Formation of Dissolved High Molecular Organic Substances in the Bleaching of Softwood Kraft Pulp, 2001 Pulping Conference Proceedings

U. Germgard, I. Boren, S. Aleksic--The study has focused on the formation of dissolved high molecular substances after D(EP)DD and D(EP) bleaching of softwood kraft pulp and especially on the possibility to influence this formation. The experiments were carried out in the laboratory.

Ultrafiltration (UF) of the generated filtrates was used to separate molecules of different sizes. The reason was that the biological degradation rate in the receiving water has previously been shown to decrease with increasing molecular weight. The cut-off for the membranes used were 1000, 3000 and 10 000 g/mol (Dalton). COD and UV absorbance at 280 nm was used to analyze the total organic substance and the amount of dissolved lignin.

The study showed that filtrates could not be stored deep-frozen prior to ultrafiltration. The study also showed that the cut-off value of a membrane increases during ultrafiltration and a special design of the UF-filtration experiments was therefore used.

The study has shown that it is possible to influence the formation of high molecular material in the different bleaching stages. To get the best mix between the amount of generated COD, the formation of high molecular COD and the bleaching efficiency modifications of the conditions in both the chlorine dioxide and the extraction stages are needed.