The development of a flow-sedimentation model for the laminar pipeline transport of slowly-settling concentrated slurries

The flow and sedimentation processes of a slowly-settling slurry flowing horizontally through a pipeline in the laminar regime are studied. The processes are investigated by developing a flow-sedimentation model and testing it using experimental data gathered from a scaled-down pipeline apparatus.; The model requires three properties of the suspension: the rate at which the solid particles settle in a static sedimentation column, the concentration of the sediment that forms when the suspension settles, and the rheology of the feed slurry. These properties are modelled using a permeability approach, a compressibility approach, and the Herschel-Bulkley rheological equation, respectively. To account for the influence of shear on the static rate of settling of the solid particles, a modified rate of settling is predicted using a solids dispersive pressure term proportional to the shear rate of the suspension at the lower pipe wall.; The parameters required in the model were obtained for a flocculated red mud suspension from static sedimentation tests, viscometric measurements, and initial pipeline flow data. The scaled-down pipeline was 28.6 m long and 1 cm in diameter.; Based on comparisons of the predicted and observed volumetric flow rate profiles, and measurements of the exiting mud concentration, from constant overall pressure gradient experiments, the blockage process took place in two stages. First, a sediment formed on the lower pipe wall and the flow rate decreased gradually as the area available for flow became progressively smaller. The mechanisms involved in the second stage of the blockage process are unclear. The final blockage process resulted in a plug being formed which occupied the full cross-section of the pipe.; Extension of the flow-sedimentation model enabled a deposit velocity criterion to be derived and a deposit velocity versus pipe diameter plot to be constructed for the 50 weight percent solids slurry. The deposit velocity was predicted to be proportional to the pipe diameter raised to the power 2.3.; It is recommended that flow experiments be performed on larger diameter pipes using different feed mud concentrations to further test the model and verify the deposit velocity criterion.