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

Подождите немного. Документ загружается.

stock usually the best dewatering stock component used at the paper machine is

selected in order to limit the size of the disc filter. In addition to cloudy and clear

filtrate, disc filter save-all applications also produce a superclear filtrate with very

low solids content. This superclear filtrate is used as a fresh water substitute for

spray applications in the paper machine. The clear filtrate is stored in a buffer

tank. The cloudy filtrate is usually fed directly back to the inlet of the disc filter, the

“used” sweetener, including the recovered stock, is fed back to the thick stock in

the approach flow system. DAF (dissolved air flotation) type save-alls are used in

older machines and nowadays when a certain degree of ash and fines removal

from the process is demanded. In this case, the sludge of the DAF is rejected. The

advantages of a disc filter save-all compared to a DAF save-all are higher filtrate

quality, no chemical consumption and less space requirements (see Sections

4.2.6.2 and 4.2.9).

There are two reasons why there is always an excess of water present in the

paper machine loop. First, the paper machine loop is continuously fed with fresh

water, used for spraying and chemical dilution. Secondly, the incoming water con-

tent of the stock is higher (consistency 12–30%) than the water content in the

sheet after the press section (consistency up to 50% and more). This excess water

is sent, in the form of clear filtrate, from the save-all unit backwards, as make-up

water, to the stock preparation, thus following the counter current principle. This

make-up water is often additionally treated in a DAF with coagulation and floccula-

tion chemicals in order to remove detrimental substances before feeding it back-

wards.

5.4.2

Water Circuit Systems in Stock Preparation

In some cases the water circuit system of a paper mill consists of only one water

loop, for example when chemical pulp is the only fiber source or in lower quality

packaging paper production. For systems using mechanical fibers and/or recov-

ered paper, the strict separation of the water loops in stock preparation from the

paper machine loop is essential in order to meet high runability and quality de-

mands because this strategy keeps detrimental substances from the paper ma-

chine. Depending on the required quality of the prepared fiber stock and thus on

the design of the stock preparation system, the water circuit system in the stock

preparation can consist of from one to three water loops. The loops are separated

by the thickening/dewatering process stages (disc filter plus wire press or screw

press). The filtrates of these stages are sent backwards for dilution purposes in the

same loop, the excess water (usually clear filtrate from the disc filter) replenishes

the preceding loop. A water buffer tank, usually fed by clear filtrate from the disc

filter in loop I, avoids uncontrolled overflow to the waste water treatment plant.

Waste water is disposed of from the first water loop as filtrate from reject and

sludge thickening, as these filtrates are of low quality. Usually they are treated

chemically/mechanically in a DAF unit. Depending on the fresh water consump-

tion, a certain amount of this treated filtrate can be recycled into the process.

5 Water Circuits212

To reduce the content of detrimental substances within a water loop, DAF units

are used as so-called kidneys for circuit water cleaning, usually in the second loop

of the stock preparation. In this case a combination of coagulants and flocculation

aids are added to part of the disc filter clear filtrate in loop II in order to precipitate

and flocculate detrimental substances into a flotatable form and subsequently re-

move them from the process in the DAF. A high ash content in recovered paper is

also problematic for some paper grades. Here washing stages in the stock prepara-

tion only make sense when large amounts of ash need to be removed. Therefore

nowadays the filtrates of screw presses are often deashed in order to achieve a

reduction in ash content in the final stock. Usually the screw press filtrate is pre-

calibrated in a spray filter for fiber saving and the fiber-free but ash-containing

filtrate is then also sent to a DAF unit after the addition of flocculation aids.

5.4.3

Examples of Mill-wide Water Circuit Systems

Three examples show in detail how the different water circuits for the various

paper grades are designed.

5.4.3.1 Water Circuit System for Graphic Papers

5.4.3.1.1 Water Circuit System in a DIP-based Newsprint Mill

Figure 5.2 shows the water circuit system for a modern newsprint mill using

deinked pulp (DIP) as the fiber source.

Fig. 5.2 Water circuit system in a DIP-based newsprint mill (source: Voith).

5.4 Water Circuits

213

The system consists of three water loops, two in stock preparation (because of

the combination of flotation – dispersion – flotation) and one in the paper ma-

chine. The water circuit design follows the previously mentioned principles of

counter current flow and thereby allows the fresh water to be used to the max-

imum effect. In addition the save-all is followed by an additional DAF unit. DAF 1

is used as a kidney for removal of detrimental substances, treating the make-up

water from the paper machine (clear filtrate) together with part of the clear filtrate

from the disc filter in loop II. DAF 2 treats the filtrates from reject and sludge

dewatering. The clarified water is partly recycled back into loop I, the rest being

sent to the effluent treatment.

5.4.3.1.2 Water Circuit System in a DIP-based LWC Mill

Figure 5.3 shows the water circuit system for light weight coated (LWC) paper also

using DIP as the fiber source.

There are again three process water loops and the counter current principle is

consequently used. Compared to the newsprint, LWC based on DIP requires a

higher DIP quality (see 4.3.2.1.1.). The lower ash content required in the final

stock and the higher demands on brightness justifying two bleaching stages neces-

sitate bigger efforts also in the water circuit system, i.e. in kidney technologies.

Therefore three DAF units are installed. DAF 1 is used for removal of detrimental

substances from the paper machine make-up water. DAF 2 provides the necessary

ash removal from the filtrates of the screw presses in loops I and II after pre-

calibration and treatment of the disc filter filtrate in loop II. DAF 3 clarifies filtrates

Fig. 5.3 Water circuit system in a DIP-based LWC mill (source:

Voith).

5 Water Circuits

214

from reject and sludge treatment, which are then partly recycled, the rest being

sent to the effluent treatment.

5.4.3.1.3 Water Circuit System in a Mill for Packaging Paper

The configuration of water loops in systems for packaging grades is less complex

than for graphic paper grades. Older or lower quality packaging paper systems are

often single loop systems. Modern systems with an integrated LC-screening in the

stock preparation plant use a thickening stage before storage and therefore consist

of two water loops, one for the paper machine and one for the stock preparation

system. Figure 5.4 shows such a water circuit system in a modern mill producing

testliner and fluting.

Again, the fresh water is added only at the paper machine. White water II is used

for redilution after thickening in the stock preparation plant. The save-all clear

filtrate is used for make-up in the fiber preparation system. Filtrate from the thick-

ening stage in the fiber preparation system is the main source of water for slush-

ing and dilution. Waste water is sent as filtrate from reject handling to the end-of-

pipe effluent treatment.

5.4.4

Current Limits on Circuit Closure

The maximum re-utilization of the water employed is limited by the various detri-

mental substances in the process water. Their concentration, measured as COD

concentration in the water (in ppm), increases at a disproportionate rate when

Fig. 5.4 Water circuit system in a mill for packaging paper

(source: Voith).

5.4 Water Circuits

215

reducing the specific effluent (in l (kg paper)

–1

). The higher the water circuit sys-

tem is closed, the more COD (in (kg COD) (t paper)

–1

) is loaded into the paper

instead of being bled into the waste water (Fig. 5.5) [5]. This leads to a dispropor-

tionate increase in various problems due to detrimental substances, as mentioned

above. Therefore the state-of-the-art systems are limited to certain specific (l (kg

paper)

–1

) effluent volume rates with respect to certain minimum fresh water con-

sumption as shown in Table 5.2.

Consequently, the multi-loop systems for graphic paper grades nowadays follow

the counter current principle and partly use circuit water cleaning according to the

kidney principle. So it is possible to reduce the specific water emissions down to

8–10 l (kg paper)

–1

. Further closing would lead to increased amounts of detrimen-

tal substances. This would negatively affect product quality, e. g. decreased optical

properties and increased stickies and dirt content as well as runnability problems

on the paper machine, such as felt and shower pluggings, reduced retention, scal-

ing and slime formation. The typically simpler board and packaging grade systems

with their somehow lower demands on water management can work with specific

Fig. 5.5 Effect of loop closure on COD values [5] (source:

Voith).

Table 5.2 Present limits of system closure.

System application Specific effluent volume/

l (kg paper)

–1

Disturbing effects

limiting further closure

Graphic paper grades

several water loops

counter current flow

circuit water cleaning (kidney)

8–10 decreasing product quality

felt/shower plugging

lower particle retention

scaling

slime

Board and packaging grades

single or two loops

water treatment only for showers

3–5 odor problems

(water and product)

corrosion

deposits

5 Water Circuits216

effluent volume rates of a minimum of 3–5 l (kg paper)

–1

. Further closing these

loops without using additional internal circuit water cleaning could lead to odor

problems both in the mill and in the paper itself, as well as corrosion and deposits

and the paper machine runnability would be significantly reduced.

5.4.5

Zero Effluent Systems

Reducing the effluent volumes to zero means that fresh water consumption is

reduced to approximately 1.5 l (kg paper)

–1

. Here problems would arise from the

extremely high amounts of detrimental substances, which will be bled out of the

process only by transferring them into the paper. The only solution to these prob-

lems is to remove the detrimental substances from the process with suitable highly

efficient kidney technologies. These include, in addition to circuit water cleaning

by coagulation and flocculation with subsequent removal in a DAF unit, anaero-

bic/aerobic combinations of biological treatment (see Section 10.1), different

membrane filtration technologies like micro-, ultra- or nano-filtration down to re-

verse osmosis or evaporation [6, 7].

Some packaging paper mills in central Europe today run with zero effluent

systems (Fig. 5.6 ) using a combination of anaerobic/aerobic biological treatment

(see Section 10.1) as kidney technology for COD reduction [6].

The driving forces for reducing the effluent to 0 l (kg paper)

–1

for these mills

were unique to each mill. In one case there was high cost-saving potential by

completely avoiding effluent fees for disposal into a public effluent treatment

plant. In another the reason for complete water circuit system closure was the lack

of available fresh water due to the local conditions when installing additional pro-

duction capacity [7].

The other mentioned kidney technologies, i.e. membrane filtration and evapora-

tion, are mainly known as pilot scale applications as they are quite cost-intensive

and still not proven state-of-the-art [7].

Fig. 5.6 Zero effluent concept for a packaging paper

mill [6] (source: Voith).

5.4 Water Circuits

217

References

1 F. Zippel, Water Management in Paper

Mills, Deutscher Fachverlag, Frankfurt,

2001, “Preface” by L. Göttsching.

2 F. Linhart, W. Auhorn, J. Degen, H. J.

Lorz, AnionicTrash: Controlling Detrimen-

tal Substances, Tappi 1987, 10, 79.

3 W. Auhorn, Das Störstoff-Problem bei Ver-

ringerung der Spezifischen Abwasser-

menge, Wochenblatt Papierfabrik. 1984, 2,

37.

4 U. Weise, J. Terho, H. Paulapuro, Stock

and Water Systems of the Paper Machine,

Paper Making Science and Technology, Book

8, Paper Making Part 1, Stock Preparation

and Wet End, Fapet OY, Helsinki, 2000,

Ch. 5, p. 180.

5 R. Mönnigmann, M. Schwarz, Abwasser-

freie Papierfabrik – Traum oder Alb-

traum?, Das Papier 1996, 6, 357.

6 M. Schwarz, Design of Recycled Fiber

Processes for Different Paper and Board

Grades, Paper Making Science and Technol-

ogy, Book 7, Recycled Fiber and Deinking,

Fapet OY, Helsinki, 2000, Ch. 6, p. 211.

7 U. Hamm, L. Göttsching, Abwasserfreie

Papierherstellung – Welche Vorteile sind

zu Erwarten?, Wochenblatt Papierfabrik.

2002, 4, 229.

References

218

Paper and Board Manufacturing

6.1

Overview of Paper and Board Machines

Herbert Holik

The objective of the process on the paper machine is to produce a continuous

paper web of the required quality, uniform in machine (MD) and cross machine

(CD) directions. The process equipment consists of the approach flow system, the

headbox and the wire section, the press section, the dryer section, often a size

press, sometimes a coating station, a calender and a pope reeler. After the web has

been reeled up this full width mother reel is cut into smaller rolls with widths and

diameters according to the customers’ requirements. The rolls are then wrapped

and made ready for shipment.

The various paper and board grades require special furnish make-ups as well as

paper machine layouts. For economic and quality reasons these machines are now

tailor-made for graphic paper, packaging paper, board, specialty or tissue paper

production. They differ for instance in wire, press and dryer section design, in

machine width and in operating speed. Machine widths are dependent on the

grade produced, since the width depends on the customer’s equipment such as

printing or corrugating board machinery. For instance, for corrugating board ma-

chines widths of 2.5 m, or 3.3 m for the new generation, are standard. So a

trimmed width of 10 m at the paper machine would fit both corrugating machine

widths (4 V 2.5 m and 3 V 3.3 m).

The technical history of paper machines shows many major changes in their

design and operation over the last three to five decades:

• Use of recycled fibers putting higher demands for instance on fabric cleaning

for high machine efficiency

• Changing the pH value of the suspension in the approach flow system from acid

to neutral, allowing the application of calcium carbonate fillers and ensuring

more durable papers

• Hydraulic headboxes enabling higher machine speeds, higher throughputs and

easy application of twin wire gap formers

• CD basis weight profiling in the headbox by dilution, resulting in improved

paper uniformity and quality, thus enabling fiber savings by lower mean basis

weights

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

ISBN: 3-527-30997-7

219

• Twin-wire formers providing more symmetrical z-structure of the paper and

high machine speeds

• Shoe presses with high dewatering capacity for high web dry content on enter-

ing the dryer section, thus saving drying energy and improving machine runn-

ability due to increased web strength

• The one-nip shoe press reducing investment costs, space requirements and op-

erating costs to a minimum

• No-draw press enabling higher machine speed and better machine efficiency

• No-draw dryer section enabling higher machine speed and machine efficiency

and changing the CD web shrinkage curve

• Synthetic forming, press and dryer fabrics to best customize the fabrics to the

requirements of application for uniform dewatering, smooth web surface struc-

ture and long lifetime

• Process control with advanced measuring techniques and control strategies al-

lowing the operators to continuously monitor the operation, see trends in paper

quality and machine condition and take appropriate action

• Machine speed: five decades ago the typical paper machine ran at a speed of

about 300–350 m min

–1

. Today maximum machine speeds are above 2100 m

min

–1

for tissue machines, more than 1900 m min

–1

in newsprint production

and above 1550 m min

–1

for woodfree grades ( Fig. 6.1)

• Machine width: The typical paper machine of five decades ago had a machine

width of about 3–3.5 m. Today machines with 10 m trimmed width (packaging

paper) or 10.35 m paper width (untrimmed) at the pope reeler (newsprint) are

found as a maximum (Fig. 6.2)

• Soft covers of calender rolls allowing integration of calenders into the paper

machine at high machine speeds for high quality paper grades

• Integration of coating (and calendering) into the paper machine even at high

speeds of up to 1550 m min

–1

Fig. 6.1 Increase in maximum speed of paper machines over

the last 100 years (source: Voith).

6 Paper and Board Manufacturing

220

When coating and calendering are integrated into the paper machine the efficiency

of the whole process decreases as there are two more sensitive process steps with

their interfaces to the pre- and succeeding process steps. Here the weakest link in

the chain limits the overall production efficiency. On the other hand in off-line

coating and calendering the loss in high quality production due to the batch proc-

ess is high. Therefore there is clear trend towards on-line coating and calendering

for most applications.

Today paper making is a continuous highly sophisticated process with high de-

mands on precision and reliability of all components. For instance the slice open-

ing of headboxes should have an accuracy of about 1/100 mm (or 10 mm) over a

width of 10 m in order to obtain the required CD basis weight uniformity and

uniform main fiber orientation in CD. Precision in calender roll grinding should

be about 3 mm, which is equivalent to 5 % deviation when comparing it for in-

stance with a paper thickness of 60 mm.

Paper machines are in operation 24 hours per day throughout the year only

interrupted by short planned or unplanned shutdowns. Both mean loss in produc-

tion which has to be minimized.

Predicted machine maintenance or wire and felt changes are examples of

planned shutdowns and should be synchronized. Unplanned shutdowns are

caused for instance by web breaks, or failure of machine components or energy

supply.

The forces which the paper web undergoes during its run through the paper

machine are as follows:

Fig. 6.2 Increase in maximum machine width of paper

machines over the last 100 years (source: Voith).

6.1 Overview of Paper and Board Machines

221