Wood pulping and papermaking means the process that dissociates wood fiber into pulp and makes paper. In a word, papermaking process includes two main steps: pulp making and paper making.
Modern pulping and papermaking started in the 19th century, and it was brought to China. The raw material for paper includes bast fiber, straw, and wood. The main pulp making and paper making countries like USA, Canada, Japan, China, Sweden, Finland, German, etc. Currently, the capacity of paper and paperboard is over 100 million tons. The paper process developed from fermentation to chemical pulping, mechanical pulping, chemi-mechanical pulping, etc.
Raw materials for paper pulp making
- Wood fiber takes 90% of papermaking raw materials. In China, wood fiber takes about 20% of the total. Common wood fiber materials include Picea jezoensis, abies nephrolepis, Korean pine, masson pine, poplar, birch, and linden, etc.
- Except for the wood, the plant with rich cellulose and suitable shape will be the raw materials of papermaking. Non-wood fiber materials include bamboo, straw, phloem fiber, cotton fiber and etc.
Wood pulp processing
Wood pulp making refers to the process that separates the pulp from wood fiber and manufactures paper with the pulp. Based on the pulp processing, wood pulp can be divided into chemical pulp (CP), mechanical pulp, semi-chemical pulp (SCP) and chemi-mechanical pulp (CGP).
Chemical Pulp
Chemical pulp mainly includes sulfite pulp and Kraft pulp. Chemical pulp is generally made by a set of pulp plant, which consists of pulp digester, blow tank, disc filter, twin roll press, vacuum drum washer and other paper pulp machines.
- Sulfite pulp is cooked under 130-150 ℃, light, easy to bleach. Apply for many kinds of paper, especially fine paper, and artificial paper. Sulfite pulp has high requirement of raw materials, and it had been replaced by Kraft pulp.
- Kraft pulp is cooked by caustic soda and sodium sulfide under 160-170 ℃. Kraft pulp is dark, hard to bleach. It is mostly applied to the sack paper, Kraft paper, paperboard, condenser paper and industrial papermaking. After bleaching, Kraft pulp also can be used for various types of cultural papermaking.
Mechanical pulp
Mechanical pulp includes ground pulp, refiner groundwood and thermomechanical pulp.
- Ground pulp has white type and brown type. White type ground pulp ground the wood log with grinder. The ground pulp is light, high-opacity, low-cost, but it has short fiber, bad strength, and short storage life. In general, ground pulp is used to make newspaper, paperboard and printing paper. Brown type ground pulp is made from steamed wood, dark, and it has long fiber and good strength. Brown type ground pulp is applied to manufacturing paperboard and package paper.
- Refiner ground pulp should wash the wood chip first, and dissolve the fiber with grinder, then accurately grind the materials into pulp. Refiner ground pulp applies for various types of softwood. Compared to the ground pulp, it has good strength, drainability, opaqueness, and printability, but high energy consumption.
- After pretreated by steam, use grinder to separate wood into pulp. Thermomechanical pulp contains much long fiber, less fiber bundle.
Semi-chemical pulp and chemi-mechanical pulp
The yield of Semi-chemical pulp and chemi-mechanical pulp is between the yield of chemical pulp and mechanical pulp. Wood chip pass through the chemical treatment and grinder finally got the pulp. The raw material of Semi-chemical pulp and chemi-mechanical pulp is generally hardwood chips.
The main difference between Semi-chemical pulp and chemi-mechanical pulp is degree of chemical treatment. The latter has mitigatory condition of chemical treatment, strong mechanical action. Its properties close to mechanical pulp, and usually apply for making corrugated board, packaging paper, newsprint, etc. The bleached chemi-mechanical pulp also applies for book paper and printing paper.
Papermaking
Papermaking is the process that uses water to dilute pulp and makes the well-formed sheet on the paper machine. Dry and wet methods are two processes of papermaking and paperboard making. The wet method takes fiber suspension as the medium, and the dry method takes air as the medium. Dry method is only used for manufacturing special type of paper like cigarette mouthpiece paper. But, the wet method has a relatively wide application. The process of wet paper making includes beating, filling, sizing, cleaning, molding, dehydration, pressing, drying, press polish and batching.
Beating and filling
Beating process can make fiber swelling, separating and fibrillating. It also increases the superficial area of the fiber, which increases the combination of fibers and strength of paper. Beating ensures the quality of paper or paperboard making. Sometimes, for enhancing the strength of paper, some kind of macromolecular organic assistant should be added, such as the high-molecular polymer, modified starch or carboxymethyl cellulose, etc. Besides, add filler into paper stock can enhance the opaqueness and smoothness of paper.
Sizing and cleaning
Sizing process increases water-resistance of paper, rosin and rosin derivative is common sizing agent.
The main function of cleaning is removing the impurities of paper like metal filing and coal cinder, and etc. The cleaning equipment separates the impurity according to density contrast.
Molding, dehydration, pressing and drying
Treated pulp pass through the pipe, flow box and spray on the molded net, there is a vacuum chamber on the back of the molded net, the chamber is used to absorb the water in the net. Finally, the pulp becomes wet paper with a certain strength, and through press roll and dryer, finally, rolls the paper. The roller is made into finished roll by rewinder according to the requirement of clients.
