FI81395B - FOERFARANDE FOER BLEKNING AV CELLULOSAMASSA. - 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

81395

METHOD FOR DETERMINING PULP I

The present invention relates to a process for the preparation of cellulose pulp obtained from hardwood, in which process a chlorine-containing silencing chemical is used.

In particular, the cellulosic pulp obtained from the pulp soup 11a is brown in color, mainly due to the lignin remaining in the pulp. According to the current understanding, the residual lignin is covalently bound to hemise11u1 o. Lignin is removed by bleaching, which makes the pulp suitable for high-quality paper and board products.

Bleaching usually takes place in a multi-stage process in which the pulp is treated alternately with oxidizing, lignin-degrading chemicals and solubilizing chemicals. The oxidizing chemicals used are ':'. chlorine-containing substances such as pure chlorine gas, chlorine dioxide or sodium or calcium hypochlorite, although other types of chlorine-free oxidizing bleaches are known. Again, alkaline solutions have been used as new chemicals to remove decomposition products.

Chlorine-containing bleaching chemicals react to cause chlorination of lignin, as well as fatty and resin acids present in the pulp. Thus, lignin generates chlorophenols in bleaching, which end up in the effluent effluent leaving the pulp. In addition, chlorine binds to those lignin and acid residues that remain in the cellulosic pulp even after bleaching. This residual chlorine has recently become increasingly seen as an environmental nuisance at the stage when a paper or board product made from pulp becomes waste, and therefore the permissible k 1 chlorine content 11e in pulp is expected to be limited in the near future. In particular, the restrictions would apply to the use of pulp destined for waste incineration plants and landfills. The problem applies in particular to pulp made from hardwood, such as birch or eucalyptus, 2 81395 with a high content of chlorine-binding extractants.

The object of the present invention is thus to provide a solution by which the residual chlorine content of a bleached pulp made of hardwood can be reduced from its current level. The invention is characterized in that the residual chlorine content is reduced by subjecting the pulp to an enzymatic treatment.

Previous studies have already shown that enzymes make it possible to release lignin from cellulose. The conclusion from this has been that when the cellulose pulp obtained from the process is first treated with an enzyme, the use of chlorine-containing bleaching chemicals in the next bleaching step is largely even completely eliminated. According to the invention, it has now been found that when the enzyme treatment is combined in a substantially normal manner with a bleaching process using chlorine chemicals, a cellulose pulp is obtained which has a substantially reduced residual chlorine content compared to what it would be without the enzyme treatment.

According to the invention, the enzyme treatment can be carried out either before or after the bleaching treatment. The pulp can thus be treated with an enzyme in connection with one of the oxidation or extraction steps (time steps) of the process, or between said steps, before or after said bleaching steps. The enzyme used is preferably hemicellulase, cellulase, esterase or a mixture thereof, and suitable enzyme products include Multifekt L 250 and Multifect K. The temperature of the enzyme hands may be in the range of 10-90 ° C, most preferably in the range of 40-70 ° C, and pH in the range 3.0-7.5, preferably in the range 4.0-6.0.

The invention further relates to the use of an enzyme for reducing the residual chlorine content of bleached cellulose pulp made from hardwood. Suitable enzymes include hemise 11u1ases, cellulases, and esterases or mixtures thereof, for example the 3,81395 enzyme products Multifect L 250 and Multifect K. The enzyme is used according to the invention as described in the description of the bleaching process above.

In particular, the use of the enzyme according to the invention is aimed at α-flying the residual chlorine content of the bleached cellulose pulp to be processed into non-food packaging board or paper. The stricter provisions on residual chlorine apply in particular to those grades of cartonboard and paper which are to be disposed of as waste in incineration plants, where the chlorine they contain may, on combustion, give rise to highly toxic dioxins.

Suitable enzyme activities (U = unit of activity) for the different enzyme species used in the invention are as follows: hemicellulases: 6 (e.g. xylanase 0-10 U / kg pulp (2 β-xy1 osidase 0-20,000 U / kg pulp; esterases: 0-100000 U / kg pulp cellulases: (3 filter paper activity 0-20000 U / kg pulp (4 CMC activity 0-500000 U / kg pulp (5 β-glucosidase 0-20000 U / kg pulp! Π-glucanase 0 -500000 U / kg mass ^ 1) Khan, AW, Tremblay, D., LeDuy, A., Enzyme Microb. Technol., 8 (1986) 373-377 2) F.Delyn, M.Claeyssens, J. van Beeumen , CK De Bruyne, Can.J. Biochem 56 43 (1978) 3) Ghose, TK, Patnak, AN, Bisaria, VS, Symposium of

Enzymatic Hydrolysis of Cellulose, Bailey, M., Enari T.M., Linko, M., Eds. (SITRA, Aulanko, Finland, 1975), 111-136 4) Mandels, M., Weber, J., Adv. Chem. Ser. 95 (1969) 391-413 4 81395 5) Berghem, Ler, Pettersson, Eur. J. Biochem 37 (1973) 21-30 6) sugar assay: Miller, G.L., Anal. Chem. 31 (1959) 426-428

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

E s i. M r k k 220 g of pulp dry matter obtained from birch sulphate soup (pulp k.a. content 10%) was slurried in 50 mM Na-citrate buffer pH 5 so that the consistency of the mixture became 2.5%. Multifect K enzyme was added to the mixture so that the xylanase activity of the mixture became 56 U / g dry matter of the pulp. The temperature of the enzyme treatment was 39 ° C and the treatment time was 20 h. After the enzyme treatment, the stock was filtered in a Biichner funnel and the resulting pulp cake was washed with deionized water to give a washing ratio of 28.

For comparison, the corresponding batch of pulp was otherwise treated in the same way but without the addition of enzyme.

The following Table Ia shows the measurement results of said two pulp stock at this stage of processing.

TABLE Ia

Enzyme-treated j no enzyme treatment j 1 (reference batch)

Chapter number 16.8 21 IDKM extract content (% of k.a.) 0.27 0.23 i viscosity j (dm3 / kg) 1065 _. 1280 ___! li 5 81395

Next, the enzyme concept 1ly 11e was carried out on the pulp pulp and the control batch11e was bleached in five successive oxidation and extraction steps. Chlorine and chlorine dioxide were used as chemicals in the first stage, sodium hydroxide in the second stage, chlorine dioxide in the third stage, sodium hydroxide in the fourth stage and chlorine dioxide in the fifth stage. The following Table Ib shows the total amounts used in bleaching and the measurement results obtained from the bleached pulp batches.

TABLE Ib

Enzyme treatment No enzyme treatment (reference batch)! : · .: dosage ak-: active - chlorine, I i i: i (mg / g mass j - k.a.) (2xkap-i!. Pa) j 33.6 j 42 yield j 93.6%! 97.5% f _ _ DKM-new-! i isity i v:% k.a. : 0.26 and 0.29 viscosity and ti 1035 I 1210

(dm3 / kg) J

____: ‘i residual chlorine content 0.845 1.270! (mg / g) ---- --------- * - - f

The results show that the residual chlorine content of the enzyme-treated cellulose pulp β 81395 after bleaching is clearly lower than that of the enzyme-untreated pulp. In addition, it is seen that there has been no decisive deterioration in the strength values of the enzyme-treated pulp.

Es i ^ merkk i__2 In this experiment, three batches of pulp pulp obtained with birch sulphate soup with the same consistency as in Example 1 above were used. Two batches were treated with the enzyme as described above and the third formed a control batch. The enzymes used differed from those in Example 1 in that the other (batch 1) gave a mixture of 56 U / g mass of xylanase activity, i.e. in addition, it has a lulase activity of 1.5 U / g of mass k.a. in addition it 11u1 assa activity 1.1 U / g mass k.a. The measurement results after the enzyme treatment are shown in Table 11a.

TABLE 11a enzyme age-j enzyme age- no enzyme hand tte- ί

• "- '- i I

processing processing (see batch) ·. *. * i (lot 1) 1 '(lot 2) * ___, __' number 15.8 15.0 18.8 ji - - - i '.....' - "-» - "- -------------—., ..- 4 (I 1; 32 0.26 i - L I ---; ------- j

viscosity I

....: (dm3 / kg) 1025 1130 1305.

_! ____ i ______________ The said batches of pulps were then bleached with chlorine chemicals and sodium hydroxide in the same manner as in Example I. In each case, the total dose of chlorine in mg / g was 2x the number of pieces. The results after bleaching are shown in Table 11 b.

7 81395 TABLE IIb --1 --—----—- f enzymatic- enzymatic- non-enzymatic manual11e processing processing (reference lot) (lot 1) (lot 2) __ ___ _ j__________________ _________ ___...... ...

yield 94.6-97.4 DKM extraction (% of k.) 0.36 and 0.35; 0.37 viscosity ti (dm3 / kg) 980 1065 (1210 "————— 'l total skloo-: ii rip concentration' 1 (mg / g) 1 1.15 | 1.15 1.46 1; _ _____________ ___.. Also in this case, the results show that the enzyme treatment substantially reduces the residual chlorine content of the bleached pulp.

For example, bleached birch wood was used in this experiment.

:: The pulp was bleached using chlorine (90%) and chlorine dioxide (10%) in the first stage, sodium hydroxide in the second stage and ‘1’ in the third stage as bleaching chemicals. oxygen.

The bleached pulp was subjected to the same conditions and using the same enzyme as in Example 1. After the treatment, the pulp was washed with water and dried at room temperature. Residual chlorine concentrations were measured from the enzyme-treated pulp and the untreated initial bleached sulphate pulp, resulting in the former 1.18 mg / g and the latter 1.56 mg / g. The enzyme treatment has thus substantially reduced the residual chlorine content of the bleached pulp.

The experiment shows that the invention achieves the desired result even if the cellulose pulp is first bleached and only then treated with an enzyme.

It will be clear to a person skilled in the art that the various embodiments of the invention are not limited to the example given above but may vary within the scope of the appended claims. For example, in the first step of conventional five-step chlorine bleaching, either chlorine alone or in a mixture with chlorine dioxide can be used. Likewise, the time for which the enzyme is allowed to act can be made considerably shorter than said 20 h. Treatment times of a few hours or even less than an hour are thus possible depending on the enzyme chosen and its activity.

Claims (9)

A process for bleaching cellulose pulp obtained from deciduous trees, in which a chlorine-containing bleaching chemical method is used, characterized in that the residual chlorine content of the bleached pulp is reduced by performing an enzym treatment for the pulp. Process according to claim 1, characterized in that the pulp is treated with enzyme before the bleaching treatment or thereafter. Method according to claim 1 or 2, characterized in that the enzyme treatment takes place in a temperature range 10-90 ° C, preferably 40-70 ° C, and in pH 3.0-7.5, preferably 4.0-6.0 . Method according to any of the preceding claims, characterized in that the enzyme used is hemicellulase, cellulase, esterase or their mixture. 5. A process according to claims 2-4, characterized in that an unbleached pulp, which is saturated by birch sulfate boiling, is added which is allowed to interact, after which the pulp is bleached by successive oxidation and extraction stages using chlorine gas and / or chlorine dioxide as bleaching chemicals in oxidation stage and sodium hydroxide in the extraction stage. Process according to claim 5, characterized in that the bleaching consists of a five-step chlorine bleaching. 7. Use of enzyme to reduce the residual chlorine content of bleached pulp of deciduous trees. Use of hemicellulase, cellulase, esterase or their mixture according to claim 7 to reduce the residual chlorine content of bleached cellulose pulp of hardwood. 12 81 395 Use of enzyme according to claim 7 or 8 to reduce the residual chlorine content of bleached cellulose pulp to be refined into food packaging board or paper. il