The effect of physical parameters on the rupture of bubble films in two-phase foams
Thesis (Ph.D. (Ing.)) -- University of Stellenbosch, 1999.
ENGLISH ABSTRACT: The effects of various physical parameters on the rupture of bubble films in two-phase foams were investigated in order to develop a better understanding of the behaviour of coarse particles in the froth phase of a novel flotation cell. This novel flotation technique is based on the fact that coarse particles, if they are selectively rendered hydrophobic by conditioning, would act as bubble film breakers. If the feed was introduced onto the surface of the froth, such particles would settle through the froth under gravity to be recovered as an underflow (concentrate) product, while the gangue would be supported by the bubble films and be recovered as a float (tailings) product. The efficiency of this technique - reverse froth flotation - depends on the interaction between various characteristics of particles and the froth. In order to simulate the experimentally observed trends, and hence investigate the various mechanisms qualitatively, a fundamental model of these interactions was developed. Various particle properties were taken into account, including surface properties, shape, size and density. To account for the changing nature of the froth at different positions in the cell, the model predicts the trajectory of a particle over discrete time events. This was accomplished by calculating bubble flow streamlines and modelling the bubble size, thickness of bubble films, air hold-up and bubble velocity at any point on the streamline. The experimental results showed that the behaviour of particles (within the size range tested) in the froth phase of the cell is primarily dependent on the mass of a particle. In general, the higher the mass, the steeper the trajectory of the particle in the froth, i.e. an increase in particle mass results in an increased recovery to the concentrate. The contact angle on the particle surface has only a secondary influence on the overall particle trajectory, in that an increase in the equilibrium contact angle will result in an increased recovery. However, the particle contact angle has very little influence on the behaviour of large, high-density particles, as well as small, low-density particles. Particles will therefore only separate on the basis of contact angle as long as their mass is between an upper and lower critical value. Any particle with a mass greater that the critical value will fall through the froth irrespective of the contact angle. Similarly, the upward force component acting on a particle with mass less than the lower critical value will dominate the force balance. The particle will therefore remain supported by the froth, irrespective of the particle contact angle and bubble film rupture time. For particles within these mass limits, the effect of the contact angle increases with a decreased mass. It was further concluded that these mass limits are dependent on the operating conditions of the cell as well as the particle shape. The particle shape influences the mass to cross-sectional surface area ratio (M/Ao). Where particles therefore . have the same mass, the M/Ao ratio would govern the particle trajectory. The higher the M/Ao ratio, the more particles would be recovered to the concentrate, while a decrease in the M/Ao ratio would result in flatter particle trajectories in the froth, thereby increasing the probability of a particle reporting to the tailings. The model provided a unique understanding of the interrelationship between the various parameters which would not have been gained to the same extent by alternative modelling methods. Two potential industrial applications for the reverse froth flotation process were evaluated. As a coarse particle flotation technique for sulphide bearing ores, it was found that Xanthate pre-conditioning of the ore results in concentrating the sulphur bearing particles to the concentrate. By increasing the Xanthate addition, the relative % mass recovery as well as % sulphur recovery increases. This increase in mass and sulphur recovery, however, is not linear. The greatest concentration effect was obtained by the initial Xanthate addition. An economically optimum Xanthate addition point therefore exists. These results were very promising in terms of finding a coarse particle flotation technique for the pre-concentration of sulphide bearing minerals. HOl(Vever, it is recognised that several potential problems might exist with the technique. The results further indicated that the reverse froth flotation process is not suitable for the replacement of grease belts for fine (-3 mm) diamond recovery. The major concern is the high potential losses of valuable material to the tailings. These losses are mainly due to the fact that the major separation process in the reverse froth flotation cell is based on particle mass. The surface properties of the particle account only for a secondary separation process.