A method and an apparatus for removing extractives from fibrous suspension

Field of the invention

The invention relates to a method according to the preamble of the appended claim 1 for removing extractives from fibrous suspension. The invention also relates to an apparatus for implementing the aforementioned method in accordance with the preamble of the appended claim 11.

Background of the invention

In the production of cellulose pulp, the chips are cooked in an alkaline water solution. As a result of the cooking, the fibers and lignin of the wood separate from each other, and lignin dissolves in the cooking solution. After cooking the pulp is washed, wherein the fibrous pulp can be separated from the lignin-containing cooking solution.

Wood also contains a number of other substances besides lignin and fibers, such as fatty acids and resins, which are commonly called extractives. Most of the extractives dissolve in the cooking solution during the cooking, but a large amount of them still remain in the finished pulp. Especially in hardwood pulps contain large amounts of neutral extrac- tives after the cooking. The extractives in hardwood pulps are mainly formed of unsaturated fatty acids, which are for the most part located in the small-sized parenchymatous cells among the pulp. The extractives can also appear on the surface of the fibers or freely in the solution.

Extractives cause numerous problems in the further processing of pulp. They cause yellowing of the pulp in the following pulp processing stages and odour and taste nuisance in paper made of pulp. The free extractives in the solution can accumulate and form deposits, stickiness and precipitations in the pulp processing equipment and its parts. In addition to disturbing the function of the process equipment, the deposits can suddenly disintegrate into the pulp and cause soiling of the

pulp. Furthermore, the agglomerated extractives disturb paper manufacturing, because they cause greasy spots in the paper, which hinder the surface-sizing and printing of paper because they do not absorb glue and reject printing ink.

To eliminate these problems, the aim is to remove the extractives from the pulp at some stage of the pulping process, before pulp is conveyed to drying or production of paper or paperboard. For this purpose, additives have been used, which are added in the pulping process. They function as surfactants, and make the extractives soluble to the solution in the pulp suspension. As a result, it is possible to remove the extractives from the pulp with the solution, and they do not remain in the pulp. In the process of cooking hardwood pulp, especially birch- wood pulp, resin acids obtained from the process of cooking softwood, such as tall oil and tall oil soap have been used as additives. These additives have been added into the cooking solution of the pulp. At present, synthetic resin soap is mainly used as an additive in the process of removing extractives, the ratio of the resin acid and fatty acids therein being selected in such a manner that they dissolve extractives efficiently. Such methods are disclosed for example in the Finnish patent application No. 20022270 (corresponding WO publication 2004/057107), in which a mixture of resin acid/fatty acid is added in the cooking solution of pulp, especially pulp made of birchwood, and in the Finnish patent application No. 20030846 (corresponding European patent 1484375) in which the addition of resin acid/fatty acid is made either in the cooking solution or washing solution of pulp.

The above-mentioned methods for removing extractives from the pulp do work as such. However, they have the drawback that because they affect the entire amount of pulp travelling through said process stage, they consume a large amount of additives used for removal of extractives. Thus, the additive costs are very high.

Furthermore, by means of these known methods it is not possible to remove the extractives in the parenchymatous cells of hardwood suffi-

ciently well, but they remain in the pulp causing problems in later process stages.

Brief description of the invention

Consequently, it is an aim of the present invention to provide a method that avoids the above-mentioned problems and by means of which it is possible to remove the extractives from fibrous pulp more efficiently and with considerably smaller amounts of additives than before when producing hardwood pulp. Furthermore, it is an aim of the invention to provide an apparatus implementing the aforementioned method.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The apparatus according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 11.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that a suspension containing fine particles, i.e. fines containing suspension is separated from the pulp obtained from the cooking, and it is treated by means of an additive for removing the extractives therein.

It has been noticed in test runs that the extractives mainly accumulate in the fines of the pulp, even up to 90% of the extractives in the pulp can accumulate in the fines. The amount of fines in the total amount of pulp is typically 3 to 10 %. When the fines-containing suspension is separated from the pulp flow and only the small fraction, in which most of the extractives is present is treated with an additive removing the extractives, it is possible to significantly reduce the amount of additives necessary for the treatment.

The removal of the extractives can be intensified further for example by refining or dispersing the fines suspension so that the size of the particles contained therein can be reduced. As a result of the treatment the parenchymatous cells in the fines disintegrate, wherein the additive can affect the extractive and increases its solubility to the solution component of the suspension.

According to an embodiment of the invention, a solution contained in the fines-containing suspension is removed from it before treating it, wherein the treatment, such as refining or dispersion is more efficient.

The fines-containing suspension separated from the pulp substantially contains all the fines existing in the pulp. Typically it contains noil (fines), parenchymatous cells, pieces of fiber, and sometimes even sand. The fines-containing suspension can be defined in such a manner that it is a suspension in which the particles therein have such a size that they penetrate through a 200 mesh wire, when the suspension has been analyzed by means of methods suitable for fractionating analysis of fibers. One such method is the Bauer-McNett analysis.

As an additive it is possible to use a side product obtained from the cooking of softwood, such as tall oil, tall oil soap or a fatty acid/resin acid mixture. The fatty acid and the resin acid can also be present in the additive in the form of their salts, i.e. in a saponified form. It is also possible to use a synthetic fatty acid/resin acid mixture as an additive.

It is an advantage of the invention that in the production of softwood pulp the amount of additive necessary for removing the extractives is reduced, wherein savings can be attained in the additive costs. Furthermore, by treating the fines containing suspension by breaking the parenchymatous cells containing extractives and contacting the extractive contained in them with the additives makes the removal of the extractives far more efficient. These advantages can be attained only by separating the fines-containing suspension from the pulp according to the invention, because by means of the invention the addi-

tives treatment can be targeted to the part of the pulp in which the extractives actually exist. It would be impossible to refine or disperse the entire amount of pulp to reduce the particles. Furthermore, the refining of the pulp would change the fiber properties, thus affecting the quality of pulp and the final product produced therefrom. The refining of the fines-containing suspension does not have such a strong effect on the quality of pulp, and especially it does not have a significant effect on the strength properties of fibers

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows schematically an embodiment of the invention for removing extractives from a fiber-containing suspension and

Fig. 2 shows schematically another embodiment of the invention for removing extractives from a fiber-containing suspension.

In Figs. 1 and 2, the same numerals refer to corresponding parts and they will not be explained separately later on, unless required by the illustration of the subject matter.

Detailed description of the invention

In this context the term pulp refers to the pulp treated in different stages of producing hardwood pulp. The term fines-containing suspension refers to a suspension separated from pulp by means of a separator, said suspension containing a solution typical for the pulp producing stage preceding the separator and fine particles. The solution can be for example cooking liquor or pulp washing solution and in addition to fine particles it may also contain for example lignin, cellulose, hemicellulose and inorganic material dissolved from wood. Fine particles are for example noil (fines), parenchymatous cells and pieces

of fiber. The term line refers to any pipe, duct or channel suitable for transferring a solution or a suspension.

Figure 1 shows schematically an apparatus according to the invention for removing extractives from a suspension containing fibers. The pulp from the pulping process is conveyed via a line 1 to separating means, i.e. a separator. The separator is a screen or a precipitator, for example a screw precipitator that functions as a pre-precipitator that is capable of separating the fines-containing suspension from the pulp. As a sepa- rator it is possible to use for example a screen, a sieve, or a filter. From the separator 2 the fines containing suspension is conveyed via a line 3 to a tank 4. The main pulp flow containing coarser material is conveyed forward in the pulping process via a line 5. Before the tank 4 an additive is led to the line 3 via first means for adding additive, i.e. an addi- tive line 6. The additive makes the extractives in the fines containing suspension more easily soluble to the solution contained in the suspension. The tank 4 functions as a retention tank that guarantees sufficient reaction time for the additive and the extractives. From the tank 4 the fines-containing suspension that has reacted with the additive is conveyed by means of a pump 7 via a line 8 to the main pulp flow. Part of the suspension that has reacted with the additive can be conveyed to the pulp in line 1 , before the separator 2.

The solubility of the extractives to the solution contained in the fines- containing suspension can be improved by treating the fines containing suspension so that the size of the particles contained therein is reduced and adding additive to the very fine particles containing suspension thus obtained. The treatment is effected by a treatment device 9 succeeding the first point of additive addition through an additive line 6. As a result of the treatment the parenchymatous cells contained in the fines break, wherein the extractive comes in contact with the additive. The treatment device 9 can be a refiner or a disperser shown schematically in Figs 1 and 2 with broken lines. After the treatment, an additive is conveyed to the suspension via second means for adding additive, i.e. a second additive line 10, which is also marked with broken lines in Figs 1 and 2.

If the apparatus comprises the above-described treatment device 9, the additive is fed both via the first additive line 6 and the second additive line 10 to the fines-containing suspension travelling in the line 3. How- ever, the amount of the additive does not increase when compared to the above-described method of removing extractives without the treatment of the fines-containing suspension, because the amount of additive is divided in such a manner that a part of the additive is fed via the first additive line 6 and the rest of the additive is fed via the second additive line 10. It is also possible that the entire amount of additive is fed only via the second additive line 10 and no additive is supplied to the fines-containing suspension before its treatment. If desired, it is also possible to feed the entire amount of additive to the fines-containing suspension via a first additive line 6 before the treatment of the suspension.

Fig. 2 shows another embodiment in which the dry matter content of the fines-containing suspension is increased before it is conveyed to treatment. In this embodiment, the fines-containing suspension ob- tained from the separator 2 is conveyed via a line 11 to a solution separator 12 which is a device removing solution from the suspension, such as a filter or a drum filter. The purpose of the solution separator is to increase the dry matter content of the fines-containing suspension led to the treatment device 9, wherein its treatment with the treatment device 9 is easier and more efficient. As a result of the reduction in the solution volume it is also possible to reduce the dimensions of the pipeworks and tank used in treating the fines-containing suspension.

From the solution separator 12 the fines-containing suspension having higher dry matter content is conveyed via a line 13 to the treatment device 9 and further to the tank 4. To remove the extractives, an additive is conveyed to the suspension before the treatment device 9 via the additive line 6 and/or after the treatment device 9 via the additive line 10. From the tank 4 the suspension that has reacted with the additive is conveyed by means of a pump 7 via a line 8 to the main pulp flow. The solution separated from the fines-containing suspension and obtained

from the solution separator 12 is conveyed via a line 14 out of the solution separator 12. According to the need, the solution separated from the fines-containing suspension can be conveyed either into the line 5 to the coarser material containing main pulp flow that is obtained from the separator 2 or via the line 15 to the pulp flowing in the line 1 , before the separator 2. It can also be conveyed via the line to the tank 4 as dilution water.

The purpose of the tank 4 disclosed in all the above-described em- bodiments of the invention is to guarantee sufficient retention time for the extractives in the fines-containing suspension to react with the additive added to the suspension. The reaction time varies from a couple of seconds to a couple of minutes. The size of the tank 4 depends on the amount of suspension to be treated and the point of adding additive before the tank. If the adding of the additive takes place far enough before the tank 4, the reaction with the additive and the extractives takes place already before the tank, wherein the tank functions as a storage or dilution tank for the rest of the process.

The separation of the fines-containing suspension from the hardwood pulp and the addition of the additive thereto can be done at any stage of the pulping process. It can be conducted either after the cooking, after the blow tank following the digester, before or after the bleaching stages, or in between bleaching stages. It can also be conducted before or after the secondary classification of the pulp. The most advantageous point to separate the fines-containing suspension from the pulp is after the knot screening following the cooking. The most essential aspect is that the reactions of the extractives after the separation take place in alkaline conditions. Thus, the fines-containing suspension must be alkaline, which is typically not a problem in pulping processes, in which process solutions are often alkaline. If the apparatus for removing extractives according to the invention is placed at that stage of the pulping process in which the alkalinity of the fines- containing suspension would be too low for extractive removal, it is possible to increase the alkalinity of the suspension by adding sodium

hydroxide or white liquor in the fines-containing suspension before adding the additive.

The invention can be applied in pulps produced of hardwood such as birch, asp, alder, maple, or a mixture of these, and eucalyptus, acacia, and so-called mixed tropical hardwood. The invention can also be applied in pulps produced of annual plants, such as straw, reed canary grass, reed and bagasse.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims.