Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes

Definitions

• the waste products of the bleaching process are known to contain BOD, organically bound chlorine and color. Thus, they contribute to the water pollution discharged from the pulp mill.
• the efficiency of the bleaching reactions is hampered by the existence of condensation reactions. This can be particularly true in the alkaline extraction step where condensation reactions block further delignification.
• a publication by Seymour (3) reports that the amount of caustic applied in the extraction stage can be doubled beyond normal with practically no reduction in bleach chemical usage in the following stages. It is a continuing objective of the pulp industry to reduce overall bleaching costs by improving efficiency in the various process steps. Improved efficiency can result in lower costs by reduction of chemical usage or reduction of the number of process steps. An additional benefit of improved efficiency can be a lowering of pollutant discharge.
• Hot alkaline extraction of the unbleached pulp has been proposed to improve bleach plant efficiency. This is sometimes referred to as pre-bleaching or pre-delignification.
• the objective is to reduce bleach costs by reducing the kappa number (lignin content) of the pulp before it enters the bleach plant. In this way a corresponding reduction in the amount of more expensive bleaching agents is achieved.
• oxygen delignification has been the subject of a number of U.S. patents (8 thru 11, 13 thru 17) as a pre-bleaching step to lower the kappa number of pulp prior to bleaching.
• the unbleached pulp is contacted with oxygen and alkali under conditions of elevated temperature and pressure for time periods which are typically about 15 to 30 minutes.
• the "cold soda” process was developed (1) principally as a means to remove hemicellulose and thus improve alpha-cellulose content of dissolving grade pulp. This process can be applied to pulps at any stage in the bleaching or purification sequence including pulps which have been hot alkaline-extracted. Optimum temperatures for cold caustic extraction range between 15 and 25oC and treatment times, between 15 and 60 minutes.
• Kemph and Dence (5) reported significant reductions in permanganate number after extraction of chlorinated pulp in an oxygen atmosphere. Tests which they conducted in an air atmosphere also showed an improvement although it was only about 1/5th as large as the effect noted with oxygen. More recently, based on improved methods of mixing pulp and oxygen such as disclosed in U.S. Patents 3,832,276 and 4,451,332, the commercial use of oxygen in the extraction stage has grown rapidly worldwide.
• Elton describes the two most common systems for oxygen extraction (6).
• sodium hydroxide is added to the pulp after it leaves the chlorination stage washer and prior to the addition of oxygen to a mixing device.
• the alkaline pulp suspension containing a fine dispersion of oxygen, is either introduced into the bottom of the extraction tower or, when extraction is in a downflow tower, into a pre-retention tube.
• oxygen extraction is effective at improving efficiency, it does create some added problems. These are the need to handle oxygen, a potentially hazardous chemical; the added expense of the oxygen; and the need to provide adequate ventilation to prevent buildup of toxic and combustible gases.
• the use of hydrogen peroxide has also been used to enhance lignin removal in the extraction stage (2). While this is a relatively simple method, its application does require the added expense of peroxide.
• This invention teaches an improved method of conducting alkaline extraction of pulp. It is preferably employed in one or more of three locations:
• the present invention is based on a surprising discovery relating to alkaline treatment of pulp. It was found that pulp delignification can be improved if a portion of the liquid phase is removed from the reacting mixture after only a short time (0.5 to 10 minutes) of reaction. The pulp is then allowed to continue to react with the remaining liquid solution for a normal period of time (30 to 90 minutes). This suggests that during the initial phase of reaction, substances are formed which either reverse or inhibit pulp delignification. It was further learned that the liquid phase, after being removed from the pulp suspension, can be treated to alter, remove, or otherwise deactivate those substances which reverse or inhibit the delignification process, thus making the liquid phase suitable for reuse in delignification or extraction.
• One such method of treatment is to heat the liquid phase for a period of time ranging from about 5 minutes or longer, depending on reaction temperature.
• the reuse of the treated liquid phase can be accomplished either by adding it to fresh pulp or by re-adding it to the original pulp. This is not intended to imply that reuse of the liquor is limited to these two means.
• Another object of this invention is to provide an improved process of pulp bleaching and delignification in a paper making process by reducing condensation reactions between dissolved and undissolved lignin, by the adding of an alkaline mixture to the pulp and, after a short period, withdrawing a major portion of the liquid phase of the alkaline mixture and continuing to react the remaining pulp solution.
• Another object of this invention is to provide a process whereby in a paper making system a portion of the liquid phase of an alkaline mixture which is added to the pulp is withdrawn from the pulp suspension and the withdrawn liquid is reused as an additive with the alkaline mixture that is combined with the pulp.
• Fig. 1 is a flow diagram which illustrates how one process of the invention can be practiced.
• Fig. 2 - 7 are flow diagrams, similar to Fig. 1, but illustrating how alternate processes of the invention can be practiced.
• Fig. 1 illustrates the first, most simplified embodiment of the invention, wherein either unbleached chlorinated or partially bleached wood pulp is blended with a suitable alkali, such as NaOH, in a mixer 1 at a pulp consistency between about 0.01% and 3.0%, preferably about 7 to 15%.
• a suitable alkali such as NaOH
• the alkali can be combined with the pulp by distributing it on a sheet of pulp so that the natural capillary forces will distribute the alkali throughout the pulp.
• the amount of alkali added can be the same as, greater or less than the amount normally used for extraction, delignification or oxygen delignification.
• the alkaline pulp suspension is next carried into the reactor 2 where it is treated for 0.5 to 10 minutes or longer depending upon the reaction temperature. Table II shows the approximate relationship between optimum reaction time and temperature. TABLE II
• a portion of the liquid phase is removed by filtration of the alkaline pulp suspension in the filter 3; and the thickened pulp slurry is conveyed to the reaction vessel 6.
• the amount of filtrate removed from the pulp slurry at the filter 3 is adjusted to be less than approximately 90% of the liquid phase of the mixture and preferably between 40 and 70% of the liquid phase present with the pulp in the reactor 2.
• the pulp slurry which passes to the reaction vessel 6 should contain sufficient entrained chemical to complete the delignifcation reaction in the vessel.
• the conditions of time and temperature applied in vessel 6 can be those normally applied to the pulp for the stage of processing at which this invention is being practiced.
• vessel 6 could be operated at 60 to 70oC and 30 to 90 minutes; and if practiced with oxygen delignification, vessel 6 would be operated at about 100oC and 100 psi for 15 to 45 minutes.
• vessel 6 if operated as an alkaline pre-delignification, vessel 6 might be operated between 70 and 100oC for 15 to 45 minutes.
• the amount of alkali present in vessel 6 can be considerably less than is normally used for the corresponding process practiced without the improvement of this invention.
• the final washer 7 is optional. It is included because it represents good bleaching practice. It is not meant to limit this invention to systems which include washing after the reaction vessel 6.
• FIG. 2 A second embodiment of the invention is shown in Fig. 2. This differs from Fig. 1 by the inclusion of a second mixer 5 in the process line between the filter 3 and the reaction vessel 6.
• the alkali required for reaction is added in two parts, the first part at the mixer 1 and second part at the second mixer 5.
• the relationship between optimum time of treatment and temperature in reactor 2 is substantially the same as that given in Table II for the first embodiment of this invention. Best results are obtained when between 50 to 80%, and preferably about 55 to 70%, of the alkali requirement is added at the mixer 1 and the remainder at the second mixer 5. Conditions in the reaction vessel 6 and washer 7 are similar to those for Fig. 1.
• a washing step 4 is added between the filter 3 and the mixer 5. This improves the degree of removal of the liquid phase.
• the filter 3 and washer 4 can be combined into one unit by using a conventional pulp washer employing a filtration step followed by a displacement wash.
• the wash liquid used can be either water or fresh alkali solution. Filtrate from the final washer 7 can be reused as the wash liquid if it is suitable.
• the optimum dosage of alkali at mixer 1 is between 50 and 80% and preferably between 55 and 65% of the total alkali charged, with the remainder applied at the second mixer 5. Conditions in the reaction vessel 6 and washer 7 are similar to those for Figs. 1 and 2.
• the fourth embodiment of this invention is shown in Fig. 4.
• the arrangement is the same as in the embodiment illustrated in Fig. 2 with the exception that the entire charge of alkali is added at the mixer 1 and filtrate from the filter 3 is collected in a tank 8.
• Some of the filtrate is treated in the filtrate reactor 9 and re-added to the pulp either at the mixer 5 or between reactor 2 and filter 3 or at both places.
• the portion of the filtrate which is not treated in reactor 9 can be discarded.
• the amount of filtrate removed from the system at this point is determined by the consistencies of the feed pulp and the pulp entering reactor 6.
• pulp will enter reactor 2 at between 8 to 15% consistency and have a consistency of between 8 and 25% as it enters reactor 6.
• Reaction conditions in the reactor 2 are similar to those previously given in Table II. Valves (not shown) in the lines of Fig. 4 can be used to control the flow.
• Heater 12 comprises a provision for adding heat to the filtrate reactor 9 as shown in Fig. 4. Most of the heat requirement for the reaction in vessel 6 could be added to this point.
• reaction vessel 6 and washer 7 Conditions in the reaction vessel 6 and washer 7 are similar to those for Figs. 1, 2 and 3.
• FIG. 5 A fifth embodiment of this invention is shown in Fig. 5. This is the same as Fig. 4 except for the inclusion of a washer 4 between the filter 3 and the mixer 5 in the line of process flow.
• the pulp is washed with treated filtrate from the filtrate reactor 9 to remove additional traces of entrained liquid phase which remain in the pulp after filtration. It is possible to use the treated filtrate only at the washer 4 in Fig. 5.
• additional treated filtrate can be added to the pulp either at the mixer 5 or between reactor 2 and filter 3 or at both places. Valves (not shown) in the lines of Fig. 5 can be used to control the flow.
• the filter 3 and washer 4 can be combined into one unit. Additional treated filtrate is added to the pulp at the mixer 5 if needed, or alternately, the mixer 5 can be eliminated and the pulp conveyed to the reaction vessel 6 for further processing.
• reaction vessel 6 and washer 7 Conditions in the reaction vessel 6 and washer 7 are similar to those for Figs. 1, 2, 3 and 4.
• the sixth and seventh embodiment of this invention are shown in Figs. 6 and 7.
• chlorinated or unbleached pulp is blended with treated filtrate from the filtrate reactor 9 in the mixer 1.
• the treated filtrate is sprayed or otherwise distributed on a sheet of pulp allowing the natural capillary forces to distribute the filtrate.
• the temperature of the pulp suspension at this point will depend on the temperatures of the streams entering the mixer and usually will be in the range of 40 to 60°C.
• the slurry is then filtered, or dewatered at filter 3.
• treated filtrate may be used to dilute the pulp although this dilution is optional.
• the optimum liquid removal by filter 3 in the system of Fig. 6 is between 70 and 90% removal, b ut 30 to 70% liquid removal would still provid e significant benefits. However, liquid removal rates of 75 to 90% are easily ach ieved commerically. Table III shows, for the system of F ig . 6, some values for consistency entering filter 3 and entering the reactor 6 that will result in 67%, 80% and 90% removal of the liquid phase at the filter 3.
• the process operates best when the alkali charge to the first stage is maximized. After filtration at filter 3, the resulting thickened pulp slurry will carry forward sufficient alkali in the entrained liquid phase to complete the delignificaton reaction.
• the conditions of time and temperature in the reaction vessel 6 and washer 7 can be the same as those stated in the first embodiments of this invention (Figs. 1 thru 5).
• Heater 12 comprises a provision for adding heat to the filtrate reactor 9. This provides the elevated temperature desired for filtrate treatment. Holding periods in filtrate reactor 9 ranging from 8 to 60 minutes at 50°C have been used successfully. The short treatment time is preferred because it requires the smallest reactor size for implemention. As stated in the discussion of embodiment four, at a temperature of 60° in filtrate reactor 9, a holding period between 5 and 12 minutes is sufficient.
• the temperature of the pulp mixture entering reactor 2 of Fig. 6 is determined by the temperatures and consistencies of the streams entering the mixer 1. Since it is an advantage to use elevated temperature to treat the filtrate in filtrate reactor 9, the temperature of the pulp stock entering the reactor 2 will be correspondingly high. Typical of the values which might be encountered would be pulp stock at 35oC and 15% consistency moving to mixer 1 and filtrate at 60oC recirculating from filtrate reactor 9 to mixer 1 which results in the feed from mixer 1 to reactor 2 having a temperature of about 50oC and a consistency of 5.6%.
• fresh alkali can be added to the system either at the tank 8 (point A), at the inlet to the filtrate reactor 9 (point B) or at the outlet of the filtrate reactor 9 (point C). Wherever added, sufficient agitation from normal flow conditions in the system should be available to disperse the alkali evenly throughout the filtrate. If not, it would be desirable to provide a means for agitation. It is, of course, possible to add fresh alkali to the pulp at a point before it enters the Mixer 1, for example, by adding it in the pulp conveyor or spraying it on the washer or in the pulper of the previous stage (not shown).
• the seventh embodiment of this invention shown in Fig. 7 differs from the sixth by the inclusion of a washer 4 in the process flow between the filter 3 and the reaction vessel 6.
• the washer uses treated filtrate from the filtrate reactor 9 to displace liquor remaining in the pulp after filtration by filter 3.
• the inclusion of the washer 4 allows more complete removal of the liquid phase by replacing it with treated filtrate. It is desirable to operate according to the guidelines of embodiment six with alkali charge to the reactor 2 as high as practical. This is accomplished by maximizing withdrawal of liquid phase between reactors 2 and 6.
• the washing step improves efficiency of liquid phase removal without requiring low consistency entering the filter.
• the addition of treated filtrate to the pulp between the reactor 2 and the filter 3 is optional. Valves (not shown) in the lines of Fig. 7 can be used to control the flow.
• fresh alkali could be added to the pulp upstream of the mixer 1 of Fig. 7.
• Conditions applied to the pulp suspension in the reaction vessel 6 are the similar to those specified in the other embodiments. Further, as in all of the other embodiments, the washer 7 is optional.
• the mixers 1 of Figs. 1-7 and 5 of Figs. 2-5 can be chosen from equipment already available to the pulp industry including, but not limited to, static mixers, high shear mixers, and stirred tank mixers.
• the reactor 2 can be any vessel of appropriate size to provide sufficient residence time for the first stage reaction.
• the vessel should ideally be designed to minimize backmixing. Therefore, a long tubular reactor such as a pipeline, tall tower or stand pipe would be suitable. It would be desirable to have the flexibility to adjust the residence time in reactor 2 to allow response to changes in operating temperature. Numerous methods to do this are known to those skilled in mechanical design of reactors.
• the filter 3 could be chosen from equipment already available to the industry including but not limited to such devices known as sidehill screens, extractors, deckers, drum filters and belt filters. It will be apparent to one skilled in the art that for the embodiments in which the filter 3 and washer 4 are used together (Figs. 3, 5 and 7), these can be combined by using a conventional pulp washer employing a filtration step followed by displacement (not shown). If a separate washer is used, those commonly employed by the pulp industry such as diffusion washers, pressure washers or wash presses are acceptable.
• the reaction vessel 6 can be any one of the types commonly used for extraction. Its major purpose is to provide sufficient residence time and temperature for completion of the extraction reaction. If the process is to be used in conjunction with oxygen delignification, the reaction vessel 6 can be any of the oxygen delignification systems commonly used for that purpose.
• the tank 8 (Figs. 4-7) can be any standard filtrate or seal tank commonly used in the pulp industry. Its purpose is to serve as a collection point for filtrate and provide a barometric seal whenever a vacuum filter is used for filter 3. Tank 8 could be eliminated from the systems without significantly altering the efficiency of the system.
• the filtrate reactor 9 (Figs. 4-7) is constructed to provide the necessary residence time (5 to 10 minutes) for filtrate treatment with a minimum of backmixing.
• Filtrate reactor 9 includes a heater 12 to add the heat to the filtrate, which will raise the temperature of the filtrate to its reaction temperature of 50 to 60oC.
• a pipeline reactor with indirect steam heating would be acceptable as a filtrate reactor.
• reaction vessel 6 As a control, one sample of the same pulp was well washed, blended with sodium hydroxide solution to a consistency of 7.7% in a plastic bag, and placed in a constant temperature bath for 30 minutes to simulate normal alkaline extraction.
• pulps were dispersed in deionized water to 1% consistency, well washed, formed into sheets and analyzed for kappa number using TAPPI Method T236 m-60. The conditions used and results are listed in Table IV.
• the dosage of NaOH applied to the pulps is expressed as a weight percentage based on oven dry pulp. Lignin removal is recorded as the change in kappa number of the pulp as a result of treatment.
• Run No. U3 shows that the process of this invention can also be used to effect a reduction in operating temperature of the extraction while still achieving a small improvement in delignification.
• Example 1 A sample of the same unbleached softwood kraft used for Example 1 was delignified with oxygen using the process of Fig. 1. Treatment conditions were the same as in Run No. U3 of example 1 with the exception that after 67% of the liquid phase was removed on the buchner funnel, the sheet was blanketed with oxygen of 99.5% purity. The oxygen was allowed to permeate the sheet under the action of the vacuum. The sheet was lifted carefully from the filter to preserve its porosity, placed in an oxygen atmosphere inside a plastic bag and treated at 70oC for 30 minutes. Table V shows the results of this test.
• Examples 3 through 12 show the use of this invention for alkaline extraction of chlorinated pulps.
• the hardwood kraft pulp chosen for these tests had a kappa number of 15.8, and the softwood kraft pulp (kappa 25.2) was the same as used for feed stock in Examples 1 and 2.
• the pulps were chlorinated for 60 minutes at 3.5% consistency and 35oC.
• a measured quantity of concentrated chlorine/water solution was diluted with sufficient water to give the desired test consistency and immediately blended with 50 gm (o.d. basis) of the prewashed pulp.
• the reaction mixture, in covered containers was then placed in a constant temperature bath to carry out the chlorination. Periodic mixing of the pulp suspension was provided during the initial heating up period.
• the chlorine dosage used for the tests was varied and is reported in the examples to follow. All samples were well washed prior to being used.
• Example 12 After treatment of the pulps according to the methods used in Examples 3 through 11, the pulps were well washed, formed into sheets and analyzed for extracted permanganate (CEK) number using TAPPI Method T214 m-50. In Example 12, the pulp was well washed and its response to sodium hypochlorite bleaching was measured. Unless stated otherwise, the dosages of chlorine, alkali, and hypochlorite reported in Examples 3 through 12 are expressed as a weight percentage based on oven dry pulp.
• Example 3 To demonstrate the process of Fig. 1 on chlorinated pulp, two samples of hardwood which had been chlorinated with 3.2% chlorine were blended with identical amounts of NaOH solution. The first was allowed to react for 1.25 minutes at 40oC and 10.4% consistency after which 69% of the liquid phase was removed and the thickened pulp, now at 27.2% consistency, was treated for, an additional 60 minutes at 60oC. As a control, the second sample was simply treated at 10.4% consistency for 60 minutes at 60oC without removal of the liquid phase. The amount of alkali blended with the pulps was the same in both cases, 1.91% based on oven dry pulp weight. The results are shown in Table VI.
• Fig. 1 The process of Fig. 1 was used for oxygen extraction of chlorinated softwood kraft pulp.
• the pulp which had been chlorinated with 4% chlorine, was diluted to 1% consistency and formed into a pad on a buchner funnel. The pad consistency was estimated to be 25%.
• the pulp was then saturated to 11% consistency by distributing preheated NaOH solution on its surface.
• the alkali solution contained 3.3% NaOH based on ovendry pulp weight.
• the thickened pulp was then treated in an atmosphere of pure oxygen gas for 60 minutes at 60oC and 1 atmosphere total pressure.
• Example 5 To show the effect of higher alkali dosage on the process of Fig. 1, softwood kraft pulp chlorinated with 4% chlorine was used. The procedure was identical to that used in example 4 with the exceptions that higher alkali dosages (9.2% vs. 3.3%) were used, the pulp was saturated to 8.3% consistency on the buchner funnel instead of 11%, and after removal of 67% of the liquid phase, the pulp pad had a consistency of 20% instead of 25%. Treatment time in the second stage was 90 minutes instead of 60, and second stage treatments with and without oxygen were tested. The control was reacted at 3.1% alkali and 8.3% consistency for 90 minutes at 60oC. The results are given in Table VIII. TABLE VIII
• Hardwood kraft pulp was chlorinated with 3.5% chlorine and used in another demonstration of the process of Fig. 1.
• first stage consistency, first stage time, and alkali charge to the first stage were varied. The procedure differs somewhat from that used in the previous examples.
• reaction was conducted in polyethylene bags instead of by flooding the buchner funnel. This allowed the use of lower consistencies in the first stage and simulated the use of the mixers.
• the first stage treatment was conducted at ambient temperature (23 to 24oC) followed by partial removal of the liquid phase by filtration on a buchner funnel. Enough liquid was removed to give a pulp consistency of about 30% for the second stage of reaction.
• the second stage reaction was conducted in polyethylene bags at 60oC for 60 minutes.
• Fig. 3 The pulp had been chlorinated with 4.4% chlorine and well washed. Sodium hydroxide solution and pulp were blended in plastic bags at 10% consistency, and 25oC and immediately placed in a constant temperature bath at 60oC for periods ranging from 1 to 5 minutes. The pulp was then promptly filtered on a buchner funnel, diluted to 1% with deionized water and filtered again removing approximately 95% of the residual first stage liquid. A second aliquot of NaOH was then blended with the pulp at 10% consistency and 25oC followed by treatment at 60oC for 60 minutes. The total charge of NaOH was 3.3% which was divided between the two stages. In one test, the entire alkali charge was added to the first stage with water only added to the second stage. After the second stage the pulp was well washed, formed into sheets and analyzed for CEK number.
• HW3 F 1.15 2.0 40 0.76 2.0
• results show no apparent difference between operation at 1% or 10% consistency in the first stage.
• the effect of time in the first stage is small but shows a slight preference for the longer time of 4 minutes .
• Example 10 Using the process of Fig. 5, a series of runs were made to demonstrate the reuse of first stage filtrate.
• the chlorinated pulps were identical to those used in Example 8.
• the alkali charge was 3.3% for the softwood and 1.91% for the hardwood.
• Preheated NaOH solution was blended with the pulps in plastic bags and the mixture allowed to react for 1.5 minutes at 40oC and 10% consistency.
• the slurry was then filtered on the buchner funnel and washed with treated first stage filtrate from a previous run on the same species.
• the filtrate had been treated by holding it at 60oC for a period of time between 5 and 12 minutes.
• the filtrates from these two operations were combined and treated at 60oC as before.
• Example 8 Comparing these data with Example 8 shows about the same result for hardwood and better results for softwood.
• First stage time 1.0 min.
• First state consistency 3.5%
• Second stage consistency 23%
• Second stage temperature 60oC
• Second stage time 90 min.
• the results show a stable value of CEK number of 3.3 using the process of Fig. 6. This represents an 18.5% reduction in CEK number and shows that the process of this invention can achieve excellent results.
• the result also shows that the process of Fig. 1, when used at high alkali dosage, can be used as a valid simulation of the process of Fig. 6.
• a high yield kraft pulp was delignified with oxygen and alkali for 30 minutes at 100oC and a pressure of 100 psig.
• the resulting pulp which had a kappa number of
• Run No. OX3 preheated sodium hydroxide solution was blended with the pulp at 3% consistency in a Pyrex beaker and allowed to react for one minute at 52oC and an alkali charge of 12.87%.
• the pulps were well washed with deionized water and bleached with sodium hypochlorite.
• the hypochlorite bleach was conducted at 10% consistency and 50oC for 60 minutes at a starting pH of 11.5. Hypochlorite dosage was 0.70% expressed as active chlorine and was the same for all tests.
• the samples were filtered, well washed, formed into pads and analyzed for Elrepho brightness according to TAPPI method T452 om-83 and cupriethylenediamine (CED) viscosity by TAPPI method T230 om-82.
• the spent liquor from the filtration of the hypochlorite bleaches was analyzed for residual hypochlorite. This enabled calculation of the amount of hypochlorite consumed during the bleach. This is reported in Table XV along with the brightness and viscosity results.
• Fig. 1 1.5% H 2 O 2 added * as active chlorine, o.d. pulp basis
• the results show that the process of this invention can be used to reduce the amount of chemical consumed in subsequent bleaching steps.
• comparison of Runs OX2 and OX3 shows that the hypochlorite reduction is equal or better than that obtained by adding a 0.5% charge of peroxide to a normal extraction stage.
• Run No. OX4 shows that even greater effectiveness is achieved when peroxide is used in the process of this invention.
• the improvements in brightness and viscosity achieved by the process of this invention over the corresponding control test shows the product benefits gained by application of the process.
• the process of this invention has been demonstrated by the Examples 1 through 12 to be an effective method to improve the efficiency of delignification of unbleached softwood kraft, chlorinated softwood kraft, chlorinated hardwood kraft and chlorinated oxygen delignified kraft pulps by extraction with sodium hydroxide. It is further shown in Example 12 that the process is also effective when hydrogen peroxide and sodium hydroxide are used together.
• the process of this invention will also improve the efficiency of delignification when other alkaline substances are used.
• alkaline substances as have been used in the prior art for delignification are ammonium hydroxide, lithium hydroxide and other alkali metal hydroxides.
• lignocellulosic materials can be used effectively with a broad range of lignocellulosic materials.
• a partial list of these lignocellulosic materials would include but should not be limited to the following: nonwood fibrous materials such as bagasse, kenaf, bamboo, grass and other vegetable fiber, unbleached hardwood kraft pulp, unbleached softwood sulfite pulp, unbleached hardwood sulfite pulp, chlorinated softwood sulfite pulp, chlorinated hardwood sulfite pulp, unbleached and chlorinated pulps from all pulping processes on all types of lignocellulosic material, and partially bleached pulp which has had 3 or more stages of bleaching such as CEH, CED and others.
• This invention improves the efficiency of pulping and bleaching by providing a technique which achieves greater extraction of lignin without the use of additional chemicals.
• the improvement in efficiency results in a net reduction in chemical usage and additionally produces a product pulp of higher brightness and higher viscosity.
• the process When used for alkaline extraction of chlorinated pulp the process can be operated in a manner to reduce alkali consumption while maintaining the same amount of extraction as measured by the CEK number. This will reduce the operating costs of the bleach plant by an amount equal to the reduced alkali usage.
• the process has the flexibility which allows it to be operated at alkali consumptions equal to or higher than used in normal bleach plant practice. This enables reductions in CEK number considerably greater than can be achieved by either low presure oxygen extraction or by applying comparable increases in alkali charge to state of the art extraction systems. Only pressurized oxygen systems have reported reductions in CEK number as high or higher than the 36.8% obtained in Example 5, and these require expensive pressurized equipment. Further flexibility is also offered by the fact that the process is effective when peroxide is used in the extraction.
• the process can also reduce costs and pollution in another way. It is possible to use the process to reduce chlorine consumption in the first stage of bleaching while maintaining normal levels of alkali charge in the extraction stage. In addition to reducing chlorine costs, this also enables a reduction in pollution from the chlorination filtrate, which is highly toxic to aquatic life.
• the process can also be used for both applications simultaneously, the delignification of unbleached pulp and the extraction of chlorinated pulp. This allows the benefits of the process to be realized for both.
• the process behaves as if one of the materials which is extracted from pulp by alkali can undergo reactions with the remaining lignin which inhibit its further removal. Condensation reactions are known to occur in delignification, and these are likely the reactions responsible for the inhibition. When the pulp is first contacted with alkaline solution, these interfering substances are rapidly dissolved. This provides them with greater mobility than they had in the solid phase and condensation reactions begin to occur. The condensation reactions occur somewhat more slowly than the initial dissolution process. Therefore, if the liquid phase is promptly removed from the pulp suspension after the initial dissolution period, the condensation reaction with pulp lignin is effectively blocked by physical separation of the pulp and liquor. The best time to separate the pulp and liquor is when the competition between the condensation reactions and dissolution process begins to favor condensation.

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