Visco-elasticity and Consolidation of the Fibre Network during Free Water Drainage

Abstract

Measurements of specific permeability and compressibility of pads of synthetic and wood pulp fibres have been made in an apparatus in which rate of flow of water through the pad, pressure drop across the pad, external applied load and pad thickness can be controlled and measured. For non-swelling fibres at high porosity, with no applied load, the Emersleben-lberall drag treatment was found to provide estimates of permeability in reasonable agreement with observed values. The relationships between solids concentration C and applied stress P and between C and pressure drop p have been studied separately in terms of the empirical compressibility equations C= MP^N with p=0 and C=m(p)ⁿ with P=O. The connections between mean compacting pressure during flow and the total pressure drop, between the exponents N, n and between the coefficients M, m, are discussed. Factors influencing the compressibility and consolidation of the fibre network include the flexibility and lateral conformability of the wet fibres, which apparently affect not only their capacity to deform elastically, but also the extent to which irreversible relative movement can take place. These effects are illustrated by the properties of pads from two series of pulps, each covering a range of lignin content-Pinus radiata sulphate pulps at various stages of bleaching and Eucalyptus regnans NSSC pulps cooked to different degrees. When compacting pressures that are due to flow and external loads are applied simultaneously, the compressibility equations in Pand Ophold, within limits, for constant values of p and P,respectively.