PT95615A - PROCESS FOR THE PRODUCTION OF WHITE POWDER WITH A REDUCED PROPORTION OF TOTALLY ORGANICALLY CONNECTED CHLORINE AND REDUCED SHINE INVERSE - Google Patents

Description

INTERNATIONAL PAPER COMPANY " PROCESS FOR THE PRODUCTION OF WHITE POWDER WITH A REDUCED PROPORTION OF TOTALLY ORGANICALLY CONNECTED CHLORINE AND REDUCED SHINE INVERSION "

TECHNICAL FIELD OF THE INVENTION The present invention relates to a process employing xylanases to reduce the level of organically bound chlorine and to reduce the reversal of whiteness of chemically bleached lignocellulosic pulp for use in the pulp and paper pulp industry.

GENERAL BACKGROUND OF THE INVENTION

The main components of the lignocellulosic materials (for example wood) are cellulose, hemicellulose and lignin. These compounds comprise, respectively, about 35% to 50 Z, 20 Z to 30 Z and 20 Z to 30 Z of the dry weight of the plants supplying wood. Cellulose is a polymeric saccharide composed of D-glucose units arranged in a linear arrangement. In woody plants, linear cellulose molecules are arranged in densely compacted bundles of fibrils. Hemicelluloses are linear and branched saccharides, homopolymeric and heteropolymeric, composed of several pentagonal and hexagonal sugars. These sugars include, for example, xylose, arabinose, mannose, galactose and glucose. Hemicelluloses consisting of a homopolymer of one of these sugars are designated, respectively, xylan, arabinane, mannan, galactan and glucan. Hemicelluloses participate in the cross-linking of cellulose molecules and fibril bundles. The other important component of the lignocellulosic material is lignin. Like cellulose and hemicelluloses, lignin is a natural polymer. However, lignin is more complex than both cellulose and hemicelluloses. It consists principally of methoxy-side phenylpropane units randomly linked by a variety of carbon-carbon and ether bonds, resulting in a three-dimensional matrix. This matrix encloses cellulose fibrils, imparting mechanical strength and stiffness to the composition. Thus, lignin can be considered a kind of " cement " that holds together and surrounds the cellulose fibers. The paper is basically a two-dimensional network of randomly arranged cellulose fibers connected by hydrogen bonding between the polysaccharide units. In woody plants, the cellulose fibers are " cemented " by lignin forming ordered parallel arrangements. Accordingly, in order to render useful the material of woody plants for the manufacture of paper, it has to be separated into individual cellulosic fibers -3-

0 '% capable of bridging hydrogen bonds with other cellulose fibers. In the conventional processing of the material of the woody plants, the fibers are released by grinding or mechanical refining, by chemical modification or by removal of the lignin or by combinations of these methods. The separation of the fibers results in the formation of paper pulp - a slurry or a suspension of wood fibers. The deposition of these fibers so as to form a woven mat results in the paper. Pulp formation by mechanical means is the physical separation of the woody fibers, producing the so-called high yield pulp. A high yield is obtained because the lignin is not removed during the pulping process, thus contributing to the formation of the pulp. its mass to the pulp.

In the chemical pulping process, the lignocellulosic material is treated with aggressive chemical oxidizing agents that degrade lignin. The chemical pulping process is most commonly achieved with the Kraft (sulfate), sulfite, soda and modified sulfite processes. These treatments remove most of the lignin matrix, releasing the cellulosic fibers and resulting in the formation of the pulp. For example, the Kraft pulping (sulfate) process produces a pulp containing only 5% to 8% by weight residual lignin, a reduction of approximately tris to five times the original lignin content.

Hybrid processes have also been used, such as the process for the formation of thermomechanical, chemical-mechanical and chemical-mechanical pulp.

Each of the pulping processes described above results in obtaining an intense color pulp, this color being due mainly to the residual lignin. The color intensity of the pulp depends on both the total amount of the residual lignin and its chemical state. For example, chemical pulps from which most of the lignin has been removed during the formation of the pulp are especially dark in color because the remaining lignin is extensively oxidized and modified. The residual lignin remaining in the chemical pulps is particularly refractory to removal. This difficulty has been attributed to the covalent attachment of residual lignin to hemi-cellulose (e.g., xylan) and perhaps to cellulose. Some of these bonds may be present in the wood. It is believed, however, that most forms during the chemical pulping process [see, for example, Matsumoto et al., &Quot; The Role of Sugars Remaining in Residual Lignin ", Fourth International Symposium on Wood and Pulping Chemistry, Paris, France, April 1987, Vol. 2, pages 305-311; Iversen et al., &Quot; The Formation of Lignin-Carbohydrate Bonds During Kraft Pulping ", Fourth International Symposium on Wood and Pulping Chemistry, Paris, France, April 1987, Vol. 2, pp. 163-165; Iversen and -5-

Wannstrom, " Lignin-Carbohydrate Bonds in a Residual Lignin Isolated from Pine Kraft Pulp ", Holzforschung, 40, pp. 19-22 (1986)]. There are many types of paper, ranging from high quality white paper to that used in corrugated paper. In order to produce a desirably white paper, the pulp has to be bleached (leached) prior to its transformation into paper. The whiteness quality is commonly measured in terms of 1 of G.E. - a measure of reflectance. The lignin content of pulp or paper products is usually quantified in terms of kappa index. Although the dark color of the pulp is due to lignin chromophores, the whiteness (% of G.E.) is not directly proportional to the lignin content (kappa number). For example, a bleaching operation that reduces the pulp kappa number from 25 to 15, without destroying the lignin chromophores, does not whittle significantly the pulp. The kappa number should drop to a value less than about 8 to 12 so that significant increases of% of G.E. are observed as a result of the elimination of the lignin. Thus, extensive delignification may be necessary before a measurable bleach can be achieved. Bleaching of the pulp is generally a multi-story process. The sharpening is not necessarily observed at -6 every stage of the bleaching process or even after some of the first floors. It is the sum total of the bleaching stages which results in a bleached and shiny pulp. Single-story or multi-story processes that destroy lignin chromophores, remove lignin (dele-nhificam) or perform these two operations may be termed bleaching stages.

The most commonly used chemical bleaching operations employ chlorine or chlorine-containing compounds, such as chlorine dioxide, calcium hypochlorite, or sodium hypochlorite. Chemical bleaching stages employing hydrogen peroxide, oxygen and ozone have also been used. In commercial bleaching sequences, an alkaline extraction stage, followed by a water wash stage, should commonly follow each of the chemical bleaching operations in a particular bleaching sequence. Most commercial bleaching sequences comprise at least one chlorine treatment plant or a chlorine containing compound. Chemical whitening stages whiten the lignocellulosic pulp primarily by removing their lignin, rather than destroying the lignin chromophores.

Lignocellulosic pulps which have been subjected to bleaching sequences which comprise bleaching operations with chlorine or with chlorine-containing compounds generally contain

organically bound chlorine; and the paper products produced from these pulps contain chlorinated organic compounds. There is a growing demand for paper products with reduced levels of organically bound chlorine.

Bleaching sequences which do not comprise a chlorine treatment stage or with chlorine containing compounds produce bleached pulps with little or no organically bound chlorine. However, such chlorine-free chemical bleaching sequences have not been used on a large scale because the bleaching operations they employ (eg treatment stages with oxygen, ozone or hydrogen peroxide) tend to be more expensive than bleaching operations with chlorine or with a chlorine compound. In addition, some of these operations tend to cause greater degradation of cellulose fibers than bleaching processes with chlorine or with compounds containing chlorine. This degradation of the cellulosic fiber results in a pulp of low viscosity, which produces paper with poor mechanical properties.

For the above reasons, most commercial bleaching sequences comprise at least one chemical bleaching operation using chlorine or a chlorine-containing compound. Accordingly, a bleaching process is required which comprises at least one bleaching operation with chlorine or with a chlorine compound and which yields bleached lignocellulosic pulps with reduced content of organically bound chlorine (" TOCl ").

All paper products produced from lignocellulosic pulps which have been bleached by known bleaching processes are subject to the reversal of whiteness (i.e., yellowing) in response to the action of heat, light and aging. This reversal of bleaching is undesirable, especially in high quality white paper products. Therefore, a commercially practicable bleaching process which yields a lignocellulosic pulp with reduced shade inversion is also necessary.

Com- patible with work indicating that residual lignin from the Kraft pulp is crosslinked with hemicellulose, several publications refer to the treatment of wood pulp with hemicellulases (eg xylanase), sometimes in combination with chemical bleaching treatments , for the purpose of delignifying, bleaching and / or producing pulp which produces paper having the improved mechanical properties.

None of these publications state that xylanase treatment decreases TOCl or reduces bleach reversal. What the publications actually suggest is that hemicellulases, such as xylanase, delignify by degradation of hemicellulose thereby extracting the cleaved hemicellulose and the lignin which is crosslinked therewith; see, for example, -9- and

Viikari I; Paice et al., &Quot; Bleaching Hardwood Kraft Pulp with Enzymes from Cloned Systems ",Proceedings; 74th Annual Meeting of the Canadian Pulp & Paper Association, Montreal, Canada,

January 1988, pages A133 - 136; Chauvet et al., &Quot; Assistance in the Bleaching of Nerver-Dried Pulps by the Use of Xylanases, Consequences on Pulp Properties ", Fourth International Symposium on Wood and Pulping Chemistry, Paris, France, April 1987, Vol. 325-327; Noé et al., &Quot; Action of Xylases on Chemical Pulp Fibers ", J. Wood Chem. Technol., 6, pp. 167-184 (1986); Viikari et al., &Quot; Bleaching with Enzymes ", Proceedings: Third International Conference on Biotechnology in the Pulp and Paper Industry, Stockholm, Sweden, June 1986, pages 67-69; French Invention Patent Number 2,557,894; and U.S. Patent Number 2 280 307.

SUMMARY OF THE INVENTION The present invention relates in one embodiment to a practical method for the production of bleached lignocellulosic pulps with reduced organically bound total chlorine content (" T0C1 ") in comparison with that obtained using the known bleaching sequences. According to a second embodiment, the present invention relates to a process for the production of bleached lignocellulosic pulp with reduced bleaching inversion compared to that obtained using the known bleaching sequences.

Accordingly, it is an object of the present invention to provide a method for the production of bleached lignocellulosic pulp having a high viscosity and producing paper having superior mechanical properties and having a reduced TOC content and a reduced reversal of whiteness.

These and further objects and advantages of the present invention, evident from the detailed description and the claims which follow, are achieved in the first embodiment by subjecting the lignocellulosic pulp to at least one treatment plant with xylanase in combination with one or more chemical bleaching floors, wherein the chemical treatment floor or floors comprise at least one floor using chlorine or a chlorine-containing compound.

In the second embodiment, the objects and advantages of the present invention are achieved by subjecting cellulosic pulp to at least one xylanase treatment plant in combination with at least one chemical bleaching stage.

DETAILED DESCRIPTION OF THE INVENTION

LIGNOCELLULOSE POLYPS

Suitable lignocellulosic pulps for the practice of the present invention include pulps prepared from materials such as azzoz straw, wheat straw, bagasse, paper waste, hardwood and softwood; hardwood pulps and softwood pulps are preferred. By way of example, such wood pulps include those which are prepared by the well known processes of sulfite, sulfate or Kraft, soda and modified sulphite. Also suitable are mechanical pulps, thermomechanical pulps and chemothermomechanical pulps. The list referred to above is a partial list of lignocellulosic pulps which can be used to good advantage in the process according to the present invention. For example, the use of pulps produced by processes not currently known in the art is also excluded.

PREPARATIONS OF XILANASE

Xylanase preparations derived from both fungi and bacteria are useful for the purposes of the present invention. Preferred are xylanase preparations which comprise an endoxylanase. The particular microorganism used as a source of xylanase is not part of the present invention. In contrast, any microorganism which produces xylanases, and preferably endoxylanases, is useful for the purpose of the present invention. Many such microorganisms are known in the art. [See, for example, Bali and McCarthy, " Production and Properties of

Xylanases from Actinomycetes ", J. Appl. Bacteriol. 66, pages 439-444 (1989); Bastawde, " Studies on Xylanase from Chainia Sp. &Quot;, PhD thesis (in Biochemistry). University of Poona, Division of Biochemical Sciences, National Chemical Laboratory, Pune-411008, India, pages 9-37 (May 1987) (" Bastawde Thesis "); Dekker, " Biodegradation of the Hemicelluloses ", in Biosynthesis and Biodegradation of Wood Components, Academic Press, NY, pages 505-533 (1985)].

Useful fungi species include those extracted from, inter alia, Aspergillus, Chaetomium, Sporotrichum, Sclerotium, Schizophyllum, Trichoderma and Thermoascus. Particular species of fungi which are believed to be useful include, among others, Aspergillus ochraceus, Aspergillus niger, Aspergillus awamori, Sporotrichum dimorphosporum, Schizophyllum radiatum, Trichoderma reesei, Trichoderma harzianum and Thermoascus aurantiacus.

Useful bacterial species include those derived from *

Chainia -, Streptomyces, Bacillus and Clostridium. Particular bacterial species which are believed to be useful include, among others, Streptomyces olivochcomogens, Bacillus subtilis, Bacillus stearothermophilus, Clostridium thermocellum and Clostridium acetobutylicum.

It was proposed that the genus Chainia become a junior synonym of the genus Streptomyces and that the species it contains -13-

if they assigned new names accordingly. Goodfellow et al., &Quot; Transfer of Chainia sp. to the Genus Streptomyces with Emended Description of Species ", System. Appl. Microbiol. 8, pages 55-60 (1986). Approving this suggestion, the American Type Culture Collection reclassified most of the strains of Chainia to the genus Streptomyces. The Northern Regional Research Laboratory apparently does not agree with this proposal; did not reclassify its species Chainia.

Chainia species and Streptomyces species previously classified as being of the genus Chainia are especially preferred as sources of xylanase preparations.

Strains which produce xylanases but not cellulases are preferred.

Many strains of the genera and species mentioned above are available in the deposits of public microorganisms. Useful (ie, xylanase-producing) strains can be identified by the simple expedient of culturing the microorganisms by isolating the supernatant from the extracellular culture and detecting the xylanase activity according to procedures known in the art (see, for example, Khan et al., " Assay of Xylanase and Xylosidase Activities in Bacterial and Fungal Culture ", Enzyme Microb. Technol., 8, pp. 373-377 (1986)]. The applicant has studied, for xylanase activity, sixteen strains of Streptomyces and Chainia, obtained from the American Type Culture Collection (" ATCC "). All but three were found to produce xylanases.

Lyophilized microorganisms obtained from ATCC in culture media recommended by ATCC (ATCC media number 5j hereinafter referred to as " sporulation media ") (50 ml sporulation medium in 250 ml Erlenmeyer flasks) were grown at 26 ° C, on a rotary shaker set at 150 revolutions per minute. After accumulating a large biomass (approximately two weeks), 10 Âμl of inoculum (5 ml) was placed in 50 ml of 3 x Spore Media (contains all soles of the Spore Media at a concentration of three times larger) in 250 ml Erlenmeyer flasks and incubated for seven days at 28 ° -30 ° C on a rotary shaker set at 150 revolutions per minute. After seven days of incubation, 10% of inoculants (5 ml) of each seed of the cultures were placed in the 3% Xylan Fermentation Medium [3% xylene (Sigma Chemical Co. N2 X3875) and 1% yeast extract (Difco) in water from the distribution network] (set A) and 5%

Wheat bran [12.5% post-natural meal flakes (40 1 wheat bran) and 1 t yeast extract (Difco) in water from the network] (set B).

The cultures were incubated in 3% Xylan Fermentation Media (Set A) in 250 ml Erlenmeyer flasks at 30øC in a rotary shaker assembly at 220 revolutions per minute. Aliquots (3 ml) were removed from each culture of Set A on each of days 3 to 8 after inoculation.

Cultures were incubated in Wheat Bran Meal at 51 (set B) for four days in 250 ml Erlenmeyer flasks at 28øC in a rotating shaker assembly at 150 revolutions per minute. On the fourth day of incubation, a 10% (5 ml) inoculum from each culture of Set B was transferred aseptically into 3% Xylan Fermentation Means and incubated in 250 ml Erlenmeyer flasks at 30ø C in a rotary shaker assembly at 220 revolutions per minute. Aliquots of each culture from set B were withdrawn on each of days 3 to 8 after inoculation.

The aliquots were centrifuged at 70 x g for twenty minutes at room temperature and the supernatants were assayed for xylanase activity as described below. It should be noted that, since xylanase production has peaks on different days according to the different strains, it is important to test the activity on at least every 3 to 8 days after inoculation.

Set B was incubated in the Wheat Bran Medium at 51 because this medium was said to induce the production of xylanase [see Srinivasan et al., Studies on Xylan Degrading Enzymes from Chainia, Biotechnol. Lett., 6, pp. 715-718 (1984)].

However, we did not observe significant differences in xylanase activity between the duplicate cultures of Set A and Set B.

Using each of the selection protocols described above, we have identified ten strains as positive xylanase producers. A strain was classified as a positive producer if only on any of days 4 to 8 it was characterized by a xylanase activity greater than 2 U / ml. The ten positive producer strains (and their ATCC accession numbers) are as follows: Streptomyces sclerotialus (ATCC 15896); Streptomyces flaviscleroticus (ATCC 19347); Streptomyces fumigatiscleroticus (ATCC 19345); Streptomyces minutiscleroticus (ATCC 17757); Streptomyces niger (ATCC 17756); Streptomyces ochracilscleroticus (ATCC 15814); Streptomyces poonensis (ATCC 15723); Streptomyces roseiscleroticus (ATCC 17755); Streptomyces sp. (ATCC 27946); and Chainia hygroatrocyanea (ATCC 43962). -17-

In addition to the above ten positive producing strains, the applicant identified three other strains as the marginal producers of xylanase. A strain was classified as a marginal producing strain if, on its maximum production day between days 4 and 8, it was characterized by 1-2 U / ml xylanase activity. The three marginal producer strains (and their ATCC accession numbers) are: Streptomyces olivaceiscleroticus (ATCC 15722 and 15897) and Streptomyces purpurogeniscleroticus (ATCC 19348). Only three of the sixteen strains tested were found to be negative producers. A strain was classified as a negative producer if, on its maximum production day between days 4 and 8, it was characterized by less than 1 U / ml xylanase activity. The negative producer strains (and their ATCC accession numbers) are as follows: Streptomyces ruber (ATCC 17754); Streptomyces violens (ATCC 15898); and Chainia vaxianesis (ATCC 43139). The most preferred species as endoxylanase source is Chainia sp. (NCL 82-5-1) (ATCC 53812). The xylanase II of this strain is preferred over xylanase I. The hemicellulases of Chainia sp. (NCL 82-5-1) are studied in the Bastawde thesis and in Srinivasan et al., MStrudies on Xylan Degrading Enzymes from Chainia, Biotechnol. Lett., 6, pp. 715-718 (1984). For further characterization of this strain, see Srinivasan et al., &Quot; High Activity Extracellular Glucose (Xylose) Isomerase from a Chainia Species ", Biotechnol. Lett., 5, pages 611-614 (1983); Vartak et al., &Quot; Characterization of Extracellular Substrate Specific Glucose and Xylose Isomerases of Chainia ", Biotechnol. Lett., 6, pp. 493-494 (1984); and Pawar et al., " Purification and Characterization of Glucose (Xylose) Isomerase from Chainia sp. (NCL 82-5-1) ", Biochem. Biophys. Res. Commun., 155, pp. 411-417 (1988). An advantageous feature of this strain of Chainia is that it does not also produce cellulases.

Therefore, unfractionated extracellular culture media or their concentrates can be used without the appearance of deleterious effects of cellulases (i.e., reduced pulp viscosity and reduced yield of pulp).

Generally, the desired xylanases are secreted by the microorganism into the extracellular culture medium. The extracellular culture medium is then harvested and processed to achieve the desired degree of concentration and the desired enzyme purity. The culture conditions and the time of harvesting influence the total amount of enzyme activity recovered, as well as the relative proportions of each of the various xylanases, if more than one is present. In addition, the various strains differ in their total production of xylanases and, if more than one xylanase is produced in proportion to the various xylanases. Accordingly, one skilled in the art can optimize the production of xylanase over the intended end use of the enzyme preparation. Processes for the initiation and development of fungal and microbial cultures and for harvesting the extracellular development medium for optimum xylanase production do not form part of the present invention and may conveniently be carried out by methods known in the art.

The culture media harvested may be used directly. Preferably, however, the culture medium is concentrated to yield an unfractionated xylanase concentrate using the process of the present invention. The desire to use extra-cellular development media or an unfractionated concentrate thereof decreases when these preparations comprise significant amounts of cellulases or active proteases. Significant amounts of active cellulases cause, due to their cellulose attack, undesirable decreases in pulp viscosity and, if present in sufficient quantity, a measurable decrease in pulp yield. Significant amounts of active proteases may, on account of their attack on other proteins, cause undesirable decreases in xylanase activity in the reaction mixture. The undesirable effects of cellulases or contaminating proteases can be reduced or prevented by the addition of inhibitors of these enzymes to the reaction mixture. Alternatively, the enzyme preparation can be fractionated to separate some -20 λ-5 - ν 5 ν or all contaminating cellulases or proteases. Any concentration method which preserves the enzymatic activity is suitable. Suitable concentration methods include freeze-drying and evaporation under vacuum, as well as precipitation with a high content of salt, polyethylene glycol, acetone and alcohol. If any of these concentration procedures are used, the xylanase can be reconstituted in water or an appropriate buffer or in a pH solution adjusted to 5 to 7, yielding an unfractionated xylanase concentrate.

Alternatively, the lyophilized or precipitated product may be added directly to the slurry.

In certain circumstances, it may be desirable to use partially purified subfractions of the culture media harvested. For example, the use of a xylanase-containing subfraction which does not also contain significant amounts of cellulases or proteases is preferred. Alternatively, it may be desirable to separate the various xylanases so that these enzymatic activities may be used independently in the enzyme treatments. Finally, the use of xylanases which have been purified to near homogeneity, as well as the use of mixtures of purified xylanases is envisaged.

The methods used to subfraction extracellular culture media harvested to achieve the desired xylanase purification degree -21 are not part of the present invention and may be carried out by any effective combination of methods known in the art. For example, chromatographic separation operations based on differences in charge, PI, hydrophobicity or size may be employed. Also various methods of selective precipitation, for example with ammonium sulfate, are suitable. In addition, use is made of affinity chromatography using, for example, monoclonal antibodies to purify selected xylanases. See, for example, U.S. Patent No. 4,725,544 to Tan et al., Which describes a process for separating the xylanases from their mixtures with other hemicellulases and cellulases produced by culturing hemicellulitic microorganisms.

Recombinant or synthetic xylanases may also be useful in the process according to the present invention. These enzymes can be produced by methods known in the art.

Typically, the desired xylanase preparation is added directly to the pulp suspension. However, it may be advantageous to immobilize the enzymes on a solid support, and then add this derivatized solid support. The manner of proceeding used to immobilize the enzymes depends on the carrier used and may be carried out by any suitable method which does not destroy the enzymatic activity. An advantage of using xylanases coupled to solid carriers is that the enzymes may, under certain circumstances, be more conveniently recovered from the delignification reaction mixture after incubation with the pulp suspension. The recovered enzyme can then be reused.

Xylanase Treatment Phase The xylanase treatment step (" phase X ") may be performed in any container of the desired size for which preferably the setting of the mixture and the temperature of the contents have been provided. The order of addition of the reaction components is not critical. The basic reaction mixture comprises lignocellulosic pulp and an active xylanase preparation, in water or aqueous solution at appropriate pH.

If the lignocellulosic pulp is a wood pulp, it should be present in the reaction mixture with a consistency of from about 0.1% to 15%, and preferably from about 2% to 10%. One skilled in the art is of course able to routinely determine the optimum consistency of the pulp for other lignocellulosic pulps. The xylanase preparation is present in the reaction mixture in a ratio of about 0.1 to 200 units of xylanase activity per gram of dry weight of pulp. Preferably, the xylanase preparation is present between about 0.1 and 50 and more preferably between about 1 and 25 units of xylanase activity per gram of dry weight of pulp.

A unit of xylanase activity is defined as the amount of enzyme which causes the production, from xylan, of 1 micromole of xylose per minute under standard reaction conditions. Cleavage of xylan by xylanase results in reducing sugar pools, which then react with dinitro-salicylic acid (" DNSA ") in the test solution to produce a color change which is detected at 540 nm.

A standard curve of xylose concentration (micromoles / milliliter) as a function of absorbance at 540 nm was prepared using various dilutions of 100 mM xylose (Pfaltz and Bauer, Inc.) in 50 mM sodium acetate, pH 5.0. From the standard curve, 1 micromole of xylose was determined to have an absorbance at 540 nm approximately equal to 0.0128 under normal reaction conditions. This constant was used to calculate the xylanase activity of the sample solutions (for example culture supernatants) which were assayed. The assay was linear for sample solutions with an absorbance at 540 nm to about 0.2. Sample solutions with absorbance above this value were diluted so that their absorbances were within the linear zone. -24-

For each sample solution (for example, culture supernatant) assayed, a " reference solution " which contained the same amount of sample solution (for example, culture supernatant) and sufficient amount of 50 mM sodium acetate, pH 5.0, to give the final assay volume equal to 1 ml. To each sample tube was added a 1% solution of larch wood xylan (Sigma Chemical Co. No. X3875) in 50 mM sodium acetate, pH 5.0 (0.5 ml). Subsequently, the sufficient volume of 50 mM sodium acetate, pH 5.0 was added to the tubes containing the samples to give a final test volume of 1 ml, after addition of the sample solution culture supernatant) and the xylan solution. Analogously, a sufficient volume of 50 mM sodium acetate was added to the reference tubes to give a final test volume of 1 ml after the addition of the sample solution (for example, culture supernatant). Thereafter, the desired volume of sample solution (maximum volume 0.5 ml) was added to each sample tube and reference tube. The contents of the tubes were thoroughly mixed and incubated at 50 ° C for thirty minutes. After incubation, 1% DNSA in distilled water (1 ml) was added to each tube and the tubes incubated for five minutes at room temperature. Then, 5 N NaOH (0.2 ml) was added to each tube (to promote color development) and the tubes were incubated for thirty minutes at room temperature. The absorbance at 540 nm of each sample solution was then measured with the spectrophotometer calibrated to 0 in the case of the reference solution corresponding to the sample. The pH of the reaction mixture is maintained within the range of about 4 to 8 throughout the incubation of the xylanase preparation with the lignocellulosic pulp. Preferably, the pH is maintained within the range of about 5 to 7.5. Generally, the pH of the reaction mixture is within the desired pH. However, if affirmative action is required to maintain the desired pH (for example, where the pulp to be used has a particularly high or low pH value) the pH regulation may be employed by chemical means. Maintenance of pH by chemical means is the preferred method for maintaining the pH in large scale incubations. Alternatively, the pH can be maintained (if necessary) by employing a buffer in the reaction mixture. Any suitable buffer that is effective at the desired pH, e.g., phosphate, may be used. When a buffer is added, it is generally at a concentration of 0.1 to 100 mM in the reaction mixture. The reaction mixture comprising the xylanase preparation and the lignocellulosic pulp is incubated at about 20 ° C to 70 ° C and preferably at about 40 ° C to 65 ° C. The incubation time is comprised between about 0 , 25 and 18 hours, preferably between about 0.5 and 6 hours, and most preferably between about 1 and 4 hours.

Preferably, the reaction components are mixed at the beginning of the incubation; no further mixing is required.

Those skilled in the art will readily be able to optimize the reaction conditions for the particular preparation of xylanase used, without the need for much experimentation.

If the xylanases are to be used again in the reaction mixture, they must be separated from the lignocellulosic pulp after incubation. Appropriate methods for separating the xylanases from the pulp include vacuum filtration, precipitation and sedimentation; filtration is preferred. Chemical bleaching operations The choice and number of chemical bleaching operations used in combination with the X operations are not part of the present invention. In contrast, any sequence of chemical bleaching operations can be combined with one or more X operations according to the present invention, with the result that the bleached pulp thus produced has reduced T0C1 and a reduced reversal of brightness in comparison - With the pulps subjected to comparable bleaching sequences, but without any X operations. Any kind of chemical bleaching operations - many of which are well known - are useful in the practice of the present invention. Also useful are chemical bleaching operations not known in the art.

Further, the process according to the present invention need not result in a fully bleached pulp. For example, a sequence according to the present invention producing a pulp with a whiteness of only about 60% GE may still have a lower T0C1 index than the pulp subjected to the same bleaching sequence without the X operations. as used herein, the term " bleached pulp " refers to a lignocellulosic pulp which has undergone at least one chemical bleaching operation and at least one X operation and is therefore measurably de-signified. No bleaching is required " complete " nor the brightening.

Commercial bleaching processes generally comprise a sequence of various types of chemical bleaching operations. Useful examples of well-known chemical bleaching include, but are not limited to, treatment operations with chlorine and chlorine dioxide (C)), chlorine dioxide (D) and ozonation (Z). Generally, an alkaline extraction (E) is carried out following each of the C 1, D 2 and Z operations. These extraction operations may be increased with oxygen (EQ), hydrogen peroxide (E 2) or oxygen and hydrogen peroxide (EQ + p). The pulp is often washed with water between many of the above operations.

It should be noted that the above indication of chemical bleaching operations is given by way of example only; is not exhaustive. The objects of the present invention are achieved by combining one or more of any type of chemical bleaching operations with one or more xylanase treatment operations in accordance with the present invention. The particular sequence of chemical bleaching operations and operations X is not part of the present invention. In contrast, in the first embodiment of the present invention, any sequence comprising at least one X operation and at least one chemical bleaching operation using chlorine or a chlorine containing compound gives a bleached pulp with a reduced TOC value when compared to a pulp subjected to the same chemical bleaching operations but not to any X operations. Similarly, in the second embodiment of the present invention, any sequence comprising at least one X operation and at least one chemical bleaching operation produces a reversed bleached pulp reduced brightness when compared to a pulp subjected to the same chemical bleaching operations, but not to any X operations.

Preferred are the processes according to the present invention which comprise an X operation.

Operations X or operation X may advantageously be performed at any point in the process sequence. For example, operations X may be the last operations of the bleaching sequence. Similarly, the chemical bleaching operations can be chosen and combined in any desired order. Examples of the many factors that those of ordinary skill in the art routinely consider when choosing a particular sequence of operations in accordance with the present invention include, but are not limited to, the desired degree of bleaching, cost and compatibility with existing bleach plant equipment and the processes carried out. Preferably, however, the pulp is washed with water immediately prior to any operation X. In order to better understand the present invention, the following Examples of the process according to the present invention are described. These Examples are for illustration only and the present invention is not to be construed as limited by any of the Examples described herein.

EXAMPLES EXAMPLE 1

Preparation of xylanase from Chainia sp. (NCL 82-5-1)

The microorganism Chainia sp. (NCL 82-5-1) (ATCC 53812) in potato dextrose agar tubes at 4 ° C. The microorganisms were transferred to a sterilized culture medium containing a xylan source (e.g., 5% bran wheat or 1% xylan) and the conventional nutrients (e.g., 1% yeast extract). Cultures were incubated at 30 ° C in shaken flasks vigorously shaken until a peak of xylanase activity (as measured by the assay described above, above) (generally after three to five days) appeared. After reaching the peak of activity, the cells were separated from the microbes and other solids by conventional means (e.g., filtration or centrifugation) to provide a clear filtrate or a clear supernatant. The filtrate or supernatant was typically concentrated, prior to use, by ultrafiltration or by evaporation under vacuum. The yield of the xylanase obtained from the cultures was approximately 10 U / ml if a medium containing 5 7 wheat bran and approximately 25 U / ml was used if 1 7 of xylan was used. -31 -31

EXAMPLES 2 TO 7

Southern pine kraft pulp (softwood) was subjected to six different bleaching sequences, each consisting of one X operation in combination with various chemical bleaching operations.

The softwood kraft pulp used in Examples 2 to 7 was extensively washed with distilled water in a Buchner funnel and stored at 28% consistency at 4øC prior to use. The water-washed pulp had a kappa number equal to 34 and a viscosity equal to 0.033 Pa.s (33 cP).

In Examples 2 to 4, treatment sequences with X " mock " ("X ^ OCk") served as controls The X ^ Ock operations were performed as the corresponding X operations, with the difference that no xylanase was used. In Examples 5 to 7, the treatment sequences without an X or an Xk-Ock operation served as controls The other operations of each control treatment sequence were performed as in the corresponding non-control sequence.

After each treatment operation in each Example, the pulp was isolated by vacuum filtration on a Buchner funnel and then washed in the funnel with distilled water (6 liters). -32- .cri »

After each bleaching sequence, the whiteness and the organically bound total chlorine (T0C1) of the bleached pulp were measured. These results are shown in Table I. Table I also shows the total available chlorine load applied during the completion of each bleaching sequence.

To determine the T0C1 value of a pulp sample, the pulp (5 grams, dry weight basis) was extensively washed with deionized water to remove any residual organic or inorganic chloride. The washed pulp was then flushed in a closed flask according to the method of Schoniger [see Kolthoff et al., Quantitative Chemical Analysis, page 586 (4th Edition, 1969)]. The chloride content of the sample which was subjected to combustion was quantified by ion chromatography [see Franklin and Fitchett, " Fast Chemical Characterization of Pulping and Bleaching Process Liquors by Ion Chromatography ", Pulp & Paper Canada, 83, pages J40-44 (1982)].

To determine the whiteness of a pulp sample, a pulp cushion was prepared according to the procedure of T218. The degree of whiteness (% G.E.) of the pulp pad was then measured according to the methods of proceeding ΤΑΡΡΙT452 and T217 [TAPPI West Methods, Vol. 1, TAPPI Press, Atlanta (1989)]. EXAMPLE 2

Bleaching Sequences and XCpEgD

Operation X was carried out according to the following procedure.

Chainia xylanase was prepared and dried by evaporation under vacuum as described in Example 1. A preparation of that xylanase (30,000 units dissolved in 1 liter of 50 mM sodium acetate solution, pH 5.0) was added to a slurry of 200 grams (based on dry weight) washed with water and 9 liters of sodium acetate, pH 5.0. The resulting slurry pulp, having a consistency of 2% pulp, was incubated for three hours at 50 ° C with continuous stirring with magnetic stirrer. The X ^ QCK operation was performed essentially as described for step X, except that no xylanase was applied.

After step X, a portion of the treated pulp, washed with water (60 grams, dry weight) was subjected to chlorination in a Cp operation. The pulp was placed in a polyester film bag (Scotch Pak NQ 5, 3M Company). Chlorine and chloro dioxide were then added to give a pulp consistency of 37%, 8.35% chlorine and 0.1% chlorine dioxide, with chlorine percents and chlorine dioxide based on the weight of the dried pulp. The bag was then sealed, kneaded for a short time until the reaction mixture became homogeneous, placed in a water bath at 45 ° C and incubated for one hour. The bag was occasionally removed from the water bath during incubation and kneaded for short time intervals to ensure proper mixing.

After the CD operation, part of the treated pulp was washed with water (30 grams, dry weight) under alkaline extraction in the presence of oxygen (Eq). The EQ operation was run in a Parr reactor, mixing at 320 revolutions per minute, at 70øC, for fifty minutes, using a pulp consistency of 4% and a 4% sodium hydroxide solution (based in dry weight). The oxygen pressure was equal to 2.8 kg / cm (40 psig) for the first twenty minutes and then equal to 2.1, 1.4 and 0.7 kg / cm2 (30, 20 and 10 psig) for the subsequent periods of ten consecutive minutes.

After operation Eq, the treated pulp, washed with water (30 grams, dry weight) with chlorine dioxide, was bleached in step D. The bleaching operation D was carried out in a polyester bag, with the mixture provided by kneading as described above in the case of operation Cq. The reaction composition before incubation was as follows: 10% consistency of the pulp; chloro dioxide: 1% sodium hydroxide: 0.4 l (based on dry pulp). The bag was incubated in a water bath at 70 ° C for three hours. The pH of the slurry at the end of step D was 3.0. EXAMPLE 3

Bleaching sequences xmoCKCDED and xcdED

In this Example, operations X, XM0CK, CD and D were performed essentially as described in Example 1. Operation E following the operation was carried out in a polyester bag, as described above with respect to step C, at 70 ° C for one hour, with the pulp consistency equal to 10% and 4% (based on the dried pulp). EXAMPLE 4

Bleaching sequences xm0CKCDE0D and xcuEoD

In this Example, the operations X, XI, QCK 'E0 and D were carried out essentially in the same manner as described in Example 2. The operation was carried out essentially as described in Example 2, except that a lower quanti-

(6.8% chlorine and 0.1% chlorine dioxide based on the weight of the dry pulp) applied to the pulp (60 grams, dry weight). EXAMPLE 5

Bleaching sequences ZEqD and XZEqD

In this Example, operations Eq and D were performed essentially as described in Example 2. Operation X was also carried out as described in Example 2, except that less xylanase (10,000 units per 200 grams dry weight of pulp).

After step X, the pulp slurry was filtered and a portion of the treated pulp (100 grams dry weight) was resuspended with pulp consistency of 2% with 0.1% diethylene triamine pentaacetic acid (based on the dry weight of the pulp) and diluted sulfuric acid was added to lower the pH of the suspension to 2.5. The slurry was adjusted to pH adjusted using a Lightning mixer (1000 revolutions per minute) for one hour at 50 ° C and then the pulp was recovered by vacuum filtration. The pulp was treated with dilute acid to remove metal ions that are thought to have an adverse effect on ozone bleaching. -37-

To perform step Z, water washed pulp (pulp consistency equal to 1%) was suspended in water at pH 2.5 and placed in a closed reactor equipped with a mechanical mixer. Oxygen (100%) was passed through a Welsbach ozone generator (model No. T408), which converted about 1.5 to 3% of the oxygen into ozone. This oxygen / ozone mixture was then bubbled to the bottom of the reactor through an inlet at the rate of 2 liters / minute and at a pressure of 6 psig. The outlet at the top of the reactor allowed the oxygen / ozone mixture to exit after passing through the slurry of pulp. The concentration of ozone in the ozone / oxygen mixture was measured at the inlet and outlet with a Dasibi ozone monitor (model N9 1003HC). Based on these measurements, 2% ozone (based on the dry weight of the pulp) was determined to be consumed during the reaction. The ozonation was carried out at room temperature for about 0.5 hour with continuous stirring.

After operation Z, part of the ozone treated pulp (30 grams dry weight) was subjected to EqD operations as described in Example 2.

Following control bleaching (ZEqD), operation X was not performed. Other operations were performed as described immediately above. EXAMPLE 6

Bleaching sequences ODED and OXDED Oxygen assignment with oxygen (step 0) was carried out as described below. Washed pulp was incubated with water (290 grams, dry weight) having a pulp consistency of 10æg, with 3% sodium hydroxide and 0.5% magnesium sulphate (based on the dry weight of the pulp), in a Quantum reactor, with mixing at 600 revolutions per minute (5 seconds every three minutes),

(80 psig) for one hour at 95øC. The kappa number value of the resulting pulp was 18.72 after it was washed with water as was described above.

Part of the washed washed and oxygen-delaminated pulp (200 grams, dry weight) was subjected to an X operation performed essentially as in Example 5. Thereafter, part of the pulp (30 grams, dry weight) was subjected to a first step D, which was carried out essentially as described in Example 2, but with 1.78% chlorine dioxide (based on the dry weight of the pulp). Following the first step D, operation E was performed essentially as described in Example 3, but with 2.8% sodium hydroxide. Finally, a second run D was performed essentially as described in Example 2, but with 1% chlorine dioxide (based on the dry weight of pulp). Both operation D and operation E were carried out in polyester bags as described in Example 2.

Following control bleaching (ODED), no X operation was performed. Other operations were done as described immediately above. EXAMPLE 7

Bleaching sequences C ^ E ^ DO and C ^ XE ^ DD

Water-washed pulp (100 grams, dry weight) was first washed in a C18 operation, which was carried out essentially as described in Example 2, except that 7.5% of chlorine and 0.1 % of chlorine dioxide (based on the dry weight of the pulp).

Chlorinated pulp was washed with water in step X essentially as described in Example 5, except that half of the xylanase (5000 units) was used in order to maintain constant the ratio of xylanase to pulp dry weight . Subsequently, part of the pulp (30 grams, dry weight) was subjected to an Eq run, performed essentially as described in Example 2. Finally, the pulp was subjected to two -40-40. , successive operations D, with washing with water between the operations, using 1.5% and 0.5% of chlorine dioxide (based on the dry weight of the pulp), respectively.

In the control bleaching sequence (CjjEqDD), operation X was not performed. Other operations were carried out as described immediately above.

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Hardwood kraft pulps and softwood, bleached and commercially available were subjected to a number of different X operations.

The pulps used in Examples 8 to 10 were washed extensively with distilled water in a Buchner funnel before use. They were stored at 4 ° C, with a consistency, respectively, equal to 19.9% (softwood) and 12% (hardwood).

Control samples of the bleached hardwood and softwood kraft pulp were subjected to the X-ray operations performed under the same reaction conditions as the corresponding X operations, but without xylanase.

In Examples 8 and 9, the TOCl value of each pulp was determined after step X or according to the procedure described in Examples 2 to 7.

In Examples 8 to 10, the whiteness of the water-washed (i.e. pre-treatment) starting pulps and the pulps after treatment with xylanase or with xylanase mock were determined as described above in Examples 2 to 7. After determination the pulp pads were subjected to two aging treatments intended to accelerate the reversal of whiteness: 1) heating in an oven at 105 ° C for one hour [Rapson and Spinner, &quot; Bringhtness Reversion in Bleached Pulp &quot;, in Singh, The Bleaching of Pulp, pages 359-360 (TAPPI Press, 1979); and 2) exposure to water vapor for one hour according to the TAPPI T260 procedure. The whiteness of the pulps were determined after this aging treatment.

Table II contains the results of the respective analyzes. EXAMPLE 8

Treatment with bleached softwood pulp xylanase

A first sample of bleached softwood kraft pulp (50 grams, dry weight basis) was suspended so as to obtain a pulp having the consistency of 2% in 0.5 mM sodium acetate solution, pH 5, 0. Chainia xylanase was prepared and dried by evaporation under vacuum as described in Example 1. A preparation of this xylanase (2500 units dissolved in 250 ml of 50 mM sodium acetate, pH 5) was added to the suspension of pulp. -44-

A second sample of bleached softwood kraft pulp (50 grams, dry weight) was suspended as the first sample. More concentrated Chainia xylanase preparation (7,500 units dissolved (in 250 ml of 50 mM sodium acetate, pH 5.0) was added to this second pulp sample.

A third sample of bleached softwood kraft pulp (50 grams, dry weight), such as the first sample, was also suspended but no xylanase was added. This sample served as a control for XÎ ± Î ± CK '

The above three pulp suspensions were incubated for two hours at 50øC with magnetic stirring using a Lightning mixer. The pulps were then isolated by vacuum filtration in Buchner funnels and washed with distilled water (6 liters per charge). EXAMPLE 9

Treatment with bleached hardwood pulp xylanase

Treatments were carried out with bleached hardwood kraft pulp xylanase essentially as described in Example 8. â € ƒâ € ƒâ € ƒâ € ƒâ € ƒâ € ƒâ € ƒâ € ƒ EXAMPLE 10

Treatment with bleached softwood pulp xylanase

Bleached softwood kraft pulp (50 grams, dry weight) was suspended in distilled water so as to obtain a pulp consistency of 10 in a polyester bag. The xylanase (250 units) was dissolved in 10 ml of distilled water, added to the pulp slurry, and the bag was sealed. The resulting pulp slurry had a pH value of 6.8. The suspension was kneaded in the sealed bag for mixing and then incubated for four hours in a water bath at 60 ° C. The bag was occasionally removed from the water bath and kneaded for a short time to mix. A control pulp sample was incubated without xylanase.

After incubation, the pulps were washed with distilled water (6 liters) as described above. -46- -

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Claims (8)

A process for the production of bleached lignocellulosic pulp with a reduced proportion of organically bound total chlorine, characterized. in that it comprises: a) one or more xylanase treatment operations comprising the step of incubating lignocellulosic pulp with an effective amount of a xylanase preparation; and b) one or more chemical bleaching operations wherein at least one of the chemical bleaching operations employs a compound selected from the group consisting of chlorine, chlorine-containing compounds and mixtures thereof. -4- 2. A process for the production of bleached lignocellulosic pulp with a reduced gloss reversal comprising: a) one or more xylanase treatment operations comprising the step of incubating the lignocellulosic polypeptide with an effective amount of a preparation of xylanase, and b) one or more bleaching operations. 3. A process according to claim 1 or 2, characterized in that the lignocellulosic pulp is chosen from rice straw pulps, wheat straw, bagasse, paper waste, hardwood and softwood. 4. A method according to claim 3, wherein the lignocellulosic pulp is selected from hardwood kraft pulp and softwood kraft pulp. 5. A process according to any one of claims 1 or 2, characterized in that the xylanase preparation is derived from a microorganism selected from the group consisting of strains of Aspergillus, Sporotrichum, Sclerotium, Chaetomium, Schizophyllum, Chainiá, Clostridium, Streptomyces , Bacillus and Trichoderma. A process according to claim 5, characterized in that the xylanase preparation is derived from a strain of Chainia or Streptomyces. 7. A process according to claim 6, wherein the strain is Chainia sp. (NCL 82-5-1) has the identification characteristics of ATCC 53812. 8. A process according to claim 6, wherein the strain is selected from the group consisting of Streptomyces sclerotialus having the identification characteristics of ATCC 15896; Streptomyces flaviescleroticus having the identification characteristics of ATCC 19347; Streptomyces .fumigatiscleroticus having the identification characteristics of ATCC 19345; Streptomyces. minutiscleroticus having the identification characteristics of ATCC 17757; Streptomyces niger having the identification characteristics of ATCC 17756; Streptomyces ochracilscleroticus having the identification characteristics of ATCC 15814; Streptomyces poonensis having the identification characteristics of ATCC 15723; Streptomyces roseiscleroticus having the identification characteristics of ATCC 17755; Streptomyces sp. having the identification characteristics of ATCC 27946; and Chainia gygroatrocyanea having the identification characteristics of ATCC 43962. 9. A process according to any one of claims 1 or 2, characterized in that it comprises a xylanase treatment step. 10. A process according to claim 9, characterized in that the xylanase treatment step is the latter. Lisbon, October 17, 1990; ·; wmogí dc rrcpr.edcde indosfrkjj -I _ t