Method for pitch control during bleaching

Abstract

The invention relates to a method for pitch control and/or removal during bleaching of cellulosic fibres. Cellulosic fibres are obtained from a pulping process and formed into an aqueous cellulosic fibre suspension, and bleaching of the cellulosic fibre suspension is performed in a bleaching process comprising at least one acidic stage, where pH of the cellulosic fibre suspension is <6. A pitch control agent is added to the cellulosic fibre suspension before the acidic stage, wherein the pitch control agent comprises at least one cationic polymer comprising cationic functional groups which are hydrolysed at pH 9.0

1 . A method for pitch control and/or pitch removal during bleaching of cellulosic fibres, comprising:
 obtaining cellulosic fibres from a pulping process and forming an aqueous cellulosic fibre suspension;   bleaching the cellulosic fibre suspension in a bleaching process comprising at least one acidic stage, where pH of the cellulosic fibre suspension is <6; and   adding a pitch control agent to the cellulosic fibre suspension before the acidic stage, wherein the pitch control agent comprises at least one cationic polymer comprising cationic functional groups which are hydrolysed at pH 9 or higher.   
     
     
         2 . The method according to  claim 1 , wherein the pitch control agent comprises at least 25 weight-%, preferably at least 50 weight-%, more preferably at least 75 weight-%, of the cationic polymer comprising cationic functional groups which are hydrolysed at pH 9 or higher, calculated from a dry weight of total polymers of the pitch control agent. 
     
     
         3 . The method according to  claim 1  wherein the cationic polymer has a weight average molecular weight MW of 1 500 000 -6 000 000 g/mol, preferably 2 000 000 - 5 000 000 g/mol, more preferably 2 500 000 - 4 500 000 g/mol, even more preferably 2 700 000 - 4 300 000 g/mol. 
     
     
         4 . The method according to  claim 1 , wherein the pitch control agent comprises polyacrylamide obtained by copolymerisation of (meth)acrylamide and cationic monomers. 
     
     
         5 . The method according  claim 4 , wherein the pitch control agent comprises a cationic polyacrylamide which is obtained by copolymerisation of (meth)acrylamide and 4 - 60 mol-%, preferably 5 - 50 mol-%, more preferably 5 - 40 mol-%. 
     
     
         6 . The method according to  claim 4  wherein the cationic polyacrylamide has a total ionicity in a range of 4 - 15 mol-%, preferably 5 - 13 mol-%, more preferably 6 - 12 mol-%, even more preferably 6 - 10 mol-%. 
     
     
         7 . The method according to  claims 4 , wherein the cationic polyacrylamide is a net cationic amphoteric polyacrylamide, preferably obtained by gel polymerisation of (meth)acrylamide together with anionic and cationic monomers. 
     
     
         8 . The method according to  claim 7 , wherein 53 - 90 %, preferably 50 - 85 %, more preferably 60 - 80 %, of the charged groups of the amphoteric polyacrylamide are cationic. 
     
     
         9 . The method according to  claim 4 , wherein the cationic polyacrylamide is obtained by copolymerisation of (meth)acrylamide and at least one cationic monomer selected from a group consisting of 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-(acryloylamino)propyl] trimethylammonium chloride (APTAC), and [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC). 
     
     
         10 . The method according to  claim 1 , wherein the cellulosic fibres originate from chemical pulping process, such as Kraft pulping process. 
     
     
         11 . The method according to  claim 10 , the wherein pitch control agent is added after a separation of used cooking chemicals and washing of the cellulosic fibres. 
     
     
         12 . The method according to  claim 1 , wherein the pitch control agent is added in amount of <500 g/ton dry pulp, preferably <300 g/ton dry pulp, more preferably <200 g/ton dry pulp or <100 g/ton dry pulp. 
     
     
         13 . A bleaching process of cellulosic fibres, wherein cationic polymer comprising cationic functional groups which are hydrolysed at pH 9 or higher is added into the process as pitch control agent. 
     
     
         14 . The bleaching process of  claim 13 , wherein cationic polymer is cationic polyacrylamide obtained by copolymerisation of (meth)acrylamide and at least one cationic monomer. 
     
     
         15 . The bleaching process of  claim 13 , wherein the cationic polymer is used as pitch control agent for steryl esters, sterols and/or betulinol.