Dead-end filtration with torsional shear: Experimental findings and theoretical analysis

A range of constant pressure dead-end filtration experiments with additiona l torsion shear are analysed, Two cases are considered: 1) suspensions of n eutral particles (i.e. zero zeta potential); and 2) suspensions of double l ayer interacting particles (high zeta potential). In both cases the data, w hich have been recorded in the form of outflow volume of the filtrate as a function of time, show an increase in the cake density when shear is applie d at the final compression state. In the case of suspensions of neutral par ticles increasing the shear rate has practically no effect on the filtratio n rate until the final stage of the filtration process, while suspensions o f interacting particles display effects due to shear in the intermediate st age as well. This behaviour is explained in terms of the state of stress in the filter. For neutral particles suspended in an aqueous solution the pro blem is simplified by modelling the suspension/cake system as two Newtonian fluids of different viscosities. It is argued that this is an acceptable a pproximation for suspensions that consist of neutral particles. A more elab orate model is needed for suspensions consisting of interacting particles, allowing for a reduction in skeletal stress at higher shear rates. The anal ytical model put forward identifies fluctuations in field parameters; it is verified by means of a lattice-Boltzmann numerical simulation. Finally, it is shown that the higher cake density established towards the end of the f iltration process is due to a nonhomogeneous compaction mechanism, which ca uses the cake to form vertical structures that are orientated in the direct ion of the major principal skeletal stress directions. Shearing the cake ca uses the orientation to change with respect to the apparatus axis, thus wea kening the one-dimensional response of the cake material.