Patent Publication Number: US-6659636-B1

Title: Method and apparatus for feeding a chemical into a liquid flow

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention is related to a method and apparatus for feeding a chemical into a liquid flow. The method and apparatus of the invention are particularly well applicable to homogeneous adding of a liquid chemical into a liquid flow. Preferably the method and apparatus according to the invention are used for feeding a retention aid into fiber suspension going to the headbox of a paper machine. 
     Naturally, there is practically an innumerable amount of prior art methods of feeding various chemicals into liquid flows. These methods may be divided into a few main categories, though, as seen from the following. Firstly, it is quite possible to just let the liquid to be added flow freely into a second liquid without employing any special regulation or mixing means. This method of adding can not be employed in situations where the mixing ratio or homogeneity is of significance. Neither can it be employed in situations where the price of the chemical to be added is of significance. The next applicable method is to feed the chemical in a strict proportion to the liquid flow, whereby correct and economical proportioning is obtained. However, even in this case one has to take into account that usually the proportion of the chemical is slightly excessive compared to the optimal proportioning, because the mixing is known to be inadequate. The mixing may be improved, though, by feeding the chemical e.g. through a perforated wall of a flow channel, whereby the chemical to be mixed may at least be spread throughout the liquid flow. Lastly, a situation may be considered, where the chemical is fed in a strict proportion either into the liquid flow on the upper-flow side of the mixer or through the mixer itself into the liquid. In that case, the efficiency of the mixing of the chemical into the liquid flow is totally dependent on the mixer design. 
     Papermaking is in its own way a very demanding special field when chemical mixing is concerned. When using paper chemicals, it is good to bear in mind that their precise and homogeneous mixing is of vital importance in the short circulation of a paper machine. Homogeneous mixing means in a direct sense better quality and homogeneity of paper. At the same time, the process may be carried out without disturbances and problems. Poor mixing, on the other hand, requires chemical overdosing, which may increase the production costs remarkably. It is self-evident that in case of poor mixing, the quality of the paper and the operation of the process are not satisfactory. The existing mixing technique utilizes, on the one hand, clean water fractions both as dilution waters and as so-called “whip-water” which is used in order to intensify the mixing. On the other hand, efforts are made to close the water circulations of paper mills, whereby the feeding dosage of clean water into the system should be decreased, and internally clarified fractions or some non-treated direct flow from the process, such as e.g. filtrates, should be used instead. The existing systems for the mixing of chemicals do not allow or allow only to a small extent the use of water fractions of internal processes. 
     An essential case of mixing relating to paper manufacture is the mixing of a retention aid into fiber suspension flow going to the headbox of a paper machine. In paper manufacture, retention chemicals are used especially in order to improve the retention of fines at the wire part of the paper machine. As retention aid a chemical is used, long molecular chains of which bind together solid matter particles of the pulp and thus prevent the fines from passing, during the web formation stage, together with water through the wire. The retention aid should be mixed into the pulp as homogeneously as possible in order to gain the maximum effect of the chemical and to avoid variation of paper characteristics caused by retention fluctuations. Mixing, on the other hand, means that the liquid is subjected to a turbulent flow, the shearing forces of which break/may break long molecular chains, which naturally weakens the effect of the retention aid. Nevertheless, there are different kinds of retention aids. Sensitive to the effects of a turbulent flow are, e.g., polyacrylic amides, broken molecular chains of which are not known to be restored to their former length after the turbulence has attenuated, but there are also retention aids (e.g. polyethyleneimines), molecular chains of which are restored to their essentially original length shortly after the turbulence has attenuated. 
     In the short circulation of a paper machine, the feed point of the retention aid depends to a great extent on the retention aid used, the state of the flow from the feed point to the headbox lip, and the pulp used. The introduction of retention aids sensitive to shearing forces usually takes place immediately after a means (that may be a pump, a screen or a centrifugal cleaner) that causes shearing forces and is placed prior to the headbox, the feeding being carried out either into one spot or e.g. into the accept pipe of each pressure screen. It is also possible to use several retention aids of various types at the same time and introduce them into the fiber suspension by stages. The part of retention aids which is resistant to shearing forces may be fed as early as into the high-consistency pulp or prior to the headbox feed pump, and the part of retention aids which is sensitive to shearing forces is usually introduced not until the fiber suspension feed pipe prior to the headbox. 
     At present, as feeders of retention aids two types of apparatus are mainly used. A simpler apparatus (FIG. 1 a ) comprises an annular manifold placed around the pulp flow channel in a distance therefrom, connected by a number of feed pipes (at least four feed pipes) with the pulp flow channel so that the retention aid is discharged via said feed pipes in an even flow to the pulp flowing in the channel. A second possibility (FIGS. 1 b  and  1   c ) is to take e.g. two feed pipes crosswise through the flow channel and provide the part of the feed pipes which is left inside the flow channel with retention aid feed holes or slots, through which the retention aid flows in an even stream into the pulp, whereby the mixing result is to some extent better. At present, retention aids are fed into the fiber suspension flow under a relatively small pressure difference, whereby the retention aids form their own flow channels or at least a distinct danger exists that they are channeled inside the fiber suspension flow. In other words, in retention aid feeding it is commonly presumed that after the feeding point of the chemical there is a mixing apparatus that mixes the chemicals homogeneously into the fiber suspension. On the other hand, the amount of retention aid that is fed into the fiber suspension is chiefly based on practical knowledge from experience. This means that in practice retention aids are mixed into fiber suspension in an amount big enough to ensure the desired effect. In fact, this means a remarkable overdosing of retention chemicals (sometimes even by tens of percents) due to not homogeneous mixing. 
     It is characteristic of retention aids and their introduction that the retention aids are delivered to paper mills, in addition to liquid form, also as powders which are used depending on the paper to be made and the material to be used in an amount of about 200-500 g per one paper ton. A retention aid in powder form is mixed into fresh water in a special mixing tank in a proportion of 1 kg of powder to about 200 liters of clean water. This is because retention aids are known to react with, that is to stick onto, all solid matter particles in the flow very quickly, in about a second, which means that the dilution liquid has to be as clean as possible. In other words, in this stage, per 1 ton of produced paper 40-100 liters of clean water is used for retention aid production. Consequently, the consumption per day is, depending on the production of the paper machine, 10-100 cubic meters (here the production is estimated to be 250-1000 tons of paper per day). Nevertheless, this first dissolution stage is not the stage where water is used at the most, as in prior art processes this retention aid solution is further diluted into, e.g., one fifth of its concentration, which in practice means that for this so-called secondary dilution 200-500 liters of clean water is used per 1 paper ton. This results in a calculated daily consumption of 50-500 cubic meters of clean water per one paper machine. 
     In other words, until now it has been accepted that for the dilution of the retention aid per one paper machine hundreds of cubic meters of clean water is needed per day. Nevertheless, this has to be understood as a clear drawback, especially in cases when the paper mill is known to have great amounts of various circulation waters available, which might be utilized for this purpose, too. The only precondition for the use of circulation waters is that there should be a way to prevent retention chemicals from reacting with the solid matter in the circulation waters. 
     On the one hand, one has to bear in mind that the short circulation of a paper machine employs, due to large amounts of liquid, large-sized pipes. For example, as a feed pipe of the headbox of a paper machine, a pipe with a diameter of about 1000 mm may be used. This is one of the reasons why mixing a relatively small additional flow, such as a diluted retention aid, homogeneously into a wide flow channel is problematic. 
     On the other hand, the construction of the above described, presently used retention aid feeding apparatuses is very simple. When considering their operational efficiency, i.e. the homogeneity of the mixing, one might even say that they are too simple. In other words, the simplicity of the apparatus and the feeding method of chemicals, resulting in non-homogeneous dosing and also degradation of chemical molecules, inevitably lead to remarkable overdosing of chemicals, as the basic goal inevitably is to achieve a certain wire retention on a paper machine. 
     A further evident problem discovered in prior art processes is connected with the most traditional way of mixing the retention aid into the fiber suspension, that is prior to the headbox screen. Because the reaction time of a retention aid was known to be short, the headbox screen was considered a magnificent place for homogeneous and quick mixing of the retention aid into the pulp. And so it was when headbox screens of old art where used, which had a hole drum as a screening member. But now, with slot drums conquering the market, it has been discovered that the retention aid is capable of forming flocks prior to the slot drum, and thus a great amount of both the retention aid and the fines of the fiber suspension otherwise usable is, at best, rejected or, at worst, clogs the fine slots of the slot drum. 
     As noticed from above, numerous drawbacks and disadvantages have been discovered for example in the feed of retention chemicals. For solving e.g. the above mentioned problems of prior art, a new method and apparatus have been developed, which allow feeding into the liquid flow even chemicals consisting easily degrading polymeric chains, for instance retention chemicals, so that the polymeric chains remain non-degraded to a remarkably larger extent than before. As another advantage of the method and apparatus according to the invention we may mention, e.g., a substantial decrease in the consumption of fresh water in a paper mill, when desired, and an essentially more efficient and homogeneous mixing of retention aids into the fiber suspension. 
     According to one aspect of the invention there is provided a method of mixing a first liquid chemical into a second liquid using a mixing apparatus having a mixed-liquid discharge, comprising: (a) Introducing the second liquid into the mixing apparatus so that a second liquid flow is formed. And (b) introducing the first liquid chemical into the mixing apparatus so that the first liquid chemical is substantially simultaneously mixed with the second liquid with the discharge of the chemical and second liquid from the mixing apparatus into a fourth liquid. 
     According to another aspect of the invention there is provided a method of mixing a first liquid chemical into a second liquid substantially free of solid matter, comprising: (a) Feeding the first liquid chemical into the mixing apparatus so that a spiral flow of the liquid chemical is established. (b) Introducing the second liquid into the mixing apparatus into communication with the spiral flow of liquid chemical. And (c) discharging the second liquid mixed with the liquid chemical, from the mixing apparatus into a fourth liquid. 
     According to another aspect of the invention there is provided mixing apparatus for mixing a liquid chemical and a second liquid comprising: A casing with inlet conduits therein for the chemical to be mixed and the second liquid and one outlet conduit. A member located inside the casing essentially concentrically with the casing, the member having an outer shell which defines inside the casing an annular space outside the shell and a space inside the shell. And a chemical conduit connected to the space inside the shell. 
     According to another aspect of the invention there is provided Mixing apparatus for mixing a liquid chemical and a second liquid comprising: A casing having an inlet conduit for the liquid chemical, an inlet conduit for the second liquid, an open interior and a single outlet conduit. And the inlet conduit for the liquid chemical connected to and opening into the casing interior so that chemical ted into the liquid chemical inlet conduit flows spirally within the casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the method and apparatus according to the invention are disclosed in more detail with reference to the appended figures, where 
     FIGS. 1 a ,  1   b  and  1   c  illustrate prior art retention aid feeding apparatuses, 
     FIG. 2 illustrates a retention aid feeding process according to a preferred embodiment of the invention connected with the short circulation of a paper machine, 
     FIG. 3 illustrates a retention aid feeding-/mixing apparatus according to a preferred embodiment of the invention, 
     FIG. 4 illustrates a retention aid feeding-/mixing apparatus according to a second preferred embodiment of the invention, 
     FIG. 5 illustrates a retention aid feeding-/mixing apparatus according to a third preferred embodiment of the invention, 
     FIG. 6 illustrates an arrangement of a retention aid feeding-/mixing apparatus in connection with the fiber suspension flow channel according to a preferred embodiment of the invention, 
     FIG. 7 illustrates an arrangement of a retention aid feeding-/mixing apparatus in connection with the fiber suspension flow channel according to a second preferred embodiment of the invention, 
     FIGS. 8 a  and  8   b  illustrate an arrangement of a retention aid feeding-/mixing apparatus in connection with the fiber suspension flow channel according to a third preferred embodiment of the invention, 
     FIG. 9 illustrates a detail of the retention aid feeding process of FIG. 2 according to a preferred embodiment of the invention, 
     FIG. 10 illustrates an alternative to a detail of the retention aid feeding process of FIG. 9 according to a second preferred embodiment of the invention, and 
     FIG. 11 illustrates an alternative to some details of the retention aid feeding process of FIGS. 9 and 10 according to a third preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to FIG. 1 a , prior art feeding arrangement of retention aid comprises a fiber suspension flow channel  2  surrounded by an annular retention aid manifold  4 , into which retention aid is introduced through conduit  5 . Therefrom a number of feed pipes  6  (in the figure four feed pipes) lead to the flow channel  2 , which feed pipes open into the flow channel  2  so that the retention aid from feed pipes  6  may freely flow into the fiber suspension. As already mentioned, the feeding according to prior art is carried out so that the chemical is allowed to flow into the fiber suspension at a relatively low pressure difference, whereby the final mixing is presumed to take place in a mixing apparatus, such as e.g. the headbox feed pump or the headbox screen. FIGS. 1 b  and  1   c  illustrate a second, alternative solution. In this solution, two retention aid feed pipes  16  are arranged inside flow channel  2 , said feed pipes having feed holes or feed slots  18  in the area inside the flow channel. In the latter alternative, retention aid is more efficiently mixed with the flowing fiber suspension, because the retention aid may be proportioned also into the center of the flow. 
     FIG. 2 illustrates an arrangement of the short circulation of a paper machine partially according to both prior art and a preferred embodiment of the invention, mainly in view of retention aid introduction. In a process according to FIG. 2, the fiber suspension to be fed to the paper machine is diluted to applicable consistency in a wire pit  20  with white water from the paper machine  22 , although a separate mixing tank may be utilized. Other adequate liquids may be used for dilution too, if desired, as for instance filtrate from a white water filter. From the wire pit  20 , the fiber suspension is guided by means of a pump  24  to centrifugal cleaning  26  and further to a gas separation tank  28 . Gas-free fiber suspension is pumped by means of a headbox feed pump  30  into a headbox screen  32 , and after that in a feeding-/mixing apparatus  34  a retention aid is added into the fiber suspension prior to transporting the fiber suspension to the head-box  36  of the paper machine  22 . The process arrangement described above may be considered as prior art. 
     In FIG. 2 there is also a schematic illustration of the treatment of a retention aid prior to the retention aid is fed into the fiber suspension. The retention aid in liquid or powder form is mixed into fresh water, clean water in order to avoid flocculation, in a container  40 , wherefrom the retention aid solution is proportioned by means of a pump  42  directly into a feeding-/mixing apparatus  34 . In arrangements according to prior art, the retention aid solution was either taken into a second mixing container where it was further diluted to a final concentration of about 0.05-0.1%, or the corresponding dilution was carried out in the flow channel. FIG. 2 shows further a pipe  44  leading from the wire pit  20  of the paper machine to the mixer  34 . In other words, in an arrangement according to this embodiment, white water is applied from wire pit  20  into the mixer  34  for further dilution of the retention chemical, which white water thus contains fines filtrated off the fiber suspension through the wire. Naturally, for instance filtrate from white water filter or some other filtrate obtained from the process may be used for the dilution. Another additional possibility shown in FIG. 2 is a pipe  48 , through which more clean water or fresh water may be introduced into the retention aid solution in order to dilute the solution, if desired. 
     FIG. 3 illustrates schematically a mixing apparatus according to a preferred embodiment of the invention. The mixing apparatus  34  according to FIG. 3 is, in fact, a nozzle comprising preferably an essentially conical casing  50 , flanges  52  and  54  arranged into it and preferably, but not necessarily, placed at its opposite ends, and a conduit  56  for the retention chemical. The mixing apparatus  34  is connected via flange  52  to a dilution medium pipe (whip water pipe) and via flange  54  to the fiber suspension flow channel. In the arrangement according to the fig., the casing  50  of the mixing apparatus  34  is converging from flange  52  towards flange  54  inside of which is the opening  58  of the mixing apparatus. A purpose of the conical form of the casing  50  is to accelerate the medium flow in the mixing apparatus  34  so that the velocity of the jet discharging from the mixing apparatus  34  into the fiber suspension flow is at least three times, but preferably about five times the velocity of the fiber suspension flow. This velocity difference ensures that the retention chemical jet penetrates quickly enough and deep enough into the fiber suspension flow to be mixed with the fiber suspension essentially more homogeneously than in prior art embodiments. In the embodiment according to FIG. 3, the retention chemical feeding conduit  56  is preferably tangential in order to ensure that retention aid discharging through opening  58  of the mixing apparatus  34  into the fiber suspension flow is distributed homogeneously at least on the whole periphery of the opening  58 . At the same time, tangential feeding ensures that the retention chemical is mixed into the whip water under minimum possible shear forces in order to prevent the polymeric chains of the chemical from degrading. 
     FIG. 4 illustrates as an additional embodiment of the mixing apparatus  34  of FIG. 3 a hollow annular member  60  arranged centrally inside the mixing apparatus  34 , into which member the retention aid is guided via conduit  56 . In this embodiment, the member  60  essentially comprises two rotationally symmetrical shells  59  and  61  and possibly one end wall  62 . Further, at the end of member  60 , on the fiber suspension flow channel side, there is a preferably annular opening  64  provided, through which the retention chemical is allowed to be discharged into the fiber suspension. The retention chemical conduit  56  pierces the wall of the conical casing  50  of the mixing apparatus  34  and further leads via the annular space between the conical casing  50  and the member  60  into the member  60  through the outer shell  59 , at the same time preferably carrying the member  60  in its place. In this embodiment, the inner shell  61  restricting the member  60  is cylindrical and forms or comprises a pipe  62 , through which part of the dilution medium flow i.e. whip water is allowed to discharge into the fiber suspension flow. In this embodiment, the retention aid flow guided tangentially into member  60  turns in form of a spiral flow towards its own annular opening  64 , through which the retention aid is discharged as a fan-shaped jet into the fiber suspension together with the dilution liquid discharging in this embodiment both from outside the opening  64  through the annular opening  58 , and from inside the opening  64  through pipe  62 . An additional purpose of member  60  is to further throttle the cross-sectional flow area of the mixing apparatus in order to insure a sufficient velocity difference between the retention aid flow and the fiber suspension flow. A second purpose of member  60  is to enable the mixing of the retention aid with the dilution liquid to take place essentially at the same time that the retention aid is being fed into the fiber suspension flow. The figure clearly shows that the retention aid need not necessarily be in any contact with the dilution liquid before it is discharged through its opening  64  into the fiber suspension flow channel. 
     FIG. 5 illustrates a retention aid feeding-/mixing apparatus according to a third preferred embodiment of the invention. In principle, the apparatus is exactly similar to the one of FIG. 4, but it clearly differs from previous apparatuses by both its coupling to the process and by its operational characteristics. In the apparatus of FIG. 5, the inner pipe  62  of member  60  is connected to the process via its own flow path  162  and the outer pipe of the apparatus  34 , forming the wall of the conical casing  50 , via its own flow path  144 . Both flow paths  144  and  162  are provided with flow regulation devices  146  and  164 , preferably valves. The flow pipe  144  functions as already stated before, but into the inner pipe  62  of member  60  it is now possible to introduce e.g. either clean water, some circulation water from the paper mill, white water, clear filtrate or some other non-clean liquid suitable for that purpose, even fiber suspension fed into the headbox. Further, through flow path  162  it is possible to introduce, if desired, a retention aid component, especially in question of a retention aid containing several components. As an example, a short-chain retention chemical might be mentioned, in case the retention aid is formed of a long-chain and a short-chain chemical. In that case, the long-chain chemical is supplied tangentially into member  60  earlier, through conduit  56  illustrated in FIGS. 3 and 4. That is, liquids introduced through flow paths  144  and  162  may be of similar or different character, depending on the application. 
     An advantage of separate feeding through flow path  162  is that by changing the amount of the feed, the effect of the liquid discharging from inner pipe  62  on the mixing of the chemical may be regulated. For instance, by introducing a large amount of liquid through inner pipe  62 , the retention chemical is made to penetrate deeper into the fiber suspension flow. Accordingly, by feeding in a smaller amount of liquid through inner pipe  62 , the penetration of the retention chemical is reduced, too. 
     Further, it is worth mentioning that in a solution according to both FIG.  4  and FIG. 5, the retention chemical feed is very gentle compared to prior art methods of retention chemical introduction. As the retention chemical in any case is formed of molecules composed of polymeric chains, these should be fed with additional water introduction as gently as possible, in order to prevent the very sensitive polymeric chains from breaking and, subsequently, in order to avoid a remarkable reduction in the effect of the retention chemical. When the chemical is supplied in the apparatuses according to FIGS. 4 and 5 as a fan-shaped jet into the water discharged through the annular opening  58 , shearing forces between the water and the chemical solution are reduced to minimum. The desired functioning of the feeding-/mixing apparatus according to the invention is proved by the test results, which show that the utilization of the apparatus according to the invention improves wire retention by at least 10%. The only explanations for the advantageous test results are more precise and more efficient mixing of the chemical and reduction in the degradation of the polymeric chains of the chemical during the mixing. 
     As a further preferred embodiment of the apparatus according to the invention, the improvements made in the feeding-/mixing apparatuses of FIGS. 4 and 5 are worth mentioning. Our tests showed that the position of both the inner pipe  62  of member  60  and the outer shell  59  of member  60  in the axial direction of member  60  in relation to the end of the casing  50  of the feeding-/mixing apparatus  34  has an effect on the efficiency and accuracy of chemical mixing. Thus, in the most advanced version both said shells  59  and  61  are made separately movable in the axial direction of member  60 . One possibility of doing this is to arrange the inner pipe  62  totally separate so that it slides along the inner surface of the inner shell  61  of member  60  and further in relation to the member  60  itself so that the member  60  slides in relation to the inner pipe  62 . In that case it is, naturally, advantageous to supply the liquid into both the inner pipe  62  and the member  60  in their moving direction i.e. in the axial direction, whereby the liquid feed pipes (corresponding to conduit  56  and flow path  162  of FIG. 5) are arranged slidably sealed in relation to the member  60  and the inner pipe  62 . 
     A further additional modification of the feeding-/mixing apparatus according to the invention is to arrange at the end of the inner pipe of member  60  or at the end of pipe  62  arranged inside member  60  a nozzle head which closes the opening of pipe  62  at the axis, leaving an essentially annular slot between itself and the rims of the pipe opening. This construction insures that the liquid jet discharging from pipe  62  is well-spreading and of essentially conical form. 
     FIG. 6 illustrates schematically a possible arrangement of the feeding-/mixing apparatuses  34  of FIG. 3 in connection with the fiber suspension feed pipe  70 . In principle, this is carried out in a way demonstrated in FIG. 1 a . The only difference from the prior art method according to FIG. 1 a —excluding the feeding of dilution liquid into the mixing apparatus and the point that as dilution liquid something else than clean water is used—is, in practice that the retention chemical solution discharging from the mixing apparatus  34  is planned to penetrate so deep into the fiber suspension flow in the feed pipe  70  that the retention chemical is mixed practically into the whole fiber suspension flow. 
     FIG. 7 illustrates a second preferred method of feeding a retention chemical from the mixing apparatus  34  into the fiber suspension flow. In this embodiment, the mixing apparatuses  34  are arranged staggered opposite each other e.g. at the accept outlet  72  of the headbox screen or at another pipe of corresponding shape. The end of said outlet  72  facing the screen housing is arranged as essentially rectangular, from which point on, towards the feed pipe  70  leading to the headbox, it takes a round shape. The mixing apparatuses  34  are placed at the side walls of the outlet conduit  72  so that the retention aid jets discharging from the mixing apparatuses cover an essential part of the total cross section of conduit  72 . Only at two comers of conduit  72  there is a small uncovered space left, which is not significant in respect of the mixing of the retention aid, as the fiber suspension flow when discharging from the screen is in such a heavy turbulence that the retention aid is mixed practically completely into the fiber suspension during the short interval available for that. 
     FIGS. 8 a  and  8   b  illustrate still a further alternative solution for the construction of a mixing apparatus according to the invention. The solution is mainly based on a round pipe according to FIG. 6, whereby there is a problem, especially in question of big pipes that liquid jets of mixing apparatuses penetrate into the pulp flow in a round pipe only to a restricted depth. Thus, jets from mixing apparatuses placed on the periphery of the pipe do not necessarily, in all circumstances, get into the center of the pipe, and the chemical is not mixed therein. And, if all jets from mixing apparatuses placed on the periphery of the pipe do get into the center of the pipe, the crossing areas may be subjected to chemical overdosing. The said problem has been avoided in the embodiment according to the figure by changing the shape of pipe  78  at the mixing point to be elliptical (preserving advantageously the same cross-sectional flow area). The mixing apparatuses  34  are placed on the periphery of the ellipse so that their jets are directed through the narrowest part of the ellipse, as shown in FIG.  8 . In the embodiment according to the figure, the distance from the mixing apparatus  34  to the opposite side of pipe  78  is reduced by half compared to an analogous situation in a round pipe (FIG.  6 ). The amount and location of the mixing apparatuses  34  are chosen so that jets from the mixing apparatuses  34  form an essentially even cover on the cross section of the elliptic pipe  78 . As advantages compared to a round pipe, it is worth mentioning that practically 100% of the pipe cross section is covered by the jets, and further the fact that in an elliptic pipe, just as in a rectangular pipe according to FIG. 7, overlapping, crossing jets are not formed. As a result, no local overdoses occur and neither passing through of untreated pulp, i.e. pulp which has not come to contact with the retention chemical. An elliptic flow channel is arranged separately in a longish direct pipe line, for example according to FIG. 8 b , or e.g. the accept opening of the headbox screen is made elliptic or rectangular. FIG. 8 b  illustrates an arrangement of the mixing apparatus/es in an elliptic pipe section  78  between cylindrical pipe sections  80 ′ and  80 ″. Preferably the reshaping of the cross section of a pipe from elliptic to cylindrical and vise versa is performed so that the cross sectional area remains constant, which means that also the flow speed, accordingly, remains constant. 
     FIG. 9 illustrates the coupling of a mixing apparatus  34  fixed in a flow channel leading to the headbox with various pipe lines. As seen already from FIGS. 3 and 4 and partly from FIG. 2, retention aid solution produced in a solution tank  40  (FIG. 2) is transported to conduit  56  of the mixing apparatus  34  through pipe  43 . Pipe  43  is provided with a filter  74  for separating from the solution the insoluble materials possibly left therein. If desired, additional dilution water, preferably clean water, may be brought into the retention chemical solution through pipe  48 . In this embodiment, that is illustrated to take place between filter  74  and the mixing apparatus, but it is naturally possible to introduce the additional dilution liquid into the upper-flow side of filter  74 . This is not necessary, though. Additionally, a suitable feeding liquid is introduced into the mixing apparatus  34  through pipe  44  fixed on flange  52 , which feeding liquid may be white water from the wire pit according to an embodiment of FIG. 2, clear or turbid filtrate or some other liquid suitable for the purpose. 
     FIG. 10 illustrates an alternative to the feeding liquid of FIGS. 2 and 9. FIG. 9 illustrates a minor side flow from feed pipe  70  into pipe  44 , which side flow is fed at an increased pressure by means of a pump  76  into the mixing apparatus  34 . In other words, as feeding liquid the same fiber suspension that is already being fed into the headbox is used. 
     FIG. 11 illustrates further the coupling of the feeding-/mixing apparatus of FIG. 5 with the rest of the process. The figure shows how white water from the wire pit, clear or turbid filtrate or some other liquid suitable for the purpose, or fiber suspension being fed to the headbox in principle exactly in accordance with FIGS. 9 and 10, is supplied into the apparatus through flow path  144 . But, according to the embodiment of FIG. 5, the inner pipe  62  of member  60  of the apparatus  34  is connected to an outer flow path  162  which may lead either to a retention chemical solution tank  140 , various sources of additional liquid, e.g. white water, clear or turbid filtrate etc., or to a source of clean liquid. Further the figure illustrates how both flow paths  144  and  162  are provided with valves  146  and  164  for regulating the liquid flow in said flow paths in a desired way. 
     As for the feeding-/mixing apparatus described above, one has to understand that, although it is most preferably operating and located when fastened directly in the flow channel wall, whereby the mixing of the retention chemical into the “whip water” may be carried out practically at the interface of the feeding-/mixing apparatus and the flow channel, it is, of course, possible to place the feeding-/mixing apparatus according to the invention further away from the fiber suspension flow channel. A precondition for this is, however, that all the liquids used in the mixing are clean waters, i.e. without suspended matters that the retention chemical might react with. In other words, by essentially increasing the consumption of clean water, the mixing of the retention chemical into the whip water may be arranged to take place further away from the fiber suspension flow channel leading to the headbox. At the same time, almost all advantages mentioned above may be obtained. The only disadvantage, apart from the increasing consumption of clean water, is a slightly harder treatment of the retention chemical in the stage when it is actually mixed into the fiber suspension. 
     When the mixing apparatus is placed further away from the fiber suspension flow channel, the retention aid has time enough to be completely mixed into to the so-called whip water, whereby, when this discharges into the fiber suspension flow duct, part of the retention chemicals is subjected to shearing forces strong enough to cause part of the polymeric chains to degrade and the retention chemical to possibly lose some of its effect. 
     Nevertheless, when the mixing of the retention chemical into the so-called whip water in the actual feeding-/mixing apparatus has been carried out gently, i.e. by feeding the retention chemical in a tangential flow through an annular opening  64  into the whip water discharging from an outer annular opening  58  at an exactly appropriate speed so that practically no injuriously great shear forces are generated between the liquids, the retention chemical is not damaged prior to the actual mixing into the fiber suspension, whereby practically the whole retention aid with its total effect is still usable when being mixed into the fiber suspension. 
     In addition to the embodiments described above, it is, of course, possible to arrange a special mechanical mixer in connection with the mixing apparatus, by means of which mixer the retention chemical solution is mixed into the feeding liquid. When applying this method, a mixing apparatus according to FIGS. 3 and 4 with its tangential feeding of retention chemical is not necessarily needed. Accordingly, a high-pressure pump for transporting the retention chemical solution into the mixing apparatus is not necessarily needed, either, because the mechanical mixer that is used may be a mixer that increases the feeding pressure. 
     As may be seen from the above, a new method of feeding and mixing a retention chemical into fiber suspension flow has been developed. Referring to what has been stated here, one has to notice that the figures illustrate many different embodiments of the invention suitable to be used together depending on what is needed. Further, one has to notice that although the invention has been illustrated in the text only in connection with the mixing of retention chemicals in paper manufacturing, the invention may be utilized also in other connections demanding homogeneous and, at the same time, gentle mixing of a chemical into a liquid. Further one has to notice that none of the embodiments illustrated in the figures excludes the possibility that the arrangement to be applied and protected by the patent claims might be simpler than the entity illustrated in the figures. Thus, the field of application and the scope of protection of the invention are described by the appended patent claims only.