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6.1 Introduction 561

6.2 Screening Theory 563

6.2.1 Introduction 563

6.2.2 Flow Regime 564

6.2.3 Fiber Passage and Reject Thickening 566

6.2.4 Selective Fiber Passage 570

6.3 Screening Parameters 572

6.3.1 Equipment Parameters 572

6.3.1.1 Screen Basket 572

6.3.1.2 Rotor 573

6.3.2 Operating Parameters 575

6.3.2.1 Reject Rate 575

6.3.2.2 Accept Flow Rate 575

6.3.2.3 Feed Consistency 577

6.3.2.4 Temperature 577

6.3.2.5 Rotor Tip Velocity 577

6.3.3 Furnish Parameters 578

6.3.3.1 Pulp Fibers 578

6.3.3.2 Contaminants 579

6.3.3.3 Entrained Air 579

6.4 Centrifugal Cleaning Theory 579

6.4.1 Introduction 579

6.4.2 Flow Regime 580

6.4.3 Sedimentation 581

6.4.4 Underflow Thickening 584

6.4.5 Selective Separation 585

6.5 Centrifugal Cleaning Parameters 586

6.5.1 Cyclone Parameters 586

6.5.2 Operating Parameters 587

6.5.2.1 Flow Rate and Pressure Drop 587

6.5.2.2 Feed Consistency 587

6.5.2.3 Temperature 587

6.5.3 Furnish Parameters 587

6.5.3.1 Pulp Fibers 587

6.5.3.2 Contaminants 588

6.6 Separation Efficiency 588

Contents XI

6.6.1 Screening and Cleaning Efficiency 588

6.6.2 Fractionation Efficiency 590

6.6.2.1 Removal Efficiency 590

6.6.2.2 Fractionation Index 591

6.7 Screening and Cleaning Applications 592

6.7.1 Selective Contaminant Removal 592

6.7.1.1 Knots 593

6.7.1.2 Shives 593

6.7.1.3 Bark 593

6.7.1.4 Sand and Stones 593

6.7.1.5 Metals and Plastics 594

6.7.2 Fractionation 594

6.8 Systems for Contaminant Removal and Fractionation 594

6.8.1 Basic System Design Principles 594

6.8.2 Systems for Contaminant Removal 596

6.8.2.1 Arrangement 596

6.8.2.2 Fiber Loss versus Efficiency 598

6.8.3 Systems for Fractionation 599

6.9 Screening and Cleaning Equipment 601

6.9.1 Pressure Screens 601

6.9.2 Atmospheric Screens 604

6.9.2.1 Secondary Knot Screens 604

6.9.2.2 Vibratory Screens 605

6.9.3 Hydrocyclones 605

Volume 2

7 Pulp Bleaching 609

Herbert Sixta, Hans-Ullrich Suss, Antje Potthast, Manfred Schwanninger,

and Andreas W. Krotscheck

7.1 General Principles 609

7.2 Classification of Bleaching Chemicals 610

7.2 Bleaching Operations and Equipment 613

Andreas W. Krotscheck

7.2.1 Basic Rheology of Pulp-Liquor Systems 614

7.2.2 Generic Bleaching Stage Set-Up 616

7.2.3 Medium Consistency Pumps 617

7.2.4 Medium Consistency Mixers 619

7.2.4.1 High-Shear Mixers 620

7.2.4.2 Static Mixers 621

7.2.4.3 Atmospheric Steam Mixers 622

7.2.5 Medium Consistency Reactors 623

7.2.5.1 Atmospheric Upflow Reactors 623

XII Contents

7.2.5.2 Atmospheric Downflow Reactors 624

7.2.5.3 Pressurized Reactors 625

7.2.6 Blowtank 627

7.2.7 Agitators 627

7.2.8 Washing 628

7.3 Oxygen Delignification 628

7.3.1 Introduction 628

7.3.2 Chemistry of Oxygen Delignification 632

Manfred Schwanninger

7.3.2.1 Bleachability 634

7.3.2.2 Lignin Structures and their Reactivity 634

7.3.2.3 Oxygen (Dioxygen) and its Derivatives 641

7.3.2.4 A Principal Reaction Schema for Oxygen Delignification 649

7.3.2.5 Carbohydrate Reactions in Dioxygen-Alkali Delignification

Processes 657

7.3.2.6 Residual Lignin–Carbohydrate Complexes (RLCC) 666

7.3.2.7 Inorganics (Metals) and their Role in the Protection/

Degradation of Cellulose 668

7.3.3 Mass Transfer and Kinetics 671

Herbert Sixta

7.3.3.1 Kinetics of Delignification 672

7.3.3.2 Kinetics of Cellulose Chain Scissions 685

7.3.3.3 Application of Surfactants 687

7.3.4 A Model to Predict Industrial Oxygen Delignification 688

7.3.4.1 Theoretical Base of the van Heiningen Model 690

7.3.4.2 Case Study 695

7.3.5 Process Variables 701

7.3.5.1 Temperature 701

7.3.5.2 Retention Time 702

7.3.5.3 Alkali Charge 703

7.3.5.4 pH Value 704

7.3.5.5 Final pH 705

7.3.5.6 Alkali Source 706

7.3.5.7 Oxygen Charge, Oxygen Pressure 707

7.3.5.8 Consistency 708

7.3.6 Pulp Components and Impurities 708

7.3.6.1 Effect of Metal Ion Concentration 708

7.3.6.2 Residual Lignin Structures 713

7.3.6.3 Carry-Over 716

7.3.6.4 Xylan Content 719

7.3.6.5 Selectivity of Oxygen Delignification 720

7.3.7 Process and Equipment 721

7.3.7.1 MC versus HC Technology 721

7.3.7.2 Process Technology 722

7.3.7.3 Process Equipment 731

Contents XIII

7.3.8 Pulp Quality 733

7.4 Chlorine Dioxide Bleaching 734

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