14 Apr How paper is made
Paper is the flake fiber product used for writing, printing, painting or packaging, which is a powerful tool or material in the fields of idea exchange, culture diffusion, science & technology development and production. Generally by the pulping processing, the vegetable fiber in aqueous suspension is interwoven and evenly distributed on the net. And then through the processes of initial dehydration, compression and drying, papermaking is finished. Modern papermaking process can be divided into major steps of pulping, washing, bleaching, further process, refining and papermaking process and others.
The following is the detailed whole process of papermaking:
Preparation for paper pulping:
- Storage effect of raw wood: to maintain normal continuous production of the plant; to improve the quality of raw material. Storage way of raw wood: water storage and land storage
- Wood sawing: raw wood shall be needed be sawed into wood strips with different certain lengths that are 2m-2.5m for the chipper and 0.6m-1.2m for the grinder.
- Wood debarking: Because of low content of fiber and high content of impurity of wood barks, they will reduce cooking efficiency, increase cooking chemical consumption. The pulp dust will also increase, but its strength will decrease. Therefore, in the production of making high quality pulp for relatively higher requirements, the wood should be generally debarked.
- Wood knot removing: in order to avoid the influence of wood knots to grinding stone and the quality of wood pulp, wood strips for the grinder should be needed to remove the knots.
- Wood chipping: in order to fit the cooking of chemical pulp and meet the production needs of wood chips grinding into wood pulp, raw wood should be chipped using the chipper. The wood chips should be shaped uniform with the same length and thickness. Generally, their specification is 15mm-20mm long and 3mm-5mm thick. Their width is not required, but not more than 20mm. After chipping they should pass round screen to select the required one. Their qualified rate should reach more than 85%.
Pulping refers to the production process to make the vegetable fiber (raw material) disassociate into raw pulp (unbleached pulp) or further bleach into bleached pulp using chemical method or mechanical method or both two.
Chemical pulping is to use chemicals, under certain conditions, to make the most lignin from vegetable fiber of raw material dissolve out and disperse into the pulp. The cooking process is mainly a process of delignification. In the meanwhile, this will inevitably cause part of cellulose and hemi-cellulose to degrade to some extent. Therefore, the yield of chemical pulp is generally lower, which is about 50%.
It is the process to use mechanical method to grind raw fiber material into pulp.
Mainly divided into two categories:
- (Wood chips) Disc grinding: after wood chips are sent into the grinder, first they will be destroyed to match shapes
- (Wood strips) SGW grinding: a certain length of raw wood into the grinder will become a single fiber or a fiber bundle under a certain pressure and temperature through compression, shearing, crushing and friction of grinding stone.
Chemical pretreatment (impregnation) grinding
Main task of chemical pretreatment is to achieve the softening of the fiber so as to make the fibrous tissue loosened and more suitable for grinding.
Paper pulp washing
After cooking raw fiber material, nearly 15-50% of the substances have dissolved in the cooking liquor. The cooking liquor discharged in final is called waste liquid. The purpose of washing is referred to extract high concentration and temperature of cooking liquor as well as fully wash the pulp. Through the alkali recovery process organic and inorganic substances can be recycled from waste liquid. After the washing, chemical pulp still contains uncooked substances, for example, crude fiber bundle. The grinding pulp also contains large wood strips, big wood chips and crude fiber bundle and so on. In addition, in the processes of collection of raw material, transportation, storage and production, sand, fly ash, iron wire, plastic and other debris may enter them. These substances can not only affect the quality of paper and but also damage machine equipments, resulting that production cannot be carried out smoothly. Therefore, the impurities must be removed through screening and purification.
Paper pulp bleaching
The main purpose of bleaching process is to remove colored materials within the fiber to obtain a certain degree of brightness and appropriate physical and chemical properties of the pulp.
Two main types of pulp bleaching are as follows:
- Lignin dissolving bleaching: to use proper bleaching agent to make the lignin dissolved out by the oxidation in order to achieve the purpose of bleaching, for example, chlorine bleaching, hypo hypo-chlorine bleaching, chlorine dioxide bleaching, chlorine peroxide bleaching, etc. For the relatively higher requirements on pulp brightness, this method is often applied. This method is commonly used in chemical pulp bleaching.
- Lignin containing bleaching: to keep the lignin and only make chromophore decolorized. This method has very small bleaching loss and can retain the pulp characteristics, for example, hyposulphite bleaching or sodium borohydride bleaching. This method is often used to the bleaching of high yield pulp.
Further process of paper material is the following crucial link, which is directly related to the strength, color, printing quality and lifespan of the finished paper. The common further process can be roughly divided into three steps:
- adhesive adding and filling
Refining and papermaking process
The main work of the refining and papermaking department is to make the dilute paper material evenly interleaved and dehydrated, and then through the procedures of drying, pressing, rolling, cutting, sorting and packaging become the finished paper products. The general process is as follows:
- Screening of paper material
To dilute the paper material after the further processing into a lower concentration, and then through screening equipment, screen and remove the debris and undissociated fiber bundle in order to maintain the quality and protect the equipments.
To make the paper material to flow out of first box and be evenly distributed and interweaved on copper wire net or plastic net on the circulation.
To make wet paper after the net moving away enter two rolling rooms with the felts, and then through pressing and the suction effect of the felts, wet paper will be further dehydrated to get more compact to improve paper strength.
The moisture content of wet paper after the pressing is still as high as 52-70%. In this case, the paper has been unable to use mechanical force to remove moisture. It shall be further dried through the method of passing the surface of many cylinders filling internal heating steam.
- Rolling, cutting, sorting and packaging
Paper is the most commonly used item in our daily life for reading, writing and painting. In the fields of industry, agriculture and national defense industry, we also cannot be without paper. Today we can’t imagine without it.