- •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
Introduction
Among the oxygen-based bleaching chemicals, ozone is the most powerful oxidizing
agent, reacting readily with almost any organic material. The good delignifying
and brightening properties make ozone an attractive candidate to replace chlorine-
based bleaching agents. The use of ozone as a bleaching agent results in an
effluent which is free from organochlorine compounds and can be completely
recirculated to the chemical recovery system. Thus, ozone bleaching may be a prerequisite
for a closed-loop bleaching process. However, there are some difficulties
concerning the application of ozone bleaching in industrial practice. First, ozone
is an unstable gas which must be produced on site, most commonly by passing
oxygen gas through an electrical discharge where some of the oxygen molecules
are dissociated into oxygen atoms. In turn, oxygen atoms unite with oxygen molecules
to form ozone. Ozone generation technology in the early stages could produce
only 2–4% ozone by weight in an oxygen carrier gas. Later developments in
ozone generation technology could produce 5% by weight. In the early 1990s, concentration
of ozone could be raised to 8–12% by weight with power efficiency.
Recent advances in ozone generation which enable ozone concentrations up to
16% by weight, as well as the lowering of oxygen cost by means of on-site production,
have established ozone as a highly competitive bleaching chemical. The
ozone concentration can be further increased by compressing the gas mixture;
this improves the mass transfer from the gas into the liquid phase, which is a prerequisite
for an efficient bleaching process. Second, the high oxidation potential
of ozone makes it also prone to depolymerize and to degrade pulp polysaccharides.
In fact, its delignification selectivity is significantly lower than that of chlorine
dioxide. The prevalent view attributes this lack of selectivity to the generation
of highly reactive and nonselective hydroxyl radicals during the bleaching process.
The formation of hydroxyl radicals is usually ascribed to ozone self-decomposition
7.5 Ozone Delignification 777
in an aqueous system, to ozone decomposition catalyzed by transition metal ions,
and mostly to reactions between ozone and lignin structures, preferably containing
phenolic hydroxyls. Based on a huge research effort within the past decade,
the performance of ozone bleaching has been significantly improved with respect
to both selectivity and production costs, making ozone a competitive bleaching
agent. However, it has not yet been possible to increase the selectivity of ozone to
the same level exhibited by chlorine dioxide. This is a severe drawback for the production
of pulps where the high molecular weight of cellulose is a prerequisite to
attain the desired properties (paper-grade pulp: high-strength properties; dissolving-
grade pulp: high solution viscosity). Special emphasis will be given in future
research work to further improve the efficiency and selectivity of ozone bleaching.
Although the first implementation of ozone on industrial scale was until 1990,
when the first installation of an ozone bleach plant came on stream in Lenzing,
ozone has long been known as an efficient bleaching agent.
The reaction of ozone with textile fibers such as cotton and linen was studied as
early as 1868 [1]. In 1889, a method for bleaching “fibrous substances”, including
those used in the making of paper, with a mixture of chlorine and ozone gases
was patented by Brin and Brin [2]. Cunningham and Doree reported in 1912 that
ozone would preferably attack the lignin part in jute, but cellulose was also
affected [3]. In 1934, Campbell and Rolleston patented a process for bleaching
pulp by sequential treatment with chlorine and ozone [4]. Since the studies of Brabender
in 1949, in which he investigated some of the variables involved in ozonation
and patented a high-consistency ozone bleaching process, many reports and
patents on ozone bleaching have been published [5]. The breakthrough of ozone
bleaching was the invention and development of a technology to compress ozone
gas, and this is the prerequisite to apply ozone in medium-consistency technology.
Since the first industrial installation of an ozone plant in 1990, more than 25 pulp
mills with an annual production of about 8 million tons of pulp have implemented
ozone bleaching on industrial scale (see Tab. 7.39).
7.5.2
Physical Properties of Ozone
Ozone (O3) is an allotropic form of oxygen. At ambient conditions, it is a pale blue
gas (= 2.1415 g L–1 at 0 °C and 101.3 kPa). It condenses into an indigo blue liquid
(–112 °C) and freezes to a deep blue-violet solid (–195.8 °C). Ozone has a bent
structure of C2v symmetry with an apex angle of 116°49′and equal oxygen–oxygen
bond distances which are more closer to that of molecular oxygen as compared to
that of hydrogen peroxide. Hence, the bonds in ozone have considerable doublebond
character [6]. Data obtained from the microwave spectrum of the ozone molecule
have shown it not to be significantly paramagnetic [7]. Thus, the ozone molecule
can be pictured as a resonance hybrid consisting of four mesomeric structures,
as shown in Fig. 7.77.
778 7Pulp Bleaching
+O
O
O- O
+
O
O- -O
+
O
O -O
O
O+
1 2 3 4
Fig. 7.77 Resonance structures of ozone.
The contributions of forms 1 and 4, which have a positively charged terminal
oxygen with only six electrons, have been used to explain the electrophilic character
of ozone. As such, ozone falls into a moderately large class of 1,3-dipolar compounds
and will in certain reactions follow mechanisms typical of this class as a
whole [8]. In a simple molecular orbital representation of the ozone molecule,
each of the oxygen atoms is considered to be an sp2 hybrid and thus overlap of the
p-orbitals provides a molecular orbital containing four p electrons. The UV spectrum
of ozone shows an absorption maximum in 0.01M HClO4 at 260 nm with an
extinction coefficient of 2930 L.M–1.cm–1 [9]. In acid aqueous solution, the oxidizing
power is exceeded only by fluorine, atomic oxygen, OH radicals, and a few other
species [6]. The oxidation potential in aqueous solution is expressed by Eq. (83):
O3 _ 2H_ _ 2e_ _ O2 _ H2O_ E0 _ 2_07eV _83_
Ozone is thermodynamically unstable, and 1mol decomposes exothermically to
1.5 mol of molecular oxygen.
The solubility of ozone is an important criterion for ozone bleaching. The solubility
of ozone in equilibrium with its partial pressure is usually defined by
Henry’s law according to the following expression:
xO3 _
PO3
kH _84_
where xO3 is the dissolved ozone molar fraction (mol mol–1), PO3 is the ozone partial
pressure (kPa), and kHis Henry’s law constant (kPa mol fraction–1).
The ozone molar fraction can be transformed to a concentration of ozone, cO3
(in mol L–1 or mg L–1) by multiplying the molar fraction by 55.51or by 2.664 . 106,
respectively. Ozone solubility is influenced by several factors, such as temperature,
pH, ionic strength and dissolved matter. Henry’s law constant, kH, depends on the
temperature, T, according to Eq. (85):
dLnkH
d_1_T__ _
DH
R
kH _ k0
H _ Exp _
DH
R
1
T _
1
T0 __ _85_
where R is the gas constant and DH is the heat of solution of the gas. The parameters
k0
H and T0 refer to kH and T at standard conditions. Equations (84) and (85)
show that an increase in temperature is connected with a decrease in the dissolved
7.5 Ozone Delignification 779
780 7Pulp Bleaching
ozone concentration. The reduced ozone solubility at higher temperature can be
explained by a drop in the liquid phase driving force, and by a higher decomposition
rate. The pH is the predominant parameter which determines the stability of
ozone in aqueous solution (see Section 7.5.4), and hence also its solubility in
water. It is agreed that the dissolved ozone concentration increases with decreasing
pH. This is one of the important reasons why ozone bleaching is conducted
under acidic conditions, preferably in the pH range of 2–3. Quederni et al. have
determined the apparent Henry’s law constants for ozone solubility in water as a
function of temperature at pH 2 and pH 7 [10]. The relationship between kH and
the temperature for these two different pH levels is expressed in Eq. (86):
kH _ 101_3 _ Exp 20_7 _
3547
T __pH _ 7_
kH _ 101_3 _ Exp 18_1 _
2876
T __pH _ 2_
_86_
The solubility of ozone in water at 1atm, pO3 = 101.3 kPa and 0 °C calculates to
(101.3/1.966 . 105) . 2.664 . 106 = 1 . 37 g L–1 at pH 2, and to (101.3/
2.270 . 105) . 2.664 . 106 = 1.18 g L–1 at pH 7. Figure 7.78 illustrates the course of
the equilibrium dissolved ozone concentration as a function of temperature in the
range of 15 to 50 °C for the two pH levels, considering typical conditions for medium
consistency ozone bleaching:
_ Generated ozone concentration in oxygen gas: 200 gm–3 = 9.3 Vol%
_ Total pressure in the mixer: 8 bar = 0.81MP a
_ Ozone partial pressure, pO3: 0.093 . 810.4 = 75.4 kPa
10 20 30 40 50
100
200
300
400
500
600
p
O3
= 75.4 kPa
pH = 7 pH = 2
Dissolved O
3
conc. [mg/l]
Temperature [.C]
Fig. 7.78 Influence of temperature and pH on the dissolved
ozone concentration in water assuming a partial pressure,
pO3, of 75.4 kPa (according to results determined by Quederni
et al. [10]). The ionic strength is kept constant at 0.13mol L–1.
The ratio between the ozone solubility at pH 2 and pH 7 further increases with
rising temperature. The dependency of the dissolved ozone concentration on pH
was even more pronounced, according to results obtained by Sotelo et al. [11].
These authors found an increase by a factor of 1.6 (3.1) when comparing ozone
solubilities at pH 2.5 and pH 7.0 (pH 9.0) at 10 °C and an ionic strength of 0.15 M.
As already mentioned, gas absorption is also dependent on the ionic strength.
Generally, the dissolved gas concentration decreases as ionic strength increases.
The effect of ionic strength on the solubility of ozone is most pronounced in the
presence of phosphates, chloride or carbonate ions, whereas the addition of sulfate
ions exerts practically no change in solubility. According to Sotelo et al., the
dissolved ozone concentration is decreased by half when increasing the ionic
strength from 0.04 mol L–1 to 0.49 mol L–1 in the presence of sodium chloride (pH
5.94 and 1.1 kPa →4.8 mg L–1 versus 2.4 mg L–1) [11].
In more general terms, both the influence of temperature and ionic strength is
described by Eq. (5) [12]:
kH _ Exp _
2297
T _ 2_659 _ l _
688 _ l
T _ 16_808_ _87_
where kH, Henry’s constant is kPa.L.mol–1, T, temperature in Kelvin, and l is the
molar ionic strength. The temperature-dependence of the dissolved ozone concentration
is more pronounced using Eq. (86) than with Eq. (87), with an almost perfect
correspondence at temperature higher than 35 °C (Fig. 7.79).
0 10 20 30 40 50
0
200
400
600
800
1000
1200
1400
p
O3
= 101.3 kPa
Oyama: μ= 0.13 MQuederni et al.: μ= 0.13 M
μ= 1.00 M
Dissolved O
3
-conc [mg/l]
Temperature [.C]
Fig. 7.79 Comparison of calculated dissolved ozone concentration
as a function of temperature using Henry’s constants
from different literatures sources: Quederni et al. [10] versus
Oyama [13]. The influence of ionic strength was assessed
using Eq. (87).
7.5 Ozone Delignification 781
7.5.3
Ozone Generation
Ozone is produced at the site of use because it is unstable and cannot be stored.
The ozone-generating system is selected according to the requirements on site,
including the ozone bleaching technology (medium- or high-consistency), the
source of oxygen (cryogenic or adsorption), the temperature of cooling water, and
the possibilities to recycle the vent gas for oxygen delignification. Figure 7.80 illustrates
the principal elements of an ozone bleaching system, including the oxygen
source, the ozone-generating system, the ozone delivery system with an ozone
compressor in the case of medium-consistency ozone bleaching technology, the
mixer or reactor, the off-gas destruction system and the vent gas recovery and
recycle loops.
Oxygen
Source
Ozone
generator
Ozone
compressor
Mixer /
Reactor
Ozone
Destruction
Cooling
water
O2
O2/O3
Pulp O3 treated Pulp
Vent gas
O2 gas to recycle
or reuse
Fig. 7.80 Principal course of ozone in a pulp
bleaching system (according to [14]).
Ozone is produced from oxygen-containing gases in ozone generators by means
of silent electrical discharge in the so-called “corona discharge process”. To date,
in bleaching operations only oxygen gas is used to achieve a high ozone concentration
and to avoid the formation of reactive byproducts such as nitric acid. Oxygen
is passed through two electrodes which are separated from each other by a
dielectric and two discharge chambers (Fig. 7.81). When a high voltage is applied
between two concentrically arranged electrodes, and the voltage exceeds the ionization
potential of the dielectric material, then electrons flow across the gap and
782 7Pulp Bleaching
provide energy for the dissociation of oxygen molecules; these then combine with
oxygen molecules to form ozone. The key element of a corona discharge ozone
system is the dielectric. The electrical charge is diffused over this dielectric surface,
creating an electrical field where high-energy electrons bombard gas molecules
so that they are ionized and a light-emitting gaseous plasma is formed,
which is commonly referred to as a “corona”. Many different materials in a variety
of configurations are used for the dielectric, including scientific-grade glass (e.g.,
borosilicate) and nonglass materials such as silicone rubber. The quantity of
ozone produced is related to a number of factors, such as the voltage and frequency
of the alternating current applied to the corona discharge cells, the cooling
system, and the design of the ozone generator.
O2
O2/O3
Outer ground
electrode
Discharge
gap
HV electrode
Cooling
water
dielectric tube
Fig. 7.81 Schematic diagram of an ozone generation system.
The generally accepted technologies can be divided into three types: low-frequency
(50–100 Hz); medium-frequency (100–1000 Hz); and high-frequency
(1000+ Hz). Medium-frequency ozonators are now favored as they provide many
benefits over the older low-frequency technology. An example of this is a greater
ozone production with less electrode surface area, so that the equipment can be
smaller for a given ozone output, and the power consumption per kg ozone produced
is also reduced.
Since ozone generation by corona discharge is an exothermic physico-chemical
reaction, and ozone decomposition increases as the gas temperature and ozone
concentration increase, correct cooling is an important factor in generator design.
Moreover, oxygen entering the ozone generator must be very dry (minimum
65 °C), because the presence of moisture affects ozone production and leads to the
formation of nitric acid. Nitric acid is highly corrosive to critical internal parts of a
corona discharge generator, and this can lead to premature failure and a significant
increase in the frequency of maintenance. Besides the destruction of the
ozone generator itself, transition metal ions are released from the stainless steel
electrodes, and this can be very harmful to the pulp during the course of ozone
bleaching. Depending on the strength of the electric field, cooling and the design
of the ozone generator, ozone yields of up to 16% by weight (~240 g m–3) can be
7.5 Ozone Delignification 783
achieved in the production gas. The specific energy consumption for the production
of 1kg ozone is usually between 6 and 10 kWh, depending on the desired
concentration. The efficiency of medium-consistency ozone bleaching is limited
by a certain gas void fraction, Xg (according to Bennington, the upper operating
limit is reached at Xg = 0.13 [15], and according to industrial experience at Xg ~0.25
[16]). The gas void fraction is defined by Eq. (88):
Xg _
Vg
Vg _ VL
XR _
Vg
VL
Xg _
VR
1 _ VR
_ XR _
Xg
1 _ Xg
_88_
where:
Vg,T,P= Vg_T0_P0 _ 101_3 _ T
P _ 273_15 is the volumeof the gas fraction, with P in kPa and T in K;
VL= Prod
_con _ qsusp_
is the volume of the aqueous pulp suspension;
XR is the volume ratio;
Prod is the standardized pulp production (e.g., 1odt pulp);
qsusp = 1
con
1_53 _ _1 _ con_
qliquid _
is the density of the pulp suspension;
con is the pulp consistency, expressed as a fraction; and
qliquid ~1is the density of the liquid.
Both high-concentration ozone feed and compression of the feed gas are required
to ensure an efficient ozone consumption rate in medium-consistency
ozone bleaching. Compression is exclusively carried out isothermally by means of
liquid-ring compressors to avoid ozone destruction. The influence of ozone concentration
in the feed gas to the compressor on the ozone charge being efficiently
consumed in a medium-consistency mixer at a constant pressure of 8 bar at typical
industrial conditions (T = 50 °C, Xg,max = 0.25) is shown in Tab. 7.36.
The data in Tab. 7.36 indicate that the ozone charge in a medium-consistency
ozone mixer is limited to 3.2–4.2 kg odt–1. Clearly, the efficiency of medium-consistency
ozone bleaching also depends on the specific energy dissipation, e, and
on the retention time (see Section 7.5.5.2, Mixing). However, in the case of a modern
medium-consistency mixer the addition of higher ozone charges is connected
with decreasing amounts of ozone consumption rates (see Fig. 7.107).
784 7Pulp Bleaching
Table 7.36 Effect of ozone concentration in oxygen gas prior and
after compression to 0.8 MPa on the limit of ozone charge in a
medium consistency ozone mixer.
Ozone concentration in oxygen Xg
c Max. O3-charged
wt% Vol. % C at STa
[g Nm–3]
c at I NDb
[g m–3]
for 1 kg
[O3 odt–1]
[kg O3 odt–1]
5.0 3.4 72.6 491 0.17 1.4
6.0 4.1 87.4 591 0.15 1.7
6.8 4.7 100.0 676 0.13 1.9
7.0 4.8 102.3 692 0.13 1.9
8.0 5.5 117.3 793 0.12 2.2
9.0 6.2 132.5 896 0.10 2.4
10.0 6.9 147.7 999 0.09 2.7
10.2 7.0 150.0 1014 0.09 2.7
11.0 7.6 163.0 1102 0.09 2.9
12.0 8.3 178.5 1207 0.08 3.2
13.0 9.1 194.0 1312 0.07 3.4
13.4 9.3 200.0 1352 0.07 3.5
14.0 9.8 209.7 1418 0.07 3.7
15.0 10.5 225.4 1524 0.06 3.9
16.0 11.3 241.3 1632 0.06 4.2
16.5 11.7 250.0 1691 0.06 4.3
19.6 14.0 300.0 2029 0.05 5.1
a. ST = standard conditions: T0 = 273.15 K, P0 = 101.3 kPa.
b. IND = industrial conditions: T = 323.15 K, P = 810.6 kPa = 8 bar.
c. Xg = Vg/(Vg+VL) at 10% pulp consistency and IND.
d. Assuming an upper limit of Xg = 0.25 to obtain a reasonably
high ozone consumption rate.
7.5.4
Chemistry of Ozone Treatment
Manfred Schwanninger
Ozone treatment is a very effective way to remove residual lignin that remains
after pulping. The structure and reactivity of the residual lignin have already been
described (see Section 7.3.2.2). One of the major disadvantages of ozone as a
7.5 Ozone Delignification 785
bleaching agent is its moderate stability in aqueous solutions [1–4]. It has the tendency
to decompose in water, generating some very reactive, highly unselective
radical species [1–5]. Hydroxide ions are known to catalyze ozone decomposition
and to promote hydroxyl radical formation [1,3,4,6], whilst another drawback is
the undesired degradation of cellulose [7–23].
7.5.4.1 Ozone Decomposition
The pathways and kinetics of the decomposition of aqueous ozone are of interest
for a wide range of topics, not only for pulp bleaching, and have therefore been
studied intensively [1–5,24]. The chain mechanism of ozone decomposition
(Scheme 7.28) is based on the studies of Buhler et al. [5], while Staehelin et al. [1]
showed the decomposition of ozone and the formed intermediates (Scheme 7.28).
O2
-
HO2
O2
HO4
O3
OH
HO3
O3
-
1O2
O3
H2O
OH-
O2
H2O
H+
HO4
+
H2O2 + 2O3
HO3 H2O2 + O3 + O2
+
Termination
HO4
HO4
Scheme 7.28 Chain mechanism of ozone decomposition
according to Staehelin et al. [1].
The decomposition occurs by a radical chain mechanism, which in pure water
is initiated by the reaction between hydroxide ions (OH–) and ozone [Eq. (89). Superoxide
_O2
– then reacts with ozone rather selectively as part of a chain cycle [1].
O3_OH_→HO2 _ _ _O_2 _89_
The hydroxide-ion-catalyzed decay of ozone is expressed in the following general
rate equation (1):
d_O3
dt _ _k _ _O3a _ _OH_b _90_
The reaction order b with respect to hydroxide ion concentration varies from
0.36–1.0 and the reaction order a for ozone is reported to vary between 1.5–2.0 [aa,
bb]. Pan et al. have studied the decomposition rate of ozone in a pure aqueous
786 7Pulp Bleaching
7.5 Ozone Delignification 787
2 4 6 8
0
30
60
90
120
150
second order reaction rate
O
3
-concentration [mg/l]
Time [min]
Second Order Rate Constant
[mol-1*l*min-1]
pH
0 5 10 15 20 25 30
5
10
15
20
25
30
[O
3
] at pH 3 [O
3
] at pH 7
Fig. 7.82 Effect of pH on second order rate constant of ozone
decomposition and the course of ozone concentration in aqueous
solution at 25 °C according to Pan et al. [25].
system as a function of pH at 25 °C. The results revealed a rapid increase of the
second order rate constant from pH 4 to 7 as seen in Fig 7.82.
The concentration of dissolved ozone decreases rapidly at a neutral pH, while it
remains quite stable under acidic conditions (pH 3) within a time frame relevant
for industrial ozone bleaching applications.
The first propagation step of the chain reaction proposed by Weiss for the
decomposition of aqueous ozone can be described with the intermediates _O3
–
and HO3_ [5]. The elementary reactions of these species and their constants are
presented in Eq. (91) [5]:
O3__O_2 →_O_3 _1O2 k_ 1_6 _ 109M_1s_1
_O_3 _H_
ka
kb
HO3 _ ka _ 5_2 _ 1010M_1s_1 kb _ 3_7 _ 104M_1s_1
HO3 _→HO_ _ O2 k_ 1_1 _ 105M_1s1_1
_91_
In water, the decay of HO3_ is very rapid, with a half-life of about 6 ls. Since
_O2
– reacts rapidly with ozone, but relatively slowly with organic compounds, the
latter will interfere with ozone decomposition in aqueous solutions by scavenging
OH radicals rather than _O2
– [5]. The second propagation step and the reactions of
these species and their constants are presented in Eq. (92):
HO_ _ O3→HO4 _ k_ 2_0 _ 109M_1s_1
HO4 _→HO2 _ _ O2 k_ 2_8 _ 104M_1s_1
HO2 _ _ _O_2 _H_ pKa_ 4_8
_92_
The transient HO4_ might be a charge-transfer complex (HO_O3) [1]. The lifetime
of HO4_ was found to be much longer than its accumulation rate, and therefore
it acts as a carrier reservoir within the chain cycle. As a consequence, HO4_
is the important transient for chain termination reactions (Fig. 7.82). The termination
reactions shown in Eq. (93) are the dominating ones in the presence of
high ozone concentrations [1].
HO4 _ _ HO4 _→H2O2_2O3
HO4 _ _ HO3 _→H2O2_O3_O2 _93_
If, at low ozone concentrations, organic solutes are present, their dominant
effect will be to withdraw OH radicals; at high ozone concentrations this will similarly
occur with HO4_. Some organic materials are thereby able to regenerate _O2
–
in order to sustain the chain.
As the initial step of the ozone decomposition is the reaction between ozone
and the hydroxide anion [Eq. (89) and Scheme 7.28], strong pH dependence is
expected [6]. Gurol and Singer [4] investigated the kinetics of ozone decomposition
in the pH range from 2 to 10. They found that ozone decomposes rapidly at a pH
above about 6.5, but remains quite stable under acidic conditions; indeed, this
finding has been confirmed by several authors [6,25,26]. Hydrogen peroxide, also
used as a bleaching chemical, is incapable of initiating ozone decomposition;
however, its deprotonated form, the hydroperoxy anion (HO2
–) has such ability [6].
At pH < 12 and hydrogen peroxide concentrations >10–7 mol L–1, HO2
– has a
greater effect on the decomposition rate than the hydroxide anion (OH–) [3], and
this might be important for bleaching sequences. In the radical-type chain reaction
decomposition of ozone, inorganic and organic compounds can be divided
into three categories: (a) initiators; (b) promoters; and (c) inhibitors [6,24,27,28].
Initiators are substances that are capable of initiating the decomposition of ozone
to the superoxide anion radical [Eq. (89)], while promoters are radical converters
forming the superoxide anion radical from the hydroxyl radical (Scheme 7.29).
Inhibitors are substances that react with the hydroxyl radical without the formation
of superoxide anion radical, called radical scavengers, such as bicarbonate
and carbonate, leading to the corresponding radicals [24]. Some examples are given
in Tab. 7.37.
CH3OH
OH H2O
H2C
OH
H2C
OH
O2
OO
O2
-
B- BH
H2CO
Scheme 7.29 Reaction of methanol as a typical hydroxyl radical
to superoxide anion radical converter acting as a promoter [24].
788 7Pulp Bleaching
Tab. 7.37 Typical initiators, promoters, and inhibitors for
decomposition of ozone by radical-type chain reactions
[6, 24–28].