| Green liquor from alkali recovery process was used in the pretreatment of poplar wood chips. The delignification reaction course and the consumption process of alkaline during subsequent kraft pulping were studied. After green liquor pretreatment poplar wood chips were cooked under conventional kraft cooking conditions. Black liquor from cooking process was used to analyze the lignin content and the concentration of NaOH and Na2S by acid precipitation method and titration method, respectively. For kraft pulping after green liquor pretreatment, the yield was 73.1%, delignification rate was 32.54%. Contrasted to the traditional kraft pulping, delignification velocity was almost steady during the cooking process. On the initial delignification phase, delignification rate was 14.52%. On the bulk delignification phase and residue delignification phase, delignification rates were 42.62% and 3.41%, respectively. Compared to the traditional kraft pulping, delignification rates of three delignify phases reduced by 3.55, 19.38 and 9.11 percent points, respectively. NaOH was almost consumed at the initial delignify phase and bulk delignify phase with the consumption of 57.71% and 42.29%, respectively. The concentration of Na2S decreased firstly, and reached to a minimum of 1.09 g/L in 1.25 h, but after 1.25 h, since sulfur precipitated from ligninsulfonate, the concentration of Na2S began to increase gradually. Kraft pulping with green liquor pretreatment could not only improve the selectivity of delignification, but also reduce the degradation of carbohydrates. The total cooking yield was 50.8%, increased by 2.9 percent points,compared to the traditional kraft pulping. |
