Method for reducing starch content of an aqueous phase removed from fibre stock preparation

Abstract

The invention relates to a method for reducing starch content of an aqueous phase, which is removed from a fibre stock preparation in a manufacturing process of paper, board, tissue or the like. The fibre stock preparation comprises a thickening step, where a fibre stock comprising cellulosic fibres originating from recycled fibre material and/or broke as well as starch dispersed in an aqueous phase is thickened from a first concentration to a second concentration by removing a part of the aqueous phase from the fibre stock. A cationic polymer obtained by copolymerisation of (meth)acrylamide and at least 25 mol-% of solely cationic monomer(s), having a standard viscosity SV of at least 1.7 mPas, is added to the fibre stock at the latest at the thickening step in order to associate the starch with the cellulosic fibres of the fibre stock.

1 . A method for reducing starch content of an aqueous phase removed from a fibre stock preparation in a manufacturing process of paper, board, tissue or the like, wherein the fibre stock preparation comprises a thickening step, where the fibre stock comprising cellulosic fibres originating from recycled fibre material and/or broke as well as starch dispersed in an aqueous phase is thickened from a first concentration to a second concentration by removing a part of the aqueous phase from the fibre stock,
 wherein a cationic polymer obtained by copolymerisation of (meth)acrylamide and at least 25 mol-% of solely cationic monomer(s), having a standard viscosity SV of at least 1.7 mPas, is added to the fibre stock latest at the thickening step in order to associate the starch with the cellulosic fibres of the fibre stock.   
     
     
         2 . The method according to  claim 1 , wherein the cellulosic fibres of the fibre stock comprise at least 50 weight-%, preferably at least 70 weight-%, more preferably 100 weight-%, of recycled fibre material, calculated from total amount of fibres, as dry. 
     
     
         3 . The method according to  claim 1 , wherein the recycled cellulosic fibre material comprises at least 40 weight-%, preferably at least 50 weight-%, of fibre material originating from old corrugated containerboard (OCC). 
     
     
         4 . The method according to  claim 1 , wherein at least one additional dose of the cationic copolymer is added to the fibre stock after the thickening step and before a wire section, preferably before a dilution step, where the fibre stock is diluted from the second concentration to a third concentration of <2 weight-%. 
     
     
         5 . The method according to  claim 1 , wherein the first concentration is 0.6-1.4 weight-% and the second concentration is 2-13 weight-%. 
     
     
         6 . The method according to  claim 1 , wherein the cationic copolymer added to the fibre stock prior to the thickening step is obtained by polymerising 5-75 mol-%, preferably 20-75 mol-%, more preferably 30-70 mol-%, even more preferably 40-70 mol-%, of (meth)acrylamide and 25-95 mol-%, preferably 25-80 mol-%, more preferably 30-70 mol-%, even more preferably 30-60 mol-% of cationic monomers. 
     
     
         7 . The method according to  claim 1 , wherein the cationic monomer(s) are selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acrylamido)propyl] trimethylammonium chloride (APTAC), or [3-(methacrylamido)propyl] trimethylammonium chloride (MAPTAC). 
     
     
         8 . The method according to  claim 1 , wherein the cationic copolymer is obtained by solution polymerisation, suspension polymerisation, dispersion polymerisation, emulsion polymerisation or gel polymerisation. 
     
     
         9 . The method according to  claim 1 , wherein the standard viscosity of the cationic copolymer is in a range of 1.7-7.0 mPas, preferably 2.5-6.0 mPas, more preferably 3.0-5.0 mPas. 
     
     
         10 . The method according to  claim 1 , wherein the cationic copolymer is added in an amount of 0.2-1.5 kg/ton, preferably 0.3-1.2 kg/ton, more preferably 0.4-1 kg/ton, even more preferably 0.5-0.8 kg/ton. 
     
     
         11 . The method according to  claim 1 , wherein preceding the addition of the cationic copolymer the method is free of addition steps of synthetic organic coagulant or inorganic coagulant. 
     
     
         12 . The method according to  claim 1 , wherein the fibre stock comprises starch at least 2 weight-%, calculated from the dry solid matter, before the addition of the cationic copolymer. 
     
     
         13 . The method according to  claim 1 , wherein the cationic copolymer is added to the fibre stock in two or more dosage locations, preferably in different amounts in each dosage location. 
     
     
         14 . The method according to  claim 1 , wherein the cationic polymer is added at least one additional dosage location, situated after the thickening step and before a wire section of a web forming machine. 
     
     
         15 . The method according to  claim 1 , wherein the fibre stock preparation comprises a fibre fractioning step, where a long fibre fraction is separated from a short fibre fraction, and the cationic polymer is added at least to the short fibre fraction. 
     
     
         16 . The method according to  claim 15 , wherein the cationic polymer is added to both the long fibre fraction and the short fibre fraction, and the cationic polymer is added to the short fibre fraction in an amount that is higher than the amount of the cationic polymer added to the long fibre fraction.