Method for reducing pulp viscosity in production of dissolving pulp

Abstract

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.

Claims

1. A method for reducing pulp viscosity in production of dissolving pulp, the method comprising producing cellulose pulp, whereby the obtained cellulose pulp has a cellulose content of at least 90%, bleaching the obtained cellulose pulp, wherein the cellulose pulp after bleaching is treated with performic acid.

2. The method according to claim 1, wherein performic acid is used in amount of 0.1-7 kg/ton pulp, preferably 0.5-5 kg/ton pulp, more preferably 1-3 kg/ton pulp.

3. The method according to claim 1, wherein the pH during treatment with performic acid is at least 5, preferably at least 6.

4. The method according to claim 1, wherein during the treatment with performic acid pH of the pulp changes less than one pH unit.

5. The method according to claim 1, wherein the temperature during the treatment with performic acid is in the range of 30-100? C., pref erably 40-70? C.

6. The method according to claim 1, wherein the method is free from additional heating or cooling of the pulp before the treatment with performic acid.

7. The method according to claim 1, wherein the pulp is treated with performic acid directly and immediately after the end of the last bleaching step.

8. The method according to claim 1, wherein the pulp from bleaching has a consistency of 5-15%, preferably 7-12%, calculated as dry content.

9. The method according to claim 1, wherein the performic acid is produced on-site and used directly for treating the cellulose pulp after bleaching.

10. 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, preferably 10-20 kg/ton dry pulp.

11. The method according to claim 1, wherein the cellulose pulp is produced by a kraft process with a prehydrolysis step.

12. 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%, preferably 95%.

13. The method according to claim 1, wherein the method further comprises the steps of, measuring the viscosity of the pulp after the bleaching, treating the pulp with performic acid in case the viscosity value of the pulp is higher than a predetermined viscosity value.

14. The method according to claim 1, wherein the cellulose pulp is treated by continuous addition of performic acid to the pulp after the bleaching.

15. The method according to claim 1, wherein the viscosity decrease of the pulp is at least 30 dm.sup.3/kg, preferably at least 50 dm.sup.3/kg.

Description

EXAMPLE 1

[0028] Off-spec dissolving kraft mill pulp was subjected separately to a viscosity reducing treatment after bleaching. The viscosity reducing step was performed at temperature of 60? C., residence time 120 min. The pulp consistency was 10% and used peracid dose was 2 kg/ton dry pulp. pH level of the pulp was 8. Two different peracids were tested, namely, performic acid (PFA) and peracetic acid (PAA).

[0029] The results of Example 1 are shown in Table 1.

TABLE-US-00001 TABLE 1 Results of Example 1. Reference Sample Sample 2 Sample 3 Used peracid PAA PFA Final-pH 8 5.7 7.0 Residual-PAA, kg/ton pulp 0.69 0 Viscosity, dm.sup.3/kg 507 470 447 Brightness, % ISO 90.4 91.3 90.8 (standard ISO 2470) Yellowness 5.5 5.1 5.4 Whiteness CIE D65/10+UV 80.1 81.8 80.9 (standard CIE 15:2004) L* 97.8 98 97.9 a* ?0.36 ?0.32 ?0.28 b* 3.23 2.97 3.13

[0030] It can be seen from Table 2 that treatment of pulp with performic acid reduced the viscosity by 60 dm.sup.3/kg, which is more than enough to turn an off-spec pulp into an in-spec pulp. The same significant viscosity reduction could not be observed when peracetic acid was used.

[0031] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.