Studies on hindered settling and related topics

The stability of some suspensions, and the settling-out of others,both natural and man-n~de, are of widespread occurrence andimportance. Nonetheless, in spite of the efforts of numerousworkers, published theories and mathematical representations ofsedimentation behaviour are relatively rudimentary. Some of thesetheories have been developed from model systems which are markedlydifferent from suspensions of practical significance. Othertheories, although based on studies of more-realistic mixtures,nonetheless have failed to consider chemical and physical propertieswhich may be thought to influence sedimentation.This study has been essentially an attempt to improve upon thisposition, and the work has been carried out in a number of ways.The scope and limitations of some existing theories of sedimentationhave been examined, and additional useful interpretation hasbeen proposed. The relationship of sedimentation rates to the concentrationsof the suspensions under consideration has beeninvestigated in some detail for a number of systems, and newmathematical models suggested to represent the observed behaviour.It has been concluded that the relationship of sedimentation rateto suspension-concentration is better represented by a combinationof sigmoid and exponential curves, rather than by the purelyexponential expressions previously used. The experimental evidencehas been used to try to identify chemical and physical propertiesof solids and liquids, which affect sedimentation phenomena. Ithas also been possible to suggest some improvements in the use ofsedimentation data to estimate mean particle sizes for suspended materials, and some consideration has also been given to theusefulness of observations of settled sediment volumes. It hasbeen concluded that there are two fundamental causes of hindranceto sedimentation; one is flocculation of the sedimentingmaterial, and the other is association of liquid round the sedimentingparticles and floes. Evidence is presented that thesecond effect is more important that the first.The suspension concentration necessary for maximum sedimentationmass transfer rate has been identified as the threshold concentrationfor the onset of the phenomenon known as hinderedsettling, in which particles sediment together at one rateirrespective of their sizes. The fact that this threshold concentrationvaries conSiderably from one experimental system toanother has been interpreted in terms of a linear-up flow model ofthe sedimenting mass.

Level dc:type.qualificationlevel

Doctoral (Level 8)

Year dc:date.issued

1977

Author dc:creator

• Davies, L

Language dc:language

en

Identifier

oai:salford-repository.worktribe.com:1336343

OAI identifier oai:identifier

oai:salford-repository.worktribe.com:1336343