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    Topic review

    Froth Flotation

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    Submitted by: Yao Ning

    Definition

    Froth flotation is an important operation for the recovery of valuable minerals in the mining industry. The essence of flotation processes lies in the interactions between particles and bubbles, which can be categorized into three successive sub-processes which are bubble-particle collision, attachment and detachment. These three sub-processes determine the overall performance of flotation processes for treating a particular ore.

    1. Introduction

    There are limitations of flotation operation in treating particles of different sizes [1][2]. This size limitation can be varied for different type of ores and flotation works well for particles of base metal ores in the range 20 to 150 μm in diameter [3]. Recovery of fine and coarse particles declines progressively and underlying reasons are different depending particle size [4].

    Fine particles with small inertia will more likely follow streamline and therefore have lower chance of colliding with a bubble [5][6]. Though this is not a problem for coarse particle flotation, coarse particles with higher inertia would more likely detach with modest disturbances from surrounding liquid motion [7]. Therefore, studying interactions between particles and bubbles are fundamental to improve flotation performance. It is unavoidable that fine particles are produced in the grinding processes for the beneficiation of minerals [8]. Moreover, it is desirable to treat coarse particles using flotation for the early rejection of gangue as large amount of energy can be saved in the comminution processes [9]. It is highly necessary for studying particle-bubble interactions under the effects of turbulent flows as they are limiting factors in determining flotation performance [10][11].

    Studies have been directed towards expanding size limit [12][13] and manipulating particle-bubble capture [14]. Studying particle-bubble interactions in turbulence is key to expand size limit and improve flotation performance. We have done a systematic endeavor to understand the effects of turbulence, especially on particle-bubble collision and particle detachment. Critical literature reviews on separate sub-processes have been presented on particle-bubble collisions [15][16], attachment [17][18][19] and detachment [20]. It is desirable to combine these three sub-processes together and consider the effects of turbulence on the flotation process as a whole. The effects of turbulence on the particle-bubble interactions in general is considered to be a stochastic process [21]. It can be attributed to the dynamic interactions between particles or bubbles with the turbulent eddies in flotation environment. On one side, hydrodynamics in flotation machines need to be characterized [22], and on the other side, how turbulence control sub-processes in flotation machines should be examined [11]. In this way, hydrodynamics in flotation machines can be optimized to improve flotation performance [22]. Though some classical work in old days have been cited, we aim to collect literature in this century.

    2. Particle-Bubble Collision, Attachment and Detachment

    2.1. Effects of Turbulence on Collision

    Due to the limitations of current experimental techniques for studying particle-bubble collisions in turbulent flows, to the best of authors’ knowledge, there is no experimental work reported on the collisions between particles and bubbles in turbulent flotation processes. Simplified experiments were designed to study particle-bubble collision efficiency where single bubbles rose in the quiescent slurry [23][24]. Single bubble flotation experiments were designed to study. Particles were considered to stay attached to bubbles upon collision and collision efficiency could be represented by particle collection efficiency. Particle-bubble collision was directly observed using a high-speed camera [25][26][27][28]. Collisions were represented by direct observations of the falling particles around a positioned bubble. It should be mentioned that these experimental studies did not consider the effects of turbulence on particle-bubble collisions.
    Most recent studies used computational fluid dynamics (CFD) to study the effect of turbulence on collision efficiency in flotation [29][30][31]. A single bubble was considered either rise at constant velocity or stationary. The Reynolds averaged Navier–Stokes (RANS) model was used to simulate turbulence. Note that this model cannot simulate small turbulent flow structures in the scale of bubbles or particles. To study the effect of turbulence on the collisions between particles, a direct numerical simulation (DNS) model was built in our group [32][33]. Collisions between a group of particles and bubbles were studied in a forced homogeneous isotropic turbulence (HIT) as is shown in Figure 1. The turbulence intensity of this background flow was modulated in terms of turbulent energy dissipation rate. Due to most collision models were developed for quiescent liquid, collisions between particles and bubbles in quiescent liquid were simulated and compared to the predictions calculated using widely accept collision models [33]. Results showed collision efficiency as a function of particle diameter and bubble diameter. The collision efficiency between particles and bubbles decreased with increasing bubble diameter and increased with increasing particle diameter, which was in accordance with the general trends of model predictions. In the study of particle-bubble collisions in HIT turbulence, the effects of preferential concentration of particles and bubbles on collision were studied. The collision kernel was found to increase with turbulent dissipation rate as is shown in Figure 2. This was due to the increase of increasing radial relative velocity in more turbulent flows. The effects of preferential concentrations on the particle-bubble collision kernel were ineffective compared to the effects of radial relative velocity.

    This entry is adapted from 10.3390/min11091006

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