این بخش به منظور آشنایی دانشجویان صنایع چوب و کاغذ با متون تخصصی کاغذ می باشد .
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Introduction to chemimechanical pulping
Wood pulp fibers are manufactured by chemically dissolving those components (mainly lignin) from the wood that keep wood cells together to form the original wood structure. The aim is that whole cells are liberated with least possible damage. These tracheid cells are called fibers. The quality of fibers produced strongly depends of the morphology of the wood material used. In softwoods, long and slim fibers (tracheids) are dominant. Hardwoods have short and slim fibers and also vessel elements. Trees from boreal forests gow seasonally, - intensively in early summer, slower towards autumn, and not at all during winter. This results in heterogeneous wood structure within the tree. Early wood tracheids are wider in diameter and thin walled, while late wood tracheids are narrower but with relatively thicker cell walls. Clearly visible annual rings over the cross section of a stem are a result of seasonal growth. The tracheid dimensions vary also by location in the tree trunk. Tracheid length increases from the root up towards the middle of the trunk and then decreases again towards the tree top. Fiber length increases and cell wall thickness decreases radially from the centre outward. Chemical pulp fibers are therefore not homogeneous in dimensions. Fiber size distribution strongly depends on wood species and growth rate.
Wood is mechanically debarked and cut into chips, and the chips are screened before they are treated chemically to form fibrous pulp.
Lignin and some carbohydrate material are dissolved from wood chips during cooking in aqueous solutions of several alternative alkaline, neutral, or acidic components at elevated temperature and pressure. The dominant process is the strongly alkaline Kraft process, in which the active components are the hydroxide (OH-) ion and the hydrosulfide (HS-) ion. Neutral or acidic processes mainly use the HSO3- ion as the active component. These processes are called sulfite or semi-chemical processes.
Organic solvents such as alcohols or strong organic acids can also be used as delignifying agents alone or in combination with inorganic alkaline or acidic solvents. Organosolv processes are new and not yet applied commercially.
Fiber properties are set by wood morphology and also by the process used in their manufacture. Alkaline delignifying processes produce strong and flexible but brown fibers, while fibers made in acidic processes are weak, stiff, and brittle, but much lighter in colour.
Chips maintain their wood structure during chemical pulping despite the loss of most of their lignin content and about half of their total solid mass. But the structure has become so weak that they will break down to individual fibers by modest mechanical action. Delignifying chemicals cannot easily penetrate directly to the compact lignin-rich middle lamella which binds cells together in wood. Chemicals travel mainly from the hollow fiber lumens through the semi porous cell walls towards the middle lamella, dissolving lignin and carbohydrates along the way. The material in the middle lamella is dissolved last. Chips come apart into individual fibers by shear forces if delignification is taken far enough; this occurs upon discharging the digester either by blowing or pumping the diluted material out. Refining is used to complete disintegration in cases where modest delignification is sufficient for the desired fiber quality, such as linerboard base stock.
Washing, screening and bleaching
The spent inorganic and the dissolved organic materials are separated from the pulp in multistage counter-current washing systems operated to achieve the cleanest pulp and least dilution of the spent liquor. The spent liquor is processed in the mill's chemical regeneration system.
Discharged pulp will also contain solid impurities, mainly incompletely delignified wood, some knots, and other solid materials. These have to be separated out by screening, and the rejects are usually reprocessed to reduce raw material losses.
The discharged, washed pulp can as such be the raw material for unbleached paper and board products. Alternatively, it can be bleached for use in high-quality white paper and board products. Bleaching is a purification process in which coloured substances are modified so that they either loose their light absorption ability or they dissolve. Other aims of bleaching are to further reduce solid contaminants that have not been separated out in previous operations, and to improve the hygienic properties of the product.
Native cellulose is crystal clear, and so are most of the other polysaccharides. It is lignin that gives native wood its colour. Chemical pulp, especially pulp made by alkaline processes, is darker in colour than the original wood, despite a high degree of lignin separation. The light absorption properties of residual lignin increase during the cook. Cellulose and hemicellulose also gain some light absorption properties. Chromophoric groups in these substances are dissolved or modified in bleaching, a multistage process of alternating oxidizing and extractive treatments and interstage washing operations. Bleaching effluents contain the dissolved organic material ( 2-8% of fiber mass) and the spent chemicals from bleaching. These effluents do not go to the mill's chemical recovery system, rather they are treated and discharged. It is therefore important that effluent volumes are minimised and that effluents receive proper physical and biological treatment before being released to receiving water bodies.
Bleached pulp is either dryed and baled for shipment, or is pumped as a slurry to an integrated paper mill.
Recovery of chemicals
Spent kraft pulping liquor recovered as washing filtrate is too dilute to be used directly as a fuel. So the excess water is removed in a multistage evaporation station to dry solids concentrations well above those needed for burning without support fuel. The strong black liquor is then burned in a recovery boiler where the organic substances are decomposed to carbon dioxide and water and the inorganic substances are converted to a regenerable form. Sodium and sulfur salts are converted under reductive conditions to a smelt of sodium carbonate and sodium sulfide which flows from the boiler floor to a dissolving tank for further processing. Heat released from burning the organic material is converted to high-pressure steam for power generation in a turbo-generator.
Sulfite spent liquors (e.g from a magnesium base sulfite cooking) are oxidatively incinerated to form magnesium oxide dust and sulphur dioxide gas. The dust is separated from flue gases in wet scrubbing devices or electrostatic precipitators for further processing and is recombined with sulphur dioxide.
The smelt from a kraft recovery boiler is dissolved in weak wash liquor from the recausticizing plant. The resulting green liquor is clarified. Calcium oxide and water are reacted in a slaker to form calcium hydroxide, which in turn reacts with sodium carbonate in the green liquor to form sodium hydroxide and calcium carbonate. The calcium carbonate, which is insoluble, is separated by filtering and is washed free from sodium salts. The calcium carbonate is calcined in a lime kiln to calcium oxide and reused. The regenerated, clarified white liquor is used in cooking.
In the case of the magnesium base sulfite process, magnesium oxide is slaked with water to form magnesium hydroxide. This hydroxide is contacted with sulphur dioxide rich flue gases from the recovery boiler in a multi-stage wet scrubbing system to form magnesium bisulfite, the base acid for sulfite cooking.
The basis of all chemical pulping systems is the effective utilization of dissolved organic material for energy production and the regeneration of spent cooking chemicals. A modern kraft pulp mill producies more energy from its wastes than it needs for own production purposes. This net energy surplus will increase in the future as technologies evolve.
Chemical pulping proceses can also produce organic by-products such as turpentine, tall oil, methanol, ethanol and, various proteins.
در صورت داشتن مشکل در زمینه های لغتهای هر متن تخصصی می توانید با گذاشتن سوالات خود در بخش نظر دهید به جوابهای خود برسید.
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