The effect of physical parameters on the rupture of bubble films in two-phase foams

The effects of various physical parameters on the rupture of bubble films in twophase 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. Stellenbosch University https://scholar.sun.ac.za