EP0496782A1

EP0496782A1 - High efficiency chlorine dioxide pulp bleaching process.

Info

Publication number: EP0496782A1
Application number: EP19900915570
Authority: EP
Inventors: Hou-Min Chang; Geoffrey Eugene Seger; Hasan Jameel
Original assignee: North Carolina State University; University of California
Current assignee: North Carolina State University; University of California
Priority date: 1989-10-19
Filing date: 1990-10-17
Publication date: 1992-08-05
Anticipated expiration: 2010-10-17
Also published as: CN1052157A; ZA908341B; PL287415A1; AR243945A1; EP0496782A4; DE69026105D1; CA2069322A1; NZ235754A; AU6537690A; DE69026105T2; MX172853B; PL164745B1; WO1991005909A1; ES2085358T3; EP0496782B1

Abstract

Procédé de blanchiment hautement efficace de cellulose technique, permettant de produire des celluloses techniques d'une blancheur supérieure à l'aide d'une quantité de dioxyde de chlore égale, ou d'une blancheur égale à l'aide d'une quantité de dioxyde de chlore réduite de manière significative. Le procédé consiste à faire réagir le dioxyde de chlore avec de la cellulose technique à un pH compris entre environ 5 et 10 pendant environ 5 à 40 minutes, puis à acidifier le mélange jusqu'à un pH compris entre environ 1,9 et 4,2. Ensuite, on laisse réagir le mélange pendant environ deux heures ou plus, afin d'achever le procédé de blanchiment à pH haut/faible en deux étapes.Highly efficient process for bleaching technical cellulose, to produce technical celluloses of greater whiteness using an equal amount of chlorine dioxide, or an equal whiteness using an amount of dioxide significantly reduced chlorine. The process consists in reacting the chlorine dioxide with technical cellulose at a pH between approximately 5 and 10 for approximately 5 to 40 minutes, then acidifying the mixture to a pH between approximately 1.9 and 4, 2. Then, the mixture is allowed to react for about two hours or more, in order to complete the two-step high / low pH bleaching process.

Description

HIGH EFFICIENCY CHLORINE DIOXIDE PULP BLEACHING PROCESS

Technical Field

The present invention relates to the bleaching of pulp and more particularly to an improved process for bleaching wood pulp with chlorine dioxide in a manner whereby the wood pulp is subjected to a 2-step high pH/ low pH bleaching stage which results in a substantial decrease in the usage of chlorine dioxide required to brighten wood pulp.

Background Art

As is well known in the wood pulp bleaching art, the main objectives of wood pulp bleaching are to increase the brightness of the pulp and to make it suitable for the manufacture of printing and tissue grade papers by removal or modification of some of the constituents of the unbleached pulp, including the lignin and its degradation products, resins, metal ions, non-cellulosic carbohydrate components, and various types of flecks. The bleaching of chemical wood pulp is normally carried out in multiple processing stages utilizing elemental chlorine, caustic soda, hypochlorites, oxygen, hydrogen peroxide, and chlorine dioxide. The number of stages required in a particular bleaching process is dependant upon the nature of the unbleached pulp as well as the end use to which the pulp will be put. A sulfate or kraft pulp is today most typically bleached in a five stage sequence which is designated as (CD) (EO)DED. In the

(CD) (EO)DED designation, D denotes chlorine dioxide, C denotes elemental chlorine, E denotes caustic extraction, and 0 denotes oxygen gas. The multi-stage process in essence comprises a chlorination step (CD) , a first oxidative extraction stage (EO) , a first bleaching stage (D,) , a second caustic extraction stage (E₂) , and a second and final bleaching stage (D_) .

In the conventional (CD) (EO)DED multi-stage bleaching process, each of the two chlorine dioxide bleaching stages is carried out in a one-step process at an end pH of about 3.8 for three hours at 70° centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the wood pulp mixture, and this particular pH has heretofore been considered optimal for each of the two chlorine dioxide bleaching stages in the (CD) (EO)DED sequence. It should also be appreciated that although the (CD) (EO)DED sequence has been specifically addressed, the one-step chlorine dioxide bleaching stage can be used in any D stage for most other three, four, five, or six-stage bleaching processes known to those familiar with the art of wood pulp bleaching.

A shortcoming of the one-step chlorine dioxide bleaching stage presently used in the pulp and paper industry is that approximately 30% of the chlorine dioxide is lost to the formation of the unreactive species chlorite and chlorate, and this is very undesirable in view of the relatively high cost of chlorine dioxide. The present invention solves this well-known deficiency in state of the art chlorine dioxide bleaching by significantly reducing the chlorine dioxide loss during the chlorine dioxide bleaching process. The advantages of the reduced loss of chlorine dioxide are a very significant reduction in the cost of the wood pulp bleaching process as well as the reduction of pollution levels.

Disclosure of the Invention

In accordance with the present invention, applicant provides an improved process for bleaching wood pulp in an aqueous suspension using chlorine dioxide which substitutes a two-step bleaching stage for the conventional one-step bleaching stage known to those familiar with the wood pulp bleaching art. The novel process comprises first subjecting the aqueous wood pulp suspension to a first bleaching step by mixing it with an aqueous solution of chlorine dioxide and maintaining the mixture at a pH between about 5-10 for about 5-40 minutes. Next, an acid or acid gas is introduced into the mixture in order to bring the pH down to a pH between about 1.9-4.2, and the mixture is then subjected to a second bleaching step at the reduced pH for 2 or more hours, most suitably between about 2.5-3.9 hours. This novel process can be used in the D, or D₂ stage of the (CD) (EO)DED bleaching sequence as well as in any D bleaching stage of other three, four, five, six, and seven-stage bleaching sequences. The operating temperature during the novel process should be between about 55-85°C, and the pulp's final consistency should be between about 3-12%.

It is therefore an object of the present invention to provide more efficient chlorine dioxide bleaching in the wood pulp bleaching process.

It is another object of the present invention to significantly reduce the conversion of chlorine dioxide to non-bleaching chemicals during the wood pulp bleaching process.

It is still another object of the present invention to reduce the cost of the wood pulp bleaching process.

It is yet another object of the present invention to achieve a higher wood pulp brightness with a selected chlorine dioxide charge than has heretofore been possible. Description of the Drawings

Some of the objects having been stated, other objects will become evident as the description proceeds, when taken in connection with the accompanying drawings, in which:

Figure 1 is a graph of the effect of pH on chlorate and chlorite formation in chlorine dioxide bleaching of kraft pulp (reprinted from "The Bleaching of Pulp", Ed. R. P. Singh, p. 137) ;

Figure 2 is a graph of D.. brightness for the pulp of Figure 2 when the D.. charge is varied on the pulp for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention;

Figure 3 is a graph of D_ brightness versus chlorine dioxide charge for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention wherein the D_ charge is 0.2% C10₂ on pulp;

Figure 4 is a graph of D, and D_ brightness versus chlorine dioxide charge for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention;

Figure 5 is a graph of D, brightness versus percentage (%) chlorine dioxide on the pulp (D. charge) for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention;

Figure 6 is a graph of D_ brightness for the pulp of Figure 5 when the D_? charge is 0.2% chlorine dioxide on the pulp for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention;

Figure 6(a) is a graph of final brightness versus C10_ charge for the conventional one-step process and the novel two-step bleaching process of the present invention using a (CD) (EO)D sequence. Reverted brightness is also shown after 24 hours at 105°C;

Figure 7 is a graph of D. viscosity versus D, pH for the conventional one-step bleaching process and high pH for the novel two-step bleaching process of the present invention;

Figure 8 is a graph of total organic chlorine (TOC1) or (AOX) in D, plus E₂ effluents versus chlorine dioxide charge in D. for the conventional one-step bleaching process and the novel two-step bleaching process of the present invention;

Figure 9 is a graph of chlorate formed in the D, stage versus end pH; Figure 10 is a graph of chlorate formed versus D. charge and CE kappa number for conventional bleaching;

Figure 11 is a graph of chlorate formed versus D. charge and CE kappa number for the novel two-step high/low pH bleaching process of the present invention;

Figure 12 is a graph of chlorate formed as a percentage (%) of chlorine dioxide converted to chlorate versus percent (%) chlorine dioxide in D.. for the conventional one-step bleaching process and the novel two-step high/low bleaching process of the present invention;

Figure 13 is a graph of D.. pulp brightness versus the percentage of chlorine dioxide on the pulp (D. charge) for the conventional one-step bleaching process and the novel two-step high/low pH bleaching process of the present invention (wherein the middle line is the calculated brightness due to reduced chlorate formation) ;

Figure 14 is a graph of chlorate formation versus D- brightness for the conventional one-step bleaching process and the novel two-step high/low pH bleaching process of the present invention; and

Figure 15 is a schematic representation of two (2) different process systems for a wood pulp bleaching plant for incorporating the two-step high/low pH bleaching process of the present invention. Best Mode for Carrying Out the Invention

Chlorine dioxide bleaching of kraft pulps is typically carried out at an end pH of 3.8 for 3 hours at 70° centigrade. It is commonly known that pH has an important bearing on brightness and strength properties as well as the chemical species present in the mixture. As shown in Figure 1 of the drawings, the formation of chlorate increases as the pH of the solution is decreased. Below pH 5 a major loss of oxidizing power occurs since the chlorate formed is inactive as a bleaching agent. Conversely, as the pH is increased, the conversion of chlorine dioxide to the chlorite anion is increased which is also inactive toward lignin. The sum of chlorite plus chlorate is lowest at end pH 3.8 which is found to be optimal for chlorine dioxide bleaching. However, formation of chlorite is not actually lost oxidizing capability since acidifying the chlorite solution forms chlorous acid which is known to be very reactive toward lignin.

In order to increase the efficiency of chlorine dioxide bleaching, a new two-step process has been discovered. The process is as follows:

1. Pulp is mixed with sodium hydroxide and subsequently mixed with chlorine dioxide in a conventional manner. The pH is maintained between about 6 and 7.5 for optimum brightness and viscosity although beneficial results are also found in a pH range of about 5-10. Reaction time is varied between about 5-40 minutes, and the reaction temperature is between about 55-85° centigrade, most suitably about 70° centigrade.

2. After the initial bleaching step, the pulp mixture is acidified to an optimum end pH of 3.8 with sulfuric acid, hydrochloric acid, or other suitable acid. Although a pH of 3.8 is optimal for brightness, end pH values of 1.9-4.2 have been recorded with substantial brightness gains over conventional bleaching methods. Final consistency of the pulp is between about 3-12%, most suitably about 10%, and reaction time in this second step is 2 or more hours, most suitably between about 2.5 and 3.9 hours. Reaction temperature is between about 55- 85° centigrade, and most suitably about 70° centigrade.

To prove the efficacy of the new process generally described above, detailed bleaching experiments were carried out by applicant on southern pine kraft pulp. The furnish was obtained from the decker before the bleach plant, and to insure maximum mixing CD stage bleaching was done in plastic Nalgene bottles which rolled on a ball-mill type apparatus for the full reaction time. All other bleaching stages were carried out in sealed polyester bags which were kneaded at various times throughout the bleach to insure proper mixing. Processing parameters used by applicants for the multiple bleaching stages are listed in Table 1 below. Chlorination stage charges were varied to achieve target (CD)E kappa numbers, and all charges are on OD brownstock pulp. Optimum high/low pH values are 6-7.5 and 3.8, respectively. Large batches of (CD)E pulp were made and then divided into individual DED runs for comparison. All comparisons were made on pulps from the same (CD)E batch, and all water used in bleaching and washing was distilled. Chlorine dioxide solutions used in testing were generated on site by acidifying sodium chlorite solution and absorbing the C10₂ gas in cold distilled water. Chlorine content in the solutions was kept between 7 and 10% (active basis) .

Processing parameters for the bleaching experiments and the analytical methods used in the experiments are as follows:

TABLE 1

Stage Charge Time Temperature Consistency End pH

CD .17-.22 x Kappa 1 hour 30-40^υ C 3% <1. 8 % Available Chlorine on Pulp (10% C10₂ Substitution)

E, 0.7 x Cl₂

% Of Caustic on Pulp

D 1 varied H/L D varied

E„ 0. 75% . varied

BRIGHTNESS Elrepho 2000 ISO VISCOSITY TAPPI T230 os-76 KAPPA NUMBER TAPPI T236 hm-85 TOC1 (AOX) EPA method 9020 CHLORATE Ion Chromatography

Brightness

On the basis of the results achieved in the bleaching tests, a substantial increase in brightness is always found using the high/low pH bleaching method as compared to conventional bleaching methods. As seen in Figure 2, the D, brightnesses achieved were higher than those of the present C10₂ bleaching techniques. In Figure 2, high pH values are between 8 and 9.5, and low pH values are from 1.9-2.1. The control had end pH values of 3.3 to 3.7. At a brightness level of 76 ISO, a charge of 0.9% C10_ on OD pulp was needed for conventional bleaching while only 0.68% was needed using the high/low bleaching method. This accounts for a 24% savings in chlorine dioxide. In 5-stage (CD) (EO)DED bleaching, however, the effect of the brightness gain is reduced in the final bleaching stage (D₂) . This is shown in Figure 3, where the pulps of Figure 2 are further bleached in the E₂ and D₂ stages wherein the D₂ stages are run conventionally. After the final bleaching stage, a 15% savings in chlorine dioxide is realized at a brightness of 88.3 ISO.

Regardless of the incoming (CD)E kappa number (lignin concentration) , chlorine dioxide savings are always found using the high/low pH bleaching process. This is illustrated in Figure 4 for pulp with a (CD)E kappa number of 8.5. Again a savings of approximately 0.2% C10₂ on pulp is realized in the D, stage, and the agnitude of savings is lower at a comparable D₂ brightness. Thus, even at high (CD)E kappa values, substantial reductions in chlorine dioxide use are realized by the bleaching method of the invention.

Similar brightness ceilings are reached in the D-. stage irrespective of which method of bleaching is used. This occurs around 84.0 ISO for both methods for an incoming (CD)E kappa of 4.4 (see Figure 5). In Figure 6, D₂ pulp from Figure 5 was found to have an 11% savings in chlorine dioxide even at a very high brightness of 90.5

ISO, but eventually the ceiling is reached at 91.4 ISO at a total charge of 1.2% C10₂ on OD pulp.

One of the major applications of the novel high/low pH bleaching process is in a three stage sequence (see Figure 6(a)). Current trends toward reducing operational and capital costs of pulp mills have led to the development of short sequence technologies in the pulp and paper industry. The major three-stage sequences are (CD) (EO)D and (CD) (EOP)D, and with high/low pH bleaching it is possible to decrease chlorine dioxide usage by as much as 29% in these processes.

Pulp Viscosity

Pulp viscosity measurements were made using TAPPI standard T 230 os-76. Earlier experimental work has indicated that chlorine dioxide at a pH of less than 5 reacts selectively with lignin, and at a pH greater than 7 chlorine dioxide reacts with the carbohydrate and lignin in the pulp vigorously, which in turn degrades the cellulose chain. As shown in Figure 7, pulp viscosity depends heavily on the pH of the reacting mixture. Pulp viscosity decreases slowly from pH 6 to 7, then falls rapidly at pH values higher than 7. The decrease in viscosity at the high pH for the two-step high/low pH bleaching process is not significant because of the low reaction time in the high pH step. From viscosity and brightness data obtained, a pH of 6-7.5 and a pH of 3.8 is optimal for the high pH and low pH, respectively, in the two-step high/low pH bleaching process.

Table 2 below gives an example of pulp qualities measured from a bleach run performed on a pulp of (CD)E kappa = 4.4 and viscosity 25 cp. An average viscosity drop of 0.6 centipoise was detected for the two-step high/low pH bleaching process as compared to conventional bleaching results. Other bleach runs performed showed a similar effect.

HIGH/LOW pH BLEACHING

Charge High pH Low pH CED Bright (CD)EDED Viscosity ISO Bright ISO CP

3.8 67.5 87.6 24.5

3.7 78.5 89.9 24.1

3.2 82.2 90.9 24.0 3.0 84.5 91.4 24.1

Total Organic Chlorine (TOCl) or (AOX)

TOCl (AOX) measurements in applicant's tests were made on both the D, and E₂ for one data set. The values were added together and are shown in Figure 8 of the drawings. Surprisingly, conventional bleaching TOCl values were parabolic versus an increasing C10₂ charge while TOCl values with the high/low pH bleaching method varied only slightly. A greater decrease in TOCl from bleaching with the two-step high/low pH bleaching process can be realized by substituting the chlorine dioxide saved in the D.. stage back into the chlorination stage (CD) of the multi-stage bleach sequence. This would result in a decrease in TOCl (AOX) in effluents from the bleach plant.

Chlorate

Chlorate (CIO ^') is a well known herbicide, and discharge of chlorate from paper mills has been gaining more attention from environmentalists now that possible detrimental effects on various microalgaes have been observed. Thus, improving the efficiency of chlorine dioxide bleaching by lowering chlorate production may have a favorable impact on both economic and environmental issues. Conversion of chlorine dioxide to chlorate can be lowered by the two-stage high/low pH bleaching method for most chemical charges on pulp. At very high chemical charges (or lower lignin concentrations) , chlorate formation is independent of whether the new or conventional bleaching method is used, because a brightness ceiling is reached.

Thus, it is important to determine if the chlorine dioxide saved using the two-step high/low pH bleaching process is due to a subsequent decrease in the formation of chlorate. The two possible pathways of forming chlorate are set forth in Equations 1 and 2 below:

Equation 1 2C10₂ + 20H~ > C10₃~ + C10₂ ^" + H₂0

Equation 2 2HCK>₂ > H⁺ + HCIO + C10₃ ^~

Equation 1 is not a very prominent reaction in bleaching carried out at pH 7 since only a small concentration of hydroxyl ions are present. Under typical bleaching conditions, the pH starts around 5 and drops to less than 4 by the end of the bleaching process. At pH 5, less than 1% hydroxyl ions would be present for reaction, and at pH 4 only 0.1% exist. Supporting evidence for this observation is shown in Figure 9 of the drawings. The trend indicated shows that as the pH is increased up to 9, the formation of chlorate decreases. The major pathway for chlorate formation is Equation 2 above. In principle, chlorous acid reacts with itself to form chlorate and hypochlorous acid. This is a biomolecular reaction which is considered to be slow at low concentrations. Chlorous acid, as stated above, is very reactive toward lignin. Chlorous acid oxidizes lignin and is reduced to hypochlorous acid according to Equation 3:

Equation 3 HC10₂ + LIGNIN > HCIO + OXIDIZED LIGNIN

During chlorine dioxide bleaching, a competitive pathway is present for consumption of chlorous acid. A high chemical charge would increase the rate of reaction of Equation 2, and a high lignin concentration would increase the rate of reaction of Equation 3. Figure 10 shows a plot of D. charge of chlorine dioxide versus % chlorine dioxide converted to chlorate for conventional chlorine dioxide bleaching. As the lignin concentration is increased (low chemical charge or higher kappa number) less chlorate is formed. Likewise if a high concentration of chemical is present (low kappa number) , the higher the formation of chlorate. The same trend also holds true for the two-step high/low pH bleaching process as can be seen in Figure 11. From Figures 10 and 11, it is evident that the two-step high/low pH bleaching process significantly lowers chlorate formation at most chemical charges. However, little difference is seen at high charges where the brightness ceiling is reached. Corresponding chlorate measurements for the brightness shown in Figure 5 are plotted on Figure 12. Again, as the charge is increased, the formation of chlorate rises. In order to determine the chlorine dioxide savings in terms of chlorate reduction, the chlorate measurements are expressed as available chlorine. At a brightness of 78.3 ISO, the high/low pH bleaching process and conventional bleaching required 0.6% and 0.8% C10₂ on pulp, respectively. These charges correspond to 1753 parts per million (ppm) and 2338 ppm, respectively, as available chlorine. The difference provides a savings of 585 ppm available chlorine. Chlorate measurements were found to be 351 ppm and 423.3 ppm as available chlorine for the high/low pH bleaching process and normal bleaching, respectively, at a charge of 0.6% on pulp for a 17% reduction. Subtraction yields a savings of 72.3 ppm available chlorine, which corresponds to only 17% of the total savings realized of 423.3 ppm. Figure 13 of the drawings demonstrates this effect by replotting Figure 6 with the calculated savings due to chlorate reduction. It is apparent that a decrease of chlorate is not sufficient to explain the total C10₂ savings. A change in lignin structure and/or greater solubilization of the lignin may be possible explanations for the total savings in the C10₂ observed in the tests. A larger reduction in chlorate is realized at a comparable D, brightness. As shown in Figure 14, it is possible to reduce chlorate by as much as 45% (at 78.3 ISO) using the two-step high/low pH bleaching process as compared to a conventional C10₂ bleaching stage.

Chlorate formation in the D₂ stage is identical for either bleaching process since they are carried out identically.

Process Apparatus

The two-step high/low pH bleaching process can be implemented in both a new plant or an existing pulp bleaching plant. The optimum design schematic is shown in Figure 15, where C10₂ and caustic are added to the first mixer. The pulp flows into a J or U tube (Figure 15A) or upflow tower (Figure 15B) with a retention time of approximately 5-40 minutes. A second mixer is provided to mix the acid for pH adjustment of the wood pulp. The pulp can then be discharged directly to a downflow tower. The retention time in the downflow tower is 2 or more hours and most suitably between about 2.5- 3.9 hours. In an existing bleach plant the simplest method for implementing the two-step high/low pH bleaching process technology would be to install a mixer on the discharge from the upflow leg of the tower to the downflow leg of the tower. Typical chemical charges for conventional bleaching process and high/low pH bleaching process stages are listed in Table 3 below. The chlorine dioxide savings is 4 lb/ton, while the caustic and the acid charge increase by 3 lb/ton and 3.6 lb/ton, respectively.

The following conclusions can be drawn about the novel 2-step high/low pH bleaching process described herein from the bleaching of mill southern pine kraft pulps:

1. The high/low pH bleaching process reduces chlorine dioxide usage by as much as 24% in the D.. stage;

2. The formation of chlorinated organic material characterized by TOCl can be decreased by the use of the high/low pH bleaching process if the C10_ saved is substituted into the CD stage;

3. The formation of chlorate is decreased by as much as 45% in the D. stage using the high/low pH bleaching process at a target D. brightness;

4. The high/low pH bleaching process can be easily implemented in either a new mill or an existing mill; and

5. The formation of chlorate at acidic bleaching conditions is due to the biomolecular reaction of chlorous acid with itself. Formation of chlorate can be reduced by lower bleach chemical charges or higher kappa number pulps.

It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.

Claims

Claims What is claimed is:

1. A bleaching process for bleaching wood pulp in an aqueous suspension using chlorine dioxide, comprising the steps of:

subjecting said aqueous wood pulp suspension to a first bleaching step by mixing it with of chlorine dioxide and maintaining the mixture at a pH between about 5- 10 for about 5-40 minutes;

decreasing the pH to a pH between about 1.9-4.2;

subjecting said mixture to a second bleaching step at the reduced pH for about 2 or more hours.

2. A bleaching process according to claim 1, wherein caustic is also mixed with said aqueous wood pulp suspension during said first bleaching step.

3. A bleaching process according to claim 2 wherein said caustic comprises sodium hydroxide.

4. A bleaching process according to claim 1 wherein the pH of the mixture during said first bleaching step is between about 6-7.5.

5. A bleaching process according to claim 1 wherein the temperature during said first bleaching step is between about 50-85 centigrade.

6. A bleaching process according to claim 5 wherein the temperature during said first bleaching step is about 70° centigrade.

7. A bleaching process according to claim 1 wherein the pH is decreased with an acid.

8. A bleaching process according to claim 1 wherein the pH of the mixture during said second bleaching step is about 3.8.

9. A bleaching process according to claim 1 wherein the temperature during said second bleaching step is between about 55-85° centigrade.

10. A bleaching process according to claim 9 wherein the temperature during said second bleaching step is about 70° centigrade.

11. A bleaching process according to claim 1 wherein the final consistency of the mixture after the second bleaching step is between about 3-12%.

12. A bleaching process according to claim 11 wherein the final consistency of the mixture after the second bleaching step is about 10%

13. A bleaching process according to claim 1 wherein said bleaching process is used in a three stage sequence process.

14. A bleaching process according to claim 1 wherein said bleaching process is used as the first bleaching stage in a five stage sequence process.

15. A bleaching process according to claim 14 wherein the five stage sequence process is a (CD) (EO)DED process and the first bleaching stage is followed by conventional extraction and bleaching stages.

16. A bleaching process for bleaching wood pulp in an aqueous suspension using chlorine dioxide, comprising the steps of:

subjecting said aqueous wood pulp suspension to a first bleaching step by mixing it with caustic and chlorine dioxide and maintaining the mixture at a pH between about 6-7.5 and a temperature of about 70° centigrade for about 5-40 minutes;

introducing an acid into said mixture so as to bring the pH down to about 3.8;

subjecting said mixture to a second bleaching step at a temperature of about 70° centigrade for 2.5-3.9 hours.

17. A bleaching process according to claim 16 wherein said caustic comprises sodium hydroxide.

18. A bleaching process according to claim 16 wherein said acid is selected from the group consisting of sulfuric acid and hydrochloric acid.

19. A bleaching process according to claim 16 wherein the final consistency of the mixture after the second bleaching step is about 10%.

20. A bleaching process according to claim 16 wherein said bleaching process is used in a three stage sequence process.

21. A bleaching process according to claim 16 wherein said bleaching process is used as the D, bleaching stage in a five stage sequence process.

22. A bleaching process according to claim 21 wherein the five stage sequence process is a (CD) (EO)DED process and the D, bleaching stage is followed by conventional extraction (E₂) and bleaching (D₂) stages.