Bleaching of cellulose pulp in a first chlorine dioxide bleaching step

Abstract

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.

Description

[0001] The present invention concerns a method for bleaching cellulose pulp in a first chlorine dioxide step. THE PRIOR ART [0002] In association with the bleaching of cellulose pulp with chlorine dioxide, sequences of several steps have most often been used, with two or more bleaching steps in which chlorine dioxide is added batchwise. A typical bleaching sequence of several steps can be: D0(EO)D1D2, D0(EOP)D1D2 or solely D0(EOP)D1 with washes between the steps, where the first chlorine dioxide step is equivalent to the D0-step and where the bleaching sequence is normally preceded by oxygen gas delignification, O2. Other bleaching agents can sometimes be used, such as ozone, Z, which can be batchwise added before or after D0, either with or without an intermediate wash. A step using pure peroxide, P, may be used as a final bleaching step. [0003] U.S. Pat. No. 3,745,065 reveals a chlorine dioxide bleaching stage for cellulose pulp with a tower with upward flow and an immediately sub...

AI Technical Summary

Benefits of technology

"The patent describes a method for improving the bleaching of pulp by using two phases of chlorine dioxide treatment at different temperatures. The first phase is carried out at a low temperature and for a short time, while the second phase is carried out at a high temperature and for a long time. This method results in a higher viscosity of the pulp compared to a modified method that destroys inorganic acids. The technical effect of this patent is to achieve a better bleaching process with higher pulp strength."

Problems solved by technology

In the same manner, there is normally present a high level of organic acids, which gives a poor light-stability for the finally bleached pulp, in the cellulose pulp when bleaching hardwood that has been produced according to the sulphate cooking process (either in batches or in continuous cooking, conventionally followed by oxygen gas delignification after the cooking) but before the first chlorine dioxide step.

However, a conventional D0-step with a long retention time is used in this case.

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