The effect of shear on the evolution of floc properties was investigated to analyse the flocculation mechanisms. Little fundamental attention has been given to the shear influence that often creates compact aggregates, while the floc characteristics might differ in other aggregating conditions. It is thus crucial to understand how flocs evolve to steady state, if their properties are to be 'tailored' to suit subsequent solids-liquid separation processes. In this work, flocculation of monodisperse latex particles of various sizes (60, 380, and 810 nm diameter) via electrolyte addition was carried out in a couette-flow and also in shear fields generated by an axial-flow impeller (Fluid foil A310) and a radial-flow impeller (Rushton R100) in standard mixing tanks. A small-angle light scattering technique was used to acquire information regarding the time variation of floc properties in a non-intrusive manner. The structure was quantified by a measure of fractal dimension, signifying the degree of floc compactness. Estimates of the average floc mass were also obtained from the aggregate scattering patterns. By monitoring the changes in floc structure and mass, corresponding to the size evolution; mechanisms of floc formation, fragmentation, and restructuring were identified. Aggregates of 60 and 380 nm particles were observed to grew larger initially, before decreasing to their equilibrium sizes at moderate shear rates (32 - 100 s-1) in a homogeneous shear environment. Floc restructuring at large length scales occurred extensively, and was responsible for the drop in size, particularly at the early stage of the process. Aggregates of 810 nm particles did not, however, display this behaviour. Flocs of larger primary particles were presumably susceptible to breakage rather than deformation, as they were weaker under comparable conditions. Denser aggregates were found when restructuring transpired, while comparatively tenuous flocs were observed when formation and breakage kinetics were the governing mechanisms. The disparity in floc behaviour at higher shear rates (246 s-1 - 330 s-1) was less apparent. The intense hydrodynamic stresses in those instances inevitably caused fragmentation, regardless of the intrinsic particle properties; hence the observed floc compaction was the product of break-up and re-aggregation. A population balance model, incorporating variation in floc structure, displayed comparable trends in size evolution; verifying that restructuring indeed took an important role under certain flocculation conditions. Similar phenomena were likewise observed with the flocculation in stirred tanks. The results reinforced findings in literature; that while circulation time controlled the process kinetics; the floc size was determined by the turbulent stresses. In addition, the maximum shear levels also influenced the floc structures, with denser aggregates produced in a shear field generated using the radial-flow impeller at equivalent energy dissipation per-unit mass. A correlation between non-dimensional floc factor that embodied the aggregate size and structure, and aggregation factor comprising the significant parameters from flocculation conditions, was proposed. The proposed relationship takes into account aspects such as the aggregate structure, interparticle forces, and particle concentration that are often overlooked in existing relationships, which usually only relate the maximum floc size to the applied energy dissipation rate. It thus provides an improved manner of presenting general flocculation data, as well as a means to predict floc properties produced under a specific aggregation condition. Future studies with increasingly complex systems that resemble real conditions are recommended in order to establish a practical understanding of the flocculation mechanisms, for the purpose of optimising the aggregate properties.
| Identifer | oai:union.ndltd.org:ADTP/187785 |
| Date | January 2002 |
| Creators | Selomulya, Cordelia, Chemical Engineering & Industrial Chemistry, UNSW |
| Publisher | Awarded by:University of New South Wales. Chemical Engineering and Industrial Chemistry |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Cordelia Selomulya, http://unsworks.unsw.edu.au/copyright |
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