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Manufacture of wood pulp Wood pulp is made in several stages: History Using wood to make paper is a fairly recent innovation. In the 1800s, fiber crops such as linen fibres were the primary material source, but a shortage led to experimentation with other materials. Around 1850, a German named Friedrich Gottlob Keller crushed wood with a wet grindstone to obtain wood pulp. Further experimentation by American chemist C.B. Tilghman and Swedish inventor C.F. Dahl enabled the manufacture of wood pulp using chemicals to break down the fibres. The use of wood pulp (and the introduction of steam power to the printing and paper making processes) led to cheaper paper and to the arrival of a new literary genre which we would later coin pulp fiction but were known to their contemporaries as dime novels or penny dreadfuls. Environmental impact The major environmental impacts of wood pulping come from its impact on forest resources and from its waste by-products. The number of trees consumed depends on the type of paper, whether made by using the groundwood process or the kraft process. It has been estimated that based on a mixture of softwoods and hardwoods 40 feet tall and 6-8 inches in diameter, it would take a rough average of 24 trees to produce a ton of printing and writing paper, using the kraft chemical (freesheet) pulping process. On the assumption that the groundwood process is about twice as efficient in using trees, it takes about 12 trees to make a ton of groundwood and newsprint. * However, kraft pulp mill is self-sufficient in bioenergy. When the paper is bleached with elemental chlorine, byproducts such as chlorinated compounds even such as dioxins and furans are formed, and in high pulping areas such as British Columbia, high concentrations of these contaminates led to the closures of some fisheries in 1992. However, improvements in technology have either eliminated the use of elemental chlorine through Elemental Chlorine Free (ECF) or Totally-Chlorine Free (TCF) technology, combined with oxygen delignification. These technologies reduced the amount of chlorinated compounds released into the environment. Elemental Chlorine Free technology utilizes chlorine dioxide (ClO2) in place of chlorine (Cl2). Total chlorine free bleaching utilizes no chlorine in the bleaching process. The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. Reducing the environmental impact of this effluent is accomplished by closing the loop and recycling the effluent where possible, as well as employing less damaging agents in the pulping process. The most important way to mitigate the impacts is the biological effluent treatment. In the Kraft process, the largest volume byproduct from the pulping process is weak black liquor. This liquor contains the pulping chemicals and the lignin from the trees. The lignin is high in heat content, so this weak black liquor (about 15% solids) is concentrated into heavy black liquor (usually 68% to 75% solids) by use of multiple effect evaporation. Multiple effect evaporation is a process in which one pound of steam is used to boil 4.5 to 5.5 pounds of water. The heavy black liquor is burned in a recovery boiler and the chemicals fall to the bottom of the boiler in a semi-liquid state called smelt. The smelt then flows out of the boiler and is dissolved in water or weak wash to form green liquor. The green liquor is then clarified. Quick lime (CaO) is added to the clairified green liquor to convert a majority of the sodium carbonate (Na2CO3) to sodium hydroxide (NaOH). The green liquor with the quick lime is then clarified and the resulting liquid is white liquor. The white liquor is used as pulping chemicals and the process begins again. The spent lime (CaCO3) is then calcined at approximately 1800 degrees Fahrenheit (1000 degrees Celsius) to yield quick lime to be used again in the clarified green liquor. Paper made from wood pulp can typically be recycled four to seven times before the fibres become too short. To solve this problem recycled paper is usually mixed with virgin wood pulp to ensure a high quality paper. Alternatives Today, some people and groups are advocating using field crop fiber or agricultural residues instead of wood fiber as being more sustainable. However, wood is also a renewable resource which takes generations to grow. See also | ||||||||||
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