Enhancing bubble-liquid segregation in flotation using multiple parallel inclined channels

Research Doctorate - Doctor of Philosophy (PhD)For the first time the use of multiple parallel inclined channels is investigated to enhance the segregation rate between bubble and liquid mixtures. This study has evolved from recent advances made in solid-liquid gravity separation studies using a device known as the Reflux Classifier. This system consists of a number of closely spaced parallel inclined channels, set to an angle of inclination of 70° to the horizontal, that are positioned above a vertical fluidization chamber. The channels act to increase the effective sedimentation area of the system, thus allowing enhanced solid-liquid segregation and solid classification when fluidizing from below a bed of solid particles. The aim of this work was to examine the processing advantage afforded to flotation processes by enhancing bubble-liquid segregation. The study firstly examines the effect of the inclined channels in enhancing liquid rejection in foam of relatively high bubble volume fraction. A series of foam fractionation experiments were conducted using the standard configuration of the Reflux Classifier, with the inclined channels located above the vertical vessel. Drift flux theory was used to describe the performance of the conventional vertical system in terms of surfactant recovery, volume reduction and enrichment. The theory demonstrated a highly constrained hydrodynamic process that is predominately one-dimensional and limited to two practical control parameters: the selection of the bubble size and gas flux employed. A further degree of freedom in the drainage of the interstitial liquid held in the foam was introduced by the inclined channels, thus permitting “drier” foam to leave the system. Experiments showed the inclined channels delivered up to a 4 fold increase in enrichment over foam flowing in the vertical direction. By increasing the residence time in the system, a 21 fold improvement in enrichment was obtained. Under certain conditions the flow rate of foam through the individual channels was found to fluctuate and transition between no flow and excessive flow. The fluctuation in flow was found to be dampened using a compressible-inflatable capacitor. The processing advantage given to foam fractionation by the inclined channels became negligible, in terms of the volume reduction and enrichment achieved, when using high gas fluxes. However, in terms of the liquid flux leaving the inclined channels, enhanced bubble-liquid segregation remained evident. Thus the second part of the thesis is concerned with operations involving lower bubble volume fractions, with a focus on bubble retention in bubbly-liquid flows rather than liquid rejection from dry, high bubble volume fraction foam. Here, an inverted fluidized bed approach was used by inverting the Reflux Classifier and applying a downwards flow of fluidization water to produce a bubble-liquid mixture. The inclined channels beneath the fluidized bed of bubbles acted to segregate the bubbles from the downwards flow of liquid, thus preventing their loss to the underflow. A concentrated bubbly zone, essentially a spherical wet foam, above the inclined channels exhibited very high permeability, permitting very high and uniform fluidization water fluxes, an order of magnitude greater than wash water fluxes used in conventional froth flotation. Fine silica particles were used to model hydraulic entrainment of hydrophilic gangue material in the froth flotation process. More than 99 % of the silica was rejected from froth product using extreme fluidization water fluxes, and gas fluxes beyond the flooding condition. Furthermore, bubble surface fluxes, S_b, as high as 140 s^-1 were obtained using very fine bubbles of order 350 μm. By comparison conventional flotation systems are constrained to an optimum bubble diameter of about 1 mm and a S_b generally below 60 s^-1 in order to avoid the flooding condition (Yianatos and Henríquez, 2007, Wace et al., 1968). An interesting theoretical conundrum was revealed when considering the placement of the fluidization water boundary condition. The conundrum was examined using drift flux theory. When the wash water was added at the free-surface the process is said to be flux curve constrained when using relatively high gas fluxes. In this case the wash water is transferred directly to the overflow with little or no benefit to the process. At lower gases fluxes, the process becomes flux curve unconstrained and a portion of the wash water is transferred down through the froth. However, when wash water is injected an infinitesimal level below the free-surface, at any level in the froth, the added water is transferred downwards. Fluidization water distributed in the inverted Reflux Classifier, referred to the Reflux Flotation Cell (RFC), is shown to conform to the latter theoretical solution