Paper pulping is the process of separating pulp from plant fiber materials. After the plant fiber material is chemically cooked by digester, 50 to 85% of the pulp is obtained, and about 15 to 50% of the substance is dissolved in the cooking liquid, and the cooking liquid is discharged by the cooking, which is collectively referred to as waste liquid.
Principle of cooking liquid permeability: According to the driving force of liquid permeability, there are two types of permeation including pressure permeation and diffusion permeation, driving force of pressure permeation is pressure difference, and mass transfer driving force of diffusion permeation is the difference in concentration of chemical liquid. The level of impressed pressure and the viscosity of chemical liquid will affect the pressure permeation. The capillary action in the axial direction of the fiber is always 50 to 100 times greater than that in the transverse direction. The effect of diffusion permeation mainly depends on the effective cross-sectional area of the capillary and the concentration difference of the chemical solution, and is also related to the activity and size of the drug molecule or ion, and is affected by the temperature and the moisture of the raw material.
Influence factors: composition temperature of the chemical solution, type of pressure fiber raw material, and grid of the tablet.
The pulp screening is also an indispensable part of the pulping process. The cooked pulp contains 1 to 5% undigested material. For wood pulp, it mainly refers to bark, knots, strips, wood chips, etc. For straw pulp, it mainly include grass knots, vines, etc. In addition, there are non-fibrous impurities such as sediment, gravel and iron filings.
Depending on the requirement of the pulp quality, these fibrous and non-fibrous impurities must be separated from the acceptable pulp fibers by a pulp screening and purification process.
The so-called pulp screening refers to a process that uses some paper pulp machine to remove fibrous impurities of pulp. It can be divided into fine screening or coarse screening. Purification is based on the removal of non-fibrous impurities.
The pulp washing process is used to separate the waste liquid from the pulp and ensure the cleanness of the pulp, create a good operating condition for the next process of screening and bleaching. It is necessary to protect the washing waste liquid as much as possible to facilitate the recovery or comprehensive utilization of the chemicals. Therefore, it is required that use less water while ensuring the quality of pulp washing.
The unwashed pulp has a waste liquid portion which is freely present in the flow space between the fiber and the fiber, about 80-85%, and about 15-20% of the waste liquid is present in the fiber cell cavity and the cell wall. common pulping washing equipement include twin roll press, disc filter, single screw press, and etc.
During the pulp washing process, part of the free waste liquid is relatively easy to separate. Actually the free waste liquid can be extruded by pressing force, which can be the static pressure of washing or the applied mechanical pressure. For the waste liquid in the cell cavity and the cell wall, it is not feasible to adopt simply squeezing, the diffusion method is the only way to solve this problem. To be specific, the concentration difference between the washing water and the waste liquid is used to diffuse the dissolved matter contained in the waste liquid.
Therefore, the pulp washing process must use a variety of principles of action. It mainly includes diffusion, filtration, extrusion and so on. A certain type of washing equipment is often dominated by a process of action, supplemented by other functions.
Although there are various insoluble impurities in the pulp, the two principles of action are generally used to distribute these impurities:
- The first one, separate the impurities by screening according to different size shape and types. This process is called the pulp screening;
- The second one is known as purification, according to the proportion of impurities in the paper pulp, use gravity drop or centrifugal separation to separate impurities.
In the actual pulp making process, it is common to combine the process of screening and purifying the pulp.