Reference Manual

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Chapter 14

Bleaching and Brightening

If the kraft pulp being produced is going to be

bleached, then pulping is allowed to proceed until

90% or more of the lignin originally found in the

wood is removed; however, the small amount of

lignin that is left gives unbleached pulp its

characteristic light brown color. Bleaching is the

way to remove the residual lignin while causing

minimal damage to the fibers and produce white

pulp.

The differences between bleaching and

brightening are as follows:

Bleaching — This process removes the lignin and

is used to increase the brightness of chemical

pulps.

Brightening — This process converts chemical

groups in lignin to forms that do not darken pulp,

thereby making it whiter. This process is used for

mechanical or chemi-mechanical pulps that still

contain vast amounts of lignin.

The Process

Oxygen Delignification

This process is typically found midway between

pulping and bleaching. Oxygen can be used in

sodium hydroxide (NaOH) solution under pressure

to delignify (i.e. remove lignin for the wood)

unbleached pulp (see figure 14-1). Up to one-half

Figure 14-1. Oxygen Delignification Diagram

Drawing is from TAPPI’s Making Pulp and

Paper Series and is used with permission.