Biermann Ch. Handbook of Pulping and Papermaking

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204 8. STOCK PREPARATION AND ADDmVES FOR PAPERMAKING

ions.

Since the molecular weight of EDTA is 292

and calcium sulfate is 136, fairly large amounts of

EDTA must be used. On the other hand, poly-

phosphates, polyacrylates, and phosphonates work

by the threshold effect where less than stoichiomet-

ric amounts offer protection against scaling.

Organic deposits, i.e., stickles or pitch, are treated

by emulsification with nonionic surfactants or

dispersants, attached to fibers with cationic fixa-

tives such as polyamines or ammonium zirconium

carbonate, or made not sticky by adsorbents such

as talc or bentonite clays, which are magnesium

and aluminum silicates, respectively. Sequestering

agents for excess aluminum and addition of syn-

thetic fibers such as polypropylene have also been

explored for stickles control. Biological deposits

(slime) are controlled by biocides.

Biocides

Microorganisms, particularly bacteria and

fungi, will grow around the paper machine and

produce slime consisting of proteins and poly-

saccharides. This slime may break off in pieces

and lead to pitting of paper, actual holes in lighter-

weight papers, and even breaks in the web which

lead to very expensive downtime. A number of

bacteria and fungi will grow in various raw mate-

rials used to make paper. To grow, microor-

ganisms need the proper pH (as a rule, pH 4-6.5

is ideal for fungi and pH 6-8 is ideal for bacteria),

the proper temperature (40°C or 105°F is ideal,

but little slime is produced above 60°C or 140°F),

water, food, and oxygen. Some anaerobic micro-

organisms grow in conditions where oxygen is

absent and produce methane, hydrogen sulfide, or

hydrogen, which have caused fatal explosions at

mills;

anaerobic bacteria produce a characteristic

foul odor, and the sulfate using bacteria, living

under sulfate scales, can cause considerable corro-

sion. Recycled fiber and large amounts of starch

may aggravate the problem of microorganism

growth, as does poor housekeeping. Too, alkaline

papermaking may require more expensive or

higher levels of biocides to achieve control.

Good housekeeping and biocides can be used

to control slime. Typically they are added from

0.05-1

kg/t (0.1 to 2 lb/ton) on paper. Oxidizing

biocides (which are usually inorganic chemicals)

include chlorine or chlorine dioxide. As shown in

Fig. 17-1, in papermaking systems below pH 6

chlorine exists as the highly effective HOCl spe-

cies;

in alkaline systems approaching pH 9 the

active species is the less active species OCl".

Chlorine dioxide is added at 500-2000 ppm at

various application points, although it quickly

disappears from the system. There are several

commonly used organic biocides whose structures

are shown in Fig. 8-10. Quaternary ammonium

salts contain alkyl, aryl, or heterocyclic

substituents of Cg-Cjs; they are most effective

under alkaline conditions, although they lose their

effect in systems with lots of contaminants, espe-

cially nonpolar oils and related materials. Methy-

lene his-thiocyanate is

particularly effective against

the sulfate-reducing bacteria of the Desulfovibrio

genus but decomposes in Whitewater systems if the

pH is much above 8. Brominated propionamides

are potent, broad spectrum biocides. Carbamates

. R3 J

Quaternary ammonium salts

N=CSCH2S(^N

Methylene bis-thiocyanate

Br 0

I II

N=C—C—C

Br NHr

Dibromonltrile-

propionannide

R' ^S^

R' = H for monoalkyi

Carbamates

0 0

II II

HC—CH2-CH2-CH2CH

Glutaraldehyde

Isothiazolin

Fig. 8-10. Structures of some commonly used organic biocides.

N—CH,

CONTROL ADDITIVES 205

are particularly effective above pH 7 and occur as

dialkyl or monoalkyl, with longer alkyl chain

members having lower toxicity than those contain-

ing short chain alkyl groups.

Glutaraldehyde

is an

important crosslinking agent, as it has two reactive

aldehydes that readily react with amines, and may

react with proteins of the microorganisms.

Isothiazolin is a mixture of

two

compounds; it has

acute and broad activity. Because of the complex

chemistry of all of these materials, one should be

very careful before saying a certain microorganism

is actually resistant to a particular biocide. Some-

times two or more biocides are used together to

develop a synergistic effect. For more informa-

tion on biocides is available in Purvis and Tomlin

(1991).

Biocides are, by their very nature, toxic

chemicals; as when handling any toxic chemical be

sure to follow the manufacturer's directions,

including suggestions for protective equipment and

clothing, and read the Materials

Safety

Data Sheet.

Occasional

boilouts

of some paper machines using

hot water at 60°C (140°F) and pH 12 with a

dispersant can reduce the amount of biocides

required to control slime.

Other additives

Other additives include pH control agents and

corrosion inhibitors. Sulfuric acid is often used to

lower the pH of stock for the paper machine.

Carbon dioxide (a byproduct of fertilizer manufac-

turing and refineries) is being used at some loca-

tions to replace sulfuric acid.

8.6 WET END CHEMISTRY

Flocculation

There is relatively little information on fiber

flocculation in the literature. Formation is clearly

dependent on fiber flocculation, which makes it

very important. One fundamental study with a

good evaluation of the literature is the work of

Jokinen and Ebeling (1985). They used a pilot

plant machine to evaluate numerous factors on

fiber flocculation. Mechanical properties of fibers

were much more important than chemical aspects

of the slurry. For example, temperature from 18-

35°C had no effect, nor did pH from 3.9 to 10.7.

Fiber length had an important effect although the

magnitude depends on the pulp consistency; under

one set of typical commercial conditions, floccula-

tion would change by 1.2 times the relative change

in fiber length. A flow velocity change of 100%

would cause a 14% effect on flocculation in the

range of 0.15 to 0.6 m/s. Fiber flexibility had

little effect on flocculation, although there was

more of an effect at high consistencies and floes

formed from stiff fibers were more difficult to

break apart. The effects of beating on pulp are

similar to the effects of the component changes of

beating-namely the changes in fiber length and

flexibility. A variety of deflocculation aids were

tested, and anionic polyacrylamides (PAM) of high

molecular weight had the most pronounced effect,

with

0.5

causing up to

65 %

decrease in floccula-

tion. Chemicals could not be used alone to in-

crease flocculation above 5%. With fillers,

however, retention aids can greatly increase

flocculation.

Alum chemistry, coordination chemistry

The behavior of alum in aqueous solutions is

critical to many aspects of wet end chemistry.

Alum is used in rosin and ASA sizing, as a reten-

tion aid, and as a drainage aid. Alum is the

central chemical in wet end chemistry and has

received much attention. There are numerous

papers describing the behavior of alum during

papermaking that generally contradict each other.

Many of these studies are meaningless or simply

wrong. Most aspects of wet end chemistry are

attributed to ionic charge interactions; however,

coordination chemistry is probably more impor-

tant, at least in the final product where free water

is absent. The classic Advanced Inorganic Chem-

istry text by Cotton and Wilkinson (4th edition,

1980) shows the absurdity of many published

works describing the chemistry of alum in the pulp

and paper "literature".

This role of alum coordination chemistry in

retention has been clear for decades to textile

chemists that describe the mordant action of alum

in textile dyeing-the same thing that occurs with

rosin sizing. In the textile field a mordant is a

substance that fixes a dye to a material. A mor-

dant works by combining with the dye to form an

insoluble complex that fixes the dye to the materi-

al.

Mordants are metal atoms that attach to the

206 8- STOCK PREPARATION AND ADDITIVES FOR PAPERMAKING

dye at the oxygen and nitrogen atoms through

formation of coordinate bonds. Actually alum is

a common mordant used in textile dyeing. In pulp

and paper literature the alum rosin complex is

depicted in terms of ionic interactions (which must

form exceedingly hydrophilic linkages). In fact

coordinate structures are formed that are nonpolar

and hydrophobic in nature, although ionic interac-

tions are important for retention.

In a series of experiments Subrahmanyam and

Biermann (1992) showed unequivocally that

coordination chemistry is central to how alum

achieves its function in rosin sizing. Rosin sizing

with numerous highly coordinating transition and

lanthanide elements was efficacious at a variety of

acid and alkaline pH's depending on the formation

constant with hydroxide of each individual ele-

ment. Formation constants with rosinate, sulfate,

chloride, and other ligands were also relevant to

the degree of sizing.

Aluminum ions in solution form polymers by

sharing hydroxide groups. Two adjacent alumi-

num ions share two hydroxide groups to form the

chain; these linkages are termed hydroxo linkages

and are relatively nonpolar linkages, unlike the

polar linkages often shown that would be

destroyed by water. The chemistry of aluminum

is developed in more detail in Section 14.7.

The results of the first study allowed

Biermann (1992) to take rosin sizing to a pH of 10

using polyamines with very high charge densities.

This indicates that alkaline rosin sizing is possible,

something that used to be debated.

8.7 ANNOTATED BIBLIOGRAPHY

Auxiliary equipment

Ingraham, H.G. and E.E. Forslind, Auxiliary

apparatus and operations preliminary to paper

machines, in Pulp and Paper Manufacture,

Volume3,

MacDonald, R.G., Ed., McGraw-

Hill, New York, 1969, pp 185-243. This is

a good description of equipment and opera-

tion of vacuum systems, stock chests, white

water flow with material balance, savealls,

centrifugal separators, deculators, and

screens.

TAPPITIS 0410-01 gives fan pump calcula-

tions.

Gooding, R.W. and Craig, D.F., The effect

of slot spacing on pulp screen capacity, Tappi

J. 75(2):71-75(1992). Blinding, accumula-

tion of fibers near the screen plate apertures,

occurs when more than 7% of the fibers are

longer than the slot spacing. When 30% of

the fibers are of this length severe blinding

may occur, decreasing the capacity of the

screens appreciably.

Fillers

Schwalbe, H.C., Fillers and loading, in Pulp

and Paper Science and Technology, Volume

Libby, C.E., Ed. McGraw-Hill, New

York, 1962, pp 60-89. This is a good over-

view on the physical properties of fillers.

Zeta potential

The zeta potential is a colloidal effect having

to do with charge distributions on the surface of

suspended particles. The zeta potential is the

charge density on the surface of colloids and

suspended particles. It varies from about -50 mV

to + 50 mV. Retention is often at a maximum

when the charge density is near zero. This is

probably where the solubility is lowest. At one

time,

it was thought to be the panacea for wet end

control. While not meeting this expectation, it

does have some use for wet end control. Wet end

reactions such as retention are looked at primarily

from the point of view of charge distributions. As

discussed in several sections of this book, howev-

er, this is only part of the picture.

TAPPI TIS 0106-05 (1988) gives trade

names, % solids, brightness, and median

particle sizes on a variety of North American

calcium carbonates from various suppliers.

TAPPI TIS 0106-06 (1988) does this for

kaolin clays, and TAPPI TIS 0106-07 (1989)

does this for titanium dioxide.

Internal

sizing—general

aspects,

6. Reynolds, W.F., Ed., The Sizing of Paper,

2nd ed., TAPPI, Atlanta, 1989. 156 p. This

is a good overview of internal sizing with

rosin,^ AKD, ASA, stearic acid, and

fluorochemicals. Surface sizing and testing

of paper and board for sizing are also cov-

ered. The volume is fairly general.

ANNOTATED BIBLIOGRAPHY 207

7. Grouse, B.W. and D.G. Wimer, Alkaline

paperaiaking: an overview, Tappi /.

74(7):

152-159(1991). This has a good intro-

ductory discussion on sizing agents used in

the alkaline region including ASA, AKD, and

rosin, with 53 references.

Rosin sizing

8. Davidson, R.W., Retention of rosin sizes in

papermaking systems, /. Pulp Paper Sci.

14(6):J151-J159(1988). This is a fundamen-

tal study on rosin retention during sizing.

9. Wilson, W.S. and H.E. Duston, Paper

Trade

/. 117(21):223-228(1943).

Alkaline

sizing agents

10.

Markillie, M. A., Effects of starch properties

on sizing in alkaline papermaking systems,

TAPPI

Notes,

1989 Advanced Topics in Wet

End

Chemistry

Short Course, pp 7-13. This

is a very useful article to help form a good

emulsion without hydrolysis of the sizing

agents.

AKD size

11.

Davidson, R.W. and A.S. Hirwe, Retention

of alkyl ketene dimer size in papermaking

systems, TAPPI Notes, 1985 Alkaline

Papermaking,

pp 7-16.

This reference, which uses AKD labeled with

the radioisotope carbon-14, describes AKD

retention under alkaline papermaking condi-

tions,

using the Dynamic Drainage Tester

with 15% kaolin clay or calciimi carbonate

filler or no filler. The study found that

cationic potato starch (0.35% N) or an un-

specified cationic resin (5.2 meq/g) (both

used at 0.1

on pulp) was suitable for AKD

(0.1%

addition) retention. Use of post-addi-

tion of an unspecified high molecular weight

anionic polymer (at 0.01-0.02%) greatly

improves retention of AKD and fines. The

level of sizing (HST) in handsheets correlates

well to AKD retention, which, in

turn,

corre-

lates well to fines retention. The presence of

mineral fillers, especially the clay, greatly

reduces sizing, which for calcium carbonate

can be partially circumvented by higher

levels of AKD addition.

ASA

size

12.

Wasser, R.B.. The reactivity of alkenyl suc-

cinic anhydride, TAPPI Notes,

1985 Alkaline

Papermaking, pp 17-20. This is a useful

paper on hydrolysis of

ASA

and development

of sizing in paper.

13.

E. Strazdins, Role of alum in alkaline

papermaking,

TAPPI

Notes,

1985 Alkaline

Papermaking, pp 37-42. Alum is a useful

additive to develop a high level of sizing with

ASA. Some useful data on the use of alum

in ASA sizing is presented here.

Formation

aids

14.

Wasser, R.B., Formation aids in paper. An

evaluation of chemical additives for dispers-

ing long-fibered pulps,

Tappi

61(11):

115-

118(1978).

Defoamers

15.

Keegan, K.R., Defoamer theory and chemis-

try, Notes, 1991

Chemical

Processing Aids

Short

Course,

Tappi Press, pp 27-36.

Biocides

16.

Purvis, M.R and J.L. Tomlin, Microbiologi-

cal growth and control in the papermaking

process, 1991 Chemical Processing Aids,

TAPPI press, pp 69-77.

Flocculation

17.

Jokinen, O. and K. Ebeling, Flocculation

tendency of papermaking fibres, Paperi ja

Puu - Papper och Trd (5):317-325(1985).

This is highly reconmiended reading and is a

fundamental study with a good evaluation of

the literature.

18.

Allen, L.H. and I.M. Yaraskavitch, Effects

of retention and drainage aids on paper ma-

chine drainage: a review,

Tappi

J. 74(7):79-

84(1991) with 42 references. Retention and

drainage aids can increase or decrease the

rate of dewatering on paper machines. The

effect of

candidate additive on your furnish

208

8. STOCK PREPARATION AND ADDITIVES FOR PAPERMAKING

should be determined by performing labora-

tory tests prior to a mill trial.

Alum behavior

19.

Subrahmanyam, S. and C.J. Biermann, Gen-

eralized rosin soap sizing with coordinating

elements, Tappi J. 75(3):223-228(1992).

Polyamines in wet end chemistry

20.

Biermann, C.J., The use of polyamine mor-

dants in rosin sizing from pH 3 to 10, Tappi

75(5):

166-171(1992).

EXERCISES

Stock approach

Name two methods of cleaning pulp prior to

the paper machine.

What filler would you use in linerboard?

Sizing

8. Penetration (sizing) of liquids is determined

by what properties of the liquid and base

sheet?

9. What sizing agent would one use for tissue

and towel grades?

10.

What is the mechanism whereby rosin and

alkaline sizing agents size paper? How do

the alkaline sizing agents differ from rosin

size?

11.

Can rosin size be easily used in papers filled

with PCC? What is the contradiction be-

tween these two?

Additives

What are the differences between functional

and control additives?

Fillers

Give several advantages for using PCC in

papers.

Which filler is used in very thin papers of

high opacity? Why is this filler used?

5 What are the major advantages of alkaline

papermaking?

6. What filler enjoys the most use by weight?

12.

What is the mechanism of sizing with starch

on the size press? Is starch hydrophobic?

Miscellaneous

13.

What material is used as an adhesive in

corrugated boxes? Hint: this material differs

from the principal constituent of paper by

one chemical linkage. What is the mecha-

nism of adhesion?

14.

What is the difference between wet strength

additives and dry strength additives.

15.

Why is it important to limit growth of micro-

organisms? What problems do they cause?

PAPER MANUFACTURE

9.1 INTRODUCTION

Paper

Paper consists of a web of pulp fibers (nor-

mally from wood), usually formed from an aque-

ous slurry on a wire or screen, and held together

by hydrogen bonding. The same basic steps are

involved for either hand- or machine-made paper

and were shown in Figs. 1-5 for handmade paper

and 6-16 for laboratory handsheets. The steps are

as follows:

Forming - applying the pulp slurry to a

screen.

Draining - allowing water to drain by means

of a force such as gravity or a pressure

dif-

ference developed by a water column.

Pressing - further dewatering by squeezing

water from the sheet.

Drying - air drying or drying of the sheet

over a hot surface.

The first American paper mill was built in

the late 17th century in the Northeast. The pro-

cess was very slow, being performed entirely by

hand. The early development of the fourdrinier

paper machine allowed the production of paper to

increase tremendously and has been chronicled

(page I). By 1850 there were 191 machines and

Byran Donkin was honored at the Great Exhibition

for his contribution. The width of machines was

stabilized at 160 in. until an American engineer,

William Hulse Millspaugh, developed the centrifu-

gally cast couch with a cycloidal vacuum pump in

1911.

From this development the width, length

and speed of paper machines increased rapidly.

Maximum paper machine speeds by grade

It is interesting to compare paper machine

speeds and increases in machine speed over the

last 10 years by paper grade and basis weight

(Ely, 1981; Mardon et al, 1991) as shown in

Table

9-1.

With increasing basis weight, a slower

machine speed is necessary. However, the pro-

duction weight tends to increase with increasing

basis weight. Glassine paper has a low production

speed for its basis weight because the fibers are

highly refined.

9.2 THE PAPER MACHINE

Fiber

slurry

preparation

Fibers must be properly slurried and mixed

with additives. The slurry must be treated to

remove contaminants and entrained air. Consis-

tency regulation is also important. These subjects

were discussed in the previous chapter.

Overview of

the

paper machine

The paper machine is a device for continu-

ously forming, dewatering, pressing, and drying a

web of paper fibers. Until recently, the most

common type of wet end was the fourdrinier,

where a dilute suspension of fibers (typically

0.3-0.6% consistency) is applied to an endless

wire screen or plastic fabric. Water is removed

by gravity, or the AP developed by table rolls,

foils or suction equipment, and the drilled couch.

The web at this point is

18-23%

consistency.

More water is squeezed out in the press section to

a consistency of 35-55%. Finally the sheet is

dried with steam heating in the dryer section.

Machines that use two wires to form and

drain water from the dilute, pulp slurry are called

twin wire formers. These have become popular

since the late 1960s for printing and lightweight

papers. During the 1970s, many multi-ply for-

mers were developed for heavyweight board.

These formers use up to seven wires consecutive-

ly and are modifications of another type of paper

machine, used for heavy-weight board, the cylin-

der machine, which has been used since the early

1800s. Modem paper machines can be quite

expensive, costing up to several hundred million

dollars each.

209

210 9. PAPER MANUFACTURE

Table 9-1. Maximum paper machine speeds and production by paper grade (Ely, 1981; Mardon,

Yyse, and Ely, 1991).

Paper grade

1 Towel, dry-creped

Towel, wet-creped

Glassine

II Towel, wet-creped

Coating rawstock

II Newsprint

II Printing, writing

Corrugated medium

1 Kraft linerboard

Kraft linerboard

II Kraft linerboard

Liquid packing

Basis weight

lb/3000

ft^ '

54.5

126

207

270

470

g/m^

13 1

41 1

127

126

204

337

438

765

Speed, fpm

1991

6560

3000

2800

4000

4503

3650

1 2505

1 2550

1 2290

1 1700

1 1260

1 330

1981

5750

3000

1215

2800

3100

3700

2175

2180

2300

2100

1630

Production^

ton/in./d ||

1991

1.05

1.49

1.52

1 1.96

2.70

3.97

3.90

3.98

5.77

7.29

6.80

1 3.10

1981

0.91 1

1.49 1

0.60 1

1.52 1

1.47 II

2.22 II

2.37 II

3.40 1

3.59 1

5.29 1

6.75 1

'Production is a direct ftmction of paper machine speed and basis weight assuming no down time.

^Many of these grades normally use basis weights for 1000 ft^, so divide the table value by three. It

appears there were errors from this difference in some of the basis weights from the 1991 paper.

Pulp must be applied to the screen at low

consistencies to give good formation, that is, an

even distribution of fibers so the paper has uni-

form thickness. Softwood pulp slurries at 3%

consistency do not even flow well. Therefore, the

entire purpose of the paper machine is to remove

all of this water that one is forced to use to give

paper that is uniform. For every 1 lb of fiber 200

lb of water are used. For this one lb of fiber

about 195 lb of water (98%) are removed at the

wet end with the web leaving at a consistency of

20%.

Another 2 to 3 lb of water are removed in

the press section

to 1.5%) with the web leaving

the press section at 35-50% consistency. The

remaining 1 to 1.5 lb (0.5 to 0.75%) of water are

removed by the dryer section. Although the dryer

section removes the least water, the high energy

requirements make this the most expensive section

to operate. The white water removed in the wet

end of the paper machine is reused to dilute pulp

slurry for the headbox. The fiber and fillers of

excess white water are removed by savealls such

described in Section 10.3. Pulp sweetener stock is

sometimes added to the excess white water to help

recover the fines and fillers by filtration.

Vacuum systems use about 30% of

the

power

of the paper machine with almost 75% of die

vacuum power going to the press section.

O L

THE PAPER MACHINE 211

Differential Pressure Sight Glass

Inlets to Headbox Across the Paper Machine

A A A A A A A A A A A A

Tapered Header

Recirculation

Control Valve Pump

n r~L__

1 r

Inlet Flow

~ Control Valve

Fig. 9-1. Tapered inlet headbox approach now used on virtually all paper machines.

The dewatering

process

The key to good paperaiaking with long

fabric life, good retention, and minimized sheet

two-sidedness is control of the process (Hansen,

1991).

It is important to delay sheet sealing when

forming since this will lead to extra drag. Sheet

sealing occurs at around 0.8-1.4% consistency

unless precautions are taken.

9.3 THE HEADBOX

The headbox

approach

The inlet to the headbox must insure an even

fiber slurry consistency and pressure across the

width of the headbox to insure cross direction

uniformity of paper. A variety of techniques in

the past has been used to insure this, including

complex distribution piping or various types of

manifolds. The invention of the tapered inlet by

J. Mardon in the 1950s greatly simplified this

goal. All modern headboxes use a tapered inlet

pipe (Fig. 9-1) to accomplish this purpose.

even

pressure across the headbox is main-

tained by controlling the rate of

recirculation.

For

example, by increasing the recirculation, i.e., by

reducing the pressure at the narrow end of the

tapered inlet, the pressure and, therefore, flow

rate at the headbox over the narrow part of the

tapered inlet is decreased.

Headbox

The headbox is a pressurized device that

delivers a

uniform

pulp slurry on the wire, through

the slice, at approximately the same velocity as

that of the wire. Original headboxes were open,

unpressurized, and used a hydrostatic head for the

necessary pressure

Almost all headboxes on paper machines

operating below speeds of 2500 ft/min have two to

five perforated rectifier rolls (holey rolls) inside

the headbox that create microturbulence to keep

the fibers in suspension, giving even formation.

Fig. 9-2 shows this type of headbox with the

standard three-roll

design.

New headboxes use the

three-roll design, never the five-roll design that

was once common. The rolls are

0.2-0.8

meters

(8-32 in.) in diameter with holes

1.5-2.5

cm (0.6-

1.0 in.) in diameter that occupy 40-52

the

roll

area and rotate at 5-20 rpm. Fig. 9-3 shows a

rectifier roll.

secondary headbox

may be used part of

the

way down the table to give a top coat of high

quality fiber relative to the rest of the sheet. This

is done, for example, to produce a white printing

surface on linerboard or to put secondary fiber in

the middle layer of

linerboard,

where contaminants

such as polymers and wax are hidden. Fig. 9-4

shows a fourdrinier paper machine with a second-

ary headbox.

212

PAPER MANUFACTURE

Slice opening jacks

Slice setback

£i

'i!^*'

jacks

Slice setback

jacks

' :

'-1

I'^H

':\:Oy^[

•'*S~:''^-j'^

niTxS^

L,;0;;-

:''^1%

j^l

>J'fr-|^lig^}^~/%^V^

I:^^I!?IM

Fig.

9-2. A

closed, pressurized headbox with air cushion

for

relatively slow paper machines. This

standard three-roll rectifier roll headbox. Courtesy

Beloit Corp.

Originally,

the

pressure

accelerate

the

stock

to the

speed

of the

paper machine

was

supplied

by a

hydrostatic head

liquid.

But as

Fig.

9-5

shows, high speed paper machines require

too much pressure

for

this

to be

practical,

paper machines

now

used closed, pressurized

headboxes

supply

the

necessary pressure.

Paper machines operating above 2500 ft/min

require

special, high pressure headbox known

a hydraulic headbox. These headboxes

do not use

rectifier rolls because

the

high turbulence that

generated with these rolls causes formation prob-

lems beyond

the

headbox.

Slice

The slice

is a

rectangular slit

in the

headbox

where

the

pulp slurry

applied

to the

wire.

consists

of a

lower, fixed apron

and an

upper,

adjustable

lip

controlling

the

slice height.

The

slice height controls how much stock is applied

the wire

(and

therefore

the

basis weight);

the

slice

height

variable across

the

width

of the

paper

machine

insure uniformity

of the

paper across

the width

of the

paper machine. Paper machines

now use velocity formation where the lower apron

Fig.

9-3. A

rectifier roll

for a

headbox.

Re-

printed from Making Pulp

and

Paper^

Crown Ziellerbach Corp., with permission.

THE HEADBOX 213

Fig. 9-4. A two headbox fourdrinier machine for linerboard.

lou-

Af\

14U-

izu-

IIHJ-

OU"

4U~

/ r

1000

5000

hSO

6000

•30 I

•20

2000 3000 4000

Wire

Speed,

ft/min

Fig. 9-5. Relationship

between

the

wire speed

and the

theoretical height of a gravity feed headbox

Qeft)

pressure of

pressurized headbox (ri^t). Frictionallosses

are not

included. H= Wi2g)