In traditional flotation practice, the ore is ground to a top size of 100 to 200 µm, to liberate the valuable components from the host rock. However, even when the ore is free-milling and does not require fine grinding, the same top grind size is used, because conventional flotation machines are incapable of producing high recoveries of coarser particles.

The drop in recovery as the particle size increases, may be attributed to the fact that in mechanical flotation cells, high shear rates are found which produce eddies of high rotational speed. If a bubble should attach to a coarse particle, the doublet will find itself in the centre of a rapidly rotating eddy, and the particle experiences a centrifugal force that detaches it from the bubble. The performance of mechanical cells deteriorates as the size of the final grind increases, because higher and higher rotational speeds are required to keep the particles in suspension off the bottom of the cells.

This paper will explore alternatives to mechanical cells, for the flotation of coarse particles, by which we mean particles of base metal sulfides of the order of a millimetre in diameter or more. The driving concept is to create an environment that is favourable for the rapid capture of hydrophobic particles, without the creation of undue turbulence that may lead to particle-bubble detachment.