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Video observation of pulp pad formation and properties in brownstock washing: defoamers primarily help speed up drainage

Pulp & paper Canada, 2002-11, Vol.103 (11), p.23

Copyright Southam Business Communications, Inc. Nov 2002 ;ISSN: 0316-4004 ;EISSN: 1923-3515

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  • Title:
    Video observation of pulp pad formation and properties in brownstock washing: defoamers primarily help speed up drainage
  • Author: Pelton, R ; Flaherty, T ; Hrymak, A ; Shawafaty, N
  • Subjects: Pulp cleaning
  • Is Part Of: Pulp & paper Canada, 2002-11, Vol.103 (11), p.23
  • Description: In the case of ideal displacement washing, the minimum volume of wash water for complete washing is equal to the volume of liquor initially between the fibres. Real brownstock washers, however, are much less efficient. An important cause for poor washing efficiency is that the fibre pad formed on the face of the washer drum is not uniform. Poor pad uniformity means that some areas of the pad will be much more permeable than others. Thus the fibre pad on washer drums contains channels through which more wash water can pass than through the less permeable parts of the pulp pad (6)(7). In the extreme case, clean wash water passes through these channels, during the washing, while other areas of the pad are not exposed to wash water at all. The following paragraphs describe our new technology -- polymer-enhanced washing -- a process designed to plug the most open channels. Lignin in black liquor is present as soluble polymer molecules and micro-gel particles that are both negatively charged polyelectrolytes. In polymer-enhanced washing, the displacing wash liquor is a dilute aqueous solution of cationic polyelectrolyte. Figure 4 shows the washing experiment in which polyDADMAC, a cationic polyelectrolyte, was present in the wash water. In contrast to Fig. 3, dark regions form and remain in the coarse-bead centre channel at the end of the washing experiment. These dark regions are precipitated polyDADMAC-lignin complex, Fig. 5. The precipitate has the effect of lowering permeability of the channel by about a factor of two, causing less wash water to be wasted by flowing through the centre channel. The remarkable feature of this experiment is that the lignin polymer complex selectively forms in the coarse bead channel. If complex precipitates had formed in the entire bed, the pulp would be contaminated with precipitate and displacement would be inhibited. Polymer-enhanced washing has been evaluated in commercial mill trials. Our initial work involved the addition of Channel Block[Symbol Not Transcribed] [registered symbol], commercial cationic polymer especially formulated for this application, to the showers of a brownstock washer. This work was hampered by the difficulties in obtaining meaningful data from short-term brownstock washer trials. Because of multiple feedback loops and the inherent noise in the continuous kraft pulping process, it was difficult to obtain reproducible results. To circumvent these difficulties, Channel Block[Symbol Not Transcribed] [registered symbol] was applied with a special device called the Mini-shower bar that only treated part of the washer, Fig. 9. The advantage of this approach is the effectiveness of polymer enhanced washing can be evaluated by comparing the quality of washing in the treated areas against the untreated areas (11). Fluctuations in the feed pulp suspension properties are accounted for by continually measuring washing efficiency in the untreated (control) zone of the washer.
  • Publisher: Westmount: Annex Publishing & Printing, Inc
  • Language: English
  • Identifier: ISSN: 0316-4004
    EISSN: 1923-3515
  • Source: ProQuest Central
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