Flocculation and consolidation in thickening processes

The use of flocculation and sedimentation processes for thickening of fine-particle suspensions has been investigated. Flocculation can be performed in either continuous or batch systems. In general, information on batch flocculation is more readily available than that on the continuous systems, especially those involving synthetic organic flocculants. A systematic study of the effects of process variables on continuous flocculation was carried out. Settling rate and supernatant turbidity were used as indicators of process effectiveness.;Some experiments on the effects of agitation speed, solids concentration, suspension pH, polymer addition rate and concentration on batch flocculation were performed to provide data comparison and to establish standard procedures for the continuous flocculation studies.;The continuous flocculation tests were conducted using a stirred tank and a Kenics static mixer. The results showed that the effects of flocculation variables were qualitatively similar for the batch and continuous tanks. However, the magnitude was generally larger for the batch system.;For the Kenics static mixer, it was found that increased polymer dosage generally increases settling rate and decreases turbidity; while increased flowrate decreases settling rate and increases turbidity. A model for flocculation in a Kenics static mixer has been developed and shown to be consistent with experimental results.;Residence time distribution measurements were used to study flow behavior in the continuous mixing systems. Flow behavior in a stirred tank follows closely a fully mixed tank reactor, while that in the Kenics static mixer generally follows a large extent of dispersion model.;Continuous sedimentation tests were conducted on the flocculated suspensions using a laboratory-scale column thickener. Both dynamic and steady state conditions were evaluated. A mass-balance steady state, in which the solids flowrates in the feed and the underflow are equal, could be achieved in the continuous column for the experimental conditions used. However, a steady state sediment height was not always achieved even when the mass balance steady state was accomplished. Several process models from the literature were applied to the results. The calculated sediment height at steady state generally is lower than the experimental value for the same underflow solids concentration.