Bridging flocculation studied by light scattering and settling

Polymers are used increasingly in solid-liquid separation processes. Bridgi ng flocculation is the most common particle aggregation mechanism in these processes. However, little is known about the structure of the aggregates f ormed. This paper presents a critical comparison of two of the techniques t hat can be very useful tools for the characterisation of aggregate structur es, i.e. static light scattering and settling. Of particular interest was t heir applicability for bridging flocculated aggregates. Both techniques wer e tested on two model systems: salt-induced fast coagulation and polymer-in duced bridging flocculation of colloidal alumina particles. For diffusion-l imited cluster-cluster aggregation at a high salt concentration, aggregate mass fractal dimensions of 1.75 and 1.65 were obtained from the light scatt ering and settling experiments, respectively. For bridging flocculation whe reby flocs were formed using dual polymers, light scattering and settling g ave mass fractal dimensions of 2.12 and 1.85, respectively. It was conclude d that each of these techniques has certain advantages and disadvantages, t herefore, it is best to view them as complementary. The settling method may be better suited for studying aggregates in bridging flocculation where fl oc sizes can be quite large which may cause the light scattering technique to become inapplicable. (C) 2000 Elsevier Science B.V. All rights reserved.