Froth, production

In the D-R Denver machine (Fig, 19-7,5), the pulp enters the top oF the recirculation well A, while the low-pressure air enters through the air passage B. Pulp and air are intimately mixed and thrown outward bv the rotating impeller C through the stationaiv diFFiiser D. The collector-coated mineral particles adhere to be removed in the Froth product. 

In view of the above adverse effects a safety factor should be applied where flammability is assessed using flash point. For pure liquids in containers the vapor should be considered potentially flammable if the liquid temperature is upward of at least 5°C below the reported flash point. For mixtures whose composition is less certain, such as petroleum mixtures, the safety factor should be about 15°C relative to the flash point [55]. Where combinations of adverse effects are identified the safety factors should be increased accordingly. A simple but very conservative approach is to assume that all liquids having a flash point <141°F may produce a flammable atmosphere under some ambient conditions, even where no mist or froth production is involved. A more practical approach is to assume that liquids handled in air at least 5-15°C below their closed cup flash points will not present ignition risks unless... 

Concentrate froth product recovered in coal froth flotation. 

One technology to be tested at the CCTF will be a microbubble flotation process developed by Bechtel National, Inc., of San Francisco. Microbubble flotation is a further development of conventional froth flotation of coal. Froth flotation is a physicochemical process that uses the difference in the surface properties of coal and its associated mineral impurities to effect a separation. An aqueous coal slurry is fed into an aerated tank, where the hydrophobic coal particles become attached to, and are buoyed to the surface by, finely dispersed air bubbles and are collected as a clean-coal-froth product. The mineral matter, being hydrophilic, is wetted by water and remains in suspension to be carried off as refuse. 

Very little information about the rheological properties of froths has been published because the froths are not very stable and collapse with time. Also, great care is needed because froths, which are a multiple dispersion of four phases, are very subject to phase rearrangements during the conduct of experimental measurements. Some approximate froth viscosities for commercial-scale froth production mentioned in the patent literature are... 

Another novel concept is the Air-Sparged Hydrocyclone developed at the University of Utah. In this device, the slurry fed tangentially through the cyclone header into the porous cylinder to develop a swirl flow pattern intersects with air sparged through the jacketed porous cylinder. The froth product is discharged through the overflow stream. 

Froth production is rare in nature, apart from fermentations, but an interesting example is afforded by the plant parasite which protects itself by means of a foam derived from leaf sap. Pathological and often fatal examples of froth formation in the higher animal are seen in air embolus, and pulmonary oedema. 

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