On the Application of Polyelectrolytes to White Water Treating by Dissolved-air Flotation.

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Dissolved-air flotation has been applied as a separation technique to recover suspended solids from the paper mill white water system. Polyelectrolytes are generally used to facilitate the flotation. Main components of the white water are pulp, filler and size, though their contents differ according to the kind of paper to be manufactured. The purpose of this study is to determine various factors involved in flotation with Polyethylenimine (PEI) and Polyacrylamide (PAM), and to evaluate their applicabilities. Laboratory tests were conducted with artificially prepared white water samples, simulating pulp alone, pulp-filler, pulp-aluminum sulfate or pulp-filler-aluminum sulfate systems to compare the effectiveness between PEI and PAM for the above each system. For the flotability of pulp the following results were obtained ; Flotability of pulp fibers varied with the kind of pulp, and that of coniferous wood pulp fiber was poor. Floating velocity increased when white water contained filler up to about 50% of pulp but the quality of treated water degraded in these cases. Flotability was particularly impeded by the presence of aluminum sulfate. For the effects of PEI and PAM, it was found that, the affectiveness of PEI and PAM for the same kind of deciduous wood pulp differed depending on the manufacturers. PAM was more effective than PEI when filler was contained. PEI was superior to PAM when aluminum sulfate content was less than about 11 ppm (as Al), while PAM was better at the content more than that. When both filler and aluminum sulfate were contained other than pulp, the flotability of pulp was poor but most clear treated water was obtained at aluminum content of about 11 ppm. PAM was better for this system. In conclusion, aluminum sulfate has significant effect on flotability and either PEI or PAM should be carefully chosen according to the composition of white water to be treated.