Radial gas holdup profile and mixing in the collection zone of flotation columns

Xu, Manqiu; Xu, Manqiu

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Radial gas holdup profile and mixing in the collection zone of flotation columns

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Xu, Manqiu

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Radial gas holdup profiles were determined in 50 cm and 91 cm dia. flotation columns. The local gas holdup was measured using an electrical conductivity technique, which is described in detail. The effect of gas rate and axial location on the profiles was investigated. The relative variation in gas holdup was about 20%. The profiles were axially symmetric but of complex shape: parabolic, saddle and 'W' shapes were observed. For the case of a parabolic profile, the shear stress model was solved to give liquid circulation velocity profiles; no solution was found for the other gas holdup profile shapes. Measurement of radial gas holdup profiles may prove useful in evaluating sparger systems. For example, in the pilot column, one off-centre sparger gave a non-symmetrical shape but nevertheless did distribute the gas relatively evenly. Liquid residence time distributions (RTD) were measured for various operating and design conditions using the pulse tracer technique. Application of the one-dimensional plug flow axial dispersion model and the use of the vessel dispersion number $N sb{ rm d}$ to quantify the degree of mixing was evaluated. A numerical solution to the axial dispersion model with closed-closed boundary conditions using the finite difference method is recommended for column RTD studies. Compartment models were also evaluated: the N perfect tanks-in-series model did not fit the data but a backflow compartment model was successful. The effect of gas rate, liquid rate, column length, feed percent solids and column verticality on $N sb{ rm d}$ was studied. It was found that $N sb{ rm d}$ increased with gas rate and decreased with liquid rate and column length. Percent solids and verticality had a minor effect on $N sb{ rm d}.$ New correlations to predict $N sb{ rm d}$ were developed and compared with previous correlations.

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