The present invention relates to a process according to the preamble of claim 1 for producing bleached lignocellulosic pulps.

In particular, the invention relates to a process for enhancing the enzymatic treatment of lignocellulosic pulps which are to be bleached. According to such a method lignocel¬ lulosic pulp is treated with an enzyme preparation containing mannanase activity in connection with the bleaching.

The invention further relates to a process according to the preamble of claim 17 for producing bleached lignocellulosic pulps.

The main aim of chemical pulping is to defibre the lignocellulosic raw material by removing lignin from the fibre matrix as completely and specifically as possible (delignification). Particularly in sulphate cooking, residual dark lignin remains in the pulp, giving rise to a brown colour of the pulp. Traditionally, dark residual lignin has been removed by chlorine bleaching using chlorine gas or chlorine dioxide as the bleaching chemical.

Nowadays, however, the trend is towards totally chlorine-free bleaching sequences where oxygen-based chemicals, such as oxygen gas, ozone or hydrogen peroxide or combinations thereof, are used. These chemicals, however, are less specific in delignific- ation than chlorine chemicals, and accordingly, the brightness of a pulp bleached totally without chlorine chemicals generally remains lower than that obtained using chlorine chemicals.

It is known in the art that the bleachability of pulps cooked using a conventional pulping process can be increased by means of a hemicellulose treatment (Kantelinen et al. 1988; Viikari et al. 1991). Thus far, pulp bleachability has been enhanced by a partial enzymatic hydrolysis of the pulp xylan. As enzymes, mainly xylanases have been used in combination with different bleaching sequences in laboratory (Viikari et al.,

1986; Viikari et al., 1987; Viikari et al. 1990) and in mill scale experiments (Viikari et al., 1991). Xylanase treatment has also been used prior to totally chlorine-free bleaching sequences (van Lierop et al., 1993; Malinen, 1993).

In addition to xylanases, the use of mannanases of bacterial and fungal origin for pretreatment of conventional kraft pulps during bleaching has been studied (Clark et al., 1990; Clark et al., 1991). Although purified mannanase preparations alone and together with xylanase are reported slightly to increase the bleachability of the pulps studied, the effect of the mannanases always is smaller than that of the xylanases.

The aim of the present invention is to provide a process for enhancing the effect of mannanase treatment on chlorine-free bleaching in particular.

This invention is based on the unexpected observation that the effect of a mannanase treatment is considerably intensified if the kappa number of the pulp is lowered to a small enough value prior to enzymatic treatment. Typically the kappa number is lowered below 20 in the case of softwood and below 15 in the case of hardwood sulphate pulps.

More specifically, the process according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

Moreover, the bleaching process according to the invention is characterized by what is stated in the characterizing part of claim 17.

Within the scope of the present invention, the term "hemicellulolytic enzyme" or "hemi- cellulase" denotes an enzyme which is capable of cleaving and modifying hemicelluloses. Furthermore, "xylanase" denotes an enzyme capable of cleaving polyose chains containing xylose units (xylans). Xylanases may be exemplified by endo-l,4-β-D- xylanase. The term "mannanase" is used to refer to an enzyme which is capable of cleaving polyose chains containing mannose units (mannopolymers) such as glucomannan, galactoglucomannan and galactomannan. Endo-l,4-β-D-mannanase is an example of mannanases.

The term "enzyme preparation" designates any product which contains at least one hemicellulase. Thus, the enzyme preparation can, for instance, be a culture medium or its concentrate containing a xylanase and/or a mannanase, an isolated xylanase, mannanase or a mixture of two or more xylanases and/or mannanases.

The term "enzyme preparation containing mannanase activity" denotes an enzyme preparation which contains at least one mannanase enzyme.

"New cooking methods" and "modified cooking methods" (the terms are used interchan- ge-ably in the following text) are cooking processes which differ in their cooking conditions or method from conventional batch or continuous kraft cooking and in which delignificat- ion can be enhanced resulting in the production of low kappa number pulps without considerable losses in pulp properties. In new cooking methods the cooking conditions (uniform alkalinity) are advantageously modified in such a way that, for example, re- precipitation of xylan is prevented. New cooking methods have been described in the literature (Malinen, 1993).

In the context of the present invention, the term "low kappa number pulp" refers to a softwood pulp having a kappa number below 20 and a hardwood pulp having a kappa number below 15.

Conventional pulping techniques do not allow for easy lowering of the lignin content of a pulp to the level described herein. The specificity of the cooking chemicals used in conventional batch pulping is reduced along with prolonged cooking time resulting in increased degradation of the carbohydrates in the pulp which in turn are relevant for the strength properties of pulp (Sjδstrom, 1993). However, by using pulping aids such as anthraquinone (a redox catalyst), conventional batch cooking delignification can be enhanced and the cooking can be continued even to relatively low kappa numbers without extensive losses in pulp strength properties.

According to the present invention it is, however, advantageous to cook a lignocellulosic pulp to the desired kappa number using a pulping process known as a modified cooking method whereby it is possible to delignify softwood pulps to kappa numbers below 20

and hardwood pulps to kappa numbers below 15, while simultaneously achieving relatively low carbohydrate losses, thus retaining the strength properties of the pulp. Examples of the new cooking methods include Modified Continuous Cooking (MCC), modified batch-type cooking (Superbatch cook), an RDH (Rapid Displacement Heating) method combined with continuous cooking as well as an isothermal cooking method and extended MC cooking (EMCC cook) (Andrews, 1989; Dillner, 1989; Hiljanen et al., 1990).

The new cooking methods aim at preventing and retarding the degradation of pulp carbohydrates by modifying the alkaline profile, the chemical dosage and the circulation of the cooking chemicals of the cook, as well as by altering the cooking temperature. The kappa number of the pulp can be further reduced by oxygen delignification after cooking. In the case of low lignin levels, however, oxygen is a relatively unspecific bleaching chemical and can generally be used to achieve a 50 % delignification only (Axegard et al., 1991).

The process according to the invention is particularly well suited for pulps produced by these new modified cooking methods because, according to one preferred embodiment of the invention, a process is provided which can be used to increase the bleachability of low kappa number pulps even when totally chlorine-free bleaching is used.

Thus, according to the invention, treatments based on the use of mannanase and/or mannanase-containing xylanase preparations provide an advantageous improvement of the bleachability of pulps produced by the new cooking methods and of oxygen delignified pulps in totally chlorine free bleaching.

These features are surprising because, so far, no significant increase of the bleachability of pulps produced by modified cooking methods has been considered obtainable by means of hemicellulase treatments. Hemicellulases, mainly xylanases, are generally known to hydrolyze primarily the xylan on the surfaces of pulp fibres blocking the removal of residual lignin (Kantelinen et al. 1992). This surface xylan mainly consists of xylan which is dissolved during the initial cooking phase and which, as a result of decreased pH towards the end of cook, precipitates on the pulp fibres. In conventional

pulps where such xylan is not present, a xylanase treatment cannot be used to provide significant improvements in the bleaching result. For instance, a xylanase treatment does not notably enhance the bleachability of sulphate pulp produced by continuous cooking in a flow-trough digester. The same is true for sulphite pulps produced in alkaline or acidic conditions. As indicated above, the alkaline profile of the new and modified cooks is controlled; for example, the alkalinity of the cooking liquid is maintained at a relatively high level by, e.g., liquid displacement (Superbatch) or by adding alkali to the cook in several stages (MCC). Thus, it is apparent that pulps produced by such modified cooking methods only contain minor amounts of reprecipitated hemicellulose which could be attacked by hemicellulases.

According to the present invention, however, it is even possible to increase the bleachability of pulps which do not contain significant amounts of reprecipitated and recrystallized xylan or mannan if the pulp is cooked to a sufficiently low kappa number. The invention is especially suitable for pulps produced by the superbatch and MCC methods, as described in the examples below.

The combined hemicellulase treatments, especially mannanase and xylanase treatments together, have also been found to increase the bleachability of pulps produced by modified cooking methods. It is remarkable that the effect of mannanase treatment on the bleachability of oxygen delignified pulps increases with decreasing kappa number of the pulps. Surprisingly, we have found that in low kappa number pulps the effect of a mannanase treatment on pulp bleachability exceeds that of a xylanase treatment.

The concept according to the invention is particularly well suited for treatment of pulps where a totally chlorine-free bleaching is desired, allowing for the use of oxygen, hydrogen peroxide and ozone alone or in different combinations as bleaching chemicals. Hemicellulases can be used as a mixture, in purified form or as a culture medium or a concentrate thereof.

According to the invention hemicellulase treatments are carried out in connection with bleaching, meaning that they are effected prior to, during, or after the bleaching. According to a preferred embodiment of the invention, a pulp of the alkaline phase of a

superbatch cook is first delignified by oxygen before being subjected to an enzymatic treatment. After a mannanase treatment, the pulp is contacted with oxygen gas so as to enhance the removal of lignin, whereafter the actual bleaching is carried out using, e.g., peroxide. Alternatively, oxygen delignification can be carried out after the hemicellulase treatment.

The effect of mannanase treatment increases with decreasing kappa number of the pulp. Thus, although mannanase treatment typically only has a relatively limited effect on the bleachability of conventional kraft pulps, the reduction in kappa numbers achieved by mannanase treatment of sulphate pulps modified according to the invention is considerable. The effect is clearly observed in softwood pulps whose kappa number is below 14, preferably below 10, and in hardwood pulps whose kappa number is below 10, preferably below 6. Mannanase treatment is especially effective for softwood pulps. As is clear from Example 5 below, a kappa number reduction of over 25 % is achieved after mannanase treatment in the case of softwood pulp cooked to a kappa number of

6.4. At the same time, a three-unit increase of brightness can be obtained with respect to a reference mass. As stated above, the brightness increase of low kappa number pulps (kappa number < 10 in softwood pulp) is clearly more pronounced after mannanase than after xylanase treatment.

In the enclosed Figure 1 , the same results are graphically presented by means of a bar diagram. The effects of mannanase and xylanase treatments, respectively, on the bleach¬ ability of pulps produced by superbatch, MCC and continuous cooking methods are compared in the diagram.

The use of mannanases together with xylanases is especially advantageous. In Example 6 the combined effect of mannanase and xylanase on kappa number and brightness is presented.

The conditions applied in a hemicellulase treatment are presented below.

Hemicellulases used in the method can be produced by different micro-organisms. Thus, as host producers of xylanases, bacteria, yeasts, and molds can be used. Such producers

can for example belong to one of the following genera: Trichoderma (e.g., T. reeseϊ), Aspergillus (e.g., A. niger), Phanerochaete (e.g., P. chrysosporium), Penicillium (e.g., P. janthinellum, P. digitatum), Streptomyces (e.g., S. olivochromogenes, S. flavogriseus), Humicola (e.g., H. insolens) and Bacillus (e.g., B. subtilis, B. circulans). Other white-rot fungi belonging to species such as Phlebia, Ceriporiopsis, and Trametes can be used for producing mannanase.

Examples of suitable xylanases include two xylanases from Trichoderma reesei with pi values of about 5.5 and 9, and molecular weights (measured by SDS-PAGE) of about 19 kDa and 20 kDa, respectively (Tenkanen et al., 1992).

However, as far as the mannanase is concerned, the unexpected observation has been made that not all mannanases can function effectively in a fibrous matrix. Thus, the required hydrolysis of mannopolymers cannot be obtained with bacterial mannanase. For this reason, the mannanase used is preferably produced by molds or fungi belonging to, for example, the following genera: Trichoderma (e.g., 7. reesei), Aspergillus (e.g., A. niger),

Phanerochaete (e.g., P. chrysosporium), and Penicillium (e.g., P. janthinellum, P. digitatum). The white-rot fungi presented above can also be used as hosts for mannanase production.

Examples of suitable mannanases include two major mannanases from Trichoderma reesei with pi values of about 4.6 and 5.4 and molecular weights of about 51 kDa and 53 kDa (SDS-PAGE), respectively (Stahlbrand et al., 1993). It should be kept in mind that the SDS PAGE method has an accuracy of about ±10 %.

Xylanases and mannanases can also be produced by strains genetically modified for production of these proteins only or by other genetically modified host organisms to which genes coding these proteins have been transferred. After the cloning of the genes of the required protein (Teeri et al., 1983), the protein can be produced by a chosen host organism. The chosen host organism can be a T. reesei -mold, a yeast, a mold of a different origin, for example from the genus Aspergillus, a bacterium or any other micro-organism with sufficiently well-known genetics.

After the isolation of the gene, the production of hemicellulase by the original production strain can also be enhanced or modified using well-established genetical methods, i.e. by transferring several copies of the chromosomal mannanase gene to the mold or by connecting the gene under a (for instance stronger) promoter of another gene. The expression of the gene can thus be obtained in the desired cultivation conditions, e.g., in a growth medium in which the strain does not naturally produce hemicellulases.

According to a preferred embodiment of the invention, the desired enzymes are produced by the mold Trichoderma reesei. This strain is a widely used production organism and its hemicellulases are relatively well known. T. reesei can synthesize several xylanases, two of which have been described in more detail in PCT Publication No. WO 92/03541, and at least five mannanases.

Xylanases and mannanases can be isolated from the culture medium of the mold

Trichoderma reesei by several known methods. Typically, in these isolation methods various purification techniques such as precipitation, ion exchange chromatography, affinity chromatography and gel chromatography are used in combination. By means of affinity chromatography, hemicellulases can be rapidly separated even directly from the culture medium. However, the gel material used in affinity chromatography is difficult to produce and is not commercially available. In the preferred embodiment of the present invention, xylanase and mannanase enzymes were Separated from other proteins in the culture medium by a fast anionic exchange purification method. The purification method is described in more detail in Examples 1 and 3 below. However, the invention is not limited by this enzyme isolation method; instead, the isolation of the desired enzymes can be carried out using other known methods.

To sum up, the method according to the present invention for enhancing the bleachability of low kappa number pulps comprises the following steps:

Pulp is cooked to a desired low kappa number, advantageously by a modified cooking method, in a manner known per se. Examples of the alternative cooking methods include modified continuous cooking (MCC), batch-type superbatch cooking, an RDH

(Rapid Displacement Heating) method combined with continuous cooking, isothermal cooking (ITC), and extended MCC cooking (EMCC).

After the cook the pulp can further be delignified by means of an oxygen chemical, in particularly oxygen gas, in order to reach a kappa number level below 20 in the case of softwood pulps and below 15 in the case of hardwood pulps. However, a superbatch method, for example, can be used to cook the pulp to a low enough kappa number even without oxygen delignification. The next step is to treat the pulp with hemicellulase. The treatment is typically carried out at a 0.1 to 20 % consistency, preferably at a 1 to 10 % consistency, and at a temperature of about 30 to 90 °C, preferably 40 to 60 °C.

The pH value of the enzymatic treatment varies according to the optimal pH of the hemicellulase used. Typically, the treatment is conducted in slightly acidic or neutral conditions with a pH value ranging from 3 to 8, preferably from 4 to 7. The pulp pH can be adjusted by adding acids. The enzyme dosage is about 1 to 10.000 nkat/g, preferably about 10 to 1.000 nkat/g. The time of treatment ranges from 1 min to 20 h, preferably from 30 min to 5 h.

After the enzymatic treatment the content of such metals which could hamper chlorine- free bleaching is reduced in the pulp. The metal content is preferably reduced by treating the pulp with a complexing agent such as EDTA or DTPA.

After this treatment the pulp is bleached to a target brightness value which typically lies between about 50 and 89. Bleaching is carried out by a chlorine chemical free bleaching sequence using oxygen gas, peroxide and/or ozone as the bleaching chemical. Between the oxidizing stages of the bleaching sequence, the pulp can be treated with an alkali, such as sodium hydroxide. Examples of suitable partial and complete bleaching sequences include the following: XQP _n , OXQP _n , (EOXQP)QP _n , OXP _n , EOXO, EOXZ and OXQPZEP _n (X = hemicellulase treatment, Q = chelating treatment, O = oxygen delignification, E = alkaline extraction, P = peroxide treatment, Z = ozone treatment, and n = integer number between 1 and 10).

Considerable advantages are achieved by the invention. Thus, by means of the enzyme preparation of the present invention, an increase of brightness is attainable in pulps

produced by modified cooking methods, especially after oxygen delignification, which cannot be obtained by peroxide bleaching or a corresponding chemical treatment alone without loosing the strength properties of the pulp. Furthermore, enzymatic treatment also enhances the leaching of residual lignin during bleaching, whereby, for instance, the amount of bleaching chemicals and consequently also environmental loads can be reduced.

In the following, the invention is described in more detail with the help of a number of non-limiting examples. In Figure 1 , the effects of enzymatic treatment on the bleachability of the oxygen delignified MCC and superbatch pulps of Example 1 are shown.

The brightness values of the pulps presented in the examples have been measured accor¬ ding to SCAN Cl l and the viscosity values according to SCAN C15:1988. The kappa numbers have been obtained by means of the SCAN Cl :1997 method. All percentage values have been calculated by weight.

Example 1. Isolation of a xylanase enzyme

Xylanase was isolated as previously described by Tenkanen et al. (1992).

The fungus Trichoderma reesei (strain VTT-D-86271, Rut C-30) was cultivated in a 1 dm ³ fermentor on a culture medium containing 6 w-% of Solca floe cellulose, 3 % distilled spent wheat grain, 0.5 % KH ₂ PO ₄ , and 0.5 % (NH ₄ ) ₂ SO ₄ . The cultivation temperature was 30 °C and the pH value was adjusted to a value between 4 and 5. Cultivation was carried on for 6 d after which the fungal mycelium was removed from the culture medium by centrifugation.

The enzyme isolation was initiated by buffering the centrifuge-treated culture medium to pH 4 by gel filtration (Sephadex G-25 coarse). At this pH value, the enzymatic solution was applied to an anion exchanging chromatography column (CM-Sepharose FF) onto which some of the proteins in the sample were adsorbed. A major part of the adsorbed

proteins was eluted from the column by a pH 4 buffer to which sodium chloride was added to develop a linear concentration gradient between 0 and 0.15 M. The xylanase was collected into these fractions (in a NaCl concentration of 0.07 to 0.15 M).

The xylanase-containing solution separated during the previous run was buffered to pH

8.0 by gel filtration (Sephadex G-25). At this pH value the enzyme was first bound to a cation exchange column (CM-Sepharose FF) and then eluted from the column by a buffer at pH 8, adding sodium chloride to develop a linear concentration gradient between 0 and 0.05 M. The purified enzyme was collected into these eluted fractions.

Finally, the enzyme was purified by gel filtration on a Sephacryl S-100 HR column. The concentrated xylanase-containing liquid obtained during the previous run was fed to the column and was eluted with a sodium citrate buffer at pH 5.5 containing 0.1 M of NaCl.

Example 2. Characterization of the xylanase

The protein properties of the enzyme preparation purified according to Example 1 were determined by standard methods used in protein chemistry. Isoelectric focusing was carried out by Multiphor II System equipment (Pharmacia) in accordance with the manufacturer's instructions using a gel containing 5 % polyacrylamide. Carrier- ampholyte Ampholine, pH 3.5-10 (Pharmacia), was used for creating a pH-gradient between 3.5 and 10 in isoelectric focusing. A traditional gel electrophoresis was carried out in denaturating conditions (SDS-PAGE) using a gel containing 10 % polyacrylamide. The accuracy of the SDS-PAGE method is known to be about + 10 %.

The proteins in both gels were stained by silver staining (Bio Rad, Silver Stain Kit).

According to the results the molecular weight of the xylanase was 20 kDa and the isoelectric point was 9.

Example 3. Isolation of a mannanase enzyme

The Trichoderma reesei (RUT C-30, VTT D-86271) culture medium was subjected to

bentonite treatment for enzyme isolation, as described by Zurbriggen et al. (1990). The medium was subsequently concentrated by ultrafiltration and the concentrate was dried by spray drying.

The isolation of the enzyme was initiated by solubilizing the spray dried culture medium into a phosphate buffer. The unsolubilized material was separated by centrifugation and the enzyme solution was buffered to pH 7.2 by gel filtration (Sephadex G-25). At this pH value the buffered solution was applied to a cation exchanging chromatography column (CM-Sepharose FF) onto which some of the proteins in the sample were adsorbed. The target enzyme was collected through the column from the unadsorbed fractions.

The enzyme-containing liquid at the above-mentioned pH value (pH 7.2) was applied to an anion exchange column (DEAE-Sepharose FF) onto which most of the proteins in the sample were adsorbed. The required enzyme was collected from the unadsorbed fractions.

The fractions containing the enzyme were further purified by hydrophobic interaction chromatography (Phenyl Sepharose FF). The enzyme was bound to the column at a salt concentration of 0.3 M (NH ₄ ) ₂ SO ₄ . The adsorbed enzyme was eluted by a buffer at pH

6.5 using a decreasing linear (NH ₄ ) ₂ SO ₄ concentration gradient between 0.3 and 0 M whereafter the elution was continued with a buffer at pH 6.5. The mannanase enzyme was collected to the fractions eluted in the end of the gradient and after the gradient.

The enzyme solution was buffered by gel filtration (Sephadex G-25) to pH 4.3. The enzyme was adsorbed to a cation exchange column (CM-Sepharose FF) at this pH, and part of the adsorbed proteins (among others, most of the remaining cellulases) were eluted from the column by a buffer at pH 4.4. The mannanase enzyme was eluted from the column by a buffer at pH 4.3, adding sodium chloride to develop a linear concentration gradient between 0 and 0.05 M. The target enzyme was collected into these eluted fractions.

Finally, the enzyme was purified by gel filtration (Sephacryl S-100 HR). The

mannanase-containing solution from the preceding purification step was applied to the column and then eluted by a 0.1 M sodium phosphate buffer at pH 6 containing 0.2 M of NaCl.

Example 4. Characterization of the mannanase enzyme

The protein properties of the enzyme preparation purified according to Example 3 were determined by standard methods used in protein chemistry using the equipment described in Example 2. The molecular weight of the protein was measured by the SDS- PAGE method.

The enzyme preparation contains two mannanases (Stahlbrand et al, 1993) which were found to exhibit similar biochemical and functional properties. The pi of the one enzyme is 4.6 and that of the other is 5.4. The molecular weights are 51 kDa and 53 kDa, respectively. The pH optimum of both enzymes is between 3 and 5.3, and the optimal temperature at activity measurement conditions is about 70 °C.

Example 5. Experimental bleaching of lignocellulosic pulps

Industrial softwood kraft pulps (pine/spruce) cooked and oxygen delignified to different kappa numbers (kappa numbers ranging from 19.8 to 6.4) were hydro lyzed with Trichoderma reesei mannanase and xylanase at 5 % consistency, pH 5, at a temperature of 45 °C for 2 hours. After such treatment the pulps were washed twice with distilled water (2 x 10 x pulp d.w.). Metals were removed from the pulp whereby the following conditions and chemical dosages prevailed: 0.2 % EDTA, 5 % consistency, 1 h at pH 5 and a temperature of 45 °C. The pulps were washed with water (2 x 10 x pulp d.w.) prior to a one-stage peroxide bleaching. The chemical dosages and conditions applied were: 3 % H ₂ O ₂ , 1.5 % NaOH, 0.5 % MgSO ₄ , 0.2 % DTP A, 10 % consistency for 1 h at 80 °C. After the peroxide stage the pulp was acidified and the brightness, kappa number and viscosity of the pulp were measured. The bleaching results are shown in

Table 1.

Table 1. Peroxide bleaching of mannanase and xylanase treated sulphate pulps produced by different cooking methods

Pulp Initial Enzyme Dosage Brightness Kappa Viscosity production kappa nkat/g % (ISO) dm ³ /kg method number extended batch- 13.1 MAN 500 59.1 7.6 610 +AQ

XYL 500 59.3 6.3 630

REF - 57.6 7.1 620

EMCC 19.8 MAN 500 53.4 12.1 950

XYL 500 53.1 11.3 970

REF - 51.9 12.5 970

Superbatch + O ₂ 6.4 MAN 500 67.2 3.6 560

XYL 500 65.7 5.6 580

REF - 64.2 4.9 580

MCC + O ₂ 13.4 MAN 500 59.5 7.6 950

XYL 500 60.9 6.7 1010

REF - 57.8 7.8 1010

AQ = Anthraquinone addition

MCC = modified continuous cook

EMCC = extended modified continuous cook

O ₂ = oxygen delignification after pulping

REF = reference

MAN = mannanase

XYL = xylanase

The mannanase treatment increased the final brightness by 2.6 to 2.9 % for the pulps produced by new pulping methods and by 2.9 to 4.7 % for the oxygen delignified pulps as compared to the final brightness values of the reference treated pulps. The corresponding increases in final brightness values obtained by xylanase treatment were 2.3 to 3.0 % and 2.3 to 5.4 %. The reduction of kappa number obtained by mannanase treatment was between 0 and 3.3 % for the pulps produced by new cooking methods and between 2.6 and 26.5 % for oxygen delignified pulps as compared to the kappa

numbers of the reference pulps. The corresponding reductions in final kappa numbers obtained by xylanase treatment were 9.8 to 11.8 % for the pulps produced by new pulping methods and 0 to 14.3 % for the oxygen delignified pulps.

Example 6. Experimental bleaching of lignocellulosic pulps

Softwood kraft pulps cooked by different new cooking methods and oxygen delignified to low kappa numbers (superbatch, kappa 9.8) and oxygen bleached superbatch (kappa 5.9) and MCC (kappa 11.8) were treated with mannanase and xylanase at 5 % consistency and at pH 5 for 2 h at 45 °C. After the treatment the pulps were washed, metals were removed, and the pulps were washed again according to Example 5. The pulps were then peroxide bleached and their final brightness and kappa number were measured. The chemical dosages and conditions used during bleaching were: 3 % H ₂ O ₂ , 1.8 % NaOH, 0.5 % MgSO ₄ , 0.2 % DTP A, 10 % consistency, 3 h, 80°C. The bleaching results are presented in Table 2.

Table 2. Peroxide bleaching of mannanase or xylanase treated low kappa number pulps

The mannanase and xylanase treatments together decreased the kappa number of the MCC+O ₂ pulp by 28 %, that of the superbatch pulp by 13 % and that of the superbatch+O ₂ pulp by 9 % compared to the kappa number of the reference treated pulps when measured after peroxide bleaching. The combined enzymatic treatment increased the brightness values of the pulps by 5.3 %, 3.9 % and 3.9 %, respectively, compared to the reference brightness values.

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