Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Patent
1988-06-20
1989-11-07
Alvo, Steve
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
162 62, 162 78, D21C 916
Patent
active
048789984
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of controlling peroxide bleaching of mechanical, thermomechancial or chemi-mechanical pulp.
For several products, such as soft tissue, paperboard and different types of fine paper, it has started to become more and more common to use bleached mechanical or chemi-mechanical pulps instead of fully bleached chemical pulps. Besides the fact that the production of mechanical pulp is much more attractive from an environmental point of view than the production of chemical pulp, the raw materials are also more efficiently utilized. This means that mechanical pulp can be produced at a considerably lower cost and, in several aspects, mechanical pulp also has better properties than the chemical pulp. However, up to now a disadvantage of the mechanical pulp has been a lower brightness which has limited its use in several types of products.
As a consequence of the development of the peroxide bleaching process, for example by beaching in several stages and at high pulp concentrations, it has been possible to increase the brightness and at the same time reduce the costs for chemicals. Previous bleaching systems, both one and two stage systems, have, however, shown a considerable disadvantage in that the possibilities of controlling, regulating and optimizing the bleaching have been limited.
In existing bleaching plants the control is in the simplest case based on measurement of the brightness of the incoming pulp and the brightness value is then used directly for adjustment of the addition of bleaching chemicals. According to another system, which is more common, the brightness of the pulp is measured after the addition of the chemicals and after a defined reaction time of between 1 and 5 minutes. The brightness value is then used for "feed-back " regulation of the addition of the chemicals.
The brightness of the unbleached pulp is, however, not a satisfactory measure of the bleachability of the pulps and changes in the brightness can depend on several factors which influence the relation between the chemical addition and the brightness of the finished pulp in various ways. The raw material can thus vary with regard to content of rotten material, storage time, bark content and blends of different types of wood. The process conditions vary with the blends of chemicals, differences in degree of beating, the temperature and the treatment times and these and other factors influence the relation between the addition of chemicals and the brightness of the finished pulp in different ways.
The present invention will now be disclosed in more detail with reference to the appended drawings.
FIG. 1 shows the brightness of pulp at bleaching according to a previously known method.
FIG. 2 shows the brightness of pulp at bleaching according to the present invention.
FIG. 3 shows the control of a peroxide bleaching system in two stages according to the invention.
In FIG. 1 the brightness of pulps bleached in laboratory is shown as a function of the brightness of the unbleached pulp. The peroxide addition has in all cases been 40 kg/t H.sub.2 O.sub.2 and the addition of alkali has been optimized. The bleaching has been carried out on pulps produced in different manners, TMP, CTMP and groundwood pulp, and from different types of wood, birch, aspen, eucalyptus, spruce and pine wood. All the pulps were bleached under identical conditions and the poor correlation between unbleached and bleached brightness is clearly evident.
In closed systems, for example groundwood mills and TMP-plants, wherein the white water from the bleaching plant is used for dilution after the defibration the brightness of the incoming pulp will of course be an even poorer basis for the control. The brightness of incoming material to the bleaching plant will in these cases be strongly dependent on the amount of residual bleaching chemicals which are recycled with the white water and this residual amount is in turn set by the degree of system clsorue and the amount of residual chemicals from the bleaching. An increased brightness in the
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Yamashita; "Control of the Bleaching Process by Cormec", Pulp & Paper Canada, 33:8 (1982), pp. 19-20.
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Akerlund Gorgen
Hook Jan-Erik
Alvo Steve
Eka Nobel AB
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