Flocculation studies in papermaking

Abstract

The flocculation process induced by polymeric additives has been studied extensively and is well reported in the literature. However, from the point of view of the papermaking process, still few studies relate flocculation behaviour and flocs characteristics with retention, drainage and sheet formation under various process conditions and for different retention aid systems. This correlation is of great importance in order to understand, predict and optimize retention and drainage performance and thus, sheet formation and quality. In this study, a strategy that allows obtaining information about flocculation kinetics, flocs characteristics, flocs resistance and reflocculation capacity in a single test and in turbulent conditions was developed. The light diffraction scattering technique (LDS) was used to monitor the flocculation process due to its advanced capabilities that allow one to extract information on both the particle size distribution and the fractal dimension of the flocs. Monitorization of the flocculation of precipitated calcium carbonate particles with new cationic polyacrylamides allowed assessing how the polymer characteristics, namely the charge density and the degree of branching affect flocculation, flocs characteristics, flocs resistance and reflocculation capacity in distilled and in industrial water. It was shown that the optimum flocculant dosage decreases and flocs produced are smaller, denser and more resistant as the polymer charge density increases. However, independently of the charge density, the flocs strength decreases as the flocs size increases. Furthermore, when flocculation takes place by bridging, flocs restructuring occurs during flocculation. When branched polymers are used, flocculation is slower and the flocs produced are larger and have a more open structure when comparing with linear polymers. Reflocculation is very small or practically inexistent for all the polymers studied with the exception of the linear polymer of high charge density that produces flocs that partially reflocculate. The structure of the reflocculated flocs is more compact than before flocs break up and more open as the charge density decreases. The use, simultaneously, of a microparticle retention aid improved significantly the reflocculation process and, in this case, the reflocculated flocs have a more open structure than reflocculated flocs without microparticles. However, the action of the microparticles is reduced as the charge density of the polymer decreases and as the degree of branching of the polymer increases. The high cationic content of the industrial water enhances the flocculation kinetics. Nevertheless, the optimum flocculant dosage becomes higher in industrial water than in distilled water. Flocculation kinetics and flocs characteristics are less affected by the cationic content of the water when highly branched polymers are used. The effect of the degree of polymer branching on retention and drainage performance of flocculated kraft pulp fibre suspensions containing precipitated calcium carbonate (PCC) was investigated in the dynamic drainage analyser (DDA) and the results have been correlated with flocs properties obtained by LDS. The results show that polymers of medium charge density are more adequate to be used as retention aids. The results also demonstrate that it is possible to correlate the flocculation process evaluated by LDS with the flocculant’s performance in the drainage test. The effects of the chemical flocculation on the rheological behaviour of the pulp suspension have been studied correlating flocculation data obtained by LDS with the rheological behaviour obtained with the rotational viscometer developed by UCM. It was shown that the choice of the flocculants is important for reducing the power consumption in papermaking. Flocculants with high charge density and without branches seem to be those that more reduce the resistance of the pulp suspension to shearing. As a whole, LDS and DDA results have shown that medium charge density highly branched polymers can be promising additives for papermaking. They offer good retention and drainage with low flocculant dosage and with relatively fast flocculation kinetics due to the formation of small flocs with an open structure, mainly at the secondary aggregates level. Furthermore, highly branched polymers are less affected by the water cationic content in all the flocculation stages (flocculation and break up) leading to similar flocs properties independently of the suspending medium. A population balance model for the flocculation of PCC particles with polyelectrolytes of very high molecular weight and medium charge density is presented. The model describes successfully the flocculation kinetics of both linear and branched polymers. Correlations of the optimized parameters (maximum collision efficiency, kinetic parameter for flocs restructuring and parameter for fragmentation rate) with flocculation data show well the effects of flocculant concentration, flocs structure and polymer structure on these parameters as well as on the flocculation kinetics and flocs restructuring.