Hydrocyclone separation, and reject refining, of thick-walled mechanical pulp fibres

Kjell-Arve Kure; Göran Dahlqvist; Jenny Ekström; Torbjørn Helle

doi:10.3183/npprj-1999-14-02-p100-104

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Hydrocyclone separation, and reject refining, of thick-walled mechanical pulp fibres

Kjell-Arve Kure
Kjell-Arve Kure
PFI – Norwegian Pulp and Paper Research Institute, Trondheim, Norway

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, Göran Dahlqvist
Göran Dahlqvist
Norske Skog Research, Halden, Norway

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, Jenny Ekström
Jenny Ekström
Norske Skog Research, Halden, Norway

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and Torbjørn Helle
Torbjørn Helle
Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway

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Published/Copyright: July 19, 2018

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From the journal Nordic Pulp & Paper Research Journal Volume 14 Issue 2

100Nordic Pulp and Paper Research Journal Vol 14 no. 2/1999Keywords:Mechanical pulping, Hydrocyclones, Reject, Refi-ning, Fibre structureSUMMARY:The use of hydrocyclones for fractionation ofmechanical pulp fibres on the basis of structural fibre characteris-tics such as fibre wall thickness, was studied in two differentmills using commercially available hydrocyclones. The resultsindicate that the technique may be used for removal of thick-wal-led fibres, harmful for paper smoothness. The fraction of thin-walled earlywood fibres were significantly larger in the acceptthan in the reject flows. Fibres with broken circumference arepredominately thin-walled, and were accumulated in the acceptflow. By reject refining at high and low consistency, the wallthickness of the reject fibres was significantly reduced.ADRESSES OF THE AUTHORS: Kjell-Ar ve Kure, PFI – Nor we-gian Pulp and Paper Research Institute, Høgskoleringen 6B,7491 Trondheim, Nor way. Göran Dahlqvist, Jenny Ekström,Norske Skog Research, P.O. Box 53, 1756 Halden, Nor way.Torbjørn Helle, Department of Chemical Engineering, TheNor wegian University of Science and Technology, 7491Trondheim, Nor way.The demand for print quality and thus surface smooth-ness of wood containing printing paper is raising. Thick-walled latewood fibres represent a difficult problem in thisrespect (Høydahl, Dahlqvist 1997; Fjerdingen et al. 1997).The coarse fibres may be collapsed by calendering, butwill regain tubular shape upon remoistening (Forseth,Helle 1997; Forseth, Wiik, Helle 1997; Reme, Helle,Johnsen 1997). The small surface area per unit weight ofsuch fibres even means poor bonding capacity and lowlight scattering (Braaten 1998). Sandberg et al. (1997)fractionated fibres by screens and hydrocyclones, claimingthese two technologies to be complementary. A good over-view of the fluid dynamics and separation efficiency ofhydrocyclones was published by Sevilla and Branion(1997). A fibre population has a wide range of wall thicknessand fibre perimeter combinations (Fig. 1). The fibres mayhave similar outer dimensions and different wall thickness.This indicates that separating thin and thick-walled fibres,based on shape, like in a screen, is not a promising avenue.Screens are well suited for separation of large particles,like shives, and long fibres, based on width and orientationrelative to the entrance aperture. Screens fractionate fibresprimarily on the basis of length, but even stiffness willplay a role, and stiffness is related to cross-sectional fibredimensions. For separation according to fibre wall thick-ness, other principles have to be applied. Hydrocyclones fractionate fibres based on size, shape(specific surface) and density of the particles. Rehmat andBranion (1995) showed that specific surface area per unitweight of fibre can be related to fibre coarseness andhydrocyclones is thus an interesting technique. Høydahland Dahlqvist (1997) showed that to some extent it is pos-sible to separate thick-walled fibres using commerciallyavailable hydrocyclones.Generally, Norway Spruce fibres are thin-walled (1–8μm, Kure 1997), however, the distribution have somethick-walled fibres, affecting the paper surface negatively. The present study explored the potential for improvingpaper properties, by fibre separation with subsequent refi-ning of coarse reject fibres, applying a special hydrocyclo-ne system in two TMP installations. To investigate the bestway of reducing fibre wall thickness of the coarse rejectfibres, the reject fibres was exposed to both low and highconsistency refiningExperimentalFractionation and reject refiningFibre fractionation was undertaken in two Norske SkogASA mills. Mill A produces newsprint and Mill B SC-Amagazine paper. Fractionation was carried out on pureNorway Spruce blends, using a mobile rig with NOSSRADICLONE hydrocyclones. Fig. 2depicts the systemdesign, using 8 and 4 hydrocyclones respectively in a twostage system. System feed consistency of screened pulpwas 1.0% and first stage inject (Inject 1) was 0.6%. Thesystem reject rate was varied. Feed freeness was 130 and35 CSF in mill A and B respectively. System feed flowra-tes were held at some 3.5–4.0 o.d. kg/min. Reject was refi-ned in four stages at “high consistency” (some 21%) in a91 cm Sunds/Bauer atmospheric double disc refiner opera-ting at 1500 rpm, and at “low consistency” (some 3.5%) ina Jylhä Conflo JC01 conical LC refiner, at 910 rpm, at88 ̊C.Fibre analysisCross-sectional fibre dimensions were measured on SEMmicrographs of the Bauer McNett +50 mesh fibre fraction,using the methods described by Fjerdingen et al. (1997).All fibre cross-section images were thinned to a skeletonof width 1 pixel. For intact fibres, this yielded the meanfibre wall perimeter, P, for broken fibres, the length of thethinned image, L. Fibre wall thickness, WT, is calculatedAw/P, where Awis the cross-sectional fibre wall area. Wallthickness of split fibres is calculated by replacing Pwith L.Fibre length was measured using PQM1000. Other testingPulps were tested according to SCAN-standards. Hand-sheets were made while recycling the white water. TheHydrocyclone separation, and reject refining, of thick-walled mechanical pulp fibres Kjell-Ar ve Kure, PFI – Nor wegian Pulp and Paper Research Institute, Trondheim, Nor way. Göran Dahlqvist, Jenny Ekström, Norske Skog Research, Halden, Nor way. Torbjørn Helle, Depar tment of Chemical Engineering, Nor wegian University of Scienceand Technology, Trondheim, Nor way

Published Online: 2018-07-19

Published in Print: 1999-05-01

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