Settling rate, polymer property effect

Among the properties measured here, the settling rate is mainly a measure of the size of the floes and in later stages the compressibility of floes and floe networks, and the supernatant clarity is a measure of the size distribution of floes and size dependent capture of the particles and floes by the polymer. The sediment volume and the pulp viscosity on the other hand, are direct measures, not only of floe size and structure but also of adsorbed polymer layers. It is to be noted in this regard that it is this latter aspect which makes it possible to estimate the thickness of adsorbed polymer layers by measuring the viscosity of the medium and the suspension in the presence of polymers (20,21). This combination of effects is another reason one cannot always expect correlation between various flocculation responses. 

In a recent study, the necessity for understanding the effects that molecular probes (e.g., pyrene) can have on colloidal system has been clearly illustrated. " Mixtures of pyrene-labeled and unlabeled polyethylene oxide (PEO) were adsorbed on silica and the properties of the resulting suspension were monitored. Settling rate results (Eigure 7.29) clearly demonstrated that pyrene-labeled PEO has a marked effect on the flocculation of silica suspension. It has been shown that even relatively small amounts of labeled polymer, when mixed with unlabeled polymer, can dramatically affect the behavior of their mixtures. On the other hand, it has also been shown that when labeled polymer was used in sufficiently low amounts (<3%), the mixture behavior does not differ from that due to the unlabeled polymer. These findings once again underscore the fact that while certain measurement techniques are invaluable for investigating colloidal phenomena, potential effects caused by introduction of various molecular probes into the system should always be noted. 

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...

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