US3884752A

US3884752A - Single vessel wood pulp bleaching with chlorine dioxide followed by sodium hypochlorite or alkaline extraction

Info

Publication number: US3884752A
Application number: US398165A
Authority: US
Inventors: Robert T Campbell; Laurence B Ritter; Jr Joseph G Land
Original assignee: International Paper Co
Current assignee: International Paper Co
Priority date: 1973-09-17
Filing date: 1973-09-17
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20
Also published as: DE2407542A1; SE7400192L; LU69421A1; FR2245817B1; FR2245817A1; NO740054L; BE810468A; IT1004819B; FI105174A; JPS5058302A; CA1019906A; SE408069B; BR7407461D0

Abstract

A wood pulp bleaching process wherein an alkaline extraction treatment or a sodium hypochlorite treatment is conducted after a chlorine dioxide bleaching treatment in the same tower as the chlorine dioxide bleaching treatment.

Description

United States Patent 1191 Campbell et al.

International Paper Company, New York, NY.

Filed: Sept. 17, 1973 Appl. No.3 398,165

Assignee:

U.S. Cl 162/89; 162/88 Int. Cl. D2Ic 9/14 Field of Search 162/89, 88, 87

References Cited UNITED STATES PATENTS 5/1935 Rue 162/89 2/1936 John et al. 162/88 X 1451 May 20, 1975 2,477,631 8/1949 Levy et al. 162/87 2,558,054 6/1951 Martin et al.. 162/89 2,587,064 2/1952 Rapson 162/88 X 3,120,424 2/1964 Ruedi 162/87 X 3,294,624 12/1966 Sloman 1 162/88 X 3,313,678 4/1967 Rydin 1 162/88 X 3,433,702 3/1969 Jack et al 162/88 X 3,501,374 3/1970 Jack et a1 162/89 3,595,743 7/1971 Sepall 162/89 3,622,444 11/1971 Andrews v 162/89 X 3,630,828 12/1971 Liebergott et al. 162/88 X Primary Examiner-S. Leon Bashore Assistant ExaminerArthur L. Corbin Attorney, Agent, or Firm-Alfred L. Michaelsen [57] ABSTRACT A wood pulp bleaching process wherein an alkaline extraction treatment or a sodium hypochlorite treatment is conducted after a chlorine dioxide bleaching treatment in the same tower as the chlorine dioxide bleaching treatment.

31 Claims, 3 Drawing Figures PATENTEDHAYZOIHYS 3,884,752

SHEET 2 OF 2 SINGLE VESSEL WOOD PULP BLEACHING WITH CHLORINE DIOXIDE FOLLOWED BY SODIUM HYPOCI-ILORITE OR ALKALINE EXTRACTION BACKGROUND OF THE INVENTION FIELD TO WHICH THE INVENTION PERTAINS The overall process of manufacturing paper can be divided into two major segments, viz. the process of transforming wood into pulp and, subsequently, the

process of transforming the pulp into paper. The process disclosed herein generally relates to the manufacture of wood pulp and more particularly, to a specific process used in the manufacture of wood pulp, viz. the bleaching of wood pulp.

The manufacture of wood pulp basically entails the alteration of the chemical constituents of wood. That is to say, wood is primarily comprised of cellulose fibers and a complex chemical generically referred to as lig nin. From the point of view of providing a wood pulp useful in the manufacture of paper, the pulp maker desires to retain the cellulose fibers and either dispose of the lignin initially combined therewith or alter its chemical form so as to yield desired properties. Thus, the initial steps in the production of wood pulp usually comprise the reduction of the wood to chips and the subsequent chemical treatment of the chips thus formed.

In more detail, after the wood has been reduced to chips in order to optimize the exposed surface area, the chips are loaded into a vessel and mixed with various chemicals, usually under conditions of elevated pressure and temperature, whereby the chemicals act upon the lignin and dissolve a portion thereof. In the paper industry, this process of initial delignification is referred to as digesting.

After the pulp has been cooked or digested, the resulting material is generally a dark colored cellulose fiber. The dark color is attributable to the fact that not all of the lignin has been removed in the digesting process and, as a result of the digestion, the residual lignin is darker than in its natural state, The pulp thus provided is referred to as unbleached pulp and may pass directly to the paper making operation for use in the manufacture of paper which does not have to be white or colored other than the dark color of the pulp, e.g. Kraft paper for paperbags, paper for use in manufacturing corrugated board, etc.

Obviously, many types of paper require that the pulp which forms the paper be essentially white. Thus, there arises a need to transform the unbleached of dark colored pulp into a bleached or essentially white pulp. The process of bleaching pulp has been practiced for a number of years and as a result of scientific investigation a number of bleaching processes have been developed. While it is instructive to consider a number of these processes, it may generally be observed that the overall objective of all of the processes is to either l achieve further delignification of the pulp, or (2) chemically alter the lignin included in the pulp so as to provide the desired optical qualities. Moreover, it must be appreciated that these objectives can be deemed to be successfully achieved only if the achievement thereof does not result in an excessive concomitant attack upon the cellulose fibers. In other words, since the overall objective of the pulping process is to provide cellulose fibers, a bleaching process which improves the optical properties of the pulp but severely attacks the fibers (fiber degradation) would be unacceptable. Of course, in addition to the constraint that any successful bleaching process should not result in excessive fiber degradation, the cost thereof should be at a minimum for the corresponding improvement in optical qualities.

With respect to optical qualities, the common measure or index thereof is brightness and, more specifically, the so-called GE brightness number. As a measure of relative brightness, pulp supplied directly from a Kraft digestion process, after washing, commonly has a GE brightness of 15 to 30 whereas highly bleached pulp has a GE brightness in the range of to 90.

Considering briefly some of the more common bleaching processes which have been developed, one of the oldest such process which is still in common use is chlorination. In modern pulping practice, chlorination is commonly the first step in multiple-step bleaching processes. In the chlorination process, the unbleached pulp is mixed with a chlorine and water combination which has the effect of chlorinating, oxidizing, or otherwise solubilizing the lignin either in the acid chlorination system or a subsequent alkaline extraction process.

Alkaline extraction is a process which is commonly used in the bleaching of pulp and most commonly is used as a second step following an initial treatment such as chlorination. Briefly stated, in an alkaline extraction process the pretreated pulp is contacted with a basic solution which is commonly a sodium hydroxide solution. The effect of this operation is the further removal of lignin. Thus, although alkaline extraction may be used as a first step in a bleaching process, it is most expeditiously used subsequent to a step such as chlorination since the chlorination step causes the lignin to be particularly soluble in an alkaline solution. More over, the alkaline extraction step removes a significant amount of material which would impede the bleaching action of subsequent steps, e.g. subsequent oxidative bleaching steps.

A third bleaching process is referred to as hypochlorite bleaching. This bleaching technique is one of the oldest forms of chemical bleaching of pulp and is attractive to the manufacturer of pulp because the associated chemicals are effective and economical. Although in earlier applications hypochlorite bleaching was used alone, the more common practice today is to use it after chlorination and alkaline extraction. Hypochlorite bleaching is particularly useful with pulp that contains a significant amount of lignin since, at the start of the hypochlorite stage, the hypochlorite will react faster with lignin than the cellulose and normally most of the hypochlorite will be consumed before cellulose degradation commences to a substantial degree. Typical hypochlorites utilized are calcium hypochlorite and sodium hypochlorite.

One of the most significant pulp bleaching processes presently employed is chlorine dioxide bleaching. This bleaching process is a relatively recent developmentit was not practiced to any significant commercial extent prior to approximately 25 years ago. Today, its use is widespread. The desirability of this bleaching process resides in the fact that (1) it will bleach almost any type of pulp to a high brightness with good color stability (little brightness reversion); the bleaching action is achieved withinsignificant damage to the pulp strength; and several methods of manufacturing chlorine dioxide within a pulp mill have been developed.

ln chlorine dioxide bleaching, chlorine dioxide is first generated as a gas and is then absorbed in water. The resulting solution is then brought in contact with the pulp resulting in the oxidation and chlorination of lignin into water soluble compounds. In some instances, chlorine dioxide is used together with chlorine as a first bleaching step.

Although chlorine dioxide bleaching can be used as the sole step in a bleaching operation, it is more commonly used in combination with other bleaching steps such as described above. For example, it is common practice to have an alkaline extraction or hypochlorite treatment follow a chlorine dioxide treatment.

Since bleaching operations as commonly practiced today are usually multiple step processes wherein more than one type of bleaching step is employed, the industry has developed a shorthand nomenclature for designating particular sequences of bleaching steps. Thus, a chlorination step is designated by the letter C, an alkaline extraction step is designated by the letter E, a hypochlorite bleaching step as H, a chlorine dioxide bleaching step as D and a combined chlorine and chlorine dioxide treatment as C Using this nomenclature, a typical bleaching sequence would be designated as CEDED which would indicate a bleaching sequence comprised of chlorination, alkaline extraction, chlorine dioxide bleaching, alkaline extraction and chlorine dioxide bleaching stages.

An improved pulp bleaching process employing chlorine dioxide bleaching and alkaline extraction or hypochlorite treatment is the field to which this invention pertains.

PRIOR ART As previously indicated, modern bleaching practice generally employs a multi-step process, for example processes such as C EDED, CHDED, CEDHD or CEDED. Referring to FIG. 1, the DED portion of a bleaching process is schematically shown. As shown in FIG. 1, pulp from a previous alkaline extraction or hypochlorite stage is provided to a thick stock pump 11. The pulp handled by the thick stock pump is in the form of an aqueous suspension or slurry and typically has a pulp concentration (consistency) in the range of 8 to 14 percent. When the pulp slurry is discharged from the thick stock pump 11, an aqueous chlorine dioxide solution is added thereto and mixed with the pulp slurry in the pulp mixer 12. Upon discharging from the pulp mixer 12, the pulp slurry-chlorine dioxide mixture enters a first chlorine dioxide bleaching apparatus or unit generally indicated as 20. The unit 20 is representative of modern, chlorine dioxide bleaching units and includes an upflow, preretention tube 13 and a downflow tower 14. Further, the downflow tower 14 is generally divided into two sections, an upper section 15 referred to as the retention zone and a lower section 16 referred to as the dilution zone. Chlorine dioxide bleaching towers such as shown in FIG. 1 are known to those skilled in the art to which this invention pertains and, as well, are described in existing literature, e.g. The Bleaching of Pulp, Tappi Monograph Series No. 27. However, the tower could be downflow without a preretention tube, or upflow without a preretention tube.

In accordance with prior art practice, the pulp supplied to the tower is subjected to a chlorine dioxide bleaching action while in the preretention tube 13 and in the retention zone 15 of the downflow tower 14 or in some other form of retention vessel. As the pulp slurry enters the dilution zone 16, water or other liquid is generally added thereto, as at 18, in order to reduce the consistency from approximately 8 to 14 percent to approximately 2 to 4 percent. One of the purposes of the consistency change achieved in the dilution zone is to permit the use of a fan type pump 17 in lieu of a more expensive thick stock pump.

In addition to adding water in order to achieve a dilution or consistency reduction in the dilution zone, it is common practice to also make a chemical addition at the beginning of the dilution zone. The objective of this chemical addition is to neutralize the pulp slurry by increasing the pH from 25 (which normally is present in the retention zone) to a pH in the range of approximately 6 to 8. The purpose of this pH change, which is normally achieved by the addition of sodium hydroxide with the dilution water, is to insure that any residual chlorine dioxide is converted to sodium chlorite. If residual chlorine dioxide were present in the pulp slurry leaving the chlorine dioxide tower, expensive corrosion resistant processing equipment would have to be utilized down stream therefrom. The prior art has recognized that the need for such expensive equipment can be avoided through the addition of sodium hydroxide (or other chemicals) which convert any residual chlorine dioxide into noncorrosive sodium chlorite. Alternatively, sulfur dioxide can be added to reduce chlorine compounds to the chloride form.

Upon exiting from the downflow portion of the chlorine dioxide tower 20, the low consistency (approximately 24 percent) pulp slurry is pumped to a washer 19, which is typically a drum type washer. Commonly, the filtrate from a down stream washer (e.g. the D filtrate) is used to wash the pulp. Additionally, sodium hydroxide is added to the pulp slurry as it exits from the washer 19. Sodium hydroxide is added at this point to raise the pH of the pulp slurry being discharged from the washer 19 to a pH in the range of approximately 10 to 13 which is required for the following alkaline extraction treatment. Thereafter the pulp slurry at a consistency of 8 to 14 percent and a pH of 10 to l3 is pumped to the extraction tower 22. In the lower part of the extraction tower 22, washer filtrate is again added to the pulp slurry as at 23 in order to once again reduce the consistency. Thereafter, the pulp slurry exits from the caustic extraction tower and is pumped by pump 24 to washer 25. After undergoing a wash and consistency reduction, the pulp is then supplied to thick stock pump 27. Thereafter, the pulp is subjected to a second chlorine dioxide treatment (the D stage), as indicated.

Another prior art, multi-step bleaching process which has received attention follows a CEDHD sequence and is described in US Pat. No. 3,595,743. In this process, a hypochlorite stage is substituted for the second caustic extraction stage shown in FIG. 1.

SUMMARY OF THE INVENTION In accordance with the invention disclosed herein, an alkaline extraction treatment or a hypochlorite treatment is conducted in the downstream portion of a chlorine dioxide bleaching apparatus. In this manner, a separate alkaline extraction unit or hypochlorite stage unit and a washer are not required. Additionally, the cost and operating expense of a washer and an alkaline extraction tower or a hypochlorite tower is avoided.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a DED. prior art bleaching process.

FIG. 2 is a schematic representation of the process disclosed herein.

FIG. 3 shows an apparatus arrangement for practicing the process of the instant invention.

DETAILED DESCRIPTION OF THE INVENTION Briefly stated, we have discovered that an alkaline extraction or hypochlorite treatment of wood pulp may be conducted in the downstream portion of a conventional chlorine dioxide bleaching apparatus.

Referring to FIG. 2, there is shown therein a schematic representation of different embodiments of our invention. Considering FIG. 2 in more detail, it may be noted that a number of process units shown therein are identical to corresponding units shown in FIG. 1. Thus, in FIG. 2, the thick stock pumps 11 and 27, the chlorine dioxide pulp mixers and the two chlorine dioxide towers 20 and 40 are the same as shown in FIG. 1.

In accordance with one embodiment of our invention and similar to prior art practice, sodium hydroxide is added to the pulp slurry at the beginning of the dilution zone 16. However, in this embodiment of our invention and contrary to prior art practice, sufficient alkaline material, for example sodium hydroxide, is added to raise the pH of the pulp slurry to a pH of at least about 10. Illustratively, the pH is raised to approximately to 13, and preferably within the range of, approximately, 10 to 12. Surprisingly, we have found that if the pH of the pulp slurry in the dilution zone of a conventional chlorine dioxide tower is thus adjusted, sufficient alkaline extraction treatment occurs in the dilution zone to obviate the need for a subsequent alkaline extraction tower.

In another embodiment of our invention, alkaline material, for example sodium hydroxide, is added at the top of the downflow portion of a conventional chlorine dioxide tower, i.e. at the beginning of the retention zone, as indicated by the dotted line representation 30. Once again, alkaline material is added in an amount sufficient to raise the pH of the pulp slurry to a pH in the range of, approximately, 10-13 and preferably, ap-

proximately 10-12. When alkaline material is added at the top of the downflow portion of the tower, a pulp mixer, not shown in the drawings, should be included at the top of the tower. Using this approach, the need for a subsequent alkaline extraction tower and an intermediate washer is obviated while nevertheless obtaining an overall bleaching action which is equivalent to the prior art practice. In either event, the chlorine dioxide bleaching apparatus is otherwise operated in accordance with prior art procedures, e.g. a dilution stream is added at the beginning of the dilution zone to reduce the consistency of the pulp slurry to approximately 2-4 percent. The consistency of the pulp slurry in the preretention tube 13 and the retention zone 15 of the downflow tower 14 may be maintained in the range of, approximately, 8 to 14 percent.

In accordance with still another embodiment of our invention, we have found that it is possible to achieve a hypochlorite bleaching action in the dilution zone of a conventional chlorine dioxide bleaching apparatus. When practicing this embodiment of our invention, an alkaline material, such as sodium hydroxide, is added to the pulp slurry entering the dilution zone 16 of the downflow tower l4. Sufficient alkaline material is added to raise the pH of the pulp slurry to a pH in the range of, approximately, 10 to 13 and, preferably, approximately lO-l2. Additionally, as suggested by the dotted line representation 31, sodium hypochlorite is also added to the wood pulp slurry at the top of the dilution zone 16. In this manner, a sufficient hypochlorite treatment occurs in the dilution zone 16 of the downflow tower 14 to achieve a DH" bleaching sequence and thus obviate the need for a subsequent hypochlorite tower. As those skilled in the art will appreciate. the precise amount of sodium hypochlorite which must be added to a wood pulp slurry in order to achieve hypochlorite bleaching action cannot be predicted since the required amount depends upon such diverse factors as the initial brightness of the pulp, the previous bleaching treatments to which the pulp has been subjected, the temperature of the pulp slurry in the hypochlorite stage and the desired final brightness. Thus, the amount of sodium hypochlorite added is generally empirically determined. Typically, however, the amount of sodium hypochlorite added is in the range of 0.1 to 0.6 percent and preferably 0.2 to 0.4 percent as available chlorine based on oven dry pulp.

The need for raising the pH of the pulp slurry when conducting a hypochlorite treatment as described above arises from the fact that the temperature of the pulp slurry in the downflow portion 14 of a conventional chlorine dioxide bleaching apparatus is generally approximately to F and, at these temperatures, a high pH is necessary in order to avoid fiber degradation during a hypochlorite treatment. With further regard to the question of fiber degradation, it is believed that a hypochlorite treatment cannot be commenced at the top of the downflow portion 14 unless temperature is significantly reduced because, in such event, an extended residence time would be encountered and the combination of the high temperature and the hypochlorite treatment would result in prohibitive attack upon the cellulose fibers. Thus, the relatively short residence time of the pulp in the dilution zone, for example approximately ten minutes, apparently permits a rapid hypochlorite bleaching to occur without a corresponding attack upon the cellulose fibers.

To determine the efficiency, from an optical brightness point of view, and the cost savings, if any, which result from conducting an alkaline extraction treatment in the dilution zone of a chlorine dioxide bleaching apparatus such as shown in FIG. 2 (a D/E treatment) or conducting a hypochlorite treatment in the dilution zone of a chlorine dioxide bleaching apparatus as shown in FIG. 2 (a D/H treatment), experiments were conducted using, as a starting material, pulp which had been subjected to a C E bleaching sequence. The results of these experiments are summarized in the following table:

Table 1 Variables Studied DED D/ED D/HD Chlorine dioxide (D or D" phase of D/E or D/H pH before chlorine dioxide addition 1 1.6 1 1.6 l 1.5 pH final 2.6 3.0 2.8 pH after neutralization 7.6 N.A. N.A. Caustic Extraction (E or E" phase of D/E or H phase of D/H Consistency, 92 4 4 Retention, min. 60 10 10 pH initial 11.6 10.8 10.8 pH final 11.4 10.6 10.8 Chlorine Dioxide (D pH before chlorine dioxide addition 1 1.1 10.8 1 1.0 pH final 4.9 4.1 3.8 pH after neutralization 8.5 7.4 7.0 Fully Bleached Pulp G.E. brightness, TAPPI, 7:

Initial 89.8 89.5 89.4 Reverted 84.2 85.0 83.6 Post Color Number 0.915 0.699 0.992 Total chemicals required, "/1

Chlorine dioxide 1.03 1.03 0.73 NaOH (includes amount combined with hypo) 1.56 1.32 1.49 NaOCl as available chlorine 0.00 0.00 0.25

The bleaching conditions employed in the various stages set forth above are as follows:

Considering the information set forth in Table 1 above, a number of interesting results may be noted, First, the initial brightness for the D/E and D/H treatments corresponds favorably to the initial brightness obtained from the conventional DED treatment. Similarly, the reverted brightness for the D/E and D/H treatments compared favorably with the reverted brightness of the pulp from the conventional DED treatment. Indeed, it may be noted that the pulp treated in the D/E process had a higher reverted brightness than the pulp treated in the conventional DED process. The Post Color Number indicated in Table l is an index of brightness stability wherein a lower Post Color Number indicates increased color stability.

Another result of interest which is reflected by Table l is the retention time of the pulp slurry for the E phase of the D/E treatment or the H phase of the D/H treatment. Thus, it may be noted that the retention time for either of these phases was ten minutes whereas the retention time for the E phase in the DED sequence was 60 minutes. It is apparent, therefore, that the process time associated with the D/ED or the D/HD sequence is significantly shorter than the process time for the conventional DED sequence although substantially thesame brightness is achieved in either event.

Table l reflects still another fact of considerable interest, viz. the reduced chemical use associated with either the D/ED sequence or the D/l-ID sequence compared to the DED sequence. As may be noted, the bleaching sequences in accordance with our invention should provide a reduced chemical operating cost.

Summarizing the more apparent facts reflected by the information in Table 1 above, the bleaching se quences which embody our invention provide pulp of a brightness equivalent to a conventional bleaching sequence yet a reduced process time and a reduced chemical operating cost attends the use of our invention. Of course, it is evident that the information presented in Table 1 above does not reflect the substantial reduction in initial equipment cost which would be realized if a bleach plant was initially constructed so as to take full advantage of our invention. For example, in such bleach plant, the initial capital cost thereof would be reduced by the cost of an alkaline extraction tower or hypochlorite tower and the washer which would normally be associated witheither such tower. Alternatively, in the event that our invention is practiced in conventional bleach plants, the operating expense associated with an alkaline extraction tower or a hypochlorite tower and the associated washer would be avoided. Additionally, as pointedout above, the practice of our process reduces the process time and the operating cost.

As previously indicated, the chlorine dioxide bleaching towers shown in FIGS. 1 and 2 are commonly used in the industry and include a preretention tube which may be of the upflow type. Although the use of a preretention tube is often desirable, the prior art has recognized that pulp may be subjected to a chlorine dioxide bleaching treatment in a tower which does not include a preretention tube and which may be either an upflow tower or a downflow tower. An upflow chlorine dioxide bleaching tower which does not include a preretention tube is shown in FIG. 3. As suggested by FIG. 3, a chlorine dioxide bleaching tower without a preretention tube may also be used to practice our invention.

Referring to FIG. 3, it will be seen that the tower 49 includes an upstream portion 50 and a downstream portion 52. By way of example, the tower 49 of FIG. 3 is shown as an upflow tower wherein a pulp slurry passes through a thick stock pump 11, is mixed with chlorine dioxide, passed through a mixer 12 and enters the tower 49 at the bottom. After entering the tower 49, the pulp-chlorine dioxide mixture passes up through the upstream portion or retention zone 50. Thereafter, the mixture passes through the downstream portion or dilution zone 52. In accordance with our invention, either an alkaline extraction or a hypochlorite treatment may be effected in the downstream portion 52 of the tower 49. Thus, to secure an alkaline extraction, sufficient aklaline material, e.g. sodium hydroxide, is added to the pulp entering the dilution zone to raise the pH of the pulp slurry to a pH in the range of approximately, 10 to 13 and preferably 10 to 12. If it is desired to perform a hypochlorite treatment, the pH of the pulp slurry is raised to a pH in the range of approximately lO to 13 or preferably ID to 12 and a sufficient amount of sodium hypochlorite is also added. In either event, a diluent, such as the filtrate from a washer, is also added in an amount sufficient to lower the consistency of the pulp slurry to, approximately, 2 to 4 percent. After passing through the downstream potion 52, wherein the residence portion may be approximately 10 minutes, the treated pulp exits from the tower 49 and may be supplied to other units for further processng.

While specific examples and embodiments of our invention have hereinbefore been disclosed, it will be appreciated that such disclosure is by way of example, and not by way of limitation. For example, Table I reflects an application of our invention with respect to a C EDED or a C,,EDHD bleaching process. It is evident, of course, that our invention would be applicable to any bleaching process which includes a DE, or a DH sequence. Thus, it will be understood that the scope of our invention is to be defined by the claims appended hereto.

We claim:

1. In combination with a conventional chlorine dioxide pulp bleaching process wherein chlorine dioxide is added to a wood pulp slurry in an amount such that substantially all of saidchlorine dioxide will be consumed in said conventional chlorine dioxide bleaching process, the improvement which comprises adding to said pulp, after substantially all of said chlorine dioxide has been consumed and while said pulp is in the same vessel in which said conventional chlorine dioxide bleaching process was conducted, and without draining said pulp, an alkaline material in an amount sufficient to raise the pH of said wood pulp to a pH in the range of 10 to 13.

2. The process of claim 1 which further includes the step of adding sodium hypochlorite to the pulp in said downstream portion in an amount sufficient to effect a hypochlorite treatment of said pulp.

3. The process of claim 2 wherein the amount of sodium hypochlorite is in the range of 0.1 percent to 0.6 percent as available chlorine based on oven dry pulp.

4. The process of claim 2 wherein the amount of sodium hypochlorite is in the range of 0.2 percent to 0.4 percent as available chlorine based on oven dry pulp.

5. The process of claim 2 wherein the pH of the pulp is raised by adding an alkaline material.

6. The process of claim 5 wherein said alkaline material is sodium hydroxide.

7. The process of claim 6 wherein chlorine dioxide treatment and said hypochlorite treatment are conducted as the third and fourth steps, respectively, in a CEDHD bleaching sequence.

8. The process of claim 1 wherein the step of raising the pH to approximately 10 to 13 effects an alkaline extraction in said downstream portion and said chlorine dioxide treatment and said alkaline extraction treatment are the third and fourth steps, respectively, in a CEDED bleaching sequence.

9. In the process of bleaching pulp wherein the pulp is sequentially subjected to a conventional chlorine dioxide treatment and then an alkaline extraction treatment, wherein in said chlorine dioxide treatment chlorine dioxide is added to said pulp in an amount such that substantially all of said chlorine dioxide is consumed in said chlorine dioxide treatment, the improvement which comprises sequentially conducting said conventional chlorine dioxide treatment and said alkaline extraction treatment in the same vessel and without draining the pulp between said treatments.

10. In the process of bleaching pulp wherein the pulp is sequentially subjected to a conventional chlorine dioxide treatment and then a hypochlorite treatment, wherein in said chlorine dioxide treatment chlorine dioxide is added to said pulp in an amount such that substantially all of said chlorine dioxide is consumed in said chlorine dioxide treatment, the improvement which comprises sequentially conducting said chlorine dioxide treatment and said hypochlorite treatment in the same vessel and without draining the pulp between said treatments.

11. In a pulp bleaching process which includes the steps of mixing a pulp slurry with chlorine dioxide in a conventional chlorine dioxide treatment, passing the pulp slurry-chlorine dioxide mixture through a bleaching tower having a preretention tube and downflow tower portion which includes a dilution zone and, without draining the pulp, subjecting the pulp slurry to an alkaline extraction treatment, the improvement which comprises conducting said alkaline extraction step in the downflow portion of said bleaching tower after substantially all of said chlorine dioxide has been consumed.

12. The process of claim 11 wherein said alkaline extraction step is commenced at the top of said downflow tower.

13. The process of claim 11 wherein said alkaline extraction step is commenced at the top of the dilution zone of said downflow tower.

14. In a pulp bleaching process which includes the steps of mixing a pulp slurry with chlorine dioxide in a conventional dioxide treatment, passing the pulp slurry-chlorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower portion which includes a dilution zone and, without draining the pulp, subjecting the pulp slurry to a hypochlorite treatment, the improvement which comprises conducting said hypochlorite treatment in the dilution zone of said bleaching tower after substantially all of said chlorine dioxide has been consumed.

15. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with a wood pulp slurry, passing the pulp slurrychlorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone, the improvement which comprises, without draining the pulp, the step of raising the pH of the plup slurry in the downflow portion of said tower to a pH in the range of approximately 10 to 13 whereby an alkaline extraction occurs in said downflow portion of said tower after substantially all of said chlorine dioxide has been consumed.

16. The improved bleaching process of claim 15 wherein the pH of said pulp slurry is raised by the addition of an alkaline material.

17. The process of claim 16 wherein said alkaline material is added at the top of said retention zone and further including the step of mixing said alkaline solution and said pulp.

18. The process of claim 16 wherein said alkaline material is added at the top of said dilution zone.

19. The process of claim 18 wherein the consistency of the wood pulp slurry in said preretention tube and said retention zone is approximately 8-14 percent and further including the step of reducing the consistency of the pulp slurry in said dilution zone to approximately 2-4 percent.

20. The process of claim 17 wherein said alkaline material is sodium hydroxide.

21. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with an aqueous wood pulp slurry, passing the pulp slurry-chlorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone, the improvement which comprises, without draining the pulp, the steps of raising the pH of the pulp slurry in the dilution zone of said tower to a pH in the range of approximately lO to 13 and adding to the wood pulp slurry sodium hypochlorite in an amount sufficient to effect a hypochlorite bleaching treatment of the pulp in said dilution zone after substantially all of said chlorine dioxide has been consumed.

22. The process of claim 21 wherein the amount of sodium hypochlorite added to the pulp on said dilution zone is in the range of 0.1 to 0.6 percent as available chlorine based on oven dry pulp.

23. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with a wood pulp slurry having a consistency in the range of approximately 8-14 percent, passing said pulp slurry-chorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone and reducing the consistency of said pulp slurry in said dilution zone to approximately 2-5 percent, the improvement which comprises, without draining the pulp, the step of adding sodium hydroxide to said pulp slurry in said downflow tower in an amount sufficient to raise the pH of said pulp slurry to approximately 10-1 2, said addition occurring after substantially all of said chlorine dioxide has been consumed.

24. The process of claim 23 wherein said sodium hydroxide is added at the top of said downflow tower.

25. The process of claim 24 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of, approximately, to F.

26. The process of claim 25 wherein said sodium hydroxide is added at the top of said dilution zone.

27. The process of claim 26 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of, approximately, 150 to 195F.

28. The process of claim 27 wherein the residence time of the pulp slurry in said retention zone is at least approximately 10 minutes.

29. The process of claim 26 which further includes the step of adding sodium hypochlorite to said pulp slurry at the top of said dilution zone in an amount between approximately 0.1 percent to 0.6 percent as available chlorine on oven dry pulp.

30. The process of claim 29 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of approximately, 150 to 195F.

31. The process of claim 30 wherein the residence time of the pulp slurry in said retention zone is at least approximately 10 minutes.

UNITED STATES PATENT OFFICE CERTIFIOATE OF CORRECTION PATENT NO. 3,884,752 0 DATED y 1975 |NVENTOR(S) Robert T. Campbell et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 9, line 7, "potion" should be --portion-- Q Col. 9, line 8, "portion" should be time-- Col. 9, line 11, "processng" should be processing Col. 10, line 36, after "conventional" insert chlorine Col. 11, line 25, "on" should be in Col. 11, line 33, "chorine" should be -chlorine- I a I Signed and Sealed this second Day Of September 1975 [SEAL] Attest: C

RUTH C. MASON C. MARSHALL DANN Arresting Officer ('mnmr'ssrmu'r of Patents and Trademarks f

Claims

1. In combination with a conventional chlorine dioxide pulp bleaching process wherein chlorine dioxide is added to a wood pulp slurry in an amount such that substantially all of said chlorine dioxide will be consumed in said conventional chlorine dioxide bleaching process, the improvement which comprises adding to said pulp, after substantially all of said chlorine dioxide has been consumed and while said pulp is in the same vessel in which said conventional chlorine dioxide bleaching process was conducted, and without draining said pulp, an alkaline material in an amount sufficient to raise the pH of said wood pulp to a pH in the range of 10 to 13.

6. The process of claim 5 wherein said alkaline material is sodium hydroxide.

10. IN THE PROCESS OF BLEACHING PULP WHEREIN THE PULP IS SEQUENTIALLY SUBJECTED TO A CONVENTIONAL CHLORINE DIOXIDE TREATMENT AND THEN A HYPOCHLORITE TREATMENT, WHEREIN SAID CHLORINE DIOXIDE TREATMENT CHLORINE DIOXIDE IS ADDED TO SAID PULP IN AN AMOUNT SUCH THAT SUBSTANTIALLY ALL OF SAID CHLORINE DIOXIDE IS CONSUMED IN SAID CHLORINE DIOXIDE TREATMENT, THE IMPROVEMENT WHICH COMPRISES SEQUENTIALLY CONDUCTING SAID CHLORINE DIOXIDE TREATMENT AND SAID HYPOCHLORITE TREATMENT IN THE SAME VESSEL AND WITHOUT DRAINING THE PULP BETWEEN SAID TREATMENTS.

15. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with a wood pulp slurry, passing the pulp slurry-chlorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone, the improvement which comprises, without draining the pulp, the step of raising the pH of the plup slurry in the downflow portion of said tower to a pH in the range of approximately 10 to 13 whereby an alkaline extraction occurs in said downflow portion of said tower after substantially all of said chlorine dioxide has been consumed.

20. The process of claim 17 wherein said alkaline material is sodium hydroxide.

21. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with an aqueous wood pulp slurry, passing the pulp slurry-chlorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone, the improvement which comprises, without draining the pulp, the steps of raising the pH of the pulp slurry in the dilution zone of said tower to a pH in the range of approximately 10 to 13 and adding to the wood pulp slurry sodium hypochlorite in an amount sufficient to effect a hypochlorite bleaching treatment of the pulp in said dilution zone after substantially all of said chlorine dioxide has been consumed.

23. In the process of bleaching wood pulp with chlorine dioxide in a conventional chlorine dioxide treatment which includes the steps of mixing chlorine dioxide with a wood pulp slurry having a consistency in the range of approximately 8-14 percent, passing said pulp slurry-chorine dioxide mixture through a bleaching tower having a preretention tube and a downflow tower which includes a retention zone and a dilution zone and reducing the consistency of said pulp slurry in said dilution zone to approximately 2-5 percent, the improvement which comprises, without draining the pulp, the step of adding sodium hydroxide to said pulp slurry in said downflow tower in an amount sufficient to raise the pH of said pulp slurry to approximately 10-12, said addition occurring after substantially all of said chlorine dioxide has been consumed.

25. The process of claim 24 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of, approximately, 150* to 195*F.

27. The process of claim 26 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of, approximately, 150* to 195*F.

30. The process of claim 29 wherein the pulp slurry in said downflow portion of said bleaching tower is at a temperature in the range of approximately, 150* to 195*F.