Filtration of flocculated suspensions

Thin, compact filter cakes tend to result from the filtration of stable, well-dispersed particles. The permeabihty of compact filter cakes is quite low, so the rate of the filtration process tends to be self-slowing in this case. In contrast, thick, porous filter cakes tend to result from the filtration of flocculated suspensions. 

Garrido R, Concha F. and Burger R., 2003. Settling velocities of particulate systems 14. Unified model of sedimentation, centrifugation and filtration of flocculated suspensions, Int. J. Min. Proc., 72, 57-74. 

Filtration of flocculated suspensions can be assessed using the filterability test method described in chapter 11. Using the filterabihty number F against the product Gt an optimum flocculation can be determined for filtration through a sample of porous granular material as exemplified in Figure 4.16. 

Smellie RH, La Mer VK. Flocculation, subsidence and filtration phosphate slimes VI a quantitative theory of filtration of flocculated suspensions. J Colloid Sci 1958 13 589-599. 

Landman K.A., White L.R., Eberl M. Pressure filtration of flocculated suspensions. AIChE J. 1995 ... 

Landman K.A., Sirakoff C. and White L.R., 1991. Dewatering of flocculated suspensions by pressure filtration, Phys. Fluids A, 3(6), 1495-1509. 

Kos, P., Theory of gravity thickening of flocculant suspensions and a new method of thickener sizing . Proceedings of the Second World Filtration Congress 1979, Olympia, London, Sept. 18-20, Filtration Society, 595-603 (1979)... 

Precipitation and accumulation of clay. Precipitation of clay particles takes place at some depth in the soil as a result of 1. flocculation of clay particles, or 2. (mechanical) filtration of clay in suspension by fine capillary pores. 

The re-filtration technique of La Mer (3) involves filtering the flocculated suspension and then passing the filtrate once more through this pre-formed filter cake. 

The procedure used to obtain the results in Figure 3 is not entirely satisfactory, since transfer of the flocculated suspension to the syringe inevitably causes some break-up of floes which may affect the filtration behaviour. In order to investigate this point, two different procedures were adopted ... 

The method described here provides a convenient means of determining the specific filtration resistance of fairly dilute suspensions. Results for clay suspensions flocculated by cationic polymers show that the specific resistance gives a sensitive indication of flocculation and is a useful guide in the selection of optimum flocculant concentrations. In a series of trials not reported here, it has been shown that the specific resistance results are very well matched by re-filtration rate data, as expected. The results also agree well with other, unrelated techniques. For more concentrated suspensions, some discrepancies have been found between permeability methods and other measures of flocculation (4). 

A factor of very great importance in the handling of colloidal dispersions is the structure of floes and sediments. Thus, in the filtration of a flocculated suspension it is necessary to have as open a structure as possible so that liquid can flow through the filter cake. 

In clarifiers based on separation of suspended particulates by the cloud of floccules, the combined effect of sedimentation and filtration through a fluid layer formed by hydroxide floccules is utilized. Suspended solids are removed from such water in the form of sludge, either from the space occupied by the cloud of floccules, or from the clarifier bottom. The water flow in such a clarifier is vertical. Several processes take place in the clarifier at the same time formation of suspensions by coagulation, trapping of suspension in the cloud of floccules, separation of floccules from clarified water, condensation and removal of the excess of floccules. Figure 3.55 illustrates a scheme of a clarifier with imperfect flotation. Flotation of the cloud of floccules is mechanical. 

Oirect Filtration (Contact Flocculation). This is a mixture of filtration and flocculation in which the suspension entering the bed has been flocculated, but the floes have not fiiUy formed prior to bed entry. The technique is applied to suspensions of low concentration of colloidal matter. The shear induced inside the deep bed provides excellent conditions for orthokinetic flocculation. The main advantage of direct filtration is a reduction in the capital cost over the provision of a flocculator and separate vessel for solid-liquid... 

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