IMPROVEMENT OF OXYGEN BLEACHING OF PULP

Abstract: An improved method for oxygen bleaching of pulp comprises treatment of the pulp with hemicellulose-hydrolyzing enzyme.

Description: Improvement of oxygen bleaching of pulp Field of the invention This invention relates to an improvement of oxygen bleaching of pulp by enzyme treatment. Specifically, this invention relates to the use of hemicellulose hydrolysing enzyme with little or no cellulase activity. Background of the invention Oxygen bleaching of pulp in its different forms is known and it has been described e.g., in the publication The Bleaching of Pulp, by Singh . , P., Tappi Press 1979,pages 159-209. The increasing price of energy and the demand for protection of the environment have made it desirable to substitute chemical bleaching processes of pulp with pro¬ cesses demanding less energy, which would, in addition, make it possible to burn all or at least the main part of the waste liquor from bleaching plants in the con- ventional burning process of alkaline cooking. Oxygen bleaching of pulp by using sodium hydroxide as an effec¬ tive alkali directly after cooking is used today in many paper mills. This process makes it possible to diminish the amount of chlorine containing agents and sodium hydroxide used in bleaching plants and to recycle and burn about half of the dry substances of the bleaching effluents. Extended oxygen bleaching, however, causes extensive depolymerization of carbohydrates, thus dimin¬ ishing the paper properties of pulp. It has not, as yet, been determined how to extend the delignification of pulp - by using diminished amounts of chlorine, sodium hydroxide and oxygen and make it possible to burn as much as possi¬ ble of the spent bleaching liquors. Summary of the invention The object of this invention is to improve the oxygen bleaching of pulp by means of enzyme treatment. The invention is characterized by adding to the oxygen bleaching process a hemicellulose hydrolyzing enzyme or enzyme preparation. Preferably, the he icellulase enzyme or enzyme preparation is substantially free from cellulase activi¬ ty. According to the invention, hemicellulase enzymes are used in conjunction with oxygen treatment to reduce the amount of chlorine-containing agents and sodium hydroxide used in pulp bleaching processes. The enzyme treatment can be performed before, after or at the oxygen stage. Description of the invention This invention is based on the discovery that pure hemicellulose-hydrolyzmg enzymes or enzyme preparations containing hemicellulose-hydrolyzmg enzymes can be used in conjunction with traditional oxygen bleaching of pulp to decrease the amount of chlorine-containing and alka- line agents used during the process. According to the invention, enzyme preparations, most preferably containing hemicellulase, are used in conjunction with the standard bleaching procedure. It is preferred that the hemicellulase enzyme or preparation used is substantially free from cellulases. As used herein, the term cellulases refers to enzymes which are able to dissolve crystalline cellulose and to liberate therefrom sugars or oligosaccharides. The hemicellulase derived from Chainia sp. NCL 82-5-1 (ATCC No. 53812) is a good example of such an enzyme. The hemicellulase produced by that strain, which is the subject of copen- ding U.S. application Serial No. 224,590, has a low molecular weight active xylanase unit capable of good penetration into wood fibers where it can selectively degrade xylan. Other hemicellulase enzymes, as are known in the art, can be obtained from, for example, actinomy- cetes (such as Streptomyces olivochromoσenes) , bacteria such as Bacillus sp. and fungi such as Pencillium stec- kii. The enzyme according to the invention can also be obtained in such a way that a cellulase/hemicellulase preparation produced by Trichoder a lonαibrachiatum is purified by removing all or the main part of the cellu- lase activity. The suitable enzyme dosage calculated as xylanase is about 0.1-100 Units (U) per gram of dry solids (d.s.) of the pulp. The preferred level of enzyme is 0.5-25 U/g d.s. of pulp. The xylanase activity of the enzyme preparation is determined as follows: To 1 ml of xylan solution (1%, Sigma No.: X-0376, prepared in 50 C mM Na citrate buffer, pH 5.3), 1 ml of ' an enzyme suitably diluted in the same buffer is added. The solution is incubated at 50 C in a water bath for 30 minutes. The reaction is stopped by adding 3 ml of DNS reagent (3,5-dinit-rosalicylate reagent) and the color was developed by cooking the sample for 5 minutes. The absorbance was measured at a wave length of 540 nm. One enzyme unit liberates one micromole of reducing sugar calculated as xylose per one minute under the assay conditions. The cellulase activity of enzyme preparations was determined as filter paper activity (Ghose, T. , K. et al., Symposium of Enzymatic Hydrolysis of Cellulose, Bailey M. , Enari, T.M. , Linko, M. , Eds. (SITRA, Aulanko, Finland, 1975, p. Ill to 136) as follows: A piece of filter paper (Whatman 1, 50 mg) was added to 1 ml of acetate buffer (0.05 M NaAc, pH 4,8). 1 ml of a suitably diluted enzyme solution was added. The solution was incubated for one hour at 50 °C. The reac¬ tion was stopped by adding 3 ml of DNS reagent, and the color was developed and measured as in the xylanase determination. One activity unit liberates 1 micromole of reducing sugars calculated as glucose per one minute. The hemicellulase enzyme or enzyme preparation can be added before, during or after treatment with oxygen, most preferably, it is added before the oxygen bleaching stage. The treatment can be carried out within the pH ranges from about 2 to about 10, preferably from about 4 to about 8, depending e.g. on the origin and the proper¬ ties of the hemicellulase enzyme. Treatment time depends on the enzyme dosage and the treatment conditions, and it can range from 10 minutes to ' one day, preferably from 30 minutes to 8 hours. The temperature during the enzyme treatment may vary from about 10 to about 90 C, prefera¬ bly from about 25 to 70°C. The method according to the invention will be illus- trated by means of the following examples. The meaning of the examples is not to restrict the invention but to show the working of some embodiments according to this invention. Examples Enzyme of the example 1 and 2 was prepared by Chainia sp. NCL 82-5-1 strain. This strain was deposited with the ATCC as No. 53812 on July 27, 1988. Spores of Chainia sp. NCL 82-5-1 strain preserved on PDA slants face was transferred to sterile medium (e.g. 5% wheat bran, 1% yeast extract, pH 7.0). The medium was cultured in shake flasks for about 3 to 5 days at 30 °C in vigorous shaking speed (50-100 ml liquid, in 250 ml flask, 200 rpm) . The liquid was filtered or centrifuged clear. About 10 U/ml of xylanase was produced when wheat bran was used, more xylanase (at least 25 U/ml) was produced when pure xylan base was used. The enzyme thus produced was substantially free from cellulase. The enzyme used in Example 3 was produced by acti- nomycete Streptomvces olivochromocrenes using the method described in Evaluation of Different Microbial Xylanolytic Systems, Poutanen, K. , Ratto M. , Puls, J., and Viikari, L. , Journal of Biotechnology, 6 (1987) p. 49-60; the activity of enzymes: xylanase 2,500 U/g, cellulase 0 U/g. The enzyme used in Example 4 and 5 was Multifect K (Cultor Ltd.) produced by the fungus Trichoderma longib- rachiatum. The xylanase activity in the preparation was 5,200 U/ml and cellulase activity 4 U/ml. Example 1 Oxygen Bleaching I of Birch Pulp Finnish birch sulfate pulp was bleached; the origi¬ nal kappa number of the pulp was 20.5. The bleaching sequence was 0 -D-E-D-E-D. The chlorine dioxide used comprised about 8% chlorine. The control pulp was conventionally bleached but in order to make the conditions as comparable as possible to the enzyme treatment, the control pulp was held at 55 C and at pH 6 for four hours. Enzyme treatment of the pulp was performed before bleaching in the following conditions: - temperature: 55 C - time: 4 hours - pH: 6 - consistency: 10% - enzyme dosage as xylanase: 25 U/g of pulp d.s. After enzyme treatment (or incubation of control pulp) the pulp was washed and the normal oxygen bleaching was performed. In the oxygen stage, the following conditions were used; temperature: 90 C time: 30 min. 0 : 3 bar consistency: 10% NaOH: 2% In the first chlorine dioxide stage, an active chlorine dosage of 0.2 x kappa % of pulp d.s. was used. In the four last stages the dosages of chemicals were as follows: El, 0.1 x kappa % of pulp d.s.; Dl, 2.5% of pulp d.s.; E2, 0.8 % of pulp d.s.; D2, 1.0% of pulp d.s. The results of the experiments are shown in Table 1. Table 1 Bleaching Experiments/Oxygen Bleaching I/Birch Pulp Enzyme Control Treated Kappa number after enzyme treatment 20.3 18.3 Kappa number after oxygen stage 14.8 12.2 Brightness % 90.3 92.1 Viscosity kg/dm 3 1080 1170 TCI mg/kg 573 500 Yield enzyme treatment 91.0 90.7 + bleaching Despite the fact that the time of enzyme treatment was moderate (4 hours) , the brightness of the pulp increased a remarkable 1.8% units, even though the original brightness of the pulp was over 90%. Addition¬ ally, there were no yield losses. Example 2 Oxygen Bleaching II of Birch Pulp Finnish birch sulfate pulp was bleached; the origi¬ nal kappa number was 20.5. A bleaching sequence 0 2 ~ enzyme-D/C-E-D-E-D was used. The amount of chlorine dioxide was 90% and the amount of chlorine 10% calculated as active chlorine in the D/C stage. The oxygen stage was performed by using the follow- ing conditions: - temperature: 90 °C - time: 30 in. - 0 : 3 bar - consistency: 10% - NaOH: 2% After the oxygen stage, the pulp was divided into two parts. The control pulp was normally bleached, but in order to make the conditions of the treatment as comparable to the enzyme treatment as possible, the control pulp was held at 55 C and at pH 6 for four hours. The enzyme treatment of the pulp was performed after oxygen bleaching but before the D/C stage by using the following conditions: After enzyme treatment (or after reference treatment of the control pulp) the pulp was washed and the final bleaching using the sequence mentioned above was per¬ formed. At the D/C stage an active chlorine dosage of 0.2 x kappa % d.s. of pulp was used. In the four last stages the dosages of chemicals were as follows: El, 0.1 x kappa % d.s. of pulp; Dl, 2.5 % d.s. of pulp; E2, 0.8 % d.s. of pulp; D2, 1.0 % of d.s. of pulp. The results of the experiments are shown in Table 2. Table 2 Bleaching Experiments/Oxygen Bleaching II/Birch Pulp Enzyme Control Treated Kappa number after oxygen stage 14.6 14.6 Kappa number after enzyme treatment 13.6 11.7 Brightness % 89.6 92.1 Viscosity kg/dm 3 1015 1045 TCI mg/kg 291 252 Yield enzyme treatment + bleaching 91.8 90.9 The time of enzyme treatment was moderate (4 hours) , yet the brightness increased 2.5 % units, even though the original brightness was almost 90%. Additionally, the yield loss was insignificant. Example 3 Oxygen Bleaching III of Birch Pulp Finnish birch sulphate pulp was bleached according to Example 1 with the exception that enzyme prepared from Streptomyces Olivochromoorenes was used and the time of enzyme treatment was 8 hours. The results of the experi¬ ments are shown in Table 3. Table 3 Bleaching Experiments/Oxygen Bleaching Ill/Birch Pulp Enzyme Control Treated Kappa number after enzyme treatment 19.9 18.0 Kappa number after oxygen stage 14.7 12.0 Brightness % 90.2 91.1 Viscosity kg/dm 3 1080 1010 TCI mg/kg 560 510 Yield enzyme treatment + bleaching 91.5 87.7 ' Example 4 Oxygen Bleaching IV of Birch Pulp Finnish birch suifate pulp (original kappa number 20.8) was bleached after enzyme treatment and for refer¬ ence without enzyme treatment (control pulp) with the bleaching sequence 0 -D-E-D-E-D. The chlorine dioxide used comprised about 8% chlorine. The enzyme preparation used in the experiment was Multifact K produced by the fungus Trichoderma lon ibrac- hiatum. The reaction conditions used in the enzyme treatment before conventional bleaching were as follows: - temperature: 45 °C - time: 4 hours - pH: 5 - consistency: 10% - enzyme dosage as xylanase: .5 U/g pulp d.s. The control pulp was treated before bleaching using the same conditions as were used in the enzyme treatment but no enzyme was added to the mixture. After the enzyme treatment (or incubation of the control pulp) the pulp was washed and the oxygen bleach¬ ing was performed as described in Example 1. The results of the experiments are shown in Table 4. Table 4 Bleaching Experiments/Oxygen Bleaching IV/Birch Pulp Enzyme Control Treated Kappa number after enzyme treatment 20.5 19.5 Kappa number after oxygen stage 14.9 12.9 Brightness % 90.1 91.5 Viscosity kg/dm 3 1080 1120 TCI g/kg 570 500 Yield enzyme treatment + bleaching 91.1 90.9 Despite the fact that the time of the enzyme treat¬ ment was moderate (4 hours) , the brightness of the pulp increased a remarkable 1.4% units, even though the ■ original brightness of the pulp was over 90%. Addition- ally, there were no yield losses. Example 5 Oxygen Bleaching V of Birch Pulp Finnish birch sulfate pulp (original kappa number 20.8) was bleached. The bleaching sequence used was 0- enzyme-D/C-E-D-E-D and for the reference pulp 02- D/C-E-D-E-D. The ratio of chlorine dioxide to chlorine in the D/C stage was 90:10 calculated on the basis of active chlorine. The oxygen stage was performed as described in Exam¬ ple 2. After the oxygen stage the pulp was divided into two parts. One part of the oxygen treated pulp was treated before the D/C stage with enzyme by using the following reaction conditions: The enzyme used in the treatment was Multifect K produced by the fungus Trichoderma longibrachiatum. The other part of the oxygen treated pulp (the con¬ trol pulp) was treated before the D/C stage using the same conditions as were used in the enzyme treatment but no enzyme was added to the mixture. After the enzyme treatment (or after the reference treatment of the control pulp) the pulp was washed and the final bleaching was performed as described in Exam¬ ple 2. The results of the experiments are shown in Table 5. Table 5 Bleaching Experiments/Oxygen Bleaching V/Birch Pulp Control Treated Kappa number after oxygen stage 14.8 14.8 Kappa number after enzyme treatment 13.7 12.0 Brightness % 89.3 91.3 Viscosity kg/dm3 1015 1020 TCI mg/kg 280 270 Yield enzyme treatment + bleaching 91.7 91.1 Despite the fact that the time of the enzyme treat¬ ment was moderate (4 hours) the brightness of the pulp increased 2.0%, even though the original brightness was almost 90%. Additionally, the yield loss was insignifi- cant. Example 6 Oxygen Bleaching of Pine Kraft Pulp Finnish oxygen delignified pine kraft pulp (original kappa 17,2) was bleached after enzyme treatment and for reference without enzyme treatment (control) with the bleaching sequence D-E1-D1-E2-D2. Other conditions as mentioned in the example 1. The enzyme preparations used in the experiment were Multifect K (Cultor Ltd.) produced by the fungus Tricho¬ derma longibrachiatum. The reaction conditions used in the enzyme treatment before conventional bleaching were as follows: - temperature 45 C - time 2 hours - pH 5,7 - consistency 10% - enzyme dosage 5 xylanase U/g pulp d.s. The control pulp was treated before bleaching using the same conditions as in the enzyme treatment but no enzyme was added to the mixture. The results of experiments are shown in table 6. Table 6 Bleaching Experiments/Oxygen Bleaching/delignified pine pulp Control Trichoderma Enzyme dosage 0 5 (xylanase U/g pulp) Brightness (%) 87.6 88.7 The enzyme treatment improves the bleachability of oxygen delignified pulp even when only chlorine dioxide is used in the first bleaching stage. Oxygen delignified pulp is difficult to bleach to high brightness levels with high chlorine dioxide amounts in the first necessary stage. High amounts of chlorine dioxide in the first stage are needed if the generation of chlorinated organic compounds is to be decreased during bleaching. A conclusion can be drawn especially from examples 1 and 2 that by using the enzyme according to this inven¬ tion in connection with oxygen bleaching, it may be possible to omit the stages E2 and D2. This would result in remarkable savings in investments when building a new bleaching plant. From the examples it can also be concluded that a target brightness of 85-90% can be reached without the first D/C stage in the conventional bleaching if the pulp is treated with enzymes before or after the oxygen treatment. If so, the final bleaching after the enzyme treatment and the oxygen stage (or after the oxygen stage and enzyme treatment) would be the following: E-D-E-D. This means that more of the material dissolved in the bleaching process could be burned because there would be no drawback to burning the material dissolved in the El stage.

Claims: CLAIMS 1. In a process for the oxygen bleaching of pulp the improvement comprising treating the pulp with a hemi- eellulosehydrolyzing enzyme or enzyme preparation. 2. A method according to claim 1 wherein said hemi- cellulose-hydrolyzing enzyme or enzyme preparation is added prior to the oxygen-bleaching stage of the bleach¬ ing process. 3. A method according to claim 1, wherein said hemicellulose-hydrolyzmg enzyme or enzyme preparation is added after the oxygen-bleaching stage of the bleaching process. 4. A method according to claim 1, wherein said hemicellulose-hydrolyzmg enzyme contains less than 0.5% (i.e. 1/200) of cellulase activity. 5. A method according to claim 1, wherein the hemi¬ cellulose-hydrolyzmg enzyme is produced by a species belonging to the genus Streptomyces or Trichoderma. 6. A method according to claim 1 wherein said hemi- cellulose-hydrolyzing enzyme is a low molecular weight xylanase produced by Chainia sp. ATCC 53812. 7. A method according to claim 6, wherein said xylanase is substantially free from cellulose-hydrolyzing enzyme. 8. A method according to claim 6 wherein said xylanase is added in an amount of about 0.1-100 U/g calculated on the dry solids of the pulp. 9. The method according to claim 8 wherein the xylanase is added in an amount of about 0.5-25 U/g calculated on the dry solids of the pulp. 10. A method according to claim 6 wherein the xylanase treatment is carried out within the pH range from about 2 to about 10; at about 10 to 90 °C and the time of enzyme treatment is about 10 min. to 24 h. 11. The method according to claim 10 wherein the xylanase treatment is carried out at a temperature of from about 25 to about 70 C for a time period of from about 0.5 to about 8 hours and at a pH range from about 4 to about 8.

Inventors: KOLJONEN MARJA, KRUUS ILKKA, LAINE JAAKKO, KOLJONEN, MARJA, KRUUS, ILKKA, LAINE, JAAKKO
Applicants: CULTOR OY
Application year: 1990
Main classification: D21C9
IPC codes: C12N9, C12P19, C12R1, C12S3, D21C5, D21C9