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

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• Mixing and storing: To generate a uniform blend of suspension components,

and to prevent suspension components from de-mixing.

Separation processes are of high importance in recovered paper processing as this

material contains

• Additives used in the previous paper production process, such as fillers and

dyes, coating components and functional and process chemicals

• Substances added according to the application such as printing inks, varnishes,

coatings, foil laminations, adhesives and waxes

• Materials mixed with the paper during its life cycle, subsequent collection, and

handling in the mill including wires and strings, glass, sand and stones, or

paper clips and folders.

Solid particles comprise a wide range of materials with a wide range of properties.

Particle characteristics used as separation criteria must clearly differ from fiber

characteristics. These include particle size, shape and deformability, density, and

surface properties (hydrophobicity). Table 4.1 shows as an example the character-

istics “specific gravity and size” for various types of contaminants. Most of these

contaminants have to be removed for quality and machine runnability reasons,

fillers may be tolerated to a certain degree. Figure 4.2 shows the efficiency scatter

of the separation processes vs. particle size. Nonselective flotation (dissolved air

flotation) separates small particles very efficiently from water. Washing removes

small particles, the efficiency depending on the amount of extracted water. Se-

lective flotation effectively removes particles over a wide size range, but only hydro-

phobic ones. Screening is most successful in removing larger and cubic particles.

For small, flat or deformable particles the separation efficiency is lower. Cleaners

have high efficiency in the removal of large particles with densities differing from

water and wetted fibers.

Fig. 4.2 Efficiency ranges of unit processes for contaminant

removal in recovered paper processing.

4 Stock Preparation

152

The main unit processes, their principles and the machinery applied will be

described in the next sections followed by a description of fiber stock preparation

systems for different kinds of fiber raw material and final application of the fin-

ished stock.

4.2

Main Unit Processes and Equipment

4.2.1

Fiber Materials Feeding

The objective of fiber material feeding is to supply a predetermined and measured

amount of fiber stock to the repulping unit. The feed is either continuous or batch-

wise. The fiber material is delivered to the paper mill in the form of bales (virgin

pulp, recovered paper) or as loose material (recovered paper). Only in the so-called

integrated mills (pulp mill and paper mill integrated in one location) is the fiber

material supplied to stock preparation by pumping the stock suspension directly

from the pulp mill to the paper mill. An extra feeding system and repulping is only

necessary for shutdown periods of the pulp mill.

Virgin pulp is delivered in bales (mostly sheeted material), which are bound

together by wires into stack units of six or eight bales. The first step is to dewire the

units by cutting the centrally arranged binding wires, to remove them automat-

ically and to wind them to coils for easy disposal. The next step is to de-stack the

unit into single bales thus preparing the bales for dewiring, i.e. to cut and remove

the wires and wind them to coils as above. A metal detector may follow to detect

any uncut wires which would then be cut and removed by hand. The wire removal

efficiency is better than 96% depending on the unit and bale quality. A further

Table 4.1 Density and particle size of contaminants in

recovered paper processing.

Type of contaminant Density, g cm

–3

Particle size range, mm

metal 2.7–9 > 1 000

sand 1.8–2.2 >1–> 1000

fillers/coating particles 1.8–2.6 <1–1000

ink particles 1.2–1.6 < 1–> 1000

stickies 0.9–1.1 < 1–> 1000

wax 0.9–1.0 <1–10

styrofoam 0.3–0.5 > 100–> 1000

plastics 0.9–1.1 > 100–> 1000

4.2 Main Unit Processes and Equipment 153

handling device may turn the bales through 180° for removal of the bottom packag-

ing material. The bales are then fed to the repulping unit. In batch-wise working

systems a weighing system will be installed. The capacity of such a virgin fiber

material feeding line is up to 180 bales per hour. Figure 4.3 gives an overview of a

handling system for virgin pulp units and bales. Figure 4.4 shows a closer view of

a dewiring station for units and bales of virgin pulp.

Recovered paper is supplied to the mill in (individual) bales or as loose material.

Often a mill has to handle both types. Automatic dewiring of recovered paper bales

is more demanding than that of virgin pulp bales since these bales may vary in

Fig. 4.3 Overview of a handling system for virgin pulp units

and bales (source: B+G Fördertechnik).

Fig. 4.4 View of a dewiring station for units and bales of

virgin pulp (source: B+G Fördertechnik).

4 Stock Preparation

154

shape, size and kind of wiring. As in virgin fiber feeding systems the wires are cut

and usually removed, the probability of bales not being dewired is less than 4%,

depending on the bale quality. Only in cases when raggers are installed in low

consistency pulpers (mainly in board and packaging paper lines), are wires needed

to build up the tail and to entangle plastic foils, strings, and textiles. On automatic

cutting of the wires the bales open up extensively. After wire removal the bale

structure is opened into loose material by a bale opener. Now the flow-stream of

loose paper is equalized to the required height by a levelling drum. The levelled

loose material on the conveyor belt is weighed by a radiometric weighing system.

In combination with a conveyor speed control this gives a constant mass flow of

fiber material into the repulping unit. The capacity of such a fiber material feeding

line for recovered paper bales is up to 120 bales per hour depending on the quality

of the recovered paper. In Figure 4.5 the schematic of a complete feeding system

for baled and loose recovered paper is shown.

4.2.2

Disintegration

4.2.2.1 Repulping/Slushing

The purpose of repulping or slushing is to break down the dried primary fiber pulp

or recovered paper into individual fibers or, at least, to form a suspension which

can be pumped. In the latter case the remaining flakes have to be broken down in

subsequent deflaking machinery. Repulping is needed not only at the beginning of

the stock preparation system but also for the wet or dry broke from the paper

machine.

During pulping the applied disintegrating forces have to be greater than the raw

material strength. Wetting reduces the strength by breaking the fiber-to-fiber hy-

drogen bonds. Strength reduction by wetting is about 85–98% for primary fiber

pulp and nonwet strength recovered paper, and < 60–80 % for wet strength re-

Fig. 4.5 Feeding system for baled and loose recovered paper

(source: B+G Fördertechnik).

4.2 Main Unit Processes and Equipment

155

covered paper. Recovered wet strength paper grades which are difficult to repulp

may be slushed at elevated temperatures of more than 75 °C. Addition of chem-

icals – acidic or alkaline, depending on the wet strength agent – further assists wet

strength reduction.

The relevant forces in repulping seem to result from viscosity, acceleration and

mechanical clinging. Viscosity is mainly a matter of suspension consistency, to-

gether with velocity difference it creates shear stress. Acceleration of a particle

results in inertia forces. Clinging of a flake e.g. around the rotor may induce

viscosity, acceleration, or mechanical forces.

The steps in repulping are:

• To feed the system with a predetermined rate of raw material and water

• To wet the fibrous raw material rapidly and completely

• To apply sufficient force to break the material down into individual fibers

• To discharge the suspension.

In the case of recovered paper repulping further steps may be necessary:

• To remove solid contaminants such as foils, stickies, and printing ink from the

fibers

• To remove solid contaminants from the process at an early stage before they are

broken down into too small particles which are difficult to remove in subsequent

machinery

• To mix process chemicals (such as deinking and bleaching agents) into the

suspension

Depending on the raw material, the amount of production and the contaminants

content, repulping is done in different types of pulpers or drums at consistencies

between < 6% and < 28 %. Slushing time is between about 5 and 40 min. Pulpers

are usually stainless steel vats with a vertical axis. A concentric impeller is the

slushing tool, vertical elements at the cylindrical wall and guide elements at the

bottom redirect the rotating suspension flow to the vat center.

Low consistency (LC) pulpers (Fig. 4.6) comprise a flat impeller with circum-

ferential speed of about 15–20 m s

–1

. They operate at consistencies of up to about

6%. At the bottom they have a screening sieve with hole sizes of 6–20 mm for

suspension extraction. Operation is either continuous for slushing of recovered

paper (fluting and liner, high wet strength grades) and most of the primary fiber

materials, or periodic for certain primary fiber applications. In recovered paper

processing ongoing removal of trash has to be ensured in order to prevent ex-

cessive trash concentration which would reduce the production and quality and

might even stop the pulper rotor. Figure 4.7 shows a LC pulper trash removal

system. Part of the suspension in the pulper is extracted and fed to a junk separator

to remove heavy contaminants. The following disk screen has two functions. It acts

as a deflaker to reduce the number and size of the flakes and as a coarse screen for

removal of remaining trash and oversized flakes. The reject is sorted in a drum

screen, its accept being recirculated to the pulper and rejects being disposed. Often

raggers are used for additional trash removal such as for bale wires, plastic, foils,

and textiles.

4 Stock Preparation156

Stock consistencies at medium consistency (MC) pulpers are up to about 12%,

those at high consistency (HC) pulpers up to about 19%. Both pulper types have a

helical rotor and usually no screen plate. Circumferential speed is about

12–17 m s

–1

. They generally operate intermittently and are used in processing re-

covered paper such as newspapers and magazines. Figure 4.8 shows an HC pulper

and Fig. 4.9 an HC pulping system for recovered paper processing including

dumping and dilution water feed system. For each batch unwired baled or loose

raw material and water are fed to the pulper. After its reduction to the desired flake

content and size at high consistency – and detachment of ink to a certain degree –

dilution water is added. The suspension is then fed to a disk screen with deflaking

and coarse screening functions. Its reject goes via a buffer tank to a drum screen,

Fig. 4.6 LC (low consistency) pulper (source: Voith).

Fig. 4.7 LC pulper trash removal system (source: Voith).

4.2 Main Unit Processes and Equipment

157

its accept to a dump chest. The reject of the drum screen leaves the system via a

dewatering screw, the accept is recirculated.

Drum pulpers operate at consistencies of about 14%-28 %. The drum is driven

on the periphery, the axis is declined to the drum end. Drum pulpers are used in

repulping of recovered paper of lower wet strength such as newspapers and maga-

zines, fluting and liner as well as liquid packaging board. Due to the lower forces

the size reduction of sensible contaminants such as stickies or thin foils is limited.

Drum pulper systems combine the functions of slushing and coarse screening.

There are two types on the market. One (Fig. 4.10) has a single drum body with a

first zone for slushing and a second zone for coarse screening. The rotation speed

is about 100–120 m min

–1

, the drum diameter 2.5–4 m, length up to 30–40 m, the

slushing zone being about two thirds of the length. Slushing consistency is about

14–20%, accept consistency of coarse screening (hole diameters about 6–9 mm) is

about 3–5%. As the drum rotates the raw material is lifted with the help of lifting

Fig. 4.8 HC (high consistency) pulper

(source: Voith).

Fig. 4.9 HC pulping and detrashing

system (source: Voith).

4 Stock Preparation

158

baffles mounted in the axial direction. Disintegration occurs mainly by two princi-

ples: (i) During lifting part of the material rolls and slides back thus generating

shear forces and (ii) the remaining part of the material lifted to higher position

falls back to the pond. The resulting impact exerts effective slushing.

The second type distributes the two functions of slushing and of coarse screen-

ing between two individual drums, each operating at different circumferential

speeds and consistencies adopted to the different functions (Fig. 4.11). Further-

more the slushing drum is equipped with a D-shaped “displacement core”. Both

displacement core and drum are equipped with bars in the axial direction. The

length of the slushing drum is about 7–15 m, that of the coarse screening drum

7–17 m. The rotation speed of the slushing drum is about 1.5 m s

–1

, that of the

coarse screening drum about 2.5 m s

–1

. Consistency in the slushing part is about

Fig. 4.10 Drum pulper with a single drum body incorporating

slushing and coarse screening (source: Andritz).

Fig. 4.11 Drum pulper system with separate drums for

slushing and coarse screening (source: Voith).

4.2 Main Unit Processes and Equipment

159

23–28%, that in the accept of coarse screening 3–5 %. The filling level of the

slushing drum is adjusted to the actual production and is about 30–60% of the

drum volume. As the drum rotates the stock is exposed to shear forces in the up-

going channel between the drum and the fixed displacement core. The impact of

the stock falling down from the top further supports effective slushing.

Broke pulpers beneath the paper machine are found at the end of the wire

section and the press section where the web is wet and easy to slush. Slushing in

broke pulpers in the dryer section, at the size press or in coating stations needs

more energy and time, as the web is dry. (Broke from other places outside the

paper machine are treated in pulpers as described earlier.) Broke pulpers extend

across the whole width of the paper machine and have to treat the full production.

In the case of a web break, the broke pulper located upstream of the break position

has to start its full operation almost immediately. Water showers direct the web

into the pulper and provide the necessary amount of dilution water. A consistency

of 3–5% together with an optimized system of vat geometry and rotor ensure good

stock circulation and slushing in the pulper. Circulation energy and defibering

forces are exerted either by agitators with a horizontal axis and propellers mounted

on these or by impellers such as found in pulpers in stock preparation. The disin-

tegrated part of the pulper content is extracted from the pulper through a screen

plate. Figure 4.12 shows an example of a broke pulper.

4.2.2.2 Deflaking

The objective of deflaking is to break down small pieces (flakes) of undisintegrated

paper or pulp sheets into individual fibers. The residual flake content after the

deflaker should be zero, in special cases at least below 5%. Deflaking helps to

avoid paper quality problems, to save fiber raw material and to ensure improved

operating conditions for the succeeding machines in the process e.g. screening or

cleaning. Deflaking is carried out in deflakers after slushing in a pulper or a drum,

in the preparation of recovered paper, virgin pulp or broke. A remarkable deflaking

effect also occurs in disk screens. Cylindrical screens or pumps have a lesser ef-

fect.

Fig. 4.12 Broke pulper beneath the paper machine (source:

Voith).

4 Stock Preparation

160

Deflaking is done at stock consistencies of about 3 to 5%. The shear forces

necessary for disintegration are applied to the fiber bundles and flakes when they

pass radially through the fillings slots of the intermeshing teeth of the rotor and

the stator (Fig. 4.13). The peripheral speed of the rotor fillings is 25 to 40 m s

–1

Depending on the entering flake size, flake content, and trash content, coarser or

finer fillings for the deflaker are selected (Fig. 4.14).

For stocks with low deflaking resistance the specific energy demand is 20 to

40 kWh t

–1

. Deflaking is mostly done in a single pass. Two or more passes may be

required for stocks which are more difficult to deflake. Flakes with high wet

strength must be disintegrated in a disperser which can apply higher shear

rates.

Fig. 4.13 Schematic of a deflaker (source: Voith).

Fig. 4.14 Coarse and fine fillings for deflaking (source: Voith).

4.2 Main Unit Processes and Equipment

161