Manufacturing processes of a multilayer web are known from prior art, in which processes, in order to return wire waters, in connection with multilayer head boxes the wire water from a wire section is passed to that layer of the head box from where it originally came from.

European patent application 0 664 356 discloses a method and an apparatus for improving the quality of multilayer papers, in which arrangement the waters from the former are divided into parts so that the waters from the surface layers are sup- plied in the multilayer headbox onto surface layers and the waters from the middle layers are passed to the middle layer.

In the method and apparatus for producing a multilayer paper web disclosed in published US patent application 4, 477, 313 the collection of wire waters from the former is arranged so that the water collected first is passed to the lowermost layer and the water collected next is passed to the middle layer.

Finnish patent application 992670 discloses a method for the manufacture of a multilayer web and a multilayer former having two formers, at least the other headbox being a multilayer headbox, and the wire waters being collected from the former such that the waters from the surface layers are passed to the surface layers in the headbox and the waters from the middle layers are passed to the middle layer.

In prior art short circulation systems the material flow from stock preparation runs via the short circulation process to the machine and possible rejects from the fol- lowing processes are returned to the previous process or passed to fiber recovery.

Since the end product, for example board, is made up of several layers, unwanted substances accumulate in the short circulation in different layers depending on the materials introduced into each layer. It has been necessary to introduce different additional processes, e. g. dispersing of the surface layer stock of recycled board, into the circulation systems in order to diminish the disadvantages caused by the unwanted substances. While the additional processes serve to disintegrate/remove unwanted matter in order to reduce the detrimental effects caused by it, they also increase costs and energy consumption.

In addition to recycled fiber material recycled fiber contains other materials of various kinds, e. g. box treating agent, surface treating agent such as wax, bitumen and different dirt particles, etc. generated in the treatment of rubbers and adhesive tapes, which particles form stickies. A particular problem is that, when recycled fiber is used in the manufacture of paper/board pulp, without the expensive addi- tional processes of the type described above, both the machine and the end prod- uct will be subjected to problems caused by detrimental materials coming from the raw materials. When manufacturing a multilayer web out of recycled fiber, problems are caused especially by dirt particles conveyed with the recycled fiber when passing the wire waters via short circulation to be used again. For example, different particles, stickies, e. g. waxes and bitumen originating from the recycled fiber raw material stick to machine parts. This means that the surface quality of the end product will not be what was desired, because flecks caused by the stickies and/or the machine parts damaged by them are formed in the surface.

In the prior art processes the stock flow is treated and light particles such as plas- tic, heavy particles, sticky particles and coloured particles are removed from it.

These particles go through different screening processes in which the fiber raw material is also separated into short, long and middle-length fiber. Depending on the end product, e. g. long fiber is selected for the surface layer, thus providing strength. If it is desired to provide the surface with material containing long fibers, stickies and other detrimental particles are usually also conveyed with the recy- cled fibers. Due to this, additional processes have been used in connection with the separation of long fibers in order to enable grinding of, e. g. stickies and other detrimental substances. However, this kind of additional processes take up a lot of energy.

An object of the invention is thus to reduce the amount of energy used in the processing.

In prior art arrangements the stickies and other detrimental materials accumulate in the system and thus cause problems. If they get to the surface or back layers, they come into contact with the machine parts, which causes problems in the proc- ess and cleaning and also creates a need for cleaning.

An object of the invention is to reduce the effects of the detrimental materials con- veyed with recycled fibers in the paper or board machine and in the end product.

An object of the invention is to provide a solution in order to eliminate or at least minimize the above-described problems.

To achieve the afore-mentioned objects and those that come out later the method according to the invention is mainly characterized in what is presented in the characterizing part of claim 1.

By means of the method a situation is achieved in which there are fewer problems with the cleaning of the machine and fewer defects in the end product. This in turn reduces the amount of energy needed to manufacture the product. Since the proc- ess produces less reject, harmful effects on the environment also diminish. A bet- ter utilization degree of the machine is also achieved by means of the invention, thanks to fewer process and cleaning problems.

The basic idea underlying the invention is that stickies and other detrimental ma- terials are removed in the wire section at least partially with the wire water and are passed to a layer, preferably the middle layer or to the back layer, if necessary.

This means that the stickies and other detrimental materials will not accumulate in the wire water of the surface layer and will thus not stick to machine parts or cause surface defects in the finished web. The wire water discharging from the forming of the middle layer is supplied to the surface layer, and the water dis- charging from the forming of the surface layer and containing stickies and other detrimental materials is passed to be used in the forming of the middle layer, and the water discharging from the forming of the back layer is returned to the back layer. In a two-layer web, for example, fresh water is supplied to the surface and the dirtier waters as well as the stickies and other detrimental materials are sup- plied to the back.

According to the invention stickies and other detrimental materials conveyed with the water flows are passed to layers where they cause fewer problems. This way the harmful effects of unwanted matter such as stickies and bitumen can be re- duced e. g. on the surfaces of a fibrous web product. This in turn makes it possible to decrease the amount of additional processes such as dispersing in the manufac- ture.

The process of the invention is devised and designed in a new way giving further new opportunities to optimize the process and the end product what comes, for example, to the filler used as the raw material of a desired layer. An advantage achieved thus in the invention is the relocation of couplings performed in the process. Forming serves as part of the cleaning process and at the same time new functions are provided in the wire section.

The invention will now be described in more detail with reference to the accom- panying figures, to the details of which the invention is, however, by no means intended to be narrowly confined.

Figure 1 schematically shows an embodiment of the invention.

Figure 2 schematically shows an embodiment of the invention.

Figure 3 schematically shows an embodiment of the invention.

In an embodiment of the invention shown in Figure 1 a pulp tower containing middle-length fiber has the reference number 10, that containing short fiber the reference number 20 and that containing long fiber the reference number 30. From the pulp tower 10,20, 30 the pulp is pumped by means of a feed pump 11,21, 31 to a machine chest 12,22, 32. From the machine chest 12,22, 32 the pulp is passed to a mixer 13,23, 33. Overflow of the screen 12,22, 32 of the machine chest is passed back to the corresponding pulp tower 10,20, 30. In the mixer 13, 23,33 the pulp that came from the pulp tower 10,20, 30 via the screen 12,22, 32 of the machine chest is diluted with wire water taken from a wire water pit 17,27 37 into headbox consistency and passed further via a screen 14,24, 34 to a head- box 15,25, 35. The pulp suspension is fed from the headboxes 15,25, 35 to a wire section 16,26, 36, where the web being formed is dewatered. The web being formed in the wire section 16 is passed to be used as the back layer of the web being formed. The web being formed in the wire section 26 is passed to be used as the middle layer of the web being formed and the web being formed in the wire section 36 is passed to be used as the surface layer of the web being formed. The wire water removed from the web is passed from the wire section 16,26, 36 to the respective wire water pit 17,27 and 37. The overflow of the wire water pit 17,27, 37 is passed via a container 18,28, 38 to the pulp tower 10,20, 30 or to be dis- charged, and the wire water of the back layer from the wire water pit 17 is passed back to the mixer 13 leading to the headbox layer 15 of the back layer. The wire water of the middle layer from the wire water tank 27 is passed to the mixer 33 leading to the surface layer. And from the wire water pit 37 of the surface layer the wire water is passed to the mixer 23 of the middle layer. This wire water con- tains the possible stickies and equivalent substances that have become released from the surface layer, and the substances are thus passed to the middle layer of the web in order to prevent them from hampering further processing.

The same reference numbers as in Figure 1 have been used in Figure 2 for parts corresponding to those illustrated in Figure 1. The embodiment of the invention shown in Figure 2 differs from that of Figure 1 in that from the wire water pit 17 of the back layer the wire water is passed to a channel leading to the mixer 23 of the middle layer. From the wire water pit 27 of the middle layer the wire water is passed to a channel leading to the mixer 33 of the surface layer, and from the wire water pit 37 of the surface layer the wire water is passed to a channel leading to the mixer 13 of the surface layer.

The same reference numbers as in Figures 1 and 2 have been used in Figure 3 for parts corresponding to those illustrated in Figures 1 and 2. What is shown in Fig- ure 3 differs from the above-described embodiments in that from the wire water pit 27 of the middle layer the wire water is passed to the mixer 13 of the back layer. From the wire water pit 17 of the back layer the wire water is passed to the mixer 33 of the surface layer, and from the wire water pit 37 of the surface layer the wire water is passed to the mixer 23 of the middle layer as in the embodiment according to Figure 1.

In the embodiments shown in Figures 1-3 the stickies are passed, at least partially, with the wire water to the desired layer, preferably to the middle layer, to the back layer, in necessary. The stickies will thus not be conveyed to the surface layer and do not stick to machine parts or cause surface defects in the finished web. The screened, fresh water is supplied to the surface, the dirtiest water to the middle and the semi-dirty water to the back layer. The embodiments shown in Figures 1-3 relate to the manufacture of a three-layer web, but when manufacturing a two-layer web, for example, the surface layer is supplied with the fresh water and the back with the dirtier waters.

The invention has been described above with reference to some of its advanta- geous embodiments only, to the details of which the invention is, however, by no means intended to be narrowly confined.