The ‘froth factor’ is a measure of the air contained in the froth. It is quantified by filling a measuring cylinder or bucket, of known volume, with froth and measuring the froth column. After air dissipation the remaining water and solids volume is measured. The ratio of the original volume of froth to the remaining combined volume of water and solids is the ‘froth factor’. Measured ‘froth factor’ values are not employed by the flotation cell or pump designers. These are modified based on experience and application.
Froth transfer between flotation stages requires either gravity flow or pumping. The characteristics of the froth depend to a large degree on the type of ore being treated, the fineness of the particles, the solids concentration, the amount of air in the froth, the type of reagents used and the froth ‘brittleness’. Froth factors for launders and sumps are dependent on reagent addition rates and reagent type, froth brittleness, the ability to add spray water, the nature of the sprays, the design of the launder or sump and the fineness and nature of the particles. Froth factors up to eight to ten have been measured/reported for ores with stable froth structure. These froths require high volume launders, energy to release air and effective addition of spray water (to avoid excessive dilution). Open launders assist in effective froth transport and air/slurry separation. A minimum width of 300 mm to 400 mm is recommended.
Froth can vary from brittle, that is easily broken down and the bubbles are generally large, to tenaciously stable, where the air is tightly bound usually as very fine bubbles in which case it is difficult to break down even with time. Froth characteristics will vary from day to day and even hour to hour depending on these many parameters, so it is important to select froth pumps for the worst pumping scenario. The main requirement is to select the correct size of pump and impeller and then the correct pump speed for the froth encountered. Overspeeding is one of the parameters that will affect pump performance dramatically and deleteriously when handling froths. As a guide, vertical pumps are good for pumping brittle type froths. For medium type froths to very tenacious froths, horizontal froth pumps are generally superior.
Recommended froth factors for pumps are shown below:
| Froth Type | Froth Factor |
Brittle Froth | 1.1 to 1.25 |
Medium Froth | 1.25 to 1.5 |
| Tenacious Froth | 1.5 to 1.6 |
Interpretation of Flotation Data for the Design of Process Plants: G S Lane, G D Richmond, J Dioses