- •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
50 °C, is illustrated graphically in Fig. 11.29.
0 2 4 6 8
0
5
10
15
20
HW-Sulfite HW-PHK
Chain Scissions [104/DP
w,t
-104/DP
w,0
]
Ageing time at 50 °C [h]
Fig. 11.29 Course of chains scissions (based on weighted
molecular weight) of alkali celluloses prepared from hardwood
sulfite and PHK dissolving pulps as a function of time
at 50 °C.
The ageing of alkali cellulose follows a pseudo zero-order reaction kinetics
based on the number-average degree of polymerization DPn, according to the following
expression [56]:
104
DPn_t _
104
DPn_0 _ _ kA t _1_
where kA is the reaction rate of the ageing process.
1057
11 Pulp Properties and Applications
The deviation from linearity, which is particularly discernible for alkali cellulose
made from a hardwood sulfite pulp, can be attributed to the change (decrease) in
polydispersity during degradation. Based on today’s knowledge of the reaction
mechanism, chain scission is initiated by the reducing endgroups. In agreement
with these considerations, oxidative alkaline degradation of sulfite pulp proceeds
faster as compared to the more narrowly distributed PHK dissolving pulp (see
Fig. 11.29).
The arguments in favor of electron beam treatment of dissolving pulp are a better
control of viscosity degradation following a strict random scission mechanism,
which results in better reactivity towards derivatization due to a better accessibility
In the crystalline regions.
Hardwood sulfite and PHK dissolving pulps were irradiated by means of a 10 MeV
accelerator, applying dosages of between 0 and 30 kGy (Fig. 11.30).
0 10 20 30
0
5
10
15
20
25
HW-Sulfite: untreated after steeping
HW-PHK: untreated after steeping
Chain Scissions [104/DP
w,j
-104/DP
w,0
]
Radiation dose [kGy]
Fig. 11.30 Course of chain scissions (based on weighted
molecular weight) of hardwood sulfite and PHK dissolving
pulps, before and after steeping as a function of radiation
dose.
After irradiation, the molecular weight was determined without any further
treatment and after alkalization with a caustic solution of 17.5% NaOH (steeping).
Degradation kinetics can be described by means of zero order, whereby using the
model equation from Sakurada:
104
DPn_t _
104
DPn_0 _ _ kD Dn _2_
1058
11.3 Dissolving Grade Pulp
where kD is the reaction rate of the radiation degradation.
The exponent n takes into account the fact that the polydispersity changes during
radiation degradation. According to Fig. 11.30, the course of chain scissions is
comparable for both pulps investigated. Because of a larger amount of carbonyl
groups (including reducing end groups), the degradation rate of a sulfite pulp is
more enhanced after a subsequent steeping step as compared to a PHK pulp. As
mentioned previously, the statistical character of molecular weight degradation
has been proposed as one important advantage of the electron beam treatment
compared to chemical degradation processes. If random chain scission is
assumed, then nonuniformity (U = PDI – 1) would approach unity, as indicated
by Kuhn [89].
The data in Fig. 11.31 show that the MWD curves through progressive degradation
are practically equal for both degradation processes, namely electron beam
treatment followed by steeping and oxidative alkaline ageing. Furthermore, the
results suggest that the statistical degradation performance applies to both degradation
processes. The conclusion is that radiation degradation and alkaline ageing
successively reduce the molecular weight, making the MWD progressively narrower.
In both processes, the reaction kinetics is governed by the content of carbonyl
groups, indicating that the degradation rate of sulfite pulps is higher than
that of PHK pulps.
600 500 400 300 200
1
2
3
4
5
6
HW-Sulfite: Ageing E-beam&steeping
HW-PHK Ageing E-beam&steeping
Polydispersity Index [M
w
/M
n
]
Viscosity [ml/g]
Fig. 11.31 Polydispersity index (PDI) as a function
of cellulose viscosity indicating the course
of degradation: comparison of electron beam
treatment with oxidative alkaline
treatment for both hardwood sulfite and PHK
dissolving pulps. The PDI was determined
from GPC-MALLS measurement (according to
[68]).
1059
11 Pulp Properties and Applications
11.3.2.8 Overview of Pulp Specification
As stated previously, the suitability of dissolving pulps can be adequately determined
simply by simulating the conversion processes to the final products, at
least on a laboratory scale. The most important property of virtually all dissolving
pulps can be expressed by the term “chemical reactivity”. Pulp reactivity, however,
cannot be described by a single structural feature, but rather by both the physical
structure of the cellulosic material and the type of chemical interaction with the
reagent. Additionally, all three structural levels – the molecular, supramolecular,
and fibrillar – must be considered when using the term reactivity [90].
Reactivity is related to the accessibility of chemicals to the cellulose, which
means the relative ease by which the hydroxyl groups can be reached by the reactants.
Structure and morphology of the fiber determines the homogeneity of the
conversion process and final product quality [91].
One of the most informative parameters in commercial specification sheets
(quality card) is that of alkali solubility tests at room temperature. Here, the pulps
are subjected to extractions with 10% (highest alkaline solubility) and 18% NaOH
(concentration of steeping lye), respectively. Thereby, differentiation must be
made between methods based on the gravimetric determination of the extraction
residue (R-values) and determination of the soluble fraction (S-values) using
potassium dichromate oxidation of the filtrate, followed by titration. The results
are specified as a percentage based on the dry starting material. The concentration
of NaOH is given as a subscript index (R10, R18 or S10, S18). The alkali resistances
are directly related to alkaline processing of dissolving pulps, as for viscose and
etherification processes. There, the R18 or (in some cases preferred) R21.5 values
have been cited as being representative of the yield of alkaline-processed products
(viscose fiber and cellulose ether) [92]. It has been shown that the cellulose content
corresponds well with the R18 value. For sulfite pulps with low molecular
weight, the R18 value lies below the cellulose content, because low molecularweight
material becomes dissolved. For PHK pulps and high-viscosity sulfite
pulps (ether application), the R18 value exceeds the cellulose content because high
molecular-weight, alkali-stable hemicelluloses remain in the pulp [93]. The difference
between the two extraction results is sometimes used as a measure for low
molar mass cellulose (R18 – R10 or S10 – S18).
Moreover, the S18 or (100 – R18) values are good indicators for estimating the
organic wastewater load associated with the production of viscose fibers or cellulose
ethers.
The data in Tab. 11.16 represents a simplified specification profile of the most