Fluidized bed flotation of coarse particles

Research Doctorate - Doctor of Philosophy (PhD)Froth flotation has been in use since the beginning of the 20^th century. It is a well-established process, yet many of the fundamental mechanisms that governs the capture of particles in flotation systems are not fully understood. One of those mechanisms is the detachment due to the turbulent conditions inside the mechanical flotation cells. Turbulence itself is an active research area with many unknowns and therefore its application to the flotation is somehow limited. Nevertheless, the detachment of coarse particles and therefore the limit of a floatable upper particle size limit in mechanical flotation cells is attributed to the turbulent eddies that form in the cells due to strong agitation provided by the impeller. A workaround for coarse particle floatability problem is grinding. However, grinding processes that govern the final product size of particles are very energy intensive. There are countries that exists that the 15% of the total energy consumed in that country, is solely used by the mining industry. It is also known that grinding takes a significant portion of the costs of an operating mine due to the energy requirements. For a sustainable environment in the future, the energy consumption must be reduced. Since grinding takes a large portion of the energy requirements of an operating mill, even a slight reduction in the consumption will result in a large savings in operating costs and will contribute to the sustainable environment aim in the long run. To reduce the energy costs related with grinding, a method must be utilized that can float coarse particles. A solution has been suggested by Jameson (2010), known as the fluidized bed froth flotation. A fluidized bed provides a quiescent environment, due to the lack of eddies, which assists large particles to float to the concentrate without detaching from bubbles in the pulp phase. Since the fluidized bed flotation is a fairly new process, the literature lacks studies regarding the behaviour of fluidized bed flotation systems. This thesis examines the effects of operating conditions such as the fluidization ratio, the particle size, the bubble size, the air flowrate, the values concentration, the bed height and the contact angle of particles on the performance of a specifically constructed laboratory scale fluidized bed flotation device by using single bubble experiments to gain understanding on the mechanisms that govern particle collection in fluidized bed flotation. This thesis also provides a model for particle accumulation on the surface of the bubbles rising in a fluidized bed. To complement the model predictions, and to be able to evaluate the goodness of the model, the results of carefully designed experiments are described that utilizes a new method to determine the rise velocity of bubbles inside the particulate zone in a fluidized bed. From photographic evidence, it is seen that the particles attached to a bubble slide down to the rear of the bubble as it rises in the fluidized bed. Particles continue to accumulate in the rear up to a point, beyond which newly arrived particles are able to exert a force on the layer of those that had previously arrived, dislodging individual particles. Thus, once a bubble has reached a surface coverage of approximately 45 to 50% as seen in the experiments, it cannot collect further particles. It should be noted that the packing fraction, the fraction of a given geometrical segment on the bubble’s surface actually covered by solid mineral, was found to be of the order of 0.2 For uniform spheres in a square array, the packing fraction is Π/4 = 0.785. It can be concluded that the particles in the surface layer are relatively loosely packed, compared with the close packing that can be achieved by uniform spherical particles in contact with all their neighbours. The observation that bubbles rising in the fluidized bed are never fully covered with particles when they emerge, leads to two separate conditions that limit the mass that a bubble can lift out of the bed. One is the well-known buoyancy limit, according to which the mass of particles that can be carried must be such that the buoyancy of the particle- laden bubble must be zero. This limit still applies, subject to a second limit, that the surface coverage of the bubble when the buoyancy limit applies, must be less than a number specific to the system under examination and theoretically cannot be larger than 1. The observed limit for the experiments carried out during this study was approximately 50%. In general, it has been found that the model is a good first approximation that allows the determination of mass accumulation on the surface of a bubble rising in a fluidized bed flotation device. It has also been found that the collision and the attachment times are extremely fast in fluidized bed flotation. The fitted collection efficiency term has been found to be consistent with what has been reported in the literature previously. It is concluded that to maximize the collection of particles, and therefore recovery, the operating parameters such as particle size, bubble size, the height of the fluidized bed and the fluidization ratio U/Umf, must be selected carefully for the system in question