WO2007063182A2

WO2007063182A2 - Method for manufacturing paper

Info

Publication number: WO2007063182A2
Application number: PCT/FI2006/050522
Authority: WO
Inventors: Juan Cecchini; Antti Poikolainen; Hannu LEPOMÄKI; Petteri Soini
Original assignee: Metso Paper, Inc.
Priority date: 2005-12-01
Filing date: 2006-11-28
Publication date: 2007-06-07
Also published as: EP1954873A2; CN101321910A; WO2007063182A3; FI20055635A0; CN101321910B; ATE470753T1; EP1954873B1; FI20055635A; DE602006014861D1

Abstract

A fiber raw material is fractionated into a long fiber fraction, which is rich in long fibers, and a fine fiber fraction, which is rich in short fibers and fines, after which additives are mixed into the fiber fractions. The long fiber fraction and the fine fiber fraction are combined to form a mixed stock while controlling the mixing ratio in order to keep the fines content of the mixed stock at a desired value. Additives are mixed into the mixed stock after which paper is manufactured from the mixed stock on a paper machine.

Description

Method for manufacturing paper

The invention relates to a method for manufacturing paper or board from a fiber raw material.

The properties of paper or board are to a great extent determined by the type of the fiber raw material used and the manner in which the paper machine and the web forming process treat these raw materials. Only a few paper grades are produced from a single fiber type alone. In most cases, at least two types of fiber raw material are used for paper. Several paper grades, in particular those intended for printing, contain considerable amounts of mineral fillers. Moreover, dry-strength additives, sizes, retention aids and several other papermaking chemicals are often added to the paper stock in an amount of a few percent. The percentage of the latter kind of additives is, however, small as compared with the amount of fillers, whose proportion is most generally 10 to 40% of the weight of the paper.

The stock used for making paper is a complex, multi-phase mixture of fibers, fines, fillers and other additives. The properties of all these components are continually changing, which may lead to upsets in the process. To make matters more complex, a significant portion of the paper stock does not become paper in its first pass down the production line. The amount of broke and white water that must be recycled varies considerably. Dewatering of the stock on the wire section may cause an amount of valuable components, above all fines and fillers, to be lost along with the white water removed from the stock through the forming wires, resulting in poor retention.

Pulps used as fiber raw material are in general more or less heterogenous. The fibers of pulp are widely differing in their morphology, mechanical properties and chemical composition, some of them having a high papermaking potential that can be exploited as such for the production of high quality papers, while others have no potential at all unless extensively processed. Appropriate classification of pulp into grades having different properties is considered an attractive possibility for utilizing its full potential. Savings in energy, chemicals and raw material costs can be envisaged, and further mechanical treatment (e.g. refining) can be focused selectively on those fibers that need it, while the chemical treatment of each fraction (e.g. bleaching) can be performed with the proper agents and dosages. Thanks to fiber fractionation, pulp quality targets can be achieved with lower energy and/or chemical consumption and without damaging the fibers through excessively se- vere processing.

US 5,133,832 discloses a process for preparation of waste paper stock. After pulping and cleaning, the fiber raw material is fractionated by filtering through a continuously moving mesh belt of a predetermined mesh size which will retain sub- stantially all of the fibers of greater than a predetermined length as a first fraction, a second fraction comprising the majority of the liquid component of the feed suspension along with small contaminant particles and the remaining fines. These two fractions are then treated separately by bleaching the first fraction and recovering the fines from the second fraction, after which the fines and the first fraction are recombined for further treatment and use.

US 4,731,160 discloses a method and apparatus for the bleaching of mechanical pulp. Mechanical pulp directly from a secondary refiner is separated by first and second centrifugal separators into a fines fraction, which is under about 200 mesh, and into a fiber fraction. The fiber fraction and the fines fraction are then bleached separately, after which they are recombined before passage to a paper products production machine. Because of the poor drainage characteristics of the fines it would be impractical to use displacement bleaching or washing techniques with the fines fraction. US 4,781,793 discloses a method for improving properties of paper, in particular its retention. In the method, a basic stock is divided into two separate fractions on the basis of fiber length and the fibers in each fraction are treated in accordance with specific objectives. The stocks are then passed into distinct layers of a multi- channel headbox.

In the long run the composition of certain fiber raw materials, such as hardwood pulps, recycled fibers or agricultural fibers, may vary significantly. Such variations tend to cause fluctuations in the properties of the paper and problems in the runnability of the paper machine. Another problem resulting from varying fines content is the difficulty of obtaining a stable and constant level of retention.

Therefore, it is an object of the present invention to improve the management of fiber fractions in papermaking.

In order to attain this object, the new method comprises the following steps:

- fractionating the fiber raw material into a long fiber fraction, which is rich in long fibers, and a fine fiber fraction, which is rich in short fibers and fines, - mixing additives into the fiber fractions,

- combining the long fiber fraction and the fine fiber fraction to form a mixed stock while controlling the mixing ratio in order to keep the fines content of the mixed stock at a desired value,

- mixing additives into the mixed stock, and - manufacturing paper from the mixed stock on a paper machine.

Management of fiber fractions is most desirable when fiber raw material is being used whose fines content is naturally suffering from serious fluctuations. In order to obtain stable and relatively constant fines content and fiber length distribution, the mixing ratio of the fine fiber fraction to the long fiber fraction should be kept constant in spite of any fluctuations in the composition of the fiber raw material. The advantages achieved by fractionation, separate treatment of fractions and controlled recombination of the treated fractions are quite many depending on the character of the treatment and the stage at which the fractionation is carried out. Generally speaking, the results are the better the earlier in the process the fractionation and the separate treatment of long and fine fiber fractions are carried out.

Most preferably, fractionation is carried out at the stock preparation department before the different fiber raw materials are mixed to form a mixed stock. Typi- cally, more than one fiber raw material is used. It may be sufficient that only the most difficult fiber raw material, which has the largest variations in its fiber and fines content, is fractionated. Alternatively, two or more of the fiber raw materials may be fractionated separately, after which the long fiber fractions are combined and treated together and the fine fiber fractions are combined and treated together. Alternatively, the fiber raw materials may be fractionated as a mixture of different fiber raw materials. In that case fractionation may be carried out as late as in the approach flow system of the paper machine.

Fractionation may be carried out by any suitable means, using e.g. hole, slot or wire mesh screens, gap washers, hydrocyclones, etc. It is advisable that the fractionation system is modular so that it comprises two or more steps.

Refining only the long fiber fraction will produce savings in the form of smaller size of refiner equipment and lower consumption of energy. It may also improve the strength of the paper.

Additives that are mixed into the long fiber fraction, fine fiber fraction and/or mixed stock comprise conventional papermaking chemicals, such as fillers, anionic trash catchers, dry-strength additives, sizing agents, retention aids, etc. By choosing properly the place and the process stream into which each additive is mixed, the performance of additives can be improved. Anionic trash comprises anionic dissolved polymeric and colloidal materials, which mostly originate from wood raw material and get released during pulping. Anionic trash interferes with the performance of cationic retention aids, cationic dry-strength agents, and wet-strength resins. Adding fixatives before the dosage of cationic starch has been found to produce considerable synergistic benefits. One of the most common benefits is better adsorption of starch onto fibers and fillers, resulting in higher overall retention. The strength of paper is also improved because the starch is to a greater degree adsorbed into the fibers and in a lesser de- gree into the anionic trash.

After fractionation, most of the anionic trash is contained in the fine fiber fraction. In the system according to the present invention, problems due to anionic trash are avoided by mixing anionic trash catchers (ATC) into the fine fiber fraction. Treat- ing only the fine fiber fraction enables reduction of the ATC dosage. The efficacy of the ATC adsorption is improved by prolonged retention in a fine fiber storage.

Cationic starch is primarily used for increasing dry-strength of the paper but it also affects retention and sizing. Due to their large specific surface area, fines and fillers tend to adsorb most of the cationic starch added to a papermaking stock. It has been found that the strength properties of paper may be improved by adding a major part of the starch to the long fiber fraction and only a minor part of the starch to the fine fiber fraction, instead of adding the whole dosage of the starch to the mixed stock. The dose mixed into the long fiber fraction may be 50 to 99%, preferably 70 to 80% of the dry-strength additive, and the dose mixed into the fine fiber fraction or to the mixed stock may be 50 to 1%, preferably 20 to 30% of the dry-strength additive.

Likewise, the sizing performance can be improved by dosing part of the sizing agent into the long fiber fraction and part of the sizing agent to the fine fiber fraction. Poor retention may be a big problem when producing paper that has a high content of filler. Retention can be improved by mixing part of the filler with the fine fiber fraction and by adding retention aids, such as starch, to the mixture. As a result, fines and filler are agglomerated or pre-coagulated, which promotes their retention during the dewatering process.

Measuring the average fiber length of the long fiber fraction and the fine fiber fraction gives information about the performance of the fractionation devices and the long fiber refiner. It is also possible to control the mixing ratio of the fine fiber fraction to the long fiber fraction on the basis of the average fiber length measured from the long fiber fraction and the fine fiber fraction.

Furthermore, the mixing ratio of the fine fiber fraction to the long fiber fraction may be controlled on the basis of retention measured from the wire section of the paper machine. Controlling the addition of the fine fiber fraction is a new way of managing retention on the paper machine.

The new system can be applied to greenfield installations, i.e. new projects, as well as to rebuilds. In greenfield installations, fiber fractionation can be carried out early enough in a most suitable way for each particular type of fiber raw material. In that case the capacity and the energy requirements can be adjusted to particular type of fiber raw material.

A second option is to carry out fiber fractionation after the different fiber raw materials have been mixed into a paper stock. Most probably this would require a different degree of separation efficiency as well as multi-stage screening or a suitable device according to the type of raw material, e.g. Gap Washer™. Sometimes it might be advantageous to refine the different fiber raw materials together, after which the long and fine fiber fractions may still be exposed to separate chemical treatments. A third option is to carry out fiber fractionation in the approach flow system after the thick stock has been diluted to the headbox consistency. In that case the possibility of separate refining of the long fiber fraction is lost. This kind of arrange- ment is mainly focused on improving retention. The strength properties of paper may also be somewhat improved by selective addition of dry-strength additives. A moderate anionic trash control can probably be obtained, too.

In rebuild cases fiber fractionation can be carried out in a modular way, adapted to the particular requirements. For example, fractionation may be performed on the most demanding fiber raw material in the process. All the options discussed above in connection with a greenfield case are also valid in a rebuild case.

The inventive concept can as well be applied to production of multi-ply paper by using a multi- layer headbox. In that case the outer layers of the web may be produced from mixed stock having a higher percentage of fine fiber fraction in order to give the paper improved surface properties whereas the middle layers of the web are produced from stock having a higher percentage of long fiber fraction in order to improve the strength of the paper.

There is a continuous need to reduce costs and to improve quality in an integrated pulp and paper mill that produces SC paper. High consistency refining is an important source of cost, and so is the raw material, too. Replacing HC refiners with LC refiners could lead to a 50% reduction in energy consumption when aiming at the same freeness level. Fiber properties may be somehow different depending on the type of refiner used and the procedure that precedes the low consistency refiner. Another source of saving could be introduction of cheaper raw materials, such as deinked pulp (DIP), recycled fiber (RCF) or hardwood bleached kraft pulp, in a suitable percentage, which would lead to improvements in quality and efficiency. According to one embodiment of the invention, fractionation may be adjusted in accordance with the fines content of the fiber raw material, after which the two fiber fractions are refined separately and selectively in order to improve their desired properties. The long fiber fraction may be refined in a conical refiner, the aim being to reduce the amount of coarse fibers and to improve the strength properties of the paper. The fine fiber fraction may be refined in a double disc refiner employing low refining intensity, the aim being to improve the wet strength of the paper. Both refiners may operate at low consistency so that energy is saved in comparison with conventional HC refining.

Fractionation may be carried out in two steps so that the first step comprises a screening device, the separating capacity of which can be adjusted according to the fines content of the fiber raw material, and the second step comprises hydro- cyclones, arranged to separate fibers according to their coarseness. From the hy- drocyclones the coarser fibres are passed to a conical refiner and the finer fibers are mixed with the accept fraction of the preceding screening stage. When the fiber raw material comprises DIP, the second step enables separation and selective treatment of chemical and mechanical fibers. When the fiber raw material comprises only mechanical pulp, the second step enables separation and separate treatment of mechanical fibers according to their coarseness and stiffness.

After fractionation the two fiber fractions comprise fibers that are different in their properties and that would benefit of different kinds of treatment. It is preferable to refine the long fiber fraction that is rich in coarse and stiff fibers in a conical re- finer, after which dry-strength additive should be mixed with the long fibers. The aim of this treatment is to reduce the coarseness of the fibres and to improve the dry strength and smoothness of the paper. It is as well preferable to refine the fine fiber fraction that is rich in fines and short, flexible fibers in a double disc refiner employing low refining intensity. By refining the fine fiber fraction with low en- ergy consumption and by adding selected papermaking chemicals (e.g. fixatives, retention aids, fillers) to this fraction it is possible to improve the wet strength of the paper web, thus improving the runnability of the process, and the smoothness of the paper.

The effects attainable by fractionation and selective treatment of fiber fractions may be further improved by measuring fiber length distributions at different points of the process and by controlling the operation of certain process devices on the basis of these measurements. The operation of the fractionation device may be controlled on the basis of the fiber length distributions measured from the input flow, the reject flow (long fiber fraction) and the accept flow (fine fiber fraction). It is also possible to feed an adjustable part of the long fiber fraction back to the fine fiber fraction on the basis of the measured fiber length distributions. Moreover, the operation of the double disc refiner may be controlled on the basis of fiber length distributions measured from the stock before and after the refiner.

It is possible to further reduce the raw material costs in the production of SC paper by using cheaper raw materials such as hardwood bleached kraft pulp (e.g. eucalyptus, birch). Including hardwood bleached kraft pulp in the paper stock in a suitable percentage will lead to improvements in quality (smoothness and strength) and in production efficiency (wet strength and runnability). Part of the added hardwood bleached kraft pulp may be refined together with the fine fiber fraction whereas the major part of the hardwood pulp is utilized without ordinary refining. The addition of hardwood bleached kraft pulp is proposed to be 8 to 15% of the main pulp mass flow (GW or TMP). The portion passed to refining could be 20 to 30% of the total addition and the portion added to the final blend without refining could be 70 to 30% of the total addition of hardwood bleached kraft pulp.

In the following, the invention will be described with reference to some preferred embodiments of the invention illustrated in the figures of the accompanying drawings. The invention should not, however, be confined to the illustrated or de- scribed embodiments of the invention alone. Fig. 1 is a flow diagram illustrating one embodiment of the invention.

Fig. 2 is a flow diagram illustrating another embodiment of the invention.

Fig. 3 is a flow diagram of a system in which fractionation is carried out after mixing of different fiber raw materials.

Fig. 4 illustrates the principle of multi-phase fractionation.

Fig. 5 illustrates adjustable fractionation followed by selective refining and chemical treatment of the fractions in an integrated SC paper mill.

Fig. 6 illustrates another modification of the inventive process for an integrated SC paper mill.

Fig. 1 shows a system that aims at improved control of fiber fractions in paper- making. The system is suitable in particular for TMP and chemical pulp having a broad fiber length distribution.

A fiber raw material 10 is passed to fractionation equipment F where it is divided into two fractions. A first fraction 12, which is rich in long and coarse fibers, is passed to a long fiber storage 13. A second fraction 16, which is rich in short fibers and fines, is passed to a fine fiber storage 17. An example of possible fractionation equipment F is shown in Fig. 4.

The long fibers from the long fiber storage 13 are passed to a low consistency refiner 20 in order to refine the fibers to make them more suitable for papermak- ing. Chemical pulps are typically refined before they are passed to the paper machine. If no further refining is needed, which is the case with TMP, the refiner 20 may be omitted or bypassed. Refining only the long fiber fraction gives reduced size of equipment needed as well as savings in energy consumption. After fractionation, most of the anionic trash brought along by the fiber raw material 10 is either retained in the fine fiber fraction 16 or washed away during the fractionation. Problems caused by anionic trash can be avoided by adding fixa- tives, such as anionic trash catchers (ATC), to the fine fiber fraction 16. In the present case, fixatives 21 are added to the fine fiber fraction 16 at a point preceding the fine fiber storage 17.

Dry-strength additive 22, most often cationic starch, is added to the long fiber fraction 23 at a point that precedes its mixing with the fine fiber fraction. In the present case, the main part of the cationic starch, about 75% of the total amount, is added to the long fiber fraction, and a minor part, about 25% of the total amount, is added either to the fine fiber fraction or to the mixed stock. This arrangement ensures that an adequate amount of starch may be adsorbed by the long fiber frac- tion. Splitting the starch dosage in two parts that are added to different fiber fractions increases the efficiency of starch as a dry-strength additive.

Next, the long fiber fraction 23, the fine fiber fraction 24 and broke 25 recycled from the paper machine are combined to produce a mixed stock. Fig. 1 shows a blending chest 26, which is not, however, compulsory. The different stock components 23, 24 and 25 and filler 27 are mixed, either in the blending chest 26 or in a tube mixer (OptiFeed™), into a thick stock 31. After mixing the thick stock 31 is passed to the approach flow system, either via the blending chest 26, a control refiner 28 (optional), and a level box 29, or directly as indicated by a broken line 30 (OptiFeed™). In this connection, sizing agents 32 are added to the thick stock 31.

A control block 33 is arranged to control the mixing ratio of the fine fiber fraction 24 to the long fiber fraction 23. The target is to achieve stable and constant feed of fine fibers and to compensate for any fluctuations in the composition of the original fiber raw material 10. The control block 33 may be connected to an optional measuring device 34 that is arranged to indicate the average fiber lengths of the long fiber fraction 23 and the fine fiber fraction 24.

In the approach flow system, the thick stock 31 is first diluted with white water, which is taken from a wire pit 35. In an alternative system (OptiFeed™), indicated by line 36, the thick stock 31 is diluted in a tube mixer. Diluted stock 37 is then fed by a first pump 38 to centrifugal cleaning equipment 39 and further to a deaerator 40. After dearation the stock is fed by a second pump 41 to a machine screen 42 and finally to a headbox 43. The headbox 43 distributes the diluted stock to a wire section 44, where water is drained from the stock through forming wires in order to form a paper web, which is further transported to a press section and a dryer section (not shown).

Filler is contained in a filler storage 45, from which it is added either to the di- luted stock in the approach flow system, as indicated by reference number 46, or to the thick stock in the pulp preparation department, as indicated by reference number 27. Alternatively, the filler dosage may be split into two parts, the first part 46 being fed to the thick stock and the second part 27 being fed to the diluted stock.

Retention aids 47, 48 are added to the diluted stock flow in two steps in such a way that the first retention aid component 47, e.g synthetic cationic polyacryla- mide (C-PAM), is added before the machine screen 42 and the second retention aid component 48, e.g. nanoparticles, is added after the machine screen 42.

Fig. 2 illustrates another system that aims at improved fines and filler retention when the fine fiber content of the fiber raw material is fluctuating. The system is suitable for chemically dirty fiber raw materials, such as thermomechanical pulp (TMP), deinked pulp (DIP), recycled fiber pulp (RCF), and chemical pulp with bleaching residues. The fiber raw material 10 is fractionated by means of fractionation equipment F. The long fiber fraction 12 is passed to a long fiber storage 13 and the fine fiber fraction 16 is passed to a fine fiber storage 17. If needed, anionic trash catchers 21 are added to the fine fiber fraction 16 at a point preceding the fine fiber storage 17. If needed, the long fiber fraction is passed to a low consistency refiner 20. Alternatively, refining may be omitted as indicated by line 50. Dry-strength additive 22, such as cationic starch, is added to the long fiber fraction at a point preceding a blending chest 26. After that the long fiber fraction 31 ' is passed to the approach flow system, either via the blending chest 26, an optional further refiner 28 and a level box 29, or directly via a pipe 30.

In this embodiment, the long fiber fraction and the fine fiber fraction are not combined until at the approach flow system of the paper machine. The two fiber fractions may be mixed either in connection with a wire pit 35 or in a tube mixer, in which case the wire pit 35 may be bypassed or omitted as illustrated by line 36.

Filler is added to the paper stock in two parts. A first part 51 of the filler is mixed into the fine fiber fraction in a blender 52 while a second part 46 of the filler is mixed into the diluted paper stock in the approach flow system. Preferably, 20 to 50% of the filler is fed to the blending chest 52, and 50 to 80% of the filler is fed to the mixed stock in the approach flow system. Furthermore, starch 53 or some other cationic polymer suitable for pre-coagulation is fed to the fines and filler blender 52. By blending filler and fines and by adding starch to this blend it is possible to pre-agglomerate the fines and filler and thus improve their retention on the paper machine. When starch is used as a pre-coagulant, the amount of starch fed to the fine fiber fraction is about 25% of the total dosage while the amount of starch 22 fed to the long fiber fraction is about 75% of the total dosage.

Furthermore, the system comprises a retention control block 54, which receives consistency information from the headbox 43 and from the wire section 44. On the basis of the gathered information the control block 54 controls, on the one hand, mixing of the stream 55 of pre-coagulated fines and filler with the long fiber fraction 31 ' and, on the other hand, addition of retention aids 47 to the diluted stock 56. This provides twofold retention control. Controlling the amount of the fine fiber fraction 55 represents coarse tuning of retention, whereas controlling the addition of retention aids 47 represents fine-tuning of retention.

Sizing agents 32, 57 are added separately to the long fiber fraction 31' (flow 32) and to the fine fiber fraction 55 (flow 57). Separate addition points allow division of the dosage in two parts as desired. This is recommended especially in the case of fine paper with high PCC addition to evaluate the sizing reactivity.

The ash content of the paper may be controlled by means of a control block 58 which receives information about the ash content of the dried paper and controls the addition of filler 46 to the diluted stock in the approach flow system.

Fig. 3 illustrates a system that is suitable for pulp blends with a constant or variable fines content. Fiber raw materials suitable for this system include TMP, DIP, RCF and chemical pulp with bleaching residues.

In this method, different fiber raw materials 60 and 61 and broke 25 are first blended in a blending chest 62, after which the blended stock 63 is fractionated by fractionation equipment F. Fractionation can be carried out by any suitable fractionation method known as such.

After fractionation the long fiber fraction 12 is passed to a low consistency refiner 20, where the long fibers are refined to make them more suitable for papermaking. If no refining is needed, refiner 20 may be omitted or bypassed as indicated by dashed line 50. Dry-strength additive 22, such as starch, is added to the long fiber fraction, after which the long fibers are fed to a long fiber storage 64. Anionic trash catchers 21 are added to the fine fiber fraction 16 before it enters a fine fiber storage 17. Fine fibers from the fine fiber storage 17, filler 51 from the filler storage 45 and pre-coagulation agent 53, such as starch or cationic polymer, are blended in a fine fiber and filler blender 52 in order to pre-coagulate filler and fines and to improve their retention on the paper machine.

The long fiber fraction 31' and the fine fiber fraction 55, which also contains flocculated filler, are mixed and diluted in the approach flow system to produce diluted paper stock 37 that is fed to the paper machine. Before mixing the different fiber fractions, sizing agents 32, 57 are separately added to both fractions 32. The retention control system 54 and the ash control system 58 are similar to those described above in connection with Fig. 2.

Fig. 4 shows one example of equipment that may be used for fractionation of fiber raw material. The fractionation equipment F comprises two fractionation devices 11 and 15 which may be of any type that is able to divide the fiber raw material into two fractions having different average fiber lengths. As an example, the first fractionation device 11 may be a coarse screen of 100 or 125 mesh while the second fractionation device 15 is a fine screen of 200 mesh. The coarse fraction 12 from the first fractionation device 11 is passed to a long fiber storage while the fine fraction 14 is passed to the second fractionation device 15 for further purification. From there the purified fine fiber fraction 16 is passed to a fine fiber storage while the filtrate 18 is recycled for further use. A minor stream of long fibers 19 may be passed to the second fractionation device 15 to act as a sweetener that en- ables filtering by forming a cake that retains the fines on the screen.

Fig. 5 illustrates a system that aims at improving the properties of supercalendered (SC) paper and the production of it. The fiber raw material 70 to be fractionated in this process comprises groundwood (GW), thermomechanical pulp (TMP), deinked pulp (DIP), or a mixture of them. The fiber raw material 70 is passed to a fractionation device 71 that divides the pulp into two fractions 72 and 73. Advantageously, the fractionation device 71 is a screen the speed of rotation of which is adjustable according to the fines content of the fiber raw material 70. A first fiber fraction 72, which is rich in long and coarse fibers, is passed to hydrocyclones 74 that comprise one or more separation steps. A second fraction 73, which is rich in short fibers and fines, is passed to a fine fiber storage bin 75.

When the fiber raw material 70 comprises deinked pulp, hydrocyclones 74 are used for separating the fibers of chemical pulp from those of mechanical pulp. The more resilient fibers 76 of chemical pulp are passed to the fine fiber storage bin 75 and the stiffer fibers 77 of mechanical pulp are passed to a conical refiner 78 for further refining. On the other hand, when the fiber raw material 70 comprises mechanical pulp, hydrocyclones 74 are used for separating coarse and stiff fibers from flexible and resilient fibers. Also here the more resilient fibers 76 are passed to the fine fiber storage 75 and the coarser fibers 77 are passed to the conical refiner 78 for further refining.

The conical refiner 78 operates at low consistency, reducing shives and coarseness of the long fiber fraction. After refining the long fibers are passed to a long fiber storage bin 79 where dry-strength additive 80, such as starch, is added to the stock. In this way the strength and smoothness of the paper can be improved by reducing the coarseness of the long fibers and by adding starch to the fraction that benefits most from it.

The fine fiber fraction 73, supplemented with resilient fibers 76 from the hydrocyclones 74, is passed to a double disc refiner 81 that acts at low intensity and low consistency. After refining the stock is passed to a container 82 where fixatives 83 (anionic trash catchers) are added to the stock. In the next container 84 fillers 85 and retention aids 86 are added to the stock. Finally the two fiber fractions 87 and 88 that have been refined separately and that contain different additives are mixed in a mixing device 89, after which the mixed stock 90 is fed to the paper machine.

Fig. 6 shows another system that aims at improving the properties of SC paper and the production of it. The fiber raw material 70 to be fractionated in this process is groundwood or TMP produced in the paper mill. Additionally, hardwood bleached kraft pulp is blended with the fiber raw material in order to improve the properties of the SC paper.

The fiber raw material 70 is passed to a fractionation device 71 that divides the pulp into two fractions 72 and 73. The first fraction is a long fiber fraction 72 that is rich in long and coarse fibers and the second fraction is a fine fibre fraction 73 that is rich in short fibers and fines. A measuring device 91 is arranged to measure the fiber length distributions of the fiber raw material 70, the long fibre fraction 72 and the fine fiber fraction 73. The speed of rotation of the fractionation device 71 may be adjusted on the basis of the fines content of the fiber raw material 70 in order to adjust the division of fibers into the long fiber fraction 72 and fine fiber fraction 73.

A major part of the long fiber fraction 72 is fed to a conical refiner 78 whereas a minor part of the same is joined with the fine fiber fraction 73. The conical low consistency refiner 78 is used for reducing the proportion of coarse fibers and shives in order to improve the strength and smoothness of the paper. Preferably the portion of long fibers passed to the fine fiber storage bin 75 is 10 to 20% of the total input of fiber raw material 70.

The amount of hardwood bleached kraft pulp added to the process is advantageously 8 to 15% of the amount of fiber raw material 70. The chemical pulp is slushed in a pulper 92, deflaked in a deflaker 93 and fed to a chemical pulp storage bin 94. Preferably, 20 to 30% of the hardwood bleached kraft pulp is then fed to the fine fiber storage bin 75 whereas the rest of the pulp is fed directly to a final mixer 95.

In the fine fiber storage bin 75 the fine fiber fraction 73 is mixed with a portion of the long fiber fraction 72 and a portion of the hardwood bleached kraft and fixatives 96 are added to the stock. Next the stock is passed to a double disc refiner 81 for low intensity refining carried out at a low consistency. After refining retention aids 97 are added to the stock by means of an mixing device 98, such as a Re- taMix™ retention aid mixer, produced by Metso Paper, Inc.

A measuring device 99 is arranged to measure the fiber length distribution of the hardwood bleached kraft pulp and of the stock before and after the double disc refiner 81. The operation of the double disc refiner 81 is controlled on the basis of the fiber lengths measured.

Dry-strength additive 100, such as starch, is added to hardwood bleached kraft pulp before it is fed to the final mixer 95. The mixing device 101 is preferably a RetaMix™ retention aid mixer of Metso Paper, Inc.

The final mixer 95 that mixes the refined long fibers 87, hardwood bleached kraft pulp 102 and mixed stock 88 is preferably a LobeMix™ stock mixer, which is produced by Metso Paper, Inc. Fillers 103 are also added to the stock at this point.

Instead of a tube mixer, conventional mixing in connection with a wire pit may also be used at this point.

The hardwood bleached kraft pulp is preferably made out of birch or eucalyptus.

This kind of pulp is cost-effective and able to improve the strength and optical properties of the paper produced. The examples provided above are not meant to be exclusive but many other variations of the present invention, which would be obvious to those skilled in the art, are contemplated to be within the scope of the appended claims.

Claims

1. Method for manufacturing paper or board from a fiber raw material, characterized by the steps of - fractionating the fiber raw material into a long fiber fraction, which is rich in long fibers, and a fine fiber fraction, which is rich in short fibers and fines,

- mixing additives into the fiber fractions, combining the long fiber fraction and the fine fiber fraction to form a mixed stock while controlling the mixing ratio in order to keep the fines content of the mixed stock at a desired value,

- mixing additives into the mixed stock, and

- manufacturing paper from the mixed stock on a paper machine.

2. Method according to claim 1, characterized by using two or more fiber raw materials and fractionating only one of the fiber raw materials.

3. Method according to claim 1, characterized by using two or more fiber raw materials and fractionating at least two of them separately, after which the long fiber fractions of the fiber raw materials are combined and the fine fiber fractions of the fiber raw materials are combined.

4. Method according to claim 1, characterized by using two or more fiber raw materials and fractionating a mixture of said fiber raw materials.

5. Method according to any one of claims 1 to 4, characterized by adding anionic trash catchers to the fine fiber fraction in order to control anionic trash.

6. Method according to any one of claims 1 to 5, characterized by refining the long fiber fraction.

7. Method according to any one of claims 1 to 6, characterized by adding a major part of dry-strength additive to the long fiber fraction and a minor part of the dry- strength additive to the fine fiber fraction or to the mixed stock.

8. Method according to claim 7, characterized by dosing 50 to 99%, preferably 70 to 80% of the dry-strength additive to the long fiber fraction and 1 to 50%, preferably 20 to 30% of the dry-strength additive to the fine fiber fraction or to the mixed stock.

9. Method according to any one of claims 1 to 8, characterized by adding sizing agent to the long fiber fraction and to the fine fiber fraction.

10. Method according to any one of claims 1 to 9, characterized by adding a first part of filler to the fine fiber fraction and a second part of filler to the mixed stock.

11. Method according to claim 10, characterized by adding 20 to 50% of the filler to the fine fiber fraction and 50 to 80% of the filler to the mixed stock.

12. Method according to claim 10, characterized by blending fillers and pre- coagulation agents with the fine fiber fraction in order to flocculate the filler and fines.

13. Method according to any one of claims 1 to 12, characterized by measuring average fiber lengths from the long fiber fraction and from the fine fiber fraction and controlling the mixing ratio on the basis of the measurements.

14. Method according to any one of claims 1 to 12, characterized by measuring retention on the wire section of the paper machine and controlling the mixing ratio of the fine fiber fraction to the long fiber fraction on the basis of the measured retention.

15. Method according to claim 1, characterized by separate and selective refining and chemical treatment of the long fiber fraction and the fine fiber fraction.

16. Method according to claim 15, characterized by refining the long fiber frac- tion at a low consistency in a conical refiner (78) and refining the fine fiber fraction at a low consistency in a double disc refiner (81) employing low refining intensity.

17. Method according to claim 1, characterized by adjusting the operation of the fractionation device (71) on the basis of the fines content of the fiber raw material

(70).

18. Method according to claim 17, characterized by two-step fractionation where the first step is carried out by a screen (71) and the second step is carried out by hydrocyclones (74).

19. Method according to claim 18, characterised by adjusting the speed of rotation of the screen (71) according to the fines content of the fiber raw material (70).

20. Method according to claim 1, characterized by measuring the fiber length distributions of the fiber raw material (70), the long fiber fraction (72) and the fine fiber fraction (73) and controlling the operation of the fractionation device (71) on the basis of the measured fiber length distributions.