Process to obtain fully oxidized white liquor for use in the fiberline of a kraft pulp process

Abstract

The present application relates to a process to obtain fully oxidized white liquor from white liquor used in a Kraft pulp process. Said fully oxidized white liquor is suitable to be used in alkaline steps of certain stages of the Kraft pulp process fiberline in order to partly or totally substitute the purchased sodium hydroxide used in those stages. The presently disclosed process also allows to purge non-process elements (NPE's) by purging alkaline wash filtrates to a wastewater treatment plant.

What is claimed is: 
     
         1 . A process to obtain fully oxidized white liquor for use in the fiberline of a Kraft pulp process characterized by comprising the following steps:
 Oxidizing between 1 and 100 m 3 /h of white liquor from the Kraft pulp process with oxygen, obtaining fully oxidized white liquor comprising NaOH, Na 2 SO 4 , Na 2 CO 3  and Na 2 S 2 O 3 ;   Feeding the fully oxidized white liquor to the bleaching stages and/or the delignification stages and/or the alkaline extraction stages of the Kraft pulp process;   Purging non-process elements by purging the filtrates from washing steps after the bleaching stages and/or alkaline extraction stages to a Wastewater Treatment Plant;   Feeding up to 100% of the alkaline filtrate obtained in the washing steps after the bleaching stages to an evaporation plant;   Feeding the alkaline filtrate obtained in the washing steps after the delignification stages to the evaporation plant; and   Purging non-process elements from a recovery boiler in the Kraft chemical recovery process of the Kraft pulp process.   
     
     
         2 . The process of  claim 1 , wherein the white liquor is oxidized with 20 to 60 kg of O 2 /m 3  of white liquor. 
     
     
         3 . The process of  claim 1 , wherein the oxygen is used as pure oxygen or as a gas mixture comprising oxygen. 
     
     
         4 . The process of  claim 1 , wherein oxygen is used with a purity degree above 70% v/v. 
     
     
         5 . The process of  claim 1 , wherein the fully oxidized white liquor substitutes up to 100% of the amount of purchased NaOH used in the bleaching stages, alkaline extraction stages, delignification stages of the Kraft pulp process and combination thereof. 
     
     
         6 . The process of  claim 1 , wherein the oxidation step occurs at a temperature above 100° C., pressure above 1 barg, and the total Hydraulic Residence Time is at least 30 minutes. 
     
     
         7 . The process of  claim 1 , wherein the oxidation step occurs at a temperature above 100° C. 
     
     
         8 . The process of  claim 1 , wherein the oxidation step occurs at a pressure above 1 barg. 
     
     
         9 . The process of  claim 1 , wherein the total Hydraulic Residence Time is at least 30 minutes. 
     
     
         10 . The process of  claim 1 , wherein up to 100% of the filtrates from washing steps after the bleaching stages and/or alkaline extraction stages are sent to the Wastewater Treatment Plant.