Few inventions in the history of civilizations have been as pervasive and impactful as that of paper, some 2,000 years ago. But the sheets used in documents just a few centuries ago seem like a different material compared with the paper products of our times. The refined stock that makes anything from egg trays and tissue paper to diaper linings needed other inventions to come about. These innovations are their own class of process equipment: paper pulp manufacturing technology.
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Grade the wood chips into even sizes
Paper pulp is derived from wood cores obtained from harvested forest products. Timber-based pulp can be produced from all kinds of heartwood and sapwood, but coniferous trees like pines, firs, and spruce contain longer fibers.
Start by debarking the wood and chipping the core material. Use specialized shredding technology for paper pulp to grade your final wood chips to a consistent size. Uniform chips facilitate a homogeneous separation process that translates into higher-quality paper.
Separate cellulose fibers from lignin to obtain the pulp
Half of the mass of chipped wood is water. The other half is divided mainly between cellulose fiber and lignin. All of which get processed in a blender.
Lignin serves a vital function in holding the cellulose fibers together in the tree. But for papermaking purposes, the fibers need to be loose. Paper pulp equipment is precisely designed to separate cellulose fibers from lignin.
Digesters are critical manufacturing technology for chemical paper pulping
A method of removing lignin involves cooking the wood chip slurry in a digester. The paper pulp process subjects the raw pulp to heat pressure, typically using steam. Add an alkaline solution to dissolve the lignin while the mixture is heating up.
Known as ‘white liquor’, this solution is formulated from chemicals like sodium hydroxide and sodium sulfide that react with the lignin and break it down. Conventional digester systems operate batch processing, but continuous solutions have become more widely available.
Recover cooking chemicals with the Kraft paper pulp method
A variant of this chemical process is called the Kraft technique and is suitable for high-strength paper and board products, often used for packaging applications.
Kraft pulping recovers the cooking chemicals and the dissolved lignin as black liquor. Evaporate the water to turn the mixture into a green liquor, leaving behind the base chemicals. Treated with lime, the ingredients may be reused as white liquor in a new cycle of chemical pulping.
Add limited chemicals to prepare wood for mechanical abrasion
Besides chemical pulping, wood chips may also be defibrated mechanically. The raw pulp is pressed against rotating disks at atmospheric temperature, separating the cellulose fibers by abrasion.
Although primarily mechanical, this technique may incorporate thermal or chemical processes to soften the wood. Thermomechanical pulping injects pressurized steam to force out the lignin, while the chemi-mechanical process introduces additives to enhance the hydrophilic properties of the fibers.
Bleach away the lignin residues after the pulping process
Neither chemical nor mechanical separation processes remove all lignin content from the raw pulp. To obtain free fibers, the pulped material is bleached with chemicals to transform lignin into an alkali-soluble form.
The use of chlorine in the paper industry, for this purpose, was traditionally widespread. But modern paper mills employ total chlorine-free (TCF) bleaching methods using oxygen and alternative chemicals.
Correct the color of the material if it’s manufactured for print quality
Mechanical paper pulping forms strong bonds between the fibers, producing crisper paper products. However, the natural color range is grey to brown. Bleaching the processed pulp adds whiteness and brilliance to produce printing-grade paper and boards.
If the final product is intended for applications that do not require high brightness, such as cartridge paper for newsprint, you may forgo the bleaching process.
Bring water content down to 10% with industrial drying solutions
At every step of the way, the paper pulp is washed with water to remove any impurities. In the end, the cellulose material forms a wet web with excess water content.
There are several techniques for drying the moisture down to 90% fiber and 10% water. Standard drying techniques include drum dryers, fluidized-bed driers, and flash driers. Each method is suitable for different production parameters such as capacity and product type.
Apply superheated steam drying systems for fully sterile results
Conventional drying equipment does a job for generalist products like cardboard or paper bags. More delicate paper materials, however, such as those used for sanitary purposes or food products, demand stricter hygiene processes.
Superheated steam dryers keep the slurry away from direct heat. The technology treats the slurry with superheated steam that instantly vaporizes water molecules trapped in the pores of the wet web. This solution generates no smells or tastes, producing a dry, sterilized pulp ready for the most demanding applications.
Paper pulp processing from recycled materials
Besides fresh sawdust or wood chips, paper pulp may be sourced from existing paper products. The process converts used paper and board back to cellulose fibers.
Separate the collected material into a clean stock and mix it with water into a slurry. Specialized paper pulp processing technology (a repulper) separates the fibers into individual strands. Rewash the final product to remove residues, and remove the water content with an industrial dryer.