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Thesis for the degree of Doctor of technology, Sundsvall 2011
ATMP PROCESS: IMPROVED ENERGY EFFICIENCY IN TMP REFINING UTILIZING SELECTIVE WOOD DISINTEGRATION AND
TARGETED APPLICATION OF CHEMICALS Dmitri Gorski
Supervisors:
Prof. Per Engstrand MSc. Jan Hill Dr Lars Johansson
FSCN ‐ Fibre Science and Communication Network Department of Natural Sciences
Mid Sweden University, SE‐851 70 Sundsvall, Sweden
Norske Skog Industries, nsiFOCUS AS, Pulp Team NO‐1756 Halden, Norway
ISSN 1652‐893X
Mid Sweden University Doctoral Thesis 108 ISBN 978‐91‐86694‐34‐0
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Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av teknologie doktorsexamen torsdagen den 5 maj 2011, klockan 13.15 i sal M102, Mittuniversitetet Sundsvall.
Seminariet kommer att hållas på engelska.
ATMP PROCESS: IMPROVED ENERGY EFFICIENCY IN TMP REFINING UTILIZING SELECTIVE WOOD DISINTEGRATION AND TARGETED APPLICATION OF CHEMICALS
Dmitri Gorski
© Dmitri Gorski, 2011
FSCN ‐ Fibre Science and Communication Network Department of Natural Sciences
Mid Sweden University, SE‐851 70 Sundsvall Sweden
Telephone: +46 (0)771‐975 000
Printed by Kopieringen Mittuniversitetet, Sundsvall, Sweden, 2011
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Dr. Alf de Ruvo
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I would like to ask you, dr. Atack, about the relationship between energy and properties that we have in refining. As you know we have improved the properties of mechanical pulps due to TMP, CTMP, etc., but the disadvantages seem to be that we always increase the energy input. Do you think there is any chance that we can break this vicious circle, so as to reduce the amount of energy and still get better properties in refining?
Dr. Douglas Atack
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Yes, I do think this can be done. But we need to do further work to be certain.
Fibre‐Water Interactions in Papermaking Symposium, Oxford, UK, 1977
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ATMP PROCESS: IMPROVED ENERGY EFFICIENCY IN TMP REFINING UTILIZING SELECTIVE WOOD DISINTEGRATION AND TARGETED APPLICATION OF CHEMICALS
Dmitri Gorski
FSCN ‐ Fibre Science and Communication Network, Department of Natural Sciences, Mid Sweden University, SE‐851 70 Sundsvall, Sweden
ISSN 1652‐893X, Mid Sweden University Doctoral Thesis 108;
ISBN978‐91‐86694‐34‐0
ABSTRACT
This thesis is focused on the novel wood chip refining process called Advanced Thermomechanical Pulp (ATMP) refining. ATMP consists of mechanical pre‐
treatment of chips in Impressafiner and Fiberizer prior to first stage refining at increased intensity. Process chemicals (this study was concentrated on hydrogen peroxide and magnesium hydroxide) are introduced into the first stage refiner.
It is known that the use of chemicals in TMP process and first stage refining at elevated intensity can reduce the energy demands of refining. The downside is that they also alter the character of the produced pulp. Reductions in fibre length and tear index are usually the consequences of refining at elevated intensity. Addition of chemicals usually leads to reduction of the light scattering coefficient. Using statistical methods it was shown that it is possible to maintain the TMP character of the pulp using the ATMP process. This is explained by a separation of the defibration and the fibre development phases in refining. This separation allows defibration of chips to fibres and fibre bundles without addition of chemicals or increase in refining intensity. Chemicals are applied in the fibre development phase only (first stage refiner). The energy demand in refining to reach tensile index of 25 Nm/g was reduced by up to 1.1 MWh/odt (42 %) using the ATMP process on Loblolly pine. The energy demand in refining of White spruce, required to reach tensile index of 30 Nm/g, was reduced by 0.65 MWh/odt (37%).
Characterizations of individual fibre properties, properties of sheets made from long fibre fractions and model fibre sheets with different fines fractions were carried out. It was established that both the process equipment configuration (i.e.
the mechanical pre‐treatment and the elevated refining intensity) and the addition of process chemicals in the ATMP process influence fibre properties such as
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external and internal fibrillation as well as the amount of split fibres. Improvement of these properties translated into improved properties of sheets, made from the long fibre fractions of the studied pulps. The quality of the fines fraction also improved. However, the mechanisms of improvement in the fines quality seem to be different for fines, generated using improved process configuration and addition of process chemicals. The first type of fines contributed to better bonding of model long fibre sheets through the densification of the structure. Fines which have been influenced by the addition of the process chemicals seemed in addition to improve bonding between long fibres by enhancing the specific bond strength.
The improved fibre and fines properties also translated into better air permeability and surface roughness of paper sheets, properties which are especially important for supercalendered (SC) printing paper. The magnitude of fibre roughening after moistening was mainly influenced by the process equipment configuration while the addition of process chemicals yielded lowest final surface roughness due to the lowest initial surface roughness. There was no difference in how fines fractions from the studied processes influenced the fibre roughening. However, fines with better bonding yielded model fibre sheets with higher PPS, probably due to their consolidation around fibre joints. Hence, the decrease in PPS can probably be attributed to the improvements in the long fibre fraction properties while the improvement of fines quality contributed to the reduction of air permeability.
The process chemicals, utilized in the ATMP process (Mg(OH)2 and H2O2) also proved to be an effective bleaching system. Comparable increases in brightness could be reached using the ATMP process and conventional tower bleaching.
Maximum brightness of the pulp was reached after approximately 10 minutes of high‐consistency storage after refining or 40 minutes of conventional bleaching.
This study was conducted using a pilot scale refiner system operated as a batch process. Most of the experiments were performed using White spruce (Picea glauca). In Paper I, Loblolly pine (Pinus taeda) was used. It is believed that the results presented in this thesis are valid for other softwood raw materials as well, but this limitation should be considered.
Keywords: ATMP, TMP, Hydrogen Peroxide, Magnesium Hydroxide, Mechanical Pre‐Treatment, Fibre Characterisation, Refiner Bleaching, SC paper, Newsprint
vi SAMMANFATTNING
Avhandlingen är fokuserad på massaegenskaper och energiförbrukning hos den nya processen för raffinering av mekanisk massa, ATMP (Advanced Thermomechanical Pulp). Processen består av mekanisk förbehandling av flis med Impressafiner och Fiberizer före förstastegsraffinering. Intensiteten i förstastegsraffineringen har ökats genom att använda matande raffinörsegment och/eller högre rotationshastighet hos raffinören. Processkemikalier (här har främst väteperoxid och magnesium hydroxid använts) satsas i förstastegsraffinören efter defibrering av flis.
Det är känt att tillsats av kemikalier och förstastegsraffinering vid högre intensitet leder till sänkt energiförbrukning i raffineringen. Nackdelen är dock att de samtidigt förändrar karaktären hos producerad massa. Reducerad fiberlängd och rivindex är vanliga vid raffinering med högre intensitet. Tillsats av kemikalier i raffineringsprocessen leder vanligen till att producerad massa får mer ”CTMP‐
karaktär”, det vill säga minskad ljusspridning jämfört med TMP vid samma dragindex. Med hjälp av statistisk databehandling (varians‐ och principalkomponentsanalys) har det visats att det är möjligt att behålla TMP‐
karaktären hos ATMP massan även när kemikalier och högre raffineringsintensitet används. Det beror på att defibrering av flis och utvecklingen av fibrer sker i olika processteg. Separationen medger att genomföra defibrering av flis till fibrer och fiberknippen under TMP‐liknande förhållanden utan tillsats av kemikalier eller ökning av raffineringsintensiteten. Kemikalierna appliceras endast under fiberutveckligsfasen (förstastegsraffinören och vidare). Med bibehållen TMP‐
karaktär hos producerad ATMP‐massa minskade energiförbrukningen till dragindex 25 Nm/g med 1.1 MWh/odt (42 %) när ATMP processen tillämpades på sydstatstall (Pinus Taeda). Energiförbrukningen vid raffinering av gran för att nå dragindex 30 Nm/g minskade med 0.65 MWh/odt (37 %) när White spruce (Picea glauca) användes.
Karaktärisering av fibrer och ark inklusive långfiberfraktions‐ och finfraktionsark – de senare tillverkade med en blandning av TMP‐långfiber och olika finfraktioner – har utförts. Både processutformningen för ATMP (det vill säga mekanisk förbehandling och raffinering vid högre intensitet) och kemikalietillsats påverkar fiberegenskapsutvecklingen exempelvis intern‐ och externfibrillering samt andel splittrade fibrer. Bättre fiberegenskaper hos ATMP gav bättre arkegenskaper hos ark tillverkade av långfiberfraktion från ATMP än från TMP.
Både processutformning och kemikalietillsats påverkade och förbättrade kvaliteten
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hos ATMP finfraktion. Förbättringarna till följd av ändrad processutformning respektive kemikalietillsats tyder på att olika mekanismer bidrar.
Förbättrade egenskaper hos fibrer och finmaterial leder också till minskning av luftpermeabilitet och ytråhet hos pappersark. Dessa egenskaper är viktiga för SC tryckpapper. Graden av fiberresning i pappersytan vid förhöjd fukthalt (mätt som ytråhetsdifferens före och efter befuktning) påverkas främst av processutformningen. Ark, tillverkade av ATMP med kemikalietillsats, har dock lägst ytråhet efter befuktning till följd av lägsta ytråheten för torrt ark. Både TMP och ATMP finmaterial hade samma inverkan på fiberresning. Inblandning av finfraktion med bättre bidningsförmåga leder till högre ytråhet. Förmodligen kan detta förklaras med förhöjd koncentration av finmaterial kring bindningspunkter mellan fibrerna. Följaktligen är minskningen i ytråhet för ATMP‐massor sannolikt relaterad till de förbättrade långfiberegenskaperna samtidigt som den bättre finfraktionen bidrar till viss reduktion av luftpermeabilitet.
ATMP tillverkad med bruk av peroxid och magnesiumhydroxid i förstastegsraffineringen visade sig ge ett effektivt system för massablekning.
Jämförbar ökning i ljushet kunde nås med ATMP processen och konventionell peroxidblekning i torn vid samma kemikaliesatsning. Maximal massaljushet nås för ATMP efter cirka 10 minuters lagring vid hög massakoncentration respektive efter 40 minuter för konventionell tornblekning vid laboratoriestudium.
De rapporterade försöken genomfördes i pilotskala med ett raffinörsystem som kördes satsvis. White spruce (Picea glauca) användes som råvara för de flesta försöken. Sydstatstall (Pinus taeda) användes i försöken beskrivna i Paper I. Det är sannolikt att resultaten som presenterats här, gäller för de flesta barrvedssorter.
Denna begränsning bör dock tas i beaktning.
viii TABLE OF CONTENTS
ABSTRACT... IV SAMMANFATTNING... VI LIST OF PAPERS... X AUTHOR’S CONTRIBUTIONS TO THE MANUSCRIPTS...XI RELATED PUBLICATIONS... XII ABBREVIATIONS ...XIII
1 INTRODUCTION... 1
1.1BACKGROUND... 1
1.2OBJECTIVES OF THE STUDY... 2
1.3HYPOTHESES... 2
1.4OUTLINE OF THE THESIS... 3
2 LITERATURE OVERVIEW... 4
2.1RAW MATERIAL... 4
2.1.1 Wood ultrastructure ... 5
2.1.2 Chemistry and reactivity of wood ... 8
2.2CONVENTIONAL CHIP REFINING... 9
2.2.1 Softening of wood... 9
2.2.2 Mechanisms of chip refining... 11
2.2.3 Character and quality of refined pulp ... 15
2.3MECHANICAL PRE-TREATMENT... 18
2.4REFINING AT INCREASED INTENSITY... 20
2.5COMBINATION OF MECHANICAL PRE-TREATMENT WITH HIGH-INTENSITY REFINING... 23
2.6PEROXIDE AND MAGNESIUM HYDROXIDE CHEMISTRY... 23
2.7USE OF CHEMICALS IN REFINING... 26
2.8RELATIONSHIP BETWEEN FIBRE AND FINES PROPERTIES AND PAPER PROPERTIES... 28
2.8.1 Fibre development and paper quality... 28
2.8.2 Contribution of fines to the paper quality ... 29
3 MATERIALS AND METHODS... 32
3.1PILOT SCALE TRIALS... 32
3.1.1. Conditions during the trials... 33
3.1.2. Accuracy of the specific energy demand measurement... 34
3.1.3. The ATMP process ... 35
3.2STATISTICAL METHODS... 36
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3.2.1 ANOVA ... 36
3.2.2. PCA ... 37
3.3LABORATORY TESTING... 37
3.3.1 Whole pulp testing... 37
3.3.2 Physical testing of laboratory sheets... 38
3.3.3. Fibre characterisation ... 39
3.3.5. Laboratory bleaching... 40
3.4ATMP PROCESS IN MILL SCALE... 41
4 RESULTS AND DISCUSSION... 42
4.1MECHANICAL PRE-TREATMENT... 42
4.2CHARACTER OF ATMP ... 47
4.3FIBRE AND FINES PROPERTIES... 54
4.3.1. Properties of individual fibres ... 54
4.3.2. Physical properties of long fibre sheets ... 58
4.3.3. Influence of fines on the paper quality ... 60
4.4ENERGY EFFICIENCY IN REFINING... 66
4.5OPTICAL PROPERTIES AND BLEACHING EFFICIENCY... 72
4.5.1. Brightness improvement... 72
4.5.2. COD generation ... 74
4.5.3. Light scattering ... 77
5 SUMMARY AND CONCLUSIONS ... 79
6 RECOMMENDATIONS FOR FUTURE WORK... 81
7 ACKNOWLEDGEMENTS ... 83
8 REFERENCES... 85
APPENDIX 1: DATA FROM PILOT TRIALS AND LABORATORY TESTING ... 105
x LIST OF PAPERS
This thesis is mainly based on the following six papers, herein referred to by their Roman numerals:
Paper I Review: reduction of energy consumption in refining through mechanical pretreatment of wood chips
Gorski D., Hill J., Engstrand P., Johansson L.
International Mechanical Pulping Conference, Sundsvall, Sweden, 2009 Nordic Pulp and Paper Research Journal, 25(2), 2010, p. 156
Paper II Improvement of energy efficiency in TMP process through selective wood disintegration and targeted addition of chemicals
Johansson L., Hill J., Gorski D., Axelsson P.
Nordic Pulp and Paper Research Journal, 26(1), 2011, p. 31
Paper III Peroxide‐based ATMP refining of spruce: influence of chemical conditions on energy efficiency, fibre properties and pulp quality Gorski D., Mörseburg K., Axelsson P., Engstrand P.
Nordic Pulp and Paper Research Journal, 26(1), 2011, p. 47
Paper IV Role of equipment configuration and process chemicals in peroxide‐
based ATMP refining of spruce Gorski D., Mörseburg K., Johansson L.
Nordic Pulp and Paper Research Journal, 26(2), 2011
Paper V Using ATMP technology to improve energy efficiency and pulp quality in production of SC magazine paper
Gorski D., Kure K.‐A., Hill J.
Submitted to Nordic Pulp and Paper Research Journal
Paper VI Brightness improvement in peroxide‐based ATMP process compared to conventional bleaching
Gorski D., Johansson L., Engstrand P.
Submitted to Holzforschung
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AUTHOR’S CONTRIBUTIONS TO THE MANUSCRIPTS The author’s contributions to the papers in this thesis are as follows:
Paper I Literature survey, review of the literature and writing the paper
Paper II Literature survey, analyzing the results in cooperation with the other authors and writing the paper
Paper III Literature survey, planning the trials together with Lars Johansson, Patrik Axelsson and Jan Hill, helping to conduct the trials, analyzing the results in cooperation with the other authors and writing the paper
Paper IV Literature survey, planning the trials together with Lars Johansson and Jan Hill, helping to conduct the trials, analyzing the results in cooperation with the other authors and writing the paper
Paper V Literature survey, planning the trials together with Lars Johansson and Jan Hill, helping to conduct the trials, analyzing the results in cooperation with the other authors and writing the paper
Paper VI Literature survey, planning the trials together with Lars Johansson and Jan Hill, helping to conduct the trials, analyzing the results in cooperation with the other authors and writing the paper
xii RELATED PUBLICATIONS
Mg(OH)2‐based hydrogen peroxide refiner bleaching: influence of extractives content in dilution water on energy efficiency
Gorski D., Engstrand P., Hill J., Axelsson P., Johansson L.
64th Appita Conference, Melbourne, Australia, 2010 Appita Journal, 63(3), 2010, p. 218
Combining selective bleaching chemistries and ATMP technology for low energy mechanical pulping at higher brightness
Hill J., Sabourin M., Johansson L., Mörseburg K., Axelsson P., Aichinger J., Braeuer P., Gorski D.
7th International Symposium on Fundamental Mechanical Pulping, Nanjing, China, 2010, p. 164
On the relationship between improved energy‐efficiency in high consistency refining, fibre and fines properties and critical paper properties
Gorski D., Mörseburg K., Kure K.‐A.
International Mechanical Pulping Conference, Xi’an, China, 2011
xiii ABBREVIATIONS
TMP Thermo Mechanical Pulping
CTMP Chemo Thermo Mechanical Pulping
RTS© TMP, where first stage refining is conducted at elevated intensity by using higher refiner rotational speed
ATMP Advanced Thermo Mechanical Pulping, a novel thermo mechanical pulping process consisting of the following modifications to the conventional TMP process:
‐ Mechanical pre‐treatment of chips in Impressafiner and Fiberizer units
‐ Elevated first stage intensity (using higher refiner rotational speed and/or feeding segment pattern)
‐ Addition of process chemicals after chip defibration ATMP (aq.) No process chemicals added into the dilution water ATMP (B) With the addition of NaHSO3 into the first stage refiner
ATMP (Mg+P) With the addition of Mg(OH)2 and H2O2 into the first stage refiner ATMP (Na+P) With the addition of NaOH and H2O2 into the first stage refiner ATMP (AA) With the addition of CH3COOH into the first stage refiner
ATMP (AA+P) With the addition of CH3COOH and H2O2 into the first stage refiner
ATMP (P) With the addition of H2O2 into the first stage refiner
SEC Specific Energy Consumption (also specific energy demand) S3A Surface area index, used in this study to estimate the external
development of individual fibres
COD Chemical Oxygen Demand
SC‐paper Super Calenderd printing paper usually used for heatset‐offset and rotogravure printing wish especially high demand on the surface quality
LWC‐paper Light Weight Coated printing paper
PPS Parker Print Surface, a measure of surface roughness of paper
odt Oven Dry Tonne
PFI Paper and Fibre Research Institute
1 INTRODUCTION 1.1 Background
TMP (Thermo Mechanical Pulp) refining is a very energy‐intensive process. For example, the electrical energy demand in mechanical pulping mills in Sweden, most of it utilized in TMP refining, amounts to approximately 5% of the total electrical energy used in the country (www.skogsindustrierna.org). Substantial improvement of electrical energy efficiency in refining is needed in order to secure the long term operation of many TMP mills. With rapidly growing energy costs the problem of high energy demand has become acute in recent years.
Norske Skog ASA is a major producer of mechanical pulp in Norway. It has cooperated with institutes and universities in the areas of energy‐efficient mechanical pulp production for more then ten years (Kure 1999, Reme 2000).
During a pre‐study for a new pulping line at Norske Skog Pisa mill in Brazil in 2002, a decision was made to look into a refiner‐based solution that would satisfy the mill’s pulp quality specifications using local raw material (Pinus taeda). Earlier trials showed that the conventional TMP process produced pulp with inadequate strength properties at too high energy demands. The research conducted by Norske Skog, Paper and Fibre Research Institute (PFI) and the equipment manufacturer Andritz resulted in the idea of a novel mechanical pulping process named ATMP (Advanced Thermomechanical Pulp). A partial ATMP process is operational in the Pisa mill since 2006.
The ATMP process consists of three main features:
¾ Defibration of chips into fibres and fibre bundles, achieved during mechanical pre‐treatment in Impressafiner and Fiberizer units.
¾ Utilization of elevated refining intensity during primary refining using higher rotational speed and a high‐intensity segment pattern.
¾ Selective addition of chemicals after pre‐treatment during primary refining, aimed at improving the fibre development while not influencing the chip defibration.
FSCN (Fibre Science and Communication Network) is a multi‐disciplinary research centre at Mid Sweden University (Sundsvall, Sweden) which has an aim of creating leading‐edge knowledge and foresights for innovative products and production systems for the forest products industry. In 2007, cooperation was established between FSCN, Andritz, PFI and Norske Skog ASA resulting in a PhD
project related to the ATMP process. The ATMP technology was commercialized for softwood in 2010 and the first full scale line has now been built in the Steyrermühl paper mill in Austria (UPM Kymmene Corporation).
1.2 Objectives of the study
The goal of this thesis is to find a way to produce mechanical pulp fibres with improved quality suitable for printing papers while at the same time minimizing the energy demand in mainline high‐consistency refining. Large improvements in refining energy efficiency were shown to be possible using the ATMP process (Hill et al. 2009, 2010). The main objectives of this study were:
¾ To evaluate the potential for energy reduction using the ATMP process compared to the conventional TMP process for pine and spruce raw materials.
¾ To investigate if significant energy demand reduction is possible while preserving important mechanical pulp and printing paper properties such as strength combined with optical properties and surface characteristics on similar level for the ATMP and the TMP.
¾ To study how fibre and fines properties, important for printing paper quality, were developed in the mainline refining in the ATMP process compared to the TMP reference.
¾ To evaluate the bleaching efficiency of ATMP process with hydrogen peroxide and magnesium hydroxide as process chemicals and to compare it to conventional high‐consistency tower bleaching simulated in
laboratory.
1.3 Hypotheses
¾ By separation of chip defibration and fibre development process it is possible to optimize those two phases of TMP refining separately. Thereby it is possible to achieve better energy‐efficiency in the development of the fibre properties and produce pulp with better quality at lower energy demand.
¾ First stage refining of wood material, which is fiberized during
compressive pre‐treatment, can be conducted using higher intensity and chemical treatment without altering the character of the produced pulp, i.e. preserving strength and optical properties typical for TMP.
¾ The chemical system used to soften the fibre material is also able to bleach the pulp with efficiency, comparable to conventional high‐consistency tower bleaching.
1.4 Outline of the thesis
Chapter 1 includes introduction, objectives of the study, hypothesis and the outline of the thesis. In Chapter 2, literature related to this study is reviewed. This chapter also includes results from Paper I, which is a literature review of mechanical pre‐treatment, one of the key unit operations in ATMP refining
Chapter 3 describes the experimental methods, used in this study. Detailed description of statistical methods can be found in Paper II and fibre characterisation methods in Paper III.
In Chapter 4, main findings in this study are presented. The influence of the ATMP process on the energy efficiency and pulp quality in pilot scale refining of Loblolly pine (Pinus taeda) and White spruce (Picea glauca) is described. An attempt is made to link the influence of different mechanisms, employed in the ATMP process, on individual fibre properties to pulp quality and electrical energy efficiency in refining. The influence of long fibre fractions and fines from different pulps, produced in this study, on properties of laboratory sheets is also described.
Further, a possibility to use the ATMP refining process for production of pulp for SC printing paper with its specific demands on pulp quality is explored. Finally, the bleaching efficiency and COD (Chemical Oxygen Demand) generation in ATMP process is studied and compared to conventional high‐consistency tower bleaching.
Chapter 5 contains a summary and conclusions and Chapter 6 recommendations for future work.
2 LITERATURE OVERVIEW
In this chapter, a review of related literature is conducted. The choice of reviewed literature is based on its relevance for the ATMP process and this thesis. In‐depth descriptions of wood raw material, refining and papermaking processes can be found elsewhere (Panshin and de Zeeuw 1980, Fellers and Norman 1992, Sundholm 1999, Fengel and Wegener 2003).
2.1 Raw material
Wood consists of highly heterogeneous anisotropic elements with a lot of variations both within and between the trees (Lundqvist et al. 2003). Large differences in such fibre properties as length, width, content of extractives and microfibril angle can also be found in fibres originating from the same tree. These differences depend on what height from the ground and where in the trunk the fibres originate from (Atmer and Thörnqvist 1982). Moreover, the age of the tree plays an important role; younger trees consist of fibres with different properties as compared to older trees (Sundholm 1999). Thus, representative sampling is very important in evaluation of fibre and pulp properties. The differences in fibre structure of Norway spruce are illustrated in Fig. 1 where it can be clearly seen that fibres from different periods in a tree’s life have different fibril alignment and wall layer thickness.
Figure 1. Cell wall models of Norway spruce tracheids. a). earlywood tracheid, b).
latewood tracheid from juvenile wood, c). latewood tracheid from mature wood. Different microfibril orientation is indicated in the layers of the models (Brändström 2002).
This thesis is based on trials performed using softwood raw material, mainly White spruce. In one of the trials, Loblolly pine was used. There are some differences between those two softwood species, see Table 1. As can be seen, pine and spruce fibres have approximately similar length but there are considerable differences in fibre width, cell wall thickness and specific gravity. Pine fibres are thus more rigid and coarse, have higher ratio of latewood (thick‐walled) fibres to earlywood (thin‐walled) fibres as well as higher amount of extractives (Sundholm 1999).
Table 1. Properties of White spruce and Loblolly pine (Sundholm 1999)
Property White spruce Loblolly pine
Average fibre length (mm) 3.5 3.6
Average fibre width (μm) 25‐30 35‐45
Cell wall thickness (μm) 2.4 3.3
Specific gravity (odg/cm3) 0.42 0.54
Extractives content, EtOH‐Benz. (%) 2.0 3.2‐5.4
2.1.1 Wood ultrastructure
Wood is a highly hierarchical composite material, Fig 2. 90‐95% of softwood material consists of tracheids, commonly referred to as fibres (Huber and Prütz 1938). The tracheids function as mechanical support structure for the wood trunk and also transport liquid and minerals up the stem. This transport is more important during the initial phase of a tree’s life (fibres have higher fibril angles, thinner walls and larger inner diameters), while mechanical support of the growing trunk is more important in the later phase (fibres have smaller fibril angles and thicker walls). This is one of the reasons for juvenile wood fibres having different fibre dimensions compared to mature wood fibres. In the centre of each fibre there is a cavity, called the lumen. A thin layer of the secondary wall called S3 separates the lumen and S2. The main function of the S3 is to withstand the negative pressure in the lumen caused by the transport of liquid upwards in the trunk, which can be up to 20 bars (Booker and Sell 1998). This conclusion is supported by a study which showed that S3 layer thickness and fibril orientation are important for the transverse properties of fibres (Bergander and Salmén 2000).
S2 is the thickest layer of the secondary wall and contributes with as much as 70‐
80% to the total fibre mass (Vehniäinen 2008). It determines most of the stiffness and other mechanical characteristics of the fibre due to its high cellulose content and axially oriented fibrils. The S1 layer separates S2 layer from the primary wall, P. Middle lamellae (M) is the amorphous layer separating wood fibres from each other. Middle lamellae determines most of the rheological characteristics of wood due to its low glass transition temperature.
Figure 2. Schematic illustration of a softwood fibre (Persson 2000)
Approximate content of the different polymers contained in wood fibres is shown in Fig. 3. This is historically the figure used most often to describe the polymer composition of the fibres; however, considerable advances have been made in characterisation of wood fibre wall polymer structure since this figure was produced in the 1970‐s.
Figure 3. Different polymers, contained in wood fibre wall (Panshin and de Zeeuw 1980)
It was recently proposed that a double fibre wall should be considered as fundamental building block of wood (Booker and Sell 1998, Corson 2001). The double wall layer acts as an effective energy dissipation structure when a tree is subjected to mechanical stresses. The main source of mechanical stress in a tree is the swaying of the trunk due to wind, which causes axial compressions and vibrations. The stiff cellulose matrix in the S2 layer provides mechanical support.
This matrix, with fibrils oriented at an angle from the fibre axis, flexes upon the axial compression of the tree, see Fig. 4.
Figure 4. Flexing of the cellulose microfibril matrix upon axial compressions, caused by swaying of a tree (Corson 2001).
The flexing of the S2 layers of two adjustent fibres causes shearing of the middle lamellae, which dissipates this energy as heat due to its amorphous structure.
Stresses, activating this naturally evolved mechanism of energy dissipation in a tree, are similar in many ways to the stresses originating from refining of wood chips. In mechanical pulp refining, the wood structure is also subjected to repeated stressing aimed at liberating individual fibres (i.e. breaking up the material). It is thus important to optimize the frequency and intensity of this cyclic stress so that the liberation of fibres from the wood matrix happens in an energy‐efficient way, not trigging the natural defensive mechanisms which took millions of years to evolve.
2.1.2 Chemistry and reactivity of wood
Wood polymer composition, previously described in Fig. 3, has been characterized in a lot more detail in the previous 30 years. Therefore, an updated version, based on the literature data, is suggested, see Fig. 5.
Figure 5. Approximate distribution of different polymers in wood fibre wall (drawn based upon Lindgren and Mikawa 1957, Marton and Adler 1961, Panshin and DeZeeuw 1970, Sorvari et al. 1986, Westermark et al. 1986, Heitner and Min 1987, Bacic et al. 1988, Suckling 1991, Peng and Westermark 1997, David and Hon 2001, Fengel and Wegener 2003, Rowell 2005, Stevanic 2008)
The main difference, compared to Fig. 3, is a much more detailed composition of the polymer group, previously named “hemicellulose”. An increased concentration of the coniferyl‐type lignin structures in the P1 and S1 fibre wall layers is also accentuated. The main purpose of the figure is not to depict all details in cell wall composition, but to illustrate that there is considerable difference in reactivity of the different areas in the fibre wall. Coniferyl lignin structures, pectin and protein, which are all highly reactive compounds compared to relatively inert cellulose, are all concentrated to the P1 and S1 fibre wall layers, i.e. the outer layers in the fibre structure. Thus, the outer fibre layers can be considered to be the most reactive parts of the fibres. Reactions of fibre components with hydrogen peroxide and alkali are described later in this thesis.
The reactivity of different chemicals towards fibre components is also influenced by the transport mechanisms of those chemicals in the fibres. As far as traditional
chemical and chemi‐mechanical pulping is concerned, there are two possible mechanisms of chemical penetration into the chip and fibre structure, see Fig. 6:
¾ Penetration into the fibre lumen and then to the middle lamellae through the pit pores (Westermark et al. 1987, Peng et al. 1992).
¾ Penetration into the fibre lumen and then through S3, S2, S1 and P1 fibre wall layers, perpendicular to the fibre axis.
Figure 6. The two different ways of penetration of chemicals into fibre wall structure:
through the secondary wall layers and through the pit pores (Konn 2006)
Note that both of these mechanisms imply penetration of chemicals firstly through the lumen of the fibres and then outwards, through S3, S2, S1 and P1 fibre walls. Penetration of chemicals through pit pores and into the amorphous middle lamellae regions was previously reported to be faster compared to penetration through the secondary fibre wall layers (Konn 2006).
2.2 Conventional chip refining 2.2.1 Softening of wood
Wood at room temperature is brittle and stiff; all three main groups of polymers are below their glass transition points. In order to process wood material without damaging it, the wood structure needs to be softened so that relatively intact fibres can be liberated from the wood matrix. In a traditional TMP process, this is done by means of increasing temperature. Increased moisture content was found to influence the softening temperature of lignin and hemicellulose but not cellulose, due to its crystallinity (Goring 1963). The plastization of lignin by water was proposed to happen due to the replacement of intramolecular hydrogen bonding within the lignin by lignin‐water bonding linkages (Irvine 1984). Both the
hemicellulose and amorphous phases of the cellulose matrix are above their softening points under wet conditions already at 20 °C (Cousins 1978). Thus, it is proposed that the softening behaviour of lignin influences the refining process and the quality of produced pulp at the conditions under which mechanical and chemimechanical pulps are manufactured (Norgren 2008).
Wood is a viscoelastic material, thus its softening properties are influenced by the frequency of the mechanical action it is subjected to. The softening temperature of lignin is around 90 °C under normal conditions. However, at higher loading frequencies, the lignin softening is shifted towards higher temperature. In a chip refiner, where frequency can be around 30‐40 kHz, the softening temperature of lignin is approximately 130 °C (Becker et al. 1977).
The softening temperature of lignin is influenced by chemical modifications. The mechanism is thought to be swelling of the polymer structure through the introduction of charged groups, which leads to higher moisture content. Water uptake in native lignin is restricted to approximately 5% (Back and Salmén 1982).
Chemical modifications, such as sulphonation, carboxylation and introduction of other types of ionic groups, lead to increased charge and increased swelling (Salmén and Berthold 1997). The softening temperature of a polymeric material can thereby be lowered in proportion to the extent of the chemical treatment, see examples in Figs. 7 and 8 (Atack and Heitner 1979, Corson and Fontebasso 1990, Salmén 1995).
Figure 7. Softening temperature of residual lignin in spruce as a function of the degree of sulphonation (Atack and Heitner 1979).
Figure 8. The softening index for wood having different counterions to the charged groups as a function of the content of charged groups in wood subjected to sulphonation or peroxide treatment (Salmén 1995)
The wood softening also depends on the counter ion to the charged groups (Scallan 1983, Salmén 1995, Hammar et al. 1995). More then 50% of the carboxylic groups in softwood were shown to be bound to metallic ions (Hammar et al. 1995).
In the natural state of the wood the counter ions are calcium, if the wood was treated with chemicals they can for example be sodium (conventional peroxide beaching) or magnesium (bleaching using magnesium hydroxide). Swelling of wood fibres is influenced by the counter ions in the following order:
Li+ > Na+ > K+ > Ca2+ > Al3+
A metal counter ion with higher charge generally leads to less softening, less swelling and also an increased energy demand in refining (Hammar et al. 1995).
2.2.2 Mechanisms of chip refining
Refining of wood chips into pulp can be described by three distinct events (Luhde 1962):
¾ Decomposition of wood chips into a coarsely reduced form, at the entrance of the refining zone, to ensure uninterrupted feeding
¾ Disintegration of those coarse fibre bundles into papermaking fibres
¾ Refining of the separated fibres
The conventional TMP refining process can be concluded to consist of two distinct phases: defibration of wood chips into fibres and fibrillation (internal and
external) of those fibres (Campbell 1934, Koran 1981, Kano et al. 1982, Marton and Eskelinen 1982). Thus, the goals of the refining process are (Atack 1981):
¾ To reduce wood into its constituent fibres
¾ To retain the integrity of a considerable fraction of these fibres
¾ To induce a desired amount of flexibility and fibrillation into the separated fibres and fine fibre fragments
In conventional refining, defibration occurs in the breaker bar zone of primary refiner, where wood chips are transformed into coarse fibre bundles. This process involves large plastic deformations where fibres are liberated from the wood matrix. Only a small fraction of the total refining energy is converted during defibration of chips into fibres and the energy demand in defibration was reported to be almost the same for all raw materials (Corson 1989). Conditions during defibration are crucial for determining the final properties of the produced pulp (Miles and Karnis 1995). There are two main aspects of how defibration influences the final pulp properties. The magnitude of the plastic deformations during defibration (i.e. refining intensity) influences the particle size distribution of the liberated fibres. A certain intensity is required to initiate the plastic deformations and separate the fibres from the wood matrix. Increased intensity leads to increased amount of plastic deformations but also to a decrease in average fibre length of liberated fibres (Strand 1997). Where in the chips defibration occurs also has a strong influence on the final pulp properties. If defibration proceeds mainly through the middle lamellae, coarse thick fibres with intact lignin layers on the surfaces will be liberated from the wood matrix. If the defibration proceeds deeper inside the fibre structure, through S1 or S2 fibre walls, the resulting fibres are thinner and more fibrillated. At the same time, more fine material, originating from the outer parts of the fibres is created. Thus, the mode of defibration influences the starting material for further fibre development in refining. Different modes of chip defibration are depicted in Fig. 9. Softening of wood material prior to defibration has major influence on where the fracture zones will be located. In a conventional TMP process, where increased moisture content and temperature are the mechanisms of wood softening, the fracture zones are located within the secondary walls of the fibres (Kibblewhite 1981, Franzén 1984, Johnsen et al. 1995). If wood chips are impregnated with chemicals, as in the CTMP process, the softening due to swelling of the reactive middle lamellae and the primary wall regions is predominant and the defibration fracture zones are located there. Thus it can be concluded that softening of wood has a major influence on the properties of fibres liberated from the wood matrix during the defibration of chips.
Figure 9. Fracture zones forming upon defibration of wood chips under different conditions (Franzén 1984, modified by Htun and Salmén 1996).
When wood chips are separated into individual fibres, the second phase of refining called fibre development can start, Fig. 10. Those two stages overlap to some extent. The vast majority of the energy applied in the refining process is converted during fibre development (Campbell 1934, Neill and Beath 1963, May 1973, Höglund et al. 1976, Leider and Nissan 1977, Kurdin 1979, Atalla and Wharen 1980, Koran 1981, Eskelinen et al. 1982, Kano et al. 1982, Jackson 1985, Karnis 1994). During the fibre development phase, P and S1 layers are peeled off the surface of individual fibres, exposing the S2 layer (Reme 2000). This has two implications; firstly, the fines fraction is created from the peeled‐off material and secondly, fibrils are created on the surfaces of the long fibres which also decrease in their transverse dimensions. Fibre wall thickness was shown to be reduced through progressive refining (Jang et al. 1995, Johnsen et al. 1995, Kure 1997, Mohlin 1997). Fibres are also developed internally through induction of small cracks and delamination points within the fibre wall structure. Internal fibre development proceeds through elastic, viscoelastic and small plastic straining (Reme 2000). All of the mechanisms mentioned above lead to development of fibre properties needed in papermaking. Flexible, easily conformable and collapsible fibres with good bonding surface are created. The fines fraction formed in the refining process contributes to the papermaking properties of the pulps through increasing the bonding and light scattering power of produced paper.
Figure 10. Phases of mechanical pulp refining (Sundholm 1993)
First stage refining was on many occasions claimed to create a “fingerprint” in the final pulp quality (Leask 1981, Corson 1989, Höglund and Wilhelmsson 1993, Karnis 1994, Heikkurinen et al. 1993, Stationwala et al. 1993, Høydahl et al. 1995).
Since it is the defibration phase that influences both the particle size distribution and the character of fibres, liberated from wood matrix, it can probably be claimed that it is more precisely the defibration phase that creates that “fingerprint”.
Although both defibration and fibre development are today conducted in the same equipment (first stage refiner) it would seem beneficial to separate these two refining phases. When defibration and fibre development are considered separately, they can be optimized independently to reach better energy‐efficiency and quality of produced pulp at the same time as the “fingerprint” or, rather, character of the final pulp can be controlled in a more efficient way.
It was earlier concluded that the degree of softening of wood matrix has a major influence on the properties of fibres liberated during defibration of chips. Softening of already liberated fibres should also influence their development in the process of refining. This has not been studied so extensively in the past due to the simple reason that, as mentioned earlier, both defibration and fibre development proceeds today in the same equipment. It was therefore difficult to obtain fibres that were liberated from the wood matrix (i.e. defibrated chips), but where the development of their properties had not yet started. The energy needed for defibration, was calculated to be in the order of magnitude of 0.05‐0.4 MWh/odt or about 10‐20% of the total refining energy (Van der Akker 1958, Atack et al. 1961, Lamb 1962, Neill and Beath 1963, Leider et al. 1977, Kano et al. 1982, Karnis 1994). In an earlier study, where development of already separated fibres in a PFI beater was enhanced with the help of different chemicals, reduction of energy demand in
beating by over 50% was reported (Chang et al. 1979). Fibres were liberated from wood matrix using 470 kWh/odt of energy, which is in the range of the theoretical estimation of defibration energy demand. Studies where fibre softening was increased prior to reject refining can also offer a clue as to how much effect increased softening has on energy demand in fibre development. Both chemicals which introduce sulphonic acid groups (Gummerus 1987, Goel 1987, Nurminen and Sundholm 1995) and carboxylic acid groups (Sferrazza 1988, Strunk et al. 1986, 1990) as well as increased refining temperature (Höglund et al. 1997, Norgren 2008) can be used to increase fibre softening. Results suggest improvement in energy efficiency in refining by 0.2‐0.5 MWh/odt or 10‐25 % and improvement in the quality of produced pulp. It can be concluded that softening of individual fibres after chip defibration has a positive effect on refining energy‐efficiency.
Carboxylation during hydrogen peroxide bleaching usually increases the content of carboxylic groups in the pulp from approximately 100 to 150‐200 μekv/g. This increase should reduce the softening index from 97 to 94‐96 RH° depending on the counter‐ion in the wood (Salmén 1995). In order to selectively influence the softening of wood material prior to fibre development but after defibration, those two refining phases obviously need to be conducted in separate process stages.
This has been suggested on multiple occasions in the literature during the last 30 years (Salmén 1982, Sabourin 2003), but no such process exists today.
2.2.3 Character and quality of refined pulp
Paper made from mechanical pulps can be characterized by high light scattering ability, good strength and smoothness, fairly high brightness and high bulk (Sundholm 1999). The combination of good strength and very high light scattering ability is what makes thermo‐mechanical pulp unique. High light scattering ability is obtained through a high content of fine material, produced during refining of chips into pulp. Figs. 11 and 12 illustrate the difference between different mechanical and chemical pulps. Increased content of fine material leads to increased light scattering ability due to increase in the amount of available surface.
Note that the light scattering ability of pulps produced using purely mechanical methods (TMP, PGW and GW) is considerably higher compared to CTMP, which was also produced using refining. The reason for this difference was previously illustrated in Fig. 9. Impregnation of chips with chemicals leads to defibration in the middle lamellae; this is unfavourable from the printing paper point of view due to decreased fines content and therefore also light scattering ability. It can be suggested that in mechanical pulping, the conditions under which the defibration of chips is carried out defines the character of produced pulp.
Figure 11. Fibre fraction distributions of different mechanical pulps (Sundholm 1999)
Figure 12. Relationships between the light scattering coefficient and freeness for various pulps (Sundholm 1999).
Mechanical pulps are mainly used for production of printing papers. These papers can be characterized by good opacity and printability at low basis weight.
The major grades are newsprint, LWC (light weight coated) and SC (supercalendered) papers. CTMP is mainly used for board grades, soft tissue and
absorbent where high bulk is required and high light scattering ability is not that important. The relationship between light scattering coefficient and tensile index for TMP and CTMP is illustrated in Fig. 13. As can be seen, light scattering decreases at an equal tensile index with increasing degree of chemical pre‐
treatment of chips; the TMP pulp looses its TMP character and becomes more and more “CTMP‐like”. This happens because chemical pre‐treatment of chips influences the defibration mechanisms, as discussed earlier.
Figure 13. Relationship between light scattering coefficient and tensile index for mechanical pulps without chemical pre-treatment with sulphite (TMP) and with various degrees of sulphite pre-treatment (CTMP) (Atack et al. 1980)
Chemical treatment in CTMP manufacturing also has an effect on the content of shives in mechanical pulps. As can be seen in Fig. 14, the content of shives in CTMP is considerably lower than TMP. This is explained by increased defibration of chips and easier liberation of fibres from the wood structure when chips are softened by the swelling action of chemicals.
Figure 14. Development of shive content as a function of pulp freeness in mechanical (TMP) and chemimechanical (CTMP) pulping (Åkerlund and Jackson 1984).
The most common raw material used for production of mechanical pulps today is spruce. Pine has several disadvantages when compared to spruce as a raw material for mechanical pulping: it requires 10‐30 % more energy to refine and paper produced from it has 10‐25 % lower tensile and tear strength (Lindström et al. 1977, Härkönen et al. 1989). These properties can be explained by thick‐walled fibres (Yuan et al. 2006, Fernando and Daniel 2008) and higher extractives content (Reme 2000). In an earlier study, it was discovered that the initial defibration mechanisms differ significantly for spruce and pine (Fernando and Daniel 2008). Pine defibrates easier, but further out in the fibre wall towards the middle lamellae while spruce requires more energy to defibrate, but the fracture occurs closer to the lumen.
2.3 Mechanical pre-treatment
There are a number of publications dealing with compressive pre‐treatment prior to refining but few of them give a comprehensive overview of what has been done in this field. A review paper (Paper I) was written on this subject.
There exists a certain contradiction in the description of the goals of conventional chip refining. In the defibration phase, large plastic deformations in the chips are desired since these are the mechanism of chip defibration. During the fibre development, on the other hand, large plastic deformations often mean that fibres are damaged and average fibre length is decreased (Koran 1980). Carefully induced small plastic and elastic cyclic deformations are better suited for fibre development (Salmén et al. 1985, 1997). This is called fatigue and it is the main mechanism of refining used today both for defibration and fibre development (Hartler 1980). However, fatigue is obviously not the most effective way to induce
large plastic deformations (i.e. to defibrate chips) since a lot of energy is converted to heat in a cyclic fashion. It would be theoretically possible to achieve defibration in a more effective process than refining, via a process that would utilize a lower number of stressing cycles (thereby minimizing energy conversion to heat) and higher stress amplitude to induce permanent plastic deformations. This would also probably be more energy‐efficient in respect to the natural energy‐dissipation mechanism that exists in a tree and prevents the structure from being damaged by low‐amplitude high‐frequency stresses originating from swaying of the tree trunk.
Effects of compressive pretreatment on wood morphology and energy demand in refining were first described in a study of axial pre‐compression of Western hemlock blocks (Frazier and Williams 1982). Reductions in energy demand by 0.16 MWh/odt (9 %) for TMP and 0.7 MWh/odt (40 %) for CTMP were achieved.
Energy‐efficient separation of chips through cracks within S1 or S2 fibre walls, resulting from the pre‐compression, was thought to explain the better pulp properties and reduction of energy demand observed. If the fibres are more separated from one another already at the refiner inlet, more of the refiner energy can be directed to developing fibre properties instead of separating the fibres in a less efficient way. Formation of cracks within the S1 and S2 fibre walls as a result of compressive pre‐treatment was described also in later studies (Sabourin 1998, Kure et al. 1999, Johansson and Dahlqvist 2001), see example in Fig. 15.
Figure 15. Cross-sectional SEM images of wood chips subjected to compressive pre- treatment. The chips in image “b” and “d” are treated with bromium, making the lignin-rich middle lamellae to appear bright compared to the rest of the fibre wall (Kure et al. 1999)
Axial pre‐compression was proposed to lead to shearing of S1, P and middle lamellae between the S2 layers of adjacent cell walls (Booker and Sell 1998). Upon the shearing action, fibre wall delamination occurs at the weakest point which depends on fibril angle, chemical composition, fibre wall thickness and ambient conditions. Other studies, where reduction of electrical energy demand was achieved, were conducted using different equipment such as plug screw feeder (Thornton and Nunn 1978), Bi‐Vis twin screw (de Choudens and Anglier 1987, Kurdin and Tantalo 1987, de Choudens et al 1989) and PREX impregnator (Peng and Granfeldt 1996, Johansson et al 1999). A number of studies of compressive pre‐
treatment were conducted using the Andritz RT Impressafiner, Fig. 15, which was also used as part of compressive pre‐treatment in this study (Kure et al. 1999, Robertsen et al. 2001, Sabourin 1998, Sabourin et al. 2003). Reduction of energy demand in subsequent refining by 5‐15 % or 0.1‐0.3 MWh/odt could be achieved.
Better results were reported to be obtained with pine raw material compared to spruce (Robertsen et al. 2001). This could have been explained by the differences in defibration behaviour between pine and spruce chips (Fernando and Daniel 2008).
Other advantages of using the compressive pre‐treatment of chips prior to refining include more even moisture content (Johansson et al. 1999), more homogeneous chip size distribution (Kurdin and Tantalo 1987) and improved mass transfer of chemicals (de Choudens et al. 1985). Compressive pre‐treatment of wood chips is also an effective way of removing extractives (Tanase 2009). Extractives in wood are contained in resin canals and in parenchyma cells. Compressive pre‐treatment of wood leads to a release of extractives from the wood structure (Thornton and Nunn 1978). It was earlier determined that more then 70% of the parenchyma cells were intact after compression of wood, while more then 90% were damaged after 1st stage refining (Cisneros and Drummond 1995). Thus, pre‐compression of wood leads to a release of extractives, contained mainly in the resin canals (Tanase 2009).
Extractives are known to interfere with refining and possibly have a negative impact on energy consumption in refining and pulp quality (Engstrand et al. 1995, Reme 2000, Svensson 2007).
Figure 16. RT Impressafiner (Andritz AG).
2.4 Refining at increased intensity
The most common quantitative description of refining action is the Specific Energy, i.e. applied net power per fibre mass throughput. However, this
