What Is Involved in Bleaching Pulp?

As a significant element of creating paper products, bleaching pulp is a multi-step process in which residual lignin is destroyed or removed in order to visually brighten the final product. Lignin, an organic compound that serves to rigidify the cells of woody plants, plays a significant role in the color of pulp. Thus, bleaching pulp involves breaking down lignin into smaller molecules to achieve brightness. In general, the more lignin remaining in the final product, the more likely it is to gradually yellow due to exposure to air or light. The act of delignification tends to involve several distinct processing steps, as a one-step bleaching process would likely adversely affect the overall pulp strength.

There are two possible approaches to the chemical process of bleaching pulp. If the end product is desired to maintain a high percentage of pulp and achieve up to 70 percent of potential brightness, the best approach is to break down the lignin to minimize its effects on pulp coloration. For brightness of 90 percent or higher with some loss of fiber qualities, the lignin must be almost completely removed. Brightness is defined as the degree of ability to reflect light. In either process, dissolved lignin is washed out of the pulp between processing stages.

Traditionally, the common process for bleaching pulp was referred to as CEHDED or CEDED, in which the letters indicate the order of application of specific chemicals. The CEHDED process involved the following steps: chlorine (C); alkaline extraction (E); hypochlorate (H); chlorine dioxide (D); more alkaline extraction (E); and more chlorine dioxide (D). For CEDED, the hypochlorate (H) step was removed.

The processes used in bleaching mechanical pulp have not been viewed as having a negative environmental effect, because the chemicals typically used in these processes tend to produce byproducts that are relatively benign. In contrast, bleaching chemical pulp can result in damage to the environment, particularly due to the organic materials released into neighboring bodies of water. Before the 1930s, household bleach or sodium hypochlorite was commonly the means of bleaching pulp; later, chlorine was the bleaching element of choice. Since 1990, delignification of pulp has more often used newer processes that do not utilize chlorine. These processes are commonly known as Element Chlorine Free (ECF), which can contain certain amounts of chlorine dioxide, and Totally Chlorine Free (TCF), which uses chemicals such as hydrogen peroxide, oxygen, or ozone in place of chlorine.