A technique for on-line simultaneous determination of gas and solids holdup, based on a combination of conductivity and pressure difference measurements, was developed and tested in a laboratory flotation column (10 cm in diameter and 447 cm in height). The actual holdup was determined by an isolating technique. It was found that the shape of dispersed particles could significantly affect the conductivity of the dispersion. For flake-shaped particles, this resulted in unacceptable estimates of solids holdup in solid-water systems when the classical models for a spherical dispersed phase (e.g., Maxwell (1892) and Bruggeman (1934)) were used. Fricke's model (1924), which accounts for particle shape, was found suitable. A critical ratio of dispersed to continuous phase conductivity, CCR, was introduced beyond which the dispersion conductivity is insensitive to the dispersed phase conductivity. Two equations to approximate CCR for any given dispersion were proposed. The effect of concentration, size and type of solid particles on gas holdup was investigated. The presence of solids significantly decreased the gas holdup, by up to 40% relative. Possible mechanisms to explain the effect of solid particles were evaluated based on bubble coalescence, slurry density/viscosity changes, radial profiles and wake structure effects. It was experimentally shown that bubble coalescence was not responsible for the gas holdup reduction. It was proposed that the effect of solids on reducing gas holdup is a combination of an increase in the rise velocity of bubbles due to stabilization of the bubble wake and a change in the gas holdup profile from flat to saddle-shaped.

Laplante, A. R. (Supervisor)

Finch, J. A. (Supervisor)