Rapid determination of compressive yield stress of dense suspensions by a mean-phi (Φ^-) model of high pressure filtration

Abstract

The compressive yield stress plays a crucial role in consolidation of concentrated suspensions in sedimentation and thickening, filtration under pressure or vacuum, shaping of ceramic bodies by slip casting, and in disposal of mining and industrial particulate wastes. Measurement of compressive yield stress is ordinarily a time consuming undertaking. Fast and demonstrably reliable techniques for measuring compressive yield stress are needed for screening dispersant/flocculent reagents and establishing their optimum dosage for improving the dewatering characteristics. Fast characterization of yield rheology is also essential in case of biologically active sludges whose rheological behavior changes with time and in monitoring of slurry characteristics for on-line control of industrial dewatering processes in face of fluctuating feeds. We propose rapid methods for determining compressive yield stress from truncated single pressure or truncated step pressure test data by a recently proposed mean-phi (Φ^-) model of high pressure filtration. Because filtration rate slows down drastically as the end point is approached, considerable saving in measurement time is possible in truncated tests where the experiment is terminated in single pressure tests or pressure is stepped up to next higher level in multi pressure tests before the filtration system reaches equilibrium or steady state. Based on a recently proposed Φ^--model, we derive and validate a mathematical model for simulation of conventional and truncated step pressure filtration tests. In the former case, the process is permitted to run to equilibrium before the pressure is stepped up. The simulation model is employed to extract compressive yield stress as a function of solid volume fraction from truncated single pressure and step pressure test data. The proposed fast technique is used to demonstrate the effect of pH, polymers and surfactants on compressive yield stress of flocculated and dispersed suspensions.