Polysulfide Pulping

The impregnation of polysulfide liquor leads to the oxidation of the accessible reducing

end groups, provided that the [OH– ]ion is sufficiently high (see Eq. (139)

below) [169]. The ability of a polysulfide solution to stabilize carbohydrates

increases with the concentration of elementary sulfur, and with the ratio of elementary

sulfur to sulfide sulfur. The pretreatment with polysulfide solution can

be carried out in different ways. One way would be to impregnate the wood chips

with pure polysulfide liquor (Na2S4) at temperatures between 100 and 130 °C prior

to conventional cooking. After the pretreatment, the excess liquor is withdrawn

and stored for reuse [170].

The polysulfide solution can be prepared by dissolving elementary sulfur in the

white liquor. In the presence of [HS– ]ions, elemental sulfur is simultaneously

converted to polysulfide sulfur already at rather moderate temperature:

nSs HS _ OH _ __ SnS 2_ H 2 O _131_

306 4 Chemical Pulping Processes

According to Eq. (131), the dissolution of sulfur in the [HS– ]ion-containing solution

consumes alkali. To maintain a target EA charge, it is necessary to add additional

alkali to compensate for the alkali consumed. The polysulfide solution consists

of an equilibrium mixture of different polysulfide ions, with n between 1 and

5. A second method for polysulfide production comprises a direct catalytic oxidation

in which part of the hydrogen sulfide in the white liquor is oxidized to polysulfides

according to the following reaction [Eq. (132)]:

_ n 1__ HS _

n

2 _ O 2 ___ n _ 1__ OH _ SnS 2_ H 2 O _132_

Equation (132) represents the main reaction in white liquor oxidation according

to the MOXY process [171]. About 30% of the initial hydrogen sulfide will react to

thiosulfate according to the following expression:

2 HS _ 2 O 2 __ S 2 O 2_ 3 H 2 O _133_

One catalyst for the MOXY system is a granular, activated carbon which has

been treated with a wet-proofing agent to provide areas on the carbon surface

which are not wetted by the liquid phase.

Applying the MOXY process to the whole white liquor would substantially

reduce the amount of [HS– ]ions; this must be considered when choosing the

appropriate conditions for modified cooking. It is thus recommended to raise sulfidity

in the white liquor (i.e., in chemical recovery) from 30% to 40%, or even to

50%. It is then possible to maintain the [HS– ]ion concentration at a high level

during the cook, which is important for maintaining a high intrinsic viscosity of

the pulp. The MOXY process utilizes only that sulfur content normally present in

the mill’s liquor supply, and thereby does not alter the sulfur:sodium ratio in the

black liquor, as would be the case when adding sulfur to the white liquor to produce

polysulfide liquors.

Impregnation with the polysulfide-containing solution should be performed

below 110 °C, as polysulfide easily decomposes to thiosulfate and sulfide under

these conditions according to the Eq. (134) [172,173]:

SnS 2_ _ n _ 1__ OH _ 1 _

n

_ 4__ H 2 O → 1

n

_ 2__ HS _

n

4 _ S 2 O 2_ 3 _134_

The rate of decomposition of polysulfide solutions increases with increasing temperature,

increasing [OH– ]ion and decreasing [HS– ]ion [172,174].

A recent kinetic study revealed that polysulfide disproportionation depends only

on the hydrogen sulfide, polysulfide ion concentrations and on temperature

according to the following rate expression [Eq. (135)][174,175]:

d _ S _0_

dt _ _7_7 _ 1013 _ Exp _

140 000

8_314 _ T _ ___ S _0_ 1_6__ S __ II _ _0_8 _135_

4.2 Kraft Pulping Processes 307

Equation (135) was derived for [S(0)]/[S(-II)] ≤ 0.15, but validity can be assumed

for [S(0)]/[S(-II)] as high as 0.5.

From this kinetic equation it can be concluded that a higher sulfidity in the

cook will be in favor of a higher polysulfide concentration. A decrease in temperature,

as employed in the latest generation of modified cooking processes (e.g.,

CBC, ITC), will lead to a slower decomposition of the polysulfide present. The

overall result will be governed by the difference in activating energies between the

production and decomposition of polysulfide sulfur.

Unbleached pulps from Pinus sylvestris from both conventional kraft and polysulfide

cooking were compared with respect to their amounts of gluconic acid end

groups. Polysulfide pulp contains significantly more glucometasaccharinic end

groups than simply kraft-cooked pulps. Thus, it can be concluded that part of the

reducing end groups are oxidized to aldonic acid groups during polysulfide cooking,

which at least partly explains the higher yield of polysulfide cooking (50.9%

at kappa number 31.5) when compared to conventional kraft cooking (45.9% at

kappa number 28.1) [176].

Polysulfide also reacts with certain lignin structures, rendering them more soluble

due to the introduction of, for example, carboxylic groups. Studies with

model compounds revealed that polysulfide can oxidize coniferyl alcohol not only

to vanillin 1 and acetovanillone 2 as shown by Nakano et al. [177]and Brunow and

Miksche [178], but also to (4-hydroxy-3-methoxyphenyl)-glyoxylic acid 3 [179].

OH

OCH3

CHO

OH

OCH3

O

OH

OCH3

O

O

OH

1 2 3

The formation of the latter requires temperatures above 135 °C, in contrast to

other oxidized structures such as vanillin and acetovanillone, which already generate

at lower temperatures. The formation of these compounds indicates that polysulfide

can introduce not only a-carbonyl groups into free phenolic structures but

also carboxylic acid groups into the side chain. The oxidation products 1 – 3 have a

strong UV-absorbance at about 350 nm. The UV spectra of black liquors from

cooks with a polysulfide pretreatment show a strong absorbance at 353 nm for

spruce and at 370 nm for birch [179,180]. The compound responsible for absorbance

at 370 nm is identified as (4-hydroxy-3,5-dimethoxyphenyl)-glyoxylic acid.

The introduction of carboxylic acid groups into lignin structures explains why

more lignin is removed when part of the sulfide sulfur is added as polysulfide

[175].

308 4 Chemical Pulping Processes

At a low EA concentration (e.g., 0.125 mol L–1), the rate of bulk and residual

phase delignification is increased by the addition of polysulfide, which may be led

back to the better solubility of oxidized lignin structures. It has also been suggested

that the amount of residual phase lignin is reduced by the polysulfide treatment,

though this observation might be connected with the findings that polysulfide

attacks and degrades phenolic enol ether structures at only moderate temperatures

[181]. These compounds are rather stable under standard kraft cooking conditions,

and consequently they are constituents of the residual phase lignin. The

removal of enol ethers creates new phenolic b-O-4 structures which, in turn, contribute

to further degradation of the lignin.

The ability of a polysulfide treatment of wood to stabilize the carbohydrates to

achieve a yield increase is dependent on the ratio of elemental to sulfide sulfur,

S(0)/S(-II), the hydroxyl ion concentration, [OH– ], and on the concentration of

excess S, S(0). The wood was pretreated with a solution containing elemental sulfur

(0.1–0.5 M), effective alkali (0.3–0.01 M [OH– ]) and hydrogen sulfide ions at

90 °C for 1 h prior to conventional kraft cooking (l:s = 4:1, initial concentration of

NaOH was 0.8 M, of NaHS 0.2 M corresponding to an EA charge of 13% and sulfidity

of 40% at 170 °C for 90 min) [169]. The redox potential of the polysulfide

solution could be predicted by the following expression:

E 0_ mV _606 _ 49 _ Log _ OH _ _37 _ Log _ S _0_ _56 _ Log

_ S _0_

S ___ II _ _

4 _ _ _136_

The oxidation of reducing end groups to aldonic acids is highly dependent upon

the hydroxyl ion concentration:

RCHO 3 OH _ _ RCOO _ 2 H 2 O 2 e _

S 3 S 2_

H 2 e _ _

HS _ _137_

Total redox equation:

RCHO

S 3 S 2_

OH _ _ RCOO _

HS _

H 2 O _138_

The ability to oxidize the reducing end groups can be predicted from the redox

potential (E) by adding separate terms to consider the influence of hydroxide ion

and excess sulfur concentrations to the Nernst equation according to Eq. (139):

D Y ≈ E 0

R _ T

F _

_ Ln _ OH _ Ln _ S _0_ _ _139_

where DY corresponds to the yield increase compared to kraft cooking without

polysulfide pretreatment.

4.2 Kraft Pulping Processes 309

The ability of a polysulfide pretreatment to achieve a yield increase can be

described without using the redox potentials by combining expressions of the type

displayed in Eqs. (136) and (139). The yield increases (in percent on wood),

obtained as a result of the polysulfide pretreatment, can be calculated by Eq. (140):

D Y _ 5_0 1_5 _ Log _ S _0_ 2_4 _ Log _ OH _ _2_2 _ Log

_ S _0_

S ___ II _ _

4 _ _ _140_

The experimental results plotted against Eq. (139) are shown in Fig. 4.89.

-800 -750 -700 -650 -600 -550

Yield increase [%]

Redox Potential, E [mV]

Fig. 4.89 The increase in yield based on wood, obtained by

pretreating wood with polysulfide at 90 °C prior to kraft pulping

as a function of the redox potential, E, which is composed

of E0 derived from Eq. (136) and the Nernst equation derived

from Eq. (139) (according to [169]).

The yield increases shown in Fig. 4.89 are dependent on the concentration of

excess sulfur, the ratio of excess sulfur to sulfide sulfur (Xs), and the alkalinity of

the polysulfide solution. The conditions, as well as the calculated and measured

results, are detailed in Tab. 4.38.

In Tab. 4.38, Xs is the ratio of excess sulfur to the sulfide sulfur [S(0)]/[S(-II)],

E0,m and Ec the measured redox potentials and E0,c and Ec the calculated redox

potentials according to Eqs. (136) and (139), DYm the measured and DYc the calculated

yield increase according to Eq. (140).

At a given kappa number of 35, yield increases about 1.5% for every percent of

elementary sulfur added in the pulping of pine and spruce. The effect can be

enhanced to about 2% if the sulfur is introduced only in that part of the white

liquor which is adsorbed by the wood during impregnation. According to carbohydrate

analysis, polysulfide pulps have proven that the yield increases can be attrib-

310 4 Chemical Pulping Processes

Tab. 4.38 The increase in carbohydrate yield based an wood

obtained as a result of 1-h polysulfide pretreatment of wood at

90 °C prior to kraft pulping [ 169].

[S(0)]

[mol L–1]

Xs [OH– ]

[mol L–1]

E0,m

[mV]

E0,c

[mV]

RT/F*Ln[x]a)

[mV]

Em

[mV]

Ec

[mV]

DYm

[%]

DYc

[%]

0.1 2.0 0.32 –509 –511 –126.1 –635 –637 3.3 3.6

0.1 2.0 0.10 –485 –486 –180.2 –665 –666 2.7 2.4

0.1 0.5 0.32 –552 –558 –126.1 –673 –684 2.2 1.8

0.1 0.5 0.10 –538 –534 –180.2 –716 –714 0.4 0.6

0.5 2.0 0.32 –559 –537 –75.7 –606 –612 4.9 4.7

0.5 1.0 0.10 –555 –539 –129.8 –678 –669 2.1 2.4

0.5 1.0 0.01 –512 –490 –237.9 –725 –728 0 0.0

a) right part of equation (139).

uted to improved retention of glucomannan for softwood [182]and xylan for hardwood

[183].

In laboratory trials a spruce-lodgepole-fir blend (80:15:5) was pulped by using a

conventional batch-type schedule [184]. The polysulfide-containing cooking liquor

was produced according to the MOXY process using an industrial white liquor in

a pilot plant. The composition of the white liquor before and after the oxidation

treatment is compared in Tab. 4.39.

Tab. 4.39 Oxidation of white liquor according to the MOXYprocess [ 184].

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