Explain the application of defoamers in the paper industry
In the papermaking industry, defoamers are mainly used in the following three aspects: 1. Use of defoamers in the pulp manufacturing process. In China, general paper mills no longer use defoamers. However, in the production of sulfite bleached wood and reed pulp, high temperature and strong alkali resistant defoamers are required. Defoamers can be used in this process. Adding the defoamer in a certain proportion evenly to the head trough of the pulp washer or to the outlet of the pulp screen can effectively control the foam, facilitate pulp transportation, prevent pulp loss, and make the pulp easy to wash and filter, shorten the pulp washing time, improve pulp production and quality, eliminate the phenomenon of sticking the net, and extend the service life of the net skin. 2. Use of defoamers in papermaking process Internationally, coated paper is often divided into two categories: coated paper containing mechanical pulp and coated paper without mechanical pulp. Coated paper without mechanical pulp mainly includes coated paper, which is divided into No. 1, 2, and 3 coated paper, with a basis weight of 80~170gsm. Such paper has excellent quality and good durability because it does not contain mechanical pulp. Coated paper containing mechanical pulp includes medium weight coated paper (abbreviated as MWC paper), i.e. No. 4 coated paper, with a basis weight of 80~115gsm; light weight coated paper (abbreviated as LWC paper), i.e. No. 5 coated paper, with a basis weight of 51~70gsm; and ultra light weight coated paper (abbreviated as ULWC paper), i.e. No. 6 coated paper, with a basis weight of 35~48gsm. In addition to the difference in basis weight, the amount of coating is also an important feature that distinguishes coated paper from light weight coated paper. The coating amount of coated paper is generally 15~20gsm per side, which is usually completed after multiple coating operations. The pulp ratio of the base paper is less straw pulp (referring to domestic coated paper), and does not contain mechanical pulp; while the coating amount of LWC paper is only 6~12gsm per side, which is usually completed after one coating on each side. The base paper contains more straw pulp. Due to the shortage of wood used for papermaking in China, LWC generally does not contain mechanical pulp, which is different from the base paper of foreign LWC paper. Defoaming in the papermaking process is mainly Defoaming of coated paper. The difference in coating process is related to the amount of defoamer used. Defoamers can be used in this process. Adding defoamers directly to the entrance of the mesh box in a certain proportion can eliminate foam during papermaking, help disperse paper fibers, and prevent “fish eyes” and “wet spots” on paper. It shortens the drying time, saves energy, improves paper quality and sizing, and has a certain anti-adhesive effect. 3. Use of defoamers in papermaking wastewater treatment The amount of wastewater treated by papermaking wastewater is large, and it is necessary to use a defoamer with low cost, high efficiency, and no new pollution.
