Patent Publication Number: US-2007107861-A1

Title: Method for producing a paper web, and paper machine

Description:
SUMMARY OF THE INVENTION  
      The present invention provides that the paper web is produced by using a fibrous stock suspension which contains fibers that are partially loaded with ash.  
      When supplying ash with the assistance of the Fiber-Loading-Process, for example according to DE 102 04 254A1 the ash is precipitated directly in the fibers and adhered to them. As detectable through microscope recordings, ash is also deposited in the cavities of the fiber. Blackening is avoided since even with collapsed fibers a fiber-ash interface and an ash-fiber interface exist.  
      This means that according to the current invention the blackening index, however, at the same final roughness is reduced by at least 10%. Vice versa, the final roughness in the paper is reduced by at least 12% at the same blackening index.  
      One particularly advantageous embodiment provides that the paper web is run through a calender in the paper machine which is equipped with a multitude of nips wherein line loads greater than 50 kN/m are applied to the paper web in the nips.  
      It is especially advantageous if line loads greater than 100 or greater than 350 kN/m are applied to the web.  
      It is also advantageous if the paper web is dried to a moisture content of less than 5%, and is subsequently remoistened prior to the calender to a moisture content of more than 7%.  
      The invention also relates to a paper machine for the production of supercalendered paper according to one of the aforementioned embodiments.  
      The paper machine is characterized in that it includes a device for loading a fibrous stock suspension with ash. The advantages of said device apply to online as well as to an offline production of the paper web, in other words, even if the device for loading the fibrous stock suspension and the machine for the production of the paper web are not arranged in tandem and the paper manufacturing process occurs immediately following the loading of the fibrous stock suspension with ash. Due to the fiber loading the paper can endure higher line loads in the calender and a greater smoothness of the paper web can be achieved.  
      It is advantageous if the device includes a static mixer, a processing unit for the addition of calcium oxide or calcium hydroxide, a press or dewatering screw, an equalizing reactor or an equalizing screw, a container serving as crystallizer, an additional static mixer, a carbon dioxide storage tank or an additional device for the recovery of carbon dioxide.  
      One advantageous embodiment provides for a high consistency cleaner and/or a carbon dioxide heater and/or a storage tank for press water or for water which was removed in the dewatering screw.  
      It is also advantageous if filtrate from the fiber stock suspension which is recovered in the dewatering screw is returned through a line to a header tank or to another upstream device for processing of the fibrous stock suspension.  
      Another advantageous provision would be an additional washer unit for cleansing of the fibrous stock suspension following the container that is serving as crystallizer.  
      In addition it is also advantageous if the paper machine also includes a twin wire former. The press section which is located downstream from said twin wire former would include at least one shoe press. The paper machine includes one calender with a multitude of nips in order to reduce roughness and increase smoothness. An arrangement for remoistening of the paper web is located before the calender. The remoistening is advantageously accomplished by way of a steam blow box which also increases the gloss of the paper.  
      In the production of LWC and MWC papers (LWC=light weight coated; MWC=medium weight coated), a coating unit is of additional advantage.  
      A completely new product possessing new and improved characteristics compared to products which are already on the market will result if a fibrous stock suspension is treated with the fiber loading technology during the paper manufacturing process. The process described below permits precipitation of a filler (calcium carbonate) which is evenly distributed and deposited only on and in the fibrous stock—especially the paper fiber—to occur directly in the stock preparation in the paper mill.  
      Due to the fact that only fibrous stock loaded with precipitated calcium carbonate is produced, wherein the calcium carbonate is attached on or in the fibers, or is embedded in them the formation of loose precipitated calcium carbonate (PCC) is prevented. An additional wash cycle before a refining process and/or after the refining process and/or before the crystallization process in a crystallizer and/or before a headbox chest or prior to delivery into the paper machine or recycling the press filtrate to a header tank or to another storage arrangement located on the infeed side provides that a constant calcium hydroxide content is adjusted or regulated in the infeed system of the fiber loading device. The calcium hydroxide can be added directly in a fiber stock pulper. The press filtrate can be recycled into the pulper system. Calcium hydroxide which does not attach on or into the fibers is recycled back to the upstream process.  
      Only that filler which is not attached on or in the fibers, in other words loose precipitated calcium carbonate, is washed out. The fibers themselves which are provided with filler on the inside and the outside do not lose said filler through the wash process and the recycling of the press filtrate, so that the positive effects of the fiber loading technology are maintained.  
      The present invention especially also provides that the fibrous stock suspension is fed into a press arrangement intended to squeeze out a filtrate. Subsequently, the filtrate is recycled back, at least partially into an arrangement for pulping of the fibrous stock suspension, specifically into a reservoir which is located on the infeed side, for example a header tank. The calcium hydroxide is added at least partially in the arrangement for pulping of the fiber stock. In the complete pulper system, specifically in the arrangement for pulping of the fibrous stock, a pH value of between 7 and 12, especially between 9 and 12 is maintained.  
      Calcium hydroxide in aqueous or in dry form, or calcium oxide are mixed into the aqueous paper fiber stock in a range of between 0.01 and 60% of the existing solids content. A static mixer, a header tank or a pulper system are utilized for the mixing process; a pH value in the range of between 7 and 12, preferably between 9 and 12 is applied. The reactivity of the calcium hydroxide is between 0.01 seconds and 10 minutes, preferably between 1 second and 3 minutes. Dilution water is added according to predetermined parameters.  
      Carbon dioxide is added into the moist paper stock suspension according to the reaction parameters. In doing so calcium carbonate precipitates in the carbon dioxide atmosphere.  
      At the same time a refining energy in the range between 0.1 and 300 kWh/ton dry paper pulp is applied. Compared to conventional processes for the production of a fibrous stock suspension, the current invention provides energy efficient attainment of a higher level of freeness; according to the current invention as much as 50% of refining energy can be saved.  
      The high mechanical strengths in the end product which are achieved through the high freeness value positively affect the production of all paper grades since, due to process based mechanical loads in the various sections of the paper machine, such as in the press section, the dryer section or in the section where the web is wound, the produced intermediate product and the end product which is to be produced bear a high mechanical load due to utilization of winders, rewinders and converting machinery. The energy supply during the refining process, specifically the heat volume and the resulting warming effect are controlled. Crystals in various forms can be produced, according to the control.  
      The pre-treatment of the fibrous stock suspension also creates the basis for improved drying, thereby increasing the efficiency level in the production of all paper grades. Residual moisture contents in the range of 1 to 20% are advantageous.  
      With the current invention greater brightness and/or higher optical values with an improved brightness of up to 15 or more lightness points are also achieved on all grades of paper.  
      It may also be provided that an expenditure of energy of between 0.3 and 8 kWh/t, especially between 0.5 and 4 kWh/t is used for the precipitation reaction, especially if no refining process is utilized.  
      The process temperature is preferably between −15° C. and 120° C., especially between 20 and 90° C. Rhombohedral, scalenohedron and spherical crystals are formed, wherein the crystals measure between 0.05 and 5 μm, especially between 0.3 and 2.5 μm.  
      Static and/or moving, especially rotating, mixing elements are utilized for the production of a fibrous stock suspension which is loaded with calcium carbonate.  
      The process may be conducted in a pressure range of between 0 and 15 bar, especially between 0 and 6 bar. Also, the process is carried out at a pH value that is preferably between 6 and 10, especially between 6.5 and 8.5. The reaction time is between 0.01 seconds and 1 minute, especially between 0.05 seconds and 10 seconds.  
      An additional advantage when utilizing the present invention with the above referenced paper grades is that these can also be processed further in a calender. Blackening is avoided due to the fact that when using the fiber loading technology, fiber loading particles are deposited in, around and on the fibers.  
      Compared to a fibrous stock which is produced according to conventional methods a fibrous stock produced in accordance with the fiber loading combination process technology possesses a superior dewatering capacity which is in the range of between 5 to 100 ml CSF or 0.2 to 15° SR, depending upon the required level of freeness and filler content. This fibrous stock possesses a lower water retention capacity of approximately 2 to 25%, depending upon the raw material that is used in the paper manufacturing process. Compared to conventional fibrous stock the water can be removed more quickly from the fibrous stock suspension, and the fibrous stock dries accordingly faster. This also has a positive effect on remoistening which, as a result, is diminished in the paper manufacturing process, and upon the printability of the produced paper grades. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a schematic drawing illustrating the preparation of a fibrous stock suspension for application in a machine for the production of a fiber web;  
       FIG. 2  shows blackening as a function of the roughness in a paper web; and  
       FIG. 3  shows a paper machine for the production of calendered paper according to the present invention. 
    
    
      Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate an embodiment of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a pipe line system  1  that is equipped with control valves  2 ,  3  which is provided for a fibrous stock suspension. The control valve  2  is located in a line  4  through which the pipe line system  1  is connected with a static mixer  5 . Dilution water is added to the mixer  5  via a valve  6 . A tank  7  or a container for storage of the fibrous stock suspension is located following the mixer  5 , viewed in a direction of the fiber stock flow direction. From the tank  7  the fibrous stock suspension is pumped via a pump  8  to an additional static mixer  9 . Dilution water is also added to the mixer  9  via a valve  10 . Likewise, the inflow of a calcium hydroxide suspension is controlled through a valve  11  which is located in a line  12 .  
      This is supplied by a processing unit  13  where solid calcium oxide or calcium hydroxide is added to water. For this purpose the processing unit  13  is supplied with water via a line  14  which is equipped with a valve  15 . The suspension that is produced in the processing unit  13  is supplied through a pump  16  into the line  12 .  
      The fibrous stock suspension to which calcium hydroxide was added flows from the mixer  9  into a line  17  which is equipped with a valve  18  to a dewatering screw  19  where water is removed from the fibrous stock suspension. Said water can, for example, be returned through a line  20  to the mixer  5  as dilution water. Alternatively, or in addition the water that was removed in the dewatering screw  19  may also be routed to a storage tank  21  for the fibrous stock suspension, or it is returned to the mixer  9 . Due to the return flow of calcium hydroxide containing water the pH value can be increased and adjusted in all instances in the units that are located preceding the dewatering screw  19 .  
      In order to equalize the fibrous stock suspension, said suspension is delivered via a line  22  from the dewatering screw  19  to an equalizing screw  23 . A downstream container  25  (crystallizer) is connected through a line  24  with said equalizing screw. For the purpose of supplying carbon dioxide this container is connected with a carbon dioxide storage container  30  via a line  29  which is equipped with valves  26 ,  27  and a pump  28 . Carbon dioxide is supplied from this container into the crystallizer  25  in order to produce the desired precipitation reaction of calcium hydroxide and carbon dioxide for the formation of calcium carbonate as a filler in the fibers of the fiber stock.  
      In addition the carbon dioxide storage container  30  is connected with the equalizing screw  23  via an additional line  31  which is equipped with a valve  32  and which branches off line  29 . Carbon dioxide can herewith also be supplied to the equalizing screw in order to achieve at least a partial precipitation already at this point.  
      Line  29  is also connected via an additional valve  33  with a static mixer  34 . This serves to add additional carbon dioxide to the fibrous stock suspension which is flowing from the crystallizer  25  via a line  36  which is equipped with a valve  35 .  
      The fibrous stock suspension flows from the mixer  34  into a mixing chest  37 . A storage tank  38  which additionally serves as a filtration unit may be provided between the mixer  34  and the mixing chest  37 . From the storage tank  38  the filtrate which has been enriched with calcium carbonate is recycled back into the header tank  7  or into another upstream unit for the processing of the dilution water or the fibrous stock suspension.  
      The mixing chest  37  is equipped with a rotor  39  to thoroughly mix the fibrous stock suspension. The fibrous stock suspension then flows from the mixing chest  37  either immediately to a head box in a paper machine, or will be subjected to additional mechanical processing, for example in a refiner feed chest.  
      Fibrous stock suspension to which calcium hydroxide has not yet been added can also be supplied to the mixing chest  37  through a pipe line system  1  via the valve  3  and a line  40  in which said valve is installed.  
      It is further provided that white water or process water which has been recovered from the machine for the production of the fibrous stock suspension, especially from the paper machine, for example in the wire area of the paper machine or, as already described previously, for the fibrous stock suspension from the dewatering screw  19 , is supplied to the tank  21 . Dilution water may for example be supplied to this tank through a line  41  which is equipped with a valve  42 .  
      From the container  21  dilution water which is mixed with process water flows through a line  43 , a pump  44  and a valve  45  to the crystallizer  25 . According to the design of an arrangement for loading of the fibrous stock suspension with a filler, especially with calcium carbonate as depicted in  FIG. 1 , a multitude of possibilities arises to influence the composition of the fibrous stock suspension that is being produced in various stages of the manufacturing process.  
      The installation of a high consistency refiner  46  inside line  4  is advantageous. A heater  47  for the carbon dioxide which is supplied by the storage tank  30  is optional. The heater  47  operates with superheated steam which is supplied via an inlet  49  and discharged via an outlet  48 .  
      In the production of an SC paper web (SC=super calendered) ( FIG. 2 ) and depending upon the quality of the paper—SC-A-paper or SC-B paper—certain upper limits for the roughness R for the blackening S (blackening index), which is measured in micrometers according to the PPS measuring method, must be cited as a function of the roughness. Only papers that have roughness and blackening values below the limits are usable.  
      Calendered papers are especially suitable for the rotogravure printing process. These papers are calendered in calender nips in order to achieve the desired smoothness of the paper surface, since otherwise flaws would occur in the print. However, if a calender with an insufficient number of nips is utilized and if excessive nip pressures are applied upon the paper web, a portion of the fibers would be squeezed in such a way that they would take on a transparent appearance. In order to achieve a high paper quality this malfunction must not exceed the limits illustrated in  FIG. 2  which are determined for various paper grades through the limit curves G which are also depicted in  FIG. 2 . Also a too high a waste paper (DIP=deinked paper) component reduces the paper quality to lower values. Great roughness causes many printing flaws (“missing dots”). However, the production line becomes unprofitable if a multitude of nips are utilized during the printing process while at the same time working at a low pressure or low line pressure. In accordance with the current invention the paper quality is therefore improved by loading the cavities of the fibers with ash. This makes substantially greater line pressures in the calender nips possible and the number of calender nips can be reduced compared to the current state of the art. This also permits an increase in efficiency of the calender and at the same time a reduction of the blackening.  
      A paper machine ( FIG. 3 ) which is suitable for the production of calendered papers comprises a twin wire former  50 , a press section  51  with two pairs of rolls arranged in tandem, wherein in each instance one press roll is in the embodiment of a shoe press roll.  
      A single row dryer section  52  is located adjacent to the press section  51 . The paper web is calendered in a calender  53  which is equipped with a multitude of roll pairs. The paper web is subsequently wound in a winder  54 .  
      While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.