Wood pulp washing 2. Displacement washing of aqueous lignin from model beds with cationic polymer solutions

Cationic poly(diallyldimethylammonium chloride), polyDADMAC, increased the efficiency of the displacement of aqueous kraft lignin from a bed of glass beads. The bed consisted of a cylindrical bed, 5 cm in diameter, of fine (97 μm) glass beads surrounding a center channel, 1.5 cm in diameter, filled with coarse (1200 μm) glass beads. The bed was designed to model the displacement washing characteristics of pulp fiber pads on kraft brownstock washers.The effects of polyDADMAC molecular weight and concentration on lignin displacement were investigated. The maximum washing efficiency corresponded to the polymer concentration giving the maximum yield of lignin/polyDADMAC precipitate. Low molecular weight polyDADMAC was not as effective as high molecular weight polymer whereas improved washing with polyamine was not dependent upon molecular weight. It was postulated that improved washing arose from the deposition of lignin/polyDADMAC complex in the center channel. This mechanism is called “clotting”.