Method for washing pulp in a bleaching line

Abstract

The method is for bleaching cellulose pulp in a bleaching line that has at least two bleaching steps including a first (D 1 ) and a second (D 2 ) bleaching step. The bleaching steps have wash apparatuses (W 4 , W 5 ) for the pulp arranged after the first and the second bleaching step. Wash liquor and where appropriate dilution liquor is led in principle in counter-current to the pulp flow through the bleaching steps in the bleach line (W 1 -D 0 -W 2 -EO/EOP-W 3 -D 1 -W 4 -D 2 -W 5 ). At least one of wash liquor and dilution liquor is taken to the subsequent wash (W 5 ) of the second bleaching step (D 2 ) and at least a part of the wash filtrate from the subsequent wash of the second bleaching step is led to a second branch position (A 2 ) in the main conduit.

1 . A method of bleaching cellulose pulp in a bleaching line, having at least two bleaching steps comprising: 
 providing a first and a second bleaching step, as seen in a flow direction of the pulp through the bleaching line,    the bleaching steps having wash apparatuses for the pulp arranged after the first and the second bleaching steps, respectively,    leading wash liquor counter-currently to a pulp flow through the bleaching steps,    supplying the wash liquor in a main conduit that is pressurised during steady state, taking the wash liquor to a subsequent wash of the second bleaching step from a first branch position in the main conduit,    leading at least a part of the wash filtrate from the subsequent wash of the second bleaching step to a second branch position in the main conduit,    taking the wash liquor to a subsequent wash of the first bleaching step from a third branch position (A 3 ) in the main conduit,    leading at least a part of the wash filtrate from the subsequent wash of the first bleaching step to a fourth branch position (A 4 ) in the main conduit,    arranging the first branch position in a first position and the second branch position and the fourth branch position in subsequent succession relative to the first branch position as seen in the flow direction.    
   
   
       2 . A method according to  claim 1  wherein a base level of pressure in the main conduit is established at a level in a range of 1,5-3,5 bars.  
   
   
       3 . A method according to  claim 2  wherein dilution and wash liquids taken from the main conduit to dilution vessels or wash apparatuses operating at pressures above the base level is pressurised by a second pressurising means.  
   
   
       4 . A method according to  claim 2  wherein the base level of pressure in the main conduit is established at a level in a range of 4,5-6,5 bars.  
   
   
       5 . A method according to  claim 1  wherein the main conduit is connected to receive and distribute acidic filtrate from and to, respectively, acidic bleaching steps.  
   
   
       6 . A method according to  claim 1  wherein the main conduit is connected to receive and distribute alkaline filtrate from and to, respectively, alkaline bleaching steps.  
   
   
       7 . A method according to  claim 5 , wherein upstream the first branch position (A 1 ) in a first end of the main conduit, a main pressurising device or a pressurised wash liquid tank, is provided which pressurizes the main conduit and establishes a basic flow in the main conduit in a direction reverse to a formed flow of cellulose pulp in a bleaching line.  
   
   
       8 . A method according to  claim 7 , wherein before the second and fourth branch positions, filtrate is led to the main conduit via pump devices.  
   
   
       9 . A method according to  claim 1 , 
 wherein at least one additional bleaching step is provided before the first and second bleaching steps, as seen in the flow direction of the pulp, and    at least one liquor of wash liquor and dilution liquor is taken to a subsequent wash of an additional bleaching step, from a fifth branch position (A 5 ) in the main conduit ( 1 ) and at least a part of the wash filtrate from the subsequent wash of the additional bleaching step is led to a sixth branch position (A 6 ) in the main conduit.    
   
   
       10 . A method according to  claim 9 , wherein an extraction step is provided after the additional bleaching step and before the first bleaching step, as seen in the flow direction of the pulp through the bleaching line, and that a wash apparatus is arranged after the extraction step.  
   
   
       11 . A method according to  claim 10 , wherein the wash filtrate from the subsequent wash of the extraction step, at least partly is used as dilution liquor for the wash step subsequent to the additional bleaching step, and a part of the wash filtrate is drawn off.  
   
   
       12 . A method according to  claim 9 , wherein the cellulose pulp is washed in a wash apparatus before the additional bleaching step, as seen in the flow direction of the pulp through the bleaching line, and at least one liquor of wash liquor and dilution liquor is taken to the wash apparatus from a seventh branch position in the main conduit.  
   
   
       13 . A method according to  claim 1  wherein chlorine dioxide is used as active bleaching agent in the bleaching steps, which chlorine dioxide is added to the pulp in a blending apparatus.  
   
   
       14 . A method according to  claim 1  wherein at an other end of the main conduit, an outlet is provided, from which wash liquor and filtrate are drawn off.  
   
   
       15 . A method according to  claim 14 , the outlet is controlled by a pressure and/or flow controlling control valve.