Studies on hindered settling and related topics

Davies, L 1977, Studies on hindered settling and related topics , PhD thesis, University of Salford.

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The stability of some suspensions, and the settling-out of others, both natural and man-n~de, are of widespread occurrence and importance. Nonetheless, in spite of the efforts of numerous workers, published theories and mathematical representations of sedimentation behaviour are relatively rudimentary. Some of these theories have been developed from model systems which are markedly different from suspensions of practical significance. Other theories, although based on studies of more-realistic mixtures, nonetheless have failed to consider chemical and physical properties which may be thought to influence sedimentation. This study has been essentially an attempt to improve upon this position, and the work has been carried out in a number of ways. The scope and limitations of some existing theories of sedimentation have been examined, and additional useful interpretation has been proposed. The relationship of sedimentation rates to the concentrations of the suspensions under consideration has been investigated in some detail for a number of systems, and new mathematical models suggested to represent the observed behaviour. It has been concluded that the relationship of sedimentation rate to suspension-concentration is better represented by a combination of sigmoid and exponential curves, rather than by the purely exponential expressions previously used. The experimental evidence has been used to try to identify chemical and physical properties of solids and liquids, which affect sedimentation phenomena. It has also been possible to suggest some improvements in the use of sedimentation data to estimate mean particle sizes for suspended materials, and some consideration has also been given to the usefulness of observations of settled sediment volumes. It has been concluded that there are two fundamental causes of hindrance to sedimentation; one is flocculation of the sedimenting material, and the other is association of liquid round the sedimenting particles and floes. Evidence is presented that the second effect is more important that the first. The suspension concentration necessary for maximum sedimentation mass transfer rate has been identified as the threshold concentration for the onset of the phenomenon known as hindered settling, in which particles sediment together at one rate irrespective of their sizes. The fact that this threshold concentration varies conSiderably from one experimental system to another has been interpreted in terms of a linear-up flow model of the sedimenting mass.

Item Type: Thesis (PhD)
Contributors: Dollimore, D (Supervisor)
Schools: Schools > School of Computing, Science and Engineering
Depositing User: USIR Admin
Date Deposited: 08 Sep 2021 08:27
Last Modified: 15 Feb 2022 14:46

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