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Pulp Purification

8.1

Introduction

The production of dissolving pulp involves the removal of short-chain carbohydrates,

denoted as hemicelluloses, which negatively influence either the processing

behavior of the pulp or the quality of the final product. (The technical definition

of hemicelluloses comprises both alkali-soluble heteropolysaccharides and

degraded cellulose soluble in the steeping lye.) Purification processes for dissolving

pulps include both the removal of noncellulosic material (e.g., extractives, lignin,

hemicelluloses), and the change of the molecular distribution to a narrow,

monomodal type of distribution with a minimum amount of low molecularweight

carbohydrates. The extent of purification should thus be adjusted to the

need of the dissolving process, and pulp grades of varying purity level are available.

It is a well-known fact that the mechanical properties of the viscose fibers

correlate quite well with the amount of short-chain molecules. As early as 1941,

Hermans stated that the chain-length distribution in the dissolving pulp is a crucial

property in the production of rayon fibers [1]. In addition, by using sulfite and

prehydrolysis-kraft (PHK) pulps of different purity levels, Avela et al. were able to

demonstrate that all strength characteristics are significantly reduced with an

increase in the low molecular-weight fraction [2]. The short-chain molecules represent

the weakest part in the fiber; this means that, the shorter the molecules, the

lower will be the number of molecules linking the crystalline regions. In a recent

study, a correlation between the strength properties of rayon fibers and the

amount of low molecular-weight fraction (expressed as the DP50-fraction) was

established, using a set of dissolving pulps prepared by different organosolv processes

[3].

In general, caustic extraction steps are conducted to remove short-chain carbohydrates

from wood pulp that resisted the pulping process, in order to obtain

favorable product characteristics such as improved material properties (e.g.,

increased fiber strength), higher brightness and brightness stability. These alkaline

purification procedures can be carried out in two different ways – as either

cold or hot caustic extractions. While the cold process, which is conducted at 20–

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

40 °C and high sodium hydroxide concentration (1.2–3.0 mol L–1), involves mainly

physical changes, the hot purification process, operated in the range between

70 °C and 130 °C and low sodium hydroxide concentration (0.1–0.4 mol L–1),

induces multiple carbohydrate degradation reactions. In addition to cleavage of

the terminal glycosyl groups, one by one via b-alkoxy elimination (peeling reaction)

until the reducing end group is converted into a corresponding aldonic acid

(alkali-resistant metasaccharinic acid end group), a series of fragmentations to

mainly short-chain organic acids (mainly C2 and C3 hydroxy acids) occurs at elevated

temperatures. This explains why the alkali consumption does not correspond

to 1 mol per degraded monosaccharide, but rather to 1.6 mol, indicating

that fragmentation to smaller acids takes place [4].

Unlike PHK pulps, acid sulfite pulps require the application of both technologies

to achieve purification levels appropriate to produce high-tenacity regenerated

fibers (e.g., continuous-filament industrial rayons), cellulose acetate or cellulose

ethers of pure quality. Cold alkali purification is certainly the most selective way

of increasing the alpha-cellulose content of the pulp. The yield losses are in the

range of 1.2–1.5% per increase of 1% in alpha-cellulose content [4]. In the case of

hot caustic extraction, a yield loss of about 3% per 1% increase in alpha-cellulose

content is experienced. However, cold caustic extraction is rarely used on a technical

scale because of the huge amounts of alkali needed. When working at 10%

consistency and 10% NaOH concentration, 1 t NaOH odt–1 pulp is necessary to

charge. In combination with a PHK process, part of the press-lye can be re-used

in the cooking process or, alternatively, white liquor can be used for the cold

extraction process. Another means of employing the excess lye is to use it for hot

alkaline purification, with the prerequisite that the production of hot alkali-purified

pulp considerably exceeds that of cold alkali-purified pulp. Recirculation of

the lye (after pressing) significantly deteriorates the result of the purification, due

to an accumulation of impurities derived from short-chain carbohydrate degradation

products, being characterized as beta- and gamma-celluloses. Beta-cellulose

is defined as the precipitate formed upon acidification of an aqueous alkaline solution

containing the dissolved pulp constituents, while gamma-cellulose comprises

the carbohydrate residue in solution. The former consists of higher molecular-

weight, the latter of lower molecular-weight material.

These compounds can be (partly) removed by means of dialysis of (part of) the

press-lye [4,5]. In addition, inter- and even intramicellar swelling of pulps under

the conditions of cold caustic extraction (low temperature combined with high

alkali concentration in the vicinity of the swelling maximum) impedes the

removal of excess lye during the course of subsequent washing. An optimum between

purification performance and limitation of fiber swelling can be found by

adjusting the temperature and caustic charge.

The treatment of pulp with aqueous sodium hydroxide solution still represents

the principal means of producing highly purified dissolving pulp. When applying

these caustic treatments, the extent of purification can be controlled by adjusting

the appropriate conditions. The relationship between the process conditions, involving

both sodium hydroxide concentration and temperature, and the course of

934 8 Pulp Purification

reaction comprising the carbohydrate constituents of a selected hardwood sulfite

dissolving pulp is described in the next section.

8.2

Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

Wood pulp obtained by the acid sulfite process still contains considerable amounts

of low molecular-weight carbohydrates (hemicelluloses). These make the pulp less

suitable for many purposes as known for the production of cellulose acetate, highpurity

cellulose ethers or high-tenacity regenerated fibers. As mentioned previously,

the pulp is refined with alkali either at temperatures below 50 °C whereby

strong solutions of sodium hydroxide are used (characterized as cold caustic

extraction, CCE), or at higher temperatures using weaker alkaline solutions (characterized

as hot caustic extraction, HCE). In some cases, both processes are

applied subsequently (in any order: CCE before or after HCE) to obtain the highest

purity dissolving pulp derived from the sulfite cooking process. It is well

known that the extraction of wood pulp with strong sodium hydroxide solutions at

low temperatures produces higher levels of alpha-cellulose than with dilute solutions

at high temperatures, while the yields obtained are considerably higher. The

basis of both purification processes was developed during the 1940s and 1950s.

Hempel studied the solubility of viscose pulps at 20 °C in the range of NaOH concentration

between 1 and 20%, with the emphasis on maximum solubility [6].

Shogenji and associates treated chlorinated sulfite pulp at 25 °C with 3 to 12%

NaOH and investigated the alkaline solutions after treatment for total and combined

alkali [7]. Wilson and coworkers tested the alkali solubility of pulp in relation

to the alpha-cellulose determination, and stated that wood originally contains

appreciable amounts of gamma-cellulose of low degree of polymerization (10–30),

but no beta-cellulose [8]. The latter is formed during the pulping processes from

alpha-cellulose. Many studies have been conducted to determine phase-transition

during the treatment of pulp or cotton linters with alkaline solutions of varying

concentrations, using X-ray diffraction. Ranby studied the appearance of cellulose

hydrate when treating different cellulose substrates at 0 °C with increasing concentrations

of sodium hydroxide [9]. With cotton, the first indication of hydrate

cellulose occurs at 8% NaOH, whereas with wood pulp it occurs already at 6%

NaOH. The NaOH concentration necessary for transition is related to the water

sorption of the original cellulose, which means that cellulose undergoing transition

at low NaOH concentration has a high water sorption. An electron-microscopic

study of spruce holocellulose indicated that alpha-cellulose is built up of

micelle strings about 8 nm wide, whereas gamma-cellulose contains no strings

[10]. The beta-cellulose fraction appears to be a mixture of short string fragments

and small particles. An X-ray investigation showed that both alpha- and beta-celluloses

show the same type of lattice (cellulose II). The gamma-cellulose seems to

consist of several phases different from cellulose II. The beta-cellulose is assumed

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution 935

to originate from alpha-cellulose by degradation during the pulping and bleaching

processes.

The composition of the beta- and gamma-celluloses fractions removed from the

wood pulp during cold and hot extraction processes with respect to the amount of

unchanged carbohydrates has been the focus of few studies. Corbett and Kidd

studied the degradation of a mixture of beta- and gamma-celluloses extracted by

hot alkali from spruce pulp [11]. These authors found that the insoluble residue

essentially consists of glucan, and whereas the beta-cellulose fraction is made predominantly

of xylan, the gamma-cellulose originates from a mixture of glucan

and mannan. In a recent study, the change in composition of the alpha- (residue),

beta- and gamma-celluloses fractions created during treatment of a beech sulfite

dissolving pulp with aqueous NaOH of various concentrations ranging from 20 to

340 g L–1 at 20 °C, 50 °C and 80 °C, was investigated [12]. The pulp consistency was

kept constant at 5%, which is a typical value for the industrial steeping process.

The profile of the xylan content of the residue (alpha-cellulose) and the weight

fraction of the dissolved hemicelluloses (sum of beta- and gamma-cellulose)

related to the initial amount of pulp is illustrated graphically in Fig. 8.1.

As expected, xylan removal is more efficient at 20 °C than at higher temperatures.

To obtain the lowest possible xylan content in the pulp residue (about 0.7%

appears to be alkali-resistant), the NaOH concentration must be increased from

0 100 200 300

0 100 200 300

15 20.C 50.C 80.C

Xylan content [%od]

Dissolved

Hemicellulose [% od]

NaOH concentration [g/l]

Fig. 8.1 Profiles of xylan content in the pulp

residue (upper) and the amount of dissolved

hemicelluloses (sum of beta- and gamma-cellulose)

(lower) during alkaline treatment of a

beech sulfite dissolving pulp (93.4%R18, 4.0%

xylan) at different temperatures [12]. Caustic

treatment: 5%consistency, 30 min reaction

time, NaOH concentrations: 20, 40, 60, 80,

100, 140, 180, 280, and 340 g L–1.

936 8 Pulp Purification

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

100 g L–1 to about 140 g L–1 when raising the temperature from 20 to 50 °C. The

alkaline treatments at 50 °C and 80 °C reveal a comparable pattern of xylan

removal up to a lye concentration of about 280 g L–1. The xylan removal efficiency

remains unchanged at 80 °C and also at NaOH concentration up to 340 g L–1, but

is slightly reduced at lower temperatures.

The profile of the amount of hemicelluloses dissolved during alkaline treatment

resembles the swelling behavior of cellulose in dependence on lye concentration,

as experienced by Saito [13,14]. At low temperature (20 °C), the amount of dissolved

hemicelluloses increases rapidly with increasing NaOH concentration, and

passes through a maximum at 100 g NaOH L–1. While the residual xylan content

remains fairly constant with increasing lye concentration, the amount of dissolved

hemicellulose decreases significantly to values less than half of the amount determined

at maximum solubility. In the low lye concentration range up to 170 g

NaOH L–1, the solubility of pulp constituents is significantly lower at 50 °C as compared

to 20 °C, whereas the maximum solubility is shifted to 140 g NaOH L–1. At

higher NaOH concentrations, the pattern of the solubility of hemicelluloses develops

quite comparably for both temperatures, 20 °C and 50 °C, respectively. In contrast,

alkaline treatment at 80 °C causes a steady increase in hemicellulose solubility

up to a NaOH concentration of 280 g L–1. Beyond this lye concentration, the

amount of dissolved hemicelluloses experiences a slight reduction (see Fig. 8.1,

lower). In hot alkali treatments (80 °C), the removal of short-chain carbohydrates

is essentially governed by chemical degradation reactions involving endwise depolymerization

reactions (the peeling reaction). With increasing temperature, the

peeling reaction becomes the dominant pathway for the degradation of pulp carbohydrates.

This explains the different pattern of hemicelluloses removal as compared

to the alkaline treatment at lower temperatures (20 °C and 50 °C). In contrast,

cold alkali treatment at 20 °C induces intermicellar and intramicellar swelling,

permitting short-chain material to dissolve. The physical interaction between

cellulose and aqueous sodium hydroxide proceeds in several steps. According to

Bartunek [15] and Dobbins [16], the addition of low amounts of electrolytes (e.g.,

NaOH) seems to create unbound or “monomeric” water by shifting the equilibrium

between clustered and free water. Swelling can thus be explained by the penetration

of the unbound water molecules into the cellulose structure, while destroying

intermolecular hydrogen bonds. Moreover, swelling facilitates the accessibility

of the hydrated ions into the crystalline structure. The degree of swelling is governed

by both the number of water molecules present as hydrates of the alkali

ions entering the cellulose structure, which decreases with increasing lye concentration,

and the penetration depth of these alkali ions into the structure, which

increases with lye concentration until the conversion to alkali cellulose is completed.

Thus, swelling passes through a maximum at a lye concentration that is

sufficient to ensure complete penetration of the whole structure. The decrease in

swelling beyond this value can be explained by a disproportionally large reduction

of the hydration number when further increasing the NaOH concentration.

It can be assumed that the extent of hemicellulose dissolution proceeds parallel

to the swelling behavior of the pulp. The monomeric sugar composition of the

8 Pulp Purification

0 100 200 300

γ

[% of total hemi removed]

Proportion of Xylan removed

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

20 °C

dissolved as Xylan total removed Xylan

0 100 200 300

γ

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose Degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

50 °C

[% of total hemi removed]

Proportion of Xylan removed

dissolved as Xylan total removed Xylan

0 100 200 300

80 °C

γ

[% of total hemi removed]

Proportion of Xylan removed

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose Degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

dissolved as Xylan total removed Xylan

Fig. 8.2 Profiles of carbohydrate composition

of the gamma- and beta-celluloses fractions dissolved

during alkalization of a beech sulfite dissolving

pulp (93.4%R18, 4.0%xylan) at three

different temperatures: (a) 20 °C; (b) 50 °C; (c)

80 °C [12]. Caustic treatment: 5%consistency,

30 min reaction time, NaOH concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

dissolved hemicelluloses was analyzed by anion-exchange chromatography (AEC)

with pulsed amperometric detection (PAD) after separation into beta- and

gamma-cellulose fractions [17]. It is noted that the proportion of beta-cellulose

decreases with increasing temperature, particularly above 50 °C. While the absolute

amount of gamma-cellulose remains fairly constant at 20–50 °C throughout

the whole range of NaOH concentrations investigated, the increase in the total

amount of dissolved hemicelluloses at 80 °C is mainly attributed to an increase in

the gamma-cellulose fraction (see Fig. 8.2). The fact that up to 90% of the gammacellulose

fraction consists of degraded carbohydrates (equal to non-neutral sugars)

clearly indicates that the removal of hemicelluloses through alkaline treatment at

80 °C is mainly governed by chemical degradation reactions (e.g. peeling reaction).

As stated previously, the extent of chemical degradation reactions decreases with

decreasing temperatures. Accordingly, the amount of degraded carbohydrates

decreases at lower temperatures. The beta-cellulose fraction originating from any

alkaline treatment consists almost exclusively of neutral sugars (except for uronic

acid side chains and oxidized end groups). As anticipated, the maximum yield of

beta-cellulose corresponds with the maximum solubility of the hemicelluloses

(sum of beta- and gamma-celluloses) at an alkaline treatment at 20 °C and 50 °C,

and with the lowest xylan content in the pulp residue at any temperature investigated.

Parallel to its highest yield, the beta-cellulose consists of the highest glucan

content (74 wt.%, 59 wt.% and 47 wt.% based on beta-cellulose for 20 °C, 50 °C,

and 80 °C, respectively). It can be assumed that the glucan fraction derives from

degraded cellulose and comprises the highest molecular weight within the betacellulose.

Surprisingly, the treatment at 80 °C also produces a beta-cellulose fraction

enriched with degraded cellulose at the same conditions where a complete

removal of alkali soluble xylan occurs. This indicates that at a lower lye concentration

the cellulose structure is opened by inter- and intramicellar swelling, even at

high temperatures. Apart from degraded cellulose, the predominant hemicellulose

fraction in beech sulfite dissolving pulps is made up of xylan, while the glucomannan

content is almost negligible. Therefore, the main objective of the alkali

purification processes comprises removal of the residual xylan content.

By comparing the amount of xylan removed from the pulp with the amount

recovered in both the beta- and gamma-cellulose fractions, it can be concluded

that most xylan is recovered in oligomeric and polymeric structures. The proportion

of degraded xylan is greater only in the lower NaOH concentration range (up

to 80 g L–1) where the easily degradable fraction is removed. Apart from the minimum

at a NaOH concentration of 100 g L–1 at 20 °C and 140 g L–1 at 50 °C and

80 °C due to the increased dissolution of degraded cellulose, the beta-cellulose

becomes increasingly enriched with xylan as both the NaOH concentration and

temperature are raised (Fig. 8.3). This means that the xylan part in the hemicelluloses

is clearly more resistant to alkaline degradation than the other carbohydrate

components. The major part of the xylan remains stable even after hot caustic

extraction (100 °C, 0.25 N NaOH, 1–4 h) as exemplified in a study conducted by

Corbett and Kidd [11].

8 Pulp Purification

0 100 200 300

20.C 50.C 80.C

Xylan content in Beta-Cellulose [%od]

NaOH concentration [g/l]

Fig. 8.3 Xylan content in beta-cellulose as a function of

NaOH concentration and temperature [12]. Caustic treatment:

5%consistency, 30 min reaction time, NaOH concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

Model compound studies using aldobiouronic (4-O-methyl-b-d-glucuronic acid-

(1→2)-xylose) (4OMeGlcA) and aldotriouronic acid (4-O-methyl-b-d-glucuronic

acid-(1→2′)-xylobiose), confirmed that substitution at position 2 of the terminal,

reducing xylose unit strongly inhibits alkaline degradation [18]. In the absence of

a C-2 substituent, the xylose chain is rapidly shortened according to classical peeling

pathways, until the next C-2 substituted xylose unit is reached. The results

explain the observed higher stability of the xylan fraction as compared to the glucan

fraction isolated from the steeping lye. Thus, the decreased alkaline degradation

of the xylan isolated from the beta-cellulose fraction can be attributed to the

presence of side branches consisting of 4-O-methyl-glucuronic acid as detected by

FT-IR-spectra and by MALDI-MS with a 4OMeGlcA:Xylose-ratio of 5:100 at the

maximum [19].

The interaction between aqueous NaOH and cellulose also affects the supramolecular

structure of cellulose. Increasing the NaOH concentration beyond 70–

80 g L–1 at room temperature leads gradually to a change from the native cellulose

I structure into the Na-cellulose I structure. Thereby, the plane distance of the

101-lattice planes is widened from the original 0.61 nm to more than 1.2 nm due

to incorporation of the sodium hydrate ion [20]. At a NaOH concentration between

160 and 190 g L–1 the lattice transformation to Na-cellulose I is completed.

This structure gives rise to a better reactivity with chemical reactants due to the

better accessibility of the hydroxyl groups on C6 and C2 (e.g., CS2 in the case of the

viscose process). It is well known that the transition curve from cellulose I to Na-

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

cellulose I depends also on the supramolecular structure of the dissolving pulp.

Sulfite pulps generally require a lower lye concentration to achieve full lattice conversion

than do PHK pulps [21]. The somewhat higher mercerization resistance

may be due to the less degraded primary cell wall of the latter, restricting swelling

by NaOH [20]. The changes in supramolecular structure upon alkali treatment of

two dissolving pulps, beech acid sulfite and eucalyptus PHK pulps, have been

investigated using solid-state CP-MAS 13C-NMR spectroscopy (Fig. 8.4).

0 100 200 300 400

Eucalyptus-PHK: Cellulose I Na-cellulose I Na-cellulose II

Beech-Sulfite: Cellulose I

Proportion [%]

NaOH concentration [g/l]

Fig. 8.4 Lattice transition from cellulose I to Na-cellulose I

and Na-cellulose II of beech sulfite and eucalyptus-PHK pulps

depending on NaOH concentration. Data were recorded

using solid-state CP-MAS 13C-NMR spectroscopy (according

to [22]).

Over the range of NaOH concentration from about 160 g L–1 to 270 g L–1, the

structure of Na-cellulose I prevails, while beyond this concentration level a further

lattice conversion to Na-cellulose II arises. The NMR-spectrum of this lattice type

indicates cleavage of the intramolecular hydrogen bond between O-3-H and O-5′,

and thus the coordination of an additional Na+ ion to O-3 [23]. A series of comprehensive

reports provides further information on the changes in supramolecular

structure that occur during the treatment of cellulose with aqueous solutions

[20,22,24–26].

8 Pulp Purification

8.3

Cold Caustic Extraction

The extent of purification, measured in terms of R18 and R10 values and residual

hemicellulose content (xylan in case of hardwood pulp), depends primarily on the

NaOH concentration and the temperature (see Section 8.2). Additionally, the reaction

time, the position of the cold caustic extraction (CCE) within the sequence,

and the presence of dissolved hemicelluloses may have an influence on the efficiency

of purification. In industrial CCE treatment, emphasis is placed on efficient

washing. The pulp entering the CCE stage must be thoroughly washed and

dewatered to a high consistency (>35%) in order to avoid dilution of the added

caustic solution through the pulp slurry. The conditions of CCE include the

homogeneous distribution of pulp in 5–10% NaOH for at least 10 min at temperatures

between 25 and 45 °C in a downflow, unpressurized tower. Due to the rapid

interaction between alkali and cellulose, a separate retention tower is not really

needed (in industrial praxis, a tower would perfectly act as a surge tank). Removal

of the lye from the highly swollen pulp is rather difficult, and requires efficient

post-CCE washing in a series of more than three washers [27]. Special attention

must be paid to the washing concept in order to avoid reprecipitation of dissolved

polymeric hemicelluloses (beta-cellulose) during the course of washing.

The most important parameters influencing the degree of purification are presented

in the following section.

8.3.1

NaOHC oncentration

As anticipated, the hemicellulose content in the pulp, determined as xylan,

decreases linearly with increasing NaOH concentration in the aqueous phase of

the pulp suspension, up to a value of about 100 g L–1 (Fig. 8.5).

Parallel to the decrease in residual xylan content, an increase in R18 content

can be observed. The course of R18 content during CCE treatment as a function

of NaOH concentration is illustrated graphically in Fig. 8.6.

The purification efficiency of both sulfite and PHK pulps is quite comparable,

provided that the initial xylan contents are at the same level. The xylan content of

the unbleached sulfite pulp was reduced by a mild hot caustic extraction followed

by oxygen delignification without interstage washing ((E/O)-stage). Purification

during CCE proceeds for both pulps to levels close to 1% xylan (or slightly below),

even at NaOH concentrations significantly lower than 100 g L–1, which prevents

the conversion of significant parts to Na-cellulose I (see Fig. 8.4). A subsequent

change of the crystalline lattice to the cellulose II-type alters the fiber structure

and thus deteriorates pulp reactivity towards acetylation [29]. A xylan content of about

3% in the untreated pulp must be ensured in order to avoid a change in the supramolecular

structure while attaining a sufficiently low xylan content to meet the required

specifications for high-purity pulps (see Section 11.3, Tab. 11.7, Pulp properties).

The relationship between initial pulp purity (R18) and final xylan content

8.3 Cold Caustic Extraction

0 20 40 60 80 100

unbleached HW-S (E/O) treated HW-S O-Z treated E-PHK

Xylan content [%]

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