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NaOH concentration [g/l]

Fig. 8.5 Purification of hardwood sulfite pulps

(HW-S) and eucalyptus prehydrolysis-kraft

pulp (E-PHK) with cold aqueous NaOH solution

of varying strength [28]. HW-S:

unbleached, kappa number 6; (E/O) pretreated:

kappa 1.6. E-PHK: OZ pretreated,

kappa 0.6. CCE-treatment: 10%consistency,

30 °C, 30 min.

0 20 40 60 80 100

90.0

O-Z treated E-PHK

R18 content [%]

NaOH concentration [g/l]

Fig. 8.6 Purification of eucalyptus prehydrolysis-kraft pulp (EPHK)

with cold aqueous NaOH solution of varying strength

[28]. E-PHK: OZ pretreated, kappa number 0.6. CCE-treatment:

10%consistency, 30 °C, 30 min.

8 Pulp Purification

through alkaline treatment, depending on NaOH concentration, is further illustrated

in Fig. 8.7. The different levels of R18 content after cooking and subsequent

oxygen delignification (O) of the eucalyptus PHK pulps have been adjusted by prehydrolysis

intensity (P-factor). Even though xylan removal efficiency increases

with increasing initial hemicellulose content, the initial purity must exceed a certain

level in order to achieve a sufficiently high purity without approaching a

change in the supramolecular structure.

0 20 40 60 80 100

0.0

Initial R18 content:

96.6% 97.2% 97.6%

Xylan content [%]

NaOH concentration [g/l]

Fig. 8.7 Purification of eucalyptus prehydrolysis-

kraft pulps (E-PHK) of three different initial

purity levels with cold aqueous NaOH solution

of varying strength [28]. O-pretreated E-PHK

pulps: (a) R18 = 96.6%, kappa number 3.4; (b)

R18 = 97.4%, kappa number 2.5; (c)

R18 = 97.6%, kappa number = 2.2. CCE-treatment:

10%consistency, 30 °C, 30 min.

8.3.2

Time and Temperature

The influence of temperature on the performance of caustic extraction has been

discussed in detail in Section 8.2. It is well established that lower temperatures

cause a high degree of swelling, and this enhances the solubility of hemicelluloses.

The effect of temperature and retention time in the range 30–50 °C and 10–

60 min, respectively, while keeping the NaOH concentration constant at 70 g L–1,

is illustrated in Fig. 8.8.

It can be seen that extraction time has no influence on the purification efficiency

in the range investigated. The retention time during alkalization is not a

critical parameter because swelling takes place almost instantaneously [4]. However,

the increase in temperature from 30 °C to 50 °C induces a decreased removal

of xylan content of 0.4% (from 1.5% to 1.9% in the residue). This temperature

8.3 Cold Caustic Extraction

0 20 40 60

0.0

30.C 40.C 50.C

Xylan content [%]

Time at Temperature [min]

Fig. 8.8 Influence of time and temperature during CCE treatment

of eucalyptus prehydrolysis-kraft pulp (E-PHK) at a constant

NaOH concentration of 70 g L–1 [28]. E-PHK: OZ pretreated,

kappa 0.6. CCE-treatment: 10%consistency, 30 min,

70 g NaOH L–1.

increase equally affects purification, as would a decrease in NaOH concentration

by 17 g L–1, from 70 to 53 g L–1, at 30 °C, respectively. The processability of the

CCE treatment is worsened at low temperature because washing is deteriorated

due to an increased lye viscosity. At a given washer capacity, this may result in

additional alkali losses. In industrial praxis, a compromise must be found between

economic considerations and pulp quality demand. In most cases, the temperature

level is adjusted to about 35 °C, which might fulfill both targets.

8.3.3

Presence of Hemicelluloses in the Lye

CCE treatment requires a comparatively high dosage of NaOH. Maintaining a

NaOH concentration of 80 g L–1 (74 kg NaOH t–1) at 10% pulp consistency

requires a total NaOH charge of 666 kg odt–1. The total alkali loss to the sewer is

economically by no means acceptable, however, and consequently methods to reuse

the entire quantity of the lye must be evaluated. In the case of cold caustic

purification of a PHK pulp, the excess lye may be completely recycled to the kraft

cook, provided that the demand of alkali for cooking is not lower than the amount

of alkali originating from the CCE treatment. In this particular case, white liquor

must be used as the alkali source. The efficacy of the white liquor with respect to

purification efficiency is equal to a pure NaOH if the strength of the white liquor

8 Pulp Purification

is calculated as effective alkali (EA). Assuming total EA losses (including EA consumption

through CCE treatment and washing losses of about 50 kg odt–1), an

amount of 616 (equals 666–50) kg odt–1 of EA is recycled to the cooking plant

(note that the CCE filtrate must be evaporated in order to reach the white liquor

EA-concentration). Supposing a bleached yield of 35% (o.d.), this amount of alkali

corresponds to an EA charge of 216 kg odt–1 wood which, for cooking, seems to be

a rather too-low than a too-high amount (depending on the wood species, cooking

technology and intensity of prehydrolysis, the required EA amount for cooking

ranges from 22% to 26% on o.d. wood). This brief example shows that the excess

lye of cold alkali purification balances quite well with the demand in PHK cooking.

However, the situation is different when combining acid sulfite cooking with

a CCE treatment. There, the opportunities to re-use the excess lye quantitatively

are limited to special cases. For example, one possibility of disposing of the excess

lye from the CCE treatment would be to use it for hot caustic extraction, provided

that the production of hot alkali-purified pulp considerably exceeds that of cold

alkali-purified pulp. If this is not the case, the only chance of preventing too-high

losses of alkali would be to recirculate the pressed lye to the sodium hydroxide

circuit for re-use in CCE treatment. A closed loop operation, however, inevitably

leads to an accumulation of dissolved hemicelluloses in the lye circulation system.

Depending on the amount of hemicelluloses removed from the pulp and the leaks

from the circuit (e.g., the discharge with the press cake), a certain level of dissolved

hemicelluloses is allowed to be reached under equilibrium conditions. It

has been reported that the extent of purification is much deteriorated by the presence

of dissolved hemicelluloses and other impurities [4]. Surprisingly, if an (E/O)

treated hardwood acid sulfite dissolving pulp is subjected to mild cold caustic

extraction at 5% NaOH concentration in the presence of 10 g L–1 hemicelluloses,

the xylan content even slightly increases, clearly due to xylan reprecipitation

(Fig. 8.9).

As expected, the purification efficiency increases when raising the NaOH concentration

to 90 g L–1 while keeping the ratio to the hemicellulose concentration

constant at 5:1. Nonetheless, the presence of hemicelluloses significantly impairs

pulp purification (see Fig. 8.9). In the light of the previous discussion about the

nature of hemicelluloses, it was interesting to examine which of the two hemicellulose

fractions would have the greater impact on purification efficiency. The

gamma-cellulose fraction was separated by nanofiltration, while the beta-cellulose

was prepared by precipitation upon acidification. The data in Fig. 8.10 show that

the presence of the low molecular-weight gamma-cellulose fraction during CCE

treatment does not affect purification, whereas the presence of the high molecular-

weight beta-cellulose clearly impedes xylan removal.

This observation strengthens the presumption that xylan, when exceeding a certain

molecular weight, precipitates onto the surface of the pulp fiber even at rather

high NaOH concentration (2.5 mol L–1). Xylan redeposition is clearly the main reason

for a reduced purification efficiency, if CCE is carried out with a lye containing

dissolved beta-cellulose.

8.3 Cold Caustic Extraction

0 20 40 60 80 100

0.0

hemicellulose containing lye: 10 g/l at 50 g NaOH/l; 18 g/l at 90 g NaOH/l

pure lye

Xylan content [%]

NaOH concentration [g/l]

Fig. 8.9 Effect of hemicelluloses in the lye on the xylan

removal efficiency during CCE treatment of hardwood acid

sulfite dissolving pulp (HW-S) in the range of 0 to 100 g L–1

NaOH [28]. HW-S: (E/O) pretreated, kappa 1.6, 2.7%xylan.

pure Lye 20 g/l Gamma 20 g/l Beta

0.0

0.5

1.0

1.5

2.0

Xylan content [%]

Lye Purity

Fig. 8.10 Effect of the presence of low (gamma) and high

(beta) molecular-weight hemicelluloses on xylan removal efficiency

during CCE treatment of hardwood acid sulfite dissolving

pulp (HW-S) at 100 g L–1 NaOH and 25 °C [28].

Unbleached HW-S: kappa number 5.7; total bleaching

sequence: CCE (E/O)ZP.

8 Pulp Purification

8.3.4

Placement of CCE in the Bleaching Sequence

The efficiency of cold alkali purification is reported to be improved by a preceding

hot caustic extraction stage in the case of a sulfite dissolving pulp [4]. More

recently, it has been shown that the position of the CCE stage within a bleaching

sequence has no significant impact on the degree of purification, provided that

washing takes place between both purification stages (Fig. 8.11). In contrast,

when oxygen delignification (O) follows hot caustic extraction (E) without interstage

washing, denoted as (E/O) sequence, a CCE treatment preceding (E/O)

seems to be advantageous over the reversed sequence with respect to delignification,

as illustrated in Fig. 8.11. The simple reason for the higher overall delignification

efficiency of the latter is that unbleached pulp exhibits a higher level of

alkaline-extractable lignin than an (E/O) pretreated pulp, while the efficiency of

oxygen delignification appears to be unaffected by the prehistory of pulp treatment.

Likewise, the position of CCE within a final bleaching sequence of a hardwood

TCF-bleached PHK pulp proved to have no influence on the purification efficiency,

as shown in Fig. 8.12. Nevertheless, placing CCE before the ozone stage

(Z) is preferred compared to both other alternatives because of a loss in viscosity

(in the case of CCE after Z) or higher capital costs (in the case of CCE after P).

Moreover, CCE treatment on the bleached pulp might be disadvantageous with

Untreated (E/O)-CCE CCE-(E/O)

Kappa number

Xylan content [%]

Xylan

CCE

(E/O)

(E/O)

CCE

Kappa number after First Stage after Second Stage

Fig. 8.11 Influence of the positions of CCE and

(E/O) stages in a sequential treatment on the

purification and delignification performances

of a hardwood acid sulfite dissolving pulp [28].

Unbleached HW-S: kappa number 5.7; CCE

(E/O) versus (E/O)CCE; CCE-treatment:

100 g L–1 NaOH, 30 min, 30 °C; (E/O)-treatment:

E: 30 kg NaOH odt–1, 85 °C, 120 min; O:

85 °C, 90 min, pO2,t = 0 = 8 bar (abs).

8.3 Cold Caustic Extraction

regard to possible impurities of the final product. It is also reported that pretreating

pulp with cold alkali prior to hot caustic extraction reduces the amount of

alpha-cellulose being degraded during the latter process [11]. The reduction in viscosity

loss is most pronounced when the pulp is partly converted from cellulose I

into cellulose II.

However, in cases where oxidative degradation is desired to reduce pulp viscosity,

the CCE treatment should be placed immediately after ozonation.

The placement of CCE in the bleach sequence is open to debate, but depends

ultimately on the prevailing circumstances in industrial practice.

O-treated A-Z-CCE-P A-Z-P-CCE CCE-A-Z-P

Xylan

Viscosity [ml/g]

Xylan content [%]

Intrinsic viscosity

Fig. 8.12 Influence of CCE placement within an AZP sequence

on xylan removal efficiency and final viscosity of a hardwood

PHK pulp [28]. O-treated E-PHK: kappa number 2.4; CCEtreatment:

70 g L–1 NaOH, 30 min, 30 °C.

8.3.5

Specific Yield Loss, Influence on Kappa Number

Cold caustic extraction is a rather selective purification process because it mainly

involves physical changes in the corresponding pulp substrate. The yield losses

reported in the literature are 1.2–1.5% per 1% increase in alpha-cellulose content

[4] or 1.2–1.8% for a 1% gain in R10 [27]. These values are in close agreement

with recent results obtained from hardwood sulfite and PHK dissolving pulps

[28], as illustrated in Fig. 8.13.

On average, the yield loss calculates to 1.6% for a 1% reduction in xylan content.

Closer examination of the results shows that CCE treatment on HW-PHK pulps is

slightly more selective as compared to that of HW-S pulps, as indicated by a specific

yield loss per 1% decrease in xylan of 1.4% for the former, and 1.8% for the latter.

8 Pulp Purification

0 1 2 3

CCE of unbleached&(E/O)-treated HW-S CCE of O-treated HW-PHK

Xylan Removed [% od]

Yield Loss [% od]

Fig. 8.13 Yield loss as a function of the

amount of xylan removed from hardwood sulfite

(HW-S) and hardwood prehydrolysis-kraft

(HW-PHK) dissolving pulps during CCE

treatment [28]. CCE-treatment for HW-S: 50–

100 g L–1 NaOH, 25–30 °C, 30–60 min; CCEtreatment

for HW-PHK: 40–70 g L–1 NaOH,

30–50 °C, 10–60 min.

It has already been pointed out that a certain lignin fraction is removed through

CCE treatment. It may be speculated that the delignifying performance of CCE

exceeds that of normal alkaline extraction (E) at elevated temperature, known as

operation to remove leachable residual lignin (see Section 7.3.7.2, tables 7.24 and

7.25, Process technology: oxygen delignification), because part of the xylan being

removed during CCE may be covalently linked to the residual lignin. The rather

high delignification removal efficiency of CCE (14–25%) despite the very low initial

lignin content (2.1. 0.15 – 6.0. 0.15 = 0.3% – 0.9%) is demonstrated in

Tab. 8.1.

Tab. 8.1 Average kappa number values before and after CCE

treatment of differently pretreated hardwood sulfite and

hardwood PHK dissolving pulps [28]. For details of CCE

treatments, see Fig. 8.13.

Treatment HW-Sulfite HW-PHK

Unbleached (E/O) O

Untreated 6.0 2.1 2.8

CCE 4.5 1.8 2.3

8.3 Cold Caustic Extraction

Clearly, delignification is most pronounced for the unbleached pulp. However,

significant parts of the residual lignin structures are even removed after oxygen

delignification through CCE, but this may be attributed to the dissolution of xylan

linked to residues of oxidizable structures (degraded lignin and/or HexA?).

8.3.6

Molecular Weight Distribution

The aim of CCE is selectively to remove short-chain carbohydrates and other alkaline-

soluble impurities, and this leads to a narrowing of the molar mass distribution.

The effect of CCE on molecular weight distribution (MWD) has been investigated

using a standard hardwood sulfite dissolving pulp (HW-S). The data in

Fig. 8.14 show that the main part of the short-chain carbohydrates with molecular

weights ranging from 2.5 to 12 kDa (maximum at 5 kDa) is removed through

CCE. At the same time, the mid-molecular weight region between 30 and 380 kDa

becomes enriched. CCE treatment at low temperature (23 °C) proves to be rather

selective. Only a very small proportion of the very high molecular-weight fractions

(>1000 kDa) is degraded through CCE. Numerical evaluation of the MWD confirms

the removal of short-chain material (Tab. 8.2). It should be noted that the

polydispersity and amount of low molecular-weight fractions (below DP50 and

DP 200) are significantly decreased, while the high molecular-weight fraction

remains largely unchanged (beyond DP2000).

103 104 105 106 107

0.0

0.2

0.4

0.6

0.8

1.0

HW-S CCE treated HW-S Δ (CCE-Untreated)

dW/dlogM

Molar Mass [g/mol]

Fig. 8.14 Molar mass distribution of a hardwood-sulfite dissolving

pulp (HW-S) before and after CCE treatment [12].

CCE-treatment: 80 g L–1 NaOH, 23 °C, 45 min.

Tab. 8.2 Numerical evaluation of molecular weight distribution

of HW-S pulp before and after CCE treatment [12]. DP: degree of

polymerization, I: weight fraction.

Pulp DPw DPn PDI I< P50

[wt.%]

I< P200

[wt.%]

I> P2000

[wt.%]

HW-S 1950 245 8.0 5.1 17.4 27.9

HWS-CCE 1880 355 5.3 2.0 13.4 26.6

8.4

Hot Caustic Extraction

The purpose of hot caustic extraction (HCE) is to remove the short-chain hemicelluloses

(determined as S18, S10 fractions) for the production of reactive dissolving

pulps based on acid sulfite cooking. In contrast to cold caustic purification, which

relies on physical effects such as swelling and solubilization to remove short-chain

noncellulosic carbohydrates, hot alkali extraction utilizes primarily chemical reactions

on the entire pulp substrate for purification.

The treatment is carried out at low caustic concentration, typically 3–18 g L–1

NaOH, with pulp consistencies of 10–15% at temperatures ranging from 70 °C to

120 °C (occasionally 140 °C). As mentioned previously, HCE is carried out solely

for sulfite pulps, because the same carbohydrate degradation reactions are

involved in alkaline cooks (kraft, soda), at less severe conditions and thus avoiding

alkaline hydrolysis reactions. Therefore, HCE does not contribute much to the

purity of pulps derived from alkaline cooking processes. From the chemistry point

of view, HCE should be placed before any oxidative bleaching stage, as the efficiency

of purification is impaired as soon as aldehyde groups are oxidized to carboxyl

groups. It has been found that the gain in alpha-cellulose is related to the

copper number (or carbonyl content) of the unpurified pulp [30]. Consequently, if

measures are undertaken to stabilize the carbohydrates against alkaline degradation

either by oxidation (HClO2) or reduction (sodium borohydride), virtually no

purification is achieved [31,32]. However, for the production of low-grade dissolving

pulps with a focus on viscose applications, hot caustic extraction (E) and oxygen

delignification (O) are often combined into one single stage (EO) to reduce

costs. The reduction in purification efficiency is negligible, provided that the

degree of purification is limited to R18 values well below 94–95%.

952 8 Pulp Purification

8.4.1

Influence of Reaction Conditions on Pulp Quality and Pulp Yield

8.4.1.1 NaOHCharge and Temperature in E, (EO), and (E/O) Treatments

The NaOH charge is the most important parameter controlling the degree of purification

during HCE. At a given alkali dosage, the consistency determines the

alkali concentration in the purifying lye. According to Leugering [30], the gain in

alpha-cellulose is accelerated with increasing consistency at a given alkali charge,

as shown in Fig. 8.15.

0 1 2 3 4

5% consistency 10% consistency

Alpha-Cellulose [%]

Time [h]

Fig. 8.15 Alpha-cellulose versus time, calculated according to

the empiric formula developed by Leugering [30], with the following

assumptions: Initial alpha-cellulose content 90%, 4%

NaOH charge, 90 °C.

The relationship between the gain in alpha-cellulose content (Da) and NaOH

concentration multiplied by retention time has been derived on the basis of

spruce acid sulfite pulps:

Da _ __3_3 _ 0_1 _ con_ _ 0_13 _ _T _ 80__ _ _NaOHch _ con

100 _ con _ t__ 1

2_0_2 con_ _1_

where: con = consistency (%; validity range 5–15%); T = temperature (°C; validity

range 80–97 °C);NaOHch= NaOHcharge (kg odt–1; validity range 34–228 kg odt–1);

and t = time (h; validity range 0–4 h).

8.4Hot Caustic Extraction 953

HCE is usually carried out at medium consistency of 10–18%, though in some

cases a consistency of 25–30% is practiced. The presence of oxygen at elevated

pressure during HCE, aiming to reduce the kappa number parallel to pulp purification,

clearly impairs the degree of purification (Fig. 8.16).

0 30 60 90 120

E-stage EO-stage

R18 content [%]

NaOH charge [kg/odt]

Fig. 8.16 R18 content as a function of NaOH charge comparing

E- and (EO)-treatments of hardwood sulfite dissolving

pulp (HW-S) [33]. HW-S: kappa number 5.1, 91.8%R18 content.

Process conditions: E: 90 °C, 0–120 kg NaOH odt–1,

90 min; (EO): equal to E plus oxygen: 8.4 bar (abs) at t = 0.

The data in Fig. 8.16 indicate clearly that purification levels off at about 94%

R18 if (EO) is applied. At a given alkali charge, temperature and time are adjusted

to achieve a minimal caustic residual. The amount of NaOH consumed relates to

both the gain in R18 and pulp yield. The curve characterizing the increase in R18

as a function of the caustic consumed is comparable for spruce and beech sulfite

pulps; these data are in agreement with the report of Leugering [30].

When oxygen delignification follows HCE treatment without interstage washing

[characterized as (E/O)], the relationship between R18 and the amount of

caustic consumption proceeds parallel to pure HCE treatment (E), with a shift to

higher NaOH consumption due to an additional consumption during oxygen

delignification (Fig. 8.17). When oxygen delignification and HCE occur simultaneously,

the degree of purification is leveled off at ca. 95% R18. By further intensifying

the reaction conditions during (EO) treatment through increased temperature

and caustic charge, no additional gain in R18 content can be attained while

caustic consumption continues to increase. This unselective behavior of (EO) is

also reflected in the relationship between purification yield and R18 content (see

Fig. 8.18). As anticipated, E and (E/O) treatments with hardwood sulfite pulps

954 8 Pulp Purification

0 30 60 90 120

HW-Sulfite SW-Sulfite

E-stage EO-stage (E/O)-stage E-stage

R18 content [%]

NaOH consumption [kg/odt]

Fig. 8.17 R18 content as a function of the

amount of NaOH consumed comparing E-,

(EO)- and (E/O)-treatments of hardwood sulfite

dissolving pulp (HW-S) and E-treatment of

spruce sulfite dissolving pulp (SW-S) [33]. HWS:

kappa number 4.6–7.1, 91.4–92.0%R18 content;

SW-S: kappa number 4.6–12, R18 content:

90–91.6%. Process conditions: E: 82–110 °C,

40–120 kg NaOH odt–1, 90–240 min; (EO): 85–

110 °C, 150–300 min, 35–145 kg NaOH odt–1,

8.4 bar (abs) at t = 0; (E/O): 90–110 °C, 30–

120 kg NaOH odt–1, 90–240 min, 8.4 bar (abs)

at t = 0.

follow the same pattern in terms of yield versus R18 content. The gain in R18 content

during HCE of spruce sulfite pulps appears to develop slightly more selectively

as compared to beech sulfite pulps (see Fig. 8.18). The reaction of purification

can be divided into two phases: first, a more-selective course; and second, a

less-selective course. Transition between the two phases appears for E and (E/O)

stages at R18 values of 95.5–96.0%, and in the case of (EO) treatment at R18 values

of 94.0–94.5%.

A yield loss of about 3% per 1% increase in alpha-cellulose content has been

reported elsewhere [4,27,30]. Recent studies on beech and spruce dissolving pulps

have confirmed this “rule-of-thumb” in general. However, small deviations are

experienced as the yield loss is related to R18 content which, in contrast to alphacellulose

or R10 values, is rather independent of viscosity in the range investigated.

A summary of the specific yield losses and NaOH consumption values is

provided in Tab. 8.3.

The data in Tab. 8.3 show that HCE is very unselective at R18 values greater

than 96%. The one-stage hot purification and oxygen delignification behaves

slightly less selectively when exceeding R18 values of 94%. NaOH consumption is

a good indication for the degree of purification. Similar to kraft cooking,

8.4Hot Caustic Extraction 955

92 94 96 98

HW-Sulfite SW-Sulfite:

E-stage EO-stage (E/O)-stage E-stage

Purification yield [%]

R18 content [%]

Fig. 8.18 Purification yield as a function of R18 content comparing

E-, (EO)-, and (E/O)-treatments of hardwood sulfite

dissolving pulp (HW-S) and E-treatment of spruce sulfite

dissolving pulp (SW-S) [33]. Pulps and conditions are as in

Fig. 8.17.

Tab. 8.3 Specific yield losses and NaOH consumption values in

the course of E-, (EO), and (E/O) treatments of beech and

spruce sulfite pulps.

Pulp Purification Yield loss per 1%

R18 increase

NaOHcons per

C6 sugar dissolved

<96% >96% <96% >96%

[% o.d. pulp] [mol mol–1]

HW-S E 3.7 5.0 1.4 1.6

HW-S EOa 4.0 2.0

HW-S (E/O) 3.2 5.0 2.0 2.0

SW-S E 3.3 4.1 1.4 1.6

a. Max. 95% R18.

the alkali consumption in pure E stages amounts to between 1.4 and 1.6 mol mol–1

monosaccharide unit (calculated as C6) dissolved, indicating that the end products

of degradation must be fragmented to smaller units than isosaccharinic acid, such

956 8 Pulp Purification

as glycolic, lactic, pyruvic and 3,4-dihydroxybutyric acids, as described by MacLeod

and Schroeder [34]. As anticipated, specific alkali consumption increases to a value

of about 2 mol mol–1 monosaccharide unit when oxygen delignification is integrated

into the purification reaction, either in the same (EO) or in a separate stage

(E/O).

An elevated temperature between 80 and 120 °C is necessary to activate peeling

reactions in the presence of sufficient alkali to achieve an increase in R18 and

R10, and a decrease in hemicelluloses. As shown in Fig. 8.19, cellulose degradation

begins at temperatures exceeding 140 °C, as suggested by a decrease in R10

content, indicating the fragmentation of microfibrils. Clearly, temperatures

beyond 140 °C do not contribute to further purification due to alkaline hydrolysis.

0 2 4 50 100 150

80.0

R18 R10

R values [%]

Temperature [.C]

Fig. 8.19 Development of R18 and R10 contents as a function

of temperature of a high-viscosity spruce sulfite pulp during

HCE [4]: HCE-conditions: 120 kg NaOH odt–1, 4 h.

8.4.1.2 Xylan versus R18 Contents

Prolonged acid sulfite cooking causes both the removal of hemicelluloses (e.g.,

xylan) and the degradation of cellulose, resulting in a low-viscosity pulp. HCE

treatment of low-viscosity sulfite pulps allows reduction to a very low xylan content,

while the R18 content remains rather close to that of pulps with a higher

initial viscosity at a comparable yield level (Fig. 8.20). These data conclude that the

R18 content of medium- to high-viscosity pulps partly contains alkaline-stable

hemicelluloses. On the other hand, part of the degraded cellulose is not included

in the R18 fraction of the low-viscosity pulp.

8.4Hot Caustic Extraction 957

81 84 87 90 93

Xylan content [%]

Medium Viscosity: R18 Low Viscosity: R18

R18 content [%]

Purification yield [%]

Xylan Xylan

Fig. 8.20 R18 and xylan contents related to purification yield

during (E/O)-treatment of hardwood sulfite dissolving pulp

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