Capture of impacting particles on a confined gas–liquid interface

Film flotation is highly dependent upon how well wanted and unwanted particles are separated at the gas–liquid free surface. Single particle experiments and modelling analysis can be undertaken to determine a critical impact velocity at which the optimum separation is likely to occur. However, in commercial film flotation systems the higher loading density of the free surface can result in interactions that change the critical impact velocity. To investigate this influence, film flotation experiments have been undertaken with 4, 5 and 6 mm diameter spherical polypropylene particles with water, sucrose and surfactant (CTAB) solutions contained within vessels of varying dimensions and wettability (static contact angle). Experimentally it was observed that for a given particle diameter the critical impact velocity was found to decrease with decreasing vessel diameter, especially when the particle-to-vessel diameter ratio increased beyond about 0.2. Conversely, the critical impact velocity was found to be relatively independent of the liquid depth; but did decrease with decreasing static contact angle in the region where the vessel wall had an influence. The experimental system was also modelled using the Young–Laplace equation using both static and advancing contact angle measurements for both the particle and vessel surfaces. The model predictions were generally in good agreement with the experimental observations, including showing an increase in particle penetration depth with increasing vessel diameter and meniscus profiles, both at the particle impact point and the wall of the vessel. The predictions were improved when the advancing contact angle was used, especially for the smaller diameter vessels where there was more liquid motion. Finally, a model to determine the critical (minimum) diameter of vessel required so that the cavity profile generated by the impacting particle is unlikely to be influenced by the vessel walls is presented.