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Introduction

Hydrocyclones are gravity separators of relatively simple mechanical design, and

have no moving parts. In order to function, they require an internal vortex flow as

well as a difference in densities between the liquor and the particles to be separated.

Figure 6.17 shows the streams around a hydrocyclone. The pulp is fed with the

feed stream QF at the concentration cF. The fraction containing the heavier particles

is concentrated in the underflow of the cyclone in the stream QU at the consistency

cU. The lighter solids are discharged with the overflow stream QO. The

underflow is also referred to as “apex flow” and the overflow as “base flow”.

QF, cF

QU, cU

QO, cO

Fig. 6.17 Streams around a hydrocyclone.

579

6.4.2

Flow Regime

The gravitational forces which drive the separation in a hydrocyclones are generated

as pressure energy is converted into rotational momentum. In the very basic

design (see Fig. 6.18), the feed flow enters the cyclone tangentially at the upper

end of the cylindrical section and induces a vortex around the axis of the cyclone.

As the suspension swirls downward at the perimeter of the cylindrical and conical

sections, heavy-weight material is concentrated near the wall and is eventually

dragged to the underflow at the apex of the cone. The balance of the liquor together

with the light-weight material rotates towards the axis of the cyclone and proceeds

to the overflow at the opposite end of the cyclone. The overflow escapes

through the vortex finder, a piece of pipe extending into the body of the cyclone

which helps to limit the short-circuit flow from the feed inlet to the overflow.

Overflow (Base)

Underflow (Apex)

Feed

Fig. 6.18 Flow pattern in a hydrocyclone.

Hydrocyclones develop an air core when one of the outlets discharges into atmosphere.

Modern cyclones used in cleaning operate at a backpressure and do

not have an air core.

The geometrical form of the hydrocyclone has a major influence on the separation

efficiency. While manufacturers have developed different designs mainly based

on experience, all of them target at maintaining a largely laminar flow regime.

Unlike pressure screens, hydrocyclones cannot profit from turbulence and the

resulting fluidization. They must be operated at low consistencies in order to

minimize particle–particle interactions. Both turbulence and higher consistencies

will considerably jeopardize the cleaning efficiency.

580 6 Pulp Screening, Cleaning, and Fractionation

The tangential velocity profile observed in a cyclone starts with a forced vortex

flow at the axis. It then passes into a free vortex flow via a transition zone before

the wall effect reduces the velocity to zero again (Fig. 6.19). The axial velocity profile

shows the flow towards the apex at the perimeter of the cyclone body and

towards the base around the center. The pressure loss in the cyclone is a function

of the friction, mainly at the cyclone walls and at the inner wall of the vortex finder.

Locus of zero

axial velocity

Free vortex

v T · r = constant

Forced vortex

v T : r = constant

v tangential

r

v axial

r

Fig. 6.19 Tangential (left) and axial (right) velocity profiles in a hydrocyclone [21].

6.4.3

Sedimentation

Let us make some basic considerations about sedimentation to better understand

what is happening in a hydrocyclone. A characteristic parameter describing sedimentation

is the terminal settling velocity. It describes the state where a particle

moves at constant speed under the influence of frictional and gravitational forces.

Figure 6.20 shows the main forces which act on a settling particle; these are the

weight FW, the buoyancy FB, and the drag force FD.

vsettling

Weight force

Buoyancy force

Drag force

Fig. 6.20 Forces acting on a settling particle.

6.4 Centrifugal Cleaning Theory 581

Weight and buoyancy depend on the specific weight of the particle and the displaced

liquor, respectively. The drag force is a function of the particle movement

and the particle shape. It is always directed in the opposite direction of the velocity

vector. In the Earth’s gravitational field, the three forces are defined as follows:

FW _ _S V g _12_

FB _ _L V g _13_

FD _ cDAP _L

v2s

2 _14_

where qS = density of the particle (kg m–3); qL = density of the liquid (kg m–3);

V = volume of the particle (m3); AP = area of the particle as seen in projection

along the direction of motion (m–2); g = acceleration due to gravity (9.81 m s–2);

cD = drag coefficient; and vS = settling velocity (m s–1).

In the steady state, the forces are in equilibrium, which means that

FW _ FB _ FD _ 0 _15_

Combining Eqs. (12–15) and solving for vS yields Newton’s law for the terminal

settling velocity in the Earth’s gravitational field:

vS _ _2_________________

cD

__S _ _L_

_L

V

AP

_ g _16_

In the special case of a spherical particle with the diameter d (m), where

V = d3p/6 and AP = d2p/4, the above expression becomes:

vS _ __4________________

3 cD

__S _ _L_

_L

_ d g _17_

Note that the settling velocity increases with the density difference between the

particle and the liquor and with the particle diameter. The drag coefficient

depends on the size and shape of the particle, on the viscosity and density of the

fluid, and on the settling velocity itself. When the sphere settles in a creeping,

laminar environment, Eq. (17) converts into Stokes’ law:

vS _

__S _ _L_d2

S g

18 l _18_

where l is the dynamic viscosity of the liquid (Pa·s).

582 6 Pulp Screening, Cleaning, and Fractionation

The solids contained in a pulp stream are very different in shape and size.

While sand particles may come close to spherical shape, pulp fibers obviously do

not. Likewise, there are wide ranges of particle densities from plastics to metals.

In addition, the reinforced gravitational field in the cyclone adds complexity to the

matter. Consequently, meaningful theoretical models for the settling of solids in a

pulp suspension during centrifugal cleaning are not available. We will therefore

use the general form of Newton’s law, as per Eq. (16), for the qualitative evaluation

of separation in a hydrocyclone.

vtangential

vradial

vsettling

r

Net gravitational force Drag force

Fig. 6.21 Forces acting on a particle in a hydrocyclone.

So, what is happening to a particle in a hydrocyclone? The tangential feed provokes

a tangential liquor velocity which makes the particle move along a circular

path around the axis of the cyclone (Fig. 6.21). A radial flow vector describes the

transport of the liquor from the feed inlet at the outer perimeter to the centrical

vortex finder. There is also an axial flow vector which is directed towards the apex

at the cyclone perimeter and towards the vortex finder around the axis. The forces

acting on the particle in a plain perpendicular to the axis are a drag force pointing

against the direction of the settling velocity, and gravitational forces as a function

of the different solid and liquid densities.

Clearly, the settling velocity must be larger than the radial velocity in the cyclone

for a particle to be separated to the underflow. Nevertheless, the tangential velocity

represents the most important flow vector in the hydrocyclone because it controls

the gravity forces acting on the particle.

The acceleration term is determined by the tangential velocity vT (m s–1) and the

distance between the particle and the center of rotation, r (m). When substituting

the acceleration due to the Earth’s gravity g by the centripetal acceleration vT

2/r,

Eq. (16) can be rewritten to give:

vS _ vT _2_________________

cD

__S _ _L_

_L

V

AP

1

r _ _19_

6.4 Centrifugal Cleaning Theory 583

Apparently, higher tangential flow velocities vT and smaller distances r increase

the settling velocity. This means that a cyclone of a smaller diameter is more efficient

for the removal of small particles than a large-diameter cyclone. Likewise,

higher tangential flow velocities improve the efficiency. Both the cyclone diameter

and the tangential velocity are physically limited by the necessity to maintain the

typical laminar flow pattern.

The density difference between the liquid and some particles (e.g., plastics or

light-weight wood components) may be very low. This means that high velocities

and small radii are needed for cleaning to be efficient. In a typical cleaner, the

centrifugal force is so much larger than the Earth’s gravity that it does not make

any difference whether the cleaner is installed vertically or horizontally.

For the cleaning of pulp, the relevant solids density qS is the apparent fiber density

– that is, the density of the swollen fiber consisting of the liquor-saturated

fiber wall and liquor-filled lumen. It has been suggested that for chemical pulp,

the influence of the fiber shape on the drag force and consequently on cD is not

significant [22].

The derivations described above are valid for particles which have a larger density

than the fluid. When a particle is lighter than the fluid, its weight becomes

smaller than the buoyancy, and the vector for the settling velocity shown in

Fig. 6.20 is directed upwards. This is when the particle begins to float to the surface

rather than settle to the bottom. Consequently, the drag force points downwards.

When then the terminal settling velocity is calculated in analogy to

Eq. (19), the solid and liquid densities in the numerator of the density term

change place:

vS _ vT _2_________________

cD

__L _ _S_

_L

V

AP

1

r _ _20_

So, the separation of light-weight particles to the overflow is controlled by the

same factors as the separation of heavy-weight particles to the underflow, the difference

being that there is no need to overcome the radial velocity for separation

to occur. In theory, this circumstance facilitates the separation of light-weight particles

compared to heavy-weight particles. However, in practice the density difference

between light-weight material and liquor is often very small, and any support

for obtaining a reasonable separation efficiency is welcome.

6.4.4

Underflow Thickening

In all cleaning operations, pulp fibers are heavier than liquor. Consequently, fibers

become concentrated in the underflow of the hydrocyclone. In an analogy to

screening, the thickening factor T is defined by:

T _

cU

cF _21_

584 6 Pulp Screening, Cleaning, and Fractionation

6.4 Centrifugal Cleaning Theory 585

The underflow thickening depends on the specific design of the cyclone applied,

with typical thickening factors ranging between 1.5 and 3.0.

Thickening leads on the one hand to poorer separation when particle–particle

interaction hinders the free movement of material to be separated. On the other

hand, thickening may cause plugging of the cone which takes the cyclone out of

operation. If underflow thickening is of major concern for a certain application,

special cyclones with a dilution near the apex can be employed.

6.4.5

Selective Separation

As in screening, the selective separation of different types of solids contained in

the feed stream is of major importance for all contaminant removal and fractionation

applications. As described earlier, separation in a hydrocyclone depends

mainly on differences in the particles’ densities and specific surfaces. Hence, the

selectivity of separation in a contaminant removal application improves with the

density difference between debris and pulp.

Likewise, the density difference between individual pulp fibers determines how

selectively they can be separated in a cyclone. The apparent density of a pulp fiber

results from its diameter and wall thickness. Thick-walled, smaller-diameter fibers

(as found in softwood latewood) have a higher apparent density than thin-walled,

larger-diameter fibers (as found in softwood earlywood). When pulp is subjected

to fractionation in a cyclone, the thick-walled fibers will proceed preferably to the

underflow, forming the coarse fraction. Despite their lower apparent density, thinwalled

fibers are still heavier than the liquor. They report not only to the overflow (fine

fraction) but also to the underflow. It is therefore easier to obtain a relatively pure fine

fraction in the overflow than to obtain a pure coarse fraction in the underflow [23].

0

5

10

15

20

25

30

0 2 4 6 8 10

Proportion in each class, %

Fiber wall thickness, μm

Fine fraction

Feed

Coarse fraction

Fig. 6.22 Example of fiber wall thickness distributions of feed and fractions

after several stages of centrifugal cleaning;bleached softwood kraft pulp [23].

Separation in a hydrocyclone is influenced by a variety of factors such as the

complex fiber morphology, particle–particle interactions and short-circuit flows

within the cyclone. In practice, separation in a single cyclone is far from ideal,

and several stages of cleaning are needed to obtain fractions of significantly different

character, such as those illustrated in Fig. 6.22.

6.5

Centrifugal Cleaning Parameters

In this subsection, we will review those parameters that affect the operation and

determine the performance of a cleaning system, and their qualitative influences

on the cleaning efficiency.

These parameters include operating conditions, such as flow rate and pressure

drop, feed consistency and temperature. They also include equipment-specific parameters,

mainly the cyclone diameter. In addition, we need to observe the furnish

characteristics of both the pulp fibers and the contaminants.

Some of the above parameters can be adjusted, but some are intrinsic to a special

process step or piece of equipment. The chosen combination of adjustable

cleaning parameters depends on the individual requirements of the application,

and is usually a compromise within performance limits and operating constraints,

because the optimization of single parameters often leads in opposite directions.

Due to the complexity of the involved mechanisms and the fact that system

design is usually based on rules of thumb with supportive testing, the discussion

of parameters below is of qualitative nature only.

6.5.1

Cyclone Parameters

Since there are no moving parts, the performance of a hydrocyclone is determined

by its geometry. Design details vary between cyclone manufacturers and target,

for instance, at the minimization of the short-circuit flow from the feed to the

overflow, at lower or higher reject thickening, or at the prevention of cone plugging.

The major parameter affecting cleaning efficiency is the cyclone diameter. At a

given pressure drop, cyclones of smaller diameter generate higher centrifugal

forces, but they also process lower flow rates. Hence, the cyclone size is subject to

an economical restriction given by the number of units to be installed for handling

a particular production capacity. Smaller units are also more sensitive to plugging

due to the small diameter of the underflow opening.

586 6 Pulp Screening, Cleaning, and Fractionation

6.5.2

Operating Parameters

6.5.2.1 Flow Rate and Pressure Drop

A higher pressure drop, which is a synonym for an increased flow rate and a higher

tangential flow velocity, improves the separation efficiency. Care must be taken

not to increase the tangential velocity beyond a point where turbulence occurs.

Since turbulence destroys the controlled flow pattern in the cyclone, it is highly

unwelcome in cleaning and must be avoided. In addition, the pressure drop influences

the operating costs of centrifugal separation, which are mainly determined

by the pumping energy required to overcome the pressure drop.

6.5.2.2 Feed Consistency

In order to limit flocculation, hydrocyclones are normally operated below about

0.6% feed consistency. An increase in feed consistency above this level leads to

reduced cleaning efficiency.

6.5.2.3 Temperature

Higher temperatures can have a positive effect on the cleaning efficiency due to

the reduced liquid viscosity. The maximum operating temperature of a pressurized

hydrocyclone is limited to 70–80 °C.

6.5.3

Furnish Parameters

6.5.3.1 Pulp Fibers

With respect to cleaning, pulp fibers are characterized mainly by their density, surface

texture, size, freeness and disruptive shear stress of the fiber network. Together

with the consistency, these properties determine the performance of the furnish

in a hydrocyclone.

The main parameter affecting separation is the apparent density of the fiber.

Depending on the nature of the furnish, this can mean that fibers are separated

according to wall thickness or coarseness. At the same fiber diameter, fibers with

thicker walls tend to be rejected to the underflow. At the same coarseness, fibers

with smaller diameter tend to be rejected. With regard to size, larger fibers and

fiber bundles go to the underflow. The influence of length alone is inferior to the

influences of other fiber properties [24].

Compared to a nonfibrillated fiber, a fibrillated fiber exposes a larger specific

surface area which offers more resistance to the relative flow in the gravity field of

the cyclone. The larger the resulting drag force, the more likely the fibrillated fiber

reports to the overflow.

6.5 Centrifugal Cleaning Parameters 587

6.5.3.2 Contaminants

The nature of a contaminant decides the preferred technical solution for its

removal. The most important contaminant parameters for cleaning are the contaminant

density and the contaminant shape.

Contaminants with densities that deviate far from the apparent fiber density are

easier to remove. Irregularly shaped contaminants can pose a challenge to cleaning

due to their inherently higher drag forces. Large contaminants may plug the

underflow of the cyclone.

A categorization of contaminants and selective ways for their removal are discussed

in Section 6.7.

6.6

Separation Efficiency

A variety of parameters are being used to describe the separation efficiency of

screening and cleaning operations. While overall parameters are usually sufficient

for characterizing the separation of impurities, a more refined approach becomes

appropriate especially for the purposes of fractionation.

6.6.1

Screening and Cleaning Efficiency

The very basic definition of the separation efficiency E is

E _

amout of debris in reject

amount of debris in feed _22_

Traditionally, this equation is employed generally for screens and more or less

exclusively for cleaners. There are some limitations to Eq. (22), however. E turns

unity when all debris is rejected, irrespective of the reject ratio. Likewise, the

operation of merely splitting a feed flow by a plain pipe tee yields a separation

efficiency larger than zero. In total, E disregards the good fiber loss with debris in

the reject stream.

The efficiency of a screen is usually plotted against the reject ratio due to its

overwhelming influence on the efficiency. Nelson has introduced a screen performance

parameter, the screening quotient Q, which can be easily determined by

just two analyses [25]:

Q _ 1 _

cd_A

cd_R _23_

where cd,R = mass concentration of debris in oven-dry reject (kg kg–1); and

cd,A = mass concentration of debris in oven-dry accept (kg kg–1).

588 6 Pulp Screening, Cleaning, and Fractionation

The screening quotient becomes zero for the pipe tee, and unity for ideal separation.

When applied to measurements from a given screen, Q was found to vary

only insignificantly over the range of industrially practiced reject ratios. Under

consideration of the mass balance over the screen, the screening efficiency is

obtained by:

E _

Rm

1 _ Q _1 _ Rm_ _24_

where Rm is the mass reject ratio – that is, the oven-dry reject mass divided by the

oven-dry feed mass. Figure 6.23 shows the screening efficiencies calculated for

different values of Q over the mass reject ratio. Since the performance of a given

screen is characterized by a particular Q, the screen’s operating point will, in theory,

move along a curve of constant Q. Typical values of Q for shives are 0.9 and larger.

0%

25%

50%

75%

100%

0.0 0.2 0.4 0.6 0.8 1.0

Efficiency, E

Mass reject ratio, Rm

0.0

0.5

0.7

0.9

Q = 1.0

Fig. 6.23 Screening efficiency as a function of the mass reject

ratio and screening quotient Q.

Using their plug-flow model, Gooding and Kerekes [1] have derived the screening

efficiency by combining Eqs. (5) and (22):

E _ RPc

V _25_

where Rv and Pc are the volumetric reject ratio and passage ratio of the contaminants,

respectively. Figure 6.24 illustrates screening efficiencies calculated for different values

of Pc over the volumetric reject ratio. Again, the performance of a given screen is

characterized by a particular Pc, and the screen’s operating point will move, in theory,

along a curve of constant Pc. Typical values of Pc for shives are 0.1 and smaller.

6.6 Separation Efficiency 589

0%

25%

50%

75%

100%

0.0 0.2 0.4 0.6 0.8 1.0

Efficiency, E

Volumetric reject ratio, Rv

1.0

0.5

0.3

0.1

Pc = 0.0

Fig. 6.24 Screening efficiency as a function of the volumetric reject

ratio and debris passage ratio Pc.

When comparing Fig. 6.23 with Fig. 6.24, the constant-Q curves expose a steeper

inclination at low reject ratios than the constant-Pc curves. This hold true even

after correction between mass reject ratio and volumetric reject ratio. The superiority

of the plug-flow model over the mixed flow model suggests that Eq. (25) is

more appropriate to describe a screen’s performance than Eq. (24) [10].

It must be remembered that all efficiencies calculated from Eqs. (22), (24) and

(25) above are actually contaminant-removal efficiencies. Each of these becomes

100% when the reject ratio is unity – a case which is of no industrial relevance.

Clearly, the economy demands that the amount of good fibers lost with the reject

from a separator is kept at a minimum. Therefore, any contaminant removal efficiency

calculated as per these equations must always be evaluated in conjunction

with the loss of good fibers.

6.6.2

Fractionation Efficiency

6.6.2.1 Removal Efficiency

In the basic case, the screening yield can be adopted for the purposes of fractionation.

The fiber removal function e(l) is defined as the mass of fibers with length in

the interval [l, l+dl] in the reject stream divided by the mass of fibers with the

same length in the feed [26]:

e_l_ _

QR cR_l_

QF cF_l_ _26_

590 6 Pulp Screening, Cleaning, and Fractionation

where cR(l) and cF(l) are the concentrations of the fibers with length in the interval

[l, l+dl] in the reject and feed streams, respectively. Assuming a plug-flow model

and constant passage ratio, this expression can be rewritten using Eq. (5):

e_l_ _ RP_l_ V _27_

While the fractionation objective above is determined by fiber length, other

pulp parameters, such as wall thickness, freeness or coarseness, may be assessed

similarly. In a more general form, the yield of a fiber fraction can be defined for

either the accept or the reject stream, with the selection depending on which

stream is of interest [11]. Then, the fractionation yield, Y, for any property of interest

is defined by:

Y _

QStream of interest cStream of interest_Property of interest_

QF cF_Property of interst_ _28_

6.6.2.2 Fractionation Index

In most fractionation applications it is important to remove as high a portion of

the one fraction while removing as little a portion as possible from the other fraction.

Therefore, the quality of the fractionation is characterized by the removal

functions of both fractions.

This can be quantified by the introduction of a fiber fractionation index, U. In

case of length-based fractionation, U is defined as the average e(l) for long fibers,

EL, minus the average e(l) for short fibers, ES [9]:

U _ EL _ ES _29_

where EL is basically the long fiber removal and ES is the short fiber loss. Unlike

removal efficiency, the fractionation index is penalized by removal of the fraction

which ought to be accepted, in the above case by the fraction of short fibers. U = 1

applies when the reject stream is composed only of long fibers and the accept

stream is composed only of short fibers – that is, perfect separation. In addition,

U = 0 means that the fiber length distribution remains unchanged – that is, no

separation.

The fractionation index increases as the hole size is reduced below the targeted

marginal fiber length, but deteriorates again as the hole size becomes smaller

than about half the marginal fiber length [9]. At similar reject thickening, the fractionation

index is almost twice as high for holed screen plates as for slotted ones [8].

The plug-flow model delivers a fractionation parameter a which is defined in

terms of the passage ratios of long fibers, PL, and short fibers, PS [10]:

a _ 1 _

PL

PS _30_

6.6 Separation Efficiency 591

Since the passage ratios are independent of the reject ratio, a reflects the performance

of a specific screen and can be used to anticipate the effect of changes in

reject ratio. Applying Eq. (25) to long and short fibers and eliminating Rv yields

EL _ E1_a

S _31_

Both the fractionation index and fractionation parameter are plotted within the

field of long fiber removal versus short fiber loss in Fig. 6.25. The solid lines calculated

for different values of a represent the curves on which a screen’s operating

point will move. For a given screen, the optimum point for fractionation lies

where the constant-a curve is tangent to a line of constant fractionation index.

Typical values of a are in the range of 0.4 to 0.7 [10].

0%

20%

40%

60%

80%

100%

0% 20% 40% 60% 80% 100%

Long fiber removal, EL

Short fiber loss, ES

0.0

0.4

0.7

Φ = 0.8 0.6 0.4 0.2 0.0

α = 0.9

Fig. 6.25 Screen operating curves (solid lines of constant a)

and fractionation index (dashed lines of constant U) plotted

in a field of long fiber removal versus short fiber loss [10].

6.7

Screening and Cleaning Applications

6.7.1

Selective Contaminant Removal

The selective removal of solid pulp impurities is by far the predominant application

of screening and cleaning in the production of chemical pulp. An overview

over the most common contaminants and their removal is provided below.

592 6 Pulp Screening, Cleaning, and Fractionation

6.7.1.1 Knots

Typically, knots represent the largest fraction of impurities in the pulp coming from

the digester. Knots originate from the dense sections of branches and heartwood, as

well as from oversized chips which have not been cooked down to their center. Knots

are rather large in size and of dark color. They can cause the failure of downstream

equipment in the pulp mill if they are not efficiently removed from the pulp.

Thus, knot removal (knotting) is normally carried out before washing. Knot separation

from the main stream of pulp is performed in a pressure screen. The separated

knots are then subjected to removal of good fibers in a secondary, atmospheric

screen. Both operations are governed by a barrier screening mechanism.

6.7.1.2 Shives

Shives are smaller impurities consisting of fiber bundles from incompletely

cooked wood. Their removal during screening is more difficult than that of knots.

Shives cause operational problems on the paper machine. In contrast to knots,

shives are mostly bleachable, but they consume higher amounts of bleaching chemicals

and may still remain of darker color than the bulk of the pulp after bleaching.

Shives should be removed before bleaching. Shive separation is carried out in a

system consisting of a number of pressure screens. Whether shive removal follows

barrier or probability screening depends on the aperture size of the screens.

As the use of very narrow slotted screens becomes common, shives tend to be

removed increasingly by the barrier principle.

6.7.1.3 Bark

Bark originating from incomplete debarking of the wood represents one of the

most challenging impurities. Bark is of dark color, has a similar density as wood,

and disintegrates easily.

There is normally no dedicated process for the removal of bark from pulp, but

the primary removal of bark should take place in the woodyard before chipping.

The remaining bark is removed from the pulp, together with other contaminants

during the course of screening and cleaning.

6.7.1.4 Sand and Stones

Sand and stones mainly come along with the wood chips, but may originate also

from tiling or concrete tanks. Rocky material can cause equipment failure and is

responsible for the wearing of equipment. The removal of stones and sand is

therefore best carried out as soon as possible in the fiberline.

Larger stones can be separated from the pulp by screening. Cleaning takes care

of any type of rocky material including sand. When narrow slotted screens are

used in a screening application, sand is rejected on a barrier principle and carried

through the subsequent screening stages. Special precautions must be taken in

such a case to minimize wear in the system caused by sand accumulation.

6.7 Screening and Cleaning Applications 593

6.7.1.5 Metals and Plastics

Metals and plastic can enter the fiberline with the wood, they may break away

from equipment, or they may enter the process accidentally. Like stones, metals

can cause the breakdown of equipment and must be removed to protect sensitive

machinery. Plastic contaminants adversely affect the quality of the final product

by causing operational problems in paper-making.

Because of the large density difference, metals can be easily separated from

pulp by centrifugal cleaning. Plastics are generally more difficult to remove, but

as certain types of plastic are less dense than pulp they can sometimes be separated

by reverse centrifugal cleaning.

6.7.2

Fractionation

Fiber fractionation generally follows the probability mechanism of separation.

Despite the limitations placed on fractionation efficiency by currently available

screening and cleaning equipment, fractionation applications are gaining increasing

attention and the prospects of value-added, tailor-made fibers have stimulated

the imagination of product developers.

With regard to paper-making properties, pulps containing long and thick-walled

fibers generally produce a higher tear index. Pulps with thin-walled fibers, and

those containing fines, have better optical properties, higher tensile strength,

internal bond strength, elongation and density [23,27].

The different fractions can be separately refined or treated otherwise, and may

then be recombined, or not. A market pulp producer with two dewatering

machines may fractionate his pulp to increase the long fiber content of the furnish

sent to one machine in order to produce a high-value reinforced pulp. A

paper producer with a multi-layer headbox may direct the shorter fibers to the surface

layers to improve sheet smoothness and optical properties, while placing the

longer fibers in the core to provide strength [10]. Besides strength, fiber fractionation

can also substantially improve the porosity of a pulp by removing the short

fibers and fines that reduce porosity [28].

In total, the fractionation of pulp creates a multitude of new opportunities for

the alternative utilization of the fiber raw material. Nevertheless, fractionation is

practicable only in mills which can make use of all the obtained fractions.

6.8

Systems for Contaminant Removal and Fractionation

6.8.1

Basic System Design Principles

We have seen above that there are various purposes for operating a screening or

cleaning system. While fractionation is of increasing interest, most applications

594 6 Pulp Screening, Cleaning, and Fractionation

still target the removal of large, heavy-weight, or light-weight contaminants. There

is a fundamental difference between contaminant removal and fractionation with

respect to the amount of material to be separated. After fractionation, the smaller

pulp fraction is seldom less than 20% of the pulp in the feed stream. In contrast,

the contaminants to be removed during screening or cleaning are typically no

more than 3% of the feed stream pulp.

Both contaminant removal and fractionation are subject to the condition that

the rejected portion contains only a minimum of the acceptable portion. Modern

screening and cleaning equipment removes unwanted matter quite efficiently

from the feed stream and produces an accept stream of high purity. In order to

achieve this, the reject stream must contain a relatively large amount of acceptable

material in addition to the matter to be rejected.

In a contaminant-removal system, economic reasons call for the minimization

of good fibers lost with the removed contaminants. Such systems usually consist

of a number of separators which can be operated in different arrangements. On

the one hand, contaminant removal is usually most efficient in a cascade feedback

arrangement. On the other hand, generally accepted rules for designing fractionation

systems are yet to be developed. In fact, it is uncertain if such rules will ever

exist, as fractionation tasks are custom-designed for a particular application.

In many cases, the design of separation systems is based on experience and

rules of thumb, because the interrelation of equipment, operating and pulp furnish

parameters is not yet fully understood. The resulting systems are often safe

to operate, but do not necessarily represent the best process solution and economy.

Screening and cleaning systems tend generally to be complex because of the

large number of design and operating parameters. Their function is challenged by

the circumstance that the optimum performance of the system is typically

achieved with equipment working near its point of failure (i.e., plugging). As

mechanistic models are further developed, the basic understanding of effects on

screen capacity, reject thickening and screening efficiency will improve. Computer

simulation provides valuable support in this respect [19,29].

In the following sections we will examine some common systems for contaminant

removal, as well as a few potential fractionation systems. However, before

doing this it may be appropriate to highlight some general aspects regarding the

design of separation systems.

Slotted screen baskets are quite susceptible to damage by junk material such as

metal bolts or rocks. A damaged screen basket leads to inferior screening efficiency

and requires costly replacement. Therefore, it has proven advantageous to

protect slotted screens from junk by the installation of an upstream perforated

screen. A protective screen is also highly recommended for cleaning systems to

avoid damage or blocking of hydrocyclone cones. When the amount of junk material

is low, protective screens can be operated with intermittent reject discharge.

As a result of reject thickening, industrial separation techniques involve dilution

at various points, both in the form of internal dilution to the equipment and

in the form of dilution between stages. The objective of dilution is first, to avoid

6.8 Systems for Contaminant Removal and Fractionation 595

plugging at the reject outlet and second, to adjust the feed consistency between

stages. It should be noted that most of the illustrations in this chapter lack such

dilution streams in order to avoid unnecessary complexity.

6.8.2

Systems for Contaminant Removal

6.8.2.1 Arrangement

As mentioned above, the contaminant level in chemical pulp is far below the

mass reject ratio of industrial separation equipment. Consequently, a large portion

of acceptable fibers can be found in the reject of a single separator, together

with the contaminants. Economical constraints of pulping, however, require that

undesirable contaminants taken from the screening system carry along as few

good fibers as possible.

Hence, it is common to use a combination of separators, where, for instance, a

second screen is used to reduce the amount of good fibers in the reject of the first

screen, and a third screen to remove good fibers in the reject of the second one.

Such a simple cascade arrangement is shown in Fig. 6.26.

A

A

R

R

R

A

F

Fig. 6.26 Three-stage screening in cascade feedback arrangement.

F = Feed;A = Accept;R = Reject.

In a cascade system, the reject from one screen passes on to the feed of the

screen in the next stage. In a cascade feedback arrangement, only the accept of the

first stage proceeds to the downstream step in the pulp production process, while

the accepts of the other stages are in each case brought back to the feed of the

preceding stage (Fig. 6.26).

It should be noted that, in a cascade feedback screening system, sand accumulation

can lead to substantial wear and to the need for frequent exchange of screen

baskets. As screen slots become narrower, an increasing portion of the sand com-

596 6 Pulp Screening, Cleaning, and Fractionation

ing with the feed to the first screening stage is rejected. If the following stages

have screens of similar aperture size, the repeated rejection of sand effects a relative

increase in the sand concentration in the reject of each stage. If one of the

following stages has a screen of larger aperture size, sand may be accepted by this

screen and flow back to the preceding stage, where it is rejected again. Both of

these phenomena are inherent to screening systems operating with narrow slots.

Depending on the sand contamination of the pulp furnish, the installation of special

sand cleaners in between stages may be required to reduce the accumulation

of sand in the system.

Similar to pressure screening systems, hydrocyclones are normally arranged in

feedback cascades (Fig. 6.27). At four to five stages, cleaning systems often have

more stages than screening systems with two to four. This is stimulated by a lower

quantity of contaminants in the feed of cleaning systems and more difficult separation

tasks.

J

R

D

D

D

A

F

Fig. 6.27 Four-stage cleaning in cascade feedback arrangement

preceded by protecting pressure screen. F = System feed;J = Junk;

A = System accept;R = System reject;D = Dilution.

In a cascade feed-forward scenario, accepts from other stages are mixed with the

primary accept. Figure 6.28 illustrates a simple two-stage feed-forward system,

which is common for the barrier screening application of knot removal. The secondary

screen of the knot removal system is usually a piece of equipment which

combines several unit operations, including screening, washing and dewatering.

6.8 Systems for Contaminant Removal and Fractionation 597

A

A

R

R

F

Fig. 6.28 Two-stage screening in cascade feed forward

arrangement. F = Feed;A = Accept;R = Reject.

In a cascade feed-forward system for shive removal, the reject from the secondary

screen could be treated in a refiner, after which the accepts of the tertiary

screen could be combined with the accepts of the primary screen, while the rejects

of the tertiary screen go back to the refiner. However, the quality requirements of

chemical pulps do not, in most cases, allow feed-forward operation of shive

screening and, in some cases, not even reject refining.

6.8.2.2 Fiber Loss versus Efficiency

For an exemplary shive screening application where an incoming pulp contains

1% of shives, Fig. 6.29 shows the mass balance for pulp over a single screen,

assuming a 20% mass reject rate and a 90% shive removal efficiency. The reject

stream contains a huge amount of good fibers (in fact 95% of the rejected pulp)

and almost one-fifth of the good fibers from the feed pulp are lost to the reject.

Total pulp

Shives

500 t/d

5.0 t/d

Total pulp

Shives

400 t/d

0.5 t/d

Total pulp

Shives

100 t/d

4.5 t/d

Feed Accept

Reject

Fig. 6.29 Mass balance for single screen;20% mass reject

rate, 90% shive removal efficiency.

Keeping the same assumptions (i.e., 20% mass reject rate and 90% shive

removal efficiency in the primary screen), we can consider a three-stage screening

system operated in cascade feedback mode (Fig. 6.30). Due to the repeated screening

action, the amount of good fibers in the system reject is reduced to 1% of the

feed pulp. In general, the good fiber loss can be reduced by adding another

screening stage or by decreasing the reject ratio. However, the flow regime in the

pressure screen places a physical limit on both the reject ratio and the number of

stages in a multi-stage screening system. That is why there is a minimum loss of

598 6 Pulp Screening, Cleaning, and Fractionation

good fibers with the system reject from the last stage of a pressure screening cascade.

When the economic feasibility of equipment and operating costs versus the

loss of good fibers is taken into consideration, the number of stages in a screening

system for shive removal is typically three or four. As an indication, the related loss of

good fibers in everyday operation seldomfalls below the amount of rejected shives.

Total pulp

Shives

500 t/d

5.0 t/d

System feed

Total pulp

Shives

491 t/d

0.55 t/d

Primary accept

Total pulp

Shives

613 t/d

5.55 t/d

Primary feed

Total pulp

Shives

151 t/d

5.49 t/d

Secondary feed

Total pulp

Shives

123 t/d

5.0 t/d

Primary reject

Total pulp

Shives

28 t/d

0.49 t/d

Tertiary accept

Total pulp

Shives

38 t/d

4.94 t/d

Secondary reject

Total pulp

Shives

113 t/d

0.55 t/d

Secondary accept

Total pulp

Shives

9.4 t/d

4.45 t/d

Tertiary reject

PRIMARY SCREEN

SECONDARY

SCREEN

TERTIARY

SCREEN

Fig. 6.30 Mass balance for three-stage feedback cascade;20% primary

mass reject rate, 25% secondary and tertiary mass reject rates, 90% shive

removal efficiency in each screen.

When comparing the single-stage and three-stage screening balances depicted

in Figs. 6.29 and 6.30, another observation relates to the screening efficiency. Due

to the internal circulation within the three-stage system, the accepted pulp contains

10% more shives than the accept from the single-screen case. It should be

noted that multi-stage screening helps to minimize the loss of good fibers but at

the same time reduces the screening efficiency.

6.8.3

Systems for Fractionation

The wide range of tasks achievable by fractionation has been repeatedly indicated

above. Because of the specialty of each case, general design principles for the fractionation

of chemical pulps are not yet established. Thus, the information in this

subsection is restricted to some general comments.

6.8 Systems for Contaminant Removal and Fractionation 599

The flow rates of the different fractions are defined by the particular application,

and a low reject ratio is not necessarily part of the fractionation requirements. It

may be advantageous to perform fractionation in a multi-stage system. In contrast

to contaminant removal, the efficiency of fractionation can be improved by multistage

systems. Two simple fractionation systems using screens are illustrated in

Fig. 6.31.

A

A

R

R

F A

R R

F A

(a) (b)

Fig. 6.31 Two-stage fractionation systems with feedback (a) cascade and (b) series.

Remember that holed screens fractionate better than slotted ones. While feedback

is clearly important for obtaining a higher fractionation efficiency, it has

been shown that both cascade and series arrangements may yield similar fractionation

results at a given mass reject rate [19]. According to the example shown in

Fig. 6.32, the best achievable fractionation occurs between about 30% and 60%

mass reject ratio. Note that the location of the fractionation index peak shifts

along the mass reject ratio axis dependent on of the relative amount of fractions

of interest in the feed pulp.

0.0

0.2

0.4

0.6

0.8

0.0 0.2 0.4 0.6 0.8 1.0

Fractionation index, Φ

Mass reject ratio, Rm

Single screen

Two-stage

cascade

feedback

Two-stage

series

feedback

Fig. 6.32 Fractionation index as a function of the mass reject

ratio for single-stage and two-stage fractionation with holed

screens;length-ba sed fractionation, simulation results [19].

600 6 Pulp Screening, Cleaning, and Fractionation

6.9

Screening and Cleaning Equipment

There is an abundance of different types of commercial separation equipment,

most of which are available as several variants. Some examples of more recent

design are detailed in the following section.

6.9.1

Pressure Screens

In the Impco HI-Q Fine Screen, the pulp feed enters the unit tangentially in the

upper section. Heavy contaminants separate centrifugally into the junk trap.

Then, as the pulp suspension enters the screening zone, prerotation vanes

increase its tangential velocity to improve screening efficiency. The accept passes

through the screen apertures, which are kept clean by pulsation provoked by the

special bump elements attached to the closed rotor. The reject proceeds to the bottom

of the screen where it is diluted and discharged through the reject nozzle [30].

Fig. 6.33 The GL&V Impco HI-Q Fine Screen [30].

In Metso’s DeltaScreen, the pulp suspension is fed tangentially into the bottom

part of the unit, where the heavy debris is trapped and can be removed through

the junk nozzle. The pulp proceeds upwards through the rotor to the screening

zone. The screening process takes place as the pulp moves downwards between

the foil rotor and the fixed screen basket. As with some other pressure screens,

the DeltaScreen can be equipped with a cyclone-type separator on top of the

machine, which offers the possibility for removal of light-weight contaminants

[31].

6.9 Screening and Cleaning Equipment 601

Fig. 6.34 The Metso DeltaScreen [31].

In the Impco HI-Q Knotter, the pulp feed enters the unit tangentially in the

upper section. Heavy contaminants are separated centrifugally into the junk trap.

The accept passes from the outside of the screen through the screen apertures to

the inside and proceeds to the accept nozzle at the bottom of the screen. Hydrofoils

rotating at the accept side provide the pressure pulses which keep the screen

apertures open. At the same time, the rotor action does not break up the knots.

On the reject side outside the screen basket, the knots are diluted and washed as

they proceed downwards to the reject nozzle [32].

Fig. 6.35 The GL&V Impco HI-Q knotter [32].

602 6 Pulp Screening, Cleaning, and Fractionation

Metso’s DeltaCombi is a combined knotter and fine screen that operates in similar

fashion to the DeltaScreen. In the lower section, the screen is equipped with

an additional rotating screen basket with holes for knotting. The feed pulp must

first pass this coarse screen basket from the outside to the inside. The pulse-generating

stationary foils of the knotting section are located on the accept side of the

screen basket. The coarse reject is taken out from the bottom part. The accept

which has passed the coarse screen basket is led upwards through the rotor of the

fine screen, and then enters the fine screening section between the foil rotor and

stationary fine screen basket [33].

Fig. 6.36 The Metso DeltaCombi [33].

The Noss Radiscreen features a different design, with the pulp feed entering

the unit in an axial direction. Accepted fibers pass through the two conical screens

plates fixed in the housing, while the reject proceeds to the reject nozzle at the

housing perimeter. As the rotor vanes pass along the screen plates, their peripheral

velocity increases by the radius towards the reject end of the screen plates, and

this leads to increased turbulence in the critical zone of higher consistency.

Radiscreens are available with perforated screens for both knotting and screening

applications. Their design does not require internal dilution, and features a comparatively

small pressure drop and low power consumption [34].

The abundance of screen designs makes it impossible to present all variations

offered by screen suppliers in this book. Tailor-made solutions are available for

special applications, with recent developments including, for example, screen baskets

with intermediate dilution [35] or intermediate deflocculation [36] half-way

down the screen basket to reduce the effects of reject thickening, or different surface

profiling along the length of the screen basket providing increased turbulence

towards the reject end of the screening zone [37].

6.9 Screening and Cleaning Equipment 603

Fig. 6.37 The Noss Radiscreen [34].

6.9.2

Atmospheric Screens

6.9.2.1 Secondary Knot Screens

Secondary knot screening is a barrier screening application targeted at the recovery

of good fibers from the knot stream coming from the primary knotter. Modern

secondary knot screens are equipped with a screw rotating inside a vertical or

inclined perforated screen cylinder. The pulp feed enters the screen near the bottom.

As the knots are transported upwards by the screw, accepted fibers pass

Fig. 6.38 The Noss Raditrim [38].

604 6 Pulp Screening, Cleaning, and Fractionation

through the screen apertures to the annular accept chamber. Shower liquid is

added to the knots, and assists separation by washing good fibers from the knots

to the accept side. A certain liquor level is maintained inside the screen, and after

the knots emerge from the liquor they dewater by gravity before being discharged

through the reject nozzle. The enclosed design of such secondary knot screens

avoids emissions to atmosphere.

The Noss Raditrim is an example of a vertical atmospheric knot screen

(Fig. 6.38). In line with other manufacturers’ screen designs, shower liquid is

introduced into the lower end of the screw shaft and becomes distributed through

holes in the shaft.

Secondary knot screens are typically fed with an inlet consistency between 1.0%

and 1.5%, and deliver knots at a consistency of 25–30%. The amount of good

fibers carried along with the knots is in the range of 10% of the total reject.

6.9.2.2 Vibratory Screens

The number of vibratory screens in use in the pulp industry is continuously

diminishing. This may be due to the fact that vibratory screening is connected to

a number of drawbacks, such as the unsuitability for fully automated control, the

rather dilute accept consistency, and the mostly uncovered design impairing vent

collection. However, if operated in the last stage of screening, the vibratory screen

has the advantage of delivering a reject stream which contains only a minor

amount of acceptable fibers.

6.9.3

Hydrocyclones

Since efficient centrifugal cleaning requires low pulp consistency and small-sized

hydrocyclones, a large number of units is required to deal with the considerable

flow rates. This has formerly resulted in long rows of cleaners with atmospheric

reject discharge. Today, the established arrangement of large numbers of pressurized

hydrocyclones is in canisters.

Figure 6.39 shows a Noss Radiclone, where the hydrocyclones are installed

radially in a pressurized cylindrical canister with vertical axis. Depending on the

cleaning capacity, one canister can hold several hundred cyclones. The feed enters

the canister centrically from the bottom and the pulp flows to the individual

hydrocyclones, where the separation takes place. The rejects and accepts from the

individual cyclones are then collected in separate compartments and leave the canister

through nozzles at the bottom. Typical cyclone diameters range from 80 to

125 mm. The pressure drop from the feed side to the accept side is between 1 and

2 bar [39]. In order to reduce the fiber loss from the last stage of cleaning, hydrocyclones

can be equipped with apex dilution.

The above-mentioned type of hydrocyclone used for the separation of heavyweight

particles is also called a forward cyclone. Cleaners for the separation of

light-weight contaminants are often termed reverse cyclones, accounting for the

6.9 Screening and Cleaning Equipment 605

inversion of accept and reject positions. Reverse cleaners can also be arranged in

canisters. The flow pattern in such canisters is similar to that illustrated in

Fig. 6.39, but the dimensions of the flow channels are adapted to the comparatively

larger apex flow rate and smaller base flow rate. In contrast to forward

cyclones, reverse cleaners do not thicken the reject, but lift the accept consistency

considerably above the feed consistency. Typical thickening factors are between

1.5 and 3.0 [40].

Larger-diameter, individual cyclones are sometimes employed for the separation

of heavy-weight contaminants to protect screen baskets or refiners from detrimental

feed components. These cyclones are typically 200–500 mm in diameter, and

may extend some meters in an axial direction. An example of a larger-diameter

cyclone separator, the Metso HC cleaner, is shown in Fig. 6.40. The cleaner can be

operated either on a continuous basis or with intermittent reject discharge as a

junk trap. HC cleaners are designed to work with feed consistencies up to 5%, but

are normally operated in the 1.5–2.5% range [41].

606 6 Pulp Screening, Cleaning, and Fractionation

Fig. 6.39 The Noss Radiclone AM [39]. Fig. 6.40 The Metso HC cleaner [41].

References 607

References

1 Gooding, R.W., R.J. Kerekes, Derivation

of performance equations for solid-solid

screens. Can. J. Chem. Eng., 1989;

67(10): 801–805.

2 Gooding, R.W., R.J. Kerekes, The

motion of fibers near a screen slot.

J. Pulp Paper Sci., 1989; 15(2): 59–62.

3 Yu, C.J., J. DeFoe, Fundamental study of

screening hydraulics. Part 1: Flow patterns

at the feed-side surface of screen

baskets; mechanism of fiber-mat formation

and remixing. Tappi J., 1994; 77(8):

219–226.

4 Walmsley, M., Z. Weeds. Concentration

and flow variations in a wood pulp fiber

separator. In APCChE Congress and

CHEMECA. Melbourne, 2002.

5 Yu, C.J., B.R. Crossley, L. Silveri, Fundamental

study of screening hydraulics.

Part 3: Model for calculating effective

open area. Tappi J., 1994; 77(9): 125–131.

6 Gooding, R.W., R.J. Kerekes, Consistency

changes caused by pulp screening.

Tappi J., 1992; 75(11): 109–118.

7 Olson, J., G. Wherrett, A Model of Fiber

Fractionation by slotted screen apertures.

J. Pulp Paper Sci., 1998; 24(12):

398–403.

8 Olson, J.A., Fiber length fractionation

caused by pulp screening, slotted screen

plates. J. Pulp Paper Sci., 2001; 27(8):

255–261.

9 Olson, J., B. Allison, N. Roberts, Fiber

length fractionation caused by pulp

screening. smooth-hole screen plates.

J. Pulp Paper Sci., 2000; 26(1): 12–16.

10 Gooding, R., J. Olson, N. Roberts.

Parameters for assessing fiber fractionation

and their application to screen

rotor effects. International Mechanical

Pulping Conference. Helsinki: TAPPI,

2001.

11 Дmmдlд, A., Fractionation of thermomechanical

pulp in pressure screening. University

of Oulu, 2001.

12 McCarthy, C., Various factors affect

pressure screen operation and capacity.

Pulp Paper, 1988; 62(9): 233–237.

13 Pinon, V., R.W. Gooding, J.A. Olson,

Measurements of pressure pulses from

a solid core screen rotor. Tappi J., 2003;

2(10): 9–12.

14 Feng, M., J. Gonzalez, J.A. Olson, C.

Ollivier-Gooch, R.W. Gooding, Numerical

simulation and experimental measurements

of pressure pulses produced

by a pulp screen foil rotor. J. Fluids Eng.,

2005; 127: 347–357.

15 Amand, F.J.S., B. Perrin. Fundamentals

of screening: effect of rotor design and

fiber properties. Tappi Pulping Conference.

Orlando, FL, USA, 1999.

16 Rautjдrvi, H., A. Дmmдlд, J. Niinimдki,

Effect of entrained air on the performance

of a pressure screen. Tappi J.,

2000; 83(9).

17 Gooding, R.W., D.F. Craig, The effect of

slot spacing on pulp screen capacity.

Tappi J., 1992; 75(2): 71–75.

18 Kumar, A., R.W. Gooding, R.J. Kerekes,

Factors controlling the passage of fibers

through slots. Tappi J., 1998; 81(5):

247–254.

19 Friesen, T., et al., Pressure screen

system simulation for optimal fractionation.

Pulp Paper Can., 2003; 104(4):

T94–T99.

20 Olson, J.A., S. Turcotte, R.W. Gooding,

determination of power requirements

for solid core pulp screen rotors. Nordic

Pulp Paper Res. J., 2004; 19(2): 213–217.

21 Slack, M. Application Challenge Cyclonic

Separator. 2nd QNET-CFD Workshop.

Lucerne, Switzerland, 2002.

22 Li, M., et al., Characterization of hydrocyclone-

separated eucalypt fiber fractions.

J. Pulp Paper Sci., 1999; 25(8):

299–304.

23 Vomhoff, H., K.-J. Grundstrцm, Fractionation

of a bleached softwood pulp and

separate refining of the earlywood- and

latewood-enriched fractions. Annual

General Meeting. Baden-Baden, Germany:

ZELLCHEMING, 2002.

24 Statie, E., et al., A computational study

of particle separation in hydrocyclones.

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25 Nelson, G.L., The screening quotient: a

better index for screening performance.

Tappi, 1981; 64(5): 133–134.

26 Olson, J.A., et al., Fiber length fractionation

caused by pulp screening. J. Pulp

Paper Sci., 1998; 24(12): 393–397.

608 6 Pulp Screening, Cleaning, and Fractionation

27 Panula-Ontto, S., Fractionation of

unbleached softwood kraft pulp with wedge

wire pressure screen and hydrocyclone.

Helsinki University of Technology,

2003.

28 Olson, J., et al., Fiber fractionation for

high porosity sack kraft paper. Tappi J.,

2001; 84(6).

29 Weckroth, R., et al. Enhanced pulp

screening using high-performance

screen components and process simulation.

APPW, Durban, South Africa:

TAPPSA, 2002.

30 IMPCO HI-Q Fine Screen (product leaflet).

GL&V Pulp Group: Nashua, USA,

2001.

31 DeltaScreen (product leaflet). Metso

Paper: Sundsvall, Sweden, 2001.

32 IMPCO HI-Q Knotter (product leaflet).

GL&V Pulp Group: Nashua, USA, 2001.

33 DeltaCombi (product leaflet). Metso

Paper: Sundsvall, Sweden, 2001.

34 Radiscreen-F Fine Screen (product leaflet).

Noss: Norrkцping, Sweden, 2002.

35 Fredriksson, B., Increased screening

efficiency with belt dilution. 90th PAPTAC

Annual Meeting. Montreal: PAPTAC,

2004

36 McMinn, T., A. Serres. Intermediate

deflocculation and dilution device (ID2):

a new technological decisive step in the

screening processes. APPW. Durban,

South Africa: TAPPSA, 2002.

37 AFT VariProfile (product leaflet). AFT:

Montreal, Canada, 2003.

38 Raditrim Secondary Knotter (product

leaflet). Noss: Norrkцping, Sweden,

1999.

39 Radiclone AM80 (product leaflet). Noss:

Norrkцping, Sweden, 2002.

40 Radiclone BM (product leaflet). Noss:

Norrkцping, Sweden, 2000.

41 HC Cleaners (product leaflet). Metso:

Valkeakoski, Finland, 2002.

609

7

Pulp Bleaching

Herbert Sixta, Hans-Ullrich Sьss, Antje Potthast, Manfred Schwanninger,

and Andreas W. Krotscheck

7.1

General Principles

Unbleached chemical pulps still contain lignin in an amount of 3–6% on o.d.

(oven dry) pulp in the case of softwood kraft (sulfite), and 1.5–4% on o.d. pulp in

the case of hardwood kraft (sulfite) pulps. Lignin in native wood is colored only

slightly, whereas residual lignin of a pulp after cooking – particularly kraft cooking

– is highly colored. Moreover, unbleached pulps also contain other colored impurities

such as certain extractives (resin compounds) and dirt which is defined as

foreign matter having a marked contrasting color to the rest of the sheet. Dirt may

originate from wood (bark, incompletely fiberized fiber bundles, sand, shives,

etc.), from cooking itself (carbon specks, rust, etc.), and from external sources

(grease, sand, other materials, etc.).

A continuation of cooking to further reduce the noncarbohydrate impurities

would inevitably lead to a significant impairment of pulp quality due to enhanced

cellulose degradation. Therefore, alternative concepts must be applied to selectively

remove chromophore structures present in the pulp. Various chlorine- and

oxygen-based oxidants have proven to be efficient bleaching chemicals which,

being applied in sequential steps, progressively remove chromophores and impurities.

As a result of the concern about chlorinated organic compounds formed

during chlorine bleaching at the end of 1980s, conventional bleaching concepts

were rapidly replaced by the so-called elemental chlorine-free (ECF) bleaching process,

and this became the dominant bleaching technology. The complete substitution

of chlorine by chlorine dioxide was the key step in reducing the levels of organochlorines

(measured as adsorbable organic halogen; AOX) in pulp mill effluents.

Further pressure, particularly fromthe environmental organization Greenpeace, and

especially in the German-speaking regions of Europe, led to the development of

totally chlorine-free (TCF) bleaching processes with a main emphasis on the use

of oxygen (O), hydrogen peroxide (P) and ozone (Z) as bleaching agents.

Bleaching is defined as a chemical process aimed at the removal of color in

pulps derived from residual lignin or other colored impurities, as outlined above.

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim