Oxidative processes.

An example of the brightness gains achieved in bleaching with dithionite or

Hydrogen peroxide is shown in Fig. 6.1. Based on the initial brightness levels, the

range between 70% ISO to about 80% ISO is accessible for softwood. Hardwood

pulps can be bleached to >85% ISO. The top brightness requires more than one

Bleaching stage. The higher the brightness target, the more complicated the

Bleaching technology becomes. The simplest way to improve brightness is to treat

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

0 1 2 3 4 5

Dithionite Peroxide

Brightness gain [% ISO points]

Chemical charge [%]

Fig. 6.1 Typical response of mechanical pulp to oxidative or reductive

Bleaching with hydrogen peroxide or dithionite (hydrosulfite).

The pulp with sulfur dioxide. The reduction of transition metals decolorizes the

above-mentioned catechol–metal complexes. Metal ions reduction not only decolorizes

The chromophores but also solubilizes the metals and allows their removal

By washing and subsequent dewatering. Because the reduction is easily reverted

by air oxidation, all reductive bleaching treatments require “closed” systems.

Because air can be excluded from pulp slurries more easily at a lower consistency,

reductive bleaching in the past was conducted in up-flow towers at only 3–5% consistency.

The de-aeration which takes place in modern medium-consistency

pumps permits reductive bleaching at consistencies from 8% to 14% (see Section

II-6.3). This allows a smaller size of reactors to be used (tubes instead of towers)

And a lower volume of process water.

6.1

Bleaching with Dithionite

The impact of the reduction process is intensified with the application of dithionite,

Which in addition reduces quinones and quinone methids. (In Englishspeaking

countries, dithionite is frequently still labeled as “hydrosulfite”, an incorrect

Description of the product, but this was rectified in 1881.) The dominant compound

Used in bleaching processes is sodium dithionite, and in the description of

bleaching processes the normal abbreviation for dithionite bleaching is “Y”.

Sodium dithionite is produced using several processes, but historically the zinc

Dust process is the most important. The reaction of zinc dust in an aqueous slurry

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

Дата добавления: 2015-10-21