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1、Lesson 13 Pulp bleaching Bleaching is the treatment of wood and other lignocellulosic pulps with chemical agents to increase their brightness. Brightness is increased by decolorizing or dissolving the colored components in pulp, primarily lignin. Two approaches are used in the chemical bleaching of
2、pulps. bleach v. 漂白 lignocellulosic 木质纤维素的 agent 【化学】药剂 brightness 亮度 decolorize vt. 使脱色 dissolve vt.使溶解 component n. 组分 Lesson 13 Pulp bleaching One is to almost totally remove residual lignin, which is used for bleaching of chemical pulps and leads to greater fiber-fiber bonding strength in paper
3、but weaker fibers due to the strong chemical used to decrease the length of cellulose molecules. The other approach is to utilize chemicals that selectively destroy some of the chromophoric groups in mechanical pulps, which is referred to as lignin-preserving or brightening.residual 残余的，剩余的 bond v 结
4、合，使粘合 strength 强度 chemical 化学药品 molecule 分子 chromophoric 发色的，发色团的 group 基团，基 mechanical pulp 机械浆 brightening 显白，显亮Lesson 13 Pulp bleaching Since lignin is a complex molecule with different linkages, the breakage of various types of bonds requires the use of different chemicals. Therefore, all the co
5、lored material in pulps can not be eliminated by any single chemical and bleaching is a multi-step process. complex 复杂的，多元的 linkage 【化】键合，成键Lesson 13 Pulp bleaching The principal commercial bleaching chemicals consists of the oxidants: chlorine (C), oxygen (O), hypochlorite (H), chlorine dioxide (D)
6、, hydrogen peroxide (P), and ozone (Z); the only reductant: hydrosulfite; and the only alkali: sodium hydroxide (E, or caustic extraction). oxidant 氧化剂 chlorine 氯 oxygen 氧气 hypochlorite【化】次氯酸盐 chlorine dioxide 二氧化氯 hydrogen peroxide 过氧化氢 ozone 【化】臭氧 reductant 还原剂 hydrosulfite【化】 亚硫酸氢盐 alkali 碱 sodiu
7、m hydroxide 氢氧化钠 caustic extraction 碱抽提 Lesson 13 Pulp bleaching It is possible to rank each of the oxidizing chemicals for economy, selectivity, and particle bleaching, although it must be remembered that the particular conditions of use will greatly effect on ranking. The rankings are: Economy: OC
8、DHPZ; Selectivity: Delignification DCOZ Brightening: D, PH Particle bleaching: DH, C, O, PZdelignification 脱木素作用Lesson 13 Pulp bleaching Because of their greater economy, chlorine and oxygen are used in the early stages of bleaching for lignin removal, but can not be used for more extensive treatmen
9、t because their poor selectivity results in cellulose degradation. Chlorine dioxide, hypochlorite, and peroxide can be used in the later stage due to their selectivity and also their higher cost. Selectivity 选择性 degradation 降解作用Lesson 13 Pulp bleaching We can divide bleaching sequences into two part
10、s: delignification and brightening. In the delignification part, lignin removal is the objective and kappa number is used as the principal control parameter. In the brightening part, higher brightness is the objective and brightness is the main control parameter. Oxygen can be used either before chl
11、orination or after as an additive to the E stage (Eo). In the delignification part of the sequence, 80%-85% of the residual lignin in unbleached pulp is removed. kappa number 卡伯值 parameter 参数 chlorination【化】氯化(作用) additive 添加的,附加的; 添加物 unbleached 未漂的Lesson 13 Pulp bleaching The brightening part of t
12、he sequence can be single stage, i. e., H to reach 70% brightness. Or this part can be 3 or 4 stages to reach 90% brightness. Hypochlorite and chlorine dioxide are the principal bleaching agents in this part.Sequences more typical of modern mills for bleaching softwood kraft pulp are (D+C)(E+O)DED a
13、nd O(D+C)(E+O)D. The most widely adopted short sequence is (D+C)(EO)D. In most cases, each bleaching stage is followed by a washing stage to remove reaction products. Lesson 13 Pulp bleaching Sulfite and hardwood kraft pulps are “easier bleaching” than softwood kraft pulps. Both have lower lignin co
14、ntent, and the lignin residues in sulfite pulps are partially sulfonated and therefore more readily solubilized. Consequently, a somewhat simpler process can be used to achieve a comparable brightness level. sulphonate 磺酸盐，使磺化 solubilize v.溶解Lesson 13 Pulp bleaching For mechanical pulp bleaching or
15、pulp brightening, in order to retain the advantage of high yield, pulps must be decolored or brightened by methods that do not solubilize any appreciable amount of lignin. Commercially used chemicals for this purpose today are sodium hydrosulfite which has a reductive action, and hydrogen peroxides
16、which have an oxidative action. Single-stage treatments are sufficient for improving the eye appeal of newsprint- type pulps. Brightness level over 80 can often be achieved by a modern two-stage peroxide or peroxide / hydrosulfite sequence.decolor 脱色 appreciable 可观的,可以感到的 reductive 还原性的 oxidative 氧化性的课堂练习 列出与氧有关的词 列出与氯有关的词 列出与氢（水）有关的词 列出与
