| 摘要 |
The invention relates to a process for bleaching preoxidized paper pulp enabling a pulp having a very high degree of whiteness to be obtained with a limited number of treatment steps and enabling pollution to be reduced without degrading the qualities of the pulp; such a process essentially comprises the combination of the three successive treatment steps, namely: (a) a first step of hot chlorination with gaseous chlorine at a temperature of between about 30 DEG and 80 DEG C., and preferably between 35 DEG and 50 DEG C., in the presence of urea used at a rate of 0.5 to 5% by weight of the chlorine used, and preferably from 1 to 3%, with a concentration of pulp of between 3 and 30%, and a duration of retention of the chlorine of 30 seconds to 45 minutes; (b) a second step of alkaline extraction in the presence of an oxidizing agent of the hypochlorite or peroxide type, at a temperature of between about 30 DEG and 70 DEG C., and at a pH of between about 8 and 12, and preferably between 10.5 and 11.5, and (c) a third step of treatment with chlorine dioxide, at a temperature of between 60 DEG and 90 DEG C. and preferably between 70 DEG and 80 DEG C., at an initial pH of between 5 and 11, and preferably between 7 and 9, a recycling of the effluents of the second and/or the third treatment step for intermediate washing preceding the corresponding step being provided. |
