Flotation columns are counter-current bubble columns that have recently been applied to froth flotation separation of minerals. A unique feature of a flotation column is the water stream applied at the top of the column for the purpose of washing entrained nonfloatable particles from the floatable bubble-particle aggregate, consequently permitting high upgrading in a single separation stage. The objective of our work is the development of a scale-up methodology for flotation columns, and as a first step toward this objective we have measured the dispersion parameters of large industrial columns. Pulsed tracer tests were performed with both liquid and solid tracers on 0.46 and 0.91 m square columns, each 13 m high. The liquid axial dispersion coefficient appears to be linearly dependent upon column radial dimension. As well, it is confirmed that the axial dispersion coefficient of mineral particles is the same as that of the liquid, and that the residence time of solid particles is unaffected by a bubble swarm. lNTRODUCHON An inherent limitation with the flotation of fine particles in conventional froth flotation cells is recovery of hydrophilic (gangue) particles by mechanical entrainment in the water reporting to the froth[ 11. The method of minimizing entrainment is to create a 5530 cm thick froth at the slurry surface. The froth permits the gangue to drain back to the pulp while retaining the hydrophobic particles which are eventually discharged over the cell lip. This cleaning action is seldom sufficient and sequential stage flotation is necessary. The alternative approach to the cleaning of entrained particles has been introduced with column flotation. Industrial flotation columns are typically 13 m high by 0.45 to 0.91 m square. A schematic diagram of a flotation column is shown in Fig. 1. Three distinct zones are indicated. Feed slurry enters the column at the top of the recovery zone, 9-l 1 m high, and flows downward against a counter-current flow of l-2 mm dia. air bubbles. After collision, particles that are sufficiently hydrophobic will adhere to the bubbles and the bubble-particle aggregate will rise into the cleaning zone. The cleaning zone incorporates a unique feature; water is added from an array of perforated pipes located about 5-10 below the discharge lip. This washwater drains downward and gives the cleaning zone the appearance of a packed bubble bed. The downward velocity of the washwater is regulated by the difference between the tailings flowrate and the feed flowrate (or bias). Gangue particles that are entrained in bubble wakes or that shortcircuit from the feed point are washed back from the cleaning zone to the recovery *Now at Dept. Metallurgy and Materials Science,