Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With classifying – separating or screening of pulp
Patent
1992-07-02
1995-06-27
Alvo, Steve
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With classifying, separating or screening of pulp
162261, D21D 502
Patent
active
054276519
DESCRIPTION:
BRIEF SUMMARY
This invention relates to novel cellulose pulps having abnormal fibre-size distribution, and also to the use of said pulps in the manufacture of paper, including writing and printing paper, paperboard, soft paper, absorption pulp and other products in which cellulose fibres are included. The pulps are primarily produced by sulphate cooking of softwood, and bleaching the pulp to brightnesses from 45% ISO and higher (normally beneath 95% ISO) (SCAN-C 11:75). The pulp can be manufactured with environmentally-friendly methods embraced by the invention.
The expressions pulp and cellulose pulp are used synonymously in the present document and refer to cellulose fibre material with or without lignin.
In the manufacture of bleached cellulose pulps, efforts are made to remove lignin, resin, etc. In order for cellulose pulps to be suitable for the manufacture of paper products, it is considered that their limit viscosities shall be greater than 900 cm.sup.3 /g (In description and claims it is intended, that the limit viscosity is measured in accordance with SCAN-C 15:62). However, the most important criterion is that when the pulp is beaten in a PFI-mill, the pulp will behave in an acceptable manner with regard to such properties as beating degree development (.degree.SR, SCAN-C 19:65) and changes in density (kg/cm.sup.3, SCAN-P 7:75), tensile index (Nm/g, SCAN-P 38:80) and tear index (mNm.sup.2 /g. SCAN-P 11:73) and that tear index and surface roughness (Bendtsen ml/min, a given manner. In the majority of cases, the beating degree, i.e. .degree.SR, shall increase as little as possible during the beating process.
The requirements on mentioned properties are contingent on the use and the type of pulp concerned. Thus, the demands are mutually different for sulphate pulp, groundwood pulp, thermomechanical pulp, sulphite pulp etc.
In the case of bleaching softwood sulphate pulp, e.g. with oxygen gas, it has been considered that good paper properties are incompatible with limit viscosities beneath 900 cm.sup.3 /g. Subsequent to cooking, the kappa number of the pulp (washed and screened) is normally between 20-40. In order to manage the viscosity and to remove further lignin, bleaching has therefore been carried out in stages with different bleaching chemicals. An example of this is HClO/ClO.sup.- Cl.sub.2, O.sub.2, O.sub.3, ClO.sub.2, H.sub.2 O.sub.2, BH.sub.4.sup.-, S.sub.2 O.sub.4.sup.-, NO.sub.2 etc., each of which requires its own specific conditions (e.g. different pH-values and the addition of cellulose protectors). NO.sub.2 is directed primarily to activation of pulp with NO.sub.2 (in the presence of H.sup.+ and NO.sub.3.sup.-) and optionally in the presence of oxygen gas, so as to convert formed NO to NO.sub.2. Chlorine is a classic bleaching chemical, although it has the drawback of producing chloro-organic compounds which are considered to be toxic. Chloro-organic compounds which remain in the pulp subsequent to processing the pulp can result in degradation of cellulose and yellowing. It is normal to classify the bleaching agents as chlorine-containing and non-chlorine containing substances. Another method of classification is reducing bleaching agents and oxidizing bleaching agents. Of the aforesaid bleaching agents, BH.sub.4.sup.- and S.sub.2 O.sub.4.sup.2 - are reducing, whereas the remainder are oxidizing.
There has long been an interest to fractionate and to study the properties of the different pulp fractions obtained. These studies have been directed primarily to so-called high yield pulps. i.e. thermomechanical pulp (TMP), stone groundwood pulp (SGW), chemithermomechanical pulp (CTMP), etc. Fractionation has been carried out industrially in screen rooms and has involved the enrichment of long fibres and also of so-called shives, which subsequent to treatment in a disc refiner are again mixed with the original pulp. Fractionation has also been carried out with the intention of producing two mutually different pulp qualities from one original pulp (one quality with longer fibres and one quality with short
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patent: 4562969 (1986-01-01), Lindahl
patent: 4776926 (1988-10-01), Lindahl
patent: 4781793 (1988-11-01), Halme
Lindqvist, B. et al., "Nitrogen Dioxide . . . for the Future?", Journal of Pulp & Paper Science, vol. 12, No. 6, pp. J161-165, Nov. 1986.
Tasman J. E., "The Fiber Length of Bauer-McNett Screen Fractions", TAPPI, Jan. 1972, vol. 55, No. 1, pp. 136-138.
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English language version of the measuring method SCAN-M6:69 discussed in "Fiberfraktionering av mekanisk massa med McNett-apparat" (Fiber Fractionation of Mechanical Pulp in the McNett Apparatus), Scandinavian Pulp, Paper and Board, SCAN-M6:69, 1969.
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Alvo Steve
Mo och Domsjo Aktiebolag
Nguyen Dean T.
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