EP0303962A2 - Procédé d'extraction en milieu alcalin en présence d'oxygène pour la production de pâte blanchie - Google Patents

Procédé d'extraction en milieu alcalin en présence d'oxygène pour la production de pâte blanchie Download PDF

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Publication number
EP0303962A2
EP0303962A2 EP88113019A EP88113019A EP0303962A2 EP 0303962 A2 EP0303962 A2 EP 0303962A2 EP 88113019 A EP88113019 A EP 88113019A EP 88113019 A EP88113019 A EP 88113019A EP 0303962 A2 EP0303962 A2 EP 0303962A2
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Prior art keywords
pulp
oxygen
stage
oxidant
bleaching
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EP88113019A
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German (de)
English (en)
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EP0303962A3 (fr
Inventor
John Federick Cirucci
Augustine Ivanhoe Dalton
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Air Products and Chemicals Inc
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Air Products and Chemicals Inc
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Publication of EP0303962A2 publication Critical patent/EP0303962A2/fr
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    • 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 present invention is concerned with the manufacture of bleached pulp, particularly for the production of white cellulosic paper products.
  • the unbleached brown pulp (“brownstock”) from the pulp mill is directed to the bleach plant where it is subjected to a series of delignifying/bleaching process steps, each involving distinctly different bleaching chemicals and/or process conditions.
  • the process objective of pulping is to chemically delignify wood or other ligno-cellulosic material (remove the lignin "glue” that binds the cellulosic fibers together)
  • the primary objective of bleaching is to whiten the pulp, albeit some residual delignification occurs.
  • the measure of effectiveness is the content of the remaining lignin and lignin residues, which is commonly expressed as the Kappa or permanganate number.
  • the primary analytical parameters are the pulp brightness and viscosity.
  • bleach pulp to the desired brightness level, which usually ranges from 75-93 brightness units (I.S.O), typically greater than 79.
  • I.S.O brightness units
  • Many different sequences have been studied, but some representative, commercially practiced examples are: CEDED, C D EDED, OCEDED, CEHD, CEHED, OCEH, CEHDED and CHHD.
  • Each sequence can be different not only with respect to the order of chemical addition but also in process configuration.
  • the pulp becomes progressively brighter with each bleach step and contains less residual lignin.
  • the actual sequence utilized by a given plant is a reflection not only of target brightness, but also local process economics, feedstock type, end use of the bleached pulp product, and age of mill.
  • pulp strength and yield after bleaching are important to the pulp manufacturer.
  • Multistep bleaching utilizing progressively milder and more selective bleaching agents, which tend to be relatively more expensive chemicals, is necessary with the present state-of-the-art to maintain the pulp strength and yield.
  • typical brownstock pulp of 28-35 Kappa can be bleached in a CE sequence (two stage) to a Kappa less than 6; but if one attempted to achieve a brightness level of 88 with this chlorine, the chlorine would begin to react with the cellulosic pulp (modify structure and depolymerize cellulose) reducing both the yield and fiber strength.
  • Oxygen Alkali Extraction has been symbolized in bleach sequence terminology as E o .
  • E o oxygen-based process technology
  • Some representative examples are: CoEoD, CoEoHH, CoEoDED, CE o H, CE o HDP, OC D E O HD, and OC D HE O DEH.
  • the primary effect of the E o process technology is to reduce consumption of chlorine dioxide and hypochlorite in the subsequent bleach stages while retaining pulp brightness and strength. This effect has been utilized to attain a net cost savings benefit among other useful beneficial advantages, such as:
  • dynamic mixer In present practice of the E o stage a dynamic mixer, to which the oxygen is directly added to effect efficient 0 2 /pulp mixing, is incorporated between the C- and E-stage vessel/tower.
  • dynamic mixer is defined as equipment which provides the high-shear stress necessary for the intimate mixing of oxygen with medium-consistency pulp. It shall include mechanical mixers of the "high-shear” type, medium-consistency centrifugal pumps which create a turbulent, high-shear zone at their discharge suitable for mixing, refiners, and "static” or “motionless” mixers in the case where they impart high-shear stress on pulp suitable for oxygen mixing.
  • a retention tube is also added between the Chlorine and Extraction stages at a point after alkali and steam addition and the added dynamic mixer.
  • a thick stock or medium consistency pump is also required if not already present in-line.
  • Oxygen is added using the dynamic mixer to effect good mass transfer of the oxygen into the alkyline pulp. Conventional wisdom holds that oxygen mass transfer is the limiting factor; i.e., to get acceptable process results, the oxygen must be rapidly and intimately contacted with the alkaline pulp.
  • the function of the "retention tube” is that of an oxygen reactor; i.e., to allow sufficient time and pressure for the oxygen to react/bleach the pulp before entering the E-tower. A minimum reaction/retention time of 2-2 minutes under pressure is required. If the E-stage operates in the pulp upflow mode, or a displacement bleach tower is used, a retention tube is not required.
  • U.S. Patent 4,451,332 is concerned with multi-stage bleaching of ligno-cellulose containing fiber material in which oxygen-containing gas is employed in an alkali extraction step.
  • the oxygen-containing gas is mixed with the cellulose fiber material in a manner so as to form a foam of these, which foam, without an intervening oxidation step, is subjected to alkali extraction in upward flow.
  • the method is aimed at production of a bleached delignified fiber product without bleaching the extracted lignin, and to suppress the problem of lignin condensation causing pulp discoloration.
  • the oxidative extraction can be employed as a step in various multi-stage sequences of the prior art.
  • the present invention provides a process which achieves the desirable benefits of oxygen alkali extraction (E o ) without being committed to the requirement for oxygen addition quickly after alkali addition or to the costly investment of a dynamic mixer.
  • the beneficial advantages of oxygen alkali extraction are obtained in the absence of a dynamic mixer and/or without the requirement for oxygen addition quickly after alkali addition.
  • This unexpected result is achieved by mixing alkaline pulp with molecular oxygen and a small amount of a co-oxidant, at a point either before or within an alkaline bleaching stage of a ligno-cellulose pulp delignification and bleaching process carried out in order to increase pulp brightness and/or to reduce chlorine-based chemical consumption of subsequent bleaching stages.
  • FIG 1 illustrates application of the invention in a typical prior art multi-stage pulp bleaching system having a retention tube just prior to the alkaline extraction stage (E).
  • the unbleached pulp enters the bottom of tower 10 in which it is treated with chlorine (C) and discharges from the top of tower 10 into a washing section 12 wherein the chlorinated pulp is washed with water in conventional manner.
  • the washed pulp is discharged from 12 via line 14 into which line sodium hydroxide is added.
  • the resulting thick stock is then introduced by means of a thick stock pump or medium-consistency pump 15 into the bottom of a retention tube 16, the pulp being contacted with molecular oxygen in the line after the thick stock or medium consistency pump, at the bottom of the retention tube, or within tower 18.
  • the thick stock leaving pump 15 flows directly into the bottom of retention tube 16 and discharges from the top of that tube directly into extraction tower 18.
  • Molecular oxygen is introduced either immediately after pump 15, into the bottom tube 16 or directly into tower 18.
  • the co-oxidant employed in accordance with the invention may be introduced into the stock upstream of pump 15, into the stock discharged from pump 15, into the bottom of tube 16, or directly into the top of tower 18.
  • the alkaline pulp having been treated with molecular oxygen and co-oxidant is withdrawn from the bottom of tower 18 by a pump 19 and washed at 20.
  • the washed pulp is discharged from 20 via line 22 and pumped into the bottom of tower 25 by pump 24.
  • the pulp is treated with C10 2 (D) introduced into the line entering tower 25.
  • the pulp flows upwardly through treating tower 25 and discharges at the top of the tower into a wash station 26.
  • the treated pulp is pumped by means 35 to a wash station 36.
  • the washed pulp discharged from 36 via line 37 is pumped at 38 into a second upflow tower 39. Additional C10 2 is introduced into the pulped stream entering the bottom of tower 39. Leaving the top of tower 39 the pulp is again washed at 40 and discharged via line 41 as bleached pulp.
  • the system illustrated in Figure 1 operates in the bleaching sequence C E o D E D.
  • the introduction of molecular oxygen and co-oxidant in the E-stage can similarly be practiced in plants having a bleach sequence other than that illustrated in Figure 1, as for example, in systems in which the C Stage is substituted by a C D or D/C stage with or without a preceding 0 stage, as well as in systems having a lesser number of stages, e.g. C E o D, CE o HD.
  • the oxygen in the embodiments of Figures 1 and 2 as well as in other arrangements employing molecular oxygen/co-oxidant in the E o -stage the oxygen can be added nearly simultaneously with the co-oxidant or after the co-oxidant but preferably not before the co-oxidant.
  • employing at 15 a medium consistency centrifugal pump it is not necessary to add the gaseous oxygen into the high-shear zone at the pump discharge as taught by prior art, but, rather, the oxygen can be applied directly into the pulp at a location convenient to the plant-specific configuration.
  • the illustrated system is one employing an upflow E-stage, but otherwise operating in the same sequence as that illustrated in Figure 2, CE o DED. Similar parts of the illustrated apparatus have the same numbering as in Figures 1 and 2.
  • the thick stock discharged by pump 15 is discharged into the bottom of upflow tower 18.
  • Oxygen is introduced directly at or near the bottom of tower 18; the co-oxidant may be introduced at the same level as the oxygen or upstream of that level in tower 18 or into the line entering the bottom of that tower.
  • FIG. 4 illustrates a typical prior art system for practice of displacement bleaching modified in accordance with the invention for introduction of oxygen gas and co-oxidant.
  • Unbleached pulp is pumped by means of 40 and mixed in a known manner at 41 with chlorine, chlorine dioxide and water.
  • the pulp is discharged into the bottom of multilevel displacement bleach tower 45.
  • tower 45 the pulp flows upward through a series of zones where treatment with bleaching chemicals occurs. Each zone replaces an individual stage or tower in a conventional bleach plant.
  • Bleaching chemicals are transported to the appropriate zones via central shaft 42.
  • Extraction zone 43 replaces the first extraction stage of conventional bleach plant.
  • zone 43 the pulp can be treated with oxygen and co-oxidant in a manner chemically identical to that described previously for a conventional bleach plant.
  • Methods of introducing oxygen and a co-oxidant into the extraction zone are depicted in figure 4.
  • Oxygen and co-oxidant can be dispersed into the sodium hydroxide (extraction) solution at location 44 which passes down through shaft 42 and is diffused radially into zone 43 or added into the extraction zone through the side of the tower.
  • Figure 4 depicts only one bleaching sequence that can be performed by displacement bleaching.
  • the invention for introduction of oxygen gas and co-oxidant can be applied to a displacement bleach plant regardless of the bleaching sequence if an alkaline zone is present.
  • FIG. 5 is a schematic flow diagram for introduction of oxygen into a conventional multi-stage pulp bleaching system employing an existing alkaline hypochlorite treating tower (H).
  • the unbleached pulp is introduced into the bottom of tower 50 in which it is treated with chlorine introduced into feed line 51.
  • the chlorinated pulp discharged at the top of tower 50 is washed at 52 and the washed pulp leaving the washer via line 54 is pumped at 55 into the top of tower 58.
  • Sodium hydroxide is introduced into line 54 downstream of washer 52.
  • Initial alkaline extraction (E) takes place as the pulp descends in tower 58.
  • the pulp discharged from the bottom of tower 58 is again washed at 60 and the washed pulp discharged via line 62 is passed to the top of the hypochlorite treating tower 64, the hypochlorite treating solution being introduced into line 62.
  • the oxygen can be added as described for the CE o DED sequence (Fig. 3) or without additional co-oxidant.
  • the oxygen can be added prior to the H-stage (after hypochlorite application) or directly to the H-stage.
  • oxygen is introduced prior to or directly into tower 64 in which the pulp is undergoing treatment with alkaline hypochlorite.
  • the oxygen can be added nearly simultaneously with the co-oxidant or after the co-oxidant.
  • the oxygen may be added at any designed dosage; generally six to eleven pounds of oxygen per ton of oven-dry pulp is satisfactory.
  • the oxygen may be introduced in any convenient manner such as by one or more diffusers after pump 15, or it may be sparged directly into existing equipment.
  • the co-oxidant may be alkali or alkaline earth hypochlorite, chlorine, or hydrogen peroxide. Alkali hypochlorite is preferred due to its relative cost effectiveness. The choice between hypochlorite and peroxide can be based on existing on-site availability and economics.
  • the co-oxidant may be added in dosages up to 5 weight percent on pulp, but preferably in the range of 0.2-2.0% (oven-dry basis) with pulp of medium consistency.
  • a co-oxidant may employ that piped in from another part of the bleach plant, if available.
  • caustic sodium hydroxide
  • the oxygen reaction vessel was a direct-steam heated pressure vessel containing a removable rack upon which seven circular stainless steel mesh trays are arranged, one above the other.
  • the trays allow thin layers of pulp to be dispersed within the vessel so as to provide intimate contact with oxygen in order to simulate good 0 2 /pulp contact provided by a dynamic mixer.
  • the reactor was preheated while the alkali and pulp were mixed as described above for conventional extraction (E). After the pulp samples were placed on the vessel's removable trays, the assembly was placed in the preheated reactor which was then bolted closed. Oxygen was immediately added to the reactor to 25 psig and the temperature raised immediately ( ⁇ 1 min) to 70°C. After 4-10 minutes exposure to oxygen, the oxygen was vented, and the extraction was allowed to proceed an additional 35-45 minutes without oxygen in order to simulate the extraction stage. The separate pulp samples were then removed, washed with water, and prepared for the dioxide bleaching stage.
  • This procedure involving delayed oxygen addition, is to simulate the adverse impact of not using a dynamic mixer and/or not contacting the alkaline chlorinated pulp with oxygen quickly after alkali addition.
  • the same reactor, conditions, and procedure was utilized as described above for conventional oxygen alkali extraction except admission of oxygen was delayed for 15 minutes after the pulp had been brought to 70 °C, i.e., 15 minutes after caustic was added.
  • the oxygen contact time remained 4-10 minutes as before, at which point the reactor was vented and the extraction allowed to proceed at 70°C for a total period of 45 minutes.
  • the extracted pulp sample was placed in a polyester bag and a calculated amount of aqueous chlorine dioxide added (0.1 to 2.2% on pulp) followed by sufficient water to bring pulp consistency to 12%.
  • the bag was sealed and rapidly brought to 70 C and maintained at this temperature for 3 hours. At this time, an aliquot of bleach liquor was removed and analyzed for residual dioxide. If the dioxide treatment was the last stage of the particular bleach sequence, the pulp was treated with sulfur dioxide to bring the pH to 3.
  • Kappa number, viscosity, handsheets, and GE brightness determinations were made in accordance with the respective TAPPI Standard Test Procedures. Chemical charges are on a weight percent basis; pulp weight is air dry basis.
  • the purpose of this example is to provide comparative base case data on the effectiveness of a conventional bleaching sequence that incorporates a conventional oxygen alkali extraction (E o ) stage with a dynamic mixer; i.e., oxygen is added quickly after the alkali under conditions of intimate contact with the pulp.
  • E o oxygen alkali extraction
  • brownstock was subjected to a C D E O D bleach sequence.
  • the experiment was identical to that described in Example 1 for the C D ED sequence except for the oxygen stage which was carried out as described in the general procedure in a manner to simulate conventional E o .
  • alkali is added to washed chlorinated pulp and heated to about 70. C with steam.
  • the hot pulp is then rapidly pumped to a point at which oxygen is immediately contacted intimately with the pulp using a dynamic mixer, which effects good mass transfer of the oxygen into the pulp slurry.
  • the oxygen/pulp slurry then passes through a length of process pipe ("retention tube") sized to give the oxygen at least 2-4 minutes reaction time under pressure with the pulp prior to introduction to the alkaline E-stage/tower.
  • Retention tube a length of process pipe
  • chlorinated pulp from the C D -stage was charged with alkali and dispersed in a thin layer over several trays in the oxygenation reactor to maximize intimate contact of the 0 2 /pulp.
  • the reactor vessel was heated to 70° and brought to 25 psig with oxygen. After either a 4 or 10- minute oxygen contact time, the oxygen was removed and the caustic reaction was allowed to proceed an additional 40 or 35 minutes, respectively, to simulate conditions within the extraction (E) tower.
  • Example 3 The purpose of this example is to demonstrate that the pressence of low levels of hypochlorite will reverse the negative effect demonstrate in Example 3, overcome the requirement for quick and efficient mixing of alkali/pulp/ O 2 . and allow the successful practice of direct, in-line oxygen injection technologies in the absence of dynamic mixers at any point after alkali addition including directly into the pulp transfer lines or directly into the E-tower.
  • the experiment described in Example 3A was repeated except a 0.5% charge of hypochlorite was added along with the alkali. As before, the system was brought to temperature and after a 15-minute delay, the pulp was then contacted with oxygen for 10 minutes followed by the usual alkali extraction. The results obtained are set out in Table 4 below and are compared in Plot IV of Figure 7 with the results of Example 2 and 3 (curves II and III).
  • Example 2 The purpose of this example is to demonstrate that the effectiveness of this process is not due simply to adding an additional oxidant to the bleach process.
  • the experiment described in Example 2 which simulates conventional oxygen alkali extraction (E o ) with a dynamic mixer, was repeated in the presence of a 0.5% and 1.4% charge of hypochlorite. The results obtained are set out in Table 6 below and are plotted in Figure 9 along with those obtained in Example 2.

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
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EP19880113019 1987-08-17 1988-08-10 Procédé d'extraction en milieu alcalin en présence d'oxygène pour la production de pâte blanchie Withdrawn EP0303962A3 (fr)

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US8632487A 1987-08-17 1987-08-17
US86324 1987-08-17

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EP0303962A2 true EP0303962A2 (fr) 1989-02-22
EP0303962A3 EP0303962A3 (fr) 1991-06-26

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EP (1) EP0303962A3 (fr)
JP (1) JPS6485387A (fr)
BR (1) BR8804126A (fr)
FI (1) FI883811A7 (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4568420B1 (en) * 1984-12-03 1999-03-02 Int Paper Co Multi-stage bleaching process including an enhanced oxidative extraction stage
US4657633A (en) * 1985-05-24 1987-04-14 Westvaco Corporation Delignification and bleaching of a cellulose pulp with an alkalioxygen-hypochlorite single stage sequential extraction

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EP0303962A3 (fr) 1991-06-26
JPS6485387A (en) 1989-03-30
FI883811L (fi) 1989-02-18
FI883811A7 (fi) 1989-02-18
FI883811A0 (fi) 1988-08-17
BR8804126A (pt) 1989-03-07

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