Method for washing pulp in a bleaching line and 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 and second bleaching steps. The bleaching steps have wash apparatuses for the pulp arranged after the first and second bleaching steps. The dilution liquor is led in counter-currently to the pulp flow through the bleaching steps in the bleach line. A substantial portion of the wash liquor is supplied by means of a main conduit so that each of the wash apparatuses is independently supplied by means of first branch conduits connected to the main conduit. Each of the wash apparatuses independently bleeds out wash filtrate by means of second branch conduits connected to the main conduit. The main conduit has an outlet end which bleeds out at least some of the wash filtrate from the wash apparatus.

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 a flow direction of pulp, 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 a substantial portion of the wash liquor by means of a main conduit, independently supplying the wash apparatuses by means of first branch conduits connected to the main conduit,  
     each of the wash apparatuses independently bleeding out wash filtrate by means of second branch conduits connected to the main conduit, and  
     the main conduit having an outlet end for bleeding out a portion of the wash filtrate from the wash apparatuses.  
   
   
       2 . A method according to  claim 1 , wherein main conduit has an inlet end which under steady state conditions continuously is fed with wash liquid and an outlet end which under steady state conditions continuously bleeds out at least some of the wash filtrate from the main conduit.  
   
   
       3 . A method according to  claim 2 , wherein the inlet and outlets ends are arranged at opposite ends of the main conduit with the branch conduits connected to the main conduit between the inlet and the outlet ends.  
   
   
       4 . A method according  claim 1  wherein the main conduit is connected to receive and distribute filtrate that is mainly acidic or mainly alkaline.  
   
   
       5 . A method according to  claim 4 , wherein there are two main conduits, one to receive and distribute mainly alkaline filtrate and one to receive and distribute mainly acidic filtrate.  
   
   
       6 . A method according to  claim 1  wherein the method further comprises providing a first branch position in an upstream end of the main conduit and a main pressurising device that supplies fresh wash liquid and pressurizes the main conduit and establishes a basic flow in the main conduit in a direction in the main conduit in reverse to a formed flow of the pulp in a bleaching line, the pressurising device being connected to the main conduit at a position at an opposite end in the main conduit in relation to the outlet end.  
   
   
       7 . A method according to  claim 1  wherein filtrate is led to the main conduit via a pump device.  
   
   
       8 . A method according to  claim 1  wherein the main conduit is maintained at a pressure of at least 0.5 bar above atmospheric pressure.  
   
   
       9 . A method according to  claim 8 , wherein the pressure within the main conduit is maintained, during steady state, at a pressure below 3 bar.  
   
   
       10 . A method according to  claim 9 , wherein wash liquor is led from the main conduit to the wash apparatuses via supply lines by means of pump devices.  
   
   
       11 . A method according to  claim 9  wherein dilution liquid is supplied directly via a branch line connected to the main conduit.  
   
   
       12 . A method according to  claim 8 , wherein the pressure in the main conduit, during steady state, is maintained within a range of 3-20 bar above atmospheric pressure.  
   
   
       13 . A method according to  claim 1  wherein an intermediate bleach step is provided intermediate the bleach steps.  
   
   
       14 . A method according to  claim 1 , wherein the outlet end is controlled by a pressure and flow control valve.  
   
   
       15 . A method according to  claim 14 , wherein the pressure and flow control valve provides teed back control of a main pressurising device to secure a predetermined pressure and/or flow through the main conduit.  
   
   
       16 . A method according to  claim 15 , wherein a flow at the outlet end, during steady state, is within a range 0.1-12 m 3 .  
   
   
       17 . A bleaching line for bleaching cellulose pulp in a bleaching line, comprising:  
     at least two bleaching segments having a first and a second bleaching step, as seen in a flow direction of pulp, the bleaching segments having wash apparatuses for the pulp arranged after the first and the second bleaching segments wash liquor is led via bleach lines counter-currently to the flow direction of the pulp through the bleaching segments  
     each one of the wash apparatuses being independently connected to a main conduit by means of first branch conduits,  
     each one of the wash apparatuses (W 4 , W 5 ) being independently connected to the main conduit to bleed out wash filtrate by means of second branch conduits, and  
     the main conduit being arranged with an outlet end to bleed out at least some of the wash filtrate from the wash apparatuses.  
   
   
       18 . A bleaching line according to  claim 17 , wherein the outlet end is arranged opposite an inlet end of the main conduit, the branch conduits being connected to the main conduit between the inlet end and the outlet end.  
   
   
       19 . A bleaching line according to  claim 17  wherein there are two main conduits, one to receive and distribute mainly alkaline filtrate and one to receive and distribute mainly acidic filtrate.  
   
   
       20 . A bleaching line according to  claim 17  wherein a first branch position is arranged in an upstream end of the main conduit and a main pressurising device to supply fresh wash liquid and to pressurise the main conduit.  
   
   
       21 . A method according to  claim 17  wherein a pump device is arranged within at least one of the branch conduits to pump the filtrate to the main conduit.  
   
   
       22 . A bleach line according to  claim 17  wherein there is arranged at least one branch line connected to the main conduit to supply dilution liquid to at least one position of the bleach lines.  
   
   
       23 . A bleaching line according to  claim 17  wherein the outlet end is arranged with a control device.  
   
   
       24 . A bleaching line according to  claim 23  wherein the control device is connected to a pressure sensing device to control pressure and/or flow in the main conduit.