What is the paper pulping? The paper pulping refers to the process of dispersing raw materials into single fiber, the obtained products are called paper pulp. Generally, paper pulping is used to dissolve the lignin of fiber materials. Under the precondition of no fiber loss or less fiber loss, paper pulping requires less power consumption, certain specific surface area and felting quality, provides condition for fibers reintegration.
Like we all know, there are various pulping processes, such as chemical pulping, mechanical pulping, Bio-pulping, etc. Chemical pulping is the most widely applied one among all paper pulping processes. So we take chemical pulping in the first place.
Chemical pulping process
Before taking about the pulping process, we must learn some features of chemical pulping. Chemical pulping uses chemical liquid to cook wood chips (or other fibers) and defibers the materials into pulp. According to different chemical agents, chemical pulping can be classified into acid pulping and alkaline pulping, the comparison of acid pulping and alkaline pulping as below:
| Types | CP | chemical agent | Raw material | uses |
| Alkaline pulping | lime process | Ca(OH)2 | Wheat straw, cotton, jute | Paper board |
| Soda pulping | NaOH | Reed, hardwood | Cultural paper | |
| kraft process | NaOH+Na2S | Softwood, bamboo, reed | high-grade culture paper and package paper | |
| Acid pulping | sulfite process | MSO3 | All fiber materials | Cultural paper |
The paper pulping process is commonly divided into these steps, including materials preparation, pulp cooking, pulp washing, pulp screening, and pulp bleaching.
Material preparation
Before pulp cooking process, the long wood log or straw should be cut into small sizes and remove impurities in the fiber materials. Material preparations include three parts like stocking, treatment, transportation and storage.
As an important step of paper pulping, materials preparation has some requirements for the raw material plant as following:
- Fire Protection and Safety: prevent autoignition, lightning strike, accident and etc.
- Drainage: keep the drainage trench
- Ventilation: ventilatting duct, spacing between stacks, length of stack.
- Transportation: equipment and material type.
Pulp cooking
Pulp cooking is mainly a process of delignification. Cooking temperature is about 155-180℃, the cooking process continues 1.5-2 h. as for the pulp cooking equipment, there are common digester, continuous digester, and displacement digester, blow tank, and etc.
Pulp washing
Pulp washing process separates waste liquor from the pulp by pressing and filtration. Black liquor usually means the waste liquor of alkaline pulping, while in the acid pulping, waste liquor is called red liquor. Multiple stage washing is one of the popular washing methodss. Vacuum drum washer, pressure pulp washer are widely used in the pulp washing process.
Pulp screening
Pulp screening is a process of removing impurities from paper pulp. Pulp screening equipment includes vibrating screen, centrifugal screen, hydrocyclone, pressure screen, etc.
Pulp bleaching
Pulp bleaching removes residual lignin and change chromophoric group and improve the whiteness of pulp. The pulp bleaching methods include oxidative bleaching and reduction bleaching.
- Oxidative bleaching: use oxidizing agent to remove residual lignin and colorful things. High whiteness and good stability, sustain permanently, so paper or paper board cannot fade for long storage. Common bleaching agents include CIO2, NaCIO, H2O2, and Na2S2O3.
- Reduction bleaching: use chemical agent to destroy the chromophoric group of pulp and make pulp white. In the reduction bleaching,the bleaching agent is only used to fade, not delignification. Bad stability and easy to be yellow. Common bleaching agents include Zn2S2O4, sodium borohydride, and etc, reducing bleaching agent is expensive.
Mechanical pulping
Mechanical pulping adopts physical method to separate plant materials into fibers, there is no use of chemicals in the pulping process. Mechanical pulping includes stone ground wood pulp and chip mechanical pulp. Mechanical pulp has high pulp yield and less pollution. Due to retaining impurities and lignin and cutting the fibers, mechanical pulp is low-strength, and easy to be yellow.
Chemi-mechanical pulping
As the name applied, Chemi-mechanical pulping has two stage of pulping process including chemical pretreatment and mechanical treatment. Firstly, use chemical pulping to treat raw materials, dissolve the non-fibers of materials and soft the fibers, and then, ground them into pulp. APMP is a typical chemi-mechanical pulping, combine the pulping cooking and pulp bleaching together.
Bio-pulping
Do pretreatment of wood chips (or other fiber materials) with microorganism like bio-enzyme, and make pulp by mechanical pulping or chemi-mechanical pulping. Bio-pulping selectively dissolve the lignin of raw materials try to reducer the loss of carbohydrate. Reduce energy consumption and waste water pollution, improve the strength of pulp.
