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Pumpar, Omrörare; Luftare till Pappersbruket
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Hinweis der Redaktion
- The composition of wood generally consist of 30% Hemicellulose, 25% Lignin and 5% Extractives. When manufacturing chemical pulp, the aim is to remove the lignin, which works like a binder, by cooking without damage the wood fibres in the process too much. When manufacturing mechanical pulp no lignin is removed, the fibres are separated into different fractions. In the wood yard of a pulp mill, round wood and chips are sto- red and processed to meet the requirements of the pulping processes. Thus, the logs are cut, debarked and sometimes chipped. The chips are screened and stored in silos or in piles until transported into the mill. The chemical pulp process can be divided into two major processes - the sulphate process and the sulphite process plus some variations of these. Each process is designed to produce pulp with specific characteristics. The wood yield of unbleached chemical pulp is 46-60% of the dry wood weight depending on the raw material and the intended use of pulp. Mechanical pulp is the term used for groundwood pulps and all kinds of refiner pulps. Groundwood pulp is produced by grinding spruce logs. Refiner pulps are produces mainly from chips of spruce, but some hard woods and pines may also be used. The wood yield of mechanical pulp is between 85- 97%, depending on whether the chips are preheated, which lowers the yield. Recycled paper has gained importance in recent years. The fibre preparation is particular tough as the waste contains not just old paper. Sulphate Process In sulphate pulping the cooking liquor is alkaline, i.e. is has high pH. The active chemicals are sodium hydroxide (NaOH) and sodium sulphide (Na2S). The process is named after sodium sulphate, which is used to replace losses of sodium and sulphur. The raw material may be pine, spruce, birch or eucalyptus. Sulphate pulp is brown. The fibres must, therefore, be treated with chemicals which can dissolve the remaining lignin - the cause of the brown colour. If white pulp is desired, it is screened and pumped over to the bleach plant. There it is treated in several stages with oxygen, peroxide and chemicals containing chlorine. Sulphite Process The sulphite process is named after the active chemical of the cooking liquor, the hydrosulphite ion HSO-(also called the bisulphite ion). The base can be calcium, sodium or nowadays mostly magnesium. It is then usually referred to as the magnefite process. In sulphite pulping, the cooking liquor is usually acid, i.e. it has low pH. The raw material is mainly spruce, birch or beech. Sulphite pulp is significantly lighter in colour than sulphate pulp and can e.g. be used in newsprint without being bleached. Sulphite pulp for fine papers can be produced at a higher wood yield than sulphate pulp, but has a somewhat lower strength. It exists a neutral sulphite process based on sodium sulphite Na SO3, with a pH at about 7-8. The reason for sulphite pulps decline are: the longer cooking time, it produces a weaker pulp and that the stronger but browner sulphate pulp can now be bleached effectively. Neutral Sulphite Process It is the most common method of producing semi-chemical pulp for fluting. The cooking is done in a continuous digester. The cooking liquid is sodium sulphite, which is produced by blowing sulphur dioxide into a solution of sodium hydroxide. Cooking temperature is 160-180oC and the yield is high, about 80%. The wood chips are after the cooking sequence so called ”uncooked”. They must be ground in order for the fibres to be separated. Sodium sulphite pulp is always manufactured in connection with sulphate mills. This is because spent liquor from the neutral sulphite cooking is recovered jointly with black liquor from the sulphate mills. Mechanical pulp process The mechanical treatment of the wood is either with a grindstone, i.e. groundwood pulp (SGW, GWP) or between rotating and stationary discs, i.e. refiner pulp (TMP, CTMP), where the chips may have been heated or chemically treated before refining. The methods are designed to keep the best possible fibre length and brightness. After grinding or refining, the pulp must be cleaned by screening and centri-cleaning. The reject fraction which is obtained must also be upgraded before the pulp can be pumped to the paper mill. Sometimes, the brightness of the pulp must be raised by bleaching. This process is not the same as the bleaching processes used for the chemical pulps. It is much simpler and does not remove lignin. Mechanical pulping has fewer operational stages, and is much simpler, than the chemical pulping processes. The wood yield is also much higher than in chemical pulping. On the other hand, mechanical pulping needs much more electric energy. However, this need is constantly being reduced by more efficient heat recovery. Present pulping techniques make it possible to vary the pulp characteristics within wide limits. Mechanical pulp can today replace chemical pulp in more and more products, as long as strength is not the critical consideration. Apart from papermaking and hygiene products, mechanical pulp (in the form of refiner pulp) is also used in the production of fibreboard. Recycled paper is a suitable raw material for newsprint, soft paper, cardboard and Kraft liner. If it is cleaned thoroughly, it can provide fibres which are almost as strong as new fibres. However, repeated dryings, grindings and other heavy handed treatments, the fibres can be assumed to be quite worn after four or five rounds. At this point, the fibres begin to be sorted out as fine fraction in the cleaning process. Recycled paper which is to be decoloured (deinked) should not be too old. Certain printing inks used in paper can oxidise which makes it more difficult to separate them from the fibres. Paper with laser or Xerox print can be particularly difficult to deink. A major problem is ”stickies”. These are elastic and sticky pollutants which come from glue in self-adhesive envelopes and glued spines in magazines (they fasten in wires and press felts. Recycled paper can otherwise contain many different pollutants. These may be plastics etc., light weight contaminants, or staples, bolts, etc., heavy weight contaminants.