APPARATUS AND PROCESS FOR THE SORTING OF A PULP SUSPENSION

Abstract: The invention relates to a centrifugal separator and a process for sorting a fibre suspension. Said centrifugal separator (10) comprises a rotor (24) coaxially inside a housing (12). The mantle of said rotor (24) has slots (26, 30) through which part of said suspension can be caused to pass from a space (25) inside of said rotor to a space (28) outside said rotor and from said space (28) outside said rotor to said space (25) inside said rotor. Said housing (12) comprises means (32) for introducing dilution water into the suspension. The separator according to the invention is particularly preferred for the use as a cleaner in the short circulation of a papermaking machine.

Description: Apparatus and process for the sorting of a pulp suspension. The present invention relates to a process and an apparatus for sorting a fibre suspension by separation of a relatively lighter and a relatively heavier fraction by means of centrifugation. Particularly the invention relates to the use of said process and apparatus for the cleaning of papermaking stock in the wet end short circulation of a paper machine. According to the invention the function of cleaners widely used in the pulp and paper industries can be optimized and made more efficient so that the fibre losses remain small and the cleaning effi¬ ciency high while cleaning the stock in one single cleaning stage. Cleaners known in the prior art are usually conical vessels, into the larger end of which the material to be cleaned is fed tangentially. A heavier fraction is removed from the narrower end of said vessel and a lighter fraction is removed centrally from the larger part of said vessel. In such cleaners the material can be split into two fractions, or in other words, either heavy particles can be removed from the suspension and rejected, or light particles, as for example plastics, may be removed. In either case a cleaned pulp suspension is obtained from the other outlet. Some times multiple outlets are arranged, aiming at the simultaneous separation of more fractions, whereby, however, the cleaning efficiency is not at its best. The problem by conventional cleaners thus remains the separation efficiency and the cleanliness of the desired fraction. It is clear that a cleaner can be built so, that it removes essentially all heavy debris from the processed material, but in this case also a comparably large proportion of acceptable material is lost. A corresponding problem can be observed when removing light debris. So for example in the pulp and paper industry solutions can be seen, where mul¬ tiple stage centrifugal cleaning is used, where the accept as well as the reject of the first cleaning stage are further treated even in multiple stages, in order to achieve the desired degree of cleanliness. So, the cleaning systems of a paper mill end up using huge cleaner batteries, consisting of up to tens of cleaners in every stage, and thus already the space required is very large. In addition because, as known, the cleaners are susceptible to wear due to the fast movement and high g-forces of sand and other heavy material along their surface, the huge number of cleaners lead to an increase of the damage sensi- tivity and maintenance need for the whole cleaning plant. Among others F.J. Saint-Amand has attempted to resolve said problems and in his US Ppatent 4,443,331 he discloses a centrifugal cleaner. Said cleaner consists of an essentially cylindrical rotating chamber, in which the light frac¬ tion moves to a vacuum zone forming in the center of said chamber and the heavy fraction moves towards the mantle of said rotating chamber. The light fraction is removed from the center of the separator either at its inlet end or from the outlet end opposite to the inlet end and the heavy fraction collecting at the mantle is removed from the periphery of the chamber. According to a particular embodiment of the apparatus, a separate liquid is fed along the mantle of the apparatus, which liquid dilutes the suspension collecting at the mantle, washing out lighter components from the heavier fraction. In his international Patent Application published as WO 90/02839 said inventor discloses an improved apparatus, where a rotating body consisting of two conical parts has been installed in the center of the rotating vessel, with the aim of improving the function of the apparatus and act as a discharge for light reject. In this embodiment the inventor has not suggested to introduce dilution and washing liquid at the mantle. The feeding of dilution liquid is suggested only for washing fibres from heavy reject already separated in order to reduce fibre losses. In both solutions mentioned above the cylindrical mantle of the centrifugal cleaner rotates at a high speed, whereby the junctions to the stationary fee¬ ding and outlet pipes require solutions which are technically inconvenient and subject to wear, which has obviously for its own part influenced the lack of success of said apparatuses. The traditional paper machine wet end involves huge volumes of circulating backwater and many feedback loops for secondary cleaning and screening stages. Copendϊng Patent Application FI 922285 by the present inventor dis¬ closes a solution to the resulting problems regarding controllability and clean¬ liness by providing a process eliminating the huge water volumes and the feedbacks of a traditional paper machine short circulation. The object of the present invention is to avoid the inconveniences of the prior art and to provide a technically well functioning apparatus, with an excellent separation efficiency and sorting capability. The object is to improve the function of cleaning systems used in the pulp and paper industry so, that instead of the prior use of multiple consecutive cleaners, the cleaning can be made efficiently in one single apparatus, so that the suspension can be fed through one cleaner rapidly while saving energy and effectuating repeated washing of the heavier fraction integratedly during the cleaning process. In the most preferred embodiment of the invention, acceptable fibres are continually separated from the already separated heavy fraction and from the already separated light fraction and transferred into the accept flow constituted by the intermediary fraction. The invention is particularly favorable for the use in the short circulation of a paper web forming process, where a controlled stock dilution and fast recirculating can provide an essential improvement of the process. The particular features of the invention are defined in the appended claims and appear from the following description and accompanying drawings. Thus the object of the invention is a centrifugal separator for the sorting of a fibre suspension, said separator comprising an essentially cylindrical housing at one end of which is arranged a feeding inlet for fibre suspension to be cleaned, and at the opposite end of which there are outlets for a lighter fraction and a heavier fraction. It is characteristic for the apparatus according to the inven¬ tion, that inside said cylindrical housing a coaxial hollow rotating rotor is arran¬ ged for receiving said fibre suspension or an essential part of the same, that the mantel of said rotor has slots or openings, through which a part of said suspension can be brought to pass from a space inside said rotor mantle to a space outside, and from said space outside said rotor to said space inside, and that said separator has means for introducing a dilution and washing medium into said suspension for improving the separation efficiency between said ligh¬ ter and said heavier fractions. In a particularly favorable embodiment said dilution and washing medium is fed close to the mantle of said housing in multiple places. In a favorable embodiment of the invention, a tube is arranged coaxially in the center of said rotor acting as a sorting channel, for separating a light fraction. In the process according to the present invention for sorting a fibre suspension in a centrifugal separator said fibre suspension is fed through an essentially cylindrical housing in said separator and said suspension is caused to rotate in said separator so that said suspension due to a centrifugal force is separated according to the density of its components into at least two fractions. Said process comprises the steps wherein a) said fibre suspension is fed into the inlet end of said separator; b) said suspension flow or a substantial part thereof is caused to pass into a hollow rotor rotating coaxially inside said cylindrical housing; c) the heavier fraction of said suspension is brought to move toward the mantle of said rotor; d) the heavier fraction accumulating at said mantle of said rotor is conducted out from said rotor into a space between said rotor and said housing to form a heavy fraction; e) a dilution / washing medium is introduced into said heavy fraction; f) a lighter subfraction of said heavy fraction in said space between said rotor and said housing is conducted into the lighter fraction in said rotor; g) at least a part of the stages c) to f) is repeated if needed; and h) said heavy fraction cumulating outside said rotor at said housing and said lighter fraction cumulating inside said rotor are separately discharged from said separator. In a preferred embodiment according to the present invention an inject suspen¬ sion is divided into three fractions, an intermediate accept fraction constituting the main suspension flow. A heavy fraction and a light fraction constituting heavy and light reject, respectively, are separated from said suspension. Further, a lighter subfraction is separated from said heavy fraction and a hea¬ vier subfraction is separated from said light fraction, the two subfractions being brought back into said intermediate fraction. In said preferred process said heavy fraction is collected outside said rotor and said light fraction is collected inside a central tube installed inside said rotor. The invention will be explained in greater detail in the following description making reference to the attached drawings, wherein Fig. 1 represents a schematic section of a centrifugal separator according to a preferred embodiment of the invention; Figs. 2 to 7 represent sections of the centrifugal separator according to Fig 1 , along lines A-A, B-B, C-C, D-D, E-E and F-F correspondingly; Fig. 8 represents a schematic section of a centrifugal separator according to another embodiment of the invention; Fig. 9 represents a flow sheet of a papermaking process in which the use of the invention is particularly preferred. In the following reference is made particularly to the embodiments presented in the Figures, whereby the corresponding parts are numbered by the same numbers in all the Figures. It should, however, be remembered that the Figures are only clarifying examples of solutions according to the invention, and that they are by no means intended to limit a the invention. Referring to Fig 1 the separator 10 according to the invention, has an essen¬ tially cylindrical housing 12 and in one end 14 of said separator an inlet 1 6 for the suspension to be cleaned. In the opposite end 1 8 of said separator 10 there is an outlet 20 for the lighter fraction and correspondingly an outlet 22 for the heavier fraction. Inside said cylindrical housing 12 there is installed a hollow rotating rotor 24, which functions as the principal sorting channel 25 for the fibre suspension. In the presented embodiment said rotor is cylindrical and its mantle has slots 26, which permit part of the suspension flowing in sorting channel 25 to flow into the space 28 between rotor 24 and housing 1 2 which space forms a channel for the heavy fraction. The mantle of said rotor 24 there are other slots 30, which are directed so, that part of the suspension flowing in the heavy fraction channel 28 can easily transfer into the sorting channel 25. The edges of channels 26 and 30 are shaped to form oblique smoothly streamlined channels in the desired direction of suspension flow. The rotor 24 can also be of a different than cylindrical shape, for example a polygon, whereby openings for letting the lighter fraction pass into the sorting channel 25 in the rotor are arranged in the parts of the edged mantle closest to the center of the separator 10. Correspondingly, openings for letting the heavy fraction separating from the intermediate fraction in channel 25 pass into the heavy fraction channel outside the rotor are arranged in the parts most distant from the center of the edged rotor. In the embodiment represented in Fig 1, the mantle of rotor 24 consists of multiple cylindrical parts, between which the slots 26, 36 remain. The cylind¬ rical parts are united by radial separation walls 31 seen in Fig 5. At the outside of the housing 12 are arranged feeding pipes 32 for dilution and/or washing liquid, which in the presented embodiment are formed as annular inlets 35, through which liquid can be evenly distributed into the heavy fraction accumulating close to the inner surface of housing 12. Thus a laminar zone forms and retains lighter material, which forms a lighter subfraction and flows with the liquid into sorting channel 25 through slots 26, whereas heavy material transfers the laminar layer, accumulating at the inner surface of the housing. The fluid fed through feeding pipes 32 may, preferably, be water, like backwater recycled from the process. The fluid may also be clean water or for example gas, like air, which, being lighter than the suspension will strive towards the center of the separator, carrying lighter material with it. At the outlet end of the separator is formed an expanded zone between the rotor 24 and the housing 12, which forms a chamber 34 for the heavy reject. An outlet 22 for heavy reject is installed at the chamber 34. At said heavy fraction chamber 34, preferably close to its inlet end there is arranged an inlet 32' for dilution/washing medium. Between rotor 24 and the outlet end of housing 12 there is a slot 36, which allows the lighter subfraction separating from the heavy fraction in chamber 34 to flow from the chamber into the sorting channel 25. According to the inven¬ tion, the slot 36 is preferably adjustable for controlling the flow of the lighter subfraction separating from chamber 34 to the sorting channel. The slot 36 can be adjusted by moving the rotor 24 in relation to the housing 12, by means of levers or other suitable means {not shown) . In the embodiment of Fig 1 the housing 12 of the separator 10 is not evenly cylindrical, but has expansions 38 at which the channel 28 for the heavy frac¬ tion is more spacious. The expansions 38 are preferably arranged in connec¬ tion with the fluid inlets 32. The section of housing 12 is correspondingly restrained after the expanded zone 38 so, that a throttling of heavy fraction channel 28 results. By the throttling of the heavy fraction channel 28 a better guiding into the sorting channel 25 of the lighter subfraction, separating from the heavy fraction is effectuated. In the presented embodiment the housing 12 of the separator 10 is static, which is preferred in view of the technical execution of the apparatus. The housing can, however, be made rotating without disturbing the essential func¬ tion of the process according to the invention. In the center of rotor 24 functioning as a sorting channel 25 inside the housing 12 there is preferably a tube 40 which forms a channel 42 for . the light fraction and/or gases. The central tube 40 rotates together with the rotor 24 and in the presented embodiment functions as a shaft for the rotor 24. The rotor 24 is turned by a drive (not shown), installed at the upper or lower end or both of the rotor. Alternatively the rotor may be free wheeling without mechanical drive. In this case the kinetic energy of the suspension fed into the inlet end and/or extrac¬ ted at the outlet end is transformed into rotating energy by means of a turbine effect, which turns the rotor in order to reach a separation effect. As can best be seen from Fig 5, in the presented embodiment, having a drive turning the rotor 24, the rotor is connected to the central tube 40 by radial partitions 31 , which divide the sorting channel 25 into axial sorting chambers. In the wall of tube 40 are slots or openings 37, 39, the edges of which are streamlined oblique for promoting the flow of suspension between the sorting channel 25 and the light fraction channel 42. Thus, during the sorting process, the light fraction separating from the intermediary fraction in sorting chamber 25 flows through slots 39 into the tube 40 and correspondingly a heavier sub- fraction can flow through slots 37 from the light fraction channel 42 back to the sorting channel 25. Even if it is not shown in the Figure, the tube 40 can be throttled in the same manner as explained before regarding the heavy frac¬ tion channel for guiding the fractions into and out from the light fraction chan¬ nel 42. In the presented embodiment, the light fraction channel 42 ends in a central light reject outlet 43. In the embodiment of Fig. 1 there is a solid center piece 44 in the housing 12 at the inlet end of separator 10. Said center piece 44 forms a continuation of the tube 40 and acts as a rotation shaft. Due to said center piece 44 an annu- lar channel 45 is formed at the inlet end of the separator. A first sorting of the suspension will take place in said annular channel between the housing 12 and the center piece 44. In this area the suspension is brought to rotate firstly by the tangential direction of the inlet 16 at the inlet end 14 and secondly by the vanes 46 attached to the center piece 44, shown in Fig 3. At the joint bet¬ ween center piece 44 and the hollow central tube 40 is a slot or opening 41 , through which the light fraction cumulated at the center piece 44 can transfer into the central tube 40. In the presented embodiment, the rotor 24 reaches from the outlet end 18 of the separator toward the inlet end 14, ending however a certain distance from the inlet end, so that said annular channel 45 is formed. The function of the apparatus is independent of its measures or proportions, which can be varied according to the particular needs. Thus for suspensions of different types and of different consistency the preferred proportions may dif¬ fer from each other. The fibre suspension fed to the paper web forming is usually in the range of 0.2 to 2%. Because the amount of heavy reject in this connection is usually small, the space between housing 12 and rotor 24 is preferably only a fraction of the diameter of the very rotor. As can be seen in Fig. 2, the inlet 16 of the separator 10 is shaped as a tangentially directed, narrowing pipe which enables the transformation of the energy of the suspension into rotation energy. In certain operating condition the turbine effect thus reached may be sufficient for turning especially a light rotor, and no separate means for turning the rotor may be needed. Fig. 3 shows the said solid center piece 44, and the attached radial vanes 46. Close to the inlet, the vanes 46 are bent to form turbine blades so that as much rotation energy as possible is recovered from the fed suspension flow. Further off the inlet end the vanes straighten and accelerate the rotation of the suspension, helped by the driven shaft 44. The vanes 46 divide the sorting channel 25 into separate axial channels, in each of which the centrifugal force causes a separation of a lighter and a heavier fraction. Fig. 4 represents the section of the separator, where the first inlet 32 for dilu¬ tion/washing fluid joins the housing 12. As can be seen the inlet 32 is arranged in an annular way, distributing the fluid evenly around the mantle of housing 12. After inlet 32, the mantle is somewhat expanded so, that the fluid can be caused to flow as laminarly as possible into the heavy fraction flowing past the inlet for washing out the lighter fibres from said heavy fraction. A fluid can be introduced into the separator 10 also earlier, for example close to the inlet end, forming a thin fluid layer at the surface of the mantle of hou¬ sing 12, so that the heavy fraction striving toward the mantle has to pass this fluid layer. The fluid can be fed into the separator tangentially or radially, lami¬ narly or turbulently, around the mantle or locally, depending on the effect desi¬ red by the fluid feeding at each point. Fig. 5 represents a section of the separator 10 at the sorting zone. It shows the mantle of housing 12, the rotor 24, the central tube 40 and the partitions 24, joining tube 40 and rotor 24 and reinforcing the construction. The parti¬ tions 31 divide the cannel 25 into separate channels, in which the sorting proceeds efficiently. Each of the separate channels formed in sorting channel 25 opens to the heavy fraction channel 28 as well as to the light fraction channel 42 through said slots or openings 26, 30, 37, 39. Fig. 6 represents the section of the separator 10 at the expanded section of outlet end 18. Wing-like means 33 reach from rotor 24 into the heavy fraction chamber 34, accelerating the heavy fraction, and improving the separation efficiency. The discharge pipe 22 for heavy fraction is arranged at the outer¬ most end of the chamber 34. The inner channel of central tube 40 may be throttled at this point in order to reduce the proportion of the lightest fraction. As seen in the Figure, it can also be somewhat expanded at its outer side for obtaining a desired suspension flow pattern. At the outlet end, the partitions 31 should preferably be bent as turbine blades in order to recover the rotation energy as efficiently as possible. Fig. 7 represents the extension of sorting channel 25 at the outlet end 18, formed to an energy preserving outlet 20. Even if the Figures 1...7 represent a preferred embodiment of the invention, the invention is not limited to this embodiment, but can have several other forms and arrangements. Thus, for example the separator can be built without any channel for the light fraction, separating the suspension in two fractions only. The light fraction can also be brought to flow toward the inlet end of the separator as shown in Fig 8, representing one of many possible alternative embodiments of the invention. For instance when the heavy fraction constitutes the accept and thus the flow of heavy fraction is relative large, the heavy fraction channel 28 is relatively large and equipped with vanes for keeping the heavy fraction in rotation. No reject chamber 34 is needed. Fig. 8 represents an alternative embodiment of the present invention which like Fig. 1 has an inlet end 14 and an outlet end 18 and inlets and outlets 16, 20, 22 arranged at these. In this embodiment the rotor 24 reaches quite close to the inlet end 14 and the light fraction channel 50 is arranged in the inlet end of the separator 10, so that the light fraction is brought to accumulate close to the outer surface of a hollow tube 52 extending from the inlet end 14. The central tube 52 has a slot or opening 54, through which the light fraction can pass into tube 52. The light fraction is removed from the separator 10 through a channel 50 in the central tube 52, in the direction opposite to the suspension flow. In the shown embodiment the channel 50 has no other connection to the sorting channel 25 than said slot 54. It is, however possible to arrange multiple slots 37, 39 as mentioned in the description of Fig. 1 also in tube 52, which slots make possible the returning into sorting channel 25 of a relatively heavier subfraction from the light fraction channel 50. In Fig. 8 there is a dilution liquid inlet 56 feeding fluid forming a liquid layer at the inner sur ace of housing 12. In the presented embodiment the central tube 52 and a connected solid central piece 58 function as a shaft of the rotor 24. In the following the process according to the present invention is explained making reference to Fig. 1 , where the suspension fed into separator 10 is split into three fractions, i.e. an intermediary accept fraction, a heavy reject fraction and a light reject fraction. It should, however, be observed that the process according to the invention comprises also processes, wherein the suspension is split into two fractions only, of which the outermost one is relatively heavier and the innermost one relatively lighter. Depending on the situation and the suspension to be sorted, the light fraction, the heavy fraction or the interme¬ diary fraction may be accept or reject in respect to the specific sorting process. It is essential for the process according to the invention that an integrated multiple stage sorting is provided in one continuous operation. The process yields a favorable sorting result partly because a dilution/washing medium is introduced into the suspension and partly because a lighter subfraction can transfer back into the intermediary fraction from the already separated heavy fraction. Correspondingly, according to the preferred embodiment of the inven¬ tion a heavier subfraction can transfer back from the already separated light fraction into the intermediary fraction and the separation of the heavy fraction and light fraction, respectively, continue throughout the length of the separator. In the presented example the suspension to be sorted in the separator 1 0 according to Fig. 1 is a fibre suspension to be brought to the web forming part of a papermaking process. The suspension has been diluted to a consistency of 0.2 to 2 %, preferably about 0.5 to 1 .5 %. The suspension is fed into sepa¬ rator 10 through inlet 1 6 so that its kinetic energy is transformed into rotation energy in channel 45. The center piece 44 is turned so that vanes 46 cause the suspension to rotate with a rotation speed, sufficient for causing an effi¬ cient separation, preferably in the range of 300 to 1000 revolutions per minute. In order to keep the surfaces of the separator clean, the axial speed of the suspension shall preferably be maintained at a minimum of about 3 meters per second. The centrifugal forces caused by the rotation of the suspension cause the suspension to separate according to specific weight, so that a heavy fraction strives toward the mantle of the housing 12 of separator 10, and a lighter frac¬ tion cumulates at the surface of center piece 44. Thus an initial sorting of the suspension occurs in channel 45. When the suspension flows forward in the separator, it encounters the rotor 24 rotating in the separator. A substantial part of the suspension is brought to flow inside the rotating rotor. Only the heavy fraction accumulated at the inner surface of the housing 12 remains still outside the rotor. The rotor 24 forms a main sorting channel 25, in which the separation of a heavier and a lighter fraction continues so, that the heavier fraction strives toward the mantle of rotor 24. The heavy fraction accumulated at the surface of housing usually contains a considerable proportion of acceptable fibres. In order to wash out these accep¬ table fibres from the heavy fraction, washing liquid is fed into this fraction through the inlet 32 discharging annularly at the inner surface of the housing 12. Whereas the washing liquid is fed around the mantle as a laminar flow, and especially whereas the diameter of housing 12 preferably simultaneously expands, the washing liquid forms a relatively laminar flow along the inner surface of the housing, through which the heavy material fraction transfers, striving toward the mantle. Hereby lighter subfraction following the heavy frac¬ tion is released and suspended into the washing liquid. In certain conditions it is preferred to utilize a turbulent liquid flow, in order to obtain an efficient mixing. In the sorting channel 25, the flow should, however, constantly remain essentially laminar. The subfraction released from the heavy fraction follow the washing water through the slots 30 in the rotor 24 from the heavy fraction channel 28 into sorting channel 25. The flow from channel 28 into channel 25 can be promo¬ ted by throttling the suspension flow in the heavy fraction channel 28. The feeding of a diluting washing liquid into the heavy fraction cumulating at the inner surface of housing 12 is preferably repeated along the length of separa¬ tor 10 so that an integrated multistage washing of the heavy fraction is obtai¬ ned. Preferably backwater obtained from a forming process is used as the washing liquid. Simultaneously the separation of a heavier and a lighter fraction inside the rotor 24 continues, and the heavier fraction accumulating at the " mantle of the rotor 24 flows through the slots 26 from the sorting channel 25 into the heavy fraction channel 28 being cleaned by influence of the water into said channel 28 so that preferentially only the heavy particles accumulate at the inner sur¬ face of housing 12. It is to be noted, that the washing liquid or part thereof can be fed into the heavier fraction flowing from the rotor for example at the discharge slots 26. in the presented embodiment the suspension is separated into three fractions during the same sorting action, whereby the light fraction accumulated at the central piece 44 together with any contained gas is conducted into the light fraction channel 42 in the center of rotor 24. In the central tube 40 forming the light fraction channel 42 are slots 39 similar to the ones of rotor 24, which permit the heavier subfraction separating from the light fraction to flow back into the sorting channel 25, from which channel correspondingly the separa¬ ting lighter subfraction can transfer into the channel 42. The slots 39 have an obliquely streamlined shape promoting the desired flow. As the separator 10 has multiple inlets 32 for washing liquid, the rotor 24 has multiple slots 26 and 30, and the central pipe 40 has multiple openings 39, the sorting will occur continually and manifold so that at the end of the separator very efficiently sorted suspensions are obtained. At the outlet end of the heavy fraction channel 28 the heavy reject is further concentrated in a reject cham¬ ber 34, where the reject is extracted through heavy fraction outlet 22 at the outermost end of said chamber, whereas acceptable fibres separated from the reject in the reject chamber as a lighter subfraction are brought into the inter¬ mediate fraction constituting the accept through a slot 36 between rotor 24 and housing 12. In a preferred embodiment of the invention, the width of said slot 36 can be adjusted, affecting the flow of suspension flowing from the reject chamber into the accept and thus also the flow of heavy reject. For a certain suspension consistency and type optimum sorting conditions can thus be sought by controlling the suspension flow, the washing fluid flow and the width of slot 36. In the presented embodiment, the intermediate accept fraction is fed into a tangentially narrowing outlet 18 so, that the rotation energy is recovered as pressure in the accept outlet and as rotation energy through the vanes shaped as turbine blades at the outlet 20. The heavy reject cumulating outside rotor 24 is extracted through outlet 22 and the light reject cumulating in the center of rotor 24 is removed through the light reject outlet 43. The centrifugal separator according to the invention, functioning in one stage is particularly preferred for being used in the short circulation of the web for¬ ming of a papermaking process for obtaining a fast and controlled function and for avoiding the multiple feedback loops of centrifugal cleaners according to prior art. Fig. 9 represents the use of the separator according to the present invention in connection with a papermaking process disclosed in copending Patent Application FI 922285 by the present inventor. However, it is obvious for persons skilled in the art that a separator according to the present inven¬ tion provides essential improvement also in a traditional papermaking process. When backwater recycling is accelerated and simplified the time needed for grade changes is reduced and process control is improved. Figure 9 represents a forming process for a paper web, where the advantages of a separator according to the invention, functioning efficiently in one single stage are utilized particularly efficiently. In the presented process, which is explained in detail in said above mentioned Patent Application FI 922285, the paper stock is fed as a controlled flow at about 3 to 5% or higher consistency from stock preparation 124 to the short circulation. The stock is first diluted to a consistency of about 0.5 to 1.5%, whereafter it is brought to a separator 10 according to the present invention, which functions in a single stage, without recycling of reject. In the separator backwater recycled free of air from the forming part is used for dilution and washing of reject. The cleaned stock is then brought forward to a screen 121 , preferably of a type disclosed by the present inventor in copending Patent Application FI 922284. Said screen functions in one single stage and also uses recycled air free backwater as dilution water. From the pressurized screen the stock is brought to the paper machine head- box 100. From the headbox 100 the stock is fed to a sheet forming part, which can be of different known types. During sheet forming the major part of the water contained in the fibre suspension is drained into separate draining boxes 101 , 102, 103 in connection with the forming wire or wires. The back¬ water, collected in the draining boxes is recycled back into the main process flow, as separate flows, without passing through open tanks by means of multiple pumps, of which at least a part are preferably air separating pumps 1 10, such as gas separation pumps to copending Patent Application FI 922283 by same inventor. According to the presented favorable process the backwater to be recycled is brought to the various dilution points of the short circulation as separate flows, so that the dilution water required by the separator according to the present invention, and by the screen following the same in the stock flow, flows directly to the stock main flow, without tube ramifications or upstream recircu¬ lating. In the process it is further preferred to feed the backwater first drained through the wire and which thus contains the highest proportion of fibre mate¬ rial as close to the headbox as possible. The single stage function of the separator according to the present invention, without feed back of reject and without recycling of backwater or fibre suspen¬ sion essentially accelerates reaching of a new state of equilibrium in connec¬ tion with a change of paper grade or process adjustment, and thus considerab¬ ly reduces the amount of waste paper produced at a grade change and impro¬ ves the process controllability. The process according to the present invention has been explained referring to a cleaning process for papermaking stock, which is considered a preferred embodiment. It is, however, obvious for persons skilled in the art the process and the apparatus according to the invention can be used also for the sorting of other types of suspensions, and also for removing only a heavier fraction or a lighter fraction from a certain suspension.

Claims: Claims 1. A centrifugal separator for sorting a fibre suspension, said separator (10) comprising an essentially cylindrical housing (12) having an inlet end (14) with a suspension inlet (16) and outlets (20, 22) for a lighter and a heavier fraction, respectively, at the opposite end (18) c h a r a c t e r i z e d in that - a coaxial hollow rotating rotor (24) is arranged in said housing (12) for recei¬ ving the flow of said suspension or a substantial part thereof, - said rotor (24) has slots or openings (26, 30) through which a part of said suspension can be brought to transfer from a sorting channel (25) inside said rotor (24) to a heavy fraction channel (28) surrounding said rotor, and from said heavy fraction channel (28) into said sorting channel (25), and - said separator comprises means (32) for introducing dilution/washing liquid for improving the efficiency of separation between said lighter fraction and said heavier fraction of said suspension. 2. A centrifugal separator according to claim 1 wherein along the length of said separator 10 there are multiple means (32) for introducing a dilution/was¬ hing liquid, said means being shaped as essentially annular liquid inlets (35). 3. A centrifugal separator according to claim 1 or 2 wherein a coaxial, hollow, essentially cylindrical tube (40; 52) functioning as a channel (42; 50) for a light fraction and/or gas, is arranged in the center of said rotor (24) functioning as a separation channel (25) in the center of said housing ( 2). 4. A centrifugal separator according to claim 3 wherein said central tube (40; 52) rotates together with said rotor (24) preferably acting also as a shaft for said rotor, whereby there are preferably oblique, streamlined slots or openings (37, 39; 54) enabling the flow of suspension between said light fraction channel (42; 50) and said sorting channel (25). 5. A centrifugal separator according to any one of the preceding claims 1 to 4 wherein the section of said heavy fraction channel (28) formed between said rotor (24) and said housing (12) changes at least in one place along the length of said separator (10) in order to cause a part of said suspension to pass between said separation channel (25) and said heavy reject channel (28). 6. A centrifugal separator according to any one of the preceding claims 1 to 5 wherein the mantle of said rotor (24) consists of at least two, preferably more parts, between which remain slots or openings (26, 30) for the suspension flow, said slots being preferably oblique so, that smoothly streamlined chan¬ nels in the desired direction of suspension flow result. 7. A centrifugal separator according to any of the previous claims 1 to 6 wherein vanes (46, 29, 31 ) are attached to the rotating shaft of said rotor (24) with the objective of recovering energy from said suspension, causing said suspension to rotate, divide said suspension into separate axial channels, and / or reinforcing the construction. 8. A centrifugal separator according to one any of the preceding claims 1 to 7 wherein at least one expansion (38) is arranged to said housing (12) preferably in connection with an essentially annular liquid inlet (35). 9. A centrifugal separator according to any one of the preceding claims 1 to 8 wherein said channel (28) formed between said rotor (24) and said housing (12) is expanded to form a chamber (34) close to the outlet end (18) of said separator (10), said chamber (34) having an outlet (22) for said heavy fraction. 10. A centrifugal separator according to claim 9 wherein between the outlet end (18) of said rotor (24) and said housing (12) a preferably adjustable slot (36) is formed for providing a controlled flow from said chamber (34) of a lighter subfraction separating from said heavy fraction. 1 1. A process for sorting a fibre suspension in a centrifugal separator, in which process said fibre suspension is fed through an essentially cylindrical housing in said separator and said suspension is caused to rotate in said separator so that said suspension due to a centrifugal force is separated according to the density of its components into at least two fractions, c h a ¬ r a c t e r i z e in that said process comprises process stages whereby a) said fibre suspension is fed into the inlet end of said separator; b) said suspension flow or a substantial part thereof is caused to pass into a hollow rotor rotating coaxially inside said cylindrical housing; c) the heavier fraction of said suspension is brought to move toward the mantle of said rotor; d) the heavier fraction accumulating at said mantle of said rotor is conducted out from said rotor into a space between said rotor and said housing to form a heavy fraction; e) a dilution / washing medium is introduced into said heavy fraction; f) a lighter subfraction of said heavy fraction in said space between said rotor and said housing is conducted into the lighter fraction in said rotor; g) at least a part of the stages c) to f) is repeated if needed; and h) said heavy fraction cumulating outside said rotor at said housing and said lighter fraction cumulating inside said rotor are separately discharged from said separator. 12. A process according to claim 1 1 wherein essentially the whole of said suspension flow is conducted into said rotor and a lighter fraction of said suspension together with any gas contained therein, is conducted into a channel coaxially installed in said rotor to form a light fraction and at least part of said light fraction is removed as light reject. 13. A process according to claim 12 wherein a heavier subfraction is caused to separate from said light fraction and at least part of said heavier subfraction is caused to flow back into said suspension flow, preferably through slots or openings in said light reject channel. 14. A process according to any one of the preceding claims 11 to 13 wherein at least a part of said suspension flow is caused to flow between the space inside and the space outside said rotor through slots or openings in said rotor, by throttling the flow inside or outside the rotor and/or introducing dilution / washing liquid into said space between said rotor and said housing. 15. In a papermaking process the improvement comprising circulating back¬ water draining through a forming fabric as essentially air free separate flows without passing open vessels to said dilution means (32) of a separator (10) according to any one of the preceding claims 1 to 10.

Inventors: MEINANDER PAUL OLOF, MEINANDER, PAUL, OLOF
Applicants: MEINANDER PAUL OLOF, POM DEV OY AB, POM DEVELOPMENT OY AB
Application year: 1993
Main classification: D21D5
IPC codes: B04B5, B04C3, D21D5, D21F1