Formation of Organochlorine Compounds

The negative environmental impact associated with the use of elemental chlorine

is primarily related to the formation of chlorinated organic compounds. A large

variety of individual chlorinated compounds are formed during the chlorination

reactions, and the major part of these are released to the aqueous phase where

they are summarily detected as AOX (adsorbable organic compounds). Another

part of the chlorinated organic compounds remains in the bleached pulp; this is

denoted organic chlorine content, known as OCl or OX. The AOX fraction can be

classified into two categories of different molecular weight: (a) The high molecular

fraction (molecular weight >1000 Da), which constitutes about 80% of the

AOX and contains mainly hydrophilic and nonaromatic compounds; and (b) the

low molecular fraction, which consists of highly chlorinated compounds (e.g.,

polychlorinated phenolic compounds, etc.) that are potentially problematic and

toxic to aquatic organisms due to their ability to penetrate cell membranes. The

substitution of elemental chlorine with 100% chlorine dioxide during the first

7.4 Chlorine Dioxide Bleaching 771

bleaching stage (D0) significantly reduces AOX formation, and virtually eliminates

levels of polychlorinated phenols in the final effluents to below the limits of analytical

detection [21]. The generation of organically bound chlorine is linearly

related to the charge of active chlorine according to the following expression [22]:

AOX _ 0_1 __ C _ D _5_ _82_

where AOX is adsorbable organic compounds (in kg odt–1), C is the amount of

chlorine (in kg odt–1), and D is the amount of chlorine dioxide (in kg, calculated as

active chlorine odt–1).

Equation (82), which is valid for softwood kraft pulps, indicates that chlorine

dioxide introduces only about one-fifth of the AOX formed during chlorine

bleaching. In the case of hardwood kraft pulps, less AOX is generated due to the

different chemical structure of hardwood lignin (syringyl units) as compared to

softwood lignin (guaiacyl units). The amount of AOX evolving from chlorine and

chlorine dioxide bleaching of hardwood kraft pulps can be estimated from Eq.

(82) by replacing the factor 0.1through 0.05 to 0.08, depending on the hardwood

species and reaction conditions.

Almost all of the chlorinated organic substances in the effluent of a multi-stage

ECF sequence comprising at least two D stages are formed in the D0 and E1 stages.

Kinetic studies have revealed that the generation of organic chlorine occurs very rapidly

[23], with the final amount of total chlorinated organic material (AOX+OX) being

produced within the first 10 min of reaction with chlorine dioxide (Fig. 7.71).

0 50 100 150

0,0

0,2

0,4

0,6

0,8

1,0

Kappa number

Organic Chlorine, kg/odt

Reaction time, min

OX in pulp AOX in Liquor

Kappa number

Fig. 7.71 Kinetics of organic chlorine formation (AOX and

OX) during D0 treatment of spruce kraft pulp, kappa number

28.7 (according to [23]). D0 conditions: 45 °C, 1% consistency,

kappa factor 0.22.

772 7Pulp Bleaching

The data in Fig. 7.71show that all the organic chlorine attached to the pulp

(OX) is formed within a very short time, while the increase in AOX in the bleaching

filtrate is predominantly due to increasing solubility of the chlorinated lignin

in the pulp throughout chlorine dioxide treatment. The same study revealed that

86% of the sum of AOX and OX originates from the reaction with hypochlorous

acid which is formed in situ through the step-wise reduction of chlorine dioxide

[see Eqs. (61), (63) and (64)]. Hypochlorous acid reacts with the chemical structures

present in lignin in a different way as compared to elemental chlorine,

which is created simply by shifting the pH below 2 [Eq. (66)]. In principle, the

extent of chlorination is lower for reactions with hypochlorous acid as compared

to those with elemental chlorine. As an example, hypochlorous acid reacts with

olefinic structures to form chlorohydrin, while chlorine converts them to dichlorinated

compounds [24]. The covalently bound chlorine is more easily eliminated

from chlorohydrins during subsequent alkaline extraction (by a SN reaction) than

from the dichlorinated structures derived from reactions with elemental chlorine.

Alkaline extraction following a D0 stage generally reduces the AOX and OX level,

depending on temperature and sodium hydroxide concentration. The elimination

of a washing step between D0 and E1 provides a reduction of 65% in the total level

of AOX in the effluents. This was demonstrated for an existing ECF bleaching

sequence processing E. globulus kraft pulp, kappa 13, where a DE pre-treatment

was replaced by a (DE) delignification unit, while keeping the final DED sequence

unchanged [25]. Unlike the Ultim-O process described above, the temperature

and pressure in the extraction stage were not altered. The sodium hydroxide in

the E1 stage was sufficient to neutralize the acidic carry-over in the effluent of the

D0 stage while maintaining the pH above 11.

Surprisingly, it was found that the AOX levels generated in a D0(EO)D(EP)D

sequence were higher for the oxygen-delignified softwood kraft pulps as compared

to the non-oxygen-delignified pulps when compared at the same kappa numbers

of the pulps entering the D0 stage [26]. The relationship between AOX and kappa

number for both types of pulp is shown graphically in Fig. 7.72.

The main difference between the unbleached and the oxygen-delignified pulps

is reflected in the higher content of HexA (4-deoxy-b-l-threo-hex-4-enopyranosyluronic

acid) in the latter, compared at the same kappa number, due to its resistance

towards oxygen delignification [27]. This indicates that the AOX formation

in the D0 stage is more dependent on the HexA content than on the kappa number,

as depicted in Fig. 7.73. HexA probably forms chlorinated dicarboxylic acids

in the presence of chlorine dioxide, which however is easily decomposed by

means of alkaline post-treatment [28].

The rule-of-thumb Eq. (82) is only valid within the conventional temperature

range used in D0 or D1 stages. The implementation of ECF bleaching in existing

bleach plants very typically was made by simply replacing chlorine with chlorine

dioxide. Some mills even today still operate a low-consistency D0 stage, because

the equipment was not modified. Similarly, the temperature was kept at the low

level required to run a C stage, or increased only moderately. Thus, typically D0

stages are operated between 45 °C and 70 °C (at best), and D1 or D2 stages at

7.4 Chlorine Dioxide Bleaching 773

0 10 20 30

0.0

0.5

1.0

1.5

2.0

SW-Kraft SW-Kraft-O

AOX, kg/odt

Kappa number

Fig. 7.72 AOX formation in the D0 stage as a function of the

kappa number of both oxygen-delignified and non-oxygendelignified

softwood kraft pulps (according to [26]).

0 10 20 30 40 50 60

0.0

0.5

1.0

1.5

2.0

SW-Kraft SW-Kraft-O

AOX, kg/odt

HexA content, ìmol/g

Fig. 7.73 AOX formation in the D0 stage as a function of the

HexA content of both oxygen-delignified and non-oxygendelignified

softwood kraft pulps (according to [26]).

774 7Pulp Bleaching

70–80 °C. The application of a hot D0 stage, as described by Lachenal [29], alters

not only the bleaching results but also the effluent characteristics. Figure 7.74

compares the AOX load resulting from the treatment of a eucalyptus kraft pulp

with increasing amounts of chlorine dioxide in a hot D0 stage. An increase in the

chlorine dioxide, from 1% to 2% active chlorine, does not result in a doubling of

the AOX load. For comparison, the other technological alternative for a combination

of hot acid hydrolysis and chlorine dioxide delignification [30], hydrolysis for

110 min and addition of ClO2 (without intermediate washing), was tested. The

short retention of only 10 min at 90 °C results in a significantly higher AOX residual.

This is a clear indication of decomposition reactions taking place during the

2-h period at high temperature. Hydrolysis to inorganic chloride ions also occurs.

If such hydrolysis is conducted well ahead of the chlorine dioxide addition, and

the time following the addition is short, then degradation will not take place.

1,0 1,5 2,0

0,0

0,1

0,2

0,3

AOX formation [kg/odt]

Active Chlorine Charge [%]

D

hot

A

hot

/D

Fig. 7.74 Impact of active chlorine amount and addition

point in hot chlorine dioxide delignification on AOX load. Oxygen-

delignified eucalyptus kraft pulp, kappa 10. D0 at pH 3,

90 °C, 2 h; Ahot/D with 110 min acid hydrolysis at pH 3, 90 °C,

addition of ClO2 additional time 10 min.

It is therefore not surprising to see similarly lower AOX and OX values also in

high-temperature softwood pulp bleaching. The decrease does not require an

extreme residence time, as in this example 1h was applied to the D0 stage. The

effect is clearly the result of the very high temperature.

This impact is shown graphically in Figs. 7.7.5 and 7.76. In comparison to conventional

ECF bleaching [31], the amount of dissolved halogenated compounds

(AOX) is cut by more than half by increasing the temperature in the D0 and D1

stages. Similarly, the application of high temperature in other D stages reduces

the amount of halogenated compounds remaining in the pulp.

7.4 Chlorine Dioxide Bleaching 775

50.C+70.C 90.C+85.C

0.0

0.2

0.4

0.6

0.8

1.0

AOX formation [kg/odt]

Temperature in D stages [. C]

D

D

Fig. 7.75 Impact of high temperature on the

AOX load generated in bleaching oxygendelignified

(kappa 13.4) softwood kraft pulp.

Constant 3.35% active chlorine (kappa factor

0.25), 1 h and pH <3in D0, variable temperature

(50 °C or 90 °C); Eop with 1.8 %NaOH,

0.5% H2O2 and 0.4 MPa O2; D1 with 1% active

chlorine, 2 h, and 70 °C or 90 °C.

D1(70.C) D1(90.C) D1(70.C) D1(90.C)

D

at 90.C

OX in pulp [g/odt]

D

at 50.C

P D

(90.C) D

(70.C)

Fig. 7.76 Impact of the temperature in D stages on the residual

of halogenated compounds in pulp bleaching with the

sequences D0EopD1D2 or D0EopD1P. For conditions, see

Fig. 7.75; D2 with 0.5% active chlorine, P with 0.25% H2O2.

776 7Pulp Bleaching

When not only the D0 stage but also all other all D stages are operated at higher

than “normal” temperature, the residual of halogenated compounds remaining in

the pulp (“OX”) also decreases. When a final alkaline peroxide stage is added,

which results in additional saponification and extraction, the OX level of the pulp

reaches a level that would be accessible under conventional conditions only in

ECF “light” bleaching – that is, with a much lower input of active chlorine [15].

The explanation for the lower AOX and OX values is the reactivity of quinones

(see Section 7.4.4). A sequence with a final P stage is certainly more attractive for

reaching low OX values compared to the addition of sulfamic acid. Although the

addition of sulfamic acid similarly lowers the level of OX, a 25% higher charge of

chlorine dioxide is required [13].

7.5

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