1124 6 Bleaching of Mechanical Pulp

with sulfur dioxide generates zinc dithionite which is reacted further with caustic

soda:

Zn + 2SO2 → ZnS2O4

ZnS2O4 + 2NaOH → Zn(OH)2 + NaS2O4

Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,

after concentration of the liquor by evaporation, sodium dithionite is precipitated

by the addition of sodium chloride. The salt, which is dried before shipment, contains

up to 300 p.p.m. zinc.

The dominant process is the reaction of sodium formate with sulfur dioxide

and caustic soda under pressure. The reaction is described by:

HCOONa + 2SO2 + NaOH → NaS2O4 + CO2 + H2O

Commercial grades of sodium dithionite powder typically have a content of

about 88% sodium dithionite, with the sodium salts of bisulfite, sulfite, sulfate

and carbonate as the byproducts. Solutions of dithionite in water must be prepared

on-site. In pulp bleaching, the use of returnable steel containers with up to

2000 Kg of technical-grade product is common. Typically, a small amount of a chelant

(e.g., ethylene diamine tetra-acetic acid; EDTA) is added. This avoids scaling

(precipitation of calcium carbonate) caused by the hardness of the dilution water.

An analysis of the stability of dithionite solutions in the presence of iron and manganese

[33] contradicted the speculation about the metal ion (Fe, Mn) -induced

decomposition of dithionite [34]. The fine powder reacts exothermically with atmospheric

oxygen; the heat of oxidation can lead to ignition. Sodium dithionite is

therefore classified as “spontaneously combustible goods”, and the corresponding

transport and storage regulations must be applied.

Alternatively, some mills operate an on-site process using a solution of sodium

borohydride (~12% by weight) with caustic soda (~40%). This mixture is reacted

with sulfur dioxide to yield dithionite solutions:

NaBH4 + 8NaOH + 8SO2 → 4Na2S2O4 + NaBO2 + 6H2O

Some pulp mills are supplied with an alkaline solution (pH > 12) of sodium

dithionite. Transportation requires cooling to a temperature below 10 °C to maintain

the content at ca. 11–12% dithionite. Cooling of the storage tank is not required

when there is a rapid turnover of the product. Exclusion of air is required

to avoid product losses, as dithionite reacts rapidly with oxygen in the air to yield

sulfite. In the absence of air, decomposition reactions take place, with one of the

products being thiosulfate; this, in turn, may accelerate corrosion reactions.

Bleaching with dithionite typically is conducted at moderately acidic pH, between

pH 4.5 and 6.5. The temperature in tower or tube bleaching is maintained

at 60–80 °C, although a higher temperature produces a faster response to the

chemical addition. The reduction of chromophores occurs very quickly; so the

6.1 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

acids. However, because hydrogen peroxide will not react easily with the

aromatic systems of lignin, its level of removal is moderate. In mechanical pulp

bleaching, this is an advantage, and the yield and optical properties (opacity) are

only moderately affected by the bleaching process. An example of the response of

mechanical pulp to hydrogen peroxide is shown in Fig. 6.1.

Peroxide addition yields increasing brightness with charges up to about 5%. The

plateau is reached at about 82% ISO for softwood pulp, spruce (Picea spp.) and pine

(Pinus spp.), and 86% ISO for hardwood mechanical pulp, poplar and aspen (Populus