David Harbottle
1
, Dominic Rhodes
2
, Mike Fairweather
1
and Simon Biggs
1
1
Institute of Particle Science and Engineering, University of Leeds, UK, LS2
9JT pre0dh@leeds.ac.uk
2
Nexia Solutions, Sellafield, Cumbria, UK
ABSTRACT
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.
Through consideration of the flow properties of super-fine particulates and the role of interparticle forces moderating the behaviour of the particle system, this paper explores areas that have been neglected in classical Hydrotransport studies and starts to focus on the issues of transporting sludges and slurries containing colloidal material in small diameter pipes. The resulting work will therefore start to provide a basis for an understanding of the transportation properties of nuclear material.