Retention of PCC and GCC fillers on chemical pulp fines surfaces

We studied the retention of precipitated and ground calcium carbonate fillers (PCC and GCC, respectively) on chemical pulp fines surfaces by measuring the amounts of adsorbed fillers on the surfaces of the primary and secondary fines and long fiber fractions of eucalyptus pulp in the presence of a polymeric flocculant (CPAM). The mechanism of filler particle retention and adsorption on fines surfaces was clarified in more detail by measuring the kinetics of PCC adsorption in the absence of flocculant. The results showed that the primary and secondary fines fractions adsorb more PCC and GCC fillers on their surfaces (per gram of pulp) than the long fiber frac-tion in both the absence and presence of flocculants. In the absence of flocculants the adsorption of PCC both on fines and on the fiber fractions follows Langmuir kinetics, implying that the adsorption is in dynamic equilibrium with filler detachment and the maximum possible amount of filler that can be adsorbed corresponds to the monolayer coverage of particles. At low flocculant doses, i.e., doses that are relevant to papermaking, the fines fractions adsorb significantly more filler than the fiber fractions. In this case, their high surface charge density promotes CPAM adsorption onto fines, which in turn increases the adsorption of fillers to the fines surfaces through polymer bridges.