DK175185B1 - Process for bleaching cellulose pulp and using enzymes - Google Patents

Abstract

The present invention concerns a procedure for bleaching cellulose pulp obtained from hardwood, wherein bleaching chemical containing chlorine is used. The essential feature of the invention is that the residual chlorine content of the bleached pulp is reduced by subjecting the pulp to enzyme treatment. With the enzyme especially the removal from the pulp of chlorine-binding extractive substances present in hardwood pulp is promoted. The invention furthermore concerns the use of enzyme towards lowering the residual chlorine content of bleached cellulose pulp made from hardwood. Such cellulose pulp is particularly contemplated which will be converted into foodstuff package cardboard or paper, which are commonly disposed of by burning. Lowered chlorine content reduces the risk of producing toxic dioxin at the incineration step.

Description

DK 175185 B1

METHOD OF BLELLING CELLULOSMASS AND

USE OF ENZYMES

The prior art 5

The invention relates to a process for bleaching cellulose pulp, cellulose pulp, obtained from deciduous wood, in which bleaching is used chlorine-containing chemicals, and the use of enzymes to reduce the residual chlorine content of baked cellulose pulp made from deciduous wood.

10

Cellulose pulp recovered by sulfate boiling in particular is particularly brown, primarily due to lignin which remains in the pulp (pulp). It is believed today that this residual lignin is bound to hemicellulose with covalent bonds. Lignin is removed by bleaching, making the pulp usable for high quality paper and cardboard products.

Bleaching is usually carried out in a multi-step process in which the pulp is alternately treated with oxidizing, lignin-degrading chemicals and with chemicals that dissolve the substances formed as decomposition products 20 Above all, as oxidizing chemicals, chlorine-containing substances are used. , such as pure chlorine gas, chlorine dioxide or sodium or potassium hypochlorite, although other types of chemical bleaches which do not contain chlorine are also known. Extraction chemicals for the removal of the decomposition products have been used alkali solutions.

During the course of the reaction, the chemicals containing chlorine, lignin as well as fatty and resinous acids which may be present in the pulp are chlorinated. Therefore, the bleaching process results in the formation of chlorophenols of lignin, and they end up in the pale wastewater flowing from the chlorine pulp, further being bound in the residues of lignin and 30 acids still present in the cellulose pulp after bleaching is completed. Recently, this residual chlorine has increasingly been regarded as an environmental damage at the stage where paper or cardboard products made from the pulp end up as waste, which is why DK 175185 B1 2 is expected to be laid in the future restrictions on the permissible chlorine content in cellulose pulp. Such restrictions will in particular apply to pulp for food packaging that ends up in waste incineration plants and landfills.

This problem is particularly serious in the case of pulp made from hardwood, 5 for example birch or eucalyptus trees, which have a high content of chlorine-binding extractants.

The object of the invention

It is therefore the object of the invention to provide a problem solution by which the residual chlorine content of bleached cellulose pulp made from hardwood can be reduced from the present level. This is achieved according to the invention by reducing the residual chlorine content of the bleached pulp by subjecting the pulp to an enzyme treatment.

15 It is already known from previous studies that it is possible by enzymes to separate lignin from cellulose. The conclusion is that when the pulp produced by the boiling process is first treated with enzyme, the use of chlorine-containing bleaching chemicals in the subsequent bleaching steps are eliminated in whole or in part. According to the invention, it is recognized that when the enzyme treatment is combined with a bleaching process which uses chlorine chemicals in a generally known manner, the result is a cellulose pulp with a residual chlorine content which is substantially reduced in comparison to what it would According to claims 2 and 3, the enzyme treatment can be carried out at any stage of the bleaching process. The pulp can thus be treated with enzyme in connection with one of the process's oxidation or extraction steps (lute steps) or between the respective steps, before the bleaching steps or only after these steps. The enzyme used is advantageously hemicellulase, cellulase, esterase or a mixture thereof, and as suitable enzyme products may be mentioned "Multifect L 250" 3 The temperature of Multifect K can be in the range of 10 to 90 ° C, preferably 40 to 70 ° C, and pH in the range of 3.0 to 7.5, preferably 4.0 to 6.0

Another object of the invention is the use of enzyme to reduce the residual chlorine content of bleached cellulose pulp made from hardwood. Suitable enzymes, according to claim 4, include hemicellulases, cellulases and esterases or mixtures thereof, for example, the enzyme products "" Multifect L 250 "and" "Multifect K". The use of the enzyme as claimed in claim 5 is as described in claim 5. the above description of the bleaching process.

10

In particular, the use of enzyme as claimed in claims 7-9 relates to reduction of the residual chlorine content in bleached cellulose pulp, which must be included in food packaging in the form of paper or cardboard. This is due to the ever-tighter precautionary measures with regard to residual chlorine content, especially for cardboard and paper types that end up as waste and which are fed to waste incineration plants, where chlorine content during burning can develop toxic dioxin.

The appropriate enzyme activities (U = activity units) for various enzyme categories that can be used according to the invention are within the following limits.

Hemicellulases for example xylanase 0-10 U kg of mass 1} β-xylosidase 0-20,000 U per kg of mass 2) 25

Esterases: 0-100 000 U per kg mass

cellulases:

Filter paper activity 0-20,000 U per kg mass 3) CMC activity 0-500,000 U per 4) 30 β-glucosidase 0-20,000 U kg) mass 5) DK 175185 B1 4 β-glucanase 0-500,000 U per kg mass 6) 1) Kahn, A.W., Tremblay, D., LeDuy, A., Enzyme Microb. Technol., 8 (1986) 373-377.

2) F Delyn, M. Claeyssens, J. van Beeumen, C.K. de Bruyne, Can. J. Biochem.

56 43 (1987) 3) Ghose, T.K., Patnak, A.N., Bisaria, V.S., Symposium of Enzymatic Hydrolysis of Cellulose, Bailey, M., Enari, Τ.Μ., Linko, Μ, Eds. (SITRA, Aulanko, Finland, 1975), 111-136.

4) Almonds, M, Weber, J., Adv. Chem Ser. 95 (1969) 391-413.

5) Berghem, Ler, Pettersson, Eur. J Biochem 37 (1973) 21-30.

6) Sugar determination: Miller, G.L., Anal. Chem. 31 (1959) 426-428)

Description of Embodiments 20 The invention is described in more detail below by means of two exemplary embodiments based on laboratory experiments.

Example 1 220 grams of birch sulfate boiling pulp solids (10% solids content of the pulp) were suspended in 50 mM sodium nitrate buffer pH 5 in such a way that the final concentration of the mixture was 2.5%. Multifect K enzyme was added so that the xylanase activity in the mixture was 56 U grams of pulp solids. The temperature during the enzyme treatment was 39 ° C and the treatment time 20 hours. The pulp was filtered after the enzyme treatment with a Buchner funnel and the pulp cake thus recovered was washed with ion-exchanged water to reach a wash ratio of 28.

5 DK 175185 B1

A similar pulp portion was treated as control in the same way, but without enzyme addition.

5 Table 1a below shows measurement results from the two pulp portions in this treatment step.

TABLE la 10 Enzyme Treated Non Enzyme Treated (Control portion)

Kappa number 16.8 21 15 DKM extract content (% of dry matter) 0.27 0.23

Texture (dmykg) 1065 1280 20 -—-----—-

The enzyme-treated portion of cellulose pulp and the control sample were then subjected to bleaching by five successive oxidation and extraction steps. In the first step, chlorine and chlorine dioxide 25 were used as chemicals, in the second stage sodium hydroxide, in the third stage chlorine dioxide. in the fourth stage sodium hydroxide and in the fifth stage chlorine dioxide. Table Ib below shows the total chlorine quantities used for the bleaching and the measurement results found for the bleached pulp portions.

DK 175185 B1 6 TABLE Ib

Enzyme Treated Not Enzyme Treated (Control portion) 5 -

Dosage of active chlorine (mg / g pulp solids) (2 x kappa) 33.6 42 10 Yield 93.6% 97.5% DKM extract content (% solids) 0.26 0.29 15 Consistency (dma / kg 1035 1210

Residual content of chlorine (mg / g) 0.645 1.270 20

The results show that the residual chlorine content after bleaching has been clearly lower in the enzyme treated cellulose pulp compared to the mass that did not receive enzyme treatment. It is also seen that no impairment of the strength values of the enzyme treated mass has been produced.

Example 2 In this experiment, three stock pulp portions from birch sulfate boiling were used which had the same consistency as the portions in the above Example 1. Two portions were treated with enzyme as indicated above, while the third portion was a control sample. The enzymes were different from those used in Example 1 in that in one sample (sample 1), in addition to a 56 U xylanase activity of 56 U grams of pulp solids also established a cellulase activity of 1.5 U per gram. in the other (sample 2), in addition to a xylanase activity of 56 U per grams of pulp solids established a cellulase activity of 1.1 U per grams of pulp solids. The results of the enzyme treatment are shown in Table IIa

Enzyme-Enzyme-Not Enzyme-Processed Processed Processed 10 (Tested) (Sample 2) (Control)

Kappa number 15.8 15.0 18.8 DKM extract content 15 (% of dry matter) 0.32 0.32 0.26

Consistency (dms / kg) 1025 1130 1305 20

The pulp portions were then bleached with chlorine chemicals and sodium hydroxide as in Example 1. The total chlorine dosage in mg / g was 2x the kappa number in each case. The results after bleaching are shown in Table IIb.

0 DK 175185 B1

ISLAND

TABLE 11b

Enzyme-Enzyme-Not Enzyme-Processed Processed 5 (Sample 1) (Sample 2) (control)

Yield 94.6 - 97.4 DKM extract content (% of dry matter) 0.36 0.35 0.37

Consistency (dm3 / kg) 980 1065 1210 15

Total chlorine content (mg / g) 1.15 1.15 1.46 In this case, the results also show that enzyme treatment significantly reduces the residual chlorine content of the bleached cellulose pulp.

Example 3 Bleached birch cellulose pulp was used in this experiment. The pulp had been bleached using as bleaching chemicals for bleaching in the first stage chlorine (90%) and chlorine dioxide (10%), in the second stage sodium hydroxide and in the third stage. oxygen.

The bleached pulp was subjected to enzyme treatment under the same conditions and using the same enzyme as used in Example 1. After treatment, the pulp was washed with water and dried at room temperature. The residual chlorine content was determined both in the enzyme-treated pulp and in the untreated, initially bleached sulphate pulp, whereby the result was found to be 1.18 in the first and 1.56 B1 mg / g in the latter sample. Thus, the residual chlorine content of bleached pulp could be significantly reduced by enzyme treatment.

The test shows that the desired result is achieved with the invention even in cases where the cellulose pulp is first bleached and not treated with enzyme until afterwards.

The above examples of methods can be varied in different ways. For example, in the first step of a conventional five-step chlorine bleaching, either chlorine alone or chlorine can be used in admixture with chlorine dioxide. It is also possible to significantly reduce the time during which the enzyme is allowed to operate, relative to the indicated 20 hours. Thus, treatment of a few hours or even less than one hour is obtainable depending on the enzyme chosen and its activity.

Claims (9)

A process for bleaching cellulose pulp, cellulose pulp, obtained from deciduous wood, in which bleaching is used chlorine-containing chemicals, characterized in that the residual chlorine content of the bleached pulp is reduced by subjecting the pulp to enzyme treatment. Process according to claim 1, characterized in that the pulp is treated with enzyme either before or after the bleaching treatment. Process according to claim 1, characterized in that the enzyme treatment takes place within the temperature range of from 10 to 90 ° C, preferably 40 to 70 ° C, and at a pH of from 3.0 to 7.5, preferably 4.0-6. , 0th A process according to any one of the preceding claims, characterized in that the enzyme used is hemicellulase, cellulase, esterase or a mixture thereof. Process according to claims 2-4, characterized in that to the unbleached pulp 20 produced by birch suffate cognate is added enzyme which is allowed to act, after which the pulp is subjected to bleaching comprising successive steps comprising the oxidation and extraction steps, in the oxidation steps. as bleaching chemicals, chlorine gas and / or chlorine dioxide are used and in the extraction steps sodium hydroxide. 25 Process according to claim 5, characterized in that the bleaching is a five-step chlorine bleaching. Use of enzyme to reduce residual chlorine content in bleached cellulose pulp, 30 made from hardwood. DK 175185 B1 Use according to claim 7 of hemicellulase, cellulase, esterase or a mixture thereof to reduce the residual chlorine content of bleached cellulose pulp. Use of enzyme according to claim 7 or 8 for reducing the residual chlorine content of 5 bleached cellulose pulp to be processed into paper or cardboard for food packaging.