Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Patent
1993-04-15
1996-07-09
Alvo, Steven
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
162 76, 162 78, 162 80, 162 84, D21C 9147, D21C 916
Patent
active
055341152
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a process for preserving the mechanical strength properties of chemical paper pulps during treatments of these pulps for the purpose of delignifying or bleaching them. More particularly, it relates to a process where the delignification or bleaching treatment of these paper pulps comprises a sequence of two successive stages with a peroxidic reagent, the first stage being carried out in an acidic medium and the second stage in an alkaline medium.
TECHNOLOGY REVIEW
It is known to treat unbleached chemical paper pulps, obtained by digestion of lignocellulose materials, by means of a sequence of delignifying and/or bleaching treatment stages, involving the use of oxidising chemical products. Amongst these chemical products, chlorine and chlorine-containing oxidising agents such as chlorine dioxide and sodium hypochlorite have always been and still are most widely used because of their delignifying and bleaching properties with a view to the production of pulps of high whiteness and mechanical strength, which are intended for the manufacture of quality papers.
The researches carried out in recent years in the field of protecting the environment have demonstrated the polluting role of liquid wastes containing organochlorine residues, in particular those from bleaching plants in paper pulp factories. New legislation is appearing in a large number of countries, imposing a sometimes serious limitation on the quantity of organochlorine wastes in industrial effluents. This results in a research activity on the part of the paper industry for less polluting pulp-treatment sequences, leading to processes which have a reduced consumption of chlorine-containing reagents or even none at all.
Amongst the possible substitutes for chlorine and chlorine-containing reagents, peroxidic products are the materials of choice because of their harmlessness to the environment.
Delignification sequences for chemical pulps, which use two successive stages involving peroxidic reagents are known. The TAPPI document Proceedings of the 1982 International Pulp Bleaching Conference, pages 145 to 151, D. Lachenal, C. de Choudens, L. Soria and P. Monzie "Optimization of bleaching sequences using peroxide at first stage" discloses on page 146, table 3, a sequence which uses two stages which hydrogen peroxide, the first in an acidic medium and the second in an alkaline medium. In this process, only minor quantities of hydrogen peroxide (0.5% by weight relative to dry pulp) are used in each of the stages. When the quantities of hydrogen peroxide used are increased in order to obtain a more thorough delignification analogous to that achieved with the traditional bleaching/delignification sequences involving chlorine-containing reagents, a rapid decrease in the selectivity of the delignification is found, as shown by the decrease in viscosity of the pulp treated in this way.
SUMMARY OF THE INVENTION
The invention overcomes this drawback of the known processes by providing a novel delignification and/or bleaching process for chemical paper pulps, which allows high degrees of delignification to be achieved while preserving the intrinsic quality of the cellulose and the weight yield of pulp produced.
For this purpose, the invention relates to a process for preserving the mechanical strength properties of a chemical paper pulp during a two-stage delignification and/or bleaching treatment by means of a peroxidic reagent, the first stage being carried out in an acidic medium and the second stage in an alkaline medium, according to which a treatment with a reducing compound is interposed between the two stages.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, a chemical paper pulp is understood to mean pulps which have undergone a delignifying treatment in the presence of chemical reagents such as sodium sulphide in an alkaline medium (kraft digestion or sulphate digestion), sulphur dioxide or a metal salt of sulphurous acid in an acidic medium (sulphite digestion), a sulph
REFERENCES:
patent: 4400237 (1983-08-01), Kruger et al.
Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, vol. 16, pp. 724-727 (1968).
Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, vol. 4, pp. 301-311 (1992).
D. Lachenal et al, "Optimization Of Bleaching Sequences Using Peroxide As First Stage", International Pulp Bleaching Conference, 1982, pp. 145-151.
Essemaeker Paul
Hoyos Marc
Robberechts Marcel
Alvo Steven
Interox International (Societe Anonyme)
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