Method of two-stage peroxide bleaching of mechanical or semi-mec

Details Method of two-stage peroxide bleaching of mechanical or semi-mec Method of two-stage peroxide bleaching of mechanical or semi-mec

1986-01-13

1988-03-29

Alvo, Steve

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Treatment with particular chemical

162 78, D21C 916

Patent

active

047341602

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

The present invention relates to peroxide bleaching of lignocellulose-containing material, particularly mechanical and mechano-chemical pulp. Peroxide bleaching is normally carried out solely in one step and for increasing the brightness of the pulp. (In certain cases, e.g. for fluff, the intention may also be to improve the water absorption ability). It is known that the pulp properties are then altered a little, the density, smoothness and strength increase. It has also been found that strength and smoothness can be increased considerably in peroxide bleaching with high initial pH: tensile index by 50% and Scott Bond by 150%. The disadvantage in bleaching with an initial pH level above the one which is optimum for brightness, i.e. so-called hyper-alkaline peroxide bleaching (HAPB) is that brightness becomes relatively low in relation to the peroxide charge.
The present invention relates to producing a pulp by peroxide bleaching, which is both strong and bright. This result is obtained in accordance with the invention, in principle by bleaching being carried out in two stages at different pH values. In the first stage, which may be denoted HAPB, bleaching is carried out at a high pH (over 12), great strength thus being achieved. In the second stage peroxide bleaching takes place at a lower pH value more favourable to high brightness.
There are two main implementations of this method. The one consists in that an acid, e.g. sulphuric acid, is added towards the end of a HAPB, e.g. after one hour's bleaching time, to lower the pH to a level where the remaining peroxide may be used for bleaching (for about one hour). There is then obtained a pulp which has greatly improved strength and surface properties and a brightness which is (nearly) the same, in relation to the peroxide charge, as would be obtained in a standard brightness bleaching.
The second implementation which is the one preferred, is a 2-stage bleaching, strength and surface smoothness being obtained in the first stage and desired brightness in the second stage. The second stage can take place with or without intermediate washing (i.e. only withdrawal). Bleaching in the second step heavily improves brightness for very small peroxide consumption.
It will be seen from the following tables what brightness values are obtained in normal peroxide bleaching and hyperalkaline peroxide bleaching (table 1), and what brightness values are obtained after the second bleaching stage (table 2) in a method in accordance with the invention using the second implementation of the method and the same pulps.


TABLE 1 ______________________________________ Brightness after bleaching of groundwood (SGW) and thermomechanical pulp (TMP), respectively, to maximum brightness (standard bleaching) and to high strength, hyper-alkaline peroxide bleaching (HAPB), i.e. with an initial pH of 13. The peroxide consumption is given in parentheses. Peroxide charge 4%, time 120 minutes and temperature 60.degree. C. Brightness according to ISO Standard bleaching HAPB ______________________________________ SGW 78.7 (2.3%) 75.1 (3.0%) TMP 78.6 (2.5%) 74.7 (2.8%) ______________________________________


TABLE 2 ______________________________________ Brightness according to ISO after peroxide bleach- ing of a hyper-alkaline peroxide bleached (HAPB) pulp. Brightness after HAPB see table 1. Peroxide consumption in the second step is given in parenthesis. Second stage bleaching 4% peroxide charge in 1% 2% 2nd stage ______________________________________ SGW 81.9 (0.1%) 83.3 (0.1%) -- TMP -- 80.0 (0.9%) 81.6 (0.5%) ______________________________________
After the second stage bleaching, pulps were obtained with greatly improved strength and surface properties, and with very high brightness. Note that the brightness is very much higher than what may normally be obtained in a standard bleaching (table 1). The peroxide consumptions in the second stage are very small and the optimum bleac

REFERENCES:
patent: 3966542 (1976-06-01), Oldshue
patent: 4294653 (1981-10-01), Lindahl et al.
patent: 4450044 (1984-05-01), Fritzvold et al.
Singh, "The Bleaching of Pulp", TAPPI Press, Atlanta, Ga. 1979.

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