Studies on Bamboo Pulp : Multistage Bleaching Process of Bamboo Kraft Pulp (Part 1)

In this experiment, several bleaching methods of bamboo kraft pulp used chlorine, alkali and Ca-hypochlorite on 4- to 7-stage bleaching process and were discussed from the stand point of brightness. 1) Amount of addition of total chlorine The chlorine required amount for unbleached bamboo kraft pulp is about 10% from the base of wood pulp by KMnO_4 value. It is possible to obtain bleached pulp which has brightness above 85 with less amount of chlorine by this multistage bleaching process. From this fact, it is hard to say that bamboo kraft pulp is more difficult to bleach than wood pulp. Namely on the 4-stage bleaching 9% of chlorine was required, but on the 6- and 7-stage bleaching process 6% of chlorine was satisfactory and brightness was 85～90. According to O.S. SPROUT, and W. TOOREY et al., the 5-stage bleaching process, Cl-Na-Cl-H-H, required 6.63% of total chlorine to obtained brightness of 86, and the 6-stage bleaching process also needed the same amount of chlorine. From the data of this experiment, bleachability of bamboo kraft pulp showed better results than wood kraft pulp. 2) Ratio of chlorine On the multistage bleaching process, the problems are how much chlorine and in what ratio should it be added on each stage for the most effective bleaching. Principally, it is considered that the best way is to add a large amount of chlorine on the first stage and decreased gradually. However, this report shown that the principal method is not always suitable from the relationship of chemical actions, and in the discussed chlorine ratio, it was decided that the ratio of Cl: Ca-hypochlorite was divided into definite ratios. Uusually the ratio of 7:3 or 8:2 was adapted to wood kraft pulp, but in actual industry it is not established and it is differed depending on the factory. [table] In the case of bamboo kraft pulp as shown in this experiment on the ratio of 4:6 or 6:4 was given good result. The effect of Cl: Ca-hypochlorite ratio was remarkable on the 4-stage bleaching process, but less with the increase of stages in process. It was impossible to neglect this effect and about equal ratio showed better results. 3) Bleaching method If brightness was required at about 80, the 4-stage bleaching process was enough for this purpose, and farther more if well controlled, brightness would be raised above 85. Higher value of brightness pulp was obtained constantly with 6-stage of bleaching process. It is assumed that the 7-stage bleaching process is not necessary because the results were same as the 6-stage bleaching process. The most adequate process in the 6-stage bleaching process was Cl-Na-H-Cl-Na-H or Cl-Na-Cl-Na-H-H.