Method to recover chemicals in mechanical pulping

Abstract

The present invention refers to a method to recover chemicals in mechanical pulping by evaporating mechanical pulping or recycled fibre plant effluent and burning the received concentrate under oxidative atmosphere to yield sodium carbonate. According to the invention said sodium carbonate is dissolved into water together with CaO in order to convert said sodium carbonate into sodium hydroxide at least partly.

1 . A method to recover chemicals in mechanical pulping comprising the steps of:
 evaporating mechanical pulping or recycled fibre plant effluent   burning the received concentrate in a temperature of at least 850° C. under oxidative atmosphere to yield sodium carbonate,   whereby the flue gases formed during the combustion are cooled by mixing a colder medium into the flue gases to reach a temperature of 700 to 600° C. in order to prevent formed sodium carbonate to stick on surfaces,   further cooling of the flue gas and sodium carbonate fume is carried out by converting heat content of said flue gas and sodium carbonate fume into steam, and   subsequently removing sodium carbonate ash from the cooled flue gas flow and dissolving said sodium carbonate ash into water together with CaO in order to convert said sodium carbonate into sodium hydroxide at least partly for reuse in impregnation prior to refining and/or in peroxide bleaching,   
   
   
       2 . The method as claimed in  claim 1 , wherein limestone is mixed into concentrated wastewater resulting in a mixture to be fed into a concentrate furnace wherein limestone is converted into calcium oxide, and
 the produced mixture of sodium carbonate and calcium oxide is mixed into water in order to have causticizing reactions to proceed.   
   
   
       3 . The method as claimed in  claim 1 , wherein an autocausticizing agent, such as ferric oxide, titanium oxide or sodium borate is mixed into the wastewater concentrate. 
   
   
       4 . The method as claimed in  claim 2 , wherein an autocausticizing agent, such as ferric oxide, titanium oxide or sodium borate is mixed into the wastewater concentrate. 
   
   
       5 . The method as claimed in  claim 1 , wherein Na 2 CO 3  is converted into NaOH via an electrochemical method. 
   
   
       6 . The method as claimed in  claim 2 , wherein sodium carbonate and lime are separated simultaneously from a flue gas stream exiting the concentrate furnace. 
   
   
       7 . The method as claimed in  claim 6 , wherein solid sodium carbonate and lime are mixed with water to start a causticizing reaction to convert sodium carbonate into sodium hydroxide. 
   
   
       8 . The method as claimed in  claim 2 , wherein limestone is out sourced or is recycled from separation of calcium carbonate from sodium hydroxide. 
   
   
       9 . The method as claimed in  claim 8 , wherein recycled limestone is added into an intermediate concentrate before final concentration. 
   
   
       10 . The method as claimed in  claim 1 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       11 . The method as claimed in  claim 2 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       12 . The method as claimed in  claim 3 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       13 . The method as claimed in  claim 4 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       14 . The method as claimed in  claim 5 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       15 . The method as claimed in  claim 6 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       16 . The method as claimed in  claim 7 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       17 . The method as claimed in  claim 8 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge. 
   
   
       18 . The method as claimed in  claim 9 , wherein calcium carbonate formed during causticizing is separated from the solution by a filter or a centrifuge.