- •Pulp Purification Herbert Sixta
- •9.2.2.1 Introduction
- •Introduction
- •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
11.3 Dissolving Grade Pulp
equilibrium concentration level in the recycled steeping liquor. In this definition,
the hemicelluloses consist of both alkali-soluble degraded cellulose and heteropolysaccharides
such as degraded xylan or mannan. The accumulation of hemicelluloses
In the steeping lye inhibits cellulose degradation during ageing due to the
additional oxygen consumption through these degraded carbohydrates [13]. In
addition, they react preferentially with carbon disulfide in the subsequent xanthation
process, thus leading to inhomogeneously substituted cellulose which consequently
adversely affects viscose filterability [14]. A good quality of viscose solution,
characterized by low particle content (< 3 lm) and good viscose filterability,
Is governed by a low content of noncellulosic impurities, particularly pentosans,
certain inorganic substances, and resins [15].
Lenz et al. have shown that fiber tenacity is altered due to incorporation of low
molecular-weight hemicellulose if they exceed a certain concentration level in the
steeping liquor (Tab. 11.6) [13].
Tab. 11.6 Influence of hemicellulose concentration in steeping
lye on fiber tenacity [13]. Pulp substrate: ECF-bleached beech
acid sulfite dissolving pulp.
alpha-Cellulose
[%]
Hemicellulose
content in steeping lye
[g L–1]
Fibre-DPn Tenacity (cond)
[cN tex–1]
Elongation (cond)
[%]
90.5 17.3 254 27.5 15.3
90.5 20.5 253 26.6 15.5
90.5 37.0 261 26.0 15.0
91.3 17.6 254 28.6 16.8
91.3 35.4 255 28.0 16.2
Siclari reports that the use of dissolving pulps with increasing amounts of low
molecular-weight carbohydrates is closely related to a decrease in the wet tenacity
of a Polynosic-type fiber [16]. An increase of the low molecular-weight fraction determined
by gel permeation chromatography (GPC) measurement and an
Increase in the xylan content in the respective viscose fibers clearly support the
assumption that the decrease in fiber tenacity is caused by the incorporation of
the acid-insoluble hemicellulose fraction (beta-cellulose) which occurs during the
regeneration process in the highly acidic spin bath.
Hemicelluloses also contribute to discoloration of the resulting cellulose products
during both alkaline (e.g., Viscose) and acidic conversion (e.g., acetate) steps
[13,17]. The chromophore formation of the hemicelluloses under alkaline conditions
is closely associated with the presence of carbonyl and carboxyl groups.
Quite recently, it could be shown that hemicelluloses, particularly xylans, are
involved in the discoloration of Lyocell dopes [18]. Pentoses generally revealed a
significantly higher rate of chromophore formation as compared to hexoses. In
1027
11 Pulp Properties and Applications
addition, the glucuronic acid side chains have been shown to be a source of
enhanced chromophore formation, while the corresponding gluconic acid
revealed no measurable yellowing [18]. The detailed mechanism of the preferred
chromophore formation of pentoses over hexoses in both acidic and alkaline processing
is still under investigation.
Acid processing generally requires pulps of higher purity as compared to alkaline
processes. Acetate pulps are high-purity pulps containing hemicelluloses in
an amount less than 1.5% and no detectable residual lignin. Experimental findings
strongly suggest that even very small amounts of certain hemicellulose fractions
play an important role in the formation of haze and color of cellulose triand
diacetate solutions. Glucomannan from both acid sulfite and prehydrolysis
kraft (PHK) pulps is a major source for diacetate haze, false viscosity, and poor
filtration [17]. False or anomalous viscosity determines production capacity, and is
defined as the percent increase in dope viscosity compared to that of a dope prepared
from cotton linters diacetate of the same composition and intrinsic viscosity.
The effect of the pulp mannan content on false viscosity and filterability of a diacetate
solution in acetone is illustrated in Fig. 11.6.
Glucomannan is very detrimental to diacetate filterability (obtained from the filtration
of 18% diacetate acetone dope through a cotton fabric media at 275 kPa),
whether it comes from acid sulfite, PHK or conventional kraft pulp. Even small
0 1 2 3
0
100
200
300
400
500
False Viscosity Effect [%]
Filtration Value [g/cm2]
Mannan content [%]
0
100
200
300
400
500
Acid Sulfite PHK Kraft
Fig. 11.6 Effect of mannan content in various pulps on diacetate
filterability and false viscosity [17,19]. Pulp acetylation
according to the high-catalyst process using 14% sulfuric acid
catalyst based on initial amount of cellulose. Diacetate dope
consists of 18% acetate, 1.35% water, and 80.65% acetone.
1028