Equipment Description:Mechanical Pulping
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Mechanical Pulping is used for products such as Newsprint and some printing grades. It can also be mixed with chemical pulps to produce a mixture of properties. Mechanical Pulp has low strength compared to chemical pulps. Its high yield gives it an advantage although there is a huge cost in energy using this method. Mechanical pulp provides good printability but changes colour on exposure to light as can be seen in old newspapers. Mechanical pulps are mostly produced using softwood species as these suffer less damage in the harsh mechanical process. However some hardwood species can be used in a semi-chemical process which is a mixture of the chemical pulping and mechanical pulping process. |
Chemical Pulping A process of pulping using chemicals and heat rather than mechanical action Semi-chemical pulping is a pulping process using a combination of chemicals, heat and mechanical action. |
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Mechanical Pulping produces a high pulp yield of 85-95% compared to only 45% from Chemical Pulping. The process uses very little or no chemicals but is extremely energy intensive. The breaking down of the wood into fibres can be done by either grinding the logs against a revolving abrasive surface (usually stone) which gives a Groundwood pulp or by passing wood chips between one rotating (rotor) and one stationary (stator) metal discs. This process is called refining and produces a pulp often referred to as a refiner mechanical pulp. Heat can also be used in mechanical pulping to produce Thermo-mechanical pulp (TMP). The refiners themselves are subjected to heavy load and wear. Refiner pulps are generally stronger than groundwood pulps although groundwood pulps produced under high temperature and pressure conditions are stronger than those produced at ambient pressure and temperature. The main advantage of refiner mechanical pulping is the opportunity to process sawmill residues which could normally not be utilised in a grinder which is designed to process logs. Mechanical pulping in its pure form is a reasonably simple and cheap operation although rising energy costs threaten this method. Semi-chemical pulping which softens chips before refiner treatment is more flexible and has gained popularity over the pure refiner mechanical pulping process. |
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