Cake collapse in pressure filtration

Aqueous dispersions of latex particles have been aggregated by addition of Ca2+ ions and then filtered in a pressure filtration cell. The filtration c akes have been examined through small-angle neutron scattering, void volume fraction measurements, and permeate flux measurements. At all filtration p ressures (20-400 kPa), the cakes were extensively collapsed (latex volume f ractions phi = 0.27-0.46), and the remaining porosity had a tenuous structu re (fractal dimensions d(f) = 1.3-1). However, the local structures remaine d the same as in the aggregates of the original suspensions. The mechanisms that produce this collapse are made of very small relative motions of the particles, which leave the local coordination of the latex particles unchan ged but allow large voids to be reduced. These motions could be inhibited b y using particles with nonspherical shapes and by increasing the friction f orces that act between particle surfaces, thereby reducing the extent of co llapse and increasing the permeability of the cakes.