JPH05209385A - Method for bleaching pulp - Google Patents

Abstract

PURPOSE:To effectively lower the total amount of lignin contained in pulp in a short time by employing an enzyme in stead of a chemical agent from the viewpoint of environmental protection. CONSTITUTION:A method for bleaching pulp is characterized by effectively dissolving the lignin contents of pulp with the cultured product of Bacillus licheniformis or with an enzyme containing at least one kind of hemicellulases of the Bacillus licheniformis in a relative short time in comparison with conventional methods and subsequently removing the dissolved lignin from the pulp without causing any significant lowering of the viscosity, i.e., the deterioration of the cellulose fibers.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to the use of an enzyme from Bacillus licheniformis to reduce the lignin content of pulp.

[0002]

BACKGROUND OF THE INVENTION Currently, most pulp is made from wood. The wood is composed of cellulose, hemicellulose and lignin as main components, each of which is about 35-5.
It contains 0%, 20-30%, 20-30%. Among them, hemicellulose is involved in cross-linking bond between cellulose molecules and fiber bundles, and lignin is said to surround and bundle cellulose fibers to provide strength and rigidity. In the paper manufacturing process, this lignin should be removed from the wood component. This is because it reduces the strength of the paper and gives the paper undesired properties such as browning.

[0003] Generally, wood chips are pulped in a process called Kraft process in which they are treated with sodium sulfide and sodium hydroxide. Most of the lignin is decomposed in this step, but some of it remains in the pulp as residual lignin. For example, kraft pulp contains about 4 to 12% by weight of residual lignin.

To further reduce the amount of such residual lignin, the pulp is subjected to a chemical treatment called bleaching. The most commonly used pulp bleaches at present are chlorine or chlorine-containing compounds such as hypochlorites (sodium hypochlorite and potassium hypochlorite), chlorine dioxide. A disadvantage of the bleaching process with these bleaching agents is that this process produces wastewater containing a large amount of highly corrosive chlorinated lignin degradation products. These chlorinated organics are said to be toxic and highly mutagenic and carcinogenic. Therefore, the treatment of this waste water poses a serious waste treatment problem. Further, since the highly corrosive chlorine compound in the wastewater corrodes the factory machine, the wastewater cannot be circulated and used in other paper manufacturing processes.
For these reasons, there is an eager need in the industry for alternative bleaching methods that reduce the chlorine required for bleaching.

Recently, hydrogen peroxide, oxygen and ozone have been used as alternative bleaching agents for chlorine compounds. However, these compounds alone also have various drawbacks. For example, if kraft pulp is subjected to delignification to the same extent as chlorine compounds in bleaching with hydrogen peroxide, the cellulose fibers will be deteriorated by more than an allowable amount. Degradation of cellulosic fibers is likely to occur even when bleached by oxygen or ozone. Such deterioration of the cellulose fibers causes a decrease in pulp yield and a paper having mechanically weak properties due to a decrease in pulp viscosity.

In order to overcome the drawbacks of the conventional pulp bleaching agents as described above, a non-contaminating biological bleaching method using an enzyme or the like is currently being developed. For example, there is a pulp bleaching method that directly decomposes lignin using Phanerochaete chrysosporium, which is a white Bacillus subtilis (Appl. Microbiol Bio
technol., 25, 484, 1987). However, bleaching using such a bacterium is extremely slow in delignification and has a satisfactory whiteness (value indicating the degree of whiteness of pulp or paper, JIS
7074 and JISP8123) and Kappa number (values close to the lignin content in pulp, JISP
8211) to obtain at least 3
It takes ~ 7 days.

On the other hand, there are some reports that xylanase is used for delignification of pulp because lignin is bound to hemicellulose. For example, Viikari et al.
(Proceedings of the International Symposium on Woo
d And Pulping Chemistry, Paris, 1987) and Paice et al. (B
iotech. and Bioeng., 32, 235, 1988), or JP-A-2-264087, JP-A-3-40887, WO91 / 02839, WO91.
/ 02840 mag. However, in any method, in order to effectively remove lignin in pulp,
It requires a relatively long enzyme treatment of 24 hours,
Considering the time available for the current bleaching process, it is still unsatisfactory.

[0008]

The present invention is based on the Bacillus
Licheniformis culture or Bacillus Licheniformis at least one hemicellulase-containing enzyme effectively dissolves lignin-containing material in pulp in a relatively short time, which is not available in the conventional method, and the dissolved lignin can be effectively used. It is an object of the present invention to provide a pulp bleaching method which is characterized in that the pulp is removed from the pulp without lowering the viscosity, that is, the deterioration of the cellulose fiber.

Further, by reducing the chlorine-containing bleaching agent from the conventionally used amount, it is possible to reduce as much as possible the corrosive polluted waste water generated in the conventional chlorine bleaching step. The aim is to provide a more acceptable bleaching method.

[0010]

The present inventors have surprisingly found that treating pulp with an enzyme or a culture derived from Bacillus licheniformis enables delignification to be carried out more effectively in a shorter time than conventional enzyme treatment methods. I also found it.

The Bacillus licheniformis strain is used in the present invention as an enzyme source for treating pulp. However, preferred strains are those that produce relatively high amounts of hemicellulase. On the other hand, the preferred hemicellulase for treating pulp is xylanase, more preferably endoxylanase.

Bacillus producing large amounts of xylanase
The licheniformis strain is, for example, SD516, which has been deposited at the Institute for Microbial Industry of the Agency of Industrial Science and Technology (Ministry of Industrial Research, No. 10427).

As an enzyme source for pulp treatment,
Bacillus licheniformis cell-free enzyme preparations, for example purified enzymes or crude extracts of cell cultures. Preferred is a supernatant obtained from a culture of Bacillus licheniformis. More preferably, it is concentrated to an appropriate concentration with a device such as an ultrafiltration membrane before use,
It has virtually no cellulase activity. Therefore, it does not damage the cellulose in the pulp.

The supernatant of Bacillus licheniformis is
Obtained from a culture of Bacillus licheniformis grown in the presence of a suitable carbon source for inducing the production of hemicellulase by the cell. Preferred are xylan, bran (mostly derived from wheat), mechanical and chemical pulp, wood chips and the like. More preferably, bran or hardwood kraft pulp.

For use in the method of the invention, a 12-72 hour culture supernatant of Bacillus licheniformis is preferred when optimal enzyme production is achieved. Generally, the enzyme concentration is 0.1 to 1000 xylanase units / g dry pulp (U / g), preferably 10 to 200 U / g.
The range is. One xylanase unit corresponds to 1 μmol xylose produced from birchwood xylan per minute at 50 ° C.

In the case where the enzyme is directly acted on the pulp as in the method of the present invention, preferably the temperature for enhancing the enzyme activity,
It is desirable to carry out at pH and time. That is, the enzyme treatment temperature is about 10 to about 80 ° C., more preferably 40 to 60.
It is performed at ℃. The enzyme-treated pH is preferably in the range of 3-8, more preferably 5-7. On the other hand, the time for the enzyme to act on the pulp to obtain good results is from 1 to
It is 24 hours, more preferably 1 to 3 hours.

For example, as shown in Examples, xylanase 2 derived from Bacillus licheniformis used in the present invention.
Enzymatic treatment of unbleached hardwood kraft pulp with 00U / g pulp removes most of the lignin that should be removed by a short enzymatic treatment of only 1 hour. It was also shown that the same effect can be obtained by treating the unblended kraft pulp of coniferous trees with the enzyme for only 1 hour. It is considered that these results do not exist in the methods shown heretofore, and it is considered that the processing time is sufficiently available as a part of the bleaching process using the chlorine-based chemical bleaching agent which is currently performed. Also,
The pulp can be enzymatically treated with Bacillus licheniformis-derived hemicellulase used in the method of the present invention for 3 hours or more, but as shown in the examples, only a slight decrease in Kappa number is recognized. You can hardly expect any merit.

The method of the present invention can be applied to a wide range of pulps and can reduce the amount of residual lignin in the pulp. For example, groundwood pulp, sulfite pulp, semi-chemical pulp, refiner ground pulp, kraft pulp and the like.

Following treatment with the Bacillus licheniformis culture or enzymes derived therefrom, removal of the dissolved lignin from the pulp is carried out by extraction. For example, sodium hydroxide extraction is commonly performed. Typical extraction conditions are 1-5% sodium hydroxide at 40%.
It is carried out at a temperature of ~ 80 ° C and a pulp concentration of 3-20%.
The residence time is 30 minutes to 3 hours, preferably 1 to 2 hours.

In order to improve the whiteness of pulp more effectively, it is desirable to use a chemical bleaching agent after the treatment according to the present invention. However, its use can be significantly reduced compared to bleaching pulp with chemical bleach alone. For example, when chlorine dioxide is used as a chemical bleaching agent, its amount can be reduced by at least 20%, more preferably by at least 40%.

It has also been found that papers made from pulps treated with Bacillus licheniformis cultures or enzymes thereof have enhanced quality over papers made from conventional bleached pulps. It is considered that this is because xylan, which is one of the hemicellulose components in the pulp, was effectively removed by the xylanase treatment, but the details are not clear.

Next, Bacillus licheniformis SD
A method for efficiently obtaining a mutant strain of 516 having enhanced xylanase production will be described. The methods can be roughly classified into the following methods. One is a method of obtaining an artificial mutant strain using a mutagen, and the other is a method of obtaining a genetically modified microorganism by genetic engineering.

For example, in the method using a mutagen, as a mutagen for inducing mutation, an alkylating agent such as N-methyl-N'-nitro-N-nitrosoguanidine (NTG) and a base analog such as 5-bromouracil. A compound, a base synthesis inhibitor such as azaserine, a dye such as acridine orange, or ultraviolet light (UV) can be used. Of these, the acquisition of mutants by UV irradiation is widely used because the mutagenesis mechanism has been studied in detail. By this method, a xylanase-producing mutant strain of Bacillus licheniformis SD516 can be easily obtained. That is, the size of halo (the xylan turbidity around the fungus is decomposed by xylanase and becomes faint) around colonies (bacterial colonies) that have grown on agar plates containing xylan. Highly producing strains can be isolated.

It is also possible to use a genetically engineered microorganism, ie a recombinant microorganism, as a strain for further increasing the production of xylanase and / or for producing a purer xylanase. A recombinant microorganism can be produced by using a mutant strain of the genus Bacillus as a host and introducing a hybrid plasmid into this host cell. For example, the mutant strain of Bacillus licheniformis used as the host is NTG.
It is preferable that the mutant strain is a double mutant strain having no cellulase producing activity and no xylanase producing activity. This double mutant strain can be easily selected on the minimal agar plate medium containing 1% carboxymethylcellulose or xylan as a main carbon source, since the halo does not exist around the fungus.

The hybrid plasmid is a plasmid constructed by incorporating the xylanase gene into an appropriate plasmid, and includes, for example, the restriction enzyme BamH.
Vector plasmid pUB110 cut with I and restriction enzyme Sau3
Bacillus licheniformis SD51 partially decomposed with AI
The xylanase gene obtained from the chromosomal DNA fragment derived from No. 6 was constructed by ligation of T4 DNA ligase or the like.

The hybrid plasmid can be introduced into a host microorganism by a technique such as fusion of protoplasts, transduction or transformation. In addition, the host microorganism into which the hybrid plasmid is incorporated can secrete cellulase-free xylanase extracellularly. Therefore, the selection of recombinant microorganisms having xylanase-producing activity can be easily carried out by the presence or absence of halos around colonies grown on an agar medium containing 0.5% xylan.

[0027]

The present invention will be further described below with reference to examples, but the invention is not limited thereto. Example 1 Bleaching of unbleached hardwood kraft pulp using a supernatant obtained from a culture broth of Bacillus licheniformis SD516 3 g of dried unbleached hardwood kraft pulp is suspended in 100 ml of ion-exchanged water and disaggregated. The enzyme prepared from the culture solution of Bacillus licheniformis is added to the pulp slurry so that the xylanase activity is 200 U / g pulp. The enzyme reaction pH was adjusted to 6. This pulp slurry is incubated at 50 ° C. for 1 hour, 3 hours or 24 hours, and the pulp is filtered and then washed with several times the amount of ion exchange water. It is then extracted with 2% (W / V) NaOH for 1 hour at 60 ° C. and washed with deionized water to neutral pH. The effect of enzyme treatment is compared by preparing a sample without enzyme. The Kappa number of pulp in each treatment is shown below. The Kappa number is measured according to JISP82.
The method of 11 was used. The dry pulp weight after 24 hours of enzyme treatment is 2.8 g.
Met.

Example 2 Bleaching of unbleached softwood kraft pulp using a supernatant obtained from a culture of Bacillus licheniformis SD516 Dry suspension of 3% unbleached kraft pulp of softwood is suspended in 100 ml of deionized water and disaggregated. The enzyme prepared from the culture solution of Bacillus licheniformis is added to the pulp slurry so that the xylanase activity is 200 U / g pulp. The enzyme reaction pH was adjusted to 6. This pulp slurry is incubated at 50 ° C. for 1 hour, 3 hours or 24 hours, and the pulp is filtered and then washed with several times the amount of ion exchange water. It is then extracted with 2% (W / V) NaOH for 1 hour at 60 ° C. and washed with deionized water to neutral pH. The effect of enzyme treatment was compared by preparing a sample without enzyme. The Kappa number of pulp in each treatment is shown below. Enzyme reaction time Kappa number 0 hours 21.8 1 18.6 3 17.5 24 17.2 Also, dry pulp weight after 24 hours of enzyme treatment was 2.8 g.
Met.

Example 3 Bleaching of unbleached hardwood kraft pulp from a broad-leaved tree using a supernatant obtained from a culture of Bacillus licheniformis SD516 Dry suspension of 3 g of unbleached hardwood kraft pulp is suspended in 100 ml of ion-exchanged water and disaggregated. Subsequently, the enzyme prepared from the culture broth of Bacillus licheniformis was used as xylanase activity for 10, 30, 60, 120, 200, 3
It is added to the pulp slurry so that it becomes 00 U / g pulp. The enzyme reaction pH was adjusted to 6. This pulp slurry is incubated at 50 ° C. for 3 hours, and the pulp is filtered and then washed with several times the amount of ion-exchanged water. It is then extracted with 2% (W / V) NaOH for 1 hour at 60 ° C. and washed with deionized water to neutral pH. The effect of enzyme treatment was compared by preparing a sample without enzyme. The Kappa number of pulp in each treatment is shown below.

Example 4 The broad-leaved tree unbleached kraft pulp was delignified using the culture supernatant of Bacillus licheniformis SD516, and then bleached with chlorine dioxide. 3 g of the dried broad-leaved tree unbleached kraft pulp was suspended in 100 ml of deionized water. Disaggregate. Then, the enzyme prepared from the culture solution of Bacillus licheniformis is added to the pulp slurry so that the xylanase activity will be 200 U / g pulp. The enzyme reaction pH was adjusted to 6. This pulp slurry is incubated at 50 ° C. for 3 hours, filtered, and then washed with several times the amount of ion-exchanged water. Subsequently, the pulp concentration was adjusted to 3%, chlorine dioxide was added so that the effective chlorine amount was 0 to 2.0% based on the dry pulp weight, and bleaching treatment was performed at 60 ° C. for 3 hours. After treatment, filter and wash,
Ion-exchanged water was added so that the pulp concentration would be 3%.
It is then extracted with 2% (W / V) NaOH for 1 hour at 60 ° C. and washed with deionized water to neutral pH. The effects of enzyme treatment were compared by preparing samples that were treated with chlorine dioxide only. The Kappa number of pulp in each treatment is shown below.

Example 5 Acquisition of xylanase high-producing mutant strain from Bacillus licheniformis SD516 L medium (0.5% yeast extract, 0.1% polypeptone, 0.5% Na)
The cells of Bacillus licheniformis SD516 grown in Cl, pH 7.5) are collected by centrifugation and washed several times with physiological saline. After suspending the washed cells in physiological saline again, U was added so that the mortality of the cells would be 99.9% or more.
Irradiate V. After diluting the UV irradiation liquid appropriately, L
Spread on an agar plate medium containing 0.5% xylan. After culturing at 35 ° C., a strain with high xylanase production was isolated based on the size of the halo around the grown colony. The obtained mutant strain was 3% on an agar plate medium of L medium.
After culturing at 5 ° C, the cells were stored at 4 ° C.

The xylanase productivity of the mutant strain was compared with that of the parent strain by the following method. That is, 2 L of medium (pH 7.5) containing a xylan source (for example, 7% wheat bran or 1% xylan) in L medium was placed in a 5 L jar famentor and sterilized at 121 ° C. for 30 minutes. After inoculating the mutant strain, vigorous stirring (usually 800 to 1000 r) until peak xylanase activity at 35 ° C (usually 1 to 2 days)
pm) and cultured. The ventilation rate was 1 to 2 L / min. After xylanase activity peaks,
Microbial cells and other solids were removed by conventional methods such as filtration or centrifugation to obtain a clear oven liquor or supernatant. The xylanase activity of the crude enzyme solution thus obtained was about 10 U / ml when 7% wheat bran was used as the xylan source. This value is SD5
16 of the xylanase activity when cultured in the same manner was about 1.
The value was five times higher.

[0033]

According to the method of the present invention, the lignin-containing substance in the pulp is effectively dissolved in a relatively short time, and the dissolved lignin causes a significant decrease in viscosity, that is, deterioration of the cellulose fiber. Can be removed from the pulp without. Further, by reducing the chlorine-containing bleaching agent from the conventionally used amount, it is possible to reduce as much as possible the corrosive contaminated wastewater that accompanies the conventional chlorine bleaching step. An acceptable bleaching method is provided.

Front page continued (72) Inventor Noritaka Noguchi 2-24-25 Tamagawa, Ota-ku, Tokyo Showa Denko Co., Ltd.

Claims (3)

[Claims] 1. A method for bleaching pulp, comprising:
Bacillus licheniformis
Or a lignin-containing substance in the pulp is dissolved by treating the pulp with an enzyme containing at least one hemicellulase of Bacillus licheniformis, and the dissolved lignin is removed from the pulp. 2. The method according to claim 1, wherein the hemicellulase is xylanase. 3. Bacillus licheniformis SD51
The method according to claim 1, which is 6 or a mutant strain thereof.