Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Patent
1999-04-15
2000-09-26
Alvo, Steve
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
162 72, 162 76, 162 78, 162 88, 162 89, D21C 910, D21C 912, D21C 914, D21C 9153, D21C 916
Patent
active
061238090
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a process for the delignification and bleaching of chemical pulps.
BACKGROUND OF THE INVENTION
The manufacture of chemical pulp comprises two main phases, namely
a phase involving the cooking of lignocellulosic materials by means of chemical reagents intended to dissolve the greater part of the lignin and to release the cellulosic fibres, leading to an unbleached pulp,
a phase involving the delignification and bleaching of the unbleached pump, comprising in general several successive treatment stages interspersed in some cases with washing, dilution and/or concentration stages in order to achieve the residual lignin content and the whiteness that are desired.
The term chemical pulps will be taken to mean pulps which have undergone a delignification treatment in the presence of chemical reagents such as sodium sulphide in an alkaline medium (kraft or sulphate cooking) or else by other alkaline processes.
In recent years, numerous delignification and bleaching processes free from chlorine have been developed in addition to those which conventionally use chlorine and chlorine dioxide. Various kinds of delignification and bleaching agents are currently used for the treatment of the unbleached pulps. For example, it has been proposed that the chemical pulps be subjected to the action of oxygen in an alkaline medium, and then to delignification and bleaching treatments comprising treatments with ozone, peracids and hydrogen peroxide.
When chemical pulps are bleached with oxidizing agents such as ozone, peracids or hydrogen peroxide, it is advisable to remove from the pulp certain harmful metal ions. These metal ions having a harmful effect are ions of transition metals and include, among others, manganese, copper and iron, which catalyse decomposition reactions of the peroxidized reagents. They degrade the peroxidized reagents employed for the delignification and the bleaching via radical-type mechanisms and thus increase the consumption of these products while at the same time reducing the mechanical properties of the pulp.
Removal of the metal ions can be effected by a treatment with acid at the ambient temperature of the pulp. However, these treatments in an acid medium remove not only the harmful metal ions but also the ions of alkaline-earth metals such as magnesium and calcium, which have a stabilizing effect on the peroxidized reagents employed and a beneficial effect on the visual and mechanical qualities of the pulp.
It has been found recently that in chemical pulps, the metal ions are above all linked to carboxylic acid groups. Thus, the PCT patent application WO 96/12063 proposes a method for destroying selectively 4-deoxy-b-L-threo-hex-4-ene pyranosyluronic acid groups (hexene uronic groups) by treating the pulp at a temperature of between 85.degree. C. and 150.degree. C. and at a pH of between 2 and 5. The destruction of the hexene uronic groups reduces the kappa number from 2 to 9 units and reduces in a non-selective manner the adsorption of the ions of transition metals and alkaline-earth metals.
One of the major disadvantages of these processes in an acid medium is therefore that they are not selective with respect to certain metal ions, namely with respect to the harmful ions of transition metals.
A known means of selectively removing harmful metal ions from the pulp consists in the chelation of these ions. Unfortunately, this chelation stage requires a strict monitoring of the pH of the pulp often in a pH range which is close to neutral. Patent application EP 0 456 626 describes a pulp bleaching process in which a chelation stage (stage Q) is carried out in a pH range of between 3.1 and 9.0 before the treatment of the pulp with hydrogen peroxide (stage P). However, example 1 of this patent application shows that the maximum whiteness of the pulp after treatment with the peroxide comes to 66.1.degree. ISO and that this is achieved when the pH of stage Q is equal to 6.1. At higher pH values, the whiteness of the pulp declines rapidly and
REFERENCES:
patent: 5462641 (1995-10-01), Bergvist et al.
patent: 5571378 (1996-11-01), Elofson et al.
patent: 5741398 (1998-04-01), Germgard et al.
Chauveheid Eric
Devenyns Johan
Plumet Lucien
Alvo Steve
Solvay Interox (Societe Anony.)
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