Pulp bleaching processing means that make the coloring materials of the pulp are removed or modified to a certain degree. Pulp bleaching processing plays an important role in paper making, it is mainly used to improve lightness of paper pulp and get quality pulp with certain whiteness, cleanliness, purity and excellent physical and chemical characteristics. Pulp bleaching processing increases its applications. Bleached pulp can be applied to manufacture high-quality paper and refined pulp.
In the 1980s, with the discovery of harmful substances such as dioxins and furan in chlorine bleaching wastewater, the pollution of the paper industry and other issues were found, and people have developed a study of bleaching processing, involving oxygen bleaching, ozone bleaching, hydrogen peroxide bleaching, even the second Sodium bromide bleaching crane biological bleaching and other chlorine-free bleaching processing. Hydrogen peroxide is a process that widely used chlorine-free bleaching.
Principles for pulp bleaching processing
Basic principle of pulp bleaching
- It achieves pulp bleaching by using chemicals to remove lignin of pulp or change chromophoric group structure of the lignin.
- Prevent or eliminate the combination between chromophoric group and auxochrome group.
- Prevent the conjugation of chromophore groups.
- Prevent the production of new chromophore groups.
Due to the different Property of the bleaching agent, working principle for pulp is not same. Bleaching methods can generally be divided into two types, one is the delignification bleaching, and bleaching agents are the oxides, such as chlorine, hypochlorite, chlorine dioxide, peroxide, oxygen, ozone. It is used in chemical pulp bleaching. The other is to retain lignin bleaching. It usually uses hydrogen peroxide and hydrosulfite, sulfurous acid, and borohydride, which are commonly used for mechanical pulp and chemithermomechanical pulp bleaching.
Oxidizing bleaching can not only remove colored substances, also remove the residual lignin and other debris. It improves the pulp whiteness and purity and makes the whiteness lasting.
Reductive bleaching is applied to the mechanical pulp, semi-chemical pulp, and other high-yield pulp. It can only make coloring material bleaching, and will not cause the loss of fiber components. Reductive bleaching can maintain the original characteristics of the pulp, but the whiteness is not sustained, easy to be yellowing, so it is not suitable for making paper with long-term preservation.