Flocculation polymer dosage effect

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...

Polymer Concentration. The effect of polymer dosage on additional flocculation responses was measured together with adsorption... 

Addition of a surfactant can further enhance the flocculation at the ultralow polymer dosage regime. As shown in Figure 7.37, the addition of anionic surfactant (SDS) significantly improves the flocculation at very low polymer concentrations. At higher polymer levels, the surfactant addition has no effect on flocculation. This is probably due to a significant fraction of adsorption sites already occupied by polymer, making them unavailable for adsorption of the surfactant molecules. 

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. 

The effect of mechanical treatment on floe behavior is illustrated in Figure 5. In one work (40), identical slurries were treated with varying doses of the same polymer. At each dosage, it can be assumed that the same type of floe formed at the same rate. However, the dosage response was completely different depending on which parameter of the flocculated slurry was measured. Thus the term optimal flocculation caimot be appHed to any flocculant—substrate combination if the soHd—Hquid separation process or process parameter is not specified. 

Dilution. In many appHcations, dilution of the flocculant solution before it is mixed with the substrate stream can improve performance (12). The mechanism probably involves getting a more uniform distribution of the polymer molecules. Since the dosage needed to form floes is usually well below the adsorption maximum, a high local concentration is effectively removed from the system at that point, leaving no flocculant for the rest of the particles. A portion of the clarified overflow can be used for dilution so no extra water is added to the process. 

Alum dosages between 200 and 500 mg/1 were not significantly different. Table II also Illustrates that sedimentation Is effective In lowering the concentration of the pesticides to the solubility limit. Metrlbuzln Is soluble In water to a concentration of about 1200 mg/1 and the sedimentation step reduced the concentration from 2000 and 3000 mg/1 to about 1000 mg/1. At lower concentrations sedimentation was not effective. Other coagulants and flocculant aids such as hydroxide and ferric chloride, were tested but alum with an anionic polymer (Watcon 1255) was the most effective. 

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