Hemicellulase-aided bleaching is the first full-scale biotechnical application in the pulp and paper industry which truly exploits the unique specificity and safety of. () reported that the chlorine dosage can be reduced 25% for hemicellulase- assisted chlorine bleaching to obtain the similar bleached pulp. Paice et al. Elemental chlorine, while effective in bleaching pulp and low in both primary classes: hemicellulases (xylanase) and oxidases (laccase).
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This application is a continuation of application Ser. The present invention relates to processes for treating paper pulp and particularly relates to a method for enzyme treatment of paper pulp. One of the biggest challenges facing the pulp and paper industry is to reduce the use of chlorine in the bleaching process. Hemicellulasez effluent from the pulp bleaching plant, that portion of a mill that converts brown pulp to white, contains numerous chlorinated organic substances including toxic chlorinated phenols and dioxin.
Pulp and paper processors worldwide are bleachlng intense regulatory pressure to reduce these emissions.
The present invention hemicelulases to hemicwllulases improved method for using enzymes in the processing of paper pulp to boost the efficiency of the bleaching process. The process of this invention overcomes a critical and heretofore unrecognized problem that has reduced the effectiveness of enzymes in a conventional pulp mill bleachery.
The invention makes possible a three to four fold improvement in the “brightness hemicellulaaes power of these enzymes as well as reduced chlorine requirements. The starting point for making paper is wood. Wood consists primarily of cellulose, hemicellulose, and lignin. The manufacture of high quality, bright white paper largely depends on removing the lignin from the wood pulp with minimal degradation to the hemicellulaaes and hemicellulose.
Although lignin is present in lower grades of paper such as newsprint, complete lignin removal is essential for the production of fine paper. This is because lignin weakens and imparts color onto the pulp. The most common method for producing strong pulp bleacihng is light in color for high quality paper is the Kraft process. In North America, for example, After being washed with water, the cooked material contains 1. The remaining lignin is removed by a multistage bleaching process to obtain a bright, stable final product.
The first two stages of a conventional bleaching process involve treating the brownstock with chlorine, and then extracting the pulp with sodium hydroxide.
After delignification, the final remaining lignin in the pulp is removed by treating it with oxidizing chemicals such as chlorine dioxide, sodium hypochlorite and sodium hydrosulphite.
Hemicellulases in the bleaching of chemical pulps.
These treatment stages are known as the “brightening” stages because the final product is the desired bright white pulp. Unfortunately, the effluent from this chlorine-based bleaching process contains several classes of toxic compounds, namely organochlorines. These compounds are formed principally when chlorine gemicellulases with lignin in the first bleaching stage. The organochlorine inn by Kraft mills has been expressed in two ways: AOX is a nonspecific measure plp the total organochlorine production of a mill, and is generally 1.
Dioxin is one of the most acutely toxic compounds known, and has been found in mill effluents, in the pulp itself, in finished pulp products coffee filters, milk cartons, diapers, writing paperand in the food chain including trout and crab where dioxin bioaccumulates to levels thousands of times higher than in pulp wastewater.
The amount of organochlorines discharged from a pulp mill is closely related to the bleaching process used and, in particular, to the amount of chlorine used for bleaching. The following relationship between AOX production and bleaching chemical usage has been recognized:. This method involves prolonging the kraft pulping process to enhance lignin removal before bleaching.
Extended delignification techniques involve additional digester capacity, which is prohibitively expensive for existing mills. This option is only appropriate for new mills. High chlorine dioxide substitution. Clearly, these alternatives incur significant costs. One of the primary objectives of this invention is to provide an improved way of using enzymes as part of the bleaching process to make it practical to reduce AOX discharges without incurring significant capital expenses.
Enzymes are biological catalysts, i. Enzymatic catalysis involves the formation of an intermediate complex pylp the enzyme and its substrate. The region of an enzyme that specifically interacts with the substrate pul called the active site.
On binding to this site, the substrate is brought into close proximity to specific groups on the enzyme that cooperatively destabilize hemicelluoases bonds in the substrate, making them more chemically reactive. Enzymes differ most strikingly from ordinary chemical hemicellulsses in iin substrate specificity heemicellulases catalytic efficiency. Most enzymes have only a few natural substrates, which are converted to single products in remarkably high yields.
The unique structures of the active sites of enzymes provide this specificity and not only allow favorable binding of specific substrates but also exclude the unfavorable binding of many substances that are not substrates. There are strong attractive hemifellulases forces between the active site and a substrate, and enzymes may be thought to act by “attracting” the substrate into the site, where the extraordinarily unique structural transformations of the substrate occur.
For enzyme systems, a high degree of specificity is maintained, with the reaction proceeding 10 6 to 10 12 times faster than the spontaneous, uncatalyzed reaction in aqueous solution. The pH has a marked influence on the rate of enzymatic reactions. Characteristically, for each enzyme there is a pH value at which the rate of reaction is optimal, and on each side of this optimum, the rate is lower.
The influence of pH on enzymatic reactions may involve several different types of effects. Enzymes, like other hemicellulwses, are ampholytes and possess many ionic ppulp. If enzymatic function depends on certain special groupings, these may have to be present in some instances in the un-ionized state and, in others, as ions.
In some cases, the groups in the active site of the enzyme that are responsible for catalytic action have even been identified by comparing the effect of pH on enzymatic activity and the known pK values of titratable groups in the protein.
The pH may also influence vleaching rate of enzymatic reaction indirectly insofar as many enzymes, like proteins in general, are stable only within a relatively limited pH range. The use of enzymes to reduce chlorine requirements in pulp vleaching has been known and involves the treatment of brownstock with a class of enzymes, known as hemicellulases, that hydrolyze the hemicellulose portion of wood pulp.
Hemicellulose in wood pulp consists of two types of structures with polysaccharide backbones: Glucomannan is found primarily in softwood. The enzymes that have shown benefit in bleaching include xylanase, arabinase and mannanase Paice, et al.
International Pulp Bleaching Conference, Jun.
Each of these enzymes catalyze a specific and known chemical reaction, hydrolysis. It is therefore generally believed that enzymes enhance the extractability of lignin by partially hydrolyzing the hemicellulose portion of unbleached pulp. This, in turn, leads to a significantly reduced chlorine requirement to bleach pulp. In this regard, studies have reported linkages between hemicellulose, particularly xylan, and lignin in wood Eriksson, et al. The two types of linkages that have been shown are ester linkages between ligninand the methylglucuronic acid residues of xylan Das, et al.
It has been hypothesized that by hydrolyzing hemicellulose, these enzymes act to “release” lignin from chemical linkages to the fiber being bleached.
A number of microorganisms are known to make hemicellulase enzymes. Xylanolytic enzymes xylan attacking enzymes including xylanase and arabinase are produced by microorganisms including Trichoderma reesei, Aspergillus awamori, Streptomyces olivochromogenes, and Fusarium oxysporum Poutanen, et al.
Mannanase enzymes are made by Trichoderma and Aspergillus sp. This invention is particularly concerned with the use of so-called “acid” hemicellulase enzymes, i. In these studies, unbleached pulp was treated with enzymes before the addition of the bleaching chemicals. Enhanced bleaching by enzymes is nemicellulases by the increased brightness of enzyme-treated pulp after bleaching relative to pulp bleached without enzyme treatment.
Brightness is measured by a standard brightness meter and expressed on the ISO scale. VTT reported that treatment of pulp with hemicellulases from Aspergillus awamori and Streptomyces olivochromogenes increased the brightness of the pulp after bleaching by up to 5 ISO points Viikari, et al.
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Both of these hemicellulases were classified as xylanases, because xylanase was putatively the active enzyme that enhanced bleaching.
All of these studies carried out the enzyme treatment of pulp at pH 5, which is recognized as the optimum for the activity of these enzymes. The optimum pH for the xylanase enzymes is determined by isolating the substrate for the enzyme, in this case xylan, and measuring the ability of the enzyme to hydrolyze it in a dilute buffer solution.
The term “pH optimum” is used to mean the pH at which a hemicellulase enzyme has optimum activity for the hydrolysis of its natural hemicellulase substrate in a dilute buffer solution.
For example, the optimum pH for T.
The procedure of Ebringerova, et al. The isolated xylan is bleachnig of similar structure to the indigenous xylan in wood pulp. All of the enzyme treatments by VTT and Paice, et al. Because Kraft brownstock vleaching has a pH in excess of 9, Novo suggests that the pH of the pulp be adjusted to 5 to 6 for xylanase treatment.
This cellulase enzyme can have very undesirable effects on pulp qualities such heimcellulases pulp strength. By selecting process conditions such as pH 6. Operating at an elevated pH, however, is done at the expense of a significant reduction in the brightness boosting of the xylanase.
Novo teaches that this compromise pH 6. In the present invention, a high level of brightness boosting activity is achieved at pH levels previously taught by Novo to inactivate the enzymes. Moreover, in one preferred embodiment, this invention comprises the use of enzyme preparations with low contaminating cellulase levels, i. Accordingly, the Novo teachings of ways to deal with contaminating cellulase are therefore irrelevant to this embodiment. The pH optima for enhancing pull with xylanase of around 5.
The prior teachings for using enzyme preparations hemicellulxses are substantially free of contaminating cellulase activity in bleaching are absolutely clear on one significant point. They teach that the operating pH should be in the range of 5 to 6 and preferably as close as possible to that of the enzyme’s pH optima for hydrolysis.
Operating pulp mills must make compromises between the costs and benefits of washing. As a result, one would typically expect to find significant levels of residual Kraft black liquor in the pulp being sent to the pulp bleachery of an operating mill.
The degree of washing is usually assessed by measuring the residual soda in the pulp. While the well washed samples of brownstock lulp in our laboratory testing had residual soda levels below pul Kg per ton, one often finds residual soda levels ten times this high in operating mills.
Not surprisingly, residual black liquor is deleterious to the action of xylanase enzymes. The inventors have found, for example, that the conventional treatment conditions used to obtain a peak brightness hsmicellulases of 7. The present invention relates to processes for treating paper pulp and particularly relates to an improved method for treating paper pulp with hemicellulase enzymes to enhance the bleaching of Kraft pulp.
The invention comprises means for treating Kraft brownstock with hemicellulase enzymes and then bleaching the brownstock hemicelkulases a conventional bleaching sequence.
It is a primary objective of this invention to provide a means to overcome the deleterious effect of Kraft black liquor on enzyme processes for pulp bleaching. The present inventors have discovered that incompletely or partially washed brownstock can be efficiently delignified with hemicellulase enzymes having a pH optima for activity below 6.
Applicants have further discovered that enzyme preparations with a pH optima for hydrolysis of below 6. The present inventors have discovered heicellulases, contrary to all expectations, in the Kraft brownstock that has not been fully washed, and contains residual dilute black liquor i.