Influence on Bleachability

The bleachability of kraft pulps can be defined as the consumption of chemicals

required to achieve a given target brightness using a specific bleaching sequence,

either ECF or TCF. To better compare the application of different bleaching chemicals,

the bleaching chemical consumptions are calculated and expressed in oxidation

equivalents (OXE) [92]. The unit mass of a bleaching chemical (e.g., 1 kg) corresponds

to the molar equivalent of mass transferring 1 mol of electrons. The oxidation

equivalents of 1 kg of the most important bleaching chemicals applied in

ECF and TCF bleaching are listed in Tab. 4.33.

The bleachability of pulps can be expressed in several ways, for example, as the

sum of oxidation equivalents (OXE) divided by the ingoing kappa number for

each stage in a sequence or as OXE per ingoing kappa number or the kappa number

removed over the whole sequence.

Tab. 4.33 Oxidation equivalents (OXE) of the unit mass of the

most important bleaching chemicals (according to [92].)

Bleaching chemicals OXE kg–1

Chlorine dioxide 74.1

Active chlorine 28.8

Hydrogen peroxide 58.8

Oxygen 125.0

Ozone 125.0

Peracetic acid 26.3

264 4 Chemical Pulping Processes

Definitions for the bleachability based on OXE:

_

i

i _1

OXE

Kappai _1

_

i

i _1

OXEi

Kappa 0

_ i

i _1

OXEi

D Kappa

The amount of hexenuronic acid is not significantly changed during, for example,

the oxygen delignification or peroxide bleaching. Thus, it can be assumed

that HexA does not consume any bleaching chemical in these stages. For bleaching

stages where HexA is not oxidized, the specific OXE consumption must be

referred to a kappa number which is corrected for the hexenuronic acid (**means

that one kappa number unit corresponds to 11.6 lmol of HexA [68]). In case of,

for example ozone bleaching, the specific OXE consumption is referred to the

whole kappa number as hexenuronic acid is completely oxidized by ozone. The

bleachability can also be expressed by the ratio between the light absorption coefficient,

k, determined at a wavelength of 457 nm, and the corresponding kappa

number corrected (**) or uncorrected for the content of hexenuronic acids from

the pulp investigated [93]:

k 457 nm

Kappa _**_

The enhanced bleachability of modified cooked pulps can be attributed to

changes in by-product chemistry. In principle, the residual lignin in modified

cooked pulps is less condensed and therefore likely to be more readily solubilized

in subsequent bleaching stages [94]. Moreover, it has been suggested that xylan

compounds which are re-deposited during the cook form a physical barrier for the

extraction of residual lignin. The mass transfer limiting phenomenon is assumed

to hamper delignification in both the final phase of cooking and in subsequent

bleaching [95].

Pulp bleachability tends to correlate also with the content of b-aryl ether structures.

Thioacidolysis can be used to estimate the amount of b-arylether structures

in the residual lignin [96]. Pulps with a low concentration of b-O-4-structures exhibit

a high specific absorption coefficient, as seen in Fig. 4.59.

This concludes that a large amount of b-O-4-linkages in the residual lignin indicates

a low concentration of chromophores per kappa number unit. The b-O-4-

structures are degraded with prolonged cooking intensity. Thus, there is a steep

decrease of these structures until an H-factor of approximately 3000 is attained.

4.2 Kraft Pulping Processes 265

0.6 0.8 1.0 1.2 1.4 1.6

â-O-4 structures [ìmol/g residual lignin]

Specific absorption coefficient, k

457 nm

/kappa

Fig. 4.59 Relationship between the concentration of b-O-4-

structures (determined by thioacidolysis) and the specific

light absorption coefficient of the residual lignin of softwood

kraft pulps (according to [97]).

Further prolongation of cooking intensity causes only an insignificant decrease in

the amount of b-O-4-structures. A relationship between the concentration of

b-aryl ether linkages and the H-factor of kraft cooking is shown in Fig. 4.60.

0 4000 8000 12000 16000 20000

â-O-4 structures [mmol/g residual lignin]

H-factor

Fig. 4.60 Course of the content of b-aryl ether

linkages in residual lignins during kraft cooking

as a function of cooking intensity expressed as

H-factor (according to [97]). Softwood kraft

pulps delignified to kappa number of approximately

20 with different cooking conditions,

e.g., time (39 to 2602 min), hydroxide concentrations

([OH– ]= 0.55–1.5 mol L–1), sulfidities

([HS– ]= 0.075–0.5 mol L–1) and temperatures

(T = 154–181 °C).

266 4 Chemical Pulping Processes

The influence of pulping variables is clearly reflected in the specific amount of

b-O-4-structures. When the [OH– ]is increased while keeping the [HS– ]and temperature

constant, the content of b-O-4 structures is found to increase. The same

observation is made when the [HS– ]is increased with the [OH– ]and the temperature

constant, or when the temperature is increased with the other parameters

unchanged. It is well known that an increase in [OH– ]en hances the rate of delignification.

Clearly, the accelerated rate of delignification leaves a residual lignin in the

pulp with a large amount of remaining b-O-4-structures at the given kappa number.

Since hydrogen sulfide ions promote the cleavage reactions, the positive correlation

with the amount of b-O-4 structures might be interpreted as a result of the significantly

shorter reaction times at the higher [HS– ]to be the determining factor.

The color of the unbleached (kraft) pulps originates from a complex mixture of

certain unsaturated structures which are denoted as chromophores. Very small

amounts of conjugated structures with a very high specific absorption coefficient

are sufficient to darken the pulp material. Most of the chromophores are certainly

derived from lignin structures. As mentioned above, they may also originate from

unsaturated structures of the polysaccharides such as hexenuronic acid (HexA)

and aromatic structures containing catechols and chromones which evolve from

carbohydrate moieties after the elimination of water and other molecules [70].

Moreover, metal ions bound to the unbleached pulp can originate chromophores,

probably due to the formation of a complex with a catechol structure. The

contribution of certain metal ions such as Mn, Cu, Fe, Mg, Al and others to the

color of a pulp will be of increasing importance with the increasing closure of the

water cycle in the washing and bleaching areas.

The precipitation of lignin from the black liquor might also be a source of lowering

pulp brightness. The dissolved lignin shows a higher light absorption coefficient

than lignin isolated from the pulp. The extent of precipitation correlates

with the concentration of lignin dissolved in black liquor and with decreasing pH

[97,98].

The bleachability of pulps may be related to the conditions that prevail during

the kraft cook. In a detailed study, the influence of the most important parameters

in kraft cooking of softwood (Pinus sylvestris) on TCF bleachability has been investigated

[97]. It is clear that a pure peroxide-containing bleaching sequence such as

a QPQP-sequence behaves more sensitively to different chromophore structures

in the residual lignin as compared to an ozone-containing sequence (e.g., AZQP).

The latter shows no influence on cooking temperature, whereas the former reveals

an improved bleachability when the cooking temperature for pulps produced with

low and medium [OH– ]is high. It is known that the bleachability of kraft pulps is

improved when the EA charge is increased [99,100]. Interestingly, there is an optimum

EA charge with respect to bleachability. This optimum is observed in both

QPQP- and AZQP-bleaching for a hydroxide ion concentration of 1.0 mol L–1.

Beyond this EA concentration, the bleachability becomes impaired. The bleachability

in QPQP-treatment is further improved by increasing the hydrogen sulfide

ion concentration in the kraft cook. It is however almost unaffected in AZQPbleaching.

The presence of inert cations such as sodium ions in the cooking

4.2 Kraft Pulping Processes 267

liquor contributes to the ionic strength in the liquor. It can be expected that an

increased closure of the water cycles (e.g., bleach plant and recovery) leads inevitably

to higher concentrations of inert cations, such as sodium ions. If the concentration

of sodium ions is increased in the final part of the cook, the cooking time

must be increased substantially to reach the target kappa number of about 20. In

combination with low concentrations of hydroxide ion and hydrogen sulfide ion,

the QPQP-bleachability is negatively influenced at an ionic strength of only

2.9 mol L–1. The poor bleachability is also characterized by a lower brightness value

at the given kappa number of about 20. Bleachability is not adversely affected

up to a sodium ion concentration of 2.9 mol L–1 for pulps in which the ionic

strength is adjusted after the pretreatment and then kept constant throughout the

whole cook [101].

The results of the QPQP-bleaching trials reveals a clear relationship between

the content of b-O-4 structures in the residual lignin of the unbleached pine kraft

pulps and bleachability of the pulp, expressed as the consumption of oxidation

equivalents per kappa number necessary to achieve a brightness level of 87% ISO

(Fig. 4.61). The results clearly demonstrate that the higher the specific amount of

b-O-4 structures, the lower the specific consumption of bleaching chemicals.

Bleachability remains at an acceptably high level when the specific content of

b-O-4 structures exceeds a value of 5 lmol g–1.kappa**–1. However, at a value of

around 3 lmol g–1.kappa**–1, the bleachability changes dramatically and, below

this value, becomes very poor.

0 2 4 6 8 10 12

Consumed OXE / kappa**

â-O-4 [ìmol/g.kappa**]

Fig. 4.61 Consumed OXE/kappa number** for

pine kraft pulps bleached in an OQPQPsequence

to a brightness of 87% ISO as a function

of b-aryl-ether structures in the residual

lignin after cooking according to Gellerstedt

and Wafa Al-Dajani [93]and Gustavsson et al.

[97]. The peroxide charge in the first P-stage

was 3.0% H2O2, while in the second P-stage

the peroxide charge was varied between 1.5

and 6.0 % H2O2. In the second P-stage, 0.05%

Mg-ions were added.

268 4 Chemical Pulping Processes

0,2 0,4 0,6 0,8

1,0

1,1

1,5

1,6

Brownstock pulps: EA-variation at begin EA-variation at end of cook

Oxygen bleached: EA-variation at begin EA-variation at end of cook

k

457 nm

/ kappa**

Residual OH- [mol/l]

Fig. 4.62 The light absorption coefficient, k,

(measured at 457 nm) per kappa number corrected

for the content of HexA as a function of

the residual effective alkali concentration at the

end of the cook (according to [102]). The

unbleached pulps are produced according to

an ITC-type cook with EA variation in the residual

delignification phase (black liquor pretreatment

and three

stages) and according to a simple two-stage

process with EA variations in the beginning of

bulk delignification (black liquor pretreatment

and one stage). The unbleached pulps all show

a kappa number around 17. Cooking temperature

was 160 °C for all cooks, sulfidity 40% or

at constant [HS– ]. Oxygen bleaching was conducted

at 100 °C, 115 min at 12% consistency,

0.7 MPa and 2.15% NaOH on pulp.

As discussed previously, unbleached softwood kraft pulps with a high remaining

amount of b-O-4 structures can be produced by adjusting the cooking conditions

such that both hydroxide ion concentration and hydrogen sulfide concentration

are maintained at a high level, together with a low ionic strength. In any case,

cooking conditions with high H-factors must be avoided (see Fig. 4.60) [97].

The specific light absorption of the unbleached pine (Pinus sylvestris) kraft pulps

with kappa numbers close to 17 is lower for the ITC-type pulps as compared to

the two-stage cooked pulps both with black liquor pre-impregnation. As expected,

the brightness increases with raising residual alkali concentration in the black

liquor after the cook (Fig. 4.62).

The lower specific light absorption coefficient after the cook at a given residual

effective alkali concentration for the ITC-type pulps as compared to the two-stage

pulps can be attributed to both the lower concentration of dissolved lignin in the

black liquor and the lower EA concentration during the initial stage of bulk

delignification [29]. The differences in the specific light absorption between the

cooking procedures after a subsequent oxygen delignification stage diminishes, as

shown in Fig. 4.62. Oxygen delignification clearly degrades the chromophore

structures with different specific absorption coefficients equally well. The HexA

4.2 Kraft Pulping Processes 269

content is found to decrease with increasing alkali concentration. Due to the higher

EA level in the early stage of the cook, the two-stage pulps generally reveals a lower

content of HexA, which is in agreement with the results of Vuorinen et al. [28].

The intrinsic viscosity level is higher for the ITC-type pulps (pulps produced

with an alkali concentration varied in the final part of the cook) as compared to

conventional kraft pulps. In accordance with other results, the viscosity of the

ITC-type pulps remains almost constant, or even shows a slight increase with

increasing residual alkali concentration, whereas the pulps with the EA variation

in the early stage of the cook shows a declining viscosity with increasing residual

alkali concentration (Fig. 4.63) [29].

0.2 0.4 0.6 0.8

Residual OH- [mol/l]

Brownstock pulps: EA-variation at begin EA-variation at end of cook

Oxygen bleached: EA-variation at begin EA-variation at end of cook

Intrinsic Viscosity [ml/g]

Fig. 4.63 Intrinsic viscosity as a function of the residual effective

alkali concentration at the end of the cook (according to

[102]and Fig. 4.62).

The selectivity advantage for the ITC-type pulps at a given residual alkali concentration

is preserved throughout oxygen delignification and QPQP final bleaching.

The difference in viscosity at a given brightness in the range 85–91% ISO between

the two pulping procedures even increases with increasing residual alkali

concentration.

The specific OXE consumption using a QPQP bleaching sequence after oxygen

delignification to reach a target brightness level is lowest for pulps with a residual

alkali concentration close to 0.5 mol L–1, both for the ITC-type pulps and the two-stage

pulps with an EA variation in the beginning of the cook. The ITC-type pulps are

slightly easier to bleach as compared to the two-stage pulps both with kappa number

17when the comparison ismade at the sameresidual alkali concentration (Fig. 4.64).

270 4 Chemical Pulping Processes

0 500 1000 1500

EA-variation at begin EA-variation at end of cook

Brightness [% ISO]

after QPQP* bleaching

Consumed OXE/Kappa number**

Fig. 4.64 Brightness development after a

QPQP-sequence as a function of the specific

OXE consumption (OXE/kappa number**)

(according to [102]). The ITC-type pulps for

which the alkali concentration was varied in the

late stage of the cook and the two-stage pulps

for which the alkali concentration was varied in

the beginning of the cook both with a residual

alkali concentration of about 0.5 mol L–1 are

compared.

The differences in bleachability between the ITC-type pulps for which the alkali

concentration is varied in the late stage of the cook and the two-stage cook for

which the alkali concentration is modified in the beginning of the cook are, however,

very small and cannot be regarded as significant.

Recently, Olm and Tormund studied the influence of different EA profiles on

the bleachability of pine kraft pulps with a kappa number of about 20, using a

two-stage process similar to that introduced by Sjostrom, but with the difference

that the former also varied the EA concentration in the pretreatment step [102].

These authors also found a clear relationship between the residual EA and the

ISO brightness of the unbleached pulps (Fig. 4.65).

The poor brightness of the pulp originating from a cook with a residual alkali

concentration of only 0.15 mol L–1 also translates into an impaired bleachability.

The bleachability is evaluated as the total consumption of OXE per kappa number

after the oxygen stage (and per ton of pulp) necessary to reach an ISO brightness

of 89%. The specific OXE consumption amounts to 160 OXE/kappaO2 for the pulp

with the low residual alkali concentration (0.15 mol L–1) as compared to only 125

OXE/kappaO2 with the high alkali concentration (0.88 mol L–1) despite the same

kappa number of 20. Assuming a kappa number of 10 after the oxygen stage, the

additional hydrogen peroxide charge in the last P-stage comprises approximately

6 kg t–1 [(1600 – 5(kgO3/t). 125) – (1250 – 5(kgO3/t). 125) = 350 OXE t–1 = 350/58.8 = 6.0 kg

H2O2 t–1). In the range of residual alkali between 0.34 and 0.61 mol L–1, the bleachability

remains almost unaffected by the hydroxide ion concentration in both

4.2 Kraft Pulping Processes 271

0.00 0.25 0.50 0.75 1.00

Intrinsic Viscosity [ml/g]

Brightness

ISO Brightness [%]

Residual [OH-], mol/l

Viscosity

Fig. 4.65 Brightness and viscosity of

unbleached pine kraft pulps at kappa number

20 as a function of the residual effective alkali

concentration (according to [27]). Laboratory

cooking trials using a two-stage kraft process

comprising a pretreatment step (where [OH– ]

was varied from 0.1 to 0.5 mol L–1

at constant [HS– ]= 0.3 mol L–1) and a cooking

stage (where [OH– ]was varied from 1.0 to

1.6 mol L–1 at constant [HS– ]= 0.3 mol L–1).

Cooking temperature was kept constant at

170 °C, and cooking time was adjusted to reach

the target kappa number 20.

pretreatment and cooking stages. As expected, cooking with a high concentration

of residual alkali leads to a significant loss in viscosity which, however, does not

impair the strength properties measured as rewetted zero-span tensile index. On

the contrary, the strength properties are about 5% lower as compared to the pulp

originating from the cook with the high residual alkali concentration. The reason

for this might be both the higher amount of hemicellulloses (due to enhanced

xylan reprecipitation) and the significantly higher H-factor (~ 3000 versus 1000) to

attain the target kappa number at this low EA charge which negatively influences

fiber dimensions (average fiber length was only 2.36 mm with residual

[OH– ]= 0.15 mol L–1 versus 2.45 mm with residual [OH– ]= 0.88 mol L–1).

The specific OXE consumption for modified softwood kraft pulps is significantly

lower when using an A-ZQ-P-sequence following one- or two-stage oxygen

delignification stages to reach a kappa number of approximately 10 prior to ozone

bleaching. Bleachability in terms of specific OXE consumption is improved by

interrupting the ITC-type cook at a higher kappa number and alternatively extending

the delignification by applying a reinforced two-stage oxygen delignification

stage (Fig. 4.66). Completing the cook at an earlier stage might prevent the

formation of bonds between lignin and carbohydrates, which are difficult to

remove during oxygen delignification and subsequent bleaching. Treating the

272 4 Chemical Pulping Processes

50 100 150 200

ITC-type, ê = 17 ITC-type, ê = 38

Postsulphonated, ê = 15 ASAM, ê = 20.5

ISO Brightness [%]

Consumed OXE/kappa number

Fig. 4.66 Brightness gain as a function of specific OXE consumption

(OXE consumed/kappa number) for AZQP-bleached softwood kraft

(ITC-type) and ASAM pulps (according to [107]).

ITC-pulps with 0.3 mol Na2SO3 kg–1 o.d. wood in the final cooking stage to achieve

a partial sulfonation also contributes to a slightly better bleachability as compared

to the reference pulp (see Fig. 4.66).

In various pulping and bleaching experiments, it has been shown repeatedly

that alkaline sulfite pulps such as alkaline sulfite with anthraquinone and methanol

ASAM exhibit a better bleachability as compared to kraft pulps [103–106]. For

comparative reasons, an unbleached softwood ASAM pulp with a kappa number

of 20.5 is included in this TCF-bleaching study. All pulps used for TCF-bleaching

are characterized before and after oxygen delignification (Tab. 4.34).

The specific OXE requirement for a given brightness level is significantly less

for an alkaline sulfite pulp (e.g., ASAM pulp) as compared to the kraft pulps with

or without modification, as can be seen from Fig. 4.66. The accelerated brightness

development of the alkaline sulfite pulps can be led back primarily to the significantly

higher brownstock brightness as compared to the kraft pulps. The brightness

advantage is preserved throughout oxygen delignification. Alkaline and acid

sulfite pulps show equal bleachability [107]. At a given kappa number, alkaline

sulfite pulp lignin contains by far more b-O-4 structures as compared to a residual

kraft lignin, and this agrees well with the observed superior bleachability

(21 lmol g–1·kappa** in the residual ASAM lignin versus 11.4 lmol g–1·kappa**

for the residual kraft lignin, respectively) [93].

4.2 Kraft Pulping Processes 273

Tab. 4.34 Characterization of softwood kraft and ASAM pulps

used for TCF bleaching according to a AZP-sequence before and

after oxygen delignification [107].

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