Effect of shear rate on aggregate size and structure in the process of aggregation and at steady state

The paper deals with the dependence of aggregate properties on the shear rate (G) in the aggregation process and at steady state. Natural raw water and ferric sulphate were aggregated in a Taylor-Couette reactor. The methods of image and fractal analysis were used to determine the aggregate size and structure. It was observed that at the early phase of aggregation, the aggregate growth rate is higher for lower shear rates. At G <= 150 s(-1), the time aggregation curve contains the local maximum before reaching the steady state. Moreover, the different extent of break-up and restructuring was proved for different values of shear rate. At G >= 200 s(-1), the aggregation curve misses the local extreme completely. It was found that with increasing shear rate (G=21.2-347.9 s(-1)), the aggregates are smaller (d= 1504-56 mu m), more compact (D-2=1.54-1.91) and more regular (D-pf= 137-1.10). A relationship for the description of dependence of fractal dimension on the shear rate was also suggested. (C) 2012 Elsevier B.V. All rights reserved.