| 摘要 |
The bleaching process is carried out at the same temperature and comprises an oxygen delignification step, a chlorine dioxide bleaching step, a chelating agent treatment step and a peroxide bleaching step. No washing of the pulp is carried out in between chlorine dioxide bleaching and treatment with the chelating agent. - A process for treating bleached cellulose pulp comprises cooking the pulp and delignifying it with oxygen in at least one alkaline step, then washing it to obtain a pulp with a kappa value less than 15 and then bleaching in order to obtain pulp with the desired brightness. The entire bleaching sequence, including the oxygen delignification step, is carried out at a temperature greater than 90 deg.C. Final bleaching is carried out at a pulp consistency of 5-15 % in a sequence comprising the following steps : (i) transferring the washed pulp from the oxygen delignification step to a chlorine dioxide bleaching step, in which steam, chlorine dioxide and acid (for regulating pH) are added, the amount of chlorine dioxide corresponding to that needed to give a kappa value of 0.5-2.5, the amount of acid corresponding to that needed to give an initial pH of less than 5 and the amount of steam corresponding to that needed to give an initial temperature of 100 deg.C or more, the dwell time in this bleaching step being 60-200 (90-120) minutes ; (ii) treating the pulp directly, without any intermediate washing, with a chelating agent (preferably DTPA) and optionally an alkali to increase pH ; (iii) washing the pulp in a washing device (22) ; (iv) transferring the washed pulp to a pressurised peroxide bleaching step, in which alkali is added in an amount corresponding to that needed to give an initial pH of more than 9, peroxide is added in an amount of 5-25 kg/tonne pulp (kg/BDMT) and steam is added in an amount corresponding to that needed to give a temperature of at least 90 deg.C, the dwell time in this bleaching step being 30-200 (60-120) minutes ; and (v) washing the pulp in at least one washing device (32) to obtain a pulp with a brightness of more than 88 (90) % ISO. |
