Filler retention in pre-formed and forming fibrous media

Filler retention is an important issue at the wet-end of the paper machine. The efficiency with which fillers are retained by the sheet affects the printing and mechanical properties of paper, cost of the product, cleanliness of the system, and pollution load of the disposal system. Hence, a better understanding of the mechanism of filler retention in the forming of paper sheet is essential. The objective of this work is to model and predict the formation and behavior of the wet mat in the wet end of the paper making machine by simultaneously accounting for the formation of the structure from larger fibers and capture and retention of the filler particles in the slurry.;To achieve this objective, continuum equations and constitutive equations describing the process are developed from fundamental principles. Material parameters as required by the constitutive equations are determined from experimental data. Constant rate filtration experiments are run in a cylindrical column with Prince Albert softwood pulp as fiber and titanium dioxide as filler. During the filtration a gamma ray densometer measured the local porosity and pressure probes on column walls measured local pressure.;Results of fiber filtration indicate a decreasing porosity from top to bottom of cake, resulting in non-uniform pressure profiles and decreasing cake local permeability. Depth filtration results show an exponential decay of filler retention on fiber from top to bottom of cake. This is due to high concentration of inlet slurry at cake top, which decreases as it moves down the length of cake due to retention by fiber. The retention profiles in co-filtration experiments are just the opposite as the lowest layers of the fiber mat are formed during initial stages, and thus capture fillers for the longest time.;The uniqueness of this work is the simultaneous measurement of local porosity and pressure over the length of the cake. This is the first time a fundamental-physical-mathematical model is developed to predict the process of co-filtration, which represents the actual filtration process at the wet-end of the paper machine.