Biermann Ch. Handbook of Pulping and Papermaking

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714 34. UPDATES AND BIBLIOGRAPHY

and the coating is dried. Air knife coating is

limited to one side of the sheet at a time. It is

useful for carbonless papers where physical con-

tact with the paper is not possible. Some high-

end papers use two— or even three—pass coating.

Coating

pigments

The general properties of the most common

coating pigments (kaolin clay, calcium carbonate,

and titanium dioxide) were described earlier (pages

194-195 and 439-440). (Titanium dioxide absorbs

light in the near UV range and should not be used

with fluorescent whitening agents.) Particles for

coating formulations must have small size distribu-

tions,

generally below 1—2 fim, Delaminated clay

of ultrafme particle sizes are used in coatings to

give high gloss. Fine particle sizes give lower

gloss.

Coarse particles are used for matte finished

sheets. Of the additional pigments listed here,

aluminum trihydrate has appreciable use, but the

others enjoy relatively little use.

Aluminum

trihydrate,

ATH, is a high—bright-

ness pigment with a bluish color; the preferred

name is aluminum hydroxide, A1(0H)3. It is often

used as an extender for TiOj and acts as a spacer

between the TiOj particles. About 35,000 tons

were used by the world's paper industry in 1993.

Satin white, first used in the last quarter of

the 1800s, has appreciable use in premium grades

of paper in Europe, but has little use in the U.S.

It is made by reacting CaCOj with alum to give a

precipitate of needle—shaped particles consisting

of calcium sulfate. In paints it is called satin

white. Pearl filler is also calcium sulfate but is

prepared in a different manner.

Silica and silicate pigments have high bright-

ness,

small particle size, and low abrasion. The

composition is HjSiOj. They do not absorb UV

light and, therefore, work well with optical bright-

eners.

They are used on premium grades of paper

often with titanium dioxide.

Barium sulfate is an expensive filler with

good properties. Zinc oxide is occasionally used

as a coating material, but is more often used in

electrophotography.

Plastic pigments are usually spherical parti-

cles of polymers. Polystyrene is a hard plastic

and is the only material presently used for plastic

pigments. The size distribution is monodisperse,

although two (or more) different—sized particles

may be mixed together to give a high packing

density. Particle sizes are available from 0.2 to

1.0 fim. The cost is high, but high sheet gloss can

be obtained if the polystyrene is heated above its

glass transition temperature of about 100°C. The

density is about 1.05 (much lower than other

fillers and fibers, making it good for lightweight

papers) and the index of refraction is 1.60.

The rheology of the coating solutions is very

important. It is desired to have a relatively low

viscosity at high solids content so that drying costs

can be decreased. The solids content of coating

formulations is usually 50—75%. Low solids

content coatings with relatively high levels of

binders are used in the size press.

Coating

binders and adhesives

The use of coating binders must be limited so

that it does not fill in the voids between pigment

particles and, therefore, decrease the opacity of

the coating. Typical levels are 5 to 20% of the

dry solids of the coating formulation. Binder

formulations often consist of a soluble binder and

an emulsion binder. Binders must be particularly

strong for paper destined for offset printing.

Information on coating binder migration,

which causes ink mottling and poor adherence of

adhesive in many instances, is discussed on page

477.

Materials may be added to the coating

formulation to crosslink or precipitate the binder.

Starch (page 199) accounts for most of the

natural binders used in coating formulations.

Corn starch is most commonly used, but other

starches can be used. Starch is often modified to

improve its properties. Hydroxyethylated starch is

commonly used on premium grades of paper;

oxidized starch is also used and is less expensive.

Starch viscosity is decreased by enzymes (amylas-

es),

acid hydrolysis, or oxidative cleavage of

glycosidic linkages (called concersion or scission

in the paper industry). Proper cooking gives a

well—dispersed starch; cooking is accomplished

by the batch or continuous (jet) process. The

cooking process is sometimes monitored by ob-

serving the granules under a microscope with

polarized light; the granules are not polarized

after they are well dispersed. The starch is stabi-

lized by crosslinking on papers designed for offset

(lithographic) printing so that is does not dissolve

in the fountain solutions and picking is limited.

PAPER 715

Soy protein (commercially available since

1937) is used in some paper coatings and in

relatively large amounts in paperboard coatings.

The soy protein is received as a powder and is

often carboxylated, which is solubilized with the

addition of some alkali. Other formulations can

be dissolved in cold water. A wide variety of

viscosities are available. Soy protein replaced

casein in the 1950s. Casein (milk protein) has

been used widely in the past.

Synthetic binders are used as latexes (emul-

sions).

Latex originally was the term for emul-

sions of natural rubber, but the term now applies

to many emulsions of synthetic polymers. Sty-

rene—butadiene rubber (SBR) is the most

commonly used synthetic binder. Polystyrene is a

tough, strong, brittle material, and polybutadiene

is an elastic, rubbery material. The butadiene is

usually carboxylated to improve its bonding

ability. The relative amounts of these components

alters the properties of this copolymer. Styrene

content of 50% gives a tacky material with good

binding properties; styrene content of 70% gives

a material that is more brittle but gives good gloss.

The latex particles are 0.1 to 0.3 /^m in diameter.

Synthetic binders often have poorer ink receptivity

than starch or protein binders, but give enhanced

ink gloss due to improved ink holdout.

Polyvinyl acetate

(PVA)

is the major compo-

nent used in white glues for wood. PVA may be

used with other natural or synthetic binders. It is

somewhat water sensitive and gives a hard surface.

It is useful for web offset coatings because it is

somewhat porous and imparts stiffness in the paper

(useful for improving the stiffness in the cross

machine direction); these properties decrease

paper blistering during printing. Vinyl acetate is

used with acrylic monomers to make copolymers.

Polyvinyl alcohol has use in specialty coatings.

Acrylic latexes are high—quality binders with

many advantages, but the SBR binders have

limited their use until recently. They provide

good stability against aging by light, heat, and

chemicals and fold strength. Polymethyl methacry-

late is one acrylic polymer (Lucite® or Plexiglas®).

Coating additives

A wide variety of materials are added to

improve and control coating formulations (see

Chapter 21, Colloid and Surface Chemistry).

Many are similar to the papermaking additives,

although retention and drainage are not consider-

ations. One must consider additives that are

already included with the pigments, such as

dispersants, and adhesives, such as surfactants.

Dispersants are used to keep pigment parti-

cles from agglomerating, which would decrease

their opacity. They are used at about

1 %

of the

pigment weight. Polyphosphates are used with

clay calcium carbonate, titanium dioxide, and

other mineral pigments. Polyphosphates, such as

tetrasodium pyrophosphate (Na4P207, also called

sodium diphosphate) are formed by dehydrating

sodium salts of phosphoric acid (such as disodium

phosphate, Na2HP04). Polyphosphates may

hydrolyze to low molecular forms in water with

heating or low pH; this process liberates acid,

which further catalyzes the reaction. Poly-

phosphate anions are effective at sequestering

multivalent cations. Aqueous solutions of sodium

silicates (water glass) are also used as dispersants.

Polyelectrolytes, such as low molecular

weight sulfonated materials or polyacrylates, may

be used as a second dispersant in case the

polyphosphate degrades or is used improperly.

Ethoxylated fatty alcohols are used as nonionic

dispersants, which prevent the pigments from

getting close enough to agglomerate (steric

hindrance).

Defoaming agents or foam control agents are

used to prevent entrained air from forming in the

coating mixture. Entrained air increases the

viscosity of the coating formulation and leads to

defects in the coated product.

Biocides are used especially when starch, a

favorite food of microorganisms everywhere, is

used as the binder. Good housekeeping, proper

machine design, and occasional sterilization help

control microorganisms. If problems are encoun-

tered, one should isolate the offending microorgan-

ism and use a suitable biocide.

Viscosity

control agents allow the rheological

(flow) characteristics to be regulated. The use of

most binders, and the desire to have a high solids

contents, usually means that most coating formula-

tions will have a high viscosity. Some chemicals

actually reduce the viscosity of coating formula-

tions,

including urea and polyvinyl pyrrolidone

716 34. UPDATES AND BffiLIOGRAPHY

with molecular weight below 40,000. Urea, used

at 5—20% of the weight of starch, stabilize the

viscosity behavior of modified starch.

It is relatively easy to increase viscosity by

using polymers with high molecular weight

(starch, alginates, or CMC).

Antistatic (electricity) agents work by ab-

sorbing water from the atmosphere (glycerides or

glycols), by adding conductivity to the coating

(potassium chloride or quaternary ammonium

compounds and other ionic surfactants), or by

putting a charge on the surface that is opposite to

the charge that otherwise tends to build.

Other additives include lubricants or

plasticizers, such as calcium or ammonium stea-

rate,

PEG, wax, or polyethylene, which are used

to prevent dusting. Specialized coatings contain

silicone or fluorocarbon release agents for paper to

which removable labels are attached, encapsulated

pigments for carbonless papers, etc.

Additives to control the appearance of the

final product include dyes and fluorescent whiten-

ers.

Additives that absorb harmful ultraviolet

radiation (salicylates, benzophenones, and other

aromatic compounds), which would otherwise

eventually cause fading or other photochemical

degradation, may be used.

Miscellaneous

The National Cash Register Company (NCR)

obtained the original patents for carbonless paper

in 1954. Commercial coated papers range from

grade 1 to 5. Grades 1 to 3 are "free sheets";

they contain no mechanical pulp. All number 5

grade contains some mechanical pulp.

Walter, J.C., The Coating Process, Tappi Press,

1993,

260 p. This is a useful, general reference

on paper coating including materials, the prepara-

tion of coating formulations, and the various

coating processes.

34.15 FLEXIBLE PACKAGING

Hammond, P.M. and M.W. Potts, An overview of

flexible—packaging markets and structures, Tappi

J, 72(1):81-88(1989).

Thompson, K.I., A look at 20th century film

development, Tappi J. 73(6)137-140(1990). This

is an interesting history of the development of

plastic films for packaging. Cellophane was the

material that started it in 1924 and was king until

the early 1960s, when polyethylene surpassed it.

Equipment technology and film markets are also

discussed. It is too bad more detail is not given.

34.16 ENVIRONMENTAL

Cluster rules

Swan, C.E., Cluster rules update, 58(11):23-

25(1995). In late 1993, the U.S. Environmental

Protection Agency (EPA) proposed that water and

air regulations would be combines for the U.S.

pulp and paper industry. The proposal was nick-

named the "Cluster Rules". The industry has said

these regulations are unnecessarily burdensome.

As of this writing, the situation is sfill unresolved,

and future reguations are still uncertain.

Sludge disposal:

incineration,

energy recovery

Solid waste disposal by landfilling is becom-

ing increasingly expensive (with a tipping fee on

the order of $75/ton) and of concern for many

reasons (such as the possibility of a particular

landfill being targeted as a hazardous waste site in

the future for reasons presently unidentified).

Sludge disposal by incineration is one area where

landfilling of solid wastes can be decreased. For

example, sludge from the primary clarifiers typi-

cally contains over 90% wood fiber, some calcium

salts,

etc. Most mills do not use this material as

fuel because it is difficult to dewater and they do

not require the fiiel.

Jacobson, W., Fluid—bed combustion: a solution

to increasing paper sludge problems. Paper Age

106(Recycling Annual):21-22(1990). Another

major source of sludge is that generated from

secondary fiber reclamation. Although quite

variable, paper sludges have similar heating values

to wood and about 8—10% ash content; deinking

sludges have about 11—18 MJ/kg (4600—7500

Btu/lb) (dry basis) and ash contents of 15—40%.

Fluid—bed combustion of these sludges is said to

be quite efficient when they are mixed with other

fuels such as wood.

Rude, J., Sludge disposal: a problem or opportuni-

ty, PaperAge 106(7):24(1990). The author consid-

ers drying of sludge for hog fuel by rotary driers.

ENVIRONMENTAL 717

Fig. 34-4. A progressing cavity pump. Courtesy of Roper Pumps.

^ater

Upchurch, J., G. Hodge, and P. Speir, Where

does all the water go? TAPPI1989

Environmental

Conference Proceedings pp. 225-229. The paper

discusses the importance of a mill water balance

and how to obtain one for a mill. Examples of

methods to save or reuse water are given. The

process water in thousands of gallons per dry

metric ton of a state—of—the—art kraft mill are as

follows: pulping, 3—4; conventional bleaching,

8—10; bleaching system with oxygen delig-

nification,

4—7;

power—recovery—recausticizing,

3—4; paper machine, 3—5; and other uses, 2—3.

34.17 MISCELLANEOUS TOPICS

Pumps

Figure 34-4 shows a progressing

pump, which is described on page 643

cavity

Millwide Information Systems

Miklovic, D.T., and H.J. Dammeyer, Manufac-

turing automation protocol and its implications to

the forest products industry, Tappi J. 70(3):67-

74(1987). This has a four page glossary.

34.18 TESTING METHODS

Automated

(on—line)

black liquor titrations

Many manufactures make automated titrators

that are applicable to automated liquor analysis.

This is an area where much operator time could be

saved while reliability and precision could be in-

creased in the control of the kraft recovery (or

other) process, especially in these days of tighter

process/quality control. Management does not

mind buying expensive production equipment, but

is less amenable to buying relatively inexpensive

testing equipment to insure the processing equip-

ment is used to its maximum potential.

Cleveland, C, On—line analysis of pulping liquor

with a process titrator, Tappi J.

73(11):

157-

161(1990). This is used for sodium hydroxide-

sodium carbonate liquor for corrugating media.

Black liquor analysis

Busayasakul, N., J.M. Genco, and J.C. Hassler,

Carbon analysis in kraft digester control, Tappi J.

70(4):

149-153(1987).

Use of total organic carbon

(TOC) content of black liquor as a means of

controlling the cooking process, just as one uses

lignin content (kappa number) of pulp, is de-

718 34. UPDATES AND BffiLIOGRAPHY

scribed. This method is not likely to be used

routinely in pulp and paper mills.

Lignin

structural analysis

Gardner, D.J., T.P Schultz, and G.D. McGinnis,

The pyrolytic behavior of selected lignin prepara-

tions,

/. Wood Chem. Tech, 5(1):85-110(1985).

Chemical analysis of lignin by a variety of meth-

ods such as nitrobenzene oxidation, thermal

analysis, pyrolysis, and IR were investigated.

This is a good introductory article describing the

tools that tell us what is known about lignin.

Hatfield, G.R., G.E. Maciel, O. Erbatur, and G.

Erbatur, Qualitative and quantitative analysis of

solid lignin samples by carbon—13 nuclear mag-

netic resonance spectrometry.

Anal.

Chem,

59:172-179(1987). Maciel's laboratory has stud-

ied numerous aspects of NMR of lignins.

Lignin

content

infrared

UV spectroscopy

The traditional method of

lignin

measurement

using permanganate ion can be supplanted by

optical spectroscopy methods like near—infrared

(NIR) spectroscopy or visible—ultraviolet (UV)

spectroscopy. Unlike permanganate titration of

pulp,

where only a single piece of data is ob-

tained, spectroscopy methods offer reflectance

curves that offer the potential for quantifying

several components by simultaneous equations at

several wavelengths. These methods are faster

than titrations and lend themselves to continuous,

on—line measurements. In the case of IR, infor-

mation about the polysaccharides is also available,

which, in turn, allows yield determinations with a

high degree of precision.

Angeus, L. and S.—A. Damlin, Optimal control

with on—line kappa number analysis. Pulp Paper

Can.

93(2):T32-T36(1992). These workers

measured the kappa number using UV absorbance

at 280 nm with success.

Backa, S. and A. Brolin, Determination of pulp

characteristics by diffuse reflectance FTIR, Tappi

J. 74(5):218-226(1991).

Easty, D.B., S.A. Berben, F.A. DeThomas, and

P.J. Brimmer, Near—infrared spectroscopy

for

the

analysis of wood pulp: quantifying

hardwood—softwood mixtures and estimating

lignin content,

Tappi

J. 73(10):257-261 (1990).

Michell, A.J., Infra—red spectroscopy trans-

formed—new applications in wood and pulping

chemistry, Appita 41(5):375-380(1988).

Schultz, T.P. and D.A. Bums, Rapid secondary

analysis of lignocellulose: comparison of near

infrared (NIR) and Fourier transform infrared

(FTIR),

Tappi

J. 73(5):209-212 (1990).

Ion—selective

electrodes

Ion—selective electrodes (ISEs) are very

similar in use to pH electrodes. They are used for

chloride, potassium, calcium, carbon dioxide/

carbonate, oxygen, and a variety of other ions.

These methods are particularly suited for field

analysis and on—line measurements.

Lenz, B.L. and J.R. Mold, Ion—selective elec-

trode method compared to standard methods for

sodium determination in mill liquors, Tappi J.

54(12):

2051-2055(1971). Sodium ion can be

quickly measured directly with ion specific elec-

trodes, with about

1 %

error, over a wide range of

concentrations. A small sample is diluted with a

small amount of ionic strength adjuster (ISA, an

ammonium buffer solution) and the solution mea-

sured as if for pH, but in this case for p(Na'^).

This method would probably be useful for mea-

suring sodium loss in pulp by incubating the pulp

with ISA until ammonium ion has exchanged

sodium ion. It could also measure sodium concen-

trations at various stages of the recovery process.

Cooper, Jr., H.B.H., Continuous measurement of

sodium sulfide in black liquor, Tappi 58(6):59-

62(1975). This is a general article on the subject.

Schwartz, J.L. and T.S. Light, Analysis of alka-

line pulping liquor with sulfide ion—selective

electrode, Tappi J. 53(1):90-95(1970). New

electrodes often use a salicylate buffer with 1:3

sample :buffer dilution. This article has much

experimental detail.

Aluminum

ion

concentrations

Avery, L.P., Evaluation of retention aids, the

quantitative alum analysis of a papermaking

fiirnish and the effect of alum on retention, Tappi

62(2)43-46 (1979). The concentration of alum

was measured in an elegant fashion using a F'

ion-specific electrode. Fluoride complexes with

AP"^

and a titration with NaF using the ISE allows

TESTING METHODS 719

the determination of aluminum concentration

(complex formation values for F are available in

Lange).

This method can be used with poly-

aluminum chlorides (PAC), but low aluminum

values may be obtained as the polymer forms of

aluminum may react slowly. Perhaps a method

could be developed where excess fluoride is added

(if the approximate concentration of PAC is

known), the sample aged, and the fluoride back

titrated with alum (or measured directly).

Varriano—Marston, E. and H. Cheeseman,

8—Hydroxyquinoline as a stain for determining the

distribution of alum in paper, Tappi J.

69(6):

116-

117(1986).

Ion chromatography

Ion chromatography is a liquid chromatogra-

phy technique that separates ions for quantitative

and qualitative analysis. The procedure requires

a high—performance liquid chromatograph

(HPLC), usually available at corporate research

centers or university laboratories. The method is

applicable to pulping liquors, bleaching liquors,

and other process streams.

Cox, D., P. Jandik, and W. Jones, Applications of

single—column ion chromatography in the pulp

and paper industry. Pulp Paper

Can.

88(9):T318-

T321(1987).

Easty, D.B. and J.E. Johnson, Recent progress in

ion chromatographic analysis of pulping liquors:

determination of sulfide and sulfate, Tappi J.

70(3):

109-111(1987).

Murarka, S.K. and

N.S.

Fairchild, Ion chromatog-

raphy: Is it the *now' technique for monitoring

process streams in a paper mill? Part I: Principles

and instrumentation. Pulp Paper Can.

88(12):T438-T440(1987); ibid. Part II: Selected

applications. Pulp Paper Can. 88(12):T441-

T446(1987).

Bleaching chemicals

Fisher, R.P., M.D. Marks, and S.W. Jett, Mea-

surement methods for chlorine and chlorine diox-

ide emissions from bleach plants, Tappi J. 70(4):

97-102(1987). Chlorine and chlorine dioxide can

be captured from air by neutral potassium iodide

solution. A two—point titration is used to give

contents of both species.

Starch

paper

Biermann, C.J., Quantitative analysis of starch in

paper, Tappi J.

76(5):

12(1993).

A useful im-

provement over present TAPPI Standard methods

for starch analysis in paper would be a hybrid

method of enzymatic hydrolysis of starch followed

by specific detection of glucose. Specific detec-

tion of glucose could be done with glucose analyz-

ers available at drug stores or more sophisticated

ones.

Such a method would have the advantage of

taking very little operator time, being portable,

and being specific for glucose.

Imaging techniques

Paumi, J.D., Laser vs. camera inspection in the

paper industry, Tappi J.

71(11):

129-135(1988).

The title technique is used to scan paper webs

on—line at machine speeds up to 6000 ft/min for

detection of any defects.

Tomimasu, H., D. Kim, M. Suk, and P. Luner,

Comparison of four paper imaging techniques:

jS—radiography, electrography, light transmission,

and soft X—radiography, Tappi J.

74(7):

165-176

(1991).

These techniques are used for measuring

the uniformity of paper, i.e., its formation.

Pitch and

contaminant

analysis,

"stickles"

Buildup of pitch and polymer contaminants

on paper machine wires and clothing is a big prob-

lem. The stock temperature often determines

where the stickies will deposit. Often the source

is presumed to be the pulp or contaminants from

secondary fiber operations. These assumptions are

not always warranted; analysis of the residue is

instrumental to problem solving. Analytical tech-

niques include sample purification by extraction,

gas chromatography, NMR, FTIR, size—exclusion

(gel—filtration) chromatography, etc.

Biermann, C.J. and M.—K. Lee, Analytical tech-

niques for analyzing white pitch deposits,

Tappi

73(1):

127-131(1990).

This is another general

paper on the subject.

Ekman, R. and B. Holmbom, Analysis by gas

chromatography of the wood extractives in pulp

and water samples from mechanical pulping of

spruce, Nordic Pulp Paper J.

(1):

16-24(1989).

Sithole, B.B., Modern methods for the analysis of

extractives from wood and pulp: a review, Appita

720 34. UPDATES AND BIBLIOGRAPHY

45(4):260-246(1992). This is a concise review

with a lot of information and 69 references.

Sjostrom, J. and B. Holmbom, Size—exclusion

chromatography of deposits in pulp and paper

mills,/.

Chromatogr, 411:363-370(1987).

Sjostrom, J. and B. Holmbom, A scheme for

chemical characterization of deposits in pulp and

paper production, Paperi ja Puu

(2):151-

156(1988). This is a useful, general paper on the

subject.

Sjostrom, J., B. Holmbom, and L. Wiklund,

Chemical characteristics of paper machine deposits

from impurities in deinked pulp, Nordic Pulp

Paper Res. J,

(4):

123-131(1987).

Suckling, I.D. and R.M. Ede, A quantitative ^^C

nuclear magnetic resonance method for the

analysis of wood extractives and pitch samples,

Appita, 43(1):77-80(1990). Stickles can be mea-

sured qualitatively by hot press tests and other

tests.

Gaseous

pollutant measurement

Barker, N.J and J.L. Siqueira, Continuous, remote

monitoring of specific reduced sulfur compounds

in ambient air by gas chromatog-

raphy—photoionizationdetector,79SSJ?«v/ro«m^nr

Conference Proceedings, CPPA, pp. 11—15

(1988).

Ambient TRS measurements 2 and 10 km

from a mill are described in this work.

Dirt in

wood,

pulp,

and

paper

Clark, J. d'A., R.S. Von Hazemburg, and R.J.

Knoll, Paper Trade Journal 96(5):40(1933). The

authors published a chart of dirt sizes. They

included 20 particles shapes for each of 17 sizes

from 0.01 nrni^ to 5 mm^. This is the basis of the

TAPPI dirt estimation chart used in the tests

described below.

TAPPI Standards are available for quantify-

ing dirt by light reflectance in pulp (Standard T

213) and paper and paperboard (Standard T 437).

In fact, both tests are very similar. With the

widespread availability of optical character recog-

nition (OCR) systems, these TAPPI Standards un-

doubtedly will become computerized or obsolete.

• 5.00

• 4.00

• 3.00

• 2.50

• 2.00

• 1.50

• 1.25

• 1.00

• 0.80

• 0.60

• 0.50

• 0.40

• 0.30

0.25

•

0.20 •

0.15 •

0.12 •

0.10 •

0.09 •

0.08 •

0.07 •

0.06 •

0.05 •

0.04 •

0.03

0.02

Fig. 34-5. A series of circles with areas in mm^.

[Standard T 537 is similar, but merely counts dirt

particles larger than a set threshold such as 0.02

mm^ eba (explained below). It is said to be de-

signed for OCR purposes of paper [OCR is used

to determine dirt, only that dirt is determined for

an (unspecified) OCR application.]

Dirt estimation is made with the TAPPI dirt

estimation chart. The chart consists of a series of

23 pairs of black circles and rectangles (each

rectangle has a length 20 times the width) com-

prising areas from 0.02 mm^ to 5 mm^. The

rectangles are not used but are included as a

leftover of Clark et al. (1933). The equivalent

black area (eba) of a dirt particle is defined as the

area of the dot from the dirt estimation chart that

most closely resembles the visual appearance (not

the actual size) of the dirt particle. A dark dirt

particle on a brown piece of paper would give a

smaller eba than the same particle on white paper.

The limiting eba (minimum size to be counted) of

pulp is 0.08 mm^ and in paper is 0.04 mm^. Dirt

is usually reported in

mmVm^,

which is ppm. The

dirt estimation chart is not exact, and correction

values must be used for many of the spots.

Corrections are included in Standard T 213 only.

Fig. 34-4 shows a series of circles of various

approximate areas. A group of these are included

at the end of the book for practice use.

ANSWERS TO SELECTED PROBLEMS

CHAPllER 1

13.

The Abstract Bulletin of IPST for articles on

dioxins in pulp mill effluents. The Chemical

Abstracts (or medical abstracts) for the

long—term toxicity of dioxin since this is

independent of the source of dioxin.

CHAPTER 2

False.

Circle—brown, softwood, hardwood, soft-

wood, mechanical.

a) Loss of extractives b) Wood decay

1. Chip size distribution

Chip rot content

Extractives content

Age (from pith) of wood in chip

Wood species

6. Wood density

Moisture content

Fines—overcook to give a low yield and they

tend to plug the digester's liquor circulation

system. Fines tend to contain high percent-

ages of dirt and grit.

Oversize—undercook, give a large amount of

screen rejects and shives; also, larger

amounts of bleaching chemicals will be used.

6. Typically 20-30% depending on the process,

chip price, etc. Sometimes as high as 50%.

Cellulose, hemicellulose, and lignin.

8. Circle cellulose then hemicelluloses.

9. Lignin.

13.

Briefly, dissolving pulp (relatively pure cellu-

lose) is swelled in alkali and then treated with

monochloroacetic acid. The cellulosate ion

(R—0~) is a nucleophile and attacks the pri-

mary chloride of monochloroacetic acid. The

result is:

Cell-0-

+ CI-CH2-COO- ->

Cell-O-CH.COO- Na+ + Cl"

14.

The fibril angle of the S—2 cell wall layer.

15.

They adsorb and desorb water from the air

around them depending upon the temperature

and relative humidity.

16.

Hydrogen bonding.

18.

The maximum level of recycling paper on a

sustained basis is considered to be 50—60%.

At rates higher than this a significant portion

of fibers have been through one, two, or

several more uses and have lost most of their

strength.

19.

$20—40/ton. $400, or more than half the

cost of virgin pulp.

CHAPTER 3

Mechanical separation by softening and

fatigue of the compound middle lamella or

chemical dissolution of the lignin that binds

the fibers together.

Chemical pulp has higher tensile strength and

sheet density; mechanical pulp has higher

yield, unbleached brightness, and opacity.

The lignin softens to the point that fiber sepa-

ration occurs at the middle lamella (instead of

at the primary cell wall) and results in fibers

that are "coated" with lignin. Since hydro-

gen bonding does not occur with lignin,

strength is greatly reduced in the final sheet.

The higher temperature darkens the pulp.

TMP, or especially CTMP, is much stronger

than SOW since fiber liberation is much

more specific in the TMP process, leading to

longer fibers. This increase in strength

means lower amounts of chemical pulp are

required.

Batch and continuous digesters.

8. The kappa number is a measure of residual

lignin; consequently, this is a measure of

lignin content as a function of yield. The

lower the lignin content at a given yield, the

more selective (good), the pulping process.

721

722 ANSWERS TO SELECTED PROBLEMS

Other factors such as cellulose viscosity, pulp

strength, and bleachability must ultimately be

determined to make a final judgment regard-

ing the efficacy of a particular pulping pro-

cess.

9. No. If the resultant pulp has a low cellulose

viscosity and strength, it will be unsuitable

for making paper.

10.

Anthraquinone is a pulping additive used in

some kraft and alkaline sulfite pulping meth-

ods.

It undergoes a cyclic redox process.

Lignin is reduced while cellulose and hemi-

celluloses are oxidized. The oxidation of the

carbohydrates is of particular interest since

the reducing end groups (the free anomeric

carbon atoms) are oxidized from aldehyde to

carboxylic acids that prevents the alkaline

peeling reaction. (This is the same end from

which the peeling reaction occurs.)

11.

NaOHandNa2S.

12.

The pulping time and temperature are com-

bined into the H-factor. Only those two vari-

ables are considered in the H-factor; other

variables such as sulfidity and active alkali

will also influence the degree of cook.

13.

The spent liquor cannot be recovered for

reuse in the pulping process. Since there is

only a limited market for the spent liquor for

drilling muds, dispersants, and other uses,

the spent liquor is a large disposal problem

for large—scale pulp production based on this

method.

14.

Free (sulfurous acid, H2SO3) and combined

(sulfite ion, SO^^-) SO2.

CHAPTER 4

The kraft recovery process allows 1) recov-

ery of inorganic chemicals for reuse, 2)

recovery of the intrinsic energy of the waste

organic compounds to supply energy for mill

processes, 3) the disposal of the organic

materials, which would otherwise be an

environmental problem.

a) 2NaOH + CO2 -^ Na2C03 + H2O

b) NaCOs + Ca(0H)2 ^ 2NaOH + CaCOj

lOOCC

c) CaC03 -* CaO + CO2

heat

d) H20(l) - H20(g)

Turpentine is recovered from the digester

relief gases during digester heating. Tall oil

is collected by skimming the surface of the

partially concentrated black liquor.

Spent NSSC cooking liquor from the semi-

chemical pulping process is added to the

black liquor of the kraft pulping cycle to the

extent it can be used as make—up chemical.

High dilution will result in less sodium loss

through the pulp, increase the evaporation

costs as more water must be removed, and

decrease the bleaching costs as more of the

soluble, residual lignin is removed.

6. One extra ton of water per ton of pulp is

produced, and each ton of water requires

4.24""^ ton of steam, 472 additional pounds of

steam will be required per ton of pulp.

2000 lb water lib steam .«/^« , u

472 lb steam/ton

ton pulp

4.24 lb water

Long tube vertical evaporators and falling

film evaporators.

8. Concentration of black liquor by direct con-

tact with the hot flue gases of the recovery

boiler strips significant quantities of sulfur

compounds from the black liquor, causing

extremely odorous emissions, wasting sulfur,

and contributing to acid rain. Preoxidation

of black liquor prior to combustion decreases

the problem but decreases the fuel value of

the black liquor. Indirect evaporators (con-

centrators) are now used, as they decrease

sulfur emissions without the need for black

liquor preoxidation.

9. As the solid content increases beyond 65%

the viscosity increases exponentially, making

the liquor difficult to pump. On the other

hand, removal of water prior to combustion

of the black liquor increases the theoretical

amount of recoverable energy.

CHAPTER 4 723

10.

Reduction occurs in the bottom of the recov-

ery boiler and these conditions alow sulfur

chemicals to be recovered in their reduced

form (Na2S). Oxidation occurs in the top of

the recovery boiler and limits CO emissions.

11.

The ESP is used to capture particulate mat-

ter, especially Na2S04 and soot particles, for

reuse in the recovery boiler thereby decreas-

ing pollution.

12.

The sodium sulfide would be partly converted

to sodium sulfite, sulfate, and similar com-

pounds. This would precipitate as calcium

sulfite or sulfate during causticizing.

13.

The lime travels from the top of the kiln to

the bottom. Unless the lime mud is dried

separately, the lime mud first dries, is then

heated to temperature, and then undergoes

calcining as it travels through the kiln.

14.

The dregs accumulate in the lime mud, there-

by decreasing the lime availability and caus-

ing problems in lime mud settling.

15.

It is a measure of the extent of conversion of

sodium carbonate (inactive chemical) to sodi-

um hydroxide (active pulping chemical) in

the causticizing process.

CHAPTER 5

Mechanical pulps are high in lignin content.

During their bleaching the lignin is pre-

served, but the color-bearing groups

(chromophores) are altered to decrease their

light absorbance. Bleaching of chemical

pulps involves lignin removal as the primary

goal—no lignin, no lignin color.

Color reversion

a consideration in mechan-

ical pulps since large amounts of lignin are

still present.

The lignin content can be measured by

permanganate ion titration (kappa number or

k number) or by light absorption in the UV

or IR range. The latter may be converted to

an equivalent kappa or k number as if the

pulp were titrated with permanganate. This

is done since titration with permanganate is

still the most common method.

30 X 0.147 = 4.4% lignin (approximately).

Cellulose viscosity is the viscosity of cellu-

lose in solution. It is an indicator of the

degree of polymerization of the cellulose and

the strength of chemical pulps. Overly harsh

pulping and bleaching treatments decrease the

DP of cellulose and pulp strength.

6. Treatment of pulps with chlorine does not

remove lignin since this is necessarily done

under acidic conditions where the phenolic

groups are not in the salt form that contrib-

utes to water solubility. Alkali extraction

produces more phenolic groups and converts

the phenolic groups to phenolate groups that

are water soluble. In a nutshell, chlorine

"predisposes" the lignin to removal by alkali

extraction. In fact, about 75% of the lignin

present before chlorination is removed by the

subsequent alkali extraction.

If additional bleaching chemicals were used

prior to alkali extraction, most of them would

be consumed by the lignin that would other-

wise have been removed by the inexpensive

extraction stage.

Chlorine (followed by alkali extraction) is not

overly specific for lignin removal. This

treatment would not be done in the later

stages of bleaching where the lignin content

is very low.

8. If the pH is too low the cellulose degrades by

acid hydrolysis. If the pH is too high, the

cellulose degrades by oxidation. Overchlo-

rination results in cellulose degradation and

the production of large amounts of chlorinat-

ed organic compounds including dioxins.

10.

Bleaching in several stages allows different

parts of the lignin molecule to be attacked.

It is more efficient in that lower amounts of

chemical are used overall.

11.

If the pH drops to low then hypochlorite is in

equilibrium with hypochlorous acid which

degrades the pulp by oxidation.

12.

The high NaCl content increase the dead load

and corrosion in the recovery boiler.