Pulp Bleaching Process
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.
We produce the equipment for bleaching section: EOP bleaching tower, drum vacuum washer, twin roll press, medium-consistency pumps and other related equipment. Commonly, we can custom machines according to your pulp bleaching processing or needs.
The specific process flow chart is as follows:
Anyang Machinery is specialized in paper pulp making over 49 years. we provide pulp bleaching processing and related equipment, see more.
Does paper have bleach in it
Chlorine-free paper is an environmentally preferable alternative to paper bleached with chlorine. Chlorine and its derivatives (such as chlorine dioxide)—the most common bleaching agents used by the pulp and paper industry—are quite harmful to the environment, particularly the aquatic environment.
Chlorine is used to give paper its white appearance and to remove “lignin,” an element of wood fiber that yellows paper when exposed to sunlight (as occurs with newsprint). Wood-based paper is brown in its natural state, as evidenced by brown paper bags and most cardboard boxes, which are made from unbleached paper.
Both wood-based and tree-free papers are available in the chlorine-free variety, although the percentage of wood-based papers that are chlorine-free is very small, while the percentage of tree-free papers that are chlorine-free is very high.
Chlorine-free paper is categorized as either “totally chlorine-free” (TCF) or “elemental chlorine-free” (ECF). TCF paper is produced with pulp that has been bleached without any type of chlorine, or that has not been bleached at all. ECF papers are produced from pulp that has been bleached with a chlorine derivative such as chlorine dioxide (ClO2), but without elemental chlorine (Cl).
TCF papers are much more environmentally preferable than ECF papers because chlorine derivatives—while less harmful to the environment than elemental chlorine—still produce toxic chlorinated organic compounds, including chloroform, a known carcinogen. These compounds are released into waterways as effluent from the bleaching process, where they produce environmental damage. Oxygen, ozone, and hydrogen peroxide are some bleaching alternatives to chlorine and chlorine derivatives.
Recycled papers are typically “processed-chlorine-free” (PCF), which means that no chlorine or chlorine derivatives were used in the recycling process, but that papers which were originally bleached with chlorine or chlorine derivatives may have been used as feedstock. (Papers that were originally bleached with chlorine or chlorine derivatives retain some of the chemicals in their fibers.) Some paper recycling processes, such as those that produce white printing and writing papers, still use chlorine or chlorine derivatives to bleach the paper whiter. Recycled paper that is produced with chlorine or chlorine derivatives, is not PCF.
Purchasing Chlorine-Free Paper
There are very few sources of information identifying chlorine-free papers available for purchase. One known source is Conservatree’s Paper Guide, which identifies some PCF and TCF printing and writing papers.
Other Web Sites
Chlorine-Free Products Association: The Chlorine Free Products Association is a unique trade association representing companies dedicated to implementing advanced technologies, and/or, groups supporting products free of chlorine chemistry
Conservatree’s “Chlorine Free Processing”: A comprehensive information source about chlorine used in the paper manufacture process.
Mill Watch’s “Reach for Unbleached!” A national foundation and Canadian registered charity with a focus on consumer education and pulp mill monitoring.
Does paper have bleach in it Chlorine-free paper is an environmentally preferable alternative to paper bleached with chlorine. Chlorine and its derivatives (such as chlorine dioxide)—the most