What are the methods of pulping and pulp bleaching?
1. What is pulping?
Pulping is the process of separating fibers from plant fiber raw materials. The pulping methods can be mainly divided into mechanical method, chemical method and chemical-mechanical method, which respectively produce mechanical pulp, chemical pulp and chemical-mechanical pulp.
2. What is chemical pulping?
Chemical pulping is to use the aqueous solution of chemicals to treat plant fiber raw materials at a certain temperature and pressure, dissolve the lignin, non fiber carbohydrates, oil, resin, etc. in the raw materials, and retain cellulose and hemicellulose as much as possible to separate the raw materials into pulp. Among the chemical methods, the most representative pulping methods are sulfate method and sulfite method.
3. What is mechanical pulping?
Mechanical pulping is the use of mechanical methods to process fiber raw materials to dissociate fibers. Using wood as raw material is called mechanical wood pulp. The main application in the paper industry is mechanical wood pulp, also known as groundwood pulp. Mechanical pulp retains a large amount of lignin in the raw material, and the pulp yield is higher than that of chemical pulp.
4. What is the purpose of bleaching pulp?
The pulp made by chemical and mechanical methods is called natural color pulp, which has a certain color (generally yellow or brown). In order to meet the requirements of paper use, the pulp must be bleached to make the paper have a high whiteness. Bleaching is to use appropriate bleaching agent to dissolve the residual lignin in the pulp through oxidation, reduction or decomposition reactions, or to fade the colored matter while retaining the lignin.
5. There are two methods of bleaching pulp:
（1）Oxidative bleaching method
Oxidative bleaching method is to use oxidative bleaching agent to destroy the structure of lignin and dissolve it, so as to achieve the purpose of improving the purity and whiteness of pulp. Such bleaches are Cl2, Clo2 and hypochlorite. At present, oxidative bleaching is still the main bleaching method. Oxidative bleaching agents will be gradually replaced by oxygen-containing bleaching methods because of their serious pollution due to chlorine-containing bleaching.
（2）Reduction bleaching method
Reduction bleaching method is to use reductive bleaching agent to change the structure of chromophore group and make it decolorized. Since it will not cause the loss of fiber components and maintain the characteristics of the original pulp, it is especially suitable for the bleaching of high yield pulp such as groundwood pulp and chemical mechanical pulp. Such bleaching agents include zinc hydrosulfite, sodium hydrosulfite, hydrogen peroxide and sodium peroxide. Pulp bleached by reducing bleaching agent has poor whiteness stability, and will return to its original color after long-term exposure to light in the air.
Of course, in the bleaching practice, the pulp cannot be bleached to a higher whiteness by one method, and often multiple bleaching methods are used together, that is, the so-called multi-stage bleaching, such as three-stage bleaching and five stage bleaching, so as to make the pulp bleached to a whiteness that meets the use requirements.
6. Why beating?
The pulp after cooking or mechanical grinding, washing, screening and bleaching cannot be directly used for papermaking. Because the fibers in the pulp lack the necessary flexibility, the connection between the fibers is not good. If it is used to make paper, the paper will be loose, porous, rough in surface, and low in strength, which cannot meet the requirements of use. Beating is to use mechanical methods to treat the fibers in the pulp to make them broom and properly cut. More importantly, the fibers absorb water and swell during beating, so that they have high elasticity and plasticity, meet the requirements of paper machine production, and make the produced paper meet the expected quality standards.