GB2248075A - Bleaching chemical pulp - Google Patents

Abstract

A chlorine-free method for bleaching chemical pulps, particularly kraft pulp, uses a first step of treating the pulp with a hemicellulose acting enzyme of a microorganism. Subsequently, the treated pulp is extracted, treated with chlorine dioxide, re-extracted and retreated with chlorine dioxide to result in a brightness level of at least 88. The enzyme may be a xylanase or a mannanase or a mixture. The extraction steps may involve the addition of oxygen under pressure or peroxide. A hypochloride treatment may follow the first extraction step to give a six-step process.

Description

CHLORINE-FREE PROCESS FOR BLEACHING PULP This invention relates to a novel process for bleaching chemical pulp.

More specifically, it involves a process which does not require the use of a conventional molecular chlorine bleaching step in the bleaching of chemical pulp, e.g. kraft pulps.

Background of the Invention Wood is a complex material which is composed of cellulose, hemicellulose and lignin along with numerous other minor compo- nents. The lignin is associated with and perhaps even covalently bound to a matrix of cellulose and/or hemicellulose (polymers of various polysaccharides, especially xylans).

In order to reduce the lignin content of chemical pulp, and thereby reduce the brownish color imparted by the presence of lignin, pulps are subjected to a series of chemical treatments, known as bleaching. The process of bleaching involves a series of steps, representing at least two or three different types of treatments, some often repeated in the total process.Such treatments include: C Chlorination Reaction with elemental (molecular) chlorine in an acidic medium E Alkaline Extraction Dissolution of reaction products with NaOH D Chlorine Dioxide Reaction with C102 in an acidic medium O Oxygen Reaction with elemental oxygen at high pressure in an alkaline medium Eo Extraction/Oxygen Alkaline extraction in the presence of oxygen at an increased pressure CD Chlorine/Cl02 Reaction with mixtures of chlorine and chlorine dioxide H Hypochlorite Reaction with hypochlorite in alkaline solutions P Peroxide Reaction with peroxide in basic media Eop Extraction/Oxygen/ Peroxide Alkaline extraction in presense of oxygen and peroxide at an increased pressure Ep Extraction/peroxide Alkaline extraction in presence of peroxide In order to achieve pulp of a desired brightness level (usually 88 or higher) bleaching has been commonly carried out in a five or six stage procedure in which a molecular chlorine treatment is applied, typically as the first stage; CEHDED or CEDED are the more commonly used sequences. (For further descriptions of conventional bleaching processes, see Handbook for Pulp and Paper Technologists by G.A. Smook (1982), TAPPI, Atlanta, GA, especially pages 153-173.) One of the problems associated with the conventional chlorine-dependent bleaching processes is that environmentally undesirable by-products can result. Effluents from conventional bleaching processes can contain chlorinated organic compounds including some which are known to have various toxic, mutagenic, and possibly carcinogenic effects.Elimination of the chlorination treatment step would result in less undesirable effluent being produced.

Recently, the use of enzymes, eg. fungi enzymes, for the degradation and removal of lignin from pulps has become of interest. Hemicellulases, eg.

xylanases and mannanases, represent one class of interest.

Examples of such enzymes include those of the fungi Aureobasidium pullulans, Streptomyces sp., Schizophyllum commune, Sporotrichum sp. and Trichoderma spp. as well as Bacillus sp. Such hemicellulases have in particular been found to reduce the amount of chlorine needed to bleach chemical pulps. However, these enzyme treatments have been unable to eliminate the use of molecular chlorine in the typical bleaching processes.

DESCRIPTION OF THE INVENTION It has been found in accordance with the invention that a hemicelluloseacting enzyme (HAE) or enzyme system comprising one or more such enzymes can substitute entirely the elemental chlorine in the first stage of a conventional-style five or six stage bleaching process for chemical pulps to obtain a elemental chlorine-free bleaching process capable of obtaining or providing a bleached pulp of desired brightness and quality.Thus, this invention relates to an in vitro process for bleaching chemical pulps comprising: a) treating the pulp with a hemicellulose-acting enzyme system in vitro for a time sufficient to render the lignin component solubilizable; b) extracting the pulp obtained from step a); c) treating the resulting pulp with a first solution comprising only chlorine dioxide; d) extracting the pulp obtained from step c); and e) treating the resulting pulp with a second solution comprising only chlorine dioxide.

A five stage bleaching process consisting essentially of steps a) through e), above, is a preferred embodiment.

A six stage process in which a conventional hypochlorite treatment occurs between steps b) and c), above, is also provided.

In the conventional five and six stage bleaching processes, steps b) through e) and the optional hypochlorite-treatment may be carried out under various conditions, including the presence or absence of certain agents, as needed to obtain the ultimate brightness and pulp quality objectives. In carrying out the present invention, even though the enzyme treatment may not suggest a benefit or effect anywhere near that obtained from the chlorine treatment it replaces, particularly when measured by standard parameters such as Kappa number, it has been found that the conditions of the treatment for steps b) through e) and the optional hyprochlorite treatment may be varied or regulated well within practical and acceptable limits to also obtain bleached pulp of desired brightness and quality.Such variations or regulations include, for example, an increase in the amount of chlorine dioxide employed in either or both of steps c) and e) and/or conducting either or both of the extraction steps b) and d) with added peroxide and/or added molecular oxygen with such oxygen being present under pressure for at least a part of the extraction step. Preferably, one or both of the extraction steps b) and d) are carried out in the presence of added peroxide and/or molecular oxygen; for example, oxygen may be added to one or both steps or both peroxide and oxygen may be added to one or both steps. Preferably, oxygen is employed in at least one of the extraction steps. When oxygen is to be employed in only one of the extraction steps, it is usually preferred to use it in step b). In a particularly preferred embodiment both peroxide and molecular oxygen are added to both steps b) and d).

The hemicellulose acting enzymes (HAEs) are indicated to act on the hemicellulose matrix on which the lignin is bound, such that after treatment of pulp with HAEs, the lignin (or chemically modified lignin) is released and/or rendered releasable and can be separated by an appropriate extractant and removed from the pulp. HAEs which are preferred in this invention include xylanases. Other HAEs include mannanases. Cellulases may also be present, provided that the cellulases which may be present do not substantially degrade the cellulosic fraction of the pulp and result in a reduced paper quality.

The hemicellulases for use in this invention may be from any microbiological source. Microorganisms known in the art to produce HAEs useful in the removal of lignin include Aureobasidium pullulans, Streptomyces sup., Schizophyllum commune, Sporotrichum sp., Trichoderma spp.

(including T. reesei), and Bacillus sp.. Certain microorganisms may be preferred sources. For example, the fungus A. pullulans is a particularly preferred source which is rich in active xylanases and strains are also available which do not produce any appreciable quantity of an undesirable cellulase which can adversely affect the final product.

While any A. pullulans may be used in accordance with this invention, preferred strains include those which produce large quantities of EAEs, especially those strains which can be characterized as "enzyme overproducers". Preferred strains are A. pullulans Y-2311 and Y-2311-1.

A. pullulans Y-2311 and Y-2311-1 are available from the ARS Culture Collection (NRRL) as indicated in Leathers, et al. 1984 Biotechnology and Bioengineering Symp. No. 14, pg. 225-240, which is hereby incorporated by reference. Other xylanases which may be preferred in accordance with this invention are the xylanases of Trichoderma spp., including T. reesei.

The enzyme treatment step of the invention is carried out in vitro, i.e.

in the absence of the living organism from which the enzyme may be obtained, for a time sufficient to releasably degrade the lignin content of the pulp. The in vitro process comprises contacting the pulp with a lignin-releasing effective amount of an "enzyme system" comprising an HAE, preferably an endo-xylanase. Mixtures comprising xylanases, particularly endo-xylanases, with mannanases are preferred for softwood pulps.The term "enzyme system" as used throughout the specification and claims encompasses: a) one HAE present in a purified form; or b) a mixture of purified HAEs; or c) one or more HAEs in a non-purified form, but separated from the microorganism which produced them; for example, a partially purified preparation or a culture supernatant; or d) one or more HAEs in association with the microorganism which produced them, with the proviso that the microorganism which produced the HAEs is no longer viable, or e) two or more HAEs from different microorganisms, either in purified or unpurified form.

The HAEs useful as above indicated may be obtained from the organism which naturally produced them or by recombinant means.

When more than one HAE is present in the enzyme system, each HAE may be present in a varying amount. When the RAE enzyme system is in the form of a culture supernatant, the supernatant may be used as such, but it may be concentrated prior to its use. One highly convenient method of concentration involves a simple ultrafiltration designed to concentrate the supernatant 5-25 fold and in any case recover for use a concentrate containing essentially all enzymes which affect basic wood components, i.e. the cellulases (if any), hemicellulases, and any enzymes reactively affecting lignin (herein "lignolytic enzymes").

The HAE enzyme system may vary as to its activity depending on such factors as the type of organism producing the enzymes, the culture media, and other environmental cultural conditions. In general, the dosage activity of the HAE enzyme system should have an endo-xylanase activity of 0.05 to 100 U/g (units of endo-xylanase activity per gram of oven dried pulp), and preferably 0.5 to approximately 10 U/g. Enzyme dosages substantially above about 10 U/g are not indicated to usually provide substantial additional benefits to the bleaching procedure. One unit of endo-xylanase activity is defined as one micromole of reducing sugar produced per minute from oat spelt xylan substrate at 500C in a ten minute assay. The weight of oven dried pulp is determined by oven drying the pulp for two hours at 1000C.

It is preferred that the enzymatic treatment be carried out at a temperature which will enhance the enzymatic activity in vitro.

Temperatures may range from approximately 10-800C, with 30-750 being preferred. Suitable pH of the enzymatic treatment step ranges from approximately 2.5 to 11, depending on the source of the HAEs used. For A. pullulans EAEs, the pH should range from 3.0 to 6.5, and preferably from 3.5 to 5.0.

The treatment time in vitro will vary depending on factors such as the results desired, the amount of substrate used, the amount of HAE enzymes in the enzyme system, the specific activity of the enzyme system, the type of substrate used, the temperature and the like. In general, satisfactory results may be obtained after treatment for at least 0.5 hours, and treatments may range from 1 to 8 hours, more usually 2 to 4 hours, depending upon the particular pulp. The main effect of the enzyme treatment is lignin reduction and results indicate that the desired effects of enzyme treatment are usually obtained in a relatively short time, eg.

4 hours or over even a much less time, and thereafter little if any added benefits are realized.

The enzyme system is preferably obtained from the selected microorganism cultures which are grown in the presence of a carbon source. The preferred carbon sources which may be used include xylan, xylose, corn straw, corn cobs, corn leaves, hardwood kraft pulp, wheat straw, and barley straw, other hemicellulose-containing sources, or other carbon sources which produce the desired enzyme activities. Particularly preferred carbon sources in terms of amount of xylanase production include xylan, especially beech tree xylan, xylose, and corn cobs. For treatment of softwood pulps, enzyme systems which contain more generous amounts of mannanase are preferred and can be obtained, for example, when A.

pullulans is grown in the presence of an inducer for the fungal production of mannanase, such as mannan, mannose, a softwood, or softwood pulp. The microorganism is cultured on the desired carbon source for at least 1-10 days before the enzyme system is collected. Optimum enzyme production is typically achieved at 1-6 days.

The present invention may be applied to reduce or assist in reducing the lignin content (and/or chemically-modified lignin content) of any of the wide variety of chemical pulps, i.e. pulps which have been already previously treated in any of a variety of ways to reduce their lignin content. The invention is advantageously employed with any chemical pulp to which the indicated five or six stage prior art chlorine teaching process would be applicable to achieve the desired minimum brightness level of 88. The invention is particularly applicable to pulps treated by either the sulfite process or the sulfate (kraft) process and is particularly applied to kraft pulps. The particularly applicable pulps will generally already have from 80 to 97X, more usually 85X to 95X of their lignin already removed. Such treatments of chemical pulps is commonly referred to as "bleaching" and may be evidenced by a brightening in pulp color. The invention is most advantageously used with hardwood pulps or mixture thereof containing up to 25% by weight softwood.

After treating the pulp with an HAE enzyme system, the pulp may be additionally bleached to the desired level using known treatment steps of the type customarily combined with first stage chlorine treatments, provided that two extraction steps are included and at least one such step includes peroxide and molecular oxygen. The first extraction step desirably takes place immediately following the enzyme treatment step and prior to any chlorine dioxide treatment. The second extraction step desirably takes place after an intermediate treatment stage such as a chlorine dioxide treatment and prior to the final stage which may also be a chlorine dioxide treatment.For example, sequences which can be used include, but are not limited to the following (X indicates an enzyme treatment, and EOP indicates an extraction in the presence of peroxide and oxygen, the oxygen being present at increased pressure for at least part of the extraction procedure): X-Eop-Dl-Eop-D2 X-Eo-D1-Eop-D2 X-E,p-D1-E,-D2 Alternately, in the conventional six stage bleaching process (CEHDED), a chlorine-free sequence involves the following: X-Eop-H-D1-Eop-Dt X-Eo-H-D1-Eop-D2 X-E0 p -H-D1 -E0-D2 Of the above bleaching protocols, X-Eop-D1-Eop-D2 is the most preferred.

Equipment arrangements which are currently in use for conventional fivestage or six-stage bleaching processes can be readily adapted by those of ordinary skill in the art to be used in the chlorine-free process of this invention. This method has the advantage of reducing the amount of potentially harmful effluent components, resulting in decreased effluent-treatment costs and lessening environmental problems.

The extraction steps used in practicing the invention may be any conventionally employed extraction carried out in a manner essentially consistent with those useful in the conventional bleaching sequences, the basic purpose of such extractions being to remove degraded and solubilized lignin and other material from a prior chemical treatment stage. Simple extractions, ie. those without added bleaching or bound lignin-affecting substances such as peroxide and oxygen, are typically alkaline extractions. However, when extracting enzyme-treated pulps as in step b), a simple hot water (eg. 65-1000C.) has also been found useful. An alkaline extraction using dilute aqueous sodium hydroxide is generally desirable when a peroxide and/or oxygen treatment is included.

The concentration of sodium hydroxide will depend upon known factors, such as the type and condition of the pulp. Generally, the amount will provide a pH of from 10.5 to 12.5, preferably 11 to 12.

The temperature of the extraction steps preferably does not exceed 900C, e.g. 550C to 900C. The temperature is more preferably between 60-800C, and particularly when peroxide is included.

When extraction with oxygen is used, at least part of the extraction step occurs in the presence of oxygen at a pressure greater than that of ambient pressure conditions. Suitable pressures for the oxygen may range from 70,000 to 700,000 Pa, preferably 240,000 to 410,000 Pa. The increased oxygen pressure need only be present for a portion of the extraction period, i.e. less than half of the extraction period, and may be present for less than one fourth of the extraction period. Suitable results have been obtained, for example, with increased pressure for the first ten minutes of a sixty minute extraction.

The amount of hydrogen peroxide employed will generally depend upon the characteristics of the pulps and whether or not it will be employed in one or both extraction stages. In general, the total amount for both stages will not exceed about 0.8X based on the dry pulp weight, and the total amount will usually range from 0.2% to 0.8Z. Hence, an amount of 0.8X or less, eg. 0.6%, may be employed in one extraction stage and omitted in the other, preferably employed in the first extraction stage, or may be employed in varying amounts totalling 0.8Z in both extraction stages. Preferably, the peroxide is employed in both extraction stages, and more preferably in amount not exceeding about 0.3% in each stage, more preferably from 0.1 to 0.25Z in each stage.

After the first extraction step, the resulting pulp is treated with chlorine dioxide or hypochlorite depending upon whether a five or six stage process is to be used. After the second extraction step, the pulp is treated with chlorine dioxide. The amount of chlorine dioxide used in the D1 stage can vary depending on the characteristics of the pulp used, and is typically 1.2% or less, more typically 1.0% or less based on the dry pulp weight. The second chlorine dioxide treatment is used to bring the final brightness to an acceptable level (typically 88 or higher). In the second chlorine dioxide treatment, less chlorine dioxide is generally used than was present in the first chlorine dioxide treatment.It is preferred that less than half of the amount of chlorine dioxide used in the first treatment be used in the second treatment, so in general the amount of chlorine dioxide in the second treatment will be typically 0.6% or less, more typically 0.5% or less. In general, the total amount of chlorine dioxide in D1 and D2 will not exceed about 2% and will be 1.8% or less when oxygen or peroxide is used in one extraction stage.

Preferably, the amount will be 1.6% or less, for example when using oxygen in both extractions. More preferably, it is 1.3% or less. For example, good results have been obtained with 1% chlorine dioxide in the first treatment, and only 0.2 chlorine dioxide in the second treatment, and 0.85% chlorine dioxide in the first treatment and 0.2% in the second treatment, when using oxygen and peroxide in both extractions. The chlorine dioxide treatments are carried out in a conventional manner, eg. at temperatures of 60 to 800C., more usually 65 to 750C. and for times of from 2 to 4 hours, mere usually 2.5 to 3.5 hours.

In the five stage process, each of steps b) through e) are conveniently carried out within the same temperature range of 600C. to 800C.

A hypochorite (B) treatment when employed in a chlorine-free six stage bleaching process will be carried out in a conventional manner for such treatment. Sodium or calcium hypochlorite may be used, preferably sodium hypochlorite. The amount of hypochorite employed will again vary mainly with the characteristics of the pulp, and is typically 2% available chlorine or less based on the dry pulp weight. Treatment times are typically 0.75 to 2.5 hours, more usually 1 to 2 hours at temperatures of 300C. to 450C., more usually 350C. to 400C.

In general, treatments with chemicals such as peroxide, oxygen and hypochorite tend to be partially non-selective in that cellulose can be attacked and quality adversely affected. In general, the quality of a bleached chemical pulp can be indicated by its viscosity as determined by TAPPI procedure T230-OM-82 after the bleaching procedure is completed. A pulp viscosity of at least 8, preferably at least 12, is usually indicative of pulp of acceptable quality. Such viscosity levels are readily achieved by the invention process which generally provides pulp of at least equal quality to the chlorine bleaching process it replaces for the pulp in question.By the process of the invention, the HAEs may be used in place of the chlorine treatment to provide a chlorine-free bleach process attaining a pulp brightness of at least 88 together with a viscosity of at least 8, and usually 10 or higher, and, in general, the amount of and conditions of treatment particularly with peroxide and oxygen and optionally hypochlorite, may be readily regulated to attain such goals after the treatment with the HAEs in the first stage. The amount of chlorine dioxide is also kept at the efficient low levels indicated and does not adversely affect viscosity or quality. In general, paper strength, eg. tear and tensile index, is about the same or somewhat greater when the HAEs are used in the place of chlorine in the invention process.

A preferred method in accordance with this invention is a process for bleaching hardwood kraft pulp consisting essentially of the steps of: a) treating the pulp with a hemicellulose-acting enzyme system of a xylanase-producing microorganism for a time sufficient to render the lignin component solubilizable; b) extracting the resulting pulp in the presence of hydrogen peroxide and oxygen, said oxygen being present at an increased pressure relative to ambient for at least a portion of the extraction step; c) treating the pulp obtained from step b) with a first solution comprising chlorine dioxide; d) extracting the pulp obtained from step c) in the presence of hydrogen peroxide and oxygen, said oxygen being present at an increased pressure relative to ambient for at least a portion of the extraction step; and e) treating the pulp with a second solution comprising chlorine dioxide, said second solution containing less than one-half the amount of chlorine dioxide as is present in said first solution.

A further benefit of the process of this invention is that the yield of bleached pulp is equivalent to that of pulp processed with a conventional chlorine treatment.

The following non-limiting examples are presented to better illustrate the invention.

EXAMPLE 1 Chlorine-free bleaching protocol A bleaching protocol which does not involve the use of chlorine is performed on hardwood kraft pulp. The sequence used is X-Eop-D1-Eop-D2 for the enzyme-treated pulp and B-Eop-D1-Eop-D2 for the control.

Conditions used during this sequence are summarized below.

Enzyme treatment: 40 units/g A. pullulans xylanase for 3 hours at 450C in 50 mM citrate buffer, pH 4.8. Consistency is 5%.

EOP: 0.5% NaOH, 0.2% H202, 345,000 Pa oxygen which is released after 10 minutes. Total time is 60 min., at 700C and 10% consistency.

D1: Chlorine dioxide used at indicated amounts, 700C, 10 consistency and 180 min.

D2: Chlorine dioxide used at indicated amounts, 700C, 10% consistency and 180 min.

B: Buffer (for control). Pulp is incubated for 3 hours in 50 pm citrate buffer, pH 4.8 at 450C. Consistency is 5X.

Starting Kappa number is 13.5.

TABLE 1 Sample Kappa No. Chlorine dioxide Brightness after the D1 D2 D1-Eop D2 first Eop Control 11.9 0.7% 0.3X 69.3 79.1 Control 11.9 1.0% 0.2 74.2 83.8 Enzyme 5.2 0.7% 0.3% 77.9 85.8 Enzyme 5.2 1.0% 0.2% 82.3 89.6 EXAMPLE 2 Enzyme/Chemical Bleaching of Hardwood Kraft Pulp without Chlorine Hardwood kraft pulp is pre-washed with water to neutral pH and then filtered to remove the water to bring the pulp to a consistency of approximately 20%. Enzyme from various sources (10 xylanase units/g O.D.

pulp) in 50 mM sodium citrate buffer, pH 4.8, at 450C is then added to the pulp to give a final consistency of 5X. The samples, in 4.5 litre glass jars, are mixed by shaking. The jars are incubated in a 450C water bath for 3 hours and periodically shaken by hand.

After the three hour incubation with the enzyme, the samples are filtered and washed mith water. The sampler are then chemically bleached with the following sequence: Eop-D1-Eop-D2.

EOP: 0.5% sodium hydroxide, 0.2% hydrogen peroxide, 345,000 Pa oxygen which is released after 20 minutes. Total time is 60 minutes, at 700C, and 10% consistency.

D1: chlorine dioxide used at indicated amounts, 700C, 10Z consistency, 180 minutes.

D2: 0.2Z chlorine dioxide, 700C, 10% consistency, 180 minutes.

Starting Kappa number is 13.5 Following bleaching, the brightness of the samples is measured and is presented below in Table 2. As can be seen, good brightness is achieved for both enzyme preparations. The brightness of the enzyme-treated pulp is significantly higher than the control sample which is not treated with enzyme.

TABLE 2 Enzyme Chlorine dioxide in D1 Brightness Control- no enzyme 1.0 84.9 0.85 85.3 A. pullulans crude 1.0 89.8 0.85 89.6 Trichoderma reesei xylanase 1.0 89.7 0.85 90.0 Paper strength of the samples is also measured. Paper properties of the enzyme treated pulps are substantially equal to those of the control pulp for factors such as burst factor, tear factor, breaking length, and fold endurance.

EXAMPLE 3 Pulp Yield The yield of bleached pulp using the bleaching process of this invention is compared to yield using a conventional process.

The following sequences are compared: A. CDEoDED - a conventional bleaching sequence employing 2.6% elemental chlorine.

B. XEopDEopD - a three hour enzyme treatment at pH 4.8, 500C, and 5Z consistency; 1.1X chlorine dioxide used in the D stages.

In the table below, "Total Active Chlorine X" represents the total reactivity of elemental chlorine and the reactivity of chlorine dioxide.

TABLE 3 Total Chlorine Active Chlorine Dioxide Chlorine Final Yield Sequence % % % Brightness x CDEoDED 2.6 0.5 3.9 88.8 91.9+0.7 XEopDEopD 0.0 1.1 2.9 89 92.2+1.2 As can be seen from Table 3, the enzyme treated kraft pulp which was chlorine-free bleached also gives at least about an equivalent final brightness and at least about an equivalent yield as the conventionally bleached pulp. Paper made from the enzyme treated pulp has an equivalent tear factor for a given breaking length as paper from conventionally treated pulps.

Claims (13)

Claims

1. An in vitro process for bleaching chemical pulp in the absence of elemental chlorine comprising a) treating the pulp with a hemicellulose-acting enzyme system for a time sufficient to render the lignin component solubilizable; b) extracting the pulp obtained from step a); c) treating the resulting pulp with a first solution comprising chlorine dioxide; d) extracting the pulp obtained from step c); and e) treating the resulting pulp with a second solution comprising chlorine dioxide.

2. A process according to claim 1 in which a hypochlorite treatment step is added between steps b) and c).

3. A process according to claim 1 in which one or both of the extraction steps b) and d) are carried out in the presence of molecular oxygen which is present under pressure for at least a portion of the extraction.

4. A process according to claim 3 in which added hydrogen peroxide is present during one or both of the extraction steps b) and d).

5. A process according to claim 3 in which both steps b) and d) are carried out in the presence of oxygen.

6. A process according to claim 4 in which added hydrogen peroxide and molecular oxygen are present during both steps b) and d).

7. A process according to claim 1 wherein the hemicellulose-acting enzyme system comprises a xylanase produced by a microorganism.

8. A process according to claim 7, wherein the hemicellulose-acting enzyme system comprises a microorganism culture supernatant.

9. A process according to claim 7 or 8 wherein the microorganism is selected from the group consisting of Aureobasidium pullulans strain Y-2311, Aureobasidium pullulans strain Y-2311-1 and Trichoderma reesei.

10. A process according to any one of claims 1 to 9 wherein the pulp to be treated is kraft pulp.

11. A process according to claim 10 wherein the kraft pulp is hardwood kraft pulp.

12. A process according to claim 1 for bleaching hardwood kraft pulp comprising the steps of a) treating the pulp with a hemicellulose-acting enzyme system of a xylanase-producing microorganism for a time sufficient to render the lignin component solubilizable; b) extracting the resulting pulp in the presence of hydrogen peroxide and oxygen, said oxygen being present at an increased pressure relative to ambient for at least a portion of the extraction step; c) treating the pulp obtained from step b) with a first solution comprising chlorine dioxide; d) extracting the pulp obtained from step c) in the presence of hydrogen peroxide and oxygen, said oxygen being present at an increased pressure relative to ambient for at least a portion of the extraction step; and e) treating the pulp with a second solution comprising chlorine dioxide, said second solution containing less than one-half the amount of chlorine dioxide as is present in said first solution.

13. A process substantially as herein described with reference to any one of the examples.