The influence of creep on cake solid volume fraction during filtration of core–shell particles

The solid volume fraction vs. pressure relationship used in conventional filtration models is determined by measuring the cake solid volume fraction after consolidation. However, some cakes creep during consolidation, so the solid volume fraction increases at constant pressure. Thus, the conventional method for determining the solid volume fraction vs. pressure relationship cannot be used for materials with significant creep. Cake creep has been observed when core–shell particles with hard poly(styrene) cores and water-swollen poly(acrylic acid) shells are filtered. The Terzaghi–Voigt combined model has been fitted to data obtained during consolidation to determine the transition point where creep begins to be dominating for cake compression. The solid volume fraction increases by 17–35% after the transition point, particularly in the case of particles with thick poly(acrylic acid) shells and thus a high initial water content. Hence, the solid volume fraction can increase significantly during cake creep and if the solid volume fraction vs. pressure relationship that controls the initial stages of filtration is to be determined then the filtration experiments must be stopped before creep dominates. This can be done by measuring the liquid pressure at the interface between piston and sample, and stop the experiment when the liquid pressure is lower than 5% of the applied pressure.