Biermann Ch. Handbook of Pulping and Papermaking

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704 34. UPDATES AND BffiLIOGRAPHY

Agglomeration deinking is a relatively new

method of deinking that is applied to office papers

printed with laser printing inks. Betz and PPG

offer chemistry systems in this area. Surfactants

are used to agglomerate small particles of ink into

large particles that can be removed by screens or

forward hydrocleaners.

Snyder, B.A. and J.C. Berg, Liquid bridge ag-

glomeration: a fundamental approach to toner

deinking, Tappi J. 77(5):79-84(1994).

Enzymatic

deinking,

Biodeinking

Welt, T. and R.J. Dinus, Enzymatic deinking—a

review, Progress Paper Recycling 4(2):36-

47(Feb., 1995), with 81 references. Enzymatic

deinking uses cellulases (to, hypothetically, re-

move the fibrils from the surface of fibers where

the ink anchors itself to the fiber), hemicellulases,

pectinases, or, less studied, esterases (lipases),

which would saponify inks with triglycerides (soy

bean oil). The process was not commercial at the

time of this article.

Changes in ink formulations

Trends in ink formulations to decrease their

impact on the environment and make printed

papers easier to recycle include the decrease in the

use of volatile organic compounds (VOC, i.e.,

hydrocarbon and other solvents). Corrugated

flexo inks recently were 10—20% VOC, but now

range from 0—2%. The use of appreciable

amounts of heavy metals in inks is under attack.

Reductions in lead, chromium, copper, and, to a

lesser extent, barium have begun to occur.

Use of

ONP

in OCC

The use of ONP in OCC allows a higher

level of postconsumer waste to be included in

OCC,

which is important for some classifications

of paper. The strength of ONP can be increased

by chemical treatment (such as with NaOH),

refining, the use of starch adhesives, and fiber

fractionation.

General aspects

When using batch pulping, high consistency

is usually preferable; however, newsprint is

sometimes pulped in continuous pulpers at low

consistency. The hole size in pulpers is often 1/4,

1/2, or 3/4 in. in diameter.

Three inch forward cleaners are used at 0.7

to 0.8% consistency unless the fibers are fairly

short, when up to

1 %

consistency is used. Four

inch cleaners are used at

1 %

consistency.

One method to improve the quality of recy-

cled fiber is to fractionate it by size; OCC can be

fractionated into small fibers for use in corrugating

medium and large fibers to be used in linerboard.

Bobalek, J.F. and M. Chaturvedi, The effects of

recycling on the physical properties of handsheets

with respect to specific wood species, Tappi /.

72(6):

123-125(1989).

Ibid., Tappi Proceedings,

1988 Pulping Conference, pp. 183—187. Com-

mercial bleached kraft loblolly pine, eucalyptus,

quaking aspen, and northern pine were subjected

to 0—3 cycles of handsheet formation without wet

pressing (with light refining) with concomitant

strength and other properties reported. It is

unclear whether these handsheets were air dried or

oven dried, but these conditions do not seem to

duplicate commercial production practices.

Cox, J., Wastepaper growth highlighted in API's

capacity survey. Am. Papermaker 54(2):37-

40(1990).

lannazzi, F.D. and R. Strauss, Municipal solid

waste and the paper industry: the next five years,

Pulp & Pfl/7^r64(3):222-225(March, 1990).

Olkinuora, M., U.S. leads the world in the pro-

duction and consumption of wastepaper, Pulp &

Pap^r

64(3):

130-132(March, 1989). This paper

gives figures for 1986 when the U.S. was recover-

ing 27.4% of its waste paper. By 1990 this figure

was over 30% and will most likely continue to rise

as pressure mounts to reduce the amount of solid

waste going into landfills, of which waste paper is

approximately 50% of domestic waste. Japan

recovers about 50% of its paper, which is near the

sustainable maximum as fibers lose strength with

each use.

Sorenson, D., Environmental concerns, economics

drive paper recycling technology. Pulp & Paper

64(3)56-57:(March, 1990).

Gyrocleaner

Marson, M., New lightweight cleaner units solve

mill's plastic problems. Pulp & Paper 64(6):93-

96(1990). The development and operation of the

RECYCLING AND SECONDARY FIBER 705

Gyrocleaner mentioned and shown on page 271 of

this book are described.

Flotation deinking vs. flotation clarification

Doshi, M.R., Flotation deinking vs. flotation

clarification, Progress Paper Recycling 1(2):

83(Feb., 1992).

Dispersed—air flotation deinking is a separa-

tion process where ink is separated from fiber (a

selective process). Large bubbles are used, 0.1 to

1.0 mm in diameter, that quickly rise to the

surface, leaving fibers behind; particles must be

relatively large and hydrophobic to be removed.

Dissolved—air flotation clarification to clean

process water, however, is designed to remove all

particles (a collective process). Small air bubbles

are used, 0.01 to 0.1 mm, under conditions of

relatively low turbulence.

Stickles

Several mills report that the use of

0.006

in.

slotted screens has appreciably decreased the

problems associated with stickles in recycled pulp.

Doshi, M.R., Properties and control of stickles.

Progress Paper Recycling l(l):54-63(Nov., 1991).

Doshi, M.R., Quantification, control, and reten-

tion of depositable sfickies, Progress Paper Recy-

cling 2(l):83(Nov., 1992). These two articles are

the basis of the discussion in this section.

Stickles are designated as either micro or

macro because these two classes behave differently

during the papermaking process. Micro stickles

are smaller than 150 /xm and will pass through a

0.006

in. slotted screen. A second classification

is passing through a 200 mesh screen (< 76 ^m).

Microstickies are determined by tests such as

those of Buckman (plastic bottle), Berol (poly-

propylene film), Doshi (microfoam), and PIRA

(paper machine wire). Talc and zirconium are

used for the control of microstickies.

Macrostickies are determined by making

handsheets that are then hotpressed and dried.

The stickles are determined by counting with the

needle nest, counting specks with the Tappi meth-

od, or dying the fibers with a water—soluble black

dye.

Another method that requires standards dyes

the stickles with a water—insoluble dye.

Some manufacturers say talc and related

materials remove macro stickles by increasing

their density and allowing their removal in forward

cleaners, but this is probably not true.

Twin—wire machines seem to have more

difficulty with stickles (wire plugging) than

fourdrinier machines. In addition to talc, zirconi-

um compounds are also used for stickles control.

Sticky control agents should be added after

the refiners, but as far upstream as possible to

allow them ample time to work. Control agents

include talc (used at 0.6—2% on pulp),

dispersants, zirconium compounds

(0.1 %

on pulp),

and alum sequestering agents (to prevent agglom-

eration of

stickles).

Synthetic fibers were used at

one time (0.1% on pulp), but this method is no

longer used.

Dirt

counting

Glowacki, J.J., Ponderosa boosts dirt count accu-

racy with PC/scanner, Pulp & Paper 69(2):93-

94(1995). Dirt counting has come a long way

over the last several years. This system uses a

standard computer image scanner with software to

give a dirt size distribution analysis. These sys-

tems are easy to use, highly repeatable, and quick.

34.6 PROPERTIES OF PULPS

Comparison

of pulping methods

There are several papers in the literature that

compare the strength and other properties of paper

made with various pulping methods, cooking

times,

and refining levels. These studies are

usually conducted on laboratory-made pulps and

handsheets.

Liebergott, N. and T. Joachimides, Choosing the

best brightening process. Pulp Paper Can,

80(12):T391-T395 (1979).

Wegner, T.H., G.C. Myers and G.F. Leatham,

Biological treatments as an alternative to chemical

pretreatments in high yield wood pulping,

Tappi

74(3):

189-193(1991).

This paper includes fiber

classification by Bauer—McNett analysis.

34.7 MECHANICAL PULPING

Thermomechanical

methods

Prusas, Z.C, M.J. Rourke, and L.O. Uhrig,

Variables in chemi—thermomechanical pulping of

706 34. UPDATES AND BIBLIOGRAPHY

northern hardwoods,

TappiJ.

70(10):91-95(1987).

The concentrations of NaOH in the first stage and

sulfite in the second stage of Mead's CTMP

process were investigated with refining energy and

other pulp and paper properties reported. Some

good basic data for these parameters are included.

Biomechanical pulping

Biomechanical pulping is an experimental tech-

nique where wood chips are inoculated with

specific types of decay fungi (especially white rot)

or lignin—degrading enzymes to break some lignin

bonds prior to mechanical pulping; this offers the

advantages of reducing power requirements in the

refiners and giving higher strength pulps.

Akhtar, M., M.C. Attridge, G.C. Myers, T.K.

Kirk, andR.A. Blanchette, Biomechanical pulping

of loblolly pine with different strains of the

white—rot fungus Ceriporiopsis subvermispora,

TappiJ.

75(2):

105-109(1992), with 24 references.

34.8 CHEMICAL PULPING

Digesters

Horng, A.J., D.M. Mackie, and J. Tichy, Factors

affecting pulp quality from continuous digesters,

Tappi J. 70(12):75-79(1987). One important

factor that influences pulp quality from a continu-

ous digester is the rate of liquor circulation.

Kraft process, general aspects

Pulp and Paper

Manufacture,

Volume 1, 2nd ed.,

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

York, 1969, pp. 347-627 (Chap. 8-10). Espe-

cially noteworthy are the history of the process,

the liquor impregnation discussion, the demon-

stration of carbohydrate removal by type of carbo-

hydrate and pulping process, the effect of chip

quality on kraft pulping, the effect of effective

alkali charge, effective alkali concentration, and

sulfidity on the pulping of Douglas—fir sawdust,

sweet gum, and other species, and a table with

H—factor as a function of temperature in Chapter

8. Chapter 10 is an expansion of the article by

Swartz directly below with (in addition to most of

the above) information on process control, contin-

uous pulping, causticizing curves, chemical com-

positions at various points in causticizing, per-

formance of rotary kilns as a function of size,

liquor titration

calculations,

and material specifica-

tions for construction of equipment. Chapter 11

discusses the thermodynamics of the kraft recovery

cycle with an energy balance around the recovery

process based on one ton of pulp. Much of the

last chapter was from the first edition of 1950.

Swartz, J.N. and R.C. MacDonald, Alkaline

pulping, in Pulp and Paper Science and Tech-

nology, Volume 1, Libby, C.E., Ed. McGraw-

Hill, New York, 1962, pp. 160-239. This is a

discourse on alkaline pulping from an engineering

viewpoint including information on chemical

recovery, the development of

the

modern recovery

boiler, mass balances of cooking and washing,

heat transfer reactions, heat balance of multiple

effect evaporators, and tall oil and turpentine

recovery. A table relates Baume to solids content

(from 13.2—54.7% solids) to the specific gravity

of black liquor from a southern kraft mill. (Spe-

cific gravity is a very usefiil method for determin-

ing solids content of black liquor if calibrated by

gravimetric analysis for a given mill.)

Schwartz, S.L. and M.W. Bray, Chemistry of the

alkaline wood pulp processes, V. Effect of chemi-

cal ratio at constant initial concentration and the

effect of initial concentration of the rate of

delignification and hydrolysis of Douglas—fir by

the sulphate process, Techn. Assoc. Papers

22(600)1939. The comprehensive study of the

effect of effective alkali charge and effective alkali

concentration on the pulping (as yield and lignin

content) of Douglas—fir sawdust mentioned in the

reference immediately above is from this article.

Kraft pulping chemistry

Chiang, V.L., et al.. Alkali consumption during

kraft pulping of Douglas—fir, western hemlock,

and red alder,

TappiJ.

70(2):

101-104(1987).

This

study shows that 22% of the EA was used for

initial delignification of the softwoods while alder

required about 40%. Only about 2—4% NasO on

wood was required for bulk delignification of the

three species, although this accounts for about

70%

of lignin removal. (Studies like this indicate

why RDH pulping is so effective.)

Milanova, E. and G.M. Dorris, On the determina-

tion of residual alkali in black liquors, Nor. Pulp

Paper

Res.

J. 4(l):4-9, 15(1994). The concept of

CHEMICAL PULPING 707

residual alkali is important, but not easily defined.

This is a good starting point on the topic.

Tormund, D. and A. Teder, New findings on

sulfide chemistry in kraft pulping, TAPPI1989

International Symposium on Wood and Pulping

Chemistry

Proc.

pp. 247-254. The older literature

gives the second ionization constant of HjS as 10'^^

to 10'^^ According to more recent publications

cited in this work [such as Giggenbach, W. Inor-

ganic Chem.

10(7):

1333(1971) and Licht S. and J.

Manassen, /. Electrochem. Soc. 134:918 (1987)]

it is closer to 10"^^ in alkaline aqueous solutions.

This means one should always assume that NajS is

completely hydrolyzed and exists in solution as

NaHS and NaOH in any part of the kraft process;

there is negligible

S^".

This article also studied the

selective absorption of sulfide to woody materials.

Kraft

poly

sulfide pulping

Hara,

S.—i.,

New polysulfide pulping process at

Shirakawa and Hachinoche, TAPPI Proceedings,

1991 Pulping Conference, pp. 107—113. Poly-

sulfide white liquor, produced using a granular

activated carbon catalyst, gives improved pulp

yield and lowers the recovery boiler load while

maintaining pulp qualities.

Jiang, J.E., Extended modified cooking of South-

ern pine with polysulfide: effects on pulp yield and

physical properties,

TappiJ.

77(2):

120-124(1994).

Results as immediately above with an additional

claim of lowered refining energy.

Mao,

B. and N. Hartler, Improved modified kraft

cooking. Part 3: Modified vapor—phase

polysulfide pulping,

TappiJ.

77(11):

149-153.

Extended delignification

A variety of pretreatment processes (many of

which are experimental) applied to pulp lower the

amount of bleaching chemicals required, leading to

lower levels of chlorinated organic materials.

Chemicals involved in various processes include

oxygen (enjoying widespread commercial use, and

gaining), nitrogen dioxide, ozone, sulfur dioxide,

hydrogen peroxide, and sodium sulfite.

Possum, G. and A. Marklund, Pretreatment of

kraft pulp is the key to easy final bleaching, Tappi

J. 71(11):79-84(1988).

34.9 KRAFT RECOVERY

Brownstock washing

Pelton, R., Defoamers can improve brownstock

washer operation. Pulp Paper Can.

90(2):61-

68(1989). This is a well—referenced source of

information.

Multiple effect

evaporation

Bergstrom, R.E., Evaporation of alkaline black

liquor, in Stephenson, J.N., Pulp and Paper

Manufacture,

Vol. 1,

Preparation

Treatment

Wood

Pulp,

McGraw-Hill, New York, 1950, pp.

535-568.

Detailed calculations of multiple—effect

evaporators are given.

Venkatesh, V and X.N. Nguyen, 3. Evaporation

and concentration of black liquor, in Hough, G,

Ed.,

Chemical

Recovery in the Alkaline Pulping

Process, TAPPI Press, Atlanta, 1985, pp. 15-85.

A detailed discussion on black liquor evaporation

including types of evaporators, operating condi-

tions of a sextuple—effect evaporator, and strong

black liquor concentration is given here. Their

operating conditions of

the

sextuple—effect evapo-

rator is identical to that presented by Smook,

Handbook

for Pulp & Paper Technologists, pp.

126-127, which is identical to that presented by

Swartz et al. in MacDonald's Pulp and Paper

Manufacture, Vol. 1, 1969, pp. 512-513. A simi-

lar sextuple—effect evaporator example is given by

Swartz and MacDonald in Libby, C.E. Pulp and

Paper

Science

and

Technology,

Vol. 1, pp. 194-

195 (1962).

Zaman, A.A. and A.L. Fricke, Viscosity of soft-

wood kraft black liquors at low solids concen-

trations: effect of solids content, degree of de-

lignification and liquor composition, /. Pulp Paper

Sci. 21(4):J119-1126(1995).

Recovery boiler

Fujisaki, A., H. Takatsuka, and M. Yamamura,

World's largest high pressure and temperature

recovery boiler. Pulp Paper Can. 95(11):T452-

T459(46-53)(1994). This unit uses 2400 tons per

day of black liquor solids producing steam at

500°C. The fiimace is 12 m by 12 m by 55 m

high. Automation is a key feature.

708 34. UPDATES AND BIBLIOGRAPHY

Ibach, S., Conversion to high solids firing. Pulp

Paper Can. 97(3):T88-94(March 1996). This is

an excellent article on the succesful operation of

kraft recovery boilers using high solids firing. It

is required reading on the topic.

Jones,

A.K. and P.J. Chapman, Computational

fluid dynamics combustion and modelling—a com-

parison of secondary air systems design, Tappi J.

76(7):

195-202(1993).

The use of CFD to model

recovery boilers is increasing.

Thorslund, G.L., Visual recovery boiler inspec-

tion, Tappi J. 77(12):65-74(1994). An inspection

plan should include the water, steam, and fire

sides with the latter including pressure parts,

attachments, casings, and wall openings.

Recovery boiler and nitrogen oxide emissions

Nichols, K.M., L.M. Thompson, and H.J. Empie,

A review of

NO^

formation mechanisms in recov-

ery furnaces, Tappi J.

76(1):

119-124(1993).

This

study claims that most of the NO^ arises from

nitrogen in the fuel.

Prouty, A.L., R.C. Stuart, and A.L. Caron,

Nitrogen oxide emissions from a kraft recovery

furnace, Tappi J.

76(1):

115-118(1993).

Ejfect of dead load on recovery & delignification

Ahlers, P.—E., H. Norrstrom, and B. Warnqvist,

Chlorides in kraft recovery: effects on process and

equipment. Pulp Paper

Can.

79(5):T169-T

173(63-

66,

68)(1978). Any mill with over 10 g/L NaCl

in the white liquor should certainly consult these

two articles.

Mortimer, R.D. and B.I. Fleming, An empirical

relationship to describe the effect of deadload on

delignification, Tappi J. 66(1):98-99(1983).

Deadload, inactive materials in kraft white liquor,

such as NaCl, NajCOa, and Na2S04, decreases the

rate of delignification. Normal levels of these

compounds in commercial kraft pulping have a

negligible effect, but some mills with high levels

of NaCl will experience a decrease in delignifi-

cation. For example, a soda—AQ black spruce

pulp with no deadload in the white liquor has a

kappa number of 53. The kappa number is in-

creased with deadload. At 37.2 g/L as NajO (of

the chemical specified): NaCl gives 68.8, NajCOs

gives 60.3, and Na2S04 gives 64.5. The log

(kappa) is proportional to the concentration of

deadload contaminant. The slope of the relation-

ship is similar for kraft, soda, and soda—AQ

pulping methods.

Novel black liquor recovery technologies

A variety of techniques are being investigated

to recover kraft liquor, especially methods that are

amenable to small scales for incremental capacity

increases. Another goal is to eliminate the possi-

bility of smelt—water explosions.

Empie, H.J, Alternative kraft recovery processes,

Tappi J. 74(5):272-276(1991). Several of these

techniques involve methods of gasification or

fluidized

beds.

The Chemrec plasma gasifier

when operated at 1250°C (2280°F) followed by

rapid cooling offers the potential of direct recov-

ery of alkali and sulfur reduction approaching

100%.

Although there is still much development

needed on this process, Kamyr plans to build a

6000 Ib/h demonstration unit.

Paleologou, M., P.—Y. Wong, and R.M. Berry,

A solution to caustic/chlorine imbalance: bipolar

membrane electrodialysis, J. Pulp Paper Sci.

18(4):J138-J145(1992). In this article sodium

sulfate was recovered as sodium hydroxide and

sulfuric acid with the application of electricity

across two ion—selective membranes.

Although not covered in this article, the same

technique can be used to recover NaCl as NaOH

and HCl. This technique can be used to recover

alkali from the alkali extraction of pulp bleaching

with the added advantage of decolorization.

Dorr—Oliver, Inc., The fluid bed calciner, in

Hough, G, Ed.,

Chemical

Recovery in the Alkaline

Pulping Process, TAPPI Press, Atlanta, 1985, pp.

299-303.

Novel black liquor

recovery,

auto—causticizing

A variety of alternate recovery processes

have been proposed. One class of these processes

are the auto—causticizing recovery methods where

an amphoteric oxide (or related material) is pres-

ent in the black liquor prior to recovery. Upon

dissolution of the smelt after combustion of the

KRAFT RECOVERY 709

black liquor, the additive and sodium hydroxide

are recovered. (Actually Na2S behaves in this

fashion as it generates NaOH directly in the

dissolving tank.) For example, use of sodium

borate in the white liquor allows NaOH to be

regenerated directly when the smelt from the

recovery boiler is dissolved in water; this elimi-

nates the lime cycle and kiln.

In the DARS (direct alkali recovery system)

developed by G.H. Covey, iron oxide serves the

purpose. The DARS process has been used at the

Fredericia, Denmark mill, which pulps straw.

The chemistry of generating NaOH by these

methods is well known and has been used in

industry. Titanium—based recovery was also pro-

posed in the late 1960s by K. Kato. The ferric

oxide—sodium ferrite cycle, which has had some

use to produce NaOH since it was patented in

1883 by Lowig, is summarized as:

Na2C03 + Fe203 -^ 2NaFe02 + C02(t)

2NaFe02 + H2O -^ 2NaOH + Fe203(l)

Janson, J., Pulping processes based on

autocausticizable borate, Svensk Papperstidning

83:392-395(1985).

Ring formation in lime kilns

Backman, R, M. Hupa, and H. Tran, Modelling

the sodium enrichment in lime kilns. Pulp Paper

Can,

94(11):78-83(1993). Sodium and sulfur

compounds can become molten in the lime kiln

and increase the tendency for ring formation.

Notidis, E. and H. Tran, Survey of lime kiln

operation and ringing problem, Tappi J.

76(5):

125-131(1993).

A survey of 134 kilns

showed that ringing caused

20%

of kiln downtime,

although over 80% of kilns had this problem

within the last two years. Frequent scheduled

shutdowns decreased problems due to ringing.

Nearly 80% of 61 kilns had trouble with ringing

before they started burning NCG in the kiln,

indicating that NCG may not be the primary cause

of ringing. Most people felt sodium content was

the major factor.

Older kilns and kilns that switch between oil

and natural gas fuels seem to be particularly

susceptible to this problem. Two—thirds of these

burn noncondensible gases. The lime mud solids

content was

67—73%,

with an average of 70%.

The sodium content was 2% (on weight) as Na20

on dry solids before the precoat filter and 0.5%

after the mud washer. CaCOj was 90—95% of

the solids with major impurities of Na, Mg, S, P,

Si,

Al, K, and Fe present.

The ring deposit consists largely of CaCOs

and CaS04. While CaO may deposit initially,

work indicates the colder temperature of the ring

allows CaO to react with CO2 to form CaC03, in

the process of recarbonization.

Other operating parameters for mills are

available in this study. While U.S. mills average

29%

(range is 18—39%) sulfidity, Scandinavian

mills average 34%. U.S. active alkali averages

100 g/L, while Scandinavian average 121 g/L (as

Na20).

The causticizing efficiency averages 80%.

Tran, H., X. Mao, and D. Barham, Mechanisms

of ring formation in lime kilns, /. Pulp Paper Sci.

19(4):J167-J175(1993). High sodium and unstable

kiln operation are major factors in ring formation.

34.10 BLEACHING

The area of bleaching is one of the most

rapidly changing in the industry. Numerous

publications appear. I recommend using the

annual index of Tappi J. to find recent papers on

the many aspects of this topic.

Oxygen supply

Baeuerlin, C.R., M.H. Kirby, and G. Berndt,

Vacuum pressure swing adsorption oxygen for

oxygen delignification, Tappi /.,

74(5):

85-

92(1991). Oxygen for use at rates above 130

tons/day is usually produced by cryogenic distilla-

tion of air on—site. Oxygen for use at rates below

15 tons per day is best supplied by having oxygen

delivered by truck or rail. For intermediate levels

of oxygen supply, an on—site vacuum pressure

swing adsorption unit may be economical. The

system works by adsorbing N2, CO2, and H2O

from air with synthetic zeolite sieves.

Oxygen bleaching

While high consistency oxygen bleaching was

more common than medium consistency at one

time,

the trend has now switched. Most people

now use medium consistency oxygen bleaching;

710 34. UPDATES AND BIBLIOGRAPHY

although the steam and alkali costs are slightly

higher, the capital costs are lower.

Ozone bleaching

Ozone is a bleaching agent that has been

studied much in the laboratory and in pilot plant

operations. It now emerging as a commercial

process. Ozone is used at high consistency

(25—45%),

at pH 2—3, below 60°C to replace

chlorine bleaching. Several recent studies have

studied ozone bleaching after xylanase treatment.

Byrd, Jr., M.V., J.S. Grazel, and R.P. Singh,

Delignification and bleaching of chemical pulps

with ozone: a literature review, Tappi J.

75(3):207-213(1992).

Byrd, Jr., M.V. and K.J. Knoernschild, Design

considerations for ozone bleaching, A guide to

ozone use, generation, and handling, Tappi J.

75(5):

101-106(1992).

Gottlieb, P.M., P.E. Nutt, S.R. Miller, and T.S.

Macas, Mill experience in high—consistency

ozone bleaching of southern pine pulp, Tappi J,

77(6):

117-124(1994).

Liebergott, N., B. van Lierop, and A. Skothos,

The use of ozone in bleaching pulps, TAPPI

Proceedings^ 1991 Pulping Conference, pp. 1-23

(81 references). The article was reprinted in two

parts in Tappi J.

75(1):

145-152(1992) and

75(2):

117-124.

O'Brien, J., PaperAge tours Union Camp's Frank-

lin, Va., ozone bleaching facility, PaperAge

109(7):9-12(July, 1993).

Hypochlorous acid

bleaching,

M0N0X—I7^

Klykken, P.O. and M.M. Hurt, Molecular chlo-

rine free bleaching with MONOX-L,

1991

TAPPI

Pulping Conference Proceedings, pp. 523-532.

Hurst, M.M., R Korhonen, K. Vakeva, and R.

Yant, High brightness pulp bleaching with

Monoox-L, PulpPaperCan, 93(11):41-44(1992).

Hypochlorous acid is purported to be an effective

bleaching agent when used with a proprietary

additive (said to be a C—-2 monoalkyl amine) that

protects cellulose from acid hydrolysis. (One

wonders if this additive might be useful during

chlorine bleaching.) A mill trial using this process

is described here. This method helps keep chlo-

rine and caustic use balanced. The production of

dioxins is said to be low. Use of a weak hypo by-

product from the CIO2 generators is also helpful.

Chlorine

monoxide bleaching

Bolker, H.I. and N. Liebergott, Chlorine monox-

ide gas: a novel reagent for bleaching pulp. Pulp

Paper Mag. Can, 73(11)82-88(1972). Chlorine

monoxide, CI2O, was found to be an effective

bleaching agent at 15—30% consistency and could

replace chlorine dioxide in later bleaching stages.

For example, chlorinated, extracted sulfite pulp

could be brought to 94 brightness with a single

stage using 0.6% CljO on pulp. Chlorinated,

extracted kraft pulp was brought to 82 brightness

when treated with 1% CljO for one minute; a

second stage brought the brightness to 94%.

Strength properties of the pulps were as good as

those pulps bleached with chlorine dioxide. With

unbleached kraft pulp, 2% CljO was more effec-

tive than bleaching with 5.9% CI2, with 94% less

color in the effluent.

Xylanase treatment

Daneault, C, C. Leduc, and J.L. Valade, The use

of xylanases in kraft pulp bleaching: a review,

Tappi

J. 77(6):125-131(1994). Enzymatic bleach-

ing pretreatments (hypothetically) rely on breaking

lignin—carbohydrate bonds in pulp.

34.11 FIBER PHYSICS AND PREPARATION

Fiber bonding revisited

An increased level of fiber—to—fiber bond-

ing gives a sheet with higher density, lower

opacity, and increased shrinkage and swelling with

changes in moisture content. In the case of the

latter observation, the change in length of fibers

(which does not normally occur with a change in

moisture content in individual fibers) in well-

bonded papers is attributed to a change in width of

fibers laying across fibers, as demonstrated in Fig.

34-1.

Consider a fiber that lays parallel to the force

applied in a sample of paper (Fig. 34-2). This

fiber is held by a series of fibers perpendicular to

it. The force on the bonds is highest at the ends

of the fiber and lowest in the middle of the fiber

(Fig. 34-2, top graph); the tensile load on the fiber

is highest at the center of

the

fiber (second graph);

FffiER PHYSICS AND PREPARATION 711

Poorly bonded fibers Well-bonded fibers

Fig. 34-1. In poorly bonded fibers, shrinkage

width

one fiber does

not

cause

the

length

another fiber

decrease very much,

but

this

not true

well—bonded fibers.

therefore, the probability of fiber failure is highest

in the middle, especially

well—bonded papers

(third graph), and

the

probability

bond failure

is highest

the ends

the fiber (bottom graph).

Drying under restraint

is a

fact that much

increased strength in

the machine direction

paper over

the

cross

machine direction

due

drying under restrain,

and

not due to

fiber alignment.

Two

studies

this area demonstrate this with handsheets.

^-^

Force

Bonds

Tensile

Load

Fibers

Paper

Fiber

^-^

.--^

fl 1

u u u u u u u u u

Poorly

.„^ell-bonded fiber

bonded fiber

Well-bonded

Position along fiber

Fig. 34-2. Characteristics

fiber

and

bond

forces along the length

of a

fiber.

Setterholm, V.A., W.A. Chilson, and A.T. Luey,

Effect

temperature and restraint during drying

the

tensile properties

handsheets, USD A

FPL rep. no. 2265 (1962),

6 p.

Handsheets prepared from

bleached south-

ern pine kraft pulp were dried

73°, 150°,

and

220°F under four degrees of restraint. Analysis of

stress—strain data from tension tests revealed that

increasing either drying temperature

restraint

during drying increased the tensile properties, and

that restraint had the greater effect.

Setterholm,

V.A. and

W.A. Chilson, Effect

restraint during drying on the tensile properties

handsheets,

US For. Ser., Res. Pap. FPL 11

(April, 1964),

25 p.

This study shows, among

many other things, that drying under restraint

increases the density of sheets (and presumably the

degree

fiber bonding) and their tensile strength

values. Much data

given

this reference.

Fiber

physics

Chemical pulp fibers have

higher fiber

saturation point,

lower

wet

strength,

and a

higher dry strength than wood fibers.

Bolam,

F., Ed.,

The Formation and Structure

Paper, Tech. Sect. British Paper and Board Mak-

ers.

Assoc, London, 1962,

vol., 910

and

index. This

is a

highly technical look

fiber

and paper physics. Many micrographs are includ-

ed that show well on the books' coated paper.

must

for

anyone serious about paper physics.

Refining

Gentle action

refining (more impacts

lower energy each,

for

example,

reducing

the

bar or groove width thereby increasing the number

of bars) leads

higher strength properties

for

fiber refined to

given freeness. For this reason,

series refining has some advantages over parallel

refining. However, with series refining,

the

maximum pressure

of the

last refiner must

below 100 psi.

If there

too much depth between the bars

then the pressure across the refiner increases,

the

idle load increases,

and

there

more pumping

action.

the depth

is too

small, then

the

plates

wear out quickly.

The peripheral speed

disk refiners

generally in the range

4700—5700 ft/min.

712 34. UPDATES

AND

BIBLIOGRAPHY

is too low, the refiner may plug, but low speeds

are conducive to fiber cutting. If the peripheral

speed is unnecessarily high, energy is wasted.

The level of refining can be monitored by

specific freeness, temperature change, pulp free-

ness,

couch vacuum, or dry end properties.

Notice the feedback time increases with these

methods. Specific energy is a very common and

easy way of monitoring the refining.

Kraft pulping (unbleached spruce) leaves very

little uronic acid in the pulp whereas sulfite pulp-

ing leaves much higher amounts behind. This is

one reason sulfite pulps are easier to refine than

kraft pulps. These pulps differ in fiber swelling

and response to alkali and metal ions.

Knutsen, D.P. and W.J. Bublitz, Unpublished.

Energy use of the PFI mill in terms of

hp—day/ton can be calculated by the following

equation, based on the fact that the idle power of

the PFI mill (which should be determined on each

machine) is about 0.0034 watt-hr/revolution. The

amount of pulp is usually 30 grams (oven—dry

basis).

PFI watt-hr - 0.0034xPFI rev.

hp-days/ton =

grams pulp

50.67

g'hp-day

watt-hr Ion

Screening

McCarthy, C, Various factors affect pressure

screen operation and capacity. Pulp & Paper

62(9):233-237(Sept., 1988). This is a good,

general article on screening and has a useful figure

on contour types of screen cylinders.

34.12 PAPERMAKENG

Papermaking chemistry

Scott, W.E., Principles of Wet End Chemistry,

Tappi Press, 1996. This is a revision of the 1992

work Wet End

Chemistry:

An Introduction. This

book is very helpful, although the technical quality

of the computer—generated figures is fairly poor.

Papers from

mechanical

pulps

Santkuyl, R.J., SC grades without the

supercalender: hot soft—calendering can broaden

the grade offering, Pulp Paper

Can.

96(l):49-54,

T22-T27(1995). This is a very good overview of

printing papers made from mechanical pulps.

A wide variety of printing papers are made

from mechanical pulps, and the range continues to

increase. Some common acronyms include SC for

supercalendered (which is further broken down to

three commercial grades SCA, SCB, and SCC);

MF for machine finished; MFC for machine

finished coated, a relatively new process; news

for newsprint; and LWC for light weight coated.

Fig. 34-3 shows the approximate relative bright-

ness and gloss of these grades. Keep in mind that

calendering operations improve gloss, but do not

affect brightness, which improves largely by the

use of fillers and coatings.

Santkuyl points out that the names we choose

are based on the process to make them and not on

the properties of the papers themselves, which

would be much more appropriate for the end users

(i.e.,

the customer). Confusion easily arises when

SC grades can

produced without supercalender-

ing (for example, by hot soft—calendering). (We

are accustomed to calling paper made from

thermomechanical pulp as groundwood grades

when, of course, the stone groundwood pulping

process is much different from the thermo-

mechanical pulping process.)

Supercalendering must be done off—machine

because it cannot be threaded at machine speeds.

The traditional rolls for the supercalender were

cotton—filled rolls. Recently, synthetic composite

coverings are being used instead of the

cotton—filled

rolls,

allowing for better calendering

action under conditions not suited for the cotton-

filed

rolls.

Santkuyl points out a variety of rea-

sons why on—machine calendering is preferred to

supercalendering, including lower equipment

requirements and lower operating costs. Depend-

ing on the equipment used, hot soft—calendering

is carried out from 200°F (93°C) with a pressure

of 750 lb/in. (131 kN/m) (as low in Fig. 34-3) to

400°F (204°C) with 2000 lb/in. (350 kN/m) (as

high in Fig. 34-3).

Pressing and drying

A useftil general reference on this is 1989

Practical Aspects of Pressing and Drying Seminar,

TAPPI Press, Atlanta, Georgia.

FIBER PHYSICS AND PREPARATION 713

Fillers and paper strength

Fillers reduce the strength of

paper.

Finding

numbers for strength loss is not easy, but Libby,

vol.

2 (1962) gives some values on page 76.

34.13 PAPER

Surface

properties of paper

Conners, T.E. and S. Banerjee, Surface Analysis

of Paper, CRC Press, Boca Raton, Florida, 1995,

368 p. This work provides information on the

application of modern surface analysis techniques

to paper.

Handmade

paper

Hunter, D., Handmade papers, in Pulp and Paper

Manufacture, Vol 5, MacDonald, R.G., Ed.,

McGraw-Hill, New York, 1969, pp. 1-18.

Mechanical

properties

Seth, R.S., Implications of the single—ply

Elmendorf tear strength test for characterizing

pulp,

Tappi J.

74(8):

109-113(1991).

This work

points out that the tear strength of paper when

measuring a single sheet can vary by as much as

60%

compared to that of tests using multiple

sheets. The magnitude of the difference depends

on the degree of bonding in the sheet. This fact is

particularly important since the performance of

paper in usage may correlate better with

single—ply results rather than multiply results.

TAPPI TIS 0304-24 (1989), Calibration and

maintenance of the burst tester. This helps insure

a tester is giving correct readings.

Paper

abrasiveness

Lauftnann, M., G Brautigam, N. Gerteiser, and

H. Rapp, Natural ground CaCOs as copy paper

filler,

TAPPI

Notes,

1985

Alkaline

Papermaking,

pp.

61-72. This paper discusses measurement of

abrasiveness of fillers. There is some concern that

the use of calcium carbonate fillers in printing

papers will cause increased wear on the paper ma-

chine or in service such as in photocopy machines.

Pitt, R., Fillers and fabric life in alkaline

papermaking, TAPPI Notes, 1985

Alkaline

Paper-

making, 47-50. This is of related interest.

34.14 COATING AND CONVERTING

Converting

Kline, I.E., Paper and Paperboard, Manufac-

turing and Converting Fundamentals, Miller

Freeman

Pub.

Inc., San Francisco, 1982. Section

III (pp. 152-228) contains five useful chapters

corrugating operations and raw materials; packag-

ing; tissue and related grades; and cut size, bond,

copy paper, and computer paper.

Coating

Coating has been discussed on pages 250,

254-258,

and 261-262. Some additional details

are considered here. Coating operations are

usually carried out with pigments (clay, calcium

carbonate, titanium dioxide, or others) to improve

the gloss and printing characteristics of paper.

Other coatings are used to make carbonless papers

and other specialty papers. The optical properties

of coated and filled papers are considered

separately in Chapter 24.

Coatings can be applied at the size press or

after the paper is formed and dried. The coating

is applied to the paper, the coating is smoothed,

3 50

ffi30

60 75

65 70

Brightness,

Fig. 34-3. Spectrum of mechanical printing

grades after Santkuyl (1995).