Bleaching pulp is a multi-step process to remove lignin and brighten paper products. Delignifying involves breaking down lignin into smaller molecules. Two approaches to whitening paste are cutting down lignin for up to 70% brightness or removing it almost completely for 90% brightness. Chemical bleaching paste can harm the environment, but newer processes like ECF and TCF use hydrogen peroxide, oxygen, or ozone instead of chlorine.
As a significant element in the creation of paper products, bleaching pulp is a multi-step process in which residual lignin is destroyed or removed to visually brighten the final product. Lignin, an organic compound that serves to stiffen the cells of woody plants, plays a significant role in the color of the pulp. Therefore, the bleaching pulp involves breaking down the lignin into smaller molecules to obtain the brightness. In general, the more lignin left in the final product, the more likely it is to gradually turn yellow from exposure to air or light. The act of delignifying tends to involve several distinct processing steps, as a one-step bleaching process could adversely affect the overall strength of the paste.
There are two possible approaches to the whitening paste chemical process. If you want the final product to retain a high percentage of flesh and achieve up to 70% potential brightness, the best approach is to cut down the lignin to minimize its effects on flesh coloration. For a brightness of 90 percent or higher with some loss of fiber quality, the lignin must be removed almost completely. Brightness is defined as the degree of ability to reflect light. In both processes, the dissolved lignin is washed out of the pulp between processing steps.
Traditionally, the common process for bleaching pasta was referred to as CEHDED or CEDED, where the letters indicate the order in which specific chemicals are applied. The CEHDED process included the following phases: chlorine (C); alkaline extraction (E); hypochlorate (H); chlorine dioxide (D); more alkaline extraction (E); and more chlorine dioxide (D). For CEDED, the hypochlorate (H) phase has been removed.
The processes used in mechanical pulp bleaching were not considered to have a negative environmental effect, as the chemicals typically used in these processes tend to produce relatively benign by-products. Conversely, the chemical bleaching paste can cause harm to the environment, especially due to organic materials released into nearby bodies of water. Prior to the 1930s, household bleach or sodium hypochlorite was commonly the means to whiten pasta; later, chlorine was the bleaching element of choice. Since 1990, pulp delignification has more often used newer processes that do not use chlorine. These processes are commonly known as Element Chlorine Free (ECF), which may 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.
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