Applicability of Kynch Theory to Flocculent Suspensions

The performance of gravity thickeners is often analyzed using the batch sedimentation theory of Kynch. Because initial settling velocity data are most commonly used for this purpose, the accuracy with which these data predict the settleability of a suspension as it is formed during thickening was analyzed. Characteristic curves that define the ideal batch-thickening behavior were developed to facilitate the analysis. Results obtained with a flocculent calcium carbonate suspension showed that the initial settling velocity data were found to adequately predict the batch-thickening behavior for a limited concentration region. The greatest discrepancies between predicted and observed thickening behavior occurred at concentration regions in which: (1)The propagation velocities of concentration changes were high, or (2)the suspension exhibited compressibility. In both concentration regions, the settling rates of the suspension in the thickening mass were lower than the corresponding initial settling rates. The results suggested that the reason rapid concentration propagation retarded the settleability was not the inertial forces created but the accompanying changes in aggregate properties.