Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Patent
1993-09-03
1997-12-16
Alvo, Steven
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
162 76, 162 78, 162 89, D21C 9147, D21C 916
Patent
active
056980752
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for bleaching cellulose paper pulps belonging to the category of chemical pulps.
2. Description of the Related Art
It is known to treat unbleached chemical paper pulps obtained by cooking lignocellulosic substances by means of a sequence of stages of delignifying and/or bleaching treatment involving the use of oxidising chemical products. The first stage of a conventional sequence for bleaching a chemical pulp is intended to complete the delignification of the unbleached pulp in the form in which it is after the cooking operation. This first delignifying stage is traditionally carried out by treating the unbleached pulp with chlorine in an acidic medium or with a chlorine-chlorine dioxide combination, as a mixture or in sequence, so as to react with the residual lignin in the pulp and to give rise to chlorolignins which can be extracted from the pulp by solubilisation of these chlorolignins in an alkaline medium in a subsequent treatment stage.
For various reasons it is found to be useful, in certain situations, to be able to replace this first delignifying stage by a treatment which no longer makes use of a chlorine-containing reactant.
Since approximately ten years or so ago it has been proposed to replace, at least partially, the first stage of the treatment by means of chlorine or of the combination of chlorine and chlorine dioxide by a stage with gaseous oxygen in an alkaline medium. (Kirk-Othmer Encyclopedia of Chemical Technology, 3rd edition, vol. 19, New York 1982, * page 415, 3rd paragraph and page 416, 1st and 2nd paragraphs *). The degree of delignification which is obtained by this oxygen treatment is not sufficient, however, if the aim is to produce chemical pulps of high brightness.
In International Patent Application WO-79/00,637 in the name of Mo Och Domsjo it has been proposed to bleach chemical paper pulp by means of hydrogen peroxide in an acidic medium in the presence of a complexing agent. However, the brightness obtained using this technique is not very high. Moreover, the cellulose undergoes an appreciable degradation.
The invention remedies these disadvantages of the known processes by providing a new process for delignification and/or bleaching of chemical paper pulps, which enables high brightness levels to be reached without excessively degrading the cellulose.
SUMMARY OF THE INVENTION
To this end, the invention relates to a process for bleaching a chemical paper pulp not comprising any preliminary stage of delignification using chlorine in an acidic medium or using the combination of chlorine and chlorine dioxide in an acidic medium, according to which the pulp is subjected to a treatment in two successive stages comprising, in this order: treatment using peroxymonosulphuric acid and treatment using alkaline hydrogen peroxide, the stage peroxymonosulphuric acid being carried out at a temperature of between 75.degree. and 100.degree. C. for a period of between 70 and 150 minutes and at a pulp consistency of between 12 and 25% of dry matter.
According to the invention, a chemical paper pulp is intended to denote the pulps which have undergone a delignifying treatment in the presence of chemical reactants such as sodium sulphide in an alkaline medium (kraft or sulphate cooking), sulphur dioxide or a metal salt of sulphurous acid in an acidic medium (sulphite cooking). Semichemical pulps such as those where the cooking has been carried out with the aid of a salt of sulphurous acid in a neutral medium (neutral sulphite cooking, also known as NSSC cooking) can also be bleached by the process according to the invention.
The latter is intended particularly for pulps which have undergone a kraft cooking and whose residual lignin content after cooking lies in the range of kappa numbers of between 8 and 35, depending on the type of wood species from which they originate and the efficiency of the cooking process. All kinds of woods employed for the production of chemical pulps are suitable for impl
REFERENCES:
patent: 5091054 (1992-02-01), Meier et al.
"Encyclopedia of Chemical Technology"; Kirk-Othmer; 3rd edition,, vol. 19, New York 1982, p. 415, 3rd paragraph and p. 416, 1st and 2nd paragraphs.
"Paperi ja Puu"; vol. 72, No. 10, 1990, pp. 967-973; Minor, J.L.; Springer, E.L.: wood fibers with peroxymonosulfate.
Ruhanen et al, "First-stage Bleaching of Softwood Kraft Pulp With Peroxide, Instead of Chlorine", TAPPI J., Sep. 1982.
Hoyos Marc
Robberechts Marcel
Troughton Nicholas
Vrambout Gilbert
Alvo Steven
Solvay Interox (Societe Anonyme)
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