Aggregate Characteristics and Their Effect on Membrane Filtration

6 Aggregate Characteristics and Their Effect on Membrane Filtration 

Indeed, the mode size of 8 - 10 [Xm (data is shown in Chapter 4) attained is in the same size range as that estimated for the Kolmogoroff microscale in the peak energy dissipation region (within the vicinit of the radial jet) in the stirred cylindrical membrane filtration chamber (Schafer et al. (1997), Dong et al. (1994), Ciofalo et al (1996)). These estimations are explained in detail in Appendix 2. 

Given that neither a pore blocking mechanism nor a reduced cake thickness due to aggregate back-transport can account for the obsen ed differences in flux for cakes formed from rapidly-formed compared to slowly-formed aggregates, we must conclude that the differences in permeation velocity arise from differences in permeability of the cakes formed under the different aggregation regimes. The order of magnitude difference in specific resistances of cakes developed at 20 mM and 100 mM KCl concentrations (approx. 1 10 tti.g versus approx. 0.1 10 respectively) supports the 

It is clear from the results of calculations presented in Table 6.7 that both aggregate size and structure have a substantial effect on ease of fluid flow through the (assumed) ftactal aggregates making up the cake. Aggregates characterised by Case 4A (r = 0.7 Jim and D=2.4 for slowly formed aggregates and r =3.5 J.m and D=2.2 for rapidly, formed aggregates) are considered most similar to the hematite 

While compaction effects may act to minimise some of the differences, it appears Likely that the differences in size and fractal properties are induced by the kinetics of aggregation of hematite particles in suspension. Slowly formed aggregates possess a cohort of sub-micron sized aggregates of relatively high fractal dimension which form a reasonably impermeable cake while rapidly formed a regates are generally of larger size and lower fractal dimension and thus create a substantially more permeable cake. 

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