Abstract:
To increase the processing speed of a sieve mechanism ( 9 ) for extracting carrier liquid from a fiber suspension ( 39 ) during the production of paper ( 27 ), paperboard, or cardboard, the sieve mechanism ( 9 ) is provided with a first electrode ( 43 ) which is disposed above, in, or below a sieve region and is connected to a high-voltage surge generator ( 46 ). A plasma can be generated in the fiber suspension ( 39 ) or in the immediate vicinity thereof, whereby the tensile strength of the paper ( 27 ) is also increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. national stage application of International Application No. PCT/EP2006/063025 filed Jun. 8, 2006, which designates the Unites States of America, and claims priority to German application number 10 2005 028 023.4 filed Jun. 16, 2005, German application number 10 2005 049 287.8 filed Oct. 14, 2005, and German application number 10 2005 049 290.8 filed Oct. 14, 2005, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a sieve apparatus for extracting carrier liquid from a fiber suspension during the production of paper, paperboard or board. 
         [0003]    Furthermore, the invention relates to a method for the treatment of nonwoven fibrous materials in a suspension, in particular as a pulp or fibrous stock, while the suspension is being filtered or its carrier liquid is being extracted, preferably for the operation of the sieve apparatus according to the invention. 
       BACKGROUND 
       [0004]    In a paper production plant or in parts of a paper production plant, the fiber suspension leaves a head box and, from there, reaches a preferably circulating sieve (fourdrinier wire or sieve cylinder). On the sieve, the sheet is dewatered down to a dryness of preferably 16 to 25%. During dewatering, two different types of sheet formation occur: filtration and thickening. The filtration is a sharp transition between a fiber mat that is already formed and the fiber suspension lying above it. During the thickening, the concentration of fibrous materials increases continuously from top to bottom. With increasing dewatering, a strength of the sheet increases. Paper fibers are preferably composed of numerous cellulose chains with many OH groups. The strength of the paper is produced by water molecules located in between, which connect the fibers to one another via hydrogen bridges. The number of hydrogen bridges can be increased by means of pressing or slight stretching, for example in a press section. 
         [0005]    WO 2004/101891 discloses a method for the treatment of paper with plasma after sheet formation has been completed. 
         [0006]    DE 198 36 669 A1 discloses a method for surface pre-treatment on solid paper after sheet formation has been completed. 
       SUMMARY 
       [0007]    The processing speed during the paper production can be increased according to an embodiment, by a sieve apparatus for extracting carrier liquid from a fiber suspension during the production of paper, paperboard or board, comprising at least one first electrode, which is connected to a high voltage pulse generator, and arranged over, in or under a sieve area of the sieve apparatus, wherein a plasma is produced in the fiber suspension or in its immediate surroundings. 
         [0008]    According to a further embodiment, the plasma can be produced at a distance of less than 20 cm or less than 10 cm or less than 5 cm, from the fiber suspension. According to a further embodiment, a sieve can be set up as an electrode. According to a further embodiment, there can be at least a second electrode for plasma production. According to a further embodiment, the electrodes can be arranged in the immediate vicinity of a suction chamber area, in particular a wet suction area or a flat suction area. According to a further embodiment, the first electrode and the second electrode can be arranged in the immediate vicinity of the suction chamber area in such a way that the fiber suspension is led between the electrodes. According to a further embodiment, the electrodes can be set up in such a way that a gas discharge can be sucked through the electrodes or past the electrodes, in particular through the fiber suspension. According to a further embodiment, the sieve apparatus may comprise a means for the introduction of gas, or air or oxygen or pure oxygen or oxygen with noble gas, as a carrier gas, between or in the immediate vicinity of the electrodes. According to a further embodiment, at least one electrode can be configured as a plate. According to a further embodiment, at least one electrode can be configured as a sieve. According to a further embodiment, at least one electrode can be configured as a wire braid or as a wire grid. According to a further embodiment, at least one electrode can be configured as a grid or as an arrangement of round rods and/or flat bars crossing at right angles or obliquely or as a (papermaking) sieve. According to a further embodiment, at least one electrode may have one or more tip(s). According to a further embodiment, the electrodes can be arranged as at least two mutually opposite plates preferably running parallel to one another. According to a further embodiment, the electrodes can be arranged as at least two mutually opposite grids preferably running parallel to one another. According to a further embodiment, the electrodes can be arranged in such a way that a sieve or a grid is arranged as a second electrode between two plates which are interconnected via at least one plate connector and which form the first electrode. 
         [0009]    According to another embodiment, a method for the treatment of nonwoven fibrous materials in a suspension, while the suspension is being filtered or its carrier liquid is being extracted, may comprise the step of bringing the suspension into contact with a non-thermal, large-area plasma under at least atmospheric pressure, wherein the plasma is produced in the immediate vicinity of the suspension or a gas discharge or a corona discharge, is produced in the suspension or in the immediate surroundings of the suspension under at least atmospheric pressure. 
         [0010]    According to a further embodiment, the plasma can be produced at a distance of less than 20 cm or less than 10 cm or less than 5 cm, from the suspension. According to a further embodiment, the suspension may be suitable for the production of paper, paperboard or board. According to a further embodiment, the suspension used can be a moist or wet sheet. According to a further embodiment, in order to produce the plasma or the gas discharge, high voltage pulses having a duration of less than 10 us can be produced between the electrodes. According to a further embodiment, the plasma or the gas discharge can be applied to the suspension before and/or during the sheet formation, as it passes through or over a sieve apparatus. According to a further embodiment, the suspension can be brought into contact with the plasma or treated by means of the gas discharge on both sides. According to a further embodiment, the plasma or the gas discharge can be used to bleach the suspension, the pulp or the fibrous stock, or in a digester, in a bleaching container or in a feed line. According to a further embodiment, the pulp or the fibrous stock can be brought into contact with at least one electrode for producing the plasma or the gas discharge. According to a further embodiment, the plasma or the gas discharge can be produced in the suspension. According to a further embodiment, the content of carrier liquid, in the suspension may lie in the range between 40% and 99.9%, or in the range between 80% and 98% or in the range between 85% and 98%. According to a further embodiment, radicals which act on the fibrous material can be produced in the plasma or by means of the gas discharge. According to a further embodiment, radicals of different type or composition can be used for various states of suspensions in a paper, board or paperboard production process, in particular at different process stages. According to a further embodiment, the suspension can be exposed to radicals of different type or composition within a process stage in a paper or board production process. According to a further embodiment, the radicals produced can be ozone, hydrogen peroxide, hydroxyl radicals, HO 2  and/or HO 2   − . According to a further embodiment, during the bleaching in the suspension or in the pulp or in the fibrous stock, the plasma or the gas discharge can be applied in such a way that the radicals predominantly formed are ozone and/or hydrogen peroxide. According to a further embodiment, during the filtering and/or in the two-dimensionally distributed suspension or pulp or fibrous stock or in the forming or formed, as yet unpressed sheet, the plasma or the gas discharge can be applied in such a way that the radicals predominantly formed are hydroxyl, HO 2  and/or HO 2   − . According to a further embodiment, a production rate of the radicals and/or the composition of the radicals produced can be controlled by influencing an amplitude, a pulse duration and/or a pulse repetition rate of the high voltage pulses. According to a further embodiment, in order to control and regulate the production rate and/or the type of radicals produced, a concentration of the radicals produced can be measured. According to a further embodiment, in order to control and regulate the production rate and/or the type of radicals produced, a property of the suspension, preferably a quality property, in particular its opacity, gloss, whiteness, fluorescence or color locus, can be measured. According to a further embodiment, the concentration or the property can be measured “online”. According to a further embodiment, for the purpose of regulation, the amplitude of the high voltage pulses can be changed at a constant repetition rate. According to a further embodiment, for the purpose of regulation, the repetition rate of the high voltage pulses can be changed at a constant amplitude. According to a further embodiment, the suspension, the pulp or the fibrous stock can be enriched with oxygen in the region to which plasma is applied. According to a further embodiment, a high voltage pulse duration of less than 100 ns can be used in the suspension, the pulp or in the fibrous stock. According to a further embodiment, two-dimensionally distributed suspension, pulp or fibrous stock or a forming or formed, as yet unpressed sheet can be surrounded by an atmosphere enriched with water vapor in the region to which plasma is applied, in particular during filtering. According to a further embodiment, a high voltage pulse duration of 100 ns to 1 us can be applied to two-dimensionally distributed suspension, pulp or fibrous stock or a forming or formed, as yet unpressed sheet, in particular during filtering. According to a further embodiment, a high voltage amplitude corresponding to at least twice the value, and preferably at least three times the value, of a corona threshold voltage can be applied to the electrodes in the case of a two-dimensionally distributed suspension, pulp or fibrous stock or a forming or formed, as yet unpressed sheet, in particular during filtering. According to a further embodiment, in order to produce the plasma or the corona discharge, a DC voltage corona discharge is produced and the high voltage pulses are superimposed on the DC voltage corona discharge. According to a further embodiment, a pulse repetition rate between 10 Hz and 5 kHz, or of 10 kHz, can be used. According to a further embodiment, the power injection of electrical energy into the plasma can be predominantly controlled by the regulation of amplitude, pulse duration and pulse repetition rate of the superimposed high voltage pulses. According to a further embodiment, high voltage pulses with a duration of less than 3 μs, or less than 1 μs, or less than 500 ns, can be applied. According to a further embodiment, a homogeneous, large-volume plasma with a high power density can be produced without plasma constrictions or breakdowns occurring. According to a further embodiment, use can be made of a DC voltage of such a height that, in the plasma, a stable DC corona discharge is formed only in conjunction with superimposed high voltage pulses. According to a further embodiment, the DC voltage used may lie below that for stable operation without high voltage pulse superimposition. According to a further embodiment, the total amplitude used may lie above the static breakdown voltage of the electrode arrangement. According to a further embodiment, the total amplitude used may correspond to two to five times the static breakdown voltage of the electrode arrangement. According to a further embodiment, the amplitude of the high voltage pulses can be between 10% and 1000% of the DC voltage used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Preferred but in no way restrictive exemplary embodiments of the invention will now be explained in more detail by using the drawing. For clarity, the drawing is not to scale and certain features are illustrated only schematically. Mutually corresponding parts are provided with the same designations in the figures. In detail, 
           [0012]      FIG. 1  shows a schematic illustration of a paper production plant having a sieve apparatus according to an embodiment, a press apparatus and a finishing and/or drying system, 
           [0013]      FIG. 2  shows an illustration (section) of an arrangement for the production of radicals in corona plasmas in pulp or air: parallel plate or tube arrangement with wire, on which a pulsed high voltage is superimposed, 
           [0014]      FIG. 3  shows a basic illustration of pulses for the production of radicals in corona discharges in air or aqueous media when short (typically &lt;1 μs) high voltage pulses with a high pulse repetition rate are employed, and 
           [0015]      FIGS. 4 to 9  show electrode arrangements and electrode systems for the production of corona discharges: plate-plate, plate-wire-plate, coaxial wire-tube, point-plate, multiple point-plate, grid-plate (tube), grid-grid arrangements. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As a result of the treatment of the fibers with a corona plasma, preferably a cold corona plasma, on the sieve, preferably before the actual sheet formation, the molecular structure of the fiber surfaces is changed. As a result, the following positive effects are achieved:
       increasing the strength of the sheet even before a press section,   eliminating colored “molecular groups” (in particular lignin and residual dye molecules from the water circuit) on the surface and simultaneous brightening of the paper.       
 
         [0019]    In particular as a result of increasing the strength of the sheet, higher processing speeds can be achieved during the paper production. Likewise, the probability of paper breaks is reduced. In the region of the sieve apparatus, the fiber suspension is treated with plasma even before sheet formation has been completed, advantageously with regard to the subsequent material properties. 
         [0020]    It is expedient for the plasma to be produced at a distance of less than 20 cm, preferably less than 10 cm, preferably less than 5 cm, from the fiber suspension. As a result of the direct treatment of the fiber suspension, preferably pulp fibers, with cold plasma, specific radicals are preferably produced in the gas space of the fiber suspension. These radicals promote an increase in the strength of the paper. 
         [0021]    According to an embodiment, a (papermaking) sieve can be set up as an electrode. As a result of the treatment with a preferably cold plasma, more hydrogen bridge bonds are produced on the sieve at an earlier time than without the plasma treatment. The strength of the sheet on the sieve therefore increases further. The strength of the sheet, reached earlier, reduces the risk of paper breaks further. 
         [0022]    It is expedient that there is at least a second electrode for plasma production. An arrangement of at least two electrodes permits two-sided treatment of the fiber suspension or of the unpressed sheet. 
         [0023]    According to an embodiment, the electrodes are arranged in the immediate vicinity of a suction chamber area, in particular a wet suction area or a flat suction area. Advantageously, the plasma treatment of the as yet unpressed fiber stock on the sieve is carried out in the suction chamber areas (flat suction means, wet suction means). As a result, radical-containing air from a plasma reactor region above the sieve is sucked through the fiber stock or the fiber suspension and a particularly intimate connection is produced between radical-containing air and the fiber surface. 
         [0024]    It is expedient in this case if the first electrode and the second electrode are arranged in the immediate vicinity of the suction chamber area in such a way that the fiber suspension is led between the electrodes. Two-sided treatment of the fiber suspension improves the result of the treatment which is achieved by means of the sieve apparatus according to an embodiment. 
         [0025]    The electrodes are preferably set up in such a way that a gas discharge can be sucked through the electrodes or past the electrodes, in particular through the fiber suspension. 
         [0026]    Furthermore, the apparatus can be configured with a means for the introduction of gas, in particular air or oxygen, preferably pure oxygen or oxygen with noble gas, for example, as a carrier gas, between or in the immediate vicinity of the electrodes. As a result of this advantageous arrangement, preferably finely distributed air bubbles or oxygen or oxygen with a carrier gas, such as argon, is caused to flow into the fiber suspension. With the aid of this gas caused to flow in and the simultaneous treatment with plasma, the subsequent tearing strength of the paper is increased further. 
         [0027]    It is also expedient that at least one electrode is configured as a plate. In the case of a preferably flowing suspension, in particular a falling curtain of suspension, an electrode arrangement having two plates can advantageously be used for a two-sided application of plasma to the suspension curtain. 
         [0028]    According to an embodiment, relating to the method, provision is made for the suspension to be brought into contact with a preferably non-thermal, large-area plasma under at least atmospheric pressure, for the plasma to be produced in the immediate vicinity of the suspension or for a gas discharge, in particular a corona discharge, to be produced in the suspension or in the immediate surroundings of the suspension under at least atmospheric pressure. 
         [0029]    During the treatment of the raw, still largely unbonded paper surface with cold plasma, shortly before the sieve, on the sieve or immediately thereafter, for example in the first part of the press section, specific radicals are produced (e.g. OH − , HOO − , O, O 3 ), which react chemically with the paper surface and in particular the fiber suspension. 
         [0030]    Radicals are also able, inter alia, to trigger bleaching chemical reactions, in particular free oxygen O, in particular also a hydroxyl radical OH, in particular ozone O 3 , and also free functional groups such as OH groups, COOH groups. These functional groups are in turn decisively involved in particular in increasing the bonding strength of the fibers to one another, which further improves the tearing strength of the paper and therefore the possible processing speed. 
         [0031]    Preferably, in the event of simultaneous production of radicals, a series of differently oxidizing and functionalizing radicals are produced in a gas phase and used for the purpose of treating these fibers with radicals in the unpressed sheet, still on the sieve or immediately thereafter in the first part of a press section. 
         [0032]    In particular, this treatment is to be used with a content of carrier liquid of 75% to more than 98%. The strength of the paper and therefore the maximum possible working speed is already increased in good time as a result. Furthermore, by means of this type of treatment, the colored dyes located on the surface can also be bleached, for example the adhering lignin or dye residues are de-colorized by oxidation. 
         [0033]    Radicals are produced in gas discharges as a result of the fact that high-energy electrons collide with molecules and, as a result, disassociate or excite the latter and in this way lead to radical formation. 
         [0034]    In the case of disassociation, radicals are liberated immediately while, in the case of excitation, as a result of subsequent radiant transitions, UV light is produced, which in turn reacts with molecules, preferably air and water molecules, and disassociates the latter. In order to obtain sufficiently high-energy electrons in the region of about 5 eV (electron volts) up to &gt;15 eV, extremely high electric fields are needed. These high field strengths occur in particular at the top of streamers, as they are known. Streamers are discharge channels which are found in the structure and are formed because of the applied high external field strengths. The formation of such streamers takes place within less than 10 ns and then changes quickly into a thermal breakdown channel. Since no high-energy electrons are formed in a thermal breakdown channel, the object is, inter alia, to avoid these thermal breakdowns or to reduce them to a minimum. In order to obtain high energy efficiency of the production of preferably radicals in gases, it is therefore necessary to operate with very short individual high voltage pulses. The pulse duration is preferably considerably shorter than that which corresponds to a build-up time of a complete breakdown in the respective medium. 
         [0035]    A pulsed corona discharge directly above the paper or on the fiber suspension by using extremely short high voltage pulses of less than 10 μs, in particular typically of 1 μs and particularly advantageously considerably shorter than 1 μs, with voltages of a few kV to more than 100 kV, depending on a distance of the electrodes from the paper or from the fiber suspension and the properties of the paper, is applied to the paper or the fiber suspension, advantageously with regard to the quality properties. In particular, the use of such short high voltage pulses has been shown to be particularly advantageous, whereas the use of radio frequency (RF) or microwave pulses or of individual high voltage pulses with more than 10 μs duration, as described in WO 2004/101891 A1, is a far less efficient. It is suspected that the reason lies in a rapid transition from a streamer to the breakdown at atmospheric pressure, in particular given the presence of geometric irregularities on the paper surface, such as individual fibers, at which the electric field is considerably superelevated. 
         [0036]    If the paper web or the fiber suspension is located between the electrodes used for the streamer discharge, then this is particularly advantageous, since the paper or the fiber suspension acts partly as a dielectric barrier as a result. By means of the dielectric barrier, the transition from the streamer to the breakdown can be controlled better. 
         [0037]      FIG. 1  shows a schematic illustration of a paper production plant  1 , as is used in current paper mills. Its construction and the combination of different units are determined by the type of paper, board and paperboard grades to be produced and the raw materials employed. The paper production plant  1  has a physical extent of approximately 10 m in width and approximately 120 m in length. The paper production plant produces up to 1400 m of paper  27  per minute. It takes only a few seconds from the first impingement of the fiber suspension or the pulp  39  on the sieve apparatus  9  as far as the finished paper  27 , which is finally reeled up in a reel-up 15. Diluted with water in the ratio of 1:100, the fibrous material  30 , together with auxiliary materials, is applied to the sieve apparatus  9  having the sieve  10 . The fibers are deposited beside and on one another on the sieve  10 . The sieve water  23  can flow away or be extracted by means of a plurality of suction chamber areas  24 . In this way, a uniform fiber composite is produced, which is further dewatered by mechanical pressure in a press apparatus  11  and with the aid of steam heat. The entire paper production process is in this case subdivided substantially into the areas of stock preparation, paper machine, enhancement and finishing. 
         [0038]    Waste paper and, as a rule, also pulp reach a paper mill in dry form, while groundwood is normally produced in the same mill and pumped into the central stock facility  3  as a fiber/water mixture, that is to say a suspension of nonwoven fibrous materials. Waste paper and pulp  30  are likewise pulped in a fiber chest  35  with the addition of water. Constituents that are not part of the paper are removed by various screening units (not illustrated here). In the central stock facility  3 , the mixing of the various raw materials is carried out, depending on the desired paper grade. Here, fillers and auxiliary materials, which are used to improve the paper quality and to increase productivity, are also added. 
         [0039]    The headbox  7  of the paper production plant  1  distributes the fibrous material suspension uniformly over the entire sieve width. At the end of the sieve apparatus  9 , the paper web  27  always still contains about 80% water. 
         [0040]    A further dewatering process is carried out by means of mechanical pressure in the press apparatus  11 . Here, the paper web  27  is led through between rolls of steel, granite or hard rubber by means of an absorbent endless felt blanket and dewatered as a result. Part of the sieve water  23  picked up by the suction chamber area  24  is led to a screen  5  and another part is sent back to a saveall  17 . The press apparatus  11  is followed by a drying system  13 . The remaining residual water is evaporated in the drying system  13 . The paper web  27  runs in slalom fashion through a plurality of steam-heated drying cylinders. At the end, the paper  27  has a residual moisture of a few percent. The water vapor produced in the drying system  13  is extracted and led into a heat recovery system, not illustrated. 
         [0041]    For a treatment of the fiber suspension  39  in accordance with the method according to an embodiment, between the headbox  7  and the initial region of the sieve apparatus  9  according to an embodiment, a first electrode  43  is arranged under the sieve apparatus  9  and a second electrode  44  is arranged above the sieve apparatus  9 . The electrodes  43  and  44  are arranged in such a way that the two-dimensionally distributed fiber suspension  39  runs between them. In order that a large-area plasma for treating the fiber suspension  39  can be produced under atmospheric pressure in the immediate vicinity of the fiber suspension  39 , the electrodes  43  and  44  are connected to a high voltage pulse generator  46 . With the aid of this high voltage pulse generator  46 , a large-volume plasma having a large cross section and a high power density is produced between the electrodes  43  and  44 . In this case, a plasma density is distributed homogeneously over the treatment region which is covered by the electrodes  43  and  44 . According to an embodiment, this large-volume plasma with a high power density is produced by intensive, short high voltage pulses with a high pulse repetition rate of typically about 1 kHz being superimposed on a DC corona discharge. In this operating mode, an extremely homogenous, large-volume plasma having a high power density is produced without the occurrence of the plasma contractions known in the case of DC corona discharges. 
         [0042]    In order to assist the effect of the treatment exerted on the fiber suspension  39  by the cold large-area plasma, oxygen with argon as carrier gas is introduced into the treatment space between the electrodes  43  and  44  via a gas line  80  by means of a gas distributor  81 . With the aid of the oxygen-argon mixture, hydroxyl radicals are particularly advantageously produced. Hydroxyl radicals are particularly aggressive and oxidizing; as a result, in the fiber suspension remaining in the treatment region between the electrodes  43  and  44  for only a few seconds, increased strength is achieved during the subsequent sheet formation. 
         [0043]    In a manner analogous to that described previously, a large-area plasma for the treatment of the paper web  27  is produced by using an electrode system  47 ,  48  in the press apparatus  11 . The first electrode  47  in the press apparatus  11  is implemented as a half-round grid electrode. As a result of the half-round configuration of the electrode  47 , it is able to follow the course of the paper web over a transport roll  12 . The second electrode  48  in the press apparatus  11  is configured as a plate electrode and arranged in such a way that the transport roll  12  can be led between the electrodes  47  and  48 . In order to excite the radical formation in the plasma here, too, an oxygen-argon mixture is caused to flow to the plasma treatment region via the gas distributor  81  having the gas line  80 . 
         [0044]    The pressing operation compacts the paper structure, the strength is increased once more and a surface quality is decisively influenced. As a result of the treatment of the pressed paper with cold plasma, in particular with the radicals produced, the molecular structure of the paper surface is changed further. In addition to the strength of the paper  27 , printability is improved. 
         [0045]    By using the aforementioned electrode arrangements  43  and  44  and also  47  and  48 , according an embodiment, it is possible to lead the paper web  27  between streamer discharges. 
         [0046]    A streamer is a specific form of a plasma cloud moving linearly onward, or a discharge channel under development, which is formed on account of the excited high external field strength. A build-up of such streamers takes place within less than 10 ns and changes very quickly into a thermal breakdown channel. The aforementioned arrangements of the electrode systems, the paper web  27  being located between the electrodes used for the streamer discharge, are particularly advantageous since the paper  27  functions partly as a dielectric barrier as a result, which means that the transition from streamer to breakdown can be suppressed. 
         [0047]    By means of direct treatment of the pulp fiber suspension  39  with the cold plasma, the radicals OH − , HOO − , O, O 3  are preferably produced. In addition to an increase in strength, these radicals trigger a bleaching chemical reaction. The high voltage pulse generator  46  is operated in such a way that it produces high voltage pulses with a duration of typically 1 μs between the electrodes  43  and  44 . A DC voltage needed for the production of radicals and ozone in the pulp fiber suspension is around some 10 kV to more than 100 kV. The high voltage pulses are superimposed on the DC voltage and in this way form a total amplitude of typically about 100 kV. As a result of the treatment of the pulp fiber suspension  39  with a cold electric discharge, which is to say the plasma, the radicals are produced in situ. Thus, large total quantities of radicals can be introduced into the suspension  39 . For the electrodes  47  and  48 , the high voltage generator is operated in such a way that it produces high voltage pulses having a duration of typically 0.1 us up to a few μs. 
         [0048]      FIG. 2  shows, as a further exemplary embodiment, a sectional illustration of an arrangement for producing radicals. Arranged at the centre of the arrangement is a high voltage electrode  50 . The outer shell of the arrangement is set up as a mating electrode  51 . In the arrangement there is a pulp fiber suspension  39  to be filtered. A streamer  53  is illustrated between the electrodes  50  and  51 . Radicals are produced in streamers as a result of the fact that high-energy electrons collide with molecules and disassociate or excite the latter as a result. In the case of disassociation, radicals  59  are liberated immediately while, in the case of excitation, UV light is produced by a subsequent radiant transition. This UV light that is produced in turn reacts with water molecules and disassociates the latter. 
         [0049]      FIG. 3  illustrates the course of the applied voltage of the high voltage pulses. The first pulse  66  and a second pulse  67 , each having a pulse width  62 , have an interval of one pulse repetition time  63 . The time is indicated in ms on the abscissa and the voltage is indicated in kV on the ordinate. The units are chosen arbitrarily. A level of typically about 100 kV of the DC voltage coincides with the abscissa illustrated. The pulse voltage illustrated is therefore superimposed on the DC voltage. The pulses  66  and  67  have a pulse width  62  of less than 1 μs, the individual pulses  66 ,  67  having a steeply rising flank with a rise time  64  and a less steeply falling flank. The pulse repetition time  63  is typically between 10 μs and 100 ms. 
         [0050]    In this case, the individual pulses  66 ,  67  have a total amplitude such that a predefined energy density is achieved beyond the predefined direct voltage. As mentioned, the pulse rise time  64  is short here as compared with the pulse fall time. Such a type of pulses means that electric breakdowns, which would lead to temporal and spatial disruptions in the homogeneous plasma density distribution, are avoided. 
         [0051]      FIG. 4  to  FIG. 9  show examples of electrode systems for producing corona discharges in preferably aqueous media.  FIG. 4  illustrates a plate-plate arrangement of a first plate  70   a  as electrode and a second plate  70   b  as electrode. The first plate  70   a  and the second plate  70   b  are arranged parallel to each other. The first plate  70   a  forms the high voltage electrode and is connected to the high voltage pulse generator  46  via a high voltage cable. The second plate  70   b  forms the mating electrode and is connected to the high voltage pulse generator  46  as a grounded electrode. 
         [0052]    A corresponding arrangement with specific flat plate electrodes is illustrated in  FIG. 5 . Once more, there are two solid plate electrodes  70   a  and  70   c  at a fixed distance, a high voltage electrode  71  running centrally. In this plate-wire-plate arrangement, the high voltage electrode  71  is implemented as a solid wire and connected to the high voltage output of the high voltage pulse generator  46 . The grounded plates  70   a ,  70   c  are likewise connected to the high voltage pulse generator. 
         [0053]      FIG. 6  shows a wire-tube arrangement as the electrode system. A high voltage electrode  71  projects centrally into a cylindrical electrode  72 . As in  FIG. 5 , the high voltage electrode  71  is implemented as a solid wire and connected to the high voltage pulse generator  46 . The cylindrical electrode  72 , which is preferably configured as a wire braid, is grounded and connected to the high voltage pulse generator  46 . 
         [0054]      FIG. 7  shows a point-plate arrangement as electrode system. Three points  73  are connected via a high voltage line to the high voltage pulse generator  46 . The points  73  are arranged at right angles to a grounded plate electrode  74 . The distance of the point electrodes  73  from the plate electrode  74  is adjustable and can thus be adapted to different process conditions. 
         [0055]      FIG. 8  shows an electrode system arrangement which comprises 3 plates  70   a ,  70   d  and  70   e . The first plate  70   a , which is connected to the high voltage pulse generator as a high voltage electrode, is arranged centrally between two solid plates  70   d  and  70   e . The plates  70   a  and  70   d  are connected by a plate connector  70   f . Since the plate  70   d  is connected to the high voltage pulse generator  46  as a grounded mating electrode, the plate  70   e  likewise has the function of a grounded mating electrode via the plate connector  70   f.    
         [0056]      FIG. 9  shows an electrode system as a grid-grid arrangement. In a way analogous to  FIG. 4 , here a first grid  75   a  and a second grid  75   b  are arranged opposite and in parallel. The first grid  75   a  in this case forms the high voltage electrode and is connected to the high voltage pulse generator  46 . The second grid  75   b  forms the grounded mating electrode and is connected to the high voltage pulse generator  46 . 
         [0057]    A hybrid discharge, one electrode  75   a  being located completely outside a fiber suspension  39  to be treated, and a second electrode  75   b  being wholly or completely submerged in the fiber suspension  39 , is produced by an alternative arrangement, in which the (papermaking) sieve is configured as an electrode  75   a . The sieve is designed as a grid electrode and forms the high voltage electrode, which is connected to the high voltage pulse generator  46 . The grounded mating electrode  76   b  is also designed as a grid electrode and is connected to the high voltage pulse generator  46 . 
         [0058]    In order to produce pulsed discharges in the gas space close to the surface above the fiber suspension  39 , a further electrode arrangement is possible. A high voltage electrode comprising a plurality of electrically interconnected rod electrodes is arranged in the gas space of the fiber suspension  39 , close to the surface, in such a way that its rods run parallel to the surface. A grounded mating electrode is implemented as a solid plate and is arranged at equidistant intervals from the high voltage electrode, distributed over the entire area.