Ageing of aluminium hydroxide flocs

Abstract

The ageing processes of kaolinite suspensions flocculated with Al 2(SO 4) 3 under a variety of different but well-controlled flocculation conditions were studied by measuring their average diameter, diameter distribution, strength and electrophoretic mobility. Three periods could be distinguished in the ageing process. Initially shrinkage of the flocs is mainly due to cementation-aggregation. In the first 6–8 h, dependent on the flocculation conditions, the average diameter reduces from ⋍500 to ⋍220 μm. During the following 4 days a shrinkage to ⋍180 μm occurs, mainly due to condensation-polymerization and crystallization. In the third period, the average diameter increases because of Oswald ripening of the crystals. During each period the change of the diameter as a function of age can mathematically be described by: d f, i = d∞ + d′· t i + i− ω . With d ∞, d′ and ω constants dependent on the flocculation conditions. During ageing, the floc diameter distribution narrows. The shrinkage of the flocs during ageing is definitely not due to erosion of particles from the floc surface. The change of the electrophoretic mobility during ageing provides information on the thickness and homogeneity of the layer of insoluble hydroxides around the destabilized clay particles. The influence of a wide range of different flocculation process parameters on the floc ageing process is summarized, as is the influence of the ageing process on floc strength and floc density. The evolution of the effective floc density can be described by: P e| t=t i = a·d (3−K p) ƒ,0 d 3 ƒ,i . The constants K p and a are functions of the ratio quantity of suspended solids vs coagulant dose.