Bridging flocculation studied by light scattering and settling

Abstract

Polymers are used increasingly in solid–liquid separation processes. Bridging flocculation is the most common particle aggregation mechanism in these processes. However, little is known about the structure of the aggregates formed. This paper presents a critical comparison of two of the techniques that can be very useful tools for the characterisation of aggregate structures, i.e. static light scattering and settling. Of particular interest was their applicability for bridging flocculated aggregates. Both techniques were tested on two model systems: salt-induced fast coagulation and polymer-induced bridging flocculation of colloidal alumina particles. For diffusion-limited cluster–cluster aggregation at a high salt concentration, aggregate mass fractal dimensions of 1.75 and 1.65 were obtained from the light scattering and settling experiments, respectively. For bridging flocculation whereby flocs were formed using dual polymers, light scattering and settling gave mass fractal dimensions of 2.12 and 1.85, respectively. It was concluded that each of these techniques has certain advantages and disadvantages, therefore, it is best to view them as complementary. The settling method may be better suited for studying aggregates in bridging flocculation where floc sizes can be quite large which may cause the light scattering technique to become inapplicable.