Method to regenerate gas mixture in ozone-bleaching process

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product

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Details

162 3011, 162 65, D21C 1106, D21C 9147, D21C 9153

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active

060599251

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION AND DESCRIPTION OF THE KNOWN PRIOR ART

The present invention relates to a method of treating cellulose pulp using an oxygen-delignification stage and a peroxide-bleaching stage and the regeneration of the used oxygen and ozone gas mixture back to the delignification and/or bleaching stage.
Many of the known processes that are applied in the cellulose industry consume very large volumes of relatively expensive chemicals. In order for these processes to be viable economically, it is necessary to be able to reuse such chemicals to the greatest possible extent.
When cellulose pulp is bleached with a gaseous mixture that contains ozone and oxygen, for instance in amounts corresponding to about 10% O.sub.3 and 85% O.sub.2, these substances are consumed to some extent during the actual bleaching process. When coming into contact with the pulp, the gas mixture reacts therewith and carbon dioxide is formed. Ozone and possibly oxygen are consumed therewith. The gas leaving the treatment stage constantly contains large quantities of oxygen, and also carbon dioxide, nitrogen and possibly argon, among other things. The used gas could therefore be used to improve combustion or could be used in a bleaching or delignifying stage for instance, without needing to refine the gas. However, the use of the used gas mixture in an oxygen-delignification stage or in a bleaching stage is associated with certain drawbacks. The gas mixture namely consumes alkali, therewith making it necessary to adjust to a higher initial pH value. Furthermore, in comparison with the use of pure oxygen, the additional gas quantity represented by the gas mixture will probably result in channelling or tunnelling in the cellulose pulp, causing large quantities of gas to pass through the pulp to no useful end.
EP-A-406 218 teaches a method of producing oxygen and/or ozone for a consumer of these gases, wherein residual oxygen is recovered and purified in an adsorption device. The gas can then be returned to an oxygen consumer or passed to an ozone generator and thereafter to an ozone consumer. This document does not discuss the problem of carbon dioxide in the used gas mixture. The regeneration is primarily concerned with reducing hydrocarbon compound concentrations, primarily methane gas.
EP-A-526 383 teaches a method in which gas of high oxygen concentration is delivered to an ozone generator, there being generated an oxygen gas which is rich in ozone, having an ozone concentration of about 6 percent by weight. This gas is used to bleach cellulose pulp, there being obtained a used gas which contains contaminants, among other things a relatively large quantity of carbon dioxide. The used gas is regenerated by removing at least a part of the carbon dioxide. The regenerated gas can then be reused, and EP-A-526 383 suggests that the regenerated gas is mixed with fresh oxygen gas and returned to the ozone generator.
WO-A-8 804 706 teaches a method of washing alkaline pulp with the aid of carbon dioxide, which is delivered to the washing water either prior to or in the actual washing stage. This addition of carbon dioxide enables the pH value to be lowered and the washing process to be made more effective and therewith lower the water consumtion. The carbon dioxide added to the system is converted to carbonate ions and enhances the washing of organic substances (COD) and alkali from the pulp. The carbon dioxide is taken from an external source.


SUMMARY OF THE INVENTION

The object of the present invention is to improve the recovery and the use of used gas mixtures in the pulp treatment process and therewith make treatment of the cellulose pulp more effective.
This object is achieved with the initially defined method which includes the method steps set forth in the characterizing clause of the following claim 1.
Preferred embodiments of the invention are defined in claims 2-14.
When carbon dioxide is removed from the used oxygen-gas mixture in accordance with the inventive method, it is probable that the oxygen content of the mixture can

REFERENCES:
patent: 3764464 (1973-10-01), Samuelson
patent: 5145557 (1992-09-01), Peter et al.
patent: 5164044 (1992-11-01), Griggs et al.
patent: 5296097 (1994-03-01), Friend
patent: 5439556 (1995-08-01), Sethna et al.
patent: 5695604 (1997-12-01), Rounsaville et al.

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