Sulfuric acid bleaching of kraft pulp II: Behavior of lignin and carbohydrate during sulfuric acid bleaching

The purpose of this study was to investigate the behavior of lignin and car bohydrates in kraft pulps during sulfuric acid bleaching. Beech kraft pulp and red pine kraft pulp were bleached with dilute sulfuric acid at pH 1.3 w ith addition of sodium nitrate and sodium nitrite at 100 degrees C for 1h. The pulps were then extracted with aqueous sodium hydroxide solution at 70 degrees C for 1h. Lignin and carbohydrates in the acid effluents and the al kali effluents were analyzed. The carbohydrate compositions of unbleached a nd bleached kraft pulps were also determined. The residual lignin in kraft pulp was degraded to a molecular size similar to that of milled wood lignin during sulfuric acid bleaching without additives, and it was further degra ded to a much smaller molecular size during sulfuric acid bleaching with ad ditives. It was found that the amount of carbohydrate dissolved in the blea ch effluents were only about 1% of the dry weight of the kraft pulp under t hese bleaching conditions. The carbohydrates dissolved during bleaching wer e mostly of hemicellulose origin.