Holik H. (ed.) Handbook of Paper and Board

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cleaning agents are used in an appropriate manner and the washing liquor is

buffered sufficiently.

3.7.7

Flocculants and Coagulants for Clarification of Different Water Sources

In the production of pulp, paper and board a large amount of water is generally

necessary. Therefore different quantities and portions of fresh water (taken from

river/lake nearby or from fountains), and recycled process water have to be used. For

high production efficiency and good paper quality, clean water is required. On the

other hand the effluent has to be environmentally friendly and has to meet the

official legislation.

Therefore all three water sources (fresh water, recycled process water, effluent)

have to be treated and clarified, mechanical treatment (filtration, sedimentation,

flotation) always having first priority over chemical treatment. In practice, in most

cases, a combination of mechanical and chemical treatment leads to an efficient

solution.

3.7.7.1 Fresh or Raw Water

This can be colored, as turbid surface water may contain humus as well as iron

and manganese, either in the oxidized form or bound to organic matter. Water of

this type is treated by aeration + coagulation + addition of alkali + clarification +

sand filtration. The removal of color and organic compounds usually requires a

method based on chemical precipitation. Some reduction in organic matter con-

tent, of course, can be achieved by simple filtration. The removal of iron and

manganese, on the other hand, usually requires:

1. Oxidation of iron (II) and manganese (II) to oxidation state III e.g. by chlorine

or permanganate

2. Hydrolysis of the trivalent cation to produce the hydrated hydroxide

3. Coagulation of the hydrated hydroxide

4. Removal of the coagulate.

The next step is usually chemically-induced coagulation and flocculation. The se-

quence of events is addition of chemical, rapid mixing, and finally gentle stirring.

Here aluminum sulfate is one of the commonly used chemicals. A suitable rate of

addition can be calculated from zeta potential measurements. Aluminum sulfate

reacts with the alkaline compounds naturally present in the water and also with

any added lime or sodium carbonate. Another commonly used coagulant is sodium

aluminate. This is normally employed as a supplementary coagulant to ensure

proper treatment of cold waters or to coagulate residual aluminum sulfate. Supple-

mentary coagulants are used in cases where floc formation would otherwise be

unsatisfactory. Here the most common agents are activated silicic acid, certain nat-

ural organic compounds and synthetic polyelectrolytes, mainly modified polyacry-

lamides with high molecular mass. Their ionogenic character depends on the in-

3 Chemical Additives142

dividual mill conditions. Other chemicals used, mainly for pH adjustment, include

certain calcium and sodium compounds. The addition rates of aluminum com-

pounds are in the range 10 to 100 g m

–3

fresh water and those of polymers ca. 0.1

to 1 g m

–3

3.7.7.2 Recycled Process Water

Recycled process water allows one to reduce fresh water demand and to reduce the

effluent loadings from paper and board manufacture. In papermaking water con-

sumption has been reduced dramatically in the last decades. Printing papers, for

example, are manufactured at a specific fresh water consumption of about

8–10 m

–1

, packaging papers at about 3 to 5 m

–1

. Hereby, at least for graphic

papers, solids and dissolved organic matter are removed. If lowering water con-

sumption down to 2–4 m

–1

salt concentrations have to be managed. In packag-

ing paper production e.g. for testliner and corrugated medium, in more and more

cases fresh water consumption is down to the evaporated water in the dryer section

of the paper machine. This means a specific fresh water consumption of

1–1.5 m

–1

and no more waste water to the effluent. These results can only be

obtained by a very efficient mechanical and chemical treatment of the recirculated

process water. To separate and remove solids and dissolved substances from this

water, nonsalt building flocculants, e. g. polyethylenimine, polyamine and/or poly-

acrylamides, have to be used. The addition of these polyelectrolytes (solid) is in the

range 10 to 100 g m

–3

recycled process water.

In completely closed water circuits biological treatment (anaerobic-aerobic) re-

move dissolved organic matter even more efficiently. However, recycling the bio-

logically treated water can be restricted by the resulting reduction in product

brightness, which does not matter for packaging papers. With nano- and ultra-

filtration very small solid particles and some dissolved high molecular mass mate-

rial are removed. For good results the surface chemistry during the manufacturing

process has to be thoroughly watched. Internal process measures reduce loadings

at source, and direct savings can be made in raw material costs (fiber, fillers,

additives) and also in energy consumption.

3.7.7.3 Effluents

Effluents from pulp, paper and board mills contain wood materials in solid, colloi-

dal and dissolved form. The effluents also contain some chemicals used in the

production process, either in their original or modified form. The principal me-

chanical methods used to remove these compounds are filtration (including mem-

brane and nanotechnology), sedimentation and/or flotation. All these methods

require very good flocculation and coagulation of these undesirable substances in

the effluent. Only then can they be separated from the clear water. The main

chemicals used for coagulation are aluminum salts (e.g. Al

(SO

)

), iron salts (e.g.

FeCl

or Fe

(SO

)

or FeSO

) and lime with addition rates of 100–500 g m

–3

efflu-

ent. To achieve optimum flocculation results, it is often necessary to feed a suitable

3.7 Process Chemicals 143

polymer, e. g. modified polyacrylamide with high molecular mass and a certain

charge density during slow mixing. Addition rates of the polyelectrolyte (solid) are

1–50 g m

–3

effluent. The primary purpose of chemical coagulation is to neutralize

the electrical charges on the particles and hence prevent repulsion. This means

adding metal cations because the suspended organic material is nearly always

negatively charged. In most cases, the coagulant itself also precipitates (for exam-

ple, as the hydroxide), making the treatment more effective. For this reason, pH

control is important. Organic polyelectrolytes, on the other hand, normally bind

together dirt particles and in this way promote floc formation. Typical charge

measurements are anionicity determination, zeta potential measurement, and cal-

culation of particle sizes. In effluent treatment, chemical coagulation is applied

mainly to the separate treatment of concentrated effluent fractions (e.g., those

from pulp bleaching, deinking of recovered printing papers, coating color prepara-

tion, and broke). The flocs formed are removed from the water by sedimentation,

flotation, or filtration. The sludge produced is either fed back to stock preparation

or after dewatering by polyelectrolyte addition and specific presses goes to in-

cineration, depending on the paper grade produced.

Biological treatment of the effluent is particularly useful for removal of low molec-

ular mass organic matter. The process is based on the ability of microbes to live

and reproduce in effluents. In doing so, microbes break down dissolved and colloi-

dal substances by using them as nutrients. In this way, waste is converted partly

into biomass and partly into carbon dioxide and water. For effective biological

treatment pretreatment of the effluent may be necessary. In particular, tempera-

ture, pH, and oxygen and nutrient contents must be suitable for microbial growth.

For biological effluent treatment many different kinds of microbes, ranging from

the simplest bacteria to protozoa and even worms, can be simultaneously used.

Quite a large number of pulp, paper and board mills already use biological treat-

ment, very often an aerobic one, and, depending on the specific conditions, also a

combination of anaerobic and aerobic treatments.

3.8

General Remarks on the Application of Chemical Additives

A range of chemical additives are used along the process chain of paper and board

manufacturing and paper coating to provide technological, economical and eco-

logical advantages (Tables 3.10 and 3.11). They allow the reduction of fresh water

and energy consumption, as well as enabling the paper industry to increase the

recycling rate in paper and board production. Furthermore, savings in raw mate-

rial can be achieved by reducing the basis weight of paper without losing certain

quality properties. The use of new chemical additives allows one to produce paper

at neutral pH/slightly alkaline conditions instead of in an acid environment which

was mainly in use until the 1970s. This results in less corrosion of machinery parts

and also in improved paper properties, e.g. higher paper strength as well as much

better permanence of the paper. Papermaking chemicals allow large sums of capi-

3 Chemical Additives144

tal tied up in plant and equipment to be utilized more efficiently, and they can

boost productivity by reducing downtime and increasing production rates. The

productivity of subsequent coating and converting processes can also be improved

by selecting chemical additives according to their specific properties.

Whether by chance or design, many papermaking chemicals have some useful

side-effects in addition to the effects for which they have been primarily developed

(Fig. 3.25). The intelligent combination of different chemical additives can im-

prove quality, economy and ecology. On the other hand they can also interact in a

negative way so the potential interactions of all chemicals applied have to be con-

sidered (Fig. 3.26). Some integrated systems have already been established accord-

ing to the “lock and key” principle, such as combinations of fixing agents/retention

aids/drainage aids, fixing agents/retention aids/internal sizing agents and combi-

Table 3.10 Chemical additives and their different places of

addition in the paper and board manufacturing process.

Table 3.11 Chemical additives and their different places of

addition in the paper and board coating process.

3.8 General Remarks on the Application of Chemical Additives

145

Fig. 3.25 Beneficial side effects of (PEI) as a retention aid.

Fig. 3.26 Interactions of chemical additives – positive and

negative effects are possible. (RDA = retention drainage aids.)

Fig. 3.27 Impact of binder/co-binder combination on the

properties of coating color and paper quality (source: BASF).

3 Chemical Additives

146

nations of sizing agents/coating binders. Figure 3.27 shows a combination of a

binder and a co-binder in a coating formulation, which results in improved ma-

chine runnability and better printability of the paper.

Attention has to be paid to the chemical and physical conditions of the white

water circuit and to the measurement and control of the effectiveness of various

chemical additives (Table 3.12). On-line measurements and integrated process

control give the best and constant performance of multifunctional chemical com-

binations. Of similar importance are the methods and positions for metering

chemical additives to the paper production system as well as the sequence of their

addition (Fig. 3.28).

Nowadays, chemical additives are an integral part of advanced papermaking

technology. Maximum performance can be achieved by tailoring their properties in

Table 3.12 Measuring, monitoring and control of wet-end con-

ditions and effectiveness of chemical additives.

• Chemical/Physical Status of Wet-end Condition

L first-pass retention of total stock and filler

(white water consistency/headbox consistency)

L overall retention (paper production/total stock input)

L turbidity of white water and clarified water

L ph, COD, cationic demand, zeta-potential, conductivity

• Retention and Drainage Aids

L headbox and white water concentration

L cationic demand, zeta-potential,

L freeness, dewatering time of stock suspension, turbidity

• Fixing Agents

L catonic demand

L pitch particle count (number, size, distribution, deposition)

• Deaerators

L gas content (ultra sonic, compression, spec. gravity)

• Sizing Agents

L indirectly via fiber/filler retention

L directly: off-machine paper tests (Cobb. HST, BAT, Emco, DAT)

• Dry-Strength/Wet-Strength Agents

L indirectly via zeta-potential

L directly: off-machine paper tests (tensile after dry and wet conditioning)

• Dyestuffs

L chromaticity co-ordinates

L fastness of light and bleed

L resistance of rub, acid, solvent and heat

• Biocides, Slimicides

L total plate count (TPC), anaerobic and aerobic

L mould and fungus count

L sulfate reducing bacteria

L oxidative demand, catalase control

3.8 General Remarks on the Application of Chemical Additives 147

order to obtain the desired effects and by adapting products to local manufacturing

conditions. The development of innovative chemicals and intelligent formulations

and combinations of chemicals will play an even more important role in the future

in responding to the challenges facing the paper industry in terms of higher pro-

ductivity, improved and/or new quality properties and minimal environmental im-

pact. This depends on taking an integrated approach to paper manufacturing,

coating and converting and strengthening the cooperation between the paper in-

dustry, machinery manufacturers, the chemical industry and the paper converters

and printers.

Fig. 3.28 Flowsheet of paper production process with

integrated metering points and addition sequences of various

chemical additives. (DSR = dry strength resin, DSR-PAM = dry

strength resin – polacrylamide, OBA = optical brightening

agent, PAC = polyaluminum chloride, RDA = retention

drainage aid, WSR = wet strength resin.)

3 Chemical Additives

148

References

1 Auhorn, W. J., Chemische Additive für die

Papierherstellung – Kleine Mengen mit

großer Wirkung garantieren den For-

tschritt, Wochenbl. Papierfabr., 23–24, 1999.

2 Goebel, K., Thummer, R., Geller, A.N., Au-

horn, W.J., Kleemann, St., Schultz, W-St.,

Roick, Th., Köcher, M., Hamm, U., Gür-

tler, A., Chemische Zusatzstoffe – funktio-

nell unentbehrlich und ökologisch nütz-

lich, Das Papier 1997, 51 (6A), V91 – V127.

3 Kleemann, St., Götz, B., Künzel, U., Le,

P.C., Gliese, Th., Pelzer, R., Auhorn, W. J.,

Roick, Th., Schultz, W-St., Hamm, U., Ös-

terberg, F., Chemische Additive – funktio-

nell unentbehrlich und ökologisch nütz-

lich (Teil II), ipw/Das Papier 2003, Heft 9,

S. 31–46; Heft 10, S. 27–44; Heft 11,

S. 25–46.

4 Gullichsen, J., Fogelholm, C.-J., Papermak-

ing and Technology, Book 6A and 6B, Chem-

ical Pulping, ISBN 952-5216-00-4 Fapet Oy,

Helsinki 1999.

5 Sundholm, J., Papermaking and Technology,

Book 5, Mechanical Pulping, Fapet Oy, Hel-

sinki 1999 in cooperation with Finnish

Paper Engineers’ Association and TAPPI

6 Casey, J. P., Pulp and Paper – Chemistry

and Chemical Technology, Vol. I, Wiley In-

terscience, New York 1981.

7 Göttsching, L., Pakarinnen, H., Papermak-

ing and Technology, Book 7, Recycled Fiber

and Deinking, ISBN 952-5216-00-4 Fapet

Oy, Helsinki 1999.

8 Tegge, Günther, Stärke und Stärkederivate,

Behr’s Verlag, Hamburg 1984.

9 Neimo, Leo, Papermaking and Technology,

Book 4, Papermaking Chemistry, Fapet Oy,

Helsinki 1999.

10 Scott, W. E., Principles of Wet-end Chemistry,

TAPPI Press, Atlanta 1996.

11 Roberts J. C., Paper Chemistry, Blackie &

Son, Glasgow 1991.

12 Putz, H.-J., Schaffrath, H.-J., Göttsching,

L., Pulp and Paper Canada, 94(7):T193,

1993.

13 Paulapuru, H., Papermaking and Technol-

ogy, Book 18, Paper and Board Grades,

ISBN 952-5216-00-4 Fapet Oy, Helsinki

1999.

14 Auhorn, W. J., Spezialchemikalien für Spe-

zialpapiere – Chemikalien zur Erzielung

multifunktionaler Eigenschaften,

Wochenbl. Papierfabr., 8, 1999.

15 BASF Brochure of International Coating

Seminar, Ludwigshafen 05.2001.

16 Lehtinen, E., Papermaking and Technology,

Book 11, Pigment Coating and Surface Siz-

ing of Paper, Fapet Oy, Helsinki 1999 in

cooperation with Finnish Paper Engineers’

Association and TAPPI.

17 Omya Paper Handbook, Klauser Weibel

Design, Zürich 2005.

18 Kearney, R. L., Maurer H. W., Starch and

Starch Products in Paper Coating, TAPPI

Press, Atlanta 1990.

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ing, NATO ASI Series- Vol. 335, Kluwer

Academic, Netherlands, 1997.

20 Willenbacher, N., Hanciogullari, H.,

TAPPI 1997 Advanced Coating Fundamen-

tals, Symposium Notes, TAPPI Press,

Atlanta.

21 ZELLCHEMING Committee “Chemical

Additives”, ongoing investigations

2004/2006.

22 Linhart, F., Auhorn, W., Polyvinylamin –

Eine neue Klasse von Polymeren für die

Papierherstellung mit umweltfreundli-

chem Eigenschaftsprofil, Das Papier, Heft

10A, 1992.

23 Stange, A. W., Auhorn, W. G.,

Maßgeschneiderte Leistungsprofile durch

die flexible Struktur der Polyvinylamine,

Wochenblatt Papierfabr., 8, 1999.

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men Kationischer Polymerer mit partikulä-

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References

149

Stock Preparation

4.1

Overview

Herbert Holik

The objective of fiber stock preparation systems is to modify the different ingoing

raw materials in such a way that the finished stock finally supplied to the paper

machine suits the requirements of the paper machine and of the quality demands

put on the produced paper or board. The raw stocks used are the various types of

virgin pulps as well as recovered paper grades. They are available in the form of

bales, loose material or, in the case of integrated mills, as suspensions. The fin-

ished stock is a suspension of defined quality as far as the mixture and character-

istics of the fibers, additives, and impurities are concerned. This quality essentially

determines paper machine runnability and is the basis for the final paper and

board quality.

A stock preparation system consists of several unit processes that are adapted to

one another. Their individual efficiency and the reliability of processing machinery

depend on suspension properties such as consistency and flake and debris con-

tent, on the chemicals entrained and added as well as on the process design itself.

So some of the unit processes can be used several times in a preparation process

line, others just once. Most of the machines used for performing the individual

unit processes have not only one effect but also side effects that may or may not be

desirable. The systems differ considerably depending on the raw material used and

on the quality of the finished stock required. For instance, fiber design by refining

is of central importance in the preparation of chemical pulp. For recovered paper

processing the cleanliness of both the individual fibers and the suspension are the

main objectives and so the removal of interfering materials is most important.

A stock preparation system comprises essentially three levels (Fig. 4.1):

• Production level, the direct line from raw material input to the finished stock

which may include unit processes with some or all of the following objectives: To

break down the raw material into individual fibers, to separate fibers and con-

taminants, to separate fibers/solids and water, to treat the fibers, and to treat the

residual contaminants.

• Recovery level where fibers and other solids, and water are recovered from the

Handbook of Paper and Board. H. Holik (Ed.)

ISBN: 3-527-30997-7

150

rejects of the separation processes applied in the production level. Again fibers

and contaminants as well as solids and water have to be separated.

• Discharge level, for ecological and economic reasons the final rejects are sepa-

rated into wastewater and residues with high dry content.

The unit processes in fiber preparation and their objectives are:

• Slushing and deflaking: To break down the fiber raw material into a suspension

of individual fibers. Slushing should at least result in a pumpable suspension

enabling coarse separation and deflaking if required. In the case of recovered

paper, ink particles and other nonpaper particles should be detached from the

fibers.

• Screening: To separate particles from the suspension which differ in size, shape

and deformability from the fibers.

• Fractionation: To separate fiber fractions from each other according to defined

criteria such as size or deformability of the fibers.

• Centrifugal cleaning: To separate particles from the suspension which differ in

specific gravity, size and shape from the fibers.

• Refining: To modify the morphology and surface characteristics of the fibers.

• Selective flotation: To separate particles from the suspension which differ in

surface properties (hydrophobicity) from the fibers.

• Nonselective flotation: To separate fine and dissolved solids from water.

• Bleaching: To endow yellowed or brown fibers with the required brightness and

luminance.

• Washing: To separate fine solid particles from suspension (solid/solid separa-

tion).

• Dewatering: To separate water and solids.

• Dispersing: To reduce the size of dirt specks and stickies (visibility, floatability),

to detach ink particles from fibers.

Figure 4.1 Process levels in fiber stock preparation and their

functions.

4.1 Overview

151