Bleaching of cellulose pulp in a first chlorine dioxide bleaching step
The method is for bleaching cellulose pulp having an MC consistency in a first step with chlorine dioxide in a bleaching sequence with several bleaching steps. The pulp is bleached in this first chlorine dioxide step in a first phase at a lower temperature of 60±10° C. during a retention time of from 30 seconds to 30 minutes, after which the pulp is heated by at least 10° C. to a reaction temperature of 90±10° C. for a second phase, which has a retention time of 60-200 minutes, With this process modification of a first chlorine dioxide step, an efficient destruction of undesired organic acids is obtained, while at the same time improved viscosity/strength of the pulp can be obtained of the finally bleached pulp with a whiteness of approximately 90% ISO.
1 . A method for bleaching cellulose pulp, comprising:
providing a first chlorine dioxide bleaching step in a bleaching sequence where cellulose pulp has an initial kappa value for the first chlorine dioxide bleaching step that exceeds 7,
bleaching the cellulose pulp at medium consistency,
heating the cellulose pulp to a primary temperature in a range of 50-70° C.,
mixing chlorine dioxide into the cellulose pulp,
establishing the cellulose pulp at the primary temperature leading the cellulose pulp to a first phase, and the cellulose pulp being bleached in the first chlorine dioxide bleaching step for a first retention time that docs not exceed 20 minutes, and is directly after the first chlorine dioxide bleaching step, without an intermediate wash, heated by at least 10° C., after which the cellulose pulp is given a second retention time in a second step in a range of 60-200 minutes.
2 . The method according to claim 1 wherein the first chlorine dioxide bleaching step does not exceed 10 minutes.
3 . The method according to claim 1 wherein the first retention time is at least 30 seconds.
4 . The method according to claim 1 , wherein a temperature is increased in a range of 10-30° C. before the second step to provide an initial temperature for the second step in a range of 80-100° C.