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Flocculation and formation in papermaking jan 2018

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Causes and cures of fiber flocculation and paper formation

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Flocculation and formation in papermaking jan 2018

  1. 1. Pele Oy Flocculation, Formation and Paper Properties Pekka Komulainen January, 2018
  2. 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. 3. Pele Oy FLOCCULATION 3
  4. 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. 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. 6. Pele Oy 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 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 more important. Qualitative effects on flocculation are as follows:
  7. 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. 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. 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. 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. 11. Pele Oy 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. 11
  12. 12. Pele Oy FORMATION 12
  13. 13. 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. 13 Picture: Jing Yan
  14. 14. 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. 14
  15. 15. Pele Oy 15 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
  16. 16. Pele Oy 16 Some visible paper formation faults Fluting after coating Cockling Flow on wire Large scale formation
  17. 17. 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. However, these should have effect on fine material flocculation but not on fiber flocculation to improve retention without deteriorating formation. 17
  18. 18. 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. 18 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:
  19. 19. 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). 19
  20. 20. 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 20 Picture: Innventia
  21. 21. Pele Oy 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. 21 0 200 400 600 800 1000 1200 1400 0 0.2 0.4 0.6 0.8 1 1.2 Headbox consistency, % Sliceopening,mm
  22. 22. Pele Oy 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 22 Picture: Glens Falls Interweb
  23. 23. 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. 23
  24. 24. Pele Oy Valmet example on wire shake improvement  Visual appearance of FormMaster improvement on formation of 210 gsm OCC furnish sheet. 24
  25. 25. Pele Oy FormMaster improvement on OCC furnish 210 gsm  Average floc size improvement is 52%. The most improvement is on the largest flocs. 25
  26. 26. 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. 26 Bo Norman et al. Innventia PaperCon 2015
  27. 27. 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 27
  28. 28. Pele Oy 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. 28 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 %
  29. 29. Pele Oy 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). 29
  30. 30. Pele Oy 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. 30 Consistency Total mass % tons/dry ton 0,5 200 1 100 2 50 3 33 4 25 5 20
  31. 31. Pele Oy 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 31 Bulk [cm3/g] Picture: VTT
  32. 32. Pele Oy Pilot foam forming machine at VTT Finland 2013  Foam forming gives possibilities to save water, energy and material in papermaking. 32 Picture: VTT