Fluidized bed desliming in fine particle flotation – Part I

Abstract

This is the first of a series of publications concerned with a novel system that transforms the hydrodynamics of flotation. This system, referred to as a Reflux Flotation Cell, consists of a vertical flotation zone, with a system of parallel inclined channels below. The system is enclosed at the top by a fluidization distributor, while a central port is used to discharge the overflow product. The inclined channels located below the vertical section enhance the segregation of the bubbles from the tailings flow, permitting separations to be conducted at bubble surface fluxes well beyond the normal flooding condition, while also permitting extreme wash water fluxes. The system hydrodynamics produces spherical bubbly-foam, with a bubble volume fraction of order 0.5, ideal for counter-current washing, and hence desliming.This paper addresses two objectives. The first concerns the fluidization boundary condition at the top of the device. We identify for the first time a conundrum that arises when Drift Flux theory and fluidization theory are used to describe the effect of wash water addition in flotation. A subtle but nevertheless significant change in the predicted bias flux arises when the system is formally fluidized, resulting in the wash water reporting with the overflow, and hence failing to provide the desired desliming. Our experimental work, however, demonstrated that the applied fluidization leads to strong positive bias, with a downwards liquid flux and in turn powerful desliming of hydrophilic particles. Indeed the system behaved as though the wash water was introduced below rather than at the upper boundary.The second, and most important objective was to assess the system hydrodynamics with respect to extreme gas and wash water fluxes using firstly a particle-free system, and secondly assess the desliming achievable using a system containing hydrophilic particles. Thus in Part I the system was free of hydrophobic particles. The enhanced bubble–liquid segregation arising from the system of inclined channels permitted very high gas fluxes, sufficient to achieve a bubble surface flux of 144m2/m2s, well beyond the theoretical flooding limit of ~100m2/m2s (Wace et al., 1968). This high bubble surface flux was especially significant given this occurred during the application of extreme bias fluxes, as high as 2.5cm/s passing downwards. Experiments involving a silica feed were used to quantify the performance of the desliming, covering extreme gas and fluidization (wash) water fluxes. Silica rejection from the product exceeded 99%.