Abstract:
Virgin stock is conducted to a paper or board machine and further to a centrifugal cleaner installation ( 12 ) into its first centrifugal cleaner step ( 12   a   1 ). From the first centrifugal cleaner step ( 12   a   1 ) the accept is conducted into a multi-layer headbox ( 10 ) to form a layer of the web determined according to the concerned fraction. From a second step ( 12   a   2 ) and/or from lower steps ( 12   a   3   , 12   a   4  . . . ) of the centrifugal cleaner installation ( 12 ) a second fraction or more fractions are conducted into the multi-layer headbox ( 10 ) to form a second layer or other layers of the web, which are determined according to the pulp fractionation taking place in the concerned second step or lower steps.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application of International Application No. PCT/FI01/00791, filed Sep. 12, 2001, and claims priority on Finnish Application No. 20002031, filed Sep. 14, 2000, the disclosure of each application is hereby incorporated by reference herein. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a method and equipment for pulp fractionation in a paper machine or such, such as a board machine. 
     Multi-layer headboxes are already in use with many board grades and they are also on their way to printing paper machines. Layering has traditionally been done by layering the supply of either filler or retention agents. It is a weakness of this system that the pulp itself is entirely similar in all layers, so the drainability, fibre content and quantities of fines in the layers are not different. This of course limits the efficiency of layering. 
     Alternatively with e.g. tissue or board machines the different raw material components, such as short and long fibre, are treated separately from each other all the way from pulp treatment to the headbox. In such a system a double pulp system must of course be built all the way from pulp treatment to the paper machine. 
     Fractionation plants are used also in the production of pulp. Pressurized screens are generally used in the fractionation, and the fractionation is performed already at the pulp plant. In this case too a double pulp system must be built for the paper machine. 
     SUMMARY OF THE INVENTION 
     In the system according to the invention, the pulp is brought mixed into the short circulation of the paper machine. For example, in a machine using 100% recycled fibre, there is only one raw material, whereby pulp layering without fractionation cannot be done at all. 
     The centrifugal cleaners traditionally used in the short circulation of the paper machine have been used only to separate sand. The centrifugal cleaner installation separates pulp e.g. according to its density, size, shape and surface roughness. In the system according to the invention, the fractionation done by centrifugal cleaners is utilised in such a way that the accept of a certain centrifugal cleaner is conducted into a certain bypass manifold of the multi-layer headbox to form a certain web layer. In the system according to the invention, the fractionation ability of centrifugal cleaners is utilised e.g. in such a way that the fraction having more fines or long fibres is guided into the bottom and/or surface layer of the headbox. 
     In the first step of centrifugal cleaning, the pulp is divided roughly in a suitable proportion between the various layers. The final fine control of proportioning takes place only at the pump of the headbox. Surplus of pulp is circulated back to the input of the centrifugal cleaner. 
     Compared with filler layering, the quality of the pulp itself in the various layers can also be varied, and desired fibre fractions can be guided either to the surface or into the middle layer as required. 
     There is no need for any separate pulp systems before the centrifugal cleaners, but all pulp is brought in only one line all the way to the short circulation. 
     The equipment already in the short circulation is utilised and there is no need for any new partial processes. Only the operation of step  1  of the centrifugal cleaning is changed in such a way that the so-called reject ratio will correspond with the quantity of fibres needed in the various layers. 
     According to the invention, the pulp is conducted from the wire pit to the centrifugal cleaner, and from the first stage, that is, from step  1 , of the centrifugal cleaner installation the pulp is conducted forward, in one embodiment of the invention into a deaeration tank, the reject of step  1  is conducted further into the second stage of the centrifugal cleaner installation and thence the accept is conducted forward into the second part of the deaeration tank. 
     An advantageous embodiment of the invention is as follows. The accept arrived from the first stage of centrifugal cleaning into the deaeration tank is conducted from the deaeration tank into the part of the headbox forming the bottom and surface layers of the web, preferably through power screens. The pulp conducted as accept from the second stage, that is, from step  2 , of the centrifugal cleaner into the deaeration tank is conducted through a power screen located in between the deaeration tank and the headbox into the bypass manifold of the headbox, through which bypass manifold the pulp is conducted on to the formation wire to form the middle layer of the web. 
     Thus, in fractionation according to the invention, the centrifugal cleaner installation is utilised and the fractionation is carried out from various stages of the centrifugal cleaner installation in such a way that the pulp conducted from the first stage into the deaeration tank is conducted further after deaeration to form top layers of the web, and the pulp conducted as accept from the second stage or from other stages is moved further from the concerned stage/stages of the centrifugal cleaner installation to form other layers of the web, such as the middle layer of the three-layer web. However, it is not a purpose to limit the invention to the manner of forming a three-layer web described above. With the equipment according to the invention it is also possible to form two-layer paper to paper or board grades having even more layers instead of three-layer paper. 
     The system thus utilises a centrifugal cleaner installation and its fractionation in the making of multi-layer paper. The system may be applied to such short circulation already in use, which include a centrifugal cleaner. One stock is conducted into short circulation and it is treated in such a way in the centrifugal cleaner installation that the desired fraction can be conducted further through a deaeration tank to the multi-layer headbox into the pulp bypass manifold corresponding with each layer. In the system according to the invention, a power screen may also be used in between the deaeration tank and the headbox in order to achieve the final fractionation result. Such an embodiment is also possible within the scope of the invention, where there is no deaeration from the pulp. In a system where there is no deaeration from the pulp, the accepts of centrifugal cleaning may be taken directly to the suction side of the headbox&#39;s feed pump. In other respects the structure of the system is similar to the one in the embodiment shown in  FIG. 1 . 
     Such an embodiment may also be possible within the scope of the invention, wherein water leaving the wire section is conducted into the wire pit, from which wire pit the tail water is pumped into the deaeration tank and harmful air is removed from the tail water in the deaeration tank. Then the tail water is admixed with high-consistency pulp, which is conducted further into the centrifugal cleaner installation and further according to the invention from the centrifugal cleaner installation to the multi-layer headbox. 
     In an embodiment containing a deaeration tank this is preferably in two parts. From the deaeration tank there are discharge fittings for each desired fraction. The pulp fraction can then be branched off to form several layers or conducted without branching in order to form one layer containing the concerned fraction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be described with reference to the embodiments in the appended figures, but the intention is not to limit the invention to these only. 
         FIG. 1  is a schematic view of the fractionation system according to the invention. 
         FIG. 2A  is a schematic side view of the centrifugal cleaner of the first step of the centrifugal cleaner installation. 
         FIG. 2B  is a sectional view along line I-I in  FIG. 2A . 
         FIG. 3  shows a second advantageous embodiment of the invention, wherein tail water is conducted into a deaeration tank and then virgin stock is admixed with the flow conducted from the deaeration tank, and the flow is conducted further into the centrifugal cleaner installation and through this according to the invention to the multi-layer headbox. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows schematically the equipment according to the invention for pulp fractionation. The equipment includes a multi-layer headbox  10  and a deaeration tank  11 , which is preferably in many sections, being a two-section tank in this embodiment of the invention. In addition, the system according to the invention includes a centrifugal cleaner installation  12 , which includes at least two steps, steps  12   a   1  and  12   a   2 , that is, centrifugal cleaning degrees. In addition, the system according to the invention includes a wire pit  13  and a fitting b 1  leading from this to the first centrifugal cleaning step  12   a   1  of the centrifugal cleaning installation  12 . From the first step  12   a   1  of the centrifugal cleaning installation  12  there is a further fitting a 1  for the accept into the deaeration tank  11 , into the first section  11   a   1  of the said tank. From tank  11   a   1  there is another fitting a 2 , which branches off to form fittings a 2 ′ and a 2 ″. The fittings a 2 ′ and a 2 ″ include power screens  14   a   1  and  14   a   2 , from which the accept is conducted further along fittings a 2 ′ and a 2 ″ to a multi-layer headbox  10  and into its bypass manifolds J 1  and J 3 , from which the pulp is divided further into the headbox&#39;s set of pipes, through an intermediate chamber and a turbulence generator to the formation wire (not shown in  FIG. 1 , see formation wire H 1  in  FIG. 3 ) and to form the top and bottom layers of the web. The fittings, such as channels or pipes a 2 ′ and a 2 ″, include pumps P 2  and P 1  and, correspondingly, a pump P 3  is located in a fitting a 4 . Using the pumps, the pulp fractions are pumped into each bypass manifold J 1 , J 2 , J 3  of the multi-layer headbox  10 . 
     From the first step  12   a   1  of the centrifugal cleaner installation  12  there is a fitting b 2  by the reject, and further to fitting b 3 , which leads to the second stage of centrifugal cleaner installation  12 , that is, to second step  12   a   2 , from which there is further a fitting a 3  for the accept into the second section  11   a   2  of the deaeration tank  11 , and further a fitting a 4 , e.g. a pipe, into bypass manifold J 2  of the multi-layer headbox  10  to form the middle layer of the web. In this application, virgin stock is understood as being the new stock conducted to wire pit  13 . The stock includes fillers and additives and fibres. Thus, from the first step  12   a   1  of the centrifugal cleaner installation  12  there is a fitting a 1  into multi-section deaeration tank  11 , into its first section  11   a   1 , from which after the deaeration the fraction is transferred further into fitting a 2 , which branches off to form branch fittings a 2 ′, a 2 ″, which lead further into corresponding pulp bypass manifolds J 1  and J 3  of the multi-layer headbox  10 . Branch fittings a 2 ′, a 2 ″ include power screens  14   a   3  and  14   a   1 , from which the accept is conducted further to the corresponding bypass manifolds J 1 , J 3  of the headbox, and the reject is conducted along channels t 1 , t 3  back to the wire pit  13 . Correspondingly, from the second step  12   a   2  of the centrifugal cleaner installation  12  the accept is conducted into multi-section deaeration tank  11 , into its section  11   a   2  along fitting a 3 , and after the deaeration the said fraction is conducted to fitting a 4 , which is conducted further into the middle bypass manifold J 2  of the multi-layer headbox  10  to form the middle layer of the web. Fitting a 4  includes a power screen  14   a   2 , from which the accept is conducted into bypass manifold J 2  of the multi-layer headbox  10 , and the reject is conducted along fitting t 2  as a back flow back to wire pit  13 . 
     As is shown in  FIG. 1 , centrifugal cleaner installation  12  may include several steps. In the embodiment shown in  FIG. 1 , there are two actual fractionation steps, which are steps  12   a   1  and  12   a   2 , which are used for forming a three-layer web. Step  12   a   1  includes centrifugal cleaner cones  120 , of which there are five in the step and the accept outlets of which are joined together, while, correspondingly, the reject outlets are joined together. There is a corresponding arrangement in the other steps. The number of cones  120  in step  12   a   2  is four, in step  12   a   3  there are three, in step  12   a   4  two and in the last step  12   a   5  there is one cone. The reject outlet fitting b 2  of step  12   a   1  is connected to supply channel b 3  of the second step  12   a   2 . The reject outlet b 4  of step  12   a   2  is connected to supply fitting b 5  of the third step  12   a   3  and reject outlet b 6  of step  12   a   3  is connected to supply fitting b 7  of step  12   a   4 , reject outlet fitting b 8  of step  12   a   4  is connected to supply fitting b 9  of the last step  12   a   5 . The accepts of steps  12   a   3 ,  12   a   4  and  12   a   5 , for which there is a fitting d 1 , d 2 , d 3 , are connected in such a way to the system that the accept of step  12   a   3  is made to flow along fitting d 1  to the second step  12   a   2 , into its fitting b 3  to the suction side of feed pump P 5 . Correspondingly, accept fitting d 2  of step  12   a   4  is connected with supply channel b 5  of step  12   a   3  on the suction side of feed pump P 6  and, correspondingly, accept fitting d 3  of step  12   a   5  is connected with supply fitting b 7  of step  12   a   4  on the suction side of pump P 7 . The reject taken from the last step  12   a   5  is moved entirely to the discharge or to further treatment in connection with another installation. 
     Fitting b 1  from wire pit  13  includes a feed pump P 4 , and there is an input fitting f for virgin stock to the wire pit. For the tail water of the wire section there is a return fitting e to wire pit  13 , and as is shown in the figure, from deaeration tank  11  between the end walls of sections  11   a   1  and  11   a   2  there is a return fitting g for overflow to wire pit  13 . Negative pressure pump arrangements in connection with deaeration tank  11  for bringing about a negative pressure in the top section of the deaeration tank are not shown. Air is removed from the fractionated pulp with the aid of a high negative pressure brought about in the deaeration tank by a negative pressure pump. 
     Such an embodiment is also possible within the scope of the invention, where there is no deaeration of the pulp. In systems with no deaeration of the pulp the accept of the centrifugal cleaning may be taken directly to the suction side of the headbox&#39;s feed pump. In other respects the system is similar to the one in the embodiment shown in  FIG. 1 . 
       FIG. 2A  shows one centrifugal cleaner of the first step  12   a   1  of a centrifugal cleaner installation. There may be several centrifugal cleaner cones  120  in each step  12   a   1 ,  12   a   2  . . . . The accepts of the cones  120  in each step are combined with each other and the rejects are also combined and then conducted along their respective fittings a 1 , b 2 ; a 2 , b 4  . . . . As is shown schematically in the figure, the heaviest particles move along a helical path downwards in the centrifugal cleaner cone  120  and further out of the cone  120 , and from the middle at the top the accepts are conducted forward into the deaeration tank and further into that bypass manifold of the multi-layer headbox, which relates to the concerned fraction. Thus, the fractionation of the centrifugal cleaner is characterised in that fractionation takes place in the said cleaner especially as regards the pulp, whereby the heavier particles move along a helical path to the following step or stage of the centrifugal cleaning, and thus the fractionation takes place also in regard to fillers and additives and not only in regard to fibres. 
       FIG. 2B  is a sectional view along line I-I of  FIG. 2A . Fitting b 1  is joined tangentially to cone  120 . The centrifugal force thus separates the heavier particles from the pulp flow L 1  in the space  0  shaped like a truncated cone inside cone  120 , while the lighter particles and the pulp fraction separated from the other pulp are conducted (arrow L 2 ) into deaeration tank  11  of the deaeration equipment by way of fitting a 1 . 
       FIG. 3  shows an embodiment of the invention, wherein the tail water is conducted to wire pit  13  along fitting e and the tail water is conducted further from wire pit  13  pumped by pump P 10  along fitting b 1  into deaeration tank  11 , from which deaeration tank  11  the tail water is conducted further along fitting b 1 ′ pumped by pump P 20  to the centrifugal cleaner installation  12 . High-consistency pulp, that is, virgin stock, is fed into channel b 1 ′ to the suction side of pump P 20 . From the first step  12   a   1  of the centrifugal cleaner installation  12  the accept is conducted along fitting a 1  into branch fittings a 1 ′, a 1 ″, which include feed pumps P 1  and P 2 , and the pulp is conducted further through power screens  14   a   1  and  14   a   3  into bypass manifolds J 1  and J 3  of the multi-layer headbox  10 . From the first step  12   a   1  of the centrifugal cleaner installation  12  the reject is conducted along fitting b 2  to the second step  12   a   2  of centrifugal cleaner installation  12  as supply, and from the said step the accept is conducted along fitting a 3  pumped by pump P 3  to power screen  14   a   2  and further to the central bypass manifold J 2  of the multi-layer headbox  10  to form the middle layer of the web.