- •Recovered Paper and Recycled Fibers
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •2006, Isbn 3-527-30997-7
- •Volume 1
- •Isbn: 3-527-30999-3
- •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
- •1 Introduction 1149
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •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
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •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]:
- •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
- •In 1950, about 50% of the global paper production was produced. This proportion
- •4.0% Worldwide; 4.2% for the cepi countries; and 4.8% for Germany.
- •1150 1 Introduction
- •1 Introduction
- •1 Introduction
- •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
[mPa.s]
COD
[kg t–1]
Yield
[%]
DEopDD 5.0 85 28.4 37.5 96.0 0.3
DEopDP 3.8 85 27.138.2 96.0 0.25
DEophotDPhot 2.5 98 18.7 44.0 95.15 0.25
Magnesium sulfate (Epsom salt) is an additive applied in peroxide bleaching to
prevent cellulose depolymerization. The benefit of adding magnesium ions
becomes apparent in peroxide bleaching at very high temperatures and with a
high peroxide input. Thus, magnesium salts are added in TCF bleaching but not
normally in an Eop stage or a final P stage. The positive effect of magnesium sulfate
addition becomes apparent in bleaching sequences which used acidic conditions
for transition metals removal. A Q stage under mildly acidic conditions
retains the magnesium and calcium traces in the pulp, thus the impact of additional
magnesium is less pronounced. The same is valid in regions with a high
water hardness and not very narrow water loops. The mechanism of the protective
effect is not clearly understood. Magnesium will be precipitated rapidly as
Mg(OH)2 under the conditions of a peroxide stage. Theories discuss the absorption
of other metal ions during precipitation or an action as scavenger of superoxide
anion radicals [54]. Experiments with peroxide bleaching of deinked pulp in
the disperger (i.e., at high temperature) did indicate any positive effect of another
precipitation. Bicarbonate ions present in the water loop after CO2 neutralization
would decompose peroxide under the conditions of disperger bleaching. The formation
of insoluble magnesium carbonate avoided peroxide losses and improved
the bleaching result [55]. Since carbonate ions are present in caustic soda and are
generated during bleaching, their precipitation may be part of the positive effect
of magnesium salt addition.
7.6 Hydrogen Peroxide Bleaching 865
7.6.5.4 Pressurized Peroxide Bleaching
In TCF bleaching the brightness target of a final peroxide stage might require the
consumption of very large amounts of peroxide, and to achieve sufficient consumption
of H2O2 high temperature will be required. The application of pressure
is also recommended [50,51], with pressure applied ranging from 0.1 MPa to a
maximum of 0.5 MPa. These stages are frequently labeled as P(O) stage because
pressure is applied by oxygen gas addition. The positive effect was described as an
acceleration of an otherwise very slow brightening of the pulp. Later, pressurized
peroxide stages were also recommended for ECF sequences [56]. Several mills
have installed such equipment, although many typically operate without applying
pressure because bleaching is not improved by pressure or oxygen addition
[57,58]. This is in line with current knowledge of the reaction mechanism of alkaline
peroxide bleaching. The peroxide reactions are neither accelerated nor
improved by a moderate pressure increase, however, pressurized equipment is
more expensive than nonpressurized counterparts.
7.6.6
Technology of H2O2 Bleaching
Andreas W. Krotscheck
7.6.6.1 Atmospheric Peroxide Bleaching
The process flowsheet of a typical atmospheric peroxide bleaching system is
shown schematically in Fig. 7.122. Caustic soda is added to the MC pulp coming
from the previous bleaching stage, for example, to the repulper of a drum washer
or to the dilution conveyor after a wash press. The alkaline pulp falls into a standpipe
and is mixed with peroxide as it enters the MC pump.
The pump provides good mixing of the peroxide into the pulp suspension, and
a dedicated mixer is often not required. The pulp proceeds to an atmospheric
upflow reactor where the bleaching reaction takes place. Depending on the feed
requirements of the subsequent washing equipment, the pulp slurry is discharged
from the reactor either at low or medium consistency.
MC PUMP REACTOR WASHING
Pulp from
preceding
stage
Pulp to
next stage
NaOH
H2O2
Fig. 7.122 Process flowsheet of a typical atmospheric peroxide bleaching system.
866 7Pulp Bleaching
Washing after a peroxide stage is usually carried out with single-stage washing
equipment, for example, with a wash press, a single-stage Drum Displacer™, an
atmospheric diffuser, or a vacuum drum washer.
The material of construction for wetted parts in a peroxide stage is typically a
higher grade of austenitic stainless steel.
Further information regarding atmospheric peroxide bleaching equipment,
including medium-consistency pumps and atmospheric upflow reactors, is provided
in Section 7.2. Pulp washing is detailed in Chapter 5.
7.6.6.2 Pressurized Peroxide Bleaching
The equipment used for pressurized peroxide bleaching is very similar to oxygen
delignification equipment. The process flowsheet of a typical pressurized peroxide
bleaching system is shown schematically in Fig. 7.123. Caustic and peroxide are
added to the medium-consistency pulp coming fromthe previous bleaching stage, as
in atmospheric peroxide bleaching. TheMCpump forwards the pulp suspension to a
high-shear mixer which is charged with oxygen and steam, after which the threephase
mixture proceeds to a pressurized upflow reactor where the bleaching reaction
takes place.
As in oxygen delignification, it is essential that the high-shear mixer creates
stable micro-bubbles which ensure a homogeneous bleaching result, without
channeling in the reactor. After its discharge from the reactor top, the pulp suspension
is separated from the gas phase in a blowtank. The offgas from the blowtank
can normally be blown to the atmosphere. Depending on the feed requirements
of the washing equipment, the pulp slurry is discharged from the blowtank
either at low or medium consistency.
Washing after a peroxide stage is usually carried out with single-stage washing
equipment, for example, with a wash press, a single-stage Drum Displacer™, an
atmospheric diffuser, or a vacuum drum washer. Further information on pressurized
peroxide bleaching equipment, including medium-consistency pumps and
mixers, pressurized reactors and blowtanks is provided in Section 7.2. Details of
pulp washing are provided in Chapter 5
MC PUMP
HIGH-SHEAR
MIXER
REACTOR BLOWTANK WASHING
O2 Steam
Pulp from Offgas
preceding
stage
Pulp to
next stage
NaOH
H2O2
Fig. 7.123 Process flowsheet of a typical pressurized peroxide bleaching system.
7.6 Hydrogen Peroxide Bleaching 867
7.6.7
Application in Chemical Pulp Bleaching
Hans-Ullrich Suss
Hydrogen peroxide is applied in ECF and TCF bleaching sequences. Currently,
ECF bleaching is by far the most dominant bleaching technology; indeed, in 2004
over 90% of all wood pulp was bleached with chlorine dioxide as the main bleaching
agent. (In Asia, a relatively large amount of one-year-plant pulps is still
bleached using chlorine and hypochlorite; thus, in relation to all pulp production,
ECF bleaching might represent only 70%.) TCF bleaching has become a niche
specialty, notably in Sweden and in Central European sulfite mills. Its world share
in bleached pulp production is estimated at about 5%. In ECF bleaching, H2O2 is
used in the extraction stages following chlorine dioxide treatment. After the acidic
D stage, a high level of oxidized lignin remains in the pulp, due to its limited solubility
at acid pH. Consequently, the acidic and alkaline stages are applied alternately.
The effect of an extraction is a further decrease in lignin content, because
the formation of sodium salts of the carboxylic acids within the oxidized lignin
residual results in a better solubility. The demand for caustic soda depends on the
carry-over of acid from the D stage and the carboxylic acids content. Increasing
the amount of caustic soda above a certain level has only a limited effect
(Fig. 7.124). With effective washing the demand for caustic soda can decrease significantly.
The temperature in an E stage is between 75 °C and 90 °C, while the pH
is typically about 11 at the start of treatment and about 10 at the end.
1,0 1,2 1,4 1,6 1,8 2,0
2
4
6
8
10
(κ-factor)
D
0
(0.20)-E D
0
(0.20)-Ep D
0
(0.25)-E D
0
(0.25)-Ep
Kappa number
NaOH charge [%]
Fig. 7.124 Impact of increasing amounts of
caustic soda in the extraction stage following a
D0 stage; softwood kraft pulp, kappa 24.6.
Kappa factor is the multiplier for the
kappa number value to calculate the input of
active chlorine to the D0 stage. Conditions: D0
stage 50 °C, 1 h; E(p) stage 0.5% H2O2, 75 °C,
1.5 h, both at 10% consistency.
868 7Pulp Bleaching
The graph in Fig. 7.124 shows the potential for reducing the amount of residual
lignin by an addition of oxidants. The oxidation of quinoid structures improves
the solubility of lignin. In the first E stage, typically oxygen and H2O2 are applied.
Oxygen gas is mixed with the pulp in high-shear mixers, which allow a very thorough
distribution of fine gas bubbles within the fibers. The oxygen level is typically
at 0.3–0.4%. While small amounts of oxygen are consumed rapidly, too-high an
input can result in the re-formation of large oxygen bubbles that may channel
through the tower and negatively affect pulp flow. For a moderate input of oxygen,
the counter-pressure of a tower or pre-tube of 15–20 m height is sufficient. A
potential solution to the problem of higher oxygen charges is to use a pressurized
tower. However, such as investment is questionable because the number of oxidizable
sites in the remaining lignin is normally small. Therefore, a high input of
oxygen does not result in any significant benefits. The exemption are pulps with
unusually high initial kappa numbers (>20). The application of H2O2 does not
require pressure, and in most mills oxygen and H2O2 are applied simultaneously
in the first E stage. The impact of an increasing amount of H2O2 is shown graphically
in Fig. 7.125. Because of the limited availability of easily oxidizable sites, levels
of H2O2 above about 0.5% must be activated by a higher temperature. Brightness
increase is accompanied by a further drop in residual lignin levels, this being
the result of additional oxidation reactions improving lignin extraction. Peroxide
addition can be used to balance the demand for caustic soda during the E stage
(see Fig. 7.124). Increasing the addition of caustic soda has a limited impact on
0,00 0,25 0,50 0,75 1,00
70
75
80
85
90
Kappa number
brightness
Brightness [% ISO]
H
2
O
2
-charge [%]
3,0
3,5
4,0
4,5
5,0
kappa number
Fig. 7.125 Impact of the addition of H2O2 to an E stage in
bleaching eucalyptus kraft pulp, oxygen-delignified pulp,
D0 stage at 50 °C with kappa factor 0.2. Ep stage at 85 °C for
amounts of 0.25% to 0.5% H2O2, larger amounts applied at
95 °C, constant 1.4% NaOH, 1.5 h.
7.6 Hydrogen Peroxide Bleaching 869
lignin removal. Rather than apply excess caustic soda, the use of moderate
amounts of H2O2 allows the brightness to be increased and the kappa number to
be decreased, simultaneously.
In hardwood pulp bleaching, the impact of peroxide application on Kappa number
is less pronounced. Because neither H2O2 nor oxygen can degrade HexA, the
amount of HexA remaining in the pulp after the D0 stage will remain unaffected
by their addition. Both chemicals will only further oxidize the lignin residual.
Therefore, the additional decrease in kappa number is small compared with softwood
pulp. The impact of H2O2 addition to an E stage following a D0 stage at 50 °C
is shown graphically in Fig. 7.125. Despite moderate changes in kappa number,
the impact on brightness is significant. A temperature increase is required to trigger
the consumption of larger amounts of H2O2. However, despite the higher temperature,
above an input of about 0.4% H2O2 a peroxide residual will remain. The
impact on lignin removal decreases further if large amounts of chlorine dioxide
are applied in D0, or the temperature is raised. The use of a very high temperature
(>90 °C) during the first chlorine dioxide stage allows simultaneous delignification
and hydrolysis, respectively destruction of HexA. In comparison to standard D0
stage conditions (50–70 °C), this significantly reduces the amount of double bonds
measured after the Eop stage. Values between kappa 2 and 3 are achieved with
extraction only. Consequently, the impact of an oxidative support of the extraction
stage with O2 and H2O2 on the remaining double bonds becomes minimal,
though the effect on brightness is still pronounced. An example of the impact of
increasing H2O2 amounts in the E stage following a hot D0 stage with kappa factor
0.2 is shown in Fig. 7.126. It is necessary to raise the temperature to enforce
0,25 0,50 0,75 1,00
82
84
86
88
Temperature [°C]
85 95
Brightness [% ISO]
H
2
O
2
-charge [%]
Fig. 7.126 Impact on brightness of an intensified delignification
by a hotD0 stage on peroxide effectiveness in the subsequent
extraction stage. E stage at 10% consistency, with 1.4%
NaOH.
870 7Pulp Bleaching
the consumption of a higher input of peroxide. Without peroxide addition, the E
stage brightness is only at 73% ISO.
The need to add oxidants to the extraction stage might be questioned. Bleaching
with the stages DEDED is possible in theory, but this would result in a rather high
demand for chlorine dioxide with consequences for cost and effluent load (AOX).
In order to optimize effects it is important to use the potential of other chemicals
to degrade lignin and chromophores. The use of oxygen and H2O2 in the E stages
promotes the E stage from simply an extraction to a brightening and delignification
stage. The improvement in pulp brightness by up to 10 points, compared to
an Eo stage under identical conditions, is shown in Fig. 7.127. This advantage in
brightness is still apparent after subsequent D1 and D2 stages. The right-hand portion
of the graph shows, for the same input of chlorine dioxide to D1 and D2, an
advantage of about one brightness point. This represents an economical and ecological
advantage which is also beneficial with regard to the operational stability of
the bleaching process. The production of off-grade pulp becomes less likely if the
final brightness gains are smaller, because no large variations in chemical addition
are required to compensate brightness.
Eo Eop Eop
70
75
80
85
brightness (%ISO)
Eo Eop Eop
86
88
90
92
brightness (%ISO)
D1 D2
H2O2: 0.4% 0.6% 0.4% 0.6%
Fig. 7.127 Impact of the addition of H2O2 to the Eop stage on
brightness development in final bleaching.
One positive side effect of adding H2O2 to the first E stage is a significant decrease
in effluent color. Typically, an E-stage effluent is medium to dark brown in color, but
becomes light brown on addition of H2O2 [59]. Therefore, some mills apply H2O2 not
only for its bleaching effect but also to control effluent color. Another positive effect is
the higher intensity of shives bleaching [60,61]. Even when shives are not fully
bleached, they become lighter in color, which reduces their visibility.
Hydrogen peroxide is also applied advantageously in the second E stage of the
longer sequences used in softwood pulp bleaching. In a D0EopD1EpD2 sequence,
7.6 Hydrogen Peroxide Bleaching 871
H2O2 reduces the demand for chlorine dioxide in final bleaching. The substitution
of chlorine dioxide by H2O2 in the sequence follows stoichiometric rules: 1kg t–1
H2O2 replaces 2 kg t–1 active chlorine [62]. This is shown graphically in Fig. 7.128,
where the application of H2O2 results in a higher brightness with lower input of
chlorine dioxide. The resultant flat curve crosses the 90% ISO line at lower input
of ClO2. It becomes easier to achieve a standard deviation of brightness of, for
example 0.5 points around the 90% ISO value.
10 15 20
86
87
88
89
90
91 H
2
O
2
-charge in Ep: 2.5 kg/t
savings in ClO
2
Sequence: DED DEpD
Brightness [% ISO]
Active chlorine charge(D
1
+D
2
) [kg/odt]
Fig. 7.128 Substitution of chlorine dioxide by H2O2 in final
bleaching of softwood kraft pulp with the stages D1E(p)D2.
Amount of ClO2 in D1 variable, amount in D2 constant at 5 kg t–1
active chlorine; all stages at 70 °C, 2 h, 10% consistency.
Most modern mills operate an oxygen stage, and therefore have a low level of
lignin entering the bleach plant. This permits shorter bleaching sequences, such
as a four-stage sequence with a D0-Eop-D1-D2 configuration. The two D stages can
be separated by a washing step, or follow each other directly. Another alternative
is a short neutralization with caustic soda after D1, which is followed by mixing
the chlorine dioxide for D2. The target of these modifications is lower investment
costs. The five-stage version D0-Eop-D1ED2 or D0-Eop-D1EpD2 is certainly more
effective, though the differences are not pronounced enough in terms of investment
costs. The demand for chlorine dioxide in a shorter sequence can be rather
high, especially when the target is very high brightness. Several problems of these
shorter sequences are shown in Fig. 7.129. First, it becomes clear that the demand
for chlorine dioxide in a three-stage sequence would become extreme if the brightness
target were to be at 90% ISO. Whilst a level around 89% ISO is within reach,
the flatness of the curve indicates that, to reach a much higher level, would be
872 7Pulp Bleaching
very difficult. After washing the pulp, the addition of further chlorine dioxide (D2)
becomes effective once more. Nevertheless, about 1.5% of active chlorine is required
to achieve (safely) more than 90% ISO. The same brightness is achieved
with much less chlorine dioxide when the second D stage is replaced by a P stage.
The substitution is more than stoichiometric, as one part of peroxide replaces
about four parts of active chlorine. This is the result of a more effective oxidation
by two differently acting chemicals. An additional positive side effect of the P
stage is an improved brightness stability (see the next section).
0,0 0,5 1,0 1,5 2,0
87
88
89
90
91 H
2
O
2
-charge in P: 0,25 %
Bleaching stages: D
1
D
1
D
2
D
1
P
Brightness [% ISO]
Total active chlorine charge [%]
Fig. 7.129 Substitution of the final D stage with a final P
stage in a D0EopD1D2 sequence. Oxygen-delignified eucalyptus
kraft pulp. D0 with factor 0.2 at 50 °C, Eop kappa 3.0,
brightness 81.5% ISO. D stages at 75 °C, 2 h; P with 0.25%
H2O2, 0.3% NaOH, 85 °C.
7.6.7.1 Stabilization of Brightness with H2O2
Brightness stability is affected by a number of parameters, and is typically analyzed
in tests using accelerated aging. The pulp samples are exposed to elevated
temperature under either dry or humid conditions. Brightness losses during dry
heating are normally less pronounced compared with humid reversion tests. A
standard procedure is to heat handsheets over boiling water for 1h. This test
method is described as E.4P method by Paptac [63]. The changes in light absorption
and scattering are measured as post color number [64]; the smaller the number,
the less reversion has taken place. Humid brightness reversion is thought to
correlate more with natural aging occurring in pulp bales [65].
7.6 Hydrogen Peroxide Bleaching 873
The intensity or aggressiveness of the bleaching process certainly has an
impact. Compounds and conditions that affect brightness stability include transition
metals, remaining lignin, hemicelluloses, and the pulping process used [66].
Past experience with hypochlorite pointed to rather negative effects of low pH,
high temperature and high charges of this chemical on brightness stability. Losses
could be attributed to oxidation of the cellulose chain, and were often very significant.
50 years ago kraft pulp was typically not bleached above about 80% ISO
brightness using a typical CEHH sequence. The situation has now changed with
the increasing use of chlorine dioxide, initially in a final stage (CEHD) and later
with -D1E2D2 or -D1EpD2 final bleaching.
Mill experience teaches that the higher the brightness, the greater the stability,
though this applies to the same sequence and moderate changes in bleaching conditions.
The effectiveness of lignin removal or impurities removal is important for
the stability of brightness, and therefore differences between TCF and ECF bleaching
can be expected. Indeed, in TCF bleaching of birch kraft pulp HexA were identified
as a source of high reversion [67]. Likewise, poor brightness stability was
found for ECF “light” bleached softwood pulp [68]. However, in “normal” ECF
bleaching HexA were found not to be the source for reversion [69], as it was
removed completely in the process. These variances explain the importance of
bleaching conditions. The TCF sequence used to bleach birch pulp with poor
brightness stability [67] was conducted exclusively with alkaline bleaching steps.
In order to attribute correctly the reversion to certain sources, it is important to
understand how complete or ineffective potential sources for the development of
colored compounds are destroyed.
A comparison of different ECF sequences, all using sufficient chemical for lignin
oxidation and HexA hydrolysis or destruction [69], showed that cellulose depolymerization
(apparent as a lower viscosity) has no direct influence on brightness
stability. Neither hot acid hydrolysis nor ozone nor aggressive conditions and a
very high temperature in the final peroxide stage had any significant impact on
reversion. The positive impact of using more bleaching chemical in a D0-Eop-D1
sequence is illustrated graphically in Fig. 7.130. A moderate input of chlorine
dioxide (in this example, 1%) provided a reasonable brightness close to 89% ISO,
but the brightness was not stable. With a loss of about 10 points of brightness in
humid reversion, the instability was pronounced. The use of additional chemical
improves bleached brightness, but not to any great degree. A doubling of the
active chlorine input (from 1% to 2%) added only one brightness point, yet reversion
losses decreased from 10 points to only 7 points. The more intense degradation
of lignin or other “impurities” was seen to improve brightness stability.
The advantage of an additional treatment stage to improve brightness and
brightness stability is illustrated in Fig. 7.131. A small amount of active chlorine
(0.5%) applied in the second D stage lifts brightness to 90% ISO, and reduces
losses in humid reversion to about 5 points of brightness. Even more pronounced
is the improvement of a stoichiometric substitution of chlorine dioxide by H2O2
(0.5% active Cl with 0.25% H2O2). Losses in reversion decrease at best to only 3.5
points.
874 7Pulp Bleaching
1 2
act Cl in D1
78
80
82
84
86
88
90
brightness (%ISO) T260
UM200
bleached
1.5
Fig. 7.130 Analysis of brightness reversion of eucalyptus kraft
pulp after three bleaching stages with D0-Eop-D1. Active chlorine
to D0 with kappa factor 0.23at 50 °C, 1 h. D1 at 70 °C, 2 h.
Brightness analyzed after 2 h at 100 °C, 100% humidity and
4 h at 105 °C (UM 200 test).
D1: 1% act. Cl 1.5% 2%
D2 P D2 P D2 P
78
80
82
84
86
88
90
brightness (%ISO) T260 UM200 bleached
Fig. 7.131 Analysis of brightness reversion of eucalyptus kraft
pulp after four bleaching stages with D0-Eop-D1-D2, respectively
D0-Eop-D1-P.
This advantage of an application of H2O2 can be attributed to the destruction of
carbonyl groups and quinoid structures remaining in pulp after the D stage.
Using UV Raman spectroscopy, Jaaskelainen [70] detected these intermediates (or
end products) of chlorine dioxide bleaching [9] in kraft pulp bleached with a final
D stage. In peroxide-bleached pulp such structures were absent, the reason being
the rapid reaction of alkaline peroxide with quinones.
7.6 Hydrogen Peroxide Bleaching 875
The importance of the remaining quinoid structures, respectively their elimination
by alkaline H2O2, explains the rather good brightness stability of TCF or ECF
“light” bleached pulp. Despite their higher lignin residual, these pulps normally
show very moderate reversion losses in accelerated aging. The same applies to
bleached mechanical pulp, which is rather stable during heat-induced aging. Its
sensitivity against light-induced yellowing has phenols as the main source, and
follows a different reaction pathway [71].
The conclusion drawn regarding the source for reversion of the TCF-bleached
birch pulp mentioned above [67] can now be seen from a different angle. This
pulp had been subjected to a very intense peroxide treatment, but not to any acid
stage capable of removing HexA. Therefore, the source for the remaining reversion
of this pulp was HexA. It is not appropriate to generalize this specific finding,
however. In ECF bleaching with sufficient hydrolysis or oxidation of HexA,
other compounds are responsible for brightness losses in aging. The best brightness
stability results from the most effective removal of all impurities. This
includes sufficient bleaching chemical with different reactivities towards the
impurities and definitively sufficient washing. The combination of a very high
temperature in the D0 and the last D stage (90 °C) with a final peroxide stage provides
access to the brightness range above 93% ISO, and simultaneously to
extreme brightness stability. The increase in brightness, and the development of
brightness stability in humid reversion, is shown graphically in Fig. 7.132. The
peroxide-bleached pulp loses less than one point of brightness during the aggressive
aging treatment.
D1 D2 P
88
89
90
91
92
93
brightness (%ISO)
0
0.1
0.2
0.3
0.4
0.5
post color #
brightness
post color #
Fig. 7.132 Development of brightness and brightness
stability of eucalyptus kraft pulp bleached with the stages
hotD0-Eop-D1-hotD2-P, reversion analyzed after 2 h at 100 °C,
100% humidity.
876 7Pulp Bleaching
7.6.7.2 Catalyzed Peroxide Bleaching
Under acidic conditions, H2O2 reacts very slowly with pulp, the reaction being
accelerated by the presence of molybdenum and tungsten salts [72]. Such an
acidic treatment can be used to activate an alkaline peroxide bleaching stage. During
the period of intense searching for alternatives to chlorine bleaching – that is,
before settling on ECF bleaching – molybdate-catalyzed peroxide was intensely
investigated [73]. Obviously just a niche application remained [74]. In order to
achieve a sufficient turnover of peroxide with a molybdate input at 0.04%, the
temperature must be elevated above 80 °C, while the required retention time is
about 2 h.
A catalyst for alkaline peroxide bleaching was described by Patt [75]. The manganese
complex accelerates and intensifies bleaching in both ECF and TCF applications.
Unfortunately, synthesis of the manganese compound [76] (Fig. 7.133) is
complicated, and this results in high – and for an industrial application – prohibitive
costs. On a laboratory scale, the addition of 10–50 ppm of the complex
resulted in a higher brightness at a lower demand for H2O2. To date, no other
compounds with similar properties and lower production costs have been
described in the literature.
N
Mn
N
N N
Mn
N
N
O
O
H3C
CH3
CH3
H3C
2“
O O
H3C
2 [ClO4]”
Fig. 7.133 Model of the manganese complex effective in
activating peroxide bleaching of kraft and sulfite pulp.
7.6.7.3 Application in TCF Sulfite Pulp Bleaching
The brightening of sulfite pulp is rather easy with H2O2. The reason for this is a
low level of residual lignin, and very little lignin condensation occurring during
the pulping process. Today, therefore, most sulfite pulp is bleached under TCF
conditions simply with H2O2. The acidic pulping process can leave a high amount
of hemicelluloses in the pulp. This permits pulping with a high yield, though the
hemicelluloses are soluble under alkaline conditions. Because a higher input of
caustic soda is required to activate large amounts of H2O2, the intensity of bleaching
affects both yield and effluent load. An example of the impact of temperature
and caustic soda on chemical oxygen demand (COD) and yield is provided in
Fig. 7.134. The graph illustrates the high likelihood of a significant yield loss
when caustic soda is applied without care. The combination of a very high charge
of caustic soda and very high temperature is detrimental to pulp yield and effluent
load. Not surprisingly, this correlation is linear [77] within the typical range of
caustic soda addition.
7.6 Hydrogen Peroxide Bleaching 877
30 40 50 60 70 80
20
40
60
80
1
2
1
2
3
3
Temperature [°C]: 70 98
Yield Loss [kg/t]
COD [kg/t]
Fig. 7.134 Impact of alkali amount on chemical oxygen
demand (COD) and yield at two different temperature levels
and NaOH charges (1%, 2%, and 3%), 10% consistency, 1 h.
EopP(hc) Op(MgO)P(hc) EopP(mc) P(O)P(O)
90
92
94
96
98
yield (%)
Fig. 7.135 Yield in bleaching of spruce sulfite pulp to identical
brightness (88% ISO) with four different two-stage processes.
mc: medium consistency; hc: high consistency.
Bleaching to very high brightness typically begins with an MC treatment, with
the application of a moderate amount of caustic soda and H2O2, sometimes in
addition to a small amount of oxygen. Such a first step reduces the lignin level
and prepares a final bleaching step. High-consistency bleaching requires less
caustic soda because the higher concentration results in a higher pH with less
878 7Pulp Bleaching
chemical. This becomes apparent in a comparison of different bleaching technologies.
In Fig. 7.135, the yields are compared following two-stage processes. Medium-
consistency delignification followed by HC peroxide bleaching (Eop-Phc)
results in the highest yield. The aggressive alkalinity of a high-temperature pressurized
peroxide process (PO) [78] causes a significant drop in yield. Consequently,
the recommendation of such process conditions [78,79] leads in the
wrong direction. The corresponding brightness increases are shown in Fig. 7.136,
which also contains details of the applied process variables. The combination of
MC delignification and HC bleaching clearly provides the best response. Alternatively,
a combination of oxygen delignification, chelation and peroxide bleaching
(O-Q-P) is applied.
The use of magnesium oxide in an oxygen/peroxide delignification allows recycling
of the effluent into the recovery system in magnesium sulfite pulping
[80,81]. This decreases the effluent load from final bleaching, although the effectiveness
of brightening is lower – typically only the low 80s are accessible in PMgO
bleaching. A sequence applied in practice uses oxygen and peroxide together with
MgO and countercurrent washing from the acid stage after MgO treatment. This
acid stage is necessary to remove residual MgO completely. An OPMgO-A-P
sequence can reach brightness of 86–88% ISO. The effluent load is significantly
1 2 3 4
78
81
84
87
90
first P(O) stage
Eop brightness
Sequences: P(O)P(O) Eop(MC) Eop(HC)
Brightness [% ISO]
Total H
2
O
2
-charge [%]
Fig. 7.136 Brightness increase of spruce sulfite
pulp (kappa 17.1) in P stage. Pre-bleaching
with Eop, 1.5% NaOH, 0.75% H2O2, 1.5 h,
0.3MPa O2, 10% consistency. Second bleaching
stage: Pmc: H2O2 and NaOH variable, 3h,
80 °C, 10% consistency. Phc: H2O2 and NaOH
variable, 0.5% sodium silicate, 4 h, 75 °C, 25%
consistency. P(O) bleaching at 10% consistency.
1st stage with 2% H2O2, 1.8% NaOH,
0.3MPa O2, 95 °C, 1.5 h; 2nd stage with 1% or
2% H2O2, 1.6% NaOH, 95 °C, 1.5 h.
7.6 Hydrogen Peroxide Bleaching 879
decreased through the recycling procedure. Because the make-up of MgO required
in pulping can be added to the bleaching stage, this process will not lead to
additional costs – on the contrary, the costs for caustic soda are lowered.
7.6.7.4 Activators for H2O2 Bleaching
Activation steps with ozone, peracetic acid, or catalyzed acidic H2O2 have been
described in order to improve the performance of the final peroxide stage [82].
Cyanamide can be used to improve the performance of an MC peroxide bleaching
stage [83,84], but the more effective reaction of H2O2 at HC conditions [85] has
outphased this application. Dissolving pulp is bleached with an ozone step to
remove traces of lignin [86]. Peracetic acid has been used on a large scale to boost
brightness above the 90% ISO range. This process is used to bleach magnefite
pulp with the sequence O-Q-Paa/P. The activation uses on-site-mixed peracetic
acid (the mixture contains equilibrium peracetic acid); thus, peracetic acid, acetic
acid, H2O2 and water are present. Under the slightly acidic conditions of the Paa
stage, H2O2 is not consumed. The excess of H2O2 is activated for bleaching simply
by adding caustic soda. The activation with small amounts of Paa (0.15–0.3%)
allows two additional brightness points, and permits brightening to a level above
91% ISO [87].
7.7
Peracetic Acid in Pulp Bleaching
Bleaching of pulp with peracids is limited on an industrial scale to the application
of peracetic acid, CH3COOOH. In the past, other per-compounds were also promoted
for bleaching, among these being Caro’s acid, H2SO5, or mixtures of Caro’s
acid and peracetic acid. The interest in their application stems from the search for
alternatives to chlorine bleaching. The application of peracids was tested in ECF
and TCF bleaching sequences [1]. With the increasing knowledge about ECF
bleaching and its good environmental performance, peracids became less interesting.
Today, peracetic acid occupies a niche in TCF bleaching. The application of
peracetic acid in TCF bleaching was introduced because of the need to modify residual
lignin to allow its destruction in hydrogen peroxide bleaching, and to
improve the economics of TCF sequences. Typically, a peracid treatment is applied
under weak acidic conditions. This results in an improved delignification and a higher
brightness in the following alkaline peroxide stage.
Peracetic acid has a sharp pungent odor. It has a boiling point of 103 °C and a
vapor pressure of 3325 Pa at 25 °C. It is a weaker acid than acetic acid, and is produced
by mixing (glacial) acetic acid with hydrogen peroxide. The addition of a
strong acid (e.g., sulfuric acid) accelerates the formation of the equilibrium between
acetic acid, hydrogen peroxide, water and peracetic acid:
2CH3COOH _ H2O2 _ H2O _ CH3COOOH _ CH3COOH _ 2H2O
880 7Pulp Bleaching
The equation shows that the equilibrium can be shifted to the right by applying
high concentrations of hydrogen peroxide. However, there will always be an excess
of hydrogen peroxide and unreacted acetic acid in the mixture. This increases the
cost for the application of equilibrium peracetic acid in bleaching, because the
reaction conditions for peracid bleaching will not allow the reaction of hydrogen
peroxide. Another important factor is product safety. Storage and handling of
higher concentrations of peracetic acid with a high H2O2 content are restricted
due to its potential hazards. This prevents application in mill practice.
The peracetic acid equilibrium is shifted to the right by distillation under vacuum.
The resulting peroxide conversion is greater than 90%. The distillation products
are the most volatile compounds, water and peracetic acid (boiling point
103 °C). This distillate must be cooled to prevent re-formation of the equilibrium.
The ideal storage temperature for the mixture is below 0 °C; therefore, storage
tanks require both insulation and refrigeration. Cooled distilled peracetic acid is
commercially available with a content of 35–40% peracetic acid in water. Because
of the absence of a strong acid, re-formation of the equilibrium is very slow. An
accidentally higher storage temperature (e.g., ambient temperature) would not
constitute a safety hazard, but the resulting “new” equilibrium would produce a
lower concentration of peracetic acid.
Another economical alternative for peracetic acid application is on-site mixing
of peracetic acid with hydrogen peroxide. At a temperature slightly above ambient,
and with acid activation, the equilibrium is established within a few hours. Mixtures
with a content >8% peracetic acid and <40% H2O2 are commercially produced
on-site. In order not to waste the content of hydrogen peroxide in this equilibrium,
the Paa treatment must be followed by the peroxide stage, without intermediate
washing. Following an addition of caustic soda, the unused hydrogen peroxide
content in the pulp reacts in the subsequent P step.
The reactions of peracid with lignin follow mainly an electrophilic pathway.
With regard to reactivity, peracetic acid (CH3COOOH) has an advantage over
Caro’s acid (H2SO5). Peracetic acid has a pKa value of 8.2, and is only partly dissociated
at neutral or moderately acidic pH. Peracetic acid reacts via hydroxylation
(OH+), splitting into a cation and an anion, acetate (CH3COO–). In contrast, Caro’s
acid has two pKa values of 1and 9.3. Thus, it is completely dissociated
(SO5
2– + 2H+ ). An electrophilic reaction is only possible via the mono anion
(HSO5
– OH+ + SO42–), which is present only at very low concentration. This
explains the slow reaction of Caro’s acid with lignin. A comparison of both compounds
at identical active oxygen content favors Paa. The final kappa number is
lower, and the final brightness higher after the Paa-P treatment.
The demand for peracetic acid is moderate. Figure 7.137 illustrates an example
of a TCF bleaching application. An input of 0.1–0.5% peracetic acid is sufficient
for the activation. Paa is applied at moderately acidic pH and at a temperature of
about 80 °C. Because the peracid reaction is slow, a retention time of 1h is not
sufficient to consume a charge of more than 0.5% at 80 °C. On the other hand,
because of the high temperature, peracetic acid is hydrolyzed into acetic acid and
7.7 Peracetic Acid in Pulp Bleaching 881
0 0.1 0.3 0.5
peracetic acid (%)
85
86
87
88
89
90
brightness (%ISO)
P Paa-P
Fig. 7.137 TCF bleaching of softwood kraft pulp with and
without peracetic acid activation of a final P stage. Pulp prebleached
with OO-Q-OP; Paa stage at 85 °C, 1 h, 10% consistency;
final P stage with 2% H2O2, 1.4% NaOH, at 95 °C, 3h,
10% consistency.
hydrogen peroxide. After about 1h the remaining peroxo compound will be predominantly
hydrogen peroxide.
Sequences with peracetic acid activation use the Paa step ahead of the final P
stage. Thus, a TCF sequence could be OO-Q-OP-Paa/P or OO-Q-OP-Paa-P. Peracetic
acid was also recommended as a final treatment step to boost the brightness
of TCF pulp [2]. However, because of the type of reactions occurring with the
remaining lignin, brightness stability is affected negatively. There is an improvement
in brightness, but not in its stability. In addition, the high temperature required
makes application in a high-density storage tower difficult.
Figure 7.138 illustrates the impact of a Paa post-treatment of a TCF pulp with
0.5% Paa at 80 °C. The improvement in brightness by more than 2 points is significant.
Brightness stability, however, decreases. This suggests the formation of
potential chromophores with Paa oxidation. These chromophore precursors are
removed by treating pulp with alkaline peroxide. The change in pH changes the
oxidant form from an electrophile into a nucleophile and thus removes quinones,
it is likely that Paa generates phenolic intermediates. Peracid treatment is therefore
more advantageous if followed with a peroxide stage.
The moderate speed of reaction of peracids with lignin is accelerated in the
presence of chloride ions. Because of their high oxidation potential, peracids will
oxidize chloride to chlorine. This acceleration increases the effect [3], it is, however,
not environmentally sound because halogenated compounds are generated.
Higher levels of AOX in the effluent and OX in pulp result from a reaction of peracid
in the presence of chloride [4]. This contradicts the purpose of TCF bleaching.
Peracetic acid should not be applied when there is a high level of chloride in the
water loop.
882 7Pulp Bleaching
P P-Paa P-Paa/P
85
86
87
88
89
90
brightness (%ISO)
0.2
0.3
0.4
0.5
0.6
0.7
post color #
brightness post color #
Fig. 7.138 Impact of a post-treatment with peracetic acid on
brightness and brightness stability. Paa stage with 0.5% at
80 °C, 1 h 10% consistency, Paa/P at 80 °C with 0.5% Paa and
alkali treatment after 0.5 h with 0.5% NaOH and continuation
for 1 h. Stability analyzed after hot humid reversion (E.4P).
7.8
Hot Acid Hydrolysis
Kappa number – that is, permanganate demand – is generally assumed to represent
the lignin content in pulp with sufficient accuracy. Because permanganate is
a strong oxidizing compound, which reacts not only with the aromatic lignin but
also with other double bonds, this assumption applies only with limitations. Relatively
recently, a large number of “other” double bonds were identified in kraft
pulp. The first indication into the source of these double bonds was the identification
of furan-2-carboxylic acid as main product of its hot acid hydrolysis by
Marechal [1]. The origin was soon identified as hexenuronic acid, HexA,
(Fig. 7.139), a compound generated during alkaline pulping by methanol elimination
from 4-O-methylglucuronic acid on the xylan [2]. Therefore, permanganate
consumption of a pulp describes dominantly the sum of its reaction with lignin
and hexenuronic acid [3].
The ratio of substitution of the xylan backbone with 4-O-methyglucuronic acid
in most hardwoods is about 10:1 (Xyl:Me-GluU). It is higher in most softwoods
(5:1or 4:1), however, the amount of xylan being typically significantly lower in
softwood (spruce, Picea abies ~9%) compared with hardwood (birch, Betula verrucosa
~33%) [4]. Therefore, hexenuronic acids are present in softwood kraft pulp at
a low level, and their share in the kappa number is only about one unit. In
unbleached hardwood kraft pulp, about one-third of the permanganate consumption
in kappa analysis is caused by hexenuronic acid. In oxygen-delignified hard-
7.8 Hot Acid Hydrolysis 883
O
OH
O
O
O
-OOC
HO OH O
Fig. 7.139 Model of hexenuronic acid (HexA) on a xylan side chain.
wood kraft pulp, the level of “other compounds” is even higher, and can reach
almost 50%. (This explains why the degree of “delignification” achieved in an oxygen
stage of hardwood pulp seems more moderate compared with softwood pulp.
As removed “kappa number” it might reach just 30%, but as removed real “lignin”
it is typically better than 50%.)
The reason why hexenuronic acid remained undetected for decades is that the
normally applied analytical method for lignin preparation uses acidic conditions,
during which acid hydrolysis of the hexenuronic acid occurs.
The double bond of hexenuronic acid reacts with electrophilic bleaching agents,
such as chlorine dioxide and ozone. The question of how, logically, to remove hexenuronic
acid can be answered with the background of its analytical evasiveness.
At elevated temperature and low pH, hexenuronic acid is rapidly hydrolyzed. The
first report of the effect of hot acid treatment of hardwood pulp by Marechal in
1993 described the conditions required, namely a temperature above 90 °C, a pH
below 3.5, and extended time. The impact of time on the hot acid hydrolysis of an
oxygen-delignified eucalyptus kraft pulp is shown in Fig. 7.140. The time required
at 90 °C and pH 3 for complete hydrolysis of hexenuronic acid is rather long. The
sharp increase of the effluent load with prolonged time indicates an increasing
cellulose degradation. This becomes apparent from the parallel lowering of the
pulp’s viscosity. In mill applications, therefore, a “hot acid stage” will be conducted
for not more than about 2 h, in order to retain control of the pulp yield and
viscosity losses. From a practical standpoint, this also keeps the size of the towers
within reasonable limits. The process could be accelerated by using a higher temperature
or more acid, but typically the temperature is kept below 95 °C in order
to balance steam demand and to avoid pressurized conditions. A lower pH is similarly
unattractive because it would only accelerate cellulose depolymerization.
Hydrochloric acid may be a cost-attractive alternative to sulfuric acid, especially in
mills with an on-site electrolysis for chlorate. However, the potential problem of
using hydrochloric acid is corrosion of the equipment.
The high temperature and low pH used during the hydrolysis have an additional
effect, in that they allow a significant reduction in the manganese content
of the pulp [5,6]. It is speculated that manganese and iron might form complexes
with hexenuronic acid. In bleaching, hexenuronic acid destruction and metals
removal are equally important. A TCF bleaching sequence with only alkaline oxygen
884 7Pulp Bleaching
0 1 2 3 4
3
5
7
9
11
COD [kg/t]
kappa number
Kappa number
Time at 90 °C [h]
5
10
15
20
25
COD
Fig. 7.140 Impact of time in hot acid hydrolysis of eucalyptus
kraft pulp. Pulp delignified with oxygen to kappa 11, temperature
at 90 °C, pH 3(H 2SO4).
and peroxide stages (e.g., OQPP) will maintain all hexenuronic acid sites in the
bleached pulp, the consequence being a poor brightness stability [7]. The inclusion
of an acid hydrolysis stage into a TCF sequence improves the bleaching process
because it eliminates transition metal. At the same time it increases brightness
stability, due mainly to a reduction in the level of hexenuronic acids.
Despite the very substantial drop in kappa number through the hydrolysis of
hexenuronic acids, the mill application of a hot A stage is rare [8]. The reason for
this is the need for an additional treatment stage that consists not only of a tower
but also a washer. This additional equipment is expensive, and normally the
potential savings of chlorine dioxide in an ECF sequence will not pay back the
investment. Therefore, high-density storage towers are converted to a hot A stage.
The lack of a washer between storage and D0 stage can result in the connection of
a hot A pre-treatment and chlorine dioxide addition. The result is a hotA/D0 stage
combination, and this has been implemented in some mills. The alternative is to
use a reverse approach, with a combination of the D0 stage and a subsequent
hydrolysis, or hot D0 with extended retention time (see Section 7.4.5.3, modified
chlorine dioxide delignification).
7.9
Alternative Bleaching Methods
During the past few decades, there has been a constant search for environmentally
benign alternatives to pulp bleaching. The search continues today, and will
do so in the future. Besides the activation of peroxide stages, one such alternative
is offered through biotechnological means. Although hemicellulose-degrading
7.9 Alternative Bleaching Methods 885
enzymes (xylanases) were the first enzymes to be introduced on a large scale for
pulp bleaching [1], they function more as a bleaching aid than as a direct bleaching
agent. This is because they increase the efficiency of subsequent bleaching
steps by loosening the structure of reprecipitated xylans on the unbleached pulp
fibers, thereby saving on the amounts of bleaching chemicals required.
A direct approach might be to use lignin-degrading fungi (Basidiomycetes or
white rot fungi) or their enzyme systems (e.g., peroxidases, laccases), all of which
have long been recognized. These systems are able to selectively degrade lignin
not only in wood, but also in pulp. However, the time required for this process to
proceed to the desired extent is far too long for a modern pulp mill bleaching system.
This problem of extended reaction times was partly tackled by the application
of a so-called mediator. Discovered accidentally during the early 1990s by R. Bourbonnais
of Paprican during experiments with lignin model systems, the laccasemediator-
system (LMS) was found to consist of an enzyme (laccase) and a mediator
(ABTS) [2]. The mediator applied in this first LMS – a laccase substrate used
for an activity assay – was impracticable for large-scale applications, however. An
LMS suitable for pulp mill use was later patented by Call [3,4] which employed
different mediators (e.g., 1-Hydroxy-benzotriazole, HBT) [5], and initial large-scale
trials conducted with this material has shown promise.
The underlying working principle of the LMS can be summarized as follows.
The enzyme laccase, as a macromolecule, is unable to penetrate the pulp fiber,
despite such penetration being a prerequisite for lignin-degrading action. Moreover,
due to its oxidation potential, laccase on its own is only capable of oxidizing
phenolic lignin moieties, which react predominantly by dehydrogenative polymerization
rather than by lignin degradation. However, both of these difficulties were
overcome with the use of a low molecular-weight redox mediator. In this way, the
substrate range is extended to nonphenolic lignin units, as could be shown by
model compound studies, and the mediator can penetrate much more deeply into
the fibers. In the LMS redox cycle, the enzyme oxidizes the mediator to a more
reactive species, mainly of radical type, and these react in turn with the lignin
macromolecule, either via an electron transfer process or by hydrogen atom
abstraction, depending on the mediator used [6]. The reduced mediator is re-oxidized
by the enzyme, which utilizes dioxygen as a co-substrate and which, in turn,
is reduced to water.
Large-scale applications of the LMS remain inoperative, however, and some
major restraints for eventual mill usage have been identified:
_ The mediator should be a low-cost chemical which should exhibit
a minimum of side reactions. These undesired processes can
cause a reduction in enzyme activity or the production of harmful
degradation products, which in turn raises the issue of environmental
compatibility.
_ A sufficient gain in kappa number reduction usually requires several
LMS stages with additional extraction stages in between.
_ The increase in brightness is often limited, so that additional
bleaching is required.
886 7Pulp Bleaching
For further information on the LMS system, the reader is referred to some
excellent reviews [7,8].
Further developments include mediated electrochemical delignification systems
[9], and enzyme mimicking (porphyrin derivatives, manganese-based complexes,
metal–Schiff base-complexes; for a more detailed description, the reader is
referred to Ref. [10]). Mimicking the action of lignolytic enzymes is also the underlying
concept of bleaching system which was developed in the mid-1990s [11,12]
and today is receiving increased attention. A special class of metal clusters of the
Keggin-type – the polyoxometalates [13], often referred to as POMs – are utilized
as the catalyst. Polyoxometalates are metal-oxo anionic clusters with chemical
properties that can be largely controlled by transition metal substitution and the
countercation used. This, combined with their ability to donate and accept electrons
and their stability over a wide range of conditions, makes them attractive
targets for use as bleaching catalysis. To activate the cluster for delignification,
one or more structural metal atoms are donated by a first-row transition metal
atom (e.g., vanadium or manganese) [14]. Specific conditions (e.g., pH, type of
metal cluster) allow POMs to be selective towards lignin degradation and to be
thermodynamically stable in water [15]. The high-valent metal cluster anions oxidize
and thus degrade and solubilize the lignin, while themselves being converted
into the lower-valent reduced state. The re-oxidation with dioxygen is a process
that generates radicals, but this would result in undesired cellulose damage due to
highly unselective side reactions. Consequently, the POMs are reactivated in a separate
stage under conditions that effect the oxidation of dissolved lignin and other
dissolved organic matter to carbon dioxide and water. Actual delignification of the
pulp is carried out under anaerobic conditions. POMs must be applied in stoichiometric
quantities. Current types of POM are advanced products of development:
they are more easily synthesized than the original representatives of this compound
class, are not too costly, recyclable, and have the capability of self-buffering.
Recent progress has also shown that molybdovanadophosphate heteropolyanions
can be used under aerobic conditions in a single-stage process with either oxygen
or ozone as the reactivating agent [16].
Further research and development is required eventually to transfer novel
delignification principles to large-scale applications.
7.10
Bleach Plant Liquor Circulation
Andreas W. Krotscheck
7.10.1