Papermaking Science and Technology a book series covering the latest technology and future trends Book 5 Mechanical Pulping Second Edition Totally updated version Book editor Bruno Lönnberg, D.Sc. (Tech.), Professor (emeritus), Åbo Akademi University Publisher Paper Engineers Association/Paperi ja Puu Oy Paperi ja Puu Oy
Table of Contents 1 Introduction...9 2 Idea of mechanical pulping...17 3 History of mechanical pulping...23 4 Fundamentals of mechanical pulping...35 5 Wood raw materials...68 6 Grinding and pressure grinding...116 7 Thermomechanical pulping...174 8 Chemimechanical pulping...247 9 Screening and cleaning...282 10 Reject refining...327 11 Bleaching of mechanical pulps...360 12 Thickening, storage and post-refining...399 13 Flowsheets for various mechanical pulping and screening processes...419 14 Environmental impacts of mechanical pulping...430 15 The character and properties of mechanical pulps...456 16 Future outlook...515 Conversion factors...535 Index...537 8
CHAPTER 1 Introduction 1 Contents of the book...10 2 How to interpret the data...11 3 Abbreviations used...12 9
CHAPTER 2 Idea of mechanical pulping 1 Wood for mechanical pulping...18 2 Wood treatments in mechanical pulping...18 3 Brief description of the mechanical pulping...20 4 Characteristics of mechanical pulps...21 5 End uses of mechanical pulps...22 17
CHAPTER 3 History of mechanical pulping 1 Invention of the grinding process...24 2 Development of the grinding process...25 2.1 First commercial grinders...25 2.2 First reject refiner...28 2.3 Brown groundwood and chemigroundwood processes...28 2.4 Hot grinding process...28 2.5 Grindstone development...28 2.6 Development of main grinder types...29 2.7 Pressure grinding (PGW)...29 3 Thermomechanical pulping (TMP)...30 3.1 Groundwood reject refining...30 3.2 Original thermomechanical pulping process for hardboards...30 3.3 Development of RMP for printing papers...31 3.4 Development of modern TMP...32 4 Novel methods to produce mechanical pulps...33 5 Development of testing methods...33 References...34 23
CHAPTER 4 Fundamentals of mechanical pulping 1 Rheological behaviour of wood...36 2 Fundamental mechanisms in mechanical pulping, especially grinding...40 2.1 Principles of defibration of wood by grinding...40 2.1.1 Breakdown of the fibre structure by fatigue...41 2.1.2 Removal of fibres from wood by peeling action...43 2.2 Wood structure parameters affecting the breakdown process...44 2.2.1 Thin-/thick-walled fibres...45 2.2.2 Layer structure of the fibre...45 2.2.3 Wood, a viscoelastic composite polymer...45 2.3 Main physical parameters affecting wood grinding...46 2.3.1 Wood in a cyclic stress field...46 2.3.2 Influence of amplitude, frequency and temperature...47 2.4 Energy consumption in grinding...51 3 Fundamental mechanisms in refining...52 3.1 Development of fibre properties during refining...53 3.2 Traditional descriptions of refining...53 3.3 Theoretical approaches to understanding refining...54 3.4 Measured data from the plate gap...55 3.5 Present knowledge and understanding of the fundamentals in refining...57 4 Reasons for differences in energy consumption between grinding and refining...58 4.1 Energy consumption in grinding and refining...58 4.2 Reasons for high energy consumption...58 4.3 Reasons for high energy consumption in chip refining...59 References...62 35
CHAPTER 5 Wood raw materials 1 General wood properties...70 1.1 Wood basic density...71 1.2 Wood moisture content...72 1.3 Juvenile wood...74 1.4 Heartwood...74 1.5 Unsuitable parts of the wood raw material...75 2 Wood species used in mechanical pulping...78 2.1 Wood and fibre characteristics...79 2.1.1 Softwoods: spruces and pines...79 2.1.2 Hardwoods: poplars and eucalyptus...80 2.2 Properties of thermomechanical pulps...82 2.2.1 Softwoods...82 2.2.2 Hardwoods...84 2.3 Properties of groundwood pulps...84 2.3.1 Softwoods...85 2.3.2 Hardwoods...88 3 Variations in properties between trees...90 3.1 Genetic control of wood properties...90 3.2 Within-tree property variation...91 3.2.1 Fibre length...92 3.2.2 Fibre width...93 3.2.3 Cell wall thickness...94 3.2.4 Cross-sectional cell wall area...96 3.2.5 Microfibril angle...96 3.3 Within-stand variation...97 68
Wood raw materials 4 Pulp properties of different wood assortments...98 4.1 Pulpwood vs. sawmill chips...99 4.2 Butt log vs. top log...102 4.3 Heartwood vs. sapwood...103 4.4 Slow-grown vs. fast-grown wood...103 5 Seasonal variations in wood properties...104 6 Effect of wood supply practices on wood properties...105 7 Effect of wood handling operations on wood and chip properties...106 References...109 69
CHAPTER 6 Grinding and pressure grinding 1 Introduction...118 2 Grinder productivity...119 3 Two-pocket wood grinders for groundwood processes...121 3.1 Tampella two-pocket atmospheric SGW grinder...121 3.2 Metso atmospheric SGW grinder...121 3.3 Montague two-pocket SGW grinder...123 3.4 Metso pressure grinders...124 3.5 Continuous Metso two-pocket grinder...128 4 Wood grinding processes...128 4.1 Atmospheric groundwood (SGW)...128 4.2 Pressure groundwood processes...129 4.2.1 PGW95 process...132 4.2.2 PGW70 process...134 4.2.3 PGW-S120 process...135 5 Chain grinders and thermogroundwood (TGW)...136 6 Grindstones and grindstone treatments...140 6.1 Ceramic grindstones...140 6.2 Abrasive specifications and their influence on grindstone behaviour...141 6.3 New grinding surfaces...143 6.4 Grindstone treatments...143 6.5 Burr treatments for grindstones...144 6.5.1 Grindstone truing...145 6.5.2 Grindstone sharpening...145 6.5.3 Dulling of grindstone...146 6.5.4 Grindstone grooving...146 116
Grinding and pressure grinding 6.6 Grindstone surface control with ultra-high-pressure water...147 6.6.1 Water jet equipment...149 6.6.2 Grindstone treatment with water jet...149 6.6.3 Impacts of water jet conditioning on grinding process variables and pulp properties 149 6.6.4 Water Jet Control system...151 7 Grinder feeding systems...151 7.1 Feeding system for chain grinders...152 7.2 Charging system for pocket grinders...153 8 Grinding control...155 8.1 Control requirements...155 8.2 Grinding from a control viewpoint...156 8.3 Grinding process...156 8.4 Hierarchy of grinding controls...158 8.4.1 Surface wear compensation...158 8.4.2 Batch control...159 8.5 Grinding control strategy...161 8.5.1 Grinder group control strategy...161 8.5.2 Optimum operating point strategy...163 8.5.3 Water Jet Control...163 9 PGW pulps and various paper grades...164 9.1 Pulp properties...165 9.2 Suitability for various paper grades...166 References...169 117
CHAPTER 7 Thermomechanical pulping 1 Introduction to thermomechanical pulping...176 1.1 Principles of refiner mechanical pulping...176 1.2 Main refiner types...177 1.3 Thermomechanical pulping process...177 1.4 Energy consumption and yield...178 2 Handling and pretreatment of chips...179 2.1 Chip washer...180 2.2 Dewatering of chips...181 2.3 Chip wash water system...181 2.4 Chip preheating...182 2.5 Chip impregnation systems...183 3 Disc refiner...186 3.1 Refiner concepts of different manufacturers...186 3.2 Single-disc (SD) refiners...186 3.2.1 Andritz SB 150 and 170 single-disc refiners...186 3.2.2 Metso RGP SD single-disc refiners...188 3.2.3 Other single-disc refiners: SD 65 and HXD64...189 3.3 High-capacity TMP refiners...192 3.3.1 Metso RGP CD refiner...192 3.3.2 Andritz Twin refiner...194 3.4 Double disc (DD) refiners...196 3.5 Processing pulp and steam in the refiner line...198 3.6 Refining conditions and design parameters...202 4 Refiner segments the heart of the refining process...204 4.1 Refiner segment design...206 4.2 Materials of refiner segments...208 174
Thermomechanical pulping 5 Main TMP process types...209 5.1 Standard TMP line...209 5.2 Single-stage refining...213 5.3 Energy-saving TMP processes...214 5.3.1 High-speed and high-intensity TMP refining...215 5.3.2 Chip pretreatment...217 5.3.3 Third-stage post-refining...218 5.3.4 Energy savings with refiner plate development...218 6 Refiner type and process conditions and their impact on pulp properties...221 6.1 Intensity of refining...221 6.2 Influence of refiner type...222 6.3 Refiner speed...223 6.4 Pulp consistency...224 6.5 Production rate...226 6.6 Preheating and steaming of chips...226 6.7 Temperature in preheating and refining...228 7 Process control...229 7.1 Control requirements...229 7.2 Basic controls...230 7.3 Plate gap control...231 7.4 Production rate...231 7.5 Refining consistency...232 7.6 Refining pressures...232 7.7 Pulp quality control...233 8 Heat recovery...234 8.1 Amount and composition of TMP steam...234 8.2 Reboiler for condensing TMP steam and generating clean steam...236 8.3 Auxiliary heat exchangers...236 8.4 Start-up scrubber for TMP steam...238 8.5 Surface condenser for low-pressure TMP steam...238 8.6 Turpentine recovery...240 8.7 Heat recovery as a part of the low-pressure steam net...242 References...243 175
CHAPTER 8 Chemimechanical pulping 1 Introduction...248 2 Process alternatives...249 2.1 Position of the chemical treatment stage in the process...249 2.2 Type of chemical treatment...250 3 Chemical stage...251 3.1 Sulphonation chemistry...252 3.2 Effect of process variables on the sulphonate content...254 4 The mechanical stage...255 4.1 Dynamic mechanical properties of chemically treated wood and fibres...257 4.2 Kind of rupture at fiberising...257 5 Process design and operation...258 5.1 General schemes...258 5.2 Chemical pretreatment...260 5.3 Refining...262 5.4 Drying...263 5.5 Baling...264 6 Special processes...265 6.1 Interstage sulphonation...265 6.2 Reject sulphonation...265 6.3 Chemical treatment with alkaline peroxide...266 6.3.1 APMP process...267 6.3.2 P-RC APMP process...268 6.4 Alkaline peroxide treatment of reject...268 6.5 Ozone treatment...269 7 Fibre and pulp properties...270 7.1 Fibre properties...270 7.2 Pulp properties...271 8 End uses...275 9 Market pulps...276 References...278 247
CHAPTER 9 Screening and cleaning 1 Background...284 1.1 Principle and objectives of screening...284 1.1.1 Screening and fractionation...286 1.1.2 Screening principle barrier and probability screening...287 2 Design of a pressure screen...288 2.1 General construction...288 2.2 Feed arrangement...290 2.3 Screen basket geometry...290 2.4 Rotor design...291 3 Flows within the screen...292 3.1 Axial flow in screen basket and reject removal...292 3.2 Radial flow in screen basket...293 3.3 Tangential flow in screen basket...293 3.4 Flow near basket surface and apertures...294 3.5 Turbulence, pulsation and backflow...294 4 Characterisation of screening and fractionation...296 4.1 Basic equations...296 4.2 Screening efficiencies...298 4.3 Assessment of performance with operating curves...299 5 Parameters affecting pressure screen performance...300 5.1 Design parameters...300 5.1.1 Rotor design...301 5.1.2 Screen basket geometry...302 5.2 Operating parameters...304 5.3 Furnish parameters...306 6 Energy consumption and power in pressure screening...306 282
Screening and cleaning 7 Screening systems...308 7.1 Main-line screening of SGW and PGW...309 7.2 Main-line screening of TMP and CTMP...310 7.3 Screening of rejects...311 8 Control of screening...312 8.1 Control of a single screen...312 8.2 Control of parallel screens and screens in series...313 8.3 Control of two-stage screening...314 8.4 Control of multi-stage screening...314 8.5 On-line pulp quality measurements and control of screening...314 9 Cleaning and sand removal...316 9.1 Theory...316 9.2 Cleaner types...318 9.3 Cleaning systems...319 9.4 Sand removal...319 9.5 Control of cleaning...321 References...322 283
CHAPTER 10 Reject refining 1 Fibres, equipment and systems...328 2 Reject handling...329 2.1 Thickening...329 2.1.1 Bow screen...330 2.1.2 Screw press...330 2.1.3 Twin-roll press...331 2.1.4 Twin-wire press...333 2.2 Pretreatment...334 3 Reject refiners...335 3.1 Reject refining systems...338 4 Control and operating principles...341 4.1 Main and control variables...342 4.2 Process conditions and disturbances...344 4.2.1 HC refining...347 4.2.2 LC refining...349 5 Effects of reject refining on pulp and paper properties...350 5.1 Printing paper grades...354 5.2 Board grades...355 5.3 Hardwood CTMP...355 References...357 327
CHAPTER 11 Bleaching of mechanical pulps 1 Bleaching or brightening...362 2 Factors affecting the brightness of high-yield pulps...362 3 Principles of bleaching high-yield pulps...366 4 Peroxide bleaching...367 4.1 Reactions...367 4.2 Process variables...368 4.2.1 Peroxide dose...368 4.2.2 Removal of transition metal ions...369 4.2.3 Pulp ph...370 4.2.4 Stabilising agents...373 4.2.5 Consistency...375 4.2.6 Temperature...376 4.2.7 Retention time...376 4.2.8 Acidification...376 4.3 Bleach plant design and operation...377 4.3.1 Single-stage medium-consistency bleaching...377 4.3.2 High-consistency bleaching...378 4.3.3 Medium- and high-consistency peroxide bleaching...379 4.3.4 Refiner bleaching...381 4.3.5 Flash dryer bleaching...381 5 Dithionite (hydrosulphite) bleaching...381 5.1 Reactions...381 5.2 Process variables...383 5.2.1 Dithionite (hydrosulphite) dose...383 5.2.2 Chelating agents...384 5.2.3 Pulp ph...384 360
Bleaching of mechanical pulps 5.2.4 Consistency...384 5.2.5 Temperature...385 5.2.6 Retention time...385 5.3 Bleach plant design and operation...385 5.3.1 Tower bleaching...385 5.3.2 Two-stage bleaching with peroxide and dithionite...385 5.3.3 Chest bleaching...387 5.3.4 Refiner and grinder bleaching...387 6 Other bleaching chemicals...388 6.1 Potential bleaching chemicals...388 6.2 Sodium bisulphite...388 6.3 Formamidine sulphinic acid...389 7 Bleaching of chemimechanical pulps...389 8 Brightness reversion...390 8.1 General...390 8.2 Determination of brightness reversion...391 8.3 Thermal brightness reversion...392 8.4 Light-induced brightness reversion...393 9 Effect on yield and papermaking properties...393 9.1 Dithionite (hydrosulphite) bleaching...393 9.2 Peroxide bleaching...394 10 Development of the bleaching technologies...395 References...396 361
CHAPTER 12 Thickening, storage and post-refining 1 Thickening...400 1.1 Disc filters...400 1.2 Broke deckers...402 1.3 Bow screens...402 1.3.1 Screw presses, twin-roll presses and twin-wire presses...403 2 Storage of mechanical pulps...404 2.1 Storage systems...404 2.2 Effect of storage on pulp properties...405 3 Post-refining...406 3.1 Objectives of post-refining...406 3.2 Equipment and process systems...407 3.3 Refining conditions and operations...408 3.4 Refining effects...413 References...418 399
CHAPTER 13 Flowsheets for various mechanical pulping and screening processes 1 Groundwood and thermomechanical pulping processes...420 1.1 SGW and PGW pulp screening processes...422 1.2 TMP and CTMP pulp screening processes...422 2 Mechanical pulping processes for newsprint-paper grades...422 2.1 Newsprint-grade PGW pulp screening processes...423 2.2 Newsprint-grade TMP pulp screening processes...424 3 Mechanical pulping processes for SC- and LWC-paper grades...425 3.1 SC- and LWC-grade SGW and PGW pulp screening processes...425 3.2 SC- and LWC-grade TMP pulp screening processes...426 4 Mechanical pulping processes for board grades...427 4.1 Board-grade SGW and PGW pulp screening processes...428 4.2 Board-grade TMP and CTMP pulp screening processes...428 References...429 419
CHAPTER 14 Environmental impacts of mechanical pulping 1 Woodyard, debarking, pulping and bleaching processes...432 2 Emissions from processes...433 3 Wood and chip storage...435 3.1 Trends in techniques...435 3.2 Changes in wood during storage...435 4 Debarking and chipping...436 4.1 Wet and dry debarking...436 4.2 Water circulation systems in debarking...436 4.3 Need for water...437 5 Treatment of debarking effluent...437 5.1 Composition and concentration of debarking circulation water...437 5.2 Effluent load...438 6 Release of wood components into water in mechanical pulping and bleaching...438 6.1 Discharge data for different pulping processes...438 6.2 Influence of bleaching stages...439 6.3 Chemical character of released wood components...440 6.4 Toxic components in effluents...443 7 Factors influencing release of wood components into water...443 7.1 Effect of pulping conditions...443 7.2 Effects of wood quality...444 8 Evaporation of compounds in mechanical pulping...445 8.1 Volatility of wood compounds...445 8.2 Contents of volatile compounds...445 9 Water circulation systems in mechanical pulping plants...446 9.1 Connection between pulp mill and paper mill...446 9.2 Washing stage...447 430
Environmental impacts of mechanical pulping 10 Treatment of mechanical pulping effluents...448 10.1 Purification methods...448 10.2 Potential for installing internal cleaning stages in a mechanical pulp plant...449 10.3 Zero effluent technology...449 11 Carbon footprint of a mechanical printing paper...450 References...452 431
CHAPTER 15 The character and properties of mechanical pulps 1 Origin of mechanical pulp properties...458 2 Mechanical pulp properties and paper/board grades...461 2.1 Mechanical printing papers...461 2.1.1 Newsprint...464 2.1.2 SC paper...465 2.1.3 LWC paper...466 2.2 Mechanical-pulp board...466 3 Development of pulp properties...467 3.1 Fibre shortening...467 3.2 Fibre-wall thickness and structure...468 3.3 Pulp fractions and fibre properties...469 3.3.1 Long-fibre fraction...470 3.3.2 Middle fraction...470 3.3.3 Fines fraction...471 3.3.4 Properties of fractions for pulp characterisation...474 3.4 Latency in mechanical pulps...476 3.5 Mechanical pulp fibres and paper recycling...476 4 Properties of pulp suspensions and their characterisation...478 4.1 Pulp property versus method...478 4.2 Methods for determining drainability...479 4.2.1 Canadian Standard Freeness, Schopper-Riegler value and drainage time...479 4.2.2 Water retention value...480 4.2.3 Specific filtration resistance, specific surface area and settling rate...480 4.2.4 Water quality and its effect on drainage...481 4.3 Methods for determining fractional composition...482 4.4 Methods for determining shive content...485 4.5 Methods for determining fines properties...486 456
The character and properties of mechanical pulps 5 Fibre properties and characterisation methods...488 5.1 Fibre property versus method...488 5.2 Methods for determining fibre dimensions...489 5.2.1 Fibre length...489 5.2.2 Fibre form...491 5.2.3 Cross-sectional dimensions of fibres...492 5.3 Methods for determining fibre coarseness...494 5.4 Methods for determining fibre wall structure...495 5.4.1 Fibre wall damage...495 5.4.2 Internal fibrillation...496 5.4.3 Fibre wall dimensions along the fibre length axis...497 5.4.4 Flexibility and stiffness of fibres...498 5.5 Methods for determining fibre surface properties...498 5.5.1 External fibrillation...498 5.5.2 Fibre surface roughness...499 5.5.3 Chemical composition of fibre surfaces...500 6 Handsheet properties and paper testing...501 6.1 Preparation of handsheets...501 6.1.1 Handsheets for testing physical properties except optical properties...501 6.1.2 Handsheets for testing optical properties...502 6.2 Testing of handsheet properties...502 6.3 Typical sheet properties of mechanical pulps...503 References...505 457
CHAPTER 16 Future outlook 1 Future demand for mechanical pulps...516 1.1 Development of paper demand 2010 2020...516 1.2 Furnish trends...518 1.3 Production of mechanical pulps...520 1.4 Future consumption of mechanical pulps...522 2 Paper and mechanical pulp quality development...522 2.1 General quality characteristics...522 2.2 Newsprint...522 2.3 Supercalendered (SC) papers...523 2.4 Coated mechanical and coated wood-free papers...523 2.5 Carton boards...523 3 Energy consumption of GW and TMP...524 3.1 General trends...524 3.2 Possibilities to reduce specific energy consumption...524 3.3 Consequences of electric energy savings...526 4 Impact of the increasing use of recycled fibre...527 5 Raw material availability...528 6 Process development...530 6.1 Pulping capacity development...530 6.2 Advantages and disadvantages of various mechanical pulping processes...531 6.3 SWOT analysis for mechanical pulping...531 References...533 515