Abstract:
An apparatus in the treatment of stock passed to a headbox of a paper machine or equivalent includes at least two stock chests ( 10   a   1   , 10   a   2 ). Stock (M 1 ) from a first stock chest ( 10   a   1 ) is passed along a line (a 1 , a 2 ) to a hydrococyclone plant ( 20 ) in the short circulation of the paper machine or equivalent. An accept line (a 3 ) of the hydrocyclone plant is connected with a stock line (b 1 ) of the stock (M 2 ) fed from a second stock chest ( 10   a   2 ), and a combined stock flow is passed along a line (b 2 ) to the headbox ( 100 ) of the paper machine or equivalent. A method in the treatment of the stock passed to a headbox of a paper machine or equivalent is disclosed.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application is a U.S. national stage application of international app. No. PCT/FI2004/000058, filed Feb. 5, 2004, the disclosure of which is incorporated by reference herein, and claims priority on Finnish App. No. 20030205, Filed Feb. 11, 2003. 
   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The invention relates to an apparatus and a method in the treatment of the stock passed to a headbox of a paper machine or equivalent. 
   Centrifugal cleaning is needed in paper machines for separation of sand and contaminants. With today&#39;s technology, the cleaning efficiency of centrifugal cleaning deteriorates when the fiber consistency of the pulp suspension exceeds 1 percent. This limits the increasing of the feed consistency of the stock to be fed to the headbox. In practice, the slotted screen technique has made it unnecessary to use centrifugal cleaning for separating reject fibers, such as shives. A hydrocyclone plant is placed in the short circulation of the paper machine, where the flow rates are high, as high as 2000 l/s. To be operative, centrifugal cleaning requires a pressure difference of 120-150 kPa. In that connection, all (about 5) steps of the hydrocyclone plant require pumps, which represent as much as about 25 percent of the energy consumption of the short circulation. At a flow rate of 2000 l/s, the power consumption of centrifugal cleaning is about 1200 kW. A typical amount of fiber reject from centrifugal cleaning is about 0.1-0.2 percent of production. The loss of the filler pigments coming with coated broke is at its worst about 0.5 percent of machine production. 
   A filler recovery system is often incorporated in connection with the centrifugal cleaning of the short circulation. In addition to filler, the system must also process other rejects, such as fiber reject and sand, coming from the short circulation. In that case, the efficiency of the filler recovery system is not best possible. 
   Concepts are known in which the cleaning of the stock has been transferred from the short circulation to pulp lines. The consistency (about 3 percent) of the broke system is, however, not suitable for separation of sand with hydrocyclones. 
   When centrifugal cleaning is in the pulp line (e.g. chemical pulp, DIP or TMP), these pulps need not be cleaned again any more, but the debris, sand and non-disintegrating coating sheets of paper coming to the broke system via pulpers should be treated by means of hydrocyclones. 
   SUMMARY OF THE INVENTION 
   By placing a hydrocyclone plant in accordance with the invention in a broke system line in the short circulation, the problem is solved. The fiber consistency in the headbox can be increased, when needed, to a level of over 2 percent without the fiber consistency in the centrifugal cleaning exceeding the limit of 1 percent. 
   The size and the energy consumption of the hydrocyclone plant would be only about one third of the present size and energy consumption. The size is determined based on the maximal broke percentage. 
   At the same time, better selectivity is achieved in the filler recovery process. 
   In the invention, a hydrocyclone plant is placed in a stock line which is in the short circulation and uses broke, and it is connected with another stock line, so that the bulk of the stock flow (the purer stock) bypasses centrifugal cleaning. 
   The proposal reduces the energy consumption of centrifugal cleaning by about 65 percent, which means a saving of about 17 percent in the energy need of the short circulation. On a large machine the saved power is about 800 kW. 
   The amount of reject from centrifugal cleaning is reduced to a fraction, which means that the amount of reject from centrifugal cleaning would be in its entirety less than 0.05 percent of production. In practice, it could halve the amount of reject in the area of the paper mill, thus reducing the handling capacity associated with fiber recovery. 
   The investment in equipment is reduced by about 65 percent in centrifugal cleaning and by about 10 percent in respect of the short circulation. A hydrocyclone plant is a subprocess that takes up much space. By means of the arrangement in accordance with the invention, the paper machine hall is shortened by 3 m, with the result that the saving in building costs is considerable. 
   In accordance with the invention, a system is formed that includes at least two stock chests. The first stock chest comprises a stock composition M 1  containing pulp that requires centrifugal cleaning before it is passed to the headbox of the paper machine. The stock composition M 1  contains broke pulp passed from the paper machine and, in addition, it can contain pulp coming from fiber recovery and further mechanical pulp. The second stock chest comprises a stock composition M 2  containing pulp that has already undergone centrifugal cleaning, such as recycled fiber and/or chemical pulp and/or TMP. Thus, it does not contain any broke coming from the paper machine. In the arrangement in accordance with the invention, only the stock M 1  of the first stock chest is treated in the hydrocyclone plant and at least one accept is passed from it into connection with a second stock chest line and its stock M 2 . There can be more stock chests than two. 
   The apparatus in accordance with the invention thus includes a hydrocyclone plant that is much cheaper in capital expenditure and takes up less space than that of the prior art because its capacity need not be as high as that of the prior art arrangements in which all stock is passed through a hydrocyclone plant. In the arrangement in accordance with the invention, it is only the stock M 1  which has come as broke that is passed through the hydrocyclone plant in the short circulation of the headbox. 
   In the following, the invention will be described with reference to some advantageous embodiments of the invention shown in the figures of the appended drawings, but the invention is not meant to be exclusively limited to them. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a prior art apparatus for passing stock to a headbox of a paper machine. 
       FIG. 1B  shows an arrangement in accordance with the invention. 
       FIG. 2A  shows a first embodiment of the invention in which broke-containing stock is passed from a first stock chest to a hydrocyclone plant, and in which embodiment the stock is passed through a wire pit. 
       FIG. 2B  shows a second embodiment of the invention. 
       FIG. 3  is an illustration of principle of the operation of a hydrocyclone plant. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A  shows a prior art stock system in which all stock M 1 +M 2 +M 3  is passed to a hydrocyclone plant  20 , which means that a high capacity is required from the hydrocyclone plant. 
     FIG. 1B  shows an arrangement in accordance with the invention. A stock chest  10   a   1  contains stock, i.e. a pulp fraction M 1 , which contains broke passed from a paper machine and said pulp fraction M 1  is treated in a hydrocyclone plant  20 . The cleaned stock, its accepts are passed further into connection with stocks M 2  and M 3  that do not contain broke and further to a headbox  100 . The pulp fractions M 2  and M 3  that do not contain broke in stock chests  10   a   2  and  10   a   3  thus bypass the centrifugal cleaning  20 , and the accept of the stock M 1  from the hydrocyclone plant  20  is passed into connection with said stocks M 2  and M 3 . The hydrocyclone plant  20  is not required to have as high a capacity as that of the embodiment of  FIG. 1A . 
   In the embodiment of  FIG. 2A , stock M 1 , or a pulp fraction, of a first stock chest  10   a   1  also comprises a stock composition that requires centrifugal cleaning before it is passed to a headbox of a paper machine. The stock M 1  contains broke coming from the paper machine and, in addition, it may contain pulp coming from fiber recovery, and further mechanical pulp. 
   Stock M 2  of a second stock chest  10   a   2  comprises a stock composition that has already undergone centrifugal cleaning, such as recycled fiber and/or chemical pulp and/or TMP. 
   In the embodiment of  FIG. 2A , the stock M 1  is passed from the stock chest  10   a   1  through a stock line a 1  to a lower part of a wire pit  11 . The line a 1  includes a pump P 1 . In the lower part of the wire pit, the stock M 1  is diluted with wire water obtained from a wire section of a paper machine (not shown) along a line d 1  to a consistency required by a hydrocyclone plant  20 . A line a 2  leads from the lower part of the wire pit  11  to the suction side of a pump P 2  and a line a 2  leads from the pressure side of the pump P 2  to a first centrifugal cleaning step  20   a   1  of the hydrocyclone plant  20  situated in the short circulation of the paper machine. In the figure, the centrifugal cleaning steps are designated with  20   a   1 ,  20   a   2 ,  20   a   3  . . . An accept line from the centrifugal cleaning step  20   a   1  of the hydrocyclone plant  20 ; a line a 3  is passed further to join a line b 1  of the stock M 2  of the second stock chest  10   a   2  via a mixing device  12 . The mixing device  12  is also supplied with wire water from the wire pit  11  along a line e 1  for diluting the stock M 2  to be fed to the headbox  100  to a suitable consistency. 
   From the upper part of the wire pit  11  there is further a line c 1  for dilution water, said line c 1  including a pump P 3 . The line c 1  leads further from the discharge side of the pump P 3  to a deaeration tank  13   a   1 . The dilution water passed through the deaeration tank  13   a   1  is conducted further after the deaeration treatment to a discharge line f 1  and further while pumped by a pump P 4  to a machine screen  14   a   1 , whose accepted fraction, i.e. accept, is passed to a dilution inlet header J 2  in the headbox  100 . 
   The stock chest  10   a   2  is provided with the line b 1  for the stock and further to the suction side of a pump P 5 . On the discharge side of the pump P 5 , the line b 1  is connected to the mixing device  12 , after which there is a pump P 6  in a line b 2  for pumping the combined stock further along the line b 2  to a deaeration tank  13   a   2 , from which a discharge line f 2  leads further to the suction side of a pump P 7 . On the discharge side of the pump P 7 , in the line f 2  there is a machine screen  14   a   2 , from which an accepted fraction, i.e. accept, is passed to a stock inlet header J 1  of the headbox  100 . 
   In the device arrangement in accordance with the invention, only the broke-containing stock M 1  passed from the stock chest  10   a   1  is treated in the hydrocyclone plant  20 . An accept line a 3  leads from said hydrocyclone plant further into connection with the stock line b 1  of the stock M 2  of the second stock chest  10   a   2 . Since the stock M 2  of the second stock chest  110   a   2  comprises stock that has already previously undergone centrifugal cleaning, said line can be connected directly to the headbox  100  of the paper machine, via its deaeration tank  13   a   2  and machine screen  14   a   2 . 
   In the embodiment of  FIG. 2B , stock M 1 , i.e. a pulp fraction, of a first stock chest  10   a   1  also comprises a stock composition that requires centrifugal cleaning before it is passed to a headbox of a paper machine. The stock M 1  contains broke coming from the paper machine and it can additionally contain pulp coming from fiber recovery and further mechanical pulp. 
   Stock M 2  of a second stock chest  10   a   2  comprises pulp that has already undergone centrifugal cleaning, such as recycled fiber and/or chemical pulp and/or TMP. 
   Also in this embodiment of the invention, only the stock M 1  passed from the stock chest  10   a   1  is treated in a hydrocyclone plant  20 . In the embodiment of the figure, the stock is passed from the stock chest  10   a   1  through a line a 1  while pumped by a pump P 10  to a mixing device  120 , in which the stock is diluted to a centrifugal cleaning consistency with wire water obtained from a line f 4 , and the stock M 1  is passed further through a line a 2  to the suction side of a pump P 20 . The line a 2  on the pressure side of the pump P 20  is connected to the hydrocyclone plant  20  to form the feed of its first centrifugal cleaning step  20   a   1 . 
   In the embodiment of  FIG. 2B , the hydrocyclone plant  20  situated in the short circulation of the paper machine includes centrifugal cleaning steps  20   a   1 ,  20   a   2  and  20   a   3 . An accept line a 3  leads further from the first hydrocyclone, i.e. the centrifugal cleaning step  20   a   1  of the hydrocyclone plant  20  into connection with a stock line b 1  of a second stock chest  10   a   2 . 
   In the embodiment, wire water from the paper machine is passed to a wire pit  110  through a line d 1 , which wire pit  110  in this embodiment is formed by a planar wire pit structure, a so-called flume, which comprises a horizontal flow path for wire water. Said wire pit  110  removes effectively air in bubble form from the wire water, and pre-deaeration of the wire water is accomplished by means of said wire pit type. The wire water is passed from the wire pit  110  through a discharge line d 2  and a pump P 30  to a deaeration tank  13   a   3 , from which there is further a discharge line f 3  leading into connection with the line b, of the stock M 2  of the second stock chest  10   a   2  via a mixing device  12 . The line f 4  leads further from the discharge line f 3  of the deaeration tank  13   a   3  into connection with the line a 1  of the stock M 1  of the first stock chest  10   a   1  via the mixing device  120 . A branch line f 5  leads further from the line f 3  to a pump P 40  and further from the pressure side of the pump P 40  to a machine screen  14   a   3 , which conducts the wire water further as accept from the machine screen  14   a   3  to a dilution water inlet header J 2  of a headbox  100 . 
   The stock M 2  is passed from the stock chest  10   a   2  through a pump P 50  along the line b 1  to the mixing device  12  in order to be combined with the stock coming as accept along the line a 3  from the hydrocyclone plant  20  and with the dilution water coming along the line f 3 . After that the diluted stock is pumped by means of a headbox feed pump P 60  through a machine screen  14   a   4  to a stock inlet header J 1  of the headbox  100 . 
   As shown in  FIG. 3 , the hydrocyclone plant  20  includes several centrifugal cleaning steps  20   a   1 ,  20   a   2 ,  20   a   3 , so that, as shown in the figure, accept from the first step  20   a   1  is passed through the line a 3  further into connection with the line b 1  of the stock M 2  of the second chest  10   a   2 . As shown in  FIG. 3 , the stock is passed through the line a 1  as a feed to the first centrifugal cleaning step of the hydrocyclone plant  20 , i.e. to the hydrocyclone  20   a   1 . The stock flows along a spiral-shaped path inside the hydrocyclone  20   a   1  and heavier particles separate as reject from the bottom of the hydrocyclone and lighter particles rise as accept further through the line a 3  into the line b 1  of the stock M 2  passed from the second stock chest  10   a   2 . There can be several hydrocyclones  20   a   1 ,  20   a   2 ,  20   a   3  . . . and the reject from the first hydrocyclone  20   a   1  can be passed further to the second hydrocyclone  20   a   2  as its feed and the accept from it in one embodiment can be passed further to the line b, of the stock M 2  of the second stock chest  10   a   2 . 
   The figure shows a headbox  100 . The headbox  100  in accordance with the invention is advantageously a so-called dilution headbox, which means that the dilution water passed to the dilution water inlet header J 2  is passed further across the width of the headbox to different points of the stock passed from the stock inlet header J 1  In this way, dilution makes it possible to regulate the basis weight of the web across the width of the web. The dilution water passed from the dilution water inlet header J 2  is passed to ducts which are provided with dilution water valves, by means of which the supply of dilution water can be regulated as desired across the width of the headbox, thus enabling the basis weight of the web to be regulated to be even across the entire width of the web. 
   As shown in the figure, the hydrocyclone plant can also include several accept lines, the stock passed through them being conducted into connection with another stock or with stocks passed from other chests. In accordance with the invention, it is also possible to use several stock chests, but in the invention only that stock, such as the broke-containing stock M 1 , which shall be treated in the hydrocyclone plant is circulated through the hydrocyclone plant  20 . The pulp fraction M 2  which need not be cleaned with hydrocyclones is passed directly to deaeration and, after a machine screen, to the stock inlet header J 1  of the headbox  100 . The accept derived from the stock M 1  in the centrifugal cleaning  20  is conducted into connection with said stock. 
   When the stocks M 1  and M 2  of the chests  10   a   1 ,  10   a   2  are referred to in this application, it is also possible to call them a pulp fraction M, and a pulp fraction M 2 . In this application, the paper machine is understood to mean paper, board and tissue machines. 
   The broke can be formed of paper broke, which can include trimmings or paper passed to a pulper in connection with web breaks. 
   The present application refers to lines by which are meant stock lines, pipes, ducts along which stock/wire water is passed.