I II III

Bleaching chemicals

Chlorine-containing Cl2 ClO2 NaOCl

Chlorine-free O3 O2 H2O2

Type of reaction electrophilic electrophilic nucleophilic

pH level acid acid/alkaline alkaline

Reaction sites in

lignin structures

olefinic and

aromatic

free phenolic groups,

double bonds

carbonyl groups,

conj. double bonds

Reaction sites in

carbohydrate structures

hexenuronic

acids

hexenuronic acid

(only ClO2)

The data in Tab. 7.1show additionally that each chlorine-containing chemical

has an equivalent chlorine-free counterpart. Ozone and gaseous chlorine are

grouped together because they react as electrophilic agents with aromatic rings of

both etherified and free phenolic structures in lignin, as well as with olefinic

structures. The hexenuronic acids which contribute to the kappa number, predominantly

in the case of hardwood kraft pulps, are degraded solely by electrophilic

reactants in an acid environment. Chlorine dioxide and oxygen under alkaline

conditions are placed in the same category because they both attack primarily free

phenolic groups. Compared to chlorine dioxide, oxygen behaves rather unselectively

because molecular oxygen gradually reduces to highly reactive radicals (e.g.,

hydroxy radicals) which also attack unchanged carbohydrate structures. Nucleophilic

agents such as hypochlorite and hydrogen peroxide attack electron-poor

structures (e.g., carbonyl structures) with conjugated double bonds, which are

often highly colored. Nucleophilic agents thus decolorize (brighten) the pulp

while being less efficient with respect to delignification as compared to electrophiles.

However, peroxide bleaching changes to an efficient delignification stage

when applying reinforced conditions (high temperature, high charges of sodium

hydroxide and hydrogen peroxide).

Bleaching is most efficient when the bleaching sequence contains at least one oxidant

fromeach category. Froma process point of view, however, it is not advantageous

to change between electrophilic and nucleophilic bleaching stages, because this is

connected with a change in pH (see Section 7.10).

The classification is of course rather simplistic because it cannot take into

account the fact that most of the bleaching chemicals are not stable in aqueous

environment, and change to species with different reactivity towards the pulp

components.

Bleaching reactions of chemical pulps are equivalent to oxidation reactions. To

date, sodium borohydride is the only reductant used in chemical pulp bleaching.

It is primarily applied to stabilize the carbohydrates either after hot caustic extraction

or after ozone treatment, and also exerts a slight brightening effect. Oxidants

accept electrons from a substrate and are thereby reduced. For example, a chlorine

dioxide molecule accepts five electrons and forms one chloride ion. The equivalent

weight corresponds to the weight of an oxidant transferring 1mol of electrons.

With the concept of oxidation equivalent (OXE), the oxidation capacity of any

bleaching chemical can be expressed [5]. An OXE is equal to 1mol of electrons

being transferred during oxidative bleaching. A list of the most important bleaching

chemicals involved in conventional, ECF and TCF bleaching is provided in

Tab. 7.2 [5].

Historically, the active chlorine concept was used to quantify the oxidizing

power of the different chlorine-containing chemicals such as elemental chlorine,

hypochlorite and chlorine dioxide into chlorine gas equivalents. In ECF bleaching

sequences this concept still prevails. The intention of the OXE concept was to

compare various ECF and TCF sequences with regard to their efficiencies simply

by summing all the OXEs used in the different stages. This concept is widely

accepted today, although it does not allow the bleachability of a certain pulp to be

612 7Pulp Bleaching

7.2 Bleaching Operations and Equipment

Tab. 7.2 Oxidizing equivalents (OXE) of the most important

bleaching chemicals involved in conventional, ECF and TCF

bleaching of chemical pulps [5].

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