Mechanical flotation machine design is typically based on hydrodynamic analysis, with scale-up calculated by geometric proportionality. End users often specify and purchase machines on the basis of retention time, characterised by the $/m3' approach; they may also consider the manufacturer's installed base. In some applications, end users also consider specific energy and the impact of energy consumption on operational costs. In other cases, a decision may be based on machine type (forced-air versus self-aspirating) and perceived metallurgical or operational advantages of one machine type over the other.
With regard to metallurgical performance efficient bubble-particle contact is very important and is primarily influenced by two factors:
the concentration and size distribution of bubbles in the machine's contact zone (the volume enclosed by the disperser); and
the number of times that slurry is pumped through the contact zone.
The second factor is a function of the machine's pumping rate and its retention time. Field data show that pumping efficiency is independent of aeration rate.
In this paper, a new dimensionless number for the characterisation of flotation efficiency is presented. The contact frequency number (CFN) is defined as the number of times a given volume of slurry passes through a machine's active contact region, and is given by:
CFN Qp/Qf,
where:
Qp
is the pumping rate through the contact region
Qf
is the volumetric feed rate to the machine