Method for reducing pulp viscosity in production of dissolving pulp
The invention relates to a method for reducing pulp viscosity in production of dissolving pulp. The method comprises producing cellulose pulp, whereby the obtained cellulose pulp has a cellulose content of at least 90%, and bleaching the obtained cellulose pulp. The cellulose pulp is treated after bleaching with performic acid.
1. A method for reducing pulp viscosity in production of dissolving pulp, the method comprising:
producing a cellulose pulp having a cellulose content of at least 90%;
bleaching the obtained cellulose pulp;
and
decreasing
the viscosity of the bleached pulp by treating the bleached pulp with performic acid, while at the same time, keeping a brightness change of the pulp to less than 1% ISO,
wherein the brightness change is a difference between a brightness of the pulp measured before a treatment with the performic acid and a brightness of the pulp measured after a treatment with the performic acid, and
wherein the pH during treatment with the performic acid is in a range between 6-8.
2. The method according to claim 1 , wherein performic acid is used in amount of 0.1-7 kg/ton pulp.
3. The method according to claim 1 , wherein during the treatment with performic acid pH of the pulp changes less than one pH unit.
4. The method according to claim 1 , wherein the temperature during the treatment with performic acid is in a range of 30-100° C.
5. The method according to claim 1 , wherein the method is free from additional heating or cooling of the cellulose pulp before the treatment with performic acid.
6. The method according to claim 1 , wherein the pulp is treated with performic acid directly and immediately after the end of the bleaching step.
7. The method according to claim 1 , wherein the bleached pulp has a consistency of 5-15%, calculated as dry content.
8. The method according to claim 1 , wherein the performic acid is produced on-site and used directly for treating the obtained bleached cellulose pulp.
9. The method according to claim 1 , wherein distilled peracetic acid is added to the pulp after the bleaching in amount of 0.01-30 kg/ton dry pulp.
10. The method according to claim 1 , wherein the obtained cellulose pulp is produced by a kraft process with a prehydrolysis step.
11. The method according to claim 1 , wherein the obtained cellulose pulp, which is treated with performic acid, has a cellulose content of at least 92%.
12. The method according to claim 1 , wherein the obtained bleached cellulose pulp is treated by continuous addition of performic acid to the pulp after the bleaching.
13. The method according to claim 1 , wherein the viscosity decrease of the cellulose pulp is at least 30 dm 3 /kg.