The influence of sedimentation during downward cake filtration

Data for near incompressible cake formations with simultaneous settling are presented. Aqueous calcite suspensions exhibiting similar median particle size, but different size distributions, were filtered over a range of constant pressures. For each experiment the time dependent history of filtrate removal and the particle size distributions of cake samples at different spatial positions were measured. These data were compared with predictions from a new mathematical model that divides cake formation into a range of discrete time steps. Cake growth due to filtration and sedimentation were considered to proceed simultaneously, but separately, with the additive results predicting the change in cake thickness during a time step. Account was taken of the changing effects of suspension concentration on settling rate and the transient influence of size distribution on specific cake resistance. The model is shown to quantitatively predict the influence of feed particle size distribution on cake formation and filtrate removal rates and favourable comparisons are made with values recorded in experiments. For the experimental conditions investigated, sedimentation is shown to contribute up to one third of the cake resistance in a filtration test. At lower pressures and with wider size distributions, larger particles from the feed tended to accumulate near the filter medium and in some cases a minimum cake resistance was observed toward a mean cake height. For higher pressures, however, the effect of particle