Biermann Ch. Handbook of Pulping and Papermaking

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134 5. PULP BLEACHING

Ozone bleaching

Young (1992) describes a mill in Austria that

uses ozone bleaching. The mill is able to use

ozone since magnesium sulfite pulp is produced

(as opposed to kraft pulp that is inherently difficult

to bleach), hardwood is pulped, which has less

lignin than softwood, and the mill makes dissolv-

ing pulp where a low pulp viscosity is desired.

Ozone bleaching promises to become increasingly

important and is discussed in Section 34.10.

Other stages

Nitrogen oxides, and other chemicals are

being considered for bleaching stages, but do not

enjoy commercial significance as yet.

Biobleaching

Biobleaching is a term used to describe each

of two completely different methods. One method

is the use of decay fiingi or enzymes isolated from

decay fungi to selectively degrade lignin. It is

akin to biopulping, and, like biopulping, is still

only in the laboratory stages.

Biobleaching is also used to describe the

application of hemicellulases to pulp to help the

bleaching process. One example is the use of

xylanase enzymes. These enzymes hydrolyze the

xylans into short chains. Their mode-of-action is

explained as the breaking of lignin carbohydrate

linkages so that the lignin is removed from the

pulp.

The method has been largely promoted by

manufacturers of enzymes. It is difficult to know

if this method will ever have widespread use.

Effluent

reuse in the

bleach

plant

Up to 20,000 gallons of water per ton pulp is

required to bleach pulp in a five-stage sequence if

there is no reuse of effluents. The use of counter-

current washing of pulps in the bleaching stages

has reduced this value to less than

half.

The reuse

of chlorination filtrate in the chlorination plant has

reduced the figure further. Some mills use the El

effluent in the brown stock washers, but relatively

high amounts of chlorine will cause corrosion and

dead load in the recovery cycle. Use of the

effluent from first stage oxygen bleaching in the

brown stock washers, since it does not contain

large amounts of NaCl, also conserves water.

5.5 ANNOTATED BIBLIOGRAPHY

General

aspects

O'Neil, F.W., K. Sarkanen, and J. Schuber,

Bleaching, in Pulp and Paper Science and

Technology, Vol. 1, Libby, C.E., Ed.

McGraw-Hill, New York, 1962, pp 346-374.

A good overview of bleaching chemistry.

Singh, R.P., Ed., The Bleaching of Pulp,

Tappi Press, Atlanta, 1979, 694 pp. While

many recent developments in oxygen

delignification, decreased use of elemental

chlorine, dioxin formation, etc. are not cov-

ered, this text is an excellent reference for

the topic. It is well illustrated and also

covers equipment.

Kappa

number,

permanganate

number,

number

Alander, P., I. Palenius and B. Kyrklund,

The relationship between different cooking

degree numbers, Paperi ja Puu (8):403-

406(1963).

Chiang, V.L., H.J. Cho, R.J. Puumala, R.E.

Eckert, and W.S. Fuller,

Tappi

70(2):

101-

104(1987).

5. Guillory, A.L., 1982

Brown

Stock

Washing,

Tappi Press, Atlanta, Georgia, p 29.

Chlorination,

organochlorine compounds

6. Giertz, H. W., Delignification with bleaching

agents, in E. Hagglund, Ed., Chemistry of

Wood,

Academic Press, New York, 1951, pp

506-530.

Pryke, D.C., Mill trials of substantial substi-

tution of chlorine dioxide for chlorine: Part

II,

Pulp Paper

Can,

90(6):T203-T207(1989).

8. Reeve, D.W., Organochlorine in paper prod-

ucts,

Tappi J. 75(2):63-69(1992). The chlo-

rinated organic compounds in bleached pa-

pers do not seem to be a health hazard due to

the low concentration of mobile species and

low fraction that might be transferred to the

user.

ANNOTATED BIBLIOGRAPHY 135

9. Hise, R.G., R.C. Streisel, and A. M. Bills,

The effect of brown stock washing, split

addition of chlorine, and pH control in the C

stage on formation of AOX and chloro-

phenols during bleaching,

Tappi

J, 75(2):57-

62(1992). (Also in 1991 TAPPI Pulping

Conference

Proceedings.) Split addition of

chlorine and other modifications of pulping

and bleaching can decrease the production of

dioxins and other chlorinated organic com-

pounds. This is a useful work with 20 refer-

ences.

Oxygen bleaching

and

delignification

10.

Coetzee, B., Continuous sapoxal bleaching-

operating, technical experience, Pulp Paper

Mag. Can, 75(6):T223-T228 (1974).

The first commercial mill to use oxygen

delignification was the Enstra mill of SAPPI

in South Africa where, notably, water is in

short supply. The successful operation of

this process at SAPPI led to the widespread

use of oxygen delignification and is reported

in detail here.

There are well over 100 articles on the sub-

ject in the primary literature of the last 20

years.

Some recent general articles on oxy-

gen bleaching follow.

14.

Althouse, E.B., J. H. Bostwick, and D.K.

Jain,

Using hydrogen peroxide and oxygen to

replace sodium hypochlorite in chemical pulp

bleaching, Tappi J.

70(6):

113-117(1987).

This is a review article with ten references

that concludes, in general, that using less

hypochlorite at equal brightness gives higher

pulp viscosity for kraft and sulfite pulps.

15.

Tench, L. and S. Harper, Oxygen-bleaching

practices and benefits: an overview,

Tappi

70(11):55-61(1987). This is a worldwide

survey of mills using the title method.

16.

Idner, K., Oxygen bleaching of kraft pulp:

high consistency vs. medium consistency,

Tappi 7. 71(2):47-50(1988). This article

concludes that a slightly higher pulp viscosity

is achieved with medium consistency than

with high consistency oxygen bleaching.

17.

Homsey, D., A.S. Perkins, J. Ayton, and

M. Muguet, A mill survey of oxygen utiliza-

tion in extraction

and

delignification process-

es,

Tiqfpi J. 74(6):65-72(1991). This is a

practical discussion

oxygen delignification

from mill experiences, especially for medium

consistency mills, the growth of which has

been spurred by the development of mixers

for dispersing oxygen into pulp.

11.

Shackford, L.D. and J.M. Oswald, Flexible

brown stock washing eases implementing

oxygen delignification. Pulp & Paper

61(5):

136-141(1987); no references.

12.

Kleppe, P.J.

and

S. Storebraten, Delignifying

high-yield pulps with oxygen and alkali,

Tappi

68(7):68-73(1985). This publication

reports the results of four years production

experience at a mill using mediimi consisten-

cy oxygen bleaching.

13.

Enz, S.M. and F.A. Emmerling, North

America's first fiiUy integrated, medium-

consistency oxygen delignification stage,

TappiJ.

70(6):

105-112(1987). Thisisavery

good summary of operating conditions and

results for hardwood pulp.

The chemistry

oxygen delignification

18.

Hsu, C.L. and J.S. Hsieh, Advantages of

oxygen vs. air in delignifying pulp, Tappi J.

69(4):

125-128(1986).

19.

van Lierop, B., N. Liebergott, and G. J.

Kubes, Pressure in an oxidative extraction

stage of a bleaching sequence, Tappi J.

69(12):75-78(1986).

20.

Gellerstedt, G. and E.-L. Lindfors, Hydro-

philic groups in lignin after oxygen bleach-

ing,

Tappi

70(6):

119-122(1987).

21.

Hsu, C.L and J.S. Hsieh, Oxygen bleaching

kinetics at ultra-low consistencies, Tappi /.

70:(12):

107-112(1987).

This is a laboratory

study conducted at 0.4% consistency.

136 5. PULP BLEACHING

Chlorine dioxide bleaching

22.

Schmidt, E., Ber. 54:1860(1921);

56:25(1923).

What is cellulose viscosity, and what is its

ibid., significance?

23.

Wilson, R., J. Swaney, D.C. Pryke, C.E.

Luthe, and B.I. O'Connor, Mill experience

with chlorine dioxide delignification, Pulp

Paper

Can.

93(10):T275-T283(1992). This

article is a good starting point with 42 refer-

ences.

Miscellaneous topics

24.

Young, J., Lenzing mill bringing second

ozone bleaching line onstream. Pulp & Paper

66(9):93-95(1992).

25.

Senior, D.J. and J. Hamilton. Biobleaching

with xylanases brings biotechnology to reali-

ty, Pulp & Paper

66(9):

111-114(1992).

EXERCISES

What is the fundamental difference in ap-

proach to bleaching chemical pulps versus

mechanical pulps?

In what types of pulps is color reversion a

problem and why?

How is the lignin content of pulp measured

as a quality control measure for the pulping

process?

To approximately what percentage of lignin

in pulp does a kappa number of 30 corre-

spond for a softwood pulp?

6. Why is chlorine bleaching always followed

by alkali extraction?

Would chlorine be used as a bleaching agent

in stage 5 of a bleaching sequence? Why or

why not?

8. Why are pH control and chlorine addition of

critical concern during chlorine bleaching?

9. At what consistencies, temperatures, chemi-

cal usage etc. is oxygen delignification car-

ried out for the medium consistency and high

consistency methods? What are the advan-

tages of oxygen delignification? What are

the disadvantages?

10.

What are the advantages and disadvantages of

bleaching in several stages?

11.

Why is buffering action important during

bleaching with sodium hypochlorite?

12.

Why is the effluent from Ej after chlorination

not normally used in the brown stock wash-

ers?

13.

Discuss how water can be conserved in the

bleach plant.

14.

Discuss how oxygen bleaching can conserve

water, decrease color from the mill effluent,

and save on chemical costs.

REFINING AND PULP CHARACTERIZATION

6.1 D^TRODUCTION TO REFINING

Introduction

Pulp refining is a mechanical treatment of

pulp fibers to develop their optimum papermaking

properties. The optmium paper properties, of

course, depend on the product being made. Fur-

thermore, there is always a tradeoff between vari-

ous properties. Refining of fibers is very impor-

tant before making paper from them. Refining

increases the strength of fiber to fiber bonds by

increasing the surface area of the fibers and

making the fibers more pliable to conform around

each other, which increases the bonding surface

area and leads to a denser sheet. During refining,

however, individual fibers are weakened and made

shorter due to cutting action. With very long

fibers of a few species this cutting action improves

the formation of the sheet on the paper machine,

but in most cases, it is an undesired effect of

refining (even if formation is improved some-

what);

consequently, refining is generally a trade-

off between improving fiber to fiber bonding and

decreasing the strength of individual fibers.

Most strength properties of paper increase

with pulp refining, since they rely on fiber to fiber

bonding. The tear strength, which depends highly

on the strength of the individual fibers, actually

decreases with refining. After a certain point the

limiting factor of strength is not fiber to fiber

bonding, but the strength of the individual fibers.

Refining beyond this point begins to decrease other

strength properties besides tear. Refining of pulp

increases their flexibility and leads to denser

paper. This means bulk, opacity, and porosity

values decrease with refining.

Fig. 6-1 shows four paper types made from

increasingly refined fibers. Two of these samples

are glassine papers which are typically refined to

a much higher degree than most printing and

packaging papers. It is apparent from these

figures that fiber to fiber bonding increases with

refining. Also, the volume of space between the

fibers decreases; hence, refining increases the

density of the sheet.

Fiber Brushing

Refining at high consistency with a relatively

large distance between the refiner plates increases

fiber-fiber interactions that are termed fiber brush-

ing. This tends to roughen the fiber surface, with

minimal fiber cutting for improved fiber-fiber

bonding.

Fiber

Cutting

Operating refiners at low consistencies with

a minimal distance between the refining surfaces

increases fiber-bar contact, resulting in fiber

cutting. This is desired with certain woods (espe-

cially redwood with 7 nmi fibers and cotton with

long fibers) and other materials containing long

fibers to increase the quality of formation on the

paper machine. In most cases, however, it is

desirable to minimize fiber cutting to maintain

high paper strength.

Drainage

Drainage is the ease of removing water from

pulp fibers, either by gravity or mechanical

means. CSF is a measure of drainage and a useful

means for determining the level of refining.

Fibrillation

Fibrillation is the production of rough surfac-

es on fibers by mechanical action; refiners break

the outer layer of fibers, i. e., the primary cell

wall, causing the fibrils from the secondary cell

wall to protrude from the fiber surfaces.

Average fiber length

The average fiber length is a statistical

average length of fibers in pulp. Fiber length is

measured microscopically (nimiber average), by

classification with screens (weight average) or by

optical scanners (number average). Two such

averages are expressed here, akin to molecular

weight averages of polymers. L,. is the length of

the fraction of fibers with a length of /,

N,-

is the

number of fibers of length /, and W/ is the weight

of the fraction of fibers with length

The weight

average fiber length is equal to or larger than the

number average fiber length.

137

138 6. REFINING AND PULP CHARACTERIZATION

Fig. 6-1. Paper of increasingly refined pulps. Bleached kraft softwood fibers (top left); same pulp

with refining (top right); machine glazed florist tissue (bottom left); and glassine weighing paper.

INTRODUCTION TO REFINING

139

nimiber average length

weight average length =

EiNT.xL.

TW.xL^

Theory

One action of refining is the "pumping" of

water into the cell wall making it much more

flexible. This is sometimes compared to the soften-

ing of spaghetti upon cooking, which is relevant in

that in both cases internal hydrogen bonds are

broken with the addition of water molecules.

While this would occur whenever fibers are

thoroughly wetted, to the extent that refining

disrupts the crystallinity of fibers, more water can

be added to cellulose fibers.

A second action of refining is fibrillation,

that is, exposure of cellulose fibrils to increase the

surface area of fibers, thereby improving

fiber-fiber bonding in the final sheet. For exam-

ple,

the surface area of kraft, softwood fibers is on

the order of

m^/g at 750 CSF, but it increases to

about 5 mVg at 350 CSF.

A third action in refining is delamination of

the cell wall, such as between the primary and

secondary layers, which increases the fiber flexi-

bility. The analogy of greased leaf

springs,

which

are much more flexible than the equivalent amount

of solid metal, has been used to explain this effect.

Refining decreases the pulp freeness, the rate

at which water will drain through the pulp.

Refined pulp, therefore, has a low freeness. The

extent of refining is monitored by measuring the

pulp freeness and the strength properties of

handsheets made from the refined pulp.

Pulp used to be treated in

beaters,

such as the

Hollander beater that is explained below, but now

refiners are used. The terms beating and refining

are often used interchangeably, but refining is

applicable to most modem equipment.

Refining variables

Pulps low in lignin (low yield pulps) and high

in hemicelluloses are relatively easy to refine in

terms of development of strength

properties.

High

temperature and high pH during refining decrease

the refining energy requirements. Refining at high

pH offers the advantages of an increase in the rate

of hydration due to fiber swelling (Scallan, 1983),

an increase in the ultimate strength of the paper,

and an increase in sheet bulk. It also produces a

lower stock freeness, reduces equipment corro-

sion, and produces a softer sheet. Refining at low

pH hardens the fibers, leads to a denser sheet that

is hard and snappy, and runs better on the paper

machine. Refiner plate speed, plate clearance, and

type of tackle are other important variables.

Major changes in refining have resulted in

the development of equipment that allows refining

to occur at high consistency. High consistency

refining leads to more fiber fibrillation and less

fiber cutting. Refining at high consistency, how-

ever, may lead to fiber curling. Fig. 6-2 shows

two pulps, one refined at low consistency (3%) in

a Jordan refiner and one refined at high consis-

tency in a double disk refiner. The fibers refined

at high consistency have much more fibrillation

and less fiber cutting.

Canadian

Standard

fireeness,

CSF

Refining is easily monitored by the drainage

rate of water through the pulp. A high drainage

rate also means a high freeness. Obviously free-

ness is of utmost importance in the operation of a

paper machine. A low freeness means that the

paper machine will have to operate relatively

slowly, a condition that is usually intolerable.

The CSF test measures the drainage of

liter

0.3

consistency

pulp

slurry through a calibrat-

ed screen. The test is shown in Fig. 6-3. The

device is shown in Fig. 6-4. The 1992 TAPPI

Standard T 227 revision describes a change in

design that was made in 1967, where the angle

and shape of the side orifice was changed. The

CSF test was developed for use with groundwood

pulps and was not intended for use with chemical

pulps;

nevertheless, it is the standard test for

monitoring refining in North American mills. It

tends to be most influenced by the fines content of

the pulp and to a smaller extent by the degree of

fibrillation and other fiber properties. The water

is diverted around a spreader cone where, if it

drains quickly, some of it overflows to a side

orifice and escapes collection from the bottom

140 6. REFINING AND PULP CHARACTERIZATION

Fig. 6-2. Refining at

consistency (left) shows more fiber cutting with less fibrillation than at

30%

orifice. The water from the side orifice is collect-

ed and measured in a graduate cylinder and report-

ed in ml. Unrefined softwood might be 700 ml

CSF,

whereas mechanical pulp is about 100-200

ml CSF (Fig. 6-5).

AIR

COCK

Ir^

TOP

LID

HINGES

BOTTOM

LID

CALIBRATED

SCREEN PLATE

SPREADER

CONE

SIDE

ORIFICE

1000

GRADUATE

CYLINDER

CALIBRATED ORIFICE

Fig. 6-3. Schematic diagram of the Canadian Fig. 6-4. The Canadian Standard freeness

Standard freeness test in progress. device.

INTRODUCTION TO REFINING 141

650

80 100 120 140

CSFFreeness,ml for

grams

100 200 300 400 500 600 700

CSF

Freeness, ml for

grams

Fig. 6-5. Comparison of freeness scales for mechanical and chemical

pulps.

Data from US 0809.

142 ^- REFINING AND PULP CHARACTERIZATION

There are other freeness tests that are used

around the world. Perhaps the most common one

is the Schopper Reigler test, which is similar in

concept to the CSF test. TAPPI TIS 0809-01

gives inter-conversions of CSF, Schopper Reigler

units,

Williams

Precision,

and

Drainage Factor

for

various types of

pulp.

Fig. 6-5 has two graphical

presentations from the first table of TIS 0809.

Since the conversions are not exact, the two lines

of each set represent the bounds of

the

conversion.

See Section 17.5 for CSF correction equations for

temperature and consistency.

6.2 REFINING

Beater

A beater is an early device (the Hollander

Beater was invented in the 1700s) used to treat

pulp to improve the papermaking properties.

Beating is a batch process where the pulp slurry

circulates through an oval tank around a midsec-

tion and passes between a revolving roll with bars

and a bedplate with

bars.

The pulp is at about 6%

consistency and emphasizes fiber brushing. The

use of these had been phased out at most mills by

the late 1970s because they are slow and expensive

to operate. Fig. 6-6 shows a Hollander beater that

was in use until the mid 1970s. Some mills retain

these for use in mixing stock for small paper

machines.

Refiners

Refiners are machines that mechanically

macerate and/or cut pulp fibers before they are

made into paper. There are two principal types:

disk and conical refiners. Disk refiners have

superseded

the

conical refiners for many purposes,

as they offer many advantages.

The operation of a conical refiner (Fig. 6-7)

is similar to the operation of a disk refiners,

except for the geometry of the refiners. In conical

refiners, the refining surfaces are on a tapered

plug. These surfaces consist of a rotor that turns

against the housing and the stator, both of which

contain metal bars mounted perpendicularly to

rotation. The Jordan refiner, patented by Joseph

Jordan in 1858, is one type of conical refiner with

a 12° angle on the rotor (with respect to the

longitudinal axis); it is suited to low consistency

refining (2% consistency) with much fiber cutting.

The Claflin refiner uses a rotor with a 45° angle

with respect to the longitudinal axis that revolves

Fig. 6-6. The Hollander beater for mill beating of pulp.

REFINING 143

Jordan

Claflin

Fig. 6-7. The Jordan and Claflin conical refiners. Reprinted from Making Pulp and

Paper^

Crown Zellerbach Corp., with permission.

inside the mating shell. The inlet is at the small

end of the taper. The Claflin refiner is intermedi-

ate in operation to the Jordan disk refiners. These

two refiners are shown in Fig. 6-7.

Disk refiners became available for

papermaking in the 1930s, after the conical refin-

ers.

Disk refiners had enjoyed widespread use in

food processing. For example, they are used to

process peanuts into peanut butter and corn into

cornstarch and flour. The pulp slurry makes one

pass between rotating plates equipped with teeth or

bars.

There are three common configurations:

one fixed plate with one rotating plate (Fig. 6-8),

two rotating plates that turn in opposite directions.