THE EFFECT OF PARTICLE-PARTICLE INTERACTION FORCES ON THE SEDIMENTATION AND RE-SUSPENSION PROPERTIES OF SILICA SLURRIES
Advancing the chemical engineering fundamentals
Multifase Flows - I (T2-5a)
Keywords: Particle-particle interaction, Sediment bed structure, Transitional flow
This paper reports experimental observations on the sedimentation and re-suspension characteristics of colloidal silica as a function of the strength of the particle-particle interaction forces. Silica slurries are encountered mainly in mining operations, with efficient transportation leading to substantial environmental and cost savings. In this study mono-dispersed silica is used to produce model slurries where the physical and chemical properties are easily controlled and understood. Stability of the silica slurries have been studied using a variety of analytical techniques, investigating the effect of particle surface charge on the settling rate and sediment packing structure. The transition from a stable to a flocculated state is induced by a pH change and/or a change in the strength of an appropriate electrolyte. Through controlling the strength of the particle interactions, the packing structure of the sediment can be manipulated. When there is strong attraction between the particles, the resulting sediment bed consists of a random arrangement of particles with a low bed density and high bed porosity. For a weakly attractive system, a denser sediment is observed due to the individual particles packing more efficiently. The sediment bed yield stress then becomes a function of the packing structure of the particles, ultimately influencing the re-suspension and transport properties of the particles in the pipe.
A 25mm horizontal pipe loop applying techniques such as Particle Image Velocimetry (PIV) and Ultrasonic Velocity Profiling (UVP) was used to look at the re-suspension properties of consolidated sediment beds. In order to enable movement of particles settled on the pipe invert, the driving forces which are acting to set the bed in motion and lift the particles (pressure gradient, shear stress) have to exceed the resistive forces (mechanical and fluid friction) which are acting to hold the bed in place. The reaction forces are assessed in this study through analysis of the re-suspension characteristics in the laminar and turbulent flow regimes, along with consideration for re-suspension in transitional flow, where a sharp peak is observed in the axial velocity fluctuations.
See the full pdf manuscript of the abstract.
Presented Monday 17, 11:55 to 12:15, in session Multifase Flows - I (T2-5a).
