US3832276A

US3832276A - Delignification and bleaching of a cellulose pulp slurry with oxygen

Info

Publication number: US3832276A
Application number: US00338862A
Authority: US
Inventors: S Roymoulik; K Brown
Original assignee: International Paper Co
Current assignee: International Paper Co
Priority date: 1973-03-07
Filing date: 1973-03-07
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27
Also published as: FR2220620A1; CA1007807A; FI59434C; AR202919A1; JPS5747798B2; FR2220620B1; JPS5024503A; FI59434B; NO144930C; IT1011546B; GB1445276A; PH10177A; AU6602374A; NO144930B; DE2410934C2; NO740795L; ZA741287B; SE397983B; BR7401573D0; DE2410934A1

Abstract

COLLULOSE PULP IS CONTINUOUSLY BLEACHED AND DELIGNIFIED BY OXYGEN, WITHOUT REQUIRING THE USE OF A CELLULOSE DEPOLYMERIZATION PROTECTIVE AGENT, WHEREBY AN ALKALINE AQUEOUS PULP SLURRY OF A CONSISTENCY OF FROM ABOUT 2 TO 10%, HAVING A PH OF BETWEEN ABOUT 9 AND 14, A REACTION TEMPERATURE OF BETWEEN ABOUT 70* AND 120*C, WITH THE OXYGEN DISSOLVED AND INTIMATELY DISPERSED AND SUBDIVIDED INTO THE SLURRY SO THAT NO AGGLOMERATED BUBBLES ARE FORMED AND THE OXYGENATED PULP SLURRY HAS SUBSTANTIALLY NO BUBBLES EXCEEDING ABOUT 1/16 INCH IN DIAMETER, AND THEREAFTER CONTINUOUSLY INTRODUCING THE SLURRY INTO THE LOWER REGION OF A VERTICAL ELONGATED TOWER AND FLOWING THE SLURRY UPWARDLY THROUGH THE TOWER WITHOUT ANY SUBSTANTIAL AGITATION, SO AS TO GRADUALY DECREASE THE PRESSURE TO WHICH THE SLURRY IS OBJECTED AND CONTINUOUSLY WITHDRAWING TREATED SLURRY FROM THE UPPER REGION OF SAID TOWER.

Description

K. RoYMouLlK ETAL s. 3,8325276 DELIGNIFICATIQN AND BLEACHING 0F A cnLLuLosE PULP-f v Aug. 27, 1974 SLURRY WITH OXYGEN 3 shuts-Shut 1 Filed latch 7. 1973 Au 27, 1974 s. K. RoYMouLlK ETAL 3,832,27

DELIGHIFICTION AND BLECHING OF CELLULOSE PULP- SLUKRY WITH OXYGEN Filed latch '7. 1973 3 Sheets-Shut 2 TO FURTHER TREATMENT WASH T0 BROWN STOCK WASHERS FIG2.

NdOH

MAKE-UP AU!- 27 1974 s. K. RoYMoULlK HAL 3,832,276

DELIGNIFICATION AND BLEAGHING oF A csLLULosE PULP swam WITH OXYGEN Filed latch 7. 1973 5 Sheets-Sheet 3 'dUAilsoDsm FIGB.

United 'States Patent Otce 3,832,276 Patented Aug. 27, 1974 3,832,276 DELIGNIFICATION AND BLEACHING OF A CELLULOSE PULP SLURRY WITH OXYGEN Sunanda K. Roymoulik, Newburgh, and Kenton J. Brown,

Suffern, N.Y., assignors to International Paper Company, New York, N.Y.

Filed Mar. 7, 1973, Ser. No. 338,862 Int. Cl. D21c 9/10 U.S. Cl. 162--65 15 Claims ABSTRACT OF THE DISCLOSURE Cellulose pulp is continuously bleached and delignied by oxygen, without requiring the use of a cellulose depolymerization protective agent, whereby an alkaline aqueous pulp slurry of a consistency of from about 2 to 10%, having a pH of between about 9 and 14, a reaction temperature of between about 70 and 120 C., with the oxygen dissolved and intimately dispersed and subdivided into the slurry so that no agglomerated bubbles are formed and the oxygenated pulp slurry has substantially no bubbles exceeding about 1/16 inch in diameter, and thereafter continuously introducing the slurry into the lower region of a vertical elongated tower and owing the slurry upwardly through the tower without any substantial agitation, so as to gradually decrease the pressure to which the slurry is subjected and continuously withdrawing treated slurry from the upper region of said tower.

BACKGROUND OF THE INVENTION It has been conventional for many years to delignify and bleach wood pulp by the use of various chlorination procedures. These are sometimes referred to in the paper industry as the CDE stages in the 5-stage CDEDEDy or 6-stage CDEHDED sequences. The use of chlorine gas is not inexpensive and the removal of unused chlorine gas and the chlorine-containing by-products from the efuent streams requires expensive chemical recovery systems so as to abate stream and environmental pollution problems.

Over the years suggestions have been advanced to replace the conventional chlorine delignication and bleaching treatments by replacing the use of chlorine with oxygen. Although a number of processes for bleaching and delignifying pulp with oxygen have been proposed, such as Richter U.S. Pat. 1,860,432, Grangaard et al. U.S. Pats. 2,926,114 and 3,024,158, Gaschke et al. U.S. Pat. 3,274,049, Meylan et al. US. Pat. 3,384,533, Watanabe U.S. Pat. 3,251,730, Rerolle et al. U.S. Pat. 3,423,282, Farley U.S. Pat. 3,661,699, French Pats. 1,130,- 248 and 1,387,853 and articles by Nikitin et al. in Trudy Leningradshoi Lesotekb. Nickeskoi Akad. i.S.M. Korova (Transactions of Leningrad Academy of Forestry), Vol. 75, pp. 145-155 (1956), Vol. 801, pp. 65-75, 77-90 (1958) and Bumazh. Prom., Vol. 35, No. 12, pp. 57 (1960). None of these has been commercially practicable. Many of these processes require protective agents to prevent depolymerization of the cellulose, such as magnesium carbonate disclosed in Meylan U.S. Pat. 3,384,533, and preserve pulp viscosity. In addition to the imparting of scale and encrustation problems on the process equipment, the use of such chemicals has a serious disadvantage in that they present pollution abatement problems. If pollution is to be avoided, expensive recovery treatments must be employed to remove such protective agents from the effluent streams.

The only process for delignification or bleaching of wood pulp by oxygen which has ever been sufficiently satisfactory to consider for development of potentially commercial use is the type disclosed in Verreyne et al.

Pat. 3,660,225. That process, however, requires the use of high consistency pulps and high pressure procedures necessitating sophisticated and expensive high pressure apparatus, as well as dewatering and shredding equipment. Another disadvantage of this high pressure process, over and above its high capital expenditures for equipment, is that the process causes severe pulp (cellulose) degradation and this can only be abated somewhat by using cellulose depolymerization protectors, such as magnesium carbonate. As pointed out hereinabove, the use of cellulose depolymerization protectors increases the cost and requires expensive recovery procedures to remove these agents from the efuent streams.

It is, accordingly, an object of the present invention to provide a commercial low cost continuous process for the delignication and bleaching of wood pulp.

It is anther object of the present invention to provide a low-cost continuous process for delignication and bleaching wood pulp which does not require chlorination treatment.

It is a further object of the present invention to provide a process for delignication and bleaching of wood pulp which does not require the use of cellulose depolymerization protectors, such as magnesium carbonate.

It is an additional object of the present invention to provide a low-cost continuous process for delignication and bleaching of wood pulp by the use of oxygen which does not require high pressure equipment.

It is a further object of the present invention to provide a continuous process for delignication and bleaching of wood pulp, using oxygen, which can utilize standard, conventional bleaching towers.

Other objects will be apparent to those skilled in the art from the present specification, taken in conjunction with the appended drawings, in which:

FIG. 1 is a ow diagram illustrating one embodiment of the present invention; Y

FIG. 2 is a flow diagram illustrating another embodiment of the present invention;

lFIG. 3 is a graph in which viscosity of the bleached pulp is plotted versus its Kappa No. where pressure is held constant throughout delignification and bleaching with oxygen as against gradually reducing the pressure during the delignification and bleaching with oxygen, as in the present invention.

GENERAL DESCRIPTION OF THE PROCESS We have discovered that when employing an alkaline pulp slurry of low consistency, such as less than about 10% wood pulp, oxygen bleaching and delignication starting at mildly elevated pressures, such as less than about p.s.i.g., with the pulp containing no more than a minimal amount of undissolved oxygen, the pressure is gradually reduced, excellent bleaching and deligniiication characteristics, as represented by low Kappa numbers, can be obtained without serious loss in viscosity. This result was unexpected. Indeed, our professional associates did not believe that oxygen bleaching and delignication could be achieved to any satisfactory degree without the use of high pressures and a concomitant excessive loss in viscosity whichy would signify excessive depolymerization of the cellulose. This is achieved without the use of a cellulose depolymerization protective agent, such as magnesium carbonate. Thus the present invention provides several significant advantages:

l. Economical bleaching and delignifcation is achieved without the cellulose undergoing serious depolymerization or degradation. The process therefore produces a purer and more valuable cellulose for the paper industry and related industries.

2. High oxygen pressures and high pressure equipment are not necessary.

3. Present bleaching equipment can be employed satisfactorily.

4. The aqueous bleaching solution can be readily regenerated by adding additional sodium hydroxide or other alkaline agent and recycled. Consequently, unconsumed reagents are reused in this continuous process and ecological contamination is eliminated.

In accordance with the process of the invention, an alkaline aqueous pulp of low consistency, such as less than about by Weight of wood pulp, preferably between about 2 to 6%, and most desirably between about 3 and 4%, is employed. Sufficient alkali is introduced to elevate the pH of the pulp to between about 9 and 14, and preferably between about 11.5 and 12.5. When sodium hydroxide is employed, it is usually desirable that about 1 to 10 grams per liter are employed, or to constitute between about 0.1% and 1.0% by weight of the pulp slurry.

The alkaline pulp is desirably mixed with oxygen in a high-shear mixing device so that no large bubbles exceeding about 1/16 of an inch in diameter are present. Preferably, substantially no undissolved oxygen gas is present in the pulp. Ordinarily, oxygen is introduced in an amount of between about 0.1 and 4% by weight of aqueous pulp, with amounts of about 0.2 to 0.8% being preferred for softwood, and between about 0.2 and 0.4% by weight giving best results for hardwood pulp. Any undissolved bubbles of oxygen of substantial size are to be avoided, since they cause channeling to disrupt the upward flow of pulp through the bleaching tower, thereby causing non-uniform bleaching, which is highly undesirable. Also larger bubbles tend to agglomerate and this is to be avoided. Any undissolved bubbles should be so nely dispersed as to avoid any substantial agglomeration.

Any undissolved oxygen, such as bubbles exceeding about V16 inch in diameter, are vented from the system before the oxygenated pulp is introduced into the bleaching tower.

The distribution of oxygen through the pulp is desirably achieved through any high-speed, high-shear mixing device or gas absorber. Among such devices are the Lightening In-line mixer or Line-Blender of Mixing Equipment Co. Inc. However, any high-shear mixer may be employed.

Optionally, the alkaline aqueous pulp may be briefly subjected to a high pressure oxygen pretreatment before introducing the pulp into the bleaching tower. Momentary pressures of up to 300 p.s.i.g., or preferably 2 to 10 atmospheres, are desirable. This pretreatment is done briefly in small volume pressure apparatus which does not add substantially to the cost of the apparatus employed.

During the bleaching treatment it is desirable that the reaction temperature of the aqueous pulp be about 70 to 120 C., with about 90 to 110 C. preferred. Of course, where reaction temperatures substantially in excess of 100 C. are employed, some means of providing pressure are required. For this reason, maximum reaction temperatures to be employed will be somewhat dependent on the height of the bleaching tower or initial pressure employed. Desirably the temperature should not exceed the boiling point of the pulp slurry at the pressure involved.

During the bleaching operation the pressure on the aqueous alkaline pulp is gradually reduced by a diferential of at least about one atmosphere, with a maximum differential being about 10 atmospheres. This differential in pressure during the bleaching operation may be represented by the height of the bleaching tower, although any means for gradually and constantly reducing the pressure during treatment may be employed. Thus a 300 foot bleaching tower provides an initial pressure of about 135 p.s.i.g. and a 40 foot bleaching tower provides an initial pressure of about 17 p.s.i.g. It is desirable that a bleaching tower be employed which is not higher than about 300 feet with the minimum height being about 40 feet.

The residence time of the aqueous pulp in the bleaching tower may vary depending upon the pressure on the system and on the degree of bleaching required for the particular pulp employed. Some pulps require more drastic bleaching treatment than others, Generally speaking, from about 5 to 120 minutes is suflicient. With a higher initial pressure provided by a higher tower, the time can be reduced to a period of from about 1 minute to 60 minutes. With a 40 foot tower, providing a pressure differential of roughly about one atmosphere, about 30 to 60 minutes, preferably about 40 minutes is satisfactory.

One of the advantages of the present invention is that the bleaching and deligniiication process does not require any additional equipment. The bleaching tower 8 may be one of the pre-existing towers which have been conventional in the paper industry for ybleaching by chlorination. These towers are relatively inexpensive since they require no high pressure specifications and are therefore easily constructed.

Referring to FIG. 1 of the drawings, describing one form of apparatus and embodiment of the process, a pulp slurry of the desired consistency is produced by mixing in make-up tank 1 sufficient pulp, sodium hydroxide or other alkaline agent, such as ammonia, sodium carbonate, etc., and desirably, recycled bleach liquor transferred from stream 2. This recycled liquor is introduced so as to economically reuse the alkaline values of the bleach liquor. Pump 3 carries the alkaline pulp of desired consistency into oxygenator or mixer 4, which is a chamber having a high-speed, high-shear mixing device, such as a Lightnin Mixer, to incorporate and disperse oxygen into the alkaline pulp. Optionally, oxygen can rst be mixed with the warm alkaline spent liquor stream 2 as shown at 2a in FIG. 1 and recovered at the make-up tank. The oxygenated pulp is then carried to heat exchanger 5 where steam is employed to elevate the temperature to the desired value. The heated alkaline oxygenated pulp is then optionally subjected momentarily to a pre-pressurizing chamber 6 where the pressure is momentarily elevated in that chamber, using oxygen, for a brief period of time. Any undissolved, undispersed oxygen is then removed from the liquid by vent 7 and thereafter the vented oxygenated alkaline pulp is introduced into the bottom of bleaching tower 8. The flow of the alkaline oxygenated pulp is upward through the tower, as shown, with suflicient retention time to permit the desired bleaching and delignification to take place. Agitation of the pulp slurry in the tower is to be avoided. The initial pressure and differential in pressure during the bleaching treatment is determined by the height of the tower 8.

The effluent from the tower is then carried though pipe 9 to washer 10. The residual warm alkaline liquor recovered at the first washer is collected in container 11 and part of it is returned through pipe 2 to pulp make-up tank 1. Another part is returned to washer 10. The pulp from washer 10 is then carried through conduit 13 to second washer 14 where wash water is applied and the washed pulp is then carried to succeeding stages of bleaching, such as represented by chlorine dioxide treatments. The effluent is collected in container 15 from which a portion is used for brown stock washing and the remainder carried through conduit 16 .to be used in washing the pulp in iirst washer 10.

The embodiment of FIG. 2 and the apparatus described therein differs from that of FIG. 1 in that the pre-pressurizing chamber 6 of FIG. 1 is eliminated. The oxygenator 4 of FIG. 2 has an in-line Lightnin Mixer or other high-shear mixer having the facility of readily dispersing oxygen throughout the pulp.

One of the unexpected advantages of the present invention is shown by FIG. 3 of the appended drawings. That figure represents a graph plotting the pulp viscosity versus the Kappa Nos. for two runs, one of which employed constantly decreasing pressure during bleaching in accordance with the present invention (represented by the circled points) (representing a decline of pressure from 40 p.s.i.g. to zero p.s.i.g.) compared with constant pressure (represented by points encompassed by squares) in which the pressure was maintained constantly at 40 p.s.i.g. As shown in FIG. 3, the two curves are represented by straight lines.

Viscosity represents a measurement of the average degree of polymerization of the cellulose in the pulp sample, i.e., the average chain length of the cellulose. Thus, decreases in viscosity values represent the extent of depolymerization or degradation caused by the bleaching process. Excessive degradation is to be avoided since it provides undesirable physical properties in any paper made from t-he pulp.

Kappa No. is determined by the potassium permanganate consumed by a sample of pulp and represents a measurement of its retained lignin content. The higher the Kappa No., the less bleached and delignitied is the pulp. By comparing Kappa Nos. of samples before and after bleaching treatment, one can obtain an evaluation of the extent of delignication which has taken place.

As shown in FIG. 3, for the same Kappa No., oxygen bleaching with declining pressure gave a viscosity value which was consistently approximately 21/2 centipoises greater than the same pulp bleach at constant pressure. Thus, the process of the present invention, employing constantly decreasing pressure produces substantially less of the unwanted depolymerization of the cellulose. This shows that the process of the present invention produces substantially less deploymerization for the same degree of bleaching and delignification.

It has also been discovered that the present invention, employing gradual reduction in pressure during bleaching and delignification, produces less pulp shrinkage for the same degree of bleaching and delignilication than does oxygen bleaching conducted at constant pressure. This means that there is less damage to the cellulose and hemicellulose as represented by reduced loss of bleached pulp. This is a consequence of the fact that the process of the present invention is more selective in lignin removal than is oxygen bleaching conducted at constant pressure. As a consequence, the process of the present invention is more economical than a similar process conducted at constant pressure.

DETAILED DESCRIPTION OF THE INVENTION In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow, and throughout the specification, the quantities of material are expressed in terms of parts by weight, unless otherwise speciiied.

Example 1 Employing apparatus illusrtaated by FIG. 1 of the appended drawing, except that no high pressure pre-retention chamber 6 was employed, several pulp slurries having a consistency of 2 or 3% by weight of pulp, in which the pulp was slurried using 0.1 normal sodium hydroxide, were subjected to the process of the invention. The pulp slurry was heated to boiling. Oxygen gas was mixed with the heated slurry and pressurized to 40 p.s.i.g. and vigorosuly agitated in the oxygenator 4, equipped with a highshear propeller, during a residence time of about 5 minutes. Excess undissolved and undispersed oxygen was vented through the vent 7 and the oxygenated pulp slurry introduced near the bottom of bleaching tower 8, which was approximately 90 to 05 feet high. The slurry was permitted to rise without agitation so that it had a residence time in the tower of approximately 60 minutes. During its rise through the tower, the pressure head on the slurry decreased -from about 4'0 p.s.i.g. to atmospheric pressure. Throughout the passage of the slurry through the tower, no bubbles were seen to rise out the top of the slurry. The results are shown in Table 1 below:

The results shown in Table 1, above, demonstrate that the delignication and bleaching of a commercial hardwood kraft pulp with oxygen can be carried out in a simple tower, using low-consistency pulp without the use of a depolymerization protector, such as magnesium carbonate.

Example 2 Employing the apparatus system of FIG. 2, wood pulp was slurried with 0.1N sodium hydroxide to a pH of 11.5 and a consistency of approximately 3% and the slurry heated to a temperature of about C. Oxygen was mixed through oxygenator 4 equipped with Lightening -In-line propeller-equipped mixer Model No. 4LB`C operating at a velocity of 1700 r.p.m. to finely divide the oxygen in the slurried pulp. Residual undispersed and undissolved oxygen was vented at vent 7. The oxygenated alkaline pulp was then introduced near the bottom of bleaching tower 8, comprising a tower 84 feet tall providing a static pressure at the bottom of approximately 36 p.s.i.g. The slurry was permitted to ow slowly upward through the tower without any agitation to provide a retention time of approximately 42 minutes. Throughout the operation no bubbles of oxygen were seen to escape at the top of the slurry. The results are shown in Table 2 below:

Example 3 Apparatus illustrated by FIG. l of the appended drawings, both with and without the use of the chamber 6, was employed. A pulp slurry of 3% by weight of softwood paper pulp, in which the pulp was slurried using 0.1 normal sodium hydroxide, was subjected to the process of the invention. The pulp slurry was heated to boiling at atmospheric pressure. Oxygen gas was mixed with the heated slurry and pressurized to 40 p.s.i.g. and vigorously agitated in the oxygenator 4, equipped with a high-shear propellar, during a residence time of about 5 minutes. During part of the run in which the high pressure retention chamber 6 was in operation, the slurry was subjected to a pressure of oxygen of about 100 p.s.i.g. for a short time of one minute to 30 minutes. Excess undissolved and undispersed oxygen was vented through the vent 7. The oxygenated pulp slurry was then introduced into bleaching tower 8 which was approximately 90 to feet high. The slurry was permitted to rise lWithout agitation so that it had a residence time in the tower of approximately 60 minutes. During its rise through the tower the pressure head on the slurry decreased from 7 about 40 p.s.i.g. to atmospheric. No bubbles of oxygen were seen to rise out of the top of the slurry in the tower. No depolymerization protector was employed. The results are shown in Table 3 below:

Example 4 The procedure of Example 1 was repeated, employing a pulp slurry consistency of 3% by weight of pulp using two samples of hardwood kraft pulp and softwood kraft pulp, The results obtained are set forth in Table 4 below:

TABLE 4 Original unbleached Viscosity, Kappa Bright- 0.5 CED Pulp sample N o. ness (EL) (cp.)

Hardwood kraft:

Sample 1 13. 7 34. 0 25. 05 Sample 2 16. 2 34. 7 36. 90 Sotwood kraft:

Sample 1 31. 4 27. 7 35. R3 Sample 2 21. 5 29. 9 21. 74

Oxygen bleached Hardwood kraft:

Sample 1 8. 3 50. 5 15. 47 Sample 2 8.7 52.0 21.00 Softwood kraft:

Sample 1 15. 2 33. 2 '15. 00 Sample 2 11.4 38. 4 l2. 50

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modications are possible within the scope of the invention claimed.

What is claimed is:

1. A continuous method of bleaching and delignifying cellulose pulp which does not require the use of a cellulose depolymerization protective agent, which method comprises preparing a cellulose pulp slurry comprised of an alkaline aqueous suspension of a pulp having a reaction temperature of between about 70 and 120 C., a consistency of from about 2% to about 10% by weight, and having a pH of between about 9 and 14, dissolving and intimately dispersing oxygen into said slurry so that no agglomerated bubbles are formed and the oxygenated cellulose pulp slurry has substantially no bubbles exceeding about 1/16 inch in diameter, continuously introducing said slurry into a vessel, subjecting the slurry in said vessel to an initial pressure of less than about 135 p.s.i.g. and thereafter gradually reducing that pressure, without subjecting the slurry to any substantial agitation, so that there is a pressure gradient of between about 1 and 10 atmospheres between the initial pressure and nal pressure, and continuously withdrawin-g treated slurry from said vessel.

2. A continuous method of bleaching and delignifying cellulose pulp which does not require the use of a cellulose depolymerization protective agent, which method comprises preparing a cellulose pulp slurry comprised of an alkaline aqueous suspension of a pulp having a reaction temperature of between about 70 and 120 C., a consistency of from about 2% to about 10% by weight, and having a pH of between about 9 and 14, dissolving and intimately dispersing oxygen into said slurry so' that no agglomerated bubbles are formed and the'oxygenated cellulose pulp slurry has substantially no bubbles exceeding about 1/16 inch in diameter, continuously introducing said slurry into the lower region of a vertically elongated tower, subjecting the slurry in said tower to an initial pressure of less than about 135 p.s.i.g and flowing said slurry upwardly through said tower without any substantial agitation while maintaining a pressure gradient of 10 between 1 and l0 atmospheres between the point of introduction of said slurry into said tower and the point of withdrawal of said slurry from said tower, and continuously withdrawing treated slurry from the upper region of said tower.

l 3. A continuous method in accordance with claim 2, wherein substantially no undissolved oxygen is present in said slurry when introduced into said tower.

4. A continuous method in accordance with claim 1, wherein any undissolved oxygen is not of sufficient size to agglomerate.

5. A continuous method in accordance with claim 1, wherein the consistency of the pulp slurry is between about 2 and 6% by Weight.

6. A continuous method in accordance with Claim 1, whrein the pH of the pulp is between about 11.5 and 12.

7. A continuous method in accordance with Claim 1, wherein the amount of oxygen introduced is between about 0.1 and 4% by weight of the pulp slurry.

8. A continuous method in accordance with Claim 1,

wherein the temperature is between about 90 and 9. A continuous method in accordance with Claim 2, wherein the tower is not higher than about 300 feet and the oxygenated slurry is introduced near the bottom of said tower.

10. A continuous method in accordance with Claim 9, wherein said tower is at least about feet tall.

11. A continuous method in accordance with Claim 2, wherein the residence time of the slurry in said tower is between about 2 and 120 minutes.

12. A continuous method in accordance with Claim 2 wherein the residence time of the slurry in said tower is between 5 and 60 minutes.

13. A continuous method in accordance with Claim 1, Iwherein the oxygenated slurry is subjected to a pre-pressurizmg treatment of about two atmospheres up to about 300 p.s.i.g. for a period of about one to 30 minutes.

14. A continuous method in accordance with Claim 1 wherein the amount of oxygen introduced is between about 0.2 and 0.4% by weight of the pulp slurry.

15. A continuous method in accordance with Claim 1, wherein the oxygen is intimately dispersed and subdivided into said slurry by means of a high-shear agitator.

References Citedv UNITED STATES PATENTS 3,024,158 3/1962 Grangaard et al. 162-65 X 1,653,730 12/1927 PeetZ 162-61 1,831,206 11/1931 Swanson et al. 162-61 X 2,993,827 7/1961 Bennett 162-61 2,926,114 2/1960 Grangaard et al. 162-65 X 2,137,779 11/1938 Olsen 162-61 X S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner U.S. Cl. X.R.

UNlTED STATES PATENT OFFICE CERTFICATE OF CGRRECTION Patent No. 3,832,276 Dated August 2'?, 197# 1nventor(s) Rovmoulk et al.

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

Column 2, line l?, "anther" should be "another".

Colmnn 5, line 2l, after "bubbles", the ollrming has been omitted, "of oxygen remain in the aqueous pulp. Desirably no oxygen bubbles" Column 5, ,linel 511, insert e. comme. after "Also".

Column Uf, line 69, "Lightnin" should be "Lightening".

Column 5, line 60, the word "illusrtaated should be "illustrated.

Column 5, line67, the word "vigorosuly should be "vigorously".

column 5, line 73, "o5" should be "95". K Y

Column 6, line M6, the word "Beppe," should be "Kappa".

column 6, line 5o, the word "Krigntness" should be "Brightness". Column 6, line 61%,- the word "propeller" should be "propeller".

Column 7, line lll, after the word "samples", insert the word "each".

Signed and sealed this 29th day of October 1974.

(SEAL)- Attest:

MccoY M. GIBSON JR. C. MARSHALL DANN Attestng Officer Commissioner of Patents