SEDIMENTATION OF COARSE PARTICLES

Coarse particles have a much lower specific surface, and consequently surface forces and electrical interactions between particles are of very much less significance than in the fine particle systems considered in the previous sections. Flocculation will be absent and, generally, the particles will not influence the rheology of the liquid. The dividing point between coarse and fine particles is somewhat arbitrary although is of the order of 0.1 mm (100 pm). 

There has been considerable discussion in the literature as to whether the buoyancy force acting on a particle in a suspension is determined by the density of the liquid or by that of the suspension. For the sedimentation of a coarse particle in a suspension of much finer particles, the suspension behaves effectively as a continuum. As the large particle settles, it displaces an equal volume of suspension in which there will be comparatively little relative movement between the fine particles and the liquid. The effective buoyancy may, in these circumstances, be attributed to a fluid of the same density as 

In a sedimenting suspension, however, the gravitational force on any individual particle is balanced by a combination of buoyancy and fluid friction forces, both of which are influenced by the flow field and pressure distribution in the vicinity of the particle. It is a somewhat academic exercise to apportion the total force between its two constituent parts, both of which are influenced by the presence of neighbouring particles, the concentration of which will determine the magnitude of their effect. 

Several experimental and analytical studies have been made of the sedimentation of uniform particles. The following approach is a slight modification of that originally used 

For a single spherical particle settling at its terminal velocity hq, a force balance gives  

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Aquatic sediments are formed in all surface waters by the settling of coarse and fine inorganic and organic particles. They are present in rivers, in lakes and in the oceans, and radionuclides deposited on the surface of the earth will sooner or later come into contact with these sediments. They may enter the sediments by sorption of molecularly-dispersed species (ions, molecules), by precipitation or coprecipitation, by coagulation of colloids (in particular carrier colloids) followed by sedimentation of the particles formed, or by sedimentation of coarse particles (suspended matter). By desorption, the radionuclides may be remobilized and released again into the water. 

Submarine pipelines are widely used in the oU and gas industry. For slurries, difficult accesses with possible sedimentation of coarse particles tend to be discouraging. Submarine slurry pipelines are typically limited to nonsetfllng slurries for tailings disposal, or to relatively short distances. 

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