This paper examines the kinetics of Fast Flotation of very fine, hydrophobic particles using a rectangularsectioned downcomer in the Reflux Flotation Cell (RFC). Three separate downcomers of equal height and breadth, but different channel widths of 2, 4.5, and 9 mm, were used to investigate the recovery of particles ranging from 1 to 100 mu m in diameter. The RFC utilises parallel inclined channels to enhance the segregation rate of rising bubbles in liquid, a phenomenon known as the Boycott Effect. This work was focused on the contribution of the downcomer kinetics to the Fast Flotation. Feed fluxes over the range of 0.9-8.6 cm/s were examined, with residence times in the downcomer of less than 1 s. Recovery due to entrainment was made consistent by fixing the portion of liquid reporting to the overflow to 10% of the feed volumetric rate, independent of the gas flux employed. Kinetic rate constants for the particle recoveries were analysed by varying flotation parameters such as particle size, gas flux and feed flux, obtaining downcomer kinetic constants of up to 419 min(-1). Distinct kinetic behaviour was found for the 1-10 mu m and 10-100 mu m coarser particles, with the kinetics of finer particles being more dependent on the particle size. The kinetics using the narrower downcomer widths (4.5 mm and 2 mm) relied more on the gas flux than the feed flux, while the kinetics using the wider downcomer width showed more dependence on the feed flux. Scaling laws for the kinetic rate constant were derived empirically based on the downcomer channel width, particle size, gas flux and feed flux. These scaling laws provided insight into the underlying mechanisms and a basis for exploring the potential flotation performance over a wide range of system operational conditions. The narrower downcomers exhibited better performance at the feed fluxes <10 cm/s. Above this feed flux, the wider downcomer of 9 mm performed better, especially for coarser particles above 10 mu m. A basis for downcomer system design is provided based on the kinetic scaling laws. (C) 2018 Elsevier Ltd. All rights reserved.