US3575795A - Rapid high consistency bleaching with continuous diffusion - Google Patents

Abstract

A METHOD OF RAPID BLEACHING OF CELLULOSIC MATERIAL, SUCH AS WOOD PULP, IS PROVIDED WHEREIN A HIGH CONSISTENCY LIQUID SUSPENSION OF CELLULOSIC MATERIAL IS INTRODUCED INTO A VESSEL, A BLEACHING SOLUTION IS INTRODUCED INTO SAID LIQUID SUSPENSION THROUGH A PLURALITY OF INLET DUCTS, MOVED RAPIDLY RELATIVE TO THE CELLULOSIC MATERIAL, AND THE SUSPENSION LIQUID EXTRACTED THROUGH A PLURALITY OF EXTRACTION SCREENS.

Description

p 29, 371 A.W. BRINKLEY, JR. ETAL 35%,?

RAPID HIGH CONSISTENCY BLEACHING WIlfl CONTINUOUS DIFFUSION Filed Aug. 14, 1967 4 Sheets-Sheet 1 Ami 2Q; 1% w, g Y, JR" ET Al. 3,575,75

RAPID HIGH CONSISTENCY BLEACX-IING WITH CONTINUOUS DIFFUSION Filed Aug.

4 Sheets-Sheet 2 prifi 2Q, 1971 A. w. BRINKLEY, JR,.. ET AL 3,575,795

RAPID HIGH CONSIS'IENCY BLEACHING WITH CONTINUOUS DIFFUSION Filed Aug. 14, 1967 4 Sheets-Sheet 3 Apzi! 2Q 1971 Filed Aug. 14,

A. W. BRINKLEY, JR. ETAL WASHING OCI BLEACHING WASHING NOOH EXTRACTION and or Clo 2 BLEACIHNG 4 Sheets-Sheet 4 a A n- 7270 #z-75 WASHING clo 8 70 BLEACHING g WASHING EXTRACTION WASHING 70\ ,4- Clo 2 BLEACHING United States Patent 3,575,795 RAPID HIGH CON SISTENCY BLEACHING WITH CONTINUOUS DIFFUSION Amiel W. Brinkley, Jr., George G. Nelson, Jr., R. Terry Campbell, and Owen M. Golf, Mobile, Ala., assignors to International Paper Company, New York, N.Y. Continuation-in-part of application Ser. No. 583,349,

Sept. 30, 1966. This application Aug. 14, 1967,

Ser. No. 663,920

Int. Cl. D21c 3/26 US. Cl. 162-19 2 Claims ABSTRACT OF THE DISCLOSURE A method of rapid bleaching of cellulosic material, such as wood pulp, is provided wherein a high consistency liquid suspension of cellulosic material is introduced into a vessel, a bleaching solution is introduced into said liquid suspension through a plurality of inlet ducts, moved rapidly relative to the cellulosic material, and the suspension liquid extracted through a plurality of extraction screens.

This invention relates generally to improvements in processes for bleaching cellulosic materials in the form of pulp and particularly to rapid bleaching of a moving mass of high consistency pulp. This application is a continuation-in-part application of our co-pending application, Ser. No. 583,349, filed Sept. 30, 1966, for Rapid High Consistency Bleaching, now abandoned.

Pulp, as it comes from the digester, whether produced from hardwood or softwood contains residual coloring matter. While unbleached pulp may be used for the manufacture of certain grades of paper, for example, heavy wrapping paper and paper for use in bags, pulp which is to be used for printing or writing paper or paper which is to be dyed, must be bleached. Furthermore, bleaching may be required in order to remove impurities in the pulp which would otherwise interfere in the use of the pulp as a raw material for the production of rayon, gun powder and other cellulose products.

Depending upon the nature of the raw pulp and the end use for which the pulp will be employed, various chemical bleaching stages and various sequences of these stages have been used heretofore. Among the principal chemical bleaching stages which have been used are the chlorination stage (designated C), the caustic extraction stage (designated E), the hypochlorite stage (designated H), and the chlorine dioxide stage (designated D). In addition, both chlorine and chlorine dioxide may be used in the same stage (designated C and the chemicals may be used as a mixture or added sequentially. Various combinations of the above stages have been employed depending upon the specific conditions and bleaching requirements. For example, common bleaching sequences may include the following: CEH, CEHD, CEHED, CEHDED, CEDED and CEDH.

In the initial chlorination stage, chlorine is added to the washed pulp received from the digester. Ordinarily, the chlorination stage is performed at temperatures in the range of 60 to 90 F. with a pulp consistency of about 3%. Under these conditions, the retention time in the chlorination tower is about 60 and 70 minutes. The chlorine reacts directly with the lignin and other impurities in the pulp and causes moderate damage to the carbohydrate fraction of the pulp. Chlorine dioxide may be used in conjunction with or in place of chlorine for the initial chlorination stage.

Following the chlorination stage, a caustic extraction stage using a dilute aqueous solution of sodium hydroxide (0.5 to 5.0% NaOH based on ovendry weight of pulp) Patented Apr. 20, 1971 is performed to dissolve the chlorinated and oxidized lignin as well as some of the resin. The extraction stage is usually performed at temperatures of to 180 F. for a period of about 60 to 120 minutes with a pulp consistency of 10 to 20%.

The next stage of bleaching is commonly a hypochlorite stage although a chlorine dioxide stage is sometimes preferred. In the hypochlorite stage, either sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl) is used to oxidize the remaining lignin and other impurities in the pulp. Some degradation of the pulp as a result of shortening the chain length of the cellulose molecule occurs in the hydrochlorite stage. Normally, the hypochlorite stage is performed at temperatures between 90 and F. and at a pulp consistency of 3 to 15%. The time required for the hypochlorite stage varies inversely with the pulp consistency and the temperature ranging from 1 to 2 hours at a 15% consistency up to 5 hours at a 3% consistency. A third stage hypochlorite tower may commonly be operated at a temperature of 95 F. for a period of 90l20 minutes at a pulp consistency of 12%.

Following the hypochlorite stage, there may be a second alkaline or caustic extraction stage or a chlorine dioxide stage. The chlorine dioxide stage is usually designed for a 3 to 5 hour operation at about 11% consistency and a temperature around F. As chlorine dioxide is a relatively mild bleach and will produce a good pulp over a fairly wide range of conditions, it is particularly effective late in a multistage operation since the high tem perature will tend to soften shives and a residual of chlorine dioxide will bleach out the thus softened shives.

In addition to some combination of the various bleaching stages outlined above, it is usually considered necessary to provide a washing stage intermediate each bleaching stage to remove the spent bleaching agent and the products of reaction from the pulp prior to the beginning of the next bleaching stage so that the chemical requirements may be minimized. Washing is ordinarily carried out by diluting the pulp to low consistency (usually 0.5 to 1.25%) followed by thickening to 10 to 15% consistency and washing on a drum type washer wherein an excess of wash water displaces the liquid in the pulp.

The present practice of multistage bleaching thus re quires a period of 12 to 18 hours to bleach pulp to the desired brightness and viscosity. Moreover, it is apparent that both the temperature and the consistency or dilution of the pulp is varied from stage to stage and for the interstage washing operations necessitating a high consumption of water and steam. Furthermore, the long periods of time required for each bleaching stage introduce problems in the control of the pulp quality, especially during periods of varying production rates, since a long period of time must elapse from the time a change or adjustment in the operation is made until the effect of that change may be observed. Consequently, it is evident that if a significant reduction in bleaching time could be effected, and if the consistency of the pulp could be maintained relatively constant throughout the multistage bleaching process, improved control of the bleaching process as well as substantial savings in time would result. Additionally, the adaptability of the process to advanced control techniques, including the use of process control computers, would be enhanced.

Consideration has been given to the mechanism of bleaching in an effect to reduce the time required for the process.

Professor W. H. Rapson of the University of Toronto (in a paper entitled Dynamic Bleaching published in TAPPI, August 1966, volume 49, No. 8, pp. 324-334) has observed that for widely different types of pulp and various bleaching agents the optimum retention time of the pulp in the bleaching agent falls within relatively narrow limits. Since it would be expected that the widely different chemical reactions involved would have correspondingly different reaction rates, it was concluded that the limiting factor in the bleaching process was not the chemical reaction rate but instead was the rate of dif fusion of the reactant to the reaction sites. Prior to Professor Rapsons work it had also been assumed that if the reactant Were thoroughly mired with the pulp, then the reaction rate would be determined by the time required for the reactant to diffuse through the cell walls of the pulp. However, while thorough mixing of the pulp and reactant does improve the bleaching action, no substantial reduction in optimum bleaching time occurred. Professor Rapson therefore assumed that the limiting factor in the reaction was not the rate of diffusion through the cell wall but was, instead, the rate at which the bleaching agent was diffused through the relatively thick water layer which surrounds the pulp fiber. Laboratory work performed by Professor Rapson provided support for his assumption and showed that when the reactant was diffused through the pulp at a relatively rapid rate, instead of being permitted to remain static relative to the pulp fiber, the time of the bleaching process was reduced from a matter of hours to a matter of minutes.

The following examples of rapid high consistency bleaching confirming the work of Professor Rapson have been performed by the applicants in a laboratory apparatus:

EXAMPLE 1 Pine kraft pulp that had been bleached for one hour in a conventional chlorination stage and treated for two hours in a conventional caustic extraction stage so as to have a viscosity of 22.8 centipoises (TAPPI Standard T- 230) was further bleached with sodium hypochlorite according to the rapid high consistency bleaching process. The pulp at consistency was treated with a 4% solution of hypochlorite (expressed as percent available chlorine based on ovendry pulp weight) for 3 minutes at a temperature of 118 F. Thereafter the pulp, still at a consistency of 10%, was washed by diffusing wash water at a temperature of 180 F. through the pulp to displace the hypochlorite solution for a period of 3 minutes. The amount of hypochlorite consumed was 0.8% as available chlorine based on ovendry pulp. The brightness of the bleached pulp at the end of the hypochlorite stage was 63% (TAPPI Standards T-217 M-48, T-218 M-59) and the viscosity was 16.5 centipoises.

EXAMPLE 2 Pine kraft pulp having a permanganate number of 21 was bleached in a CEHD sequence according to the rapid high consistency bleaching process, the pulp being maintained at a 10% consistency throughout the bleaching and washing sequence. In the chlorination stage, pulp at a temperature of 80 F. was bleached with a 3.8% solution of chlorine, based on ovendry pulp weight for a period of 3 minutes. The pulp was then washed for 3 minutes with water at a temperature of 160 F. Following the wash, the pulp was treated with a 3% sodium hydroxide solution (based on ovendry pulp weight) for a period of 3 minutes at a temperature of 160 F. Again, the pulp was washed for 3 minutes with water at a temperature of 160 F. The pulp was then treated with a 4% hypochlorite solution (expressed as available chlorine based on ovendry pulp) at a temperature of 140 F. for a period of 3 minutes, during which process 3.0% chlorine was consumed. Again, the pulp was washed with water for a period of 3 minutes at a temperature of 160 F. In the final bleaching stage the pulp was treated with a 2.5% solution of chlorine dioxide (based on ovendry pulp weight) for a period of 3 minutes at a temperature of 160 F., during which process 0.6% chlorine dioxide (based on ovendry pulp weight) was consumed. Following the final bleach, the pulp was washed with water for a period of 3 minutes at a temperature of 160 F.

The CEHD bleaching sequence of Example 2 was completed during an elapsed time of 24 minutes. A pulp brightness of 74% and a viscosity of 12 centipoises was attained. Similar pulp was bleached in a conventional system using a CEHD sequence to a brightness of and a viscosity of 16.5 centipoises. However, to attain those results the elapsed time was 8 hours compared with 24 minutes for the rapid high consistency procedure.

EXAMPLE 3 Washed pine kraft pulp that had been bleached by the CEHD sequence described in Example 2 was maintained at a consistency of 10% and then treated in accordance with the rapid high consistency process with a 1% sodium hydroxide solution (based on ovendry pulp weight) at a temperature of F. for a period of 3 minutes. The rapid extraction stage was followed by a washing stage in which water at a temperature of 160 F. was diffused through the pulp for a period of 3 minutes. The washed pulp was then treated with a 2.5% solution of chlorine dioxide (based on ovendry pulp weight) for a period of 3 minutes at a temperature of 160 F. During this bleaching stage 0.6% chlorine dioxide was consumed. A final wash with water at a temperature of 160 F. for a period of 3 minutes was performed after which the pulp brightness was 82% and the viscosity 12 centipoises.

Despite the obvious advantages inherent in this new mode of operation, no commercially feasible method of carrying out the process has been proposed heretofore although Professor Rapson suggested the possibility of forming a sheet of pulp on a moving wire and applying bleaching solutions and wash water in sequence as the wire and pulp moved past the desired points of application.

It is an object of the present invention to provide a commercially feasible means for carrying out a process of rapid high consistency bleaching.

A further object of the present invention is to provide an adaptation of equipment used for pulp washing which may be used in a continuous high consistency bleaching process.

A still further object of the present invention is to provide a system for a rapid high consistency bleaching process which may perform both a bleaching and a washing operation during a single pass through the equipment.

Another object of the present invention is to provide a system for bleaching at high concentrations of pulp and chemicals whereby increased rates of reaction and shorter bleaching times can be realized.

Yet another object of the present invention is to provide a bleaching system having reduced space and capital requirements which does not employ conventional drum washers and external mixers.

Still another object of the present invention is to provide a bleaching system for a multistage bleaching process wherein the pulp is not diluted during the process and exposure to the atmosphere between stages is minimized.

Another object of the present invention is to provide a bleaching system which requires smaller amounts of external heat and water.

Another object of the present invention is to provide a bleaching system which is capable of improved control to produce pulp of consistent high quality.

The above and other objects of the present invention are accomplished by the utilization of a series of relatively small cylindrical vessels including radial diffusion equipment.

The invention will now be described in greater detail in connection with the accompanying drawings in which:

FIG. 1 is a cross-sectional schematic view of the radial diffusion equipment used in the bleaching system according to the present invention;

FIG. 2 is a cross-sectional schematic view of a radial diffusion unit for use in the practice of the present invention having two separate diffusion devices to provide two separate reaction zones;

FIG. 3 is a cross-sectional schematic view of a radial diffusion unit for use in the practice of the present invention wherein the single diffusion device has two separate reaction zones;

FIG. 4 is a schematic fiow diagram of a multistage rapid high consistency bleaching sequence according to the present invention.

Referring now to FIG. 1, the radial diffusion equipment used in the rapid high consistency bleaching system of the present invention comprises a cylindrical metal body member open at the top having a bottom portion 12 which is preferably tapered and a centrally disposed inlet duct 14 in the bottom 12 of the body member 10. The body member 10 may have a diameter of about 10 feet but may be of any desired diameter; for example, a diffuser having a diameter of about 20 feet has been used in actual operations. A cylindrical hood 15 of larger diameter than the body member 10 having an outlet duct 16 is positioned above the upper portion of the body member 10. A vertical hollow shaft 18 is rotatably mounted along the axis of the hood 15 on suitable bearings, not shown, and driven by a motor 20 together with a suitable gear reduction unit mounted on the upper end of the hollow shaft 18. The shaft 18 is preferably designed to be rotated at a speed of about 10 rpm. The shaft 18 carries at its lower end a hollow horizontal shaft 22, the length of which is intermediate the diameters of the hood 15 and body member 10. A plurality of arcuate scraper blades 24 are fixed to the shaft 22 at intervals along the shaft 22 to direct pulp leaving the body member 10 through the hood 15 and into the outlet duct 16. It may be found desirable to form the outermost of the scraper blades 24 with a straight instead of an arcuate section and extend the blades close to the bottom of the hood member 15 so as to facilitate removal of the treated pulp through the outlet duct 16. Depending from, and communicating with, the hollow shaft 22 are a series of hollow tubes 26 which extend into the upper portion of the body member 10. The lower ends of the hollow tubes 26 are preferably formed so as to be relatively narrow in a direction radial to the body member 10 and elongated in a direction tangential to the body member 10. A series of apertures 28 are formed in the lower trailing end of each of the hollow tubes 26 through which liquid may be forced into the pulp contained in the upper portion of the body member 10.

A group of concentric hollow extraction screens 30 are also positioned in the upper portion of the body member 10. The extraction screens 30 are relatively narrow in a direction radial to the body member 10 and elongated in the vertical direction. Preferably, the extraction screens have a vertical dimension between 2 and 3 feet. As shown in FIG. 1, the hollow tubes 26 are positioned along the hollow shaft 22 so as to be intermediate the extraction screens 30. A series of screen outlet ducts 32 located near the lower end of the extraction screens interconnect the extraction screens 30 and also position the extraction screens within the upper portion of the body member 10. Suitable sealing means 33 are provided between the ducts 32 and the body member 10. A vertical support rod 34 is connected at its lower end to each of the outlet ducts 32 and at its upper end to an hydraulic or mechanical lift mechanism 36. The lift mechanism 36 is designed cyclically to raise and lower the group of extraction screens 30 in a vertical direction through a distance of, for example, between 4 and 6 inches and is arranged so that the upward rate of travel of the extraction screens correspond approximately to the rate of movement of the pulp through the body member 10 while the downward rate of travel of the extraction screens is relatively faster.

Alternatively, the lift mechanism may comprise hydraulic lift cylinders, positioned inside the body member 10, which include hollow shafts through which the liquid extracted through the screens 30 may be withdrawn. With this construction, the sealing means between the outlet ducts and the body member may be simplified.

In operation, pulp at a consistency of 5 to 15% is introduced into the body member 10 through the inlet duct 14 and passed upwards through the diffusion apparatus so that a given cross-section of the pulp passes the extraction screens 30 during a period of between 2 and 12 minutes. During the upward travel of the pulp, water or solutions of chemical bleaching agents are forced into the pulp through the apertures 28 of the rotating tubes 26. Simultaneously the extraction screens 30 are periodically raised and lowered. Liquid is extracted from the pulp through the extraction screens 30 and the screen outlet ducts 32 during the upward travel of the extraction screens but not during the relatively faster downward travel of the extraction screens. With the equipment as illustrated in FIG. 1, the water or bleaching solution forced into the pulp is not maintained static with respect to the pulp but diffuses more or less radially through the pulp at a relatively high rate forcing ahead of it the liquid previously contained in the pulp. Since there is a relatively short distance between the region of entry and the region of extraction of the water or bleaching solution, relatively high rates of production are achieved.

As shown in FIG. 1, the basic equipment unit comprises a single set of delivery tubes for the reactant or wash liquid and a single set of extraction screens. However, since it is generally desirable to follow each bleaching stage with a washing operation, the basic unit is preferably designed with two independent reaction zones so that the pulp may be bleached and 'washed in the same unit. In this way, washed pulp may be introduced directly to the succeeding bleaching stage. This construction is illustrated schematically in FIG. 2. Alternatively, the basic equipment unit may be designed so as to have a single set of delivery tubes divided into separate upper and lower zones so that different reactants may be simultaneously applied at two different cross-sections of the pulp. In this event, the extraction screens are also divided into separate upper and lower zones so as to extract separately the liquids contained in the pulp. This alternative construction is shown schematically in FIG. 3. As with the construction shown in FIG. 2, the bleaching stage is preferably conducted in the lower reaction zone while the washing operation is performed in the upper reaction zone. The Kamyr continuous diffuser, described in an article entitled Pulp Washing by Continuous Diffusion by Johan Richter, published in TAPPI, June 1966, volume 49, No. 6, pp. 48A49A, may be used practicing the method of the present invention. Where a multistage bleaching operation is to be conducted, it will be apparent that it is necessary to provide additional basic diffuser units for each stage desired.

A more thorough understanding of the practice of rapid high consistency bleaching according to the present invention may be had by reference to the following description of a multistage bleaching operation shown schematically in FIG. 4. The term high consistency as used herein with respect to suspensions of cellulosic material such as wood pulp suspensions shall be understood to mean suspension having a consistency ranging between 5 and 15% as disclosed in the examples referred to above and in the following detailed description.

Unbleached high consistency pulp enters the lower end of the first diffuser at 40 at a consistency of 5 to 15%, preferably 8 to 12% and moves upward through the unit. As shown in FIG. 4, a bleaching solution, preferably a solution of chlorine, chlorine dioxide or chlorine and chlorine dioxide is forced into the first diffuser at 42. The bleaching solution may have a concentration of 1 to 10% but is preferably in the range of 2 to 8%, based on ovendry pulp weight, the precise concentration depending upon the bleach demand of the pulp. The temperature in this first stage may range between 60 and 160 F. depending upon the degree of bleaching desired, the specific chemical used, and the quantity of the bleaching chemical. Liquid extracted from the pulp is withdrawn from the diffuser at 44. This liquid may contain trace amounts of chemicals and could be sewered, but preferably the extracted liquid is used to transport additional pulp into the diffuser at point 40.

The pulp passes through the first bleaching stage, preferably in 2 to 12 minutes, though a somewhat longer time could be used if desired. After leaving the first bleaching stage, the pulp enters a washing operation which is preferably conducted in another zone of the diffuser unit in which the chlorine bleaching stage was performed. Wash water at a temperature of 140 to 200 F. is forced into the diffuser at 46 and the liquid displaced from the pulp extracted from the diffuser at 48. The displaced liquid leaving the diffuser at 48 is preferably refortified with bleaching chemical and reintroduced into the diffuser at 42 to bleach additional pulp. It is necessary, of course, to purge a volume of liquid from the system equal to the volume of makeup chemical added in each stage in order to maintain a uniform consistency throughout the bleach plant.

Washed pulp from the first diffuser unit enters the second diffuser unit at 50. As shown in FIG. 4, the second bleaching stage is a caustic extraction stage. In this stage, as in the preceding chlorine stage and washing operation, the pulp is maintained preferably at a consistency of 8 to 12%. The pulp is treated with a solution of caustic soda (NaOH) at a concentration of 1 to (based on ovendry pulp) depending on the type of pulp and the amount of chemical used in the chlorination stage, at a temperature of 60 to 200 F., preferably 160 to 190 -F. for a period of 2 to 12 minutes. The caustic soda is forced into the second diffuser at 52 and the displaced liquid is withdrawn at 5-4. The liquid withdrawn at 54 contains traces of caustic and may be sewered, but preferably is introduced into the first diffuser at 46 for washing the pulp.

After leaving the second bleaching stage, the pulp enters a second washing operation which is preferably conducted in another zone of the second diffuser. The caustic extraction liquid is displaced from the pulp by wash water which is forced into the second diffuser at 56. Washing is preferably conducted for a period of 2 to 12 minutes at a temperature of 160 to 200 F. and the displaced liquid withdrawn at 58. Due to the organic material and chlorides contained in the liquid extracted from the pulp in the second wash, the liquid withdrawn at 58 would normally be sewered or otherwise disposed of but it can be refortified with caustic and resused at 52.

The pulp leaving the wash zone of the second diffuser is introduced to the third diffuser at 60. As shown in FIG. 4, the third bleaching stage comprises a hypochlorite bleach. In this case a solution of sodium hypochlorite containing up to 5% hypochlorite as chlorine, based on the ovendry pulp Weight, and depending upon the type of pulp and the brightness required from this stage, is forced into the third diffuser at 62'. The temperature in the hypochlorite stage is maintained between 80 to 160 F., preferably 120 to 140 F. and the pulp maintained at a consistency of 8 to'12%. The liquid displaced from the pulp is withdrawn at 6'4 and may be sewered, but preferably it is introduced into the second diffuser at 56 for washing.

After leaving the third bleaching stage, the pulp enters a third washing operation which is preferably conducted in another zone of the third diffuser. Wash water at a temperature of 140 to 200 F. is forced into the third diffuser at 6'6 and the liquid displaced from the pulp is withdrawn from the diffuser at 68. The washing operation preferably continues for a period of 2 to 12 minutes. The liquid extracted at 68 comprises a diluted bleach solution and may be sewered, but preferably it is refortified with bleach chemical and introduced into the third diffuser at '62.

After leaving the third diffuser, the washed pulp, still at a consistency of 8 to 12%, is introduced into the fourth diffuser at 70. As shown in FIG. 4, the fourth diffuser includes a chlorine dioxide bleach stage which may use up to 4% chlorine dioxide but preferably 0.5% to 2% chlorine dioxide based upon the ovendry pulp weight. The chlorine dioxide solution is forced into the fourth diffuser at 72 and is diffused through the pulp at a temperature of to 200 F, preferably to F. for a period of 2 to 12 minutes. The liquid displaced from the pulp at 74 may be sewered, but preferably is used for washing or for a preceding bleaching stage. For example, the liquid extracted at 74 may be used for washing at 66 or may be refortified and introduced at 42.

The pulp leaving the fourth bleaching stage may then pass through a fourth washing operation which preferably is performed in another zone of the fourth diffuser. As in the earlier washing operations, the wash is conducted at a temperature of 140 to 200 F. for a period of 2 to 12 minutes. Wash water for the fourth wash is forced into the fourth diffuser at 76 and the liquid displaced from the pulp withdrawn at 78. This liquid contains small amounts of bleaching chemical and can be sewered, but preferably it is refortified and introduced into the fourth diffuser at 72.

As shown in FIG. 4, the fifth bleaching stage is another caustic extraction stage which may be conducted in a fifth diffuser. The washed pulp from the fourth diffuser is introduced into the fifth diffuser at 80. In this bleaching stage, a 0.1 to 3% solution of caustic soda, based on ovendry pulp weight, is forced into the diffuser at 82 at a temperature of 140 to 200 F. and diffuses through the pulp in a period of 2 to 12 minutes. The displaced liquid, which contains traces of caustic soda, is Withdrawn at 84 and may be sewered, but preferably is reused in a preceding bleaching or washing stage. For example, the liquid withdrawn at 84 may be introduced into the fourth diffuser at 76.

Again, the pulp leaving the fifth bleaching stage is passed through a fifth wash, preferably conducted in another zone of the fifth diffuser. As in the earlier washing operations, wash water at a temperature of 140 to 200 F. is forced into the fifth diffuser at 86 and the liquid displaced during the 2 to 12 minute wash is withdrawn at 88. This displaced liquid comprises a casutic soda solution and may, after being refortified, be used in a preceding bleaching stage, preferably a preceding caustic extraction stage. For example, the liquid withdrawn at 88 may be introduced into the fifth diffuser at 82 or into the second diffuser at 52.

The washed pulp from the fifth diffuser may enter the sixth diffuser at 90, still at a consistency of 8 to 12%. The sixth diffuser, as shown in FIG. 4, includes a chlorine dioxide bleaching stage. In this stage a solution of up to 4% chlorine dioxide (based on ovendry pulp), but preferably 0.1 to 2% chlorine dioxide is diffused through the pulp at a temperature of 140 to 200 F., preferably 160 to 180 F. The chlorine dioxide solution is forced into the sixth diffuser at 92 and diffuses through the pulp during a period of 2 to 12 minutes. The displaced liquid is withdrawn at 94. This liquid contains a trace of chlorine dioxide and may be sewered, but preferably it is reused in a preceding bleaching or washing stage. For example, it may be introduced into the fifth diffuser at 86.

The bleached pulp from the sixth bleach stage is then washed in a washing operation preferably performed in another zone of the sixth diffuser. Wash water for the sixth wash is forced into the sixth diffuser at 96 at a temperature between 140 and 200 F. and diffuses through the pulp for a period of 2 to 12 minutes. The displaced liquid is withdrawn from the sixth difiFuser at 98. This liquid contains chlorine dioxide and, after being refortified, may be used in a preceding bleaching stage as, for example, at 92 or 72. The washed and bleached pulp leaves the sixth diffuser at 100.

From the above description, it will be apparent that according to the practice of the present invention, a multistage bleaching process may be performed in a short period of time than was heretofore required for a single stage of bleaching. For example, the multistage process illustrated in FIG. 4 may be performed in about one hour whereas a single chlorination stage, without washing, as heretofore practiced, has required 60 to 70 minutes. Moreover, in the present rapid high consistency bleaching process, the pulp is maintained at a relatively constant consistency throughout the bleaching and washing operations, thereby reducing the requirements for dilution water whereas in the multistage bleaching process as heretofore practiced it was necessary to dilute the pulp to a low consistency for each interstage wash and the various bleaching stages were conducted at varying pulp consistencies. Corresponding quantities of steam heretofore required to heat the dilution water are also eliminated. Furthermore, any desired bleaching sequence can be performed, the sequence CEHDED illustrated schematically in FIG. 4 having been chosen to exemplify a relatively complex sequence. Other common sequences, such as CEH, CEHD, CEHED, CEDED and CEDH as well as sequencies which do not have an initial chlorination stage may also be performed in accordance with the present invention.

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 modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A method of rapid high consistency bleaching of cellulosic material in a vessel which comprises introducing a liquid suspension of cellulose material haw'ng a consistency between 5 and 15% into said vessel at the inlet end thereof, passing said suspension of cellulose material through said vessel at a controlled rate, forcing a bleaching solution selected from the group consisting of chlorine, chlorine dioxide, sodium hydroxide, sodium hypochlorite and calcium hypochlorite into a zone within said vessel through a plurality of ducts spaced from said inlet end of said vessel, moving said bleaching solution rapidly relative to said liquid suspension of cellulosic material looated with in said zone of said vessel, displacing the suspension liquid contained in said suspension of cellulose material located within said zone of said vessel, and extracting said suspension liquid displaced from said suspension of cellulosic material through a plurality of movable extraction screens located within said vessel intermediate said inlet ducts, said movable extraction screens being controlled for cyclical reciprocating movement at a first rate corresponding to the rate of movement of said cellulosic material through said vessel, and a rapid return movement to the initial position, said bleaching solution being forced into said zone Within said vessel at a rate sufficient to displace the suspension liquid contained in said suspension of cellulosic material located within said zone within a period of about 12 minutes.

2. A method of rapid high consistency bleaching, as recited in claim 1 in which the bleaching solution has a concentration, based on ovendry pulp weight, greater than 0% but not in excess of 10%.

References Cited UNITED STATES PATENTS 2,687,943 8/1954 Pete 16289X 3,313,677 4/1967 Carr 162-19 3,377,235 4/1968 Webster 162--88X 3,425,898 2/1969 Laakso 162237X HOWARD R. CAINE, Primary Examiner US. Cl. X.R.

32 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,575,795 Dated April 1971 Inventor(s) W. Brinkley, Jr., at al.

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

Col. 1, line 6 "and" should be --to-- C01. 2, line 13, "hydrochlorite" should be --hypochlorite-- Col. 2, line 65, "effect" should be --effort-- Col. 3, line 9, "mired" should be --mixed-- C01, 7, line 33, after "is", insert --then-- C01. 7, line 55, "resused" should be --reused-- C01. 8, line 50, "casutic" should be --ca.ustic-- Col. 9, line 8, "short" should be --shorter-- Col. 9, line 18, "process" should be --processes-- 001.10, line 10, "with in" should be --within-- (101.10, line 12, "cellulose" should be --ce1lulosic-- Signed and sealed this 27th day of February 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

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