The Sedimentation Process in Greater Detail

As particles settle, forming a thickened zone, the void fraction, e, decreases. At the total settling of the slurry, the void fraction is at a minimum, its value depending on the shape of the particles. For example, the minimum void fraction of spherical particles is e, in = 0.2 15 for small crystals, = 0.4. For most systems, the void fraction of a thickened sludge is approximately = 0.6 however, values should be determined experimentally for the specific system. 

For a unit volume of slurry, its weight, y. is the sum of the weights of the solid particles, Yp(I - ), and of the liquid, Y( , where Yf = specific weight of liquid and Yp = specific weight of particles  

This expression can be used to compute the void fraction from experimentally determined values of the specific weights. 

The resistance to liquid flow aroimd particles may be presented by an equation similar to the viscosity equation but with considering the void fraction. Recall that the shear stress is expressed by the ratio of the drag force, R, to the active surface, K27td. The total sphere surface is Ttd and Kj is the coefficient accoimting for that part of the surface responsible for resistance. Considering the influence of void fraction as a function I 2( ). we obtain  

For very dilute suspensions, in which the void fraction does not influence the 

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