The theory of sediment volumes of compressible, particulate structures

Specific sediment volume or its reciprocal, average volume fraction of solids, plays a significant role in determining the state of particle aggregation in a suspension. It is determined by the properties of the rising sediment (or cake) in a batch setting tube. A close relations exists between the sediment volume and sedimentation characteristics involved in design of thickeners and clarifiers. Qualitatively, the average porosity of a particulate bed is shown to be principally a function of particle size, shape, and state of aggregation. Slurry concentration, mechanical agitation, and vibration also affects the state of the sediment. Assuming that porosity is a function of the buoyed weight of solids (effective pressure) and the initial porosity of unstressed sediment, formulas are developed which give the average specific sediment volume as a function of height and bed compressibility. For highly compressible materials, the value of the specific sediment volume changes rapidly with height. The limiting value of the specific sediment volume for a cake with differential thickness or the value of the volume fraction of solids epsilon/sub s0/ for an unstressed bed is probably the best assessing the state of aggregation of a suspension. Two experimental methods are presented for its determination. Inmore » the first method, the volume of particulates is plotted as a function of thickness; and the limiting slope for a cake of zero thickness is obtained. In the second method, the initial rates of fall and rise, respectively, of the upper particle-supernatant boundary and the sediment are experimentally measured; and epsilon/sub s0/ is obtained through a material balance. 31 references, 12 figures, 3 tables.« less

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