EP0797703B1 - Peroxide bleaching process for cellulosic and lignocellulosic material - Google Patents

Abstract

A method for bleaching cellulosic and lignocellulosic pulp using hydrogen peroxide as a bleaching reagent, utilizes a three stage reaction, viz. two short high temperature peroxide bleaching steps (50,160) at a pressure sufficient to suppress boiling and an interstage treatment (90) including a longer atmospheric pressure reduced temperature bleaching step. The two short peroxide bleaching stages include the steps of introducing pulp, at a consistency of 10%-18%, to a mixer (20,130) in which the pulp is heated to a temperature above 100 degree Celsius; adding sufficient sodium hydroxide (32,142) to bring the pulp to a pH of 8.5; adding sufficient hydrogen peroxide (33,143) to equal from about 0.5%-5.0%, by weight, of the pulp; passing the pulp through a pressurized reactor column (50,160) at a rate providing a reaction time in the column of less than 45 minutes, cooling the pulp and discharging the cooled pulp to a washer. It may be desirable to introduce the pulp to a mixer and add alkali to reactivate residual hydrogen peroxide by bringing the pulp to a pH of at least 9 after the first bleaching stage, deposit the pulp in a reaction tower and allowing the reaction to proceed until a substantial portion of the residual hydrogen has been consumed. <IMAGE>

Description

BACKGROUND OF THE INVENTION

This invention pertains, generally, to the bleaching of lignocellulosic material in the pulp and paper industry and, in particular, to a process to improve the performance of a pulp bleaching sequence which uses hydrogen peroxide. It is especially valuable when used in conjunction with a bleaching sequence designed for the production of pulps bleached without the use of chlorine compounds.

The pressure to remove chlorinated compounds in the bleaching of pulps and reduce their perceived negative effect on the environment has prompted the surge of so called "chlorine free technologies". When chlorine compounds are eliminated from the bleaching sequence, it is conventional to use hydrogen peroxide as a substitute since hydrogen peroxide is generally believed to be benign to the environment. Therefore, since the use of hydrogen peroxide is increasing, there is a strong incentive to develop more efficient ways to apply it in order to maintain favorable economics of bleaching.

Hydrogen peroxide is an oxidizing chemical, commonly used in the brightening of mechanical, semimechanical, semichemical, and recycled pulps. It is also used in chemical pulp bleaching to aid in delignification.

The Pulp & Paper Research Institute of Canada (PAPRICAN) has developed several novel ways to bleach mechanical pulps. Of these, the high temperature peroxide system disclosed by Lierbergott, et al., is of interest because it pursues the same brightness development as conventional high consistency bleaching systems, but does so at medium consistency (10-14%). This is achieved by increasing the temperature of the pulp to about 85°C and lowering the pH, which is differentiated from conventional high consistency peroxide systems. Because of the faster reaction resulting, the retention time is reduced from hours to minutes (15-30 min.), and no silicate is required to stabilize the peroxide solution. The peroxide charge remains about the same as that of the conventional systems.

A high temperature peroxide system has been proposed in a copending application of Bottan, G. to be operated at 85-95°C. for very short periods of time. No one has yet proposed to superheat pulp and to operate a peroxide stage under pressure at short retention times in excess of 100°C. It is desirable to minimize the pulp retention time in order to minimize the capital investment required, while maintaining brightness similar to that which can be achieved using very long retention times.

Reactivation of Residual Peroxide has been proposed by Dominique Lachenal of the Centre Technique du Papier in Grenoble, France for use in the bleaching of mechanical pulps. This allows the reactivation of the non-consumed (or residual) peroxide (after the first reaction stage tower) by increasing the alkalinity of the pulp suspension. The aim is to eliminate expensive dewatering equipment normally used after the bleaching tower to recover the residual peroxide and to recirculate it to the point of addition of the fresh peroxide (usually at a mixer before the bleaching tower). This proposal becomes important when compared with a conventional two stage peroxide bleaching system which requires the expensive dewatering equipment between stages. The first stage reaction takes place for several hours in a conventional tower at 60°C., prior to reactivation of the residual peroxide. Caustic is then added in quantities proportional to the peroxide residual obtained after several hours of reaction at 60°C.

In chemical pulps, hydrogen peroxide has been primarily used in medium consistency systems, in which the pulp slurry, from a previous stage, is dewatered in a thickener or washer to about 10-14% consistency. The peroxide solution is conventionally added, together with alkali, at the repulper (discharge from the thickener or washer) or before the medium consistency tower in a medium consistency pump or mixer.

When used as an individual stage for bleaching of chemical pulps, conventional medium consistency peroxide bleaching stages do not provide sufficient brightness increases and are said to consume more peroxide, and require extremely long retention time for consumption of the peroxide. For more pronounced delignification or brightness effects peroxide must be applied in several towers.

The use of high consistency peroxide stages operating at more than 25 % consistency has been demonstrated to overcome many of the limitations of conventional medium consistency peroxide bleaching stages. However, its disadvantage is that it requires the installation of dewatering devices capable of achieving high discharge consistency, is more complicated to operate, and consumes more power than medium consistency systems.

On chemical pulps, fortification of oxidative extraction stages using peroxide in addition to oxygen is widely used to reduce the amount of chlorine necessary in the first stage of pulp bleaching. Conventional Eop (oxidative extraction reinforced with peroxide) typically use upflow/downflow towers, require the use of oxygen gas, and typically operate in the range of 65-85°C.

Hydrogen peroxide is also used in the final stages of pulp bleaching to achieve a high brightness stable bleached pulp. Use of hydrogen peroxide in bleaching of pulps has been limited to temperatures typically less than 90°C., as it has been believed that the peroxide would decompose, resulting in very poor utilization of the bleach chemical, reduced pulp strength, and poor economics of bleaching.

More recently, the use of hydrogen peroxide under conditions of high temperature and relatively long retention time has been proposed as a means for developing very high brightness pulps, when used at the end of a conventional or chlorine free bleaching sequence. According to the prior art, the required retention time is 1-3 hours. It has been shown that pressurized oxygen peroxide systems at long retention times can increase the brightness ceiling of the pulp over what can be obtained using conventional atmospheric peroxide bleaching stages. It is desirable to minimize the required retention time under pressure in order to reduce the capital investment required for the installation. For example, according to the prior art, the use of 100+°C. for 1-3 hours has been demonstrated in the laboratory to be very effective in reducing the amount of time required for the peroxide bleaching process. It seems though, that the economics may not be very attractive for this process as the capital required for a 1-3 hour pressurized peroxide stage is quite high.

EP-A-0 285 530 discloses a short peroxide bleaching step under pressure at a temperature greater than 100°C. This document teaches that the short peroxide bleaching process occurs at a pH of at most 9 and additionally teaches that a high efficiency washing step should follow the short pressurized bleaching reaction.

According to basic information on the economics of the prior art of conventional atmospheric peroxide bleaching stage and the prior an of pressurized oxygen peroxide bleaching stage (PO), the investment required for the retention tower for the prior art cited is calculated as follows for a single stage of about 850 madtpd capacity:

a. Pressurized peroxide tower for 2 hours retention $1,000,000

b. Atmospheric, long retention, conventional stage

1.	3 hours retention	$ 300,000
2.	6 hours retention	$ 400,000
3.	9 hours retention	$ 550,000

In order to achieve similar brightness levels with conventional atmospheric peroxide bleaching, it is necessary to have a retention time of in excess of 6 hours compared to 1-3 hours according to the prior art for pressurized oxygen peroxide systems. For example, it would be necessary to size an atmospheric tower for 8-10 hours retention to achieve similar results to a 2 hour pressurized oxygen peroxide system.

With 350 bleach plants in North America alone, if only one peroxide bleaching system is required in each plant, this would represent an investment for the industry of approximately $200-350 million. To meet increasingly stringent environmental regulations, it may be necessary to install more than one peroxide bleaching stage in each mill, which could then more than double the investment. Thus a very important disadvantage of the prior art is the very large investment required for implementing the technology according to the teachings of the prior art.

The foregoing illustrates limitations known to exist in some present cellulosic and lignocellulosic bleaching processes, and it would be advantageous to provide an alternative directed to overcoming one or more of those limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method for using hydrogen peroxide for bleaching cellulosic and lignocellulosic pulp is provided, including the steps of adjusting consistency of the pulp to 8 - 18% and introducing the pulp to a mixer in which sodium hydroxide is added to bring the pulp to a pH of greater than 9.5; adding hydrogen peroxide to preferably equal from 0.5%-5.0%, by oven dried weight, of the pulp; heating the pulp to a temperature greater than 100 degrees Celsius while maintaining sufficient pressure to prevent boiling of pulp liquor; passing the pulp through a reactor column at a rate which provides a reaction time in the column of less than 45 minutes; adding alkali to bring said pulp to a pH of more than 9.0; cooling the pulp to a temperature less than 100 degrees Celsius; depositing said pulp in a reaction tower at atmospheric pressure and allowing the reaction to proceed for 1-5 hours until a substantial portion of residual hydrogen peroxide has been consumed; and discharging the pulp for further processing.

The foregoing and other aspects of the invention will become apparent from the following detailed description, when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 and Fig. 2 show arrangements which are not in accordance with the present invention;

Fig. 3 shows one preferred embodiment of the present invention;

Fig. 4 shows another preferred embodiment of the present invention;

Fig. 5 shows a bleaching stage incorporating still another preferred embodiment of the present invention;

Fig. 6 shows another preferred embodiment of a bleaching sequence using the present invention applied to an existing bleach plant;

Fig. 7 shows another preferred embodiment of a bleaching sequence using the present invention with recovery of residual peroxide using conventional washers;

Fig. 8 shows another preferred embodiment of a bleaching sequence using the present invention with recovery of residual peroxide using presses; and

Fig. 9 shows another preferred embodiment of a bleaching sequence using the present invention with another method of recovery of residual peroxide using presses.

DETAILED DESCRIPTION

Referring to Fig. 1, pulp from a conventional washer or thickener is discharged through pipe 10 into mixer 100, where steam for heating the pulp and alkali for increasing the pH of the pulp is added through pipe 15. The pulp is neated and adjusted to a pH of more than 8.5, preferably 9.5-10.5. The heated and pH adjusted pulp is discharged from mixer 100 through pipe 20 to a conventional medium consistency pump 200 which pumps the pulp through pipe 30 to a mixer 300. Hydrogen peroxide solution is added to the mixer 300 through pipe 35 in a quantity sufficient to assure desired brightness development will be achieved at the end of the reaction. As in conventional peroxide bleaching systems, the addition of magnesium compounds for protection of cellulose viscosity, as well as sequestrants (such as SiO₂) and/or chelants (such as EDTA or DTPA) may also be added with the alkali solution through pipe 15, the peroxide solution through pipe 35, or separately through pipe(s) 16 and/or 36. As the temperature of the pulp must exceed 100°C. according to the present invention, and it is not currently practical to pump pulp at this high temperature, the final temperature rise from, for example, 85-90°C. to in excess of 100°C., is achieved by adding steam between the pump 200 and the mixer 300 through pipe 37.

The pulp, which has been heated to the desired reaction temperature, and adjusted to the desired pH, is pumped through pipe 40 into the upflow tube column 400, which is sized for the retention time desired for the reaction. According to the present invention, the upflow tube is sized to assure a pulp retention time of 1-30 minutes, preferably 5-20 minutes, after which the pulp is discharged through pipe 50 to an appropriate discharge device 500, such as a valve, and thence through pipe 60 to subsequent washing and bleaching stages. As the pulp is bleached at more than 100°C., it may be desirable to reduce this temperature to prevent "flashing" of the pulp, so water or liquor at a cooler temperature, may be used for dilution into the top of the upflow tube through pipe 70. Those skilled in the art will recognize that this invention may be practiced with upflow, downflow, upflow/downflow, or other configurations to achieve the desired retention time for the reaction. Further, those skilled in the art will recognize that there are additional means to heat and cool the pulp prior to and subsequent to the reaction, respectively. The details given in this description are net intended to limit the scope of the invention but are included for clarity of the preferred embodiment.

Fig. 2 schematically describes another arrangement which is identical to that shown in Fig. 1, except for the addition of a downflow tower 600. According to the present invention, the upflow tube is sized to assure a pulp retention time of 1-30 minutes, preferably 5-20 minutes, after which the pulp is discharged through pipe 50 to an appropriate discharge device 500, such as a valve, through pipe 60 to a conventional bleaching tower 600, preferably one which is already installed at the pulp mill and can be reused for this new bleaching stage instead of a chlorine based bleaching sequence. The pulp is retained in this tower for an additional 1-5 hours at the desired reaction temperature to consume a substantial portion of the hydrogen peroxide applied on the pulp. As the pulp is bleached in column 400 at more than 100°C., it may be uesirable to reduce this temperature to prevent "flashing" of the pulp. In the present invention, it is possible to cool the pulp prior to discharge from the upflow tube, such that the liquor with the pulp does not "flash". This embodiment may be desirable in some mills to limit potential emissions of steam and residual chemical to the atmosphere. This may be accomplished by adding cooler water or liquor to dilute the pulp through pipe 70. For example, for pulp treated in the upflow column at 12% consistency at 110°C., the addition of 1.5-2.5 m³/ton pulp of 50-60°C, liquor, will cool the pulp to less than 100°C., and result in a drop in consistency to about 10% consistency, which will not significantly decrease the performance of the second step of the reaction. It may be especially desirable to maintain the pulp at the maximum temperature allowable at atmospheric pressure.

In a preferred embodiment of the present invention illustrated in Fig. 3, the pulp is discharged from pipe 60 into tower 600 and is allowed to flash to atmosphenc pressure. The pulp entering the downflow tower will be at its maximum possible temperature, for example, 98-100°C., which will enhance the consumption of peroxide in the second phase of the reaction. Further, the flashed steam discharges through pipe 90 to heat exchanger 700. The heat exchanger is used to pre-heat the wash water applied on the washer prior to the peroxide stage, thereby reducing the amount of steam necessary to be applied to the stage through pipes 15 and 37. This increases the temperature of the wash water in pipe 101 prior to its application to the conventional washer through pipe 110.

The embodiment shown in Fig. 3 is identical to that of Fig. 2 up to the point of discharge of the pulp from upflow reaction column 400. In this case the pulp retains a substantial amount of residual peroxide upon discharge. The pulp is discharged through pipe 50 to an appropriate mixing valve 500, where additional alkali is added through pipe 55 to increase the pH of the pulp to more than 9, preferably 9.5-10.5 for the second step of the reaction.

The pulp, after flash cooling, is discharged through pipe 60 to a conventional downflow tower 600, for the second phase of the reaction. The conventional downflow tower is sized to consume a substantial portion of the remaining peroxide, and will typically retain the pulp for 1-5 hours. Bleached pulp is discharged from tower 600 through pipe 80 to subsequent process steps.

The valve 500 serves to reduce the pressure from the upflow tube 400 and allow the liquor with the pulp to flash. The alkali is added to the pulp just upstream of the valve, and the turbulence created in the valve serves to mix the alkali with the pulp. It is recognized that the installation of any mixing device at this location of the process is substantially equivalent to the function of the valve.

Fig. 4 describes another preferred embodiment of the present invention which is identical to Fig. 3, except for reactivation of pulp in mixing device 550. In this preferred embodiment, and according to the present invention, the upflow tube is sized to assure a pulp retention time of 1-30 minutes, preferably 5-20 minutes, after which the pulp is discharged through pipe 50 to an appropriate mixing device 550, in which the residual peroxide is reactivated by the addition of alkali through pipe 55, after which the pulp is allowed to flash and steam from such flashing is conducted through pipe 90 to heat exchanger 700 to preheat the water from pipe 101 to higher temperature water which is discharged through pipe 110 and used for washing of the pulp upstream of the bleaching stage.

Fig. 5 is a schematic diagram of a bleaching sequence employing the present invention. Pulp from a washer or thickener 10 is discharged through pipe 11 and is fed to a mixer 20, where steam and/or bleaching chemicals are applied through pipe(s) 12 and 13 to heat the pulp to the desired reaction temperature, and to increase the pH of the pulp to the desired level for first treatment with peroxide. The heated and pH adjusted pulp is discharged through pipe 21 to a pump 30 to transfer the pulp to a mixer 40. As the pulp will be heated to above 100°C., it is conventional to apply steam for final heating ahead of the mixer, typically through a pipe 34. Additional chemicals including alkali, peroxide, chelants, and others may be added through pipes 32 and 33, and homogeneously mixed with the pulp prior to discharge through pipe 41 to the first peroxide reaction vessel 50. The peroxide reaction vessel 50 is sized for 1-30 minutes retention time, preferably 5-20 minutes to achieve substantial delignification and/or brightness increase prior to being discharged through pipe 51 to a discharge device 52. It is preferable for the best operating cost of the sequence, but not necessary, to wash the pulp following the first peroxide treatment, and if the washing device is a non pressurized device, it will be preferable to cool the pulp prior to discharge from the first peroxide reaction vessel. Cooler dilution liquor may be added in the first peroxide reaction vessel 50 through pipe 53. It may be desirable to dilute the pulp prior to the pump with filtrate fed through pipe 54 if the washing device requires low consistency pulp for its proper operation.

The pulp is washed on washer 60, and the washed pulp is discharged through pipe 61 to a mixer 70. The second step in this preferred embodiment is an ozone stage which may be operated at either high or medium consistency, however, it is preferable that this be a medium consistency stage operating at conventional washer discharge consistencies of 10-14% to minimize the capital cost for the installation. As is known in the prior art, it is necessary to operate at a pH of less than 4 to achieve optimum results from the ozone stage. Thus, prior to the ozone stage, an acid such as sulfuric acid, may be added at the discharge of the first peroxide reaction vessel 50 through pipe 53, with the shower water applied onto the first peroxide washer 60 through pipe 56, in the feed to the pump 70 through pipe 62, or in a separate mixer (not shown). The acidified pulp is pumped with pump 70 through pipe 71 to a mixer 80, in which ozone gas is applied through pipe 72. The pulp is discharged to the ozone reaction vessel 90 and is held for up to about 10 minutes to allow nearly complete consumption of the ozone. The pulp is discharged from he ozone reaction vessel through pipe 91 to a discharge device 92, which may consist of a valve, a plurality of valves, or a mechanical device to reduce the pressure from the ozone reaction vessel. The pulp is discharged through pipe 93 to a gas separation device 220, where the gas is separated from the pulp and is discharged through pipe 202 to treatment and/or reuse in the mill. The degassed pulp is discharged through pipe 102 to a pump 110. It is preferable for the best operating cost of the sequence, but not necessary, to wash the pulp following the ozone stage prior to the next stage of bleaching It may be desirable to dilute the pulp prior to tlie pump with filtrate fed through pipe 103 if the washing device requires low consistency pulp for its proper operation. The pulp is fed through pipe 104 to washer 120, and is washed using water or filtrate applied through pipe 115.

Pulp from the washer 120 is discharged through pipe 121 and is fed to a mixer 130, where steam and/or bleaching chemicals are applied through pipe(s) 122, 123, and 124 to heat the pulp to the desired reaction temperature, and to increase the pH of the pulp to the desired level for the treatment with peroxide. The heated and pH adjusted pulp is discharged through pipe 131 to a pulp 140 to transfer the pulp through pipe 141 to a mixer 150. As the pulp will be heated to about 100°C., it is conventional to apply steam for final heating ahead of the mixer, typically through a pipe 144. Additional chemicals including alkali, peroxide, chelants, and others may be added through pipes 142 and 143, and homogeneously mixed with the pulp prior to discharge through pipe 151 to the second peroxide reaction vessel 160. The peroxide retention vessel 160 is again sized for 1-30 minutes retention time, preferably 5-20 minutes to achieve substantial delignification and/or brightness increase prior to being discharged through pipe 161 to a discharge device 152. It is preferable to wash the pulp following the second peroxide treatment; however, it may not be necessary if an additional bleaching stage is added which is not significantly affected by the presence of residual peroxide and/or dissolved organic matter. If the washing device is a non pressurized device, it will be preferable to cool the pulp prior to discharge from the first peroxide reaction vessel. Dilution of the pulp with a cool liquor may be accomplished in the second peroxide reaction vessel 160 through pipe 153. It may be desirable to dilute the pulp prior to the pump with filtrate fed through pipe 154 if the washing device requires low consistency pulp for its proper operation. The pulp is then washed on a washing device 180 using water or filtrate applied through pipe 156.

Fig. 6 is a schematic diagram of a bleaching sequence employing the present invention applied to an existing bleach plant. A typical sequence which exists in many bleach plants today is DcEoDED. An existing conventional bleach plant may be modified to incorporate the concepts according to the present invention very economically. As shown in the Figure, almost all of the existing equipment is utilized in the conversion, and the investment for conversion may be as little as three mixers, three reaction vessels, an ozone generator, and miscellaneous piping to incorporate the change. In some cases, it may be necessary to make some changes to the materials in existing bleach towers, piping, and other equipment.

In this example as shown in Figure 6, the existing chlorination tower may not be used in the new bleaching sequence, but may serve as additional storage or other pretreatment prior to the new bleach plant. The existing Eo stage will be used as a chelant stage where EDTA, DTPA, or other chelant is added and is operated at a controlled pH and temperature, preferably 5-7 pH and 10-60°C. The retention time of 30-90 minutes that typically exists in a conventional Eo stage is suitable for the chelation stage.

After washing on the existing Eo washer, the existing D1 stage will be used as a first peroxide stage (P1), by diverting the pulp to a new mixer suitable for addition of peroxide, adding the appropriate reaction vessel, and discharging to the existing D1 tower, with or without reactivation of residual peroxide according to the present invention. The first peroxide stage will operate at a pH of about 9.5 to about 12.5, with the first step of the reaction at a temperature of greater than 100°C., preferably 105-120°C., with a reaction time of 1-30 minutes, preferably 5-20 minutes.

After washing on the existing D1 washer, the existing E stage will be used as an ozone stage, by diverting the pulp to a new mixer suitable for addition of ozone, adding the appropriate reaction vessel and gas separator, and discharging to the existing E tower. The ozone stage will operate at a pH of less than 4, preferably 2-4, at a temperature of 30-70°C., preferably less than 50°C., with a reaction time of less than 10 minutes preferably less than 5 minutes. After washing on the existing D2 washer, the existing D2 stage will be used as a second peroxide stage (P2), by diverting the pulp to a new mixer suitable for addition of peroxide, adding the appropriate reaction vessel, and discharging to the existing D2 tower, with or without reactivation of residual peroxide according to the 9 present invention. The second peroxide stage will operate at a pH of about 9.5 to about 12.5, with the first step of the reaction at a temperature of greater than 100°C., preferably 105-120°C., with a reaction time of 1-30 minutes, preferably 5-20 minutes.

After washing on the existing D2 washer, the pulp may proceed to subsequent post treatments.

Figure 7 illustrates a preferred method for the reuse of filtrates in the QPZP sequence. It is noted that according to the present invention, there is substantial residual peroxide with the pulp following the peroxide treatment, for example, normally the peroxide charged is 2.5% on pulp, but the amount of peroxide consumed in the reaction is typically less than 1.5% on pulp. As peroxide is a relatively expensive bleaching chemical, it is desirable to recover this residual for reuse in the peroxide stages. This is accomplished by recirculating filtrate from the post peroxide stage washing to the washing step preceding the peroxide stage as shown in the Figure. For illustration, typical filtrate flows are shown in the Figure, for example, with each washer discharging at 10% consistency, the filtrate flow with the pulp is 9 kg liquor/kg pulp. The wash water flow to each washer, at a dilution factor of 2, which is typical in bleach plants today, is 11 kg liquor/kg pulp. The filtrate, according to the present invention, is recirculated countercurrently through the bleach plant such that the residual peroxide is applied to the pulp on the washer, and a substantial portion of this residual peroxide then carries forward to the peroxide stage. The required peroxide application in the stage is then reduced by the amount carried forward with the pulp. The amount of peroxide residual carried forward is a function of the displacement ratio (washing efficiency) of the washer. For example, if the washer has a displacement ratio of 0.85, then about 60% of the peroxide residual will be recovered. The same principle applies to the recovery of alkali used in the peroxide stages. According to this embodiment, the bleach chemical used in the sequence is reduced substantially by recirculation of filtrates as shown in Figure 7, compared to other filtrate recycle schemes which may be considered.

Figure 8 illustrates another preferred method for the reuse of filtrates in the QPZP sequence, which may be preferred if the bleach plant is to be constructed new, that is, without reuse of existing equipment in a bleach plant. Figure 8 includes the use of presses, preferably Twin Roll Washing Presses manufactured by Ingersoll-Rand Company, instead of conventional vacuum or pressure washers or vacuum or pressure diffusers. Typical filtrate flows are shown in the Figure, with each press discharging at 33.3% consistency, the filtrate flow with the pulp is 2 kg liquor/kg pulp. The wash water flow to each washer, at a dilution factor of 2, which is typical in bleach plants today, is 4 kg liquor/kg pulp. In this case, the filtrate is used for washing on the press similar to the concept described in Figure 7. However, in addition, the filtrate which contains substantial residual peroxide is also used for dilution of the pulp discharged from the press in the amount of 7 kg liquor/kg pulp. The amount of peroxide recovered is similar to that with the vacuum washer case described in Figure 7. The amount of filtrate recovered in the press is a function of the displacement ratio (washing efficiency) of the press. If the press has a displacement ratio of 0.40, a total recovery of about 60% of the peroxide residual is achieved. This may be attractive for new construction as presses have similar capital requirements as conventional vacuum or pressure washers. One additional benefit of this configuration is the ability to incorporate high consistency ozone bleaching rather than medium consistency ozone bleaching if the washing devices installed are presses. This option reduces the amount of ozone required for bleaching, and reduces the energy demand in the bleach plant.

In Fig. 9, pulp from a conventional washer or thickener is discharged through pipe 10 into mixer 100, where steam for heating the pulp and/or alkali is used for increasing the pH of the pulp is added through pipe 15. The pulp is heated and adjusted to a pH of about 11. The heated and pH adjusted pulp is discharged from mixer 100 through pipe 20 to a conventional medium consistency pump 200 which pumps the pulp through pipe 30 to a mixer 300. Hydrogen peroxide solution is added to the mixer 300 through pipe 35 in a quantity sufficient to assure substantial residual will be maintained at the end of the first step of the reaction. As is conventional in peroxide bleaching systems, the addition of magnesium compounds for protection of cellulose viscosity, as well as sequestrants (such as silicate and/or chelants {such as EDTA or DTPA}) may also be added with the alkali solution through pipe 15. The peroxide solution can be applied through pipe 35, or separately through pipe(s) 16 and/or 36. As the temperature of the pulp must exceed 100°C. according to the present invention, and it is not currently practical to pump pulp at this high temperature, the final temperature rise from, for example, 85-90°C. to in excess of 100°C. is achieved by adding steam between the pump 200 and the mixer 300 through pipe 37.

The pulp, which has been heated to the desired reaction temperature, and adjusted to the desired pH, is pumped through pipe 40 into the upflow tube 400, which is sized for the retention time desired for the first phase of the reaction. According to the present invention, the upflow tube is sized to assure a pulp retention time of 5-30 minutes, preferably 5-20 minutes, after which retention time, the pulp retains a substantial residual of peroxide in the pulp slurry. The pulp is discharged through pipe 50 to an appropriate pressing device 560, where the consistency of the pulp is raised from about 10% to about 25%. The filtrate is recycled through pipe 55 to pipe 30 to recover a portion of the peroxide not consumed in the first step of the reaction.

The pulp is discharged through pipe 60 to a conventional downflow tower 600, for the second step of the reaction which is operated at high consistency (25-30%). The conventional downflow tower is sized to consume a substantial portion of the remaining peroxide, and will typically retain the pulp for 1-5 hours. Bleached pulp is discharged from tower 600 through pipe 80 to subsequent process steps.

For clarity, pumps and mixers, as well as discharge devices, etc., which are described in Figures 1-6, are not shown, nor are heat exchangers which are necessary to maintain the optimum temperature in each bleaching stage. Heat exchange devices are incorporated in the bleach plant in accordance with good engineering practice.

Those skilled in the art will recognize that this invention may be practiced with upflow, downflow, upflow/downflow, or other configurations to achieve the desired retention time for the reaction. Further, those skilled in the art will recognize that there are additional means to heat and cool the pulp prior to and subsequent to the reaction, respectively. Further, those skilled in the art will recognize that the valve used to retain pressure in the first retention time (upflow tube) may be located either before or after the mixing device. The details given in this description are not intended to limit the scope of this invention, but merely are included for clarity of the embodiment.

Persons skilled in the art will also recognize that the use of chelants and sequestrants may be desirable or necessary to achieve the benefits of the present invention, and the addition of these types of treatments to the present invention will not limit the scope of the invention. Examples are the use of the sequences QP, QPQP, QPQZP, QPZQP, PZQP, and the like. It is further recognized by those skilled in the art that acid treatments, especially to very low pH, for example, less than 3, may substitute effectively in many case for the use of chelants in sequences of this nature; therefore, the addition of an acid stage, or the substitution of an acid stage for a chelant stage will also fall within the scope of this invention.

This technology which has been dubbed the "PAPEROXIDE" process can be applied to cellulosic or lignocellulosic materials from either softwoods, hardwoods, or varieties of non-wood fibers from chemical, mechanical, semichemical, or semimechanical pulps, or secondary fiber pulps. The benefits of such applications are demonstrated in the examples which follow.

The first step of the PAPEROXIDE process is to react the pulp with peroxide at a pH of 9.5-12.5 and at high temperature (>100°C.) for a limited time of less than about 30 minutes, preferably 5-20 minutes. It is important that this step be performed without causing metal contamination of the pulp slurry. The result of this first step is illustrated in the example below:

EXAMPLE 1

A sample of softwood kraft pulp, having a kappa number of 27.2, was oxygen delignified (0 stage) to a kappa number of 1. This pulp sample was then treated in a chelation stage using 0.6%EDTA on pulp (Q stage) at about 50°C. and for 30 minutes retention time. The sample was then further delignified using a conventional hydrogen peroxide stage (P) and an ozone stage (Z) to achieve a kappa number of 2.0 with a brightness of 77.9%ISO and a viscosity of 14.4 mPa's using the conditions and chemical charges shown in Table A.

Conditions and Chemical Charges in OQPZ stages
Stage	O	P	Q	Z
Tim, min.	60	30	240	7
Temp. °C	100	50	90	23
Consistency, %	10	2	10	40
Pressure, psig	100	-	-	-
Chemicals, %	O2,1% approx.	EDTA,0.6%	H2O2, 2.5%	O3,0.5%
on pulp,	NaOH, 2.5%	H2SO4,to pH6	NaOH, 2.5%	H2SO4,to pH2
to oven-dry	MgSO4,0.5%		MgSO4,0.05%
basis			DTPA, 0.2%

The pulp was further treated in a pressurized peroxide stage according to the prior art and compared to the first step of the new process:

Comparison of Prior Art to New Process of Pressurized Peroxide
	Prior Art Pressurized	New Process First Step
H2O2 consumed, % on pulp	2.2	0.9
End pH	10.6	11.3
Time, minutes	60	5
Temperature, °C.	110	110
Brightness, %ISO	91.6	89.6
Viscosity, mPa's	8.5	8.9

In all cases, the pulp was treated at 10% consistency, and was maintained under pressure using oxygen gas at a pressure of 5.17 bar (75 psig). The following chemicals were applied in the bleaching:

Bleaching Conditions for Table 1
Chemical	Applied, % on pulp
H2O2	2.5
NaOH	2.75
MgSO4	0.05
DTPA	0.2

This illustrates that, in the first step of the present invention, it is possible to achieve 85% of the brightness improvement, and surprisingly without a viscosity penalty and with a much decreased consumption of peroxide, at a very short retention time compared to the prior art which requires one hour or more. Although in the example oxygen gas was used to maintain the pulp under pressure, it is not necessary according to the first step of the present invention to expose the pulp to oxygen gas. This is shown in the following example.

Example 2

A sample of softwood kraft pulp, having a kappa number of 27.2, was oxygen delignified to a kappa number of 14. This pulp sample was then treated in a chelation stage using 0.6% EDTA on pulp at about 50°C. and for 30 minutes retention time. This pulp sample was then further delignified using a conventional hydrogen peroxide stage and an ozone stage to achieve a kappa number of 1.3 with a brightness of 81.8% ISO. The pulp was further treated in a conventional atmospheric peroxide stage, and a pressurized peroxide stage according to the prior art and compared to the first step of the present invention as follows:

Comparison to Prior Art Conventional and Pressurized Peroxide Stage, and Effect of the use of Oxygen Gas
	Prior Art Conventional	Prior Art Pressurized	New Process First Step
H2O2 consumed, % on pulp	1.84	1.91	1.07
End pH	11.2	11.0	11.4
Pressure, bars	--	5.17	2.41
Oxygen used	no	yes	no
Vapor pressure			2.41
Time, minutes	240	60	5
Temperature, °C	90	110	110
Brightness, %ISO	93.3	93.6	92.6

In all cases, the pulp was treated at 10% consistency, and the following chemicals were applied in the bleaching:

Bleaching Conditions for Table 3
Chemical	Applied, % on pulp
H2O2	2.5
NaOH	2.75
MgSO4	0.05
DTPA	0.2

This illustrates that according to the first step of the present invention, it is possible to achieve very similar results to the prior art of conventional long retention time atmospheric or pressurized oxygen-peroxide stages. Contrary to the prior art, these comparable results can be achieved without the use of oxygen gas in the first step of the present invention. The short, high temperature first step of the present invention illustrated in the example (5 minutes retention time) used only the vapor pressure of the pulp slurry at 110°C. which was 2.41 bar (35 psig), compared to the prior art which requires the use of high pressure oxygen gas. By these examples, and according to the present invention, results similar to the prior art can be achieved while using only a short, high temperature stage which will dramatically reduce the capital investment required for implementation of peroxide bleaching technology.

The fundamental principle of this invention is to gain rapid delignification and brightness development in a relatively short period of time, at temperatures of about 100-120°C. and retention time in a small reaction column of less than about 30 minutes, and then discharging the pulp into a conventional large retention tower for about 1-5 hours at atmospheric pressure, i.e., less than about 100°C., without washing the pulp in between the two steps. The use of subsequent retention time at atmospheric pressure for conventional retention times of 1-5 hours, will lead to further consumption of hydrogen peroxide accompanied by a further brightness increase. This may allow the incoming pulp to be received at lower brightness than the pulp in the example.

This peroxide stage design can be further improved by combining the principle of high temperature and short retention time with reactivation of residual peroxide followed by conventional bleach tower retention. Where reactivation of residual peroxide is to be practiced, the peroxide solution is added in more than sufficient quantities to guarantee a significant peroxide residual after the completion of the first step of the reaction, which is conducted for a retention time of 1-30 minutes, but preferably for 5-20 minutes, assuring therefore a peroxide residual entering the second step of the reaction. This combination is further described in a copending application of Bottan.

This invention can be applied to the rebuild of existing bleach plants and/or the construction of new bleach plants for the production of ECF or TCF pulps. The use of a short, high temperature peroxide step with or without subsequent retention and with or without reactivation of residual peroxide, and in combination with conventional bleaching chemicals allows very economical construction of short retention time bleach plants.

For example, using two short, high temperature peroxide stages with an interstage treatment using ozone gas, it is possible to produce TCF pulp at high brightness, for example, greater than 80%ISO, from an oxygen delignified softwood kraft pulp with an entering kappa number of about 14. This bleaching concept allows the full bleaching operation to take place with a total retention time in the bleach plant of less than two hours. Due to the small bleaching reactors compared to conventional bleaching technology, this approach has an unprecedented low capital cost compared to all current approaches to production of TCF pulps. This is illustrated in the following example:

EXAMPLE 3

A sample of softwood kraft pulp, having a kappa number of 25.7, was oxygen delignified to a kappa number of 13.6 and a viscosity of 22.4 mPa. This pulp sample was then treated in a chelation stage using 0.6%EDTA on pulp at about 50°C. and for 30 minutes retention time, followed by bleaching of the pulp to greater than 85 % ISO brightness in a P-Z-P sequence. According to the first step of the present invention, the peroxide stages were performed using short (5-15 minutes), high temperature (107-110°C.) conditions, but with the addition of a conventional ozone bleaching step between the two peroxide stages.

The brightness of the bleached pulp is 82.3, 87.6, and 91.0 for total retention times in the two peroxide stages of 10, 20, and 25 minutes, respectively. It should be noted that the kappa number of the bleached pulp is quite low, for example, less than 3. There are a number of pulp mills today which practice TCF bleaching of pulp using only peroxide as the active bleaching chemical, and these mills produce fully bleached pulps at an exceedingly high kappa, for example, greater than 5, which is undesirable especially from the view of brightness reversion in the paper produced from such pulps. This combination of the use of the first step of the present invention with established ozone bleaching technology, has a substantial advantage over current commercial practice for paper quality made from the pulps produced through use of this process.

It should be noted that the present invention according to the data in Table 5 was simulated using oxygen gas to pressurize the pulp mixture during the laboratory bleaching. This is a practical method of simulating bleaching in the laboratory where the temperature of the pulp is to be maintained over 100°C. According to the data presented in Table 2, however, it is not necessary according to the present invention to apply oxygen gas, and it is not anticipated to be required in commercial practice to achieve the advantages of the present invention.

The simulation of this sequence includes an ozone stage operating at high consistency for convenience in the laboratory simulation, that is, about 40% consistency; however, it is recognized that for moderate doses of ozone on pulp, that is, less than about 0.6%, essentially the same results will be achieved when using medium consistency ozone, that is, about 10-14% consistency. The present invention is therefore not intended to be limited to the use of high consistency ozone, in fact, it is preferable to use medium consistency ozone in order to utilize existing equipment in the mill, and this is thus described in the preferred embodiment.

One skilled in the art will recognize that the principle of this application of the invention, that is, two peroxide stages with an interstage non-peroxide stage, may also be employed using other interstage chemicals which preferably operate in an acid media, that is, less than about pH 7. Examples of other interstage treatments include, but are not limited to, peracetic acid. Caro's acid, various enzymatic treatments, and combinations of these types of reagents.

Evaluation of P-Z-P Bleach Sequence Employing the New Process
	New Process	Conventional
P1 stage (pres'd H2O2)
H2O2, % on pulp	2.5	2.5	2.5	2.5	2.5
H2O2, % uptake	0.5	1.1	1.1	1.3	2.1
NaOH, % on pulp	2.5	2.5	2.5	2.5	2.5
DTPA, % on pulp	0.2	0.2	0.2	0.2	0.2
MgSO4, % on pulp	0.05	0.05	0.05	0.05	0.05
Total time, minutes	5	15	15	20	60
Temperature, °C	107	110	110	110	110
O2 pressure, bars	5.17	5.17	5.17	5.17	5.17
Consistency, %	10	10	10	10	10
Final pH	11.4	11.7	11.6	11.3	10.5
Kappa no.	8.8	7.5	6.7	6.1	4.9
Brightness, %ISO	60.3	65.5	69.0	70.4	77.9
Z Stage (ozone)
O3 % on pulp	0.5	0.5	0.5	0.5	0.5
O3, % uptake	0.49	N.D.	N.D.	N.D.	N.D.
Total time, minutes	7	7	7	7	7
Temperature, °C	23	23	23	23	23
Consistency, %	40	40	40	40	40
Final pH	2.4	2.4	2.0	2.0	2.3
Kappa no.	3.7	2.0	1.1	1.1	N.D.
Brightness, %ISO	70.6	76.5	80.8	79.8	85.7
P2 stage (pres'd H2O2)
H2O2, % on pulp	2.5	2.5	2.5	2.5	2.5
H2O2, % uptake	0.9	0.8	0.9	0.9	2.0
NaOH, % on pulp	2.5	2.5	2.5	2.5	2.5
DTPA, % on pulp	0.2	0.2	0.2	0.2	0.2
MgSO4, % on pulp	0.25	0.25	0.25	0.25	0.25
Total time, minutes	5	5	10	5	60
Temperature, °C	108	110	110	109	110
O2 pressure, bars	5.17	5.17	5.17	5.17	5.17
Consistency, %	10	10	10	10	10
Final pH	11.3	11.1	11.4	11.6	10.6
Kappa no	20	N.D.	0.4	N.D.	N.D.
Brightness, %ISO	82.3	87.6	91.0	31.2	96.6

As a further example, the modification of an existing bleach plant using, for example, the existing extraction, hypochlorite, or chlorine dioxide bleaching towers as subsequent retention following the short, high temperature step of the peroxide bleaching stage according to the present invention, reduces the operating cost of the bleaching sequence compared to the previous example, and requires minimum added equipment to the existing bleaching plant.

EXAMPLE 4

A sample of softwood kraft pulp, having a kappa number of 25.7, was oxygen delignified to a kappa number of 13.6. This pulp sample was then treated in a chelation stage using 0.6% EDTA on pulp at about 50°C and for 30 minutes retention time, followed by bleaching of the pulp to greater than 85% ISO brightness in a P1-Z-P2 sequence. According to the present invention, the peroxide stages were performed using short (5-15 minutes), high temperature (107-110°C.) conditions, but with the addition of atmospheric retention time following each of the short, high temperature, peroxide stages, and in combination with a conventional ozone bleaching step between the two peroxide stages.

It is demonstrated in Table 6 that, according to the present invention, the addition of atmospheric retention following the short, high temperature first step of the present invention achieves results comparable to that achieved according to the prior art using pressurized retention in each stage for 1-3 hours, at the same peroxide dose on pulp. This is especially desirable as it is possible in many pulp mills to reuse existing bleaching towers for the subsequent retention with minimal capital investment requirements. As earlier described, the investment cost for a 1-3 hour pressurized retention vessel is relatively high, and it is further anticipated that, even if it is necessary to construct a new atmospheric retention vessel for 1-5 hours retention, that the new atmospheric retention vessel, in combination with a short (5-15 minutes) high temperature retention vessel will still be quite economically attractive for the new bleaching sequence when compared to the capital requirements of the prior art. According to the present invention, with the addition of a short 5-20 minute) upflow tube in combination with the use of an existing bleaching tower in the bleach plant for an additional 1-4 hours, the investment per peroxide bleaching stage is reduced to about $250,000 or less, or a total investment for the 350 mills in North American industry of about $90 million. Compared to the prior art, this represents a savings of between $130-430 million.

Evalualion of PZP bleaching sequence employing the present invention consisting of short time and temperature in tlie first step and a long retention time and lower temperature in the second step of each P stage
	One Step: 15 min.	Two steps: 15+250min.	One step: 60 min.
P1 stage steps
H2O2, % on pulp	2.5	2.5	2.5
H2O2, % uptake	1.1	2.1	1.1
NaOH, % on pulp	2.5	2.5	2.5
MgSO4, % on pulp	0.05	0.05	0.05
DTPA, % on pulp	0.2	0.2	0.2
Consistency, %	10	10	10
Total time, minutes	15	225	60
1st step	15	15	60
2nd step	--	240	--
Temperature, °C
1st step
	110	110	110
2nd step	--	90	--
O2 pressure, bars (1st step only)	5.17	5.17	5.17
End pH	11.6	11.1	10.5
Kappa no	6.7	5.2	4.9
Viscosity, mPa's	18.2	76.6	77.9
ISO Brightness, %	69.0	76.6	77.9
Z Stage
O3, % on pulp	0.5	0.5	0.5
Consistency, %	40	40	40
Total time, minutes	7	7	7
Temperature, °C	23	23	23
End pH	2.0	2.0	2.3
Kappa no.	1.1	NA	--
Viscosity, MPa's	12.3	11.0	14.7
ISO Brightness, %	80.8	84.6	85.7
P2 stage steps
H2O2, % on pulp	2.5	2.5	2.5
H2O2, % uptake	0.9	2.3	2.0
NaOH, % on pulp	2.5	2.5	2.5
MgSO4, % on pulp	0.25	0.25	0.25
DTPA, % on pulp	0.2	0.2	0.2
Consistency, %	10	10	10
Total Time, minutes	10	250	60
1st step	10	10	60
2nd step	--	240	--
Temperature, °C.
1st step
	110	110	110
2nd step	--	90	--
O2 pressure, bars
1st step	5.17	5.17	5.17
End pH	11.4	11.4	10.6
Kappa no	NA	NA	NA
Viscosity, mPa's	10.9	9.5	8.7
ISO Brightness, %	91.0	93.4	93.6

The data given is to quantify the performance of a few preferred embodiments, but is not intended to limit the scope of the present invention. One skilled in the art will recognize that the present invention can be practiced in numerous ways to achieve its benefits. Some examples have been given to illustrate this point, but these examples are by no means intended to limit the scope of the invention. This list describes some locations in which the present invention, "PAPEROXIDE" process, may be employed in existing or new bleach plants.

1. For delignification prior to the bleach plant.

2 For final brightening at the end of the bleach plant.

3 To replace an existing stage in the bleach plant.

4. To modify an existing stage to improve performance or reduce the use of chlorine compounds in the bleach plant.

5. As a pretreatment prior to a further bleaching stage.

6. Any of the above, using more than one new or modified stage.

7. More than one new stage in combination with one or more interstage treatments.

Claims (15)

1. A method for using hydrogen peroxide for bleaching wood pulp and other cellulosic and lignocellulosic fibrous materials comprising the steps of :

   adjusting the consistency of said pulp to 8%-18%;

   adding alkali to bring said pulp to a pH of greater than 9.5;

   adding hydrogen peroxide to provide residual peroxide after passing through a first reaction column;

   heating said pulp to a temperature greater than 100°C while maintaining sufficient pressure to prevent boiling of pulp liquor;

   passing said pulp through a reactor column at a rate which provides a reaction time in said column of less than 45 minutes;

   adding alkali to the pulp exiting said reactor column to bring said pulp to a pH of more than 9.0;

   cooling said pulp to a temperature of less than 100°C;

   depositing said pulp in a reaction tower at atmospheric pressure and allowing the reaction to proceed for 1-5 hours until a substantial portion of residual hydrogen peroxide has been consumed; and

   discharging said pulp for further processing.
2. The method of claim 1, wherein
      sufficient alkali is added to bring said pulp to a pH of between 9.5-11.5 inclusive in tlie first alkali addition step.
3. The method of claim 1, wherein
      said pulp is passed through said reactor column at a rate which provides a reaction time in said column between 5 and 20 minutes.
4. The method of claim 1, where
      said step of adding hydrogen peroxide to provide residual peroxide after passing through a first reaction column comprises adding between 0 5%-50% hydrogen peroxide, by oven dried weight of said pulp.
5. The method of claim 4 wherein said residual hydrogen peroxide is reactivated by adding sufficient alkali to bring the pulp to a pH of more than 9.5.
6. A three stage bleaching sequence for cellulosic and lignocellulosic materials comprising the steps of:

   exposing the pulp to a first stage hydrogen peroxide bleaching treatment,

   said first stage treatment comprising the steps of :

   adjusting the consistency of said pulp to 8%-18% and adding alkali and hydrogen peroxide to bring said pulp to a pH of at least 9.5 and a hydrogen peroxide content of from 0.5%-10%, by oven dried weight of said pulp;

   heating said pulp to a temperature greater than 100°C while maintaining sufficient pressure to prevent boiling of pulp liquor;

   passing said pulp, under pressure, through a reactor column at a rate which provides a reaction time in said column of less than 45 minutes;

   cooling said pulp; and

   discharging the cooled pulp to a washer in which a substantial portion of the unconsumed bleaching chemicals and dissolved material is removed from the pulp, and in which the pulp consistency is adjusted, if necessary to a preferred value for a stage of bleaching treatment;

   exposing the pulp to a second bleaching stage, said bleaching stage comprising the steps of:

   adding sufficient acid or alkali to adjust the pH of the pulp to a preferred value for the bleaching chemical;

   introducing said pulp to a mixer in which the second bleaching chemical, wherein said second bleaching chemical is not hydrogen peroxide, is mixed with said pulp;

   discharging the pulp from the mixer into a reaction vessel in which said pulp is held for a sufficient time to permit consumption of a substantial portion of the applied chemical;

   discharging said pulp from said reaction vessel to a washer, in which a substantial portion of the unconsumed bleaching chemicals and dissolved organic material is removed from the pulp, and in which the pulp consistency is adjusted, if necessary, to 8%-18%;

   exposing the pulp to a third stage of bleaching treatment, said third stage treatment comprising all steps indentical to those recited for said first stage hydrogen peroxide bleaching treatment subsequent to adjustment of pulp consistency; and

   depositing the pulp discharged from the bleaching stage in a reaction tower and allowing the reaction to proceed for 30 minutes to 3 hours until a substantial portion of the residual peroxide has been consumed,

   the filtrate from the dewatering step following the bleaching stage being used to dilute the incoming pulp, thus recovering a substantial portion of the residual hydrogen peroxide.
7. The method of claim 6, in which

   in the second bleaching stage, sufficient acid is added to adjust the pH of the pulp to a value of less than 4, and in which the step of introducing said pulp into a mixer includes the addition of ozone gas in an appropriate carrier gas which is mixed with said pulp; and

   discharging the pulp from the mixer into an ozone reaction vessel in which said pulp is held for up to 10 minutes to permit consumption of substantially all of the ozone, and discharging the pulp from said ozone reaction vessel to a washer, in which a substantial portion of the unconsumed bleaching chemicals and dissolved organic material is removed from the pulp, and in which the pulp consistency is adjusted, if necessary, to 8%-18% for the third stage of treatment.
8. The method of claim 7, in which
      the amount of ozone applied is between 0.1% and 2.0%, by oven dry weight of pulp.
9. The method of claim 7, in which
      the ozone bleaching stage is operated at 8%-18% consistency.
10. The method of claim 9 in which
       the amount of ozone applied is between 0.2% and 1.0% by oven dry weight of pulp.
11. The method of claim 7 in which
       more than one stage of ozone treatment is included without washing the pulp between stages.
12. The method of claim 7 in which additional bleaching steps are performed between the first peroxide treatment and the second peroxide treatment.
13. The method of claim 7 in which

    the incoming pulp is adjusted to a consistency of greater than 20%; and

    the discharged pulp is dewatered to a consistency of greater than 20%,
14. The method of claim 13 in which
       alkali is added to reactivate the residual hydrogen peroxide before the pulp is deposited into said reaction tower.
15. The method of claim 13 in which
       the pulp is cooled before being deposited into said reaction tower.

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