| 摘要 |
FIELD: paper-making industry. SUBSTANCE: pulp is delignified with placid or salt thereof, treated with complexing agent and finally bleached with chlorine-free bleaching agent. Delignification step is carried out with strongly oxidizing peracetic acid, which results in noticeable increase in degree of whiteness and in marked decline in kappa number after bleaching, using chlorine-free bleaching agent. The latter comprises at least one of group consisting of peroxide-containing compound, ozone or sodium dithionite which are used in specific sequence or in mixtures. Pulp viscosity is maintained at comparatively high level, since desired effect of increasing degree of whiteness is extremely selective. It is advisable that both delignification and treating with complexing agent should be carried out at pH close to neutral, thereby minimizing need for pH control and providing possibility of in-plant use of spent bleaching liquor, e.g. in pulp washing. During final bleaching with ozone and hydrogen peroxide, proposed method makes it possible to obtain soft wood pulps with degree of whiteness greater than 90% ISO index, and to remove from pulp practically all lignin, while maintaining sufficient pulp concentration level. EFFECT: higher efficiency. |
