Froth quality

Calculations can be performed to show the expected relationships between froth quality, bitumen droplet aeration, and packing density for... 

Danielson (64) measured bitumen droplet densities and corresponding froth compositions for two oil sands of differing processibility. For one oil sand, the mode of the observed distribution of aerated bitumen droplet densities was 0.31 g/mL with the recovered froth found to contain 52.9% bitumen. For the other oil sand, the mode of the density distribution was 0.51 g/mL with a corresponding froth assay of 63.2% bitumen. Although these froth compositions are somewhat higher than those predicted from Figure 13, the trend toward improved froth quality with increasing droplet density is evident. 

Figure 13. Relationship between froth quality and aerated bitumen droplet density.

Figure 29. Relationship between froth quality and aerated bitumen droplet density showing model (solid curve) and experimental results (broken curve). (Adapted from Schramm [110]. Copyright 1994, American Chemical Society.)...

The study of flotation kinetics relates to a number of mass transfer processes and these are listed in Table 2.8. The term, entrainment which figures in the mass transfer process statements made in Table 2.8 may be elaborated. It is the process by which particles enter the base of a flotation froth and are transferred up and out of the flotation cell suspended in the water between bubbles. Entrainment should be distinguished from true flotation, whereby particles come out of the cell attached to bubble surfaces. True flotation is chemically selective, while the entrainment process recovers both gangue and valuable minerals alike. Entrainment harms the product grade since recovery of the more abundant gangue mineral reduces the quality of the concentrate. This is especially true in the processing of fine ores. Much flotation research has dealt with reducing entrainment in order to improve... 

C) The DNB in a low-quality froth flow is caused by a bubble burst under the bubbly liquid layer. 

Finally, a rather unusual application of froth flotation is in the separation of damaged or poor quality fruit (see pp. 363-364 in Ref. [633]). 

Food foams are dispersions of gas bubbles in a continuous liquid or semisolid phase. Foaming is responsible for the desirable rheological properties of many foods, e.g., the texture of bread, cakes, whipped cream, ice cream, and beer froth. Thus foam stability may be an important food quality criterion. However, foams are often a nuisance for the food processor, e.g., in the production of potato starch or sugar and in the generation of yeast. Residues of antifoaming aids in molasses may drastically reduce the yield in citric acid fermentation. 

NOTE.—(a) The yield of chloroform is affected greatly by the quality of the bleaching powder used. The acetone is added slowly as the reaction is a vigorous one, and frothing is apt to occur if the flask becomes hot. 

An immense deposit of medium- to low-quality magnesite exists in Steven County, Washington, which was exploited by the Northwest Magnesite Company during World War I. Operations were centered on the towns of Chewelah and Valley. It was here that the first use of froth flotation to beneficiate magnesite was employed to reduce silica and lime content. 

Experimental studies undertaken by Schutte (61) to examine the partitioning of polar constituents of the bitumen to various process streams in the extraction circuit indicated that froth is enriched in asphaltenes. Schutte postulated that asphaltenes play a key role in promoting air attachment to bitumen. Further observations also indicated that the water and solids content of froth is directly related to the asphaltene content of the bitumen. Findings were rationalized in terms of the effect of asphaltenes on the rate of coalescence of aerated bitumen droplets at a froth interface. Schutte believed that rapid coalescence would inhibit the entrainment of water and solids and thereby favor good product quality. Experimentation with a model system using paraffin oil and process water indicated that the presence of asphaltenes substantially increases the time required for coalescence. 

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