SOLID LIQUID SEPARATION BY SEDIMENTATION

The analysis of batch sedimentation tests performed for the purpose of continuous sedimentation vessel design is described. Conventional des ign techniques employ the concept of settling flux with, under certain conditions, a constitutive equation linking solid concentration and p ressure in a unique and time-independent manner. Experimental studies employing measurement of local concentration and the liquid pressure g radient during sedimentation were used to determine the limits of the conventional design techniques. A 'maximum fluidized concentration' is defined which is the value above which it is difficult to maintain st able flux conditions and is the limit of applicability of conventional design methods based on settling flux. It is shown that the suspended solids contribute towards the liquid pressure gradient and, therefore , to the buoyancy experienced by the settling solids. However, during settlement the liquid pressure gradient reverts to the hydrostatic gra dient alone; hence the buoyancy effect is a function of sedimentation time. Additional considerations also suggest that a unique relation be tween concentration and pressure should only be used as a constituent term in a time-dependent consolidation model. Current research effort includes a suitable method of linking time-dependent consolidation the ory, under conditions of extremely low applied pressure, and sedimenta tion flux analysis.