Bleaching with Dithionite 1125

reaction time required is short – 15–60 min is sufficient. The amounts of dithionite

vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available

Chromophores for the reduction process is limited, a brightness plateau is typically

reached at an input of between 1.2% and 1.5%. An addition above these levels

Triggers decomposition and the formation of thiosulfate. A technological alternative

To tower bleaching is the application of dithionite in the refiner. Although the

Brightness gains are slightly inferior, the application is simple and can eliminate

The need for a bleach plant.

As can be imagined, the reduction reactions may be reversible. For example, the

Reduction of an o-quinone to the catechol generates the leuco form, which can be

Re-oxidized by oxygen in the air. Despite this, the brightness of dithionite-bleached

Pulp is relatively stable in heat-induced aging. Stability is affected by light, UV radiation

Or transition metal-induced reactions of the phenolic groups within the

lignin, easily generating chromophores [35].

6.2

Bleaching with Hydrogen Peroxide

The application of hydrogen peroxide in bleaching mechanical pulp is by far older

Than its use in chemical pulp bleaching. A prerequisite of peroxide bleaching is

the “neutralization” of peroxide-decomposing transition metals (e.g., Mn). Chelation

Of the metal ions is achieved with compounds such as diethylene diamine

Penta-acetic acid (DTPA) or ethylene diamine tetra-acetic acid (EDTA) (see Section

II-5.4.2.4.5.1, Prerequisites of hydrogen peroxide application). The chelants are

Added most beneficially during screening after the refining process. This allows

Sufficient residence time and typically has the correct pH level (slightly acidic) for

effective chelation. The demand for chelant is between 0.1% and 0.3% on fiber (of

the commercial product with an active content of ~40%). The addition of chelant,

Together with the other chemicals (H2O2, NaOH, silicate) is another option,

Though it is slightly less effective because of the higher pH.

The limited aggressivity of hydrogen peroxide might be a disadvantage where

Delignification is required, but it may be advantageous if an improvement of

Brightness is the only target. In the past therefore, hydrogen peroxide bleaching

was often labeled as being “lignin-conserving”. As described in previous chapters,

This it not the case, and lignin is in fact removed during peroxide bleaching. Lignin

Side chains are cleaved and quinones oxidized to more water-soluble carboxylic

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