- •2006, Isbn 3-527-30997-7
- •Isbn-13: 978-3-527-30999-3
- •Isbn-10: 3-527-30999-3
- •Volume 1
- •1.1 Introduction 3
- •Isbn: 3-527-30999-3
- •2.2 Outlook 59
- •4.1 Introduction 109
- •4.2.5.1 Introduction 185
- •4.3.1 Introduction 392
- •5.1 Introduction 511
- •6.1 Introduction 561
- •6.2.1 Introduction 563
- •6.4.1 Introduction 579
- •Volume 2
- •7.3.1 Introduction 628
- •7.4.1 Introduction 734
- •7.5.1 Introduction 777
- •7.6.1 Introduction 849
- •7.10.1 Introduction 887
- •8.1 Introduction 933
- •1 Introduction 1071
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and
- •III Recovered Paper and Recycled Fibers 1147
- •1 Introduction 1149
- •2.2 Inorganic Components 1219
- •2.3 Extractives 1224
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •4680 Lenzing
- •Isbn: 3-527-30999-3
- •4860 Lenzing
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1.2 The History of Papermaking
- •1 Introduction
- •1.2 The History of Papermaking
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.3 Technology, End-uses, and the Market Situation
- •1 Introduction
- •1.5 Outlook
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •1.5 Outlook
- •1 Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Void volume
- •Void volume fraction
- •Xylan and Fiber Morphology
- •Initial bulk residual
- •4.2.5.1 Introduction
- •In (Ai) Model concept Reference
- •Initial value
- •Validation and Application of the Kinetic Model
- •Inititial
- •Volume.
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •4.3.4.2.1 Cellulose
- •Influence of Substituents on the Rate of Hydrolysis
- •140 116 Total so2
- •Xylonic
- •Viscosity Brightness
- •Xyl Man Glu Ara Furf hoAc XyLa
- •Initial NaOh charge [% of total charge]:
- •864 (Hemicelluloses), 2004: 254.
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Xylosec
- •Xylan residues
- •Viscosity
- •Introduction
- •Viscosity
- •Viscosity
- •Introduction
- •Initiator Promoter Inhibitor
- •Viscosity
- •Viscosity
- •Viscosity
- •Introduction
- •Viscosity
- •Introduction
- •Intra-Stage Circulation and Circulation between Stages
- •Implications of Liquor Circulation
- •Vid Chalmers Tekniska
- •Introduction
- •It is a well-known fact that the mechanical properties of the viscose fibers
- •Increase in the low molecular-weight fraction [2]. The short-chain molecules represent
- •Isbn: 3-527-30999-3
- •In the cooking process or, alternatively, white liquor can be used for the cold
- •Is defined as the precipitate formed upon acidification of an aqueous alkaline solution
- •934 8 Pulp Purification
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution 935
- •Is essentially governed by chemical degradation reactions involving endwise depolymerization
- •80 °C [12]. Caustic treatment: 5%consistency ,
- •30 Min reaction time, NaOh concentrations:
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution
- •80 °C is mainly governed by chemical degradation reactions (e.G. Peeling reaction).
- •Investigated using solid-state cp-mas 13c-nmr spectroscopy (Fig. 8.4).
- •Indicates cleavage of the intramolecular hydrogen bond between o-3-h and o-5′,
- •8 Pulp Purification
- •Interaction between alkali and cellulose, a separate retention tower is not really
- •In the following section.
- •3% In the untreated pulp must be ensured in order to avoid a change in the supramolecular
- •8.3 Cold Caustic Extraction
- •Xylan content [%]
- •8 Pulp Purification
- •Is calculated as effective alkali (ea). Assuming total ea losses (including ea consumption
- •Xylan content [%]
- •8.3 Cold Caustic Extraction
- •120 °C (occasionally 140 °c). As mentioned previously, hce is carried out solely
- •Involved in alkaline cooks (kraft, soda), at less severe conditions and thus avoiding
- •8.4Hot Caustic Extraction 953
- •954 8 Pulp Purification
- •120 Kg NaOh odt–1, 90–240 min, 8.4 bar (abs)
- •8.4Hot Caustic Extraction 955
- •956 8 Pulp Purification
- •Into the purification reaction, either in the same (eo) or in a separate stage
- •960 8 Pulp Purification
- •8.4.1.5 Composition of Hot Caustic Extract
- •8.4Hot Caustic Extraction 961
- •Isbn: 3-527-30999-3
- •Xyloisosaccharinic acid
- •Inorganicsa
- •Inorganic compounds
- •Value (nhv), which better reflects the actual energy release, accounts for the fact
- •968 9 Recovery
- •It should be noted that the recycling of bleach (e.G., oxygen delignification) and
- •9.1 Characterization of Black Liquors 969
- •9.1.2.1 Viscosity
- •9.1.2.3 Surface Tension
- •9.1.2.5 Heat Capacity [8,11]
- •9.2 Chemical Recovery Processes
- •Is described by the empirical equation:
- •9 Recovery
- •Vent gases from all areas of the pulp mill. From an environmental perspective,
- •9.2.2.1 Introduction
- •In the sump at the bottom of the evaporator. The generated vapor escapes
- •Incineration, whereas sulphite ncg can be re-used for cooking acid preparation.
- •9 Recovery
- •Values related to high dry solids concentrations. The heat transfer rate is pro-
- •9.2 Chemical Recovery Processes
- •9.2.2.3 Multiple-Effect Evaporation
- •7% Over effects 4 and 5, but more than 30% over effect 1 alone.
- •9.2 Chemical Recovery Processes
- •Increasing the dry solids concentration brings a number of considerable advantages
- •9.2.2.4 Vapor Recompression
- •Is driven by electrical power. In general, vapor coming from the liquor
- •Vapor of more elevated temperature, thus considerably improving their performance.
- •9 Recovery
- •Is typically around 6 °c. The resulting driving temperature difference
- •Is low, and hence vapor recompression plants require comparatively large heating
- •Vapor recompression systems need steam from another source for start-up.
- •9 Recovery
- •Its temperature is continuously falling to about 180 °c. After the superheaters,
- •In the furnace walls, and only 10–20% in the boiler bank. As water turns into
- •9.2.3.1.2 Material Balance
- •Is required before the boiler ash is mixed. In addition, any chemical make-up
- •In this simplified model, all the potassium from the black liquor (18 kg t–1
- •Values for the chemicals in Eq. (11) can be inserted on a molar basis, equivalent
- •9.2 Chemical Recovery Processes
- •Input/output
- •9 Recovery
- •9.2.3.1.3 Energy Balance
- •In the black liquor, from water formed out of hydrogen in organic material, and
- •9.2 Chemical Recovery Processes
- •9.2.3.2 Causticizing and Lime Reburning
- •9.2.3.2.1 Overview
- •9.2.3.2.2 Chemistry
- •986 9 Recovery
- •Insoluble metal salts are kept low. Several types of filters with and without lime
- •Is, however, not considered a loss because some lime mud must be
- •988 9 Recovery
- •In slakers and causticizers needs special attention in order to avoid particle disintegration,
- •9.2 Chemical Recovery Processes 989
- •Ing disks into the center shaft, and flows to the filtrate separator. There, the white
- •9.2.3.2.4 Lime Cycle Processes and Equipment
- •It is either dried with flue gas in a separate, pneumatic lime mud dryer or is fed
- •990 9 Recovery
- •Its temperature falls gradually. Only about one-half of the chemical energy in the
- •9.2.3.3.2 Black Liquor Gasification
- •Inorganics leave the reactor as solids, and into high-temperature techniques,
- •In the bed. Green liquor is produced from surplus bed solids. The product gas
- •992 9 Recovery
- •Incremental capacity for handling black liquor solids. The encountered difficulties
- •10% Of today’s largest recovery boilers. When the process and material issues are
- •9.2 Chemical Recovery Processes 993
- •9.2.3.3.3 In-Situ Causticization
- •Is still in the conceptual phase, and builds on the formation of sodium titanates
- •9.2.3.3.4 Vision Bio-Refinery
- •Into primary and secondary recovery steps. This definition relates to the recovery
- •994 9 Recovery
- •Is largely different between sulfite cooking bases. While magnesium and
- •Introduction
- •In alkaline pulping the operation of the lime kiln represents an emission source.
- •Isbn: 3-527-30999-3
- •Is by the sophisticated management of these sources. This comprises their collection,
- •Ions, potassium, or transition metals) in the process requires the introduction
- •Industry”. Similarly guidelines for a potential kraft pulp mill in Tasmania [3]
- •Initially, the bleaching of chemical pulp was limited to treatment with hypochlorite
- •In a hollander, and effluent from the bleach plant was discharged without
- •In a heh treatment and permitted higher brightness at about 80% iso (using
- •Increasing pulp production resulted in increasing effluent volumes and loads.
- •10.2 A Glimpse of the Historical Development 999
- •It became obvious that the bleaching process was extremely difficult to operate in
- •In a c stage was detected as aox in the effluent (50 kg Cl2 t–1 pulp generated
- •1% Of the active chlorine is converted into halogenated compounds (50 kg active
- •In chlorination effluent [12] led to the relatively rapid development of alternative
- •1000 10 Environmental Aspects of Pulp Production
- •10.2 A Glimpse of the Historical Development
- •In 1990, only about 5% of the world’s bleached pulp was produced using ecf
- •64 Million tons of pulp [14]. The level of pulp still bleached with chlorine
- •10 000 Tons. These are typically old-fashioned, non-wood mills pending an
- •In developed countries, kraft pulp mills began to use biodegradation plants for
- •10 Environmental Aspects of Pulp Production
- •Indeed, all processes are undergoing continual development and further improvement.
- •Vary slightly different depending upon the type of combustion unit and the fuel
- •10.3Emissions to the Atmosphere
- •Volatile organic
- •In 2004 for a potential pulp mill in Tasmania using “accepted
- •10 Environmental Aspects of Pulp Production
- •Is woodyard effluent (rain water), which must be collected and treated biologically
- •10.4 Emissions to the Aquatic Environment
- •Is converted into carbon dioxide, while the other half is converted into biomass
- •Into alcohols and aldehydes; (c) conversion of these intermediates into acetic acid and
- •10 Environmental Aspects of Pulp Production
- •In North America, effluent color is a parameter which must be monitored.
- •It is not contaminated with other trace elements such as mercury, lead, or cadmium.
- •10.6 Outlook
- •Increase pollution by causing a higher demand for a chemical to achieve identical
- •In addition negatively affect fiber strength, which in turn triggers a higher
- •Introduction
- •2002, Paper-grade pulp accounts for almost 98% of the total wood pulp production
- •Important pulping method until the 1930s) continuously loses ground and finds
- •Importance in newsprint has been declining in recent years with the increasing
- •Isbn: 3-527-30999-3
- •Virtually all paper and paperboard grades in order to improve strength properties.
- •In fact, the word kraft is the Swedish and German word for strength. Unbleached
- •Importance is in the printing and writing grades. In these grades, softwood
- •In this chapter, the main emphasis is placed on a comprehensive discussion of
- •1010 11 Pulp Properties and Applications
- •Is particularly sensitive to alkaline cleavage. The decrease in uronic acid content
- •Xylan in the surface layers of kraft pulps as compared to sulfite pulps has been
- •80% Cellulose content the fiber strength greatly diminishes [14]. This may be due
- •Viscoelastic and capable of absorbing more energy under mechanical stress. The
- •11.2 Paper-Grade Pulp 1011
- •Various pulping treatments using black spruce with low fibril
- •In the viscoelastic regions. Fibers of high modulus and elasticity tend to peel their
- •1012 11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •Viscosity mL g–1 793 635 833 802 1020 868 1123
- •Xylose % od pulp 7.3 6.9 18.4 25.5 4.1 2.7 12.2
- •11 Pulp Properties and Applications
- •Inorganic Compounds
- •11.2 Paper-Grade Pulp
- •Insight into many aspects of pulp origin and properties, including the type of
- •Indicate oxidative damage of carbohydrates).
- •In general, the r-values of paper pulps are typically at higher levels as predicted
- •Is true for sulfite pulps. Even though the r-values of sulfite pulps are generally
- •Is rather unstable in acid sulfite pulping, and this results in a low (hemicellulose)
- •11 Pulp Properties and Applications
- •Ing process, for example the kraft process, the cellulose:hemicellulose ratio is
- •Increases by up to 100%. In contrast to fiber strength, the sheet strength is highly
- •Identified as the major influencing parameter of sheet strength properties. It has
- •In contrast to dissolving pulp specification, the standard characterization of
- •Is observed for beech kraft pulp, which seems to correlate with the enhanced
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Is significantly higher for the sulfite as compared to the kraft pulps, and indicates
- •11.2 Paper-Grade Pulp
- •Xylan [24].
- •11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Introduction
- •Various cellulose-derived products such as regenerated fibers or films (e.G.,
- •Viscose, Lyocell), cellulose esters (acetates, propionates, butyrates, nitrates) and
- •In pulping and bleaching operations are required in order to obtain a highquality
- •Important pioneer of cellulose chemistry and technology, by the statement that
- •11.3 Dissolving Grade Pulp
- •Involves the extensive characterization of the cellulose structure at three different
- •Is an important characteristic of dissolving pulps. Finally, the qualitative and
- •Inorganic compounds
- •11 Pulp Properties and Applications
- •11.3.2.1 Pulp Origin, Pulp Consumers
- •Include the recently evaluated Formacell procedure [7], as well as the prehydrolysis-
- •11.3 Dissolving Grade Pulp
- •Viscose
- •11 Pulp Properties and Applications
- •11.3.2.2 Chemical Properties
- •11.3.2.2.1 Chemical Composition
- •In the polymer. The available purification processes – particularly the hot and cold
- •11.3 Dissolving Grade Pulp
- •In the steeping lye inhibits cellulose degradation during ageing due to the
- •Is governed by a low content of noncellulosic impurities, particularly pentosans,
- •Increase in the xylan content in the respective viscose fibers clearly support the
- •11.3 Dissolving Grade Pulp
- •Instability. Diacetate color is measured by determining the yellowness coefficient
- •Xylan content [%]
- •11 Pulp Properties and Applications
- •Xylan content [%]
- •11.3 Dissolving Grade Pulp
- •11.3 Dissolving Grade Pulp
- •Is, however, not the only factor determining the optical properties of cellulosic
- •In the case of alkaline derivatization procedures (e.G., viscose, ethers). In industrial
- •11.3 Dissolving Grade Pulp
- •Viscose
- •Viscose
- •In order to bring out the effect of mwd on the strength properties of viscose
- •Imitating the regular production of rayon fibers. To obtain a representative view
- •11 Pulp Properties and Applications
- •Viscose Ether (hv) Viscose Acetate Acetate
- •Xylan % 3.6 3.1 1.5 0.9 0.2
- •1.3 Dtex regular viscose fibers in the conditioned
- •11.3 Dissolving Grade Pulp
- •Is more pronounced for sulfite than for phk pulps. Surprisingly, a clear correlation
- •Viscose fibers in the conditioned state related to the carbonyl
- •1038 11 Pulp Properties and Applications
- •In a comprehensive study, the effect of placing ozonation before (z-p) and after
- •Increased from 22.9 to 38.4 lmol g–1 in the case of a pz-sequence, whereas
- •22.3 To 24.2 lmol g–1. The courses of viscosity and carboxyl group contents were
- •Viscosity measurement additionally induces depolymerization due to strong
- •11 Pulp Properties and Applications
- •Increasing ozone charges. For more detailed
- •11.3 Dissolving Grade Pulp
- •Is more selective when ozonation represents the final stage according to an
- •11.3.2.3 Supramolecular Structure
- •1042 11 Pulp Properties and Applications
- •Is further altered by subsequent bleaching and purification processes. This
- •Involved in intra- and intermolecular hydrogen bonds. The softened state favors
- •11.3 Dissolving Grade Pulp
- •Interestingly, the resistance to mercerization, which refers to the concentration of
- •11 Pulp Properties and Applications
- •Illustrate that the difference in lye concentration between the two types of dissolving
- •Intensity (see Fig. 11.18: hw-phk high p-factor) clearly changes the supramolecular
- •11.3 Dissolving Grade Pulp
- •Viscose filterability, thus indicating an improved reactivity.
- •11 Pulp Properties and Applications
- •Impairs the accessibility of the acetylation agent. When subjecting a low-grade dissolving
- •Identification of the cell wall layers is possible by the preferred orientation of
- •Viscose pulp (low p-factor) (Fig. 11.21b, top). Apparently, the type of pulp – as well
- •11 Pulp Properties and Applications
- •150 °C for 2 h, more than 70% of a xylan, which was added to the cooking liquor
- •20% In the case of alkali concentrations up to 50 g l–1 [67]. Xylan redeposition has
- •11.3 Dissolving Grade Pulp
- •Xylan added linters cooked without xylan linters cooked with xylan
- •Viscosity
- •In the surface layer than in the inner fiber wall. This is in agreement with
- •11 Pulp Properties and Applications
- •Xylan content in peelings [wt%]
- •Xylan content located in the outermost layers of the beech phk fibers suggests
- •11.3.2.5 Fiber Morphology
- •11 Pulp Properties and Applications
- •50 And 90%. Moreover, bleachability of the screened pulps from which the wood
- •11.3.2.6 Pore Structure, Accessibility
- •11.3 Dissolving Grade Pulp
- •Volume (Vp), wrv and specific pore surface (Op) were seen between acid sulfite
- •11 Pulp Properties and Applications
- •Irreversible loss of fiber swelling occurs; indeed, Maloney and Paulapuro reported
- •In microcrystalline areas as the main reason for hornification [85]. The effect of
- •105 °C, thermal degradation proceeds in parallel with hornification, as shown in
- •Increased, particularly at temperatures above 105 °c. The increase in carbonyl
- •In pore volume is clearly illustrated in Fig. 11.28.
- •11.3 Dissolving Grade Pulp
- •Viscosity
- •11 Pulp Properties and Applications
- •Increase in the yellowness coefficient, haze, and the amount of undissolved particles.
- •11.3.2.7 Degradation of Dissolving Pulps
- •In mwd. A comprehensive description of all relevant cellulose degradation processes
- •Is reviewed in Ref. [4]. The different modes of cellulose degradation comprise
- •11.3 Dissolving Grade Pulp
- •50 °C, is illustrated graphically in Fig. 11.29.
- •11 Pulp Properties and Applications
- •In the crystalline regions.
- •11.3 Dissolving Grade Pulp
- •Important dissolving pulps, derived from hardwood, softwood and cotton linters
- •11.3 Dissolving Grade Pulp 1061
- •Xylan rel% ax/ec-pad 2.5 3.5 1.3 1.0 3.2 0.4
- •Viscosity mL g–1 scan-cm 15:99 500 450 820 730 1500 2000
- •1062 11 Pulp Properties and Applications
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •1072 1 Introduction
- •Isbn: 3-527-30999-3
- •Inventor of stone groundwood. Right: the second version
- •1074 2 A Short History of Mechanical Pulping
- •In refining, the thinnings (diameter 7–10cm) can also be processed.
- •In mechanical pulping as it causes foam; the situation is especially
- •In mechanical pulping, those fibers that are responsible for strength properties
- •Isbn: 3-527-30999-3
- •In mechanical pulping, the wood should have a high moisture content, and the
- •In the paper and reduced paper quality. The higher the quality of the paper, the
- •1076 3 Raw Materials for Mechanical Pulp
- •1, Transversal resistance; 2, Longitudinal resistance; 3, Tanning limit.
- •3.2 Processing of Wood 1077
- •In the industrial situation in order to avoid problems of pollution and also
- •1078 3 Raw Materials for Mechanical Pulp
- •2, Grinder pit; 3, weir; 4, shower water pipe;
- •5, Wood magazine; 6, finger plate; 7, pulp stone
- •Isbn: 3-527-30999-3
- •4.1.2.1 Softening of the Fibers
- •1080 4 Mechanical Pulping Processes
- •235 °C, whereas according to Styan and Bramshall [4] the softening temperatures
- •Isolated lignin, the softening takes place at 80–90 °c, and additional water
- •4.1 Grinding Processes 1081
- •1082 4 Mechanical Pulping Processes
- •1, Cool wood; 2, strongly heated wood layer; 3, actual grinding
- •4.1.2.2 Defibration (Deliberation) of Single Fibers from the Fiber Compound
- •4 Mechanical Pulping Processes
- •Influence of Parameters on the Properties of Groundwood
- •In the mechanical defibration of wood by grinding, several process parameters
- •Improved by increasing both parameters – grinding pressure and pulp stone
- •In practice, the temperature of the pit pulp is used to control the grinding process,
- •In Fig. 4.8, while the grit material of the pulp stone estimates the microstructure
- •4 Mechanical Pulping Processes
- •4.1 Grinding Processes
- •Is of major importance for process control in grinding.
- •4 Mechanical Pulping Processes
- •4.1.4.2 Chain Grinders
- •Is fed continuously, as shown in Fig. 4.17.
- •Initial thickness of the
- •75 Mm thickness, is much thinner than that of a concrete pulp stone, much
- •4 Mechanical Pulping Processes
- •Include:
- •Increases; from the vapor–pressure relationship, the boiling temperature is seen
- •4 Mechanical Pulping Processes
- •In the pgw proves, and to prevent the colder seal waters from bleeding onto the
- •4.1 Grinding Processes
- •In pressure grinding, the grinder shower water temperature and flow are
- •70 °C, a hot loop is no longer used, and the grinding process is
- •4 Mechanical Pulping Processes
- •Very briefly at a high temperature and then refined at high
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •Intensity caused by plate design and rotational speed.
- •4.2 Refiner Processes
- •1. Reduction of the chips sizes to units of matches.
- •2. Reduction of those “matches” to fibers.
- •3. Fibrillation of the deliberated fibers and fiber bundles.
- •1970S as result of the improved tmp technology. Because the key subprocess in
- •4 Mechanical Pulping Processes
- •Impregnation Preheating Cooking Yield
- •30%. Because of their anatomic structure, hardwoods are able to absorb more
- •Is at least 2 mWh t–1 o.D. Pulp for strongly fibrillated tmp and ctmp pulps from
- •4 Mechanical Pulping Processes
- •4.2 Refiner Processes
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa;
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •In hardwoods makes them more favorable than softwoods for this purpose. A
- •4.2 Refiner Processes
- •Isbn: 3-527-30999-3
- •1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning 1115
- •In refiner mechanical pulping, there is virtually no such coarse material in the
- •1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.3 Reject Treatment and Heat Recovery
- •55% Iso and 65% iso. The intensity of the bark removal, the wood species,
- •Isbn: 3-527-30999-3
- •1124 6 Bleaching of Mechanical Pulp
- •Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,
- •2000 Kg of technical-grade product is common. Typically, a small amount of a chelant
- •6.1 Bleaching with Dithionite 1125
- •Vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available
- •1126 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •70 °C, 2 h, amount of NaOh adjusted.
- •6.2 Bleaching with Hydrogen Peroxide
- •Is shown in Fig. 6.5, where silicate addition leads to a higher brightness and a
- •Volume (bulk). For most paper-grade applications, fiber volume should be low in
- •Valid and stiff fibers with a high volume are an advantage; however, this requires
- •1130 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •Very high brightness can be achieved with two-stage peroxide bleaching, although
- •In a first step. This excess must be activated with an addition of caustic soda. The
- •Volume of liquid to be recycled depends on the dilution and dewatering conditions
- •6 Bleaching of Mechanical Pulp
- •6 Bleaching of Mechanical Pulp
- •Is an essential requirement for bleaching effectiveness. Modern twin-wire presses
- •Is discharged to the effluent treatment plant. After the main bleaching stage, the
- •6.3 Technology of Mechanical Pulp Bleaching
- •1136 6 Bleaching of Mechanical Pulp
- •Isbn: 3-527-30999-3
- •7.3 Shows the fractional composition according to the McNett principle versus
- •1138 7 Latency and Properties of Mechanical Pulp
- •7.2 Properties of Mechanical Pulp 1139
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •In Fig. 1.2, the development of recovered paper utilization and paper production
- •Is split into the usa, the cepi countries, and Germany. It is clear that since 1990,
- •5.8% For Germany and worldwide, and 5.9% for the cepi countries.
- •1150 1 Introduction
- •1 Introduction
- •Industry, environmentalists, governmental authorities, and often even the marketplace.
- •It is accepted that recycling preserves forest resources and energy used for
- •1 Introduction
- •Incineration. The final waste (ashes) can either be discarded or used as raw
- •Virgin fibers
- •74.4 % Mixed grades
- •Indonesia
- •Virgin fibers
- •Inhomogeneous sample Homogeneous sample
- •Variance of sampling Variance of measurement
- •1.Quartile
- •3.Quartile
- •Insoluble
- •Insoluble
- •Insoluble
- •Integral
- •In Newtonion liquid
- •Velocity
- •Increasing dp
- •2Α filter
- •0 Reaction time
- •Increasing interaction of probe and cellulose
- •Increasing hydrodynamic size
- •Vessel cell of beech
- •Initial elastic range
- •Internal flow
- •Intact structure
- •Viscosity 457
- •Isbn: 3-527-30999-3
- •1292 Index
- •Visbatch® pulp 354
- •Index 1293
- •1294 Index
- •Impregnation 153
- •Viscosity–extinction 433
- •Index 1295
- •1296 Index
- •Index 1297
- •Inhibitor 789
- •1298 Index
- •Index 1299
- •Impregnation liquor 290–293
- •1300 Index
- •Industries
- •Index 1301
- •1302 Index
- •Index 1303
- •Xylose 463
- •1304 Index
- •Index 1305
- •1306 Index
- •Index 1307
- •1308 Index
- •In conventional kraft cooking 232
- •Visbatch® pulp 358
- •Index 1309
- •In prehydrolysis-kraft process 351
- •Visbatch® cook 349–350
- •1310 Index
- •Index 1311
- •1312 Index
- •Viscosity 456
- •Index 1313
- •Viscosity 459
- •Interactions 327
- •1314 Index
- •Index 1315
- •Viscosity 459
- •1316 Index
- •Index 1317
- •Xylose 461
- •Index 1319
- •Visbatch® pulp 355
- •Impregnation 151–158
- •1320 Index
- •Index 1321
- •1322 Index
- •Xylan water prehydrolysis 333
- •Index 1323
- •1324 Index
- •Viscosity 459
- •Index 1325
- •Xylose 940
- •1326 Index
- •Index 1327
- •In selected kinetics model 228–229
- •4OMeGlcA 940
- •1328 Index
- •Index 1329
- •Intermediate molecule 164–165
- •1330 Index
- •Viscosity 456
- •Index 1331
- •1332 Index
- •Impregnation liquor 290–293
- •Index 1333
- •1334 Index
- •Index 1335
- •1336 Index
- •Impregnation 153
- •Index 1337
- •1338 Index
- •Viscose process 7
- •Index 1339
- •Volumetric reject ratio 590
- •1340 Index
- •Index 1341
- •1342 Index
- •Index 1343
- •1344 Index
- •Index 1345
- •Initiator 788
- •Xylose 463
- •1346 Index
- •Index 1347
- •Vessel 385
- •Index 1349
- •1350 Index
- •Xylan 834
- •1352 Index
Viscosity
[mL g–1]
DKappa/CS
0 34.0 1280
19 Z 17.6 920 12.5
19 ZE 12.6 905 15.3
19 ZEP 11.9 885 14.7
19 ZE0 9.5 850 14.2
The use of an E-stage following an ozone stage reduces the ozone charge by 25–
45% when bleaching to a certain kappa number target. Intermediate washing or
neutralization does not affect the extent of lignin removal during subsequent alkaline
extraction. However, neutralization directly after the ozone stage appears to
improve selectivity when followed by alkaline extraction.
7.5 Ozone Delignification 825
In the case of oxygen prebleaching, being the more realistic alternative, the saving
of ozone reaches almost 50% [82]. The viscosity values of the OZE-bleached
pulps correspond to those determined for the OZ-bleached pulps after reduction
with borohydride. Fiber strength (zero-span tensile index) is almost not impaired
by the E-stage (in relation to the Z-treated pulp), at least when using LC bleaching
technology. There are indications that more lignin is removed after LC and MC
ozone bleaching than after HC bleaching [90], but as yet this observation is not
understood.
7.5.6
Technology of Ozone Treatment
Andreas W. Krotscheck
7.5.6.1 Medium-Consistency Ozone Treatment
The process flowsheet of a typical medium-consistency ozone delignification system
is shown schematically in Fig. 7.99. MC pulp coming from the previous
bleaching stage falls into a standpipe after sulfuric acid has been added to adjust
the pH. The pump forwards the pulp suspension to a high-shear mixer which is
charged with compressed ozone/oxygen gas.
MC PUMP
HIGH-SHEAR
MIXER
BLOWTANK WASHING
O3
Pulp from
preceding
stage
H2SO4
Pulp to
next stage
Offgas
Fig. 7.99 Process flowsheet of a typical medium-consistency
(MC) ozone delignification system.
It is of utmost importance that the ozone and pulp are mixed intensively,
because the predominant portion of the delignification occurs inside the mixer.
This is why the medium-consistency ozone system does not have a reactor comparable
to other bleaching applications. Instead, the mixing time is prolonged at
high power dissipation and, on occasion, a second high-shear mixer is installed
for that purpose. Additional time for the reactions to complete after the mixer is
usually provided by the pipe to the blowtank. This pipe may be increased slightly
in diameter to offer about 1min of retention time.
826 7Pulp Bleaching
The pressurized three-phase flow coming from the mixer expands into the blowtank,
where the pulp suspension is separated from the gas phase. The offgas is
cleaned of fibers in a scrubber and proceeds to the ozone destruction unit. The
pulp slurry is discharged from the blowtank either at low or medium consistency,
depending on the feed requirements of the subsequent equipment.
Washing after the ozone stage is often omitted, and the pulp is forwarded to the
subsequent stage at medium consistency. Otherwise, washing can be carried out
with single-stage washing equipment, for example with a single-stage Drum Displacer
™, a wash press, or a vacuum drum washer.
The material for the construction of wetted parts in an ozone stage is typically a
higher grade of austenitic stainless steel.
Further information regarding ozone delignification equipment including medium-
consistency pumps, mixers and blowtanks is provided in Section 7.2. Details
of pulp washing are provided in Chapter 5.
7.5.6.2 High-Consistency Ozone Treatment
High-consistency bleaching requires a press to be utilized before the stage of efficient
pulp dewatering. A plain dewatering press is preferable as it achieves higher
consistencies (up to 40%) than a wash press. It is necessary to adjust the desired
pH in the press feed, because there is no means by which sulfuric acid can be
mixed in between the press and the reactor. The fiber mat leaving the press nip
must be thoroughly disintegrated in order to ensure good accessibility for the
bleaching chemicals to all fiber surfaces.
The Metso ZeTrac ozone delignification system is illustrated in Fig. 7.100. Although
former HC systems required a screw feeder and pulp fluffer, the Metso
ZeTrac does not. Instead, the specially designed shredder screw of the press delivers
well-fluffed pulp which falls into the horizontal reactor and is brought into
contact with the ozone/oxygen gas mixture [100]. Paddles keep the pulp in motion
as it travels concurrently with the gas through the reactor. The reactor operates at
a slight vacuum, thus ensuring that no gas can escape to the ambient air [101].
The delignified pulp is discharged into a dilution screw conveyor, where it is
alkalized. The medium-consistency slurry then falls into a tank before being
pumped to the subsequent stage.
Fig. 7.100 The Metso ZeTrac high-consistency (HC) ozone bleaching system [100].
7.5 Ozone Delignification 827
7.5.6.3 Ozone/Oxygen Gas Management
The oxygen gas needed for the generation of ozone can be supplied from a liquid
oxygen storage tank, or it can be produced on site by oxygen plants using either a
pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) process.
Oxygen is by far the predominant gas in any commercially generated ozone/
oxygen gas mixture. When entering the pulp delignification process, each kilogram
of ozone is accompanied by 6–9 kg of oxygen. This amount of oxygen, together
with oxygen which has been created by the decomposition of ozone, forms
the major part of the offgas leaving the delignification process.
The flow scheme of the common once-through gas system is shown in
Fig. 7.101. Oxygen is fed to the ozone generator and, if used for medium-consistency
delignification, is subsequently compressed. The compression step can be
omitted in the case of high-consistency delignification. After addition to the pulp,
ozone is partly consumed during the delignification reaction and partly decomposed.
Only small concentrations of ozone are left in the offgas, and these are
destroyed in a dedicated destruction unit. Catalytic destruction is the most popular
approach, followed by thermal destruction. The residual gas after destruction is available
for other applications which normally require its re-compression. In the bleaching
plant there are several points where residual oxygen from ozone delignification
can be re-used. These include oxygen delignification, oxygen-reinforced extraction or
peroxide bleaching, and white liquor oxidation. Other opportunities for re-use may
exist in other areas of the mill.
Offgas
separation
Ozone
generation
(Ozone
compression)
Ozone
delignification
Ozone
destruction
Pulp Pulp
Oxygen
compression
Oxygen to oxygen
delignification and
Oxygen other utilisation
(Excess oxygen
to atmosphere)
Fig. 7.101 Once-through gas management.
If the oxygen supplied with the offgas exceeds the mill’s demands, the most appropriate
option is to exhaust some offgas into the atmosphere. Although at first sight
this may not be the most economic solution, the ambivalent operational experiences
fromthe so-called long loop system(Fig. 7.102) seembarely worth the effort. The long
loop contains an additional purification step, where gas components known to interfere
with ozone generation (including moisture) are eliminated as far as possible.
Since the ozone generators are very sensitive to inappropriate gas feed, the perfect
function of gas purification is vital for the availability of ozone from a long loop
system.
828 7Pulp Bleaching
Oxygen to oxygen
delignification and
other utilisation
Offgas
separation
Ozone
generation
(Ozone
compression)
Ozone
delignification
Ozone
destruction
Pulp Pulp
Oxygen
compression
Gas
purification
Oxygen
Fig. 7.102 Long loop gas management.
With modern ozone generators delivering 14% by weight of ozone, the oxygen
supply and demand in the mill is balanced at ozone charges between 4 and
6 kg ton–1 of pulp. Even at lower ozone concentrations in the feed gas, the majority
of ozone delignification applications features specific charges below the balanced
maximum. As a result, once-through gas management has become the system of
choice over the past years.
7.5.7
Application in Chemical Pulp Bleaching
7.5.7.1 Selectivity, Efficiency of Ozone Treatment of Different Pulp Types
7.5.7.1.1 Basic Considerations on the Selectivity of Ozone Bleaching
It is generally agreed that radical formation is a crucial factor that affects the selectivity
and performance of an ozone bleaching stage [75,102,103]. On the basis of
model compounds, the ratio of rate constants for delignification to that of carbohydrate
degradation (kL/kC) is more than 105 in the case of molecular ozone,
whereas for hydroxyl radicals a value of only 5–6 has been determined [72,104].
However, there remains much debate about the pathways of radical formation
during the ozonation of pulp. It is well-documented that radicals are formed in
aqueous medium by self-decomposition of ozone [54,104]. However, the decomposition
of ozone is rather slow in acidic media (see Fig. 7.82). Therefore, additional
reasons have been suggested as being responsible for the unselectivity of ozone
treatment. Radicals are formed in the presence of transition metal ions
[56,103,105] and in a direct reaction between ozone and aromatic lignin structures
[57,58,106]. Recent trials using the TNM (tetranitromethane) method, however,
concluded that the addition of the transition metal ions Fe(II), Cu(II), Co(II) and
Mn(II) to a lignin model compound (e.g., vanillin) do not promote additional radical
formation. It has been argued that the loss in pulp viscosity in the presence of
transition metal ions reported in the literature may be attributed to radical formation
from the reaction with hydrogen peroxide formed during ozonation
7.5 Ozone Delignification 829
[57,103,107]. Instead, radical formation is caused by a direct reaction between lignin
model compounds and ozone [57,58]. In acidic solution, syringyl structures
yield more radicals at a given ozone charge as compared to the corresponding
guaiacyl compounds, mainly due to the lower oxidation potential of the former.
However, no radicals are formed in direct reaction between ozone and carbohydrate
model compounds. Following the finding that syringyl structures yield significantly
more radicals than do corresponding guaiacyl compounds, the viscosity
loss during ozone bleaching should be much more pronounced for hardwood
than for softwood pulps. However, in practice the opposite is true. Ragnar has
shown that the better selectivity of hardwood kraft pulps can be attributed to their
higher amount of hexenuronic acids (HexA), since ozone reactions with this component
do not yield radicals [108]. Magara et al. also reported that the presence of
lignin model compounds with free phenolic hydroxyl groups enhanced cellobiose
degradation during ozonation in water [78]. However, in the presence of nonphenolic
lignin model compounds, cellobiose degradation was retarded. Based on
these model compound studies, it can be concluded that the selectivity of ozone
bleaching gradually improves with decreasing incoming kappa number of a pulp.
In fact, this may explain the superior selectivity of oxygen-bleached pulps over
unbleached kraft pulps, since oxygen reduces both the total lignin content and the
phenolic structures in residual lignin [75,107]. The latter yields more radicals as
compared to nonphenolic structures [57]. Cellulose is also degraded by molecular
ozone according to an insertion mechanism (see Section 7.5.4.3) [53,109]. It is
however doubtful if this type of reaction is responsible for the degradation reaction
of pulps in the presence of residual lignin, because the reaction rate of ozone
towards intact carbohydrate structures is very low (0.21m –1 s–1), while the reaction
rate of hydroxyl radicals towards similar structures is several orders of magnitudes
higher [104].
During the course of final bleaching, when the residual lignin content
diminishes, it seems likely that the direct reaction between ozone and cellulose
gradually becomes the predominant reaction responsible for cellulose degradation
in ozone bleaching. Model compound studies using methyl 4-O-ethyl-b-d-glucopyranoside
were conducted to elucidate the participation of radical species in the
degradation of the polysaccharide during ozone treatment [53]. From the results
obtained it was concluded that both ozone itself and radical species participate in
the glycosidic bond cleavage of carbohydrates during ozonation in aqueous solutions.
The free radical-mediated reaction may lead to both direct cleavage and to
the conversion of hydroxyl to carbonyl groups. The contribution of radical species
was estimated to be about 40–70% during ozonation in distilled water acidified to
pH 2 (the ratio of ionic to radical reactions was calculated by the relative reactivities
at C1towards ozone in anhydrous dichloromethane as a reference for pure
ionic and toward Fenton’s reagent as a reference for radical reactions). Furthermore,
the model compound studies revealed that oxidation of hydroxyl groups at
C2, C3, and C6 positions to carbonyl groups is caused predominantly by radical
species.
830 7Pulp Bleaching
7.5.7.1.2 Efficiency and Selectivity of Ozone Treatment
The use of ozone for the production of paper-grade pulps is limited to low charges
to prevent strength losses. Most of the industrial installations of ozone bleaching
operate on hardwood kraft pulps because of a better selectivity performance compared
to softwood kraft pulps; this is particularly expressed in a better preservation
of strength properties. The higher selectivity of ozone towards hardwood kraft
pulps may be attributed to the presence of a high proportion of HexA [106]. Ozone
is known to be very effective and selective in removing HexA, without simultaneously
impairing pulp properties. Therefore, it can be concluded that the use of
ozone in industrial installations is primarily focused on the removal of HexA.
Ozone is also used for the production of TCF-bleached dissolving pulps. The
ozone treatment is preferably placed between oxygen prebleaching and the final
hydrogen peroxide stage. The tasks of ozone for dissolving pulp production are
both the removal of residual oxidizable impurities (measured as kappa number)
and the controlled adjustment of viscosity. The ozone charge is predominantly
chosen to adjust pulp viscosity, while the final brightness is regulated in the subsequent
hydrogen peroxide stage. Ozone replaces the hypochlorite treatment in a
conventional bleaching sequence for dissolving pulp production. Godsay and
Pearce found a clear relationship between the number of chain scissions and
ozone consumption (in this case even a linear relationship), and this is an important
prerequisite for a controlled viscosity adjustment [99]. During the course of
the development of medium-consistency ozone bleaching, a similar shape was
recognized for the relationship between the number of chain scissions and the
consumption of both ozone and hypochlorite (as active chlorine); this latter point
was verified by Herbst and Krässig [110]. At the start of the reaction, the linear
function has a shallow slope, indicating a minimal effect on carbohydrate degradation.
During the second phase of the reaction, the slope increases and finally
becomes straight, showing that the number of bonds broken is now proportional
to the amount of chemicals consumed. The relationship between the amount of
ozone and hypochlorite consumed and the number of chain scissions in a selection
of experiments using beech sulfite dissolving pulp is depicted in Fig. 7.103.
With respect to chain scissions, the efficiency of 1kg of consumed ozone is
equivalent to that of about 2.8 kg of consumed active chlorine (hypochlorite). If
both oxidants are expressed as oxidation equivalents (OXE), 1.0 OXE of ozone corresponds
to only 0.63 OXE of active chlorine. This means that from the maximum
oxidative power of ozone, representing 6 mol electrons per mol, only 3.8 are transferred,
whereas in the case of hypochlorite all 2 mol electrons per mol are
received.
Furthermore, hypochlorite reacts slightly more selectively with the readily available
residual lignin as compared to ozone, which is characterized by the lower
slope during the first phase. The intercept with the abscissa and the slope of the
curve are characteristic parameters for each pulp. The intercept represents the
amount of ozone or hypochlorite consumed without any significant chain scissions,
while the slope depends on the efficiency of bonds broken. Both parameters
are related to the kappa number, the hemicellulose content, the amount of
7.5 Ozone Delignification 831
0 4 8 12
0
1
2
3
4
Chain scissions
Ozone charge [kg/odt]
Hypochlorite
Chain scissions
Active chlorine consumption [kg/odt]
0 2 4
0
1
2
3
4
Ozone
Fig. 7.103 Carbohydrate degradation, indicated
as number of chain scissions, depending upon
the amount of oxidants consumed (according
to Sixta et al. [41]). Pulp: EO-pretreated beech
acid sulfite dissolving wood pulp (B-AS), kappa
number 2.0, viscosity 560 mL g–1, alpha-cellulose
content 90.2%. medium-consistencyozone
bleaching: 10% consistency, pH 2, 50 °C,
10 s mixing time; hypochlorite treatment: 4%
consistency, 50 °C, initial pH = 9.5, reaction
time 60 min.
reactive groups in the cellulose chain (e.g., carbonyl groups) and the accessibility
to ordered regions under given conditions of ozone bleaching. There is no indication
that the selected wood species exerts any significant influence on the course
of degradation during ozonation, provided that the purity (measured as R18 or
hemicellulose content) and the kappa number of the corresponding pulps are at a
comparable level. The development of chain scissions as a function of ozone
charge for both beech and spruce sulfite dissolving pulps at two different purity
levels, 93% and > 96% R18, respectively, are shown in Fig. 7.104.
The results confirm that a correlation between cellulose degradation and ozone
charge is not discernible for spruce and beech sulfite dissolving pulps at a given
R18 level. The data in Fig. 7.104 also show that the presence of low molecularweight
hemicelluloses protect the pulps against cellulose degradation. Thus, highpurity
dissolving pulps are exposed to more severe carbohydrate degradation at a
given ozone charge.
832 7Pulp Bleaching
0 2 4 6 8 10
0
2
4
6
Beech-sulfite: R18 = 93% R18 = 96%
Spruce-sulfite: R18 = 93% R18 = 96%
Chain scissions
Ozone charge [kg/odt]
Fig. 7.104 Course of chain scissions as a function
of ozone charge for oxygen-delignified
beech and spruce Mg-based sulfite dissolving
pulps of two different purity levels, 93% R18
and 96% R18, respectively (according to [131]).
The remaining properties of the selected
dissolving pulps, such as hemicellulose composition
and kappa number are included in
Tab. 7.42 medium-consistency laboratory
ozone treatment: 50 °C, 10% consistency, 150 g
O3 m–3, 8 bar, 10 s mixing time.
0 2 4 6 8 10 12
0
2
4
6
Euca-PHK, κ = 2.0; Euca-PHK, κ = 4.1
Pine-PHK, κ = 6.9; Pine-PHK, κ = 4.4
Chain scissions
Ozone charge [kg/odt]
Fig. 7.105 Course of chain scissions as a function
of ozone charge for oxygen-delignified
pine and eucalyptus prehydrolysis kraft pulps
at comparable purity level, 97% R18, and different
kappa numbers. Reaction conditions see
Fig. 7.104 and pulp properties see Tab. 7.42.
7.5 Ozone Delignification 833
Table 7.42 Comparative evaluation of the degradation and
delignification behaviour during medium-consistency ozone
bleaching of oxygen delignified pulps of different origin and
composition (according to [131]).
Substrate Initital
kappa
number
R18 value
[%]
Xylan
[%]
Mannan
[%]
Ozone charge
do obtain
Kappa/O3-charge
CS* = 2.0 CS = 3.0 at j after
Z = 1
at j after
Z =0,5
Beech sulfite 1.2–2.0
1.0–1.3
93.3
96.7
3.4
1.9
0.9
0.3
3.1
2.3
4.2
2.9
1.0
0.8
Spruce sulfite 1.4–2.6
0.5–2.0
93.1
96.8
2.0
1.4
2.5
0.7
3.4
2.1
4.7
2.8
1.1
0.7
Beech PHK 4.4
2.3
1.5
1.4
95.5
95.8
96.4
97.3
15.6
5.9
3.1
2.1
0.5
0.4
0.3
0.2
4.6
2.7
2.1
1.1
3.4
2.7
1.8
Pine PHK 6.9
4.4
96.8
96.7
2.2
2.2
2.2
2.2
4.0
3.2
5.9
4.5
0.7
0.8
Eucalypt PHK 2.0
4.0
97.1
97.1
2.6
2.6
0.7
0.7
2.6
3.5
3.5
5.5
1.0
0.9
Pine kraft 3.4
17.5
87.1
86.8
7.1
7.3
6.5
6.8
3.8
9.3
4.8 0.5
CS = chain scissions given as 104
Pt _ 104
PO _ in mmol AGU–1.
The course of cellulose degradation caused by ozonation is also independent on
the wood species for prehydrolysis kraft pulps, as depicted in Fig. 7.105. Despite
major differences in fiber morphology, oxygen-delignified pine and eucalyptus
PHK pulps reveal a similar degradation pattern during ozone treatment in case of
a comparable initial kappa number.
Moreover, the data in Fig. 7.105 demonstrate that the effect of ozone charge on
cellulose degradation decreases with rising kappa number prior to ozone treatment.
Surprisingly, the applied cooking technology for the production of dissolving
pulps appears also not to have any influence on the behavior of cellulose degradation
as a function of ozone charge, provided that both pulps are of comparable
R18 content. Figure 7.106 shows that the response of spruce sulfite and eucalypt
PHK pulps on the number of chain scissions is quite comparable for a broad
range of ozone charges.
As previously indicated, cellulose purity, determined as R18 content or residual
xylan and/or mannan concentrations (see Tab. 7.42), significantly affects the degradation
pattern during MC ozonation. The progressive removal of short-chain
834 7Pulp Bleaching
0 2 4 6
0
2
4
6
Spruce-Sulfite, R18 = 97%, κ = 0.5 -2.0 Euca-PHK, R18 = 97%, κ = 2.0
Chain scissions
Ozone charge [kg/odt]
Fig. 7.106 Comparative evaluation of the
response of oxygen-delignified spruce sulfite
and eucalypt prehydrolysis kraft pulps on chain
scissions as a function of ozone charge at a
comparable purity level, 97% R18 and kappa
numbers (according to [131]). Mediumconsistency
laboratory ozone treatment: 50 °C,
10% consistency, 150 g O3 m–3, 8 bar, 10 s mixing
time.
carbohydrates leads to a growing susceptibility of the remaining high molecularweight
cellulose molecules towards ozone-induced chain scission (Fig. 7.106).
Apparently, the hemicelluloses are preferentially degraded and eventually provide
a sacrificial barrier for cellulose attack by ozone, and as a result, the fall in viscosity
of the remaining polysaccharides is somewhat suppressed.
The high resistance of the beech pulp with the highest hemicellulose content
(P-factor 50) towards chain scissions is partly due to a higher initial kappa number
as compared to the other pulps of the comparison (Fig. 7.107; Tab. 7.42). The
results demonstrate that the presence of both short-chain hemicelluloses and residual
oxidizable impurities (kappa number) protect the high molecular-weight
cellulose against degradation during ozonation. Furthermore, the laboratory
results outlined in Figs. 7.104–7.107 indicate that ozone is suitable for adjusting
viscosity, provided that the kappa number and viscosity of the oxygen-prebleached
pulp are within certain limits. It has been shown previously that, when mediumconsistency
technology is applied, the reaction of ozone with pulp constituents
occurs entirely in the mixer. Unlike laboratory conditions, the residence time in
commercial high-shear mixers is very short, with typical retention times ranging
from less than 1s to 4 s (maximum), compared to 10 s in a typical laboratory application.
The extent of reaction during medium-consistency ozone bleaching is
characterized by the ozone consumption rate inside the high-shear mixer. Parallel
to the increase in ozone charge, the gas void fraction, Xg, increases which in turn
impairs the efficiency of ozone mass transfer. In Fig. 7.108, the relationship between
ozone charge in the range from 1.0 to 5.5 kg odt–1 and the extent of ozone
consumption is compared for laboratory and industrial medium-consistency
7.5 Ozone Delignification 835
0 1 2 3 4 5 6 7
0
2
4
6
Beech-PHK:
P-Factor 50, κ = 4.4; P-Factor 500, κ = 2.3
P-Factor 1000, κ = 1.6; P-Factor 2000, κ = 1.4
Chain scissions
Ozone charge [kg/odt]
Fig. 7.107 Influence of cellulose purity of
beech prehydrolysis kraft pulps on the course
of cellulose degradation during ozonation
(according to [131]). The cellulose purity is
adjusted by prehydroly sis intensity
characterized by the P-factor. Medium-consistency
laboratory ozone treatment: 50 °C, 10%
consistency, 150 g O3 m–3, 8 bar,
10 s mixing time.
bleaching. The rather long residence time of approximately 3.5 s during high-shear
mixing in the commercial system has been obtained by the installation of two mixers
in series. The yield of reacted ozone declines in the industrialMCsystem, from about
75% at an ozone charge of 1.5 kg odt–1 to less than 50% at an ozone charge of
5.5 kg odt–1, while the laboratory mixer keeps an ozone consumption rate beyond
80% throughout the given range of ozone charges.
The lower ozone consumption in the commercial MC ozone installation is
expressed in a reduced extent of reaction between ozone and pulp constituents as
compared to the laboratory system. The data in Fig. 7.109 illustrate that, in an
industrial high-shear mixing system, the number of chain scissions levels off at
ozone charges exceeding 4 kg odt–1. A further improvement of the ozone consumption
yield in an medium-consistency installation can only be obtained by
extending the mixing time, and by reducing the gas void fraction while keeping
the specific energy dissipation, e, at a fairly constant level.
The selectivity of ozone bleaching is an important criterion not only for papergrade
but also for dissolving-grade pulp production, in order to ensure an efficient
delignification and bleaching performance. It has been mentioned previously that
the selectivity of ozone bleaching is also affected by the type and properties of the
pulps. It is well known that ozone bleaching of hardwood kraft pulp is more selective
than for softwood kraft pulp in terms of the kappa number–viscosity relationship
[106]. Moreover, Soteland established that sulfite pulps respond more selectively
to ozone treatment than do kraft pulps [111]. The better response of sulfite
pulps to ozone treatment is attributed to the lower lignin content of the
unbleached pulp [112].
836 7Pulp Bleaching
1 2 3 4 5 6
50
60
70
80
90
Gas void fraction, X
g
[-]
Ozone consumption rate:
industrial scale, τ ~ 3.5 s lab scale, τ = 10 s
Ozone consumption yield [%]
Ozone charge [kg/odt]
0.1
0.2
0.3
0.4
Gas void fraction:
industrial scale
Fig. 7.108 Comparison of industrial and
laboratory medium-consistency ozone bleaching
with respect to the ozone consumption
rate as a function of ozone charge according to
[131]). The development of the gas void fraction
in the commercial system is followed over
the range of ozone charges investigated.
The set-up of the commercial system comprises
the installation of two high-shear mixers
in series. Conditions of the commercial ozone
stage: pH 2.5, ozone concentration prior to
compression: 120–160 g m–3, consistency
8.5%, pressure inside the mixers 7.5 bar, 43°C.
0 1 2 3 4 5 6 7
0
1
2
3
4
5
6
7
industrial application laboratory
Chain scissions
Ozone charge [kg/odt]
Fig. 7.109 Comparison of industrial and laboratory mediumconsistency
ozone bleaching with respect to the ozone consumption
rate as a function of ozone charge (according to
[131]).
7.5 Ozone Delignification 837
The selectivity of delignification and bleaching reactions in general – and that of
ozone bleaching in particular – is defined as the ratio of the rate constant for the
desired delignification or bleaching reactions (removal of chromophores) to that
of the non-desired carbohydrate degradation reaction. A practical way to compare
the selectivity of ozone bleaching of different pulps, and of different levels of initial
viscosity, can be achieved by relating the brightness gain (D brightness) per
number of chain scissions (CS) to the brightness after ozonation. It can be
expected that the bleaching selectivity, expressed as D brightness/CS, decreases
with increasing brightness after ozone treatment. The selectivity behavior of different
types of dissolving pulps and of one softwood paper-grade kraft pulp was
studied in a laboratory medium-consistency system under comparable conditions.
The results outlined in Fig. 7.110 reveal three areas of different selectivity. The
group of highest selectivity comprises the hardwood sulfite dissolving pulps, followed
by the hardwood PHK pulps and the softwood kraft pulps, which cover the
least-selective group of pulps. The superior selectivity of hardwood and sulfite
pulps both with low initial kappa numbers is in accordance with reported values
[108]. Although ozone reacts more readily with lignin structures than with carbohydrates,
the bleaching selectivity decreases with increasing kappa number, due
to a more efficient chromophore reduction at lower residual kappa number. These
results imply that, with respect to pulp viscosity at a given kappa number, it is preferable
to intensify oxygen delignification and to apply less ozone.
50 60 70 80 90
0
10
20
30
40
B-AS: R18 = 93%, κ = 1.3 B-AS: R18 = 96% κ = 1.0 E-PHK, R18 = 97%, κ = 2.0
E-PHK: R18 = 97%, κ = 3.0 P-PHK: R18 = 96%, κ = 3.1 P-PHK, R18 = 96%, κ = 4.4
P-PHK: R18 = 96% , κ = 6.9 P-KA: κ = 3.4
Δ Brightness per
number of chain scissions
Brightness after Z [% ISO]
Fig. 7.110 Bleaching selectivity of a variety of
oxygen-prebleached dissolving pulps and of
one softwood kraft pulp during medium-consistency
ozone treatment in a laboratory highshear
mixer (according to [131]).
The pulps subjected to ozone treatment are
characterized in Tab. 7.42. Constant conditions
of ozone bleaching: pulp consistency 10%,
50 °C, pH 2.0, mixing time 10 s.
838 7Pulp Bleaching
The reason for the higher selectivity of a hardwood over a softwood kraft pulp
has been attributed to the presence of a higher amount of HexA in the former
[106]. However, dissolving pulps derived from both sulfite and PHK technology
contain only minor amounts of HexA, or are even free of HexA at high cellulose
purity levels [113]. Therefore, the presence of HexA alone is not decisive for the
superior selectivity of hardwood pulps. It may be speculated that the residual
kappa number of a dissolving pulp contains no relevant amounts of phenols of
the syringyl- and guaiacyl-type which promote radical formation in different yields
[106]. Nevertheless, ozone bleaching is less selective for both paper-grade and dissolving-
grade pulps rich in kappa number and hemicellulose content. The data in
Fig. 7.110 demonstrate clearly that the pine PHK pulp behaves more selectively
during ozonation as compared to a pine paper-grade pulp of comparable initial
kappa number (kappa number 3.4 and 3.1, respectively).
To better elucidate the influence of wood species on the selectivity of ozonation,
the performance of spruce and beech acid sulfite dissolving pulp (S-AS versus BAS)
of comparable kappa number content (1.4–2.6) and cellulose purity (R18 ~
93%) has been investigated with regard to delignification and bleaching selectivity
(Fig. 7.111).
The data in Fig. 7.111 show clearly that the selectivity of kappa number reduction
is not dependent on the wood species, provided that the compositions of noncellulosic
material in the pulps are at comparable levels. The wood species, how-
0 1 2 3
0
1
2
3
4
Δ Brightness per
number of chain scissions
Brightness after Z [% ISO]
S-AS: R18 = 93%, κ
(E/O)
= 1.4 - 2.6
B-AS: R18 = 93%, κ
(E/O)
= 1.3 - 2.0
Δ Kappa number per
number of chain scissions
Kappa number after Z
60 70 80 90
0
5
10
15
20
25
30
Brightn.
(E/O)
= 67 - 77 % ISO
Brightn.
(E/O)
= 76 - 81 % ISO
Fig. 7.111 Bleaching selectivity of a variety of
oxygen-prebleached dissolving pulps and of
one softwood kraft pulp during medium-consistency
ozone treatment in a laboratory highshear
mixer. The pulps subjected to ozone
treatment are characterized in Tab. 7.42. Constant
conditions of ozone bleaching: pulp consistency
10%, 50 °C, pH 2.0, mixing time 10 s.
7.5 Ozone Delignification 839
ever, may exhibit an influence on the selectivity of brightness gain, as indicated in
Fig. 7.111. Clearly, the beech pulp is slightly more susceptible to an increase in
brightness at a given number of chain scissions as compared to the spruce pulp.
The differences are small, but significant, and may be attributed to the different
reflectance behavior rather than to differences in the light absorption properties
of hard- and softwood fibers.
At first glance, these results appear to contradict those reported by Simoes and
Castro [64], who stated that selectivity was higher for pine than for eucalyptus
pulp when comparing the viscosity versus kappa number profiles. However, when
converting the given changes in viscosity caused by ozonation into the number of
chain scissions, in order to normalize polysaccharide degradation, the delignification
selectivity was exactly the same for both pine and eucalyptus pulps [eucalyptus
pulp: D Kappa/D viscosity = (15.5 – 4.0)/(1270 – 770) = 0.023 converted to
kappa number reduction per chain scissions = 11.5/2.27 × 10–4 = 51 ·j ·AGU·
mmol–4; pine pulp: DKappa/D viscosity = (18.1 – 4.0)/(970 – 615) = 0.040 corresponds
to 50 · j ·AGU· mmol–1) [64]. Thus, it can be summarized that the
delignification selectivity of ozonation is predominantly influenced by the initial
kappa number and the amount of noncellulosic components (e.g., the hemicellulose
content).
7.5.7.2 Effect of Ozonation on the Formation of Carbonyl and Carboxyl Groups
The formation of carbonyl and carboxyl groups has great significance in bleaching
operations. It is well known that the introduction of carbonyl groups along the
polysaccharide chains leads to cleavage of glycosidic bonds when the pulp is subsequently
exposed to alkali. Furthermore, carbonyl groups and uronosidic carboxyl
groups exert a detrimental effect on color stability. The presence of carboxyl
groups also affects the swelling characteristics and water affinity of pulp fibers,
which in turn governs the sheet formation and bonding properties.
Chandra and Gratzl monitored the formation of these groups during ozonation
of alpha-cellulose (produced from a softwood sulfite dissolving pulp further purified
by extraction with 18% sodium hydroxide at 25 °C) [60]. The carboxyl content
increased in a stepwise fashion with the degree of ozonation, while the carbonyl
content underwent steep rises followed by sharp declines throughout ozonation,
resulting in an overall increase at the end of the treatment. The observed pattern
of carbonyl groups as a function of ozone charges was explained by the assumption
that the introduction of carbonyl groups sensitizes this particular part of the
polymer towards further attack by ozone [60,114]. This may lead to excessive degradation
followed by dissolution of the oxidized fragments, and exposes previously
protected domains to continued ozonation. The results of a comprehensive study
on the generation of carbonyl and carboxyl groups along the carbohydrate chain
during the course of ozonation before or after hydrogen peroxide treatment is
included in Section 11.2 (paper grade pulps) and 11.3.2.2 (dissolving grade
pulps)).
840 7Pulp Bleaching
7.5.7.3 Effect of Ozonation on Strength Properties
Unfortunately, ozone delignification is accompanied by a concomitant degradation
of the polysaccharide fraction. As illustrated in Fig. 7.109, cellulose degradation
(characterized in terms of number of chain scissions) is clearly related to
ozone charge. The correlation between strength properties and carbohydrate degradation
(pulp viscosity) of ozone-bleached pulps was found to differ somewhat
from those of pulps subjected to conventional bleaching sequences [111,115].
Ozone-treated pulps are characterized by rapid beating, high-tensile strength but
low tearing resistance. In general, the tearing strength of ozone-bleached softwood
kraft pulps was found to be 10–20% lower as compared to conventionally
bleached pulps of the same provenance [116]. Lindholm has investigated the
impact of various ozone treatments on the tearing strength at a tensile strength of
70 Nm g–1 using a pine kraft pulp [116]. He reported that pulps subjected to Z,
OZ, and OZE treatments had comparable strength properties to those after (CD)E
and (CD)(EO) treatments, provided that the pulp viscosity of the ozone-treated
pulps was greater than 700 mL g–1 (Fig. 7.112). The kappa numbers were about 6
(range: 5–8) for both types of pulp.
400 600 800 1000 1200 1400
10
12
14
16
Pine kraft pulps, after treatments:
unbleached O (CD)E, (CD)(EO) Z, OZ, OZE
Tear Index [mNm2/g] at 70 Nm/g
Viscosity [ml/g]
Fig. 7.112 Tear index at 70 Nm g–1 versus viscosity for differently
treated pine kraft pulps (according to Lindholm [116]).
Figure 7.111 demonstrates a clear relationship between viscosity and tear index
at a given tensile index of the ozone-treated pulps. From this result it can be concluded
that strength properties of ozone-bleached pine kraft pulps are not deteriorated,
provided that pulp viscosity can be maintained above 700 mL g–1. Axegard
et al. reported that the tear strength at a given tensile index of an OAZQPbleached
softwood kraft pulp with a viscosity of 710 mL g–1 was only 5–10% lower
7.5 Ozone Delignification 841
as compared to an OD(EO)DD softwood kraft pulp with a viscosity of 890 mL g–1
[117]. Similar results have been reported by Dillner and Tibbling [118], indicating
that the strength–viscosity relationship presented for conventionally bleached
pulps by Rydholm [119] was not valid for TCF-bleached pulps, including ozone
treatment.
Strength properties of fully bleached hardwood kraft pulps with a sequence
including ozone were comparable to those of a conventionally bleached pulp, although
the viscosity of the ozone-bleached pulp was 20% lower [120]. The preservation
of strength properties despite cellulose degradation through ozone treatment
is also known for hardwood kraft pulps.
Quite recently, the relationships between the molecular weight distributions
(MWDs), intrinsic viscosity and zero-span tensile index of a birch kraft pulp subjected
to HC ozone bleaching were evaluated [121]. The relationship between
rewetted zero-span tensile strength and viscosity is shown graphically in
Fig. 7.113.
400 600 800 1000 1200
0
120
140
160
180
200
Zero-span tensile index [Nm/g]
Viscosity [ml/g]
Fig. 7.113 Zero-span tensile index versus viscosity for ozonetreated
birch kraft pulp (according to [121]). Unbleached
pulp: kappa number 15.5, intrinsic viscosity 1160 mL g–1.
A substantial decrease in fiber strength occurred only when pulp viscosity
decreased below 800 mL g–1. At the highest ozone dosage, the fiber strength was
still 75% of the initial value, corresponding to a viscosity of 510 mL g–1. Apparently,
ozone-treated pulps maintain their initial fiber strength at relatively high
level, despite a substantial reduction in molecular weight. Based on gel permeation
chromatography (GPC) measurements, it was shown that the degradation
pattern through ozonation of kraft pulp was different from that of cotton linters.
In contrast to unbleached birch kraft pulp, ozone-induced cellulose degradation
842 7Pulp Bleaching
did not generate a bimodality of the cotton cellulose peak. The different action of
ozone on MWD was attributed to the presence of lignin in the unbleached birch
kraft pulp, as lignin is known to promote the formation of secondary radicals during
ozone delignification [57]. Due to the enrichment of lignin at the surface of
fibers, it is suggested that cellulose degradation of an unbleached birch kraft pulp
occurs predominantly on the exterior of the fiber, thus generating two distinct cellulose
distributions [21,23].
7.5.7.4 Typical Conditions, Placement of Z in a Bleaching Stage
The placement of an ozone stage within a bleaching sequence must consider both
technological and chemical aspects. The low pH and high sensitivity towards
carry-over from the washing stage of an unbleached kraft pulp suggest that ozone
should not be used in a first delignification stage. Moreover, ozone degrades part
of the phenolic units and makes oxygen less reactive towards lignin in a ZOsequence.
In contrast to the observations of Lachenal et al. [122]. who found that a
single ozone stage (Z) behaves as selectively as an OZ-sequence, Brolin et al. [75],
Ragnar [106], as well as the results shown in Fig. 7.110, show that ozone bleaching
becomes more selective in terms of brightness increase per number of chain scissions
by lowering the incoming kappa number; this means that oxygen delignification
prior to the ozone stage is desirable for reasons of delignification selectivity.
In addition, OZ is favored over Z because of better process economy due to lower
chemical costs (lower ozone consumption and the possibility of recycling oxygen
from the Z-stage) and better possibilities to close the water cycle. The choice between
OZ and Z also depends on the applied ozone bleaching technology. In HC
ozone bleaching, a sufficient quantity of ozone can be reacted in order to achieve
the necessary extent of delignification in a single ozone stage, whereas ozonation
at medium-consistency is limited to a kappa number reduction of maximum 5–7
units (assuming a specific kappa number reduction of about 1unit per kg ozone
charged; see also Tab. 7.36) which in most cases is not enough to complete
delignification.
In a TCF-bleaching sequence consisting of O-, Z-, and P-stages, the use of
hydrogen peroxide (P) is essential to remove the chromophores by oxidizing the
carbonyl groups. As expected, the placement of a P-stage within such a sequence
affects the final bleached pulp properties. OZP- and OPZ-sequences show the
same delignification efficiency, while the latter appears to be more selective as
compared to OZP [122,123]. In a recent study, the effect of placing the Z-stage
prior to (ZP) and after (PZ) standard peroxide bleaching of an (E/O) pretreated
beech dissolving pulp was evaluated by charging different amounts of ozone while
all other reaction conditions were kept constant [123]. GPC measurements
revealed that cellulose degradation was more pronounced for ZP- than for PZtreated
pulps, while the latter had slightly lower brightness values (see Tab. 7.41
and Section 11.3.2.2.2). Figure 7.114 illustrates the course of cellulose degradation
in terms of weight (MW) and number (MN) average molecular weights.
7.5 Ozone Delignification 843
0 2 4 6
30
40
50
200
300
400
PZ-sequence: MW MN
ZP-sequence: MW MN
Molecular weight [kDa]
Ozone charge [kg/odt]
Fig. 7.114 Course of weight (MW) and number (MN) average
molecular weights of beech sulfite dissolving pulps as a function
of ozone charges with Z-stage prior to (ZP) and after Pstage
(PZ), applying identical conditions in each stage [123].
In contrast to the results obtained from Godsay and Pearce [99] and Berggren et
al. [121], the polydispersity index (PDI) – that is, the ratio of the weight average to
the number average molecular weights (MW/MN) – did not increase but rather
was slightly decreased, from about 6.8 in the untreated pulp to 5.5 in the most
severely degraded pulp. This may be attributed to the fact that the beech sulfite
dissolving pulps were subjected to significantly less ozone dosages (2–6 kg odt–1)
than those reported by either Godsay and Pearce (47.7–75.4 kg odt–1) or Berggren
et al. (1–35 kg odt–1).
The placement of Z within the TCF sequence also influences the shape of the
differential MWD. All samples displayed a shift of the MWD towards a lower molecular
weight range as degradation proceeded. The high molecular-weight cellulose
fraction of the pulp subjected to ZP treatment was considerably degraded in
the presence of ozone. From the high molecular-weight peak, with a peak molecular
mass (log Mp) = 5.3, a part of the pulp cellulose fraction was degraded and the
maximum shifted to the second cellulose peak, having a log Mp = 4.7. In the case
of PZ treatment, the shape of the MWD was virtually unaffected by the ozone
charge (Fig. 7.115).
It is well known that ozone treatment of pulp introduces carbonyl groups into the
AHG unit along the polysaccharide chain (see Tab. 7.41an d Section 11.3.2.2.2). In
a subsequent alkaline hydrogen peroxide stage (P), depolymerization of the oxidized
polysaccharide components in the pulp (cellulose and hemicellulose)
is favored due to b-elimination reaction. The high alkali instability of Z-treated
844 7Pulp Bleaching
3 4 5 6 7
6 kg O
3
/odt
4 kg O
3
/odt
2 kg O
3
/odt
ZP-sequence
dW/d(log M)
log Molecular Weight
3 4 5 6 7
6 kg O
3
/odt
4 kg O
3
/odt
2 kg O
3
/odt
PZ-sequence
dW/d(log M)
log Molecular Weight
Fig. 7.115 Differential MWDs of beech sulfite dissolving
pulps prepared by TCF bleaching applying different amounts
of ozone with Z-stage before (upper) (ZP) and after P-stage
(lower) (PZ), applying identical conditions in each stage
[123].
7.5 Ozone Delignification 845
pulps is also the reason why pulp viscosities of PZ-treated pulps are quite comparable
to those of ZP-treated pulps (despite the significantly higher MW and MN
values determined by GPC measurements), provided that the pulps are not subjected
to sodium borohydride reduction prior to viscosity measurements. Although
OPZ bleaching results in superior strength properties, an OZP-sequence
is preferred because of a significantly better brightness stability upon heat or light
exposure. This better brightness stability is achieved by partly oxidizing the carbonyl
groups that are introduced during ozonation. Brightness stability can also
be improved by reducing the carbonyl groups with sodium borohydride after ozonation
[92].
The effect of placing Z-stage on the generation of functional groups as a function
of ozone dosage is discussed in detail in Section 11.3.2.2.2.
Interestingly, in an ECF-sequence comprising O-, Z-, and D-stages, OZD was
found to be more selective than ODZ [124]. This can be explained by the fact that
chlorine dioxide bleaching following a Z-stage shows no adverse effect on cellulose.
The brightness stability after OZD is lower than after OZP bleaching, since a
final chlorine dioxide treatment is less effective in oxidizing or removing carbonyl
group-containing material. Contrary to a treatment in two separate bleaching
stages, a sequential application of chlorine dioxide (D) and ozone (Z) without
intermediate washing was shown to be very selective in delignifying softwood
kraft pulp [125]. This means that the Z- and D-stages are combined into one treatment
(DZ). (DZ) has been found to be more effective for unbleached pulps,
whereas (ZD) seems to be superior for oxygen-delignified kraft pulps [126]. In the
latter sequence, chlorine dioxide partially stabilizes the carbohydrate chain against
alkaline peeling reactions due to oxidation of the carbonyl groups introduced by
ozonation. In the case of an unbleached hardwood kraft pulp, however, chlorine
dioxide reacts with free phenolic groups before the highly reactive ozone is introduced,
the conclusion being that reaction kinetics clearly favors the (DZ) approach
relative to the (ZD) treatment [127]. Furthermore, after chlorine dioxide treatment
the pulp suspension is sufficiently acidic for a subsequent ozone stage. The (DZ)
concept is also advantageous with respect to AOX formation, as ozone has the
ability to destroy some AOX generated during D bleaching. Chlorine dioxide may
act as a radical scavenger, suppressing the extent of radical reactions during the
subsequent ozone treatment. However, in another study it was shown that the
selectivity was not impaired when washing was carried out between D and Z, thus
showing that the presence of residual chlorine dioxide seems not to be essential
for maintaining a high viscosity [128]. The actual reasons for improved selectivity
of a (DZ) treatment remain to be elucidated. In full ECF bleaching sequences, the
replacement of a D0 stage by (DZ) stages was shown to be particularly efficient,
since in the case of a hardwood kraft pulp 1kg charged (consumed) ozone could
replace 1.58 kg charged chlorine dioxide, as shown in Tab. 7.43 [128]. Ozone is
added to the pulp suspension 10 min after the introduction of chlorine dioxide.
846 7Pulp Bleaching
Table 7.43 Comparison of different ECF bleaching sequences of
a hardwood kraft pulp where DO is substituted either by Z or by
(DZ) stages according to [128].
Sequence Stage Chemical Chemical charge Kappa
no.
Bright-ness
[%ISO]