Method and arrangement for treating chemical pulp

Abstract

A method to pulp cellulosic fibrous material including: alkaline cooking cellulosic fibrous material in a continuous digester vessel to produce a pulp having a kappa number in a range of 50 to 120; washing the pulp discharged from the digester vessel; treating the washed pulp in the presence of oxygen and alkali to remove lignin; washing the oxygen-treated pulp such that the kappa number is recued by at least at least 30; separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and using the separated wash filtrate as a washing liquid in the washing of the pulp from the digester vessel.

1 . A method of treating chemical pulp comprising:
 alkaline cooking cellulosic fibrous material to produce a pulp having a kappa number in a range of 50 to 120;   washing the pulp;   feeding the washed pulp to an oxygen stage in which the wash pulp is treated in the presence of oxygen and alkali to remove lignin;   washing the oxygen-treated pulp such that the kappa number by at least at least 30, and   separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and   applying the separated wash filtrate as a washing liquid used in the washing of the pulp.   
     
     
         2 . The method according to  claim 1 , wherein the washing of the pulp is with a washer having an E 10 -value of at least 8. 
     
     
         3 . The method according to  claim 1 , the decrease of the kappa number is at least 35. 
     
     
         4 . The method according to  claim 1 , wherein the oxygen stage includes oxidizing with sodium hydroxide. 
     
     
         5 . The method according to  claim 1 , further comprising first screening the pulp downstream of the oxygen stage. 
     
     
         6 . The method according to  claim 5 , wherein the oxygen stage is a first oxygen stage and the method further comprises screening the washed pulp downstream of the first oxygen stage and upstream of a second oxygen stage. 
     
     
         7 . The method according to  claim 1 , further comprising knot-separation of the pulp prior to treating the pulp with the oxygen stage. 
     
     
         8 . The method according to  claim 1 , wherein the lignin is separated by precipitating the wash filtrate. 
     
     
         9 . The method according to  claim 1 , wherein the alkaline cooking includes a sulfate process. 
     
     
         10 . The method according to  claim 1 , wherein the alkaline cooking includes a soda process. 
     
     
         11 . The method according to  claim 1 , wherein the alkaline cooking is in a digester vessel and further comprising discharging the pulp from the digester vessel and feeding the to washing without treatment to a mechanical defibrator between the digester and the washing, 
     
     
         12 . The method according to  claim 1 , further comprising recovery the separated lignin. 
     
     
         13 . A system for treating pulp produced from wood chips in a cooking apparatus wherein the wood chips are cooked chemically to a kappa number in a range of 50 to 120, said system comprising:
 a continuous digester vessel including an upper inlet configured to receive cellulosic fibrous material and a lower outlet configured to discharge pulp produced from the cellulosic fibrous material in a cooking process performed in the digester vessel;   a first pulp washer coupled to the lower outlet of the digester vessel to receive the pulp, wherein the pulp washer includes an outlet for washed pulp;   an oxygen stage coupled to the outlet of the pulp washer to receive the washed pulp, the oxygen stage configured to treat the washed pulp in the presence of oxygen and alkali and including an outlet for the treated pulp;   a second pulp washer coupled to the oxygen stage and configured to wash the treated pulp from the oxygen stage;   a first filtrate conduit for wash filtrate from the second pulp washer;   a separator coupled to the first filtrate conduit, wherein the separator includes a lignin outlet for lignin separated from the wash filtrate and a treated wash filtrate outlet, and   a wash liquid conduit coupled to the treated wash filtrate outlet and to a wash liquid inlet to first pulp washer.   
     
     
         14 . The system according to  claim 13 , wherein:
 the first washer includes a pulp inlet connected to a conduit which is connected to the lower outlet of the continuous digester vessel, and the outlet for the washed pulp is connected to a conduit which is connected to a pulp inlet the oxygen stage;   the second pulp washer includes a pulp inlet connected to a conduit which is connected to the pulp outlet of the oxygen stage, and   the separator is arranged in the wash liquid conduit.   
     
     
         15 . The system according to  claim 13 , wherein the first pulp washer has an E 10 -value of at least 8. 
     
     
         16 . A method to pulp cellulosic fibrous material comprising:
 alkaline cooking cellulosic fibrous material in a continuous digester vessel to produce a pulp having a kappa number in a range of 50 to 120;   washing the pulp discharged from the digester vessel;   treating the washed pulp in the presence of oxygen and alkali;   washing the oxygen-treated pulp such that the kappa number is recued by at least at least 30;   separating lignin from wash filtrate generated by the washing of the oxygen-treated pulp, and   using the separated wash filtrate as a washing liquid in the washing of the pulp from the digester vessel.   
     
     
         17 . The method according to  claim 16  further comprising screening the pulp downstream of the oxygen stage. 
     
     
         18 . The method according to  claim 16  wherein the treatment of the washed pulp includes treating the washed pulp in a first oxygen stage in the presence of oxygen and an alkali, screening the washed pulp downstream of the first oxygen stage to extract wash liquor from the washed pulp, and treating the screen washed pulp in a second oxygen stage in the presence of oxygen and an alkali.