CA2023916A1 - Delignification of lignocellulose-containing fiber - Google Patents

Abstract

ABSTRACT

Reduction in required amounts of chlorine and optionally chlorine dioxide in the chlorination stage (C or CD) of a multistage process for delignifi-cation and bleaching of lignocellulosic pulp is achieved by increasing the temperature and O2 normally employed in the oxygen alkali extraction stage (Eo) concurrently with the addition of hydrogen peroxide to the pulp just before or after introduction of the molecular oxygen used in the oxygen alkali extraction stage.

Description

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DELIGNIFICATION OF LIGNOCELLULOSE-CONTAINING FIBER

TECHNICAL IElD
The present invention ~s d~rected to improvements ~n processes for bleach~ng and del~gni~icat~on of wood pulp and other l~gnocelluloslc materlal.

BACKGROUND QF THE INYENTI~N
In the production of bleached pulp, unbleached brown pulp ( brown-stock ) from the pulp mill ~s d~rected to the bleach plant where lt 7s sub~ected to a sequent~al serles of alternat~ng dellgnlfy~ng/bleach~ng and extraction steps, each sta~e involvlng d~st~nct~vely d7fferent bleaching chemlcals and/or process cond~tions. Whereas the process object~ve of - pulplng is to chemically del~gn~fy wood or o~her lignocellulos~c mater~al (remove the lignin glue that b~nds the cellulostc f~bers together), the prlmary objectlve of bleaching is to wh~ten the pulp, albe~t some residual dellgnif~cation occurs. In pulp~ng, the measure of e~fec~iveness is the content of the remainlng l~gnin and lign~n resldues, wh~ch ts commonly expressed as the Kappa or permanganate number. In bleach~ny, one stlll determ~nes the Kappa number, but the primary analytical parameters are the pulp br~ghtness and vlscosity.
Essentlally, all commerclally practlced bleach processes are chlorlne-based. A variety of sequences are used to bleach pulp to the des~red target br~ght- ness levels, typically greater than 79 br~ghtness un~ts (ISO) and commonly 84-88. The latter stages tend to ~nvolve mllder, and more select~ve, and correspond~ngly more expens~ve bleachlng agents like C102. The more common bleach sequences use molecular chlorine, chlorlne d~oxide, or hypochlorite. Among the more common bleach~sequences are CEDED, OCEoD, CEoD, CEDEpD, CEHD, and~CEOHD w~h the f~rs~ alkaline extract~on stage (E) commonly reln~orced with oxygen (Eo); where:

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-C - Chlorinat~on with chlorine (C12), commonly accomplished w~th co-addition ~CD~ or pretreatment (D/C) with chlor~ne d~oxide.
E = Alkali Extraction wlth NaOH.
Ep or Eo = Peroxide or oxygen-reinforced alkali extract~on.
D ~ Dioxide treatment w~th chlorine dioxide (C102).
H = Alkal~ne Hypochlorite bleaching, typically w~th sodlum hypo-chlorlte (NaOCl).
O = Oxygen bleaching with molecular oxygen ~2) The actual sequence utilized by a g~ven plant is a reflection not only of target brightness, but also local process economics, brownstock species, end use of pulps, and age of the bleach plant.
Concern over the negative impact on the environment of chlorine-based bleach plant effluents has accelerated in recent years particularly slnce the discovery of the highly toxic chlorlnated dloxlns and furans ~n some bleach plant effluents, sludge, and pulp products. Today it is generally accepted that it is critical ~o reduce the amount of chloro-organics ~n pulp and the plant effluent.
Formation of these chlorinated organlcs ls s~rongly related to the use and consumption level of molecular shlor~ne in the chlor~nat~on stage (Axegard, P., 1988 Pulplng Conference, page 307). Reduced C12 dosage results in reduced organochlorides, commonly referred ~o collectlvely as TOCl/AOX. However, in the absence of alternative technology, the required C12 dosage cannot be arbitrarlly reduced w~hout s~gn~flcant adverse effect on pulp quality. It ls of great lmportance to mtnim~ze formation of chloro-organics through identify~ng a cost-effect~ve means allowlng a reductlon ln the requlred amount of molecular chlorine utlllzed in the chlorlnatlon stage, rather than rely on post-treatment technolog~es such as advanced wastewater treatment systems for the eff1uent.
The domlnant cost items 1n the production of bleached pulps are chem~cal costs and investment/capital costs, the latter due mainly to the number of chemical treating stages lnvolved. Accordlngly any acceptable technology

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desirably would build on existing process technology and be sensit~ve to capital requirements and chemical costs. Ideally the technology would use exist~ng onsite chem~cals and equipment.
The two most accepted technologies for reduction in the required molecular chlorlne are oxygen pretreatment and partial substltut~on of chlor~ne with chlorine dioxlde ~n the chlortnat~on stage. Both are belng commercially ~mplemented (L. Tench and S. Harper, TAPPE 55 (1987);
G. E. Annergren, et al., Svensk Papperstrippin~, 90, 29 (1987). Oxygen bleaching preceding chlorinat~on can reduce chlorine requirements up to 45%
before pulp strength properties are adversely affected. Signif~cant chemical cost savings are also realized. However, payback ~s very long due to the huge capital tnvestment in the oxygen prebleach stage (e.g., CEDED -~ OCEDED) and addltional plant retrofit requirements such as add~t~onal washers or new recovery boilers to accept the added non-chlorine conta~ning load from the lS oxygen stage.
Chlorine reduction can also be achieved by substituting part of the chlorlne requirement by high levels of chlorine dloxlde in the chlorinatlon stage. Although not as common as oxygen prebleach~ng, the ch~ef advantage ls that theoretlcally no signiflcant cap~tal ~s ~nvolved. However most m~lls do not have suff~cient ClO2 capacity to handle the add~t~onal load. Although one ach~eves the envlronmental object~ve o~ re~uced chlorine consumpt~on, the chem~cal sav~ngs advantage of oxygen bleach~ng is lost since one is sub-stltuting the considerably more expensive chlor~ne dioxide (45-50¢/lb) for chlor~ne (8-10¢llb). Also, the increased consumptlon of chlorine dloxide may require investment in add~tional on-si~e diox~de generators since the diox~de is unstable and must be made on-site as needed. Furthermore, dloxide sub-st~tutlon technology is not as effective ~n short sequence bleach processes arlsing from oxygen alkali extract~on (Eo) technology when high brightness pulps are required (Annergren, G. E., et al., 1988 INTE~NATIONAL PULP
BLEACHING CONFERENCE PROCEEDINGS, pp. 37-46).
- Oxygen-reinforced alkal7 extraction (E~), is now commonly practiced ~n most bleach plants. Eo ~s a relat~vely low capltal, chemical cost-savlngs technology that simply involves mixlng/~n~ectlng oxygen ~nto the alkal~ne pulp of the ~lrst alkal~ne extraction stage whlch results ln reduced chemical : . ' . , ' - . .
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requirements in the subsequent bleach stages. The reduced chemical require-ment to achieve brightness often allows the optlon of converting a five stage bleach processes to only three stages (~short sequence bleaching"); e.g., CDEDED -~ CDEoD (J. S. Enz, et al., I~ , 143 (1984? at optimal conditions of 60C and 25 psig, an 2 contact time of no more than S minutes is required, and these systems are so des~gned. Higher pressures and longer reaction times are of no benefit (B. Van L~erop, et al., _APPI 75, December 1986); nor are they available ~n ex~sting Eo processes. Slmilarly, there ls no advantage to increase temperature above 50C (B. Van Lierop, et al., Proceed~nas 1985 International Pulp Bleaching Co~ferenc~, 83); the Eo process is run at 50-70C because that was the pr~or existlng E-stage temperature. Added reinforcement chemicals such as hypochlorite (EolH) or peroxide (Eolp) can also be beneficial to incrementally attaining higher brightness pulps or further reducing chemical consumption in the post-EO
bleach stages, (Nonni, U.S. Patent 4,568,420 (1986)).
Eo technology is opt~mized for and directed to saving bleach chemicals in the subseque~t (3rd stage and later) stages. Recently ~t was reported by S~oblom, et al. (K. S~oblom, et al., 198~ Int'l Pulp Bleachina Conference, pages 263-270) that a chlorine reduction ln the chlorination stage could be achleved by ~ncreaslng the dosage of the very expensive chlorlne dioxlde to the chlorination stage as previously disclosed by Annergr-en, et al., (1988 Int'l Pulp Bleach~ng CQnference, page 37), and also modifying the Eo stage conditions including increased temperature, pressure, oxygen contact time, added MgS04 and optionally reinforcing the Eo stage with hydrogen per-oxide. Pulp brightness could be maintained only with increased chlorinedioxide dosages to the chlor~nation stage which was dlsclosed by Annergren, et al. 1~88 Int'l Pulp Bleachinq Conference, page 37, and also by Axegard (TAPPI Journal, 54, October 1986). Even so, the ut~lity of the descrlbed process is severely limited. To reduce required chlorine by this technology, an increase in bleach chemical costs is required as well as additiona1 reinvestment in new Eo process equipment to accommoda~e the required increased 2 reaction times and additional chlorine dioxlde capaclty. To reduce relatively inexpensive chlorine, as thereln proposed, one must ~ncur the costs of the considerably more expensive chlorine d~oxide, hydrogen peroxide, MgS04 and added capltal equipment.

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J ~ 7'.'~ 3 The vast majortty of Eo systems are of the upflow-downflow conflgura-tlon and are s~zed for a 3-5 minute reaction time at 25 pslg. (B. VanLierop, et al., TAPPI 75, December 1986). The proposed "hot extractlon" technolog~
(K. Sjoblom, et al., op. clt.) requlres greater pressures and considerably longer oxygen contact reaction tlmes approachlng 30-40 minutes. Only in a system providing an upflow Eo stage, whlch ls relatively uncommon, is sufflcient residence time avallable to accommodate the technology without a major reinvestment in new Eo process equipment, as well as additional ClO2 capacity.
lo In summary, all approaches to signi~cantly reduce the requ~red amount of chlorlne require investment in significant new capital equipment or in addltlonal and expenslve bleaching chemlcals.
Among the objects of the present invention is to provide an improved bleachlng process comprising a simple and easily implemented chlorine-reduct~on technology that does not simultaneously requlre signlficant capital investment and which results ln net chemlcal savings while achleving deslred pulp brightness and strength (v~scosity).

SUMMARY OF THE INV NTION
In accordance wlth the present invention the requlred dosage/charge of chlorine and/or both chlorine and chlorine d~oxide ~s reduced without detri-ment to flnal brightness an~ viscoslty, by heat~ng the pulp to 85-100C and addltlon of a small amount o~ hydrogen peroxlde ~o the pulp ~wlthout need of ma~nesium sulfate or other stab~llzer) just prlor to or dlrectly after the 2s additlon of oxygen in the existing Eo stage. The process of the inventlon is appllcable to treatment of llgnocellulosic pulp in a ~ultistage bleach~ng process comprislng sequential chlorination and oxygen alkall extract~on stages ln whlch the chlorlnation stage optionally contains either or both chlorlne (Cl2) andlor chlorine dlox~de ~ClO2), conventionally ~ndlcated as a C, CD or DIC stage.

BRIEF DESCRIPTION OF ~ DRAW~
The inventlon w~ll be more clearly understood and lts several advantages appreclated from the descript~on which follows read ~n connectlon wlth the accompanying drawings, wherein:

Figure l is a schematic flow diagram o~ a conventional 5 step multistage pulp bleachlng sequence having a down~low alkall extractlon stage (E) follow-ing chlorlnation (C).
Figure 2 is a schematic flow dlagram of a modif~ed bleachlng system em-ploying only 3 treating stages, having an oxygen reinforced alkali extractionstage (Eo)~ that may be employed in practice of an exemplary embodiment of the present invention.
F~gure 3 ls a graphic representation of experimental data demonstrating loss of pulp brightness observed as a result of arb~trarily reducing chlorlne charge ln the initial chlorinatlon stage of a short bleach sequence ( CDEOD) ' Flgure 4 is a series of bar graphs demonstrating loss of brightness with reduced Cl2 even at varlous Eo reinforcement conditions based on data recorded in Examples l through 6.
Figure S is a series of bar graphs o~ experimental data demonstrating final brightness can be malnta~ned a~ reduced Cl2 dosage only by increaslng temperature and addltion of peroxide before excess oxygen lntroduction into underchlor~nated pulp (Examples 7 and 8) as compared to control (conventlonal) conditions (Examples l and 2).
Flgure 6 is a graphlc representation of experimental data demonstrating effect of modifled Eo stage conditlons on f~nal pulp ~rlghtness of under-chlorinated pulp ~n a short sequence (CDEoD)~ comparing Example 12 wlth a control under conventional Cl2 and Eo condi~ions (Example 9).

DETAILED DESCRIPTION
Figure l illustrates an embodiment of a conventlonal system employlng a five-stage sequence for dellgniflcat~on and bleaching of wood pulp. The illustrated system comprises 5 consecutive treating towers deslgnated C
(chlorination), E (alkal~ extraction), D (ClO2 treatment), followed by E and D. The unbleached pulp is treated with Cl2, w~th or without ClO2 reinforcement in the f~rst C tower. The pulp enters at the bottom of the first C tower, flowing upwardly through the tower and dlscharging at the top of the to~er lnto a washlng sectlon wherein lt ~s washed with water. The washed pulp is discharged from the washer vla line 14, sodlum hydroxide being added thereto. Llne 14 dlscharges the thus alkalized pulp lnto a steam mtxer 3 ç~ ; r~
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15 by which it is brought to a tempera~ure ~n the range of 40-70C. The thus heated pulp is pumped at 15 ~nto the top o~ the E tower 16, where it is sub-jected to alkallne extractlon followed by water washing as lndicated at 18.
The washed pulp ~s then pumped, at 20, into the bottom of the f~rst D
tower 22 and flows upwardly through ~hat tower in which it is treated with C102. Followlng the C102 treatment, the pulp is agaln washed with water at 24 and pumped into the top of the second E tower 25 with addition of sod~um hydroxide. Dlscharged from the bottom of tower 25 the pulp is again washed 1n water at 26 and ~ntroduced into the bottom of the second D tower 28 for a second treatment with C102; the thus bleached pulp discharged from the top of tower 28 being water washed at 29.
The conventlonal S stage system ~CDEDED) of Figure 1 ls often replaced by a short three stage sequence (CDE~D); see Enz, et al., TAPPI Pro-ceedlngs, 1983 Pulplng Conference, pages 3Q9 313; Boussard, et al., op. clt., pages 315-317). The three stage sequence wlthout loss ln pulp quallty was made posslble by introduction of molecular oxygen into the pulp enterlng the thus modified alkall extraction stage ( oxygen alkali extract~on ) designated by the ~ymbol Eo~ lllustrated in F~gure 2, llke parts bearlng the same reference characters as ln F~gure 1 wherein applicable. As shown in Figure 2, an 02/pulp mixer or 2 d~ffuser 30 is provided in the llne feedlng the Eo tower.
In practlce of the present inventlon employing the short sequence of Figure 2, the operating conditions of C and Eo stage are modified by:
(a) reductlon ln dosage of chlorine employed to KF 0.11 to 0.20, (b) increasing the temperature of the alkallne under chlorinated pulp by addlng more steam to the pulp at the steam mixer in l~ne 14, to ralse exlt pulp temperature to 85-100C, and (c) addlng at least about 0.3% hydrogen peroxlde to the pulp ~ust before or after 7ntroduction of the molecular oxygen thereto9 whlch is also lncreased to about 0.8-1.2% by welght on dry pulp. The peroxide may be added, for example, to the washed pulp ~n line 14 together with or precedlng the alkall addlt~on, or subsequently thereto at the steam mlxer, but preferably at pump 15, or alternatlvely at a po~nt in line ~ust prlor or after the pulp/02 mlxer or diffuser 30.

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Whlle in Flgure 2, the practice of the invention is descrlbed as applîed to ~he CDEoD sequence, ~t will be understood that the invention ~s not limited thereto but can be employed to advantage in the 5 stage sequence or ln any bleach sequence having an oxygen alkali extractlon stage follow1ng the chlorination stage with attending reduction of the otherwise required amount of Cl2 and/or Cl02 needed to be employed. The advantages of the invention are easily obta~ned without loss of deslrable pulp brightness, while achlev~ng the benefits of oxygen bleaching, chlorine d~oxide substitut~on, and hot alkali extract~on, without the attendant high cap~tal and added chemical costs, and w~thout requ~ring any ma~or change in the exist~ng process con-flgurat~on. Not only are environmental beneflts achieved such as reduced effluent TOCl/AOX due to the reduced amoun~ of applied C12, net chem~cal sav~ngs are also realized by the invent~on slnce the required amounts of both chlorine and also the more expensive chlorine dioxide, are slgni~cantly reduced.
A prlmary objective of the present ~nvention ~s to enable reductton ~n the required amount of chlorinating agent needed to be employed ~n dellgnlfl-cation and bleach~ng of l~gnocellulos~c pulp to obtaln the desired product wlthout adverse effect on pulp brightness or viscosity, wh~le avo1d~ng the otherwlse negat~ve impact on environment of chlorine-based bleach plant ef-fluents.
There is generally a linear relat~onship between ~ncreasing chlorlne appl~cat~on (dosage) and the pulp llgnin content, which may be expressed ~n terms of Kappa number. The hlgher the Kappa number of the pulp the greater 2S the chlorlne dosage needed to del~gn~fy the pulp. Typ~cal for kraft softwood, for example, ~ndustry employs 6 to 8% chlorlne by weight on dry pulp. The amount of molecular chlor~ne requlred for effective dellgniflcation of the pulp of a g1ven Kappa number ~s expressed by the Kappa factor ~KF~, thus % Cl2_ KF ~ Kappa No.

Use of less chlorine results in unacceptable pulp of low brlghtness.
S~nce Cl02 is frequently added to the pulp at the C stage, the quantity of actual molecular chlorlne can be reduced by substltutlon of the actlve chlorine equlvalent with Cl02, so , - . ,` ' `

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KF _ / C12 + 2-~3 (% C102) ~ Kappa number In the dellgnification of unbleached l~gnocelluloslc pulp a Kappa factor ~n the range of 0.21 to 0.23 is typically advocated and employed in m~ll practice, based on actual plant experience to provide a balance between suf-f~cient delignificdtion and minimizing subsequent/downstream chemical re-quirement to achieve target brightness.
To demonstrate the advantages afforded by the present invention, results from several bleaching sequences were compared. The basic CDEoD thrce-- stage sequence was selected for demons~rat~on purposes in all of the reported runs s~nce it is a common commercial sequence and it is an ascepted laboratory standard ln the industry. The operatlng con~it~ons employed in the demonstra-tion are represented by:
CDEoD whlch is representative of a common three-stage sequence wlth chlorine dioxide substituted for some chlorine in the chlorlnation stage as is common practice in the art and employlng a convent~onal oxygen alkali extract~on stage (3-5 minutes oxygen contact timeS
~o 20-25 psig, and 60-70C~.

This is the reference sequence in whlch it is demonstrated that total chlorine charge (C+D) in the chlorination stage can be reduced by pract~ce of the invention. In this reference experimentt a total chlorine charge of 7.5% ls used at 15% dioxide subst~tution (6.38%
C12 + 1.13% D as act~ve chlorine~.

KF , 6~382~-7~ = 0.23 30 C~EoD Also the reference three-stage sequence in which the chlorine in th~ chlorination stage was reduced without obtaining benef~t of the present invention.

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~EoD Also the reference three-stage sequence, except the reaction para-meters of the Eo stage are modifled/enhanced ~n concert with a reduction in either or both chlorine and chlorine dioxide in the chlorination stage.

In the experimental runs set out below, commercial, unbleached southern plne softwood kraft pulp tbrownstock) was used. The in~tial Kappa number was 32.7 and exhibited a 0.5% CED v~scoslty of 20.6 centipoise (cp). The operating condltions employed are summarlzed below.
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A. Standard Chlorination (CD) Stage:

Crumbled pulp was placed in a polyester bag and an amount of chlorine water and chlorine dioxide added to make the charge 6.3~% and 1.13%, respectively, as active chlorine. Dllution water was then added to bring the pulp to a consistency of 3.5%. The bag was heat sealed and the chlorination allowed to proceed at amb~ent temperature for 45 minutes.
The pulp was filtered (effluent pH = 1.7 to 1.~) and washed wlth water.

B. cQnventlonal and Re~nforced Oxygen Alkali Extract~on (Eo) Stage:

The oxygen reaction vessel was a d~rec~ steam-heated pressure vessel containing a removable rack upon which seven clrcular stalnless 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 lnt~-mate contact with oxygen in order to simulate good 02/pulp contact available in the bleach plant.

While the reactor was preheatlng, sufficient waterj chlorlnated pulp, and alkall (NaOH) were mlxed to bring the consistency to 12% and alkali charge to 3.64%. ~hen perox~de, magnesium sulfate, or hypochlorite were to be added, these chemicals were also mixed with the pulp at th~s tlme.

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After the pulp samples were placed on the vessel s removable trays, the assembly was placed in the preheated reac~or whlch was then bolted closed and the temperature raised immedlately (<2 minutes) to 70C. Oxygen was then added to the reactor at 25 psig. After 5 mlnutes exposure to oxygen, the oxygen was vented and the extract~on was allowed to proceed an addltional 55 mlnutes (total t~me never exceeded 60 minutes) wlthout oxygen to complete the extraction stage. The separate pulp samples were then re~oved, washed with water, and prepared for the dloxlde bleachlng stage.
C. Chlorine Dioxide Bleach (D~ Stages:

Each ~ndividual pulp sample removed ~rom the trays of the extractlon reactor was sub~ected separately to differen~ levels of dioxlde charge in order to determine the ~leach~ng performance profile, i.e., brlghtness vs. dioxide charge.

The extracted pulp sample was placed in a polyester bag and a calculated amount of aqueous chlorine dloxide added (usually 0.5 to 2.0% on pulp) followed by suff~clent water to bring pulp sonslstency to lO%. The bag was sealed and rapldly brought to 70C and maintalned at thls temperature for 3 hours. At th~s time, typically an allquot of bleach liquor was also removed and analyzed for resldual dioxlde. The d~oxlde bleached pulp was then treated w~th sulfur diox~de to br~ng the pH to 3 prlor to form~ng handsheets for brightness measurements.

Results are graphically represented as brlghtness versus dloxide charge which ls a measure of the abillty of this sequence to achleve target brightness.
Kappa number (T236), viscosity (T230), handsheets (T2l8), and br~ghtness (T217) determ~natlons were made ~n accordance with ~he respectlve TAPPI
Standard Test Procedure identified by ~he numbers ~n parentheses. Chem-ical charyes are on a welght percent basis; pulp welght Is reported on an air dry bas~s.

' b ~ ~' ~m~le 1: Control Experiment and Reference/Standard CDEoD Sequence with Conventional Eo Stage The purpose of thls control example ~s to determine the requ~red amount of dloxide to achieve a given br~ghtness level ~or conventlonally chlorinated pulp at KF = 0.23, and to show what maximum bright- ness level could be achieved at conventional CDEoD bleach con~itions for thls brownstock pulp.

Following the general procedures outlined above, commercial brownstock of Kappa number 32.7 and a viscosity of 20.6 cp was subjected to chlorination ~6.38% C12 + 1.13% C102 as active chlorine~ KF = 0.23) and conventlonal oxygen alkali extraction ~3.64% NaOH, 70C, 5 minutes 2 at 25 ps~g, balance of 60 total minutes with no oxygen) stages. The washed pulp from the CDEo stage was dlvided into 4 portions and subjected to different dosages of chlorine dioxide at a pH of 3.8-4.3 to complete the CDEoD~

TABLE l(a) ~_~ Brightness Viscositv 0.5 72.5 20.7 0.~ 79.8 21.0 1.0 ~.9 20.5 1.3 8~.6 20.1 :
. ' ~` ' '' ~, , Example 2: Demonstration of Loss o~ Final Brlghtness that would be Observed with and Reduction ln Chlorlne Charge without any Modification of Eo Stage (C~EoD) The experiment descr1bed in example 1 was repeated except that the molecular chlorine charge was reduced by 20%; the chlorlne diox~de charge was maintained at the amount used ~n example 1 so the effective C102 substltut~on level was of course higher ~n this experiment. The Kappa Factor (KF) was 0.18 or 22%
lower than the conventional chlorinatton of Example 1.

T~BLE l(b) ~_~ Brightness VisCositv 0.5 57.4 19.0 0.8 69.0 20.3 1.0 73.4 20.3 1.3 7~.6 19.6 These results are graphically presented in Figure 3 along with those of the control experlment (example 1). It is clearly seen that commercially unac-ceptable pulp (severe brightness loss) results from the environmentally de-sirable act o~ reducing the chlorine consumptlon in the chlorination stage even if one increases the level of dioxide substitution in the chlorination stage.

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Example 3-6: Demonstration That Pulp Brightness Canno~ be Mainta~ned at Reduced (20%) C12 Charge Even at Increased Dioxide Substi-tution and Several Common Enhancements to ~he Conventional Eo Stage (CDEoD) The purpose of this serles of exper~ments, the resul~s of wh~ch are summarlzed in Figure 4 along w~th the control tests o~ examples l and 2 for comparlson, is to demonstrate that at a reduced chlor~ne dosage of only 20%, one cannot malntain target/acceptable pulp brightness even at h~yher chlorine dioxide substitution levels, simply by modifying conventional Eo stage process con-dit~ons to longer oxygen contact tlmes ~5 minutes to 20 minutes~ or, even at this extended contact tlme, by increased Eo stage temperature (70-100C), or addltlon of hypochlorite or hydrogen peroxide to the Eo stage.
Examples 3-6 were performed as described in example 2 where the chlor~ne charge alone was reduced by 20~o w~th the chlor~ne dioxide level maintainQd at the same (example l~ chlorinat~on stage level so, in effect, to ~ncrease the dioxide substitutlon level to greater than the init~al 15X~ Otherwise, only the Eo stage parameters were changed as descrlbed bPlow, w~th the results compared ln Table 2.

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- ~ ' ' '.' ~' , ' , Brightnçss at Respective D~oxide Dosage Exampl~ 0.5% 0.~% 1.0% 1.3%

Ex. 3 Extend 2 contact time of 62.5 73.2 76.2 81.7 graph Eo stage from conventional 4c 5 to 20 minutes.
Ex. 4 Eo stage temperature ~n-graph creased from conventional 70:
4d (1) to 10VC, 2 con- 68.6 78.2 81.5 84.2 tact t~me - 20 minutes.
(2) to 110C, 2 con- 53.6 75.0 79.4 82.0 tact t~me - 20 mlnutes.

NOTE: H~gher temperature (above 100C) glves poorer results.

Ex. 5 0.7% sodlum hypochlor~te 5g.5 70.0 73.4 78.4 graph added to Eo stage; 2 4e contact t~me - 20 minutes.

Ex. 6 0.7% hydrogen peroxide added 69.4 77.2 81.5 83.8 graph to Eo stage; 2 contact tlme 4f - 20 minutes.

As seen in Flgure 4, bar graphs 4a and 4b show the brlghtness of pulp a~ter receiving treatment under condit~ons o~ Examples 1 and 2, respectlvely, (83.9 at fu11 chlorine dosage and 73.4 at 20% rQduction ~n chlorlne dosage).
Bar graph 4c shows that the loss in brightness at lowered chlor~ne dosage ls not compensated by extension of the t~me~of exposure to added oxygen (Example 3). Bar graph 4d shows that some further increase in br~ghtness ls had by .
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raising the temperature of the Eo stage to 100 or 110C (Example 4) wh~le maintaining the oxygen exposure at 20 minutes. Example 5 (bar graph 4e) carried out at the conventional Eo temperature (70C) at extended time of 2 exposure (20 minutes) but with addition of hypochlorlte to the Eo stage does not achieve acceptable brightness. Nor ~s desired brightness achieved by uslng hydrogen peroxide additive in the Eo stage (Example 6). The results of Examples 3 to 6 are summarized in Table 2.

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Examp~e 7-8: Demonstration that Pulp Br~gh~ness can be Maintained with 20%
Less Chlorine by Both Add~ng Peroxide and Increasing Alkaline Pulp Temperature Prior to Addition of Oxygen ~n the Eo Stage The experiment described in example 2, which reduced ch1Orine by 20% while maintaintng the same dioxide level in the chlorinatlon stage, was repeated except 0.7% hydrogen peroxide was added to the alkal~ne pulp which was then heated to 100C pr~or to contacting with pressurized oxygen at 25 psig for 5 minutes ~example 7) and 20 minutes (example 8>.

The results are tabulated below ln Table 3 and summarized in Figure 5 along w~th the control tests of examples 1 and 2 for comparison. The results clear-ly show th~s unique and specific condltion allows a reduction ~n chlor~ne charge wlthout resorting to long oxygen contact times unavallable to existlng convent~onal Eo systems. Furthermore, if longer 2 contact times were made available, only incrementally hlgher brightness levels could be ach~eved at the reduced chlorine charge.

Table 3 Pulp Br~gh~ness Example 7 Example 8 ~_~ (5 mins 02 Contact~ ~2Q mins 02 Conta~
o.5 70.1 80.1 .8 80.5 86.1 1.0 84.9 86.4 1.3 86.2 87.4 .., .~, r?

Example 9-11: Demonstration that the Pulp Properties o~ Brightness, Vis-cosity, Cleanliness, and the Quality of the Eo Stage Effluent can be Ma7ntalned with a 30% Reduct~on in Chlorine and 100%
Reduct~on/Elim~nation of Chlorine Dioxide in the Chlorinatlon Stage (CDEoD vs. C~oD) For purposes of this demonstration, a new control (base case, example 9) was simultaneously completed along with two experiments (examples 10 and 11) that demonstrate the technology at 0.4 and 0.5% hydrogen perox~de charges (prior examples used 0.7% perox1de). The results are tabulated below and summar1zed in Table 7 showing that within experimental error, final pulp quallty (brlght-ness, viscosity, and shives) and effluent quallty from the Eo stage are maintained at 30% reduction of chlorine and w~th elimlnation of the C102 in the chlorinatlon stage. The data also sugges~s 0.3% peroxide will be minimum required dosage.

Example 9 - New Control The bleach sequence experiment descr~bed in example 1 was repeated except at a molecular chlorine charge of only 6.19% and a dioxide charge of only 0.414%
~KF = .20~. Results are reported ln Table 4.

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Table 4 Eo Sta~e D-Stage 0.5% 0.8% 1.0% 1.3/0 ~ 2.0%
Viscosity 20.5 Color 35,250 COD 1,424 Brightness 71.7 82.4 85.4 87.3 88.2 88.5 V~scos~ty 20.2 --- 19.0 19.6 19.4 18.~
Shives --- --- --- 42S 400 325 Example 10 Experiment~example 9 was repeated except with 30% less C12 (4.33% C12) and no chlorlne dioxlde in the chlorlnatlon stage. The Kappa Factor was only 0.13, or 35% less than the conventlonal chlorinat~on in Example 9. The temperature of the Eo stage was increased to only 90C and only 0.4%
peroxide added prior to the oxygen. Oxygen contact time and pressure were maintained at 5 minutes and 25 psig. Results are reported in Table 5.

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Table 5 ~ç~ Q _ D-Stage O.5% 0.8% 1.Q% 1.3% ~ 2 Q~
V~scosi~y ~0.9 Brightness 60.8 73.1 78.2 83.1 85.1 85.6 Viscosity 18.9 19.3 19.0 19.0 17.6 lb.8 Shlves ___ ___ ___ ___ 375 575 Example 11 Experi~ent/example 10 was repeated excep~ with 30% less C12 and no chlorlne d~oxide ln the chlor1natlon stage (KF = 0.13). The temperature of the Eo stage was increased to only 90C and only 0.5% peroxlde added pr~or to the oxygen. Oxygen contact time and pressure were maintained at 5 minutes and 25 pslg. Results are reported in Table 6.

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Table 6 Eo Stage D-Staae 0.5% ~ ~_ 1.0% 1.3% 1.~% 2.0/~
Viscosity 21.3 Color 36,375 COD 1,478 Brightness 66.3 79.1 81.9 84.6 86.0 86.5 Viscosity 18.3 18.0 18.6 17.7 16.1 16.4 Shlves --- --- --- 300 22S 225 The results of Examples 9-10 are compared in Table 7. Final brlghtness is shown after 2% C102 treatment in the D stage as well as shives count.

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Effect of both increased temperature and peroxlde addition before oxygen on the final pulp properties and Eo stage effluent ln CDEoD bleachlng at reduced C12 and C102 in chlorination stage (35% lower Kappa Factor), is seen in Table 7 below:

Table 7 (Ex. 9) (Ex. 10) (Ex. 11 CDEoD _ CEoD
Conventional 30X C12 Reductlon Eo at 70C, El~mlnation of C102 5 min 2 Eo at 90C, 5 min 2 No Peroxide_ 0.4% Peroxtde0.5% Perox~de Eo Stage Pulp V~scos~ty 20.5 20.9 21.3 Effluent, Color 35,250 - 36,375 COD 1,424 -- 1,478 D-Stage at 2% C102 Pulp Brightness 88.5 85.6 86.9 Pulp V~scosity 18.2 16.8 16.4 Pulp Shlves 325 375-575 225 ,~
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~mple 12: Demonstration that Observed Results are not a Tradeoff on the Beneflts to Conventional Eo (CDEoD vs. CDEoD) Th~s experlment follows Nonni, U.S. 4,568,420 of reinforclng the con-ventlonal Eo stage with H202, except that the temperature of the Eo stage ls lncreased to 90C.
The experimental bleach sequence descr~bed ~n example 9 (full charge of C12 + C102 in chlor~natlon sta~e) was répeated except the temperature o~
the Eo stage was ~ncreased to 90 and 0.5% perox~de added. The results are tabulated below and summar~zed ln Figure 6 along with the control, examp)e 9, accompllshed at conventlonal E~ conditlons. The simllarlty of thls brlght~
ness vs. dloxlde dosage and bleach~n~ curve show, as expected, that this pro-posed new chlorlne reductl~n technolo~y is different and not slmply a varia-tlon on relnforced oxygen alkali extraction, as seen ln Table 8.

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Control Thls Example 12 ~_~ Ex. 9 Brlqh~nessVL~Cosltv 0.8 82.4 85.2 20.4 1.0 85.~ 86.8 20 1 1.3 87.3 87.8 19.3 1.6 ~.2 88.2 18.1 2.0 88.5 88.6 16.9 :
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No advantage is gained in brightness at the elevated temperature as expected from the Nonni examp1es and B. Van Llerop et al. Proceedings 1985 nternational Pulp Bleachinq Conference page 83 which states there is no advantage to operating an Eo system above 50C.
As demonstrated in our invent~on a surprising temperature effect is observed only when chlorinating agent ls reduced.
The benef~clal advantages of the present invent~on are realized when the hereln advocated process conditlons are u~ilized in the treatment of unbleached pulp (CEo... sequence) as well as in the del~gni~catlon/bleaching of a pulp wh~ch has been oxygen prebleached (OCEo...). Moreover the addition of a magnesium compound (MgS04) or other stabil~zer deemed essential in prior pulp treatment to avo~d pulp degradat~on may be om~tted when practicing the present invention.
: In general the desired reduced chlorination bleach process of the present invention can be achieved by reduclng the needed chlorinat~ng agent (molecular chlorine with or wlthout chlor~ne d~ox~de) dosage to that required to maintain a Kappa factor in the range of 0.11 to 0.20 employing selected operating cond~tions not conventional in the oxygen alkali extractlon stage (Eo) following the init~al under chlorination at reduced KF sa~d sondit~ons including:
(1) lncreasing oxygen to the total amount of 0.8 to 1.2 percent by weight of pulp (dry basis) and at a pressure of 20 to 70 psig; higher oxygen dosage (beyond about 1.5%) can be used without added advantage. Oxygen contact tlme should be 3 to 5 minutes at 20-25 pslg.
(2) use of temperatures above that commonly employed in the art pre-ferably above 85C`and up to about 100C. Temperature above 100C obtaln no added advantage and can have adverse effect on pulp quality.
(3) add~tion of at least 0.3% and no more than about 1% hydrogen per~
oxlde by welght of pulp (dry basis).
All of the above conditions (1) higher oxygen dosage (2) higher temper-ature (3) minimum peroxide dosage ind~cated must be observed to malntaln desired brlghtness at the reduced chlorination dosage (reduced Kappa Factor).
No benefit is observed in bleachlng without reduced chlorine dosage as shown in Example 12.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the delignification and bleaching of lignocellulosic pulp wherein the unbleached pulp or oxygen pretreated pulp is treated in successive stages with various delignification and bleaching chemicals including in sequence an initial chlorination stage with a chlorinating agent comprised of chlorine dioxide or of molecular chlorine with or without added chlorine dioxide followed in turn by alkali extraction in the presence of added molecular oxygen the improvement which comprises:
utilizing in said initial chlorination stage an amount of chlorinating agent corresponding to a Kappa factor equal to or less than 0.2 performing said alkali extraction at a temperature of at least 85°C
with the molecular oxygen being at a dosage of at least 0.8% by weight of air dried pulp and with the addition therein of at least 0.3% hydrogen peroxide by weight of pulp on a dried pulp basis.

2. The improvement as defined in Claim 1 wherein the initial chlorination stage is performed with an amount of chlorinating agent corresponding to a Kappa factor in the range of 0.11 to 0.18.

3. The improvement as defined in claim 1 wherein said alkali extraction is carried out at a temperature in the range of 85° to 100°C.

4. The improvement as defined in Claim 3 wherein the hydrogen peroxide addition is no more than 1% by weight of the pulp.

5. The improvement as defined in Claim 4 wherein the hydrogen peroxide is added to the pulp prior to the molecular oxygen.

6. The improvement as defined in Claim 5 wherein following the recited alkali extraction stage the pulp is subjected to one or more additional bleaching stages.

7. The improvement as defined in Claim 6 wherein at least one of said bleaching stages is performed using chlorine dioxide.

8. The improvement as defined in Claim 7 wherein during said alkali extraction stage molecular oxygen is introduced for up to five minutes at a pressure of about 25 psig.

9. The improvement as defined in Claim 8 wherein said alkali extraction is effected during a total period of 60 minutes.

10. The improvement as defined in Claim 1 wherein the initial chlorination stage is performed with molecular chlorine without addition of chlorine dioxide, said molecular chlorine being applied in an amount corresponding to a Kappa factor of 0.13, and wherein 0.4 to 0.5% hydrogen peroxide by dry weight of pulp is used in the alkali extraction stage.

11. The improvement as defined in Claim 1 wherein said initial chlorination is carried out under conditions corresponding to a Kappa factor of 0.20 and said alkali extraction is carried out at 90°C with the addition of 0.5% peroxide by weight of dry pulp, followed by at least one bleaching step using chlorine dioxide.

12. The improvement as defined in Claim 11 wherein in said alkali extraction the molecular oxygen is introduced for about five minutes at a pressure of about 25 psig, after which the extraction is continued in the absence of additional oxygen for up to about a total period of about sixty minutes.

13. The improvement as defined in Claim 1 wherein said alkali ex-traction is performed in the absence of magnesium compound or other added stabilizer for retarding pulp degradation.