This application discloses a process for delignifying and bleaching pulp that does not use chlorine or chlorine derivatives.

BACKGROUND OF THE INVENTION

Wood is a lignocellulosic material that is predominantly cellulose, hemicellulose, and lignin. Cellulose is a saccharide polymer composed of linear D-glucose units. Hemicelluloses are linear and branched saccharide homo- and heteropolymers of five and six carbon sugars such as xylose, arabinose, mannose, galactose, and glucose. Lignin is a polymer composed primarily of methoxylated phenylpropane units that have been randomly linked by a variety of carbon-carbon and ether linkages, resulting in a three-dimensional matrix.

In wood, the linear cellulose molecules are arranged in densely packed fibril bundles, crosslinked by hemicellulose. Lignin matrices encase the cellulose fibrils, imparting strength and stability to the wood.

As discussed in EP 0 406 617 A2, paper is a two- dimensional meshwork of randomly arranged cellulosic fibers linked by hydrogen bonds between the polysaccharide units. To make paper from wood, the cellulosic fibers must be separated from the lignin and deposited into a hydrogen-bonded tangled mat . The strength of paper and paper products prepared from pulp depends on the strength of the intertwined cellulosic network, and therefore, it is important that the process that isolates the fibers from the lignin and that treats the fibers thereafter not unduly degrade, or shorten, the cellulosic polymer. Fibers are typically isolated from lignocellulosic material mechanically or

chemically, or through a combination of these processes. Pulps are prepared chemically by treating lignocellulosic material with chemical oxάdants, such as sulfate and sulfite, that degrade the lignin. After the lignin is oxidized, it is separated from the cellulose. A small fraction of the oxidized lignin, however, typically remains in the pulp covalently bound to hemicellulose. The oxidized lignin has a dark color that discolors the resulting pulp and products made from the pulp.

When the residual lignin is bleached and allowed to remain in the pulp, brightness reversion (i.e. yellowing) can occur as the bleached lignin oxidizes over time in contact with air. When the residual lignin is completely removed from the pulp, brightness reversion can be minimized.

The standard unit of measure for the lignin content of pulp or paper products is the kappa number. The kappa number is determined by a permanganate oxidation test according to TAPPI

(Technical Association of the Pulp and Paper Industry) standards (TAPPI Method T-236 CM-85) . In general, hardwood pulps have a lower kappa number than softwood pulps, and therefore, are easier to bleach.

The standard unit of pulp brightness is the International Organization for Standardization (ISO) , which is evaluated by TAPPI Method T-525 OM- 86. Pulps of a brightness of 83-92% are considered fully bleached and are sometimes necessary for the production of good quality paper.

Extensive research and development efforts have been undertaken to determine the optimal conditions for the bleaching and/or removal of the residual oxidized lignin from pulp. These methods are generally referred to as pulp bleaching processes. Most bleaching processes are multistep procedures

in which the pulp is treated with a number of agents in very specific sequences.

The conventional method for bleaching pulp includes treatment of the pulp with chlorine or chlorine-containing compounds such as calcium hypochlorite, sodium hypochlorite, and chlorine dioxide. These processes work by removing, as opposed to bleaching, the lignin. The chlorine compounds, however, also degrade a portion of the cellulose, reducing the chain length of the polymer, and the viscosity of the pulp. Further, and importantly, the effluent from these processes constitutes a potential environmental hazard since it contains a large number of chlorinated lignin breakdown products.

Of the total world production of 23.5 million tons of bleached Kraft pulp in 1990, only approximately 0.1 million tons were bleached without chlorine containing compounds. During the last decade, public concern for the environment has greatly increased. This increased environmental concern has become a significant driving force for stricter regulation of potentially hazardous compounds in effluents from pulp bleaching. In the same time span, an unprecedented modernization of pulping and bleaching processes has taken place. Installations of extended Kraft cooking, oxygen based delignification and substantial replacement of elemental chlorine with chlorine dioxide have significantly reduced the formation of dioxin and adsorbable organic halogen (AOX) in the process . In spite of the enormous efforts to upgrade and modernize their operations, the pulp and paper industry still finds itself the focus of a debate over possible negative environmental impact from pulp bleaching processes.

The use of enzymes in the bleaching process has been explored as a means to modify or degrade the lignin without attacking the cellulose. U.S. Patent No. 4,687,745 discloses a process for enhancing the strength and brightness of mechanical pulps that includes treating the pulp with a lignolytic enzyme mixture from a Phanerochaete chrysosporium fermentation process. EPO 0 406 617 A2 discloses a method to delignify lignocellulosic material that includes treating the material with a ligninase enzyme, or ligninase in combination with xylanase. U.S. Patent No. 4,923,565 discloses a method for treating a paper pulp with cellulase and hemicellulase enzymes. PCT WO 91/02840 discloses treating the pulp with a hemicellulose-hydrolyzing enzyme.

Methods to bleach pulp have also been advanced that include oxidizing the pulp with oxidants such as ozone, oxygen, and hydrogen peroxide instead of chlorine or chlorine dioxide in very specific sequences in the absence of an enzyme step in the process.

Canadian Patent No. 1 132 760 discloses a process for bleaching chemical pulp that includes, in specific sequence, a peroxide bleaching stage, wherein the peroxide is selected from the group consisting of alkaline hydrogen peroxide, acid hydrogen peroxide, and peracid; and at least one ozone bleaching stage. .An oxygen stage is optionally included as the first step.

Conventional caustic extraction steps can follow the ozone treatment stages.

PCT Publication No. WO 92/12289 discloses a chlorine-free process for bleaching lignocellulosic pulp that includes an ozone step followed by a peroxide step.

U.S. Patent No. 5,164,043 discloses a process for bleaching lignocellulosic material that includes, in specific sequence, an oxygen step, an ozone step, and then a chlorine-dioxide or peroxide step.

Liebergott, et al. , "Bleaching a softwood kraft pulp without chlorine compounds, " TAPPI Journal, August 1984, compares the economics and efficiency of the conventional chlorine process for bleaching pulp (C/DEDED) with two non-chlorine containing processes that utilize sodium hydrosulfite as an oxidant .

Liebergott, et al. , in "Lowering AOX Levels in the Bleach Plant," Proceedings of the 1992 Environmental Conference, 1065-1075, discusses the use of oxygen, ozone, and peroxide, with or without chelation and extraction steps, as alternatives to chlorine bleaching of pulp.

U.S. Patent No. 5,179,021 to du Manior, et al. , discloses that chemical pulp can be bleached to an ISO brightness level of approximately 50-60% by treating the pulp with oxygen or an oxygen- containing gas in alkaline medium, followed by treatment of a sufficient amount of a substantially cellulase-free xylanase to hydrolyze xylosidic linkages in the material. To further increase the brightness level of the pulp, the patent teaches that one or more additional steps be employed selected from the group consisting of treatment in an aqueous medium with: (i) chlorine, chlorine dioxide, or mixtures thereof; (ii) chlorine dioxide alone; (iii) peroxide; (iv) hypochlorite; (v) ozone; or (vi) nitrogen dioxide. In the preferred embodiments, the oxygen and xylanase steps are followed by steps (i) and (ii) , or steps (i) and (iii) . Importantly, du Manior does not disclose any specific sequences of process steps that

provides a high pulp brightness level and which do not include chlorine or chlorine dioxide.

While extensive efforts and advances have been made in processes to bleach pulp without the use of chlorine or chlorine derivatives, no one has to date disclosed a specific, optimized, sequence of process steps to bleach pulp that include the use of an enzyme treatment in combination with oxygen or oxygen derivative bleaching steps. Therefore, it is an object of this invention to provide a specific, optimized, sequence of process steps to bleach hardwood and softwood pulp that include the use of an enzyme treatment in combination with oxygen or oxygen derivative bleaching steps.

It is also an object of the invention to provide a method for bleaching pulp that has a minimal effect on the environment and that is economically competitive with the present bleaching processes.

SUMMARY OF THE INVENTION

A method for the bleaching of hardwood and softwood pulp without chlorine or chlorine derivatives is disclosed that has a short sequence of steps and that provides a pulp with high brightness, good physical strength, and low degree of brightness reversion. The specific sequence in which the treatment steps are carried out is important. It has been discovered that, for example, xylanase has little effect when used after an ozone step. Further, if ozone is used as the last step of the bleaching process, a significant amount of brightness reversion can occur, whereas if peroxide is used as the last step, brightness reversion is minimized.

Specifically, it has been discovered that hardwood pulp can be bleached without chlorine or chlorine derivatives to a high brightness level using a specific, optimized, sequence of process steps that includes the use of an enzyme treatment in combination with oxygen or oxygen derivative bleaching steps. The specific steps are, in sequence: (i) treatment with oxygen; (ii) treatment with xylanase; and (iii) treatment with ozone; followed by (iv) treatment with peroxide. This specific sequence of bleaching steps is referred to below as the OXZP process. It has been discovered that this specific combination of steps can provide a brightness level of up to 90% ISO or greater, without undue detrimental effect on the physical properties of the pulp. The combination is superior to other tested sequences of enzyme steps and oxygen or oxygen derivative treatment steps, as discussed in detail below. Optionally, the pulp can be subjected to alkaline extraction or enhanced alkaline extraction steps between the treatment steps to assist the removal of the lignin, as necessary.

It has also been discovered that softwood pulp can be bleached without chlorine or chlorine derivatives to a high brightness level also using a specific, yet different, optimized, sequence of process steps that includes the use of an enzyme treatment in combination with oxygen or oxygen derivative bleaching steps. The specific steps are, in sequence: (i) treatment with oxygen; (ii) treatment with xylanase; (iii) alkaline extraction; and (iv) ozone treatment; followed by (v) treatment with peroxide. This optimized sequence for the bleaching of softwood pulp is referred to below as OXE _p ZP. In an alternative embodiment, an additional ozone step is included

before the alkaline extraction step (OXZιE _p Z ₂ P) . This alternative sequence can be used to bleach those pulps with significantly higher initial lignin content, within one ozone step may be inadequate alone to achieve sufficient delignification without impairing pulp physical strength. In another embodiment, softwood RDH (rapid displacement of heat) pulp is bleached with the sequence XE _p ZP. Optionally, softwood pulp can be also subjected to alkaline extraction or enhanced alkaline extraction steps between the treatment steps to assist the removal of the lignin, as necessary.

Eucalyptus, pine, and pine RDH kraft pulps, delignified with the specified combination of xylanase and oxygen or oxygen derivative steps set out above, were found to have consistently lower kappa numbers and higher brightness at the same level of ozone charge as compared to control pulps without the enzyme step. When the eucalyptus pulp was bleached in the sequence OXZP using 0.8% ozone, a brightness of 90.0% ISO was readily obtained compared to a brightness of 84.7% for the control, OZP pulp. When pine kraft and pine RDH pulps were bleached in the sequences OXZ.E _p Z ₂ P and XE _p ZP respectively, brightness values of 81.0 and 85.7% were obtained compared to the brightness of the control pulps (without xylanase treatment) of 71.3 and 76.3% respectively. The total ozone consumption for pine kraft pulp was 1.38%, while for RDH pulp it was 0.96% based on o.d. pulp.

As a reference, pulps were also bleached to the same brightness levels using the conventional sequence DEDED. The pulps bleached with the specific enzyme and oxygen-based chemical steps have good brightness stability, similar tensile

index and slightly lower tear index than the reference DEDED pulps.

In alternative embodiments, the oxygen step is removed from the OXZP, OXE _P ZP and OXZιE _p Z ₂ P processes for bleaching hardwood and softwood pulps that have very low initial lignin contents such as pulps from the RDH cooking process.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a graph illustrating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and kappa number of the oxygen- bleached eucalyptus Kraft pulp with and without xylanase treatment. The open triangles indicate the brightness level (% ISO) of the oxygen- bleached, ozone treated eucalyptus Kraft pulp. The solid triangles indicate the brightness level (% ISO) of an oxygen-bleached, xylanase and ozone treated eucalyptus Kraft pulp. The open circles indicate the kappa number of an oxygen-bleached, ozone treated eucalyptus Kraft pulp. The solid circles indicate the kappa number of an oxygen- bleached, xylanase and ozone treated eucalyptus Kraft pulp.

Figure 2 is a graph illustrating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and viscosity (mPa.s) of an oxygen-bleached eucalyptus Kraft pulp with or without xylanase treatment. The open triangles indicate the brightness level (% ISO) of the oxygen-bleached, ozone treated eucalyptus Kraft pulp. The solid triangles indicate the brightness level (% ISO) of the oxygen-bleached, xylanase and ozone treated eucalyptus Kraft pulp. The open circles indicate the kappa number of an oxygen- bleached, ozone treated eucalyptus Kraft pulp. The

solid circles indicate the kappa number of an oxygen-bleached, xylanase and ozone treated eucalyptus Kraft pulp.

Figure 3 is a graph indicating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and kappa number of an oxygen- bleached pine Kraft pulp with or without xylanase treatment. The open triangle indicates the brightness of the oxygen-bleached and ozone treated pine Kraft pulp. The solid triangle indicates the brightness of the oxygen-bleached and ozone treated pine Kraft pulp that has also been treated with xylanase. The open circle indicates the viscosity of the oxygen-bleached and ozone treated pine Kraft pulp. The solid circle indicates the viscosity of the oxygen-bleached and ozone treated pine Kraft pulp that has also been treated with xylanase.

Figure 4 is a graph indicating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and viscosity of an oxygen- bleached pine Kraft pulp with or without xylanase treatment. The open triangle indicates the brightness (% ISO) of the oxygen-bleached pine Kraft pulp treated without xylanase. The solid triangle indicates the brightness (% ISO) of an oxygen-bleached pine Kraft pulp treated with xylanase. The open circle indicates the viscosity (mPa.s) of an oxygen-bleached pine Kraft pulp treated without xylanase. The solid circle refers to the viscosity of an oxygen-bleached pine Kraft pulp treated with xylanase.

Figure 5 is a graph showing the effect of ozone consumption on brightness and kappa number of a RDH pine Kraft pulp pre-treated with XE _p or E _p stages. The open triangle indicates the brightness (% ISO) of the RDH pine Kraft pulp treated with enhanced alkaline extraction, ozone, and peroxide. The

solid triangle indicates the brightness (% ISO) of ^* the RDH pine Kraft pulp treated with xylanase, enhanced alkaline extraction and ozone. The open circle indicates the kappa number of a RDH pine Kraft pulp treated with enhanced alkaline extraction and ozone. The solid circle refers to the kappa number of a RDH pine Kraft pulp treated with xylanase, enhanced alkaline extraction and ozone. Figure 6 is a graph showing the effect of ozone consumption on brightness and viscosity of the RDH pine Kraft pulp pre-treated with XE _p or E _p stages. The open triangle indicate the brightness (% ISO) of the RDH pine Kraft pulp treated with enhanced alkaline extraction and ozone. The solid triangle indicates the brightness (% ISO) of a RDH pine Kraft pulp treated with xylanase, enhanced alkaline extraction and ozone. The open circle indicates the kappa number of the RDH pine Kraft pulp treated with enhanced alkaline extraction and ozone. The solid circle indicates the kappa number of an RDH pine Kraft pulp treated with xylanase, enhanced alkaline extraction and ozone.

Figure 7 is a graph indicating the tear and tensile relationship of eucalyptus Kraft pulps bleached in the sequences of OXZP and ODEDED. The open circle refers to eucalyptus Kraft pulp treated in sequence with oxygen, xylanase, ozone, and peroxide (OXZP) . The solid circle refers to eucalyptus Kraft pulp treated in sequence with oxygen, chlorine dioxide, alkaline extraction, chlorine dioxide, alkaline extraction, and chlorine dioxide (ODEDED) .

DETAILED DESCRIPTION OF THE INVENTION ^*

The term hardwood, as used herein, refers to angiosper s. Examples of hardwoods include eucalyptus, birch, oak, beech, aspen, gum, maple, poplar and sycamore.

The term softwood, as used herein, refers to gymnosperms. Examples of softwoods include pine, spruce, hemlock, Douglas-fir, larch and redwood. The Beloit Rapid Displacement of Heat (RDH) process used to produce the pine RDH pulp is reported in TAPPI, Vol. 66 (3) , 120 (1983) .

The following abbreviations are used herein: oxygen (0) , ozone (Z) , alkaline or sodium hydroxide extraction (E) , enhanced alkaline extraction (sodium hydroxide extraction combined with peroxide or oxygen) (E _p or E ₀ ) , chelation of metal ions (Q) , hydrogen peroxide (P) , hypochlorites (H) , sodium hydrosulfite (Y) , chlorine (C) , mixtures of chlorine and chlorine dioxide (C10 ₂ ) (C/D) , chlorine dioxide (C10 ₂ ) (D) , and xylanase (X) . As used herein, the term "ISO" refers to International Organization for Standardization which is a basic brightness unit.

The term DTPA refers to diethylene triamine pentaacetic acid.

The invention as described herein is an optimized process for the delignification and bleaching of hardwood and softwood chemical pulps that uses no elemental chlorine or other chlorine containing chemicals, and results in pulp with good strength and high brightness levels. The specific steps for the bleaching of hardwood pulp are, in sequence: (i) treatment with oxygen; (ii) treatment with xylanase; and (iii) treatment with ozone; followed by (iv) treatment with peroxide. This specific sequence of bleaching steps is referred to

below as the OXZP process. The specific steps for ^' the treatment of softwood pulp are, in sequence: (i) treatment with oxygen; (ii) treatment with xylanase; (iii) alkaline extraction; and (iv) ozone treatment; followed by (v) treatment with peroxide. This optimized sequence for the bleaching of softwood pulp is referred to below as OXEZP. In an alternative embodiment, an additional ozone step is included before the alkaline extraction step (OXZ^^P) . Softwood RDH (rapid displacement of heat) pulp is bleached with the sequence XE _p ZP. Optionally, both the hardwood and softwood pulps can be subjected to alkaline extraction or enhanced alkaline extraction steps between the treatment steps as necessary to assist the removal of the lignin.

Chelation of metal ions in the pulp solution prior to the peroxide bleaching step results in pulp with higher brightness levels than when the peroxide stage is performed without prior chelation of the metal ions. Performing the ozone step before the peroxide step also generally results in significantly higher brightness levels and less brightness reversion than pulp bleached with these steps reversed.

I. Process Step Conditions

The process step conditions used to bleach hardwood and softwood pulp according to the process described herein are set out below. These conditions are merely exemplary, and are not intended to limit the invention. The conditions can be varied as necessary to obtain the desired product. All percentages are by weight unless otherwise specified.

Oxygen Bleaching Treatment (O)

The oxygen bleaching of pulp is typically carried out in.an alkaline aqueous media with a pH above 7, and preferably above 11, a pulp consistency of approximately 10-15%, a partial pressure of oxygen of between approximately 80 and 120 psi, and a temperature between approximately 80 and 110 °C. The oxygen treatment step generally takes between approximately 30 and 90 minutes. The oxygen containing gas can be air, or can be purified oxygen.

Xylanase Treatment (X)

The xylanase treatment is typically carried out at a temperature ranging between 30 and 70'C, preferably 50°C, a pulp consistency of between approximately 5 and 20%, preferably 10%, with a xylanase concentration of 0.1-5 U/g o.d. (oven dried) pulp, preferably 2.0 U/g o.d. pulp, for approximately 30 minutes to 4 hours, preferably 90 minutes. The pH of the xylanase treatment typically ranges between 3-7, and preferably 5.

In a preferred embodiment, xylanase is derived from the fungus Aureobasidiu pullulans . Xylanase from this source is essentially free of cellulase activity, which weakens the strength of the wood pulp. Other xylanases which do not show appreciable cellulase activity can also be used in this process, such as xylanases from thermophilic and alkalophidic bacteria.

Ozone Treatment (Z)

The ozone bleaching step is typically carried out at a temperature ranging between 15 and 60 °C, preferably 25 ^° C, a pulp consistency of 30-45%, with an ozone concentration of 0.1-5.0%, typically 0.1- 0.5% for approximately 3 to 15 minutes. The pH of

pulps during ozone treatment is generally between 2 and 4, and more typically, around 3.

Enhanced Alkaline Extraction (E _p or E ₀ )

The enhanced alkaline extraction is typically carried out at a temperature ranging between 30 and 90°C, and preferably approximately 70 ^β C, at a basic pH, preferably greater than 11; and at a pulp consistency of 5-20%, preferably 10-15%, for a time period ranging between 30 and 120 minutes, and generally approximately 90 minutes. The charge of sodium hydroxide is usually between 0.5 and 2.0%, and contains hydrogen peroxide ranging between 0.2 and 1.0, preferably 0.5%, or oxygen gas with a partial pressure 50-100 psi.

Chelation (0)

The optional chelation step is usually carried out at a temperature ranging between 40 and 100°C, preferably approximately 80'C, a pulp consistency of between 5 and 20%, preferably 10%, with a chelating agent concentration of 0.1-0.5%, preferably 0.2%, and with the addition of 1-5% sodium bisulfite, preferably 3% as S0 ₂ on o.d. pulp.

This step is typically carried out for between approximately 5 and 90 minutes, and generally, for approximately 15 minutes. The chelation stage can be performed before the enhanced alkaline extraction as well as before the peroxide stage.

Chelating agents that can be used in the process include, but are not limited to DTPA, EDTA (ethylenediamine tetra acetate) and NTA (nitrilotri acetic) .

Peroxide Treatment (P)

The peroxide step is typically carried out at a temperature ranging between 50 and 90'C, preferably

i 70°C, a pulp consistency of between 5 and 20%, typically 12%, with a peroxide concentration of between 1.0 and 3.0%, and a sodium hydroxide concentration of between 1.0 and 2.0%, for between approximately 1 and 5 hours, preferably 3 hours.

The pH of the solution used for the peroxide treatment step is basic, and typically greater than

11. After the reaction has been completed, the pulp is filtered and washed with water. Residual hydrogen peroxide is checked by iodometric titration of the filtrates. Examples of typical bleaching conditions for the different stages are presented in Table 2.

It is preferred to use hydrogen peroxide as the source of peroxide in this step. Other peroxides that can be used in place of hydrogen peroxide that perform equivalently.

II. Examples of Treatment of Pulp with

Seguenced Enzyme and Oxygen-based Chemical Steps

To illustrate the process described herein, three different types of kraft pulps have been bleached with enzyme and oxygen-based chemicals. Eucalyptus, pine, and pine RDH kraft pulps, delignified with a combination of xylanase and ozone stages, were found to have consistently lower kappa numbers and higher brightness at the same level of ozone charge compared to the control pulps without the enzyme stage. When the eucalyptus pulp was bleached in the sequence OXZP using 0.8% ozone, a brightness of 90.0% (ISO) was readily obtained compared to a brightness of 84.7% for the control, OZP pulp. When pine kraft and pine RDH pulps were bleached in the sequences OXZιE _p Z ₂ P and XE _p ZP respectively, brightness values of 81.0 and 85.7% were obtained compared to the brightness of the

control pulps 71.3 and 76.3% respectively. The total ozone consumption for pine kraft pulp was 1.38%, while for RDH pulp it was 0.96% based on o.d. pulp. As a reference, pulps were also bleached using the conventional sequence DEDED to the same brightness levels. The pulps bleached with enzymes and oxygen-based chemicals have good brightness stability, similar tensile index and slightly lower tear index compared to the reference DEDED pulps.

MATERIALS AND METHODS

Pulp Samples

Commercially produced unbleached and oxygen- bleached kraft pulps, as well as a RDH (Rapid Displacement of Heat) pulp were obtained from different pulp mills. The pulps were further washed with water before use. The kappa number, viscosity, and brightness of these pulps after washing are presented in Table 1.

TABLE 1: Characteristics of the mill-produced kraft pulps

Kappa Viscosity Brightness Pulp number (mPa.s) % (ISO)

Eucalyptus kraft pulp

Unbleached 12.1 29.5 34.0 Oxygen-bleached 8.4 23.1 53.0

Pine kraft pulp

Unbleached 28.1 34.0 20.1

Oxygen-bleached 19.3 24.1 27.0

Pine RDH pulp Unbleached 10.6 8.6 31.4

Enzyme Production and Preparation

The xylanase was produced in batch fermentation by the fungus, Aureobasidium pullulans . The cultivation and subsequent preparation of the culture filtrate has been described previously. Yang, J. and Eriksson, K-E., Hotzforschung, 46(6) :481 (1992) . Evaluation of the enzyme against birchwood xylan, mannan, carboxylmethylcellulose and arabinogalactan indicated that the hydrolytic activity of this culture filtrate consists solely of endo-xylanase activity (480 units/ml) . One unit of xylanase activity was defined as the amount of enzyme releasing one μmol of xylose equivalent per minute at ambient defined (30°C) temperature. Pulp Bleaching Procedure

Pulps were disintegrated by 10,000 revolutions in a disintegrator (Noram, PQ, Canada) at room temperature before bleaching. The pulp suspensions were filtered on a Buchner funnel and then placed in double-layer polyester bags. The xylanase treatment (X) was carried out at a pulp consistency of 10% in 50 mM sodium acetate buffer with a pH 5.0 and a xylanase charge of 2 units/g pulp at 50'C for 90 minutes. After the enzyme treatment, the pulps were washed with water. Treatment of pulp in the control sequence was carried out under identical conditions as the treated pulp but without enzyme.

Ozone delignification was carried out batchwise at high consistency conditions. Pulp samples were first acidified with sulfuric acid to a pH range of 2-3 and then dewatered with a handpresser to a pulp consistency of approximately 40%. The pulps were subsequently fluffed in a hammer mill prior to the ozone treatment. The ozone generator used in this study had a production capacity of 80-200 g ozone/hr, yielding ozone concentrations of 1-5% (w/w) . A typical batch ozone treatment was carried

out with 50 g o.d. (oven dry) pulp, and an ozone charge of between 0.1 to 3.0% (w/w) for 3-15 minutes at ambient temperature and pressure. Ozone concentration was determined by iodometric titration.

Before entering the peroxide stage (Ep or P) , pulps were pretreated with 0.2% diethylene triamine pentaacetic acid (DTPA) and 3.0% NaHS0 ₃ calculated as S0 ₂ on o.d. pulp at a consistency of 10%, pH 4.5 for 15 minutes at 80°C (Q) .

For the hydrogen peroxide (E _p , P) stages, pulps and water were heated to the desired temperature and mixed with the bleaching chemicals to a final consistency of 10%. This procedure was also used when chlorine dioxide (D) and alkaline extraction (E) stages were applied. After the reaction was completed, pulps were filtered and washed with distilled water. Residual hydrogen peroxide and chlorine dioxide were checked by iodometric titration of the filtrates. Bleaching conditions for the different stages are presented in Table 2.

20

TABLE 2: The conditions used to bleach kraft pulps

Symbol Chemical Reaction Treatment pH Consistency charge (%) time (min! ) temp.f C) I (%)

Eucalvotus kraft oulo

X 2 U xylanase/g pulp 90 50 5.0 10

Z Oz: 0.1-3.0 3-15 ambient 2-3 35-45

P H2O2: 1.0 180 70 > 1 1 10

NaOH: 1.0

Na2SiO3: 2.0

Pine kraft and RDH pulps

X 2 U xylanase/g pulp 90 50 5.0 10

Ep H2O2: 0.5 180 70 > 1 1 10

NaOH: 2.0

Z 03: 0.1-3.0 3-10 ambient 2-3 34-45

P H2O2 180 70 > 1 1 10

NaOH: 2.0

Na2SiO3: 3.0

.Analysis of Pulp Chemical and Physical Properties

The chemical and physical properties of the pulps were analyzed according to the following Tappi test methods: T205 om-88 (forming handsheets for physical tests of pulp) , T227 om-85 (freeness of pulp) , T230 om-89 (viscosity of pulp) , T236 cm- 85 (kappa number of pulp) , T238 cm-85 (laboratory beating of pulp) , and T525 om-86 (brightness of pulp) . The physical properties of the bleached kraft pulps were evaluated at the Institute of Paper Science and Technology (IPST) , Atlanta, Georgia.

RESULTS Bleaching of Eucalyptus Kraft Pulp

The mill produced oxygen-bleached eucalyptus pulp was treated with Aureobasidium xylanase. This treatment decreased the kappa number from 8.4 to 7.5, increased the viscosity from 23.1 to 25.1 mPa.s and brightness from 53.2 to 55.4% (ISO) . Xylanase treatment of unbleached eucalyptus pulp decreased the kappa number from 12.1 to 10.9, increased the viscosity from 29.5 to 31.0 and brightness from 34.0 to 36.6% (ISO) .

The effect of ozone consumption on oxygen bleached and xylanase treated pulp were extensively investigated. The results are summarized in Figures 1 and 2. Figure 1 is a graph illustrating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and kappa number of an oxygen-bleached eucalyptus Kraft pulp with and without xylanase treatment. Figure 2 is a graph illustrating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and viscosity (mPa.s) of an oxygen- bleached eucalyptus Kraft pulp treated with or without xylanase. As shown in Figure 1, the kappa number decreased readily from 8.4 to below 2 with gradually increased ozone charges up to approximately 1%. Figure 2 indicates, however, that the viscosity of the pulp was simultaneously reduced. Pulp bleached with the OXZ sequence has a consistently lower kappa number and higher brightness than pulp bleached without the enzyme stage, i.e., OZ only, at the same levels of ozone charges. Brightness of the xylanase treated pulp developed much faster and the brightness ceiling was higher compared to the OZ bleached pulp.

Bleaching of Pine Kraft Pulp

The mill produced, oxygen bleached pine kraft pulp was also treated with Aureobasidium xylanase. Xylanase treatment of this pulp decreased the kappa number from 19.3 to 18.0, increased viscosity from 22.2 to 23.4 and brightness from 28.7 to 30.0% (ISO) . Xylanase treatment of unbleached pine kraft pulp decreased the kappa number from 28.6 to 26.8, increased viscosity from 34.0 to 34.2 and brightness from 23.8 to 26.1% (ISO) .

The effect of ozone charge on the oxygen bleached and xylanase treated pulp was studied. The results are provided in Figures 3 and 4. Figure 3 is a graph indicating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and kappa number of an oxygen- bleached pine Kraft pulp with or without xylanase treatment. Figure 4 is a graph indicating the effect of ozone consumption as a function of pulp percentage (w/w) on brightness and viscosity of an oxygen-bleached pine Kraft pulp treated with or without xylanase.

The kappa number of the xylanase treated pulp was reduced from 18 to 6. As indicated in Figure 3, when xylanase was not used in pulp treatment, the kappa number decreased from 19 to 8 with gradually increased ozone charges up to approximately 2.5%. As indicated in Figure 4, however, the viscosity of the pulp declined initially and then stabilized at approximately 10 mPa. s. Pulp bleached with the OXZ sequence has consistently higher brightness than OZ bleached pulp at the same levels of ozone charges. Brightness of the xylanase treated pulp developed much faster and the brightness ceiling was higher than the OZ pulp.

Due to the substantially higher kappa number of the particular oxygen-bleached pine kraft pulp used compared to the eucalyptus oxygen-bleached kraft pulp, two ozone stages were necessary in order to delignify the pulp to an acceptable level before entering the final brightening stage. It can be seen in Table 3 that the pulp bleached according to the OXZ _i E _p Z^ sequence has a brightness of 81.0% ISO while, if the enzyme stage is excluded, a brightness of only 71.3% ISO was reached.

Bleaching of Pine RDH Kraft Pulp

The mill produced unbleached pine RDH pulp was also treated with Aureobasidium xylanase. Enzyme treatment of this pulp decreased the kappa number from 11.3 to 10.6, increased the viscosity from

7.9 to 8.6 mPa.s and brightness from 30.1 to 31.4% ISO.

The effect of ozone charge on xylanase treated pine RDH pulp is illustrated in Figures 5 and 6. Figure 5 is a graph showing the effect of ozone consumption on brightness and Kappa number of the RDH pine Kraft pulp pre-treated with XE _p or E _p stages. Figure 6 is a graph showing the effect of ozone consumption on brightness and viscosity of the RDH pine Kraft pulp pre-treated with XE _p or E _p stages. As indicated in Figure 5, the kappa number of the pulp decreased from 8.2 to approximately 2 for the XE _p treated pulp while a decrease in kappa number from 9 to approximately 3 was obtained for the E _p treated pulp with gradually increased ozone charges up to approximately 1%. The viscosity of the pulp was reduced from 7.5 to 5.3 and from 7.5 to 4.8 for the XE _p Z and E _p Z bleached pulps respectively. The XE _p Z bleached pulp had a considerably higher brightness than the E _p Z pulp at the same level of ozone charge. The brightness of

the XE _p Z bleached pulp was approximately 77 while it was only approximately 67 for the E _p Z bleached pulp.

A comparison of the ozone consumed on w/w pulp percentage, the viscosity, and brightness of hardwood and softwood pulp are summarized in Table 3. Noticeably, the brightness level of the pulp is consistently higher when the enzyme step is used. For the eucalyptus pulp the brightness is 5.5 units higher, for pine kraft pulp, 9.7 units and for pine RDH pulp, 9.4 units higher.

TABLE 3 : Comparison of some properties of three types of kraft pulps bleached in different bleaching sequences

Bleaching Ozone consumed Viscos ity Brightness sequence on pulp (%) mPa .s % ISO

Eucalyptus kraft pulp

OXZP 0.79 9.2 90.2

OZP 0.76 9.3 84.7

Pine kraft pulp

OXZ,E _p Z ₂ P Z _ι: 0.80/Z ₂ :0.58 8.0 81.0

OZ _j E^P Z _ι: 0.84/Z ₂ :0.58 8.0 71.3

Pine RDH pulp

XE _p ZP 0.96 5.4 85.7

E _P ZP 0.95 5.0 76.3

Physical Properties of Bleached Eucalyptus Kraft Pulp

Physical properties of eucalyptus pulp bleached in the sequence OXZP and the reference sequence

ODEDED were measured at the Institute of Paper

Science and Technology (IPST) . Figure 7 is a graph indicating the tear and tensile relationship of eucalyptus Kraft pulps bleached with these sequences. The freeness which development (a

measurement of drainability or water holding capacity of a given pulp) during the beating of the OXZP bleached pulp is very similar to that of the

) reference pulp (ODEDED) , even though the viscosity of the OXZP bleached pulp is only 9.2 mPa.s, much lower than the viscosity of the reference pulp, 24.6 mPa.s. However, tensile index values of both pulps are very similar while the OXZP bleached pulp shows a lower tear index value compared to the reference pulp, Figure 7.

III. Effect of Acid Wash and Chelation Steps

The presence of transition metal ions such as Mn ⁺ and Fe ⁺ in pulp is known in general to have a detrimental effect on the efficiency of an alkaline peroxide bleaching step. In light of this, the effect of the treatment of pulp with acid or a chelating agent on the kappa number, viscosity, and brightness of the pulp was evaluated. The peroxide step was carried out with 1% H ₂ 0 ₂ , 1% NaOH, and 2% Na ₂ Si0 ₃ for 180 minutes on 10% pulp consistency. The acid wash was carried out by acidifying the pulp to pH 2.0 with 10 mM of sulfuric acid, and then washing the pulp with water. The chelation step was carried out with 0.2% DTPA and 3.0% NaHS0 ₃ calculated as S0 ₂ on o.d. pulp at a consistency of 10%, and pH of 4.5 for 15 minutes at 80°C.

The results are summarized in Table 4. As indicated in Table 4, pretreatment of pulps with acid or DTPA improves the efficiency of the peroxide step and results in a higher brightness and lower kappa number of the pulps. It should be noted that the ozone step itself is carried out at a pH of 2-3 and therefore has the effect of an acid wash. It appears, however, that chelating agents

are more effective than acid washing in improving the efficiency of the peroxide step .

TABLE 4 Ef fect of Pretreatment of Pulp by Acid

(A) or Chelating Agent (Q) prior to the Peroxide - Stage on Residual Peroxide, Kappa Number, Viscosity and Brightness .

Sequence H ₂ 0 consumed Kappa Viscosity Brightness on pulp number mPa . s % ISO

OXZP 100 1.4 10.9 81

OXZAP 94.6 1.3 9.8 82

OXZQP 47 1.2 10.9 83

OXP 100 5.4 21.7 65

OXAP 94.6 5.1 20.1 72

OXQP 56.5 4.7 22.0 73

OP 100 5.5 21.4 62

OAP 99.3 5.3 18.9 67

OQP 83.0 5.1 21.8 71

IV. Comparison of Properties of Pulp Treated with Different Sequences of Enzymes and Oxygen-based Chemicals

A comparison of the brightness, viscosity, and ozone consumed on w/w of eucalyptus pine and pine RDH kraft pulps bleached with different sequences of enzymes and oxygen-based chemicals is presented in Table 5. The eucalyptus pulp was bleached to a brightness range of 83 to 92% ISO with sequences of OXZP and OXPZ. As a reference, the eucalyptus pulp

was also bleached using a sequence of ODEDED to a ' brightness of 90.2%. The pulp bleached in the OXZP sequence has the highest brightness compared to the OXPZ and OZP at the same level of ozone charge. The viscosity of the bleached pulps were generally low, ranging from 9.2 to 12.4 mPa.s compared to the viscosity of the reference pulp (ODEDED, 24.6 mPa.s) . The effect of xylanase on the brightness of the pulp was significant. Similar results were also obtained with softwood kraft pulps. When pine and pine RDH pulps were fully bleached, the most noticeable effect is that brightness level is consistently higher when the enzyme stage is used.

TABLE 5: Comparison of Some Properties of Three Types of Kraft Pulps Bleached in Different Bleaching Sequences

Bleaching Ozone Consumed Viscosity Brightness Sequence on pulp (%) mPa.s (% ISO)

Eucalyptus kraft pulp

OXZP 0.79 9.2 90.2

OZP 0.76 9.3 84.7

OXPZ 0.81 12.8 83.0

OPZ 0.82 11.1 83.0

ODEDED -- 24.6 90.2

Pine kraft pulp

OXEpZP 1.30 7.1 88.1

OEpZP 1.31 6.8 85.3

ODEDED -- 19.1 86.2

Pine RDH pulp

XEpZP 0.96 5.4 88.1

EpZP 0.95 5.0 81.8

DEDED — 9.3 86.2

The brightness stability of the fully bleached eucalyptus (pulp #1) , pine (pulp #2) and pine RDH (pulp#3) pulps was also evaluated on heating the treated pulp at 105°C for 4 hours. The results are presented in Table 6. The brightness was very stable when alkaline peroxide was applied as the final stage, as in the sequence OXZP compared to the reference pulps bleached with ODEDED or DEDED.

TABLE 6. Brightness reversion of bleached kraft pulps (kept at 105°C for 4 hours) .

Bleaching Bright;ness % (ISO) sequence Before aging After aging p.c no. *

Eucalyptus kraft pulp

OXZP 90.2 87.8 0.32

ODEDED 90.2 87.0 0.44

Pine kraft pulp

OXEpZP 86.4 85.2 0.22

OEpZP 85.4 83.2 0.45

ODEDED 86.2 82.8 0.68

Pine RDH kraft pulp

XEpZP 88.0 86.6 0.22

EpZP 81.8 80.4 0.36

DEDED 86.2 84.0 0.42

* p.c. no. (post color number) used as a measure of color reversion

This invention has been described with reference to its preferred embodiments. Variations and modifications of the process for delignifying and bleaching pulp will be obvious to those skilled in the art from the foregoing detailed description of the invention. It is intended that all of these variations and modifications be included within the scope of the appended claims .