Method of producing pulp using single-stage cooking with formic

Details Method of producing pulp using single-stage cooking with formic Method of producing pulp using single-stage cooking with formic

1999-05-03

2000-12-05

Alvo, Steve

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Treatment with particular chemical

162 78, D21C 304

Patent

active

061561568

DESCRIPTION:

BRIEF SUMMARY
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 utilized for modification of residual lignin. The method of the invention provides an ISO brightness exceeding 90% without chlorine chemicals. The method can be applied to both herbaceous plants and wood.
The production of pulp from herbaceous plants (common reed, goat's rue) is described in the publication Laamanen J. & Sundqvist, J., Agrokuidun tuotanto ja kaytto Suomessa [Production and use of agrofibre in Finland]. Final report on the study, Part III, Vaihtoehtoiset kuidutusmenetelmat [Alternative defibration methods], Jokioinen 1996, Agricultural Research Centre, Publication 5 series A, p. 69 to 88. Common reed is cooked using single-stage, two-stage and three-stage Milox methods. In single-stage Milox cooking, a mixture of formic acid and hydrogen peroxide at a maximum temperature of 80.degree. C. is used. In the two-stage method, the grass is first cooked in formic acid alone and then in a mixture of formic acid and hydrogen peroxide. The three-stage method first comprises a peroxy formic acid cooking stage, followed by two-stage Milox cooking.
After Milox cooking the pulps are bleached with alkaline peroxide. The publication states that the common reed pulp reaches its target brightness 80 to 85% with alkaline peroxide alone, but, in spite of its low kappa number, uses more peroxide than Milox pulp produced from e.g. birch chips. The yield of the bleaching is stated to be quite low too, about 80%. The best brightness 83.4% was reached using five peroxide stages and a peroxide consumption of 3.3% calculated on pulp.
As to cooking goat's rue, it is stated that goat's rue Milox pulp can be bleached to a brightness exceeding 85% with alkaline peroxide alone and that peroxide consumption is in the same range as with common reed Milox pulp (4%). Four peroxide stages have provided an 84.5% brightness with an about 3% peroxide consumption calculated on pulp.
The summary of the publication states that the best results, both when using common reed and when using goat's rue, have been achieved by a two-stage Milox method first involving cooking in formic acid alone and then in a mixture of formic acid and hydrogen peroxide at 80.degree. C. for 3 hours.
Brightness values exceeding 90% have not been reached with the above methods.
Finnish Patent 74750 discloses a method of producing bleached pulp from lignin-containing cellulose raw material, such as softwood or hardwood chips. Organic peroxo acids, such as peroxo acids derived from formic acid, are used as defibration reagents. The defibration reagent can be produced by e.g. adding hydrogen peroxide to formic acid. An alkaline solution containing hydrogen peroxide is used for bleaching. In the example of the patent, the highest achieved brightness was 90.3%. The hydrogen peroxide amounts used were, however, relatively high, preferably 5 to 20% calculated on raw material, and the cooking times were long.
It is known from the publication Sundqvist J., Chemical Pulping Based on Formic Acid--Summary of Milox Research, Paper and Timber 78 (1996)3, p. 92 to 95, to feed hydrogen peroxide into the first and third cooking stages of multi-stage formic acid cooking. In the method, the chips are treated with formic acid and a small amount of hydrogen peroxide at 60 to 80.degree. C. in the first stage. The main delignification is carried out in the second stage at a temperature of 90 to 100.degree. C. In the third stage, the pulp is cooked in a solution containing formic acid and hydrogen peroxide. In all three stages, the formic acid content in the cooking solution exceeds 80% and the cooking times in each stage vary between 1 and 3 hours. Liquid circulation is planned such that after the cooked pulp has been washed with acid, the acid enters the third stage, from where it enters t

REFERENCES:
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.

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