| abstract |
The present invention relates to a method for dewatering sludge, for example deinking sludge, from a pulp, paper or board manufacturing process. The method comprises obtaining an aqueous sludge comprising an aqueous phase and a fibrous material suspended in the aqueous phase. The sludge is subjected to a pre-thickening step in which a first portion of the aqueous phase is removed from the sludge and a compaction step in which a second portion of the aqueous phase is further removed from the sludge. A dry pressed sludge is thus obtained. Before or during the pre-concentration step, a polymer composition having a charge density of at most 1.7 meq/g, preferably at most 1.5 meq/g, more preferably at most 1.1 meq/g is added to the sludge. The polymer composition comprises a cationic synthetic first polymer having a charge density of at least 1.0 meq/g at pH 2.8 and a cationic second polymer obtained by polymerization of (meth)acrylamide with at least one cationic second monomer. a polymer, wherein the amount of cationic monomer is 2 to 19 weight percent calculated from the total amount of monomers, and the second polymer is polymerized in the presence of the cationic first polymer. The first polymer has a higher charge density than the second polymer. |