Method and a device for preparing cellulose pulp

Abstract

A method and an apparatus for preparing cellulose pulp, in which a lignin-containing raw material is impregnated with impregnation liquor and the impregnated raw material is cooked. The black liquor obtained by cooking is concentrated in an evaporation plant integrated in a chemical recovery plant. In the impregnation, the lignin-containing raw material is impregnated with a circulating alkaline impregnating liquor, which impregnating liquor is concentrated by evaporating water from it in at least one evaporator unit of the evaporation plant arranged in the chemical recovery plant of a pulp mill.

Description

CROSS REFERENCES TO RELATED APPLICATIONS [0001] This application claims priority on Finnish Application No. 20041288, filed Oct. 4, 2004, and Finnish Application No. 20055289, filed Jun. 6, 2005, the disclosures of which are incorporated by reference herein. STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT [0002] Not applicable. BACKGROUND OF THE INVENTION [0003] The invention relates to a method for preparing cellulose pulp and to an apparatus for implementing the aforementioned. [0004] The purpose of pulping is to remove, from a lignin-containing raw material, the lignin which binds fibers to each other, and to detach the fibers from each other so that they can be utilized in the manufacture of cellulose and / or paper. When wood is used as the raw material, cooking is preceded by chipping of the raw material, i.e. chopping of the wood to the optimal size required by the pulping process. In sulphate cooking, or so-called kraft pulping, the...

AI Technical Summary

Benefits of technology

[0013] Consequently, it is an aim of the present invention to provide a method and an apparatus avoiding the problems of the above-presented methods and raising the sulfide concentration of the impregnation liquor used in the impregnation stage as high as possible. As a result, the delignification reactions become faster and the carbohydrate yield of the pulping is improved.

[0014] The invention is based on the idea that the hydrogen sulfide concentration of the impregnation liquor used for impregnation is raised by concentrating the impregnation liquor used for impregnation and by recirculating the concentrated impregnation liquor back to the impregnation stage. Evaporation of water will raise the hydrogen sulfide concentration of impregnation liquor high. Also, the hydroxyl concentration of impregnation liquor will remain sufficiently high in spite of the reactions of dissolution of carbohydrates. This will prevent the condensation reactions of lignin. In the concentration of impregnation liquor, free water is evaporated from the impregnation liquor, wherein the concentration differences required by diffusion remain high and the diffusion rate increases. Thanks to the recirculation of the impregnation liquor, the carbohydrates dissolved in connection with the hydrolysis at the beginning of the impregnation are returned to the impregnation, which improves the total yield of the process according to the law of mass action. Furthermore, the carbohydrates thus saved have been found to have a catalyzing effect on the pulping.

[0015] All of this makes it possible to use a higher impregnation temperature in the method according to the invention than in methods of prior art, namely a temperature ranging from 100 to 170° C., preferably from 100 to 130° C. at the beginning of the impregnation and from 120 to 150° C. at the end of the impregnation. Thanks to the high temperature of impregnation, the hydrogen sulfide in the impregnation liquor reacts with the lignin and binds chemically to the wood matrix of the chips. As a result, the actual cooking becomes faster and the total cooking yield increases. The high impregnation temperature does not only make the absorption more efficient, wherein no absorption takes place in the cooking stage any longer, but it also provides that the acids formed in the dissolution of carbohydrates are neutralized already in the impregnation stage and not in the cooking stage, wherein less alkali is needed for digesting the cellulose in the cooking stage, that is, the cooking is more selective.

Problems solved by technology

In particular, the thickness of a chip affects the distance and rate of penetration of chemicals into the central parts of the chip.

If the impregnation is insufficient and the rate of raising the temperature is too high, delignification will only take place in the surface layers of the chips, and the alkali concentration will decrease before it reaches the inner parts of the chips.

Furthermore, undesired reactions of condensation of lignin will be possible in the areas with an alkali deficiency, and the separation of the fibers will be incomplete.

However, this method only provides. a relative increase in the concentration of hydrogen sulfide ions when compared to the content of hydroxide ions, and the absolute hydrogen sulfide concentration is not necessarily very high compared to white liquor, for example.

This method increases the absolute hydrogen sulfide concentration in the cooking stage but fails to provide a real benefit.

A problem in this method is that the hydrogen sulfide concentration of black liquor from the cooking stage is not very high in practice, because it is reduced by the chip water in the impregnation stage preceding the cooking stage.

To obtain a sufficiently high hydrogen sulfide concentration in the black liquor after the cooking for it to be beneficial in the impregnation stage, the system according to the method of the SE patent requires that a lot of white liquor is added into the cooking stage, which has a negative effect on the yield of the pulping, because both a high hydroxyl content and a high temperature occur simultaneously in the bulk step.

Images