Bubble swarm velocities in a flotation column

Shen, Gang, 1953-; Shen, Gang, 1953-

Thesis

Bubble swarm velocities in a flotation column

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Creator

Shen, Gang, 1953-

Contributors

Finch, J. A. (Supervisor)

Abstract

English

A new fast response conductivity meter was developed and tested. The "five time constant" of the meter is 0.08 s which meets the requirement for measurements under the dynamic conditions relevant to this work. In a laboratory column, a bubble interface was created by introducing a step change of gas flow, and the rising velocity of this interface, $u sb{in},$ was measured using a conductivity method with the new conductivity meter. A measurement of the three-dimensional bubble swarm velocity in the column was obtained by interpolation from the $u sb{in}$ measured as a function of $J sb{g2} vert J sb{g} sb1 ,$ where $J sb{g} sb1$ and $J sb{g} sb2$ are the superficial gas velocities before and after a step change of gas flowrate, respectively. This velocity was referred to as the hindered velocity, $u sb{h}.$ The buoyancy velocity, $u sb0 ,$ was readily determined by switching off the gas, i.e. $u sb0 = u sb{in}$ at $J sb{g} sb2 = 0.$ The average gas velocity, $u sb{g},$ was corrected to the local average gas velocity, $u sb{g,loc},$ to obtain the average gas velocity under the local pressure conditions at a given vertical position in the column. The experimental results showed that $u sb{h}$ was significantly less than $u sb{g,loc}$ (and $u sb{g}).$ This is because the $u sb{h}$ is the three-dimensional bubble swarm velocity and $u sb{g,loc}$ is the one-dimensional bubble swarm velocity. Unlike $u sb{g,loc},$ the $u sb{h}$ was constant along the column, which was supported by theoretical momentum analysis. The $u sb{h}$ is proposed as the key characteristic swarm velocity of the system. For the air-water only system in the two-dimensional domain, using parabolic models for gas holdup and liquid circulation velocity profiles over the cross section of the column, the $u sb{h}$ could be fitted to the experimental data. For the air-water-frother system, the $u sb{h}$ could not be fitted to the experimental data which is attributed to the air bubbles adopting a circulatory flow pattern. In the air-water only system under batch operation, Nicklin's derivation (1962), i.e. $u sb{g} = u sb0 + J sb{g},$ was supported only under restrictive conditions, namely $u sb{g}$ and $J sb{g}$ must be measured at atmospheric pressure. Considering the local values, the experiments showed that $u sb{g,loc}$ was not equal to $u sb0 + J sb{g,loc}.$ In the presence of frothers under batch or countercurrent operation, the experiments showed that Nicklin's derivation was not applicable even if atmospheric values of $u sb{g}$ and $J sb{g}$ were used.

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