) on the time-dependent gibbsite particle inter actions that occur during Bayer process crystallisation. The temperature, s upersaturation and seeding levels investigated simulate those experienced i n industrial crystallisers. Gibbsite agglomeration was shown to occur by re versible aggregation followed by irreversible cementation. These two sub-st eps were individually characterised by careful choice of seed surface area and liquor supersaturation during batch crystallisation. At seed loading le vels less than 10% w/w aggregates are rapidly cemented into agglomerates, t his is more pronounced in sodium than potassium-based liquors. These suspen sions were Newtonian and the extent of agglomeration correlated with their viscosity. At seed loading levels greater than 20% w/w particle aggregation resulted in extensively time-dependent and non-Newtonian rheology. However , the aggregates did not undergo cementation into agglomerates and no irrev ersible size enlargement was evident. Yield stress development with time wa s used to probe the kinetics of aggregation and quantify the particle inter action behaviour. The rate and extent of the particle network formation is more pronounced in sodium rather than potassium-based liquors, supersaturat ion dependent, alkali concentration dependent, but only weakly temperature dependent. These findings are discussed with respect to the chemical and ph ysical mechanisms of agglomeration in Bayer crystallisation and the role of cation. (C) 2000 Elsevier Science B.V. All rights reserved.