1. Introduction
Froth flotation is a method that is used to separate useful minerals and gangue by utilizing air bubbles to carry solid particles upward, which provides an effective method for sorting various complex refractory ores [
1]. The success of a flotation process benefits from the provision of an appropriate fluid dynamics environment and the perfect particle suspension by the flotation equipment [
2].
Particle suspension or mixing not only determines the method of interaction between particles and bubbles but also has an important impact on entrainment; therefore, it has always been a concern in laboratory and industrial flotation machines. Manjunath and Sanja investigated particle behavior in a flotation tank with a 457 mm diameter using electrical resistance tomography (ERT) and found that the impeller size and off-bottom clearance had significant influences on the particle suspension characteristics [
3]. In Zheng et al.’s research, a mathematical model for solids suspension was proposed to evaluate the degree of entrainment in some flotation cells [
4]. Moreover, they found that experimental data collected from Outokumpu tank flotation cells of three different sizes (3, 100, and 150 m
3) were fitted using the proposed exponential model, and the solids content in the top region was dependent on the cell size. Yianatos found that the short-term mixing in a 130 m
3 industrial flotation machine could evaluate the probability of particle–bubble contact near the impeller zone [
5], which obtained a better understanding of how the mixing takes place in a large-sized flotation cell. In order to make the particles in different tank sizes have similar suspension effects, the critical suspension speed (N
js) has usually been used as the magnification parameter in the scale-up process of a flotation machine [
6,
7,
8].
In recent decades, with the rapid scale-up of flotation machines, the requirement for realizing efficient particle suspension inside their flotation cells has gradually increased [
9]. Generally speaking, solids suspension is superior in the impeller zone with the high-intensity turbulence but poorer in the upper static zone. Therefore, solids suspension must be taken into consideration when the laboratory flotation machine with high turbulence is expanded to industrial cells with less turbulence.
Solid particles concentrate near the bottom of the flotation cell in a flotation machine [
10], and some of them that are not adsorbed by the bubbles will be sucked into the impeller agitation region by the lower circulation driving. During this process, most of the hydrophobic particles will rise to the foam zone along with the bubbles, whereas the hydrophilic particles will fall to the bottom of the tank and gradually accumulate over time. Concurrently, some coarse mineral particles will tend to be mixed in the accumulated materials, and thus, will not be captured by the bubbles. The collision and adsorption processes between the solid particles and the bubbles can occur only when the particles are fully suspended [
3]. Fundamentally, solid particle suspension in a flotation machine depends on the flow characteristics at the clearance between the impeller bottom and the bottom of the flotation tank [
11].
Until now, a relatively limited number of studies have been published on the effect of the structural parameters on the solids suspension in a flotation machine compared to the research on stirred tanks. It is worth noting that the structures of a flotation machine and a stirred tank are similar, except for the presence of a stator in the former. Although a stator weakens the rotational strength of the liquid flow in a flotation machine to a certain extent, the loop structures of the upper and lower circulation formed by a flotation machine and a stirred tank are almost identical. Therefore, optimization of the flow field characteristics in a flotation machine can be realized based on the influence of its structural parameters on the flow field characteristics in a stirred tank.
Over the last 20 years, researchers have focused on studying the influence of different structural parameters on the N
js in a stirred tank. Armemante et al. [
12] found decreasing the impeller off-bottom clearance can significantly reduce N
js when using a radial impeller. Montante et al. [
13] first discovered that the flow pattern in a stirred tank showed a transition from a double loop to a single loop as the impeller off-bottom clearance decreased. A similar experimental phenomenon was also reported by Galletti et al. [
14]. Further studies by Ochieng et al. [
15] and Zhu et al. [
16] suggested that a single-loop flow can shorten the mixing time of the materials and reduce the energy consumption in a stirred tank. On this basis, Basavarajappa et al. [
17] compared the influence of a Rushton turbine and a flotation impeller on the flow characteristics in a lab-scale stirred tank. They suggested that the abovementioned flow pattern transition from a double-loop to a single-loop flow can also be observed using a flotation impeller. Moreover, they found that the flow pattern is mainly affected by the impeller off-bottom clearance and the impeller diameter and that an impeller with a larger diameter is more prone to promote the flow pattern transition. These studies indicate that the structural parameters of an impeller are critical to the flow pattern of the fluid flow characteristics and that the flow pattern transition has a further impact on the particle suspension and mixing in a stirred tank. However, for flotation machines, there is still a lack of research on the influencing factors of local flow pattern transitions and the mechanism of different flow patterns for solid particle suspension.
Here, a brief analysis of the motion characteristics of the bottom flow in a flotation machine is provided. The impeller in a flotation machine acts like a partially open pump [
18]. The bottom liquid flow is continuously sucked into the impeller because of the pressure drop in the center of the impeller. Under the action of agitation and pumping of the impeller, the bottom flow forms an ascending spiral swirl. The axial velocity directly reflects the pumping capacity of an impeller. The increase in the axial velocity can increase the impeller circulation volume [
19]. The tangential velocity of a swirl tends to initially increase and subsequently decrease in a radially outward direction [
20], and its direction depends on the rotation direction of the impeller. Until now, there are few reported studies on the radial flow velocity near the bottom of a flotation machine. However, considering the complex three-dimensional nature of this bottom flow, clearly, the radial velocity cannot be ignored.
Figure 1 shows the effect of the radial velocity on the bottom rising vortex.
In an ideal case, a fluid particle rising along a spiral path is assumed to be below the impeller. As shown in
Figure 1a, the fluid particle undergoes a centripetal spiral ascension motion, with the direction of the radial velocity being toward the axis. In contrast, as shown in
Figure 1b, when the direction of the radial velocity is away from the axis, the fluid particle presents a centrifugal spiral ascension motion. The difference in the flow pattern determines the trajectory of the particles to a certain extent, which will affect the suspension and the mixing of the particles near the bottom. Actually, fluid particles do not necessarily move around the axis. Hadane et al. investigated the influence of a stator on the flow field characteristics of a flotation machine using computational fluid dynamics (CFD) simulations [
20]. It was observed from their simulation results that the rotational motion apparently disintegrates near the stator in the tank center and that the center of the vortex formed below the impeller also deviates from the axis. Clearly, the center of an ascending vortex spiral is dominated by the vertical ascending motion when the main suction region is located at the bottom of the impeller. The misalignment of the center of the vortex and the tank center suggests that the main suction position deviated from the center of the impeller, which will be detrimental to the uniform dispersion of the particles. This phenomenon might be caused by the instability of the bottom flow field. Concurrently, the bottom flow instability adds complexity to the measurement of the hydrodynamic parameters inside a flotation machine. Theoretically, the radial velocity at the center of a vortex is zero. Once the flow at the axial center has a radial velocity, it can be considered that the center of the vortex is not coincident with the axis, i.e., the bottom flow is unstable.
In order to obtain the flow field information of the fluid, optical techniques, such as laser doppler anemometer (LDA) and particle image velocimetry (PIV), are generally adopted [
21]. As they are non-intrusive measurement devices, LDA and PIV will not affect the flow pattern of the fluid; therefore, higher measurement accuracy can be obtained. Darelius et al. [
22] used an LDA to measure the distribution of velocity and turbulence characteristics in a high shear mixer and found that the axial velocity component near the wall region was directed upward and the fluctuating velocities were at the same level in the tangential and axial directions. Kysela et al. [
23] also utilized an LDA to measure the mean axial ensemble-averaged velocity and analyzed the turbulence energy spectrum for three impeller speeds. Darabi et al. [
24] investigated the influence of cell geometry on the flow pattern and turbulence characteristics in a lab-scale Denver flotation machine based on the high-speed stereo-PIV technique. Furthermore, the flow velocities measured using LDA and PIV are often utilized to verify the numerical simulation of fluid inside flotation machines [
19,
25,
26]. Although LDA and PIV have been proven to be reliable for the measurement of the flow velocity of a liquid phase system, it cannot be applied to measure non-transparent systems, such as a high concentration slurry in the flotation machine.
As a typical representative of aerating mechanically agitated flotation machines, KYF (Kuang Yuan Flotation) flotation machines have been extensively used in mineral processing plants [
27]. Compared to the tank designs of cylindrical and inverted cones [
26,
28], a U-shaped tank structure is adopted in a KYF flotation machine to enhance the circulation of the bottom pulp. This unique U-shaped tank significantly reduces the bottom space of the flotation machine, which allows the particles to accumulate at the center of the tank bottom. Therefore, the flow characteristics at the impeller suction area play a key role in particle suspension.
In this study, the flow velocities of the fluid near the bottom of a lab-scale KYF flotation machine were measured using an LDA to investigate the influence of the impeller diameter and the impeller off-bottom clearance on the flow velocity distribution. Based on the experimental results, the flow patterns at different impeller parameters were analyzed. Moreover, the difference in several flow hydrodynamics parameters (vortex stability, radial and axial velocities, velocity vectors, and axial fluctuating root mean square (RMS) velocity) were discussed to identify the more advantageous parameters for a dispersed phase suspension.
4. Conclusions
(1) The present study focused on the flow velocity distribution near the bottom of a KYF flotation machine to obtain the detailed flow hydrodynamics parameter variation with impeller variables. Centripetal and centrifugal vortices, as well as transitional spiral ascending vortices, were observed in different cases of the impeller diameter and the impeller off-bottom clearance, respectively.
(2) The impeller diameter and the off-bottom clearance were found to have significant effects on the flow patterns, radial and axial velocities, velocity vector distribution, and axial fluctuating RMS velocity characteristics. In the case of a large impeller diameter or a small off-bottom clearance, the fluid flow passage at the lower circulation inside the flotation machine shrunk. This resulted in a remarkable improvement in the vortex stability, which was beneficial for producing a uniform solid particles suspension.
(3) The axial velocity of the fluid near the bottom inside the flotation machine was higher than the radial velocity, particularly in the case of a large impeller diameter and a large off-bottom clearance. However, when the impeller off-bottom clearance was increased, the velocity amplification effect was not as remarkable as that caused by the impeller diameter increase. Moreover, the high-velocity area located near the impeller bottom was notably extended in the case of a large off-bottom clearance, which will cause inevitable damage to impeller blades.
(4) The low-velocity region distributed at the suction zone inside the flotation machine weakened the solid particle suspension. It varied with the flow patterns. When a centripetal flow was generated at a large impeller diameter and a small off-bottom clearance, the low-velocity distribution region area was significantly reduced near the bottom center.
(5) The fluctuating component of the turbulence, namely, the axial RMS velocity, was higher in the centripetal flow than that in the centrifugal flow. The distribution area of the high axial RMS velocity was broader and more consistent in the centripetal flow generated with a large impeller diameter and a small off-bottom clearance. Thus, forming a centripetal flow near the bottom inside the flotation machine via the adjustment of the impeller variables is of extreme importance for the suspension of the solid particles, as well as the dispersion of air bubbles.