Causes and cures of fiber flocculation and paper formation

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Flocculation, Formation and Paper
Properties
Pekka Komulainen
Pekka.Komulainen@clarinet.fi
January, 2018

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Flocculation and formation
 Flocculation of fibers occur in the approach flow. Headbox tries to destroy flocs and
disperse fibers.
 Fiber flocs from the headbox and on the wire fix to the sheet when water removes. This
determines sheet formation, which is measured as small scale basis weight variation
(e.g. 1x1 mm2) by using beta ray absorption.
 On the wire fibers reflocculate and disperse very fast again. Dewatering time has a great
effect on this process. Long dewatering time means that there will be more flocculation.
 The extent of fiber flocculation or dispersion directly influences the resulting paper
formation.
 Good formation may be the only paper property, which has no negative effects on the
final paper properties.
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Flocculation
in approach flow
Dispersion
in headbox
Dispersion
on wire
Reflocculation
on wire

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FLOCCULATION
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Flocculation environment
 Ideal suspension of fibers would be so dilute
that no collision between individual fibers
could be possible. Each fiber would then
occupy a sphere, where the sphere diameter
is same as fiber length.
 In practice consistencies are higher and there
are always collisions between the fibers.
 However, this thinking is the basis of different
theories about flocculation and also very
useful in practice to understand flocculation.
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Picture: Hubbe

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Mechanical flocculation structure
 Flocks can be formed without any bonds
between fibers. A fiber may only become a
part of a network if it is in contact with at
least three other fibers.
 It is easy to make a rigid flock structure from
four wooden sticks, each having three
contact points. Elastic energy between the
bent sticks and friction forces hold the sticks
together.
 If fibers are totally dispersed this kind of
flock requires turbulence to be formed.
 Turbulence forces can form but also destroy
these flocks and disperse fibers.
 Accelerating flow destroys effectively flocks
without forming new flocks. This is very
important in the headbox.
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Flock structure without bonding
d = fiber diameter, L = length

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Flocculation variables
Increased flocculation
 High fiber length/diameter ratio
 Low fiber coarseness
 Persistently curled fibers
 Wide fiber length distribution
 Equal sized objects
 Fibrillated fiber surface
 Stiff fibers
 Low fluid viscosity (warm water)
 Slow dewatering
 Small shear forces
 Fiber charge close to zero
 Poor mixing of chemicals
 Dead ends of flow
Decreased flocculation
 Low fiber length/diameter ratio
 High fiber coarseness
 Straight fibers
 Narrow length distribution
 Unequal sized objects
 Low external fibrillation
 Flexible fibers
 High fluid viscosity (cold water)
 Fast dewatering
 High shear forces
 High fiber charge
 Good mixing of chemicals
 No dead ends of flow
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 The criteria of flocculation for the papermaker is, how high mass consistency can be
used in the headbox. However, in theory volumetric concentration is more important.
Qualitative effects on flocculation are as follows:

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Formation and jet to wire speed ratio
 Best formation is normally achieved when jet
and wire speeds are same.
 Some other studies conclude that best
formation is, when there is a very small
difference in the jet and wire speeds.
 Jet-to-wire speed ratio can have curved CD
profile. This is the reason that formation can
vary very much in the cross machine
direction.
 In laboratory sheets good formation correlates
with good tensile strength.
 On a paper machine, where good MD tensile
is made with higher jet-to-wire speed ratio,
good tensile strength correlates with bad
formation.
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Pic: JURAJ GIGAC and MÁRIA FIŠEROVÁ

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Flocculation tendency of different pulps
 Pictures of Huawei Yan after headbox nozzle. Fiber concentration 5 g/l, flow speed 8 m/s.
 A = BSKP, B = BHKP, C = TMP and D = SGW.
 Formation of groundwood fibers is best and softwood kraft worst. This is not only effected
by fiber length but also by fiber coarseness.
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 Crowding factor
where Cm = mass concentration, L= fiber length, ω = fiber coarseness
 Pulps formed at 0.5 % consistency. Fiber properties:
 Fir: length 2.7 mm, inverse specific perimeter 0.72
 Aspen: length 0.8 mm, inverse specific perimeter 0.38
Wood fibers and flocculation
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Picture: Kerekes et Schell , TAPPI Oct 1994
Fir
Crowding factor 95
Aspen
Crowding factor 17

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Effect of fiber charge on formation
 Adding anionic PAM to the pulp improves formation by increasing negative
charge and preventing flocculation.
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No A-PAM A-PAM
1.7 mg/g
Picture: Lindström et Christiernin,
NPPRJ, Jan 2006

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TrumpJet® flash mixing to improve formation
 It is believed that a fast and dynamic mixing of chemicals and additives can give better
combination of formation and retention.
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FORMATION
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Flocculation and dispersion
 As shown in the top of the figure, the
random fiber distribution is generated by
the stochastic distribution of fibers in the
plane of paper.
 One can see how regions of low and
high grammage are formed by this
natural process.
 There is a certain level of flocculation
within random fiber distribution, but they
are not necessarily generated by a
tendency of fiber aggregation through
physical or chemical forces.
 The other two figures show flocculated
(left) and dispersed (right) fiber
distributions.
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Picture: Jing Yan

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Formation measurement
 Real formation is measured by small scale beta ray absorption (Ambertec).
 Normally standard deviation of grammage (g/m2) is calculated.
 Formation number normalized with respect to the grammage is called
specific formation number, since the formation number is statistically
inversely proportional to the square root of the mean grammage.
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Paper quality and formation
 It is possible to study paper quality by taking pictures against window and
then treating these digital pictures by adjusting size, colour contrast etc.
 These examples are of a Chinese newsprint mill.
Typical formation
Wire markWire mark

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Some visible paper formation faults
Fluting after coating
Cockling
Flow on wire
Large scale formation

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Retention, drainage and formation
 Conventional wisdom is that the relationship between retention / drainage
and sheet formation is a tradeoff: Increasing retention produces a decrease
in formation quality and low retention results in better formation.
 Frequently when the drainage is improved the retention falls and poor
formation is obtained.
 Through the phenomena of adsorption and electrostatic interactions,
retention chemicals are able to develop chemical aggregation mechanisms
by which fillers, fiber fines, and other functional additives are retained in the
sheet.
 Chemical retention and flocculation topics are not much discussed in this
presentation. However, these should have effect on fine material flocculation
but not on fiber flocculation to improve retention without deteriorating
formation.
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Example of refining effects on formation
 Sometimes it is not clear how e.g. refining effects on formation.
 Normally formation is improved in refining. However, if there is very little
cutting in refining and fines material have more effect on dewatering,
refining can have negative effect on formation.
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Refining effects Explanation Effect
Removal of primary fiber wall Lower fiber coarseness –
Delamination and swelling of fibers (internal fibrillation) More flexible fiber ++
External fibrillation Higher surface friction –
Shortening of fibers (cutting) Shorter fibers need less space +++
Creation of fines Small flocs from fines +
Longer dewatering time –
Dissolving of material (hemicellulose) Lower fiber coarseness –
++Total refining effect on formation:

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Flocculation, formation and paper properties
 The extent of fiber flocculation or dispersion directly influences the resulting paper
formation. Good formation may be the only paper property, which have no negative
effects on the other paper properties.
 Refining produces fine material which is not flocculating, but it increases dewatering
time and can increase flocculation.
 Small scale basis weight variation is fixed after wire section and cannot be improved
after that.
 Optically measured formation can be improved also in calendering but not mass
formation measured by beta radiation.
 Optically measured formation is possible to measure online and also very fast in
laboratory. It is a very common measurement. However, correlation to printing
quality can be very poor, when paper is calendered. Also problems will arise for
highly bleached products and heavy weight products. (Robert Tolkki, KTH).
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Effects of good formation on paper properties
 More even print result, less mottling in
offset, less missing dots in rotogravure.
 Less print-through
 Better paper smoothness
 Higher paper gloss
 Lower air permeability
 Better tensile strength and stiffness
 Due to lower calendering need to the
desired smoothness:
 Better bulk and stiffness
 Better strength properties
 Less calender blackening or higher
moisture in calendering
 Less dusting and linting
 Better opacity and brightness
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Picture: Innventia

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High dilution forming to improve formation
 There are several paper grades which require high dilution forming to get the required
paper formation uniformity. This is due to long special fibers, synthetic or natural.
 The picture below is a calculation of headbox opening of 100 gsm paper and 80%
retention as a function of consistency.
 It is impossible to use slice opening over one meter with a conventional headbox. This is
one of the reasons to use inclined wire for long fibers.
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0
200
400
600
800
1000
1200
1400
0 0.2 0.4 0.6 0.8 1 1.2
Headbox consistency, %
Sliceopening,mm

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Inclined wire technology (Deltaformer)
 Inclined wire former with angle of 15° to 35°, consistencies from 0.01 to 0.2%.
 Higher stock dilution is needed to keep long fibers from entangling.
 Fiber lengths from 5 up to 38 mm.
 Water removal capacity up to 600 l/min/cm, width up to 5 m, speed up to 600 m/min
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Picture: Glens Falls Interweb

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Wire shake example Valmet FormMaster 120
 FormMaster 120 shakes the breast roll in the cross direction and breaks flocs by
creating shear forces to the web.
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Valmet example on wire shake improvement
 Visual appearance of FormMaster improvement on formation of 210 gsm OCC
furnish sheet.
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FormMaster improvement on OCC furnish 210 gsm
 Average floc size improvement is 52%. The most improvement is on the largest flocs.
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Three-layer SC paper formation
 It is very difficult to get at the same time good formation and retention.
 With Aqua-vane headbox, where filler is dosed through the Aq-vanes this is
possible. Lower number in the picture means better formation.
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Bo Norman et al. Innventia
PaperCon 2015

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SC-paper
MD
Filler
Filler
Filler
Filler
Filler
Filler
Filler
Filler
Four A4 samples, one from each configuration. The
conclusion of a large set of pretrials was that the most
promising dosage strategy was dosing fillers through the
Aq-vanes only. This strategy was study with reference to
uniform filler dosage across the thickness of the paper.
Bo Norman et al. Innventia, PaperCon 2015
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High consistency forming
 The main improvement of papermaking should be reduction of water usage of the
internal circulations. Conventional solution to this is high consistency forming (1-3%).
 In addition to the flocculation tendency the CD accuracy is demanding in high
consistency forming.
 It is easy to calculate what would be the slice opening for different headbox
consistencies. When grammage is low slice opening is just some millimeters, which is
demanding for CD accuracy. The calculation below is for 100% retention. In practice
lower retention increases slice opening.
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SliceOpening,mm
Grammage, gsm
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
0 50 100 150 200 250 300 350
HB cons. 1 %
HB cons. 3%
HB cons. 2 %

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High consistency forming and paper quality
 Fibers in high consistency forming are
randomly oriented in all directions rather
than in the plane of the web making this
forming process unsuitable for printing
papers.
 The random grain orientation is believed
to be due to collision during drainage of
the densely packed fibers. In addition,
the formed web has high bulk, high
porosity, grainy formation, increased z-
direction strength (out of the plane of the
web) and reduced in-plane strength.
While this web is suitable for some
board grades it is not suitable for thin
publication papers.
 The picture on the right shows what is
the difference between filtering (normal
paper) and thickening (high consistency
paper).
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High consistency forming headbox
 There is a very interesting patent idea (WO 2013024205 A1) of Matti Luukkanen on High
Consistency (HC) forming (2-5%). This could be very suitable for pulp drying machines and
several board grades, especially for the filler ply of three-layer board.
 The picture below shows how rotating drum inside a curved chamber produces turbulence,
pressure and flow to the water removal gap between a solid apron and a moving wire on a
dewatering box.
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Consistency Total mass
% tons/dry ton
0,5 200
1 100
2 50
3 33
4 25
5 20

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Foam forming to solve flocculation problems
New possibilities with increased headbox consistency by foam forming:
 New paper properties by using special long fibers with good formation
 High bulk products with good strength by combination with nanofibrillated
cellulose for insulation materials, filters and tissue products
 High bulk with good z-strength for e.g. middle ply of cartonboard
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Bulk [cm3/g]
Picture: VTT

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Pilot foam forming machine at VTT Finland 2013
 Foam forming gives possibilities to save water, energy and material in papermaking.
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Picture: VTT