METHOD IN FORMING HIGH CONSISTENCY PULP IN A PAPER OR BOARD MACHINE AND A CORRESPONDING HIGH CONSISTENCY PULP FORMER OF A PAPER OR BOARD MACHINE This invention relates to a method in forming high consistency pulp in a paper or board machine, wherein high consistency pulp, with the headbox consistency exceeding 1. 5t, is supplied onto the forming fabric in a section between the successive rolls of a roll assembly supporting it using a sliding element adapted to contact the forming fabric, wherein the forming high. consistency pulp layer is thereafter prepressed with the form- ing fabric to form the web. The invention also relates to a corresponding high consistency pulp former of a paper or board machine.

Today the paper machine formers are so called gap formers, wherein the gap is formed between two rolls supporting the forming fabric. Pulp is supplied from the headbox slice channel over an open draw to the gap between the two forming fabrics.

Between the forming fabrics, water is removed from the pulp, after which the web is guided to the actual pressing.

These gap formers and other formers can be used to make sin- gle-layer web only. When it is desired that the web has more than one layer, it is necessary to place several formers one after the other. That is, pulp is supplied onto the previously formed web using a second former to obtain a double-layer web.

Another method is to join the webs formed with different formers only afterwards. The end product quality in the methods presented above is, however, uneven. For example, the paper sides are different and the bonds between the layers are weak.

These have an adverse effect on, for example, paper printing and reduce the paper strength. In addition, the gap former requires a low pulp consistency. Furthermore, several formers

and in particular their forming fabrics require a large mount- ing space as well as a lot of maintenance.

A Finnish publication print No. 69329 sets forth a paper ma- chine former differing from the previous description, wherein pulp is supplied with a sliding element onto the forming fabric in the section between the rolls supporting it. The pulp layer formed after the supply is prepressed. This has the effect of removing water from the pulp, and a web is formed, which is guided further to the actual pressing and further processing.

The prepressed web is removed from above the forming fabric, after which another pulp layer can be supplied onto the same forming fabric. After this the formed webs are joined and guided to the pressing operation to form a double-layer web.

The second pulp layer can also be supplied to the surface of the first web.

The layers formed with this former, too, remain at least par- tially separate also in the final product. In addition, the layers are different from one another, which weakens the qual- ity of the final product. Practically, the layers are on top of each other. Furthermore, the former set forth is poorly suit- able for the forming of high consistency pulp mainly due to the sliding element design.

The object of the invention is to provide a new method in forming high consistency pulp in a paper or board machine, which can be used to form multi-layer web with symmetric sides but with a low splitting tendency. Another object of the inven- tion is to provide a new high consistency pulp former in a paper or board machine, which is less complex than heretofore, but which can utilize pulps at various consistencies. The characteristics of the method according to this invention appear from the appended claim 1. Similarly, the characteris- tics of the high consistency pulp former according to the invention appear from the appended claim 6. In the method

according to the invention, several pulp layers are formed and prepressed simultaneously. This provides a final product with a more uniform quality and higher strength than heretofore. The method can be used to adjust forming with greater versatility than is known heretofore allowing the method thus be applied for various pulps. At the same time, the properties of the final product can be influenced more than heretofore. A former according to the invention can be used to produce multi-layer web in a simple and reliable manner, allowing thus to obtain a high basis weight for the final product whenever required..

Similarly, the need for the dewatering capacity can be reduced by using high consistency pulp, which, when using a former according to the invention, can be kept at a sufficiently mixed state until to the forming fabric. In addition, the former requires significantly less mounting space than the formers presently used.

The invention is described below in detail by making reference to the enclosed drawings, which illustrate some of the embodi- ments of the invention, in which Figure 1 shows a principal drawing of a high consistency pulp former according to the invention, Figure 2 shows a principal drawing of a part of the pulp feeding point in the high consistency pulp for- mer according to the invention, Figure 3 shows a variation of the embodiment shown in Figure 2, Figure 4 shows a principal drawing of a second applica- tion of the high consistency pulp former accord- ing to the invention, Figures 5a-b show structural applications for the slice chan- nel of the high consistency pulp former accord- ing to the invention, Figure 6 shows a modification of the high consistency pulp former shown in Figure 1.

Figure 1 shows the high consistency pulp former according to the invention. In addition to the paper machine, the high consistency pulp former according to the invention, later referred to as'former'for simplification, can also be used with other machines, such as board machines. The former is designed to form a web from high consistency pulp with a head- box consistency exceeding 1. 5%. In practice, the consistency can exceed as high a percentage as 5, ranging, however, most commonly from 2 to 4%. The use of high consistency pulp is of a great importance for the dewatering capacity of a paper machine, because a conventional paper machine uses a pulp, whose headbox consistency usually ranges from 0.5 to 1%.

The former comprises a forming fabric assembly 10 and a roll assembly 11 adapted to support it, and including several rolls 12-15. The former is further equipped with a headbox 17, which contains sliding elements 16, for supplying high consis- tency pulp onto the forming fabric assembly 10. The design of the sliding element is described in greater detail in connec- tion with Figures 2 and 3. According to the invention, pulp is supplied onto the forming fabric assembly 10 in the section between the successive rolls 12 and 13 of the roll assembly 11.

Adapted after the said headbox 17, the former also includes a prepressing nip 18, which consists of two rolls 12 placed against each other. The prepressing nip 18 can also consist of a shoe press nip, in which one roll 12 is rigid and the other is a resilient shoe roll 12' (Figure 6). The shoe roll is also referred to as extended nip roll. The forming fabric assembly and the high consistency pulp supplied onto it is adapted to travel through the prepressing nip to form the web. The web is pressed primarily in the press section, to which the web 33 is transferred with the pick-up roll 20 (Figure 2).

According to the invention, the forming fabric assembly 10 comprises two forming fabrics 21 that are placed against each other at least for a part of their travel. The forming fabric

is later referred to as fabric for simplification. In addition, one of the rolls 12 from both roll assemblies 11 supporting the fabrics 21 are arranged against one another to form the pre- pressing nip 18. Correspondingly, the headbox 17 includes two sliding elements 16 for forming a high consistency pulp layer 19 on each forming fabric 21 prior to the prepressing nip 18, which is adapted to form a single web 33 from the high consis- tency pulp layers (Figure 2). Using this design, high consis- tency pulp can be supplied onto the two fabrics substantially symmetrically. Thus the both sides of the final product are uniform. In practice, the layers face each other in a way that differs from the prior art formers, in which the layers are on top of each other. In addition, the separately formed, symmet- rical high consistency pulp layers are prepressed against each other in the prepressing nip to form a single web. Due to the former design, the pulp layers are prepressed simultaneously, whereby the final web will also be symmetrical as regards its surfaces and structure. For clarity, the pulp layers 19 and the web 33 are shown only in Figure 2.

Figures 2 and 3 show the pulp feeding point in a greater de- tail. High consistency pulp, later referred to as pulp for simplification, is supplied with the headbox 17, which includes a sliding element 16 adapted to counter the fabric 21. Pulp is discharged from the slice channel 22 of the sliding element 16 at a high speed, and therefore, to keep a constant distance between the sliding element 16 and the fabric 21, the fabric 21 is arranged to a high tension. Stability can also be increased by arranging the fabric 21 to form a curvature in the area between the rolls 12 and 13. According to the invention, the sliding element is adapted against the fabric in such a way that the fabric forms an angle ß at the sliding element, the angle being 100-170°, more advantageously 140-160°. In addition, the sliding element 16 surface facing the fabric 21 is advantageously curved. The fabric 21 tension is adjusted for example by moving the rolls 14 of the roll assembly 11. Gener-

ally the fabric tension varies from 3 to 15 kN/m. Similarly, the curvature of the fabric 21 is adjusted by moving the rolls 13 preceding the supply point. In Figure 3 these rolls 13 are further away from each other than in Figure 2, in which case the angle formed by the fabric 21 increases at the sliding element 16. The same reference numbers are used for function- ally similar parts.

The operation of the former can be further adjusted by changing the pulp supply angle. According to the invention, high consis- tency pulp is supplied towards the fabric at an angle a, which ranges from 10° to 80, more advantageously from 50° to 60°. A relatively steep angle provides a uniform, yet dynamic pulp distribution on the fabric. The pulp supply can be further adjusted by changing the pulp supply speed. According to the invention, pulp is supplied onto the fabric at a speed, which is by 5 to 80%, more advantageously by 10 to 40% lower than the fabric speed. Thus the fabric moving faster than the pulp produces internal shearing forces in the pulp, which slows down fiber flocculation.

In practice, the fibers start to flocculate immediately on hitting the fabric. To obtain a good adhesion of the pulp layers, pulp is supplied onto the fabric as close as possible to the prepressing nip. Thus the fibers of the different pulp layers flocculate with one another at least partially, in which case a formation of a clear dividing line between the pulp layers is avoided. Consequently, the above mentioned partial travel is formed between the two rolls 12 and 13, successive in the travel direction of the fabric 21, of the roll assembly 11 of the fabric 21. According to the invention, the distance between these rolls 12 and 13 is 500-4000, more advanta- geously 1500-3000 mm. Using large rolls also advances the adhesion of the pulp layers. According to the invention, the radius of curvature of the roll 12 of the prepressing nip 18 is 200-1000 mm, more advantageously 250-750 mm. Instead of a

rotating roll, a curved slide shoe can be used whose radius of curvature is easy to make higher than that mentioned above. The fabrics are guided against the slide shoe, thus forming an extended functional prepressing nip between the fabrics. That is, the slide shoe is used without a counter roll. The pre- pressing nip can also consist of a shoe press, which, due to its resilience, avoids any cross profile problems.

Each sliding element 16 is supported to a construction 23 in the paper or board machine. At the same time the sliding ele- ments 16 appropriately placed against each other compensate forces produced due to the operation of one other. Figure 2 also shows a slice bar 24, arranged in the sliding element. 16 and adapted to limit the outlet end of the slice channel 22.

According to the invention the slice bar is adapted to move in the direction of the slice channel. In this way the thickness of the pulp layer can by adjusted in a simple manner. Generally the adjustment ranges from 1 to 20 mm, which is sufficient for supplying different pulps at varying consistencies with one type of former.

Consequently, pulp is supplied onto the fabric 21 with the headbox 17, which in addition to the sliding elements 16 in- cludes a header 25 and a slice channel 22 extending from it to the sliding element 16. According to the invention, the height of the slice channel ranges from 2 to 10 mm in average, more advantageously from 3 to 6 mm. Shearing forces are thus pro- duced in the pulp flow, which correspondingly produce strong turbulence in the slice channel. This prevents flocculation of fibers in the slice channel that is longer than in a conven- tional design. In addition to the low-construction slice chan- nel 22, guiding elements 26 adapted to influence the pulp flow, can be arranged in the inner surface of the slice channel 22.

In Figures 5a and 5b the guiding elements 26 are provided using specific slice channel wall constructions. The guiding elements can also be separate units attached to the slice channel (not

shown). Even the inner surfaces of the slice channel can be replaceable.

Figure 3 shows another embodiment of the former according to the invention. According to the invention, pulp and/or filler material is supplied between the fabrics prior to prepressing.

Pulp is advantageously supplied to the closing gap of the prepressing nip. That is, the center-most slice channel 22 is detached from the fabrics 21. This former provides a three- layer web in a simple manner. For example, the center-most header supplies unbleached recycled pulp and the outermost- headers supply bleached pulp, in which case the final product is symmetric both for its surfaces and internal structure. At the same time, for example the printability of the final prod- uct is good with the total costs remaining lower than usually.

Deviating from Figure 3, the header can be placed above the sliding elements if required, in which case the third slice channel is formed advantageously short. Dilution water equip- ment 27 can also be combined in connection with the headers 25 for controlling each pulp flow zone-wise in the cross direction of the machine.

When supplying pulp onto the open fabric, water is removed from the forming pulp layer. The area between the rolls is advanta- geously closed and a vacuum is created therein at the supply point using a suitable protection 28, in which case the dis- charging water can be collected in a controlled manner. On the back side of the fabric 21, it is also possible to arrange separate dewatering elements 29, which in Figure 3 are based on a vacuum. A vacuum can also be used after the prepressing nip to guide the web to the correct fabric at the separation point of the fabrics.

The dewatering capacity can be further improved by using in prepressing at least one press felt and/or shoe press in addi- tion to the fabrics. In Figure 4 two felts 30 are used. Accord-

ing to the invention, in addition, the felt's moisture profile is controlled prior to prepressing. In this case prepressing can still influence the CD moisture profile of the currently forming web in a paper or board machine. The control is pro- vided using for example a steambox 31 with zones, or similar.

Similarly, the felt 30 can be conditioned using cleaners 32 located after the prepressing nip 18. For a similar purpose, steam can be supplied also to the prepressing nip between the pulp layers.

The method and former according to the invention provide a multi-layer web in a simpler way than heretofore. In addition, the former can be used to supply pulps, whose consistency can vary widely. The basic final product is symmetrical in struc-<BR> ture. On the other hand, it is easy to produce a web, whose layers are different on purpose. Independent of the number of the pulp layers, only one prepressing operation is required, in which the pulp layers are simultaneously formed to a web.