Froth stabilizer

Foam compositions, including a latex and a polynitrile oxide such as 2,4,6-triethylbenzene-l,3-dicarbonitrile oxide, or a latex and an epoxy silane, or a latex and a mixture of the two crosslinkers have been prepared (526). The compositions may also contain additional components, including fillers, surfactants, cell detackifiers, froth stabilizers, froth boosters, viscosity reducers, and compounds to improve resilience, and antioxidants. The compositions are particularly useful in the manufacture of flooring, wall covering, shoe lining and nonwoven materials. 

Ekmekci, Z., Bradshaw, D.J., Harris, P.J., and Buswel, A.M., Interactive Effect of Milling Media and CuS04 Additions on the Flotation Performance of Sulphide Minerals from the Morensky ore, Part II Froth Stability, International Journal of Mineral Processing, Vol. T8, pp. 164-174, 2006. 

As discussed in Section 1.2.2 the bubble shapes in fairly dry foams and froths (4 gas > 0.83, approximately) are not spheres or distorted spheres, but polyhedrons. In practice there will be distributions of both gas-cell sizes and shapes. In addition to the gas bubbles, froth contains the floated particles, pulp liquor, and a fraction of (hydrophilic) particles that did not float due to bubble attachment, but which were mechanically entrained in the froth. The pulp liquor and these latter particles all have to be allowed to drain back out of the froth. The rate of this drainage will be greatest at the froth-pulp interface (i.e., the bottom of the froth layer) and slowest near the top of the froth layer. Froth drainage equations are discussed elsewhere [53]. The froth needs to be a stable enough foam that some time can be allowed for these drainage processes, and also so that the upper layer(s) of the froth can be swept out of the flotation cell. On the other hand, the froth should not be too stable as a foam so that it will break easily after collection. In addition to the role of the frother, froth stability is also promoted by increasing liquid viscosity. 

FIGURE 16.4-1 Diagram illustrating the correlation between froth stability and surface tension lowering due to the addition of a surfactant. (After Cooke 1 conrtesy of John Wiley Sons.)... 

Froth stability and also froth rheology exhibit varying time-dependencies, some have quite remarkable stability (no change in viscosity over 20 to 40 h), and some have marked instability (50% reduction in viscosity within 20 h). 

Even more confusing is the picture resulting from the comparison of a widely used surfactant and an experimental product called EP-H-18 by Burkhart and co-authors [42]. Both of the products again resulted in identical values in the froth stability test, but differed significantly on an industrial production machine. Whereas the competitive material resulted in a coarse cell structure, the experimental product yielded a fine and regular foam. 

In addition to inducing froth stability, frother specfes can take part in the overall process of adsorption on the mineral surface. Like the collector species, the ftother species also can be expected to migrate to the particle-gas interface during the time of contact and assist in establishing the attachment of the bubble to the particle. Coadsotption of ftother along with the collector species can be favorable for flotation, possibly because the neutral molecules adsorbed between charged collector ions can reduce the repulsion between the latter species and thereby enhance the overall surfactant adsoiption. 

However, for particles with rough edges, the situation then becomes somewhat more complex. Johansson and Pugh (7) have studied the stability of foams in the presence of finely ground quartz particles of different size fractions. In this work, the surfaces of the different particles were methylated to produce a range of hydrophobicities. Both the dynamic and static froth stabilities were then determined in a froth column. From this, the concept of maximum enhanced foam stability , which corresponded to a critical surface hydrophobicity shape and size, was developed. 

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