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Chapter 4

Pulp Bleaching Technology

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CONTENTSPage

THE BLEACHING PROCESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Historical Development of Bleaching Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Extent of Bleaching in the Industry.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Bleaching M ethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bleaching Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

BoxesBo x Page

4-A. What Is Pulp Brightness? How Is It Measured?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424-B. Shorthand for Describing Bleaching Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444-C. Assessing Lignin Content and Pulp Bleach ability . . . . . . . . . . . . . . . . . . . . . . . . . 46

FiguresFigure Page

4-1. Bleaching Sequences Used in U.S. and Canadian Pulp Mills. . . . . . . . . . . . . . . . . . . . . . 444-2. Sequence for the Production of Fully Bleached Chemical Pulp. . . . . . . . . . . . . . . . . . . . 47

4-3. Pulp Brightness at Stages of the Bleaching Sequence CEHDED. . . . . . . . . . . . . . . . . . 484-4. Schematic Diagram of the Chlorination Process .,.,... . . . . . . . . . . . . . . . . . . . . . . . . . . 50

TablesTable Page

4-1. Domestic Bleached Pulp Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434-2. Bleaching Chemicals: Form, Function, Advantages, Disadvantages . . . . . . . . . . . . . . . 444-3. Examples of Prebleaching Sequences for Pulp Delignification. . . . . . . . . . . . . . . . . . . . 484-4. Common Sequences Used To Bleach Kraft Pulp to Various Degrees of Brightness. 49

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Chapter 4

Pulp Bleaching Technology

THE BLEACHING PROCESS

Bleaching is the treatment of cellulosic fiber with

chemicals to increase brightness (see box 4-A).Brightness may be achieved by either lignin removal(delignification) or lignin decolonization. Ligninremains a major constituent of pulp even afterdigestion by chemical pulping. For example, kraftpulp may contain up to 6 percent lignin based on itsdry weight.

1

Unbleached groundwood spruce pulpmay contain 27 percent lignin.

If chemical pulping removes the lignin from woodfibers, why then does some lignin remain after thepulping process? The strength of paper is largely dueto the chemical bonds (hydrogen bonds) formedbetween cellulose fibers. Although longer and moresevere pulping might remove more of the lignin, thusreducing the amount of bleaching needed, thecellulose molecules might be degraded and theirbonding power diminished. Should this happen, thestrength of the pulp would be reduced. The removalof lignin by bleaching is regarded as a continuationof the pulping process, albeit somewhat gentler andless destructive, but bleaching too can degradecellulose if done improperly.

Lignin imparts a color to the raw pulp (hence itsname brown stock) and unless removed, willcontinue to darken with age (note the yellowing,darkening, and enbrittlement of newspaper exposedto sunlight). Bleaching by removing the lignin giveshigher brightness to the paper than is possible byleaving the lignin in the pulp and brightening bydecolonization, and also leads to a more durable andstable paper.

In addition to the removal and decolonization oflignin, bleaching serves to clean the pulp of dirt andforeign matter that escaped the digestion process.Bleaching also removes hemicellulose and extrac-

tives (hemicellulose is nearly completely removedfor the production of dissolved pulps). Bleachingpulp adds significantly to its value as market pulpbecause the demand for bleached paper is increasing.

Historical DevelopmentofBleachingTechnology

Bleaching of fibers for decolonization has beenpracticed since early times. Sunlight was one of theearliest bleaching agents. Japanese paper makerswere known to bleach fibers by soaking them inwater from high mountain streams that containedozone (the first use of oxygen for bleaching). In1774, Karl Wilhelm Scheele discovered chlorineand its bleaching action on vegetable fibers. Severalyears later in 1799, Charles Tennant invented bleaching powder (calcium hypochlorite),thereby converting chlorine to an easily transport-able form. For the next 130 years it remained theonly available bleaching agent. The first time a U.S.

paper mill used chlorine for bleaching was in 1804.

Rapid developments in bleaching technologyoccurred between 1900 and 1930. Multistage bleach-ing using calcium hypochlorite followed by analkaline extraction stage, then a repeat of thehypochlorite bleach stage was first adopted by theindustry. Later, technologies that allowed the use ofgaseous chlorine began to displace hypochlorite inthe first bleach stage. The use of chlorine reducedbleaching costs. New equipment developments fur-ther improved bleaching efficiency.

Improvements in the manufacture of chlorine

dioxide and dioxide bleaching technology weredeveloped in the 1940s. By the 1950s, thesedevelopments led to the adoption of the five-stagebleaching sequence that is still used extensively inthe industry: (1) chlorine+(2) alkaline extrac-

sequence allowed very bright pulp to be producedwith minor losses in fiber strength.

Oxygen bleaching was discovered in 1952 byV.M. Nikitin and G.L. Akim in the Soviet Union. Inthe late 1960s, oxygen bleaching was commercial-

ized, followed by the installation of the first dis-placement bleach plant in the 1970s. This resulted inmore rapid bleaching by displacing chemicalsthrough a pulp mat rather than mixing the chemicals

Douglas Reeve, The Principles of Bleaching, 19/37Bleach Plant Operations Seminar, TAPPI Notes (Atlama,GA: TAPP1 Press, 1987), p,3.-41

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42 Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp

Box4-AWhat Is Pulp Brightnessy? How Is It Measured?

Brightness is the reflecting properties of a sheet of pulp. It is a physical and measurable phenomenon. Somemistakenly equate whiteness with brightness, but whiteness is a physiological phenomenon, measuredsubjectively by the impression and perception of the human eye. For instance, if blue dye is added to a lightly yellowtinted paper, the sheet will look whiter but the sheet will measure less reflected light (less brightness).

Since reflectance is affected by the nature and the angle of incident light, the surface properties of the pulpsheet, and other factors, the measurement of brightness has been standardized: Brightness is the reflectance of bluelight with a spectral peak at 457 millimicrons from an opaque sample of pulp sheets compared to a specified standardsurface.

There are two recognized methods for measuring pulp brightness in North America: 1) the TAPPI method(Technical Association of the Pulp and Paper industry), Standard T-452; and 2) the CPPA method (Canadian Pulpand Paper Association).

TAPPI MethodReported in units of %GE Brightness. Illuminating light is aimed at 45 degrees to the sample and the reflected

light is measured perpendicular to the sample (90 degrees). Reflectance is compared to magnesium oxide powder(98 to 99 percent absolute reflectance). Calibrated opal glass standards are used for routine measurements.

CPPA MethodReported in units of ISO Brightness. The sample is illuminated with diffused light using a highly reflecting

integrating sphere. Reflected light measurement is taken at 90 degrees to the sample. Reflectance is compared toabsolute reflectance from an imaginary perfectly deflecting, perfectly diffusing surface. Calibrated opal glassstandards are used on a routine basis.

A third brightness standard is used throughout the rest of the world: The Zeiss Elrepho standard. It is measuredin units termed Elrepho Brightness. GE Brightness is measured with a reflectance meter manufactured by theGeneral Electric Corp., while Elrepho Brightness is measured by an instrument manufactured by Zeiss, the Germanoptical company. Since the two meters have different light geometries, there is no simple relationship between thetwo measurements. In general, Elrepho Brightness is, on average, 0.5 to 1.0 percent higher than GE Brightness.

with the pulp in the conventional way. Since the late States as well.2

Overall, the tendency has been to1970s development has taken place in the use ofoxygen enrichment in alkaline extraction stages, to

further delignify pulp after extended cooking (modi-fications of the cooking process to improve deligni-fication), and in short bleaching sequences whereoxygen is used to supplement chlorine.

ExtentofBleaching in the Industry

Nearly 55 percent of the chemical pulp currentlyproduced in the United States is bleached (table 4-l).By far the greatest proportion of bleached chemicalpulp is produced by the kraft process (about 88percent of the pulp bleached in 1987 was kraft pulp).Very little mechanical pulp has been bleached in thepast, however, this is currently changing. Mechani-

cal pulp bleaching is growing at more than twice therate of chemical pulp bleaching worldwide and it islikely that this trend will continue in the United

bleach more pulp as the demand for products usingbleached paper increases (nearly 39 percent of the

domestic paper and paperboard currently producedis from bleached pulp).

Bleaching Methods

Bleaching Agents

Pulp cooking can safely dissolve up to about 90percent of the lignin without degrading the cellulosefiber. Additional delignification is done by bleach-ing. Bleaching of high-yield chemical pulps isachieved by decolonizing with either an oxidizingagent (combines oxygen) or a reducing agent(combines hydrogen). Chlorine gas, sodium hypo-

chlorite, chlorine dioxide, oxygen gas, and hydrogenperoxide are oxidants. Sodium hydrosulfite is areductant. Alkali is used to remove the solubilized

zIbid.,p. 2.

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Chapter 4Pulp Bleaching Technology 43

Table 4-l-Domestic Bleached Pulp Capacity(thousand metric tons)

may be offset by the cost of the chemical or theequipment needed to handle it.

Grade 1971 1987

Bleached sulfite ... , . . . . . . . . . . . . . . . . 1,774 1,256Bleached kraft. . . . . . . . . . . . . . . . . . . . . . 13,364 21,259Dissolving (bleached) ., . . . . . . . . . . . . . 1,604 1,455

Total chemical pulp produced . . . . . . . 32,779 43,800

Percent of chemical pulp bleached . . 51 .lO

/O 54.70/0a

EstimateSOURCE: Oouglas W. Rowe, The Prmctples of Sfeaching, 1987EUauch PkntC@rations Seminar, TAPP/ Notes Alfanta, GA. TAPPI Press, 1987), p. 2,

lignin from the cellulose. Each has its advantages,disadvantages, and limitations (table 4-2).

Since the 1930s, chlorine gas has been thepredominant chemical used for the delignification ofpulp. Chlorine dioxide can brighten pulp withoutdamaging the cellulose. Oxygen is comparativelyinexpensive and is now coming into its own both fordelignification (immediately after digestion and

before the bleach cycle) and as a supplement in thefirst extraction (alkali) stage of the bleach sequence.Hydrogen peroxide is expensive, so it is used muchless than other bleaching agents. The effectivenessof a bleaching agent, although a major factor indetermining its use in a pulp bleaching sequence,

A critical determinant in choosing a bleachingchemical is the selectivity of the agent. Selectiv-ity refers to the capacity of the chemical to attack

lignin while doing minimal damage to the cellulosefibers. Unbleached pulp (brown stock) contains highlevels of lignin, therefore less selective chemicals(e.g., oxygen and chlorine) can be used in the initialstages of the bleach cycle. With further delignifica-tion and lower residual lignin content of the pulp,more chemical is available to react with the celluloseand pulp strength may suffer.

Chlorine dioxide and hydrogen peroxide arehighly selective, thus they react rapidly with ligninbut affect cellulose very little. The highly selectivechemicals are generally used in later bleach stages

when the lignin content is low and the cellulose issusceptible to degradation. However, both chemi-cals are expensive and are therefore used sparingly.Sodium hydrosulfite, a reducing agent, and hydro-gen peroxide are used for bleaching lignin-richmechanical pulp.

Table 4-2-Bleaching Chemicals: Form, Function, Advantages, Disadvantages

Chemicals Function Advantages Disadvantages

Oxidants

Chlorine . . . . . . . . . . . Oxidize and chlorinate Iignin

Hypochlorite . . . . . . . Oxidize, brighten and solubilizeIignin

Chlorine dioxide . . . . 1 ) Oxidize, brighten and solubilizeIignin

2) In small amounts with chlorinegas to protect againstdegradation of pulp

Oxygen . . . . . . . . . . . Oxidize and solubilize Iignin

Hydrogen peroxide . . Oxidize and brighten Iignin inchemical and high-yield pulps

Reductant

Hydrosulfite . . . . . . . . Reduce and decolonize Iignin inhigh-yield pulps

Alkali

Sodium hydroxide . . . Hydrolize chlorolignin and solubilizeligin

Effective, economical delignification

Easy to make and use

Achieves high brightness withoutpulp degradation

Low chemical cost

Easy to use. Low capital cost

Easy to use. Low capital cost

Effective and economical

Can cause loss of pulp strength ifused improperly

Can cause loss of pulp strength ifused improperly

Must be made at the mill site

Used in large amounts. Requiresexpensive equipment. Can causeloss of pulp strength

Expensive

Decomposes readily. Limitedbrightness gain

Darkens pulp

SOURCE: Oougias W Reeve, The Pnnc@as of Blaachmg, 1W7 Bleach Wanf Operatkwts Semmar, TWPI Notes Atfanta GA: TAPPI Press), p. 6.

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Bleaching Sequences

A combination of bleaching and extracting treat-ments is generally used for bleaching chemical pulps(box 4-B). The bleaching chemicals and the order inwhich they are used make up the bleachingsequence. Bleaching sequences generally contain

two phases within each sequence: 1) a delignifica-tion segment, whose function is to remove thelignin; and 2) a brightening segment, whose princi-ple function is to increase the brightness of the pulp.Examples of delignifaction stages or segmentsinclude oxidation by chlorine (C) followed by

Box 4-B-Shorthand for DescribingBleaching Sequences

The pulp and paper industry has developed aseries of shorthand descriptors for the multistagebleaching sequences. The following abbreviations

are used to designate the bleaching agents:c Chlorination

E Extraction with sodium hydroxide

H Hypochlorite (sodium or calcium)

D Chlorine dioxide

P Hydrogen peroxide

o Oxygen

N Nitrogen dioxide

z OzoneBleaching sequences are designated by listing

each treatment serially. For example, CEDEDrepresents a commonly used five-stage bleachingsequence consisting of a first-stage chlorine treat-ment, followed by a second-stage alkali extractionstage, followed by a third-stage chlorine dioxidetreatment, followed by a fourth-stage alkali extrac-tion treatment, and a final fifth-stage chlorinedioxide treatment. Washing is conducted betweeneach chemical application.

Two bleaching agents may be used in a singlestage. For instance, chlorine gas and chlorinedioxide are sometimes combined in an early bleach-ing stage. If chlorine gas is the predominant agentin the mixture, the treatment would be designated asc

D. On the other hand, if the mixture contains

more chlorine dioxide than chlorine gas, the treat-ment would be designated as Dc. Other commonlyencountered oxidative extraction treatments in-clude E

O(or E/P), E

P, E/H etc.

extraction of the dissolved lignin with sodiumhydroxide (E). Brightening segments use sodiumhypochlorite (H) and/or chlorine dioxide (D). Oxy-gen can be used for delignification and for reinforc-ing extraction of the dissolved lignin in the alkalistage.

Several of the more commonly used bleachsequences in U.S. and Canadian mills are: 1)CEDED, 2) CEDH, 3) CEHDED, 4) CEH, and 5)CED (figure 4-l). Although these are the mostprominent bleaching sequences currently in use, anincreasing number of mills are now using oxygen incombination with alkali for extraction (Eo), andchlorine dioxide (CD, Dc) in the chlorination stages.In addition, there area number of unique bleachingsequences used by some mills (e.g., CEHDH,CEHEDP, CEDPD, CEDE

HD and CEHCHDED).

Factors Affecting the Bleaching Process

Process engineers and paper chemists have a widerange of chemicals, processes, equipment, andoperating conditions to choose from in optimizing ableaching sequence. Cost of chemicals, capital costof equipment, energy requirements, and other oper-ating costs figure heavily in bleach plant decisions.While cost control is an important factor, thephysical and chemical composition of the wood rawmaterial (furnish) and the desired characteristics(brightness and strength) of the finished paper areoften more important in selecting bleaching tech-nologies. Bleaching sequences also depend on thepulping process used for initial delignification, some

Figure 4-l-Bleaching Sequences Used inU.S. and Canadian Pulp Mills

CEDED

19%

BOURCE:Darn tmm David R. Forbos, IUpgmdlng Exlsti~ BlsactI Plants, 1987 Qwsfkwts Semhw, T@P Notvs AduIta,GA: TAPPI Press,1967), p. 116.

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Box 4-CAssessing Lignin Content andPulp Bleachability

The Technical Association of the Pulp and PaperIndustry (TAPPI) has devised two standardizedprocedures for determining and reporting the lignincontent of pulp: 1) Permanganate (K) number(TAPPI Test Method T214), and 2) Kappa number(TAPPI Test Method T236). These indices are usedby the industry to control cooking within thedigester during pulping and for determining thebleachability of the pulp.

Both methods are chlorine demand tests and arebased on the amount of permanganate needed tooxidize the contained lignin, The Permanganate, orK number, is used for determining the bleachabilityof chemical pulps having lignin contents below 6percent (based on weight of oven-dry pulp). Thekappa number is applicable to all grades of chemi-cal and semi-chemical wood pulps, including high-er lignin content pulps with yields as high as 70

percent.Standard procedures have been established for

both bleaching indices. Both are based on theamount of potassium permanganate that can reactwith dry pulp samples. Most modem pulp millsnow use automated, continuous oxidation-reduction measurements or optical devices such asbrightness meters for on-line measurements togauge the progress of delignification and the needfor additional bleaching. However, permanganatetests are still used in mill laboratories for verifica-tion of the instrument reading.

Bleaching Systems

Bleaching sequences apply various bleachingagents in different orders and combinations. Be-tween each bleaching stage the pulp is generally (butnot always) flushed with an alkali extraction solu-tion to remove the dissolved lignin before it is sentto the next bleaching stage (figure 4-2). The first stepof a bleaching sequence is designed to remove thebulk of the residual lignin (delignification) andinvolves little or no improvement in the brightnessof the pulp (figure 4-3). This step, along with thefollowing extraction stage, is called prebleach-

ing. The purpose of prebleaching is to remove asmuch lignin from the pulp as possible to minimizethe volume of more expensive bleaching chemicals(e.g., chlorine dioxide, hypochlorite, and hydrogenperoxide) needed in subsequent bleaching stages.

Chlorine gas and sodium hydroxide (CE) have

been the preferred chemicals for the prebleach stageof the bleaching process. More recently, mixtures ofchlorine and chlorine dioxide have been used inplace of (or in addition to) pure chlorine treatment(table 4-3).

10Other prebleaching processes are

slowly displacing CE as the first stages of the bleachsequence at some mills. Prebleach oxygen delignifi-cation and extended cooking may shorten thebleaching sequence by reducing the amount of lignincarried forward to the bleaching process.

11

Impetus for considering alternatives to the con-ventional CE stage are based partially on reducingthe cost of bleach plant operations and partially onconcerns about environmental impacts from dis-charged bleaching effluents. These concerns resultfrom detection of chlorinated organic material con-taining chloroform and dioxins in bleached pulp millsludge. Most of the chlorinated organics containedin bleach plant effluent originate from the firstchlorine, alkaline extraction, and hypochloritestages.

Brightening stages that follow prebleaching re-move less lignin but bring out the brilliance of thepulp through bleaching action. The brighter the pulpdesired, the more bleaching stages that must be used

(table 4-2). Mill operators have a number of mix-and-match brightening processes to chose from for finalbleaching. Choices are largely determined by therelative costs and efficiencies of the bleach optionsand the required brightness of the pulp beingproduced. The entire bleaching sequence is then acombination of the prebleaching stages and thebrightening stages.

Prebleach Delignification

Chlorination-Chlorine selectively reacts withlignin and under normal bleaching conditions doeslittle harm to cellulose fibers. Because of its relative

l u ap,Wngh and Edward S. Atkinson, The Alkaline Extraction, The Bleaching of Pulp Third Editbn(Atlanta, GA: TAPP1 Press, 1979), p. 9 ll o ~ GulliChXn ThePast and Future of PulpBleaching, 1987Bleach Plant Operations Seminar, TMPI Notes (Atlanta, GA: TAPPI Press,1987), p,19.

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Figure 4-2-Sequence for the Production of Fully Bleached Chemical

Raw water, Raw water Raw water Raw water

c o n d e n s a t e o r o r o r o r

white water w h i t e w a t e r w h i t e w a t e r

Pulp

Raw water

o r

white water

Unsreeened

B I a c k B l a c k I i q u o r C h I o r i n a t i o n E x t r a c t i o n

I i q u o r lost i n e f f I u e n t e f f I u e n tt o r e c o v e r y

s c r e e n i n g o r

c h I o r i n a t i o n

SOURCE: Caribn W. Denos snd 30ran E. Annergren, Chlorination, Tbe 8kachng of Pdp h Edition Attants, GA: TAPPI Press, 1978), p. 51.

cheapness in comparison with other bleach chemi-cals it became widely used for delignification afterthe pulping process. Chlorination in the prebleachcycle begins with washed brown stock pulp slurry atlow consistency (3 to 5 percent weight of pulp towater) being pumped into a chlorination mixer.Chlorine gas, which is often dispersed in water, isadded to the pulp slurry in the mixer and isvigorously mixed (figure 4-4).

The reaction between chlorine and lignin beginsimmediately in the mixer, and the reaction iscompleted in a chlorination tower designed to givethe proper retention time. If chlorination is con-ducted at low temperatures (5 to 45 C) retentiontime may range between 15 and 60 minutes. Highertemperatures reduce the time necessary to completethe chemical reaction. The chlorinated pulp iswashed before being sent to the alkali extractionstage.

Chlorination is sometimes repeated after extrac-tion if additional delignification is needed, butbecause of possible cellulose damage a chlorine

dioxide stage is often used. Throughout the processbrightness, kappa number, and other indicators ofpulp quality are monitored.

12

There is a trend toward modification of the firstchlorination stage by including other bleachingagents (e.g., chlorine dioxide, with the chlorinecharge). Inclusion of these chemicals can reducecellulose degradation, improve pulp strength, andreduce environmental releases. Chlorine dioxide issometimes used sequentially preceding the chlorine

treatment and has been shown to be more effectivethan when the two chemicals are mixed. s Chlorinedioxide can be used to completely replace chlorinein the first delignification stage, but its gains in pulpquality do not offset the additional expense, and pulpbrightness equivalent to t-hat produced by chlorinecan not be achieved.

14

Pretreatment with sodiumhypochlorite prior to chlorination has improved thedelignification of resinous woods.

Alkaline Extraction-Hot alkaline extraction isthe second stage in the pulp bleaching process andcompletes the prebleaching delignification process.

Wouglss W. Reeve, Pulp Chlorination, 1987 Bleach Plant Operations Seminar, TAPP1Notes (Atlanta, GA: TAPP1 Press, 1987), p. 37.IJcullonW, Denceand Goran E. Annergrcn, Chlorination,The Bleaching of Pufp Third Edirwn (Atlanta, GA: TAPP1 press, 1979), p. 65.ldDougl~ C. ~k chlorine Dioxide in the~lorinationstage 1987 Bleach Piant Operation Semrwr TAPPINotes(Atlanta, GA:TAPPI Press,

1987), p. 55.

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Figure 4-3-Pulp Brightness at Stages of theBleaching Sequence CEHDED

Bleaching stage

SOURCE: N. Lkrgott and 6.Van Lierop, Oxidative Waaching A Rev ew Part 1:Oel@nificalion,< P@ Paper Carreb vof 87, No. 8, IWS, p. 58.

Moderate temperature or cold temperature alkalineextraction is also used after later bleach stages in thebrightening process of multistage bleaching se-quences. Cold alkaline extraction is particularlyimportant in the production of dissolving pulps forthe manufacture of rayon and acetate. Alkalineextraction removes the soluble colored componentsand lignin released in the preceding delignificationstage (chlorination or oxygen), therefore reducingthe amount of bleaching chemicals needed insubsequent stages and improving the durability ofthe pulp.

Sodium hydroxide has been shown to be the mostefficient alkali for decreasing the kappa number ofpulp. Efficient extraction is nearly as important asprebleach delignification in the first bleaching stage.For instance, after chlorination and washing, butbefore alkaline extraction, about 30 to 50 percent ofthe chlorinated lignin in removed. After alkalineextraction, 80 to 90 percent of the lignin is re-moved.

15A second hot alkaline extraction is some-

Table 4-3-Examples of Prebleaching Sequencesfor Pulp Delignification

CE . . . . . Chlorine - Alkali extractionCDE . . . (Chlorine+ Chlorine dioxide) - Alkali extractionCE O . . . Chlorine - (Alkali + Oxygen extraction)OC

DE . . Oxygen - (Chlorine + Chlorine dioxide) - Alkali

extractionDCE . . . Chlorine dioxide - Chlorine - Alkali extraction

SOURCE: Dougies W. Reeve, The principles of Bieaohi~,- f987 SkMch PlantQemfiorts Seminar, TAPP/Notes AUanta GA: TAPPI Press, 1987 , p. 10.

times used later in the bleach sequence afterbleaching with chlorine dioxide or sodium hypo-chorite (e.g., CEHED, CEHDED, or CEDED se-quences) to improve pulp brightness stability andconserve bleach chemicals. b

The pulp is subjected to the alkaline extractiontreatment for 60 to 90 minutes at most mills in thefirst post-chlorination extraction stage, althoughsome operate on a shorter schedule. The secondalkaline extraction usually lasts from 30 to 60

minutes.

The first alkaline extraction stage contributes thelargest potential pollutant load released from thepulp bleach plant. It may be possible to reduce thepollutant loss from the extraction stage considerablyby substituting oxidative extraction, particularlysodium hypochlorite, for the first alkaline extractionstage (e.g., CHED, bleaching sequences) .17 Sodiumhypochlorite added to the sodium hydroxide extrac-tion solution (E

H) may reduce the color (a rough

indicator of pollution load) in waste water by aboutone-half.

18

Even larger reductions in waste watercolor (about three-quarters) have resulted whenhydrogen peroxide is added at the alkaline extractionstage.

Oxidative Extraction-Oxygen gas added tosodium hydroxide in the extraction stage (E

o)

decreases the kappa number, conserves chemicals insubsequent bleaching stages (in some cases it canreduce the number of bleaching stages), and reducespulp strength loss and coloration in the wasteWater.

19

Studies have shown that the addition of

5F3 VUI Lieropetid., Caustic Extraction, Part I: Reaetion riables,1987Bleach Plaru Operatwns Seminar, TWPl Notes (Atlanta, GA: TAPPIP XXS 1987), p. 44 .

lbSln@ Atkinson, op. cit.,footno(e10, p. 91.171bid., p. 99lgLie~rgo~and van Lierop, op. cit., fOOttlote 7, p. 46.lgIbid., p,47,

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Chapter 4Pulp Bleaching Technology . 49

other oxidizers, such as hydrogen peroxide orsodium hypochorite, to an E

Oextraction stage might

allow for a shortened, three-stage bleaching se-quence capable of bleaching pulp to high brightness(88 to 90 percent ISO). The E

Oextraction stage has

gained rapid acceptance since only modest addi-tional investment in new equipment is needed.

Oxygen has been substituted for alkaline extrac-tion immediately after the chlorination stage.

20

Ifcoupled with a following chlorine dioxide bleachstage, the COD sequence can produce fully bleachedpulp with major savings in chemicals. A three-stagesequence using oxygen in the second stage follow-ing a chlorine dioxide-chlorine delignification stage(( DC)OD) has been used by the Chesapeake Corp. atWest Point, Virginia since 1972. The Chesapeakemill was the first commercial application of oxygenin the extraction stage.

Brightening Stages

Chlorine DioxideChlorine dioxide is very se-lective in attacking lignin without significantlydegrading cellulose, while producing high bright-ness pulp. In the dioxide bleach stage, chlorinedioxide is generated as a gas at the mill and dissolvedin cold water. The aqueous chlorine dioxide solutionis mixed with the prebleached pulp, heated to about70 C, and is normally held in a reaction vessel forapproximately 3 hours.

21

Because of chlorine dioxides high cost, it is mostcommonly used at or near the end of bleachingsequences (e.g., CEHD, CEHED, CEDED, and

CEHDED). Sequences using chlorine dioxide inonly one bleach stage generally produce lowerbrightness pulps.

22For instance, the CEHD se-

quence on softwood kraft pulp would probably belimited to 85 percent G.E. brightness. In order toachieve the highest brightness (90+ percent G.E.),two chlorine dioxide stages are generally required(e.g., CEDED and CEHDED sequences). Chlorinedioxide can also be used in conjunction with a

Table 4-4-Common Sequences Used To BleachKraft Pulp to Various Degrees of Brightness

Range of GE%brightness Sequence

CEH70-80 CEHH

CHEH

CEHEH80-85 CCHEHH

CE DCEHD

CHED85-92 CEHDD

C CH E DHCEDEDCEDHEDCD E O D E DO CDE H D

SOURCE. Adapted from Jlan M. springer, krdustnal Erwironrnarrkd Corrtrol:Pub andPapar/ndJstry (Naw York, NY: JohrIWiley Sons, 19S6), p. 161.

hydrogen peroxide bleach stage (CEHDP or CEDPD)

to produce 90+ percent G.E. brightness pulp.23

Peroxide-Hydrogen peroxide is a very effectivecellulose-preserving bleach agent and is well suitedfor improving the brightness of highly lignifiedpulps, such as mechanical groundwood or chemi-mechanical pulps, without significantly reducing itsyield. Hydrogen peroxide is an extremely versatiledelignifying chemical and has been proposed for useas a chip pretreatment before kraft pulping and as adelignifier in the prebleach stage prior to, or as asubstitute for, the C, C

D, or D

cprebleaching

stages.24 It is also used in association with sodium

hydroxide in alkaline extraction (EP) Because of its

high cost, hydrogen peroxide is used most often inthe later stages of pulp bleaching.

Peroxide is used in the intermediate stages of thebleaching sequence as a replacement for hypochlo-rite or chlorine dioxide. It is frequently used as thelast stage in the bleach sequence where it can add afew points of brightness to the pulp and improve itsbrightness stability. Peroxide alone is a relatively

zOR~ a P. Sin@andBjom C. Diilner, Oxygen Bleaching, TheBleaching of Pulp, Third Editwn (Atlanta, GA: TAPPI Press, 1979), p. 181.21Rapson Stt lmila op. cit., footnote 8, p. 114.zz~ug]mW. Reeve, Chlorine Dioxide Bleaching, 1987 Bleach Plant OperatiomSeminar, 7iVPlNotes (Allama, GA: TAPPI Press, 1987), p.

67. 23RWWn StIUmlla, op. cit. p. 1aZAJ.R.~es]eyand R.R, Kindron, Hydrogen Peroxide Bleaching, 1987 Bleach Plant Operations Seminar, TAPPINotes (Atlanta, GA: T PPI Press,

1987), p, 75.

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Kraft pulps, being more difficult to bleach thansulfite pulps, require that a chlorine and alkalineextraction stage be added in the prebleach segmentof the bleach sequence. Until chlorine dioxide andperoxides became available in the 1940s, kraft pulpsof 85 percent GE brightness were the brightest thatcould be produced with hypochlorite bleaching and

still maintain acceptable pulp strength, but thesepulps had poor brightness stability.

Hypochlorite is nonspecific, that is, it attackscellulose as well as lignin, therefore it requirescareful control if a reduction in pulp strength is to beavoided.

29Bleaching sequences such as CEHD,

CEHED, and CEHHD are used widely for producingpulps of 86 to 88 percent GE brightness, CEHDEDis used for pulps of 88 to 90+ percent GE brightness,and CEHDP and CEHEDP for pulps of 90 percentGE brightness using peroxides. Hypochlorite is alsoused in small amounts for oxidative extraction (seeabove). Some mills use hypochlorite as a replace-ment for the first alkaline extraction stage to reducethe color in bleach plant effluent.

30

Retention times and chemical concentrations varyfor hypochlorite bleaching depending on whichstage it is being used in the bleaching sequence.Retention times range from a low of 30 minutes atsome mills to 3.5 hours for those using hypochloritein the brightening stage. Reaction temperatures forhypochlorite stages are generally kept low (85 to 110F) to minimize cellulose degradation.

A simplified bleaching process for hypochlo-rite has recently been developed.

31Simplified bleach-

ing uses a short (l O-minute) bleach cycle at highertemperatures than normally used (180 F). Thehypochlorite treated pulp is sent without washing toa chlorine dioxide stage. This ostensibly producespulps of higher brightness at lower cost.

Studies have shown that one of the largestcontributors to the environmental release of chloro-

forms from a bleach plant is the effluent from thehypochlorite stage.

32It is hypothesized that chloro-

form is produced under specific conditions existingin the hypochlorite stage rather than simply as theresult of chlorine-based chemicals. The specificconditions and reactions contributing to the produc-tion of chloroform are not well known, however, and

more research is needed to establish causation.These early findings of the linkage between hypo-chlorite reactions and chloroform production has ledsome to propose that the release of chloroformcompounds from pulp mills could be reduced byeliminating the large-scale use of hypochlorite in thebleach sequence.

Ozone-Ozone is one of the most powerfulbleaching and oxidizing agents. It is a special formof oxygen produced by the discharge of an electricalcurrent in oxygen gas. While oxygen atoms nor-mally occur in pairs, the electrical discharge makesthree atoms associate with one another, thus givingextraordinary oxidative properties to ozone. Itsdecomposition to oxygen after bleaching producesneither a residue, nor undesirable inorganic by-products. Ozone, in a bleaching sequence withhydrogen peroxide, can produce high-brightnesspulps. Ozone is not used commercially for pulpbleaching. Some pilot plant studies have beenconducted, but additional development work wouldbe needed to permit its widespread commercial use.

Ozone, in conjunction with preliminary oxygen

delignification,holds promise for reducing the

amount of chlorine and hypochlorite used in theprebleach and brightening stages of the bleachingsequence. Ozone bleaching is particularly wellsuited to bleaching sulfite pulps because of their low

H i g h - b r i g h t n e s s ,residual lignin content.33

high-quality, hardwood kraft pulps can be produced byusing ozone in the first stage of the bleaching

Z9E.13. Althouse, J.H. Bostwick, and D.K. Jain, Using Hydrogen Peroxide and Oxygen to Replace SodiumHypochlorite in Chemical PulpBleaching, T P J, vol. 70, No 6, June, 1987, p. 113.s~uwn ~d partridge, Op. lt ~P.102.

31R.G. HiseimdH.L.Hintz,HypoehIorite Bleaching, 1987 Bleach Plant Operations Semiw TAPPI Notes (Atlanta, GA: TAPPI Press, 1987165.szIbid.

SSR Pattet al,,Laboratory and Pilot Plant Bleaching ofMriousPulps With Ozone, 1984 Oqgen elignfication Sy~osiwn T PPINotes (Atlanta,GA: TAPP1 Press, 1984), p. 33.

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sequence (e.g., ZEP, ZEZP, and ZED).34

Because itis a very powerful and nonselective chemical, its useis usually limited to the early bleaching stages.

Kraft softwood pulps must be delignified, prefer-ably with oxygen or through extended cooking toreduce the need for chlorination in the prebleachsegment, before bleaching with ozone. Low kappa

number kraft softwood pulps bleached with theOZEP sequence produced pulp comparable in bright-ness and strength to those produced from theCEHED five-stage sequence.

35Brighter kraft soft-

wood pulp can be produced with oxygen-ozone-peroxide and/or chlorine dioxide bleaching se-quences (e.g., OZEP, OZE

PP, OZEPD, OZED, and

OZEPD). For the highest brightness, chlorine diox-

ide was needed in the final bleaching stage,

Ozone is not currently used commercially by theindustry. Experimental results and pilot plant opera-

tions indicate that ozone might have future promiseas an alternative bleaching agent. Further discussionof oxygen-based, nonchlorine bleaching technology,including ozone bleaching, is found in chapter 5.

~qsleven s K Ow and RudriiP Sin@ Advances in OzoneBleaching, Part II: Blcaehingof Softwood KraftPulps With Oxygen & OzxmeCombination, OxygenDelignflcution ~~posium TAPP1 Notes (Atlanta, GA: TAPPI Press, 1984), p. 43,

J51bid.,p. 49.

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