Influence of mechanical pulp fines on clay retention

We investigated deposition of clay filler on fibrillar and flake-like fines and long fibers of mechanical pulp to study the effects of the fines on filler retention. The role of different retention mechanisms was illustrated by comparing clay retention in deposition and filtration experiments. The amount of clay deposited varied significantly between the pulp fractions in the presence of polymeric flocculants. Cationic polyacrylamide (CPAM) promoted filler deposition on fines, especially flake-like fines, more than on long fibers. Presumably, this was due to the high surface area and surface charge density of fines, which increase their bonding strength with polymer chains. Polyethylene oxide (PEO) slightly increased the deposition of clay on fines but started to disperse the clay, and especially the fines, as the polymer dose increased. PEO releases small particles from the fines that exist as bundles or entanglements in the suspension. When the deposition-based retention mechanism and the filtration-based mechanism were compared, mechanical attachment of clay in the filter cake had a significant effect on its retention. Also, long fibers are better able to form a filter layer on the screen than fines, which in turn adsorb more filler through chemical interactions.