Effective froth density

First, an effective froth density (j) (dimensionless) is calculated ... 

For evafuating the residence time 0g of gas in the froth, the volume of the froth is taken as Aohf, where the height of the froth hj is obtained by first determining effective froth density [Eq. (14-117)]. The dimensionless froth density is defined by... 

A more exact analysis of the hydraulic parameters is possible, and for this the paper by Bennett et al. should be consulted. In that woric, an effective froth density is defined as follows ... 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

In this equation, h is the weir height in cm, L is the molar liquid flow rate in kmol/s, Pl is the molar liquid density in kmol/m, l gj. is the weir length in m, and (pg is the effective relative froth density given... 

The separation of a mixture using flotation methods depends on differences in the surface properties of the materials involved. If the mixture is suspended in an aerated liquid, the gas bubbles will tend to adhere preferentially to one of the constituents-the one which is more difficult to wet by the liquid-and its effective density may be reduced to such an extent that it will rise to the surface. If a suitable frothing agent is added to the liquid, the particles are held in the surface by means of the froth until they can be discharged over a weir. Froth flotation is widely used in the metallurgical industries where, in general, the ore is difficult to wet and the residual earth is readily wetted. Both the theory and practical application of froth flotation are discussed by Clarke and Wilson 44. ... 

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface of the liquid from which they were formed, they also are called froths. Bubbles range in size from about 50 pm to several mm. The data of Table 19.8 show densities of water/air foams to range from 0.8 to 24 g/L. Some dissolved or finely divided substances may concentrate on the bubble surfaces. Beer froth, for instance, has been found to contain 73% protein and 10% water. Surface active substances attach themselves to dissolved materials and accumulate in the bubbles whose formation they facilitate and stabilize. Foam separation is most effective for removal of small contents of dissolved impurities. In the treatment of waste waters for instance, impurities may be reduced from a... 

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