Pulp and Paper Chemistry and Technology Volume 2 Pulping Chemistry and Technology Edited by Monica Ek, Göran Gellerstedt, Gunnar Henriksson Pulp and Paper Chemistry and Technology Volume 2
This project was supported by a generous grant by the Ljungberg Foundation (Stiftelsen Erik Johan Ljungbergs Utbildningsfond) and originally published by the KTH Royal Institute of Technology as the “Ljungberg Textbook”. Pulping Chemistry and Technology
Edited by Monica Ek, Göran Gellerstedt, Gunnar Henriksson Editors Dr. Monica Ek Professor (em.) Dr. Göran Gellerstedt Professor Dr. Gunnar Henriksson Wood Chemistry and Pulp Technology Fibre and Polymer Technology School of Chemical Science and Engineering KTH Ϫ Royal Institute of Technology 100 44 Stockholm Sweden
ISBN 978-3-11-021341-6
Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de.
” Copyright 2009 by Walter de Gruyter GmbH & Co. KG, 10785 Berlin. All rights reserved, including those of translation into foreign languages. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanic, including photocopy, recording, or any information storage retrieval system, without permission in writing from the publisher. Printed in Germany. Typesetting: WGV Verlagsdienstleistungen GmbH, Weinheim, Germany. Printing and binding: Hubert & Co. GmbH & Co. KG, Göttingen, Germany. Cover design: Martin Zech, Bremen, Germany. Foreword
The production of pulp and paper is of major importance in Sweden and the forestry industry has a profound influence on the economy of the country. The technical development of the industry and its ability to compete globally is closely connected with the combination of high-class education, research and development that has taken place at universities, institutes and industry over many years. In many cases, Swedish companies have been regarded as the initiator of new technology which has started here and successively found a general world-wide acceptance. This leadership in knowledge and technology must continue and be developed around the globe in order for the pulp and paper industry to compete with high value-added forestry products adopted to a modern sustainable society. The production of forestry products is based on a complex chain of knowledge in which the biologi- cal material wood with all its natural variability is converted into a variety of fibre-based products, each one with its detailed and specific quality requirements. In order to make such products, knowl- edge about the starting material, as well as the processes and products including the market demands must constitute an integrated base. The possibilities of satisfying the demand of knowledge require- ments from the industry are intimately associated with the ability of the universities to attract students and to provide them with a modern and progressive education of high quality. In 2000, a generous grant was awarded the Department of Fibre and Polymer Technology at KTH Royal Institute of Technology from the Ljungberg Foundation (Stiftelsen Erik Johan Ljungbergs Utbildningsfond), located at StoraEnso in Falun. A major share of the grant was devoted to the development of a series of modern books covering the whole knowledge-chain from tree to paper and converted products. This challenge has been accomplished as a national four-year project in- volving a total of 30 authors from universities, Innventia and industry and resulting in a four volume set covering wood chemistry and biotechnology, pulping and paper chemistry and paper physics. The target reader is a graduate level university student or researcher in chemistry / renew- able resources / biotechnology with no prior knowledge in the fields of pulp and paper. For the benefit of pulp and paper engineers and other people with an interest in this fascinating industry, we hope that the availability of this material as printed books will provide an understanding of all the fundamentals involved in pulp and paper-making. For continuous and encouraging support during the course of this project, we are much indebted to Yngve Stade, Sr Ex Vice President StoraEnso, and to Börje Steen and Jan Moritz, Stiftelsen Erik Johan Ljungbergs Utbildningsfond.
Stockholm, August 2009 Göran Gellerstedt, Monica Ek, Gunnar Henriksson
List of Contributing Authors
Göran Annergren Hans Höglund Granbacken 14 Mid Sweden University 85634 Sundsvall, Sweden Fibre Science and Communication Network [email protected] 851 70 Sundsvall, Sweden [email protected] Birgit Backlund Innventia AB Björn Johansson Drottning Kristinas väg 61 Cleanosol AB Stockholm, Sweden Industriegatan 33 Box 160 [email protected] 29 122 Kristianstad, Sweden [email protected] Elisabet Brännvall KTH Royal Institute of Technology Lars Miliander Chemical Science and Engineering Trekantsgatan 7 Fibre and Polymer Technology 65220 Karlstad, Sweden 100 44 Stockholm, Sweden [email protected] [email protected] Ants Teder Per Engstrand KTH Royal Institute of Technology Mid Sweden University Chemical Science and Engineering Fibre Science and Communication Network Fibre and Polymer Technology 851 70 Sundsvall, Sweden 100 44 Stockholm [email protected] Sweden [email protected] Göran Gellerstedt KTH Royal Institute of Technology Hans Theliander Chemical Science and Engineering Chalmers University of Technology Fibre and Polymer Technology Forest Products and Chemical Engineering 100 44 Stockholm, Sweden 412 96 Göteborg, Sweden [email protected] [email protected]
Ulf Germga˚rd Karlstad University Department of Chemistry Universitetsg. 2 651 88 Karlstad, Sweden [email protected]
ix
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1 Overview of Pulp and Paper Processes ...... 1 Elisabet Brännvall 2 Wood Handling...... 13 Elisabet Brännvall 3 Mechanical Pulping Chemistry ...... 35 Göran Gellerstedt 4 Mechanical Pulping ...... 57 Hans Höglund 5 Chemistry of Chemical Pulping ...... 91 Göran Gellerstedt 6 Pulping Technology ...... 121 Elisabet Brännvall 7 Kinetics of Chemical Pulping and Adaptation to Modified Processes...... 149 Ants Teder 8 Pulp Washing ...... 165 Lars Miliander 9 Chemistry of Bleaching of Chemical Pulp...... 201 Göran Gellerstedt 10 Bleaching of Pulp ...... 239 Ulf Germgård 11 Production of Bleaching Chemicals at the Mill ...... 277 Ulf Germgård 12 Recovery of Cooking Chemicals: the Treatment and Burning of Black Liquor ...... 297 Hans Theliander 13 The Recovery of Cooking Chemicals: the White Liquor Preparation Plant ...... 335 Hans Theliander 14 Towards Increased Closure and Energy Efficiency ...... 363 Birgit Backlund 15 Paper Recycling ...... 391 Per Engstrand and Björn Johansson 16 Pulp Characterisation ...... 429 Elisabet Brännvall and Göran Annergren
Index ...... 461 100 1
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Elisabet Brännvall Royal Institute of Technology, KTH
1.1 Introduction 1
1.2 Pulping Processes 2 1.2.1 Mechanical Pulping 2 1.2.2 Chemical Pulping 3
1.3 Recovery Cycle in Kraft Pulping 5
1.4 Bleaching 6
1.5 Papermaking 6 1.5.1 Stock Preparation 6 1.5.2 Paper Machine 7
1.6 Paper Properties 8
1.7 Distribution of Different Pulping Methods 9
1.8 Consumption of Different Paper Grades 9
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Pulp technology deals with the liberation of fibres fixed in the wood or plant matrix. Paper tech- nology is the knowledge of how to unify the fibres to form the paper web.
pulp technology paper technology
03 milk Windows system
Figure 1.1. The making of paper products from wood includes two main technological fields.
Pulp can be converted to a number of different products with a variety of applications and there- by with a variety of product demands. Paper, as a conveyor of information is perhaps the first 2 thing that comes to mind. Newspapers, magazines, and books require a paper suitable for print- ing text and pictures on, strong enough to endure browsing and folding and in many cases pref- erably able to last for generations to come so they can take part of the information. In the production of diapers and tissue paper, the preservation for future generations is not an issue, neither the strength. For these grades, the most important property is the pulp’s ability to absorb fluids. Strength, however, is the main feature when it comes to paper products aimed to keep and protect other commodities, dry or liquid. Paper has many advantages compared to other ma- terials competing with paper for package purposes. It is obtained from a renewable raw material (wood or other plants), the pulping and papermaking processes have low effluents to the recipi- ent and the paper packages are easy to recycle.
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Pulp consists of fibres, usually acquired from wood. The pulping processes aim first and fore- most to liberate the fibres from the wood matrix. In principal, this can be achieved in two ways, either mechanically or chemically. Mechanical methods demand a lot of electric power, but on the other hand they make use of practically the whole wood material, i.e. the yield of the pro- cess is high. In chemical pulping, only approximately half of the wood becomes pulp, the other half is dissolved. In a modern chemical pulp mill, however, there is no demand of external ener- gy. For a chemical process to be economically feasible, it has to consist of an efficient recovery system. Spent cooking chemicals and the energy in the dissolved organic material is recovered. The pulp obtained is coloured, the degree of colouring depending on the pulping process. For certain paper grades, the dark pulp has to be bleached. Bleaching leads to brighter (whiter) pa- per, this gives better contrast between the print and the paper. Other reasons for bleaching is cleanliness, as bleaching removes impurities that otherwise turn up in the paper as dots, and age-preservation, as bleaching can remove chemical structures in the pulp material that other- wise would in time make the paper yellow.
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By grinding wood or wood chips, the fibres in the wood are released and a mechanical pulp is obtained. In the process, some easily dissolved carbohydrates and extractives are lost, but on the whole the pulp yield is little affected. The pulp yield for mechanical pulp is about 90 to almost 100 %, depending on what mechanical pulping method is used. Mechanical pulp fibres are stiff and mostly uncollapsed. Apart from the fibres, the mechanical pulp has a large portion of small- er material called fines. These consist of fragments from the fibre wall and broken fibres and are important for the excellent optical properties of mechanical pulp. Groundwood pulp is produced by pressing round wood logs against a rotating cylinder made of sandstone. The logs are fed parallel to the cylinder axis and the fibres are scraped off. Figure 1.2 shows an example of a grinder. 3
Figure 1.2. A schematic figure of a stone grinder.
Another type of mechanical pulp is refiner pulp. In this method, chips are fed into the centre of two refining discs, Figure 1.3 One, or both, discs rotate whereby the chips are reduced in chips and fibres abraded off. The disks are grooved, coarser grooves near the centre and finer toward the perimeter. The closer the edge of the disk the wood material comes, the finer the pulp.
pulp
chips rotating part
pulp
Figure 1.3. A chip refiner with one rotating disc.
A mechanical pulp consists not only of fibres released from the wood matrix. A substantial part of the pulp is made up of so-called fines. These are smaller particles, such as broken fibres, material from the fibre surface, and give the mechanical pulp its specific optical characteristics. However, the pulp strength is enhanced if the pulp consists of a higher portion of long fibres. By softening the lignin in the middle lamella, by an increase in temperature, the fracture can take place in the secondary (or primary) wall of the fibre, thus resulting in less fines formation. The chips can be pre-treated with steam, ~120 qC, before feeding to refiner. This type of refiner pulp is called TMP, which stands for thermo-mechanical pulp. By soaking the chips in a sodium sul- phite solution the lignin becomes sulphonated and the lignin softening temperature is decreased. In a CTMP process (Chemo-Thermo Mechanical Pulp), the chemical treatment is followed by a steam pre-treatment.
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Since the fibres in wood and plants are „glued“ together with lignin, the chemical way to pro- duce pulp is to remove most of the lignin and thereby release the fibres. The delignification of 4 wood is achieved by degrading the lignin molecules and introducing of charged groups, keep the lignin fragments in solution and eventually remove them by washing. No pulping chemicals are entirely selective towards lignin, also the carbohydrates of the wood are to varying extent lost. Approximately half of the wood material is dissolved in chemical pulping. No chemical pulping method is able to remove all lignin in the pulping stage, at least not without severe dam- age to the carbohydrates. The delignification is therefore terminated with some lignin remaining in the pulp. The amount of lignin left in the pulp is estimated by determining the kappa number of the pulp. Kraft cooking is the dominant chemical pulping method globally. The cooking chemicals used are sodium hydroxide, and sodium sulphide. By leaving out the sodium sulphide and only use sodium hydroxide as the cooking chemical, the process is called soda cooking. In the sul- phite cooking process, sulphurous acid (H2SO3) and bisulphite ions (H2SO3–) are the active chemicals to degrade and dissolve lignin. Sulphite pulping can be performed at a pH ranging from 1–2 in acid sulphite pulping to 7–9 in neutral sulphite pulping. Organic solvents can be used in delignification, either as the sole degrading chemical, as in suggested organosolv pro- cesses, or as reinforcement chemical, as an addition to sulphite, sulphate or soda processes. The solvents used are ethanol, methanol and peracetic acid. Chemical pulp fibres are more flexible than mechanical pulp fibres. They conform better to each other when forming the paper and offer good strength properties to chemical pulp. The cooking procedure can be either continuous or batch-wise. Figure 1.4 shows a continu- ous cooking system with subsequent bleaching stages. Figure 1.5 shows a batch-wise cooking configuration. An oxygen delignification stage after cooking is generally applied in the produc- tion of bleached pulp. In this stage the lignin content of the pulp is reduced before the pulp en- ters the final bleaching.
oxygen delignification
bleaching digester
chip bin
washing
Figure 1.4. A continuous cooking system (Andritz).
Of the chemical pulping methods, kraft pulping dominates. The cooking liquor in kraft pulp- ing, the white liquor, consists of sodium hydroxide, NaOH, and sodium sulphide, Na2S. The ac- tive cooking species are OH– and HS–. The hydrogen sulphide is the main delignifying agent and the hydroxide keeps the lignin fragments in solution. Kraft cooking is also called sulphate cooking. The name „Kraft cooking“ derives from the German and Swedish word meaning strength. It was first used in the context of pulp from the 5 sulphate process with a high lignin content. These pulp grades have extremely high strength and are used for packages such as linerboard and for sackpaper. Kraft pulp, however, refers to all pulp processed by the sulphate pulping method, also bleachable grades with very low lignin content.
digesters
displacement tank liquor tanks
Figure 1.5. Batch-wise cooking configuration employing two digesters and accompanied with tanks for liquor and pulp (Metso Paper).
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The kraft pulping process includes a recovery cycle. The black liquor displaced from the digest- er consists of the spent cooking chemicals and the dissolved organic material. The spent chemi- cals are regenerated to active chemicals and brought back to the cooking stage. Figure 1.6 shows schematically the stages in the recovery cycle.
lime kiln
slaked burnt lime lime e e e white liquor (NaOH NaSH) green liquor (Na2CO3 NaOH NaSH) causticising
recovery evaporation black liquor thick furnace black liquor e smelt (Na2CO3 Na2S)
make up (Na2SO4)
Figure 1.6. The stages in recovery of chemicals and energy in the kraft process. 6
Evaporating off water increases the dry solid content of black liquor, which is necessary be- fore it is sprayed into a furnace and burnt. In the furnace, the inorganic chemicals in the black li- quor are recovered in molten form. The organic contents are combusted and the heat generated is used for steam production. Part of the steam can be used to produce backpressure power.
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Some paper products require a white paper. One reason is the print quality. A whiter paper gives a better contrast between the paper and the print. The cleanliness of the paper is another reason for bleaching. Impurities in the pulp may otherwise turn up on the paper as dots, deterio- rating the printing. A third reason for bleaching is the ability of paper to resist ageing. Substanc- es in the pulp can turn the paper yellow and brittle as time goes by, mainly substances connected to lignin. The bleaching agents remove these structures susceptible to discoloration of the paper. However, not all bleaching is aimed at removing lignin. Lignin removing bleach- ing is principally performed on chemical pulp, whereas the lignin-rich mechanical pulps are subjected to modifications of the lignin structures absorbing light. The bleaching is usually performed in several stages, using different chemicals in each stage. Chemicals used for bleaching of mechanical pulp are dithionite and hydrogen peroxide. Chemi- cal pulps are bleached, or delignified, with oxygen before entering the actual bleaching plant. There are a number of different chemicals used for the bleaching, for example hydrogen perox- ide, chlorine dioxide, ozone and peracetic acid.
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After liberating the fibres from the wood, they have to be consolidated again to form a web of paper. This is achieved on a paper machine. A very dilute slurry of fibres is sprayed on to a moving wire. Apart from fibres, the slurry may contain fillers, retention aid and wet strength ad- ditives. The wire is an endless woven wire cloth with a mesh size allowing the water to be drained, but retaining the fibres on the wire. From the wire, the paper web enters the pressing section of the paper machine where water is pressed out by squeezing the paper web between steel rolls. To further increase the dryness of the paper, it is dried in the drying section usually consisting of cylinders with steam within. At the end of the paper machine, the web is reeled.
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A paper can be made using only fibres. However, to obtain certain properties a number of additives are used as well. Examples are listed below. • Acids and bases to control pH • Starch to improve dry strength • Resins to improve wet strength • Fillers, such as clay, talc, titanium dioxide, to improve optical properties • Dyes and pigments to get desired colour 7
• Optical brighteners to improve the apparent brightness of the paper • Retention aids to improve the retention of fines and fillers in the paper web • Slimicides to prevent slime growth
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The paper machine is made up of a number of components with different objective. Figure 1.7 shows a Fourdrinier paper machine, with a straight outline from beginning to end. The modern paper machines have a more compact structure and the components not as easily recognised.
wet end dry end water content 99 % 80% 50% 6% press press drying headbox wire felt rolls cylinders reel
press drying section section
Figure 1.7. A simplified plot of a paper machine.
• Headbox. The dilute pulp slurry, ~0.6 % dry solids content, is to be evenly spread out to form the paper web. This is carried out by the headbox, which is a pressurised flow box, distributing the pulp stock onto the moving wire. • Wire. The wire is an endless moving belt, a woven cloth, allowing the water to be drained and retaining the fibres. Figure 1.8 shows twin-wire forming.
paper web headbox
Figure 1.8. The headbox spreads the pulp slurry between the top and bottom wires in twin-wire forming. 8
• Press section. For more efficient water removal, the paper web is couched off the wire and onto a felt and passed between pressing rolls, Figure 1.9.
heated roll
extended nip press
Figure 1.9. The extended nip makes the press section more efficient.
• Drying section. The remaining water is removed by drying the paper on steam heated hot cylinders. • Calandering. Most paper grades are subjected to calandering whereby the paper web passes through nips between iron rolls. This decreases the thickness of the paper and evens out variations in grammage along and across the paper web. The second objective of calan- dering is to improve the surface properties of the paper, mainly to make it smoother. • Coating. Many printing papers demand a very even surface. By applying a coating mixture, the hollows on the surface of the paper sheet are filled. • Reel. Finally, the paper web is wound onto a reel.
Figure 1.10. Paper wound onto reels and subsequently cut to suitable width (Metso Paper).
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In a simplified manner, the important paper properties can be divided into strength and print- ability. 9
The strength of the fibre raw material has the main influence on paper strength. The longer softwood fibres give stronger paper than the short hardwood fibres. The refining of pulp, also called the beating operation, modifies the fibres to conform better in the sheet and thereby in- crease the bonding strength of the sheet. Addition of fillers reduces the paper strength. The printability is to a great extent related to paper surface properties. Short hardwood fibres give a much smoother paper surface than long softwood fibres and therefore better printability. The purpose of calandering and coating is mainly to improve the surface smoothness and the printability of the paper. The paper strength is also a printability parameter since sufficient pa- per web strength is required to avoid web break when the paper is running through the printing press.
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Of all the pulp produced worldwide, almost three quarters are chemical pulp, Figure 1.11. Of the chemical pulp, the major part is produced by the kraft process.
distribution of chemical pulp semi-chemical pulp unbleached chemical pulp sulphite bleached kraft mechanical bleached pulp sulphite
unbleached kraft
Figure 1.11. To the left the distribution between, chemical, mechanical and semi-chemical pulp production glob- ally. The kraft pulping method is the domination pulping process worldwide, right diagram. (Figure FAO) In a semi-chemical pulping process, a smaller portion of the lignin is removed chemically and the fibres are separated mechanically in refiners.
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As can be seen from Figure 1.12, paper is primarily used as a communicating media, conveying information through text and pictures. Other media have appeared in the last decades but paper still holds its stand and even continues to increase in importance. As computers made their entry, predictions were made of the paper- less office. These fears, however, have not been realised. On the contrary, paper consumption continues to increase and to some extent, the development of electronic devices has contributed to the increased consumption, Figure 1.13. 10
tissue
newsprint printing & writing
other (ex paperboard for construction) wrapping & packaging
Figure 1.12. The global distribution of consumption of different paper grades (Figures FAO).
3
laser 2.5 writer introduction of PC 2 competitors the demand of 1. 5 to Xerox A4 copying paper in western europe first 1 (million tons) Xerox copying increase increase 0.5 machine 6.5% 4.5%
0 1950 60 70 80 90 2000
Figure 1.13. The demand of copying paper in western Europe (MoDo Paper).
500
400 forecast 300 global paper production 200 (million tons) 100
0 1950 1960 1970 1980 1990 2000 2010
Figure 1.14. The worldwide paper production (Skogsindustrierna). The consumption of paper is strongly dependent on the economical level of the society. As shown in Figure 1.15, the higher the gross domestic product a country has, the higher the con- sumption of paper 11
- 1999 Paper demand, kg per capita -
350 USA (app. cons.)
Finland 300 (final cons.) Austria Canada 250 Sweden Japan Belgium Netherlands Denmark Germany 200 UK Italy France New Zealand Spain Australia Singapore 150 Rep. of Korea
Ireland Malaysia 100 Portugal Thailand Greece South Africa Brazil Argentina 50 Mexico China Turkey Indonesia Colombia Venezuela Egypt 0 India Philippines 0 5000 10000 15000 20000 25000 30000 35000 - GDP, USD per capita -
Figure 1.15. The more money a person makes, the more paper he or she uses.
The paper industry is concentrated and dominated by a few large producers. The global mar- ket is divided into continental markets where different producers have their main market share, Fig 1.16.
Leading Global Paper & Paperboard Producers
Regional capacity distribution, 2004/1 Q
Stora Enso International Paper UPM Oji Georgia-Pacific Weyerhaeuser Nippon Unipac Holding Smurfit-Stone Container Abitibi-Consolidated Metsä Group North America SCA Western Europe Eastern Europe Asia Pulp & Paper Latin America Norske Skog Asia MeadWestvaco Oceania & Africa Mondi*
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 - Capacity 1 000 t/a -
Figure 1.16. Leading global paper and paperboard producers (Skogsindustrierna). 100 13
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Elisabet Brännvall Department of Fiber and Polymer Technology, Royal Institute of Technology, KTH
2.1 Introduction 13
2.2 Barking 14 2.2.1 Bark Morphology 14 2.2.2 Motives for Barking Wood 15 2.2.3 Barking Theory 15 2.2.4 Barking Equipment 18 2.2.5 Barking of Sawn Logs 20 2.2.6 Use of Bark 22
2.3 Chipping of Wood 23 2.3.1 The Theory behind Chipping 23 2.3.2 Factors Affecting Chip Quality 26 2.3.3 Chippers 27
2.4 Storage of Chips 28
2.5 Screening 31 2.5.1 Different Screening Equipment 31 2.5.2 Overthick Treatment 34
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Large quantities of wood are handled at a pulp mill, the wood consumption ranges between 2000 and 12000 tons per day. The management of the wood yard has a significant impact on the final pulp quality. Chip damages end up as reduced pulp quality and since the wood cost is the highest single cost component in pulp production, fibre losses have a severe effect on the econ- omy of the mill. In Figure 2.1, the wood handling steps at the pulp mill are shown. When the logs arrive to the mill, the weight or volume is determined. After de-loading the timber trucks or railway car- riages, the logs are slashed (cross-cut) to standard lengths. If the logs are frozen, de-icing is re- quired in order to facilitate barking. Sand and stones are removed in the de-icing step as well as after barking. This is necessary in order to minimise the wear on the chipper knives. For the same purpose a metal detector is usually placed before the chipper infeed to detect any metal scrap. In the chipper, the logs are reduced to smaller pieces, chips, and conveyed to the storage. Chip storage can be either open-air storage in a chip pile or in silos. Before the pulping, the chips are screened to ensure an even size distribution. The smallest fraction, pin chips and fines, 14 is discarded and used as fuel. The overthick chips are reduced in size and fed with the accepted chip size fraction to the digester or refiner.
Figure 2.1. Handling of wood at the pulp mill.
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The bark of the tree approximately amounts to 15 % of the tree’s dry weight. It can roughly be divided into outer and inner bark. The outer bark is dead tissue and acts primarily as a protec- tion for the tree. The living inner bark transports water and nutrients to the cambium layer of the wood, Figure 2.2. Bark consists of many different kinds of cells. The cork cells of the outer bark die at an early stage. They are cemented together into a tight tissue that resists water and gas. The sieve ele- ments in the inner bark transport water and nutrients whereas the parenchyma cells store nutri- ents. Two different types of cells for the mechanical support are found -bast fibres, measuring up to 3 mm in length, and small stone cells. Chemically, the bast fibres resemble the wood fibres as they are made up of cellulose, hemi- cellulose, and lignin. The other cell types have a high content of extractives. In addition, the mineral content of the bark is higher compared to wood tissue. 15
heartwood
sapwood
cambium
phloem
outer bark
Figure 2.2. The structure of the wood stem.
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Both of the terms bark or debark wood applies to the same procedure, to remove the bark from the tree. Whether to use the term barking or debarking is a matter of preference, the term bark- ing is chosen here. The motives for barking wood before pulping are many and are listed below: 1. Low yield. The fibre proportion of bark is very low. There would be little sense in trying to get hold of these fibres in the pulping process, only about 20 % of the bark can be retrieved by chemical pulping. Generally, any other types of cells present in the tree are of no interest or a nuisance in the pulp and paper mills. 2. Damage to equipment. Hard granules of sand can be embedded in the bark and cause wear in chipping equipment. 3. High content of extractives. The high content of extractives in bark gives higher consump- tion of cooking and bleaching chemicals. They give rise to pitch problems in both pulp and paper mill. 4. Drainage obstacles. The small bark cells may also cause problems in de-watering the paper web formed on the paper machine. 5. Dirt on paper. Some of the bark cells are not degraded in neither cooking nor bleaching and end up as dark spots on the paper web.
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The aim of barking is to remove outer as well as inner bark. Preferably, the separation of bark from wood occurs in the cambium layer between sapwood and bark. In favourable circumstanc- es, the cambium is rather soft and with low shearing strength making the log easy to bark. The 16 force needed to remove the bark depends for one thing on the wood specie. Table 2.1 gives ex- amples of trees and the effort needed for barking. As an example, birch is approximately twice as hard to bark than pine or spruce. It is therefore not suitable to bark birch and softwoods to- gether. q~ÄäÉ=OKNK=cçêÅÉ=åÉÉÇÉÇ=íç=Ä~êâ=ïççÇ=áë=ëéÉÅáÉë=ÇÉéÉåÇÉåíK=Em~éÉêã~âáåÖ=pÅáÉåÅÉ=~åÇ=qÉÅÜåçäJ çÖóI=dìääáÅÜëÉå=~åÇ=cçÖÉäÜçäãFK _~êâáåÖ=ÑçêÅÉ=åÉÉÇÉÇ bñ~ãéäÉ=çÑ=ëéÉÅáÉë b~ëó=íç=Ä~êâ pçìíÜÉêå=éáåÉë=j~éäÉ=l~â=jçëí=íêçéáÅ~ä=Ü~êÇïççÇë kçêã~ä=íç=Ä~êâ péêìÅÉ=_ÉÉÅÜ=eÉãäçÅâ aáÑÑáÅìäí=íç=Ä~êâ _áêÅÜ=bäã sÉêó=ÇáÑÑáÅìäí=íç=Ä~êâ _ä~Åâ=éçéä~ê=fêçåïççÇ ^äãçëí=áãéçëëáÄäÉ=íç=Ä~êâ _~ëëïççÇ=eáÅâçêó Some wood species need certain pre-treatment before the actual barking. Certain eucalyptus species and acacia have very tight fitting smooth bark that is rubbed off with difficulty. Special bark shredders are used in order to make cuts and tear in the bark creating positions where bark- ing can start. The bark of some eucalyptus types is torn off as long chunks that roll and are en- tangled into each other. These coils of bark roll around and are discarded together with the logs and may eventually end up in the chipper and continue through the whole process. The strength with which the bark adheres to the wood is related to the cutting season. In the summertime, when the tree has its growing season, the bark is much less tightly attached to the wood as compared to the season when no growth occurs, Figure 2.3.