Precipitation of Lignin and Extractives from Bleaching Filtrates - Effects on Pulp Properties, 2002 International Pulp Bleaching Conference Proceedings

Agneta Fuhrmann, Marjatta Kleen, Krista Koljonen, Monika Österberg, Per Stenius

Closing the bleaching loops leads to an increase in the amount of recirculated material (i.e. lignin, carbohydrates, organic acids, extractives and metal ions) which may lead to precipitation, not only on equipment but also on fibers. The objective was to determine what kind of precipitation occurs on fiber surfaces under unfavorable process conditions and if this precipitation can be detected by chemical and surface analysis methods. Unwashed, oxygen-delignified pulp and an acidic ozone filtrate from a softwood kraft mill were used in the laboratory study. The pH was adjusted from an initial value of 11 down to a value of 2 using the acid filtrate and/or pure sulfuric acid. The effect of precipitation pH on lignin, extractives, and metal ion concentrations in the bulk pulp, pulp sheets, and fiber surfaces was investigated using both standard methods and sophisticated surface analysis techniques. There was an enrichment of precipitated components at pH levels below 6. The precipitation was also detected as reduced concentrations of TOC and lignin in the filtrates. The papermaking properties were negatively affected by low precipitation pH. Efficient washing, however, removed the extractives and the main part of the precipitated lignin.