The invention concerns a stock feed system for a multi-layer headbox and a method in the operation of a multi-layer headbox.

From the applicant's FI Patent No. 92,729 a what is called single-stock system is known in the operation of a multi-layer headbox. The system comprises a multi-layer headbox and therein at least two separate inlet headers or equivalent. For the inlet headers the different stocks are formed out of the same fresh stock, which is derived from one and the same stock container placed in connection with the headbox. The fresh stock that is passed out of the stock container is divided preferably into two or more component flows. Into said component flows that are fed into the multi-layer headbox, the chemicals and/or additives are passed which are purposeful in view of the quality or the economy of production of different paper grades. The system is simple, and its construction is advantageous. The fillers can be added to the stock derived from one stock container in an optimal way. In the system, in direct connection with the multi-layer headbox, just one basic stock is dealt with, to which the fillers and favourably also the dilution water and other retention agents are added in compliance with the requirements of each layer.

In the present patent application, an improvement is suggested for the stock system of a multi-layer headbox. It is suggested that fractioning of the stock is added to the single-stock system known from the applicant's FI Patent 92,729, i. e. fractioning based on the fibre lengths so that the short fibres are allowed to remain in the surface layer/layers and the long fibres are transferred from the surface layer into the middle layer/layers.

In this what is called fractioning system, into a certain component flow derived from the stock container, a certain portion fractioned from the stock is passed out of some other component flow derived from said container. The quality of the printing face of paper can be improved by transferring the long fibres from the surface lay- er/layers into the middle layer/layers. It is also possible to operate so that, while long fibres are transferred into the middle layer, correspondingly short fibres are transferred from the middle layer into the surface layer/layers. On the whole, different variations of fractioning are possible in an attempt to obtain properties of certain nature of the paper. For example, by means of a particular screen of a certain sort, in a multi-layer headbox that comprises three layers, the accept from the screen in the middle layer can be passed to a surface layer, whereas the reject of the screen concerned is allowed to remain in the middle layer.

Thus, out of the basic stock of the single-stock system described in the applicant's patent FI 92,729, which stock contains substantially all the fibres to be used for the paper to be produced, at least two different components are formed, of which one component contains fibres longer than average, compared with the distribution of fibre length in the basic stock, and the other stock concept contains substantially fibres shorter than average. Moreover, the procedure is such that the short fibres remain in the surface layer/layers, and that long fibres are transferred from the stock lines of the surface layers into the middle layer/layers.

The invention is characterized in that the stock feed system of the multi-layer headbox comprises at least one such fibre distributor by whose means long fibres are transferred out of the stock that forms the surface layer of the paper into the stock that forms a layer placed in the interior of the paper, preferably the middle layer, in which connection fibres of shorter length, compared with the average distribution of fibres in the basic stock derived from the stock container, are allowed to remain in the surface layer of the paper/board, and longer fibres, compared with the average distribution of fibres, are allowed to remain in the middle layer and/or are trans- ferred into the middle layer.

In the following, the invention will be described with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying draw- ings, the invention being, however, not supposed to be confined to said embodiments of the invention alone.

Figure 1 shows a first preferred embodiment of the invention, in which the stock flow derived from the fresh-stock container is divided into three component flows, which are passed further, after supplies of chemicals and fillers, into the different inlet headers in the multi-layer headbox.

Figure 2 shows a second preferred embodiment of the stock feed system in accord- ance with the present invention for a multi-layer headbox.

Figure 3A illustrates the distribution of fibre lengths in the stock fed into the surface layers, which distribution has been achieved by separating the long fibres off by means of a prior-art technique.

Figure 3B illustrates the distribution of fibre lengths in the stock fed into the middle layer b, obtained by means of said technique.

Figure 4 illustrates the distribution operation of a conventional machine screen.

In the following description, when a middle layer is spoken of, what is meant is the stock and the distribution line or distribution lines related to said stock remaining between the surface layers of the stock so as to form the middle layer/layers in the web, such as a paper web. Thus, there can be one or several middle layers. Similar- ly, surface layers are understood as the stock and the stock line or stock lines which forms the surface layer/layers in the web, such as a paper or board web.

Fig. 1 is a schematic illustration of a first preferred embodiment of the invention, which is favourably suitable for SC-paper. In the embodiment of Fig. 1, a paper web or a board web is formed which comprises surface layers al and a2 as well as a

middle layer b. By means of the arrangement of the invention, the web is formed such that, in its middle layers ai and a2 there are shorter fibres, and in the middle layer b there are longer fibres. In this way, good surface and printing properties are obtained for the paper. When screens are spoken of in the following, what is meant is, in connection with fractioning of stock, such a stock fractioning device in general by whose means the stock is divided into different flows based on the fibre lengths, so that the long fibres are transferred into the middle portion of the web and the short fibres are allowed to remain in, or are transferred into, the surface layers al and a2 of the web. As is shown in the figure, the multi-layer headbox 10 comprises three inlet headers: the inlet headers 11,12 and 13. From the inlet header 11 the stock is passed through the distribution manifold 14a1 1, 14al 2... into the turbulence generator 15 into its turbulence tubes 15a1 1, 15al 2... and further into the slice cone 16. From the inlet header 12 the stock M2 is passed through the distribution pipes 14a2. 1, 14a2 2... of the distribution manifold 14 into the turbulence generator 15 into its turbulence tubes 15a2. 1, 15a2. 2... and further into the slice cone, and from the inlet header 13 the stock M3 is passed through the distribution pipes 14a3 1, 14a3 2... of the distribution manifold 14 into the turbulence generator 15 into its turbulence tubes 15a3. 1, 15a3. 2... and further into the slice cone 16. Thus, by means of the multi-layer headbox shown in Fig. 1, the paper is formed out of three stock concepts M1, M2 and M3. Thus, the web will comprise three layers formed out of different stock concepts.

The equipment comprises a single stock system, in which the stocks M1, M2 and M3 are formed out of the same fresh stock M. Thus, in the single-stock system the different stocks for the multi-layer headbox are prepared out of one and the same fresh stock by to said fresh stock adding the necessary chemicals/fillers in connection with the approach pipe system of the headbox. While, in conventional prior-art solutions, there were several fresh-stock lines and fresh-stock containers, in the method and solution of equipment in accordance with the present invention the different layers are formed out of the same fresh stock M, which is derived from one fresh-stock container 17. As is shown in Fig. 1, the fresh stock M is made to flow out of the fresh-stock container 17 along the line 18a and branched at the

branching point Pi into two branch lines 18a2 and 18a1. In the embodiment shown in Fig. 1, in the branch line 18a2 the chemical 3a is added to the stock M, and in the branch line 18a1 the chemical 3b is added, for example a filler or starch. In the lines 18a2, 18a1 the stocks are made to flow further by means of the pumps l9a1 and 19a2 so that along the line 18a1 the stock is passed into the machine screen 20a1. The retention agent 5b is fed into the stock before the machine screen 20a1 and the retention agent 5bb after the machine screen. In this way, good mixing of the retention agent and the stock is achieved. Along the line 21a1 the stock M2 that has been formed is passed into the middle inlet header 12 in the multi-layer headbox.

From the line 18a2 after the pump 19a2, from the branch point C, there is a line 18a2. 1 to the machine screen 20a2 and from the machine screen 20a2 a line 21a2 to the multi-layer headbox. Before the machine screen 20a2 the retention agent 5a is fed into the line 18a2. 1, and after the machine screen 20a2 the retention agent 5aa is fed into the line 21a2. The stock flow M3 is passed along the line 21a2 into the inlet header 13 in the multi-layer headbox.

From the branch point C the line 18a2 3 passes to the machine screen 20a3 and further to the multi-layer headbox. Into the line 18a2. 3 the retention agent 5c is fed before the machine screen 20a3, and the retention agent 5cc is fed after the machine screen 20a3. Along the line 21 a3 the stock flow M1 is passed into the inlet header 11 in the multi-layer headbox.

In the system illustrated in Fig. 1, the reject from the screen 200a2 of the stock line 21a2 of the surface layer, which reject consists of long fibres, is passed along the line fl into the line 21a1 of the middle layer. Similarly, the accept from said screen 200a2 of the surface layer, which accept consists of short fibres, is passed further in said line 21a2 of the surface layer. The screen 200a1 placed in the line 21ai, i. e. in the line that forms the middle layer in the web, divides the stock based on the fibres so that the accept of said screen 200a1 consists of long fibres, which are allowed to remain in the stock flow in said line 21al, and similarly the short fibres from said screen 200a1 are passed as reject along the line f3 into the line 21a2 of the surface

layer, and also, as is shown in Fig. 1, into the line 21a3 of the other surface layer along the line f4. The screen 200a3 placed in the line 21a3 operates in the same way as the screen in the line 21a2, i. e. so that the long fibres arriving from said screen 200a3 as reject of the surface layer are transferred along the line f2 into the middle layer into its line 21al, and the accept from said screen 200a3, i. e. the stock that contains the short fibres, is made to flow further in the line 21a3.

Fig. 2 shows an embodiment of the invention in which the system comprises the screens 200a2 and 200a3 in the surface lines 21a2 and 21a3 only. The screens operate in the same way as in the embodiment of Fig. 1, so that by their means the stock flow arriving along the lines 21a2 and 21a3 into the screens 200a2, 200a3 is divided based on the fibres so that stock that contains long fibres is transferred from the screens 200a2, 200a3 along the lines f, and f2 into the centre line 21a1. In stead, the short fibres are allowed to remain in said lines 21a2 and 21a3, along which they pass further into their inlet headers in the multi-layer headbox.

Above, some preferred concepts of embodiment of the invention have been described, to which alone the invention is, however, not supposed to be confined. It is obvious that fractioning of the stock in a vertical system of coordinates of the web can also be carried out so that long fibres are transferred into the middle layer/layers of the web from one surface layer only. Within the scope of the invention, embodi- ments are also possible in which the multi-layer headbox comprises a number of lines which pass into a number of inlet headers placed one above the other, in which case long fibres can be transferred, besides from a surface layer, also from one or several stock lines that form a layer near the surface layer of the web into one or several stock lines that form the middle layer/layers in the paper web or board web.

Thus, the invention can vary in many ways within the scope of the inventive idea described in the present patent application.

Fig. 3A illustrates the distribution of fibre lengths in the surface layers ai and a2.

The vertical axis represents the percentage proportion, and the horizontal axis represents the fibre length.

Fig. 3B illustrates the distribution of fibre lengths in the middle layer b. The vertical axis represents the percentage proportion, and the horizontal axis the fibre length.

Fig. 4 shows an embodiment of a machine screen. The basic stock arriving in the machine screens is divided into the reject, which comprises a what is called coarse fraction, i. e. long fibres, and the accept, which comprises a fine fraction, i. e. short fibres. The conventional machine screen shown in Fig. 4 comprises an annular screen face in the middle of the device. The basic stock is passed into the middle of the screen, and the accept fraction, i. e., in this case, the fine fraction, which comprises short fibres, is pressed by means of the pressure of a pump through the annular screen face and to the side. The non-accepted fraction, i. e. the reject, is passed through the middle of the annular cylinder down and to the side. The motor O shown in the figure rotates the rotor and the attached pulse blades, which wipe along the screen face and keep it unblocked.