The concept behind a froth pump is to avoid pumping air and to separate as much air as possible from the slurry before it reaches the pump impeller.
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The vertical froth pumps utilize the same principle as hydrocyclone separation where air is separated from the slurry in a vortex created by the impeller rotation and the tangential inlet to the pump’s conical sump.
This results in more efficient pumping at higher capacities and a smooth operation, free from pulsations caused by air locking.
Sala VF froth pumps are often used in applications that involve handling of air entrained slurries, such as flotation froth in base metal concentrators, phosphate and apatite washing plants and calcium carbonate upgrading plants.
They can also be utilized as a mixing and distribution unit, where dry powder has to be mixed with water, as well as with cement in ready mixed concrete and for grouting and injection mixing.
Typical applications
Standard pumps are supplied with parts in wear resistant Natural Rubber or High Chrome iron alloy, with a nominal hardness of 600 BHN. Other materials are also available.
The wear parts have large material sections for extra-long wear life and are designed for the toughest applications.
Parts in different materials are fully interchangeable and can be combined for optimum life.
The heavy-duty pump shaft is a cantilever design, hanging below the bearing housing. There are no submerged stuffing box, bearings or shaft seals. This design ensures minimum maintenance and eliminates any requirement for water flushing.
Impeller clearance is maintained by external axial adjustment of the shaft/bearing assembly. Bearings have double protection seals against the ingress of slurry.
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The casing on the Sala VF froth pump is supported by a beam. The beam is either lowered by hand or a hydraulic jack, depending on the pump size. The beam can be turned out so that the “wet end” parts can be accessed by a lifting device.
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 TypeFroth Factor
Brittle Froth
1.1 to 1.25Medium Froth
1.25 to 1.5
Tenacious Froth1.5 to 1.6