Pulp and papermaking process in the production of paper and board (2)

(2) Chemical wood pulp.

Chemical pulping is the use of chemical solutions (also known as cooking liquors) to dissolve lignin and other impurities in wood chips at a given temperature and pressure, to retain cellulose as much as possible, and to protect cellulose from damage as much as possible. Keep hemicellulose. According to the different chemicals used can be divided into three categories: soda, sulfate and sulfite. Another; play is also known as caustic soda law. The chemical used for caustic soda cooking is mainly sodium hydroxide, but also a small amount of sodium carbonate. This method is mainly applicable to non-wood fiber raw materials such as rice straw and wheat straw, and it can also be used for cooking hardwoods. It is generally not suitable for needle n wood. It is high in the content of softwood lignin, high alkali consumption by this method, difficulty in bleaching, and low yield.

The chemicals used in kraft pulping are mainly sodium hydroxide and sodium sulfide, and also contain small amounts of sodium carbonate, sodium sulfate and sodium sulfite, etc. Since the sodium sulfate is used as a supplementary medicine during the alkali recovery process, it is called the sulfate method. Kraft pulping has the advantages of wide adaptability of raw materials (applicable to softwood and broad-leaved wood and grass materials), high pulp strength, and mature soda recovery technology for waste liquor, so this method has become the most Important chemical pulping methods.

(3) Bovine chemical pulp and chemical mechanical pulp.

Both semi-chemical pulp and chemical-mechanical pulp are pulps that are produced both chemically and mechanically during pulping. It is a two-stage pulping process that includes chemical pretreatment and mechanical post-treatment. The difference between the two is that the degree of chemical pretreatment is different, so the yield of pulp is different, the yield of semi-chemical wood pulp can reach 65% to 85%, and the yield of chemical mechanical pulp is generally 85% to 90%. In terms of pulp yield, far more than ordinary chemical pulp (a yield of 40 to 50%). The purpose of developing bovine chemical pulp and chemical mechanical pulp is to exploit the source of pulping raw materials and make full use of hardwood which is not suitable or less used for other pulping methods. Second, in order to reduce the production cost, the production yield is equivalent. High pulp, in particular hardwood, can be used to produce pulp that is similar in quality to softwood or mechanical pulp. Semi-chemical pulp and chemical-mechanical pulp can be used to make newsprint, packaging paper, and various kinds of packaging paperboard. Bleached semi-chemical pulp can also be used to make writing paper, magazine paper, etc.

3. Washing and screening

(1) Pulp washing. After chemical fiber cooking, the plant fiber raw material obtains 40% to 85% (relatively dry wood weight) of pulp, nearly 15% to 60% of the material is dissolved in the cooking waste liquid, and the waste liquid mainly contains lignin, sugar, etc. Leachables and residual chemicals must be separated from the pulp as much as possible after cooking to ensure that the pulp is clean and waste liquids are extracted to facilitate the comprehensive utilization of the waste liquid, chemical and heat energy recovery, or other appropriate treatment. To solve the main pollution problem of the pulp mill waste cooking liquid to the environment.

Pulp washing and waste liquid extraction are carried out at the same time. The basic task of pulp washing is to use a minimum dilution factor (which is the difference between the water consumption per ton of air dried pulp and the water content of the washed pulp) and to keep the higher concentration of waste liquid. Get the highest extraction rate.

There are three main pulp washing methods: displacement washing, dilution dehydration washing and pressing washing. All three washing processes utilize the functions of diffusion, filtration, extrusion, etc. The only difference is that one type of washing is often based on a certain role, and the other functions are supplemented.

(2) Pulp screening and purification. Whether chemical, mechanical, or a combination of chemical and mechanical pulping methods, can not be completely uniform fiber dispersion, the slurry must be mixed with a variety of different degrees of coarse fiber bundles, debris, knots, small Non-fibrous cell and other non-fibrous impurities such as non-fibrous cells, and non-fibrous impurities such as resin, silt, and iron scrap. These impurities must be removed through screening and purification processes according to the requirements of the pulp quality. Otherwise, not only the quality of the pulp but also the equipment will be damaged. Increase the consumption of bleaching chemicals and create production barriers. The impurities in the pulp are separated from the fibers by their size and shape, and they are separated by sieving. This process is called screening. The second is that the proportion of impurities is larger than that of fibers, and separation is performed by means of gravity sedimentation or centrifugal separation. This process is called purification.

4. Pulp bleaching

Pulps obtained by chemical cooking or mechanical abrading methods all have a certain color, dark dark brown and pale grayish white. The production of these colors is not only related to the raw material of the fiber itself, but also to the pulping method and pulping conditions. related.

In order to increase the use of pulp and increase the whiteness of the pulp, bleaching must be carried out to remove the colored substances in the pulp and impart certain properties to the pulp. The color of the pulp is mainly derived from the lignin in the pulp, while cellulose and hemicellulose are generally not easily converted into colored substances. According to the properties of lignin in pulp, there are two major types of bleaching methods: one is the oxidation of lignin to destroy the structure of lignin and colored substances, so that it dissolves, improves the purity and whiteness of the pulp; the other is The reductive bleaching agent changes the chromophoric groups on the molecules of the chromophoric substances to a colorless group and decolorizes them. The former is suitable for removing large amounts of lignin chemical pulp, while the latter, because it does not involve the loss of fiber components and maintain the characteristics of the slurry, is particularly suitable for the bleaching of high-yield pulps such as groundwood pulp and semi-chemical pulp.

Commonly used oxidizing bleaches are mostly chlorine-containing compounds such as chlorine, hypochlorite and the like. Commonly used reducing bleaches are mainly peroxide bleaches. (to be continued)