Sedimentation Kinetics of Flocculated Suspensions I: Initial Sedimentation Region

Depending on the concentration of a suspension, it will exhibit one of three sedimentation patterns pertaining to low, intermediate, and high concentrations of solids, respectively. Of these, only the dilute region has been thoroughly investigated in the past from a theoretical and experimental standpoint. Pharmaceutical suspensions are mostly of the intermediate type. It is shown here that if the models by Kynch, Michaels, and Bolger pertain, and if the sedimented cake experiences an exponential compaction from the onset, experimental data are consistent with theory and lead to the relation:x =x0 exp [-kt] +C.[1 - exp (-kt)]. exp [-wt], where k is a sedimentation rate constant pertaining to a constant density plug, and w is a sedimentation rate constant pertaining to the cake. It has also been found that, empirically, the sedimentation heights in the initial stage may be presented by the relation: [x02 -x2] = βt, where x denotes height, and t time.