Flocculation and formation in papermaking 2

1. Pele Oy Flocculation, Formation and Paper Properties Pekka Komulainen 13 October, 2016 2. Pele Oy Flocculation and formation …
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  • 1. Pele Oy Flocculation, Formation and Paper Properties Pekka Komulainen 13 October, 2016
  • 2. Pele Oy 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. 2 Flocculation in approach flow Dispersion in headbox Dispersion on wire Reflocculation on wire
  • 3. Pele Oy FLOCCULATION 3
  • 4. Pele Oy 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. 4 Picture: Hubbe
  • 5. Pele Oy 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. 5 Flock structure without bonding d = fiber diameter, L = length
  • 6. Pele Oy Flocculation variables Increased flocculation  Long fibers  Low fiber coarseness  Persistently curled fibers  Wide length distribution  Fibrillated fiber surface  Stiff fibers  Low fluid viscosity  Slow dewatering  Small shear forces  Fiber charge close to zero Decreased flocculation  Short fibers  High fiber coarseness  Straight fibers  Narrow length distribution  Low external fibrillation  Flexible fibers  High fluid viscosity  Fast dewatering  High shear forces  High fiber charge 6  The criteria of flocculation for the papermaker is, how high mass consistency can be used in the headbox. However, in theory volumetric concentration is important Qualitative effects on flocculation are as follows:
  • 7. Pele Oy 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. 7 Pic: JURAJ GIGAC and MÁRIA FIŠEROVÁ
  • 8. Pele Oy 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. 8
  • 9. Pele Oy  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 9 Picture: Kerekes et Schell , TAPPI Oct 1994 Fir Crowding factor 95 Aspen Crowding factor 17
  • 10. Pele Oy Effect of fiber charge on formation  Adding anionic PAM to the pulp improves formation by increasing negative charge and preventing flocculation. 10 No A-PAM A-PAM 1.7 mg/g Picture: Lindström et Christiernin, NPPRJ, Jan 2006
  • 11. Pele Oy FORMATION 11
  • 12. Pele Oy 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. 12 Picture: Jing Yan
  • 13. Pele Oy 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. 13
  • 14. Pele Oy 14 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
  • 15. Pele Oy 15 Some visible paper formation faults Fluting after coating Cockling Flow on wire Large scale formation
  • 16. Pele Oy 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. 16
  • 17. Pele Oy 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. 17 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:
  • 18. Pele Oy 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). 18
  • 19. Pele Oy 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 19 Picture: Innventia
  • 20. Pele Oy 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. 20
  • 21. Pele Oy Valmet example on wire shake improvement  Visual appearance of FormMaster improvement on formation of 210 gsm OCC furnish sheet. 21
  • 22. Pele Oy FormMaster improvement on OCC furnish 210 gsm  Average floc size improvement is 52%. The most improvement is on the largest flocs. 22
  • 23. Pele Oy 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. 23 Bo Norman et al. Innventia PaperCon 2015
  • 24. Pele Oy 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 24
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