Froth mean residence time measurement in industrial flotation cells

Froth plays an important role in flotation processes preventing the pulp transport to the concentrate (short-circuit). Thus, it contributes to increasing the concentrate grade by gravity drainage of entrained particles, back into the pulp. Key parameters affecting the froth performance are the mean residence times of solids, liquid and gas in the froth. The froth mean residence time depends on the froth depth, gas flowrate, gas hold-up, and flow regime.In this work, the froth mean residence times were evaluated from direct measurements of liquid and solid time responses in the froth of self-aerated copper flotation cells of 130 m3. For this purpose the radioactive tracer technique was applied, using 82Br as liquid tracer, and non-floatable mineral particles in three size classes (+150; −150 + 45;−45 μm) as solid tracers. All tracers were injected at the cell feed entrance, which allowed the tracer to circulate first through the rotor, and become well distributed over the whole cross-sectional area before entering the froth. Each tracer time response was measured on-line below the pulp/froth interface (input signal) and at the concentrate overflow discharge (output signal). The froth mean residence time was then obtained by difference between the average times of the froth input and output tracer signals, previously modelled.For the copper rougher flotation, the froth mean residence time (9–12 s) of non-floatable solids, derived from experimental measurement, was comparable with that obtained by measuring the gas flowrate and estimating the effective gas volume in the froth. While, the froth mean residence times of liquid and floatable solid were significantly larger, 21 and 24 s, respectively.