The Paper and Pulp Industry

1 Raw materials for paper manufacture  Soft wood: Hard wood: Non wood fibres for pulping:  Textile fibres for pulping: Pulping and paper manufacturing processes Wood consists mainly of cellulose (45%), hemicellulose (30%) and lignin (25%). Lignin is the resinous material that binds the cellulose fibres together. In addition, wood has less than 5% of substances like terpenes, resins and fatty oils and acids. Wood for papermaking has to be debarked using a large, rotating drum that agitates the logs and allows the bark to be striped by the abrasive action of the neighbouring logs. The logs are then chipped using a large rotating blade prior to pulping. 2 The pulping process It involves separation of the cellulose and hemicellulose from the lignin and removes other tree oils and resins. The remaining fibres are used to produce the paper. There are two main pulping processes, chemical pulping and mechanical pulping. In mechanical pulping the debarked logs are forced against a grinding stone or rotating metal disks. As much as 95% of the wood resource is turned into pulp through this action. The high lignin content in the pulp causes it to darken when exposed to sunlight. In mechanical pulping the chips can also be steam treated to soften them prior to grinding, a process known as thermo-mechanical pulping. It is the oldest process and mostly used for newsprint paper.  disadvantage is there is lignin in the final pulp obtained. 4 Chemical pulping • This method separates the fibers from the raw material by making soluble all the non-cellulosic components in a cooking liquor at high temperature and pressure. This pulping gives better quality and greater pollution. Chemi-mechanical pulping Involve a chemical pre-treatment of the raw material before a mechanical treatment to liberate the fibers. Then the wood chips are soaked with sulphur-based chemicals prior to steaming to expedite the extraction of lignin and resin from the wood. Mechanical pulp is used for lower grade papers, such as newsprint and telephone directories, as well as for coated papers. Bleaching is used to purify and clean the pulp by removing lignin, which affects the purity of the fibre and prevent yellowing. Traditionally, Cl2 is used but now replaced by ClO2 or elemental chlorine free process. Other totally chlorine free process uses, O2/O3/H2O2 7 Paper Making • Additives (such as clay, CaCO3, TiO2) are added into pulp to improve optical and physical properties (denser, brighter, softer and smother) Cont’d… • The pulp is diluted to 1% solids aqueous suspension and sprayed onto a moving screen. • then, it dewatered by gravity and suction to 20% solids. After that, paper then pressed and dried over a steam-filled cylinders to a few percent water. • The final step in making paper is surface coatings to improve gloss, printing resolution and strength. The coating typically consists of an aqueous suspension of pigment (e.g. clay or TiO2). • Finally, paper is calendered or compressed between hot rollers to align clay platllets, improving gloss