CA2130906A1 - A method for the enzymatic treatment of chemical pulp before bleaching - Google Patents

Abstract

ABSTRACT
Provided is a method for enlarging the degree of delignification in chemical pulping without lowering the viscosity of the resulting pulp. According to the method, an unbleached pulp or an oxygen-delignified pulp is held in an acidic condition before bleaching and treated with a xylanase exhibiting the activity at pH 4.5 or less. The bleached pulp has a level of whiteness applicable to commercial use. The amount of chlorine-containing compounds to be used for bleaching may be reduced, and the generation of organic chlorine compounds harmful to environment may be reduced.

Description

2~9~
SPECIFICATION

A METHOD FOR THE ENZYMATIC TREATMENT OF
CHEMICAL PULP BEFORE BLEACHING

TECHNICAL FIELD
The present invention relates ~o a method of enzyma-tic pretreatment of chemical pulp. Especially, lt relates to -the method that reduces, or may make unnecessary the use of the molecular chlorine in the bleaching process by incorporating a combination of an enzymatic treatment and an acid treatment.
BACKGROUND ART
The molecular chlorine is used in bleaching, the process effluent may contain dioxin, which reportedly harms human beings and other living organisms. In order to eliminate such hazards, research studies ara being carried out to develop a chemical pulp bleaching process that reduces or makes unnecessary the use of molecular chlorine.
Fibrous chemical pulp is widely employed as raw materials in the paper-making industry. The chemical pulp is normally made from wood, which mainly consists of cellulose and a three-dimensional polymer, called lignin.
Lignin is thought to extend into matrixes formed of cellulosic polysaccharides and hPmicellulosic 6~ 9 ~ 6 polysaccharides.
Normally, bonds between those different components are via different chemical bonds; for ins-tance, a lignin block reportedly has a part bonding to a hemicellulose molecular chain. The hemicellulose is thought to be a secondary component of chemical pulp. The main hemicellulose in hardwood is glucuronoxylan, which includes polymers formed mainly of D-xylose and are hereinafter referred to as xylan.
In order to produce chemical pulp having sufficient strength and whiteness to be made into paper, the raw chemical pulp must, first, be processed to remove lignin.
The primary step to remove lignin from wood is conducted in a digester in the presence of chemicals, e.g., NaOH, sodium sulfide (in the Kraft pulping process), a sulfite salt (normally Na-salt or Mg-salt in sulfite pulping), or NaOH and an anthra~uinone compound (in soda-AQ pulping).
This primary step is called delignification.
The amount of lignin remaining in wood pulp is determined by an oxidation test using a permanganate and conducted according to TAPPI Test Method T-236 cm-85 standardi~ed by Nippon PUlp and Paper Industrial Technology Association, and the result is reported as a Kappa Number.
The wood pulp after the delignification step contains a significant amount of lignin, but in certain cases can be ~3~90~

used without further purification for card board paper or bag paper. In most cases, however, the wood pulp must be bleached in order to be of use for, e.g., printing paper, writing paper, sanitary paper, and the like, as it is too dark.
Conventional methods of delignification and bleaching employ 3- to 6-stage bleaching processes optionally combined with a washing step between each of the stages.
Such processes are called multi-stage bleaching processes or bleaching sequences. The object of bleaching chemical pulp is to provide a bleached pulp having a whiteness enough to make it into paper products and thin paper products. By undergoing such a bleaching se~uence, a bleached pulp having a whiteness of 85 to 90-~ is obtained.
The conventional bleaching process is based on the use of chloride or chlorine-containing chemicals, namely, chlorine dioxide and a hypochlorite salt. The stage that makes use of chlorine is called C or C-stage, that which makes use of chlorine dioxide D or D-stage and that which makes use of a hypochlorite salt H or H-stage.
The C, in certain cases C/D, meaning the C and the D
combined, normally comes first, followed by extraction using an alkaline medium and this extraction stage is called E or E-stage. In the C, the dosage of chlorine (or ~5 chlorine and chlorine dioxide combined, and represented by ~ 6~
`~ , equivalence of the total oxidative chlorine) varies in proportion to the amount of lignin (as represented by the Kappa number) remaining in the wood pulp to be processed.
Said E-stage dissolves most of the remaining lignin chlorinated and oxidized in the preceding stage. Some amount of hemicellulose is removed in this stage.
In view of ever stringent environmental regulations being enforced to reduce water pollution attributable to the chlorine-containing waste, it is desired to reduce, or most dQsirably to cease, the use of the chlorine or chlorine-containing bleaching chemicals, instead of investing in costly pollution reduction processes.
In chemical pulping, the waste liquor of the digestion step is treated by recovery processes and burned to recover the chemicals. To obtain a bleached chemical pulp, the digested and fibrillated pulp is further bleached using chlorine or chlorine-containing chemicals, as mentioned above, and the bleaching waste from naturally contains chloride ions or compounds so that it cannot be fed back to the recovery processes. This is because if it is fed back to reactors where digested wastes are recovered, the chloride ions or compounds in them will damage the evaporator and the recovery kiln. From the recovery point of Vi2W again, an alternative bleaching method not employing any chlorine has been sought.

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A noteworthy technical de~elopment of the past two decades which is in line w:ith the efforts to reduce chlorine-containing waste is the use of oxygen as a delignifying or bleaching agent.
Processing of wood pulp by oxygen mostly takes place after the digestion stage and its purpose is to remove lignin prior to the bleaching process. Oxygen is fed to the unbleached pulp under pressure and in an alXaline medium to oxidi~e, decompose and remove the lignin remaining in the unbleached pulp. Conventionally, this processing is called oxygen bleaching or oxygen delignification (hereinafter referred to as O or O-stage).
There is a limitation on the amount of oxygen to be used for the delignification, since conditions to cause excessive oxygen consumption could negatively affect the degree of polymerization (DP) of the cellulose in -the unbleached pulp.
DP is an index of the pulp strength and is represented by pulp viscosity according to the TAPPI Standard Test Method T-230 om-89). In general, the limit of delignification in the O-stage, while retaining the level of the degradation of pulp viscosity to within an allowable limit, is said to be at most an approximately 50% reduction in the Kappa number. The delignification with oxygen at this stage does contribute to a substantial reduction of ~ 3~9~

the chlorine or chlorine-containing compounds in later stages. The amount of the chlorine-containing compounds to be used in the chlorination step depends on the lignin content in pulp, so that it may be substantially reduced due to the previous delignification with oxygen.
After treatment in C-stage, the pulp is usually subjected to alkaline extraction to remove the chlorinated lignin. There are a variety of known methods to use an oxidizing agent and the alkali in combination, for instance;
(1) combination of alkaline extraction with 2 gas, hereinafter referred to as Eo.
(2) combination of alkaline extraction with hydrogen peroxide (P), hereinafter referred to as Ep.

(3) combination of alkaline extraction with hypochlorite salt (H), hereinafter referred to as Eh.
(~) combination of alkaline extraction with h~drogen peroxide and 2 gas (hereinafter referred to as Eop).
(5) combination of alkaline extraction with hypochlorite salt and 2 gas (hereinafter referred to as Eoh).
These may be referred to as oxidative extractions.
Recently, the use of enzymes for processing chemical pulp (hereinafter referred to as X or ~-stage) has been developed. For instance, it is known that ligninase, especially that to be obtained from white rotting fungi 9 0 ~

decomposes lignin. In addition, it is also known that cellulose decomposes cellulose.
Looking at the hemicellulose componen-t of chemical pulp, a variety of papers have reported on the influences of xylanase upon wood pulp. As a result of such research efforts, it has been clarified that xylanase reacts selectively with xylan in the hemicellulose. The French patent application no. 2557894 (laid open in 1985) describes the treatment of a hardwood blPached pulp, or coniferous bleached pulp, with a solution containing xylanase in order to reduce the power to beat pulp required for paper making.
The use of enzymes for pretreatment before bleaching, or as a bleaching aid, was first a-ttempted by Viikari et al. (Proceedings of International Symposium on Wood and Pulping Chemistry, Paris, lg87). They treated unbleached Kraft pulp of birch and pine with enzyme having a high level of ~ylanase and xylosidase activity which they obtained by culturing Asperqillus awamori, VTT-D-79125, or StrePtomYCeS olivochromoqens, VTT-E-82157. The thus-treated pulp, in turn, was treated with hydrogen peroxide, then further treated with a (D+C)-E bleaching sequence. They observPd that the enzyme-pretreatment promotes more purification than the untreated control by a 2-points Kappa-number reduction. The enzymatic 2~90~

pretreatment was conducted at a pH of 5.0, a temperature of 45~C and over a period of 24 hours. Under these conditions, hardly any metal salts contained in -the unbleached pulp, particularly transition metal salts, removed, so that, if excess enzyme was not added, i-t was hard -to bleach the pulp to a higher whiteness in later stages without greatly decreasing pulp viscosity, though the enzymatic pretreatment needed a long period of time of 24 hours.
Paice, Bernier, and Jurasek treated a hardwood unbleached Kraf-t pulp with enzymes produced by cloned E.
coli capable of endoxylanase or ~-xylosidase production that they had prepared by genetic manipulation of the respective genes. Then, they carried out alkaline extraction or C-E-D treatment of the pulp and compared it with the non-treated con-trol (Biotechnology and Bioengineering, Vol. 32, July, pp. 234 to 239, 1988). The pulp thus treated with enzymes and alkali substances had a high Kappa-number, so that it must be further subjected to C-E-D treatment using chlorine in order to bleach it to have a higher degree of whiteness. The use of chlorine inevitably involves the formation of toxic chlorinated compounds like dioxin, etc., and the bleach effluent cannot be recovered in a black liquor recovery system, as it contains chloride ions.

g ~ 6 Japanese Patent Application Kokai No. 2-264, 087 discloses ~ technique to obtain pulp having a practical whiteness level by treating lignocsllulose substances in pulp, first, with xylanase containing substantially no cellulase, then by treating the resulting pulp further in one or more additional states selected from C, D, E, P, H, ozone, NO2 and oxidative alkali extraction, before or after bleaching pulp with oxygen.
In this technique, the enzymatic treatment was conducted at a temperature of 20 to 80C, over a period of 1 to 48 hours, and at a pH of 4 to 8. As the xylanase activity was measured at a pH 6 . 0, thP pulp was thought to be treated at about that p~, namely, the use of enzymes having the maximum activity under neutral to acidic conditions was a premise. In all the examples in the specification, post bleaching was conducted beginning with C, D or C/D, and the examples indicate that the use of chlorine or chlorine-containing bleaching agents is inPvitable in order to obtain a high level of whiteness.
This means that the enzymatic treatment under neutral to weakly-acidic conditions does not result in the removal of metal salts with the result that the remained metal salts decompose the bleaching agents, thereby making efficient bleaching impossible.
Japanese Patent Application Kokai No. 2-293,486 _ g _ 2 ~L 3 ~ 6 discloses a bleaching process r in which unbleached pulp is enzymatically treated before or after the primary oxidation t stage with oxygen, and is then treated by repeated C, D or C/D with E or Eo in between. In this process again, the pH of the enzymatic treatment, as described, was in a range o 3.0 to 10Ø Preferably, it is in a range of 4.0 to 9Ø Namely, the use of the enzyme that exhibits the maximum activity in a neutral to weak acidic environmen-t ~ is a premise. Therefore, the subsequent bleaching ¦ 10 treatment with C or D was indispensable in order to obtain ¦ pulp having a high degree of whiteness.
¦ According to the bleaching process not using molecular chlorine at all, treatment of pulp under strong acidic conditions was not conducted throughout the process so that metal salts, especially transition metal salts remained in the system to decompose the oxidizing agent to be used for the bleaching. As a result, the bleaching effect of the agent was lowered, and the active oxygen, etc. to be formed during the decomposition attached the cellulose chains in pulp being treated to lower the degree of polymerization of the cellulose moieties therein, with the result that the quality of the thus-treated pulp was lowered. In addition, since the potency of the oxidizing agent used was lowered, some excess amounts of the agent must be added. Such noticeably detracted from the economical advantage in ~; ~
2 3L 3 ~ 3 6 ,,~, chemical pulping, and the lowering of the quality of the thus-bleached pulp was at an unacceptable level.
In order to remove or inactivate metal saits, there are hitherto known, (1) pretreatment wi-th acid, and (2) blocking with a chelating agent. However, these treatments per se have no delignification effect and the amount of the oxidi~ing agent to be used is still large to such a degree '~ that it lowers the quality of the thus-treated pulp.
The use of the D-stage as the primary stage in place of the C-stag~ may be adopted. In this case, however, the bleach effluent does contain chloride ions so that it cannot be ed back to the bleach liquor recovery processes but is drained as a waste fluid. Moreover, although the amount of chlorinated organic compounds is not so much, environmental pollution attributable to toxic chlorinated compounds cannot be eliminated.
DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method of bleaching chemical pulp which is more acceptable from an environmental point of view, in which the use of molecular chlorine is less than that in a conventionally known mathod, or its use has been completely eliminated, and which enables the transfer of at least a part of the bleach effluent to the black-liquor burning processes so A ~

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; as to recover it.
Another object of the present invention is to provide '~ a method b,v which the level of delignification of chemical pulp is improved without dec:reasing the viscosity of the pulp, and the whiteness of the pulp can be raised efficiently.
MEANS FOR SOLVING THE PROBLEMS
In order to accomplish the objects of this inven-tion, it is necessary to promote the delignification of chemical pulp and to remove metal salts that cause a decrease in the ~ viscosity. The inventors have discovered that, by applying ¦ the following two treatments in combination before bleaching, the pulp can be bleached in later bleaching stages to a regular whiteness level without causing a decrease in the viscosity and while reducing substantially, or even eliminating, the use of molecular chlorine;
(1) removal of metal salts by acid treatment and promotion of delignification.
(2) promotion of delignifica-tion through enzymes.
Furthermore, by selecting an acid-resistant enzyme, said two treatments were combined into a single stage to complete the present invention.
According to the first aspect of the present invention, a method is provided for pretreating chemical pulp before bleaching, in which the pulp is treated with 3090~
`1 an acid, washed, and then -treated with enzymes including xylanase.
According to the second aspect of the present invention, a method is provided for pretreating chemical pulp before bleaching, in which the pulp is treated with enzymes including xylanase, treated wi-th an acid, and then washed.
According to the third aspect of the present invention, a method is provided for pretreating chemical pulp before blsaching, in which the pulp is treated at a pH of 4.5 or lower with enzymes including xylanase that exhibit the maximum activity at a pH of 4.5 or lower, and then washed.
According to a further aspect of the present invention, the enzymes to be employed in the ~irst or second aspect of the invention are characterized in tha-t they include xylanase and exhibit the maximum activity at a pH 4.5 or lower.
According to a still further aspect of the present invention, the chemical pulp is either unbleached pulp or oxygen-bleached pulp and, befors being subjected to acid treatment, the pulp is treated with hydrogen peroxide or ozone. Furthermore, the acid to be employed to lower the pH is at least one selected from acetic acid, formic acid, oxalic acid, propionic acid, sulfuric acid, sulfurous acid, 2~3~g~

nitric acid and ni-trous acid.
According to a still further aspect of the present invention, the liquid separated by solid-liquid separation after the washing step is transferred back to the prior step, while the waste is eventually burned to recover the inorganic salts.
An additional bleaching process that follows may comprise any one, two or more of the following bleaching stages (1) through (7);
(l)--To treat the pulp with chlorine, chlorine dioxide, or a mixture thereof in an aqueous medium.
(2)--To treat the pulp with a peroxide in an alkaline aqueous medium.
(3)--To treat the pulp with a peroxide and oxygen in an alkaline aqueous medium.

(4)--To treat the pulp with a hypochlorite salt in an aqueous medium.

(5)--To treat the pulp with ozone in air or in an aqueous medium.

(6)--To treat the pulp with thiourea dioxide in an alkaline aqueous medium.

(7)--To treat the pulp with a hydrosulfite salt in an alkaline aqueous medium.
While the enzyme to be employed in the present invention is not specifically limited so long as it 213~9~5 exhibits ~ylanase activity, those that e~hibit xylanase activity under acidic conditions, particularly at a pH of 4.5 or lower are preferred, and as such, hemicellulases, cellulases, pectinases, esterases, and any mixture thereof may be used.
~ For the enzymatic treatment, the pulp consistency is ¦ adjusted to 2 to 12~ by weight, and the enzyme is present in an amount of 0.1 u/g to less than 1,000 u/g, preferably 1 u/g to less than 100 u/g, of the pulp in terms of xylanase activity, and the pulp is treated with the enzyme at a temperature of 0C or higher, preferably 40C or higher, for at least 10 min.
The first and second aspects of the present invention will be explained in detail.
As aforementioned, the enzymatic treatment is carried out on unbleached or oxygen-delignified pulp before the conventional C-stage. The effect of the enzymatic treatment is enhanced by carrying it out in combination with acid treatment. The enzymatic treatment may be performed either before or after the acid treatment; if it is done afterward, there is no need to control the pH; if it is done before, the waste from the acid treatment can be used to control the p~. The pH of the pulp under the enzymatic treatment is 4.5 or lower, preferably lower then 3Ø Of course, a certain effect of the treatment can be ^--2~3~90~

achieved even if the pH is higher than 4.5.
The enzymatic treatmen-t can cleave xyloside bonds in ` lignocellulosic materials to relieve lignin in-to the waste.
There are some limitations in terms of the cleaving capability; one is that the size of the enzyme molecule is too large for it to penetrate a dep-th of hemicellulose or cellulose matrix where lignin-xyloside bonds are present, with the result that the enzyme cannot act on the bonds and therefore the degree of delignification by the enzyme is limited. According to experiments conducted by the inventors, the degree of delignification achieved by the enzymatic treatment in the case of hardwood Kraft pulp is l about a 2.0 point Kappa number reduction. Attempts to achieve a higher degree of delignification result in a great decrease in the yield of the cellulose component.
Moreover, the further enzymatic reaction with lignocellulosic materials takes considerable time, so that such attempts are hardly practical from an industrial point of view.
In view of that limitation, it is necessary to develop a different approach to the delignification, and the inventors thought of applying an acid treatment (hereinafter referred to as A or A-stage) in combination with the enzymatic treatment. Chemical pulp digested essentially with an alkaline medium has a porous structure ,1 2130906 -, ~
(as it has been swollen with an alkali), and it is possible '~ to effectively and additionally dissolve out lignin from the pulp having such a structure, using various media.
Addition of an acid to the porous pulp results in acceleration of the dissolution of lignin from the pulp.
The reason is thought to be that, in the case of a mineral acid, the bond between lignin and polysaccharide is cleaved by hydrolysis and that the movement of lignin is made easy by acid hydrolysis of the low-molecular hemicellulose in lC the pulp; and in the case of an organic acid, its affinity for lignin promotes the dissolu~ion of lignin.
Acids to be used for said purpose may be an organic acid, e.g., acetic acid, formic acid, oxalic acid, propionic acid and the like, or an inorganic acid, e.g., sulfuric acid, sulfurous acid, nitric acid, nitrous acid and the like.
Another effect of the A-stage is the dissolution of the metal salts contained in the pulp. The acid decomposes . metal salts, which are removed in the later washing step.
~'~ 20 The removal of heavy metals, in particular of transition metals, inhibits the decomposition of an oxidizing agent s"' in the later bleaching stages using hydrogen peroxide and .
~- ozone, and further inhibits the formation of an active ~' chemical nucleus, like a hydroxyl radical, which attacks .~; 25 fiber. This enhances bleaching effect and prevents a .~
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decrease in the degree of polymerization of the pulp.
The present invention com:bines the effects of acid and of enzyme to enhance bleaching effect without degrading the quality of the pulp.
When the acid-resistant; enzyme and an acid are employed in combination, both of the treatments are combined into a single stage and the process can be simplified.
Experiments conducted by the inventors on the acid extraction of lignin from lignocellulosic materials, which were digested and oxygen-bleached, revealed that delignification degree by about a 2-point Kappa-number reduction can be accomplished by the acid extraction treatment. Additional enzymatic treatment on the thus-obtained lignocellulosic materials makes it possible to achieve a level of whiteness demanded from a practical point of view by applying a moderate intensity of bleaching in the la-ter stages while a decline in the viscosity is inhibited.
As for the acid treatment conditions, the period of time is at least 10 minutes, preferably 30 minu-tes to less than 3 hours; the temperature at least 20C, preferably 40C to lower than 80C; -the consistency of the pulp 3~ to less than 20~. The acid treatment, even when applied after the enzyma-tic treatment, can accomplish a specific ~13~90~
delignification comparabl~ to the same as when it is applied before the enzymatic treatment. The addition of a peroxide during the acid treatment (hereinafter referred to as Ap or Ap-stage) significantly advances delignification and bleaching. It is known that an acid and hydrogen peroxide make under certain conditions a peracid, which specifically reacts with lignin and decomposes it. The addition of a small amount of mineral acid, e.g., sulfuric acid, accelerates the generation of such peracid.
Said acid treatment is effective also for ozone oxidation of chemical pulp. Ozone reacts with most organic substances or materials and water. The reaction proceeds via intermediates, like peroxides, epoxides, hydroxyl radicals and the like, some of which have a bleaching effect. Ozone reacts preferentially with aromatic nuclei, olefin groups and the like, selectively attacking lignin having these. If chemical pulp has a few or no such nuclei or groups so that the possibility of the ozone reaction in chemical pulp is low, ozone attacks carbohydrates, namely cellulose. Particularly, ozone may intensively attack fiber unless the pH is controlled properly, even if lignin is present abundantly in pulp. This is thought to be due to an increased generation of hydroxyl radicals caused by the decomposition of ozone when the pH is high. It is 2~3~90~

therefore manda-tory in applying ozone for bleaching to control the pH of pulp stock be:Eore bleaching on the acidic side, and in view of this, said acid treatment with the enzyme exhibiting activity on the acidic side is reasonable. The presence of metals, transition metals or their salts in particular, in the o~one bleaching system accelerates the decomposition of ozone supporting the attack on carbohydrates resul-ting in a degradation of the pulp. In order to prevent such detrimental effects, either the removal of the metals by acid treatment of the use of chelating agents is required.
As another bleaching option, the use of a reducing bleaching agent, e.g., hydrosulfite, may be considered~
This agent is easily oxidized, and when the pulp consistency is high, it comes in frequent contact with oxygen in air, but such is unfavorable. Therefore, the pulp consistency is normally kept at about 4~ during bleaching. At such a low consistency, the concentration of the agent is limited. This is because even though the amount of hydrosulfite is desired to be increased so as to make the bleached pulp have a high level of whiteness, the consistency of the agent in water cannot be elevated so much. Thus, the increase in the level of the whiteness of the bleached pulp is limited and the color reversion after bleaching tends to be intense.

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Thiourea dioxide is still another reducing bleaching agent. This is a stable whi-te powder and has no reducing property at room temperature. When i-t is heated or brough-t into contact with an alkali, it decomposes to generate sulEinic acid, which enters into a strong reducing reaction. It i5 therefore used with less care in terms of oxidative decomposition as compared with hydrosulfite; it is fed to the pulp along with an alkali and heated -to accomplish an increase in the level of whiteness of the bleached pulp that would hardly be achievable by the use of the other reducing bleaching agent, hydrosulfite.
The conditions for bleaching by thiourea dioxide (hereinafter referred to as FAS or FAS-stage) are as follows: Since thiourea dioxide is more stable than hydrosulfite, i.e., its stabili-ty is not influenced by the pulp consis~ency (in water), bleaching with thiourea dioxide is preferably conducted at a medium pulp consistency of about 15%, rather than a low consistency thereof of about 4%. This is because the concentration of the chemical to be added may be increased at such a medium pulp consistency so that the level of whiteness of the bleached pulp may be elevated. A further rise in the consistency, e.g., to abou-t 30%, results in a decrease in the level o:E whiteness, so that the pulp consistency is preferably 10-to 20%. The unfavorable effect of the higher ~30~0~

pulp consistency resulting in the decrease in the level of whiteness of the bleached pulp is due to oxidation of the agent by air like in -the case of hydrosulfi-te. When the air is substituted by nitrogen, such unfavorable decrease in the level of whiteness doe not take place even if the bleaching consistency is high. The resul-ts of the experiments in which the pulp was tested at 40C, 60C and 80C, show that 80C is the most favorable temperature.
Using a temperature not lower than 100C, however, is not practical since the bleaching reac-tion has to be conducted under pressurized conditions. From a practical poin-t of view, it is preferable that for the present invention a temperature of 50 to 100C is applied. As to the reaction time, an appreciable gain in brightness is accomplished within 15 minutes and the amount of gain reaches near maximum after 60 minutes, while the brightness declined after 24 hours. It is there*ore preferable to adopt a reaction time of 10 to 90 minutes.
Now, the third aspect of the present invention will be explained in detail.
The effect of xylanase to be used in the present invention is limited by the pH condition. If the pH
condition for xylanase overs-teps the defined range, then it loses its activity accordingly. Such a characteristic ~5 of xylanase results in various disadvantages for the pH

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adjustment and, after all, in a bar to -the object for attaining the intended delignification effec-t, when the application of xylanase to the pretreatment of pulp before bleaching is taken into consicleration.
The first bleaching stage that follows the enzymatic pretreatment is essentially C, C/D or Z in which a bleaching reaction takes place under acidic to strongly acidic conditions at pH of less than 5. Therefore, if -the enzymatic pretreatment is carried out under neutral or alkaline conditions jus-t before the bleaching stage, the effluent from the following acidic processes including treatment with chlorine cannot be recycled in a counter-currentwise manner to the previous enzymatic pretreatment stage. Considering the enzymatic treatment as the pretrea-tment to be conducted prior to the bleaching treatment carried out under acidic conditions, the purpose of the enzymatic treatment cannot be accomplished sufficiently as the use of conventional xylanase favors a too high pH.
The present inventors have contrived a way of shifting the activating pH region of the enzyme to a strongly-acidic pH range to complete the present invention.
The present inventors have used an acid-resis-tant xylanase (which is resistant to strong acids) to treat an ¦ 25 unbleached or oxygen-bleached pulp, and obtained a bleached ~2~3~9~' pulp by applying the C, D, C/D or Z under acidic conditions in the later bleaching stages. The thus-obtained bleached pulp had a sufficiently high level of whiteness and was usable as a raw material for printing paper and writing paper. The inventors have confirmed that the effluent from these acidic bleaching stages can be recycled in a countercurrentwise manner to the enzymatic treatment stage.
The acid-resistant xylanase that was used showed a sufficient xylan-decomposing activity at a pH of as low as 2.0 or lower and exhibited a delignification effect even when the pulp was prepared to have a pH value falling within the range. The enzyme to be preferably used in the present invention should be one that exhibits the maximum activity at pH of 4.5 or lower. Such an enzyme is commercially available, namely as "Amano P" (manufactured by Amano Pharmaceutical Co., Ltd.~.
According to the present invention, not only a pulp with a lower lignin content which is easy to bleach is obtained by a single-stage treatment, but also the treatment can be run continuously without additional acidifying steps if the later stage is an acidic one such as ozone treatment (hereinafter referred to as Z or Z-stage). Especially, Z-stage is said to be carried out preferably at a pH of less than 3.0, so that the use of the enzyme consisting mainly of xylanase exhibiting the maximum 2~3~9~

activity at pH 4.5 or lower, preferably less than 3.0, is most effective. The amount of o~one, which is apt to degrade cellulose, is reduced. Moreover, when the later bleaching stages includa the P-stage, the heavy metals that would decompose peroxides can be removed by the acidic treatmen-t, so that the effect of the P-stage is maximi~ed.
The amount of the costly P can be reduced significantly due to the delignification effect of the enzyme.
According to the present invention, the effluent from the washing step is mostly treated in the digested black liquor recovery system and is burned therein. All the acids, enzyme, hydrogen peroxide, ozone and thiourea dioxide to be recovered from the effluent may be burned in conventional recovery boilers. If the bleaching stage following the enzymatic treatment is the Z-stage, the effluent form this stage contains a variety of organic acids and may further contain sulfuric acid used as a pH
buffer. This effluent can be made use of for the acid treatment~ and such use in a counter-currentwise manner along with the control of the acidity at the enzymatic treatment s-tage may enable a reduced dilution by fresh water and to reduce the load to the vacuum evaporator in the black liquor recovery system where the effluent is treated.
Various wastes from the present invention contain 2~3~

substances that may sufficiently be recovered in the recovery systems as mentioned above. However, if they are directly transferred to the recovery system as they are, the combustible solid content therein will be low so that the load to the recovery becomes large as a whole. As the case may be, there is a probability that the capacity of the vacuum evaporator will become insufficient. Moreover, acid contamination may result in a decline in the pH of the black liquor, which in turn may cause a rise in -the viscosity of the black liquor leading to plugging. In order to avoid this unfavorable effect, the effluents resulting from the above-mentioned acid treatment, ozone treatment, enzyme treatment and hydrogen peroxide treatment may be processèd by a separating membrane to separate and concentrate the high polymer substances in the effluents.
In particular, from various acids used in the acid treatment, useful acids may be isolated by distillation and recycled. After the recycling operation, the residues from the effluents may be burned by the burning treatment.
An embodiment of the present invention is to process the pulp, after the enzymatic treatment with xylanase active under acidic conditions and the acidic treatment which are employed in the present invention, by one or more additional stages selected from a group consisting of the following s-tages (1) through (7):

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(1)--To treat the pulp with chlorine, chlorine dioxide, or a mixture thereof in an aqueous medium.
(2)--To treat -the pulp with a peroxide in an alkaline aqueous medium.
(3)--To treat the pulp with a peroxide and oxygen in an alkaline aqueous medium.
(4)--To treat the pulp with a hypochlorite salt in an aqueous medium.
(5)--To treat the pulp with ozone in air or in an aqueous medium.
(6)--To treat the pulp with thiourea dioxide in an alkaline a~ueous mediumO
(7)--To treat the pulp with a hydrosulfite salt in an alkaline aqueous medium.
In carrying out the me-thod of the present invention, each of -those additional stages may be conducted according to a conventionally practical method generally employed in the field of pulp bleaching or any reasonable way known in the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an embodiment flow diagram for the bleaching according to the present invention. The full line indicates a flow of the pulp being bleached; (a), (b) and (c) indicate a -treatmen-t flow according to claims 1, 2 and 3, respectively. The totted line indicates the ~low of the 2~3~90~

effluent separated from the respective washing step by solid-liquid separation. As shown, it can be fed to a previous step in a counter-curre~twise manner.
DESCRIPTION OF REFERENCE NUMERALS
1.--acid treatment 2.--enzymatic treatment 3.--acid-resistant enzyme treatment 4.--washing BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is illustrated by the following examples, which, however, are by no means intended to restrict the invention. In these examples, oxygen treatment is expressed as 0, acid treatment as A, enzyma-tic treatment as X, acid-resistant enzyme treatment as Xa, a combination of alkali treatment and peroxide treatment as Ep, hypochlorite treatmen-t as H, and chlorine dioxide treatment as D. Unless otherwise noted, in the following examples and compara-tive examples, all parts are by weight;
all percent (%) referring to pulp consistency are weight percent (%) after drying in a 105C oven; and, the dose (%) of the chemicals is by-weight based on the pulp.
The testing methods to evaluate the properties of the pulp to be treated by the invention are in accordance with the following standards;
Kappa number: TAPPI Test Method T-236 cm-85 ~130~0~

Viscosity: TAPPI Tes-t Method T-230 om-89 Whiteness: JIS P-8127 (Hunter Whiteness) The xylanase activity of the enzyme to be used in the method of the present invention was obtained according to the following method:
Process:
About 30 ml of water were added to 0.500 g of xylan (manufactured by Seikagaku KK) and dissolved under heat while stirring. After this was cooled, water was added thereto to make i~ 50 ml. This was used as the substrate.
For the acid-resistant enzyme, 3 ml of 0.1 N tartaric acid buffer (pH 1.8) and 1 ml of the substrate were weighed and put into a 50 ml ground-stopper Nessler's tube, this was placed in a thermostatic water bath at 40C and left there for 5 minutes or more. Then, 1 ml of the sample solution was added thereto and immediately shaken. This was allowed to stand at 40C for 30 minutes, 2 ml of a Somogyi's solution were added thereto and shaken, and the tube was sealed and heated in a boilin~ water bath for 20 minutes and then immediately cooled. After this was cooled, one ml of ammonium molybdate solution containing arsenic was added thereto and well shaken until the red precipitates of copper suboxide were completely dissolved. Afterwards, this was allowed to stand at room temperature for 20 minutes, and water was added thereto to make 25 ml. This 2~3~9-~

was -then well shaken. About 8 ml of the resulting liquid were put in a 15 ml centrifugal tube, which was subjected to centrifugation using a cooled centrifuger (3000 rpm, 25C, 10 minutes). The absorbance (A30) of the thus-separated supernatant at 500 nm was measured through a layer length of 10 mm, as compared with that of the control sample of water. Separately, a different sample was prepared by adding 2 ml of a Somogyi's solution to 3 ml of 0.1 N tartaric acid buffer (pH 1.8) and one ml of the substrate, shaking them and further adding one ml of the sample solution thereto. This was also measured to obtain its absorbance (A0) at 500 nm.
For the xylanase "Cellulase Amano CT-4", used was 0.1 N acetic acid/sodium acetate buffer (pH 4.5) in place of 0.1 N tartaric acid buffer (pH 1.8).
Determination of Xylanase Activity:
100 enzyme units liberate 1 mg of reducing sugar as xylose per 1 min under the above-mentioned assay conditions, and the xylanase activity was calculated by the following formula;
Xylanase activity (u/g) = X x (1/30) x 100 x n where, X: amount (mg) of xylose liberated determined from the difference, A30 - Ao~ using the xylose calibration diagram, s~ o ~

n: dilution factor 1/30~ 100:--conversion factor.
Example 1 Bleaching according to O-A-X-Ep-H-D:
100 parts of an oxygen-delignified hardwood Kraft pulp was bleached by the sequence of O-A-X-Ep-H-D. The amounts of the chemicals and the conclitions of each stage are summarized as follows;
O-stage:--The consistency of the pulp, -to which 1.8 par-ts of NaO~ had been added, was adjusted to 10~ and was placed in an autoclave equipped with a high shear mixer (supplied by Nitto Autoclave K.K.). To this, 1.6 parts of 02 gas were added under pressure. The mixer was run for 1 min.
and then the pulp was allowed to stand for 1.5 hours to react, while the reactor was sealed under pressure.
Thereafter, water was added to dilute the pulp to 1-~, and then it was filtered and washed through a 5A filter paper using a Buchner funnel. The Kappa number of the pulp was 9.4 and the viscosity 24.4-cps.
A-stage:--To the thus-treated pulp, whose consistency was adjusted to 10%, 25 parts of acetic acid were added. The preparation was heated to 70C and allowed to stand for 1.5 hours to react. Thereafter, water was added to dilute the pulp to 1~, then filtered and washed through a 5A filter paper using a Buchner funnel. The Kappa number of the 2 ~

thus-treated pulp was 8.0 and the viscosity 22.2 cps.
X-stage:--0.015 parts of xylanase "Cellulase Amano CT-4"
(which is supplied by Amano Pharmaceutical Co., Ltd., has an optimum pH value of 5.5 and exhibits the maximum xylanase acti~ity of 100,000 u/g at pH 4.5) were added to the acid-treated pulp whose consistency was adjusted to 10~. The preparation was heated to 50C and allowed to stand for 3 hours to react. Thereafter, water was added to dilute the pulp to 1~, and then it was filtered and washed through a 5A filter paper using a Buchner funnel.
After the washing, the Kappa number of the pulp was 6.4, and the viscosity 20.8 cps.
Ep-stage:--0.8 parts of Na-hydroxide and 0.15 parts of hydrogen peroxide were added to the enzyme-treated pulp whose consistency was adjusted to 12%. The preparation was heated to 65C and allowed to stand for 1.5 hours to reac-t.
Thereafter, water was added to dilute the pulp to 1%, and then it was filtered and washed through a 5A filter paper using a Buchner funnel.
H-s-tage:--0.8 parts of Na-hypochlorite were added to the pulp after the Ep-stage whose consistency was adjusted to 10~. The preparation was heated to 65C and allowed to stand for 2 hours to react. Thereafter, water was added to dilute the pulp to 1~, then it was filtered and washed through a 5A filter paper using a Buchner funnel.

9`~ ~

D-stage:--0.6 parts o chlorine dioxide were added to the hypochlorite-treated pulp whose consistency was adjusted to 10%. The preparation was hea-ted to 65C and allowed to stand for 2 hours to react. Thereafter, water was added to dilute the pulp to 1%, and then it was filtered and washed through a 5A filter paper using a Buchner funnel.
The whiteness of the finally-obtained pulp having undergone the O-A-X-Ep-H-D sequence was 86.0~ and -the viscosity 16.5 cps. The amount of the total adsorbable organic halogens (AOX) in the bleaching effluent was determined to be 0.85 kg per a ton of the pulp.
Thus, it has been confirmed that a bleached hardwood Kraft pulp havlng a normal level of whiteness and viscosity can be obtained without employing molecular chlorine.
Example 2 Bleaching according to O-A-X-Ep-H-D:
The same process as in Example 1 was repeated, except that 0.15 parts of "Hemicellulase 90" (supplied by Amano Pharmaceutical Co. L-td., which exhibits a xylanase activity of 100,000 u/g) were used in the X-stage in place of "Cellulase Amano CT-4".
The whiteness of the finally-obtained pulp after the last D-stage was 85.8~ and the viscosity 16.8 cps. The amount of AOX in the bleaching effluent was 0.87 kg per a ton of the pulp.

2~3~19`~

Thus, it has been confirmed that a bleached hardwood Kraft pulp having normal whiteness and viscosity can also be obtained using hemicellulase and wi-thout employing molecular chlorine.
Reference Example 1 Bleaching according to conventional O-C-Ep-H-D:
100 parts of the same oxygen-delignified hardwood Kraft pulp as in Example 1 were bleached in the sams manner as in Example 1, except that the pulp was subjected to only the C-stage under the following conditions without being subjected to the A- and X-stages:
C-stage:--1.4 parts of chlorine were added to said pulp of which the consistency was adjusted to 3~. The preparation was heated to 50C and allowed to stand for 0.5 hours to react. Thereafter, the pulp was diluted to 1-~, and then it was filtered and washed through a 5A filter paper using a Buchner funnel.
The whiteness of the finally-ob-tained pulp after the last C-stage was 84.4% and the viscosity was 16.8 cps. The amount of AOX in the bleaching effluent was 2.1 kg per a ton of the pulp.
Thus, it is clear that use of an A-stage and an X-stage in combination (as in Examples 1 and 2) produced a pulp having a similar viscosity, but at significantly low AOX level, as compared with the conventional bleaching ~13~

process (Reference Example 1) using chlorine.
Comparative Example 1 Bleaching according to conventional 0-X-Ep-H-D:
100 parts of the same oxygen-delignified hardwood Kraft pulp were bleached in the same manner as in Example 1, except that they were not subjected to the A-stage. The Kappa number after the X-stage was 7.1 and the viscosity 23.5 cps.
The whiteness of the finally-obtained pulp af-ter the last D-stage was 82.6% and the viscosity 16.3 cps.
Thus r it is clear that the whiteness of the pulp that was subjected to only the enzymatic treatment wlthout using chlorine is lower than that of the pulp treated by the present invention (Examples 1 and 2) and that of the pulp treated by the conventional method usin~ chlorine (Reference Example 1).
Example 3 Bleaching accordiny to 0-X-A-Ep-H-D:
The same process as in Example 1 was repeated, except that the sequence A-X was reversed and the pH at the X-stage was adjusted to 3.0 or lower while making use of the washin~ effluent from the A-s-tage.
The Kappa number after the A-stage was 6.4, and the viscosity 20.8 cps.
The whiteness of the finally-obtained pulp after the 2~ 9~
last D-stage was 85.5~, and the viscosity 16.9 cps.
Thus, i-t is clear that the pulp treated in this example has a higher level of whiteness than tha-t of the pulp treated in Comparative Example 1 at the similar viscosity level.
Example 4 Bleaching according to P-Ap-X-Ep-H-D:
The sa~e process as in Example 1 was repeated, except that the A-stage was combined with hydrogen peroxide treatment to be the following Ap-stage:
Ap-stage:--To the oxygen-delignified pulp, 25 parts of acetic acid and 0.3 parts of hydrogen peroxide were added and the pulp consistency was adjusted to 10%. The preparation was heated to 70C and allowed -to stand 1.5 hours to react. Thereafter, this was diluted to 1%, and then filtered and washed through a 5A filter paper using a Buchner funnel~ .
The Kappa number of the pulp after this Ap-stage was 3.8, and the viscosity 18.8 cps.
The Kappa number after the next X-stage was 2.4, and the viscosity 16.4 cps. The whiteness of the finally-obtainad pulp after the D-stage was 89.8~, and the viscosity 14.8 cps.
Thus, it is clear that the pulp treated in this example has a much higher level of whi-teness as compared 2~3~JO~
with that in Comparative Example l.
Example 5 Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 1 was repeated, except that the 0-stage was omitted, the reaction temperature at the A-stage was varied to 40C, and the C-stage was added in accordance with the Kappa number ater the X-stage while adjusting the amount of the chlorine added.
The whiteness of the finally-obtained pulp after the last D-stage was 86.2~, and the viscosity 16.8 cps.
Reference Example 2 Bleaching according to C-Ep-H-D:
The same process as in Example 5 was repeated, except that the A-stage and the X-s-tage were omitted and the amount of the chlorine added at the C-stage was increased.
This reerence example corresponds to the conventional process not having bleaching treatment with oxygen.
The whiteness of the finallv-obtained pulp after the last D-stage was 85.5~, and the viscosity 13.5 cps.
Thus, Example 5 indicates that, by adopting the A-stage and the X-stage, the amount of molecular chlorine can be saved by about 40% in attaining an ordinary level of whiteness, as compared with the conventional bleaching process using chlorine (Reference Example 2).
Example 6 ~3~90~

Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 5 was repeated, except that the reaction temperature at the A-stage was varied to 20~C.
The whiteness of the finally-obtained pulp after the last D-stage was 85.9%, and the viscosi-ty 15.~ cps.
Thus, it is known by comparison wi-th Example 5 that the temperature at the A-stage is preferably 40~C or higher.
Example 7 Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 5 was repeated, except that the 0-stage was omitted and the reaction time at the A-stage was varied to 10 minutes.
The whiteness of the finally-obtained pulp after the last D-stage was 86.3~, and the viscosity 16.5 cps.
Thus, it is known that even if the time at the A-stage is shortened, the amount of molecular chlorine can be saved by about 36% while attaining an ordinary level of whiteness, as compared with the conventional bleaching process using chlorine (Reference Example 2); and the adoption of the A-stage and the X-stage enables such saving. In Examples 5, 6 and 7, the amount of chlorine added was so controlled that the finally-obtained pulps in these might have the same level of whiteness.

- ; ~ ` ;' ~13~9`06 Example 8 Bleaching according to A-X-C-Ep-H-D:
The same process as in Example 7 was repeated, except that the reaction time at the, A-stage was varied to 3 minutes and the amount of chlorine added was controlled.
The whiteness of the finally-ob-tained pulp after the last D-stage was 86.1~, and the viscosity 14.8 cps.
As compared with Example 7, the amount of chlorine needed for obtaining the same level of whiteness increased in Example 8. In view of this, it is known that the reaction time at the A-stage is preferably 10 minutes or longer.
Example 9 and Comparative Example 2:
The same process as in Example 1 was repeated, except that the amount of acetic acid added at the A-stage was varied to 6 parts (Example 9) and 3 parts (Comparative Example 2).
Under the conditions, there were rises in the pH of the sample at the A-stage to 4.5 (Example 9) and 7.0 (Comparative Example 2), from 2.2 in Example 1. The whiteness and the viscosity of the pulps finally obtained in these examples were as follows:
brightness (~) viscosity (cps) Example 9 84.1 16.3 Comparative Example 2 82.6 16.5 '~3~91~

Example 10 Bleaching according to 0-A-X-Ep-H-D:
The same process as in Example l was repeated, except that the acid added at the A-stage was changed to sulfuric acid.
The whiteness of the finally-obtained pulp after the last D-stage was 86.4~, and the viscosity 16.3 cps.
Thus, it is known that sulfuric acid works as good as acetic acid to attain a good result.
Comparing Example 9 with Comparative Example 2, it is known tha~ the level of whiteness of the pulp obtained in the latter is lower than that obtained in the former. From this, it is understood that the pH at the A-stage must be 4.5 or lower and is desirably 3.0 or lower.
Example ll Bleaching according to 0-Xa-Ep-H-D:
An oxygen-delignified hardwood Kraft pulp was treated in this example, according to a bleaching process of 0-Xa-Ep-H-D in which the amounts of the chemicals added and -the conditions used were as follows:
O-stage This was the same as that in Example l. After this stage, the Kappa number of the pulp was 9.4, and the viscosity 24.~ cps.
Xa-stage: 0.015~ of acid-resistant xylanase, 1'Xylanase Amano P" (supplied by Amano Pharmaceutical Co., Ltd., which r ~

exhibits a xylanase activity of 8,000 u/g at pH 1.8, and acetic acid were added together to the pulp, whose consis~ency was adjusted to 10%. The amount of the acid added was adjusted to control the pH of the pulp to 1.8.
The preparation was heated to 50C and allowed to stand fo~
3 hours to react. Thereafter, this was diluted to 1~ and then filtered and washed through a 5A filter paper using a Buchner funnel. After the washing, the Kappa number of the resulting pulp was 6.8, and the viscosity 20.2 cps.
Ep, H and D-stages: These were the same as those in Example 1.
The whiteness of the finally-obtained pulp aEter the last D-stage was 85.5~, and the viscosity 16.5 cps.
The amount of AOX from the bleaching effluent was 0.86 kg per a ton of the pulp.
Thus, it has been confirmed that, by the additional treatment with the enzyme that exhibits activity in an acidic environment, a bleached hardwood Kraft pulp having ordinary whiteness and viscosity can be obtained without employing molecular chlorine.
Example 12 Bleaching according to O-A-Xa-Ep-H-D:
An oxygen-delignified hardwood Kraft pulp was treated in this example, according to a bleaching process of O-A-Xa-Ep-H-D in which the amounts of the chemicals added ~3~

and the conditions used were as follows:
O-stage: This was the same as that in Example 1. After this stage, the Kappa number of the pulp was 9.4, and the viscosity 24.4 cps.
A-stage: This was the same as that in Example 1. After this stage, the Kappa number of the pulp was 8.0, and the viscosity 22.2 cps.
Xa-stage: 0.015% of the same enzyme as that used in Example 11 and acetic acid were added together to the pulp, whose consistency was adjusted to 10~. The amoun-t of the acid added was adjusted to control the pH of the pulp to 1.8. The preparation was heated to 50C and allowed to stand for 3 hours to react. Thereafter, this was diluted to 1% and then filtered and washed through a 5A filter paper using a Buchner funnel. After the washing, the Kappa number of the resulting pulp was 6.1, and the viscosi-ty 19.8 cps.
Ep, H and D-stages: These were -the same as those in Example 1.
The whiteness of the inally-obtained pulp after the last D-stage was 86.9%, and the viscosity 16.3 cps.
The amount of AOX from the bleaching effluent was 0.81 kg per a ton of the pulp.
Thus, it has been confirmed that a bleached hardwood Kraft pulp having ordinary whiteness and viscosity can be ~2:13~9~

obtained without employing molecular chlorine. In addition, it has also been confirmed that the effect of the enzymatic treatment with the enæyme that exhibi-ts activity at a pH of 4.5 or lower is promoted more when combined with acid treatment.
Comparative Example 3 Bleaching according to 0-X-Ep-H-D:
The same hardwood Kraft pulp as that treated in Example 11 was treated in the same manner as in Example 11, except that xylanase, "Cellulase Amano CT-4" (supplied by Amano Pharmaceutical Co., Ltd., which has an optimum pH
value of 5.5 and exhibits a xylanase activity of 100,000 u/g at pH 4.5 was used as the enzyme for the enzymatic treatment. After the enzymatic-treatment, the Kappa number of the pulp was 8.2, and the viscosity 22.3 cps.
The whiteness of the finally-obtained pulp after -the las-t D-stage was 83.4%, and the viscosity 16.1 cps.
In Comparative Example 3 compared with Example 11, the conventional enzyme having an optimum pH value of 5.5 was used and the enzymatic treatment was conducted a-t a pH
value lower than 4.5 while omitting the bleaching with chlorine, with the result that the level of whiteness of the pulp obtained therein was obviously lower than that of the pulp treated by the conventional process using chlorine.

~130~

Example 13 Bleaching according to 0-Xa-A-Ep-H-D:
The same process as in Example 12 was repeated, except that the sequence of A-Xa was reversed and the pH at the Xa-stage was adjusted to A.5 or lower making use of the washing effluent from the A-stage in Example 2. The Kappa number after the Xa-stage was 6.1, and the viscosity 19.8 cps .
The whiteness of the finally-obtained pulp after the last D-stage was 86.7~, and the viscosity 16.7 cps.
Thus, it is clear that the sequence employed in this example attains a higher level of whiteness than that attainable by an ordinary bleaching process using chlorine.
Example 14 Bleaching according to A-Xa-C-Ep-H-D:
The same process as in Example 12 was repeated, e~cep-t that the 0-stage was omitted, the reaction temperature at the A-stage was changed to 40~C and the C-stage was added.
The amount of chlorine added at the C-stage was controlled, according to the Kappa number after the Xa-stage.
The whiteness of the finally obtained pulp after the last D-stage was 87.1~, and the viscosity 16.6 cps.
Thus, it is known, as compared with the conventional bleaching process using chlorine but not employing the A-stage and the Xa-stage (Reference Example 2), that the - ~4 -21~3~0~

sequence of the present invention employing the A-stage and the Xa-stage accomplishes an ordinary level of whiteness while reducing the use of molecular chlorine by about 35%.
Example 15 Bleaching according to O-A-Z-Xa-Ep-H-D:
The same process as in Example 12 was repeated, except that the following Z-stage (ozone treatment) was added.
Z-stage: The consistency of the pulp after the A-stage was adjusted to 10~ and it was placed in an autoclave equipped with a high shear rate agitator (supplied by Nitto Autoclave K.K.), and a volume of ozone gas corresponding to 0.4~ based on ~he pulp was fed to the pulp. The ozone gas (ozone concentration: 5~) was generated by an ozone generator (supplied by Nippon Ozone K.K.). Then, the autoclave was sealed. After charging nitrogen gas into the autoclave to raise the total pressure to 5 bar, the pulp was kept at 35C under agitation for 30 seconds to terminate the reaction. The pulp was taken out of the autoclave, diluted to have a consistency o 1~, and then filtered and washed through a 5A filter paper using a Buchner funnel. The Kappa number of the pulp after the Z-stage was 3.9, and the viscosity 17.8 cps.
The whiteness of the finally-obtained pulp after the last D-stage was 88.8~, and the viscosity 15.6 cps.
Thus, it is clear that this sequence accomplishes a ~3~90~

substantially higher level of whiteness than the conventional bleaching process using chlorine.
The results of all the aforeisaid examples, comparative examples and reference examples are summarized in the following Tables.

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Table 4 Example 13 Bleaching Sequence OXaAEpHD
_ ~ .. .. . .. _ . .__ Unbleached Kappa number 18.5 Pulp Viscosity cps 28.5 O-stage 2 dosage % 1.6 NaOH ~ 1.8 consistency~ 10 temperatureDC 105 time hr 1.5 Kappa number 9.4 viscositycps 24.4 Xa-stage enzyme dosage % 0.015 peroxide ~ *l . consistency ~ 10 pH 1.8 temperatureC 50 time hr 3 Kappa number _ _ viscositycps _ A-stage enzyme dosage % 25 acetic acid% *2 consistency% 10 pH 1.8 temperature C 70 time hr 1.5 Kappa number 6.1 viscositycps 19.8 .......
Ep, H ~ D stages *3 ._ Finished pulp whiteness~ 86.7 _ viscosit~cps 16.7 *l pH adjusted using effluent of A-stage and acetic acid.
*2 addition amount was adjusted to control pH.
*3 conditions for all the examples and comparative examples are common, and are summari2ed in Table 6.

L 3 ~

Table _ _ Bleaching Sequence Example 15 OAZXaEpHD
Unbleached Kappa number 18.5 Pulp Viscositycps 28.5 O-stage 2 dosage % 1.6 NaOH % 1.8 consistency% 10 temperatureC 105 time hr 1.5 Kappa number 9.4 viscositycps 24.4 .
A-stage acetic acid% 25 peroxide %
consistency% 10 pH 2.2 temperatureC 70 time hr 1.5 Kappa number 8.0 viscositycps 22.2 . . _ ..
Z-stage O3 dosage ~ 0.4 consistency% 10 temperatureC 35 time hr 0.5 Kappa number 3.9 viscositycps 17.8 .
..
Xa-stage enzyme dosage % 0.015 acetic acid% *2 consistency% 10 pH 1.8 temperatureC 50 time hr 3 Kappa number 3.2 viscositycps 1~.9 . ._ . _ Ep, H & D stages *3 . .
Finished pulp whiteness % 88.8 viscosity cps 15.6 ~13~90~

*2 addition amount was adjusted to control pH.
*3 conditions for all the examples and comparative examples are common, and are summarized in Table 6.

Table 6 stageconditions data ... ... _ Ep NaOH ~ 0.8 H2O~ % 0.15 consistency % 12 temperature C 65 time hr 1O5 ._ . ~.
H Na-hypochlorite ~ 0.8 consistency % 10 temperature C 65 time hr 2 _ . -- _ D ClO2 % 0.6 consistency % 10 temperature C 65 time hr 2 INDUSTRIAL APPLICABILITY
According to the present invention, a bleached chemical pulp having whiteness and viscosity comparable to or higher than those obtained by conventional processes can be obtained. In addition, a much higher level of whiteness of the bleached pulp can be practically attained, using the methods embodied herein. According to the methods embodied herein, the use of chlorine or chlorine-containing compounds may be reduced while eliminating the use of molecular chlorine, to obtain a bleached pulp having an - 55 - ~ :~

-` 2 ~ 9 0 ~

ordinary level of whiteness. Accordingly, the discharge of to~ic compounds like dioxin from the pulping process of the presen-t invention can be reduced. According to the present invention, an opportunity for recirculating the organic substances to be removed from the bleaching step back to the effluent-recovering step in-the pulping process is provided, whereby a further reduction of environmental pollution becomes possible.

Claims (6)

1.--A method of enzymatic treatment of chemical pulp before bleaching, characterized in that an unbleached chemical pulp is treated with an acid, washed, and then treated with an enzyme containing xylanase.

2.--A method of enzymatic treatment of chemical pulp before bleaching, characterized in that an unbleached chemical pulp is treated with an enzyme containing xylanase, then treated with an acid and thereafter washed.

3.--A method of enzymatic treatment of chemical pulp before bleaching, characterized in that an unbleached chemical pulp is treated at pH 4.5 or less with an enzyme containing xylanase which exhibits the maximum activity at pH 4.5 or less, and then washed.

4.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1 or 2, wherein said enzyme contains xylanase exhibiting the maximum activity at pH 4.5 or less and said enzymatic treatment is conducted at pH 4.5 or less.

5.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2, 3 or 4, wherein said unbleached chemical pulp is an unbleached pulp or an oxygen-bleached pulp.
6.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2 or 4, wherein hydrogen peroxide is added at the stage at which said pulp is held in the acidic condition.
7.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2 or 4, wherein ozone is added at the stage at which said pulp is held in the acidic condition.
8.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2, 3, 4, 5, 6 of 7, wherein the acid to be employed for bringing the pH down to become acidic is at least one selected from a group consisting of acetic acid, formic acid, oxalic acid, propionic acid, sulfuric acid, sulfurous acid, nitric acid and nitrous acid.
9.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2, 3, 4, 5, 6 of 7, wherein the liquid separated from the washing step after each treatment stage by solid-liquid separation is returned back to the previous stage, and eventually fed to the step where the effluent is burned and inorganic salts are recovered.
10.--The method of enzymatic treatment of chemical pulp before bleaching, according to claim 1, 2, 3, 4, 5,

6, 7 or 8, wherein at least one kind of treatment stage selected from a group consisting of the following (1) through (7) is additionally conducted.

(1)--To treat the pulp with chlorine, chlorine dioxide, or a mixture thereof in an aqueous medium.
(2)--To -treat the pulp with a peroxide in an alkaline aqueous medium.
(3)--To treat the pulp with a peroxide and oxygen in an alkaline aqueous medium.
(4)--To treat the pulp with a hypochlorite salt in an aqueous medium.
(5)--To -treat the pulp with ozone in air or in an aqueous medium.
(6)--To treat the pulp with thiourea dioxide in an alkaline aqueous medium.
(7)--To treat the pulp with a hydrosulfite salt in an alkaline aqueous medium.