A method, use of the same, a pulp composition, and a system

Abstract

According to an example aspect of the present invention, there is provided a method comprising: providing wood chips; impregnating the wood chips with an impregnating composition to obtain impregnated wood chips; refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition; screening the refined pulp composition to obtain an accept fraction and a reject fraction; and directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.

1 . A pulping method comprising:
 impregnating wood chips with an impregnating composition to obtain impregnated wood chips;   refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition;   screening the refined pulp composition to obtain an accept fraction and a reject fraction; and   directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.   
     
     
         2 . The method according to  claim 1 , wherein the impregnating composition is an aqueous composition comprising one or more of the following: NaOH, Na 2 SO 3 , white liquor, such as oxidised white liquor, green liquor, and H 2 O 2 . 
     
     
         3 . The method according to  claim 1 , wherein in said refining, the refining energy consumption is in the range of 0.2 to 1.5 MWh/t. 
     
     
         4 . The method according to  claim 1 , wherein after the refining step, the pulp composition has a Pulmac shive content of less than 10%. 
     
     
         5 . The method according to  claim 1 , wherein after the refining step, the pulp composition has Canadian Standard Freeness (CSF) in the range of 285 to 750. 
     
     
         6 . The method according to  claim 1 , wherein, in the screening step, at least 80 wt-% of the refined pulp composition becomes directed to the accept fraction. 
     
     
         7 . The method according to  claim 1 , wherein the Pulmac shive content of the accept fraction is not more than 10 percentage points lower than the Pulmac shive content of the incoming refined pulp composition fed to the screening. 
     
     
         8 . The method according to  claim 1 , wherein the CSF of the accept fraction differs from the CSF of the incoming refined pulp composition fed to the screening by not more than ±20%. 
     
     
         9 . The method according to  claim 1 , further comprising dewatering the accept fraction output from the screening. 
     
     
         10 . The method according to  claim 9 , wherein said dewatering is carried out by a screw press and/or by a disc filter. 
     
     
         11 . The method according to  claim 9 , wherein:
 the dewatering comprises dewatering carried out by a screw press, and   the the accept fraction fed to the screw press has a consistency of at least 2%.   
     
     
         12 . The method according to  claim 9 , wherein:
 the dewatering comprises dewatering carried out by a disc filter, and   the the accept fraction fed to the disc filter has a consistency of at least 0.5%.   
     
     
         13 . The method according to  claim 9  wherein in the dewatering, the accept fraction is dewatered to a consistency of at least 20%. 
     
     
         14 . The method according to  claim 1 , wherein the dewatered accept fraction is fed to a bleaching step after decreasing the consistency of the accept fraction to less than 15%. 
     
     
         15 . The method according to  claim 1 , wherein the consistency of the accept fraction is first increased to above 20% and then decreased to about 8 to 12% to clean the pulp. 
     
     
         16 . The method according to  claim 1 , wherein the reject fraction is directed to the impregnating step at a consistency in the range of 4 to 10%. 
     
     
         17 . The method according to  claim 1 , wherein the directing comprises directing at least part of the reject fraction directly back to the impregnating step or to a step immediately preceding the impregnating step. 
     
     
         18 . The method according to  claim 1 , wherein the directing comprises directing substantially all of the reject fraction directly back to the impregnating step without any refining of said reject fraction. 
     
     
         19 . The method according to  claim 1 , further comprising dewatering of the accept fraction to obtain a dewatered pulp composition. 
     
     
         20 . The method according to  claim 1 , wherein the method produces a bleached or unbleached thermomechanical pulp. 
     
     
         21 - 26 . (canceled)