Bleaching and dyeing; fluid treatment and chemical modification – Bleaching – Chemical
Patent
1998-12-30
2000-10-24
Einsmann, Margaret
Bleaching and dyeing; fluid treatment and chemical modification
Bleaching
Chemical
162 78, D06L 302, D21C 916
Patent
active
061360414
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for the bleaching of lignocellulosic fibers wherein lignocellulosic fibers are treated with at least one oxidizing bleaching chemical in aqueous solution. The invention also relates to the application of additives for bleaching lignocellulosic fibers and to the application of an aqueous solution containing at least one additive for bleaching lignocellulosic fibers.
In the context of this paper, the term "lignocellulosic fibers" includes all sorts and types of pulp like e.g. chemical and mechanical pulp, dissolving pulp or pulp prepared from waste paper but also natural fibers like cotton or flax fibers.
BACKGROUND OF THE INVENTION
Pulps produced with alkaline pulping methods, such as the Kraft method, or produced with acid pulping methods such as the acid magnesium bi-sulfite method, or with methods which use organic dissolving agents such as methanol (ORGANOSOLV.TM., ORGANOCELL.TM., ALCELL.TM.), or with alkali pulping methods which use, in addition to aqueous alkali, sulfite, anthraquinone and/or other organic solvents such as methanol, e.g. the ASAM method (Alkali-Sulfite-Anthraquinone-Methanol) must be treated in at least one bleaching step after pulping to achieve high degrees of brightness.
The state of the art technology for the production of paper or products made from dissolving pulp is based on the use of bleached fibers containing small amounts of residual lignin. An almost completely lignin-free pulp with an .alpha.-cellulose content of 98% is required, for example, for dissolving pulps. The fiber must also be free of lignin for chemical pulps as well. The brightness requirements for paper made from recycled fibers are also continually increasing. Fibers primarily used for the production of newspaper such as ground wood, RMP (refiner mechanical pulp), TMP (thermo mechanical pulp), and CTMP (chemo-thermo mechanical pulp) are increasingly being bleached to higher brightnesses, not only with reducing bleaching agents such as hypochlorite (HClO) and dithionite (SO.sub.2 O.sub.4.sup.-2) but also with oxidizing bleaching agents containing oxygen such as hydrogen peroxide. Because bleaching is no longer conducted exclusively with elemental chlorine or chlorine containing chemicals for environmental and economic reasons, chlorine-free oxidizing compounds like oxygen, ozone or peroxo-chemicals like hydrogen peroxide or peracids and mixtures thereof are used more often.
These chlorine-free chemicals comprise mainly oxidizing bleaching chemicals like oxygen, ozone and peroxo-chemicals. Among the peroxo-chemicals, peroxides, especially hydrogen peroxide is well suited to bleach lignocellulosic fibers. However, sodium hydroxide, peracids like peracetic acid, performic acid or Caroic acid and salts thereof are also suited to increase pulp brightness. The increasing trend towards the TCF (total chlorine free) bleaching of all fibrous materials for the production of paper with oxygen, ozone and chemicals containing peroxo compounds necessitates increased efforts to more efficiently utilize and activate these chemicals in the bleaching liquor, thereby attaining higher consumption and higher brightness.
It is very difficult, however, to activate hydrogen peroxide during this procedure by adding more alkali or increasing the temperature. The higher amounts of alkali or the higher temperature can greatly effect the bleaching reaction, leading to a complete consumption of the peroxide in the alkali milieu which results in secondary yellowing. (H. Suss; H. Kruger and K. Schmidt, "Die optimale Bleiche von Holzstoffen und ihre Abwasserbelastung", Papier (34), (10), 1980, pg. 433-438). Thus it has been necessary to retain a certain residual amount of peroxide in the alkali fiber suspension after bleaching to avoid brightness reversion after final bleaching of the fibers.
Peroxide bleaching of fibrous materials used for the production of chemical and dissolving pulps has become a normal practice today. Almost all types of pulps can be bleached at least in singl
REFERENCES:
patent: 5630906 (1997-05-01), Boe et al.
Jaschinski Thomas
Patt Rudolf
Dubno Herbert
Einsmann Margaret
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