Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Patent
1995-05-17
1998-07-28
Alvo, Steven
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
162 65, 162 78, D21C 916
Patent
active
057858128
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for delignifying and bleaching lignocellulose-containing pulp, in which the pulp is delignified with a peracid or a salt thereof, treated with a complexing agent, and subsequently bleached with a chlorine-free bleaching agent. Suitably, delignification is carried out with the strongly oxidising peracetic acid, giving a considerable increase in brightness and a considerable reduction of the kappa number after bleaching with a chlorine-free bleaching agent comprising at least one of a peroxide-containing compound, ozone or sodium dithionite, or optionally sequences or mixtures thereof. The brightness-increasing effect is highly selective, i.e. the viscosity of the pulp is maintained to a comparatively great extent.
BACKGROUND OF THE INVENTION
Chlorine-free bleaching agents have long been used for bleaching mechanical pulps. In recent years, it have come increasingly common to bleach also chemical pulps with chlorine-free bleaching agents, such as hydrogen peroxide and ozone, even in the first stages. It has been considered necessary to pretreat the pulp directly after digestion and an optional oxygen-delignifying stage so as to avoid deteriorated pulp properties and an excessive consumption of the bleaching agent. Pretreatment of the pulp primarily involves acid treatment and treatment with a complexing agent or salts of alkaline-earth metals, optionally in combination. Strongly acid pretreatment removes desirable as well as undesirable metal ions from the original positions in the pulp. Treatment with suitable complexing agents primarily removes the undesirable metal ions, while the desirable ones are largely retained. Treatment with salts of alkaline-earth meals maintains or reintroduces the desirable metal ions.
EP-A-0 402 335 thus discloses the pretreatment of chemical pulp with a complexing agent directly after digestion or oxygen delignification, to make a subsequent alkaline peroxide bleaching more efficient.
EP-A-0 480 469 relates to delignification of lignocellulose-containing pulp with oxygen. The pulp can be delignified or bleached before or after the oxygen state with peroxide-containing compounds, such as hydrogen peroxide or peracetic acid, chlorine dioxide and/or ozone. Use of sequences with both peracetic acid and hydrogen peroxide, results in a significant decrease in pulp viscosity.
U.S. Pat. No. 5,091,054 describes a process where a pulp is treated with a sequence in two steps. In the first step peroxomonosulphuric acid, i.e. Caro's acid (=an inorganic acid containing sulphur), is added. A complexing agent may be added in the treatment with Caro's acid. In the second step of the pulp is bleached with peroxide and/or oxygen.
With increasingly stringent environmental standards, there is a growing need for completely chlorine-free processes for delignifying and bleaching lignocellulose-containing pulps. To produce fully bleached pulps with unaltered strength properties in a reasonable number of stages and with a reasonable consumption of bleaching agents, it has become necessary to consider using also powerful, and hence difficulty-controlled, bleaching agents having a high delignifying and/or bleaching capacity.
DESCRIPTION OF THE INVENTION
The invention provides a process in which lignocellulose-containing pulp is delignified and bleaching under the conditions disclosed in the appended claims, whereby a good delignifying and bleaching effect is obtained even before the chlorine-free bleaching.
The inventive process comprises delignifying and bleaching lignocellulose-containing pulp, wherein the pulp is delignified with a peracid or salts thereof, whereupon the pulp is treated with a complexing agent and subsequently bleached with a chlorine-free bleaching agent comprising at least one of a peroxide-containing compound, ozone or sodium dithionite, or optional sequences or mixtures thereof.
The invention process has made it possible to delignify the pulp before a chlorine-free bleaching, such that the subsequent treatment with a complexing agen
Basta Jiri
Hallstrom Ann-Sofie
Linsten Magnus
Alvo Steven
Eka Nobel AB
Mancini Ralph
Morris Louis
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