PH Value

Oxygen delignification is (always) carried out at highly alkaline conditions. In a

survey of North American mills, it was determined that the pH entering the oxygen

delignification ranges from 10.3 to 12.1 [46]. The maximum rate of degradation

for lignin model compounds such as propylguaiacol is shown to be in the

vicinity of pH 11, measured at room temperature in the range from pH 9 to pH

13.5 [39]. The rate increase at pH ≥ 9 is due to ionization of the phenolic groups,

which facilitates the redox reaction with oxygen. The maximum at pH 11 may be

due to the formation of further oxygen-containing species, such as superoxide

anions, superoxide radicals and hydroxyl radicals which contribute to the rate of

degradation. The evaluation of an industrial oxygen delignification plant revealed

the optimum viscosity–kappa number relationship (selectivity) at a blowline pH

of about 10.5 [47]. At lower pH, lignin begins to precipitate on the fiber, and this

clearly impairs selectivity.

In the case of hardwood kraft pulps, the extent of delignification is however

rather limited during oxygen delignification due to a relatively large amount of

hexenuronic acid groups. A subsequent sulfuric acid treatment would efficiently

remove the hexenuronic groups [48]. Taking these experiences into consideration,

it may be envisaged that in a two-stage process, the first stage is conventionally

run at high alkaline pH to recover the spent liquor, and the second stage at acidic

pH to remove the resistant structures. With this concept in mind, the effect of pH

in the range of 1.6 to 13.5 on the second stage of a two-stage oxygen delignification

process of a hardwood kraft pulp was investigated while the first stage was

run at alkaline pH [49].

The study revealed that the degree of delignification is highest at a pH 1.6 followed

by pH 2.7, pH 13.5, and showed at minimum at pH 7. The data in Fig. 7.42

show that both the bleachability – measured as specific OXE demand, OXE/kappa,

and pulp viscosity of the ECF-bleached pulp – are improved as the pH of the second

oxygen delignification increases from 1.6 to 13.5.

However, at a given tensile index, the apparent density and tear index decrease

with increasing the pH of the oxygen delignification, although the viscosity follows

the reverse trend.

The examination of the residual dioxane lignin revealed a negative correlation between

the extent of delignification during the second oxygen stage and the content

of total phenolic hydroxyl groups in the residual lignins of oxygen delignified

pulps. The ratio of the optical densities of the infrared bands at 1330 cm–1 to

1270 cm–1 indicates that the residual dioxane lignin in the oxygen-delignified pulp

704 7Pulp Bleaching

2 4 6 8 10 12 14

OXE / kappa

IN

Viscosity [mPas]

degree of delign. viscosity

Degree of

delignification [%]

Initital pH value

OXE / Kappa

IN

Fig. 7.42 Results of a second oxygen delignification

stage of a hardwood kraft pulp as a function

of pH (according to [49]). Initial substrate:

oxygen-delignified hardwood kraft pulp, kappa

number 13.3, viscosity 21.6 mPas delivered

from a mill; constant conditions in the second

oxygen delignification stage: 95 °C, 60 min,

initial pressure 245 kPa, 10% consistency.

produced at pH 1.6 contains fewer guaiacyl groups relative to syringyl units than

that isolated from the pulp made at pH 13.5 [49]. The lower content of phenolic

hydroxyl groups of the former residual lignin (pH 1.6) suggests that it is more

extensively degraded than the latter (pH 13.5).

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