The saturated permeability of composite pulp fiber and filler mats

The permeability of pulp mats characterizes the resistance to water flow and controls water removal in the forming and vacuum sections as well as dewatering in the press section of the paper machine. The specific surface area (S w, m2kg-1) and the specific volume (v f, m3kg-1) of the wet pulp fibers are hydrodynamic parameters which can be determined from permeability of mats of different consistency. A number of researchers have used the Kozeny Carman equation to obtain the specific surface area and the specific volume of pulp fibers. Fillers are an important component of modern paper sheets and their inclusion yields many benefits. The effect of fillers on the permeability of pulp fiber mats has not been investigated earlier and therefore the present work is aimed at understanding the influence of filler particles on the permeability of pulps.;Pulp fiber mats containing different mass fraction of filler particles were formed by slow sedimentation and then were compressed to different consistencies. The permeability of these mats was determined from the pressure drop and water flow rate under steady conditions. The specific surface area and the specific volumes of the composite pulp mats were determined from the permeability data. Representative hardwood and softwood pulps were chosen for analysis. The fillers chosen consisted of two precipitated calcium carbonates (PCC): AC -- a scalenohedral particle and AF-prismatic particle), kaolin clay and titanium dioxide particles. The permeability of the pulps was found to increase by large amounts with the addition of small quantities of PCC. Higher loadings of PCC caused decreasing permeability. The kaolin particles decreased the permeability of the pulp mats uniformly. The initial increase in permeability with PCC was also accompanied by a simultaneous increase in freeness and drainage rates of the pulps. We attributed the increased permeability to flocculation of the pulp fibers caused by the PCC particles due to the charge interactions between the cationic filler and anionic fiber surfaces. At higher filler loadings, the PCC particles clog the pore spaces in the pulp mat and decrease the permeability and cause interference during drainage. Since kaolin fillers are not cationic, they do not cause flocculation and hence decreased the permeability at all loadings.;These concepts were modeled by considering the specific surface area of the fiber mats to be composed of two contributions. The first, due to fibers alone could result in lower specific surface areas caused by large interstitial pore sizes due to flocculation. The second contribution was an increase in surface area due to clogging of the fiber mats. The specific volume of the composite pulp mats was modeled as a simple weighted mixture of the specific volumes of the individual components, namely the fibers and the filler particles. Empirical coefficients were assumed to describe the rate of permeability change due to both these effects. The model yielded permeability predictions which compared well with the experimental results. The coefficients for clogging and flocculation were similar for both the PCC types.;Keywords. Hardwood Pulp, Softwood Pulp, Permeability, Porosity, Specific Surface Area and Specific Volume, Kozeny Carman Equation.