Method of producing pulp using single-stage cooking with formic acid and washing with performic acid

6,156,156

09/297,546

May 03, 1999

The invention relates to a method of producing pulp with a high brightness using single-stage cooking with formic acid, washing with performic acid and bleaching with oxidizing bleaching chemicals. In performic acid washing, the rapid formation and rapid reactions of performic acid (HCOOOH) are utilitized for modification of residual lignin. The method of the invention provides an ISO brightness exceeding 90% without a chlorine chemicals. The method can be applied to both herbaceous plants and wood.

Rousu, Pasi (Oulu, FIX); Rousu, Paivi (Oulu, FIX); Rousu, Esa (Oulu, FIX)

Chempolis Oy (Oulu, FIX)

What is claimed is

1. A method based on single-stage formic acid cooking for producing pulp with a high brightness, the method comprising the stages of

(a) dignifying cellulose raw material in single-stage cooking with formic acid, possibly together with other organic acids, at a temperature of 105-135.degree. C.

(b) removing cooking liquor from the delignified pulp and washing the pulp with formic acid using a multiple-stage washing sequence

(c) removing the formic acid and possibly other organic acids from the washed pulp, and

(d) bleaching the pulp by using oxidizing bleaching chemicals, wherein the last formic acid washing stage in the multiple-stage washing sequence of stage (b) comprises washing the pulp, free from cooking liquor and in a medium or high consistency state, with formic acid containing performic acid produced in situ and possibly other peracids.

2. A method as claimed in claim 1, wherein the performic acid used in the last formic acid washing stage of stage (b) is prepared immediately in situ or immediately before use by adding hydrogen peroxide or ozone to formic acid.

3. A method as claimed in claim 2, wherein the amount of hydrogen peroxide used for the preparation of performic acid in the last formic acid washing stage of stage (b) is 0.01 to 1.5% of the amount of cellulose raw material.

4. A method as claimed in claim 3, wherein the amount of hydrogen peroxide used for the preparation of performic acid in the last fonnic acid washing stage of stage (b) is 0.2 to 1% of the amount of cellulose raw material.

5. A method as claimed in claim 1, wherein the performic acid treatment in the last formic acid washing stage of stage (b) is performed at a temperature of 50 to 90.degree. C.

6. A method as claimed in claim 5, wherein the pulp is washed in stage (b) in a counter-current wash using as washing liquid formic acid which is recylced from the performic acid treatment in the last formic acid washing stage of stage (b) and which contains possibly other organic acids, residual peracid and other peracids.

7. A method as claimed in claim 5, wherein the performic acid treatment in the last formic acid washing stage of stage (b) is performed at a temperature of 60 to 80.degree. C.

8. A method as claimed in claim 1, wherein pulp consistency in the last formic acid washing stage of stage (b) is 10 to 50% dry matter.

9. A method as claimed in claim 8, wherein pulp consistency in the last formic acid washing stage of stage (b) is 20 to 35% dry matter.

10. A method as claimed in claim 1, wherein the bleaching in stage (d) comprises alkaline hydrogen peroxide bleaching or combined oxygen and hydrogen peroxide bleaching.

11. A method as claimed in claim 10, wherein the bleaching in stage (d) comprises multiple stages to provide a target brightness.

12. A method as claimed in claim 11, wherein said bleaching in stage (d) comprises from 1 to 4 stages.

13. A method as claimed in claim 12, wherein said bleaching in stage (d) comprises 2 to 4 stages.

14. A method as claimed in claim 10, wherein said bleaching in stage (d) is pressure bleaching.

15. A method as claimed in claim 1, wherein the delignifing stage (a) is performed at a temperature of 110 to 125.degree. C.

162/76; 162/78

D21C 304

1995 Pulping Conference Bok 2, Sheraton Chicago, Chicago, IL, Oct. 1-5, Seisto, A, et al., Grass Pulp for Papermaking by the Peroxyformic Acid Pulping Method, 1995 Pulping Conference Chicago 1. Oct.-5. Oct. 1995, pp. 487-494.
The 8.sup.th International Symposium on Wood and Pulping Chemistry, Jun. 6-9, 1995 Helsinki, Finland. Seisto, A. et al., "Milox Pulping of Agricultrural Plants", International Symposium on Wood and Pulping Chemistry, Helsinki 6.Jun.-9.Jun. 1995, pp. 407-414.

edspgr.06156156