Filter cake, polymer-flocculated

Fig. 5. Effect of polymer dosage on different observed properties of flocculated slurry (40). Comparison of five parameters in a flocculation system (8%fluorite suspension + polyacrylamide Cyanamer P250). A, Rate of settling of floe boundary, in cm/s B, height of settled bed, cm C, height of consoHdated filter-cake, cm D, refiltration rate, arbitrary units and E, clarification, % optical transmission of 1 cm of supernatant Hquid after 3 min settling...

Eor vacuum filters, both the rate of filtration and the dryness of the cake may be important. The filter cake can be modeled as a porous soHd, and the best flocculants are the ones that can keep the pores open. The large, low density floes produced by high molecular weight polymers often coUapse and cause blinding of the filter. Low molecular weight synthetic polymers and natural products that give small but rigid floes are often found to be the best. 

Improvement in filtration efficiency of gypsum upon flocculation is determined to be due to reduction in the blockage of filter media pores and in the filter cake resistance. An optimum in polymer dosage is explained in terms of stabilization of fines and deformability of the floes formed. Anionic polyacrylamides were found to be more effective filter aids than cationic polymers. 

From the practical point of view, the change of filtration pressure, filtration area, filter medium, filtrate viscosity and solids loading is limited . To increase the filterability of the cake, most practical way, therefore, is to decrease the average specific cake resistance a. This could be accomplished by changing the particle shape, increasing the particles size and narrowing the size distribution of the particles in the cake. The latter two factors can be manipulated by polymer flocculation. 

Flocculants and surfactants (qv) are used frequently as filter aids, particularly when slimes are present or when the particles to be filtered are very fine and difficult to filter. Low molecular weight polymers are more commonly used. These form small, dense floes which provide higher cake porosity. 

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