Studies on hindered settling and related topics

Davies, L

Abstract

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.

Citation

Davies, L. Studies on hindered settling and related topics. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Sep 8, 2021
• Publicly Available Date: Sep 8, 2021
• Publisher URL: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.453138
• Related Public URLs: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.453138
• Award Date: Jan 1, 1977